BaIlY. College
, IR Part-66 Aircraft Maintenance Licence
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ICAT
nternational Centre for Aerospace Training
Barry College
ICAT International Centre for Aerospace Training
Cardiff .Airport Business Park Pon Road Rhoose Vale ofGlamorgan
CF62 3DP +4-1(0) 1••67 1982 1
[email protected]
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In the L' nUCll Kingdom
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90 TOl1cnh:tm COUrI Road. l.ondon \,'rr -4LP.
Liabiliry
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Whilst [h~~kc and infom\adon in this book are bcliC'.'c:d 10 be: true and aCCUl1l.lC at ~lrrrof going to press, neither the author !lor the publishcr can accept any • '?l';\~(\ h:b,l responsibility or liability for any enun or omissions that may be madc . ,
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Author: Chris Strike
Barry College
ICAT International Centre for Aeros pace Train ing
IR Part-66
Module 7
Maintenance Practices Knowledge Levels The basic knowledge requirements for cltegories A, B l and B2 certifying staff are indicated in the contents list of the module notes by knowledge leyci indicators 1,2 or 3 against each chapter.
Level 1 A familiarisation \",th the principal elements of the subject
Objectives The student should be familiar with [he basic elem ents of the subject. The student should be able to give a simple description o f tbe whole subject using cornmon words and examples. 1.1\1l~1e1;'tl.~dent should be able to use typical terms .
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Level 2 .A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. r ,
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Obj ectives
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The student should be able ro understand the theoretical fundamentals of the subject. The student should be able to gi,c a general description o f the subject using, as ;? appropriate, typical examples. .~ The student should be able to use mathematical formulae in conjunction with 6: p hysical laws describing the subject. :R::::': The student should be able to read and understand sketches, drawings and (~ schematics describing the subject. ,.ff' The student should be able to apply knowledge In a practical manner usin.!h: detailed procedures.
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Barry College
ICAT Intemational Centre for Aerospace Trainmg
Level 3 A detailed knowledge of the theoretical and practical aspects of rhe subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensi\-e manner.
Objectives The smdenr should know the theory of the sub ject and interrelationships with other subjects. The student should be able to gi"e a detailed description of the subject using theoretical fundamentals and specific examples. The smdent should understand and be able to use mathematical formulae related to the subject. The srudenr should be able to read, understand and prepare sketches, simple dra\vings and schematics describing the subjecL
Contents - Volume 1 Chapter Zero - Forward F orwarde. '1C\l,b
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, ,?>,<'Chapter One - Safety Precautions - Aircraft & ."?,, - Workshop ......,,,,
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Knowledge Level: A - 3, Bl- 3, B2 - 3 J lazards Inrroduction ............................................ . E-\SA Part-i45 Requirements .................... . Accidents Hazard & Risk
............ ..
Recognising I fazards Aecidenr Prc\'ention ............................ . ................ .. Health & Safety al \'(/ork Act (1974) Employer's Dury ............................. . ............... . Employee's Dury Safety Representatives & Committees Regulations ........ . Health & Safery Commission ................... . Health & SafelY Executive ................ . Associarc::d Regulacions ................................... .
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3 3 4 4 4
5 6 7 8 10 10 11
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Barry College
ICAT Ir1!emational Centre for Aerospace Training
Safet... Precautions - Aircrnf[ Airpon .-\pron Safcty Airpon Lighring Vehicles Noise Jet Blast . .. . ........ _... ..... . .......... . Fuelling Zones .... _. . . . _....... _.. . Apron Control Foreign Object D amage .... _....... _.. _............... . Line Maintcnance ......... . General Precautions - \'('orking in and Around Aircraft D ress .. . . . . . .. . ........ _...... . Elcctrical SafelY ........... . Compressed Gases - Airc raft & Workshop Storage of Compressed Gas C~'linders Precautions when Charging Gaseous Systems Compressed Air ........ . ........................ . Aviation Breathing O:'>ygen Chemical O:'>ygcn Generators ......... . Nitrogen Helium .... Acetylcne
Oils Turbine Engine Oils ... . .... .. .................... . ................. . Reciprocating Engine Oils Hydraulic Auid .. . . . . . ..... . ............................. . "1 )o.lr-ubricating O ils . . . ............. .. . .. ....................... . .,\."\6(.heci\lcals . . . . . . . . . . . . . .. . . .... . ... . ...... . . . , l" .(')\\C~e.'" Flammable Chemicals . .......... . _._ \" ;:"~~~ CorrosiYe ChCll1lcals . ~... ;,. Toxic ChcmICals .... ......... . ............... . ,
Fire O:1ssificatioll of Fires Exunguishants
13 13 14 15 16 17 18 19 20 21 21 23 23 26 27 ?_I
28 29 31 31 '? >-
32 33 33 34 34 35 35 36 36 37 38 39 41 41 42 42 43 43
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48 50 50 51
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Barry College
ICAT International Centre for Aerospace Training
Dcalingwlth Emergencies ... • _•• ___ .. _........ ..... ....... . Fire ... ....... ........ .... . .......... ....... ..... ........ .
i'.lultip!e Casualties .. ...... ......... .................. . ~Iultiple Injunes.......................... . ............... . Elecmc Shock ........... .... ............................. . Shock from I I1gh Vollage Lines .................. ........ . .".ccidenrs !m'ol,-tng Dangerous Substances ........ . POIsoning ...................... . ............ . Sel'ere Burns & Scald s .................. . .......... .. Shock .. ......... ..... . ................... . Summoning .\ssisrance
Revision
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54
55 57 57 57 58 58 58 59 59
60 62
Chapter Two - Workshop Practices Knowledge Level: A - 3, B1 - 3, B2 - 3 Tools Care o f Tools COlltrol of Tools Shadow Boards ...................................... . Centralised Tool Stores .............. . r-olamtenancc Day Tools & Eqwpmenl ..\U'Crafr M:umenance Bay Control Carriage of Tools ......... .. Lost Tool5.,\\( ... ....... .............. .. ................ . ~1Si\ l)arl-1 45 ............................................ .
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U(e--* r\X'orkshop r-.latcrials
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Certification of Materials ............................ , .. , .. . 5 Quaranonc lOre .................................. . Bonded SlOres .... . . . . . . . . .. ...... .. . ... . . . . ........ . IdencifiCllljoll ............ . . ..................... . Proof of Conformity ................................... . Inspection & Test Starus ... . ............................... . I-leal TrcaUllent of Metallic ~ [alenals ................. . Dimensions, .Allowances & Tolerances
Dimensions Allowance Tolerances Standards of \'{Iorkmanship .......................... . Standards ........ .................................... .
67 67 69
70 71
72 72
72
73 73 73 7-1
74 74 75 75 75 76 77 77 78 79
80 80
Barry College
ICAT Intern ational Centre for Aerospace Training
Calibration of Tools & Equipment, Calibration Standards Accurac\' & P reoSlOn in i\[easuremenl Measurement Error Calibration l\Icasurement Standards Physical Standa rds Narural Standards Variables Attributes ..... .... . National Ph~;slca l Laboratory (NFL) International Orgamsation for Standardisation (ISO) .......... . .. ......... .. . . . ...... . Traceability Calibration Rooms NA!\LA.S Quality Systems Calibration ReqUlfemenrs ..... . ..... ...... . ..... ... .
Revision
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82 82
83 83 83
84 85 85 85 85 85
86 87 87 87 87 90
Chapter Three - Tools Knowledge Level: A - 3, Bl- 3, B2 - 3 Gripping 'rools Bench Vice Hand Vice . ........................ ....... ... . :r oolmaker's Clamp ........ . ,('II.', . ')\"'ICl6 ~ .. Pliers . . . . . . . . ....... . ........... .
co\\c'?:-':''' Cutting & Shaping Tools .= ~",\\'l
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Steel Hammers ... ......... . Soft Faced Hammers CIUKS ' I ......................... . Hacksaws . ... . . .. . .... . . .. .... . ri k~ .. . ....... . .................. .... ..... .
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D rills TWISl Drill O ther Metal Drills \'\food Drill Bits . ............. . ..... . Countersinking Tools Counter-boring Tools Reamers
T hread Fanning Taps Dies ........ ....... . . . SUlci Fitting & RemO\·al Tool> Scrapers Snips
93 93
95 96 96 99 99 100 101 103 105 109 109 113 115 115 116 116 119 1'19
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ICAT International Cenlre
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Dism:lnding & Assembly ' J'ools Screwdri,'crs ...........................••..•.. Punches Drifts Spanners .................................... . Torque Lllnlllng \,\'renches .................. . Torque Indicaung Wrenches ................... . Torque Wrench .\da pto rs ............. .
l\larking-Out & l\lcasuring Tools
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Steel Rule~ ......................... . . . ........ . Spnng Di\'lders ................ . Scriber .................... . Scnblng Block ............ . Finer's Square ........................ . ........ .. .. . Feeler Gauges ............................... . Cahpers ................................................. . Trammel Combinanon SCI Surfacc Tables V· Blocks
Common Power Tools Drillmg fo.ladunes ............... ................ .. General Purpose ,\ir-Operated Dnlls .. .. .......... . Off-ScI & /\nglcd I le:ld Drills ........................ . Dnll AU:lchmenlS ......... ... ............ . ........................ . .............. .. _.. _ Rotary Saw S!lears '':'1 \I.'r ······· .
N'ilW' ." ............. ................. ........... .. ....... .
_ o\\i:~ nndillg i\ ladunc ................................... . . ~_;\~
~eracion & Use o f Precision l\reasuring Tools ,.\.'" Dial Gauge .................................. .... .. :\~,"'\ ~. . . ' The Vernier System ................................. .. " I... The Cm\'crsal "crruer Ca.lJper Gauge ................. . \'erruer Ilctght Gauge ................................... . Vernier Depth Gauge ................................... . Vernier Be\'e! Prouaclor ................................... . r-.Iicromcler Calipers ............................................ . External i\·!JcrOlllclers i\!Jcromctcr 1 leads TIle Imcrnal l\llcrolllclcr USing the Mlcrometcr ...................... .. Dead \'\'eighrTeslcr ................................... . Non-Adjustable Gauges ........................... .
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Lub rication Equipment & r-. rcthods
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Lubricating Oil ............................................ . Grease ..................................................... . Lubrication RequltemelHs ................. ...... . Methods of Lubrication ................ . .................. .
124 12·' 126 128 129 134 138 139 141 141 141 141
1·1:2 142 143 143 14-1 14·1 [..]5
14G 147 147 147 148 148 1·18 149 149
150 151 1-'
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182 182 184
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ICAT Intemational Cenlre for Aerospace Training
Equipment
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Operation, Function & Use of Electrical General Test Equi pment l\'ioving Coil Instrwnents Basic (Horseshoe l'\lagnet) lnstnltllent ............ . . Cylindrical i\fagn et l'\loying Coil Instrument Long-Scale i\!o\-ing Coil Instrument Volunelers & .r\mmeters .. Monng Coil Voltmeters Moving Coil Ammeter Converung to AC l\-lcasuremcnt ... ...... . Mo'\ing Iron Instruments ........... . .. ........ . Rep ulsion Type Instrumem A rtraetion Type Instrumenr . ........ . Ohmmeters ... ....... .... ........ . Series Ohnuncters Shunt Oluruneters Ratiometcr Ohnlll1cters Insulation-Resistance Testers l\ legger ..... ........ . .......... .. ...... .. .. High Tension Ignition Tester .... ........... . Safet), Ohmmeter (Insulation ReSIstance Type) Low Range Obmmeters ........ . ....... . . ................ . . Bonding Tester .... Safety Ohmmeter (Continuity ReSIstance Type) ........... .... . Bridge l\leggcr Testers E lcctro(h'namometer (\'X·arum:.ter) :-IX estmetcrs & l\'I ultimelers '10llD . . I l\ I u Inmeters . ,)(\06 - - ' l~la . (,o\\e,?::-i! -- Odler Measurement Devices .. \'\~t~~ Potentiometers -h Hot \'X'ire fnsuu mems . ~,\,'I. ,:.,<..y.~ 111enno-Couple I nstrumems U~~· Electro-Static Voluneter . Transfonncr Rectifier l\leter Tong 'fest Ammeter Oscilloscopes ........................ ..... . •••.. . ••• TinIC Domam ReOectometer ConclUSIOn Re"ision
185 188 189 189
192 193 193 194 19 5 196 197
197 198 199
200 202 203 205 205
208 209
210 210 211
212 217 218
222 223 223 224
225 ?? --?
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Barry College
ICAT International Centre for Ae rospace Training
Chapter Four - Avionic General Test Equipment Knowledge Level: Bl- 2, B2 - 3
2 . \ " ) ) ....
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Ponable Air Data Test SCI Q Codes ............. . Pilot/Stacie System Leak Tests .A.!cirude Leak Test Airspeed Leak Test Altirude & Airspeed Switch Tests . Altirude Switch Test ........ . Airspeed S\\;tch Test ............... ............... ..... . Vertical Speed Test ..... ' .... . Air Data Test Set ........... . StanC Tests Pilot System Tests .......... . Combined PiLOt/ Slatic System Tests Indicated & True I\irspeed Mach Test & Consta nt Mach Te sl Engine Pressure Ratio (EPR) Tmnsponder/ Or..IE/ TCAS Test Set General DescriptiO!1 o f Test Set .. . Transponder . . . . . . . ...... .... .... . ... .. .. ... ..... . Transponder Tests ................................... . Distance Measuring E'Iwpmem (Dl\1..E) .................• DME Test~lC . ... ... ........... ...... ............. . T qffil:!Afb.\t & Collision 1\ \'oidancc System erC.\S) . o\\C.g.C\S Tests . . . . . ............. ........... . ........ . <;V·bR/ lLS, VI-IF Communications T cst Set ........... .... ..... .
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Gencr:J.i D escription of Test Set ........ . Marker Beacons .... ......... .
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Localiser Glide Slope (G / S) VHF Omni-Directional Radio Range ('lOR) Tests ............. ................. ................... ....... . NAV COr..[J\! Test Set.... .......... ....... . .............. . VHF Commurucations Tesring ............. . VOR Testing ............... . .... ............... .
ILS Testing
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Fuel Capacitance/Quantity Test Set
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Test Set... ........ ....................... .... .. Calibr:J.t.ion
Conclusion Revision .. ...... .
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24-t 2-t6 2-t6 2-t7 2-t8 2-t9 249 250 251 251 253 254 1"'
-" 255 ?--" 256
256 257 258 259 261 262 263 263 26-t 26-t 265 265 266
266 266 267 267 267 268 268 270 271 271 272
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ICAT International Centre for AerospaCl!! Training
Chapter Five - Engineering Drawings, Diagrams & Standards Knowledge Level: A - 1, B1 - 2, B2 - 2
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Drawing Drawing Sheet Sizes Dra\ving Formats Pictorial Views Orthographic Projection First Angle Projection Third Angle Projection Views Types of Drawing ............. ........................ ' .... . .~crafl Dra\vings .. ,. . . . . . . . . . . . . . .. . .. , .. ' ..... .. Scale ....... . .. ".. .. , .............................. . Lines Dimensioning Dimensioning from a Darum Dimensioning Curvcs DimensIOning Tapers Tolerances Geometric T olcranccs .......................... . l\fachining & Surface Texture Symbols .......... . Com'eOlional Representations .......... . Abbreviations & Symbols ,l?rocess & Identification i\larkings '1(\l\ r , , '1 \)\)6 - - Dra\\"lng Amhont)' .................... .... ..... .. :;..\e,?-C Identifying Title Block Information 0 J\{icrofllm, t 'ficrofichc & Computerised Presentations .. . l\ficroGlrn ................................. . Microfiche Computcnsed Presentation ............ . Specification 100 of the ATA of :\mecca ...... . ................... . . \TA Spec 100 Zoning System ......................... .. Station Kumbenng ............................ ' Aeronautical & Othcr Applicable Standa rd s ..... ' ........... . A"iation Standards ............................ . Standards for Materials & Components ........... . "('iring & Schematic Diagrams ............. . Circuit Diagram .... ........... ...... ................. . SdlClnallC Diagrams ................................... . Location Diagrams .... ................ . Wiring Diagrams ... " , ........ . \\firing Diagram i\Ianual .................. " ........... .. Symbols .'.,......... .... ........ ... ...... , .. ... . Hydraulic System Drawings ............ "', .......... .
Revision
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277 278 278 279 280 282 283 284 285 287 289 290 292 293 294 295 295 296 298 299 301 302 302 304 308 308 309 309 310 >; 317 318 320 "0 321 0'?? __ -~ 324 .3 324 0 324 J? 3?_0 '0 3'_0 327 OJ 327 ~ 3305' 332 ~.
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Barry College
ICAT tnlerna!lOflal Centre
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Chapter Six - Fits & Clearances Knowledge Level: A - 1, B1 - 2, B2 - 1 Firs & Clearances ...................................... . Dnll Sizes for Bolt Iioles ................................... . Tap Drill Size Tables ............................. . Classes of Fits (threaded fasteners) ·Iluead Inserts Fastener Clearance I loles Classes of Fit ................... . Common sy~tems of Fits & Clearances .................. Newall Sr~tem orl...i.mirs & Fit ........................... Shnnk & Contraction Fits .................... . . . . . .. Boosh Standards SrStem ................................ Schedule of Fits & Clt:arances for ;\U"craft & Engmes Limits for Bow, Twist & \'·ear .................................... Bowmg Lum\:> ............................................ .
Twist ...... ... .......................................... . \'(/car Standard ~Icthods for C hecking Pans ............. .... Lun!1 CaUSing ..... ......... .................. ~\hgnmellt ClI(.'cks .................................... Run OUI Checks . . . ... . . . . . ......... ..... Revision
341 342 345 345
347 347 348 350 350 350 35 I 355 357 357 358
359 360 36 I 36 1 362
368
Chapte,{\\Se\}~h - Electrical Cables & Connectors "Ow. >;\!. ...
\",ID,owledge Level: A - 1, B1 - 2, B2 - 2 c
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Electrical Bonding Techniques ............................. . Bonding Conductors ...................•..•.••••••••.... Bonding of Metal.Urcraft .......................... . Bondmg of N"on·i-. letaUic ;\ucrafl ....................••..... Bond.mg. Refuelli ng/ De·Fuelling Operations ...• •••. • ................................•.•. Bondmg Connections Flc..xible Bonding Connectors ....................•.... Earth TCrn1inals ........................ . Main Earth System ReSistance Values Bond Tesling Circuit Testing - General Continuuy T esong .....•...... Milhvolt Drop Tesl .......................... . Insulation Rcsislanee Testing .......................... . l\[.illlmum Insulalion Resistance V:ducs ............. . •••. Functional TeSlS ............................................ . Fuses
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374 . 375 376 376
377 377 379
380 380 380 381 383 386 387
387 388 389 389
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Barry College
ICAT International Centre
lor Aerospace Traming
Aircraft Electrical Cable Classificaoons Conductor Pl:u..ing ................................... . Aircraft ElccUical Cable FaliUfes .......................... . Crimp Tools: Hand & Ilydraulic Crimped Connections In-Line Splices ..................................... . Preparing the Cable ........................ . I land C.rimpmg Tool ... .... . . . . ........ . ........ . .\/\IPRange .............................. ........ . I-I),drau!ic Operated Camping Tool .. .. .••....•• ..••• Testing of Crimp J oints .......................... . •. .. ..... Checking of Crimplllg Tools Connector Pm Remo\'aJ & Imeruon ....... . ••••.•• Remonl o f \\'ircd Comacts .......................... . Remonl & lnscruon of L'n-\'\ired Contacts Crimping Contacts ,\F Crimping Tools Jnscruon o f \\'ired COntacfS Tesung Contact Sockets .................... ....•.•.. .. •..• . Co-a.x.ia.I Cables ........... . Installation of Co-aXJaJ Cable Testing Co-a.Xlai Cables Fibre Opne Cable
\'\!iring Protection Tcchniclucs Introduction ................
. ...... .. _____ . .... . Cable Loommg & Loom Support ....... ....... .... ...... .. . Cable Rounng & Protection ........•••••••. Cable Clamps ................ .............••.... . . .. .... -,\\\' . . - Protecu\'e Slcenng Shielding ..... ..... . Conclusion
Revision
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390 39 1 39 1 392 393 39-1 39-1 395 3%
396 397 397
398 399 -100 400 -100 -101 -101 -402
-103 -103 -103 -103 -10-1 405 -106 -107 -108 -108 409
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ICAT International Centre for Aerospace Tralnmg
Chapter Eight - Riveting Knowledge Level: A - 1, B1 - 2 Ri"ctcd Joints ............. . Shear 5 m:ngth ..... .. .. ..... Beanng Sucnglh Shear Suength Vs Bcanng Strength Load Transfer ............................................ . ~pjoints .................. . Bun JOints .......... ............................... . Flange jouus ............................................ . Sphere of Influence ............................ . Ri"Ct Layoul TernlS ..........• . •................ Ri"ct PilCh & Spacing .... . ..... . ................ . Tools L:sed for Ri,'eting & Dimpling .................. . Action of a R1"CI Solid RI"CI Dimensions ................................... . Allowance ............................................ . Drilling Iiole Si7.e Countersinking ................. . . . .•••••• . .......... . .............. . ............... . Dimpling ............... ... ... .. ..... . . . . .. . . Sheet Crippmg Pins Jomtins Compounds & Sealants ......••••••••••• ,., ..•.... Jigs. Fboures & jury SUUts ............. . ............ .
Installing Solid "
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H ~~'&-c :l1'nlent
... .... ................ ...... .. . . . .......... . . ...)\\~'i1'i;,1C1 Riveting ........................ ....... ..• . ....... ••. lP . . . \").~(" ncumatJC Rwcllllg ............... . .. ......... ........ . _ ,,(\' ... Pneumatic Ri"cting '-Iammers ............ ... .... ...... . . ......;,...'I'iL:slIlg a H.weung Ilammer ..... .. ....... .. .......... . ................ .. \. " Reaction R1"cting Squeeze Ri\'cting ................. .. Ri\'CI Milling ................. _ .. .................... .. . Removing Solid Rl,'Cts ............. . ..................... . Removing Dome Ilcat! R,j,-ets ...................... . . . ........ . Removing Countcrsunk Rj"cts ....•..... Blind Ri veting ..... ....... ................. .............. ........ . G~
..................................................... .
Installing th'del R.lvets ................................... . Installing Chobcrl Rivets ............•....................... Installing Cherrylock Rj"crs .......................... . Inslalling I luck Rivets ........ ........................... . Tucker Pop & Imel! R..i,·cls ............ . I nSlalling Tucker Pop & I mc.x Rivcrs ................. . Removing Bllllt! Expansion Ri"cts ................. . ................. . Removing r.k-chanical Lock R.i\·ets
xii
415 4 16
·117 41 7 41 9 420 421 422 422 423 424 4'-) 4'-) ·126 426 427 -129 '129 431 432 433 43·1 434 435
435 436 437 437 438 439 440 4-11 442 442 443 443 444 447 449 450 450 451
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ICAT International Centre for Aerospace Training
Special Purpose Fasteners .. . . . .......... .. . ... . Installing lli·Shcru: RiYets ... . . ...... .... ...... ..... . lnst:llling III-Lok Bolts .... . .. .. . .. ......... . ....... . IIlST:lllingJo-Bolts ..................... . Installing I luck Bolts & ~A.\'delok Fasteners .. . . .... • Inst:llling Rinmts ........................... .. .. .. .. Ikmonng Specml Fasteners ................... . Inspectio n ...................... ............ ..... . .. ••••• . Inspecuo n o f Rj\'cted J oints Inspecung New Joints Inspecting Blind Ri\'ets Inspecung Special Fasteners POSt Repair Tests ................. . .... ...... . . Scheduled Inspection of Riyeled JOIlUS Rcyisio n .................... .............. ....... . .
454 -i34
455
-iSS -i.36 -i57
-iSS -iSS -iSS -iSS 461 461
-i62 -i62 464
Chapter Nine - Pipes & Hoses Knowledge Level: A - 1, B1 - 2 Rigid Pipe Materials
.... .............. .. .. ...... .. SiZe of Rigid Tubing .... . . ........•••. Rigid Pipe Bending . . .... .. .. ...... . Pipe Bending ~o\acll1nes ........... . Draw Bending Machines ....... .. ... . . . . . . . . .. .... ""'~,\\c . Benuulg ", '_, [ acIllIles ' .. . ' ompresslon ...... ••....•. Simple I land Bending ................ ...... .... .. Bend R.,d.ius & Method ........................ .. ........ . Fusible Allo\"5 Filling .......... . Rcmo\'lilg the Filler ................ .............. . .\!ternati\·c Pipe Bending Fillers ........ ..... . Po mio rung Bends ........... .
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Flaring Rigid Pipes
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Expandcr Conc Raring Tool .......................... . ............................ . Roller Flanng Tool Double Flaring ............ ... ......................... . Inspecting a Completed Rare ............. . Pipe Beading .... ........... ..... ......... ............. . [nspection & Testing of Aircraft Pipes & Hoses .... .... . lnspcct:ion of Locally r-.lanufactured Rigid Pipes ........ . Identification of 1llicraft Pipes .... .. .. .... . Testing Rigid Pipes . . . ... ....... . ....... . Inspcction of Jlls talled Rigid Pipes ... . ...... . Testing Installed Pipes
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Ai rcraft I-loses .................................................... . Inspection of Insralled I lose Assemblies ...... . Inspection of New Hose with Reusable End Fitungs Testing of :\lfcrnft Hoses ................... .. Installation & C lamping of Pipes .......................... . f nstaUauon of Rigid Pipes ...... ............... . Supporting Rlgid Pipes ................................... . Post lnsrallauon Tests Installing Flexible! loses Gcneral Installation POUlts Insrallauon ................... . Supporting Flcxlble I loses Post lnstallation Tes[s .......... . Insralling FleXible i\lclalhc Hoses...........................
Re\ision
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FOREWORD
Foreword \Velcome to Module Seven. '{ou could desccibe this module as being the sequel to ivlodule Si.'.;. In Module Six we were dealing predominantly with the theoretical aspect of the hardware used in aircraft engineering. In this module we will be dealing mainly with the practical use of that hard\vare. Knowledge is little more than a mental store of facts whilst understa nding has more to do with the ability to use those facts to solve problems. T hat tho ught coul d be a pretty good introduction to this module. The structure of the examination for this module will depend on the C ategory of licence examination you ha\"e applied for. Category A candidates will bc given 70 multi-choice questions and 2 essay questions (I'ime alIo\ved 90 minutes plus -1-0 minutes). Category B1 candidates will be given 80 multi-choice questions and 2 essay questio ns (Time allowed 100 minutes plus -1-0 minutes). Category B2 candida tes will be given 60 multi-choice questions and 2 essay questions (Time allowed 7S minutes plus -1-0 minutes). Note that whatevcr Category you take there will be 2 essay questions. I will include an Annex to dlls module where I will gi,-e you a few essay questions and some ideas on hO\\-" to set about answering them. Like il.lodule S1.'.;, t"llodule Se'·en is a big and \-aned subject that may, on occasions, ~ccm rather dull and heavy. If you need a self-study plan, may I sugg6tr a three-stage approach? In the first stage, set a deadline date to read ,,~ 1.0Qfh~ough the complete m<:>dulc once. Tf you get sruck at aoy point, no~e it and . CP\\e..,qUIckly move on. At !.his s rage, you can also make up a set of flip cards ,: \'~"" containing key points thaI you can use as a ready reference and guick revision ."I.~""\. tool. In the second stage, set another deadline to read through me module (,0"\':\ " again, a lirtle more carefully this time. Just as before, if you get stuck at any point, note it and then moye on. You arc likely to find that the re will not be so many notes this time . The final stage is to re~d through tbe complete mod{~e yet again, lhis time spending more time on the points you had previously not~ as being difficult. To achieve this srudy plan you need to start well before ad~ proposed examination date - three months would not go amiss . The proces&. \\rill ingrain facts in your memory and highlight areas where vou hay~ weaknesses so mat ym; can attend to them in good time. Your llip-"cards \>.;-iii. also senTe as a useful coffee break revision guide. Finallr, do make time < t9 practice answering those essay questions and do remember that you \vil}~e using a pencil and a rubber in the examination, not a word processor! Nf~ke sure that you can write legibly using these implements. ,§\.J ~
1 CQl!p ighf rg Bm?)· COlIl§ 2006 - 2007 All rights m m -rd
FOREWORD
The longest journey in a course of self-study c:tn be from :t comformble armchair to :t rable. You should not anempt to study in an amlchai r - you will f:tll asleep! Sit up :tt :t table, gi,-e yourself clear start and finish times and turn this into a daily routine. Set benchmarks for what you w:tnt 10 achieve in [enTIS of progress. \Vid, the preamble out of the way, ' ....e can concentrate on what this module is :til about. l lumans ha,-e always been fairly adept at employing lOols to help them o,'ercome the problems posed by their ,-arious undertakings. l\ \any of the hand tools that we use todar had their origins in ancient civilisations. The only obstacle to human ingenuity was the cboice of materials fl\"ail:tblc at any part.icular time. This was not such a big problem to Early i\lan because there were no long-haul pterodactyls around to worry about because Ihey had been rather abruptly withdrawn from seryice sixty-five million years earlier! I [owe\'er. Ancient Egyptians had to make do \\lith copper chisels to hew the giant component stone blocks of their pyramids. The chisel sharpening industry at th:t t time must ha'-e been a considernble one! Quire wh:n the pyramids wcre intended to achie,-c was, howe\-cr. a bit tOO obscure for us to comprehend. Today, we ha\-e an immense choice of materials and tools to assisr us in our task of getting all those cbeap charter flights [0 the hedonistic hotspots, which \VouJd have seemed an equally obscure undertaking to the Ancient Egyptians! \Vic will be smrting wi(h aircraft and workshop safety precautions, because humans also rend to be the most accident prone of species, \,\ 'e will then move 01110 the tools and equipment, that [hey im'ariably h:wc Ihe accidents wit h, before moving on to deal with the engineering processes and procedures used in aircraft maintenance. So, \\ithout further ado, ler's movc on [0 the first chapter.
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Safety Precautions - Aircraft & Workshop Introduction The title of this chapter clearly states that we are dealing with two safety related topics - the aircraft and the workshop . Although there ma:, be commonalities between the two, we must not make the mistake of concentrating wholly on the latter at the expense o f the fo rmer. \"\'e should also esrablish what the o bjectives are when \ve examine safe practices. Certainly, we can establish a clear link with the health and safe ty of people at work. Howeyer, the work that we do is surely related to the safety of those people who arc going to be affected by it. By this I mean the aircraft crews, passengers and the general public who wander beneath the flight paths. That is a lot o f people! The safety culture of an aircraft maintenance organisation is critically important and is a rdleccion o f eYCTyone working in it. \\fe need the confidence of the public or we are going to be out of a job. If we are to achieye this, we need to be sure that our work is safe. So, we are not just going to concentrate on those practices designed to protect only our colleagues and us at wo rk. \"\·c will also be gh-ing some consideration to practices that influence flight sa fety. In many cases there is a link between thcse.
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numerous publicaciom dealing with safe practices in diffe rent . C~)\\c't-c. - engineering em·ironments. Polan: of these tend to concentrate on \vorkshop \...,~f,.{' I~sues and mar unintentionally give you the impression that aircraft always 1(\1.. \,;. reside inside covered buildings . .A lot of aircraft maintenance lasks are carried - ('i~'.\ " OUl in the open, on an airport apron for example, where, notwidlsranding the L, wcather, there are specific hazards stemming from the close proximity of the aircraft manoeuvring areas and vehicle and pedestrian traffic that is n'8t necessarily associated with, or familiar with, engineering maintenance activitia . Man\' people find that their first encounter with the environment found an i. busy· airport apron is a bit like straying into unfamiliar territory and findin g" yourself on someone else's patch. For that reason, I \vill be including a sectios:> on apron, or ramp. safety as a topic in tlus chapter. _700
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The interprelation of what comprises a \vorksbop is also a fairly loose one. 1f!le diccionary definition is simply fa room or shop where work is done'. Air~aft hangar maintenance bays, mechanical and electrical com ponent bays, m;w uoe shops, cleaning plants, battery rooms and basic bench tool work are~ each have their own particular safety-related features in regard to the work Wocesses being undertaken in them. I will be drawing your attencion to [h~e where approprlate.
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CHAPTER ONE SAFET Y PRECAUTION S
As far as the aircraft is concerned, we will only be dealing with general safety related issues in this chapter. l\Iore detailed aircraft handling procedure:,;, and their related safel), precautions, are contained in Section 7-17 of the syl labus.
EASA Part 145 Requirements E . ASA Part 145 - 25 (Facility Requirements) Imposes some requirements on approved maintenance organisations that afC rele\-anr to what wc havc been discussing. I will quo tc the extracts for you. ' Facilities mUSl be provided appropriate for all planned work, ensuring in particular, protection [rom the weather elements. Specialised workshops and bays must be segregated as appropriate, to ensure that environmental and work area contamination is unlikely to occur.' (EASA Pan 1-t5 -25(a). 'Officc accommodation must be prm-ided appropriate for the management o f the planned work including in particular, the management of quality, planning and technical records.' (EASA Pan H5 -25(b). 'The working environment must be appropriate for the task carried Olll and in particular special requirements observed. Unless otherwise dicrared by the parcicuhr rask environment, the working environment must be such that the effectivcness of personnel is not impaired.' (EAS_r\ Part 145 -2S{c). You may have construed from what we have been discussing that aircraft maintenance tf!~ks arc carried au[ in a numbcr of different environments. Howcver, there is one common factor in that humans inhabit all o f these areas, and they arc the root cause of most accidents. ,'eith thar in mind, we can make a starr by taking a look at what an accident is. '!-"'~(
I\n accidenr is an unforeseen e,-ent that may result in the injury or death of people and/ or damage to equipmem or facilities. _-\ccidcnts arc not narural phenomena or 'Acts of God' - there is always a cause. The cause may be rooted in either an uns,lre human condidan or an unsafe environmental condition. In the lauer case, we should remember that humans afe normally responsible for choosing or creating tbe environments they work in so lhe cause of accidents can invariably be traced back to them_ ''{Fe normaUy describe unsafe conditions as being hazards and, in the light of what we have said - it is people who create them!
Hazard & Risk 1\ hazard can be described as an)'thing that has the potential to create danger and thus harm. The likelihood of it actually doing this is described as the risk aod tills can be evaluated as being high to low risk. The consequences of the
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danger, should it be realised, would indicate how serious the risk is. A risk assessment would involve identifying a hazard and then evalua ting the risk in the light of the precautions that are either in place, or required to be in place, to reduce the danger or rcmO\'e it.
Recognising Hazards J\hny of the more common hazards occur as a result of dar-to-day activities in and around the workplace. Slippery floors, untidy work areas, cluttered walhvays, blocked exits, unreported cqwpment faul ts, insecure handrails, defective tools and spilt chemicals arc just a small sample of the unsafe environmental conditions that can stem fro m poor husbandry by the workforce and any of these may conspire to crcate a 'window of oppornlllity' for an accident to occur. Management may also overlook d1e presence of unsafe environmental conditions . Inadequate lighting, poor "enwation, lack of equipment, uncontrolled use of dangerous materials and equipment, excessiye noise, cramped work areas, poorly maintained and defective buildings and equipment, inappropriate storage conditions and lack of emergency equipment and procedures are again a small sample of the unsafe environmental conditions that may exist as a result of management oyersights. T he way people behave in the wotk environment can also create hazards. Apathy, boredom, negligence. complacency, disobedience, tomfoolery, showing off, oycrconfidencc, fatigue, stress, la ck of fitness, inappropriate or incorrectly worn dress, long hair, poor personal hygiene, routine "iolation of work instructions. drug and alcohol abuse are all examples o f unsafe human conditions that may stem from individual behaviour and group culrure. (j'
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management can inadvertently illtroducc unsa fe human conditions. _co\\~'f:-"" Lack o f proper supervision and training, use of inexperienced sta ff, poor 'O':.~'t' morale, poor planning and organisation, inappropriate disciplinary procedures Q ,,\....<... and a lack of regard for safety arc a sam ple of fac tors that may create unsafe (f:§t,"....,.. human conditions that may in turn open a \", ndow of opportunity' for an accident to OCCllr. ~
T hese hazards may lead to accidents at the time we are working but they m~' also create 'latent fai lures'. \'\.'hen someone is careless at work and m akes :R mistake, the misrake may remain hidden for a considerablc time before reveals itself in an accident when a given set of circumstances coincide with it. ~
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In a perfect world we would all have comfortable, air-conditioned, \\'ell-lit airrl spotless hangars . T he cost of building and maintaining such facilities at loca tion llsed by airlines and maintenance organisations is not al\\fuys conm1crcially feasib le. A 1m of aircraft maintenance tas ks are carried out ~~der less tban perfect conditions. TillS includes work that is carried o ut in th~pen, in inclement weather. at rught, ill poorly lit hangars and in noisy condi~ons. \X1e need to be able [0 recognise the hazards that appear when these h3ndicions deteriorate to below acceptable levels.
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5 Copyiglit © 13m']' Colkg~ 2006 . 2007 All right; !UNTIed
CHAPTER ONE SAFETY PRECAUTIONS
\':rhen peopte become more concerned with their sur..-i,'al than the safet), of the work in hand there is bound to be a conflict of interests. Narurnttr, we should be cquaUr concerned with both, but not comptetety o\'erwhetmed by the first. Clinging on for dear life on a swaying, slippery higlHise servicing pt:uform in a Force Nine \Vintcr's gale with one hand, whilst holding a f:lst fading (orch in the other, is not conducin: to carrying OUt detailed work safety! A f.1clor that often gets overlooked is the hisrary of the buildings in which some people work. Man)' buildings were in use long before the introduction of [he tight measures that we see in force radar. Some may have been in ope ration before the Factories Act 1961. I will usc a simple example to briefly make my point. Imagine that you were maintaining aircraft in a building !1m had, sometime in lhe distant past, been used as a paint spraying faciliry. Over rime, spray dust laced with rhe chromates and isocyanares used in primers and paints would almost certainly have settled thickly on the top of roof girders and on high ledges. Opcn the hangar doors in a fresh wind and this miglll disturb [his legacy. The point is made here is nor ra alarm rou but to poilU Out lhal we should nOl be blinkered by JUSt the presem when assessing hazards at work. \',/hen we were examining samples of the variolls hazards that can appear, we were attributing responsibility to either lhe workforce or management. In realit)·, bOlh these groups ha\'e a collective responsibilit)· for the safety of people at work and lhis will become clearer when we look at the legislation contained in the Ilealth and Safety at Wrork Act ( 197..1). Defore we take a look at rhis, we should discllss how accidems might be prevented.
Accident Prevention Prm'iding th:lI ,)\,,
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Remove (he haz:lrd This should be the first principle of accident prc,'ention. Reducing noise to \\1thin sa fe limits would be one example.
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Replace (he hazard \'Ce may be able to reduce me danger by finding a safer ahernatl\·e. This could mean replacing a hazardous material with a safer one or amending a work process.
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S hield (h t..· h:lz:lrd I f we cannot remove the hazard or reduce the risk then we need to find a means of protecti.ng ourselves from it by modifying it. Fitting guards to moving machinery, or safety rails to high platforms, arc good examples of this.
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Shield (he person rf we cannot shield the hazard then we must take steps to shield the person by wcaring appropriate prOlecti\"c equipment. This is really a las( resort where we ha\'e been unabte 10 implement any of the above solutions. Providing car defenders,
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respuators, gauntlets, safety helmets and goggles arc good examples of this. :l.
Train th e perso n . Educacing people o n the nature of the dangers in the workplace and the correct procedures to follow arc vitally unportant tools in preventing accidents . TIns is now an unavoidable legal dury.
\Vhen an emergency occurs as the resul t of an accident, rour personal safety, and the safety of your colleagues, must always take priority. No pile of bricks or lump of machinery, howeyer e:.;pensi\·e it is, or however much you might admlre it, is eyer worth a human life. \'Chen dealing with emergencIes, you must never put yourself at risk. Becoming a casualt), will just add to the problems and danger faced b y others.
H ealth and Safety at Work Act (1974) The need to make an organised approach to ensuring the health and sa fety of people at work is reinforced by the fact that employers and employees are together legally obliged under dle provisions of Part 1 of the Heal th and Safety at \'·ork Act (1974) to haye a 'duty of care' for the health, safety and welfare of people at work. All nl.r craft maintenance acthirics are subject to the provisions o f this Act. Part 1 of the Acr has four basic objecti\-es:
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'To secure the health, safety and welfare of people at \vork.'
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'To protect persons, other than persons at work, against risks to health and safety arisIng out of or in connection with the activities of those at work.'
W& e that this also places a duty of care on employers and employees alike for the health and safety of VIsitors to the place of work as well as to the general public. One effeel of this is that \risitors must be brie fed on the hazards and be issued \'lith protective equipment where appropriate. This needs to be carefully considered because visitors who do not speak English, or who have sight or hearing impediments must be briefed in a wav that the,- will understand and all hazard-warning signs mUSt be ItY"a form that is clear to tbem. ~ ~.
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'To control the keeping and use of explosiyc or flammable or tOXl ~ substances and to preyent the unlawful acquisition, possession and usts of such substances.' Ef
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'To control the enusSlo n into the atmosphere of noxious or hann@l substances.' :pZ::: 0'
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.As we can now see, the Act cm-ers all people at work, in all working ac@ ties, and is extended to con.'r yisitors, sub-contractors and the general public as well. These legal obligations are expressed as general duties that are e~rceable ,
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CHAPTER ONE S AFET Y PRECAUTION S
in criminal law so there is a lot of imponance attached to them. Both the employer and the employee haye general duties under the Act.
Employer's Duty 'It shall be rhe duty of every employer to ensure, 35 far as J:" re:lsonfl b~t · pr.'Jcn·cabJe the health, safety and welfare at work of all his/her employees.'
T ills duty includes in particular: •
'The provision and maimenance of plam and systems of work that are, so far as is reasonably practicable. safe and \\ithout risks to health.' The term plan! used here refers to all macillnes and other equipment used at work. The term 'sntems' refers to work processes and procedures used at work.
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'_-\.rrangement for ensuring, so f.'l.[ :l.S is reasonablr practicable, safet~ and absence of risks to health in connection \\ith rhe usc, handling, storage and transport of artides and substances.' This is wide-ranging and incl udes for example, the safe handling of hea\"}' objects and chemic;t\:;.
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'fhe prOVISion of such infonn;tuon, mstrucDon, training and supervision as is necessary to ensure, so far as is re;tson;tbl), practicable, the healt h and safety ar work of his/her employees.'
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'So far as is reasonably practicable as regards any place of work unde r the employer's control, the maintem.nce of it in a condition that is safe and wit hout risks to health and the provision and maintenance of access a ~,d (egress from it that are s;tfe and without sllch risks.' The ~tt-~i\ta;emeOl would include fire lanes and exits.
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'So far as is reasonably practicable, the pro\'ision and maintenance of a working environment that is safe, \\ichoUl risks to health and adequate as regards facilities and arrangements for the welfare of employees at work.' TI,e i:mer statement would include the prm.-ision of adequate washing facilities and rest rooms.
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'So far as is reasonably practicable, to ensure char persons, not employed by the employer wbo may be affected by tbe aCli\·ities of thai employer are nor exposed to risks to their health and safery, 'J'his includes the provision of infonnaaon about the wa)" in which the undertaking is conducted that might affect their heahh and safety.' This would include the general public as well as ,.-isitors.
T he Act also requires that an employer must also produce a written sta1ement o f rhe comp;t!lY's health and safety policy, and the arrangements fo r ca rrying OLlt that policy, and th;t l this is to be brought to the attention of all the
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employees. The employer must nor levy any charge on employees io respect of personal protective eq uipment or ciafcty-related training that may be required . The employer is obliged to proyide this. An employer'S general dunes also require him/her to assess risks and record them, together \vith any precautions to be taken, and to make these records available to all employees at thei r place of work. 1 will include a brief description for a few of the more common assessments . Risk Assessments
To meet their obligation to prm-ide safe plant and safe systems of work, employers haye to ensure that e\"ery work process is assessed for risks in rebtion to the place the process is [Q be conductcd in and the equipmem used. These risk assessmen ts must be recorded and made a\"ailable to the employees at the place of work. An assessment must identify the hazards and the steps that employers and employees must take to remove, reduce or protect themselves from danger. This would, for example, be the procedures and p reeauuons to be observed, the personal proteetiye equipment to be worn and the limits to be observed where required
COSHH Assessments Employers han: to ensure that the use of any substance that is cons idered hazardous to health i~ assessed and controlled in relation to [he place and the process that it is to be used In. Again, these assessments have to be recorded and made available to the employees. Note that a separate assessment is required for a giyen hazardous substance in each process or location it is used In - unlversal assessments do not satisfy the requirement. T he assessments must identify the occupational exposure limits, precauuons and the specifications for the personal protective eqwpment required. \I/e will revisit tlus tOPic in more detail when we deal with chemicals.
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Employers have to ensure [hat noise Ieycls are assessed by a suitably trained specialist where these levels may cause damage to the hearing of personnel. The occupational noise threshold limits haye to be observed and the em plo:-ees must be trained in the use of any hearing protection that ~;; provided_ \"Ife will deal with the limit~ and actions later in this cbapter. ~ ~ SManual Han d ling Assessmems To mect thc general duty for safe handling, stornge and transportaoo;T methods, emplo,ers are required to assess and record the risks in relation fit, moving heavy objects in the workplace, including the safe loading of storr@! racks. The assessment record must identify the equipment to be used, the W~ d limits and the methods to be used. Empl~rees have to be warned of there sks and trained in the approved methods and tile use of any equipment #kt is provided. t\J
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Employee's Duty 'Ir shall be the dut), of evcry employee while nt work to mke reasonable care fo r the health :1nd s:1fery of himself/herself and of other persons who may be affected by his/her acts or omissions at work.' Irresponsible beha,"iour, engaging in work thar you arc nor qualified or experienced enough to do and ,"jolating procedures or taking 'short cutS' are classic examples of this. Other examples would be creating unnecessary noise, fumes or operating mO\'ing machinery that putS personnel in the vicinity in danger. This general dUly includes two further requirements: t.
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regards any duty or requirement imposed on his/her employer or any OTher person by or under aoy of the reie,-ant statutory provisions. to cooperate with him/her so far as is necessary to enable that dUly or requirement to be perfonned or complied with.'
This means that employees are legally obliged to cooperate with their employer to enable him/her to fulftlthe general duties of ao employer under the ,\Cl. 2. 'No person shall inrentionally or recklessly lnterfere Wilh or mIsuse anything provided in the interesrs of health, safe£}" and wdEl re.' Examples of infringements of this would be where an employee removed safery guards from machinery, played about irresponsibly with fire appliances or misused c(luipmem.
Safety Repre ~entatives & Safety Committees . 'H¥. R e~~W·'bns :'-.).'t&~ployers are also obliged (Q allow a recognised trade union to appoint a \' safe£}' representatiyc from among the employees who is to be consulted on all safety m.'mers. lbe safe£}' represenL'lth"e has the authority to investigate potential hazards and all dangerous occurrenccs at work. TIler may also im'estigale safety-related complaints made by an employee and make representations to the employer. The safety reprcscmati,'e may carry Ollt safc£}' inspections in cvcry area of the organisation and represent the employecs in consultation with inspectors appointed by the H ealth and Sa fery Executi ve. The employer is obliged to allocate sufficient time for thc safety reprcsenmti,"e to carry our his/her duties and to provide any training thar may bc requi red. In large organisations there will be more than one safcty represemacivc. \"'here two or more representatives requesr it, the employer must establish a safety comminee. T he safety committee is normally chaired by a senior m:lnagcr and it may carry om sttldies of nccident and disease statistics in the workforce, examine snfery inspection reports and develop safety rules and employee training programmes. It :llso monitors the effec tiveness of s:lfery tr:lining and the adequacy o f health and safety communications in the workplace.
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Health & Safety Commission The Health and Safety Commission is a goyernment appointed body that is responsible for the issue of regulations and CodCfj> of Pracdcc. The regulations are legally binding. Codes of Practice are issued for guidance o nly but observance of them docs provide evidence that an organisation is doing everything 'that is reasonably practicable' to fulfil its obligations.
Health & Safety Executive (HSE) The Health and Safety Executive (HSE) has the task of enforcing [he regulations made by the Health and Safety Commisslon in cooperation with local autho rities and other goyernmcnt enforcement agencies. The HSE has regional offices and these appoint tnspectors with wide-ranglllg powers to legitimately access any place of work, carry out any investigation they think is appropriate, question staff and collcct and take away any supporting e\-idence..
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T he HSE has the power to prosecute employers or employees and to Selye lmpro velnent Notices or Prohibition Nou·ces on employers. 1\ Prohibition Notice would be sen~ed for example if an HSE inspector \\-ere of the opinion that an employer had contrayened, or was about to contravene, a statutolJ prm-ision of the HSA \,\i Act and that there was such evidence of an imminent risk of serious personal injury that tbe acti\rity must cease llnmediare.ly. All notices have a rime limit put on them where actions required by the notice have to be completed . Tllerc is also an appeals procedure where an employer feels a notice to be un justified. :\ failure to comply with any Notice issued by th e HSE is likely to result in a criminal prosecution. The HSE does, however, operate on the principle of 'pre\·emion rather than prosecution' but this does require the full cooperation of employers. One final point is that HSE ins ~Jors will point out hazards but nOt give specific advice on how to ..,.e. :..\:I~Ve;come them in relation ro a particular workplace - that is the employer's (.1..1\\<:'-:responsibility. Under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 1995 (lliDDOR), employers also have an obligation to fo rmally report certru.n ins tancc~ of disease, injury or a dangerous occurrence at work to the HSE who may at their discretion conduct a formal inquiry into t{!e circumstances. These repons have to be submitted within a given time of t~ occurrence. In addition to this, all places of work have to maintain a separat§. record o f all accidents and injuries to employees and any first aid treatment. given. HSE inspectors haye the right to examine the accident and first-aifbooks. All places of "\york are now required to have an adeCjuate number ,riJ trained first aid personnel among the workforce.
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Associated Regulations
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~u You may h ave already guessed from the foregoi ng that the Health anctSa fety at \\fork Act (1974) invokes compliance with a number of other regulations.
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There would not be space to list all of these and rou would nO( be expected to remember them all ..\ few examples are:
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•
Factories Act I % I
•
Control of Subs tances Hazardous to Health 1988 (eOS I II I)
•
Safety Representati\"es and Safety Commirrees Regulations
•
Health and Safety (First .-\id) ReguJacions 1981
•
Reporting of Injuries, Regulations 1995
•
Manual Il andling Regulations
•
Electricity at \"'ork Regulations 1989
•
Em;ronmental Protection ~-\ct
•
Noise at \"'ork Regulations 1989
•
Manualliandling Regulations
•
Disposal ofTo:cic Waste Regulations
•
Protcction of Eyes Regulations 1974
•
Safety Signs Regulations 1980
•
Classi fication, Packaging and Labelling of Dangerous Substances
Diseases
and
Dangerous
Occurrences
R~~!~,ti~l'll I984
. l'\\":'';''<:' ... l.. Abrasive
\X 'heels Regulations 1970
I think that is sufficicnt to let you see there are: a lot of regulations! \'re will re\·isit a few haer in this chapter but, for now, we will look at some of the em;ronments thlH maintenance personnel mar wadi: in and the sa fe practices associated with them. As promised earlier. I will include some general informacion on apron sa fety on the basis that a number of employers now ask for this topic to be included in training courses so it e\~idently has significance for them. '11111t is as good a place as any to starr.
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Safety Precautions - Aircraft Airport Apron Safety /\s [ have already discussed, a lot of aircraft maintenance tasks take place on an airport apron. So, we will start by gaining some understanding of what is meanr by the various lerms used to describe the areas where aircraft may be positioned at an airport.
Airside Tlus is the area of an airpOrt outside of the mai n terminal building departures gates where access is restricted to all bur authorised persons. rr nom1ally lies within 1-11\'1 Rerenue and Customs and Hi\[ Tmmigrntion control and all persons wirl1in it are liable to search or identity checks by their representatives. In addition, at international airports in p:Jrticular, there may be a police (special branch) p resence. You must always have photographic proof of identity with you at all times. This is nonnall" your airside permil.
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T lus IS the area that accommodates aircraft for the purposes of 10ading/oft1oading passengers and cargo, refuelling, parking and maintenance. T his includes any aircraft nose-in parking stands. \"\ -irhin the apron there will be designated areas fo r the storage of aircraft handling equipment when it is not in use on aircraft and there ate strictly enforced rules in place to prevent our of use equipment being left lying loose around the apron. All apron egw \\1Cnt must be properly secured and cleared of loose articles to prevent lO ,QCVbem Geing inadvertently moved by jet blast, prop -wash or high winds. 1t is no _O)~<:.~c - fu n trying to chase after a set of aircraft steps or a pair of airborne O\'eralh that ~,;\.\1:': is heading for the adjoilung parking stand! Manoeuvring Area T his I!1cludes the area, other than the Apron . that is used for the movement ,of ~ aircraft on the ground and for take-off and landing. ~>
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Movement Area This area encompasses both the i\fanocuvring Area and the Apron. ,~
Airside Roads
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T hese arc areas that are designated as roads by the Airport Authority>
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If you are required to work on an airport apron then, for your person~'];]Jety, and the safety of others, it is essential that rou are familiar with the @'es. The apron environment is often a busy one and you need to be awafe o f the dangers posed by jet blast, aircraft movements and large moving velucles and
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equipment. The Airpo rt Operau'ons Instnlcu'on s comain the niles :ind you will h:ive to familiarise yourself with them before you are issued with a permit to access any airsidc area. I will merely givc you a few broad cxample!; of what rhcsc m:ly Iypically con min. Access You cannot enter an airport apron \\;thoul prior audlOrisation or permit from the Airport Authority. If you do not work for rhe Airpon Amhorit)'. rou nonnally ha\'e to be sponsored by your employer before a pennit is gmnted and your sponsor \\;11 be held responsiblc for your beha\"iour in addition to you should you break the rules. \'nen so authorised, rou ha\'e an obliga tion to com ply wilh the Airport O perations Instructions. If you are a visitor you arc obliged to have an :luthorised escort and you must comply wirh :lny instructions given . .\irpon authorities have to comply \\~th Ci\'i! .\irworthiness Procedure (CAP) 362 'Airside Safety A/anagem en~ so your compliance with their instructions is a legal obligation. You are also obliged to comply with the Flca/rh and Snfer), at Work A c t /9J.1 whilst eng:tged in work in the airs ide area. I f the occasion arises, entry to the manoeuvring area \\ill recluire further clearance from the Local Air Traffic Controller in each and c\'ery case and you would need a vcry good rca son before rhis would be granted . Wh ilst in rht.: manoeuvring area, aU your mo\-ements musr be notified to and clea red by the Local Air Traffic Controller. Airpo rt Mark ing s Personnel engaged in mainrenance tasks on an apron need to be aware of, and comply \\~th, the various signs and markings. There arc road tmffic signs and these comply \\~th the Tmftic Signs Regulations (1981). Vehicle rOUles arc normally identi fi ~~-br white line markings. The yellow line markings afC strictly for ai rC1'a{t\..'lm~'ements whilst green line markings will normally indicate Ptt..~; routes. "nle delineation berween [he apron and dIe manoeuvring \...,).< ~irea will also be indic.'lted by line markings such as double white lines.
Airport Lighting l>. laimcnance activities at an :Urporr are not JUSt confined to the daylight hours and good weather. ' nlis means there will be lights and they can be con fusing to the uninfomled eye. There arc likely to be StOP lights, particularly at vehicle routes OLlt of the terminal passenger and b?ggage areas and at access poims to thc manoeuni ng :!.re:!. and at holding points \\~thin it. The aircraft taxiways arc norm:!.l1y identified by blue edge lights and sometimes hayc green ccntrc·line lights. TIle aircraft taxiways also have red stop b:l.r lights at various holding points to warn you whcn aircraft arc abom to cross the path. Runways h:l\"e a sophis ticated array of aircraft visual approach indicators and dues ho ld lights fi nd afC also identified by bright white edge and centre-line lights.
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The apron is normally illuminated by an array of oyerhead pylon lights . The aircraft parking stands may ha\"C illuminated side marker boards or pressure pad indicating systems. These adapt to the type fmd size of differenr aircraft. Nose-in stands arc ohen eguipped with AGNIS (Approach Guidrl!1ce Nose-in to Stand) lights in the form of red and green bars tha t are used to direct the pilot into correct alignment \vith the stand. The operat.ion of all airside light.ing is normally under local air traffic control. Airports hfwe an emergency electrical po\\'er sys tem that switches on within seconds of a pO\ver failure.
Ve hi cles Personnel must be in possession of a ...alid .Airfield Dri"ing Permit (ADP) before they can drive a ,"chicle on the apron or the manoeuvring area. Nonnally. [here are separate permits applicable ro each area so a person with a pennit to drive o n an apron only caanO[ legitimately access a manoeuvring area . .\ll \·chicles used in these areas also haye to carry and display a pcm1it. Private \·chicles arc prohibited from usc on the apron unless specifically authorised. This indudes bicycles. ACCJuiring an AOP usually im·oh"es a wriuen test and a colour vision check. There arc strict nilcs governing rights of way when drh'ing airside.
I.
Aircraft have priorit~, over all vchicle movements.
2.
Fire and rescue vehicles on emergency callout have prlomy over all o ther road vehlCles.
3.
Pedestrians ha\·e priority over vchlcles on the apron.
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Refuelling ,·chides haY(; priority oyer all other road \·ehicles except fire ...and rescue vehicles when on emergencv callout.
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have included these priorities merely to show you that there arc some significant differences bet\yeen driring on the public highway and at an airpon .
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,\11 \·chicles used airside normally display a roof mounted nashing yellow obstruction light whilst they are mo...i.ng 011 the apron or manoeuvring areas and arc resuicted to 1Smph on the apron. The obstnlction lights are normally switched off o n stationary ychicles. Fire and rescue ,·chicles display a nashi ~ blue light when on emergency callout and are also subject to the apron speeR limit unless specifically authorised to exceed it. At night, all mo\·ing vehicles: usc dippcd headlights and display sidelights only when stationary. Vehicle£. must not approach an aircraft until the aircraft is chocked and its engines ha\"f been shut down and the anti-collision lights ha\'c been switched off. Personna arc nor permiued rn ride on any pan o f a vehicle or item of ground equipm!:!:!.~t that is not specifically designed for the carriage of passenge rs.
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'" Pllfticular care is reCJuired when to'.ving hea\·y eq uipmen t with vehides. J?'oors on ground power sets, toilet trolleys, and de-icing bowsers etc. must b q;,jecu rcd dosed to prevem collis ions occurring because of the increased width §f1ese may give when they are inadvertcnrly left open.
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Mob ile Phones Mobile phones are a '>ery significant fire hazard when they arc lcft switched on within me apron fuelling zones> Radi os >\pprO\~ed
portable radios are often carried. Radio lCanSffilSSlons require the use of a call sign and standard radio tra nsmission procedures arc rC'luired. J\ irporr ground radio freque ncies should nor be used for lengthy conversations as this jams oth er users who may h;we more urgent requirements. You will soon be reminded if you abuse this rule. Your sponsor company mar ha"e a dedicated frequcncy for your use and this should be used. There may also be security restrictions on the in formation that efl n be tmnsmiued - radios fl re not a secure form of transmission and can be heard by unauthorised pe rsons. Airports normally haw a dedicatcd emergency freCjuency for lire and rescue operations and this should nOt be used for other purpose. Radio waycs may also present a hazard when used in dose proxImity to some electronic eCluipment. Clothing Personnel working airsidc arc nonnally required "isibility :tnoraks or tabards at all nmes.
to
wear approved high
N oise "l11c noise from turbine engines can cause temporary :tnd long~term damage to lhe hearing. T empora ry damage can cause personnel ro insrant.ly lose their scnse of ba lance _and head ng. The noise also blankets the sound of approachiR~{yegWes and will drown out shomed w:trnings. All personnel on Ihc ,~1-will be wearing car protection that funher exacerbates the problem. _. \'\;\'t~fl~ese arc haza rds nor always appreciated by personnel working in and :lround _,\'\\. l,.. :tircra ft. I have included an illustration for you of a typical noise haza rd plot . ,,~..:,<''C from a single turbo-jet engine.
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Fig ure 1.1 • T~' pi c a l Noise H aozard Areas (SI:lrboard e ng ine running)
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It is \-cry unlikely that engines would be clearcd to run a[ full power o n the apron, there are usually dedicaled areas where this may bc authorised by the Local Air Traffic Controllcr. However, the lOner red circle al 30ft radius displays the area where exposure to the noise at full power will damage hearing even if ear defenders are worn . The red and amber areas up to 60ft radius indicate the area where all personnel must wcar ear defenders when the engine is operating at idle power. As power is increased, this area stretches out to 150ft. In reality, YOll will need to wear car defenders anY'.vhere on the apron when engtnes arc running
Jet Blast Apart from the effect noise has on unprotected hearing it also creales another problem. \'j/hen several engines arc running on the apron area it becomes difficult to determine the direction of the sources. Aircraft thal are being 'pushed back' out of the stands are oflen 10 the process o f starring their engines so particular care is required. \'{lith this in mind, personnd must be alert and not stra~- into the danger areas in front of cngine intakes, propellers or imo the jet blast or prop-wash danger areas. I have included an illustration for YOll of typical turbo-jet engine inlet and exhaust hazard areas. !dl~
Thrust _ I OO f r _
-10ft-
Entry
-
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1500 ft ~faximum
Thrust
Figure 1.2 - Typical E ngine Inlel and Exha ust Hazard Areas (Fo rward thrust)
co T he red areas indicate the idle po\vcr hazard areas. N ote that even at 10Qg behind the aircraft the jet blast can still be hot. T here is also an added fisk tl~t small scones and grit in this area can be flung rearwards. The red area in fr~t of the intakes and around the sides of the nacelle indicates the low-pre~re zone that exists at idle power. The 100v pressure deepens towards the inlei-This is suffiCient to lift rou off the ground and the rest is obVIOUS. The low p&ssllre may also suck in debris and loose equipment. Take a note how the h.a~\.M areas grow as power is increased from idle. There are red stripes painted o r{ the sides of an engine nacelle that warn personnel nOt to approach any further along the
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sides of the nacelle from the rear at idle power settings .. \t power settings above idle no one should be there. I will also include an illustration for you of the typical hazard areas thaT exist when re\rerse d1nJst is used just to show you rhe danger. lligh Thrus
I bgh Thrust - - - - 20(1 f , - - --
140 "
Low Thrust
Figure 1.3 - T ypic al Ellginc Inlct a nd Exhaust I-Iazard Areas (Rcvcrse thrus t)
Reverse thrust increases the forward hazard area and any debris or equipment within lhis area may be flung forwards. There is srill a significant jet blast hazard behind the aircraft. It is unlikely tbar reverse thrust would be selected on the apron bur it js well to be aware of the effect of doing it. "I,,\...I
",b Ear D t (e1\(1ers . (,0\\... ·
~ \....:-.<:~\ ll personnel working on the apron should be in possession of car defenders that comply with the apprO\red British Standard (l3S). ll1ese have to be worn whenever there arc aircraft on the apron and during the running of main engines or auxiliary power units. \\'e will discuss hearing prmccrion and the regulatory noise threshold limits later on in this chapter. Smoking Smoking is prohibited anywhere on an apron or manocU\+nng area for the ob\·ious reason that it is a fire risk.
Fuelling Zones 'nlcrc are a number of rules applicable to the area surrounding an aircraft that is being re.fuelle.d or dc-fuelled, Ir is \;tally important that fuelling vehicles are afforded unobstmcted access and egress wben attending an aircraft. The
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fuelling \"ehidcs havc ro be poslUoncd so that they do not have to rcvcrse in o rder to leave the zone. \\ ncn fuelling or de-fuclling operations arc in progress o nly aut ho rised \'chidcs and persons are allowcd inside the fuelling zonc. G round power units and "eludes must be pos itioned well deu of the aircra ft fuelling and vcnting poin ts. Thc Airport AUlhority may restri ct o r prohibit fuelling o perations during lightning risk periods. \Vc rcyisit fuclling/de-fuclling procedures in Chapter 17. Fuel Spi ll s These arc taken \'Cry seriously, nm jus t because of the fire risk thcy prescnt but also because of rhe risk to Local Authority drainage systems and the local waterways if the spill tilfearens to oycn.... helm the airpOrt drainage tmps. The fines arc significant! T he _\Upon Operations I nsrrucuons will contain the procedure to be followed and me agencies to contact in the evcnt of a fuel spill. In gcneral renns, me source of the spill should be shut off or limitcd if possible. Ali sources of ignition should be remm-ed or IIlhibilCd. All engines in ground power unitS and ychides in the area should be turned off and the area should be cordoned off to preyent furthe r access. I f the spill IS significant, effortS should be made to limit the spread and the area should be c\'acua ted if deemed necessary in the interests of safety. E m e rge ncies The emergency scn'lces internal telepho ne number at airports IS no rmally 'all the t\\'os' i.e. 2222 . The senior fire offi cer present is normally the no mi nated incident controller and rou should ens ure that you meet, direct and brief him/her and comply with any instructions that you Illay be gh-en.
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The ,\irpon .\uI:horit)' operates and malntains the airport f.'l.ci lities. In many cases, this task is sub-contacted [0 a specialist aircraft handling company. All requests for airpon controlled equipment, including fu el and towing vehicles may ha\'c 10 be passed through a dedicated control point [hat allocates these facilities on a priority basis. Some resident airlines have thei r own handling eqUIpment. The Local Air Traffic Controller deals wim clearances to tow aircraft o r run main engines and their instructions have to be complied with at all times. Thi~ realiy tells you that you cannot just please yourself on an apron - there ar£: rules. Airports employ security personnel who have a remil to en force airsidf safety so it is not a quesnon whether you \"ill be caught breaking the rules rarher whcn you will be caught. The penalties can be scvere and range fr ~il suspension of permits to instant dismissal. .?f
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Task Ligh ting
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" not 'rhe Q\'crhcad pylon lights are capa ble of illuminnting che area bur rhi~ocs help when pcrsonnel are working under aircraft o r in equipment bays. There is
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CHAPTER ONE SAFET Y PRECAUTIONS
a danger that the lighting in these areas ma), be well below what would be considered adequate for conducting maintenance msks. Hand held torches may seem to be an idcal solution bur thcir batteries gradually deplete and they do restrict the user to one-handed operations. You need to ensure that task lighting is adequate and of an approved flamepro of type. Lighting is onc area where there is a risk of complacency and a 'can do' attitude whcn personnel are working at night in the open ..\nother consideration is glare. \\then using porrablc lighting units or vehicle lights you must be conscious that the glare can affect other apron users who may require night adapted ';sion.
Foreign Object Damage (FOD)
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The detritus of maintenance work can range from off-cuts of locking wue and split pins to nuts, screws, bolts and washers. All these objects pose a serious safety risk to all aircraft types when they are carelessly dropped ooro the apron. Assorted litter in the fonn of drinks cartons and plastic bags can be blown omo the apron from rhe airpOrt surrounds and \'c!licle eyre treads can also impo rt stoncs and grit. rf any of these objects arc drawn intO the intakes of opcracing turbi ne engines they can cause cat:ls rrophic damage. They can also be picked up and drawn through rot:lting propellers causing seriolls notch damage to the blades. -nlese are the most publicised hazards but there is also a risk that FOD can be picked up in lligh velocity jet blasts and flung onto personnel or OntO neighbouring aircraft to lodge in intakes, \'ems and control hinge apermres. There is also a risk that some of these objects will become embedded in aircraft and vehicle tyres and be carried out onto the manoeu\'ring area. I have seen lengths of locking wire woven into a bird nest that was constructed in a flap track void on a parked aircraft. Aircraft eyres ha\'e burst on take-off and landing, in one \vell-reported case with very tragic results, when they have encountered foreign objects on the rUllway. All personnel who, ~ik in the vicinity o f aircraft need to be diligent in disposing of l;qp '2.~"'H~o suitable containers. It is essential that the apron is kept (-.c tll[:SUiously clean. Freezing Conditions The apron surface can become extremely slippery during freezing conditions and care is required when walking, driving or handling equipment. Particular care is required when manoeuvring vehicles or hea,,)" equipment ncar aircraft. There haye been a number of reported incidents of collisions and subsequent structural damage resulti ng from personnel underestimating stopping distances. When an apron is dc-iced, or is affected by de-icing fluid from aircraft, ir is initially very slippery and care must be exercised.
I hope that you have found the above discussion useful in orienting yourself ro
what to expect when working on an airport apron. It is one area where have 10 work alongside people who are engaged in equally important tasks also re{luire them (0 observe safe practices. I n turn , they also rely on obser...ing the rules. This alone justifies rbe inclusion of tills topic in chaprer.
you that you this
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Line Maintenance Staying wIth aircraft, we will now take a look at some of the safe pracnces associated with line maintenance. Aircraft arc comple..'.; systems that are pnma.ri1y designed to be in the air rather than sitting on the ground. Like anr machine, aircraft require maintenance and their systems need replenishing. The chocking, securing, jacking, towing, refu elling! de-fuelling, de-icing/ anti-icing and storage procedures and the associated precautions are all topics coycrcd in section 7-17 of the syllabus so we will not deal \vith these at this stage. Additionally, \ve will deal \\1th fire precautions and tire extinguishing equipment later in this chapter. The _Aircraft r.Iaintenance Manual is the guiding publication for all equipment \vhen it is ins talled in an aircraft. The ATA Spec 100 (now incorporated in to AT A i Spec 2200) format o f this manual is coyered in section 7-5 o f the syllabus. All Aircraft !"[amtenance r-.hnuals contain infonnation on s tandard practices, the safety precautio ns associated with individual maintenance processes and the specifications for the approved fluids and gases to be used. It is essential that maintenance personnel observe the authorised procedures and use only the approved products. Component !'>.faintenance Manuals arc prm~id ed for equipment that is removed from the aircra ft. \Ve will make a start b y taking a look at some general prccautions to be taken when working in and around aircraft.
General Precautions - Working in and Around Aircraft Wear appropriate clothing. \\?ear suitable foom·ear to avoid scratching surfaces or scu ffing anti-corrosi\'e treatments. Do not wear foo twear w1th steel ups that could cau~e ::;parks Ensure that overalls arc clean, '"loIS" intact and pwperl\.' fastened and that the pockets are sealed to prevent '"I\§)6 - \\ c!,!.t: loose objects dropping into the aircraft strucrure. Do not carry . Cp l' " unnecessary 1oose objects In pockets. Do not wear jewellcry. Do not '"' ~~~5" '- <..., carry any means of uncontrolled ignition such as non-sa fety matches or :-:...:..\~ lighters in or around the aircraft. Ensure you arc in possession of (r'\l apprm·ed ear defenders and are weartng these on the line when auxiliary power unitS o r main engtnes are running in the ·dcinity (S.e.-e N oise - Precautions and Limits). Do wear app ropriate cold wea~ clothing when required. "-:3.. 1.
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2.
Be aware that it can rake up to an hour to gain fully night-adaptecb. vision when moving [rom a brightly lit area to a dark area. The cye¥ adapt faster when moving from a dark area to a brightly lit area but t~ glare is initially a problem. This is a problem often overlooked \v~n working at night on a line. Do consider others when you are usfug bn ght light sources. '":l
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3.
"0 point before /l~ng,
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Aircraft mus t be connected to an effeccive earth de-fuelling, fuel transfer or \vork on o x--ygen systems is to ~v carricd out. Reliance on the conducting nose or tail wheels is noi-' normally considered sufficient in these instances. Other systems work may
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CHAPTER ONE S AFET Y PRECAUTION S
require that the aircraft is connected ro an effecrin eardl and this will normally be indicated in the Aircraft r-,'Iaimenance Manual. 4.
Approved fire exti nguishers should be loc:lled where they 3rc easily recognised and easily accessible. Check that you know how to operate them.
J .
Aircraft should be correctly chocked Wilh the appropriate clean covers. blanks and locks fitted where necessary.
6.
\"'\l1en operating aircraft systems on the ground you need to be aware of the effect this will have on the aircraft and any other systems that may be activated as fl result of yOLlr actions.
7.
\'Chen mo\-ing night controls o r selecting motoriscd cqtUpmem you must ensure that their movement is not obstructed by ground equipment and that they do nO( present a hazard LO other personnel.
8.
\'\llcn applying ground electrical, hydraulic or pnewnaci.c power to an aircraft you must ensure that the flight deck controls are not set [0 a position where equipment will be activated [0 mm-e unexpectedly and pose a hazard to other personnel or foul ground equipment.
9.
\'nen opening a component or system, the operation should be carried out in a controlled fashion to avoid dust, grit etc. emenng. _"-pproved, clean blanks should be used where appropriate.
10. Replenishment equipment sbould be kept clean and only used \\lith the fluid that it is dedicated for. AU fluid containers should be kept sealed when out of use and the contents should not be used if contamination is suspected or the identity is sllspect.
\~'l"ffiO rc~9;;lling> de-fuelling or transfercing fuel, do ensure that there o\\..:'t-Q~\ no obstructions under or around the aircraft that will foul the 11.
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strucrure when the aircraft settles or moves as a result of changes the centre of gravi[)'.
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12. Take ca rc not to introduce corrosion or fatigue risks by scratching dIe aircraft paint finis h or scoring the ai.rcraft structure through careless use of rools and equipment. 13. Ensure that alrcra ft electrical bonding wires and static dischargers nre left in tact and connected after wo rk has been completed. 14. Ensure that all work areas arc clear of tools. egwpment and debris before signing off work. Do include the surrounding floor areas in four checks. IS. Fuel and oil spillages must be cleaned up immediately. ~ta n y of these fluids present a fire risk and will also create spongy regions if lher impregnate the rubber of aircraft (yres. l>. fodern ester oils can strip cellulose and acrylic paintS and also act as an irritant ro human skin as well as bei.ng very toxic. Contact with these oils and aviatio n rurbine
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fuel is a major cause of dcnnatitis. Prolonged contact can lead to morc serious long-term health problems. Pcrsonncl should makc use of barrier cream and be diligent in wearing protectiyc clothing and washing affectcd areas of ski n. r can assure you that you will remembcr this if you cver get contarninated \vith phosphate ester-base oils such as Sk'ydrol or Chcnon B. 'l"urbine fuels can also cause vcry painful burns to sensitive areas of the skin. I ha,'e included a lot of safe practices in the foregoing bur the list is not exhaustive. }'1any of these are purely denved from COTnInon sense and that is going ro be the one human system thar needs to be permanently 'switched on'. \\"'e are now going ro rakc a look at some sa fe practices that arc spccific to parricular hazards. These apply wherever you may be working.
Dress T he form of dress worn will depend on the environment but there are a few general precautions that must be observed. A workshop may contain movmg machinery tha t can snag clothing, Cnls b fingers and throw metal swarf into the eyes. Equally, aircraft also haye sharp protrusions and moying parts that can snag or crush the unwary. Most machines, including aircraft, have \'oids that \vill trap fo reign objects in the moying parts. Personnel should not \vear jewellery. Rings, necklaces and earrings can be snagged in moying machinery or on projections and have been the cause of the most dreadful injuries. The slee\Ces of overalls should be securely fastened and tics or scan-es should not be worn for the same reasons. Overalls should be clean and not ha\-e torn or loose material dlat can be snagged and mey should hayc secure Vclcro type fastenings. Long hair is another ITap hazard ""hen \vo~ with powered equipment - if you must have it, wear a hair net. "1 \}\)0 - ...
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E lectrical Safety Virtually all the environments where aircraft maintenance is carried out will contain electrical power sources. The human body is an electrical conductor::it contains water and is co>crcd in perspiration so we must have no dOl~ about its capacity to act as a conductor. \\"11cn electrical current passes througJi tbe body it will affect the central nervous system and the muscles. This caq, produce muscular spasms that may 'freeze' tbe grip, the respiratory system anct the heart. There is also the likelihood that the 'victim' will suffer burns at d"f entry and exit points of the electrical current. Sparks from electrical equipm
::?
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People are at .ask o f becoming m-er- familiar with the use of many of th€1'lnore common portable electrical po\vcr tools and oftcn fa il to crcat th,f~ \\o;th proper care or recognise the risks associated ,,·; th using them in wet c&'ndirions. These are a major cause o f accidents involnng electrocution in the workplace.
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\,\'hen faced with a person who has been electrocuted you should nO[ touch the "'ictims' skin with your bare hands under any circumstances until you have Cut off the electricity supply. If the 'dcrim' has been electrocuted br a high voltage supply, sllch as that found in mains power external distribution lines or overhead cables, and is either still in contaCi with the source or is lyi ng within 20yds of it, you must nm approach until the power has been cut off. EleClrical power from these high voltage sources can 'a rc' over considerable distances and comact with them is invariably fatal. \~'ith those cheerful thoughts in mind we will look at a few ways of ayoiding becoming a "'icti01'. There arc quite a few 'dos and don'ts' and many of them are simply 'common sense'. 1.
Always check that the supply voltage is compatible "~th the equipment. Always ensure that the current is switched off when connecting or disconnecting electrical tools and equipment from the mains outlets . . \lways ensure lhar the tools or equipment are selected off before switching on the mains power to them.
2. _h'oid handling electrical equipment when you arc smnding on wet floors or you are wearing wet footwear. Ths just pro,·ides a path to 'eanh' and completes tbe li"f"e circuit thar you may have become a pan of. Do nm handle electrical equipment with wet hands. 3. Most 240V electrical ground equipment is fined with a three-core cable th:u includes an eanh "ire. Ensure [hat all electrical equipment is correcdy earthed. Frayed, pulled and badly kinked cables or exposed wires may produce a situation where an exposed Ih-c conductor contacts the earth conducror. \\'hen dlls occurs, [he eCluipmell1 body will become live and comact with it will put you in the live circuit. Do not use eCluipmenr that bas damaged cables or cracked connectors. -L \,\'here step-down transformers are required, ensurc [hal these arc of the corr~\\hvoltage rating and are serviceable.
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•
,,\'\.
,
. U':»":'~\~'here residual cllrrem detectors are fitted, check that these operate \\~<'t' when tested.
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6. Electrical current Oo\\"ing through a cable produces heal in proportion to lhe resistance of the cable. If the currem being drawn by the equipment is higher than the rating of the cable there is a risk that the cable insulation will o\-erhear. melt or bum and [he equipment bod" mar become lh-c as a result.
• 1....."\.
7. Neyer use extension leads that contain cable that has [00 Iowa raring for the equipment. Always unwind extension leads fully o r there is a \"ery high risk that the cable insulation will melt through overhcating. 8.
Do not Q\'crload power outlet sockets.
9.
Isolate equipmen t from the power source before attempting to disman tle or carry out adjustments to equipmenr.
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10. Do nm carry or hang electrically powered eqll1pment from the supply cables. 11. Do not leave supply cables and extension leads trailing on the fl oor where tbey may be run over by equipment or pose a trip hazard to personneL 12. Report all electrical faults immediately, disconnect the cCjuipment from the supply and do not attempt to use it until a competent person has properly repaired it. 13. Remember that the latest Buildings Regulations state that only NICE1C accredited persons can work on the mains electrical installations in buildings so do nor permit unauthorised persons to repair or modify these installaoons . 1-L l\hvays ensure that fuses or Clrcuit breakers arc o f the correct ....alue and make sure that the faults that cause them to blow or trip arc p roperly investigated. Do not keep replacing or re-setting circui r protection devices that continue to fail. Always ensure tbat replacement circuit protection devices are of the correct rating.
IS. Ensme that all electrical equipment is inspected before use for o bvious damage and that it has been re-inspected at the stipulated intervals by a competent person. 16. EleCTrical equ1pment should not be used in areas where there are
flammable gases or substances present unless the equipment is specifically cleared for use in such areas .
17. Electrically powered hand tools must never be connected to lighting _sockets because they will not be earthed. ("It! - 0\'1' ~
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18. \\fhen connecting an external electrical power source to an aircraft you must first check that the supply \'oltage, frequency and phase are compatible \,,;th the aircraft system. ·n1e ground suppl)' must be switched off prior to connection and you must ensure that the ground to aircraft connector is clean, dry and undamaged . "n1C connector must be full" mated before the supply is s\\;tchcd on. Always check that t~e aircraf~ electrical system instruments are d1en registering the requir~ voltage and freCjuc;1CY readings. -:::;.
% "
19, \'{!hen moving items o f ground equipment you must take care that the)~ do not foul overhead electrical lighting or supply cables or run ov~ electrical cables on the ground . This is a real hazard that is someti.m~ overlooked i.n busy hangar and apron environments.
p
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Agalll, J must emphasise that the list 1S not exhaustive and common must always prevail.
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25 Copyight ·:&J Bo,?)" Colleg~ 2006 - 2007 All right! IrJmed
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Compressed Gases - Aircraft & Workshop Compressed gases are found in use in hangar, workshop and apron environments. Compressed nitrogen is inert and is used to inflate aircraft lyres. landing gear struts and hydraulic systcm brake accumulators whilst compressed oxygen is used to replenish crew oxygen s}'Slems and. in older aircraft types, the passenger o"-ygcn systems. Compressed helium is used in some instruments and is also used 10 fill airships. Compressed acetylene and oxygen will be encountcrcd in welding shops whilst compressed air supplies are used to power pneumatic tools and test rigs. in rhis last case, thc compressed air is normally fed to inS[alIed storage cylinders before being distributcd around the work area. You may identify thc pipelines d1:tt carry shop compressed air by their light blue colour. l\1an~' firc cxtinguishams arc of a vaporising rype where a fluid with a low vapour pressure is stored i.n a steel cyli nder that delivers a high-p ressure flow of \"":lpour when the discharge \":tJ\'c is opened. Some fluid extinguishants arc pressurised with nitrogen or carbon dioxide. Other extinguishers contain an inert gas under pressure and the most f.,miliar of these is carbon dioxide. \'-e will deal with pressurised extinguishcrs later in this chapter.
All compressed gases present seriolls hazards. The high pressure ca n blast dust and grit into the eyes and pinhole leaks in pipes can create jets of compressed gas that can inject the skin and blood vessels \\~th potentially fatal results. Oxygen can react with petroleum products to calise spontaneous fire and cxplosions. Although it does 00[ itself burn, oxygen is highly reactivc and can chcmicall), combine with a ,-cry wide range of substances and make them flammable. All compressed gas bottle discharge stop "akes and pressure-regulating nlh-es should be operated by hand pressure onl),. YOli should nevcr attempt to m~chanica 1!y fO~Rt1~or hammer ~ seized valve. I f ~ compressed gas bottle is 1111streatc(h.w.Y:ldr6pped and dle discharge stop valvc IS fracNrcd, lhe rcaction to the. ,CSC:tpfng gas can propel the bottle like a missile. In rhc same vein, if air \\~~aeliverr hoses arc not connected, a high-pressure discharge of gas from them ~" ., can causc thcm to snake and whip around .
. ~"I.~
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.A particula.r hazard associated with fhe lISC of compressed gases during ai rcrnfr maintenance is the selection of tbe "'TOng gas. '111e thought of charging an oil filled landing gear strut with high-pressure oxygen is not a pretty onc! So, we can Ha.rt this section by learning to recognise some of me comprcssed gas cylinder.;.
.:;J
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Gas
CYlinder Colour
Lettering
CHAPT E R ONE PRECAUTIONS
Prcssure(max)
,\ir
u ght grey
.\IR (block)
4000psi (2'0 b,,-)
i'iuugcn
ught g rey wIth black neckhand
NITROGEN (black)
4000psi (rO bar)
Oxygen
Black with white Ol\)'GF.l,\: (\\-hitc) neck hand
600 psi (248 bar)
Acet\·lene ;\[aroon
.\CETYLENE (\"'hite) 225 psi (15.4 bar)
Helium
mI
Brown
2500 psi (170 ba r)
Table 1.1- Identifieation of Compressed Gas Cylinders
Storage of Compressed Gas Cylinders Compressed gas cylinders should be fined with tmnspowuion caps oyer their stop Yah-cs and be stored in unheated, well-venwared buildings and not exposed to direct sunligbL Bortles should not be in direct contact with any cover. Wfhere combustiblc gases are stored, the lighting shouJd be flameproof and there should be no sources of heat, sparks or fire. Full and empty bottles should be stored in sepanue rooms and oJo..-ygen and combustible gases sLlch as acetylene should not be stored together. Aviation breathing oxygen and welding oxygen bottles should be segregated. Bordes should not be placed on damp ground or exposed to any corrosive substa nces o r conditions. Acetylene cylinders should always be stored in an upright position. Bottles with rounded bottoms may be stacked horizontally but no more than four high and they sho1fi'\\"be wedged to prevent movement.
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General P recaution when Charging Gaseous Systems
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I will JUS t gi,e ~-ou some general precautions at chis stage. Further precautions arc required depending on the gas involved and these will be shown under t~e respective headings. 3_
,
~
1.
E nsure that you have the correct speci fication gas for the system o~ component and that the pressure remaining in the borrles is adequate.:;:
2.
When charging an aircraft system \\~th a high-pressure gas, the bot~ must be connected to a p ress ure r egu/fIlor before being connectect!0 an aircraft system or component. The ground servicing trW1-Cy normally incorporates a pressure regulator. This is essential to ~ ow control over the pressure after the storage bottle Stop vah-e is opened. Connection of a borcle directly to a component like a;:C,uyre or accumulator will inevitably carry a '-cry serious explos ion risk~
,"
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3.
Before connecting a deli"ery hose to a system. the hose should be inspected to ensure that it is clean and free of debris, moisture, oil or grease. All hose outlets should be properly blanked when not in usc. Beetles and other 'creepy crawlies' love to explo re opcn hoses but do no! ~lppreciate being blown into an m crafl system! Purge the dcli,'cl} hose with gas to clear il of ambient alI' lind moisnJre before connecting it to the charging poinr.
-1-.
All charging operations should be carried OUI slowly. Raising pressures quickly creates excessi"e heat from tbe compression and this increases me risk of explosive reactions with contaminants. Additionally, when compressed gas is expanded qUlckly through restrictions such as inflation or charging \'alves there is a high risk that any mo isture contained in tbe gas will instantly freeze and block the ,alve .
.).
FinaIl)" when charging is complete, it is a very good idea to close the aircrllfr sySTem charging \'
6.
Do nor expose gas cylinders to an external hear source, as Ihis can cause a dangerous rise in pressure in the bottle. Do not mishandle a bottle - a fracrnred stop \'a!ve can transfonn it into a gas-propelled rocket!
Compressed Air
'\~'\\ L. - (,~,\ \..
This is the cheapest compressed gas and comists of 21 % oxygen, 78%, nitrogen and 1~t)l..p.re gases plus some absorbed moisrure. The presence of oxygen h~~IJ1,acre it unpopular as a gas for many aircraft uses because of the ri~SPl1:~ explosive reaction bct:ween tbe oxygen in the air and any oil or grcase ,,~'(aeposils when the air is suddenly pur unde r \Tery high pressure. This effect is appropriately called 'dieseling'. Compressed air also has a high moisture content making it unsui.Iable for many uses. It is still used to charge the pressure storage accumulators in high-pressure pneumatic systems and the low-pressure tyres found on some light aircraft types. Care needs to be taken with all compressed air systems in that they produce free water by condensation that can pass through connected equipment and can ice up at low temperatures or pressures. In the longer tem1, the water poses a corrosion risk. \'(/hen water traps are provided 10 pneumlltlc systems they have to be drained on a frequent basis. Compressed air is also the favoured choice for workshop air systems and is used for example to power pneumatic tools, test rigs and plastic media Strip plants. In this Cflse, the supplies are at significllndy lower pressures than those contained in compressed air systems provided for aircraft use.
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Extreme care needs to be taken when high-pressure air systems spring leaks. The favoured method for leak checking is to use mild soapy water and a brush. Ho\\"cyer, in most cases. the leak can be heard in quiet surroundings . You should never check high-pressure gas systems for leaks by hand. Care also needs to be taken that workshop shop air hoses are properly connected to pneumatic LOols. The hoses may sna ke and whip if they suddenly become disconnected.
Low Pressure Air Systems Aircraft lo\v pressure pneumanc sys tems use hot air at -1-5 pSI. for the air condinoning, thermal ant-icing and engine air start system s. The only occasion that you will haye to supply this air o n the ground is when using a low-pressure air trolley. There are a couple of precautions to keep in mind. The air is very hot, over 100 degrees centigrade, and this can cause burns to the skin if you put your hands in its path or on the ducting. The supply ducting has a diameter o f J to 4 Inches and a surprising catalogue of apron wild li fe, seeking warmth in tbe cold weather, \vill easily access this if it is left open . .All low-pressure air ground supply hoses must be blanked off when not in use. Cabin conditioning trolleys are often used in hot weather to cool aircraft cabins on dle ground. These have large diameter delivcry trunking that is large enough to house feral cats! I t has happened! These ducts must always be covered when not in usc.
Aviation Breathing O",. ygen O:..-ygen is an odourless, colourless and tasteless gas. 1t is very reacti,·e and will combine with \'irtually any element or compound. Reactions between fuels and pur~ ~"'-ygen produce sufficient beat to inirutre a fire that will then be sustained "11."\c.~y- tfile oxygen. Contact with oil and grease deposits, for example, can cause . (,.....,\\co;c - spontaneous combustion. Oxygen is not in itself flammable - it merely \,;,>~(': supports combustion. The problem is that many substances that would not _,"'. . ~ normally be considcred fuel s become highly flammable when they arc in (,.....,(('<...\":'" contact with high concentrations of m.'}'gen. T
Gaseous o xygen is uscd in flight crew and sometimes passenger emcrgen!;y systems both 10 6...xed installations and in portable 'walk round' sets. T~ oxygen that is used is called Adaoon Breatlung Oxygen or, in the US, Aviator~ Breathing Oxygen. The commercial specifications of oxygen used for weldin~ and medical purposes are not suitable fo r aviation use because they have to~ high a moisrure content. A\-iation breathing oxygen has maximum moistu ~ content limit of no m ore than 0.002% or 2ml per litre. The reason why thisJ:"s so important is that any moisture condensing oul of the gas will freeze· at h~h altitudes or if it is expanded toO rapidly through restrictors. This will blod
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introduce mOisture sysTems.
111[0
the gas when you arc tasked \\;th charging oxygen
1 think timt you will have now become aware that m;ygcn requires careful handling. When this done it is as safe as ao)' other gas. I will list some of the important precautions .. \gain. c o rnfllOn sense prcYrtils. I.
Nc"cr charge an air or nitrogen system \vlth oxygen. The mixture \\;11 prescnt a significant fire and explosion risk. These systems ha\'e different charging connectors to prevent you from inadvertently doing this. Never mi.x oxygen \\·; th other gases.
2.
Ensure that you are using the correct speciticacion of oxygen. Tbe bottles arc black with white hemispherical tops and arc marked with rhe word O-xl'GEN and me chemical symbol 02 in white lenering. The bottle also carries rhe warning 'CSE NO OIL O R GRc..\SE' in red lenering on a whire background. .Americans usc green as [he identifying colour for m.")·gen bortles. If in doubt - check.
3.
Do nOt attempt ro re-charge an o:\,gen cylinder that has been fullr discharged. There is a strong possibility [hat it contains condensed moisture and it will need purging by the contracrcr..\ircmfl srs1em bottles have 1800psi ma..... so [here is little point in using a t.ransport baltic lhal has below this because you \\--iJ1 not be able to fully charge the system.
4.
Ensu re that there is no moisture, oil or grease on the cCluipmcnt and lools or in the vicinity. This includes your hands and you r clothing.
5.
Ensurc that clolhing worn during oxygen system servicing is kept clean and dry and is only worn for dlac purpose. Personnel should wear clean lint-\r.ce cotton gloves. Remove clothing and hang it in a well· "'ilWiou?a\\ area for at least a '1uarter of an hour afler completing the lP\\': -~ask. Oxygen will penetrate fabrics and make them highly flammable. If ,,).~(' rou light a cigarette, or come imo direct comaet with one, in the crew ;......\ 'rest room after work you could just as likely go up in flamesl
.~\~
, " -,'
6.
Do not expose oxygen to a source of sparks or fire. :\ small fire rapidly becomes a "ery big one in the presence of oxygen.
7.
Ensure that you ha\'e a pressure regulator in the line between the transport bottle and the aircraft or component charging poim.
8.
\Xllen charging aircraft systems ensure that the area is well ventilated and that the aircraft is earthed. Also ensure that the grollnd charging troUey is earthed. Place NO S1IOKING signs oursidc rhe aircraft and avoid all testing of electrical or radio systems. Ensure that an approprt:lte fire extinguisher is positioned where it can be easily accessed.
9.
Do not allow the open eod of a charging hose to trail in puddles or on damp ground. J\lways clean rhe charging hose connection \vlth dry
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lint-free cloth to remO\'e any m oisture. Open the regulating valve slowly and allow a low flow o f m;ygcn out of the hose to purge it of moisture and air for about 30 sec before connecting it to a charging point. Keep the flow on whilst connecting the hose to ayoid introducing air into the system. 10. Always open vah-es slowly by hand and do not altempt to force them. Keep charging rates low to prevent creating excessive heal. 11. \Vhen charging is complete, you should close the aircra ft system charging \'alve and the transport cylinder stop valve before releasing the charging line pressure by closing the regula tor valve slowly to prcyent a rapid release of pressure. 12 . .Always blank off any open ports in m...ygen system s and charging hoses when these are not in use. 13. N ever lubrica te the threads of o:-..-ygen equipment \\o;th o il or grease. 14. O xygen that emits a bad smell is mos t cerralnly contaminated with mOlsture. Do nor use it.
Chemical OX'Ygen Generatocs Chemical oxygen generators produce oxygen by burning a candle of sodium chlorate in the p resence of iron powder in a steel cylinder. The chemical reaction produces a fl ow of low-pressure oxygen . These generators are now widely used to supply passenger emergency oxygen 'drop down' masks. A generator is activated when a fi.ri.ng lanyard attached to the mask triggers a firing rpecharusm. \Vhen fired, a coloured band on the s teel container changes "l0cR91an~' to indicate timt the generator has been expended . . (,o""e~c. ". ,''loP
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C hemical oxygen generators are completely safe as there is no oxygen p resent until they are activated . It is \'ery important, however, that the correct protection cap is fined m-er the firi ng end when the generator is remm-ed, transported or stored. If tlus is not done there is a risk o f snagging the lan),ard and inadvertently activating the firing mecharnsm. 1\ m..... jor air disas ~ occurred in America when a conSignment of chemical oxygen genera tog; caused an intense fire in an aircraft baggage hold because they were n~ equipped with rhe correct protecti\-e caps. :::
Nitrogen
f 0-
"
Nitrogen is a colourless, odourless inert gas that is not flammable and \viJ(not support combustion or oxidation. f or that reason, it is now the fa Wi..l[ed choice for mflating tyres, charging landing gear oleos and brake £ ressure accumulators . Some aircraft systems require lugh purity nitrogen th:&11as low moisture content. The precautions for charging aircraft syslems with nitrogen
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arc similar to the general precautions given cartier in this section except thai (here is no fire risk associated wim this gas. It is still important to purge the charging line so as not [0 imroduce air intO a sysrem when charging and to usc a pressure regulator.
Helium This is a vcry liglu, colourless and odourless inen gas rhal is nOt flamln:lblc and docs nOt support combustion or oxidacion. Helium is notable in tha I it will nO[ combine with oxygen or water. Its use on aircraft is \"ery limited but it mar be found in some detectors, switches and instrument systems. It may also be found in ii'luid form in cryogenic processes. Helium is also supplied as Balloon Gas to fill airships and this specification has an air coment that is less than 2°10. Bottles containing this gualitJ heliwn arc brown \\;th a cream ncckband. Though heavier than hydrogen, it is the inert nature of helium [hat makes it the attracti,·c choice. For those airship buffs, It is imponam not to inadvertently introduce air when filling the bags, as this \\ill direct.lr affect buoyancy.
Acetylene /\cerylcne is a colourless gas that has a disanctive and disagreeable smell and will ignite instanlly if exposed to a spark or a hear source such as hOI metal. It also has an:lesthetic properties and this coLlld pose a hazard to personnel. I lowever, the fire and ex plosion risks are far higher. It burns with exceptional light intensity, particularly in contact with oxygen, and care should be trIkcn to protect the eyes with welding goggles to prevem 'Arc Eye' which wi ll damage the sight. II is lIsed together with welding m...ygen in the oxy·acctylene welding process. ;'i\n-: "I\'I\~6
SReO)")(f ~p;ecalitions arc required not to confuse the oxygen and acetylene ~).~t\linders. \,(/elding o""ygen cylinders are b!ack and ha"e a right h:wded stop
valvc thread :lnd hose thrcads whilst acetylene cylinders are maroon and ha"e a Icft handed stop \'al"e thread and hose threads. Thc twO cylindcrs also ha,"c distincc\"c shapes. 'nle o"""}"gen cylinder has a rounded bottom whilst the acerylene cylindcr has a flat base. Acet)"lene is '"cry reaco,'c when in contact with some mctals and alloys, for example copper :wd siln:r, and can form cxplosive compounds. The fire hazard associatcd "ith acct)'lene makes it doubly important that the equipment does not leak. You must never attempt to check for leaks with a naked flame. Soapy water and a brush is the favoured method for dctccting leaks. \Ve ha,'c now co\'cred [he gases that yOLi are most likely to encounter apart from carbon dioxidc, which we will deal with later undcr fire extinguishants. We will now movc on to examine the practices associated with oils and chemicals.
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Oils l11ere is a range of oils used on aircraft for: engine lubrication sysu:ms, ancillary gearboxes, oil filled landing gear StrutS, hydraulic s~-stems and a host of general lubrication pointS_ \'\-e need to look at these sepanHe1~'.
Turbine Engine Oils Turbine engine lubricating oils are low viscosity, synthetic ester-base otis and ace fonned from a cocktail of chemical compounds that arc toxic. First generarion turbine oils are referred to as Type I oils. These do not meet the operating requirements of modern gas turbines. Type 2 oils were dcYclopcd to withsmnd the higher operating tempcrarures of later engines. 111e two types are NOT compatible ·I.vith each other and must never be mixed together. The irritant narnre of turbine engine lubricaring oils means that it is essential that you read the safety instructions. Simply, you must avoid prolonged skin canmer with these oils and you must neyer inadvcrrencir ingest them. In the shorr [can, skin comact produces Demlaoos. In the longer Tenn, dlere is a risk of health problems occurring as a result of prolonged comact and absorption through the skin. Ester-base oils are hygroscopic in thal they tend ro absorb moisture from the air. It is imponanr that dispensing eguipmem and oil containers arc kept clean and sealed. Dispensing equipment should be dedicatcd to one oil specification and ne\"er be used for anything else. Open containers of oil should never be used and they should be disposed of. Oil systems do not contaminate themseh-es - it is people who introduce the contamination. Ester-bilse oils are also aggressive
to
most aircraft paint ftnishes. Polyurethane
"1..,\RJ'li a~~.nis hes offer the best protection from them. .
, ,,\,'1. _
,
U'\;
~,, .
i\\C.~o.:-
~~, LP
\,.~'
T he safe IJtacnces associated with turbi ne enoinc oils should now be clear. o· I. Never mL.... Type I and Type 2 oils rogether. So, check you have the correct specification oil. It is in the Aircraft Maintenance Manual and ,.... marked dose by the replenishment points. ·6 2. \Vear protective gloves, apron and face shield when handling lhe oil§. and are at risk of direct skin contact. .~
..,
3. \\·ash hands thoroughly after contact with Ihe oil and do not smokq drink or eat until you ha,e done this. Jf oil enters the eyes, seek medicjl anennon. ,. "04. Ahvays use the correct dispensing equipment and make Sll r e it is cla n. ~
,~
::>.
2
Clean up spillages immediately before they attack surf.'lce finishes~ 'u .§' U
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6.
Ensure you follow the correct repleni shment proceciurc. Turbine engine oil tanks usually han (0 be rcplenished wiThin a given time of cngine shut clown or there is a risk of overftlling the system.
Reciprocating Engine Oils Reciprocating engines usc higher nscoslI)' mineral oils that often ha\"c a detergent additivc. They are NOT compatible with rurbine cngines and, \-ice versa, rurbine oils cannot be used in reciprocating engines. Unlike rurbine oils, reciprocating oils get vcry diny with use and this increases their toxicity. J\linernl oils arc aggressive to the skin, particularly when they arc dirty and carry the products of combustion like carbon and gum deposits. The chief risk is Demlatitis bur prolonged contact with old engine oil has been suspected of being ca rcinogenic. The safe pr:tclices are similar to those we have discussed. Do not usc the wrong oil. The usc of thin turbine oil in a piston engine will quickly lead to failure .. \lways usc the correct dedicated clean container to replenish the system and do nOt introduce contaminants.
Hydrau lic Fluid The low viscosity fluids used in aircraft hydraulic systems may be either mineral based or synthetic oils. Vegetable oil was used:u one time but you arc unlikely to find it in use LOday. Fire resisrant phosphate este r-based oils arc now commonly llsed in modern systems bur mineral oil is still found in usc in some aircraft sysIR\YlS and in landing gear shock absorbers. "111ese oi ls arc NOT inrerchan!!wrblc ~~nd have a devastating effect on seals if the wrong oil is used. Tltc;.ij,l-."'Ii~ompaubility is so pronounced cllat it is e\"en dangerous 10 maintain \,;\{componems using mineral oil in the same area as components using the olher ~.\ ~ because of Ihe risk of cross comarnination. The seals and most of the flexible "- '-...\......~~" hoses used arc also not imerchangeable. Din also quickly renders many hydraulic components inopernuyc. Vegetable and miner:t! oils are flammable but (he phosphate ester-base oils carry a low fire risk in that ther are ,-ery fire resistam. They can burn in certain conditions. i\linernl oils arc not normally irritating when ther contact the skin. The phosphate ester-base oils on the other hand cause se\'crc lrritation and are also very aggrcssi\'c to all paint finishes except polyurethane. TI1e vapOUr5 from phosphate estcr-base oils is sufficient to strip cellulose paints off walls and ruso seem to be a vcr)' effective fly killer, looking at the hcaps of dead nies that sornetimes accumulate on the window sills of hydraulic component bays! These oils arc particularly irritating to the eyes and also to the respiratory system ir the mist or vapours are inhaled.
34
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Hydraulic fluids absorb moisture from the annosphere and can become contaminated by frce water as a result. It is essen,tial to keep containers tightly sealed when not in usc. Phosphate ester oils can also become acidic when overheated or contaminated. The safe practices assocIated with hydraulic fluids should now be clear. I.
Always check that you have the correct specification fluid. Consult the Aircraft Maintenance .:'Ilaoual and also check the markings on the aircraft replenishment points. Ne\'er mL"; mine ral and synthenc fl uids together.
2.
\"\lcar protective glm'es, a face shield and an apron when replenishing systems that use phosphate ester oils or when \vorking around p rcssurised systems. \'Cash affected areas of the skin with soap and water If you come into contact with either type of fluid. Do nOl smoke, cat or drink until you ha\"e done this.
3.
If the eyes arc affected, flush with water. If the eyes or respIratory system are affected by phosphate ester oils YOLI should seek medical aSSIs tance.
-I .
Alwavs use dean sealed cans for replenishment or a dedicated, clean scaled replenishment rig. If there is any suspicion that a fluid could have become contaminated you must not use it.
Lubricating Oils Lubricating oil~ are specified for components on the basis of their properties in rciau(lf to the operating conditions of the component. J t is pointless applying '\lClbW"\~scosity oil to a heavily loaded component thar runs at high temperatures . U)\l.C~c - if the oil cannot maintain a film in those conditions. That is why it is essential , 'b~~~"" to check that the correct specification oil is applied. It is also pOl!1tiess applying :'1.,\ ~ the correct oil if ,au allow it to become contaminated with dirt, moisture or ~,~\~ other oils and it b~caks down as a result. (:--"'\ . T he message is to use the correct specifications and scrupulously cle\ID equipment. Do not use ti1e same piece of equipment for different oils . AlwaS> seal containers after use to prevent dirt and gn·t entering and do not mix oils . :;.
S-
Chemicals
.-0
The very word, chemicals opens a 'Pandora's Box' of possibilities becallse~e can descri be just about any substance as being a chemical. The chemicals ?ihat we have most safety concerns about \vill fall under one o r more the following headings: flammable, corrosi\"e, toxic and chctn1cally reactiye:' They mar be solids, liquids or gases. \\·e can look at a description for eac~i5 f these groups and a few basic precautions that apply. In the latter case, you-'"\vill need to consult the relevant material safety data sheet and the COSHH assessment
i!'
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fo r all substances that arc designated as being hazardous. These sou rces of information :Ire explained :It the eod of tlus section.
F lammable Chemicals Sparks, heat or a naked flame mar ignite chemicals in this category. They will have a flash point, which is the lowest temperature at which the chemical produces enough \'apour to burn should it be ignited. Ibis is Ilonnallr the figure we would be most concerned about. Substances that we commonly meet in this category arc fue ls, oils, solYCnts and paints. Simple precautions indicated by the nature of these chemicals are: I.
Do nOt expose them to sources of sparks, heat or flame. This includes sparks generated by static electrici~'. For example, aircraft. fuel tanken: and bulk fuel conL.uncrs must be connected to eanh.
2.
Do not bring more than you need into the workplace.
3.
Ensure that there is adequate \'encilarion to prevent a dangerous concentration of vapour. Remember the flash point.
-I.
Keep containers scaled when not in use and cnsure they slored in a designated flammable store.
5.
Ensure they arc kept in a suitable container. Risks here stem from decanting fluids into unsuitable containers that are affected by the chemical.
Ensure there are adequate fire extinguishers available. Also ensure they arc s('rv~~ble and thar YOU know how to operate them . • 1.~"-\O . _ ;,,\0;..;.... ~..; (..'1. I f the chemical is ruso (Oxic \'oU should wear glO\-es and, if necessan" ~ . Well[ a respirator. 6.
.
Dust This is as good a place as anr lO draw attention [0 the danger that can be posed when high concentrations of airborne dust occur. People who operatc flour mills, grain storage tanks and wood saw mills will be more than aware that high concenrrarions of dry dust mi:-;:ed in air foml a very high e..,
Corrosive Chemicals These will rcact with and attack metals and will morc often (han not cause burn s on your skin. Etching agents, battery electrolyte solutions and corrosion
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remm-al o r passivauon fluids are a few examples of these. I"vfost corrosive chemicals arc 10 liquid fonn but you ca n encounter powder fonns such as salts. Use of these chemicals will need a few basic precautions : 1.
E nsure tha t they do not corrode the container you put them in or any containers that you store them near.
2.
Do nor inadvertently let them contae[ sensitive material.
3.
Ensure you do not leat'e anr residue when removing the chemIcal.
4.
\\iear swrable protecove clothing such as goggles or a face-shield, glm'es and apron .
J.
Check the eyewash facility.
6.
If the chemical is also toxic and/or flammable you must also take the preca utions applicable [Q these hazards.
Toxic Chemicals Toxic chemicals are poisonous and have the potential to cause injuries or diseases in the body that can appear in the shorr or long Tenn. Examples o f the diseases can range fro m illdusmal asthma to tlle deterioration of the fU Ilction of internal organs and yarious cancers. Some toxins . such as solvents used in cleaning plants, may have anaesthetic properties and produce effects similar to intoxication. In the extreme, these may have narcotic effects that can be lethal whilst others may attack the central nervous system and the immune sys tem. They have a number of routes into the bod"\'. \Ve can breathe in the \'apOllr, ~1e }i~ can absorb liquids and pass them 'into the bloodstream and we can ','o.:\!'c ~\.\qiiadverteotly carry substances onto food and into drink and ingesr them .
. CP' ' T.
. x.-\ :-:..(\~
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~
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Some substances may be hannless ill their delit-ered state but become harmful when that state is altered through a work procc~s . For example, a solid may produce harm ful dust when machined. A substance may emit hazardous fumes when it is heated o r brought into contact with reactive chemicals . Higltl~, yolatile and flammable toxins are likely to produce high concentrations 9f o vapours in poorly t"Cntilated areas. -~The precautions [0 be taken here will depend on facto rs such as the risk anet the occupational exposure limit (OEL) given in the safety data sheet, and th,f COSHH assessment applicable to the area you are working in and the wan;.: , process. 6' ,
'"'"
1. Wear the required personal protectiye equipment and observe".'t he t" exposure limits .
"
0\" 2. Ch eck that the required environmental cpntrols are availabl~nd use
them. This may lOciude extraction fans, filters etc. Refer w "COSHH asscssn1cnt.
37 Coppighl :0 BtII1)' Co/kge 2006 - 2007 All nghls wfrvtd
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3.
If the chemical is also flammable ensure [hat precautions.
yOll
also absente the fire
4. Be aware of any symp toms that may occur and eyacuate the area if the~' arc recognised. Do nor forget to warn others in the vicinity and alert the supervisor.
Reacti ve ChemicaJs Rcacu"e chcmicals are those that experience a chemical change when they arc brought into contact with other chemicals. r-. Iany chemical reactions il1"ol\'e the releasc of heal cnergy. Depending on the reactivity of the chemicals. the intensity of rhe reaction may range from a slow change in properties to a violent explosion. Some chemical rC:1ctions may produce temperatures that cause materials to ignite and burn. Examples of highly reactive chemicals arc the catalysts and accelerators that arc used in composite repairs. Peroxide is a good example of such a chemical. ~Iany surface treatmen! and plating processes such as '.\lochrom' and cadmium placing invoh-e the usc of reactive chemicals. Some reactions result in the emission of toxic ,-apours sllch as phosgene, chlorine and ferro-cyanide. Again, the precaurions will be related to [he type of chemical and what you arc using it for and where. You really must check the COSHI I assessment. I. \X 'car appropriate personal prorecti"e equipment. 2. Chcck the re'lwrcd elwironmenral comrols are in place and usc them. 3. Learn to recognise the symptoms of toxic effects . .L
E'~c. fh~\ you
follow the instructions on quantities. Explosions can
. c-..~\..:~~cur when you get this wrong with catalysIS and accelerants. \..,;\~<'
:l.
Ensure that the correCl fire appliances arc ;'inilable ... \gain, do check they are sen"iccablc and that you know how to use them. r
o
I ha,"e gi,"cn you "Cl')' broad descriptions of safety precautions associated \\~th the groups of chemicals. It would be impossible to give every precaution for every chemical in usc. This is why we h:n'e COSSII assessments to idemify the controls that arc needed. Remember, these relare to the usc of each hazardous substance in relation to where you use it and what you arc using it for. A point worth remembcring is that COSHH assessments are no use [0 )'OU if you do not read them. Wi thin the groups therc arc a lot of chemicals that belong to marc th:m one group. \X'e could find this a bit daunting when trying to idcntify the hazards. We can make a logical approach [0 this by dividing up [he properties of all the hazardous substa nces we might encounter under a few headings. These arc:
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•
Flammable
•
H ighly flamm able
•
Explosi,c
•
Toxic
•
Very roxic
•
Trrimnt
•
Harmful
•
Oxidising
•
D angerous to the enyironmcnt
O N E
PREC A UTIO NS
Hazard Warning Labels r.hm- hazardo us chemicals will share mo rc th an one of these properties. Looking at the list we have made, unlike D r. Jekyll, I would like to bave some warning b efo re usi ng the co ntents some innocent looking bottle o r can . Fortuna tely, there is a legal re
.
\'e~c ~ _l,l'\~ -
~ \,)(t~ c
.\V:'-" -
;;.\ .
-
U,'X '
Toxic
Irritant/ Harmful
l lip;hly Flammable
o
Explosive 3.
-.
-f~
CorrosIve
O xidi sing
Dangerous for the Em-ironment
Figure 1.4 - Hazard Warnin g La b els
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"nle problem is that when an accident occurs, for example to a \"Chicle carrring hazardous malerials, rhe emergency services and other specialist personnel often need to know a bir more detail about the hazards ther face. It is not always wise, or possible, to access the contents to check container labels and material safety dara sheets. In these ci,rcumstances, additional hazardous material identification labels may be displayed that carry codes and other infonnation that giY(~ more specific ad\·ice about fire hazards, health hazards, the reacth-e nature of rhe chemicals and any specific hazard that may be present such as radioactive or corrosive materials for example. Specialist chemists decide on the coding and trained persons such as fire and rescue personnel imerpret it. Jf you arc at the scene of nn accident !nYolving a vehicle or container ca rrying a hazardous subsm nce and you do nor understand the codes, keep away from it! You may have noticed these types of labels on vchicles.
2XE
1235 Specific I inzard Fig ure 1..5 • Examples ofVebiele H1za rd Information P ane ls 'l{)\)i
1\,.\1..,6
,.
...
T11e0 \tU:.1fi -illustration shows that the substance being carried is an oxidising ~;}~~~nt and that it is chemically reactive. The substa nce is identified by code and ~\,\. 1-, there is sometimes a contact telephone number for gaining specialist advice. :'I'Y.'.f,\"" You often see signs similar to this on the back of rankers on the road . The second illustration is based on tbat of the National File Protection Association in America and is an example of the type of label that would be used on vehicles in that country. It is provided here merely for your imerest and you arc not expecred to know all the definitions of the associated codes. In this example, the code figure in the red scp.mre indicates the flash point. The code figure in the yellow square warns of rhe degree of chemical stability. The Icners in the white square in this example warn tb:'ll the material is an oxidising chemical and the figure in the blue square indicates tbe lel"d o f the danger to health. There are a lot of codes related to this, too man}' to pur here. Very simply, higher the figure code is from 0 to 4: the greater the hazard associated with ir.
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Material Safecy Data Sheets Jt is a legal requirement that each hazardous substance \vill ha\'c a material safety data sheet that is pro,ided by the manu f.' lcturer. The sheet h
COSHH Regulations T he Control of Substances Hazardous to Health Regulations 1988 reCJulle th
,$' Assessments must only be carried out by gualificd person s \vho are c6 nsidered competent in accordance \vi.th the regulations. An assessment in cludes
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identifying all Ihe forms in which the hazardous substance may appear in a work process, evallL'lting risk, identifying exposure limItS and the effects on the body, introducing conrrol measures ro reduce exposure :md assessing the effect o f deterioration in these measures. It is a thorough task. In forma tion from the m:Herial sa fety dala sheers and other sources is used to support the assessmcllI. All completed assessments must be made available to the workforce and any equipmem or procedures required ( 0 suppOrt the control measures must be pur in place. '!11ere are a number of general arrangcments that would ha\'C to be made in respect of any process that im-oh-cd hazardous substanccs. Examples of thesc \\'ould includc: •
Reducing the number of personnel e,"poscd
•
Restricting access
•
Reducing the rime of exposurc
•
Prohibiting actions such as smokin& cating and drinking
•
Providing washing facilities
•
Providing changing rooms and safe storage arC!l.S for clothing
•
Inuoducing decontamination measures for the work area
LO
the area
E nvironmental P rocection The Environmental Protection _\cr is concerned with the effect that haza rdous substances have on the environment. \\'hen these substances arc pa sscd into the aunos~~c[{.'1~r imo [be ground water rable for example they nOt only prc:sS,{~vJl?t"1.~ids to other people bUl also affect the ecosystem. Asse!>smenr is t fC~ulred to es tablish the bazards and risks lind to identify the protection \"0). measures required. TIlese may include filtrati on equipment and special processing measures to effect safe disposal of subst:lnces and their byproducts. Local exhaust \-entilation ouden; (LEVs) that pass fumcs out of buildings into the air and was[C fluid storage [.'!.Oks arc also subject TO the regula tions and these ha\-c to be inspected and testcd at stannory intervals.
Personal Protective Equipment (PPE) Idcallr, employers should seek to find safer aitematiycs 10 the processes and substa nces that create the hazards. This is not always possible and steps ha\-e lO be taken to prorecr the person. Employers arc responsible for providing appropriatc PP E for all personnel who are exposed 10 the risks th:1I may be idencified in the work processes they undertake. -nle equipment must be pro\-ided, together wi.th any tta.ining required in its usc, and be properly maintained free of any charge to the employee. I n this section we will take a
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brief look at some of the PPE that is proyided for respiratory protection, contamination of the skin, eyes or clothing and noise.
Respiratory Protection Respiratory hazards include: dust, mist, fwncs, gases and \·apOLiI'S. These may nXluire the use of breathing apparatus or a respirator. There is a difference in the meanings of these terms. Breathing Apparatus
This e'luipmem is designed to supply uncontaminated ai r through a line to a regulalOr and a facemask. "nle source of the air may be a rn.ed installation or a portable cylinder that may be earned by the indj.idual. Respirator Respirators are designed to clean the ambient air in the \vork area through one or mo re filter cartridges that remm'e the harmful substances before rhe air is inhaled. The choice of respirator and the type of cartridge to be used will depend on the nanJre of rhe contaminanc, the fonn in which it is present in the air fmd its concentration in terms of parts per million. Occupational Exposure St:mdards will determine lhe class of fil ter used. When donn ing a face mask, It is important to check it for correCI fit. A badly fiumg mask offers no protection at all. An exam ple check procedure is to tlrst ca rry oul a l1eg:llil"e pressure check. The wearer covers the inlet to the respirator and then in hales and holds the breath for fi\re seconds . The flexible part of the facemask should collapse slightly and remain so unt.i.l the !luer is unco\'(:Ic(\. The second step is to carry out a POSilil'c pressure check. The ,\,,\«ea1~i covers rhe exhaust port and breathes om. The ~exible part of the mask _ \',,;.\!.":' - should bulge and there should be no leaks around the SIdes. l,.--.I\ . \"\:l-~\" ,'C:. ~ ...' --v''''' ~ .
(\,--,\
\'\/hen wearing a respirator, the wearer should be alen to any difficulty in breathing or any unusual s~' mptoms such as irricauon or dizziness. If any of thesc occur, lhe wearer should e\'acuate the area and change the respirator.
Protective Clothing Again, the nature of the clothing will depend on the hazard. Face protection: ma~' require the lise of goggles or a full -face shield. The transparent part of thi goggles or shield may be made of a tough shatterproof material such ,:<1s polycarbonatc where there is an impact hazard. T he design of a full-face s l~'Md may. also incorporate, or be a part o f, a full flow breathing apparatlls /s r a respirator. Jj
,
,~
Glovcs, gauntlets, prorecnve aprons and footwear may also be rcq~cd and these must be impermeable to the substance used and not be made of a
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mrllcria! dlrll is detcnoratcd b\, it. \\there personnel have to work in high concentrations of hannful ,·apours such as aircraft fuel tanks then a one-piece irnpenneablc suit that incorporates inJe£s for breathing app:lr:lUls ha\·e to be used. It is essential that all clothing is remo\·cd after usc and cleaned of con taminants. There is tittle point in re-using contaminated clOthing. . \dditionally, contaminated clothing should be stored in an appro\"Cd ventilaTed area as the contamination presems a hazard in itself. Gases and \-apours can penemne fab rics and remain in them for some time afterwards and be a significant fire hazard. Oxygen is a classic example of this.
,,,ill
Safeey Signs Places of work ha,e signs that have different levels of significance depending on their configuration. These signs ha,·c to comply with the Safery Signs Regularions 1980. Pro hibition Sig ns These signs arc round with a rcd border and crossbar against a white background carrying a central black symbol. YOLI must not ignore these signs. \'(ta rning Sign s These signs arc Lriangular with a black border against a retiow background that carries a centml black symbol. These signs arc cautionary.
Emergency Signs \\.'arning Signs
Prnhibicion Signs
Figure 1.6 - Safety Signs
M and atory Signs These signs arc round With a blue background that carries a central white symbol. These signs arc indicating a mandlllory action such as lhe wearing of specific personal protection equipment. E m e rgency Sig ns These signs are either squ:trC' or rectangular \\~th a green background tbat carries a central white symbol. These signs arc often used to indicate the direction of emergency exit routes etc.
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Battery Maintenance Kceping in mind the 82 students who arc e.ngaged in tillS moduJe J fcc! that we should include the safe practices associated wi th aircraft battery maintcnancc. The room that tltis is conducted In also fall s under thc heading of thaI' loosely used term 'workshop. The battery is an integral pan of the elcctrical sys tem in even' aircraft and mal' be a lead/acid banery or an alkaline batten'. These desc'riptions alone suggest that mere arc ha~ards associated with ~ervicing them. Battery chargi ng rooms are potentially Yery dangerous areas and there must be strict guidelincs for personnel working in them or visiting them. Ilydrogen is \'ellted during c\~ery stage of lead-acid banery charging and rcachcs the highest concentration towards the end of the charging cycle. Il ydrogen is also \'ented from fully charged alkaline battecies. 'Illcre arc also highly corrosi\'e fumes created when electrolytes are mixed. Battery M a inl en anc c Roo m s Battery maintenance must be carried Out In rooms that arc dedicated to that sale purpose. Therc must be separate charging rooms fo r lead/acid and alkalinc batterics, a powcr unit room for the electrical supplies Ihat supply the battery ch :1fging boards and a separate room that is used as a workshop for minor repairs and storage. Cnder no circumstances should the smne rooms be u ~ed fo r lead acid and nickel cadmlllm barrery charging. The \'cnulation o f the charging rooms must be designed so that no cross-conta.mination can occur. /\ 1\ equipment and tools should be clearly marked with the bauery type thcy
arc to be used o n, i.e., 'Acid Only' or 'Alkaline Only'. and should be kept in Lhe related rooms. ~letal tools should be of a suitably insulated design. /\11 containcrs for electrolytes and neutralising agents should be dearly marked to identify their coments and those used for lead-acid battcries should be kept segrc~ted from those used for alkali batteries. Lead-acid and alkaline ba tteries ,~~\dl0tl\a' be stored separate from each other, in di fferen t storerooms if possible. ':-'; .\!....: ...
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-
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The ambient temperature in the charging rooms must be maimained within thc tcmperarure limits specified for rhe battery type_ for example. below 2rC for !cad-acid batteries and below 21°C for alkaline batteries. The charging rooms should be cool, \vell lit with explosion proof ligbting aad be c'luipped \\;tb an air extraction system to prevent the accumulation o f diS explosiye gases that are vented off from the baneries during charging. 'n~ gases are a ml.--'::Nre of hydrogen and oll.-ygen so they are potentially ver)! d:lI1gerolis. The ventilation sntem should provide for a free flow of air around' each battery on cha rge. . :: :;
The floo r surfaces sho uld be dry, dust free and formed from a material tha SIS proof agamsr acid or alkali attack such as concrete, tiling o r birumi l)~ls compound. \,\/ooden duck boarding should be placed in from o f each cha.t;£ing b{;!lch. Floors should be lcye\ and there should be no steps o r sEt"11ped thresholds in the building, una\'oidab1e changes in floor b·ds shoul':!.:instead be linked by inclined ramps where possible.
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CHAPTER ONE S AFET Y PREC A UTION S
The benches should be constructed from hard wood with surfaces that are acid and alkali proof such as thin lead sheet. The recommended height for charging benches is lOin (O.Sm) from the floor {Q enable clear inspection and to reduce lifting effore. Batteries should not be aUowed to rest directlr on surf.'1ces such as wood, concrete or lead bur should instead be supported on l:miced panels of waxed wood during charging. Each charging room should contain a la rge sink with a draining board and an adequate fresh water supply tap and be connected to a safe disposal drainage sys tem. A bot water wp should also be made a\'ailable, Doors to the rooms should bc designcd to open outwards for casy evacuation. The baucry charging supplr is nonnally DC and this is supplied from the AC powcr distribution system through lransiormcrs and reccificrs. These are normally held in a dedicatcd power unit room. The indi"idual charging boards should be mounted adjaccnl to cach charging point :Uld be mounted directly above and to the rear of thc bench to minimise the need for long connecting cables. The cha rging cables and connectors should be properly insulated and have the correct rating for the charging current. No connections or disconnections should be made before the powcr supply has been switched off. \X 'hen disconnecting baneries, the cables should be remo"ed from the bauery first and thcn from the charging board. CO~ fire extinguishers should be strategically placed around the facilitt togcthcr ,,;th buckcts of sand.
E lcctrolyte Materials
(.~.;:'"
Lead acid ba ttery electrolyte comains sulphuric acid and distilled water. The acid is corrosi,'c and is stored in glass carboys or bottles, ea rthenware containers or lead~lined wood containers. These should be kcpt lightly sealed because the fumR~.-:1re also highly corrosive. The quantity of acid kept in the charging [~l'r1" ~\lould be strictl}' limited to that required for immediate usc. TIt~ :retTtaI"nder should be stored in a separate localion that is approved for the \....).
~\v-
.
Alkaline batteries usc a solution of potassium hydroxide and distiUed water. The potassium hydroxide is supplied in solid foml in scaled steel dnuTIs. \"ben a dnlm is opened, the whole contem should be used immediately because the compound is "ery prone to deterioration when it contacts rhe carbon dioxide in the surrounding air. Alkaline solutions are also prone to similar deterioration and , once made up, should be either uscd immediately or stored in a sealed glass container. Distilled water should be stored in dean, sealed glass containers. The stoppers of containers must be replaced immediately afte r usc. Do take note of the correct order, acid is added [0 water not the other way around. Safety Precautions:
46 CopY7ghl
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SAF ETY
CHAPTER ONE PRECAUTIONS
I.
Rubber glo\'cs and aprons should be worn when handling or maintaining baneries.
2.
PrOICCn\'e goggles should be worn when h:l ndling alkaline solution,
3.
No naked lights should be used (e xplosion risk)
4.
Spa rks should not be generated in a cha rging room (explOSIon risk)
:l.
Do n ot place batteries in positions where they can be knocked over or dropped.
G.
Do nor run or rush about when in a charging room.
7.
Keep tools and utensils clean and return not 1n use.
8.
Do not wear metal jewelle.ry such as nngs, metal watches. metal w:ltchstraps or metal bracelets when handling b:meries.
9.
Ncr 'er pour wafer into acid. (Acid is always slowly trickled :lnd mixed intO the w:lter)
11l
~lcid
or mixing
the battery charging room
to
dedicated stowages when
10. Check th:lt you know where the neutralising :lgcnrs are stored and where the fIre extinguishers arc kept and that you know how to use them.
II. Swnd on the dry, wooden insulation duckboards when carryi ng out charging procedures. "'~h~ Connect
banery terminals correctly to ayoid spa rking.
Electrolyte Spills: I. Spilled c1ectrol~,te should be soaked up with sawdust and then disposed of in an approved place. ~
"
-:>
2. Su rfaces affected by sulphuric acid should be washed in a sarur:ltcq solution o f bicarbonate of soda and ulen rinsed with fresh wateg Other neutralising agents are: ammonia powder o r borax powder.
etI
3. Surfaces affected by alkaline solutio n should be w:1shed in a satura .. solution of boric acid and then rinsed with fresh watcr. t\nod±er neuualising agent is boric acid crystals or powder. ;:j
'u
'"
#
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CHAPTER ONE SAFETY PRECAUTIONS
First Aid '111e following is for general infonnation only. You must adhere to the instnicnoJls (hat arc in force at your place of work. I f battery acid contacts the skin it should be neU[rniised \\;(hout delay using a saturated solution of bicarbonate of soda followed by washing \\;lh wann water. . \n~- burns can then be treated as burns _ _\dd splashed into the eyes should be neutralised by using eyewash solution comaining 5°/0 bicarbonate of soda followed by wann water. I f alkaline barter)T electrolyte solution contacts the skin it should be neutralised without delay using a sarurated solution of boric acid followed by was hing with wa rln water. I\ ny burns can then be treated as burns. ,\lkalinc solution splashed into rhc eyes should be neutralised by using eyewash solution containing 5% boric acid followed by warm water.
_\11 neutralising agents and eyewash soh.:tions should be ayailable in the charging room together \\<;th posters shO\\;ng their application and usc.
Hearing Protec tion \,\ 'e react to sOllnd faster than sight when we sense danger_ ~oise heightens me sensiti\-ity of the nervous system so much that it can C:luse fariguc and health problems. Noise intensity is measured in decibels (dB) and exposure to high levels can cause pennancm damage to the ears. The Noise at \,\'ork Regulacions 1989 stipulate the measures that must be taken at stated action le,'cls_ These arc: 1" Ac tio n Level
1n\r
8SdB .QE 1\lH~re - a normal conversational voice cannOl be heard clearly at a Ri$J.!l{}~~ of 2m or 6ft. T he emplorer must assess [he risk to the elllplo~-ee's \.. ~).tl,earing and inform them about the risks and what the proposed precautions _ ~,~....\ arc. 'n,c employees are to be pro\;ded with personal car protectors and briefed \. ~~': on their use. The choice whether to use them in this case rests with the cmploree.
r
d
Action Le"e1
90dB or where a nonnal conversational \'oice cannot be he:trd clearly at a distance of 1m o r 3ft. "111e employer must do all that is reasonably practicable [0 reduce exposure to [he noise by means other than JUSt pro\' iding hearing protectors. The zones where the noise exceeds lhe 2".j action le\'e1 ha\-e fO be marked wieh recognised mandatory signs that restrict entry_ Personnel entering these zones must wear ear protection and observe the occupational noise ex posure limi ts that detail me maximum time that indi\'iduals can remain exposed to lhe noise intensity. In this case, employers must enforce the wearing of ear defenders.
48 CDp)nghf
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It is possible to quantify exposure to sound in the Conn of a noi"e dose that is a combination of the noise imemity and its duration. Exposure to any sound [eye! oYer SOdB can be measured in temu of an :l\'erage nluc m-er an 8hr period. T his is called the Time nreig/Jfcd A I'Crtlgt: (l1f04). A T\v'A exceeding 85dB for Shr can result in damage 1"0 the inner ear. A 1'\\1A exceeding 90dB for Shr can cause permanent deafness. I f there is any rISk that you will be exposed to a sound intensity of IISd13 or above for any duration, howe,er short it is, you must wear ear defenders. :\ furrher prm-ision of the Noise at work Regulations is that no employee should ever be exposed to the risk of a peak sound intensity of I-lOdB on the unprotected ear. Noise experienced on an airpOrt apron can regularly exceed 85dB due to the dose proxmuty of nmning engines. Continuous exposure to these le.els will cause pcnnanem hearing damage so hearing protection must be \vorn . Ear defenders reduce the sound intensity you receive by :lbout -lOdE . Earplugs arc much less effective and only reduce intensity by up to 20dB. Noise in a hangar environment C:ln also exceed tills level, p:uticularly if pneumatic tools are being used. I will gi\'c you some example of the sound intensity lcn-ls we might encounter. Typic::t! lntcnsin' Le\-el
Source
?Odb
\\'hi sper Speech
:It ~1ll
50db
Typcwrlte r at 1m
65db
,\Iotorcarat 15m
-Udb
Hea,,' vehicle at 1Sm
-Sdb
Petr(ll lawn mower at 2m
9Ud b
Pr opeller c\ri\-en ai rcraft
"
30(lm
I (lfldb
JCt ,\ircrafr ar 300m
IIOcib
St:lnding next to a propeller dri\'cn ai rcraft
120db
Pain threshold
140db
Immcd l:lte he:ll1ng d::l.!ml.ge
150db
Table 1.2 - T~'Pic3 1 Sowld IJllc n s ity Levels
o
,1?'" .0
'
""
Before we mo\"t~ on to look at the second part of tlus ch:lpter that ~€:a ls with fi re protection and emergency procedures, there is one area th~[ is not highlighted in the syUabus but is nonetheless important and that is -lifting.
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CH A PT E R ONE S AF E T Y PRECA U TION S
Manual Lifting back injuries at work result from incorrect lifting. TIle spine is Ilot a good le,"cr or a good lifting machine. The following docs not replace the ad,;ce that will be found on the safety posters and should not be used in lieu of it. General ad,"ice is that the back and anns should be kept stmight and the leg muscles should be used to take the strain. \'(1)cn picking a load up from the floor, crouch and bend the knees not lhe back. Grip rhe load fi rmly and keeping it close to the body lift br straightening thc legs kceping lhe back and arms stmight. \X!hcn mo\"i ng the load, keep the back and aons straight, head up and the chin rucked in and keep the load close to the body. \'('hen a load is assessed as being toO great for one perSall, assistance must be souglll or a mechanical handling device should be used. r [ere is where the 'can do' attitude so often raises its head and people get injured as a result. Just StOP and think. Is somc box of bits worth a li fetime of back and hip problems? ~ Iost
Lifting Slings I ha,"c included this to bring a few poims to your attention. 111e first is simple and that is nOt to stand or < walk under a suspended load. The second is to cnsure that the safe working load of the sling is suited (0 what you intend to lift. That will be found stamped on the metal tag auached to the sling. If it is not there do not usc it. Thirdly. do nm forget 10 inspect the sling. ~Iy other point is to recognise the effect of using a sling when you set the legs at an angle.
IkN
I kl\:
O.Sk.l'\i
I
I
lk:\
lk:\
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1.7 -
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of Ang l ~ of Sling o n
th ~ T~ ns i o n
in U:g1
Imagine a r\\'o-Iegged sling is attached to a load weighing IkN.111e angle at the apex of the sling will determine the tension in each leg of the sling. As the apex angle is increased, the tension in each leg will be increased. At 120 degrees, the tension in each leg will be the same as the load i.e. I kN. At greater angles, the tension in each leg will be greater than the load!
50 CflP.Jnghl " Bany CrJ!k§ ~OO6 . :!oo -
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SA FET Y
CHAPTER ONE PRECAU TION S
Fire Fire is :I term used ro describe a process where o;\:ygen chemie:llly combines with a subs tance :It high temperature and releases more heat energy as a resulr. This mefl ns thflr there are three essential ingredients rC'luired 1'0 initiHe fl nd sustain a fi re - o.\ )'gcn, hem and a fuel All three must be present, should o ne be removed, the [LtC cannot sun·h·e. T here arc fou r classifications of fire and I will describc these for you. Cnder the current classification codes there is no separate classification for an electrical fire because all classes could equally im-ol\"(:! a li\-e electrical source. I will also gi\"e you examples of extinguishers that might be appropriate in each case but you must usc the extinguisher types prescribed by your particular orga nisation.
Classification of Fires Class A Fires Ih:ll im'oke solid materials such as paper. \\-ood and cloth. Can be extinguished with water-based excinguishers apan from cases where there is a live electrical source that cannOI be isolared. BCF (Halo n 1211 ) or C02 may be used in such cases. Class B Fires that im-oke flammable liquids such as petrol, ll\"iauon turbine fuel, solvents. lubricating and hydraulic oils . Typical extinguishers arc: C02, 13CF (11:t1on 12 11) and foam. \X'ater must never be used on a class 13 fire bcc;lUse it will instantly spread it. Some foam extinguishants are water-based but may be appro\'ed for flammable liquid fires because they do not spread a fire like plain waler does. 'I,ll
1\",t, ... \'\).~~ <
;\.~. ~
,<.\":-
U\~
..
t
• ' \ .:.\!.I.: ... ,~1'\
•
Class C
Fires that involve flammable gases. You should not attempt to fight these fires. Turn off the supply if possible and then you can figlll the subsidiary fire s depending on what rhe classification of [hem is. If tlle supply cannot be turned o ff you must call for professional help. __.
.., , '0
<
Class D
S-
Fires that im'olve metals such as titanium and magnesium. These fires areparticul:uly diftJcult to extinguish and require the usc of dry powdcE extinguishers to cut off the ol\."Tgen, ::;
Extinguishants \'\!hen faced with a fire we haye [Q decide whether to smother it and ~p[i ve it of oxygen, displace the air from around it to remove the oxygen, coo1 it ro the point where combustion will Stop or tnrermpt rhe chemical process with
51 COp'pi,l',IJI '~
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CHAPTER ONE SAFETY PREC A UTION S
another rcaction. A further common-sensc action is to cut off dlC supply of fucJ to the fire. There is a range of c.."Hinguishants 3nilable that will suit the different fire classifications. Fire extinguishers should be wall moumed and be positioned adjacent to flre exits o r along the fire exit routes. Watc r \,Iater quenches and cools the matcrial that is acting as the fuel to bclo\.... tbe combustion tempemUlrc. It is only suitable for usc on Class A fires but must nevcr be used where then: is a li,'e electrical source. T),pical water extinguishers contain around nine litres of water and may either be pressurised with stored gas or be pressurised on selection by an installed CO~ gas cartridge. In the CK these extinguishcrs arc coloured red Some aircraft carry small portablc water extinguishers in thc passcnger cabin and these carry an additive such as ethylene glycol 1"0 act as an ami-freeze agent and also inhibitors that preyent corrosion. C arbon D ioxi dc (COJ Carbon dioxide is about one and a half times hca\·icr than air and is a noncorrosi\'C agent thai is suited to electrical Fires and fires involving flammable liquids. A hazard exists in thal when large yolumes of the gas arc discharged it rapidly displaces all the air and if lIsed in a Yery confined spaee it plaees personnel in the area at risk of suffocation. It is not therefore suited for inflight use in cabins. ,\nother hazard exists in the drop in tempcramre that occurs when the gas expands through the discharge vru\"e. This is sufficient to create a mist that reduces yisibility and rua)' also freeze the users hands to the metal parts of the discharge lance and horn. Care must be taken to grip the lance by tbe insulated handholds. There is also some risk of it causing damage [Q high temperature parts such as brake discs because of tbe low temperatures It creates. '1(\0"': "l\,)l\() - -
Ct r!3tm..'ilioxide is a popular choice of ground firc extinguisher for acti\'ities , "O~(such as engine starting. It is supplied in single or double bortle appliances that ~\,\. . . are equipped with a lance and a remo\"able discharge horn. The botrles tend to _,,'\. . . ~\"!be heavy and are coloured black The type of C01 ground fire extinguisher L used for cngine starts has a telescopic metal lance thai automatically extends when the extinguisher is triggered. Care !:as to be taken not ro cause injury with this. :\ircraft engine nacelle cowls h:\.\'c sprung fire extinguisher access doors and the discharge nozzle on the lance should be used to opcn these. You should not discharge any extinguisher into the intake of a Ulrbine engine - it is pointless! Brom oc hlorodifluorom e thane (BCF) BCF (Halon 1211) is semi-wXlC but IS not considered to produce any significant problems in moderate usc. It is suitable for Class A and 13 fires and electrical fires and can be used safely all a wide mnge of materials. (BCF) is a ,'aporising fluid that converts to a gas at normal room tcmperamrc and rapidly interfcres \\oith the combustion process. It is nomlally non-corrosi,~e except when its post combustion residue is brought into contact with water. This can
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S AFETY
CHAPTER ONE PRECAUTIONS
produce halogen add thal is Ycry corrosiw. BCF is a popular cboice of ground fire extinguisher and is also used in portable extinguishers llsed in aircraft cabins and in engine fire extingu.ishing ~rstems . It has a drawback in that il is a 'greenhouse gas' and is thus now su.bject to manufacturing rCStriC lions . Extinguis hers in the UK arc normally coloured greel1. Bro mo trifluo ro m c thane (BTlvl) 13Tl\! (I lalon 130t) is semi-toxic but is considered to bc the safesl of the Iialon group of extinguishanrs . I [ is \\;dely used in commercial ayi:Hion today for power plants, auxiliary power units and cargo bays. It is stored und!.:r high
nitrogen pressure so it is not nonnally found in use in aircraft ground fire extinguishers but may be found in use in those used in component test rigs. Bro moc hloro m c lha nc (B CM)
13Cl\1 (ilalon lOll ) is highly toxic eyen in small concentrations. It has largely been replaced by the safer I lalon 1301 in for example tCSI rig facilities.
Dry Powd er This is stored as a fine powder in a blue coloured extinguisher thai is pressurlscd b~· a CO~ or a nitrogen cartridge. It is suited for lise on aircraft bnlkc fires and Class Band D fires including eleclrical fires. ,\ disad\·anlagc is lhat it has little cooli ng effect and tires may smoulder and re-ignite. It can also create significant problems wben the heated residue bakes onto equipment making it difficult to remove. Foam
,
~\.~"\.\ -
....... ' .
(~..,\
There arc fWO types of foam. fluoroprotein and aqueous film. The li rst is spccifisally intended for use on Class B fires whilst the second can be used on Sla ss \~.\ or 13 fires. Neither is safe for use on li\·e electrical fires or class 0 fIICS. '~"'\J t >IS norma), U store d to > \V hi> te or cream cxnngws > > hers > I _ 1-.\.:.,;0.::'" In 1hc UK [1aL arc ~{" 1....,1 pressurised with CO, or nitrogen.
\.... :-
-
There arc 01 her cxtin01lishants that are now either \\;thdrawn from use or brarely found. i'-[ethyl I3romide (Halon lOOt ) used to be a common aircraft engine fire extinguishant but its \-cry high toxicity and co rrosive properties hAs seen it replaced by BCF and BC~J. It was recognised br its peacock blJ1;! coloured cylinders. Carbon Tetrachloride was once popular for fightil~~ electrical fires but has no\v been withdrawn from usc because of its higlF toxicity in that it produced phosgene when heated. I mention these because rOll still hear of them e\"cn though they arc no longer used. _ .j The current alarm o\·er climate change has seen a total ban placed on the n ~' manufactu re of Halon cxtinguishants but ther can be salvaged ~l11d re-lIsed~o they will be around for a while yet. \Vben stocks are finaUy exhausted, wr~vil.l be seeing new excinguis hants being brought OntO the scene. §'
'"
53 u~'IlI.p.lrr66.,T)J"
CHAPTER ONE S AFETY PRECAUTIONS
Dealing with Emergencies I ha,-c [Quehed on Ihis topic in ~Iodu1e Nine - Iluman Faclors and much of what I dealt with there is equally applicable here. The clinical procedures for dealing wilh emergencies and applying first-aid oflen fail to take into account the fact Ihal humans ha\·e [Q implement them and they may inhabit any number of memal and physical states after an accident. \'\ ·ith Ihat in mind_ I will repeat some of the information here. I emphasise lhal you must adhere to the procedures in force at rour place of work. Infonnation on procedures and flrst-aid given herc is provided solely for preparing you for an examination and does nOI supersede anr authorised Instructions. \,\' hen you arc faced with an emergency. you ha\'e to rely on what you know al that time. You can look up Whal ~'ou didn't know after the e'-ent but it can never change the past. The problem with an emergcncy is that you get \'cry li ttle norice thar it is about to occur or what fom1 it will take. It may be an immediate inju ry to you or a colleague or a pOlcntial injury in the sbapc of a fire or a chemical spill fo r example. Injuries comc in all shapes and sizes that may include an)' combinations of: electrocution, burns, fractures, lacerations, asphyxiacion. poisoning etc. and may occur to more than one persoll. Organisations are required to proride training, guidance and equipment for dealing with emergencies and should appoint nominated first-aid personnel. This is all "cry well providing the first-aider is to hand when YOli need one and people actually know how to use the emergency applianccs. l\lurphy's Law will ha,-e somcthing to say about thac! I f you arc faced ,,;th ha\'ing to copc, you should remember tbat the main objcct:i,·es of first-aid 10 casualties is fO prcsen'e lifc. prc\"em the casualty's condition worsening and to promote recovery.
,,'lC '11 dcaling wlt 'h an emergency SInt.1.t101l, ' , wtt t I1ere arc three con~-se n sc steps yOll can take and each contains somc basic actions: If you
< ac~pl.\cca
\\,,'«( ". 1... '\'\.
....
~
c'
I. Assess the situation:
..::
"i,\......
2.
•
Remain calm ("ital)
•
Identify what has happened (quickly and accurately)
•
. \sscss the danger
•
,\ctivale warnings (shout warnings and aCD'-arc appropriate alarms)
•
Do nor put yourself a[ risk
[0
you and others
,-
l\lake the area safe:
•
Protect "l"'oursclf and others from furthcr danger (e"l"'acuatc if required)
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SA FETY
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•
ldenti£)' and protect casualties (remm·e fro m danger)
•
Remove the danger if feas ible (i.e. switch off sources)
•
Kno\\' your limitations
•
If e\·acwl.tion is necessary, ensure no one is missing
3. Summon assistance:
•
Engage hclp of others if this does not place them at risk and the,· arc co mpetent
•
C311 emergenc: sernces (do this before attempting
to
fight
a fire
~·o urs cl f)
•
Call fo r local first aid equipment if appropriate
•
i\ leer, brief and h ando n:r ro emergency services (you arc no longer in co mral)
•
Offer assistance within mur limitations
These arc rhe sort of thoughts and actions that give you a plan of acllon. You may of course not be the person dealing with the emergency b ut your participation with the person who is can be of equal importance. \Xle can now look at a few types of emergency and the common features tha t arc us ually included in the procedures to follow but rou must follow {he drill pre~cribcd for your particular organisation. .,\)\.,~
..,\,,-,b ...
..
\\ .• \!.o.: ...
,
, \ ')~\'
,\,,,-
c
....
(.\"\
Fire
... \ ......
L~-.'\;. . .
I f r Oll arc f:tced with an outbreak of fire , you must obsen' e your organisation's fire drill. T his may look something like this: •
"
~
Rais!.: lhc alarm
~
"
13-
•
Call thc fire scrvice (do this before anempting to tackle a firc yourself) -:.
•
Shut off any power isolation switches or valves C'llllpment
•
•
LO
the affectJ}
o·
""'"
O nly if feasible, trr to extinguish the fire with the correct fi r!}..f::aid extinguishcr (make sure you have called the emergency scn' ices ReTo re doing dus, if YOLl are subsequently oYcrcome you will not be abJ~~o!) ~ I f the fi re involves toxic chemical fumes o r gas - evacuate imrhediarely
55 CopJ"'i~hf C {;I1I11) Colny :!OO6 . :!oo-
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CHAPTER ONE SAFETY PRECAUTIONS
•
E,'acuarc all personnel to the nominated fire assembly pain! closing doors after you where possible, Check thai no onc is missing!
•
Report
•
\'Chen all are e,"aclI:Ited, shut off the main power isolation switch to the building
•
:\wait the emergency sen'ices and direct them to the scene
10
a supemsor and do nO[ anempl to re-enter the building
To prepare for the eventuality of an emergency involving fire there arc some common-sense things you can do: •
Learn the organisarion's fire drill
•
Know where the fire appliances are, what the\' arc used for and thelr operfLlIon
•
Know where the nearest fire alarms are
•
'\Iemorise the escape routes and fire assembly points
•
~ lemorlse
•
Observe the fire precautions
the fire emergenq telephone number
\'\ 'hen the source of a fire is elccuical, you must uy to switch o ff the source, If you cannol
Jf you are faced with an individual who has clothing on fire. rou should render assistance quickl), before severe burns occur. StOp the person from rushing around or racing out of the building into the breeze and fanning the flames. L'ly ehe person down with the burning clothing uppeonost to prevent the spread. QlllCkJ)' put out the flames by dousing with water or a non-flammable liquid or wrap Ihe person rightly in a non-flammable covering such as a coat or a blanket (not nylon!). Do nm anempt to roll the casualty on the ground as this jllSt spreads the burning [0 othe.r areas of the body. If rOll[ clothing catches Gre then wmp yourself tightly in a blanket or similar malcrial and lic down.
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SA FETY
C H AP TER ONE P REC AU T ION S
Multiple Casualties If you arc on your own, or you are the most competent person present, you may have to make an assessment of which casualties to attend to first. The ones making the most noises are not likely to be the \vorst affected . There is lilue to be gained by spcnding time skilfully bandaging someone's sore thumb if there is another casualtr, choking to death next door! On the o thcr hand, if ~·ou han! to e,-acuate casualties, there is little point in remo,·ing dead people first or people who \\ill die if you mm-e them. Get me walking casualties out first and then work down the list of priorities on who has the best chance of sur,·ival. YOLI will actually gC[ c,-eryone out much quicker if you Ihink about tlllS. 'r'ou can employ th e help of colleagues if this does not put them in danger.
Multiple Injuries In a se rious accident a casualty is most likely to bave several different in juties. You need to assess which is the most serioLls in teons of preserving life and deal \vith that first. Once that is acbic,-ed, you can deal with the others prodding there are no other priorities . Again. there is little to be gained by bandaging a sore thumb only to find that the casualty has bled to de~llb while ~·ou were doing this!
Elecrric Shock .As I have stated predously, the human body is an e!ecrncal conductor. \\"ben faced with a person who has been electrocuted and is still in contact \\;th the con ~l.ldor you should: "")\\\,(:0
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.
...
Not touch the casualtr's ski n wirh you bare hands under an\' cIrcumstances
•
Cut off the electricity supply if possible
•
I f this is not possible, move the casualty clear of conmcr with t!i.e c1cctncal supply Llsing a dry insulator (broom ha ndle or wooden chairb-. Stand on a dry insulating surface while doing this (thick newspaper or ~ wooden box)
•
I f the casual\!· is nO[ breathing, commence resuscitation straight away"J
•
Call for medical assistance and continue resuscitation until relieved
•
The two methods for applying rcsusci~ation for breathing ag.§l'v the method for applying heart compressions arc displayed by law 9n~afety posters at places of work. You would need to have familiarisccfYoursdf with these before attempting to apply [hem. ~
.~
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57 Copyi!,hf :0 Bar:J· Calk§! 2006 - 200; All rigbts mm.'td
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CHAPTER ONE S AFETY PRECAUTION S
Shock from High Volrage Lines I f the casual!)" has been electrocuted by a high voltage source such as that found in overhead powcr lines or underground power cables and is cithe_r still in contact with the conductor, or is lying wit11in 20yds of it, you must not approach until the power has been cut off. The currem from these conductors can 'arc' m-er considerable distances. Contact \\~dl them is innl.nably fatal and insulators such as dry paper and wood will oat protect you.
Accidents Involving D angerous Substances Whcn an accident involves the spIllage of dmgerous chemicals o r the emission of dangerous fumes rou must not attempt any rescue acnon that will put you into danger of coming into contact with the substance. \'\'e haye already discussed the hazard warning labels on containers and the warning nonces carried on ych icles. If you do not know the meaning of thesc you should keep your distance. Take nolC of any symbols or codes that arc displayed so that you can pass these on to the emergency services . .\1\ personnel in the ,cicinity should be evacuated and be madc to stand well upwind of the drift of any poisonous fu mes that may be escaping from tbe scene. The list of poisonous substances is too big [0 indicate the derailed acnons required to deal with each one. The most common fonus of poisoning at work actually occur as a result of exposure to industrial gases and the toxic fumes gi,"cn off by volatile chemicals. :\ casualty who may be affected by d1{~se would obviously need air but you should not attempt to rescue him/ her if this means you will also be m-ercomc. I f this is a risk. or if lhe casualty is in an enclosed space, the rescucr must be equipped with breathing apparatus and be attached to a safety line . The rescuer must be trained and experienced in the use of the breathing apl?a~tui "ill\> \ "e'" r..:.o\\o:.~ ~ ~).{~~
,
,
\~,\ ~
Poisoning
,<" I f you are presented with a casuall1· who has been poisoned, and rou are not at risk of being similarly affected, there arc a few common~sense actions YOli can take: •
Try to get infonnarion from the casual!)· and bysmnders to find out what substance is involved, how it has entered the body and how much. Bear in mind that the casualty may lose conscious ness aT anv moment.
•
Take nOte of any containers that may be nearh)" and note the substance and the warning labcls.
•
Take note of any symptoms or signs such as asphpcia or burning around the mouth
58 Copydghl 0 Barry Colltg<' 2006· 200-
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SA FETY
C HAPTER ONE PRE C AUTIONS
•
Do NOT induce ,'omiong
•
[f there arc signs of burns cool them.
•
[f the casualty is unconscious, check Airway. Breathing and Circul:nion (ABC) and if necessary resuscitate if this docs not invoke contanunating yourself with the substance.
•
Place the casualty into the recovery position whether conscious o r not and evacuate him/her to hospital without dela\".
In
the mouth, water
Or
rnilk may be used to
Severe Burns & Scalds The classification of burns is a complex topic and beyond the scope of this module. Burns can be the result of contact with naked flames o r hot objects, scalds from hot fluids and .apours, chemicals, electrical current, 10\" temperature sources or radiation. A burn coyering an area of more than lin diameter (2-3cm) or penetrating deeper than the surface layer of the skin requires medical attention. Anothcr factor to consider is t.hat the greater the burn area, the more likely it is that the casualty will be also suffering from shock. T he general advice fo r the first-aid treatment of a serious burns victim is to lay the casua lty down and then pour cold water over rhe affected :lrea and continlle to do this. Do not apply lotio ns or fa tty substa nces. [n all cases yOll mus t send for skilled assistance. Do nor let the area affected by the burn (ollch the ground. Do ensure that [he casualty is well clear of live electrical conductors before applying water or you \vill both have another problem! Rcm~ am' rings, watches or belts that ma\' cause constticnons to al1\' ~C ~\\i~~vclling rha~ might occur as a result o f the injun''-
.
(~,\\.:;':-
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,.;."\\.~v
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If a chemical has caused the burn and this is still present in the clmhing, the contaminated clothing should be remm-ed but do not remO\'e clothing in areas where it is stuck to the skin. Continue to apply a plentiful supply of cold water while doing this. Ensure that vou do not come into contact with the substance that caused the burn. Do not attempt to puncture any blisters that ha!,C formed. C0\'cr the affected area \"\1th a clean, non-fluffy dressing like a clean linen sheet. A clean plastic bag is ideal for co,'cring hand or foot burn injurie1j. [f rou covcr facial burns, do ensure the casualty can breathe! T reat the casualtY-fo r shock and get lum/her to hospital as a stretcher case. ;::-
.
~
"
6: ;"
'"
Shock
~~
'v c?
[ am incluchng tlus because it is quire important to recognise the sympt8rns as you Inight be suffering from ir yourself! People ~an die from shockc:-&hoc k is actually a failure of the body's circulatory system as a resuit of the blood pressure o r volume f:llling to a dangerous leveL This may be caused by the
59 CopJ righl
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CHAPTER ONE SAFETY PR E CAUTIONS
stress of injury bur could also be the result of imernal or external bleeding. Because dlC imernal organs rely on a supply of oxygen from rhe blood they will ultimatelr start to deteriorate ,"\;th possible fatal results. 111C body reacts by diverting all available blood away from tissue such as the skin to the \'ital orga ns and this causes the skin to become pale or grey, cold and moist with swcat. Thc victim feels cold, faint and anxious and has a weak, rapid pulse and breathing becomes shallow and rapid. 'l1le affected person may become \"cry restless and be gasping or yawning to compensate for lack of oxygen. t\ddirional symptoms may be a feeling o f thirst and nausea. The person mar lose conscious ness, ProYlcling there are no Olhe r serious injuoes that require immediate treatment, the basic guideline is to lay the person down out of draughts with the head low and to one side so that he/she will nOt choke if yomicing occurs, loosen any tight clothing around the neck, chest and waist and wrap him/her in a blanket and, proyiding they are not fractured. raise the lcgs aboyc the lc'"el o f the head and rest them on a cushion or rollcd lip blanket. You can moisten the person's Li ps but do n o t gi'tc him/ b er ;mY lhing to C1I1 o r drink Do not apply direct contact bear appliances such as hot water bottles as tllls attracts blood to the skin and srarves the organs. Unlike the image of casualties in old war films, do not offer the casualt}' the oppoftuniry to smoke as tills further restricts oxygen intake. Do nor leave the person unattended and remember [0 be reassuring. If the casualty loses consciousness, you should check airway, breatillng and circulation (AI3q and resuscitate as required. It is important to get specialist medical assistance as quickly as possible, If you do have ro e\'acuate a person who is in shock, Lise some fonn of stretcher and keep the casualty's legs propped up.
Summoning ~S"Sistance "ll.'t\.)()
c-
-
~CQiP~~ vitally important part of dealing with any emergency. \,'hen calling the YOll ~ h ould keep ca lm and not dramatise or ~bbl c . You need to pass your location and the telephone number of the place you arc calling from. If you get cut off, the emergency scn"ices can then locate yOll or call YOll back. Gh"e tIle exact location of the incident and any landmarks. Gi.e an indication of the type rmd gravity o f the incident and the number of casualties if you know it. Do mention if [here arc any specific medica.l conditions like imminent childbinh or heart attack. Additional information like the sex and age of casualties is useful. Do mention any specific hazards sllch as the presence of spilt chemica.ls, flammab le fluids or collapsing buildings. Do not replace the telephone receiyer until the emergency services controller does. Controllers rclay infomlarion on to other agencies that might be required and will indicate whemer they need to refcr back to you, They arc very good at what they do so rOll so you will be guided.
© y,~"(. e m e rgen cy services .\~"\.\ ,,' . \..~'X
If you send someone else to call the emergency scn'ices do make sure they know where they are, who to call and \vhat to say. Get thcm to repeat it back to rou if YOll ha\"e doubrs. Sending a fo reign tourist who does not understand the la nguage and barely know where thcy are may not prove to be one of your
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SAFETY
CHAPTER ONE PRECAUTIONS
bes t dcci~ionsl Do get the person to report back to you to confiml that help has been summoned. Do remain at the scene [Q guide the emergency sen"ices in and to brief (hem. Once you haye done thar, your job is oyer.
Conclusion The wording of [he syllabus for cllls chapter tends to be all encompassing rather than being specific, lea,-ing 'quizm:lstcrs' :Ill the scope in the Wo rld to ply their tr:lde. T here 15 also a lot o f scopc for posi ng ,\11 essay questio n in addition to multi-choice :lns\\"er questions. lr is fo r that rC:lson I hayc included as much infonnation as possible on a wide range of topics. I am acutely aware that many of these topics may appear 'dull' and present you with difticuhy in holding the attention for long periods of study. 111 c~" arc nonetheless importam and arc a Lcyei 3 Knowledge Requirement for all categories of candida te. Much of the knowledge comained in this chap ter ,vill be greatly reinforced with actual obse!Tation and practice in the work emiromnent and r encourage you to take ad\"antage of this during periods of practical work. Take notice of the emp hasis placed on safe practices and procedures, not just in :Ollf \'iciniry. but in as ma ny areas you Cfln \·isit. Yo u desen'c a coffee brea k for ha\·ing got tills far . Before you decide to replace this wiIh a visiT to th e pub - strictly for counselling of course - I ha\"C included a few multi-choice ques tions for you to try. They arc ani: a small sample so do not :lssumc that merely answering them will be proof of ~-our readiness to take the exam. 1 Iake up a few more.
.-';
61 CO/!J·righl C Run Gilk§ 2006 . 20fT A1I riJbls rmrrtd
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CHAPT£R ONE SAF£TY PR £ CAUTIONS
Revision Safety Precautions Q ues tions 1. The firs t p rinciple in accident prevention is to:
a) slucld the person b) shield the hazard
c) remove the hazard
2. T h e H ea hh and Sa fety at Work Act 1974 p laces duties o n :
a) Ihe employer only b) the employer and the employees c) the employees onl y
3.
'?>;\~~' ,\,.'~~\'"
A
h a~ (d)
,\\.:.-;..':'
...
'1nn-
nf'ay be d efin e d as :
l a.) something with the pOlenria.1 [0 cause hann
~
b) the likelihood that hann rna\' bc realised
(,,~
c) an unexpected
4. T he cos t o f equ ipm en t is:
C\'CIlt
that results in hann
p roviding
and
m aintaining
persona l pro tection
2=
:3 a) the emplo)'ee's responsibility b) the employer'S res ponsibility c) shared between the employer and employees
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CHAPTER ONE PRECAUTIONS
5. \'Vhe n c h a rgi ng a g as eous o")'gen sys lem th e ml c o f c hargi ng s h o uld be:
a)
(:lSt
b) slow
c) irrcle\'ant
6. Wh en c harg ing a compo n e nt with g a s fro m
hig h -prcss ure boule,
Ih e b a ili e mus l be conn ect ed thro u g h :
a) a dIver b) an oil and water separator c) a pressure regulator
7. T he hum an body:
a) does
nOl
conduct eJeClricit\·
b) will always conduct electricity
c) will ani\'. conduct electricin'. when wet
8.
O~gc n
l\.)\1
is:
, ) not flammable bur suppOrtS combustion in other materials b) flammable and will burn if ignited c) inert and will not bum or support combusuon
9. Ni trogen is : a)
nOI
flammable but suppons combustion in other materials
b) flammable and will bum if ignited c) inert and will not burn or support combusuon
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C H APTER O N E S A ,- ETY PREC A UTIONS
10. T he phosph a te este r-base grou p of o ils used system s are: a)
10
a ircra ft h yd raul ic
nOD- irritantS
b) irnranrs c) i.rritaotS ooh' when in vapour form
11. T he 1" action level ide ntified in th e Noise at Work Regulati ons is: , ) 85dE b) 90dB
c) l-lOdB
U . U nder the Safety Sig ns R egu lations 1980 a sign tha t is d esigned to pro hibit an actio n would be :
a) triangular
\\~th
a black border o n a yellow background
b) circular with a blue background
c) circular widl a rcd horder and crossbar on a wrute background
"\. k
\,~....
13. Wh en fa ced with casualti es that have been overcom e by gas YOli ')I.I\.~ sh o u '$-.,6 - \' .~e ( 0\'... · '(l';\~{' 'a) turn off the gas supply, venwatc thc area and tben evacuate (he casualties
,
" ,~~ . ...
b) c\""acuatc the casualties thcn turn off [he gas supply and \'cntilate the arca
c) "cnWate [he area, evacuate the casualties then turn off the gas supply
14. A spillage of a lkaline b a ttery electrolyte m ay be ne utralised with a saturated solutio n of:
.~
"
o
'" "
J::'
a) bicarbonate of soda
'v
c>;'
0<
b) boric acid
c) sulphuric acid
64 CIJPyighl 0 Bo'!)r Colugt JOO6 . 200-
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SAFETY
C HAPTER O N E PRE CA U T ION S
15. T he fl ash pain! of a substance is defined as t h e:
a) lowesr rempcr:nurc at \vruch the material ignites b) lowest tcmperarure at which rhe materi:ll prod uces enough \' apo ur to burn
c)
the flame temperature of a burning material
16. Electrica l fires can be fought us in g: a) water or foam extinguishers
b) foam or d ry powder cxcinguishers e) CO 2, BCF or city powder extingwshers
17. Class D metallic fi res can be fo ught using a: a} dry powder extinguisher b) fo am extinguisher
c) CO 2 extinguisher
18. When faced wit h a fire you s ho uld: first call the fi re semce before anempcing w Ilut cl1e fire o u[ ,,-,,yb . a~\I~ L)
\>,\\\..:~": . , \\)~("
,{,..,..~....\ ( P~ ·
--
b) first attempt to pur rhe fire out and then call the fi re scmce
C
c) o nly call the fire service if the fire is too big to extinguis h
19. An im portant rule when dealing wit h a person in s h ock is to : a) giye the person plenty to dcink
b) keep the person upright and mO\~ng about c)
not allow the person to eat or drink
65 1l'1l'W.parl66.r(JIJ'
CHAPTER ONE SAFET Y PRE CA UT I ONS
20. A single COSHH assessm e nt for the use of a give n hazardous substance can be used: a) in all work locations and wo rk processes where tbe substance may be used b) o nly in the related work process in the related location only c) in the related work process o nly regardless oflocaoo n
Revision Safety Precautions Answers l.
C
16.
C
2.
B
17.
A
3. A
IS.
A
4.
B
19.
C
5.
Bl\~'G " '"
20.
B
1~)\.'-
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g
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,,\\~
6. C 7. B
1... ..'1~
S.
A
9. C to. B 11. A
12. C 13. A
14. B 15. B
66 CQ/!p i,ghf C Ban] ulk,gt 2006 . JOO-
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CHAPTER TWO WORKSHOP PRACTICES
Workshop Practices Introduction to chapter two. Before we make a start I would like to sOrt OLit a few problems with the seC)ucnce in which the sylbbus for module 7 is presented. There are three parts to this chapter. The first part includes the care of tools . There arc aspects oi this topic that are related to specific tOols that arc a part of the common hand and power tools described in chapter three. It would be more logical for me to link these aspects to the descriptio n of the actual tools to avoid unnecessary duplicacion and cross- re ferencing. In this chapter, therefore, T will onl), concentrate on the general requirements of tool ca re. \ Vc!COOlC
The syllabus for chapter two also includes reference to rhe LIse of workshop materials. The identification of materials is a module 6 topic and rou wi.ll find a detailed descripoon of ierrous. non -fe rrous, composite and non~me [ailic materials and their identification in the first three chapters of module 6. There arc also a lo t of refe rences to me mo re detailed Lise of materials in sLiccessiYe chapters l!1 dus module. I will co ncentrate here o oh· o n the general requiremen ts relating to the use of workshop materials.
:.,.....'.
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,
There is also a section in this chapter on the calibration of tools and equipment. Again, the operation, function and usc of precision measuring LOols and1Alc:c tricai and avionic general test equipment appear in the syllabus for e. ~S"\~hap ters 3 and -1-. I will concentra te purely on the general calibratio n . (.1l\\C.~ requiremcnts and standards in this chapter. \\';"«' Finally, there is a refercnce to dimensions and tolerances that is rcpeated in the s,·llabus for chapter fi,"c. Hopefully, having explained these apparent it~consistencies, you wi.ll understand that we will not be lea\' ing anything our, just putting a few things in an order that will make things casier for you. ;-
..,
-.
~
Care of Tools . -\ ircraft maintenance tasks are carried out in a range of different woJ.. e!wironments but all of these im'oh-e the usc of tools and equipment in soWe form or other. Tools have one thing in common with humans, if you do-~~t tfea l them p roperly the)' will not perform as wei! as ),ou would wish fOf2aod may let you down when yOll least expect it. I f we add mis use to the ncgle~
67 Copyighl O I3tJfT)' Collfl.! 2006 - 200~ All nl,hl> rr:mlJ(n
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C HAP TER TWO WORKSHOP PRACTICES
1. \"'hen nOt in usc, tools should be properly s(Ored in an envi ronment that will not crCtlte problems with loss, corrosion or damage through impact \\~[h orner tools and equipment. 2. The old adage of 'picking the right tool for the job' was nl.·nr tnlcr. .:\lwars usc the correct sized spanners and screwdri\'ers and do not be [emp~ed to substitute these by misusing grips and pliers that will not only damage the part but the tool as welL 3.
Hard, brittle cULting tools such as drills, reamers and fues , when not in usc, sholud be kept separated to avoid contact with each other. I f these tools arc allowed to make contact they will be at risk of chipping and blunting each other . .r\ procedure followed in many stores is to rerain reamers in their individual boxes and place drills or fi les in indi\'idual stowages that partition them from each othcr. TI,e method used for storing brittle tools is always a fa\'ouritc larget for quality auditors when visiting tool stores.
-t Othe r cutting tools such as hacksaws and chisels should be s(Ored out of contact \\~th each other and other tools and equipment. :>.
i\larking out tools such as scribers and spnng dividers. when out of use, should ha\'e thcir sharpened points protected.
6. Precision measuring insrruments, when not in use, should be kept in their special to type boxes together wilh any supporting calibration dOCll1llenrS and retained in the srote. T he instnlmems should be within their prescribed calibration dares. Proteco\'e oil films should be restored where required. 111ey mUSt nOI be left lying around the work area or be intenningled wlth, or be in contact with, other tools in toolboxes. If dropped or otherwise damflged they must be withdmwn from US~t""
",,{'t'
.,~\I.-,6 \.>~.:.~pecial tools and gauges, when not in
should be kept in their specia l to type containers find retai ned in the store. They should be labell ed to identify their use. lISC,
8. The striking end of impact tools such as hand punches, drifts find chisels shou1d be inspected before usc to ensure thar these ends arc not split. peened over or burred. Using these tools in that condition risks bilS breaking off and entering the component being worked or the eres of rhe operator. The ends should be reground to the correct profile before usc. 9. :\lways use tools for the purpose they were designed for. Screwdri\"Crs are not substitute IC\-en; or chisels! Spanners are not substitute hflmmers! Socket set extension bars arc not substirute drifts! 10. Always inspect a tool before using it and, just as importantly, before reruming it. TI1C head of a hammer must be secure on its shaft and the shafl should not be split. File tangs should be righdy fitted in handles that arc not split or worn and have Lhe co rrect ferru le in pl ace. File
68 Coppighl C lin'!)' Colkgt :!006 . :u(r All n'ghfJ TtUtvffl
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CHAPTER T WO WORKSHOP PRACT I C E S
tangs ba\-e speared the hands of quite a few unfortunate people. Scre\ydriwr blades should be undamaged and haye the correct profiJe. The jaws of spanners should not be sprung - you risk skinning your knuckles if they arc. 11. E lectrical pmvcr tools must be kept clean and free from swarf and dust and be inspected by qualified persons at regular inter.als. Ther should be correctly stored when not III usc and ne,er left lying :lround. Cables and connectors should be inspected for signs of pulling or any damage to the outer sheath. Any signs of o\·crhearing or burning should be reported and the 1001 withdrawn from usc. ! 2. Bench \-ices should be secure, clean and ha\·e adecjuarc lubrica tlo n on the threads and release mechanisms.
13. Ne,er return :In unsen;ceable tool to a 1001 kit where it ca n be reissued. Repon it and return it to the store properly labelled with the defect identified. 14. Tools should be cleaned before returning them in to store. Tools that have been used for work with resins used fo r composite material repairs should be cleaned With acetone or l\[EK. 15. Tools lhat ha\·c to be used for specific tasks only may require segregation from other tools. For example, tools used for dismantling dirty h~-draulic components should be kept in the area reserved for [his and not be moved into the clean component assembly area. The same rule would apply to tools used to maintain alkaline and acid ba tteries for example.
_<"
iI,"\....
~,\'-:""
(.0'\
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~
'
",i his.lrt all fairly common sense stuff but you would be amazed al what some _ ~\\c'f-c"}.\ people do to tools to get a job done. For eX:lmpl~, there are occasions when a (.~ spanner may be deliberatelv bent to achieve a diffi cult access. If this really. has to be done, the spanner should be \vithdrnwn :tnd held as a cleady identified special tool for that job. It should not be returned to a tool kit for general uses that it is no longer able to fulfil!
.
Control of Tools I am going to start this section by painting you a preny black picture of sam5;=" of the activities in a maintenance organisation that have the potential ~ become a threat to flight safe~- if subject to poor tool control. The disco,~ that you ha'-e mislaid a tool after working on an aircraft is nOt a plea~ t experience, either for you or [he people charged with trying to locate it. ~ot knowing that you have mislaid a tool could prove to be an even ~ore unpleasant experience fo r others in the future. C"\ .(" <;
\,\'hen we start to eX~ll11ine how we can exerr control over the tool!!', and for th:lt matter the equipment. used in aircraft maintenance tasks, we may find the
69 COPJ·rigbl
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CHAPTER TWO WORKSHOP PRACTLCE S
problem extends a lot funher than first thoughL °nle [eon 'loose article' does nor specify any particular breed of article - there is a big range of things lhat could fit the description. It is pretty ob,;om thai the ream direcdy cngaged in the work will be using tools bur there still remains the issue of random visits by migrants from outside agencies such as electrical, avionic. structural and inspection bays. "111ese people may be importing tools and equipment that h:1\"e been drawn from their respecri.e stores, not yours. They may h:we indeed carried this equipment across se.. eral alrcrafr before reaching rOll. \'\!e could then ha\'e a real problem in identifying in precisely which aircraft a mislaid item might reside, if indeed is has been lost in an aircraft at all. To complicate the situation further, lhere may be items of c(luipme!1l such as drills, reamers, special tools, gauges and measuring instnullents that arc held in a cemrnl tool store serving several tcams working in different areas and on different aircrafl. Some of these mar not aCllmlly be single items but kits containing several ilems. I know that I am painling a peell)' black pic Nrc here but it is sometime surprising how many people o,oerlook exactly how many sources there are for ob taining tools in a large maintenance organisaLion, and how these tools can migrate if nm checked. The quesrion remains, how many of thcse sources are going 10 be checked when an aircrnft is being cleared for service, and when? Hearing that part of a l1luitimeter electrical test set is missing from the electrical bay the day aftcr an aircraft has departcd is not somelhing rou would wish to hear, particularly if rou had released the aircraft back to sct"'.ice! Before we fin ish with rlus preamble, I will draw }Oour attention [0 pieces of equipment that people do not often regard as being stores controlled items. Replenishment rigs and test rigs, for example, often ha,oe items such as charging hose connCClOes, blanks, caps and boule keys. \'{11ar if we find one of thosc missing? Personnel using this type of eCluipment are responsible for ca rrying out a pre-use check and a post-usc check. 111is would include checking that albaernchmems were present. hems such as borde keys are tools and theie ~~\\\~L~
"=. \"\:'~~x{ending this a bit further, what about the blanks wc lise
('-!.\'\ (~~\..,. . '
[Q close open pipelines and eleclrical connectors? Surely dlese arc just as capable of januning a control as any Olher itcm? The quesrion is, arc they tools or consumables?
The conclusion we might reach is [hat everything we bring to an aircraft is a potential 'loose article' including nurs, bolts, washers, rivCts, rubes of grease, cans and locking wire and the detrirus that our dealings with these crcate. If we cannot exercise proper control moer tbe rools and equipment we use, what hope would there be for the rest?
Shadow Boards It would be a bit chaotic ro try and centralise e\Oer)' tool and piece of equipmcnt we use in one centralised lool storc. "nIcre is oftcn a need 10 carry a range of common tools to the work site to save nwcs of foot slogging and qucuing. Where thcse tools are [0 be made available ro a rcam of people we
70 COPJ'nghl
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need to know whar tools arc in llse and b\' whom. The tool shadow board prm-ides a simple solution. The silhouette of-each 1001 is painted on a board so that it conrras{s "ith the background colour. I'- or example. black silhouettes on a light green background ..'ny tool remo\-ed from the board exposes its si lhouetTe and indicates clearly that it is not there. Each silhouette has a hook designed to accept a marker that carries a numeric or alphabetic code that rdates to the person who remm"ed the tool. \,'e now have a means of establishing what is missing and who has it. _\t cease work. the shadow board is easily checked and we know whom to question if it is not complcte. The board is cOIlL1incd within a lockable cabinet that can itself be secured in a safe area. Sounds too simple! The shadow board system that we ha ve jllst described [c(luires c;\ch tradesman to ha\-c a set of 'tags' or markers [Q hang on the board when tools arc drawn_ The question is what do we do with the spare 'tags'. If these arc carried onto the aircraft they can be mislaid and the), often fire. So, we need to check that each person has the correcr number of 'tags' fit cease work. \\:-e could, of course, keep the spare tab'S mounted on fI separate board against each named individual. 'Ilus syHem would be easy to check :lod not present a 'loose article' haz:lrd_ Now we hayc the question of tniSI. \'(lb:l1 if a person lIses someone else's tag, either \\;th or without that pcrson's pennission? -11,ere ha\-e been occasions when migram technicians require tools and 'borrow' tags or worse still dmw tools \\;thout placing a rag. \'"orse than tlus. these people may carry the 1001 across to another ai.rcraft, This is where supenisors have \0 exert strict control. These infringements would have to be treated as gross breaches of professional bchayiour. 111e alternativc would be to return to a centralised stores system and sign e\"erything out.
Centralised Tool Stores "llt\
"!\'!IPort.'lble team shadow boards are nonnally restricted to a mnge of simple hand (.\.)\\.:.~ tools, The more speci:llised range of tools and equipment is normally kept in a \\;\.«' ccntralised tool store. This reduccs the numbers of specialised tools that we are rccluired to hold and makes them available to all who wish to use them. In some org3lusations this slore also handles the storage and issue of consumables as well. :\ rube of gre2se or a can of fluid is JUSt as much a 'loose article' hazard as a tOol. Shado\,,- boards scil! ha\'c a role to play in these stOl! S but wc now ha,'e the problem of location. II is "ilfllly important that we ha\C1I: signing OLlt system thflt identifies the destination of that tool in addition to wh~ has it. 111e alternative would be a system of registering the tool onto ~ particular nircraft. Some organisations may operatc both systems. I.:
\,\'ithin the centralised tool store there must be a system that quickly indicates whar is out, who has it and where_ Tool stores will normally issue tools oj a dflih, bflsis or for a shift period. At the end of each working day or shif& he sto;c-man will check the returns and inform the app ropriate supcn,jsor rf!any discrepancies_ \\:'hen an aircraft is in the process of being cleared for §trvice the store must be checked in addition to any local shfldow boards, ,}v
,
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An important requirement within a tool store is the segregation of ullSelyiceable tools and equipment from tI:'.e serviceable stock. Unserviceable tools should nor be stored where they can Je accessed and pur back into use . . \ll unsCI".iceable tools should be dearly labelled and quarantined. The reason why a tool has been classified as unserviceable may not be apparelll by its phrsical appearance. An inslrument that is inaccurate or is our of calibration time is unserviceable. If tile calibration records appertaining to an instrument are missing then the insU'umell( is unserviceable.
Maintenance Bay Tools & Equipment l\ lany specialist maintenance bars carry specialised equipment. For example, non-dcstnlCtive testing sections carry a range of differenr probes and conneclors. Elecr..rical and avionic bays carry test sets. In addition to these, the bars oflen carry hand tools for work inside t.he bay. The situation we ha\'e here is that these items may randomly be required on aircraft work as well as for bay workshop use. Each area must ha\'c a system. Either all the tools and equipment are held in the centralised store or the areas must have a means of idencif;.;ng wbom has drawn what and where has it been taken. In this case, these slores must be included in the tool and equipment checks when an aircraft is being cleared for scn·icc. _An alternative is to register the presence of the equipment with me supcn-isor of the aircraft when the item is brought 011 to it and to register its remO\'al after work.
Aircraft Maintenance Ba,' Control Some organisations use a form of barrier conr..rol where all tools and equipment bein~ brought into the bay have to be booked in and OUI regardless of whetb%\ wer\l\~\'e been signed Out of olher stores. This is done using a re~5~'tnat makes open emries and the idemiC)' of the responsible person \.... :.{t:!ear. Again, the system requires a certain amount of tnlst and firm action when cases of abuse occur.
Carriage of Tools \"{'hen tools are carried OntO an aircraft they should be done so in a way that keeps them secure. Stuffing the pockets of o"eraUs \\;Ih tools is not recommended. Small tools carried in top pockets are at a high risk of falling our when the technician leans o,·er. .\ further problem is that projccrjng sharp tools will usu,'llly find a way of obstructing hand mo\'emcnts and impaling the owner. Tool caddies or tidies ate a solution provided that these are not then used to carry around rubbish.
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Lost Tools Organisations must ha\-e a procedure to follow when they receive a report of a lost tool. Such a procedure usually inyoh~cs a check of all sub~srores and work areas followed by a 'zone' search of the suspect aircraft by a nominated temn I f this fails to re\-eal the tooL a further and more cxrensi'-e 'zone' search ma,be implemented that miglll im-oke selected dismantling and eyen [adiographi~ search techniques. You would nor wisb to be the person responsible for causing this!
EASAPan145 There are a couple of references to tools and equipment in EA5r\ Part ! 45 25(d) and 40(a and b). Twill quote them for you. 'Secure storage facilities must be provided for aircraft componen ts, equipment, tools and material. Storage conditions must ensu re segregation of sen'iceable aircraft components, equipment. raols and material from unserviceable aircraft components, materiaL equipment and tools. 1be conditions of storage must be in accordance with tbe manufacturer's i.nstructions to prevent deterioration and damage of stored items. Access 10 storage facilities must be restricted to authorised personnel.' (EASA Part 1--15-25(d). 'The £.\5:\ Pan 1-1-5 appro..-ed maintenance organisation must have the necessary e
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(~~"\ .
,• \-...,,<.~"" .
These references are neccssarily bricf but do impose the requirement fot tool control and also calibration control. Also note the rC
Use of Workshop Materials I ha,-e mentioned earlier in trus chapter that the narure and identification of rti materials used in aircraft maintenance are module 6 topics. However, there fP! a few rather important rules go,-erning the handling of aeronautical mareriaJfin the work environment that we should discuss. ,~.
"""
The term material covers a \\;de range of products that would for ,e~mple include: metals, composite materials, oils, greases and chemicals. Ait'ft from ensuring that YOll only lise the materials stated in the approved maIntenance manual there are twO very important rules governing their use. These are the
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continuing prese lyauon of the identity and inspection status of the materials whilst we arc using them. Lose either of these and the material is sllspect and must be withdrawn from use.
Ce rtification of Materials Suppliers of materials establish and maintain procedurcs for identifying a produc, from approvcd specifications during all stages of production and dclh·cry . . \ppro\'ed manufacnlrers produce aircraft materials to scringem specifications. l3efore they issue these matcrials they are obliged to certify that lhe material confonns to specification. W'hen these materials arc delivered they :lre accompanied by a Certificate of Conformity and also the unique batch lJumber of the material. Man" matcrials are also marked \\~th the material specification code ..-\ Rclctlse Cerrific.'Jfe will accompany materials intended for ci\-il aeronautical use and this is often combined with an Appron:d Certificate that certifies rhat the materials ha\'c come from a manuf:lcturer appro\-ed by the a,-iation authority.
Quarantine Store All materi:'lis reccind into store must be iniLially inspected for damage or deterioration and then placed in a locked quamnrine store uncil the idenLifY and balch details h:lve been confirmed and they can be related to the appropriate certificate of conformity. All receipts of materials arc recorded in a 'goods receiyed' register.
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Bonded Stores u-'\\':-':"": ...
~ \"'t':,>ft-.'1falerials thar are inLendcd for aircraft usc and h:l\"c bcen satisfactorily
.\,\ ~
, <:....~\r
identified and proved to confonn to specification arc removed from the (IUarantine store and placed into a separate bonded store. \'\'hilst in the bondcd store, thc identity of the m:lterial is presen-cd :Ind an)' shelf life limitations arc obsen-ed. Petrol, oils and lubricants would be stored in a separate bonded Slore resen'cd for flammable products. Gases would be in another compound. ?Iateria!s arc issued from bonded stores on receipt of a requisition to a task :'lIld this will then be annotated with the batch number and release certificate dem.ils to presen-e traceability to source. When part of a bar, sheet Or secrion of l11etal is issued, the pan bearing the nmeria! identification mark should be retained in the store. Materials issued from a bonded store are recorded in an issues register and are traceable to tasks and arc also traceable back co theu certificatcs o f confonnity and batch numbecs.
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Identification The identity of a material must be preser,ed at all rimes throughout storage, tr:ll1sportation and usc. Solid ma terials may be CUt fro m bulk slack o r fluids may bc decanted. \'(Ihcll this is done there is a risk that the material will lose its identity as it progresses through a work process. I f this occurs, and you cannOt be sure of what it is, it must be withdrawn from use and quarantined awaiting disposal inst.ructions. The material is considered to be defecti\-e because it has no proof of identity. \X!hen replerushing aircraft with fluids :md gases we need to be sure of the identiry of these up to the poim they enter the aircraft system. Fluid conrainers have the specification number and batch number clearly marked on them. If you decant these into a replenishment rig you must ensure that the rig also identifies what is in it. I f you are not sure you cannm use it. Chemicals used in the workshop must come from containers that carry dle identity and any details regarding shelf life. \\'hen cheITllcals arc mixed they form orher compounds. This becomes a stage in a work process and we need to be sure what that stage is and whether it has been carried out success fully or not. The task record should clearly show this.
Proof of Conformi ty
.\~<.. ....
,.:.'
Using meraIs as an example. it is essential that material that is drawn from a bonded store retalns both its identity and balch number and a means of tracing it back to the certificate of confonnity should this be necessary. Certificates of conformity for maor aluminium alloys also carry heat treatment figures and times on them. You cannot use any other figures. Should this documem be lost the~ I..,y,e material cannOt be used. Even if you can identify the material c. ;,\~'ipeafication by other means, you have no documented evidence that it . (.l'\\':'~ conforms to it, or any information o n how to heal treat it. \X' hen this occurs vou must withdraw the material from use. label it clearly with the reason fo r \.....;\~~' ~vithdrawal, and quarantine it awaiting disposa.l instructions.
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Inspection & Test Status
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I t is essential that rou can establish the inspection and test status of materials whilst they arc being used in a work process. up to and when [her ars: installed in an aircraft. Let me giw you a classic example. If you hayc drawii some sheet metal from the bulk stock you would ha\"c also been gi\'en tlXe specification number. the batch number and the certificate of confo~ details. So far, so good, now you put the material through a precipitation h1"att.rcatmcnt process using the figures provided - and go to lunch! f:: ~ Your colleague arriycs and picks the material up to carry on wo rkin~-in your absence. How is he/she to know that you ha\'e heat-treated it? On ~ piece of metal looks n~ry similar to the next. That is what is meant by inspection and
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test stams. If you have heat-treated the material you must have been aware whemer rou had done this successfully or not. You may be awaiting further confirmation from me reslIiLs of a tensile test on a test-piece for example. You may e,en h:\\'e not carried au[ me helt-treatment. Your colleague needs to be able to check Ihat the material is identifi:lble, what stage the wmk p rocess is at, and whether it has passed through each stage successfully or not. If your colleague cannot eSL1blish this he/ she must quarantine the item as defecu\-e awairing disposal- or your return from an extended lunch! J magine if you had realised that you had messed up the heat treatment and had then just left the item on rhe bench. I f your colleague had assumed you did this because all was well we will now have a non-confonning part being progressed on through the process. , am using a real example here that did happen and resulted in a failed part being fined to an aircraft. FormnatcJy, the releasing engineer noticed that there was no reference to test results in the task records.
a similar vein, a pipe rhat has been pressure tested looks \·ery much like one that has not. i\ cable that has been proof-loaded looks remarkably like one that has not. I f an item has been tested there has got be an indication whether it has passed or failed. \"imout tills we run tile risk of either passing a failed item on to rhe next stage or missing rhe fact that the item was not acmally inspected. [11
\Xlhen dealing with materials in the workshop we need to ha\-c process controls that identify precisely each stage that has been reached and whether the material has passed through each stage successfully. 11lls can be done by using 'route' or 'task history' cards or adler documents that allow the o perator to certify that the item bas sllccessfully passed it stage before being passed on to the next. The next operator commences by checking the proof of inspection from the previolls stage. The system must tie items to the related documents. Someone is going to have to clear the linal release document fo r a parr rhal has been processed '1 ~i,mt someone would like some assurance thaI all the steps in the proces~\Mna oeen successful and that all the materials used were identifiable a[1,£id\'Pp~~ved. 1f YOli mink this is overkill, imagine mat rhe piece of metal we ') \'l·).q~ave been discussi.ng is going to form part o f an aircraft's primary load bearing . ;\,,,!e 5trucrure _ and 'vou have J·ust booked a discoun red fare flight in it! You migh' .\'>(.,\!. . ' ' [hen like to know that the shark repellent came from an appro\'ed source! r,
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Heat Treatment of Metallic Materials
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The heal treatmenr processes for aeronautical metallic tn..1terials are critical and sometimes change the mfltcrial spccificfltion. There are Britis h Standard tables that relate various material specifications to hear treatment nmes and temperatures. However, some aluminium alloys are 50 sensitive to change during heat treatment they have specific temperatures and times for each batch of the material. In these cases, the temperatures :md times for each p rocess are :mached to the Cercilicatc of Conformity to specificacion or App ro\-ed Certificate for the material. I f this documcnL1tion cannOt be located then you cannot under any circumstances use figures from other sources. T he material is
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C HAPTER T W O WOR KS HOP PRACTICES
no longer fit for purpose and must be quarantined :lS defecu\'e i1waiLing disposal instructions. T ha( concludes (he rather important considerauons \ve need to keep in mind when usi ng workshop m:lterials that are desti ned for :lircraft usc. Thefe is :lnothe r consideration when we are actually using the m:tterial and that is th:tt we should not wreck it through o ur actions. j\[,lterials are stored in a manner tll:ll preserves their identilv and thelf condition. A sheet of metal may have been issueu from the bond~d store in perfect condition. 'rhrow it on th~ floor and walk aU O\'er it and its condinon will no longer be fit for purpose! In the smllC n:in, jam it between the unprotected jaws of a \'ice and we will hayc achie\'ed a similar result. :\ll solid materials used in a workshop should be protec((~d from the fretting, scoring and corrosion that result from careless :lcuons. r!uids should be kept in sealed contruners to avoid cross cont:lminalion. FI:lmll1:lble fluids and gases should not be srored or used in a workshop where they m:ly create fire and explosion risks because of [he n:lnlfe of other acti\;oes taking place. W'e arc returning to chapter one here in reiternting the need for safe working practices.
Dimensions, Allowances and Tolerances Jf \\'c need TO define a finished product accurately then we need to LIse dimensions. The problem that we h:lve is that e\'(~n \vith modern engineering techniques we cannOt guarantee :lbso]ute :lccuracy \0 :l discrete figure every ti me. 1\lachines :lncl machine tools grad ually wear and material propertlcs va ry_ T he skills of people in the use of hand tools :lre even more \'ariable. Each o f Ihese factors works to create a dn ft from the nominal size o f :l dimension, 13CC:lUSC of tills we have to decide by how far we C:ln pem1it this drift from aCCUf:lCY to occur before the product is no longer fit for purpose. TillS means that we must h:lVC a system of limits and fits. \X'e need to proddc upper and . . \\\iow'l:~\limits within \vhich we are allowed to work. \\'e also need to provide _(,,\\•.::,!..c - positiYe or negaciyc allowances to achie,e required fits her-veen m:lung \:$(\ l_ components. These limits and fits are set by designers who have to consider .\.... _..., \: [he intcrchange:lbility of pans :lS well :lS the functioning of individual parts . ....... ('......
(P~
Dimensions ..\ simple definition of a dimension is that it is a l11e:lsure of the distancebetween twO fL,"cd points or planes. The three dimensions of space are: lcngth ~ breadth and dcpm (ullckness). These three dimensions are IinC:lr :lnd arf expressed qU:lnumti,-ely in discrete units or P:lf[S of those units th:lf :lrc rclatL-d 10 :l chosen scale o f measuremem. These can be millimetres in the mer:i'c system or inches in the English system. Some dimensions :lre classifiec:r~ s o lill1ctiolUl/ dimensions in that thev directly affect the functio n of:l p rQ.Q:uct. , . N on-runc lion:li dimensions arc :lpplicd to manufactltL'e or inspecuG'p b ut have no bearing on the function of the component. Au.,(jJiary dimen$..,(pns :lre used for information and h:lye no bearing on a product'S conflj'tmiry to specification.
'"
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CHAPTER TWO WORKSHOP P RACTICES
T here is, however, a wide range of featu res in a product that can be measured. I'or example, we can use linear dimensions to express: diameter, radii. ci rcumference, pitch, flatness, par.illelism. concentricity, gradient and profile. The measurement of angle, howe\·er. requires a slightly differem interpretacion. An angle is defined as [he change in distance or direction between twO lines or surfaces thac dh'erge from the same point and we h:n-e (0 employ a different measurement scale. In this case we express angles in degrees (1/ 3601lt pan of a circle) or radians (approx 57.3j.
A1Jowance When we arc required to fi r tWO mating components such as a shaft and a hole together, we need to know whether the fit should pro\'ide a positive clearance or whether it is to be an interference fit, in which case the clearance between the shaft and the hole will be negmh·e. "nle allowance is the t:Ie:mmcc thIlt exists between the slmft and tlu? hole and is expressed as being ei ther posiu\'e or ncgau\·e. In o rder to achicyc the specified fit there will be a stated dC\'iarion from the basic size of the shaft or the hole that is designed to produce that fil.
\[mm,umU
Clcamnce
:-------1;-"[aximum Clearancc
n
Clearance Fir
Interfcrcnce
n
Int(;rfen:nce Fit
Figure 2. 1 · Allowances
A tolerance is the dimensional difference between twO limitS but when we arc dealing with the fit between two components, 0111." of the limits is normally fixed whilst the other is varied by the tolerance. The teon allowance is applied to tbe clearance that exists between a shaft and a hole when the), arc brought together. This is not to be confused with the upper and lower size limits stated for eacb component. The difference betwccn the upper and tower sizc limits is the tolerancc and the shaft and me hole in this case will each have their Own tolerance.
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There are a number of fir systems that are used to achieve :l. desired fi[. Some are hole b.7scd where the shaft size is selected to fit a gi,'cn hole. Other systems arc sh.7 ft based where the size of the hole is ,-aried to fit a gi,-en shafr. \'('e will be re-yisiting the common system of fits and clearances in more detail in chapter 6 of this module. For now, you JUSt need to know what aUowancc means. Be fore we lea,·c this topic. it would be useful to take a look at rhe three common classes of fir. Clearance Fit This class of lit is achieved when there is a posio,-e difference between the smaUesl size of the hole and the largest size of the shaft. The shaft would be smaUer than the hole. Interference Fit TillS class of fit is achieved when there is a negaciye difference bc"vecn the
smallest size of the hole and the larges t size of the shaft. The shaft in this case would be bigger than the hole. Transition F it
T his class of fit rna\" be either a clearance o r interference fit because the lolc.ranccs applied [0 [he hole and the shaft oyerlap.
Tolerances
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... \ .
I think that \\'e ha'-e already re,-ea1ed what is meant by the lenn tolerance. "-e do need to be clear though. The basic size of a dimension is the size in relarion to whiSh all ]jnll tS of variation are assigned. \X'hen a basic size is given it may ~c.m:~b m panied by upper and lower limi lS of variation. For example. the ba sic ,,\\ . . . . limi- to f8 .:)!nm " . ~\.:...;..':' ~ size may be 8mm but we can accept a yanatton to an upper sIZe l·...' and a lower size limit of 7.Smm. The tolerance in tillS case is 1mm. which is ell(: difference between the upper and lo wer size limits.
-- ---- -- -- r====j:-----Cpper Limit S__ . 8.5
Luwer Ih'lC
8.S
-.3
--- ----------'" 8±O.5
Limit -.5
Sile
L_:._ _ _--'=:::::: ___ _
-
·rlllcrancc = L'ppcr 1.!llut - '..ower l.!m!l ~8.5-7.5 ~I
Fig ure 2.2 - Toleronc c
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In some cases there may be onlY one limit. For example the basic size may be 80lm with a pennined ~pper siz~ limit of 8.Smm but no lower \·ariauon. In this case the tolerance would be O.Smm (S.S - 8) When limits arc applied to dimensions in drnwings they may be shown \\~th the larger limit of size O\'er the smaller limit of size. For example: 8.50 7.50
Altcrnati\ely, the dimension may be shown as 8 ± 0.5
Standards of Workmanship \X 'hen we hear people refer to Someotle's standard of work wc nonnally associate this with a subjective opinion on whether the work is bcing carried out badlr or wcU. \,\'e sometimes bear statements like 'he achieves the h.igheH standards.' The problem is that the standard in these cases is not :lcruaJly being identified and it is therefore not possible to measure someone's perfomlance against it. In fact, these loose statements are a complete misuse of the term 'standard', If we can name a standard such as a known industrial proficiency srandard, and we can test the indi\'idual against it, we may then comment on whether he/she meers it or not. Similarly, we could reS( the whole nation against it and be satisfied that those who passed had mCI a required standard of proficiency.
Standards In cngineering, a standard is defined as a technical specification or other document approved by a recognised national, international or company stnndardising bo~r for repenred application. You either comply with the srandard ~,(yt;tJ do not. 111ere arc specifications for: materials, products, pre~&$~S; inst.'l.llation, inspection, test etc. These specifications are very ,~;\.~Uetailed. For example, a maintenance specification describes in detail the .\,\ " requirements of preveomtive and correcti\'e procedmcs to maintain a product ,~\~'~''c and specifics the materials to be used and the periodic timing of acu\Tiries. If \.. rhis sounds like a good description of an aircraft maintenance manual you would be right. You either succeed in complying with the specification or you do nOt. In both cases, we need to have the standard identified before we can comply \\;rh it, There arc thollsands of national and imernarional smndards for just about every product, sen'ice and \1lork process in existence. \'\'e met some of these in module 6. These were the British and American Standards for dungs like materials, fasteners, heat treatments CIC, TIlere are also national and international smndards for design practice, drawing practice, manufacture, maintenance, inspection, measurement, calibration etc. In d1(~ United Kingdom, the body that issues and controls national standards is the British Srandards Institution (BSI). There is also a European Standards (EN) organisation and an American Srandards organisation (AN). ~[OS t
80
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coulllnes III the world han,: their own narional standards. The ultimate aim is to bring all these differem national standards to&'Cther under one set of internationally agreed standards. -[be lmcrn:nional Organisation for Standardisation (TSO) exists fOr that purpose. ,\".arion authorities also issue standards, such as those for thc way that we opcrate appro\'Cd maintenance organisations. These standards \vill p~im to the technical spccifications that we must obsen·c whcn carrying OUt our work. Is this beginning to sOllnd familiar? \'('e work to appro,'cd dnl\vings and wc mainlain and repair aircraft and aircraft components to appro\·cd maintcnance manuals. These are the approyed manufacrurer's standa rds. \,\ 'c can giye a name to thcm and we can measure Ollr compliance \\itl1 them. The application of standards ensures that all work processcs arc ca rried out correctly regardless of location. Pro\'iding thc pcrsonnel arc correctly trained and qualified to an apprm'ed anacion amhority standard like EAS,\ Part 66. the smnda rds ensure that the work will be carried out corrcctly regardless of who undertakes it. The organisation they work for would in turn havc to comply with thc EAS1\ Part 1-1-5 joint ayianon requircments for approved maintenance organisations. That is 'standardisation' - e"eryonc docs c,'crything that is required in the appro.-ed mar-nero Paris manufactured to all appro.-ed drawi.ng arc bcing manuf.1crured 10 a standard . ill this case lhe drawing. so they should aU conform to it. In turn, the dra\\.oings arc prepared to apprO\·cd drawing and design standards. \'(11cn wc m:lIlufaerurc a part to an apprO\·ed drawing we ha"c to sa tis~· p r~scribed limi ts and fits and any special requirements. If wc do not, the pari will f.1il inspection and wc will ha,'c therefore failed to meet the standard. Aircraft maintenance tasks are ca rried out in accordance WitJl the approl·cd maintcnancc manual because that is the standard. I we fail to do this correctly wc will hal'e failed to meet the standard.
,-
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,rery simple terms, if we all agreed to measure the length of all hOllse bricks with the same ruler, that ruler would have become the sta ndard for length for hOllse bricks. \X'e arc of course reliant on the ruler being accurate so we may check it against anothcr object that we know to be correct. That object now becomes the reference standard.
1\\\ fn
(.I,,\o.:t-: \\;\(~~
Our ability to meet the standards we use for work will be hea\'ily influenced ~' the qU:l.liry and condition of the tools. equipment and facilicies a\'ailable and t~ working em'ironment. The quality and conclinon of the humans who are doini the work will also influence their ability to meet the standa rds. No\\' we can saf that a person meets or does not meet the requirements of a specified standar~ such as a drawing bccause we can cbeck [he results of a final inspection cjf his/her work against that standard. .~ o
,\ n interest.ing story shows the \'alue of standards. During the American ~~il War, all the soldier's rifles were expensively hand made by skilled guns~ths. The problem with this was that barely any of the parts wcre inrerchangc::r6Jc. If your rifle bolt fracl"Ured yOll could not borrow one from anothcr ritle Wi tb any certainty thal it would fit. f\lany weapons in use in times earlier than itis could nOl even exchange ammunition. Today, all thcsc weapons havc to meet a
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C H AP T ER TWO WORKSH O P PRACTI C E S
stringent design, manufacru ring and inspection st:lndard so, just like the people who passed our proficiency teS[, all the parts are intercbangeable - at least that is the general idea!
Calibration of Tools & Equipment, Calibration Standards I n this topic we arc going to come into contacr with the science of weighing and measuring called metrology. \'rhen we me equtpmenr for inspcction, measuring and testing we nced [Q be sure that it is accurate. This is of particular importance when the equipment is used as the sale criteria for accepting or rejecting work. Tbis brings to mind the well-known story of the wheel-tapper's hanuner. For those who do not know it, a spare of rail carriage wheels being rejected for cracks ceased when it was discovered that it was the hfl!nmer used for acoustically checking them that was cracked . If we were constantly rejecting parts for being out of limits it would be embarrassing to later discover that the measuring instrument was inaccurate. \'\'e rely on a surprising range of de\·ices [0 measure ~arious parameters. \'\"e llse gauges, instruments . sensors, specialised rest equipment and computer programmes to give us confidence that we are conforming to required specifications. \'\'e also rely on equipment like jigs, fi:xrurcs, templates and patterns during manufacture, repair and maimenance tasks. These tools nonnaUy do not measure anything but if they arc distorted or worn, they will cause us to produce work that will be re jected. \\'e also usc torque setting wrcnches and torque meters to join parts. \'(Ie need to have confidence in [he inspection, measuring and rcst equipment that we use. All measuring instruments have inherent inaccuracies and there will always be... (~ome degree of uncertainty about the accuracy of a measuremeriP. \Y/e need to know what that uncertainty is so that we can be sure th[\>--~~
Accuracy & Precision in Measurement Accuracy is de fi ned as the exactness o f a measurement. If we achieve the exact reading without error then we have achieved accuracy. How accurate a reading is depends on how close to the mean yalue it is. Precision is defined as the repeatability of accuracy. J f we keep on achieving the same reading e\Tery rime we take the measurement we have achieved precision. 1t is possible of course to haye precision without accuracy. If a measuring instrument gi\TeS an erroneous reading it is inaccurate. Jf it repeatcdly gi\Tes cxacdy the same erroneous reading then it has precision c\'en though it does not have accuracy.
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Measurement Error There are lWO lypes of error in quantir:ui,'c mea surements, sy~ !em:1 tic and (~ndom, A .<;),slcm:aic error exist, when all the readings ;"I(e shift ed to one side of the mean, There mar be precision but poor accuracy, T he readings are either consisTently high or consistenuv low, .A r:mdom error exists when successi\'c readings are scattered either ~ide of the mean, 'l1,e instrumclll never gin~s tile s:lIne reading, '111ese measurements lack accurncy and precision, The simple SOIUtiOll to our problems is [Q periodically check inspection, measuring and test equipment to establish what degree of uncertainty exists and to check for accuracy and precision, I-low often we do this would logically depend on the amount of usage we gi,"e it. The question is. what do we check it \\ith? \X 'hate,'er we choose may also become inaccuratc so we will be faced with checking that against something else, \X'c could go on ,\;,h this until we ha\'c a train of checking dences that must u1timardy lead back to a master rcference de,-ice, It would be reassuring to know that de\'ice \\':\S accurate!
Calibration Calibration is defined as detennining the true ,"alue of measurements in a way that ensures that the mea surement uncenainty is known and is consistent with the required measurement capability, For example, if an inST rument has 1O mcasure withi n an accuracy of O,002in we need to be certa in that the uncertainty is no greater than this anywhere in the measurement range o f the instrument, 'l1lis is important when you conside r that a dimension that is ncar in; upper or lower size limit could be [ejected when the instrument adds or subtracts its error to the readings,
\Vc need to be certain that the units marked on the measuring scales wc use arc truly representati\-c of the nationally agreed definition of {hose units, For example. can we rely on the metric metre in Timbuktu being exactly the same as the metric metre in London or Paris? In other words, where is the origi~ metric metre, and ",hat is it? This is a serious consideration in a glo~ marketplace where aircraft consist of parts that arc made by differen;. manufacturing countries throughout the world, If each o f these countries holdS: their own standard metric metre, can we be sure they arc all the s am~ !\merican's often claim they do not use the metric system for that reas ~ rather than the expense im'ol"ed in changing over [0 it, Sounds logicall ,..... o·
'"
You already havc some idL"a of wha t is meam by a standard, It is somecljing that is universally accepted and complied with, \'{/ith tha t in mind. we wi!l'not be surprised to know that there arc standards for each unit o f meas~ment both in the English and the metric systems, r always shy away from~ragging IIp history but in this case a ,"cry abb reviated his lOry of how we ar(i~d at the units we usc is actually helpful because it emphasises lhe importance of
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CHAPTER TWO WORK S HO P PRACTICES
smndards. 00 not try to remember tbe details just the need for haying srandards for measurement and why we seem [0 have three different systems today. The flISt standards for the English units of measurement were introduced around IS-l5. For example, the Imperi.'tI Srandard Yard (3ft) was defined by me length of a specially made bronze bar. This remained the ultimate standard for the yard until 196-l. So much importance was placed on preserving this srandard that five copies were made that were distributed around the United l(jngdom. A similar srandard was held for the pound mass. The srandard for rime, the second, was initially based on a fraction of the Earth's rotation but it is now based on a given number of cycles of radiation of the caesium :Hom. There is :uso a standard fo r the Tmperial gallon. At first you may think all dus a bit quaint but can you imagine the effect of losing these standards? All measuring scales and clocks in commercial use would guicklr become suspect because as they became inaccurate o\"er time there would be no standard to check them against except other inaccurate devices and ch:I.OS would gradually begin to reign, From 1964, the standard yard was defined as being equal to 0.9144 of a metre and the standard pound was defined as 0.45359237 kilogram, bringing them into line with the old metric centimetre, gram and second (cgs) system. The unit for the second was fortuitously the same. The current Sysleme Internationale (S J) metric system is now based on the metre, kilogram and second with the ampere and candela being additional unitS for electrical current and luminous intensity respecri,ely, Some wmdards for the metric system arc based naruml occurrences. The metre for example is based on the length of the path of light in a vacuum during a given time intervaL On the Olher hand, rhe kilogram is still based on the mass of a given block of materiaL The metric system is a coherent system and there are easily derived units that arc used for things like area, volume, velocity, acceleration, work, power and energy. The English system i,,)liirher reckless and comes up with bizarre units like 3 feet in fl yard~J 1!2'1,#ol;nds . in one hundredweight and 1760 yards in a mile. Not so bi2afft 'as dle furlongs, chains, bushels and quartS 1 had to learn at school!
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,~\~'
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The Americans did not accept the metric system but neither did they like some of the English unitS so ther haye come up with the US Cusronuuy units. For example, they ha\Te 2000 pounds in a short ton, 100 pounds in a short hundredweight and ha..-e different "alues for tbe galion, pint and fluid ounce and also use the Fahrenheit temperature scale rather than the Celsius scale. They also have rather strange units like the barrel and the kip. You would have encountered all of these in module 2 - Physics, Before we move on, we need standards that are used.
[Q
know a little bit more about the naturc of the
Physical Standards
,,
~v
These are standards that have bcen manufactured. The bronze bar defining the standard yard or the kilogram mass are good example of these.
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CHAPTER TWO W ORKSHOP PRACTICES
Natural Standards T hese are standards that are based on stable natural phenomena such as the speed of light in a vacuum or the cycles of radiation from a giyen element under specified conditions. In earlier times, the introduction o f sidereal time that \ve still use today to measure days and years was based on the rotation of the Earth and its orbit around the Sun.
Variables These are things that can be measured using a gi\'en systems of units. The measurements are desctibed as being quantitative in that they can be expressed in figures to required degrees of accuracy.
Attributes These are things that cannot be measured in quantitative terms by using units. They are properties . ror example, if you were to try to measure taste, colour, smell or feel for example you would not be able to do this with figures because they do not relate to any numencal scale. Attributes arc normally compared with a standard that represents the rCCJuircd property. I f you were checking an !tern for a particular colour for example you would have a sta ndard that \vas the required shade of colour to compare it with. A problem with attributcs is that tbey can be prone to subjccti,'c opinion depending on the person dOlng the assessment. That is why it is important to have a clear and unambiguous standard to work with . \Ve do meet attributes during inspections such as the quality of a surface finjsh or \"lstml indications of detcnoration and we need to haY~ !J1.:iclear idea of what is acceptable or not. ,\,\jb \\cg.c -
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~
...
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National Physical Laboratory (NPL) The National Physical Laboratory located at Teddington holds and maintains the CK E nglish standards and the UK COpll~S of the International met£'c standards . The onginal bronze bar fo r the yard and tbe standard pound ma~ will still be there. The UK has now adopted the metric SI system, as have man~ other countries throughout the \\Todd. This means the UK metric standard~ held at the NPL have to be checked against dIe international standards t~ ensure that thev continue to conform. .~
-
International Organisa tion for Standardisation (ISO) , The metric system standards are held by ISO at Sevres near Paris. These arc the internati~nally recognised metric sys~em standards against which ~e copies of the metric standards held b" ,arious countries arc checked.
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Traceabili ry W/e now come to the second point. \X/h at do we check our Illc:lsuring inSTruments, jigs and fixt ures against? \ XTc certainly do nor jump on :l bus and hare off to the N PL at T eddington! WThat we do is check our ins tI umentS using referencc standards that are verified in a way that can be (raccd back \0 those held at [he NPL who in rum ,-erify their standards against those held by the ISO. Refere nce Sta nd ards These arc standards of pro\-en accuracy agatnst which the accuracy of measurement devices arc checked. Working Stand ards These are thc refcrence standards that arc used in the workplace 10 verify Ihe accuracy of measuring instrumems. For example, gauge blocks and weights that arc kept locally and used in a dedicated calibration room. T rave lling Sta nda rds These arc reference standards that arc held centrally by an organisation or laboratory and arc brought to places of work. Reg iona l Sta nda rd s T hese arc reference swndards that arc held in different regions around Ihe country, usually in accredited laboratories, and arc used to veri(v the working and travelling standards. Nation al Sta nd a rds '1\)\.f
The\~....~(C;t~'fcre~ce standards
that are held at the National Physical LaboralOl")'
i(llth'e UK and arc used to verify the regional standa.rds. The smndards arc kcpl
"~\n a ,-ery strictly comroUed ennronment. 111e physical standards like the kilogram mass are kept in c\-acuated glass cabinets :lnd are never dusled or cleaned, as thcse actions alone would eventually compromise their accuracy. In ternational St:lfl da rds T hese are reference standards mat are held by the International Organisation for Standardisation III Sevres and are used to verify the various nat..ional standards.
An rhe ,·crificat..ion actions arc documented so the micrometer in your hand will ha\-e been checked for accuracy by a reference standa rd lhal has calibration record trail leading right back to the international standard.
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Calibration Roon1s Calibration rooms must meet strict environmental controls that arc specified in calibration standards. A calibration room should be dust free and have a controlled constant temperature of 20 °C (68°F) and a specified humidity. Tbe room should be free of vibration and have an adequate lighting intensity. The enyirorunental conditions of the place where mcasurement standards are stored are similarly eonrrolled. The g.wgc and the insrrumem being calibrated should both be at rhe eonrrolled rcmperature during calibration to ayoid errors clue [0 differences in expansion or contraction values. There is little point in used a wam1 gauge covered in grime [0 check a cold micrometer!
NAMAS The calibration of precision measuring instruments is often sub-contracted out to accredited metrology laboratories that operate strictly 1n accordance \vith the specifications contained in the calibration standards. These laboratories are approved by the Uruted Kingdom Accrcditation ServlCe (UKAS) under the National Accreditation of 1 1easurement and Sampling (NAJ\'lAS) reqtUrements .
Quality Systems There are a number of ~uality systems. The best known is contained in the rso 9000 series of quality system standards that coyer the requirements for deSIgn, manufacmre, installation and maintenance. These equate to the former
Bricish Standard 5750 and EN 29000 series of quality standards. These stan4.w0s include among orner things the requirements for the control of }.0i.ffieaSuring and test equipment. . "c"::'c . C.L~ .
~ \.1:1('
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Approved aircraft maintenance orgamsations work to EASA Part 145 and this also inyokes similar requirements. EASA Part 1-1- -40 states:
1
"
(~~'Y
'\Vhere necessary. tools, equipment and particularly rest equipment must be controlled and calibrated to standards acceptable to the JAA(j.it a frequency to ensure serviceabilin' and accuracy. Records of sueh.. calibrations and the standard used ~ust Ge kept by the E..ASA Part 14i approved maintenance organisation'. (EASA Part 145-40). :::!
o·
Calibration Requiremen ts
"0
" n'
The quality systems we have discussed contain or inyokc dctailed requircl~nts fat the control of inspection, mcasuring and test le quipmcnt. We necd tS>--ttkc a look at these are. 1 am using my own words to expand the meanings. ,§v
"
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CHAPTER TWO WORKSHOP PRACTICES
.-\ maintenallce organisanon is required to control, calibrate and maintain all inspection. measuring and test equipment that ther usc. This applies nOI only [0 equipment they own but also applies to any equipment that they may ha\'e on loan from other organisations. New equipmem should be subject to initial calibration prior to first using it to ascermin its accuracy and precision and to verify that these are \\~thin the required limits. The organisation must ensure that all inspection, measuring and test equipment that will affeci their compliance \\;th the quality requirements of their work is identified and that it is calibrated at periods decided by the organisation or before its usc if this is deemed to be necessary. Cnlibration must be carried out agninsl reference standards of known nccuracy that are tr:l cl.·a /)Ic by d ocum enfed e vid en c e (0 n :lfion al or inlenulIion:II s (;md:lrds. \'<'here this is not possible, the organisation must be able to produce documemed eyidence to show what criteria was used for calib racion and what was the basis for doing this. The decision on quite what items should be subject to cnlibrntion is based on what rou usc lhem for. The organisation mUSt establish the calibration status of all its inspection, measuring and tes t eCJuipmcm. If an item is used as the.:: sole criteria for accepting or rejecting work then it docs fall under the calibration re(luiremenr. In the extreme, if you used a piece of string as the sole means to make a decision of acceptance or rejection rou would need 10 register it as measuring equipment subject to check. The decision on the internJs when equipment should be calibrated is based 00 its usage. I f a micrometer were used connnuously throughout each day then the calibration interval would be short, possibly weekly. If the same instrumenr were lIsed infrequently. say once a week, then the calibration iruerval would be much longer, say three or six monthly. Some tools and equipment may fall outside the calibration rC(luircmem. Examples of th'l~,;(\Vouid be a steel ruler or a fitter's square. Again it depends on the criuc~1 nature of what the), arc being used for. Gene rally, a prc·usc cht~";B}' the opera LOr will be sufficient. These items would normally be \\.'-q~belled 'not subject to calibration' to alert operators who might be com;idering using them as the final coteria for accepting critical work.
Jigs, fixtures, templates and panerns may not necessarily be used to accept or reject work but they may become the cause for it being rejected. Again. depending on their usage, they mar have to be subjected to a periodic recalJ to esmblish that their form dimensions srill comply with manufactllrer's specifications and that they ha"e the required accuracy. In some cases the organisation may also impose a pre·use check requirement on these items. An organisation must maintain a register of all items of equipment subject to calibration. The register should det:til the equipment br type, identification number, location. calibration staNs, calibration period, calibration method and rhe accepmncc criteria. Each piece of registered equipment should carry a suitable indicator such as a label to show its calibration status. An operator using the equipmenr will need assurance that it is within its calibration period so [he date due of the next calibration may be shown on the !abel. Some o rganisations. where equipment is subject to very high utilisation, may use
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CHAPTER TWO WORKS HO P P RA CTICE S
colour-coded labels that relate to a month. For ex ample, in July the colour may be green so you would not usc an instrument that does not bear that colour
label. T he organisa tion must maintain records of rhe calibration of each item. These records should bc legible and stored in a manner where they will not deteriorate. The records should a.lsa contain the details of past calibrations and the results. The,· should also show the measurement uncertainty \·alues for ca.ch itcm. 'n1C records should also show what action was taken when the calibranon results proved to be unaccepta ble.
as
These records arc useful in highlighting trcnds well as supplying evidence that equipment is conrrolled. I f an instrument is recorded as continually having to be adj usted to bring its error into limits we might iu~ t consider scrapping it, or reducing its calibration period. o r using it for less critical work. An organisation may usc an outside agency to provide calibration sen·ices . The organisation is still res ponsible for ensuring that the requirements of the qualitr standard they are required to meet are also met by that agency. Tnat is why N.A!\L-\S accreditcd metrology laboratories arc chosen - the\- comply with the rul es . If the manufacturer of an item of test equip ment or precision meas uring equipment does not supply the calibration procedures and data, the procedures given in the British Standards fo r thc calibration o f rest and measuring c(luipment must be followed. It is the operator's responsibility to cheek the calibration status of test and measuring equipment before use. TillS normally means checking that the equipment has a current. calibration cernficate. If the calibration status cannot be ,~erified or the equipment is in any way faul ty or suspected of being so, the operator must label it as defectlYc and return it through th c stores for repair and re-calibration. , DO'- so f ar campli ed \"\olt - h all -" t b esc reqUlremems, -,OIJRI av1ilg t h e orgamsauon must c . (,l't\\o.:.'f '- ensure that all this equipment is handled, stored and transported in a manner \W~{' that will not damage it or compromise its func tion. I f there are any special . \~....<...'= requirements a~sociated with any item these should be s tated o n the calibration . o'\;.... ~ . record for thar Item. o
c-
Finally, if you lose the endencc of calibration status, or rou suspect
t!rc
accu~cr of a piece of inspection, measuring or test equi pment, including to
such as iigs, fixture, templates or patterns, you must immediately label it ~ . defective with the reason, withdraw it fro m use,1and quarantine it. A piece of equipment that has no evidence of calibration is ju st as defective as a piece tha.r· fails to o perate.
5'
Concl usion
~-:
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"'"
\'(Ie haye come to the end of chapter two. There is plenty of scope for ~ sa" questions in the topics we haye discussed here. I lwill be purring a few c;hl"nple essay questions and specimen answers into an hnnex to this modul~v In tbe next chapter we will be rensiting specific toolr and tes t equipmc&T. In the meantime, I have devised a few multi-choice answer questions for you to try.
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CHAPTER TWO WORKSHOP PRACTICES
Revision
Workshop Practices Questions 1. Brittl e cUlling too ls should be stored : a)
together
b) out of contact with each other c) in a sep:mue locarion
to
other tools
2. T eam too l control would be b es t achieved u sing: a) individual tool bags or rolls b) a team tool box c) a team tool shadow board
3. If a multimcte r has no proof of calibration s tatus you shou ld: a)
'lnili
\~~urmv
it from use and label it as defective
. ~\I;.I!.I,; . L'-1\\ • .
~).t~\.
b) assume that it is nor subject to calibration
~'\:-\ ...
,-~~\~
c) check it against a multimeter that does ha\'c proof
4. A hand c hisel that has a 'mushroom ed' impact end should b e: a) used because tills is nannal b) scrapped because it is dangerous c) reground before usc
,
,,'C~
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CHAPTER TWO W O RK S HOP PRACTICES
5. If a cons ign ment of shee l m etal is received accompanying Cenificalc of Conformity you should:
a) quaranunc
wilhout
an
It
b) es tablish ils identity before using it c) not lise it for primary structure repairs
G. Allowance is the: a) difference between the upper and lower si:r.c limits of a dimension b) clearance thai exists between a shaft and a hole c) difference between the basic size and an upper or lower limit
7. A dimension that is 20mm plus or minus 2mm has a tolerance of: a)
~mm
b) 2ml11 c)
24ml11
8. A negative allowance indicates a :
b) interference fit
c) an erfor because aJI allowances arc positive
,.. 9. Precision in measurement is the ability 10:
a) measure exact mean yalue
-'%-
"-
-0
•.
b) measurc within a gil-cn limit of uncertainty c) repeatedly achieve the same measurement
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CHAPTER TWO WORK SHO P PRACTICE S
10. Precision m eas urin g eq uipm ent must be ca libra ted using standa rds th at are: a) traceable to national or international reference standards b) issued by the manufacturer of the equipment
c) approved by the client
II. The calibratio n inten.a l relating to a measuring device is established : a) by the manufacturer
b) bv the British Standards instimtion c) by the owner based on its usage
12. The requirem ent 10 control inspection, measuring and test equipment in an approved a ircraft maintenance organisation is contained in:
a) _\ British Standard b) S\SA Part 145
c) the Aircraft Maintenance
~Ianual
Revisi,!-nb t.>"\.... ... ",.~
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Workshop Practices
I.. ~ ....
Answers 6.
1. B
B
11.
-
C
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2. C
7.
A
3.
8.
B
12.
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B
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A
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4. C
9.
C
5. A
10.
A
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u'WW.part66.,-onl
CHAPTER
THREE T OO L. S
Tools Introduction \'(Telcome to chapter three. The syllabus for this chapter coyers five topics that a rc almost chapters in their own right. The common factor is tha t in each topic we arc dealing with equipment that we use to can )' OUl maintenance and repair tasks. Important as it is to hayc a thorough knowledge of the aircraft structure and s~' stems we work on, it is eq ually as imp ortant to have a thorough knowledge of the tools we usc and their limirati0r '
Common Hand Tools
\\~(~ .
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\'{Ie can divide the ha nd tools that we use into four groups that rougWy define tbeir usc. \,\le may a rgue that some of these [Qol~ senec uses in more than one group but we are only uSIng these headings to make their presentation to you easier. \Ve will deal with each group in turn and, where possible, present the tools related to each group. b~' description, material, classification, use and any precautions rhat may apply. Tools used for crimping appear in section 7-7 (chapter 7) of the syllabus and tools llsed for riveting and dimpling appear in section 7-8 (chapter 8) of the syllabus so, to aVl id duplication, we will not be including thcm in this chapter. ,QU 1006 - ~ «1>\'::'<;c '" The groups we will be dealing with are: '
•
G ripping tools
•
Cutting and shaping tools
•
D ismantling and asscmbly lools
•
r,'larking out and mcasuring tools
....;.
Gripping Tools Bench Vice D escription
'u
~
The bench vice consis ts of a fn.ed and a fliding part, each c~.rrying a detachable, hardened, serrated steel jaw for clamping work. The axed part is
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TOO L S
secured to the workbench and it contains a retractable half nut that is springloaded into engagement with a buttress-threaded operating screw located in the sliding part. When the tommy bar handle on the screw is turned , the sliding part is driven ei th er towards or away from the fix ed part. For quick adjustment of the working ~p between the parallel jaws of the \Tice, a release lever can be moved that retr:lcts the half nut, allowing the slidi.ng part to be pulled or pushed into :lny position . The half nut will automatically re-engage with the screw when the lever is rde:lsed. The ideal working height for a bench vice is where the top of the laws are aligned with thc height of the operator's elbow when standing.
Opernting Scrcw
(~~I,~;~::"' E Thread)
H:J.lf Nm
Fig ure 3.1 - Bench Vice
Materia ls
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(: -'\
\.\'oc
The twO main bg¢y parts of the bench vice are made from malleable cast iron or cast srtC1~t""rrhe detachable steel jaws are hardened and tempered and have SetI,llW;r"work faces. "111C operating screw is made of mild steel and normally \\).<{l~as a buttress dut--ad, although some nces may ha\'e a square thread. C lassificati on
Bench \"ices are classified by their wcight and the length of their jaws. A common example is -,Sibs weight and 4.5in ja\','s. Use The bench vice is used
to
exert a finn grip on material while it is being worked.
Prec autio n s
Vice 'cl:lms' made: from aluminium, copper, lead or composite materia! should be fitt ed over the jaws to protect the sur£'lCe finish of the work from being d:lmaged by lhc serrated steel jaws. T he pans of the bench vice should never be used as an 'anvil' for hammering.
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The operating handle should ne\-er be hammered or rurned with an extension lube in an attempt to increase the grip on work. The \-ice should bc cleancd after used to removc mctal filings and the slide and the operating screw should be kept lubricated.
Hand Vice Descript ion The most common form of hand vice consists of twO spring-loaded, hinged a[l)15. each equipped with an integral jaw.. \ winged nUl and screw arc u~cd to close the jaws and grip the work.
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Hand \""ice Figure 3.2 - H o ld ing T ools
The arms are made of Heel and thc jaws are hardened and tempered. T he winged nut and operating screw are mild steel. A leaf spring is fitted to drive the arms ap:m when the winged nut is released. ,. . .
"-
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C lassification There is a variety of shapes and sizes and these arc identified bt- trade manufacturer's names. ' l11e classification is by o\"cralliength. Use
OF-
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.
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The hand vtee is mainly uscd for gripping small pans, or for holding F:l.rlS together, while they arc being drilled or ri\-ctcd.
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C H A PT ER
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T OO L S
P recauti o ns Due to the lever action o f the hinged arms, the jaws are only truly paralic! in one position so care is needed nOt to calise d:muge to work by applying roo much poim pressure.
T oolmaker's Clamp D escript io n The lOolmaker's damp consists of two parallel jaws that arc opcned or closcd by two scrcws. Final tightening is achieved by adjusting the captive screw and thcn the sccond scrcw. TIle grip of this type of clamp is parallel throughout its range of mo\·ement. ~'r a t e ri als
The jaws arc made of case hardened mild steel.
Classification Toolmaker's damps arc classified by the length of the jaws. Use
They are used for holding small items of work together for drilling. riveting etc.
Pliers D cscriRrlb'H b \\...;;-
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~ \);I.{]:'liers are supplied in a variety of sizes and configurations. The most common .\... ~\,\
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type of pliers is the combination type. These ha\'e both serrated nat jaws and serrated cunfed jaws for gripping work. They aTC also equipped with side cuners and joint cutters, making them useful for Cutting locking \\~re and trimming split pins for example. They are oftcn equipped with thick electrical insulation on rhe handles. Other common conrigumtions include: long flat-nosed, round-nosed and snipe-nosed pliers. The illustration shows diagonal cutting pliers or 'snips' and end cutting 'nippers'. There arc a few other configurations of pliers and lhese are describcdundcr their specific names. Slip·J oi nt Plie rs Slip-joint pliers gcr thelf name because of the twO connected piVOl holes that are provided 10 permit the jaw's to have two opening ranges. This allows a wider range of material th.ickness ro be g ripped.
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TOOLS
End CUllers
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." "''''Culler
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Side CUUC!TS
Round '\Josed
Snlpc 'lo5cd
Figure 3.3 - Pliers
Vice-Grip Pliers (Mole grips) Vice grip pliers have an adjustable range of jaw openings that may be sel by turning a knurled adjusting screw. They ha,-e an over-locking action tha t locks the jaws in position when lhe handles arc squeezed together. T here is a release lever provided to break the lock and release the grip when required. T hese pliers are oflen referred LO as 'grips' and they ha,·c [hc capaciry to apply ,-cry high forces onto the serrntcd jaws. Because of the high ic,-crnge that can be applied, these pliers should not be misused.
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Vice Gnp Pliers
Slip Jlllnl Plier"
InlcrloeklngJoinl Plicrs Fig ure 3.4 - Other Plie r Co nfig uratio ns
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C H APTER TOO L 5
T H REE
Interlocking J oi nt Pli e rs
Interlocking joinr pliers h;we a number of differenr pi\'ot posirions that offer a range o f jaw opening wid ths. They normally have long handles to l)I"ociuc:e ve ry high grip fo rces. Thc)" C:l n have flat or cun'cd, ~e rrat ed jaws. Again, care should be exercised in the usc of these pliers to avoid damaging work through the high grip fo rce. Wire· locking Pli ers ·rhesc diagonal cutting pliers hal"c flat, sermred jaws and are equipped with locking handles and a twisring screw mechanism designed to spin thc pliers :lnd so dcli\·cr a twist to locking wire. ;..rany organisacions now prohibit the use of these pliers because the)" tend to overstrain the locking wire. Cirelip PLi ers These arc equipped \\;th peg fittings on the nose of each ann that arc designed [0 fit the holes in a spring ciedip. They can be used for inserting and remo\-ing inrernal aod external circlips.
Fig ure 3.5 • Circlip Pliers
Plicrs arc nonnalh· made of high-carbon steel \v;th hardened and tempered Jflws. Class ificatio n Pliers are classified by type and ol"erall length. Use Uscs for pliers arc dependent on [he type. Precautions !\ hazard with mOst pliers is trapping dle skin on rhe hand between the n:lt surf:lces beneath the pivor point when the pliers arc le\'ered shut. Cnre should
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be taken lO a\'oid dOing this as it can cause painful inj urles, particularly if tbe pliers slip off the work and close up suddenly. PUers should neyer be used on pipe unions, nuts or bolt heads as a substitute for a spa nner. Ther will rollnd off the hexagon heads and can slip, crea ting a high probability of skinned knuckles . Great care should be taken not to misuse slip-jpint type pliers and vice-grip pliers. The high grip forces can easily overload and damage items .
Cutting & Shaping Tools Steel Hammers Descr.iption
The most common rypes of steel hammer are Fhe ball pein, cross pein and straight peul. The 'pem ' is the end of the head opposite the striking face. The head is normally fitted onto a handle tha t is expanded in the head with a wedge. The weights of the beads \-ary from a few o unces up to 31b. There is a specially designed bodywork hammer for removing dents and smoothing sheet metal. TIle head is light and has large, smooth, flat striking faces. This type of hammer is sometimes referred to as a 'planishing hammer' . Claw-head hammers arc not noonall), used fOr aircra ft work. ·rhey have a slightly convex striking face and a claw attachment, designed to extract nails, in place of the opposite face. The striking face is harder than mher hammers and is designed for driving nails. It is prone to chipping if used for other tasks involving harder materials. ")\)\f
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Ball Fein
Cros~
Pein
Straight Pein
Figure 3.6 - Steel H a mmers
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Materials
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The heads of steel ha.mmers arc forged from hig9 ca.rbon steel and the \\~lcing faces are hardened and tempered. The wooden handles are nonnali.S made from straight-graincd ash tl1at is frce from kn9ts. Others ma[erial~sucb as steel or fibreglas s, may be used to constrllct the bandIes.
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Classificalion Steel hammers are classified by [he type and weight of the head.
Thc cur.'ed surface of the ball pein head is used primarily for ci\'cling, whilst the flat surface is used for general work. 'TI,e cross and straight pein ends of the head are used for work where access is limited. Ilammers should always be held close to [he free end of the shaft so dmr proper control can be exercised, and a harder blow can bc achieved for minimum cffon. Precautions It is imporram to check that the head is finnlr anached to tbe shaft, and that
the shaft is not split. Do not use a hammer If Ihe wedge is missing or loose. TIlf~ correct weight head should always be chosen for a task. For examplc. 1;, and Y11b heads would be suitable for stri.king centre punches. lib head s would be suited to driving pins, 21b beads would be suited to striking metal clltting chisels, and 31b heads would be rese n-ed for heavier work.
Do not lise the side or 'cheek' of a hammer as a striking face. Do no! strikc two heads 10gcrher. Do not use claw hammers for driving punches or chisels. Do not use a hammer that has chipped faces.
Soft-faced Hamme rs Description
. ...... \' c
There arc sqKel\l.l,orypes of soft-faced hammer. T hc)' ha\'e heads with identical na~ ~\~kirtg"taces that are attached to wooden shafts in a similar fashion to steel ll,dnffiiers. Nylon and hide-faced hammers ha\'e pads affixed to lhe striking \'\\.\ . ... . faces of metal heads, whereas other types haye heads constructed wholl)' from \...~\.' a chosen material . Materials Hide-f:-tced hammers have renewable, rawlude pads afli"ed 10 the striking ends of a metal head. Nylon-f.-teed hammers have renewable, nylon pads :lffixed to rhe striking ends of a metal head. Other soft hammers have heads conslructed wholly from a choice of: lead, copper or hard rubber. Classification Soft-faced hammers arc classified by the material and weight of the he:ld. For example, a 4lb lead hammer.
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Hide Faced
[,cad o r Copper
Figure 3.7 - Soft Faced H a mmers
Soft-faced harruners are used to strike finished surfaces without lefl\-ing impact marks . .Again, hammers should be held close to the free end of the sh aft to exercise control, and achieve a harder blow for minimum effort. Precautions Again, it is important to check that the head is firmly attached to the shaft, and that the shaft is not split. Do nOt use a hammer if the wedge is loose or tTllSSlIlg.
Soft metal heads carry a risk of metal chips and flakes falling off the slriking l ends alld entering the work. There is also a risk qf metal transference onto the O,"«or"U.lthat could pose a dissimilar metal corrosion risk. Care should be taken to . (.O\\.::<;C 11.) only use these types of hammer where it is appropriate to do so .
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Chisels Description
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There are four common shapes of metal cutting chisel pOlnt: flat, cross-cll~ diamond point, and half-round. There is another special configura tion call ed asingle bevel chisel. :f: Materials
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Metal cutting chisels are forged from high carbon or nickel alloy steel bar;-;!fhe cutting edges of high carbon steel chisels are hardened and tempered \V~ the rest is left soft and tough. N ickel alloy chisels are specially heat treated ,tog-ive a durable. long lasting edge. The cutting edge is gr9und to an angle tba§ijepends o n [he material to be cut for example, 60° for l d steel, 75° fo r hard~ s[cd and 40° for brass or ocher soft metals.
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TOOL S
Convcx Edge;,'.---_..,
1'1"(;,
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I llalf Round ~
Di:J.mond Point
~E~;;~?~~~I Figllre 3.8 - Chisels
C lassi fi ca tion !'I·letal cutting chisels :lrc chssified by the shape o f the cULting edge and their ovcralllcngth. Uses Chise.ls must be used in conjuncnon with steel head hammers. They arc used for curting and chipping metals that arc softer tban the material of the chisel. Particular uses associated with eacb configuration of curting edge are: Flat
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[be chisel has a slightly COnl 'CX clIlring edgc. It is used for general work, such as dividing sheet metal, or cutong flat surfaces in readiness for filing. Cross-Cut
The cross-Cut chisel keyways.
IS
used for cutting narrow, flat borromed grooves, like
D iam ond Point Diamond point chisels are probably best known for their usc in correcting incorrect sta n s when drilling metal. Ther are used mainly for w rong oil grooves and arc particularly useful for cutting close in LO corners.
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Half-Round Half-round chisels are also known for their use in correcting d rill starts. They arc primarily used for clitting groan's .
Single Bevel Single bevel chisels are used for clltting me heads off rivets after they have been drilled through. The narrow cutting edge is well suited to this msk 111 that It does not i.nterfere with the surrounding metal. Precautions
It IS dangerous to use a chisel that has developed a 'mushroomed' impact head because it can split and chips can fly o ff it. The head should be reground. Nickel alloy chisels undergo a special hear treatment process during O1anufacru re and they must be sharpened on an oilstone. Filing or grinding methods are not appropriate for sharpening nickel alloy chisels.
A chisel should be held finnl! \,,;th one hand and struck sCJuarcIy on the head \,,;t11 a steel hammer.
Hacksaws Description A hacksaw O1a\- consist of a fixed frame, or a frame that can be adjusted to fKcept different lengths of replaceable , flexible saw blade . .A hole at each end of the blade fits over a sliding pin mounting on the adjacent end of the a ck!i3\v frame . The b lade is always mounted in the frame with its cutting teeth :'II.") - • ~Q..C ~\. :Icing forwards, away from the frame handle. One of the pin mountings {' (.0\\... • in corporates a screwed adjustment fitting that is used to tension tbe bl::lde. ;: \,:-5 ]\>[any makes of hacksaw have pin mountings that can be repositioned so that ~0~\. the blade can be mounted at right angles to t.he frame if so desired. Depending (p'Y . on their make, hacksaw frames may have either a wooden handle or a shaped metal hand grip.
~
r
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Flexible blades are supplied in a range of cuts that vary from a coarse cut, at teeth per lIlch, to a fine Cut, at 32 teeth per inch. IT he number teeth per inch i%often referred to as bei.ng the 'pied} ' of the blade. The teeth of hacksaw blade~, are alternately offset either side of the plane of dlC blade to provide a clearancp:for the non-cutting part of the blade. This is called the' set ' and it produces] CLlt that is wider than the blade plain portion to reduce sa'Wing effort aQtl prevent jamming. Typical blade lengths are 8in, lOin and 12in but there is ~ o a range of blades available for miniature backsawJ. t ~~ ",'
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• • Coarse Teeth
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Thick
Fine Teeth
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3.9· H acksaw
Materia ls Thc hacksaw framc is normally made from mild steel that incorporales either a wooden or mCl'al handle. Hacksaw blades arc made from high carbon or high· speed alloy steel. Typical alloy steels used arc tungsten and molybdenum alloys. These have a longer life but are much more expensi\·e. There arc two Iypes of bladc construction:
•
ricxiblc: The teeth are hardened and the back of the blade is annealed. This type of construction is less brittle and the blades arc no! so prone to snap l~ under beavy loads. Suit less skilled opcrativcs. "I.-'~-'{)
..
, ( _~\';'-;";VI H.1rd: Ilardencd througbout. These blades remin thei r sharpness
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longer bUI are brittle and will snap if subjected marc expcricnced operari\·es.
[Q
heavy loads. Suit
Classification Ilacksaw bladcs are classified by the length between their auachmeTlt hole outcr edges, the number of teeth per unit lengtb and wbether (hey are ' flexible' or 'all hard'.
Use The hacksaw should always be operated b}t applying cutting pressure on thc forward stroke ani), and lifting the blade dear on the backstroke. The cutting strokcs should be limited to be[\veen 40 to 50 strokes per minute and as much of lhe blade length as possible should be tlsed to avoid O\·erheating, dulling of the teeth, uneven wear and fracnlre. You should not :mcmpt ro use more than 50 cutting strokes per minure for the same reasons. \\",' hen CUlling thick mct:'ll,
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the lower ratc would be considered to be more appropriate. The saw should be p resented at approximately 30° to the cut. Flexible blades with fine teeth, for example 22 - 32 teeth per inch, are better suited for cutting thin walled rubing and thin sheet metal because coarser teeth tcnd to straddle the material and break or jam. \"Xfhen using fine teeth o n thin matenal there will be several teeth 1Il contact with the material and the cutting will be much more cffcctiye. As a guide, there should be at least three teeth in contact wah the material surface at anyone time. In the case of thin-walled tubing, that is interpreted as three teeth in contaer with each wall of the tube. You sometimes see this as interpreted as at least two teeth in contact at any time depending on wlu ch publication you pick up . Three seems to be the more generally accepted value in exam questions. All-hard blades with COarse teeth, for example 1-1- 1 18 teeth per inch, arc better suited for cutting thick materials because finer teeth tcnd to clog up wuh m aterial and then not cut effectively. Soft metals, such as brass and aluminium. should be cut with coarse toothed blades [0 avoid dle teeth clogging wltb the cut material. Hard metals, such as iron and steel, should be cut with fine toothed blades, for example 32 teeth per inch. The choice of blade is a matter of experience. The guideline is to select fine-toothed blades fo r thin or hard materials and select coarse-toothed blades fo r lhiak or soft materials. Precautions •
Choose the right blade for the task.
•
Ensure the blade IS correctly fitted, with the teeth facing forward, and correcdy tensioned to ayoid buckling in use.
•
Always cut on the forward stroke and do hot oycrload the blade.
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.Always maintain long steady cutting strokes (+0 to 50 per minute).
L0\1 •
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Files n·0
Description
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.t\ fIle consists of a blade that carries cutting tee til and a 'tang' that is designe~ to fit inro a wooden bandle. Files are supplied in a \rariery of lengths, c ross~ sections, cuts and grades. These are described in the paragraph on classificatiotL ~ given below.
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Files are made from lugh carbon steel. The blade of the file IS hardened ~Ulst the 'tang' is left soft to give it a degree of flexibility in the handle to prd;em 1l snapping. File handles are nOD1laUy made of w?od and are equipp~ with a steel ferrule at the forward end to resist any tCQ-dency for the woo8"' to split. The handle is firmlv fitted oyer the 'tang' of the flie. • I
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Class ification &Usc Files are classified by their main feaUices. These are: lengrh, cross-secLion, longi tudinal shape, cut and gr:tde. Files are: designed to cut on the lonvtlrd stroke only. Downwa rd pressure is used on the forward stroke and relieved on the rearward stroke. Leng th The length of a file is the distance from the shoulder to the forward tip of the blade. Files are avaibbie in a range of sizes from 3in to l~in.
\'\"ood Handle
•
I Flar Hand Safe-Edge
•
P,~"'t;,,~;_t.::~:=5!:=--
r lalf Round
,
Square
RIIUlld
Knife
Three-Sqllare Figurt: 3.10 - Files
Sectio n & SIHlBlil). 'l\~,6 -
T~e&*ca-range of available Iile sections and shapes, these include: \,,~{{'
1.
Fhu: .\ \'cry common section that is used for general filing work. The cross-section is normally rectangular throughout !.he blade length bUI [here arc "ar-iations in shape, such as straight. tapered or 'bellied' blades. Some flat files arc produced \\~th one smooth edge to pre\'ent damage to a finished surface when filing in a corner for example. These files arc referred to as 'hand safe edge (115£) files. The safe edge of the file is placed against the finished surface.
2.
H uff- Round: This cross-section is suited for filing la rge radii and irregular shaped surfaces. The blade \\~dth is normally curved or 'bellied' towards the tip bur parallel width \'ersions are also available.
3.
106 uP.Jri!.ht
Three-Squ:Jrc: The triangular cros;-secDon is useful for fili ng interna.l corners, pa rticularly at angles less than a right angle. The blade width is normally Clln'ed or ' bellied' towards the tip.
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Square: A sq uare cross-secoon that can be supplied in parallel, straIght taper or 'bellied' shape. "This type of file is suited for internal work.
=>.
Round: A circular cross -secnon that is sVited fo r filing small radii and holes. It is sometimes called a 'rat-tail' flie. It IS supplied in parallel, straight taper and, more commonly, 'bellied' shape.
6.
Knife: Knife ftles hayc a blade thickness that is tapered to gIve one sharp edge. They may also have a tapered width . They are primarily used for filing very small angles .
C ut
The teeth on a fue are arranged in particular patterns to suit work on different materials. The most common examples are:
Fig ure 3. 11 • Types of Cut
Single-Cui: This arrangement consists of teeth that are cut in a series of parallel grooyes that slant across the face of the blade. The pitch of the teeth is relatively broad so that the teeth do not clog easily when "1\.llfflling soft materials. Round files, and the curyed surface of half-round 'l,ab ~ files, normally have single cut teeth. ~" . Co\\C ~ \."'1"·1:(': 2. Double-Cut: This arrangement consists of two sets of sjngle cut teeth v . ,\.....'-th at cross each other at about 70° fomling diamond proftle teeth. This .,\.'type of cut is well suited to filing ferrous materials such as mild steel. C0~"" The flat surfaces of most files have double-cut teeth. 1.
J
I
('.
3. Dreadnought: This arrangement consists of deep, curved, parar@ teeth cut across the width of the bL'lde anU it is nonnally only found o! larger sized files . "Tl1is cut is parcicularlt suited to fili ng soft, btoacf surfaces. 2·
j
4.
Rasp: This aWl.l1gement consists of a pa tern of individual teeth and [ suited for filing very soft materials, for example, wood or lead. [-
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Grade The grade of a file relates to depth and spacing of irs reerh. Single and doublecut fIlcs arc produced with different degrees or 'grades' of coarseness. These grades are gi\Ten descriptive titles as follows: 1.
Btlsl:Ird: This is the coarsest grade and is designed specifically to
removc material quickly. It is often used to rough Ollt a shape before moving on co use fine r grade files.lt lea'-cs a rough finish . .,
St.·cond-Cut: '111is grade is finer and CutS at a relati,-ely slower nne. It produces a smooth finish and rna)' be the fmal choice of grade wherc this is acceptable.
3.
Smooth: As the name implies, the teeth ha,-c a shallow depth and arc closely set. TlIis grade of file is used to produce "CIY smooth finishes. II is cuts "cry slowly so It IS nonnally resen-cd for final surf..\Ce finishing work only.
Notc: Chalk is often rubbed OllIO file teeth to reduce the grade. 111is stops the ftle picking up metal chips that could scratch the surface.
Us ing a Fi le T he ball of one hand is placed on top of the free end of rhe file and is used 10 apply pressure and direct the file. The handlc of the fue is gripped between the thumb and fo refinger of the other hand. Keeping the handle in line with Ihc wris l, thc remaining th ree fingers arc used to press the handlc into rhe palm of the hand. Downward pressure is always applied on the fo rll":ud sfI"okt.' of [he file and is relieved when the file is drawn b:tck. All movement should originate from the arms and nOt rrom movements o f the body rorso.
Cro:,.<;-Iilinl!ds l lMclhod where the file is pushed at an angle across the work. i\ftc~.fwng Ln one direction, the direction is then changed to st rokes that arc ar "{f ig1N angles to the previous direction.
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Draw filing is a method often employed \vith a smoorh me to produce a finer
finish. The file is drawn across the surface of the metal at right angles to the work, ,vith equal downwa.rd p.ressure being applied by both hands. Chalk is sometimes applied to the teeth of a smooth file to produce a finer finish. Care of Files
•
New filcs should initially be used on soft materials, and then be used on progressively harder materials. A new file should not be used on a material of il1detenninate hardness, particularlr cast steels. Old mcs should be employed for this.
•
File teet h become clogged with metal chips that will scratch surfaces. 1\ wire brush, or alternatively a wire scratch card, called a ' file card', should be used to clean the tecth . The wire bristles arc drawn diagonally across the filc in the lay of the grooves.
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TOO L S
•
File blades are very brittle and must neyer be stored in contact with each other and must not be exposed to impaCl from other tools .
Precautions •
\Vhen fitting a \vooden handle to a file tang, you sh ould tap the rounded end of the handle lightly on the bench to dri,e the tang in, not the tip of the blade. Care should be taken not to split the wooden ha ndle of the file whilst doing tills .
•
Never use a tile that does not ha,·e a handle.
•
Ahvavs usc the correct size handle and ensure that it is a tu m fir o n the tang.
•
Neyer use a handle that is split or has its ferrule missing.
•
A hvays select the correct file fo r the task.
•
Ensure that the item being filed is finnl}" secured and usc "ice 'clam s' if 1n a nce.
Drills Description
Drills arc cutung tools desIgned [Q cut holes. The most common type is the spiral fluted t\ViSl drill. Other contigurations of drill are specially designed to cut non-metallic materials such as wood and composites . -Illese arc the spade or ~t~-ilrbo.ring and dagger drills. \Ve will take a 9r.ief look at each type:
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~
. U,\\c-'i-c ; T,vist Drill
.
;: .'\~'10.'5' ~"'\. ~
The parts of a twist drill are: the shank, body, flutes , land, web and tip.
~.:,\~
(~.,,,,! .
I
I
Bou\
Tip
Land
Shenk
I
FllHe
'-
,
,
L
Cle~ rancc
Figure 3.12 - T wi st Drill
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THR EE
Shank: The shank is the plain portion of a drill that is secu red in the chuck of a drilling machine. Small drills (less than 12.5mm) have a parallel shank [har is gripped in a self-cenrring chuck. Larger drills intended for use in fixed, power drilling machines h:we fI '~ Iorsc' taper (I in 20) shank and a flat driving tang designed to givc a positive drivc when fined into a l:Ipered sockeL A flXed, power drilling machine has a chuck that is mounted in a tapered socker. \"",' hen the chuck is rcmo\'ed, large drills may be fitted directly into the tapered socket of the machine. or they may need a '~lorse' taper adaptor lhat filS over the shank prior to inserting it into the machine. o
ExtnlCti(1O
Dnft (\,'edge)
ftIiI~?-F 1~~ Shank Taper 1 in 20
:\d~lPI'>r
Figure 3.13 • Morse T aper AdaplO r
2.
Body: Tl1e body of a drill is the flmed portion and includes the drill point.
3. FIllies: These are the
1.'\\'0 spiral grooves that run fllong rile body, fomling cutting edges at the tip that arc angled 10 cffectivdy cut mfllcrilll and clea r lhe swarf from the hole during the drilling process. They aI5~,~lJ ow lubricant to pass along the body of the drill to cool [he tipl."-'() ...
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'4.
.\,,, "',.:-,.,,\\Ii;
"-
L 'md: This is the surface of a drill body between the flutes. The land is ground back a little way [0 the rcar of the leading edge of the flutes to leave a raised portioo that pro\-ides a clearance between the rest of the body and the side of the hole. The raised pan of the land contacts the wall of the hole and guides the drill. The l'lfld produces a clearnnce thar reduces friction and prevents the drill jamming in rhe hole.
5.
lreb: The part of a drill tip that is not in\'olved in cUfting.
6.
Tip: The tip of a drill incorporates the cutting edges and Ihe chisel shaped point of the drill. If the cutting edges are not of eClual length, or [he point is not central, the drill will not centre and will cut an overs izcd hole.
The tip of a rwist drill is ground to form the angles that will produce an lIceuralel)" drilled hole. The angles include: the cutting angle, !lte clearance angle and the web angle.
110
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1.
ClIfting Angk': The angie formed ben'leen a cutting edge and the axis
of a driB body. There are two cutting eages of cCJual length and lhe cLltting angles arc eguaL The cutting angle most commonly adop ted is 59° gi"ing a total included poin t angle of 118° though smallcr point angles than this may be recommended for drilling some soft materials. To giye you a rough guide: the included angle for t·\Vist d.cil1s when drilling soft metals would be 90°, the included angle for drilling stainless steel at low speed ''lould be 1-1-0° and thc included angle for drilling titanium would be 105° to 120°. 2. Clearance Angle: The angle fo rmcd between the tip end of a flu te and the horizontal. The clearance angle is tiormally n °. If this angle is insufficient, the d.cil1 will cut slowly if at all. 3.
Web Angle: The angle formed between a, cutting edge and the point of a drill. There wiU be T\VO web angles, each bemg 130°.
Mate rials
Twist driUs are manufactured from either high carbon stcel (CS) or alloy steeL Alloy steel d.cil1s are often referred to as 'high speed steel' (HSS) drills because of their ability to \vithstand the higher temperatures gcnerated by operanng rhem a[ higher drilling speeds. These alloys nonnhll y contain either tungsten or molybdenum. Some 'high speed' drills arc upped ~vith mngsten carbide La maintain hardness at high tcmperamre and to extend the life of the cutting edges . Cobalt d.cil1s are also ,'eI')' bard bur are b rittle and drilHng speeds and pressures have to be Limited when using them. C las s ification
Twist drills are classified by their diameter and the material they are made
, 0.''\. ~-
~~t5oafll'The size is normaUy marked o n the shank of the drill tOgether with . 'l-.V:.'f.<:' 1\.' letters indicating the material type, i.e. CS for carbon steel, HS or HSS for 'high _(" L.'-' speed steel', Verv.. small d.cil1 shanks do not haye the room for size markings so \-.,'1>, . a drill gauge has to be used.
~~\":'
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There are tour systems uscd in Sizing driUs: fractional, numbered, lettered and metric. \'\!ith the onset of the metric system, many countries use metric sizes. America docs nOt usc the: metric system. D.cil1 sizb tables may be found in m~t engineers' handbooks though ~ny of tbem differ on ~he ranges of t~ T. fractional SLZes so we will just exarrune typical examples. ~
J
1
Fractional Siz es
'!fh
T hese stan at t / Min dia. and increase in steps f 1/64in up to l in. From dia, they increase in steps of1 / 32io up to 3.5in dia . j?:::: r-!
Numbered Sizes
,
$
"
T hese start at the minim um size No.80, equating to 0.0135in di a, to ~ximum ·N ·O??S·di ' size I o.l ,eCJuaong to .__ m a.
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Lette red These start at minimum size A, e'luating equating to OAI3in dia.
(0
0.0234in dia, to a maximum size 7.,
M e tri c These stan at the minimum size 0.35mm and increase in steps of O.05mm Smm and then by steps of 0.1 mm.
10
A drill size may sometimes be described as a decimal value, for example 0.3 125in. This is actually 5/ 16in (di'ide 5 by 16).
Uses
The use of [wist drills is well known but there are a few occasions where special requirements can arise. \",\'c will look at a few examples, Drilling T itaniu m Titanium and irs alloys ha"c extreme work hardening properties and arc rcactive when in COntact with oruer metals. They are poor conductors of heat and the tips of drills can become "cry hot whcn in contact with them. "rhis can crcatc galling or smearing when this material is being machined. Iligh-specd steel, stub (sha ft) drills should be used thal have a poilll angle of 105 0 to 120 0 and a thickened web. For larger holes, over 1/ 4in (6mm), a 900 point angle is recommcndcd. It is essemial that the drill is sharp and that plenry of cutting fluid is supplied to cool the tip, Due (0 (hc problem of work ha rdening, a centre punch mllst nc ,'cr be used [0 pfovide a drill Start on titanium and its alloys. Centre drilling should always be uscd instead of ccntre punchi ng. Note that titanium swarf is \'ery sharp and is flammable. Titanium meral fires arc extinguished wilh a dry powder cxcinguisher (coloured blue) 'll'l\,
D. (\"", "Iii\N~ \;:;;ompos " "" - lte " M a le ria "Is
'-
~)~tomposi{e
materials will blunt conl""cntional drills quickly and this can cause the composite material to delaminate as the drill then pushes rather than cutS, Spade, dagger and brad point drills are used to drill composites and these arc described later in (his secnon, Carbide drills are tbc best suited for drilling most types of composites. Diamond dust coated drills are effective on most composites except for rough fibre reinforced plastics like [he arnmid Kevlar where the dust grabs at the fibres and pulls them rather than cutong rhem. This creatcs 'fuzzing' in the drilled hole so di:lmond dust coated drills should be avoided. Especially g round, brad point carbide drills are supplied for drilling Kc" lar that will cut the fibres rather tha n break them, Kevlar should be drilled at high speed with tight pressure. Carbon fibre and fibrcglass materials can be drilled with conventional tools but these will blunt quickly. Carbidc or diamond dust coated spade o r d:tgger drills arc more effective. Carbide dagger drills give the best results when drilling carbon/grnphirc fibre composites.
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Hig h T en sile & Stain less Steels High tensile and stainless steels requIre ve~ hard, short, rigid drills. Tungsten cobalt drills are well suited to this. lfhigh carbon steel drills are used, the speed must be kepr low or the drill \"ill simply overbeat, 'blue' and blunt. Cutting Sp eeds Drills cur more efficiently when they are used au the correct speed . There are recommended speeds for various sized drills and materials. As a general guide the cutting speed for a 1/4 in diameter carbon sted drill being used to drill mild steel is -I-OORPI\I with a feed of 2i.n per minute. \'(!hen using different sized d rills, the peripheral speed of the drill should be kept around the same so a 1/8in diameter drill would be operated at 800RPM and a % in drill \vould be operated at 200RPI\I. Howe,~cr, these speeds would be doubled for brass . Lub ricants \'(!e should nor oyedook the need for lubrication when drilling materials. Lubncants cool the drill, improve the fimsh and extend the life of the drill . 'rypicallubricacion rCCJuircmcnts arc:
•
i\'Wd steel
cutting oil or soda solution, or soap and warer.
or oil and water •
High carbon sted
turpentine or paraffin (kerosene)
•
Alloy steel
mrpentine or paraffIn (kerosene)
•
Cast iron
self-lubricating self-lubricating paraffin (kerosene)
•
Aluminium alloy
paraffin (kerosene)
•
Titanium
heavily chlorinated cutong oil
£.,
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Before we move on, it would be useful drill.
to
lo ok atia few other configurations
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The flat, or spade, drill has a ,ery simple shape and has cutting angles ~t are related to the material it is used to cut. The cutting angle varie~ from a~ut 50° for ,ery hard materials, up to 200 0 for soft matbrials like brass. Th~flat drill
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will not cut holes as accurately as a twist drill will bUl it is recommended for drilling composite materials.
===<:::> ===
c. Flat (Spade) c=.
Bottommg
Dagger
Figure 3.14 - Hand Drill Brace & Drill Bits
Bottoming Drill The bottoming drill is simply a flat drill with horizontal cutting edges and it is used to level out the bottom of blind holes .
Arboring Drill _ , ~,(:, 'Ill' ... Arb\~g --drills are used to create a machined surface finish on the region
_ :1).{Stl~iounding a previously drilled hole. They incorporate a peg that locates in ,\,. . (\ {he hole. They are not used for drilling holes . ..,,;,",-,r-
1.:>''\ r >
Dagger Drill
..",
The long-tapered, o r dagger, drill is a flat, or spade, drill that has a very long taper at (he tip. They are recommended for drilling composite malerials, pa rticularly carbon/graphite composite materials.
-,
S·;:f
Brad Point Drill These are vcry similar to the spiral fluted wood drills except tha t they are g round with distinct C shaped cutting edges that are very sharp and arc ideal for cutting composite materials, particularly aramid fibres such as [
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Wood Drill Bits There are two common types of wood bits, the Auger bit and the flat bit. The Auger bit is a spiral fluted twist drill that has a Y~f)' sharp point that acts like a feed screw to pull the cutting edges into the wood. The flat wood-bonng bit is similar in shape to the bottommg drill described above. I t is a flat, paddle shaped dcill with a sharp point that acts as a pilot drill. Wood bits arc normaUy driyen by a bow type brace.
Precautions •
Use eye protection when drilling materials .
•
Do not brush away metal s\varf with r ou hands.
•
Use the correct drill speed. Generallr, the harder the material: the slower the speed. The larger the diameter of the drill: the slower the speed. L.:se the correct lubricants.
Countersinking Tools Description and Us e Countersinking tools arc used In conjullction with a tool holder to recess drilled holes [Q accept fl ush fitting countersink rivets, screws or bolts. T he uniyersal countersinking set is made up of 90°, 100 0 and 120 0 cutung hea98l5hat may be seiecti,-ely screwed into an adjustable-depth tool holder. The " 1.oQloor may be driven by a hand-operated, or a machine operated drill. Eacb ,0)\\. . ..,...1: cutting head is bored axially to accept a pilar tip that will align the cutter i.n a • \"\'':s{': drilled hole to ensure dUll the head will cut a concentric countersink. There are ,~ t\vo cutting heads provided for each countersink angle, one bored to accept . ,\' 1-,\\-";'" 2.4mm and 3.3mm pilot tips, and the other to accept -1.0mm and -l-.Smm pilot U ''\ tips. The depth of cut can be adjusted to an accuracy of 0.001 in.
,
r~
count~rsinking
~
A R ose Bir is sometimes used in lieu of a set to recess holes soft materials. Another alternative to the countersi.nking set is to use a drill thai is twice the diameter of the drilled hole and is ground to the rcquirc¢ countersink angle.
-
"
~
Countersinking tools may also be used to clear the burrs fro m the edges fif 0" drilled holes. p
'"
~
Materials
&
~ The cutting heads arc manufa.ctured from tungsten steel.
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TOOL S
Coumminki,ng Tool
Vernier, \djustmcm
Pilot Tip
Rose Bit
Counter-Bore
CC)UI1H:rSlnk
Fig ure 3. 15 _ Counter-sinking and Counter-boring T ools
Counter-boring T ools Description & Use Coumcr-boring tools arc used \\;m either hand-operatcd, or machine operated drills, to cut recesses in d rilled holes to accept cheese-head or set screws where thesc ha\'e 10 lie flush with a surface. The tool enlarges a drilled hole 10 a given depd1 whilst leaving a flat shoulder for the base of the screw to sea l on. The choice of curte r ro use will depend on the diameter of the recess reCJuired :lnd the diameter of Ihe drilled hole will determine the size of the pilol tip.
Reamers arc precision cutting tools that are used to enlarge prcviously drilled holes to achie\'c an accurate dimension and a smooth finish. ll1ey ha\'e flUies that incorporate cutting edges, Hand reamers are rumed with a wrench (hal is attachcd to the squared end thc parallel shank. The nonnal cutting d irection is clockwise. There arc machine operated parallel reamers that have lapercd shanks. Reamers cannot be re-sharpened because tills would reducc their d iamclcr. The types of reamcr commonly lIsed ace: the parallcl m'nighl reamer, thc parallel spim! reamer, the hand taper reamer, rhe hand expanding reamer and thc shell reamer.
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Shell
Hand Str:ugh t
Spi ral
Taper
A d i ll~tab le
Figure 3. 16 - Reamers
Parallel Reamers Parallel hand or machine-operated reamers enlarge boles to one size only. The flutes may be straight or spiral. Spiral fluted reamers bave a left hand spiral to reduce their tendency to screw into bored holes. Spiral reamers are primarily used to ream out boles that comain keyways or grooves because they do not jam on the edges. Spiral reamers are. also required for usc on tItanium, titanium alloys and high tensile alloy steels. Both types of parallel reamer haye tapered ends to assist Harting.
I-land taper reamers have a standard taper of 1 in 48 or a metric taper of 1 in 50 . They are ayailable in sets where each reamer has a numbered size from 0 to 10. Tbey are prirnarilr used fo r preparing holcs for tapcr pins.
Hand Ex"anding Reamers
r'.
slo~
T hese pa rallel reamers have separate blades that l re a slide fit in tapered T he blades are held in place by twO nuts . Adjustment of the cutting size of tl~ reamer is effected by loosening one nut and tigl~tcning the other. The blade~ remain parallel with rbe reamer axis throughout the adjustment r;mge. The fin ¥, l diameter of the reamer can be set by using a ring gauge or an extern~ micrometer. r "0' ~ Shell Reamers
&
"
These are fixed size, hollow centred reamers that are designed to fit .Smo a 'u mandrel or boring bar. ~ <
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M.u c ria ls Reamers arc made from high carbon steel or alloy steel. Iligh Speed Sled spiral reamers arc used for utanium, titanium alloys and high tensile alloy sreels. Class ifi cation Reamers are available in standard sizes. Parallel suaighr and spiral reamers arc supplied in fixed sizes that arc stamped on the plain shank. Taper reamers ha\"e either an English standard raper of 1 in 48 or a metric taper of 1 in 50. The tWO Iypes arc not interchangeable. The size of a hand taper reamer is clched or stamped on its plain shank. English taper reamers are normallr supplied in sizes 0 to 10. Note that I.he [racDonal size related to an English taper reamer refers to its large end diameter whilst the size of a metric taper reamer refers to the small end diamerer. The size range of hand expanding reamers is stamped on the plain shank.
Usc \'Ce h:we already discussed the primary use of reamers in enlarging holes to accurate dimensions and producing smooth finishes. Remember also, the use of spiral-fluted reamers in holes with key-ways where ther arc chosen because the curring edges cut diagonally across the keyway and do nor jam. Spiral fluted I ISS reamers arc also required for use on titanium, titanium alloys alld high I.ensile steel allo),s. The alllount of material to be removed by a reamer mus t be kepi lO a minimum. \"X!hen using ha nd reamers, the amount of material to be removed is normally wirhin thc range of O.OO3in to a maximum of 0.007in (approx O.2mm); more [han this will place toO much load on I.he reamer. When reaming a hole, the reamer should be rotated in tbe c lI{(ing directio n o nly. noqR-l\Uy 1B\\ is clochvise) even whilst the}' arc being withdmwn from I.he. l~W.lL~ "lind should be turned steadily to avoid chauering. Lubrication must ~).{ bcLuscd during cutting. The trpes of lubricam used are rhe S:UlIC : IS those ( \:. used fo r drilling Care should ·be taken to ensure [hat the axis of the reamer is aligned with the a..xis of the hole before cornmencing to cut. P recautions •
Rcnmers should not be beld by their cutting edges.
•
The reamer is very brittle and should be stored in its container and should ne\"er be allowed direct contact with O(her sharp, britue rools.
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Taps Description Taps are simply stralght fluted screws that have cutting edges . They have a plain shank portion with a squared end to receive a tap wrench. Taps arc supplied in sets of three except for BA sizes where the second tap is omItted . The three taps are: the taper tap, the second tap (not BA) and tbe plug tap .
Taper
Plug
Second
(N0l B,-\) FigtJte 3.17 - T aps
Taper Tap Tlm\.t i"p is used for starting a screw thread in a hole and is tapered from the
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~(\\l(l;th thread down from the plain shan k to rhe rip . The diameter of the tip of tlus tap is equal to the root diameter of the thread being cut. When llsed to tap a hole in sh eet material that has a thickness less than SL" threads, this tap will cut a full depth thread . Second Tap
"
I
'0
This tap, somecimcs referred to as the intermedi:ue tap, is tapered from tbe ~ up to the SLxth thread from the tip. It is used to deepen a thread that has beer;. cut by the raper tap. \'Vhen the taper tap has been used in a blind hole, o r in ik hole in thick material, the thread will require the use of the second tap. Th§: second tap is not supplied in BA tap sets.
.3 .~
.£."
Plug Tap
jJ-
"
Tlus tap, somerirnes referred to as a bottoming tap, docs nOt have a tapeb nd it is used to complete a thread in a bli nd hole or in thick material. ,CR
'0
~ ,
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M ateri a ls
Taps are made from hardened and tempered steel. The shank is lefl annealed LO
increase toughness and reduce brinlcncss.
C lassifi ca tio n Taps arc pro\'ided in sets and their size is stampcd on the plain shank.
Usc Taps arc used for cuning an internal screw thread in a hole. The bole must be d ri!led to the corc diameter of the thread being cut. The driH used for this purpose is called the tapping size drill and you can discO\'cr the size by reference to screw thread tables. Taps cannot be adjusted. \\'hen turning the lap with a [;1P wrench you should back the tap off at least every third of a rurn to break off tbe swarf. Precau tions •
Take care to clear swarf out of holes after cutting threads.
•
Store taps in thcir rack when not in Lise to proreci the cmting cdges.
•
Keep laps clean when not in use.
Dies D escri ptio n . _ .,\.~, . '1\,,-,6 ... J)ic~~ "'s pecial nuts that have their bores CUt back TO fom) four clear:lnce :',:I.~srb~\·es with cutting edges. There are [WO types of die: the circular die and the \. two-piece adjustable die. They are fined into hand turning devices known as 'STOcks'. "n)cre arc also die nuts bur these are only used to clean or restore damaged threads.
~.
~W-
Circuhr Ole
.3
~~:l=-::::::> ~:J Twn-pjecc r\djustablc Die Fig ure 3.18 - Dies
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C ircular D ie Circular dies are used for cutting small diameter external threads and have a tapered thread on one end to assist starting. One die is capable of cutting a full thread. The die is split and is fitted into a stock with the tapered thread side facing away from the stock shoulder. There are grub screws on the stock that secure and Oex the split die EO giye a limited adjustment to the dep th of cut. Two-Piece Adju stab le Die Two-piece adjustable dies are llsed for cutting larger diameter external threads. The die consists of two hah-es and these arc fitted imo the stock so that the numbers on each half are on the same side
Dies are used to cut external threads on bolts or studs. T he cylindrical portion of the bolt or stud to be cut must match the crest diameter of the thread to be cur. Tl~e;.{llpered . \:..C
" Cl1\\.... (':'I.\{ . ~ \"1'
side of the die must be p re~ented to the bolt or srud. The firs t cut carried out with the die ill the fully open position. The stock should be backed off frequently during lhe thread cutting operation to break off any f ~ \va.r .
}.i.'q? always
. ,\........ :'-,<''i-'
(~~'Q
.
If you are also tapping an lnternal hole to receive a stud or bolt that you arc cutting the thread on, make sure you tap tile hole first. Taps cannot be adjusted bur you can adjust dies to produce an external thread that \,1l.l have tbe corr% t
&
~
%
P recaution s •
Ensure that you clear swarf from threads after cutting.
•
Store dies in their protective cases to prevent damage to cutting ed!},.~
•
Keep dies clean ,vhen not in use.
,
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,
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,
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Stud Fitting & Removal Tools \'(Ie dealt with these tools in detail in i\lodule 6, Chapfer .5 under 'srud insertion and removal' but I will insert 3 brief descriprion of them here in case some 'quizmaster' decides [0 also connect them witb lbe common hand tools in this module. Fitting Studs arc fittcd with the aid of a swd b ox or a stud inserrion and ex/raelion {Dol, or by using lock nuts. The stud box is a deep hexagonal nut that has a bolt screwed into one end. It can only be used to fit studs, it cannOl remO\'c them. The box is screwed onto the smd unril it contacts tbe cnd of the bolt. 'rhe smd box is then turned \. . itb a spanner to dri\'c the stud into its tapped hole. Thc stud Insertion tool is designed to hold a stud in a friction grip. The tool hu a two-way acrion and it may be used for bmh srud insernon and femO\'a!. \'\'hen the tool is rotated with its tommy bar, cams in the 1001 operate 10 grip the stud and dri"e it into its tapped hole or unscrew it as required. Rem ova l The choice of tool used LO remm'e a srud will depend on whether lhe stud is intact or whether it is broken off abol'c or below the surface. Intact studs can be removed using lock nUls or a slud illse rlion :wd e,wr:u:tiofl tool You cannol usc :t stud box to remo\'e smds. I f the sllId threads are damaged, the srud insertion and extraction 1001 can still be used becausc it relics solely on a friction grip and not the threads. The ahernariye is to file flats on the end of the stud and usc a span ner or tap wrench to remove it. 'll,n-
If
~ ~tud '1t\;'!I.'f;rokcn
off below tbe surface, it can be remo\'cd with a suitably srud extractor known as an tEzy_olll~ This rool is tapered and has a smooth left hand spiral flute. A half stud diameter hall!: is first drilled through the centre of the stud. The 'Err-out' is then scre'\-'ed into the hole in a left hand direction unci! it bites and it is then turned with a spanner or tap wrench LO remo,'c the stud. You cannot use an
{flZc3 .. .::. \\:>
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~
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Scrapers .:J
Description Scrapers arc used to remove high spotS on flat or curved surfaces to produce an accurate finish. Scrapers have ro be \'cry sharp and the cutting edge is obtained by firS! grinding it and then honing it on an oils lOne. There arc three common types of scraper.
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T OOLS
FhlL
I lalf Round
Figure 3. 19 - Scrape rs
F lat Used for flat surfaces . Half Ro und Used fo r large. circular bearing surfaces. Three Square Used for small bushes and bearings.
Snips Description i\ letaJ cutting snips. or shears, are used for cutting sheet metal when the usc of a guillotine, or po\ver-opcrated shears, is considered impractical. There are different configurations of blade shape to suit different contour and acces s rcqtlu-~fl1ents. Tin snips are primarily used fo r cutting straight lines but they '\'\~iln 1j~ used to CLit curves. A,'iiltioo snips have a high lcvcmge action and . (.\,W.'!'-\: - sermled blades and are suited to cutting heat-treated aluminium alloys and • \\-;\\(' stainless steel. They arc supplied in straight. left-hand and right hand fonns.
,
,"\,'t -
.:.:,\y(.0'\
o
,~
00 Skin Knife
T in Snips
Aviation Snips
Fig ure 3.20 - Sheet MClal Cutting Tools
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Dismantling & Assembly Tools Screwdrive rs Desc ript io n r\ scrcwdri,"cr consists of a blade section that incorporales a plain shank with :I shaped blade at one end. The orner end of the blade is fi.."i:cd flITTlly in either a wooden or a high impact plastic bandle. The handle may incorporate a mtchet and, in some cases, a spiral dri\TC mechanism. In addition, a combination screwdriver is :wailablc IhM has a socket designed to accept a "ariery of dri\'ing bits.
There is a "aricty of blade configurations and lengths. Ilowc\'cr, there arc two main distinctions. There is rhe common flat scre\vdri~c r blade and the cruciform, or cross.poim, screwdril"cr blade.
Splnll (Pump) Scrcwdri" cr
Offset
Ratchet Figure 3.21 - Screwdrivers
,
~,
,,',
Commo n
FI ~H
Blad e r
TI,e common flat scrcwdri\"cr bladc is ground to fit a rcctanguln r scctioncd dingonal slot in n srnndnrd screw head. II is essential mat thc bladc will fit snugly b), fiUing at least 75% o f the slot to prevent it slipping and scoring the surrounding surfacc. Flnt blades must ne,'er be ground to a sharp edge because they will not achieve this required fit. Crucifoml Blades Crucifoml blades arc designed to fil snugly in the recess in meir rein ted rnnge of screw heads and nre nor prone to slipping out and damaging the surrounding material. The most common cruciform r:ype is thc Phillips screwdri\'cr that has a blade with four rnpered flutes that engage Wilh corresponding slots in a recess in a screw hend.
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CHAPTER
THREE TOO L 5
~~(~i:=J1======,=~Sd I
~~
0@3)®@~ Pbm
Phillips
C ross
Tri-\'\'ing
Figure 3.22 - Blade Configurations
There are several other cnIcifom1 types including: the Reed and Prince cross point, the Posidriw and the Tri-wing types. The cross-point configuration of the Reed and Prince blade may look like a Phillips blade but it has a sharp pmnt and a diffe rent \,,;dth rhat will damage a Phillip s screw head. T he Posidriyc and Tn-wing blades are high torgue screwdrivers and they have a clear difference in shape in that the tri-wing only has truee flutes \vhereas the Posidrive has four. The message here IS that yOll must be sure to pick the right screwdriver for the scre\\'. Offset Screwdriver 'f he offset screwdriver has a blade set at right ~ ngles at each end of a metal sha ~ ~\)These tools are used where space is limited. They are supplied in both ~ e >J.\..\ d otted and crucifonn configurations .
,,'
-,
. (,,-~\\t:.~
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.,\~""....
,
c. 0'\
.
"" "
Combination Screwdriver The combination scrcwdnyer is a very useful tool in that o ne handle can accommoda te a range of different drive bits. Each drive bit has a hexagonal ;shank that locks into a socket o n the screwdri\'er. -0 ~
Materials Scre\\-driver blades are made of high carbon or alloy steel and they mounted in wooden, or high impact plastic handles . ,~
C lassificati on
,,~
'"
p Common screwdrivers arl.: classified by their blade length . Phillips screwd..G~ers are classified by size and blade length and are supplied in four sizes fr~ the smallest at No.1 to the largest at No.4, each suiting a range of corre§lionding is screw sizes .
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Figure 3.23 • Co mbination Screwdriver
Used for installing and removing screw fasteners, the screwdriver is probably the most abused lool in a tool kit. "{lhen the blades become damaged, they exact a penalty by damaging the screw hC:lCls and often lea'-e us, or the next person ro come along, with no ahern:uive (0 drilling the screws our. Using (he wrong size and configuration screwdrinr can bring about the same problem. If this were not bad enough, screwdri,-ers. or morc precisely rhe people who use them. are responsible for more score damage on the surfaces surrounding screws than an~' other tool. Precautions •
To avoid damage and personal injury, you should always ensure that tQ.,
\.L.
(
.,'{'-is'
•
Screw head recesses often fill wirh paint and will then nor pro"ide a snug fir unless they are cleared .
•
Do not use screwdrivers for tasks they were not designed to do. TIler should nor be used as crowbars, chisels, punches or dri,ring wedges.
(\,
Punches Description & Uses A punch IS a tool designed to transmit a hammer blow to a localised area. It has a ground end that is slightly rounded to allow control and to concentrate hammer blows, and a working end that is shaped to fulfil a particular rnsk. There are number of different configurations.
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o Centre
TJper Pin
Parallel
Hollo\\"
-
Sn ap
Pin
Figure 3.24 . Punches
Centre Pun c h This punch has a sharp point tbat i~ ground [0 produce an :lpex angle [Q suit a pa rticular role. For general \vork, this angle is ideally 90° but is rcdllced [0 60° when intended for light work. The centre punch is oftcn used to apply an indcnration, or ' pop' m(lrk to the surface of malerial to locate the poin t of a drill. It is also used ro lightly 'pop' mark cut lines on material. The centre punch is made fro m high carbon sted and is har<,lcncd (lnd tempered at the tip on ly. Taper Pun ch [\lsQ.l1~ferred to as a starting or drive punch, thi~ Oat-ended punch IS used for I I _I!.t:. ;.\\l~riving out pins. tight bolts, rivets etc. It is not suited to operation in deep • ,,1', t ,l holes where the taper In..1\'. make contact with tbe wall and enlarge the hole . < L \,!. The" arc clas sified by the diameter of the face at rhe small end .
.\,'. -
"v"" (~'\
.
~
.
P a rallel P in P u n ch
Thi s punch is uscd for the same tasks undermken b)' a tape r punch. The difference is that lhe parallel pin punch can be used in deeper holes. Ir 1;. common practice to loosen a tight bolt with a taper punch and then drive fithrough the hole with a parallel pin punch. TIley are classified by the diamcte;:' of the end face. "~
:3
H ollow Punc h
..... o·
'"'"
[\Iso referred to as a gasket punch, it is used to punch out boles in soft ll1a(~ rial such as ga~kct material, !cather, sheet rubber etc. The hollow end of the l¥-Ynch fo rms a sharp circular cutting edge . \Vhen using the punch, the malerial ,Should be backed onto a hard wood block to a\'oid damage to the cutting cd!t~v
•
127 U7171,./!lIrl66. rolN
C H AP T ER
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T OO L S
Pri c k Punc h This punch is similar to a centre punch bLlt has a finer point that is lI~cd for transferring dimcnsions from a paper pattern onto the surfacc of a material. A va rinaon of this punch, called a transfer punch, has a shorr pnralld section at its end that permits irs lise in transferring the locations of holes in a templnlc directly through onto n material surface. For example, it may be used 10 trnnsfer riwc hole locations from old skin OntO a new skin. Rivet Snap Tlus punch has a profiled recess in the end face mal corresponds wirh n giycn ri\'et head shape. It is used to fonn oyet beads in the riveting process. M a te rials Punches arc made from lugh carbon sleel that has been hardened and tempered at the operating end. Class ification Taper and parnlld pin punches are classified by type and the diameter of the end face. Sizes range [rom 1/16in to 3/8in. Hollow punches are classified by the size of hole (he), cut. Odler punches are classified by the shape of their operating end. Precauti ons •
The end of the punch that receives the hammer blow graclu:1 11 y nauens and spreads. \,<' hen tbis has occurred, tbe punch should nOI be used until the end has been re-ground.
• C~~{f ;%W~hes should never be used in an anempt to drive our pins or _ ~\.;. .,ri~ets because they will merely spread the item in thc holc and jam if.
.l.o
. \.... ;..{~
............. ~
\.. ~.~
,\"\\.
L-, .
.,\.'>--
Drifts D escri ption and Use Drifts arc solid, flat-ended punches made from steel, copper, brnss or aluminium that are used to dove right-fitting components. P recautions Steel drifts only should be used for driving ball and roller bearing races. Drifts made of softer materials can shed fragments that can enter the races. Care should also be taken to maintain the shape of the head of the drift. Drifts with heads th:1l have spread or 'mushroomed' should not be used until the head has been correctly fe-fomled.
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Spanners Description The term spanner covers a multi rude of different devices that are used to tighten or loosen screws, bolts and nuts . TIle mdsr common arc those that fit onto hexagon headed screws and bolts and hexagonal nuts. These devices are referred to as being 'standard' spanners and may take the form of: open-Jawor set spanners, ring spanners, socket spanners, box spanners and tube spanners. It is useful to note tha t the Americans usuall\" refer to spanners as 'wrenches'.
o
o
G;:y~ ... ~~~~ Ed Open Endcd (Set)
(Q)~(~co~mbin,~u on )~ ~2=======O=f=~S='='R=in=g~~ ~~"~~~"'~ Flare Nut
Ring Figure 3.25 - Sp anners
Open-Jaw (Set) Spanner
.
,,,,.....
,..."'\'!-'
(.0'\
L
Qb h eSle\.1 re designed to bear against two of the SL"" faces of a hexagon nut or bolt _ .' e\!..c- "}.\"' head. They are normally supplied in a double-ended form that gives twO l ,\~ ~ 7I.t~~ ,I available sizes. Each end of the spanner is n0011aUy offs et in the same plane so that selectively turning the spanner over gives the operator two approach options when engaging the jaws with the flats of a hexagonal fastener. The torgue that is exerted is the product of the straight-line distance from the centre o f the bolt head or nut to the point where the spanner is gripped aoo the tangential load applied by the operator. Thelopen-jaw design does hav~ dra\\·back in that the jaws ha\"e a tendency co £1 x and open under load. T~ feature can create a risk that rhe spanner will either slip or distort the edges of a hexagon unless it is a snug fit at the outset. ~
\,.
Ring Spanner
'-
"
(C
These spanners have a distinct advantage m-er the open-jaw typ e in that s$ty fully enclose a hexagon bolt head or nut. Am9rtcans refer [0 them as tj}ox wrenches' because of this. The inside periphery of the aperrure ring at ca~ end of the spanner forms a 12 point bi-hexagonal contour. Each corpsr of a hexagon bolt head or nut will be firmly engaged \vith a correspondinf"k ngle in the ring. The bi-hexagonal configuration of rhe spanner gives the operator a
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choice of engagement options, allowing operation in confined places where rhere is little more than 15° arc of swing available. 'nlC spanner is nonnally supplied with a ring aperture at each end to give twO available sizes. The ends rna)' be offset above and below the plane of [he spanner or may be straight. The small drawback to th is spanner is that once a fastener is loosened it can be a tedious business to screw it off unless there is room to complctely re\-oh-c thc spanner. Combin ation Spa nne r This type of spanner has an open jaw at one end and a similar sized ring at the other. TIlls can be a useful [enrure where access is limited because you can loosen a right fastener \\~[h the ring end and then \\~nd iT off \\~th the open-jaw end. Fl a re·nut Spa nn er A type of ring spanner that has a gap set in thc periphery of its ring. ·Illis feamre pemlits the spanner to pass o,er a pipe and engage with the hcxagonal Oats of the pipc connecror. This is a very useful facility when attempting to loosen a tight pipe connector because it provides you with an ahernati\·c to the open-Jaw spanner.
Bi- Il e'\agoll:ll Sockets Crowfoot
Step-Down;'
\d~
Ad,p'm~
~~ V 0
Vb)
<:)-
")i)lr J OlOt
.U
~ ~,,«0.
, \-. \ ~
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Long Extension Bar
CC)
(0\'''''Y;
-~
C=====::{n
Short Extension Bar
m
YTBar
~
------------~D~n-v-c~Bar
Figure 3.26 - Sockel Spanne rs
Soc ke t Spann er The socket span ner has twO main parts, fI bi-hexagonal socket that fits over fI hexagon bolt hcad or nu t, and a handle that engages in a square apenure in the end of the sockel. A number o f different configurations o f handles , extensions, step-up aclfl ptors and universal joint auachmems make the socket spflnncr one
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of me most versatile spanners in relation to the locaoons It can be used in. Socket spanners are normally supplied in complete sets \v-jth a range of socket sizes and attachments to connect them to a turning handle. The most favoured turrung device is the ratchet handle that pemlits the socket to be turned without the need to connnually re-engage it. Materia ls r."fost span ners these days arc manufactured from chrome ,-anadium steel that gives extreme toughness, elasticity and corrosion resistance.
Classification of Standard Spanners The size of a spanner is marked on the jaw face, or the shank, or the side of a socket. There has to be a catch, and this it, the sizing of standard spanners. The British and Americans do it differently and t.he Metric System does not apply in America but generally follows their principle - probably because I t makes more sense! I still recalllhe British furlongs, chains and flagons here! t\ British Standard spanner size relates
the major diameter of the screw th read of the bolt whose head the spanner fits. £\ 1/ . ;10 BSF bolt would need a 1/4in spa nner. The actual \\~dth across the flats on the head of a 1/4in BSF bolt is 0.4375in but there would be littk pOint in you measuring this and trying to fit a 7/ 16in British Standard :opa rIner because i.t would be too big. You need to know the bolt size and sadly, mat is where some people end up clutching an armful of spanners, hoping for luck! to
American spanner sizes relate to the distance across the flats of the hexagon of the bah head or nut mat it will fit. A 1/ 4lll ANF polt also has a distance across the hhxagon flats of 7/16in and the appropriate spanner would be marked '")0~j-1(P~\F. The hexagon dimensions for Unified ~crew threads arc the same as ".Ll'0.C~e. "" the American hexagon . The S_AE size relating to dus is 44 and that could be . \'l"~~~' l shown instead. / 16 is 0.4375 and this is 0.-+-+ when taken to two places of . ':--.\ '=decimals in the S_lli system).
r
,~ ,;5· 0..''\
Unified sizes are a measure of the distance across the flats but arc expressed as a simple fraction followed by the letters AF, which indicates that the size ..is Across F/:I(S, i.e. 7/16AF. The following mtile shows how the no~ diameters of bolts ties up ,'lith Ulufied spanncr markino-« . :i b~ S. "
Looking at Table 3.1, we can sec for example, that a 5/ 16 diameter unified bolf \vould reguirc: a 1/2in t\F spanner. Remember th~t the abbreviauon AF meani 'Across Flats'. ~Ietri.c sizes are also expressed as a number indicating distance across the hexagon flats in millimetres, i.e. 12 would be 12mm acr~
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Nominal Bolt Diameter (in) Unified
Dimension Across r:J;1tS (A F)
SAE Size
1/ 4
7/16
44
5/ 16
1/ 2
SO
3/ 8
9/ 16
56
7/ 16
5/ 8
62
1/ 2
3/ 4
75
9/ 16
13/16
81
5/ 8
15/ 16
94
3/ 4
1 1/ 8
112
7/ 8
1 5/ 16
131
1
1 1/ 2
150
T able 3.1- Spanner Si1:eS
1 hope that was noc toO painful! We can now move on and look at adjustable and special span ners.
Adjustable Spanners
"'
\
",\"(',\'
....
These tend to be (he convenient excuse no! to trudge to the stores, or to co,'cr up the fact that tl~c spa nner size is not known. Adjustable spanners han a nasty [endeR@~ tS\spring open and slip and they should never be llsed if there is a s.taQ.d1P~ spanner available. cow-. \\).~{':
o
o
o
\. ">'\
Peg Spanner
Adjustable
Sp:lOner
-c
Box
Spanner
Spanner
Figure 3.27 .. Spanner Configurations
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Peg Spanners
These spanners have two hardened steel pegs that arc designed to engage with corresponding ho les in a non-hexagon screw platt or nut.
C Spanners T h ese spanners are designed to engage with external splines on scre,,- fitti ngs.
Strap \'{'rench
T ube Spa nner
o
l
C resce nt Adjustable Figure 3.28 · Spa nne r Config urations
Tube Spanners These spanners are mbes with internal splines that ate designed to engage with external splines on screw fittings . They pronde a more positive dnye than C :m3f1i1~~s bur are limired to components that align with the axis o f the mbe .
.... ~\O~ . "l-.V:.'f.e."L,0 • \-,,;\-~"I: \~
- ;0...""1.
5\'""
:"1...:.
U"'\
Box Spanners
Unlike the American term for a ring spanner, box spanners are tubes that have a hexagonal contour at one end and holes to receive a tOffilny-bar at the o ther. T hey provide a snug, enclosed fit on a hexagon bolt head or nut. Sm~ versions of box spanners are o ften used to access recessed adjustment screwso'
'",
.~
Strap Wrench
Tlu s wrench incorporates a plaited cotton or nylon strap that can be guickIg wrapped around a circular or irregular shaped scrc..ved com ponent. The strap~ then locked under an intcgrallc,'cr that is used ~o exert a frictional LOrque ~ the component to turn it. ·nle oil filter housing on a car is a classic exampl ~f M ItS use. r::"
"
~
,2i
Allen Keys
'U
A n Alien key is manufacnued from a steel, hexagonal section b~ that is , I cranked at 90 0 [Q foan an L shape. T he hexagonal face of the cranked end is I
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designed to engage with a corresponding recess in a bolt or screw head. Allen keys are classified by the distance across the Aats of the hexagon.
Figure 3.29 • Allen Key
Precauti ons •
Usc the correct size spanner.
•
Do not use an o pen jaw spanner
•
Always use [wo spanners when tightening or loosening pipe unions and lock nuts.
•
Do nOt apply excessive le,-eragc, or use extensions or hammer blows on spanners.
th~1t
has distorted jaws.
"IN( '"1\"\\)6 - ... _ "c,z,e'"
G"'"""\\\ "
~ \",,;o.~{forque
", "
. .'-~\~.
(. ~'\:
Limiting Wrenches
D escrlpno " n A torque-limiting wrench incorporates a mechanism tbat preyents the wrench transmitting a torque higher than a preset value. There are II number of torque wrench designs on the market together with the rigs that are necessary for setting and checking them.
.~
Turque (T) = Force (F) x D!sunce (0) Figure 3.30 - To rq ue
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E ffe c t s o f T orqu e L oading Before we look at a few examples of torque limiting wrenches, we should have some idea of why they are necessary. \\'hen a bolr or a nut is tightened, it produces two effects. First, the bolt is put under tension and stretches elastically in proportion to the increasing load applied by th e spanner. Second, [he pressure on the faces of the threads, and between tbe faces o f the bolt head o r nut and tb e material it is in contact with, increases with load. This pressure produces sufficient clamping force and limiting static friction to prevent the fastener being loosened by librarion. The correct torque load will achieve this \vithout oyerstressing the fastener.
if the torque loading on the bolt is excessive, the bolr may strctch plastically rather tban elastically, which rather defeats the 6bjecr. Failing this, the strain imposed on the faces of the screw threads, and the contacting faces o f the fas tener and material, can cause distortion leading' to fracture. The high stresses incurred can also set up an electrolytic corrosion cell al the contacting faces. It is nOl a good idea to over-tighten fasteners. On the other hand, it is not a good idea to leave them slack. rf a bolt, screw or nut has been inadvenemly overtightened, you must reject ie The nut and the bolr would both be suspect. There is no point in slackening off an oyer-r'orqued bolt or nut and retightening it because the hidden damage has already been done . Combinatio n Adaptor
/' Cam R"II]"'11 Levers
Screwed Adjustcr A dju ster
r\diu stmcnr Ke \-
r
\............... b , -0
A crarork
Britool
Sunnen
Torque Screwdriver --s. J:S-
Figure 3.31 - Torque Limiting Wrenches
.'")
As we have alreadv discussed, torque is the product of thc straight-line dism.Q1:e from the centre o'f a bolt head or nut to the point of application of forc'€on the spanner or turning handle and the value of t ha t force that is being a~ed l in a tangcI1u'al direction lO the nut or bolt. It is difficult to judge forc~ltimes distance merely by feel and sight - the 'grunt' scale is defini tely subjec# e! T hat i~ wlw we need the assistance o f a more accurAte means of establis hing the correct value of torque. \\le can do this by using a torque-limiting wrenc h that
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will no! permit rou to exceed a yerifiable preset \'alue, or we can do it by using a torque-indicating wrench that just tells you what the torque value is nHber rhan stopping rOll from exceeding it. Tn t h IS section, we arc just dealing with torque limiting wrenches. Torqu e Un its Torque loading values may be expressed as pound force foot (lb./ fl.), pound fo rce inch Qb.1 in.), gram force centimetre (gf ./cm.), kilogram force merre (Kgf'; m.) or Newton metre (Nm). Aeratork Wrenc h The !\cr.uork wrench incorporates a spring-loaded roller that sits in a profLled dnve recess on the rim of a circular cam that connects with the square dri\'e of the wrench. As the wrench is turned, me increasing torque is fed back through the cam OntO [he spring-loaded roller. At a preset load, the force on the roller overcomes dle spring pressure and it is forced out of engagement with the drive recess to ride on the smooth rim of the circular cam. 111is effecri\'ely breaks the drive from the handle of the wrench through the roller to the square drive. Any further turning of the handle merely causes the roller to ride freely over the circula r cam. The torque setOllg on the wrellch can be adjusted by resetting rhe spring pressure. This is achieved by inserting an adjusrrnent key into the end of the handle and turni ng thc screwed adjuster. T his o peration requires the additional usc of an i\cratork test rig. An importanr point TO note with this type of wrench is that, if you keep rotating it for a full circle after it has broken the driye, the roller will again reach alignment \\~th its recess and will snap back into it with a bit of a jolt. Once broken , yO~hould return the handle gently back under control unwthe 0\ ' roller re-em~nges-: le wrench is then reset and can be remO\~ed . . 1\\';''f,C. ...
;:., ,,(~\~~tber important point to note is mat the Acratork wrcnch docs drive in . ,\,\ ~ '.;. reverse but breaks at a higher tOrque \·alue when doing so. This would not be a problem if you were merely loosening a fastener but it would be a problem if _ ...,\,:"\'~ I,. , YOll were attempting to torque load a left hand threaded fastener w1th a right handed wrench. The wrench would break but at a higher \'alue than its preset value. This design featu re allows you to loosen a torque-loaded faslener; it is not intended to let rou torque left hand fasteners. You need a wrench that is specially adapted for that task. \Vhen you pick up an Acratork wrench, it is importam to check that i\ has been reset after its last usc. It is quite dangerous to use an unset wrench because the handle may unexpectedly kick back and may cause injury to the oper:ltor or damage to a rest rig or faste ner. -n1ere is a dot on the end of the cam spindle ll nd an arrow on the body of [he wrench. When the dot and the arrow flrc aligned, the wrench is set and ready for use .
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Use 1.
Check that the dot and arrow are aligned .
2. Fit the socket spanner to the square drive.in the wrench . 3.
Place on bolt head or nut and turn rhe hancUe cloch,vise until there is a sudden drop l!l resistance.
4.
Hold the handle fU111.l y and turn it gendy back through approximately 35° until you feel the roller re-engage and the dot and arrow align .
J.
RemO\'e wrench.
Britool W rench 'I11e Britool torque limiting wrench is supplied LO suit a number of torque lo ad ranges and is similar to the Akratork model in that it will brea k at a p re-set torque value. The wrench incorporates a spring loaded rocking mechanism that b reaks with an audible 'click', accompanied by a momentary loss of resistance, when the preset torque value is reached. Unlike the Akmtork, this \vrcnch will not ride freely after it breaks so no attempt should be made to C01l0nue turning the handle after it breaks. The wrench resets automatically \\·hen the operator relaxes the force on the handk. The wrench has English and l\letric scales engrayed o n its handle and the torque yalue can be set by using a tommy-bar to l turn a screwed sleeye on the hancUe. T he bCTelled edge of the slecve cutS across the graduation marks on the scale that relate to the setting of the wrench. The operation of some Britool wrencbes can be changed from right hand thread to left hand thread use by pushing the squared dri\'e on the wrench rhQggB to the opposite side of the head. An ar~ow on each side of the head 1Q.\iEi · . a f rotaDon. . I , e,C - tIl dlcates tIle di recnon
_O~\c" ""';o..\"t':
Sunnen \Vrench
,\'\. l,-
" ..\'r>
(f'';;:
Again, this wrench will brcak at a prc~ct tOrgue value. The drive is transmitted through a knuckle joint that breaks when the preset torque value is reached. This configuration is often referred to as being a 'break-back' \vrench. T hkwrench can be set using an adjusting nut located d>n the end of the handle. ~. 7;3-T orq u e Scrc\vdriver
'"
This device has a handle and a driving shank tba t are axially aligned . The shan* incorporates a square drive that is designed to accept standard socket spanne.~ . The body of the wrench contains a spring-loaded clutch mechanism rhat s)jPs when the preset torq uc valuc is reached . There is a scale and a pointer ort\:he side of the body that indicates the torgue setting. The available torque r~t$ge is low, about 5-251b fL ,C"i
'0 .§' ,
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TOO L S
Checking T orque Wre nch es t\ torque wrench th:l.l does not incorporate a setting scale, and is llsed illfrcgucntly, must be checked on a test rig before each use in order to vcrify its prcset torque value. Jf a wrench is in continuous use, then it may be subjectcd to pcriodic chccking. If the torCJue value is to be adjusted, thcn rhis must be donc in conjunction with a checking rig. Torque wrenches that do hrwe their own selling scales should be subjccted to periodic calibrntion to cnsure thc accuracy of thosc scales. Acra lo rk Un ivc rsal Tes t Rig
This rig consists of a dial that is usually scaled in English and p.lctric units and a square dri\-c sockct and adaptors to permit thc engagement of a wrench. lllC rig is capable of indicating torque ,-alues from O-..J.OOlb fe and is equipped with adaptors for the tcsting of several types of torque wrench. The dial has a zero adjuster and thc poimer should be set to zero prior to engaging a wrench. -nlC rig also has a freezc mechanism that lea,-es the pointer at an indicatcd torgue valuc unril you press a release bunon to return it to zero. \'x/hen checking a torque wrench, the wrench should be first engaged with the rig, operatcd, rcset and removed. The scale pointer should thcn be adjusted to zero. The wrcnch should then be re-engaged with the rig so that the handle is vertical and thcn rotated until it breaks . The reading can thcn be recorded from [he dial. The wrench must be reset before removing it from the rig. Care should be taken when re-sctting Acratork wrenches because an uncontrolled kick back at a high torque setting is capable of causing damage to the rig and injuring the operator.
To rq ue
I n d i~&~ ng .,~~)b
Wrenches
...
De~~,ijhlon
:, ....~~' Unlike torque l.imiring wrenches, torgue-indicating wrenches JUSt indicate the rorgue value bcing applied on a dial: the. ... do not impose any limit. ,
:\'\. b\:'
,...:.(\~
(\...,....
: Fig ure 3.32 - T orque Indicating Wre nc hes
J have illustrated twO types of 'dial type' torgue indicating wrench . The first incorpora tes a tor'lue bar that flexes under load and moves a poi mer ovcr a
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cl.librated dial. _\ $wiyel joint at me end of tbe bar allows the operator to s\\;ng the wrench around so that the dial can be read from different positions_ The dial has a scale 0- t 50 lb./ fr. The second type incorporates a torsion rod that fl exes under load to move a poimcr o\-cr a calib rated scale that has a zero setting be7.el on it.
Torque Wrench Adaptors I ha\'c includcd this topic here because torque loading docs not appear to be :Ul~'where else in the syllabus and it would come as a bit of a surprise if an examiner picked on ie If a torque wrench is fitted with an adaptor or ex tension that crfeCliydy increases its length then the torque value :;c l on Ihe wrench will nOl be lhe same as the torque that will be applied to the bolt head or nut. \X 'hen it becomes necessary to fit an adaptor [hal does this . the torque set on the wrench must be set to a "ruue that compensates for it. '111e simplest method of doing this is to fit me extension and set the torque wrench on a test rig. If this cannot be done men a 'metered' torque value will ha,'e to be calculated that will dcli,-er the correct 'actual' tor(lue :u the boh head o r nul. There is a fomlUb.
Acmal Targue
FiguH' 3.33 • Using EXIC:ns ioos
[f dlC tor
\1 Clerc< ,
Or
I "fljUC
\> I _ a uc -
\crual Torgue Value x l.ength of Torgue \'\rcnchn-, Length of Torquc \'Crench ( L) + LcnR:th of EXICnsion{E) ".
~
The metcred torc! ue yalue can then be set on the wrench and this willlJl~1 gi\-e v th e actua l or specified \ralue of torque at the bolt head or nut. [f you ai.€ usillg a ro rque-indicating w[ench, the reading on rhe d ial indicates the mcterc'd to rque. not the actual torgue o n the fas tene.r.
139 D'll'IP.p
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Examp le A 12in torque wrench is fined with a suaighl extension that increases the length br -lin. I wish to apply a torque loading of lOOlb in [0 a nul. \,\'hat metered torque figure should I set the wrench to?
'[ clercd Tlorgue
1\
\ '
[ :llle
= 100 Ib/in y 12in
12in --~5 . [b/ ,'n
+ 4in
Taking Ihis a bit further, what force would 1 have to apply on the wrench handle to produce rhe correct acttL1.1 torque on the nm?
- 100 lb /in F·orce = 6.251b 16in If I.hc torquc wrench and the c""tension fonn an angle of more than 15° \,,;th each other, the lower tem1 of lhe equation must be replaced by the straight line distance D from the point of application of force on the wrench to the cemre of the bah head or nut. Angles less than 15° arc not going to make a great deal of difference from a straight extension.
Usi ng Torque Wrenches When a torque wrench is required, you should verify that the LlIlirs used for the specified torclLle are the same as those used on the wrench or the wrench selling rig scale. The torque v:llues thar iie within the first quarter of a wrench scale can be difficult to reaqnl.$ome tool manufacturers specify that the torque wrench selected f~'dl pai:[icul:ir task shouJd have a range sllch that the speci fied torque fo.( {l1etask lies 11 7·thin the LIpper range of the wrench scale. \-..,:,1:t~
,\,\ '-
c
~...,'\."".
\'(,'hen applying torque, the wrench handle should be gripped lighd)' and the
force should be applied smoothly at ninety degrees to the axis of the wrench. \X/hen using an extension spanner, the extension spanner and wrench should be as neady as possible in line. If the alignment is out br more dmn 15° from a straight line, (he metered torque shouJd be established using the fomlUla we discussed abO\·e. All torque wrenches sr.ouJd be periodically checked for accuracy.
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THREE
TOOL S
M arking-Out & Me asuring Tools Steel Rules D esc rip ti o n & Class ificatio n Steel rules arc manufactured from high carbon steel thaI has been ha rdened and tempered. They arc classified by their length and arc graduated in English and Metric units.
Spring D ivid ers D escrip tio n, Usc & Classification Spring dividers are llsed to mark OlJt dimensions and to scribe circles and arcs. The leaf spring is manufactured from spring steel. The two legs arc made of high carbon stee! that has been hardened and tempered . The legs must be of equallengrh and the points must be kept sharp by stoning the olJ{side edges. Grinding is not recommended unless done \-ery carefully, it tends to draw the temper from the steel and soften the points. The adjustment screw and knurled mil are made of mild steel. \\-hen the dh-idcrs arc nOt being used, the points should be protected by, for example, pieces of cork. Spring d ividers are classified by the length of their legs.
Scriber
~rnng
Dl\·idcrs
Scnbmg mock
Figure 3.34 - J\hrking Out Tools
Scribe r ,~
D escriplion , Use & Classifi cation
:
'"
,0
Scribers arc used to score guidance lines on the surface of maLerials. Th~~are [l1f1!lufactured from high carbon steel that has been hardened and tell1wered. The poin ls mllst be kept sharp b. stoning and ptorectcd when no! in tgiS. . ..§ Scribers arc classified by their length .
,
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Scribing Block D escription, Use & Classification .A scribing block is used to mark off a line that is parallel to a true surface such as a surface platc or table. The base block is manuf.'lcrured from cast iron or mild steel that has been case-hardened. T he: base block has a Vee cur on the lower surface that is designed to U':Ip any dirt on the surface table that would otherwise affect conract. TIle pillar and the attachmems are made of mild Heel and the scribcr is made o f high carbon steel that has been hardened and tempered. The height and angle of the scriber are adjustable and the pillar angle has a fine adjusnnem. The base block incorporates a pair of ret.ractable dowels that can be used to act as a guide to the block on dle edge of a surface plfl te or table. Scribing blocks are classi fied br the height of their pilla rs.
;')!Ock
Fcelcr (jau,!!,cs
•
Firter's s tjUJrc
Fig ure 3.35 - Measuring Too ls
Sometimes referred to as a 'box square'. A ker-sear rule is used in conjunction with a pencil or a scriber to mark off lines that are parallel to the axis on the surface of a round bar or rube. Ther Ilonnally ha\'e a graduated scale. Key scat rules are classified by their length.
Fitter's Square D escription, Use and Classification A Fitter's or Engineer'S Square consists of a stock and a blade set ar 90 0 to each other tha t each have sides that are truly parallel. The stock may be notched at the in tersection with the inside of the blade to give a cleara nce with
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TOOL S
the corners of items being checked. The square is used for checking the truth of work and, in conjunction \\;th a pencil or scriber. for marking out lines that :\ tc:\( righ r angles to:l surface or an edge. It is milI1ufacrurcd from high c:\ rbon steel that has been hardened and tempered. E ach squa re is p rovided with its own pratecti\·e box and it should be retained in this when not in li se. j\ finer's Sll uare is classified by the length of its blade.
Checking fo r Accuracy A Fitter's square should be checked periodically for accuracy. T his can be done by checking it against a 'master' square or a \' block . . \n alternauye method is to place the stock against a known true edge on a flat surface and then draw a line on the surface usi.ng the outside edge of the blade. The square is then turned o\'er and the alignment of the blade is checked against the marked line. The (est can then be repeated using the inside edge of the blade.
Feeler Gauges Description , Use & C lassification
Feeler g;lugcs arc a set o f flexible steel blades that each have a given thickness measured in thousandths of an inch or in fractio ns of a millimetre. The blades are fa nned in a graduated stack that i ~ pi\"Q(ed at one end allowing the blades to be selectively fo lded in or out of a metal sheath. Feeler ~ uges arc used to meaSllre small clearances and gaps. Feeler gauges arc
das~ i fied
by the length of the blades.
"i11ere arc three common configurations of calipers: olltside, inside and oddleg. The~' are classified b~' length.
Outside lnsidl.: Odclkgs Figure 3.36 -
caliPcrr
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Outs ide Calipers Outside calipers are primarily used, in con junction with a steel rule, to measure outside diameters and distances. They have legs that curve inwards.
In side Cali pers Inside calipers are primarily used, in conjunction with a steel rule, loside diameters and distances. Tbey have legs that curve ourwards.
[Q
measure
Odd-Leg Calipers Odd leg calipers have o ne leg that IS sharp pointed and one rhat curves inwards. They may be used for scribing lines parallel [0 smtight edges, scribing arcs from the edge of surfaces or, finding the true cemre of a round bar.
Use The use of calipers requires a sense of 'feel'. The calipers are moved by hand to rhe approximate setting and are then adjusted by gendy tapping one leg, not the point, against a solid surface.
Trammel D escri pti on r\ trammel is simply a beam that has a scriber attached at each end. h is used
for dra\\~ng large diameter circles or for stepping off large diS(ances . One sc riber attachment is normally adjustable to give the tranune1 a range of disrances. ']..\Ilf ll",6 -
,\...,-
...{'-'-
(. ...'\.
• "I.;.'o!..(,; eLl)' . ( \\~~Combination Set
--
D escription The combination set consists of three separate instruments: A Centre Head, a Square Head and a Prouactor Head. Each is adapted to fir a specially designed high carbon steel rule that has been hardened and tempered. The bodies of rhe heads are manufactured from close-grained caSt iron. Ru le The sted rule has a central groove that aligns with a clamping screw in each head allowing it to be fi.-.;:ed at any posicion on the rule. The rule is graduated in English and Metric units.
,
,,","
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Scriber F igu re 3.37 - Combination Se t
Centre Head The Centre I-lead is used in conjunction with tbe rule to locate the centre of round bars or the centre lines of tubes. Squ are Head The SCJuare I-lead has t\vo faces at 90° to each other and a third face at 45° to the rule. This arrangement enables the SCJuare Head to be used either as a square or as a mitre. This head also incorporate~ a spirit level and has a shorr scriber housed in the bod\,.
Protractor Head Th e1~otractor Head can be used in conjunction \'iith the rule to measure or
.
;\~,\ ,.;.\
(P~
-
,,
. l ,l \..,~s{'
l'': ~W:r
1.\\i.l~ ~ ;ny angle up
to 180°. This head may also incdrporate a spirit
b-d .
Surface Tables
q
Description
." ~.
A surface, or marking-off, table is used to supp~:Jrt items whilst they are belli~ marked out and provides a true surface from which measurements can be: taken. Tt should never be used for anything else; it is nOt a workbench! surface table is manufactured from c1ose-grnined cast iron and has a true working surface and true sCJuare edges. The underside of the table has he",,'Y ribs dlat give the table rigidity. Cast iron is brit~e so the table should neve~.ic used to support items that are going to be hammered. The table is provWcd with a wooden protccth-e coyer that should alwl1.ys be fitted \vhen the t~'1e is not in use. After use, the working surface should be lightly oiled, ,'Before replacing the protective coyer. §u
.f
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,
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TOO L S
Surface Table
Vee I3 lock
Figure 3.38 - Surface Table
Surface Pl ates D escription Surface Plates arc similar to marking-off tables in that [her provide a true flat working surf.1ce and true square edges . T he}' are ffi.1nufactured from closegrained cast iron . The surface plate is much smaller and has peg hand le at each end to make it portable so that it can be positioned on a workbench. It is primarily used fo r checking flat surfaces o f items for tm th bur it cfln rtlso be Llsed as a marking-off table fo r small items. As with a marking-off rabie, lhe surface plate should be lightly oiled after use and protected by ilS wooden cover. Use
1..
W'hen checking"(wf surfaces for tmth, the working surface of the plare is first given a .r:b'i'h\\)co~ting of Engineer's Blue. The surface to be checked is then ~ou.try,,;,fubbed across the surface of the plate and a transfer of the blue coating 'O~~tvill indicate any high spots .
. ,\,,:, (\~~....
<"
v - Blocks D escri pti on r
v - Blocks are primarily Llsed
to support round items on a surface o r markingoff table. Thcy are manufactured from cast iron or mild stcel that has been case hardened. V- Blocks are supplied in matched pairs [bat arc recognised by having a similar identification number. All lhe faces on a V-I3lock are accurately machined and the angle of the 'V' is 90°. Classi fi cation V- I3locks arc classified by the maximum diameter of the round work thal can be held.
146 COPJ7ight 0 BlIl!J' Colkgt J006 . 700-
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Common Power Tools In Chap ter 7, we will be dealing \'lith the tools used fo r riveting and dimpling. Because of this, 1 \,-,-ill not include the power-operated tools associated wnh those processes in this section. T hese include: pncumaric riveting hammers, tubular rivet guns, mandrel croppers and riyet millers. Twill, howe\-er, include: air-operated drills, the rotary saw, shearing and nibbling tools beca use these are used for many sheet metal work tasks . I will not be including other poweroperated machines such as: lathes , milling machines :lnd pedestal drills becallse they cannot be described as being in common usage by aircraft maintenance engineers.
Drilling Machines Electrical and pneumatic portable drilling machines are both used in airframe repairs. Th e pneumatic, ponable drilling machines arc the tools most commonly encountered on aircraft work becau~e they are safer to use where there is a possibility of flammable vapours. Electrical machines should not be used where rlus hazard exists.
General Purpose Air-Operated Drills Description
~ c
The two commonest types of air·operated drill are the straight grip and the pistol grip. The straight grip type will accept drills up to Smm (1/ 4in) diameter whilst the pistol grip type will accept drills up t9 8mm (3/8in) diameter. Both types ~v c an adjustable, self~centring chuck td accommodate differe nt sized 7\'1utrill~~hey are air mrbine (impeller) driven, lightweight, have very positiw (.o\\c,?..c - speed control and do not oycrhcat il: continuous h eavy use. They incorporate a 'I)'t(' gearbox to produce th e correct rotanonal speed at the chuck.
A."\'- -
.\~
"..:.."\
(~..;\:
.
Chuck Collct
• Angled Pi swl Grip Srraighr Figure 3.39 - Air-O pera ted D rilling Machines
147 COPJ'right
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Off-Set & Angled Head Drills Description These variations lO the straight grip type drill are designed fo r tas ks that would not o rdinarily be accessible w general-purpose drills. Both types can accept d rills up w --I-.8I11m diameter but each size drill needs a separate chuck coller. 'fh e chuck collets require the usc of two special spanners when ins talling or rem oving the d rills from the machl!le. The angled head drill is available in nyo standard angles of -1-5° and 90°.
Drill Attachments A right-angled drill attachment can be fitted inw a straIght gnp, gen cral~ p urpose drill . The attachment accepts threaded drills mat are screwed into a coUet.
Right Angle
I T IUCOdCd
~
Drill
Figure 3.40 - Anachmeots
1\ flexible 'snake' drive can be attached to a general-purpose drill to enable
access w res tricted areas where the angled or o ffset drills cannot reach .
r
~ ,
.~
%(~
Rotary Saw
"'"
.:;'
Description The rotary saw is a hand-held, air-turbine powered machine mal is used fo r cutting sheet aluminium alloys up to 2mm thick and sheet steel up to O.8mm thick. There is a choice o f blades, each suited for cutting particular grades of materials.
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There is an electrically powered sa\v called a 'Kens Saw' that has a bead that can be adjusted to different angles making It useful for working in locations where :l.ccess is difficult. This saw is particularly useful in thal it can cut thicker sheet meta ls up to 4.5m m thick and can start cutting at any poin t on a sheet of melal. C.utting u";'~' 1
Shears
Oil Lnd Check
•
\Yindow
'\:ibblcr ~"-Pllnch Rutan· Saw Fig ure 3.4 1 - Ai r D ri ven C urling T ools
Shears Description _\ir dri\-en shears cut sheet steel up to 2rnrn thick and light :lUoy sheet up to The shears ha\·e a [eciproc:lring shea r blade th:lt pushes :lgainsr ,i.-" ILIl i~~·i l. Stellite ripped blades arc a\·ail:lble for cutting titanium and swjnless . (.~l\\e..:..t: - steel shee t materia!' 2.6rnnl~ rhick.
\\).~~ ...
-
Nibbler
..,"0 ~
Description
-"
o The air driven nibbler employs a reciprocating pWlch to cut aluminium allojr sheet LIp to 18S\\'G, and sheet steel up to 20S\'(JG. This machi ne Cuts without' defonning the material or crearing bu rrs but cannot Cut thicker gauges material than specified without suffering damage to the punch opcraurjg .~ mechanism.
c?
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Grinding Machine D escription Sometimes referred to as the 'off-hand grinder', [his electric mOlar drh-en machine is used for the shaping aod sharpening of hand curting tools. The double-ended grinder is equipped with two guarded abrasive wheels, one coarse and one fine grade. The fine grade wheel would be used for sharpening items such as HSS drills for example. Both wheels spin when the machine is In operation. Each abrasive wheel rotates to spin down past an adjustable L shaped tool rest that lies close to the wheellO prevent it dragging a hand-held tool down and trapping it between lhe tool re~[ and the wheel. The grinding machine is equipped with transparent prorective screens to deflect chips that may fly off the wheel. T he machine may be pedestal mounted on rhe floor, or it mar be mounted on a workbench. The pedestal-mounted machine nonnaUy has a quenching tank mounted on the pedeSTal. Electrical power is supplied through a wall mounted isolation switch to an on/off control switch on the front of the machine. Screens
\\"'heel
Rests Figure 3.42 · Grinding Machine
The cool rests should be adjusled so that they lie close to but not in contact with rhe wheels, the gap should not exceed 1/ 16in.\'{llen grinding, the tool should be continuaUy moved from side to side across the face of [he wheel to prC\Tcnr uneven wear creating ridges on it. Excessive pressure must ne\'er be applied to the wheel. The side of the wheel must n en'£ be used for grinding. Soft materials, such as aluminium, copper, brass or magnesium, will clog the wbeels making lhem ineffective. Prolonged grinding or excessive pressure creates sufficient heat in a rool [0 draw irs temper and ruin it. Maint ena nce Tasks involving the dressing or replacement of abrasive wbeels can only be undertaken by competent persons who arc properly authorised to carry out these tasks. A special 'Dressing Tool' is llsed to restore the face of a wheel. The operator must cbeck the adjustment of the tool rests before using the machine. A loose or badly adjusted rest can result in a tool becoming trapped and tills can burst the wheel or cause the work to be ejected at speed .
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Precautions
Before starting the machine: •
Inspect both wheels for damage, dogging and uneyen wear and check that all the guards are firmly posinoned.
•
Check that the gap benveen each wheel and its tool res t is within 1/16in and that the rest is not touchi ng the wheel.
•
Always wear protecti\'e goggles when opera ting a grinding machine .
•
Check that you know where the power isolation s'\vitches are.
Starting the machine: •
Stand to o ne side of the machine when starting it.
Operating the machine: •
Do not grind soft materials tha t will clog the wheels.
•
Do not exert excessi\'e pressure on the w heel o r use the sides.
After o perati ng the machine: •
E nsure that the wall-mounted isolation switch is turned off after shutting down the machine to preyent inadvertent operation,
,"" Operation
"f0,0 - , (.,,\\0';," -.
\,' ~ ,,~\~ .C''\;'
L·
\~').~'{'
& Use of Precision Measuring Tools
rv[easunng can be carried out either by using non-indicating measurement tools or by using indicating tools. Non-adjustable, non-indicating tools, such as gauges and test pieces erc. measure by comparison. The measurement of an item is either equal to, o r nor equal to, the size of a fi..-.;e d gauge or a test pie<;:e. These devices can be designed to offer a Go or Not G o result. .r\djustabJ;, non-indicating tools like calipers, for example, be adjusted to give a choi~ o f lengths bur are then used as a compariso6 to check if an item is thTt dimension or not. Comrersely, they can be used to transfer a measuremen t from an item back to a steel rule. Indicating feasuring tools, on the othtF" hand, such as steel rules, can be used to direct read acrual dimensions :;'
dn
le~,
There IS a big range o f properties that can be mLsured. These include: fl atness, paraJlelism, angles, profIles, roundnJss, concentricity, shape,:--lnd surface roughness. Obtaining an absolute "\"""alue for any of thes e measurements IS a physical impossibili ty, as is the chance o~ accu~atel)' matching t8~m in production processes; that is whv we han~ limirs and tolerances. A~roblem ,rim when we h,ve m ,chi,,'e high ,ecumer pmduccion hecM, t he limit>
i'1
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arc going to be fine. Tills means lhat we have to achieve a high accuracy in measurement, taking into accoum the uncerrainf)' of mechanical measuring instruments that have, themseh-es. been manufacrured to limits. \'(re need instruments that will measure ro within fine limits of accuracy and be capable of repeating this. That makes the case for having prccision measuring rools.
Di al Gauge D escriptio n The dial gauge is sometimes referred to as a dial test indicator (DTT) or a Clock Gauge. 1t is a precision instrument used to detect and measure small differences in sizes; it cannot measure actual sizes, only small variations from rhem. Tn this sense, it is a comparison too\. In the English measurement \·ersion, the dial is gradlL'ltcd in thousand ths of an inch (O.OOlin) with e'-ery ten graduations (O.Olin) being marked with a figure ie.I O, 20, 30, 40 and SO. Each full rotation sweep of the pointer registers O.lin on a smaller dial. The larger dial reads up to O.OSOio cioch"ise llnd O.OSOin anticlod,-wise. Other, more precise. ,·erstons of Lhe dial glluge h~l\"e smaller ranges graduared to read 0.0001 in. The Metric scale version has a dial thaI is graduated in O.Olmm ,\~(h every ten grnduacions (0.1 mm) marked. Each full rot:lcion sweep of the poi mer records l mm on II smaller dial. There is a more precise version that is graduated in 0.00 Imm. A further \'cl:)' precise ,·ersion has a graduation value of O.OOOSmm. Thc more precise the graduations arc, the smaller will be the range of the instrument. For example, the range of o ur last example is only O.02Smm whereas the flfst eXllmple has a range of 10mm. '1nO~
Scrcw
•
II---Zero . \djusum.·fl( Bezel
Plunger Burton Figure 3.43 - D ial Gauge
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The gauge pointer is moved by the deflection of a spring-loaded plunger operating through a geared mechanism . Tbe dial gauge has a bezel ring that is used to bring the zero mark on the scale into li ne \\~th the pointer. The ring can thcn be locked. A dial gauge is nonmilly supplied as a boxed kit thar contains the instrument together with: a base, a ,"ertical pillar, a universal elamp and a selcction of followers . Dial gauges can also be secured La scribing block pillars. "I/here a gauge is llsed to check the run-our o f components, such as an installed engine propeller shaft for example, dIeIe is usually a special to type attachment pro"lided.
A dial gauge is a "lersaruc lflStfUment and may be used for measuring: highs and lows on flat surfaces, the cooceotricity of round shafts, the plane of rotation of discs, the bowing of shafts, movement between components etc. ,Vhen suitably adapted, a dial gauge can also be used to check the bores of components for concentricity . Note that it cannot meas ure acmal dimensio ns, o nly yariations from them. \Xihen setting up the instIument, the instrument 1S ad justed on its support stand so that the plunger on the dial gauge is put in contact with th e surface of the item to be rested and is then pre-loaded by pressing it down against the surface uncil the pointer deflects by several graduations . The gauge is then zeroed by unlocking and turning the bezel ring to bring the pointer in line \\~th the zero mark. TIus sel up procedure is imporianr if the instrument is to be capable of reading lows as well as highs. 1 will give rOll descnpcions of a few typical uses of me dial gauge.
Checking
U~\\C~e ~ \l';1."\.f~
. .\.....\. ~ _o<.;;.,;5\·r
\... ,
Run~o ut
of a R ound Shaft
,?i..,QblI&ill; trument is clamped in position so that the plunger comes into vertical - contact \Vim a smooth portion of the shaft. 'lbe wuge posicion is then adjusted so that the plunger depresses and deflects the pointer. The gauge is then secured in its position and zeroed using the bqel. "Olen the shaft is rotated, the pointer will s\v;ng to indicate any de\' iation in either direction. The out of true measurement of the shaft is taken to be a half o f the maximum range of de flectio n of the pointer. ~, c
...,
Checking Truth of a Flat Surface
'"
A surface table would be required for this check. The gauge is set up in its~ stand on a surface table so that the plunger is pressed onto the surface to bf checked and the gauge is zeroed. Assuming the surface to be checked -is horizontal, the dial gauge may be moved so thla t tbe p lunger slides over ~ surface. The pointer will then deflect elocbvise or anti-clockwise in respl}JiSe to an,· high or lmv spotS dlat it detects in relation to the true surface oC the table.· It is essential to ensure that the item being lchecked is supponed cqfectly to present its surface as parallel to the table or the dial gauge will mere!t record the error in parallelism with the table. /f
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Checking the Plane of Rotation of a Disc
Tlle gauge would be secured in a stand on a (rue surface so that (he plunger was depressed at right angles against the face of the disk with (he gauge zeroed. Rotation of the disc will then cause the gauge poimer to deflect left or right in response to any denarions in the track o( the disc. Again, the out of true measurement would be raken as being a half of the maximum deviation range of the pointer. Cylinder Ga uge The cylinder gauge is a special type of dial gauge that is used for checking the amount of wear and ovality in cylinder bores. It has twO contact plungers, one of which is sp ring-loaded and fined into a T shaped head that cemralises the gauge in the bore. The second contact is one of a choice of detachable extension rods. The gauge is set to read zero at the nominal bore diameter. Any "ariation will then be registered on the dial. \,\'hen readings arc taken at right angles to each other in the bore, the difference between them is a measure of oyality. The cylinder gauge is supplied in both English and j\"[euic fo rms. Precau tions •
Ensure that the instrument you choose has the range
•
Ensure that the surface to be checked and the stylus of the plunger are clean.
•
Ensure that the plunger contacts the surface by the force of its spring only.
•
Always
%!gw
[a
suit the task.
for sufficiem deflection of the pointer left o ( right when
~e!.urlfup !.he instrument.
(,o\\C~
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,.;~
,. l"'\"
•
r\lways lift rhe plunger when a sharp recess or slot has
•
Do not clamp the instrument too tightly as dus can seize the plunger in irs housing.
•
Always treat the instrumem with care and Slore it in its box when out of use.
(0
be traversed.
\~"\\. -
.
The Vernier System Before we go any further, we should get a grasp of rhe Vernier system. Tt crops up in most of the precision measuring instruments we arc going to deal with so tlus is a good time to look at it.
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The 24/25 Vernier Scale (English) We will stan by uSlOg an example based on the English measurement scale. Imagine that Y~lI have twO lines of equal length. N ow l1nagine that we have divided each line up into equal di-dsions except that one line has o ne more division than the odler. Tn the illustration, Figure 3.+-1-, T have shown a main scale line that is 0.6in long divided into 24 diyisions, each 1/ 40,h of an inch, or 0.025in (24 X 0 .025 ::: 0.6). I haye also shown a Vernier scale line, tha t is also 0.6in long, di\Tided up into 25 divisions, each O.024in (25 x 0 .024 ::: 0 .6). "I/e now ha\-e two equal length lines diridcd so that one line has one more division than the other. :-"{ai'6Scak
I ll l
!l
111111lL
S
In
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foinch
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15
V~ r ll1er Scnle
2..J. "" b in Scale- Sub -O i\"isiom ::: 25 Ve r ~ i er Scale Di\·isions 2..J. x 0.025 0.6 lOch 25 x O.02..J. 0.6 inch
=
=
Difference III Length B eN·t:t: ~ 0 1\;S10n ;; is: 0.025 (jI bin Scale) - 0.1)2.:1 (Vernier Sql!e) = O.OO! inch Figure 3.44 - Englis h 24/25 Vernie r Scale
Now consider what the difference in length is between one main scale division and one Vernier scale division. It is : 0.025m - 0 .024in:::::; O.OOl in. lfwe were to ffim-e the Vernier scale line to the right, so that the firSt Vernier scale graduation line is lined up with the first main scale graduation line, we \vill have mm-ed exacdy O.OOlin. If we continue to mO\-c the Vernier line to the right until the sec~nd Vernier graduation line lines p with the second main seal: I3bpd~~t.ion line, we will ha\-e moycd a total of 0;002in. This sys tem is allmving (,,\\C.'f-,\!."}.'" ~s to achieve measurements accurate to 0 .00"1 10 by just using two graduated '0.~.s0 . lines. I
J
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,,"\\.~
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.
For our next step, we will look at how we might take a reading from an instrument that carried tbe scale we have discussed. The next illustration shows a Vernier scale line that bas been moved to dle right along a graduated m:p.n ~~
~
.
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9fI2.l.j.5(' ~ 1l9 1 I, I I I I i II , I I I i i i ! i i i rill: ,1 1i i i , i! I ! i j! , iJ j
2 in
~
~
1""1' 'lfI " I""I"" 1
" ; 1""l 'i ~
2U
25
_oinCldcnt Lines
+ U.3in - (2 x 11.025)10 ... O.(IOSin
::::: 2 + 1I.3 + Ii.U5 + 0.00:5 == 2.358
In
Figmc 3.45· Rcading t be E nglish 24/25 Ve mie r Scale
155 Copyright 0 B(11)' ColI~g~ 2006 . 200- All ligbt! rmrvtd
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Note, that the zero on the Vernier scale is sitting somewhete between 2.3in and 2.-1-in. \Ve would like our measurement [0 be a bit more accurate than thaI. Therefore, we have 2in p lus 0.3io plus (wo divisions at O.02Sin giving us a toral of 2.3Sin. This is not accurate enough for us. Therefore, \ve look along thc Vernier scale until wc find a Vernier graduation that aligns with a main scale graduation, rhe eighth Vernier graduation does. That reflects a shi ft of 0.008in so lct's add it on to our total. Now we have a grand [mal of 2.358io . That is a measurement accurate to one thousandth of an inch.
T he 49 / 50 Vernier Scale (Engli sh) The 49/ 50 Vcr!llcr scale is a commonly used version mainly because it is easier to read the graduations. rn this scale, -J.9 malll scale divisions arc c'luai to 2.-1-5in while the Vernier scale has 50 divisio ns. Each main scale division is 1/20 th of an inch or 0.05 in (49 X 0.05 ::: 2.-1-5) and each Vernier scale division is 0.049 (50 x 0.049 ::: 2.45). The difference between a main scale division and a Vernier scale division is: O.OSin - 0.049in::: O.OOlin. This sounds familiar!
o I '
I8' r)
, 4 5 ~ 1
t I 1
9
i\ lain Scale?lO inch ! ' . ~ 'i 6 7 8 ')
"!(I
/
-
1 ' ~ ~ 5 6 - i! 'I
I
3
II Ii \\Ifll, ll~! I! 11111
111/11UiiO'lillllllllll,IJil,I,I,llll\111\ fll) 1 10
I
2
"'I
-1\1
'ill
Vernier Scale
49 '\ Iain Scale: Sub-Di\'isions ::: 50 Vernier Scale D iyisions 49 x O.OSin ::: 2.45 in 50 x O.049in ::: 2.-1-5 in Difference in Length Between Divisions is: 0.05 CMai n Scal e) - 0.049 (Vernier Scale) ::: 0.001 ,()Il 6'>
~,.~~c 1.1.)lJ
. (.0" ......
10
Figure 3.46 _ English 49/ 50 Ve rnie r Scale
..... "'.("t')
. ,~...
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QV
Tn the next illustration, J have set up a reading 00 an English 49/50 Vernier scale. Note [hat [he zero on tbe Vernier scale is sitting between 2.1in and 2.2in. Therefore, we have 2in plus 0.1 in and [he 43rd Vernier graduation is aligned with a main scale graduation. Our graod total then is 2.lin plus 0.043in and that equals 2.1 43in. Again, our measurement is accurate to one thousandth of an inch.
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iry
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L':~"),-': Read (rom 7r·c~,~o_ _ _ _"
THR E E T O O I.. S
C(lincidellt
Lints 2in + (2 x 0.05) + 0.043ill ;::2Hl
+ O.lm -+- 0.043
= 2.143ill Figme 3.47 - Reading the English 49/ 50 Venl ie r Scale
The Metric 19/ 20 Vernier Scale You guessed it must happen; there are metric scales. There is no difference in principle. this scale, 19 main scale divisions equal 1.9c111, each didsion being exactly Imm. The Yernier scale has 20 diyisiolls, each division being equal to 0 .95mm. The difference hem'een a main scale division and a Vernier division is 0.0501111 and that wiU be the accuracy of tills scale .. \ J\[clric Vernier scale is marked in a different \Va,-. Note [hac tbere are 20 divisions but [he scale is marked from I to 10. This is because each number represents 0. 1mm (2 x 0.05 = 0.1) and it is easier for you to read.
'n
.\ b!9 Scale (mm)
o
1
2/
II I ' I' I ' I ' I ' I ' I ' I IIII1 " I I 1 1 1 I I I II I I I I I I
I
(J
I I I
3 I II
J
2 3 4 5 6 7 8 9 10 Vernier Scale
19 i\lain Scale Sub-Di,·isions ;:: 20 "crlllcr Scale Dh'isions 19 x 1mm;:: 1.9cm 20 x U.95mm = 1.9cm Difference in Length Befween Dinsions is: Imm (:'.lain Scale) - 0.95mm (\'ernier Scale) ;:: 0.0501m ~
·0
"",,
FiguCt' 3.48 - Metric 19/20 Vernier Scale
%
Let's try a measurement using this scale. 10 the next illustration, we again rea§" lhe zero mark on the Vernier. \'(le ha"\""c 37mm plus the ninth Venll~ graduation in alignment. The ninth Ve[lller graduation is 0.45mm on the s~le (0.05 x 9 = 0.45). ;f'v So, our grand total is 37nun plus 0.45mm and that equa ls 37.45mm. Jf
,
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CHAPTER
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TOO L S
,\fain Scale (mill)
4 111~rrlllllll I ~ 11111111
J
I
t ( , 'II~I'I
,pJlj
f
I 2 3 -4 5 6 -
~
9 10 Vernier Scale
37mm + (9 x O.OS)mm ::: 37111111 + OASmm
::: 37.45mm Fig ure 3.49 - Reading the Melrie 19/20 Ve rnier Scale
Merric 49 / 50 Vernier Scale T ins scale has 49 main scale diy-isions, each Imm long, which equal 4.9cm. The Vernier scale has 50 di,,;sions, each O.98mm long, winch also equal4.9cm. The difference between a main scale dj,;sion and a Vemier division is therefore O.02mm. This is morc accurate than the 19/20 scale. The Vernier scale is numbered at each fh'e dh;sioos (O.lmm) from 1 [0 10.
III~ t""",~"""" J
111 11111111' 111111l1TIIIIII ",f,mp" rrrrli lit ttU II1111111111 I" "lrlHr 'I II"I I
o
1 2
3
4
5
6 7
8 9 10 Vernier Scale
49l.\ i.RRIScale Sub-D ivisions::: 50 Vernier Scale Di"isinns 50 x 0.98111111 ::: 4.9cm
1.l",649'"x 1mill ::: 4.9cm . (.o\\,,;.'j!.c
.:. \\""{{~
,. .¢,\ .
Difference in Lenglh Belween Dinsions is: I mill (l\ lain SC:llc) - 0.98mm (Vernier SC:lIc) ::: O.02mm
",
I... ('\
Figure 3.50 - Metric 49/ 50 Ve rnier Scale r
~<
T he neXI illus (l:arion shows an example reading, Again, we read to the zero mark on the Vernier scale. \'ile have Scm plus an alignmem of lhe 38'" Vernier graduation or, 0.7mm plus O.06mm (3 x 0.02). 'nlis produces a grand total of 50.76mm.
158 Coppighf 0 Barry Colltgr
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~rain
Scale (mm)
1.""",,71'.11111 , III" [vHL! , '11'1I"" I n"~'II'~tll' . . _. 1 ' 1' . ' 1,1' n 'I))) '1 '.1 . ....5 .. '1I"ill~j~ [ll 1111111 :1
1
i
1112;.l~6- L; "IW' Vernier Scale Coincident Lines 30mm + O.7mm
+ (3 ")( 0.02)01111
::: 50mm + 0.7111111 + ll.0601m ::: 50.76!l11ll Figure 3.51 - Reading th e Metric 49/50 Vernier Scale
Exercise in Reading the Metric Vernier Before we iea\'e the Vernier system. it miglu be a good idea to try your hand at a reading. 1 will gi"\""e you a i\Iemc 2~/2S scalc rcading in the next illustracion. juSt to make life a bit more exciting. You will have to work out the accuracy of tlus. I will gh-e you the solucion to the exercise directly after Figure 3.32, so If you do not wish [Q read it you should co'"et it up. , will sa,'e another exercise in an English scale for later on when we introduce another factor.
24/25 Metric Vernier
/
:\Iain Scale mm
,j, I,f, I, f'l'IIIII_III_I_LL [,~, 1, Lill Lli o
,
q, t'115H fir uu ,0
20
Vernier Scale (23 Di,"isions ::: 1;4 r l:llf mm) Figure 3.52 - Exe rcise in Reading a Metric 24/ 25 Vemie r Scale
Did you get 30.82mm? Here is how we can arri'"c at that. 24 main sc~ di"isions, each O.Smm, equal 12mm. 2S Vernier di\;sions, each O.48mm, ~ equal 12mm. The difference betw'een the scale chisions is 0.02 (O.S - 0.48 ~ 0.02). The zero mark on the Vernier indicates a reading made up from: 5cm~ plus 0.5mm plus 0.32mm (16 x 0.02) gi\"ing a grand total of SO.82mm. ,.. ~
The 49/50 Vernier system is now the most widel\' used because the 24/25 system has a smaller' Vernier scale that is quitc difficult to read. There a,;e, howeycr, a fair number of 24/ 23 sys tem instruments still in circulation. It;.,r'1ln be difficult at times to recognise if a Vernier graduation is actually align~~ or not. Bear in mind that In the Yernier system, there can only be one alig,4fuent at a time. Lines to dle immediate left and right of the most nearly coiPcident une will give the appearance of being equaUy stepped. That is a good gfiide.
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The Universal Vernier Caliper Gauge Description The Uni\"ersal Vernier Caliper Gauge incorporates both English and Merric scales. It is an adjustable p recision meas uring instrument capable of taking external and internal measurements co an accuracy o[ O.OO lin and O.02m m. It consists of a main beam, incorporating the main scales and a 6..xed jaw, and a sliding jaw tha t incorpora tes the Vernier scales. TI1e range o f measurement of the instrument would be the difference belween the length of the main scales and the length of their related Vermer scales . The measuring faces of the ja\vs are accurately machined to be straight and parallel. Target Po ints Fixed
L.ocki ng Screws i\[ain Scale - :'Irenic
Jaw
Main Scale - E nglish Fine Adjusmlem
I.
.1
I nside Sizes (add nib wid th) Figure 3.53 - Universal Ve rnier Caliper Ga ug e
T here is a fine ~\·\:.lStment screw o n the sliding Jaw and a to ck screw [0 lock the jaw on lD:.lm't: -main beam . The sliding jaw is connecled to a clamping dence (~~~'lrc fine adjustment clipper). The damp can be locked onto the beam \...i rh ' . -p,:-';'!!:, la ck screw and turning the knurled adjus tmen t screw will then produce a fine . ~\"\\'bi adjusrment to the jaw. Each jaw may have a target point, or small recess, that _ ~'\!'(\~ may be used for accura tely setting spring dividers. i\ ' nib' at the end of each L, jaw has an accurately m achined dimensio n. The o utside edge of each ' nib' is slightly rounded to permit the accurate measurement o f rounded bores. \\1hen ir is necessary to take an internal measurement, [he combined \,ridth o f the twO 'nibs' must be added to the reading. The value of the cornbined widcll of me tWO ' nibs' is nonnally marked o n the side af [he fixed jaw.
,. '
\\fhen you arc preparing to take measurements \\rith any precision ins trument you shauld ensure that d1e instrument and the work-piece arc both at dIe same unifom1 temperature, ideally 20°C (68°F). T Ins may require you to leave them fa r a while co stabilise at the workshop tempcramre. It is also essential th at the instrument and the parts to be measured are clean.
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Before us ing a Vernier caliper gauge, you should check it by closing the jaws and holding it up to the light to ensure tbat there is fu ll conraCl between the
meaSurement surfaces. 1I1cluding [he 'nibs'. Then check Lhat the lero line on the main scale is exactly aligned with the zero line on the Vernier scale. The p:lrt of th e instmmcll t bearing rhe Ve rnier scale is sometimes referred to a being the Clm;o r . Using the Vernier caliper requires some degree of skill ' feci' when taking measurements . Owr-tightening the inaccurate readings and will damage the instrument. The gend~· onto the surfaces of the item to be measured :l!1d made to tn· 10 alLer the measurement b\- force.
to ob tain the correct caliper will produce jaws should be closed no attempt should be
I f ~·ou wish to pre-set the caliper to a measurement, it should first be set to the approximate yalue and the clamp should be locked before making the final adjustment \\;th the fint: adjusunent screw. The jaw locking screw can then be tightened to presen·e the pre-set measurcmcm. \X'hen the instrument is nor in usc. it should be kept in its case and stored in a
\\"a1111 place. Excessi,e heat will cause expansion and accuracy will be affected. The instrument should neyer be stored wtth the jaws fully closed or locked in :my position. The instrument should be kept clean and lightly lubricated with acid-free machine o il.
Exercise in Taking an Interna1 Measurement For this exercise, 1 will use an English 49/50 scale anti ask you to find the intcrn~ dimension shown in the next illustration. Again, I will proyide you ,\,\wit!f'~lC solution immediately afterwards so you may wish to cover this up . . ,'\.::.0..<:' ... .
- \..\)\
\"1)\0
\.)" '=
"",'" .
(.0'\
I
I
I
r mernal i\ 1casurcmCl1t Figure 3.34 - Exe rcise in Reading a Ve rnier Scale 49 /50 E nglis h
<
§,
161 C(Jpyighl (" &uTj Colkge 2006 . 200- All tights rmlT.fd
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TOO L S
D id you get 1.435in? If you did not, did you remember to add me width of the nibs? 11le reading is made up of lin, plus 0.1 in, plus 0.035in, plus the width of
the nibs (O.3in). T here are versions of rhis instnunent that have a pair of ' knife edges' that are positioned opposite the jaws and are designed purely for taking internal measurements. 'nus feature rcmm-es the need to consider nib widths. lnsidc Sizes 7\! ai n Scale - Enghsh
• 1'. b in Scale - :-[ctric Fixed jaw
I :--:-:~, I
Lo cking Screw -
Aiusrablc J aw
<-;:'
Outside Sizes
Fig u re 3 .55 - Vernie r C a lipe r w ith Separate J aw s for Jn te rn all\'feas ure mem
Digital Electronic Vernier Caliper This instrument removes all the fuss of reading scales and judging the alignment of graduations. There are no scales to read. The iostmment bas an electronic digital direct readout that can be selected to give English o r Ivletric readings. It is def "W;f1ed for taking external and internal measurements. ')\')\)o - \. oC-
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(, ~,'\
Fig ure 3 .56 - D igital E lectronic Verni er Cali per
ElectrOCllc instruments are now very accurate and reliable. There is one small consideration, howeyer, that is m ore to do with the operntors rather than the instruments. For example, shop assistants now have digital tills that accurately tot up figures and calculate change; students now tend to llse pocket calculators as a substitute for what were oace considered simple arithmetical skills. TillS is great until something goes wrong. Do people still have the capacity to cope with mental arithmetic? Do tbey even feel they need to? 'nlCre
162 Cop),righl lO Barry College 2006·2007
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TOOL S
is, ho\\-e\'er, a more insidious problem. I f a wrong button is pressed, o r a couple of zeroes arc inad\-crremly added. would the operator be able 10 recognise a ludicrous result? That is a Glutionary notc fo r you to approxim:Hc that digital instrumenLs arc giying you results that you would expect. Add to this the em barrassment of tIring to fi nd someone who can read a Ve rnier scale, because there is no digital Ycrsion available, and the case for learning how to do it is made. Electronic technology then becomes a \-alued aid, not a masterl
Vernier Height Gauge Description The Vern ier I !eight G :l uge is a derivation of the ca liper gauge and di ffers from it in that rhe measurement is taken as the disl:tncc between the top surface of a mo\"abJe jaw and the surface on which the gauge stand~ . There is no Ii.'\ed jaw.
Fine
External
Internal
,.
Jaw
Fig ure 3.57 - Vernie r H eig ht Ga uge ~.
~ The gauge incorporates a heavy base with an accurately machined lowq surface that is parallel with the measuring face on tOP of the mo>able jaw. .¥ detachable scriber can be fined [0 the movable jaw allowing its usc fof marking-our. TI1C main scale of the gauge does not start at zero because th? surface on which Lhe gauge sits is actually the zero po sition in thi;; configuration. For this reason, me main scalc starts at a value that reflects d~ minimum height of the measuring face over dlC surface, i.e. 1in . f-"::' ~
'"
Measurem ents arc taken from the top surface of the movable ):tw ssf'it is necessary to fit the scriber ontO thc top s·urface when taking ,
163 COPJ·right rO Bt1I?) Colk§ 2006
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movable jaw [0 enable access to an internal ctimension. In this case, the scriber is mmed upside down (sec inset). \\' hen dus is done, the thickncss of the measuring jaw must be subtracted from dlC reading.
Vernier D epth Gauge Description Thc Vernier Depth Gauge is another derivation of the Vernier caliper gauge. indus case, it consists o f a beam, incorporating the main scale, which carries a sliding measuring jaw that incorporates the Vcnuer scale. The jaw is positio ned over the hole or depth to be measured and the beam is lowered until it reaches the poinr where [he meas uremenr has to be taken. The gauge is then read in the same manner as rhe caliper gauge. There flrc several differenr configurations of depth gauge of which the Vernier gauge is bue one. The more common types are illustrated for you.
i'\fiCl'ometcr
Vernier
Figure 3.58 - De pth Gauges
Ve rni er Bevel Protractor D escriptio n The Vernier Bevd Protractor has a solid stock or base that has an underside that is machined flat to sit on a truc flat surface. The protractor head is designed to accept an adjustable srraight edge that can be adjusted to any angle to the stock. The srock c.'l.n be rotated around the protractor head and can be locked in any position. The angles fomled between the straight edge and the stock can then be read from the position of the zero index mark on the Vernier scale relative to the main scale.
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Figure 3.59 • Unive rsa l Bevel P rOIr:lClOr
i\!on:mctll o f the straight edge rotates a disc on the head that inco rporates a main scale chat is graduated in degrees. A Vernier scale on thc head permits a reading in minutes of a degree. The Vernier Protractor Scale \Vc nced
1'0 examine how the Vernier scale works on the be\·el protractor. The scale is dc~igned so that 23° on the main scale arc equivalent to 12 eq ual di\'isions on rhe ycrnier scale. Note, that the Vernicr sen lc is showing (wo scts of 12 divisions, one each side of dIe zero mark , where each third division IS numbered consecutively 15, 30, -1-5 and 60. Therefore, one division is 5 mInutes.
Vernier Scale 23
Scale D i" isions = 12 Vernier Scale Di\'isions 23 x 1°:::: :n°
~ l ain
Difference in Length nc(',.n:e [~ D i\'isiun~ IS: 2° (t>. lain Scale) . 10 55' (Vernier Scale) :::: 0° 5'
,,.
'"'" 0'
Fig ure 3.60 . Ve rnier Pro traclOr Sc a le
'0 ,i;!
'"
Eneh Vernier scale didsion is 23° divided by 12 which equals 1 0 55 ~1in u t es. The diffcl"I..:nce between two m.,un scale dinsions and a Vernier scale a"i\'ision is
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2° minus 1° and 55 minutes \vhich gi\"CS 5 minutes. This an accuracy of 5 minutes (one twelfth of a degree).
gi\-CS
the instrument
The main scale I have shown you displays a cemral 0 mark w1th the gmduarions in degrees running away on each side to rcach nine£}' degrees at each end. That com-cniently shows you how the twin Vernier scales match with 23° on each side. Alternatively, the main scale may display a central 90° mark with the graduations running away on each side ro reach zero at each end. This latter sel up is more common and T will use it in rhe neXl illustration ro describe how the scale is read.
Accuracy of 51>.linutcs Angul:i r·1\ [easurcment
20'-=:::~
55° 20' Fig ure 3.61 - Reading the Ve rn ier PrOltaClOr
.
\ ....\.
To rcad an angular measurement, YOll must first read the number of whole degrees on the main scale that have been passed by the Vernier zero mark. You [hen continue to read in the same direcaofl and locate the graduation on the Vernier scal~ rh)fiCh1igns with a main scale line and then add that number of mj~l~.:tl! rt'~ay have occurred to yOll by now that you can read the protractor (i ~l lem;er direction, obtaining an acute angle reading or an obtuse angle reading ~): . 1y. ~ respecove
.(\~
", ..:. ,,~
Taking the example shown in the illusrration and reading clod,-wise, the zero mark on the Vernier has passed through 90° and has rtavel.led anorher 3-1-° clock-wise, making a tOlal of 124°. We now look clocl..,,\;se along rhe Vernier scale from the zero and find that the eighth graduation lines up. This represents 40 minutes on the Vernier scale (8 x 5). The reading is thus 12-1-° -1-0 nunutes. Now read anti-clockwise. The zero mark on the Vernier has swept past 55°. Look anti-clockwise along the Vernier scale [rom the zero and we see that the fourth graduation is aligned representing 20 minutes (4 x 5). The reading is thus 55° and 20 minutes.
Before you use a Vernier Bevel Protractor )'OU should check it by setting the srraight edge at an angle o f 90° to the stock using a master square. The zero
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mark on the Vernier should be regtstering 90 0 on the protractor scale. 'I'ou can correct any error by adjusting the position of the Vernier scale.
Micrometer Calipers T he operatlon of the micrometer caliper is based on the pilch of a screw th read and it \Vas sometirnes referred to as the 'screw caliper' for that reason. Thcre arc two types, the external or 'outside' micromcter and the internal or 'inside' micrometer.
External Micrometers D esc ription The external micrometer consists of a frame, anviL barrel, slcc"c and a spindle. You ,,·ill come across a differcnt renninology in some books. mostly ;\merican, where the slceyc is referred to as being the 'thimble' and the barrel is refe rred 1"0 as being the 'sleeH". To sa,e con fusion, we arc going to Slick to the fust interpretation throughout this section. The measuring faccs arc on tbe ends of the anvil and tbe spindle. The barrel is engrayed with a graduated sca le that rep resents the measuring range of the instnlment, which is normally 1in or 25mm.Thc barrel has an accur.ue right hand internal thread tha t has a pitch of 0.025in (40 TPJ) on the English n~rs io n and 0.5mm on the I\[ctric version. The spindle has a matching thread and the barrel is screwed 0010 it. The sp indle incorpoffllcs a slce\-c that surrounds the barrel when th e two arc assemb led. The sleeve has a bevelled portion at the barrel end that is graduated into equal divisions and a knurled portion at the orner end to pcrmi l cas)" manual rowtlo!]. 1,1'
.;-.."\'
WII' -'"
_ :-.\.:.~":' ;.~\ 111C spindle incorporales a spring-loaded ratchet mechanism at the OUler end ~ ~(" l P ' that slips under a prcset load thus producing a standard pressure or 'feci' on ... \..,:-. Ihe measuring faces when they are in contact wiIh an item to be measured .
-
,........,\'r-
(::'''
;\(a.in
= Fr:J.mc
Scale Barrel
Cfhimble)
Stop
Adjusting Nut (Spindle Thread \'\'car)
Figu re 3.62 - E"lcrnal Mic iorn c,cr
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111C faces at [he rips of the am·il and spindle are ha rdened, or tipped with tungsten carbide, to resist wcar. The spindle can be lockcd in any position by a knurled locknut One complete revolution of dle slee"e \ \-ill alter rhe gap berwccn the faces of rhe anvil and the spindle by a distance equal to the pitch of the thread.
The English External Micrometer The main scale on the barrel of the micrometer is graduated along its axis with a scale marked in tenths of an inch, each tenth being further di\'ided into fortieths of an inch. One complete rotation o f the sleeve will move its bevelled edge oyer one division (1/ -Win) on the barrel scale. The beveUed end o f dle sleeve is divided up into 25 equal dh'isions, each fifth di\'ision being marked i.e. 5, 10, 15, 20 and 25. \Vhen the sleeve is rotated through one sleeve division, it will h:I\'e curned 1/ 25 of a nlrn. This in effect is 1/ 25 of 1/40in (0.025io), which is one thousandth of an inch (O.OOlin). The accuracy of the instrument is thus O.OO lin. Barrel Scale Divided into 1 in (0.1") and 1 in 10 40
Thimble Di\'idcd into 25 (0.001'J
\0
OS' + (2 x 0.025") + 0.Ou9" ::: OS' + 0.05" + 0.009"
::: 0.559" on a 0"
to
1" i\Ecrometer
or \.559" on a 1" to 2" r.licrometcc crc Figure 3.63 - Reading the English Micrometer
T he illuslrarioo shows that the beycllcd edge of the sleeve is positioned on the main scale of the barrel so that we can re:!d 5/ 10;0 (0.5) plus 2/-iOio (0.05). 111;S adds up to 0.55in. If we now look at the scale 00 the sleeve, we see that the ointh graduation mark is aligned with the barrel scale datum line, which i.ndicares 0.009in. This gives us a grand total of 0.559io as a reading foc this example. If there is no precise alignment of a sleeve graduation mark with the barrel scale line then me procedure is to read to the mark mat is nearest 1O iL English micrometers nonnally hayc a one-inch measurement range so the example we have looked at must be for a dimension that is less dum one inch. Jn facl, it was, and dle micrometer we used is known as a 0 - lin micrometer. LoC I us imagine that the item we had measured was bigger than o ne inch but
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less than two inches. In this case we could not tit It illlo a 0 - lin micrometer. we would need a I - 2in micrometer. The measuremen t range is still one inch so we would need to add on an inch . Therefote, the reading in our example applicable 10 a 1 - 2in micrometer would be 1.559in. T he range of a micrometer is displayed on rhe frame i.e. 2 - 3in.
The English Vernier Micrometer This is a marc accurate \-ersion, and more e'Xpensivc! This type has the addition of a Vernier scale conslsnng of ten divisions engraved circumferentialh' on the barrel adjacent to the bcn~lled edge o r the slcc\·e. These len di';sions in total equate to nine di,-isions on the slee"e. So, the difference between a slee",e dinsion and a Vernier diyision is 0.00 lin - 0.0009in = O.OOO lin, which is one ten thousandth of an inch.
==ot," DA "
+ (2
X
0,023') + 0.U17/1 + 0.0006"
+
0.03"
+ O.()!-" + 0,0006"
= 0.4676" Figure 3.64 - Reading the English Vernier Micro meler
The reading shown in lhe illusrranon IS read as foll ows: The bevelled edge of the sleevc has re,'ealed O.-l-in, plus 2/-tOths of an inch (0.03in). The sleeve scale shows that the se"cnteenth mark (0.017in) is nearly aligned. This gives us a tOlal so far of OA67in. \,\'c now look at the \' ernier scale and notc that the sixth graduation aligns with a line on dIe slec'·e (0.OOO6in). Our grand (ami is thereforc OA676in. :;
"".'.,
S-
The Metric External Micrometer The barrel scale on the metric Yasion is graduated in one-millimetre divisio.~ that are subdiyided into half millimctrcs . 'lllc bcvelled edge of the slecv~-s d ivided into 50 graduations, each division being I /SO· h of 0.5mm, whiGH is O.O lmm. The accuracy of this instrument is thus O.Olmm. 6v
"'"
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Barrel scale divided mto mm and h:ilf mm. Each 5111m is numbered
l~mm
ThImble dividcd IntO 50 (O.OI111I11)
+ O.5mm + (34 x U.Olmm)
= 18mm ..... O.5mm +
=
O.34mm
18J~4111m
Figure 3.65 - Reading the Metric Micrometer
The be\-eitcd edge of rhe slee\-e has uncovered a reading that com isIS of: 18mm plus O.5mm giving us 18.5mll1 so far. An examinacion of the sctlle on the s\cc\-e reveals that the 34th dinsion is aligned \\;th the datum line o f the barrel scale giving 0.3<1mm (34 x 0.0 I mm)_ This g1\-es us a grand lotal of 18.8-1mm fo r this reading. Obliqu e Graduati ons \'\11en reading a micrometer, rou may have trouble reading the numbers on the barrel scale when they arc partially obscured by rhe edge of the sleevc_There mtly also be difflQufty is in deciding whether the bettci edge is really jusI past a line ~~l~S't-~Bilow it, a probl_em often exacerbated by the ~mall shadow cast by [!.li! l,)j:C\-cl edge. Some mlcromctcrs arc produced \\,th thc barrel scale W,1:grnduarions slanted_ These may be shown in addition to the normal strnight lines, which may be used to indicate half ru\;sions.
Figure 3_66 - Obliq ue Graduations
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Figure 3.66 shows a metric micrometer with oblique graduations. You could t ry your band at reading ir. I will give you the solution in the follO\ving paragraph so that rou can cover it up if you so wish . D id you geL 12.65mm? You have 12mm, plus 0.5mm, plus 0.15m01 (15 x O.Olmm). You also had a chance to see if it was easier to read oblique graduations .
Large External Micrometers \'(Ie haye already discussed the range of micrometers. T he English micromeLer has a measuremcnt scale one inch long whilst the Merric yersion has a scale 25mm long. \,\l1cn we need to measure larger sizes, we haye to have micrometers to aCCoOU1lOdate them. The example T have illustrated is a 25 50mm micrometer. You ha\"c to add 25nm1 to the reacting when you use it. T he range is displayed on the frame. English standard micromcters arc supplied in eight different sizes from 0 - lin to 7 - Sin br one inch steps. i\fctric micrometers are supplied in S1., differenr sizes from 0 - 25mm LO 125 - 150mm by 25mm steps. Wfhen the micrometer is set to its uro reading, the distance between the measuring faces is equallo the low reading li.m.it for the instrument. The faces of the anvil and the spindle cannot contact each other on the larger range micrometer so you need to use a test gauge to check the accuracy of the instrument. Note that in the example shown, a 25mm test gauge has been clamped between the face s. This should coincide with a zero reading o n the barrel scale and the alignment o f the zero mark on the slee,e scale with the barrel scale.
Test
i\ ficrometer has 25ml11 range
range
~_c::~~
Figure 3.67 - Example of Large External Ivlicromeler
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Special Us e Mi crome te rs It is sometimes ncccssary to depart from the standard micromcter design to
acruc\'c certai n tasks. Some micrometers havc ,"ery deep frames to accommodate sheet materials. Some micrometers have specially adap ted anvils and spindles to enable the instrument to measure screw thread diameters or mbe thickness.
\nvil and Spindle Adapted for Sere,," Thread ~Ieasuremcnt
~<{]
~9=J Deep Frame for Sheet and Plate Material
Am"il r\ciap[(::d for Tube ThIcknes s :\lc;lsuremenr
Fig ure 3.68 - Mic ro melers Adapted fo r Special Uses
Other special types are anilablc. :\ I'.uiablc r:mge micrometer has a large frame with a sec of detachable am'ils designed to produce different ranges of measurcmem. The shortest anvil is uscd for the l:trgest measurement and the lo ngest anvil for; rh'C smalles t measurement.
. ;.('j\.)6 - .\ lr.kt~s micromete.r has an extra long, thin anvil to make it possible
10
.<' ... \):-.'"atcess recessed dimensions. A ball end micrometer is used to mcasure curred .\.j).\'I.. surfaces.
"
l -'\';
T here is a micrometer deptlJ g :wge and this was illustrated this for you earlier on, alongside the Vernier deptb gauge. 111e measurement scales on rhe barrel and slee,e on a micrometer dcpth gauge run in the opposite direcu"on to the normal micrometer head because they are recording the protrusion of the rod from the base. A caliper micrometer is very similar to the Vernier caliper except that the jaws are paf[ of a micrometer head. l\l.icrometcrs arc vcry versatile and you will find adaptations in all sorts of specialised cigs and jigs.
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M.icrometer Heads Des cription
1\ micrometer head consists of a barrel, sleeve and spindle. Heads arc used in conjunction with extension rods or pieces co give a choice of measurement r:lnges for tasks such as tool setting, gauging and fo r t:lking internal measurements. Some heads may be equipped with \-ernier SC:l1es to give an accuracy of measurement of O.OOOlin. Some heads are equipped with large diameter sJee,es that have 250 dinsians that pernur a more call\·cnient direct reading accuracy of O.OOOlin. The addi tion of n Vermer scale to this type of hcad wauld then give an accuracy of 0.00001 in.
The Internal Micrometer Descriprion
In tern:!.1 nucra metcrs arc specifically desIgned far taki ng internal mcasurements. There nrc tWO rypes, the l"\\·o-point and the thn::e-point. Both types have exacdy the same graduations as the external types except that the measurement scale length is o n.ly 0.5in. Two-Poi nt Intenlal Micro m eter T his type is o ften referred ("0 as the end measuring or 's tick' type micromete r. It is supplied as a kit that consists o f a micrometer head and n set of di fferent length extension rods supplied in onc-inch seeps from 2in up to 12io that fit into the barrel of the head to gi.e different mensurement ranges. The I' fcrnc \-crsion has a combination of hC:ld aod extension rods that giye measurement rnn~tJ/ro m sOmrn up 200mm and the English yersion has a range from 2in to 1\.v:t Oi rr: A h:lndle can be screwed into the side of rhe head 10 allow the (.l1\\C~e - micrometer to be inserted ioro deep bores. The two measuremcnt :!.nvil faces \V~f' are con vex and are located at the end of the slccn' and nt the cnd o f the ;:-,...... 'extension rod. These arc sometimes adjustable to compensate fo r wear. ",;,\'-
U'"
ExtenSIon Rod
Fig ure 3.69 _ T wo-Po int Inte rnal Mic romeler
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Reading a Metric Two-Po int Internal Micrometer
I have chosen the j\[etric type JUSt as an example. The same principles will hold fo r the Englis h type. The measurement consists of reading the micrometer head and then adding the length of the extension rod and the distance collar to that reading.
14mm + 0.5111111 + (30 x O.U I111 m) = J 4.80111m + length of e Xle n~inn rod and c o llar Figure 3.70· Reading a Two-Point lutcmal Micromcu: r (Metric)
Tn the example I ha\"e given you, the reading on the head is: l.J.mm plus O.Smm pillS O.3mm (30 x 0.01mm) giring a total of H .Smm. \,'e now have to add the length o f the extension rod and the distance collar to tbat reading.
Using the Two-Point Inte rn al Micrometer \"xThcn assembling the internal micrometer, it is essential that the eonracting faces between the parts are clean. The pan s are assembled by pressing them together with a scre\\~ng action that pushes any oil or dirt particles into radial grooyes on the faces. Alignmem marks on the head and the collar should line up. The extension rod is secured in the head with a setscrew . .After assembly, the instrument should be sel to zero and then checked for accurney with an ex ternal microqmwT. A handle can be screwed into the side of the barrel to facilimte tha)fucasurement of deep holes. _ \\ .S,!,<:-
~
.L0" .... · \.,;..1'0
,b
......-l:::'
""
C'
r
""",,
.
3-
Figure 3.71- Using the Internal Micrometer
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The instrument must be held so that its longitudinal axis is set at a right angle to the faces to be measured. Care must be taken not to jam the instrument In bores because the rncrtsurement faces and the waUs of the bore can be easily damaged. Th ree-Point Inte rnal Mi c ro m ete r This instrument has three measuremem am-ils mounted co-axially in the micrometer head at 120 0 to each O!her. The inner ends of the am-i!s are angled and threaded to engage under spring loading \\~th a conical measuring thread at tbe end of the spind le. Tlus arrangement gives a greater :lCcuracy when measuring internal dimensions . .r\ larger diameter micrometer head is normaUy used tha r is equipped with 250 didsiolls on the sleeyc to giye a direct reading accuracy of 0.0001io . There are \'ariacions such as 50 diVision sleeves together with a \ renue[ and 100 di\'ision slee\-es that gi\-C an accuracy ofO.00025in.
o~WJ~
Setting Rl1lg
Figurc 3.72 - Three-Point Internal Microm eters
E lectronic Digi tal Microme te rs Electronic Digital micrometers hale a digital direct reading display. -111C display automacicalh' sets to zero when the instrument is closed and switched on. At the push a switch, the instrument also has the ability to reset tbe z~ against a reference standard if so desired . ! n addi tion to its normal measuring fu nction, tlus added facilit), makes the instrument self-calibmting and allo\V~ you to take comparative readings, where di fferences from a re ferencedimension are displayed as pius or minus y:tr1ations. -.
of
~
Fig ure 3.73 - Electronic Digital Micromelcr
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Using the Micrometer Temperature Conditions \'\then micrometers arc being used for \-ery accurate work, the instrument and the item bcing measured should be at the Standard Reference Temperamre of 20°C (68°F'). Fine measurements should not be made in direct sunlight or in conditions where the temperature is fluctuating or near heat sources such as radiators etc. Handling The spring-loaded rntchet at the end of the sleeve should always be used when raking measurements to ensure a constant pre-set force on thc measurement faces. The sleeve should be rotated at a constant speed when screwing the instrument into contact. Irregular speeds can create false readings. The instrumem should never be forced O\'er an item or screwed down hard. This will produce false readings and "vill impose strain on the screw. Ir is surprising how ofren you see trainees doing this in an attempt to prove to someone that a job is in limits, pointless! "nlC insrrumenr should ne,et be pulled across faces; it produces scoring and wears the measurement faces. \"X'hen the instrument is not is use it should be placed in its case and srored in a wann place. Excessive heat will cause expansion that affects accuracy. Expansion can also create stress so instruments should ne\'er be stored fully dosed or locked in any position. Adjustments The zero setting of 0- 1in and 0-2Smm micrometers is checked by dosing the spindle face 0010 w-e anvil using the ratcher mechanism. The zero mark on the sleeve shgw,4 th1M align with the axial datum line on (he barrel with no pan of the p.attCr scale sbowing. When checking the larger range of micrometers. c·, ~ \,)
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Depth micrometers are checked and set against a surface plate. The base is held firmly In contact with the plate and the sleeve is rotated until the end of [he rod contacts the plate . The gauge should then be reading zero. The zero setting mar be adjusted by rotating the barreL
T apered Thread
Spindle Thread \'<'car Adjusmlenr
Zero Setting .·\d justmem Figure 3.74· Adjusnnent of OlliSide Micrometers
M icrometer Scre\"\' Ad justment
,
\'(fear on the screw thread produces end float that affects the accurnc\" o f th e instmmcnt. There is an adjusting nut in the outer end of the barrel tha t is screwed on to an external split tapered tmead. As the adj usting nut is tigh tened clochvise on to the tapered thread, the difl.meter of the barrel scre\\' is reduced, 'lC)I..~losm~ it onto the worn spindle screw. The adjustment should be continued . ,)\\c't-e:. - until the end float is rcmO\·ed. One proviso is that the 'feel' of the micrometer \. . ~(<' l, must be maintained throughout the traverse of rhe screw.
,\.\.:t:i'- \' -
(p,\! o
Dead Weight Tester
~
This might seem a strange instrument to include here rather than in the sccti0R on calibration but it is a precision instrument. Though it can measure pressureby allowing an unknown pressure to act on a known area, it is normally onl[ used to calibrate pressure gauges and pres sure sensors . The tester has a vergsimple arrangement. It has a fluid reservoir that contains a priming pump . T ttf. fluid passes from the reservoir through a prim1ng Isolation valve to a cylin# r that contains a piStOn that has a platfoon for plaCing selected weights (ma :$~s) on . The cylinder is connected through to a pressure gauge connectiorv.}har attaches to the gauge or sensor under test. The system also incorporates a:;-~aU auxiliary screw adjustable piston dlat is used for making fine adjustme~1J to the fluid pressure when setting up the instrument. ~
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\\eight Gauge Cnder Cllihr:ltion
I
Pnm1ll~
Pump
lsolation \'"h-e Screw t\djuster Figure 3.75· Dead Weigh l TeSler
Thc tCSt proccdure is to first open the priming isolation \,ah·c and then pressurise the system with the priming pump. \Thcn this has been done, the priming isolation \,ah'e is then closed. The screw adjustable pi:non is thcn
sc rewed in
lInlil
lhe pressure in rhe system is raised
jUst
sufficiently to lift the
main piston off its stops. \\'ihen a selected weight is placed on the piston plntfonn, thc combined force of the test weight and pis ron assembly creates a given pressure in the fluid in rhe system. The fluid pressure creared is the force cxe rl ed on the nuid by the piston didded by the area of the piston.
Porce = Pressure
,
X
Area
Therefore: Prcssure =
Forcl.: Area
-
I\~'~)
"(hC'-'\)'il3ication on the gauge under rest can then be compared with the « . .: \V calculated pressure created !n the sys tem. The accuracy of the rester depends ,\~'\ . on the dimensional accuracy of the piston 2nd the cylinder and the low friction ..: ~...'\.... existing betwccn thcm. This is a precision instrument so the calibration accuracy is high.
Non-Adjustable Gauges \'{' hen items ha\'c fL'(ed dimensions and ha\'e to be checked, either in large numbers or on a regular basis, It is somewha[ laborious [0 do this with adjustablc measuring instmmems. There is also a high probabilirr that different opcrators will obtain slightly different readings. Fixed size ~llges rcmovc the ch:lI)Ce of individual crror and give repeatable accuracy.
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Lim it Gauges These gauges arc more commonlr referred to as being GO, NOT GO gauges. and are designed to vcrify that componems arc \\;thin toleran ce. They are usually double ended where one end is set at the dimension upper limit and the other at the dimension lower limit. TIle GO end should fit over or into the p~lr[ being checked whilst the NOT GO end should not. Limit gauges take their description from their usc. There arc plate. gap. plug. caliper and ring gauges.
DGO NotGOD Plug Gauge
Go
O!) lON
~
N~
o
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~
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Gap Gauge
Plate Gauge
Fig ure 3.16· Lirn it Ga uges
Sc rew T hread Pitc h Ga uges
-.o,j,F
. ;.\\\'These gauges are used (0 ascertain the number of threads per inch of a l",\\L-;'-'; particular screw thread. The gauge is nonnaUy supplied as a set of folding \\~~('" profiled blades cut to match specific thread profiles. The gauge will relate to a thread s,·stcm such as J3SF. C;-';F. and !\lctric and each blade will be marked with the threads per inch.
Fig ure 3.11- Screw T hread Pilc h Ga uges
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Radius Ga uges These gauges are llsed to check the profiles of small radii in terms of size, accuracy and form. The gauge is supplied as twO sets of folding blades, each manufacrured to a specific radius. One sel of blades has conca\7e profiles to check outside radii whilst the other set has convex profiles to check inside radii.
Slip Gauges These are o ftcn referred to as being 'Johansson Blocks'. They arc often used for checking the accuracy of other gauges and as gap gauges. They arc blocks of hardened and highly polished steel that have been ground to an :lccuracy of just a few millionths of an inch in their length, breadth, thickness, tlatn css and parallelism. T he blocks are supplied as a boxed set of 81 pieces that can be assembled in combinations to produce any gauge dimension from 0.05in to l Oin in steps o r O.DOOlin.
~ t:= rl-
F
..
,
-.1 I -
•. -
"'"
".'1/
TI
'"
Figure 3.78 - Slip Gauges
The flatness and surface finish of dle blocks is to such a high smndard that atmospheric pressure will hold them together so tighdy ther cannot be pulled apart by hand. \'{' hen they arc assembled in this condition, the), are described as being 'wrung' together and they act like a solid block of steel. Before joining the blocks together, [hey should be wiped with dry ch am ois le:uher because even the finest layer of dust will stop them being 'wrung'. \'{/hen assembling a gauge lenglh with the blocks they should be joined by sliding o ne surfa ce over the other to exclude air being trapped between them. \\nen separating the blocks, you should always slide them apa n and never try to pull them apan. The sliding action used to join and separate the blocks can cause wear over time and they will require periodic checking against calibration blocks. Because the blocks are used in such high precision \vark, it is important that they and the work being checked are at the Standard Reference T emperarure of 20°C (68°F) to ensure dimensional accuracy.
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\Ve ha~;e r~ached the end of the section on precisIOn measuring equipment. There 1S gune a lot to remcmber and rou need to look for bits of information that y.ou feel would make a good multi-choice answer qucstion. The principle here !s that if you think it makes a good question, some 'quizmaster' will prabablr have thought the same. After all, ther continually 113\"C to find guesuons to present to you.
Lubrication Equipment & Methods Introduction I t is an inescapable fac t that if surfaces are permmed to man: in direct dry contact with each other the, will frct and wear as the surface layers ar~ grnduaily ground away. \V'ear 'rates will accelerate as accumulations of ground off particles stan to act as an abrasive medium. The ratc at \vhich this occurs will depend largely on the hardness of the surface materials and Lhe mechanical pressures they experience when in contact. I f the surfaces are kept apart by a film of liquid, then the layers of molecules 10 the liquid will shear as the surfaces move and no wear can occur. Ideally, each surface would have a fllin of lubricant that is tenaciously attracted to it, opposing liqlud films could then slide over each other. This is called 'film' lubrication and it is the most desirable condition. A problem occurs when surfaces are srationan' and remain in contact under load; the lub rication films may t.hen be squeezed out. \\'hen these surfaces start to moye, there will be a brief per.od \\.there they have random areas of dry contact before they can pick up sufficient lubricant to form a complete fUm. This is called 'boundary' lubrication and it is nor a desirable condition . I30undary lubrication may also occur for other reasons. High temperatures can rcduc ~_ the viscosity of most fluids and the films may start to break up. ""a\Sis2t)li1t)" is a measure of a fluid's resistance to flow. Very low temperatures . CO\\c.'t-i; - tend to increase \"lSCOSlt)' and may hinder the pIck up of lubricant so that a fIlm ~ \'I~5~': cannot fully fonn or may e\-en freeze. Excessive mechanical pressures may . ,,\""\.\.(,, break through the filins and create dry surface contact. \Vhatever fluid we . L"'Y.....,~\.,choose to usc as a lubricam, someone needs to have considered how it will creact to extremes of temperature and mechanical pressure,
c· w
Another consideration is the effect that the chosen fluid will have on surfaces it is supposed to prmect. Some types of bearing surface, for examp~ · Teflon coated bearings, must not be lubricated because the lubricant woulif deteriorate the coating and reduce rhe bearing life. Some fluids gradually turn~ acidic or mar oxidise \\o;th prolonged exposure to high temperatures and and they ma): arrack the surfaces they contact. Tf we now add the problems tHi t could ensue from introducing outside contaminants to dlese lubricants, ,£fe start to appreciate that correct lubrication is a critical maintenance action,c-FOr c example, grit will penetrate most fluid fthns:- water will create emulsions:......a nd break the ftlms up; foreign lubricants may dilute essential properties, \X7lti t we have been discussing here highlights the impOrtance of always ~ng the authorised lubricants and treating them with care. ~
at
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Lubricating Oil There arc three main sources for oil: \-cgctable, mincrnl and synthetic. Vegetable oils are no longer used in aircraft maintenance. ~fincral oils are fossil based and arc refined from crude oil. Synthetic oils are put together from products from \-arious sources such as fany acids. minerals and cellulose substances. O ils oftcn have additi\'cs to give them parucuhlr properties. These additives include detergents, extreme pressure additives, ami-oxidants, pour-point depressams and anti-corrosi\'e additiycs. Viscosity is an important property in oils and this should be appropriate for the particular task. Volatility is another considcrncion and the flas h point must not prese!ll a hazard in the intended use of rhc oil.
Grease G rease is a petroleum-based oil that has been thickened with gelling agents. The gelling agents are metallic soaps and ther have an affect on the properties and usc of the grease. Sodium or Lithium will raise the melting point of the grease. These agenrs wou ld be used for temperamre resistant greases such as those used in wheel bearings. The addition of graphite to sodium based grease produces a temperature resistant, anti-seize grease. Typical uses for this would be on plain bC:lrings subjccted to hcavy intermittent loads and 0 11 sp:lrk plug screw threads . Aluminium gelling agent produccs gre:lse that h:lS excellent :ldhesivc qU:llities. Calcium produces water-resisram properties. Other additi\·cs are used to modi~- the grease. i\folybdenum disulphide is used to produce low friction synthetic grease used on heavily loaded sliding surfaces. Silicone is added to produce a metal to rubber 111btic ~nt. Other additives produce greases for low rcmpera.!,Ure applications.
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c,;ti:la~~"is often used instead of oil in certain situations. It is more effective in h: ~:,;q6cacions where leakage is diffi cult to preveo t. It may also be used where lhere ,{\~''''' :lre lengthr periods between scheduled maintenance actions. Grease does not \,. ~~\...... . require complicated conrainment systems and lasts longer than oil; :l prepacked bearing will often contain enough lubricant to last the service life.
'fypical locations where grease would be used include: wheel bearings, flying control hinges, engine and flying control runs, universal joints, trunnions, splines, landing gear hinge pointS and on the screw rhrC:lds of reciprocating engine spark plugs.
Lubrication Requirements Lubricants arc designed to lubricate, reduce friccion, reduce wear and protect bearing surfaces from corrosion and, when required, cool thcm_ The lubrication re(luiremcnrs for an aircraft are decided upon at its design srage. Each component requiring lubrication is identified and a suitable lubricant is
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chosen together with a method fo r applying It. Tltis infonnation is rhen published in the approyed ancraft maintenance manual in the [om1 of lubrication charts and the specification nwnbers of tb e approved lubricants and their alternam"Cs . lVlimic symbols are frcguently used on lubrication charts [0 indicate the required application method, the lnbricant and the eguipment to be used, The Charts normally incorporate legends to show wh>lt each symbol means. T he airc r aft nJainlen ancc rnarw al is the overriding authority for all the materials and procedures llsed on installed aircraft equipment. Application Method
bU IB ;'I SpC';limo(n_~4A~ _~._I_--"""oc-,
Lubric:J.nl
F -
Oil Can
G~t
~~ 13\' Hand
• ~ tf
B~rS t 23
FiJI to Spccified Le\'cl
IA~Tsh;1
0
Lubricate un Assembly On ly Lubrication Unnecessan'
[f]
Pre- Packed
(
Sparingly
*
+
Not to be Lubricated Smother to Pren'm Corrosion
Figure 3.79 - Lubrication Symbols
\'I/e ha,e already discllssed some of the factors thai affect the manufacturer's choice of lubricant. Aircraft engines, fo r example, have recirculatory oil systems where oil is deli\'ered to moving parts under pressure and is then drawn back to a storage tank to begin its journey again. Engines get hot and the friction between the moving parts adds beat. Oils that are used in engiris need to have a high viscosity index, The higher the index, the less the vlscos ~ changes witlun a given temperature range. Rather like the multi-grade oils u se-~ in cars. Reciprocating engines require high viscosity oil because of the exrrem ~ mechanical pressures experienced in their plain bearings , Turbine engines, olj; the other hand, require low dscosity oils because ther have lighdy loaded, fa~ rotating, low friction ball and roller bealings. ,.:: <.
0-
Engine oil replenishmem requires particular care. The application of~he Incorrect oil specification is a ,"cry expensive maintenance b urde~~\.-Fo r example, reciprocating engine oils and turbine enhne oils are N OT comp;itible in any way. Type 1 and Type 2 turbine oils arc NOT compatible in;-.,£ny \vay. T he oils approved for use in giwn engine types are limited and g reat care needs to be taken to use tbe right oil. Replenish±nent points are marked with
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the appro"ed oil type and the tank capacity. ' nle air craff m ;/imemm ce m :mu."J is the o"erriding authority for oils to be used in installed engines and
the replenishmem procedures. Aircraft flying conlrois and landing gears are exposed ro the extreme low temperatures found at alt:itude. Greases used in Ihese areas need to have antifreeze properties. Oils would be unsuited in areas where airflow and moisture would conspire to wash it away. Control joints ma), ha"e to be pre-packed \\;th grease and sealed where there is a risk of wind-blown din and moisture entering the lubricant. Landing gear wheel bearings. on rhe orher hand, would re(luire high temperature reSIstant greases. Other high temperature components may require the application of graphite grease that is temperature resistant. Landing gear retraction systems and legs ha\'e close tolerance bushes thaI require grease co be applied under very high pressure to achieve adequate penell:auon. Some components may be packed with grease during manufacture and ne,er require further lubrication. Oilier componellls may need periodic lubrication that may be based on either calendar time or cycles of operation. Maintenance actions such as aircraft washing and de-icing can dilute and purge exposed components, such liS fla p tracks and screw jacks, of oil and grease. In these cases, the procedure will normally call for a pOS( procedurnllubrication.
Methods of Lubrication Oil Oil lLsed for general lubrication purposes is nonnally applied to specific lubrication points by the use of an oil can. Some poinrs may have oil ways or small reservoirs')Wf,vicks [hat direct oil to bearing surfaces. Other points may require ~1~\\lli.re Et application of oil to the moving surfaces. Oil may also be a r~'pl i'OtJ"tnder pressure from a uni\'ersallubricacing gun . • ,,::.«{' ~,\ '-. Engine oil replenishment is nomlaUr conducted using a replenishment rig. This _"~'-~'\ draws oil from a sealed container and delivers it to the tank through a \.. replenishment coupling. Engine oil tanks have contents indicators and dipsticks. There arc rules specific to engine type about the replenishment procedures that arc designed 10 pre"cnt m"er sen"icing. The aircraff m :u'ntenance m :w uaJprocedure must be rollowed for all installed engines. Many oils are hygroscopic in thar they are capable of absorbing moisture [rom the surrounding air. Once saturated, ther can then produce free water. Some oils may oxidise in the presence of air. Oil cOOiainers must remain scaled when not in use and you should ne,"er use oil from a container that has been left open.
Grease Grease acts as a barrier to dirt and moisture as well as proteccing against corrosion. However, grease cannot carry heat away from a bearing. Greasing
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may be applied manually or, where specified. be applied by a grease gun through specially adapted grease nipples fined to machined grease ways that pass the grease under high pressure dircctJy to bearing surfaces. The t-. laimcmll1ce Manual specifics the amount of grease [Q be applied, usually this stat cs that It is suffi cient when new grease extrudes from the bearing. Some components, such as wheel bearings, are packed with grease prio r \0 assembly. Pre· packing t-. lany conLtol bearings and electrical motor beari ngs are pre-packed and scaled during ma nufacture. The principle is tJ,at there will be suffi cielll lubricant available to last the working life of the componen l. Care has to be taken nor to expose these componenrs to dc-greasing sol\-ents that could penetrate them and wash out the lub ricant. Oi li lc Bushes Oilile bushes are manufacrured fro:n sintered bronze that is impregnated with oil. 'rhey can be re-charged with oil by either immersion in oil or by capillary action from an oil-soaked felt pad. Grease should never be applied to these bushes.
Equipment O il Can T here are many types of oil can but most of them operate on a similar principle. The can forms a resen-oir for the oil that is drawn up into :l Stogie acting pump from where it is deli'-ered through a tlex.ible application spout. 1.UI(
Flexible
Pump ---,4:~ Ilanelle
Oil C",,,aio,,--I_
Figure 3.80 - Oil Call
U ni ve rs al Lubri cation G un
f:'/
~
OJ
The Uniye rsal Lub rication Gun can be used for lhe application of o~",o r soft grease. It consists of a barrel closed at one end by a screwed cap apd at the opposite end by a screwed on pump head. The pump head incorporates a
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IC\'er-operated piston. a spring-lmded non-rerum vah-e, and a pressure release \'ake. The guo is supplied \\;th a selection of different replaceable lubricating connectors and extensions designed to fir related nipple adaptors. Some of these are designed to hook ontO a grease rupple that is screwed into a bearing housing. Others arc of a push on type. A needle grease poim is a nipple that is fined flush with the surface in the housing of a small pi\'oted joint. TIlis lype requires a speciaJ poimed lubrication connec tor. ,>,,,,·-",-rurn \'aln: I lousing
~~
Cap
;-";:ozzlc .-\sscmbly
P""",,,c R clca~c Screw Pump! lead Pump Pi~!On llnuSlOg
~ ~:=====t!J][J!.1 Fig ure 3.8 1 - Universal Lu brica lioll Gun
The procedure for filling the gun with oil differs from that used for grease. To ful the gun with oil, the screwed end cap is remo\'cd and the piston follower is pulled back O Ul of the barre.!. TIle gun is then held verUcal \\~th the pump head lowennost and oil is poured in[Q the barrel to within 1.5in frOIll the tOp. The piston followe r ~~)fr en refitted and the cap 15 screwed back on. ,.,~~\6 .. T~·, fin4l1e gun \\~Ih grease, the pump head is rernO\'ed. Thc screw cap is . ~).'\.rc.mo"ed, the piston follower is pulled back to the rear of (he b:mc1 and the .\"\\ ., cap is refitted. The gun is then held verrical with the cap end lowcnnost and ,,~,t"'" lhe barrel is then filled with grease. Care has to be taken ro compact the grease \... into the barrel \\~thout trapping air. \,(!hen I..be barrel is full, the pump head is refined. Before the gun can be used, it h.'lS (o be cleared of air-locks and the nozzle needs to be primed ..\ir-locks can be released by loosening [he press ure release valve and operating the lever until the oil or grease is seen passing ou[ of the bleed hole in the screw. Air tends to spit out of t.he bleed so keep you eres protected against splash. The pressure release screw must be kept closed while YOli arc operating the gun bur it can be loosened if the gun becomes pressure locked ontO a nipple, which does occasionally happen. 'rhe gun should be dedicated to one specification of oil or grease. The gun should be dearly marked \\~th the specification. I f it becomes necessary to change the specification, [he gun must firsr be stripped and thoroughly degreased.
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Grease Guns There are several types o f pump action grease gun. All deliver a small volwne of grease per stroke (O.Scc) aL very high pressure (up LO 8,800psi) often with single-handed operation. Ba\"o net Fi rring
Pressure Spring
Filling r randle
High Pre%ure Piston HOLlsing
Piston
Screw
t O pcnl ung Le\'er
J
Air Release
Figw:
The pistol grip ty pe shown in Figure 3.82 consists of a barrel Incorporating a pump head at one cnd and a screwed cap at the other. The barrel contains a spring-l oaded piston that has a chain connected to a filling handle at thc rcar. .,;~hiims1 used to retract the piston prior to filling the barrel with gtease.
""To fill the gun, the air release screw is first released and the handle is unlocked
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and pulled back against spring pressure. The chain is then locked in a slot in the cap before removing the pump head. TIle gun may then be filled with greasc.
r,
\Vhen full, the pump head can be refitted, rhe filling handle chain can ~ released, and the handle locked back into itS bayonet fitting in the cap. ~ pump handle may then be operated until an air free flow of grease emits [writ the air release screw bleed. The air release screw is dosed during operation of the gun but may be loosened to relieve air locks Qr (Q release a nozzle that hal become pressure loc ked on a nipple. -i The straight type grease gun in the illustration is very similar in its Precautions
•
61
operntio ~'::::
"
Always use the lubricants and procedures authorised in th§i'vaircraft , maintenance manual.
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•
Ncver use equipment dedicated for onc type of lubricant for a difCcrclH typc wlthollt. first cleaning it thoroughly.
•
Do not usc lubriCflnts from unseflled containers.
•
Keep all replenishment equipment and lubrication poims clean.
•
Do not applr more lubricant than is required, it is a debris trap.
•
,h'oid prolonged skin contact with lubricants because most arc toxic.
Operation, Function & Use of Electtical General Test Equipment Introduction The maintenance of aircraft electrical equipment and the associated rcctificatio n of electrical faulLs will frequcntly involve thc measurement of one or more \'ariablc electrical 'luantities. The four main variable cluantitics arc: voltage (volts V), current (amperes A), rCSlstance (ohms 0 ) and power (watts \'\0 . In /\C sys tems, the combined effect of rcsistance. inductive reactance and capaciti,'c rcactance makcs up the opposition to currem flow. This opposition is called 'impccbncc' and it is measured in ohms. In order to obtain a value tor each of these electrical quantities, we need a mechanical instrument that is capable of detecting a reliable foml of electrically induced signal so thar it can process it into an accurate reading for the desired "alue. The units ofli'ob:tic are: millivolts (O.OO1\~, vohs and kilovoirs (1 ,OOO\~. The un~t~\<2i..c(~r~nr fire: milli-amps (O.OOlA) and amps. TIle units of resistance arc: pHrh's, kilohms (1,000 o hms) and megohms (1,000.000 ohms). Thc units of \v{ power are: W:UlS and kilowans (1.000\\). There are megawans (1,000,000\'\) and gigawatts (1,000.000,000\'\) but these are quite a bit be~'o nd the scope o f the equipmem discussed herc! Voltmeters are lIsed to measure a potencial difference as "ohage, :unmeters are used to mefisurc current flow. ohmmeters are used to measure resistance and wattmeters arc used to measure powcr. The signal that is most commonly uscd in thesc measuring dC\'ices is the electro-magnetic effcct induced by elcctrical current flow.
o \\'hen an electrical current flO\\"S in a coil, it produces an electro-magncLic field thar is proportional to the amOunt of current flo\\;ng. By detecting t.he strength of the electro-magnetic field, a suitably calibrated instrument can rdate this to a \'alue for current. Voltage (V) and resistance (R) are both related to current (I) as expressed by V ;; I R.
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.\ddicionaU~·. apparent electrical power is the product of "ohage and currem so. if an instnlmenr can measure electrical current, it can be adapted to produce readings for \-oltage or resistance or power.
\"'hen a current carrying conducror is placed in a magnetic field, such as that produced by a permanent magnet, a mechanical force is produced that acts to mo\'e the conductor. The \-alue of the mechanical force is directly proportional to the \'alue of the current flowing through the conduclOr. The direction of movement of the conductor is dicured by the direction of the current flowing through it. This is a function of Flemi1J:r.'i L t.'{f H :md Rule. This cffeCl is an ideal meThod for converting an e.lectrical signal into mechanical mo,ement, which is jUH what we need when designing :111 instrument th:l l will mC:lsure eleclrical cu rrent. ~()w we h:l\'e learnt the principle upon which clectricalme:lsuring instmmenrs opcratc. it remains for us to look at how each instrument uses it to produce lhe desired reading. Before we do this, howe,·er. we will look :\1 some basic tnSlnllnents to sec how they are designed to react to the tlow of an electrical current.
Moving Coil Instruments . \ Illo\'ing coil is a galvanometer or current·indicating instrument is used as a measuring de\-ice in the ammeter. yoltmetcr and the ohmmeter. In its basic fo rm it o perates on OC \·olrage but it can be adapted to .\C lise and we will look :ll this later on in this section. !\!O\'ing coil instruments fire the most commonh" used instruments and are sometimes referred to as O';\rson,al meters in 'recognition of the scientist who first used the principle. "}.n\i " \\ ,b
. (.,,,\1.:..;":'" \\ ...~(...
Basic (Horseshoe Magnet) Instrument
Permanent ;.. fagnct
\.----- t-t_ Pointer
--t+""
.\luminium rorrncr
--jl1~~~~~~
Coil Control Spring Soft Iron Core (Concentr.l.tor)
Soft [ mn Pole Shoc~ Figure 3.83· Armnge mcllI of a Simplc Moving Coi.l Ins tmmc nt
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Construc tion The basic 'mo\·ing coil' instrument consists of a permanent horseshoe magnet that incorporates a pair of conroured, soft-iron pole shoes. :\ cylindrical sof[iron core, known as a concentrator, is positioned between the pole shoes so that there is a uniform air gap separating it from [hem. A light, recmnguiar aluminium former is fined oyer the concentra tor and is supported by jewelled bearings at each end. The former Cl.rries a coil of fine copper wire that is wrapped in silk Insulation. The sides of the coil are free to move in the air space betwecn the concentrator and the pole shoes as rhe concentrator rol:ttes.
Spri ng Force
-~~~~~~~I~~~~~;~~:~ Sp,i"," 1--:::oJfl ,.
Bridge Control
. \djusrer Pivot Top Perlllanent j\ lagnet
(Hor~cshoe)
----l-~---- Former
~li~'f=tF==E;;;;~om Ctlntrol Bottom Pi" .,."--,it~~5i;;;;;!;;;;;~""'_llo,,,S~p~~nng Bridge Fig ure 3.84 - Moving Co il Instrument
("~'-
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Thc coil is clectrically connected through [Wo phosphor bronze, spiral hairsprings positioned one at each end of the former. The h:ursprings are wound in optP0~nC directions so that the cffccts of thennal expansion \\~]j bc cancRlli'g1)\W. A pointer is attached to a spindle at one eod of thc fanner and ~uee~~r- ac ross a graduatcd scale. Thc adjaccnt hairspring incorporates an t,\1').\ djusrer that is used co zcro the poimer when no current is flowing in the coil. Operation The permancnt magnet crcates a magnetic field chat passes between the pole shoes. \\'hen an electrical current is passed through the coil, an elcctromagnetic field is created around the sides of the coil that inter-acts \\~th me pennancm magnet field and a force is produced that causes the coil to deflect in accordance with Fleming's Left Hand Rule. The coil has an cqual numbcr of conductors on each side, so the force s created ace equaJ on both sides of the coil. Howe\·cr, because thcy are 00 opposite sides. the direction of currcnt flow is different on each side and this creates forces that are opposite in dilcccion . The result is thar one side of the coil is forced downwards whilst the mher side is forced upwards and thc coil assembly rotarcs, c:tusing the pointer to sweep across the graduated scale .
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Detlecting Tortlue _ _ _ - - - C o n t r o l Torque -Damping Torque
11:l1fspring (Comrol) ---lirJ---,
Aluminim Former (D:UllpHlg)
s
~-~~~~j~
Soft Iron Concentrator
Figure 3.85 - Basic MO\-lng Coil MeIer
\,\ 'hen :I current is flo\\1og and the coil is deflecting, therc arc three torCJue forces:lt play. The coil is being rotatcd by a D d lectillg T o rq ue th:ll is directly proportional to the amount of current flo\dng in the coi l. .\s th is occurs, one h:lirspring is being compressed whilst the ocher is being extended. Both h:lirsprinbl'S arc storing elastic energy rhat cxerts a Comro/ Torque in o pposition to rhe deflecting torque. "nle yalue of the control torque is proportional to the distance \Iayelled by the pointcr across the sC:11e. r\ point is reached where the detlecting torque and the comrol torque bfllancc fln d the paimer wiU stabilise :It that reading. Because tbe strength of the deflecting torque is proportional to the yalue of current flow, and the strength of the control force is proportional to the pointer tra,·el the indicated reading on the r scale will be proportional to the amount of current flowing in the coil. The inst:wP'ient scale will haye equal length graduations oyer the whole I":lngc. '11.'\ ,(,
~
While thc pointer is in motion, the sides of the aluminium coil fonncr will be , l . ~...':-.,c":"c'" , 7
,
,
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L
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cutting thc magnetic field and thi5 induces a current in the former. Lcn z's Law states that the direction of an induced current in a dosed circuit will oppose the mO\·C111cnt of the conductor. Thcrefore, the induced currcnt in the fo rmer interacts with the magnetic field and creates a Dumping T orque that opposes the mO,"C111cnt of the pointer. TIus is known as 'eddy cmreet damping'. The damping torque acts as a brake on the movement of the poin~ and pre'·enrs it overshooting the point where it should come to rest, tlll-r;. reducing any tendency for il to oscillate. The damping torque is proporcional to:. the speed the pointer mo\·es at so it will always be of a su fficient val LIe t~ regulate the speed. Do remember, the damping torque can only be creatci1 when the pointer is actually in monon. ,..... o
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The sens itivity of this instrument is obtained by ha\·ing a large number of ti}rns in the coil, Jow friccion bearings and a magnet that produces a high mafC-tletic n~.
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By \·irtuc of their design, mm"ing coil instruments based on a J%rscshoe magnet have a limited afC of mm"C111cnt of around 100°. Additionally, their
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perfonnance is nOt significantly improyed by the use of the latest magnetic alloys, lbey arc useful instruments, bur other magnet configurations are available that will O\'ercome one or more of these limitations, \'Ce will look at twO examples.
Cylindrical Magnet Moving Coil Instrument Construction This \'ery compact instrumem consists of a solid cylindric:tl. 1n:lgnct [..hat is supported inside a soft iron cylinder. Two diametrically opposed, soft iron pole shoes arc attached to the face of the cylindrical magnet. The coil is wound on to a fonner tha t is mounted on bearings so that it is free to rOlare in lhe air gap ber-ween the pole shoes and the inside of the soft iron cylinder. The deflecting rorque, control torque and the damping torque arc all produced in the same wa)". \X'hen a current is passed through the coil. the magnetic field \\ill still interact \\;I.b rhe permanent magnet field to rotate the fonner and thus the pOlllter.
Soft Iron - - Ourcr Cylinde r
Cylindrical ~bgnet
em! r-,,,,n,,,-
Fig ure 3.86 • Cylindrica l ~hgn c l T ype Moving Co il Meter
This arr:tngemenl ensures that [he high intensity magneric flux of the efficient alloy magnet can operate in an air gap th:l.l is shielded from any external electro-magnetic interference, The range of pointer travel is still somewhat limited within this layour but the insmunent docs benefit from the use of more efficient magner maleri:Us.
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Long-Scale Moving Coil Instrument Permanent :\fa1-,'ll::C:':;::;::::~
Coil
-lL~~u~::11rr::~~polc
Yoke
A4'-----i:-i--core Ring
PCrll1 fl nent '\iagnt't Fig ure 3.87 - Long Scale Moving CoillnSlrum em
Construction This is another confIguration of the mO"ing coil instrument. Ie consists of two permanent magnets mounted on a cylindrical soft iron cylinder. One magnet incorporates a soft iron, yoke shaped pole extension that encloses a soft iron core ring that is attached to the second magnet. '111e coil is mounted on an extended portion of the pointer that it is posicioned within the influence of the magnetic Geld between the pole yoke and the core ring. In addition ro being able to employ highly effecti,fe magnetic illoys, the mech:ullcal arrangement of this instrument pennits a pointer sweep range of up to 300°.
Voltmeters and ammeters are measuring inS[rumenls that ha,e been fined with graduated scales to read potential difference in volts or current in amperes respectively. :\ ,-oluneter is always connected in ptlraUe! with the circuit acrqs.s which the potential difference is to be measured, Voltmeters need to ba\'e~ high resistance compared to the circuit in wbich the \-oltage is being measured, so that they will only draw a rclati\-cly small amounl of current anl;t will not significantly disrurb the circuit, Voltmeters incorporate a high valu~ resislor called a lWultiplier rhat is connected in series with the mm>1ng coil tif increase the l'alue of the potential difference they can measure. :
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7 SwnmpResistor .\mmctcr
Voluneter
Figure 3.88 - Usc of Moving Coil Meter 3S a Voltmeter & Ammeter
.\n Ammeter is nl\\'a\'s connected in seri es with the circuit dlrough which the current is to be meas·ured. Arruneters need to haye a 10 0" res t"!U.mcl.· compared 10 the circuit so that ther do nm introduce any unnecessary resistance. ' . . . . \mmeters incorporate n low yaluc resistor called a S hunt that is connected in p :lr;llId with the moying coil to by-pass most of lhe currenl around ir.
rvIoving Coil Voltmeters The mo\'ing coil instrumenl is actually a millianuneter but it is also suited for usc as a millivoltmeter when equipped wi th a suimbly calibr.lled scale. -111e coil requires nu my turns o r line copper wire to produce sufiicient deflecting torque (0 give a full-scale deflection. Copper has a high tempernture coefficient nnd that crcales significant changes of resisrnnce when the temperature \'aries. A \'olunetcr cannot remain accurate under these conditions. l\laterinls \\~th lower temperalll(~~coefficiems do not have dle low resistance property of copper eq,\~~bdler solution is necded. 111e soluuon is found by fitting n high r.f6.is~'i~e or ' m ultip lier ' in series with the coil ..\ny changes in resistnnce ~ \,\;I.(f hroughour the instrument hal"e a negligible effecI on dle value of this resistor. A resistor that is used in this way to reduce the effecI of temperature variations is called a Swam p R esistOr. The addition of this res.islor also means that the instrument can measure a larger potential difference and in this role, Ihe resistor is called a Al ultiplier. Va ri ab le Range Voltm eter The vohage range in which a mo\-ing coil instrument opera res will depend on the resistance value of the high \'alue resistor or 'multiplier ' in series with the coil. The resistnnce required to giye a desired value of poremial difference at full-scale deflection can be calculated using Volts (V) = Current (I) x Resis mnce (R). For cxnmplc, if the current were 0.01 i\mp nnd the resistance were 6 ohms at full·scaJc deflection, the potentia! difference wOLlld only be 0.06 VaILS. I [we increase the resistance to 2000 ohms, the potential difference at full- scale de fl ection would increase to 2OV. lncrcnse the resistance t.o 8000 ohms and we can achieve 80V.
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To :;:1.\-C h:n-ing a cupboard full of instruments we can modif" the inSlrumem to measurc a larger range of potential differences by connccti~g multiple \'aille resistors through a rotary selector switch_ In tlus W:lr, we C:ln selec t an appropriate measuring range on the same inSlnllllent.
7 F igur~
3_89 - Va ria ble Ra nge Voltme le r
Moving Coil Amme te r The denecting torque in a ffiO\-ing coil instrument is a function of the current. To measure the current. an ammeter must be connected in series with the circuil being tested_ The resistance of (he ins[nlmcnt \\-ill affect the \-aille of the current in rhe test circuit and this can create a problem in obtaining InIC values_ The solution to this is found in creating a b~l'ass around the instnlment coil where the majority of the current is passed Stmiglll through whilst a \'Cry small fr:lction of the current Gow is passed to the coil. The majority of the current bypasses the instrument coil through a shunt, which is a low rc s i~tance resistor that is connected in paraliel with tbe instrument. ,\n instrument used in this way has to be sensicive to the small current passing through the coil and in effect acLing as a milli\'olrmerer by measuring the potential difference across "l\\\ W1C s*unt. T illS creates a problem with temperature variation if the instrument (.,,\\..:~c. - docs not also ha\-e a 'swamp' resistor.
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T he solution to reducing the effects of tempcr:Hure variations on the instrument is to use a coil n-i{b {luck g;ll1ge copper nindings to lower the resistance, in conjunction with a suitable value swamp resistor. To increase the cu rreO! ,'alue th:ll can be measured, we can inser! a lower ,alue shu nt resistor to by-pass a greater pro portion of the current. T his means a higher current would be required to achievc full-scale deflection.
Conve rting to AC Measure m e nt Rectifie r Instrume nts The modng coil instruments that we haye bccn discussing require current to fl ow in one direction and arc thus suited to DC measurements. They cannot measure alternating current unless the current is somehow made to fl ow in one direction through the ins trument coil. Tbe problem can be O\'ercomc by fining a recrifier in series with the instnnTIent coil. ;"Iam instruments are of rbe 'average responding' type in thar they display the RAfS (roOI m e;/IJ square) villue of .r\C waveforms. The R.1\IS value is the effective or c{luivalem D C valuc of.r\C volmgc. T he instnuTIents are /es."i accur:lle when operati ng on AC c urrem. The instrument will still need tbe addition of a swamp res istor if it is to be used as a voltmeter and a shunt ifit is to be used as an ammeter.
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Output ::::::::~
AC lnpllt
Full
Figure 3.9 1 • Moving Coillns ullment Adapted fo r Usc
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AC Circ uits
T ran sfo lTIler/ Rectifi er [nstnml c nts \'\!hen modifying moving coil instruments to AC measurement, a voltage transformer IS often used instead of a series resistor and a current transformer is often lIsed in place of a shunt. This practice is more common in multi-range AC in slrUments wherc the rectifier can affect accuracy in the different ranges. Up to now, we have con fined our srudies to the measurement of voltage and current using moving coil instruments. They can also be aci-tptcd to measure
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resisrance and po\\-er. Before we mo\-e on to rhose types of instrumcIHs, \VC should look at a couple of other measurement systems tbat may be encountered inSltad of the moving coil system.
Moving Iron Instruments MO\;ng iro n instruments do nOt rely on permanetH magnets for their operation. The}' utilise the magnetic field that surrounds an energised solenoid 10 induce magnetism into an iron plate. (hat will Lhen be repulsed by . o r be allraered to, the solenoid. The imeosiry of the magnetisation is related to the \'oltage o r eurrent in the solenoid . The ad\'antagc of using this principle is that the instrument will opemlt.: u 7'th either A C or DC c urrent because re\'ersals in currelll direction arc matched by reversals in the solenoid ficld. The mechanical arrangements o f these instruments will depend on whether they arc operating on repulsive fo rces or atuaccion fo rces. \X'e will look at examples of each.
Rep uls ion T ype I nstrument Construc tion The instrument consists of a coil mounted on a non-magnenc, c\-lindrical former that has a semi~circular, iron plate attached to its inner su rface. [nside of rhis arrangement, a mo\'ing iron plate is supported on a spindle so that it is positioned in close proximity to the fL'{ ed iron plate. There is a phosphor bronze hairspring anached to each end of the spindle. These pro\'ide the l:ontrof t orque. The spindle is mounted in jewelled bearin,ht"S and tins ca rries a pointer;,. at one end and an enclosed vane at the other. The enclosed \·at1 c IS rl cs~~ d to provide the damping tOl'q lll.· when the poinler is ill m otion .
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The mo\;ng iron plate is not concentric with the fixed iron plate; it is slightly offset to provide fI larger air g1lP on one side. Operation \\;hcn an electrical current flows in the coil, an c1cctro-magnetic fidd is created around it thai induces magnetism in the fL,>;ed and mo\-ing iron plates. The effect is to create Nonh and South Poles in each plate [hat align and will thus repel each other. 'J11e force. of repulsion decrea ses with the increasing ai.r gap between the iron plates and this creates a resulta nt force that causes the monng plate [0 deflect in a clockwise direction. The deflection torque is proportional to the strength of the induced m:\gnetism in the plates that is, in tum, proportional to the current flowing in the coil. The deflecting torque is actually proportional to the square or the. current in the coil. For th:lI reason. tbe sC:lle is gmdu3tcd according to the square law i.e. me gr:lduations increase in length as rhe scale lengtll increases.
'\ir'~--+t- ~ Im·jng Tron
Fixed Iron --''''~
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e \'.;\!..':' .... ll \\ .
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\'. This moving iron instrument can be modified to produce a scale that has ncar linear rather than square law graduations . 111is im'oh'es rhe adjustment of tlle positions of the fixed and mo"ring iron plates so that the air gap between them increases the fun her the pointer is deflected.
Attraction Type Instrument Construction This instrument consists of a solenoid that is positioned in close proximity to an elliptical disc of soft iron that is moumed on the pointer spindle. Fixed position instruments haYe a counterweighted spindle \vhercl1S porrablc instruments will have a hairspring [0 provide the control wrque. The damping
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torquc is supplied by an air piston that is linked to the spindle. [ have not shown this in the illustration to a\-oid complication.
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c;r----- Counterwcight Figure 3.94 - Anraction Type Moving Iron Instrument Operation \\'hen electrical current flows in the solenoid coil it creates an electro-magnetic field dUll magnetises the soft iron disc. The disc will be attracred towards the solenoid until the counterweight force , or th~ spring force if tlus is fined. b:lianC(~s the attraction torce. -The force o f attmctio n is proportional to the strength of the electro-magnetic field that is, in turn proportional to the amOUnL of current flowin g in the coil. T he deflecting torque is propor Lional to the square of the current in the coil so th e scale of this instrument has sqll:l re l U ll" gr;ldll;uioTJS i.c. they increase in length as the scale length increases . Measu ri ng with Square Law Scales 'l\.~,-
.,\\\WhC"""nil.ture of the sguare law scale is that the graduations at the start of the . (p\\.:.s:.c'- scale arc very closely spaced. This can create problems when anempdng to _ ~:..(': measure low \-alues of yolrage or current. These instruments arc o ften ,\"..'. '" modified to gh-e a graduated scale that is closer co linea r. It is done to increase _ ~,\"5\.'-,;the length of the lo\\·cr scale graduations to make them easier to rcad.
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Measuring Voltage & Current
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The moving iron instruments suffer from similar problems (0 the mo\·ing coJ! inSlnllnents. Instruments used for \rolrage measurement require coils witfi many fine gauge copper windings and require a swamp rcsistor. Instrumems~ used· for measuring current reguire coils with a few duck gauge windings ani require: a swamp resistor and a shunt. ::: .~
Ohmmeters
ffi'u
Electrical reststance (R) is related to the yoltage (V) and the current so that R = V/ 1 In many circuits, resistance. is something that needs to be kept as low
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as possiblc because is obstructs the curren! flow. In other circuits, thcre is a need to introduce resistance to control the current flow and we do this br placing rcsislOrs of specified .alues in the circuit. l1us means tbat it i~ important to be able to accurately measure resistance. Instruments that arc designed to measure resistance actually measure current tha t is supplied to a circuit [rom an external sourcc, such as a bauery or a hand operated DC generator. For example, if a banery is connected into the circuit to be tcsted, the instrument uses the cu[!cm supplied br this to create a reading. 111e resistance of lhe circuit \vill affect the flow of this current and rhe instrument reading will reflect this by reading the actual current flow. Therefore, thesc current measuring instruments arc acting as ohmmeters and their scales arc graduaTed to read in ohms (Q).
Series Ohmmeler 'nle se ries ohmmcTer is in theory a mming coil milliammeter (hat is connected across an unknown resistance. The i.nstrument bas a self-contained dry cell bauery to supply Ihe current flow in the circuit. The instrument will have its own resistance, which includes the coil and the internal resistance of the dry cell. 11us resistance can be compensated for using a suitable fl."{ed resislOr so that the inS tnlmellt \vill give a full~scaJe deflection for the available battery e.mJ when the Output terminals are directly linked together. Dry cell battcries progressively weaken in usc so a further ,':triable resislor is needed to adjust for this. When preparing the instrument for usc, it is initially SCt up by directly connecting terminal s A and B together and, using (he variable resistor, scning the instnlll1enr pointer to register full-scale de fl ection, which corresponds \\~(h zero re s i stance .,~l'li.c unknown resistance may chcn be connected between termina~ '21-\'tJand B. The pointer will now registe r the lnlC value o f that rcs1st.\1fce.
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Se ries Ohmmete r Scale
\X'hen a cons(am c.111.f IS set l.lp in a cirCUit, the current flow will be p roponional to the resistance of that circuit. If we haye a mo\·ing coil instrument that will give a full-scale deflection for that cu rrent flow, the reading will reduce if the circuit resismnce is increased for any reason. This makes sense, because if the resistance increases, tbe currem flow will reduce and tbe instrument is merely recording this. For example. if we placed a resistor in series witb a circuit thai bad a resistance yalue equal to the original resistance \·alue of that circuit, the ohmmeter reading will fall back to the midscale pOSItion. Because the resistance of the circuit has been doubled, the current is hal,ed. If we were to increase the circuit resistance to (he point that the current flow stopped aiwgeilier, then the ohmmeter reading \vauld fall b:lck to the stan of the scale. In theory, this indicates zero current flow but the instrument \,ill actua!ly be registering infinite resistance. 'fhis is why the pointer on a series ohmmeter ,\ill register infinite resistance before tenninals :\ and B arc connected :md. \vhen connected to a circuit, will register a redUCing resistance yalue as the pointer progresses until, at full -scale deflection, it will be indicating zero resista nce.
'\:0 Current
r:!ow
Full Current Flow
Fig ure 3.96 - Example of a Series Ohmmeter Scale
Ra nge o f I\1cas urcm cn t The basic senes ohmmeter IS not suited to measuring low resistance values. The problem is that a simple mm;ng coil Instrument can register full-sql.e deflection for a current a little as 1 milliamp. I f you asswne we bave a 1..~r bauery d1en the resistance of a simple ohmmeter circuIt at full-scale deflectia~ would be 1.5V/0.00 11\ = 1,500.0 (R ::: V/l) when termina ls 1\ and B ait joined together. If rou recall from our earlier discussion, fitting a resistor with a \"alue tbat is equal to tbe circuit resistance will halve the current and the instrument pointer will drop to mid-scale, in the case in question this is ·1,500[1. Series ohmm#"rs hayc an accuracy range of about one tenth to ten rimes the mid-scale re~ng so the range of the ohmmeter we ha\"e been examining will be 1s.@h to 15,000.0. To cancel out the 1 milliamp current we would need tQ(~lace a resistor of 15,0000: between tem1inals ;\ and B. The problem is \Vc cannot obtain accurate meaSllrements below 1500.
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A parI solution can be proyidcd by connecting a low "alue, shunr resistor in parallel across the coil in the insrrument so that a higher current would be needed (O obtain fulkscale deflecrion. Say we could arrange tlus so that 10 milliamps were needed. The circuit resislance at mid·scale would then be 1500. This would giye a range from ISO [0 1,5000. \,\ 'e ha\-e improved the low reading capacity but this is at the expense of the high end of the scale. \"'\·e could push chis up by increasing me "olrage of the battery bur this would also raise the low end of the range . .A different approach is needed.
Sh unt Ohmmeter j\ shum ohmmeter is designed to measure very low resistance values. A moving coil instrument is connected in series with a self·contained battery, a fixed resistor, a variable resistor and a switch. The main difference in this inscrumenr is that tbe unknown. or test, resistance is connected in parallel across the ffiQ\ring coil. Unlike the series ohmmeter, when the ompur terminals A and B are not connected, tbe instrument pointer will deflect to fuU· scale when the switch is closed because it will be registering that all lhe available current flow from me battery is passing through the instrument coil. This IS telling you that there is maximum resistance between terminals.\ and B.
\ 'ariable Resistor
r:ixed ResiSTor
dvW-..MMA J
h -Switch
i\Joying Coil :--'feter .\
B
Test Resistance Figure 3.97 · ShUll! Ohm mete r Circuil
1f we now connect a resistor across terminals A and B, some cu rrent will be 'shunted' through it to bypass the instrument coil and me pointer will fall back. For example, if the resistor value is equal to the ohmmeter resistance value, the pointer will drop to half~scale. ff terminals A and B arc directly linked together, the pointer will drop to zero because all the available current \ViU bypass the coil through terminals A and B. This is actually telling rou that the resistance between A and B is also zero. The important point is that rhe instrument poinler on a shunt ohmmeter operates in the reverse direction to thar of a series ohmmeter and tbe scale is calibrated to reflect this.
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~rcgohms
Full Currcnr rlow
1'\'0 Current rlow
Figure 3.98 • Typical Scale Arrangement
The battery life of this instrument is rclatin,:ly short. This is because current flows from the banef)· \\"hene\'e( the s\\;lch is dosed and it will continue to flow through any low yalue shunt resistance. Series and shunt oiurunetcrs gi,·e reasonable accuracy in low resistance measurement but instruments that arc eyen morc accurate are ayailable.
Ratiometer Ohmmerer
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The simple mo\;ng coil ohnuncter is ,'cry sensitive to changes in \'ariauons in the e.m.f. from a battery. This can affect the accuracy of the instrument. The deflecting tor'lue will ,'ary 'vith changes in ,"oltage input and this ,,;11 affect the control tor'lue. which is dependent on pointer [nL\'e!. This means that a battery powered inslrument will always require preliminary adjustment before it IS used and accutacy will stray if there is any subsequent variation in input \"oltage. The siruqoo-n would not be improved if a hand-cranked generator were used unless 1.a\l~could maintain a constant ,·oltage output. If the control torque can be made to alter in conccrt ...\~th variations in input voltage in addition to the angle of pointer ttanl, then both [he deflection and comrol torques will be similarly affected. This would mean that any change of voltage affecting me defleccing torque will be matched by a change in control torque and a stable indication can then be maintained. This ensures contin~ accuracy of indication. Most single purpose ohmmeters embody a 'volta~ sensitive' control torque. ~ Const·ru cti on The ratiometer ohmmeter is a pennanem magnet, mm;ng coil inslmmem thit has twO coils mounted on a specially designed, hollow, cylindrical, soft it:Q6 concentrator. One coil serves as a deflecting coil and the second as a c09..fr'ol coil. This instrumcnr does not ha\"e hairsprings. The concentratOr is off~ so that the air gap increases from the lower edges of the pole shoes to the..~pper edges. This means that the flux density decreases in density from theA,Gwer to the upper edges. The coils are wound with fine copper wire and )tan only
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accept low current. The Instrument is equipped with suitable resistors to reduce the internal battery currem to an acceptable value.
-
Pcrmancnt Magnet
N
5
Soft lron/ Deflecting Concenuator Coil Figure 3.99 - Rariomcler
Ohmel(~r
Both sides of the deflecting coil man in the magnetic field between the pole shoes. The control coil is positioned so that only one side is in tbe magnetic field whilst the other side is shielded inside the soft iron concentrator. The tWO coils arc wound so that they exert opposing torques when a currem flows in dl(~m.
Operation When a voltage>J &m a battery or a DC generator, is supplied to the deflecting coil it wiij\\ain balanced by the control torque.
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Insulation-Resistan ce Testers The insulating material in c!ccrncal equipmeo[ experiences electrical stress that can result in a breakdown in its resistance. Ohmmeters that are used fo r measuring inslllntion resistance and o th er high resistance values must be able to replicate this stress by using an output ,-oltage that matches the operati ng \'oltage of the equipment and can increase this to achieye the break down yohage if required . The majority of insulation testers are instruments of the raliometer ohmmete r type [hat o perate o ff hand-cranked electrical generators. Typical o utput \'oltages are 250V, 500\1 and 1000\1 . These instrumen ts arc high rangt-o ohmmctcn .. caUed megohmmeters and they ha\'e a resistance measuring range that extends to i<.Iegohms. Remember. a Megohm is 1,000,000 ohms (1 million). These instnunents are more commonh' referred to as 'Al eggers '. Higher yoltage insulation resista.nce testing equipment is required for resting the high-tension leads used in engine ignition circuits. TIle H igh T ension igniu'on T ester has a hand -cranked magneto output of aro und 12,000 \'o lts. Special low voltage 's:lfct}· ohmmeters', on the other hand. arc designed with 3 ~\' genem tors and these are typicaUr employed in me testing of fire extinguisher explosive cartridge detonating circuits. \\'e will start by looking :It the '!>.Iegger'.
Megger
o
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o Figure 3.100 - Megger
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Basic Con struc tio n
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The typical insulation resislance tester, known as a 'Megger', consists of a higlJ.:. resismnce, rariometer ohmmeter that incorporarcs a hand-cr:lnked gene ra t o~ The deflecting coil is connected to the circuit under test tluough a lo\\' resistance, fix ed resistor. This series circuit is connected :IUOSS a hand-cmn ~ d generator. The control coil and a second fL.... ed resistor are connected in pa @ el to thi~ series circuit. f::,--
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Control Ca ll
Dcflccung Coil
]': anh Test Resistlnce Figure 3.101 - Basic Insulation Tesler Circuit
The current p:lssing through the conrrol coil will always be proportional [Q the ourpU( \'olmge from the generator. The current passing through lile deflecting coil is determined b)' the generator output voieage and the value of lil e test resistance. This basic instrument does not have any \'oltage limiting facility so the speed of lurning of the hnnd-crnnked generator is of impo n ance. Generally, this should no r exceed 160 RPi\{ Constant \'ohage 1l1eggcI' testers on the other hnnd ha\'c sli pping clurch mechanisms that ensure that a specified consta nt ,'oltagc will be delivered wben tbe generator is cranked over the RPi\! at which the cl urch slips. The ratio of the currents in the control and dcflccting coils is determined by the value of the tcst resistance and me position of the instrument pointer when they are baL'lOced . The acrual position of the pointer depends entirely on,.. the value of the rest resistance. ')1)0
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C \V~srhe object of insulation resistance testing is to establish the effective resistance between tWO points. Capaciti\'e circuits and components can produce unreliable readings. High resistance components berween rhe poims being rested can also affect the result and these should be disconnected or bridged and be tested separately where possible. " feggers should not be used on electronic equipment. The tcSt "oleage normally stipulated for general aircraft equipment is 2S0V. T hc 'line' and 'earth' terminals of d1e teS[er are connected across the test resistance. Jf you were connecting [0 an electrical motor for example, you would connect the 'earth' tem1inal to a suitable point on the motor cnsing flnd then you would connect the 'line' terminal to the motor winding conductor. O ne important point here is thar the tester CflO only disclose insulation faults if the points you connect to are in electrical continuity with the opposite tcrminals of the instrument. [ f there is a break in the circuit earth system for example, you will nor reveal an insulation fault. You should check continuity first. Jf aU were well, you would connect rhe leads and crank the generator slowly to check Ihat there is no swing to 'zero' indicating a short circuit. You
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would then incn:ase (he cranking to the rated speed and conunuc this for about five seconds. The pointer indication should rcmalll steady at the indicated rcsismnce nlue O"f"cr this period. I bnd Crmkcd Gcncr:uor G
1~'j~C:ro RaUOIllClC Control Resistor
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Component L'ndcr Tc:~t L-----,,----:-o>. Earth Lmc Figure 3.102 - T ypical l ns ulatioll T eSler l..:aYO UI
If the tester is operated on perfect insulation, dle poinre r will registcr a reading of infinit),. If the insulation has no resistance, the palmer will register a reading of zero. It will also register zero if you (ouch the 'li ne' and 'ea rth' te nninals lOgether. Bctween these two cxucmes, the pointer wiU Settlc at the appropriare vallIe for the resistance of the insulation under test. It is rare for the tcster to give a reading of infinity on equipment that has been in usc or in storage fo r any Icr~Bth of time. If a reading of infinity is obtained, you should recheck the kcst~tHcrmina1 connections, the leads from the Icster and the l.ester itself. "111t _ :-,..:":"c "!.\\\~~si stance indication should remain steady for a reliable reading. I f it falls off or ~,u''\' varies while the test is being carried out. it i:i most likely thaI the insulation is ... \-..,:'.{ breaking down due either to moisture penctration or a deterioration of the . . \"~'~' insulating materiaL It is important to ensure that the leads connecting the tester C-)'\ to the test circuit are in good condition and arc properly connect cd. 250V Mcggcr
l\lany aircraft electrical circuits operate within a range of low vohages. Becau~ insulation resistance test instruments use hand·cranked gcnerators. ther needto ha"c a voltage-limiting de,;ce: in [lL1ny cases. this is a clutch slip mechanisnp: I\n additional problem can be experienced if (he vohage output is not constant when the circuit being tested contains components that have capacitan(e. These can create a feedback that affects the instrument readings. 'nle mem,~ r ~hould not be used to test electronic components. The basic layout or the instrument we arc looking at is similar to the one we have already dis ~sed except for the addition of a yoltage limiting circuit and a smoothin£ ~ircuit connected in parallel with the generator. ,~
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I-bnd Cranked Generator \,'ith Split Ring Commutator
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Earth
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Figu re 3,103 - 250V Ins ulatio n TeSler
The \'oltage limiting circuit has a non-linear resistor that performs as an insul:J.tor when the generator output is less than 250V, If rhe voltage omput rises above this, the resistor progressi,ely begins to act as a conductor and acts as a 's hunt' across the generator. Tl11S ensures that the gcncra[Qr output cannOI exceed 250V if (he hand-cranked generator is turned faster than 160R.Pi'vl.
H ig h Tensio n Ignition Tester
I~l~-tension
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When enr nb ignition leads have to be tested for insulation reti_~~e. a ,·oltage in excess of the normal operating voltage has to be ~,~'t'Sl.Lpplied. TIle /-ligh Tension Ignitioo Tester incorpOrales a hand-c11lnked magneto that has an output of It,200V to 12,500V, The tester is complctcJ)' sealed to prevent inadvertcnt contact ,,,ith thc working pans, This tester will shock you in a similar way to a liyc spark plug lead so carc is needed! The tester has a spring-loaded push bunon that is designed to protect the instnlment from oyerioad if there is an earth fault o r a shon circuit in the lead under test,
Push Buunn
Figule 3.104 - H igh T e ns ion I gnition T ester
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Operation There are t\\'o leads supplied \\ith the (eSler. The line lead IS clipped to the ignition cable core conductor whilst the eanh lead is clipped (0 the cable screening. The hand-cranked magneto is then turned slowly \vhile the oper:llor checks for any audible or \'isual sign of arcing, or any sign of 'hunting' of the meter. If none of these is prescnt. the magneto is then cranked up to 300 RP1\j and the push button is depressed. The instrument reading can then be noted . The scale has a red and a green sector. The indicator should remain steady in the green sector if the l!1sulauon check is satisfactory.
Safety Ohmmeter (Insulation Resistance Type) .r\ safety ohmmeter is specially designed to test the insulation of circuits where
it would be hazardous to exceed a specified \'olrage and current. The insulation testing of rhe explosive cartridge detonating circuits in fire extinguisher systems is a good example o f this usc. A safety ohmmeter of this type would typically be limited to under 30Y and IOmA output. There is another \'ersion of the safety o hmmeter that is designed to test continuity res istance in circuits rather rhan the insulation. We will deal \V1th this latter type la ter under the heading of low range o hmmeters. f Lmd Cr:mk(!c\ Genennor \'Cith SpliL Hing Commmawr
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The generator has a single high resistance winding that is connected thro~;h a split dog commutator that has a smoothing cireuit connect ed acroiii the brushes. TIle generator will deli,er a maximum of 30V at 160 RP~l 41 nd will automatically reduce the \'oltage if the current reaches l O rnA becaU"si; of low insulation in a tcst circuit.
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Low R ange Ohnune(crs \,\1e have already discussed the usc of a shun! on mming coil ins truments to reduce the current flow from an internal battery..'\ fraction of the avalJable curren t flows through the control coil, the remainder is passed through the shun L \,\'c also discussed that these instruments had limited accuracy. The accuracy can be improved by combining a shunt with a ratiometer o hmmeter. Two instruments use this principle, the bonding tester and the connnuity resistance safety ohmmeter.
Bonding T es te r A bonding tester is used to test the coannillty resistance of the bonded connections in an aircraft. It has a measurement range of zero to 0 .1 ohm. The instrument incorporates a ratiomelcr ohmmeter and a 1.2V nickel alkaline battery housed in a wooden carrying case. There are twO connecting t\\~n core leads, one is 6ft lo ng and the other is 60ft long. The shoner lead rcrmimtes in a double prong probe whllst the longer lead has a single prong probe, The length of these leads is critical and their resistance has been accounted for in rhe calibration of the [CHer so they must not be altered or modified in any way. I f a lead is damaged, the bonding [ester must be rerurned to the manufacUlrer fo r repair and re-calibratian. l nf ~ Zero
Short Lead
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Figure 3. 106 - Bonding/Circuit Tester Ope rati on \'{'hen both probes are placed in contaCl with the aircraft StnlCnlrc, the bauery and control coil circuit is made through [he single spike probe, through the strucUlre under test lO the do uble spike pro be, and then through a resi8to r. The structure undcr rest is thercfore in series with the control coil. Because the resis tance of the structure being tested is very low, the Cll[rcnt flm.vi ng in the control coil will be constant. The deflecting coil is also connected in series through a resistor and across the strUCnlre under test.
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The curre nt flowing through the deflecting coil re!:ncs to the \'olt:lge :leross the structure under test. Since the current is constant. the voltage across Ihe deflecting coil. and the current passing duough it, :m.' proportional (0 the resistance across the structure under test. 111e instrument pointer will mO\'e to :l position rh:lt reflects the current ratio of the tW O coils. BeC:lllse this ratio is wholly dependent upon the resistance that exist be{'\\"een the IWO probes, the poi mer will be indicating the actual resistance of the structure between them. The double prong o n the short lead prevents the battery being discharged if the prongs inadvenently touch when the instrum(:nt is not in usc. Both pro ngs must connect through to the single prong to make a circuit. Usc Prior to use, you should place the single prong across both prongs on the othcr probc and then check to see that the instrument is registering zcro resiS[ance. The single prong on the long lead is then placed on a specified earth point on the Slrucrurc to be tested. The single prong must aJw:l)"S be connected first to avoid damaging the instrument. You then tOuch both prongs o f the short probe o n to the nc.'l.rby structure to check that the inslrumenl still registers zero. If this teSt is satisfactory. :ou then place both prongs of the shorr lead OntO the selected test point on the stnlcture. The insffillnent pointer will register the resistance value. ,\ high resistance reading will indicate a possible fault in a bonding connection. 1\ description of a bonding tcst is gl\'cn in Chapter 7 of this module under the heading of circuit tesnng. The bonding tester is calibrated by using test resistors o f known value between the probes.
Safety Ohmmeter (Continuity Resistance Type)
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1\'\foo,J.l~nge ohmmeters can also be used for checking the continuity of circuits. l>,~.:.'t-e --- Somc circuits may ha\"e a maximum \·oltage and current flow limit beyond ~ \\'JS<' which a haza rd may exist. The fire bottle cartridge detonating circuits are again 10. :In example of this.
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This safety ohmmeter is a low range ohmmeter that is specifically designed for testing the continuity resistance of such circuits . Unlike the insulati(Jrl resistance type, which had a resistance measutement range of zero Megohms, the continuity resistance \"ersion has a measurement range of onl!.. zero [Q S ohms. The instrument incorporates a hand-cranked DC generator: that is limited to an output current flow of lOrnA. The deflection coil and resistor arc connected in parallel with the test circuit. 'fhe control coil and it$" resistor are connected in parallel with a shunt resistor. The ratio of the curre~ in the two coils is determined hr. the resistance of the test circuir. ::
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Generator
Inf~ !!~Zero Deflecting Coil Resisror
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I Circuit Under
T'st~
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Fig ure 3.107 - Safcty Ohmmeter (Continui ty Resistance T ypc)
Usc The leads from the ohmmeter are connected to each end of the conductor in the test circuit. The hand-cranked generator is mrned at 160 RPt-.! while the rcsistance \·aluc of the test circuit is read. \Vhen resting a detonating ci rcuit on an explosi\'e cartridge, long connecting leads are used to allow the operator to be posiLioned a safe distance from the ca rtridge.
Bri dge Megger Testers tester is. a .pona~le urur ~har is d~signcd ro me~sll[c and conanmty resistance wuh \'ery high accuracy. \Vhen '" "\.o;.1."ls~d to test insulation resistance it is operated in much the same wav as a ;.."\.\ (.' \'. i\legger rester. The bridge test feature is used to accurately check continuity .,.,.5\·~ resistance in circuits of specified resistance and to establish the resistance nl.lue (~...'\ of circuits where this is not known. Ir can also be adapted to locate earth faults in concealed cables. This last fearure is known as a Varle), Loop Tesl There is quite a lot invoked in the Bcidge-llegger [ester operation and it is highly unlikely you would be asked to remember all the details. The general principle and lhe uses are much more likely topics for the 'quizmastcrs. Howc"er, it IS little use telling you what it does without giving you some idea how it docs iL !hc
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Basic ratiomctcr ohmmeters are widely used for measuring insulation resistance and concinuiry resistance and they have good acctm1cy over a restricted range. !-I o\vever, they are limited when very high accuracy is required over a wide resistance range. J f we required a range of zero to tO,OOO ohms with accuracy to within 1 o hm for example, the scale of a basic ratiometer o hmmerer instrument would be too limited to display trus. Instruments that arc cmployed to do this arc based on \Vheatstone's Bridge Principle.
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Under Test
X
Fig ure 3.lO8 - Simple Brid ge Res ista nce T esl Circuil
Wheatstone's Bridge The illusLr:l.tion depicts a pair of 'rano anns' made up of resistors R 1 and IU, a 'resistance under test' X and a .Wable resislOr R3. The connections :\ and B are joined by a galvanometer that is designed [0 give an indication if a currem flo\\'s berween them. The potential difference between .A and B is zero when X = RJ x RI/IU. In this condition, the bridge is balanced and there will be no indication of Cllrrelll flow between .t\ and B. Resisto rs RI and R2 have set \'alllCS. Tf we adjust R3 to a yalue thar brings the bridge into balance and record thar \'alue, then the value of the unknown resistance X can be detennined.
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No\\' assume that cither of the resistors R 1 and R2 can be replaced with resistors ha\;ng different \'alues of 10 ohms or 100 ohms and that the variable resjsto~R3 has a range from zcro to 10 ohms. The mtio RI /R2 can then ba\'c \i..{{jllu~~'of 1/ 10, 1/1 or 10/ 1. 111e bridge can then be balanced if the resistance II..:;\!.":;:\ under (est X is equal to 0.1 R3, R3 or IOR3. t . ,,,I ' <' . \,\;I.( R3 is variable from zero to 10 ohms in steps of I ohm. TIus nO\\I giycs alternative ranges of measurement such as 0 - I o hm in steps of 0.1 ohm,O \0 oluns in steps of I ohm and 0 - 100 ohms in steps of 10 ohms. If we werc to pro\'ide additional values for resistors R I and R2, we would be able J,i) increase the a\·ailable range of circuit resistances up to (he highest range i¥ zcro - 10,000 ohms in steps of 100 ohms. ~.
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Ratio Switch It would be a tiresome business if we had to continually replace the resistoif; Rl and R2 with the \'arious nlues required to suit desired ranges. The problG.,fn is alle\;ated by fitting a ratio switch that incorporates six resistors and:foe selections that will give RI /R2 ratios of 11 100, 1/ 10, 111. 10/ 1 and ItJl/ 1. The resistance value of X ma\' now be found by applying rhe rauo ~\'i'itch setting to the obscn'cd value ~f R3 whcn the b ridge has been bala;.l2ed by u adjusting it. >§ ,
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Fig ure 3. 109 - Ra tio Switch
D ecade Resistan ce Unit To achie,'e a high degree of accuracy, for example zero to 10,000 ohms in steps of I ohm, a single yariable resistor R3 would be inadecluate. T hc problem is soh'cd by rrm'iding four "ariable resistor units used in tandem, each containing nine idcntical resistors of either 1,000 ohms, 100 ohms. 10 ohms or I ohm. Each unit is connected to a ten-posicion rolary selector switch. l1te four switches each ha'-e ten posicions marked 0 - 9 and each have a window to displ:'ty the numerical setting. This giyCS a range selection of 0 - 9,999 in steps of 1 ohm. The "alue set on the instrumenr is eSfablished by simply reading across the four windows. This is known as a 'decade resistance unit'. The unit shown in the illustration is set to a value of 5,2-1 3 ohms,
Decade Hesistancc Cnit Test Selector Switch ~"'':!==,--.:=:r.'''~'-H:l.[io Switch
Figure 3. 1l0 - Bridge-Megger
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Bridge-Megge r Constructi o n The test \'Ollage is produced by a hand-cranked generator Lhat deli\'ers a :ons~aO[ \:ol~age. when .turned at 160 Rl?1 or more .. The generator produces .)00\ for 111s~lanon r~Slstancc te~rs. but It can be S\\'1lChcd to a lower voltage output for bridge lesang. 1lle UmtlncorpOrates a ratiometer ohmmeter that is used either to gi,e a direct indication of insulation resistance or [0 act as a gah-anometer during bridge tests. The unit test mode is selected using a reran' switch on the side of the unit that is marked r.Ieggcr. Bridge and Varle\·. . T~le ~ester incorporates a decade rcsisrancc unit for bridge leSts and an ctfecu\"e mngc of resi~tance from zero 1.0 9.999 ohms can be set using the four rotary s\\;cches on the face of the unit. TIle ranges of a\"ailablc resistance measuremem on bridge tests are set using a ratio switch o n the side of the unit. Therc are th ree terminals at one end of the unit marked 'Earth'. 'Line' :"lIld 'Varley Earth'. The instrument scale is marked from zero to infini,,". The rariomelcr is used as a gah'anome[er to gi\"c the 'zero current' indicario~ when the instrumcnt is being used for bridge tests .
Usc \'('hen the Bridge-l'-Iegger tester is selected [0 't--fegger' and is used to lest insul:-trion resistance it is operated in a similar way to a straightforward mcgger reSler. The unit 'linc' and 'earrh' cables arc held ap:lt"t and th e unit is cranked to check that the pointer registers infinity. The 'line' renninal is then connected to the conductor in the circuit being tested and the 'earth' (crminal is connected to the component casing or lhe cable screening as appropriate. The generator is dlen cranked at 160 IU}.\I and the pointcr should hold steady at the indicated insulatio n resistance \"alue. When the Bridgc-(-. Iegger is selectcd to 'Bridge'. the unit is thcn co(wencd to a \'('bc:mwne's Bridge. The deflecting coil in the rariometc.r will (ben oper,l.[e ,(\\nhc pointer rtS a galvano meter. The control coil actS to provide (he control ,~..\\..;I!..' .... torque. \'('hen the bridge is balanced and no currem is passing through (he l, deflecting coil, the instrument pointer will indicate infinity. There are twO types of bridge test. The first is to \'crify a specified circuit resistance. T he second is to establish the \"alue of an unknown resistance. ,\uluscablc Resistor
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Test "/; RcslstanccL Fig ure 3. 111 - Bridge Continu ity Res is tance Tes l Circuil
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If we simply wish to yerify the continuity resisranc.c of a circuit o~ component wilh a specified value, we sct the ratio switch to gJ\TC us the reqUIred accuracy of measurement and then sct the specified rcsistance value in me windows o f the decade resistance unit. For example, if the specified \·alue is 93 ohms, the ratio swi tch is set to +100 and the decade resistance unit is set 10 read 9300. W'e connect the 'Jjne' and 'earth' leads to thc ci rcui t. \\""'c rhen hand-crank the gencraror and check if the galvanometer is indica ring a current flow . If it rcads infinity, then the bridge is balanccd and the circuit resistance is \'er.ified. If th.e galvanometcr is indicating current Oow, we adjus t the decade resistance Unit umil the bridge balances. The reading in the windows, adiusted to the ratio switch setting, will give liS the acrual resis tance of d1(~ circuit.
If we have
find our what the unknown continuity resistance of a circuit is, wc need [0 be careful that we do not damage the circuit by passing toO much test current. The procedure is ro set the ratio s\\o;tch to x 100 and tben set the decade resistor unit to read 9999. After connecting the 'line' and 'earth ' leads', we hand-crank the genera tor steadily and adjust the left hand (thousands) rotafY s\\~tch to bring [he galvanometer pointer as dose [0 'in finity' as pos sible without going under it. Next, we adjust the second s\\~tch (hundreds) and so on dO\vn to a final adjustment with the right hand (units) switch to b.lance rhe bridge \\;th the pointer at 'infinity'. \'\"e then nare the dial reading, multiply it by 100 and we h ~!se found the resistance of rhe circuil. to
Vnrley Loop Test The Bridge-i'vfcgger can also be adapted to loca te earth faults on conce:lled cables. To do this, we need to have a length of good cable that has lhe same resistance per unit length as the cable we :ue testing.
Line Figure 3.112 - Va rley Loop Tes l
We ioin rhe good cable to lhe test cable and then connect the free ends to the 'eanh' and 'line' tenninals of the tesler. \'Ile set the selector switch to 'Bridge', the ratio switch to + I 00 and adjust the dccade resistance unit to balance the bridge, The resistance value is then recorded. \'(·c \vill call this value L. \'il e thcn move the sdector switch ro 'Varley' and connect lhe 'Va dey' terminal ro a good canh po int. L'sing the decade resistance unit, we re-balance the bridge and record th e new reading, which we \"ill call R. The distaLlce of the 'eanh fault' in lhe cable under test is a half L - R in ohms. As we know the resistance per unit length, trus locates the position of the earth fault.
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Electrodynamometer (\'Vattmeter) '!11e cleclrodynamomelcr is ,'ery sunilar in opcration [0 [he basic mo\'jog coil instrument except that an elecuo--magnet is used in place of a permanent magnet. I t will measure electrical power (wattS). lne electro-magnet consists of hea\'y gauge windings that are connected in series with the load. The Strength o f the electro-magnetic field produced is proportional (0 the amount of current (amps) tlowing in the load. 'Ine deflecling coil is connected across the load and the strength of its electro-magnetic field will be proportional 10 tbe \'oltage d rop emits) through the load. lbe dencction of the poimer is therefore proportional to the power used (,'ohs x amp~) . This instrument will o perate with either AC or DC power.
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Coil
Coil
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.\CInput or DC _
Figure 3.1U · Eleclwd yna m omelcr (Wattmc te r)
\'('hen used on :\ C circuits, the wattmeter measures the true power. 'fhe npparcnt power of an AC circuit is the product o f thc current and the voltage. ,\ppareor Power = Volts x !\mps The powcr factor of a circuit may be cn\culated by di"iding the true power by the appa rcnr power nod it is expressed as a decimal .
Power f"actor =
True Powt!r (\\'am,) Apparl.'nt Power (Volts x \mps)
11lercfore: True Power (\\) = \ pparem Power (\' X. \ x Power Factor
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Tes (rneters & Multirneters \'\'hen ),ou consider the number of different instruments that we baye discussed and the ranges of ,
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In its basic fonn, a tcstmcter, or mulrimeter, incorpor:ues a \'ery SenslO\'C mO\-ing coil instrument that o pemtes a pointcr o,er a multiple scale. The indicator normally incorporates a safcty CUtout to protect the unit from damage (rom excessi,'c current. -nlC CUlout o perates when the pointer is forced over its m.1ximum deflection point. The CUtOUt incorporates a reset burron. There \ \ill be a small dry cell banery (0 provide the current for resistancc mcasurcmCIll. Tcsoncters, or multimeters, do not ha\'e a hand~cranked
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CHAPTER
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gencr:Hor and arc therefore nor suitable for insulation resista nce checks_ B\· ~sil1g a r:lnge of shunts, resistors, changem'er and r:lOO switches, th~ ln SlrUm~nr can operatc o\'er a number of diffcrent ranges of Clirrem, vohagc :lnd reslst:lI1Ce '-alues_ All instruments incorporate a full wa\"e transformer recrifier unit [0 make them comp:aible with J\ C :lnd DC srstems. T he\' me:lsure the :\C W:l'T form as an /(ftIS I'alm.'. '
h~lfJ
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3. 115 - E"ample
or
\l
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Ammeter C ircuit The illustrated circuit shows an example of the shune resistors and switch selections. The actual instrument will ha'-e far more of tbese. -me circuit incorpor:ltes a scnsin\'(: moving coil and a full-wa,'l' transformer rectifier unit for .\C measurements. \, -hen the ClIcuit is selected for DC measurement, the .\ C 5\\;tch has to be positioned to the DC setting_The test current [hen pnsses rh.(~ the moving coil with a speci fied amount by-passing it dUOllgh the ,c "!.\)I.~f1~~t resistors. ~-\s the D C rotary selector s\\~tch is moved to a higher current _(\~W:~ setting, the resis tance value of the series of shune resistors is reduced thus bv. passing a proportionateh- g reater amount of current.
~
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f----:::::::::~·--~--__::____-__:; \fonng Coil
( pens on AC Selection
Sh~nt
Full-\\'a\'c Transformer Rectifier Second:tr), Coil Primary Coil
Rcsi!
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+ Fig ure 3.116 - Example
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or Ihe Annneic r Arm ngcm e nl in a !\-Iulumc le r
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CHAPTER TOO L S
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\'('hen the DC switch is SCt to thc AC position. the inslrument is set up for AC measurement. A pair of contacts opens 10 break the circuit through the shun! resistors. The mming coil is lhen connected through the secondary \\~nding of Ihe transfonner rectifier unit and will be controlled by the ratio of the primary' and secondary windings and the current flowing through the primary. The value of that current will depend on the range setting of the :\C switch. TIle readings will be RMS \'a1ucs. Vo ltm ete r Circ uit
When thc AC switch is sct to the DC position, the instmment is set up for DC \'oitflge measurement. The moving coil is connectcd across the multimeter terminals in series with a chain of resistors at a point selected by the DC switch setting. The pair of contacts on the chain of shunt resistors will open as soon liS rhe DC switch is selected onto a voltage r:l.Iher Ihan a current sening.
Full \,\'a\'c Trlns fr.rmer Recti tier
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-
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When the DC switch is selected to ..\C, dlC instnlmenr is set lip fo r AC voltage measurement {R.!\fS~ . ~hc moving coil is then connected through rhe rectifier 10 the secondary wlndmgs of the transformer. The primary windings of the tmnsfonner arc connected in series with the chain of resistors at a point selected by the .\C switch setting. Oh mm e ter Circ uit '111c circuit illustrated has rwo ohnune[Cr ranges that arc selected br the DC switch through a pair of contacts. In reality, a multimcter would hav~ far more ranges bur tltis would complicate our example.
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+ Fig ure 3, 118 - Exa m ple of the Ohmme ter Arrangem c nt in a f\ iuitimc tcr
The AC switch is sel to the DC position to enable the ohmmeter OIcuilS. \X'hen the DC switch is then SCt (0 eithcr a high or low resistance measurement range, the dry cell battery is connected in series with the moying coillluough a path of resistors to pro\'ide the required full-scale deflection cu rrent when the main inSlnlmem lenninals are connected through zero resisr:mce. \X'hen a resistance is then inserted between the tcnninals. thc pointcr will fe-position to show its tnle \·allle, C urrent intensity
,
[I is possible lO make a comparison between the imcnsity of current and the intensity of sound, \'('hen currcnt intensity is being compared with an audible frc~uenc}') the unit that is Llsed is the 'bel', One tenth of a 'bel' is a decibel dB ... \'/.Rnd -~f}~ mimmum intensity that can normally be distinguished when comparing (,,\\C-;'c'" tWO intensities is onc decibd. Some mllltimcters h:l,\'e a decibel dB scale of \",~\(' - 15dB to + ISdB so that comparari\'e intensity measurements ca n be made,
,\ final note on mulrimeters concerns the maintenance of the connecting leads. probes and clips that are ~upplied with them. I f care is nor taken, thcse may be damaged and seriously affect the accuracy of a reading, It can be acutclr embarrassing to discO\'er that a maintenance decision has been taken based ~ a rcading that subsequently turns out to be down to a faulty k'ad or connectiof! on the metcr. You could say it is a classic example of the ,\,('heel-tapper'~ I lammcr', One addition to the list of poims to watch is the least reliable part of any ohmmeter - the dry ccll battery. If there is any suspicion that this may bF discharged. o r there is difficulty in zeroing the meter, you should change tIi!: batten', ~ :P~ c·
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DigitaJ MuJti m eters The major difference between a digital electronic muirimeter and the nonelectronic types wc ba\-e been discussing is thc way in which the information is displayed. The analogue displays normally associated with [he non-electronic models are replaced by a liquid crystal display that shows a series of digits and an annunciator to show at a glancc whac is being measured. Thc reading accuracy of a digital mulrimcccr depends on lhe number of digits it can display, and the percentage accuracy of the readings. Meters haY"e 3- \ / 2 or 4-1/2 digit displays. :\ 3-1/2 digit meter for example will display three whole digits from zero to nine and a half digit that might display a figure 1 or a blank. This can produce a scale up to 1,999 countS; other versions may ha\'c different resolution counrs such as up to ~.OOO counts for e.-;:ample. The reading accuracy is bener than that of analogue meters because the display will show the aernal decimal fractio ns between urne s mther than leaxing you to estimate a poi nter position between rwo lines. Some meters \\i11 indicate to sc\-cral decimal places, This must be viewed in the light of the pcrcentage accuracy of the instrument of coursc.
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r
Digital multimc(crs arc very compact and incorporate a 'freeze' function on the display that allows you to take a measurement and read the result la (cr. 'nley normally have JUSt one rotary selector switch that covers both AC and DC measuremel1(S and dus is often accompanied by an auto-ranging system that automatically selects the required measurement range. This automatic range facility can be switched ( 0 manual if so required. When operating on AC selection, most of these meters display Lhe RMS (root mean squa re) nllue of the .\C \-ohage w3\·clonn. TIlese meters are dcscribcd as 'a\'cragc responding'. the R..\lS >aiue being the equivalent DC value of me A.C \·oltage. The meters may also be equipped with a rC\'erse polarity protection de ...ice so that rou cannO{ damage the meter if you inadvertenrly connect the leads incorrecur.
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Other Measurement Devices Potenliometers Potentiometers are used for the accurate measurement o f potential difference. The~' arc primarily used for measuring ,-cry low voltages. They can be adapted t~
measure resistance. -111C principle used is that an unknown potential (hfference can be measured by comparing it with a known pOlcntiaJ difference. In its simplest fonn it consists of a resistor that has truce terminals, one at each cnd and one attached to a sliding contact that runs aJong the length of the resistor. This set up can be used to change the amount of "ohagc in a circuit simply by moving the sliding contact along the resistor. Sccond:IIT Cdl
.'
•
--1.1 =-:-----j=--~ St:lndard Cell
__--,
Test Cell F ig ure 3.l20 · Simple POlcmiom elcr
The simple potentiometer illustc:ued in Figure 3.120 has a secondary cell connecled ac ross a resistance ''';re. One side of the wire is connected through a standard cell of known e.m.f to a changeon'r switch that connects to a moviRg coil voltmete r. The other side of the voltmeter is connected to a sliding spri ~ contact that can be adjusted to contact d1e resistance ,,;re at any point. T\\:g [cslleads arc used to connect across the tenninals of a cell of unknown e.m. e. ;2" The potentiometer is sct up by positioning the changeover switch to connC!(Of' the standard cell through tbe instrument to the sliding spring contact. "nib comact is pressed ontO the resistance wire and is mO\'ed until the instrum~.nt reads zero. The changeover s\\;tch is then switched to connect the (cst cell :#uo the circuil. The spring comact is again pressed and repositioned to again/zero ,the instrument. ~ The unknown resistance is c:t1culated by eguatlon:
tran~ position of
The
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E\lF of Standard Cell F.~fF of lnkno\\'n Cdl
Length 1.1 Length 12
'f1us represents the principle of how the e.m.f of an unknown cell can be found using a very basic potentiometer. The instnllnents in use arc ob"iollsly more sophistic:llcd than this example.
"1.5\'
.-
Fixed ReSIstor
1
\llIluplier
1
Zero Set
Figure 3. 121 - POIcnrio mc ter T ype Ohm m e te r
'rhe pmentiomcter ohmmete r consists of a seositi\'e moving coil ins tnln1cm thar is connected in series with a barrery, a nr:L'lble resistor and a pair of test leads ..-\ low fixed nlue resistor is connected in parallel with this series circuit. W'hen the tes t leads are joined together, all rhe baneI')' current runs through the moving coil and the pointer deflects. The instrumeEll can then be zeroed using the variable [c*\lpr. If the leads arc then separated, all the current runs throug~..:.tI~'fi xea low value resistor and the instnlmenr will read infinity. \\'hen thE Jb'ih:f!; are placed across an unknown resismnce, the curren! running through \\~(Hle fixed value resistor is directly affected by the unknown resistance and the ,j-"\\ '"' instrument scale can be calibrated to register the '·alue of tlus. -nle range of the (1.-,..;:'\ instrument can be increased by inr.roducing multiple \'alues of the f"xed resistor in a rotary, zero adjust switch.
Hot \Vire Instruments Hot wire: instruments arc based on the thermal expansion and contraction of metals. \'('hen a current passes through a meral conductor. it creates heat. A high melting point alloy is used that has a high coefficient of expansion. The linear expansion of a wire or rod is linked to a mechanical arrangemcnr [hat will move a pointer on a scale. 'nle expansion of the conductor is proportional to the s(luarc of the electrical current flowing through it. Therefore, the ins trument scale gmdualions are set our in accordance with the squa re law. They can measure AC or DC.
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T hermo-Couple Instruments \,'hen two wire cond uctors made of differem metals arc joined at their ends to form a closed loop. a thermoelectric electromoti,"c forcc (e.m.f.) ,,111 be set up in the loop if there is a difference in temperature bet\veen the joined ends . The magnitude of the e.m.f depends on the difference in temperature bet\veen the joined ends and the materials used. One end of the loop is designa ted the Cold Junction and the other the Hot Junction. TIle Cold Junction is joined through a milli\'oltmetcr that \vill measure the potential difference across that junction. 'rhe Hot Junction is connected to a current carrying conductor. \\11en an elecmcal current is passed through the conductor, it \\111 create heat in rhe Hot Junction and the instrument will measure the potential difference created at the cold junction. The heating effec! is proportional to the square of the current OO\ving in the attached conducto r so the scale is a squa re law type. They can measure AC or DC.
E Jectro-S tatic Vo ltmeter The electro-static \·oltmeler is the onl" instrument that reads \·oltagc direcdy rather than through the medium of electrical current. 11lesc instrumen[s depend on the force o f attraction that exists bet\veen plates that carry opposite charges. They are only capable of measuring yoltage, they emnot measure current. T hey are particularly suited to measuring high \·ohages . .A simple arrangement consisTS of fWO fixed metal plates positioned a shon distance apart that hav e a free mO\-ing plate positioned bet\veen them. The free moving plate is mounted on a spindle that carries a pointer at one end ll11d a ha.irs pring at the oth er to prO\~ide the control torque. The ins trument is connected so that the fixed plates acquire an equal and opposite charge to the mO\-ing pla te. The fo rce of attraction causes the modng plate to deflect, taking the poi mer across a sc~tef"They can measure AC or DC. 1\\\)O I,!,o.: -
. C.L~\C. , c
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Transfo rmer Rectifie r Meter The transfonner rcclifier meter is med to measure the current in £\C circuits where it is not practicable to disconnect cables to insen a normal ammeter Tn series. The Jaws of the instrument are closed to encircle the cable under tC~ \'·hen this is done and current is flowing in the cable, the cable acts as tl~ primary winding in a transformer. The alterndting electro-magnetic ficl~ surrounding the cable induces an e.m.f in secondary windings contained witbig: the instrumen l. The secondary windings connect \\~th a I11m-ing coil to gi ~ fuU~scale deflection at a specified \-alue of amperes (R..\IS). There is a range i f ~ instrumems, each having a specified Ampere (lu\ IS) value. ~
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Tong Test Ammeter TIle tong-test ammeter is another type of 'dip-on' instrument that is used [0 measure the CUHent flowing in cables when it is not prnccicable to disconnect them to insert a nonnal ammeter in seoes. The tester Incorporales a modng iron instrument and this enables this instrument to measure AC and DC "alues. The principle of operation is similar to the transformer rectifier meter. The jaws arc composed of soft iron laminations that connect with the magneric circuit in the moy-ing iron instrument. The instrument is provided with a number of different replaceable indicator units to provide a range of measurements.
/
Cable L·ndergoing Test
Fig ure 3.122 • Tong-T est Alll meter
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The tong-test ammeter gives the mos[ accurate readings when the pointer is registering bSt\~l!:h 40% and 80% on the scale. The indicator unit that is se!e~tQ_Je,',)\hse should be chosen on the basis that the expected reading would f~H ~'lt(lin this range.
:-.{\~
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O sciJJoscopes 'nle electron beam in a cathode-ray tube is deflected b\' the electrical current applied to the deflecting plates. TIle actual deflection of the beam is proportional to the applied l"oltage. Because there is a direct link bet\veen the posicion of the beam and voltage, rhe oscilloscope display ca n be easily calibrated to deli\·er voltage readings. A mnge of ,·olmge measurements can be obtained by pro,·iding suitable shunts to moffit)· thc ,·olcage rccei,·cd by the plates against a related scale. The fast response of the electron beam makes it fea sible to measure both .t\C and DC ,·ohages. Oscilloscopes arc also used for phase analrsis and frequenc)" comparisons. TIle problem fo r the maintenance technician is that the equipment is often complex to operate and interpret.
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Time Domain Reflec(Qmercr Time domain reflcc(ome(crs ha\'c bcen in usc for some lime. (0 idenrifv faults and [Q locatc meir position in fibre opnc cables and the principle can be applied to other mediums such as decrncal cabk·s and CO+fl:..:ial cables. The principle is that they measurc the loss. or :menuation. of powcr in a cable under test and instantly compare this with stored data on rhe expected loss in a virtual. perfect cable of the same type. The results arc displayed graphically on an oscilloscope th:ll displays the loss of power over distance and thus accurately identifi es the position of the source of that loss. In a way. this is JUSt what the
.\mmelers must be connected in series \\;th the live circuit through which the current will flow. The function of an ammeter is to measure current fl ow (milli-amps or amps) in a circui .. :\n ammeter mU$t not be connected directly ac ross the voltage source. for example the bauery connections. The resistance of ammeters is IO\\" and the high current flow can damage the meter. Ammet~ incorporate a low '{"alue resistor called a shunt that is connected in parallel arijI bypasses most of the current past the coil. The selected ammeter sbould have large enough range to give less (han full -scale deflection in the circuit unde't, test. If yotl usc a milliammeter to measu re amps, you \\ill wreck it! :\mmete r~ arc most accurate in the mid-scale range. It is very important to check that YoE connect the ammetcr to the circuit in the correct polarity. If you get tlU:s wrong, the pointer will be forced back againsr its sto p and it may be bent ~ a result. ?f
i
Ohmm e te rs !.~
Ohmmeters arc connec ted in p :/Ttllle! with a circuit with rhe powe~\Vitched off. These instruments are designed to measure the continuity resistance
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(ohms) between two points in a circuit or the circuit insulation resistance (megohms). Never ancmpt to measure resistance in a circuit while i[ is still connected 1O an clrcuical power source. 111e ohmmetcr is the only instrument to ha\'c its own, self-contained banery and ~Icgger and Iligh Tension lestcrs have Iheir own self-contained hand-crankcd generators. Always choose a measurement range that will include me resistance \'alue of the circuit. II is recommended that you choose an ohmmeter rangc in which readings woulcllie around mid-scale. A tip is to use a range where the expected reading does not exceed ten times, or is not less than onc tenth, the mid-scale reading. Always zero the instrument before usc by shorting the test leads together :lnd setting the zero adjustment. \X/hen measuring rhe resistance of components in a p\'lmUcI ci.rcuit, always disconnect one end of the component to prevent reading the resistance of parallel p:lths. Do not usc meggers on electronic components. Also, do ensure that the voltage you arc going [0 apply with a megger will not damage components in the circuit such as fuses and lamps. II is import\'lnt to check that the ohmmeter bartery charge is adequate. I t is also important to check that the connecting leads are in good condition and arc not producing excessi,'c resistance that might be mistakenly attributed to the circuit under tcst. I f the resistance of the IC\'Ids exceeds I ohm [her need replacing.
Co nclusion We ha,'c now reached the end of Chapter 3. To remind you, circuit testing in (enns of continuity, insul\'ltion and bonding [echniqucs and testing is a Chapter 7 topic. The electOcai general tcst equipment that we ha,-e discussed here plays an indispensable role in this. The syllabus for this chapter includes the oper:ttion, function and usc of electrical ground rest e{luipmeoc \X 'e h\'lve discussed the ~rntion \'Ind function of this equipment at some length. l l owe\:!!!z.~b\'lvoid duplication, the usc of rhe equipment described here will ghs;~'tSJ)' be amplified a gre\'lt deal further in the content of chapter seyen. In \ ... \\·'~ dle meantime, I ha,Te provided you with a few multi-choice \'Inswer questions ~\¢' to try before moving on to the next chapter. Do remember, you C:ln builcl up a (",(!"'" bank of questions of your own making. Tn the meantime, a coffee break is dearl)' indicated!
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Revision Tools Ques tions L Th e teeth o n a hack saw b lade s h o uld poin t: :l,}
:l,W:l,\·
from the handle
b) towards the handle
c) in either dire-crion
2. A ream er sh o uld b e tu rn ed: a) anti-clockwise when cutting and clockwis-c when being rcmO\-ed b) clockwise when cutting and anti-clock\\;sC when being rcmo\Td c) in the clIlting direction both when cutting and when being remO\'cd
,,-,i
_!l¥.ill - '1"I1 e torque loading appli ed by a wre nch is the ptoduc t of th e (.~,\\.:~o.: ... p erpendic ular dis tance from the poin t of appH cation of th e tangential \\)~~..
fo rce on the handle to th e ce ntre of the bolt head : a) dividcd by the tangential force b) mulriplicd by the tangential force c) added to the tangcllIi'l.i force
4. T he co rrect cutting ang le of a twist drill used on mi ld ste el is:
")
90"
b) 130"
c) 59°
229
CHAPTER
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S. Files a re norm ally cl assified by: a)
material, length and grade
b) length. grade. cur and cross-section c) length, width, material and cross-section
6. Steel ha mm ers a re class ified by: a)
weight and type of the heau
b) weight and shaft length c) shape of head and shaft length
7 . Th e cut o f a fil e re fe rs to the:
a)
arrangement of the: Leeth
b) number of teeth per inch c) grade
8. T o achi eve a good fit wh en e uuing threads o n a stu d and tapping a m a tching inte rn a l th read you s h ould : a)
,.,
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first and then adjust the internal thread depth
• ,\\.:.'t-~ ~~~f~ C~) cut the internal thread first and then :\djust the e:...ernal thread depth c.
c) Cut and then adjust both rhe ex ternal and internal thread depths
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9. T he s hanks o rJ arge sized d rills h ave a fMo rse' tap e r to: a)
provide a positive drh-c in the tapered socket o f a drilling machine
b) fit in a self-centring chuck
c) adapt them for usc in hand drillit1g braces
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10. A coa rse hacksaw blade wou ld be lI sed for cU lling:
a) rerrous material only b) thick section material
c) thin gauge sheet material
11. A combination s pann er consists of: a) different sized open jaws ar each end b) an open jaw at one cnd and a ring of a similar size at the OIher end
c) an open jaw at one end and a ring of a different si7.c :n the other end
U . Pliers are class ifi ed by:
a) type and b)
o,~ernlilength
jaw width and m-ernillengm
c) type and \\;drh of jaws
13 _ The purpose of th e grub sc rews in the stock o f a die is to:
a) _secure the die in the stock only 11."-1
b) hold a split die closed c) secure the die in the stock and adjust its sizc
14. A fulJ set of BSF taps consists of a :
a) taper tap and a plug tap b) tapcr tap. second tap and a plug tap c) taper tap and a second tap
231
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15. The Comb ination Set head used for locating the centre of a round bar is the:
a) Cemre Head
b) Square Head c) Protractor I-lead
16. The type of wrench t h at prevents over- tightening is calJ ed a torque :
a) indicating wrench b) limiting wrench c) setting \'Hench
17. The abrasi ve wheels on an off-hand grinding machine spi n : a) up and away from the irem being ground b) down towards the item being ground c) in either direction depending on selection
18. The sides o f the abrasive whee ls on an off-hand grinding m achine are:
b) used for sharpening drills c) used for grinding soft materials only
19. Off-set and angled head air driven drills h ave:
a) a self-centring chuck thar firs a cringe of drills b) separa te chuck collets for each size of drill
c) tapered drill attachment sockets
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20. A surface plate is u sed :
a)
[Q
pro{CCl dlC finished surfacc of work held in a ,'ice
b) to protect a surface table when using a cen ue punch c)
for checking tlat surfaces for lruth and fo r marki ng out work
21. T he pitch o fth e threads o n tbe s pindl e o f a M e tric Microm ete r is:
a) O.02.3mm
b) O.OSmm c) O.Smm
22. The pitch o f th e uuead s o n t he spindle o f an English Microm e ter is:
a) 0.02.3in
b) O.OSin
c) O.lin
23. The nib size en g raved on t h e fix ed jaw o f a Ve rnier c alipe r indicates the:
b)
me thickness of one nib
c) the nib length
24. End n Oat in a m icrom ete r can b e reduced b y: a) adjusting the barrel POSInOD with a C spanner b) greasing the spindle threads
c) adjusting the nut in the Ollter end of the barrel
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25. Th e m e as urem ent range of a stand ard external M e lri c mi cro m e ter sca le is :
a) \OOmm b) 25mm
c) 50mm
26. Th e meas urem ent o n a Vernier ca liper is ta ke n at th e:
a)
zero mark on the main scale
b) left hand edge of the Vernier scale cursor c)
7etO
mark on the Vernier scale
27. A Verni e r Beve l Pro tractor w iU meas ure to an accuracy of: a) 5 minutes of a degree
b) 2 minure of a degree c) 0.5 minute of a degree
28. A sta nda rd ex terna l M etric m ic ro meter m eas ures to :an accuracy of:
29. A D ial Tes t Ind icato r is zeroed by: a) adjusting the plunger to align the pointer \\;[h the zero b) adjusting the position of the gauge in relanon
(0
the work
c) rotating the dial bezel ring to align the zero widl rhe pointer position
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C H A P TER
30. \'(then taking an internal caliper you :
me~l su reme n t
THREE TOO L 5
with a stand a rd Ve rnier
a) subtract the \\;dlh of the nibs from the reading b) add the \\;dth of the nibs to the reading c) ignore the width of the nibs
31. The acute and ob tuse angles indkated on the Ve rni er Pro tractor sca le illus trated b elow are: 9[)
I
80
I
:I) _tOO 35 minutes :lnd 127° 25 minutes
b) 49° 25 minutes and 130° 35 minutes c) 49° 35 minutes and 130° 25 minutes
32. Th.c rc ad ing indi cated o n th e Metric M icrometer illus trated be low is : ..,\~\\:t
. "c,,",o..: l."'\ . \\,,~~.
11'11.' -
a) 12.2mm b) 12.7mm
c) 13.2mm
m-rTCmTTml==---!ZO 1::3::0--...) 15
1 - - - - - -....
~.
-
"""'-----_%
235
CHA.PTER TOO L 5
THREE
33. The dim en sion indicatcd o n the 1-2 in c h E n g lis h m icro m eter illustrated belo w is:
,) 0.46 in 1I
b) l.-4S in
c)
2
3
4 10
1.-46 in
1 .. .2
111
:-'licromc(cr
34. The reading indi cated o n the 24 / 25 Eng lish Ve rni er scale iIIustrat cd b elow is: a)
4
-I..-45S in
I
b) -1..-433 in
I I I
5
I
1
1
56 1 -
23-1.
I I I II I I II I I II I I II I I
~ I r j' r
111111 Jj II ,
c) -1..65 in
H
9
!11111 rr I 1[[11"11"ltl I [ [ I II I I
III
I.,
!I'
I
2,
35. The reading indi c ated o n th e 49/ 50 M e tri c Ve rni e r sca le illus tra ted below is:
a) 6-I.3S0101 b) 64.3-hl101
36_ \~'h en c h ecking an o utside dim en s ion wi th a di a l test indi cator: a) deflections anti-clockwise indicate O\Tcrsizc and cloch.""\\;sc undcrsi7c
b) only cloc,,",vise deflections can occur c)
deflections clock..wise indicate ovcr5izc am\ anti-clock-wise undersize
,F .:- . .~ ~
37 _ A Universa l Lubricati on g un is d es ign ed ror use w ith:
;;'" "-
a) oils or so ft g reases b) soft greases only
c) oils o nly
236
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CHAPTER
THREE TOO L S
38. Oilit c bush es
~lrc
impregna ted w ith:
a) grease only
b) o il or grease as required c) oil only
39.
~nc n
operating a grease gun, the air release screw s hould Do nnally
be:
a) opcn by
twO
[Urns
b) closed c)
fully open
40. T eflon coated bearings
a) not lubricated b) lubricated with dry grease c) lubricated with thin oil
41. Th e auth o d ty for th e use of a given lu b ricant o n an ins talled compo n ent is the : ~I.-'l\
component ovcrhaul and mail1lcnance manual
b) aircraft maintcnance manual c) rtircraft technical log
42. Ohmmeters m easu re th c resis tance in a circ uh by sen s ing :
a) mirage b) powcr c) current
237
CHAP TER
TH RE E
TOO L S
43 . The ha irsp ri ngs fill ed in a m ovi ng coil m e ier provide th e: a)
comroltorque
b) deflecting torque
c) damping torque
44. To m casu rc the cu rrent flowing in a circ uit, a n a mme te r is co n nected m:
a)
parallel with the circuit
b) series with tbe circuit c) directly across the term.inals of [he \·oltage source
45 . \Then measuring continu ity res is lan ce, supplie d b y:
:111
ohmm ete r uses cu rrent
a) the circuil power source
b) the aircraft bartery c) a self-contained banen·
46 . A 's hu nt' res is to r is used in an ammete r 10: ,\~b
'l11.1
...
. ~.:.'1'aSs most of the current through the coil " l,L
\"5~ .
~\. \,; '\ .
b) damp the monmem of the indicator
\.\
\,. " ~'
c) by-pass most of the current paSI the coil
47. An a irc raft bo nd ing test m e te r is a : a) low rangc ohmmcter
b) low resisranee vohmcter
c) low range amme(Cr
238
CAp)7ighl e BIll!) ulKgt
~006
~-
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CHAPTER
THREE
TOOL S
48. If an o h mmeter is connected across a break in a series circuit, it will register:
a) zero
b) infinilv c) mid-scale
49. In order to measure AC voltage and current, a multimeter mu st inco rpo rate ~I: a) rectifier
b) shunt c) swamp reslsror
50. The ohmm eter section of a muitimc le r is des igned to meas ure : a} insulation and continuity resistance b) insulation resistance onI\" c) continuity resismnce only
51. Currellt n ow can b e measured without breaking into a ci rcuit lIsing '1'II.,b
'I\l'
a:
\\ .,;..<:-
\'~;;~' X\~
...~,.-
- ~,'\
c·
(.11' '-.
a)
Bridge mcggcr tester
c
b) Continuity tester
c) Tong-test ammeter
52. A Megohm is: a)
1,000 ohms
b) 1.000.000 obms c)
10,000 ohms
239 R'1lTJ'.parl66.(,,,p
CHAPTER
THREE
TOOL S
53. A milli-amp is:
a) one thousandth of an amp
b) one millionth of an amp c) one Icnth of an amp
54. A milli-voltmeter can be modified to meas ure \'ohs by filting a:
a) shunt b) multiplier c) weaker hairsprings
55. If a bonding teste r lead is damaged at a connection point you:
a)
IU.'ly CUI
off lhe damaged portion and remake the connection
b) cur off the damaged portion and splice on a new length of cable c) must rCIUIn the rester to the manufacrurcr for repair and re-calibration
56. A torque indicating w renc h is 12io long and has a 4in extension spanne r fitt ed thm is aligned with the axis of the wren ch . If the actual torque Joading va lue of a bolt is 200lb in, w hat val ue w ill be " d IC%(l. " d on '1\)\\the wrench' In . l
,." <"" , .'~'
,
\,).<;{'
. L\'\ "c\!....: .. •
'a)
1501b/in
b) SO Ib/ in
57. The co rrect m ethod of producin g a s tart for a drill o n a sheet of titanium is to:
a) use a centre punch b) cemrc drill c) use a diamond point chisel
240 Cop)Tighl 0 Barry Colflgt _"006 . 200 -
All righfJ ffitrPtd
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CHAPTER
THREe TOO L 5
58. Ammeters bave: a) high resistance b) a self-contaIned batten· c) low resistance
59. Graphit e grease is used : a) as an anti-seize lubricant in high temperature applicacions b) to lubricate hIgh-speed ball and roller bea rings
c) as a metal to rubber lubricant
60. The
Hill- O ut
of a s haft can be c h ecked using:
a) a dial test indicator
b) a Vernier I-Ieight G auge c) external calipers
241
C H APT E R
THR EE
T OO L S
Revision Tools Answers A
2J.
C
41.
B
2. C
22.
A
42.
C
3.
B
23.
A
43.
A
4.
C
24.
C
44.
B
,.
B
,-,.
B
45.
C
6.
A
26.
C
46.
C
7,
A
27.
A
47, A
8,
B
28.
A
48.
9.
A
29.
C
49. A
10, B
30,
B
50.
C
11. B
3 1.
C
5 1.
C
32.
B
-, ,-,
B
13. C
33,
C
53.
A
14, B
34.
A
54.
B
15. A
35.
B
55.
C
I.
12. A')~'\l,b
;.\~\''';
. ~\..:.'\!...;. ... l·\' .
. ,\,\
"
\. \' \
""<'-~
,
\\).t{':
B
r
'c
,
-? ,
16, B
36,
C
56.
A
17, B
37,
A
57.
B
18, A
38,
C
58.
C
% ~ ,~
19, B
39,
20. C
40,
B
59.
A
60.
0
;;:::::
A
c
"
A
,, '-
"
242 Cop)right" Bmry C(}lltK
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llJuw.'.porr66.roIJI
'"""
A VIONIC
GENERAL
CHAPTER FOUR T ES T EQUIPMENT
Avionic General Test Equipment Introduc tion \,\ 'c\come to Chapter 4, the syllabus givcs no inuicacion of what test equipment it wishes us to co,er except to say it is generaL I have interpreted dlls to mcan the equipment that would usually be found in general usc on most aircraft types rather than on a select few. In this chapter, we \vill examine avionic test equipment used during ramp maintenancc and in some of the more common on-aircraft tasks in base maintenance. The equipment includes: •
Portable _m Data Test Set
•
Air Data Test Set
•
Transponder, D).IE (Distance 1\leasuring Equipment)/'J'C:\S (frn ffic A.len and Collision Avoidance System) Test Set
•
VOR/ ILS (VI IF Omni-direetion Ranging/ Instrument Landing system) VIIJ~ Communications Tes: SCt
•
Fuel Capacitance/ Quantity Test Set
I [email protected] have included the Aircraft Bonding Te~t set in rills list but we dealt that piece of equipment in detail in Chapter 7 of tills module under Bonding Techniques and Testing .
..,:.0.:. "ll'\~~ill; .
,,< c. ,;.~
U''-;:;
\\;\~('
0..,\\... ·
The usc of the [cst equipmelll I have listed rc(!uires some knowledge of the
associated a\·ioillc systems contained in Module 11.5 for B1.1 and Module 13A for 132. !-:IO\\·c\'cr, I will include abbreviated descriptions of these systems for you so tllal you can get an idca of what tCStS arc require.d. Tins is done mOte for the benefit of the BI studems: 32 srudents will know these systems. \\"f: will be dealing with general principles and descriptions of the various items test equipment rather than concentrating 00 specific proprietary makes. All means is that the illustrated [cst equipment may show controls and displays in ~ differcnt layout to those you mar observc on a particular make. 3
'" ot rim:. ~
I will remind you at rimes through the chapter thal you must abide by ~ e
instructions in the User Manu.-u associated with each item of test equip~cm that you usc. Additionally, you must complr with the. approvcd systel~tcst inslrUctions contained in the appropriate Aircraft ~Iaimenancc Manual (Ai\L\f). If you wish to look up a fcw of these, rclevant ATAIg8- j\i\ThJ chnptcrs are: ~
upJrighl
e 8(11"9 Colk§ 2006 - 200" All righl! rtSm'rd
243 1l'll:ll'.por166.rom
CHAPTER FOUR AVIONIC GENERA L
TEST
EQUIPMENT
Chapter 22 .\utollight Chapler 23 Communicacions Chapter 31 Instrumcnts Chaptcr 34 N:n-igation
Portable Air Data Test Set This is a vcry commonly used item of test c(luipmcnt so wc will nced [0 examinc it in delail. Wc will not be dealing with :my specific proprictary brand of cquipmenr in this scction. I will describe the fCHures that would be rypically incorporated into this typc of test set and how they might be uscd. Howc\"er, you must abide by the User Manual for the particular item of C(luipment you arc using and comply with rhe systcm test instrucrions in the appropriate maintenance manual. A simple. portable, Pitot/Static test set incorporates a hand-operated air pump thai can be selected to genera te posiuye or negati\'e pressure for testing aircraft systems and their components that use Pitot or Static pressures. -n1ese include: •
Altirudc indication
•
Airspeed indication
•
Alntude switch operation
•
:\irspced switch operation
•
Verr: I'Gnl 'Sp~ed Indication '1\,.\\:
.,,;
. (y\\....\"
, .'1....
\.. ,'1\
i..\.p'\.
-
~;I.«.<. .
The hand pump is regulated to limit d1C maximum pressure/vacuum output to a safe \'alue that will not cause damage to the aircraft instruments. System vennng and [he rate of change of air pressure arc achie\'cd through a R ele:lse V:111"e that is thronled to pennit a controlled vent rare. It is opened manually to gradually vcnt the sys tem pressure. It is "ery important to control the rate of change of air pressure in Pitot/Static systems to avoid creating e."-::cessive pressure or temperarure changes, which rna)' cause damage to sensitive instruments. Consequently, venting the system through the Release Valve may take several minmes. The output port incorporates a second manual Letdown Val\~e thar allows the operator the choice to control the rate of venting back to ground \e\"e1 pressure. l11c same rule applies, the system must be rerurned to ground nunosphecic pressure at a safe rate. For example. the l...ctdown Valve may be used to vent a syStem that was already down to a safe altitude or airspced.
244 CDpynghr Cl &ny CD1k6 2006 - 200- AU righ/~ fU(fJIr8
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AV IONIC
GENERAL
CHAPTER FOUR TEST EQUIPMENT
Switch Connections (Altjwde & Speed)
o o ~"'" 11(150 -30 o I ,-~k Iluo·j(l Fr:\1 ~I"r Ch",,~,'
11).1oIl
lin
DUilet to Picot Of Static
A~\l
Cuntrolled rX / VAC Relcase \'al\"c
Fine :-\djll~1l11ent ---k;,.'C .\c,,!·_ ....."-''''-to
llanclpump
---til.
Positive or Negative Pressu re Seleclnr
F igu re 4.1- Portab le Ai r O;ila Tcst SCI
The hand-operated pump is used to SCI rhe output pressure to an approximate \-aluc. A fin e pressure adjustment control is then screwed in or OUt to raise or lower the output pressure in small increments to achieyc the required yalue. The outpul port lllcorporates a standard connector to which a single flexible hose carrying the required Pilot or Static aircrafr connector may be fined. ·I1,e test set is calibrated for use wilh a specified length and bore hose, for example, Sm at 6mm bore. Th e,~ftery
operated display section of (he set incorporales a task menu fo r the .c'1I.\Qfcrection of the requi.red test option. The possible selections for Altitude arc either Leak Test or Switch Test. The possible selections for Airspeed arc either . (."...\\..:."" .: \.... ')(~' Leak Test or Switch Test. English ,alues for Altitude can be displayed in feet, .\~...'\. ft/ sec, ft/min. i\fctr1c values for .Altitude can be set in metres, m /scc, Ill/nun. C.;."" . English \·alues for Ai rspeed can be set in mph or knotS and metric yaiues in kph. The test set incorporates electrical socket connections for ,-oltage and current mea surements and for testing Altitude and Airspeed switch operatio~
-
-0
There is a facility for entering rhe ambient pressure or the day, ie QN l l o!QFE, as a correction to the display readings as an altimeter reads altitude iq.: feet not units of barometric pressure. I f you do nor want to usc local pressur~ correction, the International Srandard Aunosphere (ISA) table gives sland~ mean ba rometric pressures for given altitudes. \'\fe know thar in reality, t.1~ actual pressufe at any given altitude will \-ary from day to dar or even houJiio hour. This is why we may need to correct altitude-measuring systems~for ambient pressure changes. One problem that occurs when we consider a!§t'ude is what we use as a reference point for zero altitude. Zero altitude rb""ay be expressed as the aLmospheric pressure at Sea Level or the pressllr~&t't some point above Sea Level. \'\11(::n we express barometric pressure, wtc usually annotate it with a Q code as follows:
245 COPJrighl C Btll?)· Colu§ 2006 - 200- All right! rrmwd
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C H APTER FOUR AVIONIC G ENERAL
TEST
EQU IP MENT
Q Codes
QNH This refers to the local barometric pressure compensated back [Q Sea Leyel, which is then used as zero altirude. \'Vhen QN II is used, the aircraft's altimeter will display the aircraft's actual altitude above Sea Leye!.
QFE TIlis refers to the local barometric pressure at airfield level; at this setting the altimeter will read zero (0) feet on take-off and landing.
QNE Aboye a point known as the transition alcirude, nonnally around 6.000feet in the UK, all aircraft reset dleir altimeters 101013.25 millibars (mb), now called Hectopascals (hPa), or 29.921 inches of lIlercury (in I-Ig). This is the IS.-\ standard barometric pressure sernng. This ensures that all aircraft operating above the tmnsicion altitude arc using the same altimeter sening. QNE is more ofren referred to as 'Fligh t Level' expressed as a multiple of feet above th e lSA zero feer level. \,('hen yOli hear that an aircraft is flring at Flight Level 80 for example, it means that the alrcraft is at an altitude of 8,000feet above the ISA zerO feet level.
Pitot/ Static System Leak Tests \\fhen conducting system leak tCStS, the {eSf set must be connected to the correct aircmft sampling pain!. \,'hen carrying out an Altitude leak tcst, the flexible hose is c9flnected from the Output POrt on thc test set ro the Static pressure R\3{\b0;t'\lie aircraft. \'V'hcn carrying OUt an Airspeed leak test, the hose is L, cW\nl!t:£cd from the output port on the tcst set to the Pirot probe or tube on \\;\"tthe aircraft. Static Connectio n for Altimde Leak Test (Ncg:l.th·c Pressure)
r,
.,, '0
,~
PiLOt Connecuon for Airspeed Leak Test ---tL----(positive Pressure)
Figure 4.2 - Connect io n Poim s o n Airc m fl fo r SY'ile lfl Leak T cSls
246
uP.Jrijht e Barry Co/kgt :006 . :!OO - A ll njhlf frJtTWd
~
AV IONIC
GEN ERAL
CH APTER FOUR TEST EQUIPMENT
Before carrying Ollt an)' test 00 the aircraft, YOli must ensure that the test and aircraft system hayc been fully vented to atmosphere.
SCI'
Wait T im e
\Vhen you alter the pressure of air in an enclosed space, it will alter its temperature and this affects the pressure. The \'\"'-\!T TIi\!E is the time required to allow the air temperamre and pressure to stabilise before commencing any test that requires you to set an accurate altitude or airspeed reading by altering the air pressure in the associated system. The time wiJl nonnally be specified in the test instructions and can be as much as ii\'e nunmes 10 some tests.
Altitude Leak Test For this tcSt, the flexible hose (rom the set is conneclCd to the Sta tic porr on the aircraft. The display menu is selected 10 Altitude Leak Test together with the required units. '111e ambient pressure of the day o r QFE correction can then be entered if so required. "Ille controls on the set should be set as follows. The Fine Adjustment is set (0 me halF-va), position. The Release Valve and the Manua! Letdown Valves are e10sed and the hand-pump output selector is set to vacuum (I1CgaU\
.,\.",,\'?h~';he RUN TEST key is
pressed, the display will count dowo the \'('AIT ""W.';-c - TIME. \'Vben this expires, the START ALTITUDE will be displayed and the , \-.,;\~{' l. TEST THvIE will begin to COUnt down. On completio n of the TEST TI I\ !E, {I.,\\..b. the STO P ALTITUDE will appear in the display. -(\-.;..
-0'\' .
L,
12530
~
r\!titude IT Ti me 60 SEC S[:Ut 12550 IT Stop !2530 IT Change -20 IT Leak Ratc -2U FT / 1\1I t\!
II Static Connection
II
~ -'
I
-,
r.
"
\'J\c-@
Ncgacyc PX Fig ure 4.3 - Altilude Leak Tesl
ItJJ 'v
~
'"
If you examine the illustration. this particular test gave a STlillTAL1;!.TUDE of 1255 feet. Oyer rhe 60 sec duration of the test, the a.ltitude reduced:l:"'o 12530 v feeL This occurred because leakage of air into the system reduced dle \"HCllum
247 CO/!)'ri!,hl 0 Ban] Collfgt 2006 ·2007 All rights mmlf'd
IIIJj·flJ.part66.rom
C H AP T E R F OU R AV I O N IC GE N ERA L
T EST
EQUIPMENT
and thus reduced lhe alrimde reading. The altimde change in this case was -20 feer over 6Oscc, which gives a leak rate of -20 feet/min. Tlus leak r:ue must then be compared with the perrnined leak nlle value in the Maintenance Manual. On completion of lhe test, the '"3CUWn should be reduced to a safe "alue, certainly below 1500 feet, before slowly opening the Manual Letdown Valve to bring the sysrem pressure gradually back LO ground le\'el.
Mrspeed Leak Test For this lest, the flexible hose from the set lS connected to the Piror probe or tube on the aircraft_ The displar menu is selected to AIRSPEED LEAK TEST together wirh the required units_ The hand-pump ourput is selected to deli,-er positi\"e pressure_ The Release Val,"e and Manual Letdown Valves are closed and the Fine Adj usonent is set to the halfway position. The displayed airspeed is set to zero and the hand-pump is then used to deli\-er the instrument pressure required to set the Airspeed reading specified in the test instructions using the fine adjusrment a5 required. The desired \'X'AIT and TEST TI~rE figures arc emered and the RUN TEST key is pressed. As soon as the test commences, the START SPEED \\~ll be displayed. On completion of lhe TEST TIME, the STOP SPEED will appear in the display.
220
AIrspeed KTS 60 SEC Ime 250 KTS ~rart 220 KTS StOp KTS Change -30 , Leak Rate -30 KTS/r>. lll\l
Pitol Conncctio~
§-,
t!J Pusith-c PX Figure 4.4 - Airspeed Le:lk Test
If you ,-"amine the illustration, this particular lest ga,"e a START SPEED of 25Q1.:;tS_ This reduced to 220krs over the 60sec duration of the test_ Ths occurred because leakage of air o ut of the system reduced the positi\'e pressure resulting in a reduction in the displayed airspeed. This gave a change of -30kts o,-er a 60sec duration, which gives a leak rate of -30 krs/min, which is obviously a major exaggeration, but is used here (0 illustrate the [cst set function. The duration of the lCSt rime can be set to any value you wish, it docs nor ha,-e to be 60sec, longer duration tests will give you a mean leak rate per n11nU[e.
On completion of the (est, the system pressure should reduced to a safe value. certainly less than 80kts, before slowly o pening the Manual Letdown Valve to return the system pressure gradually back [0 ground le"e!.
248 GJPYnghf 0 Barry ulkgt ~OO6 - 200- All rigblf rmrvtd
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GENERAL.
CHAPTER FOUR TEST EQUIPMENT
AJtitude & Airspeed S,,,itch Tests Pressure-operated switches sen'e functions that include: •
Low altitude warning
•
Height-lock
•
Landing gear configuration warning
•
Speed brakes
•
Speed lock
It is important thal these switches close and open at the specdled altitude or airspeed "alues. Additionally, a Pitol or Static pressure-operated switch will often have a hysteresis between me closing and opening pressures and this must be witrnn a specified limit. Most of these switch operations activate a visual indicator or annunciator and an aural warning that is usually more pre\'alent, Sometimes these warnings may not be canceHed unless the fli ght crew reduce the airc ra ft's speed. If if is not possible to con nect the test set directly to the switch, \VC can LIse the set manually by obsen'ing the indications of switch operation,
1UU
e l..(\QAftftude Switch Tes t
(.o\\C~
\"\).~('
From the display menu, select ALTITUDE S\,\lITCH TEST and select the desired units. If so desired, enter tl1C ambient pressure of the day or Q FE correction, but these test are usually carried om at Q)JI-I, ie rSA. The flexible hose should be Connccl"ed to the Static port sen;ng the switch. \'('here the electrical connections to the switch are accessible, these should be connect~ to the switch test socket connections on the test set. T his \viU produce a vis~ display of the contact Stale of the switch . The Release Valve and the ManuiliLetdown Valve should be closed and the hand-pwnp should bc selected to:. dcli\'cr negative pressure. F Thc RUN TEST key is pressed and thc hand-pump is operated to gradu.'lJly raise the system altitude until the switch operates, The altitude at which, ~e switch operates \\-ilJ be displayed, The Release Valve is then slowly openetf to gradually reduce the vaCllum and lower the alti~ude until the switch o~.Y'ates agnin, The altitude at which tills occurs will also be displayed. C\ '0 .§' ,
249 COI!J·right © Bill!)' Colk§ 2006 - 2007 All rigb/t ffStrI'ed
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CHAPTER FOUR AVIONIC GENERAL.
TE S T
Static Connection
.. .
r-"•
•••
II " L.c VAL""@
I:'J
EQUIP M E N T
I
~
CI()~ed
looon
-
OpcneJ 9950 nosed 1000
----
Switch Test
SWllch Test
1,,,,1 ~,~.o0 ".
, 9?,,?, O'"
!tun
RUIl
F ig ure 4.5 - AJriltide Switch Test
rf you examine the illustration, you \\~ll see that the switch closed at 10,000 feet as altitude was increased and opened again at 9,950 feet as altitude was decreased. This gave a hysteresis of 50 feel belween closing and opening. These recorded values should be checked against the limits shown in the i\ [aintenance i\ [anua!.
On completion of the test, the Yacuum pressure should be allowed to reduce until a safe MriUldc is displayed, ie less than 1500 feel, before slowly opening the i\fanual Letdown ,-alve to return [\)e system pressure gradually back ground level.
[0
In a situation where it is nOI possible 10 access lhe electrical connections to the switch, the test set can be used in Manual. In this instance, the indicated altitudes have to be read in conjunction with the appearance of the switch operation indica')t&ps. ,\c\!,:~
,c.('I\\
•
"I'lfJu - -
\"\7-(('
~... &.
Airspeed Switch Test
,~\'C
I..~-'~""
From the display menu select AIRSPEED S\VlTCH TEST and select the desired unjrs. The flexible hose should be connected [0 the Pitot probe or tube serving the s\,,~tch. \'('here the electrical connections [Q tbe switch arc accessible, these should be connected to the swirch test socket connections on the tesr set. This will produce a visual display of the COnL'lct state of the switch. The Release Valve and the i\fanua.l Letdown Valve should be closed and the hand-pump should be selected to deliver positive pressure. The displayed airspeed is set [0 zero, the RUN TEST key is pressed and [he hand-pump is then used to gradually raise the system pressure until the s\\~tch operates. 111e airspeed at which this occurs will be displayed. T he Release Valve is then opened slowly to allow the pressure to graduall}r decrease until the switch operates again.
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Pi[Qr Connec tion
.•••."" I L.c
c""•
I
"
:-I',I~
C1l>'cd
180
Swrl.:h·Tcsr
180
,\ir
Run
Opene(] Closed
1~~:1
----
SwiTch
1811
Te~l
160
Air'pc:ed KTS Hun
Fig ure 4.6 "Airs peed Switc h Tes l
1f you examine the illustration, you will see that our switch closed at 180kts as speed was increasing and opened again at 160kts as speed was decreasing. Tills gi\"es a hysteresis of 20kts. which again is exaggerated juSt to show the point. These figures should be compared with the permitted limits in the i\hintenance i\1:l11ual, which will usually be between ± 1 knot and ±S knots. As before, if the switch connections are inaccessible, we Can operate the tes t set in r-..lanual by recording the airspeeds in conjunction with the suitch indications. On completion of the test. the pressure should be allowed to fall until a safe speed is indicated, ie less than SOkts, before slowly opening the Letdown V ~tl\'e to refilm the system pressure gradually back to ground level.
Veq!sa1 Speed Test "lQl~b
..
(.ll\\C~e " With this porrable test, Veru"cal Speed (VS) operation may be observed by . \1r:.~(' looking at the Vertical Sp eed Indicator ( VSI) while cbanging the alti tude ,\--.'1. " dmlng an altimeter or altitude switch test. As the pressure vacuum is created ""~'\".<'-~ using tbe hand pump, it is not possible TO get a calibrated reading of the VSI. \".. bm iT will show if it is sensing correctly in the right direction.
Air Data Test Set The Air Data Test Set can be used to carry out all the tests that we described i!F rdation to the portable Pitot/Static system tester plus a few marc. Again, \i.e will not be dealing with any specific proprietary brand of tCSt set in this secti@ . \\le will confine our examinatio n to what features would be incorporated jJ~ a typical [est seL For that reason, you do not need La memorise pa rticular k,0~ or menu selections given here to illustrate general features of the lest se~~ou must abide by the instrLlctio ns comruned in the User l\ [anual for the ~ticular item of equipment you are using. ~
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The Air Data Test Set has full pressure and vacuum comrol and mcasunng capacity for on-aircraft sense and leak testing and functional testing of air data sys tems, components and instruments. The sel has to be connected w an external electrical power supply. It incorporates two independent pneumatic systems. one for Static and one for Pitot, each sen'cd by an integral, electrical motor driven pump. The Slacic and Pilot systems mar be used independcnt of each othcr to carry out altiruclc or airspeed tests respectively or. they may be used wgcther 10 provide any combination of altitude and airspeed nlues. Typical teSls that can be conducted using this e(lwpmcm include: •
Static system rests
•
Pilot system tests
•
Combincd Pitot and Static svstem tests
•
Vertical Speed Indicator (\'SI) tests
•
Airspeed and _\ltirude leak tests
•
Airspeed and Altirude switch tests
•
Engine Pressure Ratio (EPR) tests
Earth S,Uld
r
______+=========:::;~--
Power Swi tch Unit Sekcr ;-;:====+111-- Kcys Pirot
--+-
FunCtio"o'-----l-l-J-.. ~iDooiEii KcysKeys
Stacie
I;,..._ _ _ _ _ _ _ _ _ _ _UJ--- Carrying I-bodle ~
- ?<
Figure 4.7 - Ai r Dall'! Tesl Sel
,..§-
The set can be selectively opeaued in different modes thar include: measurement, control, Icak measurement and rerum to ground. In measurement mode, you can select the parameters to be displayed and the units to be used. The control mode allows you to set rates of change to selectcd valucs of altitude, airspced,t>. lacb Number or Engine Pressure R:ltio (EPR). The display then shows the chosen parameter changing as the set controls the rate of change. The Aircraft t>.laintenance Manual will state maximum limits for pa.GI01cfers and rates of change and these need to be entered to enable the set to excrcise control.
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\,\1hen you wish LO return the system to ground, the set will register the :lirfield p ressure rind set lhe target or aim value of the parameter to zero before n.:turning the system to ground at a safe rate. \'.;'hen the system h(ls renmlCd to ground. rhe display will shmv that it is 'Safe at Ground', T he cond uct of Pitot :lnd Static sys tem leak tests (lnd airspecd and altitude switch tests is similar in principle to those we examined in the p revious section, the difference being dlat rhe parameter aim and rate of change values arc preselected and arc automatically controlled. \\'e will take a couple of examples using the illustrated test set to gi\-e typical [cst sequences .
Static Tests Leak Test For this tcst, rhe flexible hose is connected from the Stacie pOrt on the sCt to the Siaoc veOl on the aircraft. Select the required units, select Altitude and then press the Leak "leasure/Control function key. The number keys arc then used to cnter the required ahirude and the required rate of climb. The \'\':\I"r TI1\IE and TEST TIi\IE nrc then entered. On selecting Enter. the Static system pump will run 10 generate negatiye pressure. The displayed altitude will change towards the selected value at a controlled rate, slowing as it approaches the value to avoid overshoot. The test commences nfter lhe pre-set \VAIT T I1\IE nnd at the end of the timed tesr, the lea k rate is displayed . :\t the end of the test, the GROUND function key is p ressed and the set will automatically return tbe system back to local atmospheric pressure at a safe ralC.
Static ,System Test ",-,6 'Ill ~ l,W:,?-1: l' This test allows you to compare selected altitude \'(llues on the test set with .:>-,')s(' C. values indicated o n Static instruments and can be used to check the operation
,'\'-\. ~ '\ ~..""r • C'~ L·
of Static system components. The set is connected and set up as for the Leak Measure/Comrol mode above. The set will cotHral the rate of climb to tbe selected altitude. :'\lew alcitude values can be entered as required and rhe set will control the change to each new \-alue. At the end of the test, pressing the GROL':--'ID function key auromacically returns the system to local aonospb~ pressure at a safe rate, ~ Altitude Switch Test
'"
:;
The preparacion for this rest is similar except that there is a 'nudge' facility tl1M allows you to approach the target alcitude in increments on ascent to obtain ifl accuratc reading when the switch closes. T he target altitude is then r<,!i~d l1bove lhe switch operation height and is dlcn 'nudged' b:lck down to obti}n a reading for switch opening. ~
§''0
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Ve rti cal Speed Indicator (VSI) T est In a basic VSI, Slatic pressure is fed directly to the imernal capsule fllld the inside of the insrrument's case. I-Iowever, the static pressure inplll to the inSlnlmenr case goes through a restricting channel, commonly called a resrriclil'e choke. As altitude is increased or decreased, at a set rate chosen on the [cst set comrol panel, Static pressure in (he capsule changes faster than the Static pressure in the case because of the restriction. The difference in pressures is calib rated to give an indication of the rate of climb or descent in thousands of feet per minute. There is a 'I:!g' when altitude levels out after a prolonged climb where the instrument continues to register a climb until the pressures in the capsule and the case equalise. 111e modern inenial lead VSI (lLVSI) reduces dus 'lag'.
Pitot System Tests Leak Test For this test, the flexible hose is connected between the Pitot port on the set and the Pitor probe or rube on the aircraft. Select: the required units foUowed by Speed Leak J\!easurc/Comrol and then enter lhe target speed. Enter the specified \X':\IT TI~[E and TEST Tlt-.1E. On pressing Enter, the PilOt system pump in the set will generate positi\'e pressure and increase lhe displayed airspeed to the target \'alue at a fixed mte. After the pre-set wait time, the test will run and the leak rate will be displayed. On selecting GROUND, the set will automaticaU), rerum the system to airfield aunospheric pressure at a safe rate. Pi tot Sys tem T est This
pro~9Ure ilf'~imilar to
that for the Static system c.'(cept that you select Sltt,~'d'-U;k Measure/Control. \'('hen the target airspeed has been cmc.red, and \!l,;\'\.{hc Enter key is pressed, the displayed airspeed changes at a comroUed rate to ,\''1.. ~. the target value . .r\ new target value can be entered if required and the system (f,~-:.\\r will change to the new value at a controUed rare. Airspeed Swi tch Tests
~
"
Again, the procedure is similar to that used for the altitude switch test except that you select the Speed function. \'('hen dle target speed is neared, the 'nudge' facili ry may be used to raise the speed in incremental steps to obtain an accurate reading of switch closure, 'I11e target speed is then raised abo\'e the switch setting and is then 'nudged' back down to check the airspeed when the switch opens.
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Combined Pitot/Static System Tests Some system tests require the simultaneous display of speed and altitude in order to observe rates of speed and altitude changes . Before cfl rrying out this type of test, the Pitot find Static ports on the test set must be connected lhrough !1exiblc hoses to the Pitot and Static probes or vents o n the aircraft.
.\ir Data Computer -~,.L"--~ IIIj Pi
Static
.'t'-----ill:!;;] .):'11<.'J ",.:::::::-"",.\ltltU t;;--'1t;;t.~----
1\ DC T cst Set
F ig ure -1.8 • Air D a ta Test Set Counec lio n to Airc.:r:tft
The illustration shows a typical connection between the test set :lnd fin aircrflft. Note th:lt lhe position of the set in relation to the equipment being tested can result in a difference in Static pressures between the set :md the item being tested, as \\~th some aircraft this can be several tens of feet. The illustration shows tha t an altitude correction has to bc applied fo r lhe height difference between the lCSt set and the Air Data Computer. \,\Iithout this correction, the test set would be displaying an altitude "ruue lower than the e(luipmenr. 11.)~{
• ",:,..:..I!
, -0:-'\:1"
:..,'..
....:.(\.~
(~\
\.J"~
.
'1\~nclicated versus True Airspeed Pitot pressure is the sum of Dynamic air pressure (1/ 2pV7 and Static pressure. TIle Pitot method used to measure airspeed is dependent on air density (p). As we know, air density reduces with increasing altitude, which means char at altitude. the air speed indicator is not sho\\mg the !.rue speed of the a.ll:Craft,..,il is showing what is called the Indicated Airspeed (lAS). The indicat~ airspeed (l AS) at altirude will be less than the True Airspeed (IllS). ~.
"§.
Mach Test & Constant Mach Test ~
sj)~~
The l\lach speed of atl aircraft is the q uotient of its tnle airspeed and the of sound in air. If the true airspeed is the same as the speed of sound i,t air, then the aircraft is said to be flying at Mach 1.0.
.f
'0
'r he speed of sound in air varies ill proporoon ~o air tem perature. T!.fis means the speed of sound in air reduces as altitude increases and air tempera'ture falls.
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:\n aircraft flying at a gi'"CD perccntagc of me speed of sound in air at Sea Lcvel will actually srarr to increasc that percentage as it climbs to altitudc at a constant airspeed and may c\"Cn reach the speed of sound. In simplc tCnTIS, r.. lach 1.0 would be rcached at a lower TAS at altitude than it would at Sea Lc\Tcl. Commercial transport aircraft arc mainly transonic and they suffer adverse compressibility affects when airflow over thcir surfaces reaches the speed of sound. An aircraft that is designed to fly at a gi,"en l\ lach speed, say 0.75 Mach. will ha\"c a Flight Af:magemenf System (Fl\1S) acting together with the autopilot to maintain this valuc if alurude changes. Therc will also be a t-. lach warning system to alert the crew when the Mach speed reaches a critical va lue (i'- ICRfT or t-. IMo)' T his system would be activa ted by a p,fach switch or Ci\DC, the operation of which can be tested \\~[h the test set. The r>. lach test im'olvcs setting a target Mach value and thcn altering the altitude and checking that the airspeed adjustS [0 keep the t-. lach value constant. \Vc do not need to look furt her into this, it is JUSt another test facility provided by the Air Data Test SeL
Engine Pressure Ratio (EPR) The Air Data Test Set may be used to check thc operation of the EPR indicators and their sensors and to test the leak rate in the system. EPR is the ratio of rurbine discharge total pressure to intake total pressure, This is shown as Discharge Px/ lntake Px. A target EP R is entcrcd and when this has been reached, the acrual enginc inlet and outlet pressures arc displayed to confinn [hc EPR reading. For example, if the EPR reading was D,S and the intake and discharge pressures were 500 millibars (rub) and 900 mb respectively, the EPR illdication is confinncd. A leak nne test may then be conductcd o"er a specified test time, ,,~-;
i\'\ L~
That conoldHes our cxamination of the Air Data Test Set. You can sec that it is mQ(e\~':';ati.le than thc portable Pitot/StaaC testcr and is suitcd to the testing \,,;,>"tS f complex systems thar use Pitot and Static pressures.
,.5\'t; \,. ,,<;Z
-
Transponder/DME/TCAS Test Set
r.
There is a range of proprietary test sets a\"ailable that are designed for single or multiple srsrcm ramp o r bench testing. Some arc designed for Transponder/ lmerrogator tes ting whilst others also includc Dt-.·IE and TeAS testing. \Ve will be cxamining tills type o f equipment in general renns to illustrate the tests they would be used for under each system heading. The fine demil associated with each test such as: test sequences, keypad selections, display fonnals etc are peculiar to individual makes of equipment and it wowd se.t\'c no purpose to inclmle them all in tlus section. You mus t consult thc User fvfallual for the c(juipment yotl are opcrating and comply with [he test proccdllteS in the appropriatc Aircraft or Component Maintenance Manual.
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\\!hen testing SSR equipment using ramp test sets, B2 Engineers require a greater understanding of the test sets and the indications that appear on their front panels. In addition, also need to be aware of any speClal regulation, such as AirltlQrthliu:ss J\locices (A If71\? that are app~cablc from time to orne. Over the years there have several A \'i'Ns that were applicable to avionic systems, eg A \·'(TN 84 & 91, but after a defined period these are usually replaced with more permanent solutions. At the time of writing this Chapter, late 2007, only one Airworthiness N otice is currently applicable to anonic systems; A\V'N 12 Appendix 69. This Notice provides general guidance material to aircraft maintenance organisations and maintenance personnel, ie YOLl, relating to .ATC Transponder and Traffic Alert and Collisio n Avoidance Systems (TCAS). It includes 1I1formatio11 011 the ·rCAS system together with precautions to be considered when ground tesung A TC Transponders in order to minim.ise the possibility of causing nuisance advisory \varnings on TCAS eqlllpped aircraft. I have tried throughout this chapter to avoid quoting or represennng proprietal)' suppliers and I will continue in this vanc here and so similarities to any existing test equipment is accidental. Most modern ramp test sets will have combined testing facilities for .ATC Transponder and DME. The larest ones will also include facilities fot te~ting
A utomatic Dependent Surveillance - Broadcast (ADS-B), Ground I nitiated Commwu·caci ons - BrO.'1dcast (GCI B) and the currently US£\ only based Traffic Information Senrice (TIS). These all work with the Mode 5 Transponder to provide information in one form or another using the daralink.
~Iode
S
All Transponder/ DlVlE/ TCAS test sets contain a preCision simulator that allO\vs one person to functionality test aircraft Transponder (XPDR) modes .A/ ~ 6§' Distance Measuring Equipment (Dr-. IE) systems and TCAS 1 and 11. 'i:J6 - -
~e. 1.\:
(.<,\\i:._ \;,,(t"
",
Q
General Descrip tion of Test Set
" (\'?-
U'~
The equipment used for testing these systems is typically a NICAD rechargeable banery-operated, portable precision simulator that enables ~ operaror to functionally test installed aircraft systems . The set incorpora @'; Radio Frequency (RF) signa! genemtors and modulators that" operate in rl~ · frequency ranges of the associated aircraft svstems, together with a pUlsl , generator to create the interrogation and/ or reply pulses needed in most test;£.; When testing installed umts, the radio frequency transmissions from d1e reit sct are passed to [he aircraft antenna through a remote antenna connected the test set. Some makes of equipment incorporate an integral, mono-p~c antenna whereas other types require the use of a remote amenna. \Vhen teiing uninstalled equipment in an approved component repair organisation, co ~xial cables are connected to the umt tn place of an antenna. ,"i ~
iR
'"
6' [an~ o f
One stgnal generator may not cover the entire frequency the equipment under test. A nwnber of RF signal ~enerators may be required to
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achieve this. For example, one generator may cO\-er 10 kHz to SO Mllz. another 100 k1lz to t 10 MHz each increasing the frequency range in steps up to the maximum required by the set. OI\.IE for examplc operatcs in the L-Band frequency mnge from 962 r-..[Hz to 1213 r-..ll-lZ. To gain any appreciation of the equipment or tests that arc carried out, you necd to haye some idea of what the system under test acrually does. Thesc systems are discussed in 1\.10dulcs t 1 and 13 but I will give you a bricf description for the function of each system before listing the tests associated \..nth it.
T ranspond er _\ Transponder is an aircraft mounted receiver/tr.lllsmincr that cecei\'cs a ground transmitted radar signal on 1030 1\.11 Iz and rcsponds [0 it by trnnsmitting a second signal on a different frequency of 1090 MHz th:\! is in rum received by the ground based radar receiver. The system is more commonly referred to as being Secondary SUrlreilJ:mcc Radar (SSR). The signal recei"ed by rhe ground radar station is a lor stronger than the weak echo signal thar would be received from a primary radar transmission. The return signal transmissions from the aircraft arc displayed as a clear 'blip' on the radar screen. The Transponder transmits coded signa1s that positi,-ely idcnri~' the sending aircraft on the screen. The aircraft altirude can also be displayed as a number adjacen t to the image on thc radar screen Each aircraft is allocated a four (-I)-digit code (Squawk) \Ierbally by Air Traffic Control (ATC) and this is dialled into the Transponder by the pilot, which then identifies the aircraft on the _"' TC radar screen. I f requested, the pilot presses an identification button on the Transponder and this produces an enlarged 'blip' on the gr<>Nf.¥! radar display. There are also emergency codes that can be entered ,} '$V-1P pitot when it is required to alert ground radar controllers to si\i!:'lu-U bts such as an emergency, a hijack or complete radio failure. It is ... \>.,3.«therefore important that when testing Transponder systems you stay well away \J::'\\ from these codes, because when a ground ATC recei\·cs these codes, it sets off . shown on the aitera f t ........,\:~. an alann The dialled in IDENT codc (Squawk) tS Transponder and will be displayed on the interrogating ATC radar display or on a test scr when the aircraft unit replies to interrogation pulses from the ground ATC o r test set. The Transponder interfaces with a CADe or an encoding altimeter that displays an altitude code numerical readout based on height above lSA Sea Level. This code is usually transmitted automaticlilly and it appears alongside thc 'blip' on thc ATe radar screen. This rna)' be tcstcd by reading the code [rom the Transponder and verifying its receipt on a test sct.
r. >
.;,, o
3"
.~
'" " " o
The improved Mode S Transponder is capable of providing a data link between the aircraft, otber flying aircraft and the ground station. This permits the transmission of printed informacion.
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Transponder Tests Tn preparation for testing, the aircraft Transponder is s\v;tched on and allowed to run in 'standby' fo r a few minutes before dialling in the pilot's code. If the test equipment requires a remote antenna, this should be set up and connected to the tes t set, ensuring that you set dIe correct spacing between the test set and aircra ft antennas as stated in the Aircraft i\hintcnance Manual or Test Set instructions. "111e following tests may now be carried o ut:
•
Transmitter p eak power
•
Transp onder frequency
•
Receiver sensitivity
•
Reply delay
•
Correct rcply code
•
Percentage reply Operation
•
'Ident' tone
•
'Ident' pulse output
•
Correct pulse timing
•
Correct pulse spacing
•
Altitude code
•
Inadvertent Side Lobe Supp ression (sis) SLS operation
•
Receiver - decoder limits
•
l\-Iode S testing
~
~,,r
For B2 enginecrs testing a Mode Ale Transponder, the most important factor: is the initial set ~up of the test set with respect to the T ransponder antenna . TbB= test set antenna should be set up on its tripod level at a distance from t~ aircraft antelUla as dictated by the test set operations manual. Tn most cas ~") have com e across this is 21 inches (approximately 54 cm), bu t in any e~t, should not be less than 15 inches (38 cm) as this could result in damage tp) he
==
~" '0
\\lith both test sel and aircraft Trans ponder powered, leave the. Trans:~nder in standby for five (5) minutes or so for it to stabilise and work up to its normal
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opcr:uing tempcrature. Oncc srabilised, set the Transponder four (4)-digit codc, avoiding 77XX, 76XX and 7SXX for obvious reasons. The (CSt SCI will now send intcrrogation pulses to the Transpondcr and il will respond according. All test sets will have some method of allowing you to verify the Tmnspondcr codc and figure 4.9 illuslr:lles one example.
Figure 4.9 - Typical Ramp T u t SCI From Panel
As you arc aware, Transponders output their ~Iode A replies in a fOll r (4)-digit octal fomlar; the rcplies for the A pulses are shown in the following mble. the 13, C and D formats arc the same.
,
Octnl Code
Decimal N umber
Al
A2
A4
0
0
0
0
I
1
0
0
2
0
1
0
3
1
1
0
4
0
0
1
5
1
0
1
6
1
1
0
1
I
1
-
-,
'"~ . ~
Table 4.1 - Typical Ramp Test Set Front Panel
Once all Mode j\ digits ha\'c been checked, set the Transponder codc to 0000 and press the Tmnspondcr's Ident button, which may be remotely mounted in rhe cockpit, and confirm that it is bcing sent by illumination of the D4 light on the test set. T hc lest set should be set [0 Ale ALT, which changes its interrogation mode from .A to C and you should then "erify its response, which will be in thousands of feet referenced to 1St\, ie 1700 feet would read 1.7.
No te: TIle encoded altimeter readout should be unaffected by the altimeter barometric setting and will always indicate altitude referenced to lSi\.
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You should next check for the Transponder's power and fnxluency limits in accordance with the J\ laintenaoce \[anuaL Finally, you should check the Transponder's response to sidc~/obe~ suppressioll (sis), interrogation spacing and signal fram ing u~ in g the test set's variable controls. Once testing is complete, ensure that you leavc the aircraft Transponder in standby or off mode ,\;th the four (4)-digit code set to an undefined code such as 0000, -1-321 or 1600.
Distance Measuring Equipment (DME) Di\IE is anomer example of a system that uses secondary rada r techniques. A transmitter on the aircraft sends a stream of radio pulses that arc recei\"ed by a ground located SSR station. It responds by transmitting a return stream of radio pulses to the lmerrogating aircmfr on a diffe rence frc(luenc), of ± 63 Mll z, dependent all operating frequency. T he ground sta tion delays its response by 50 ~t S and so this time, plus the time elapsed between pulse transmissions and receiying a reply is automatically processed to produce and display the range of the beacon in relation to the aircraft and its approach ,·dociry. Howcycr. the yelociry reading is only accurate if lhe aircraft is flying directly to or from the station. D11£ measures the slant dis tance from the aircraft to the beacon and this produces a small error when compared to ground di stance. The crror increases as the ai rcraft approaches the beacon reaching a maximum when the aircraft is directly owrhead when its vertical altitude is displayed as a distance. Dl\IE has a range of around 300 nautical miles (Ni\Is) . .A DME ground station is l{:tJt:'Il.ly collocated with a VOH Navigatioll Beacon or an airport's ?.lv..Y tls'irument Landing System (lLS). An aircraft Dl\1E system therefore, . (,o\\C'bi:. shares a VOR/ ILS controller to provide frequency and control selection. ( \,~~('" Although the operating frequency range of a D ME system is from 960 to 121 5 ,.is''. l\n Iz, the Di\lE station docs not have to be selected specifically. All ground -0~~. based Dl\rE stations arc paired \dth a selected VOR or ILS beacon under c· T(,\O rules. \\nen a VOR or ILS frequency is selected on the controller, me frequency of the collocated DME is also selected automatically. 1; ., T he air to ground interrogation pulses arc sent Qn a different frequency [0 di. ground to air reply pulses to pre\'ent a siulallon where an aircraft receives rerlE pulses from its own transmissions 'reflecting' off the ground. Ilowcver, th~ system can operate quite happily with a 50% percentage of degraded rePl1 signal. Once synchronised to a ground station, the D ME calculates ~ range/ velocity, which then appears on the aircraft display. -nle OME testjer has three selectable ehanncls: 108.00 MHz, 108.05 MHz and 108.10 i\ Il-li?for lise with TLS. \'\'hcn the Instrument Landing System (ILS) cha nnel (10SJe) is selected, it automatically tunes in the O-ME to produce a continuous indttation of distance to touchdown. §v
""
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DME Tests Prior to conducting tests, the aircraft's OME equipmem showd be switched 00 and allowed to run for a few minures and set to the (cst frequency on the VOR controller. The test set should be SCI to freque ncy 10S.00. If a remote rest anlenna is required, this should be set lip and connected ro the test set, ensuring that you sCt the correct spacing between thc test set and aircraft anrennas as stated in the Aircraft ~[aintenance ro-Ianual or Test Set instructions. T he follo\\;ng tests may then be ca rried Out:
,\ ...\
~.5\"'"
(~,,\
'
•
DME distance operation (Distance Display ro-Iode)
•
D~lE
•
DME T ime To GO (ITG) calculations
•
D~lE
•
Transmitter peak po\ver and frequency
•
Percentage reply
•
'Idem' tone
•
Channel IOS.05 operation (repeat abo\'c rests)
•
Channel lOS. 10 operation (repeat above tests)
velocity calculations
transmitted Pulse Repetition Frequency (PIU=)
For 82 engineers re'luired to test Oro-IE o peration, the lest set should be set-up in the same 1Vil}1J~r wi th (he T ransponder system, but obviously with the tcst ..,r+-.\I:l . r set. ~\!I$:n n a localed ncar tbe Oro-IE an tenna 0 the system under test. ~&riember though that there must be at least IS inches between antennas to ~)( ... prevent damage to the test set. O nce sct up, tunc rhe VOR controUer to 108.00 i\U Iz and sel the test set mode switch to OME, if rCCIuired. as in figure 4. 12 abm-c. If there is a RANGE/ VELOC ln " switch as in the illustrated test sct, set the switch to RANGE. For VOR frequency IOS.00 MHz, the co-located Dro- IE is an 'X. channel and so if the test set has a swirchable channe.l spacing switch, this should be sct to 17X. Channe1 1SX is on frequency 1OS. IO Mf 17. while channel 1TV is on 108.05 MI fz. T he unit under test mny now be checked at various distances by using the test set controls. To lCst the OME Groundspeed (GS) and Time-To-Go (ITG) functions, the lCSt set should have a control to simulate increasing and decreasing OME distance, which will allow the O ME unit under test to calculate GS :l!1 d l TG. Jn addition, the rcst set should output a 1020 117. Idem tonc thar will be decoded by the OME unit under lest aod scnd to the Audio Imegrating System (AIS) for distribution, as rCCIuired.
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Meters on the test set should allow you to chcck the OME's po\ver output and its Pulse Repetition Frequency (PRF) while searching and locked on . Tt should also have a switch function to reduce replies from the test set by 50%, to ensure the O:\[E under test remains locked on.
Traffic Alert & Collision Avoidance System (TCAS) TCAS uscs aircra ft to aircraft interrogation betwcen l'.[odc S Transponders to pro,ide position in foffi1ation about other T ransponder equipped . The equipment scans a distance of about 15 nautical miles around the aircrafr and can measure the range, closure ratc and height separation of aIrcraft wIthin this range in relation to the interrogating aircrafl. A range of colo ured symbols is di splayed to indicate positio n, range and direction of each identified target. \,\1hen the closure tim e between an 'intruding' aircraft and the interrogating aircraft poses a leyel of threat, a series of nsual and aural proximity warnings ate gi\·en, each depending on the threat lc\"el . These may advise a\"oidancc measures in the ,ertical plane. System s that arc scheduled for introduction in the near future will include avo idance measures in the horizontal pl:ll1e. Typical aural warnings are 'Traffic, Trafuc', 'Climb, Climb', D escend, Descend' or conyCIscly, the instructions may tell the flight crew not to climb or descend.
TCAS Tests The TCAS/ Transponder test set uses the aircraft Mode S Transponder to provide up to eight di fferent sccnarios for TCAS tes ting. These are used to simulate an intruder in proximity [Q the. ailcraft under test. Tcsts include:
• rntruder closing at~ aboye or below configured altitudes •
Intruder diving, climbing or turning
The TCAS display in the aircraft is checked to verify that the correct visual and aural warnings arc being triggered in response to each test simulated by the t&.§t set. "3.
~
TCAS tc~ting is in its infancy and there are not many test sets around that ar~ capable of fully testing TC\S functions . TIle first thing to know when testing TCAS system is if it is a TCAS I or TCAS II as the tests arc different. On<§: this is es tablished you \\oill need to set up the test set and associated directio~ antenna :550 feet (15.25 metres) from and in line of sight with the top ~ d bottom aircraft antennas. \'\'hen testing the bottom antenna you should use"ithe fuselage to block the line of sight to the top ant~nna. A shield is positioR~ to block the line of sight to the b ottom antenna when testing the top anteru~ .
a;
~-Iode
'0
In order to test S Transponder and TCAS functions, the aircra,ff'n1Usr be powered with the ailcraft co nfigured for wcigh( offwhecls. T he actual testing
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of i\lode S Transponder and TCJ\S is quire complcx and so you should always follow the instructions of thc J\ircraft Mainrcn:mcc ~Ianual or test SCt instruction book. Howcver, in any event, the test set should be capable of simulating:u least four (4) intruding aircraft. Note: If aircraft is on the ground, set the feSt set to MANUAL and enter manual addresses for thc i\lode S Transponder all-calls tests as ther do not work when the aircraft is on the ground.
VOR/ ILS, VHF Communications Test Set /\gain, there are a number of proprietary makes of equipment that are designed to test some or all of these systems. You must consult dlC t.;ser i\1:lIll1al for the operation of the equipment you arc using and comply with the test proccdures in the appropriate i\ircraft or Component Maintenancc j\ianual.
General Description of Test Set i\lost ramp tCSf sets arc ponablc, rugged and powcred by rechargeable NIC.U) batteries. A typical set would incorporate a radio waye Signal Generator with a variable anenuaror and an integral amenna for transmission of signals to the aircraft. TIle signal gencra tor can be opcrated under either Cn.ed frequency comrol or in variable frc<.Juenc), mode. It may be modul:ued by LOnes from the equipment under test or may prO\-ide modulation, as appropriate. -111e set would incorporate a display that indicales the radio frequency of lh(: signal generator, the frc<.Juency of a transmitter under test, any external signal inputs or VOR be!rings. There would bc :tn integral radio freque ncy power meter suiraRLe 53'1' measuring vr IF Communication transmitter average and pe~~\P.9-"~~' An integral modulation meler would be used 10 measure the "dlertenrage modulation of the Signal Generator on any frequency band. The set ,"t. would also incorporate a 900 bearing monitor for checking VOR bearing. The test set simulaTes the signals and tones normally transmitted to the aircraft from ground-based navigaLion equipment. The rcst sel aCls as rhe sending source for tbese signals so that the aircraft equipment can be checked for correct indications and responses. These include: •
Marker signals and lanes for Outer, Middle and Ain.vays i\{arkers
•
VOR Ground Station signals
•
Localiser signals
•
Glidcslope signals
•
Transmittcd ground station Idem codes
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W/e will need to know what each of the systems does before looking at the tests associated with them. The foll mving descriptio ns have been kept deliberately brief and arc only tntended to show you what the test set simulates, m ore detailed descnptions of these systems may be found in Module 11 Chapter 5 and l\IoduLe 13 Chapter -t.
Marker Beacons l'darker beacol.1S are used in conjunction with an Instrument Landing System (lLS) and ther give an indication of distance to touch down. There are two Marker Beacons positioned along the Instrument Landing System (lLS) path. These are the Outer and Middle trarkers. The Outer Marker is approximately seven nautical miles from the runway threshold; the ivliddle Marker at about half a nautical mile. All the Marker Beacons transmit on the same freque ncy (75MHz) bur each transmits a different identification code and tone. The Outer Marker 1S heard as a series of Low tone dashes accompanied by a b lue light on the instrument panel that flashes in unison \vtth the code . The :l\liddle :l\Iarker is heard as a series of medium-tone alrernating dashes and dots accom panied by an amber light flashing in unison. The Airways Marker is heard as a seIles of high-tone dots accompanied a clear white light flashing in unison. T·he Outcr r-darkcr is often located with a low-power, "-\DF non-directional beacon (l'-TDB). The aircraft is equipped with a radio magnetic indicator CRl\!J) that incorporates Automatic Direction Finding (ADF) needles that point tmvards the NDB. As the aircraft over-flies the N DB, the needles swing around to point backwards, usually indicating the point where the aircraft should rurn onto final approach.
Localiser An Instrument Landing System (ILS) uses twO radio signals that provide runway centreline guidance and descent path guidance to touchdown. The runway centreline taclio signal is called the Localiser (LOC) and the glide p ~l signal is called the G lideslope (GIS). The Localiser transmits on the frequency band (108.10 MHz to 111.95 .MHz on the odd decimal frequenci channels) through a large antenna atray at the end of the approach. Eacll Localiser VHF frequency is accompanied by a Glideslope operating on a::.: paired U HF frequency. \'('hen an approaching aircraft drifts to one side of t1~ Localiser centreline, the change in received radio signal strength is used 'fQ indicate the aircraft position on tbe flight instrument display as being left,glr right o f the runway centreline.
vr-ft
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Glideslope (G I S) We ha,·c already introduced this. The Glideslope antcnna is positioncd adjaccfll to thc lOuch down poinr on the runway. \\'hcn an approaching aircraft is above o r below the Glideslope, the change in received radio signal strength is lIsed to display the aircraft's positio n on the flight instrumcflI display as being high or low of the glidepath. A simple display uses rwo necdles. o ne vcrticaland one horizomaL D eflections of the vertical needle indicate positions lefl o r right of lhe nmway centreline and deflections of the horizontal necdle indicale positions abo"e and below the glidepath.
VHF Omni-Directional Ranging (VOR) VOR beacons arc strategically placed along airways and othcr mwigarional reporting poinls likc holding panerns to allow the night crew to navig:He the aircraft along a chosen flighrpath. _\ VOR beacon r.ransmits a signal in the VI IF band frcquency modulated (F"I) at 30 \-Iz from an antenna that rO[a[es at 30 Rc"oluoons Per Second (RPS) so providing amplitude modulation (A~l) at 30 \I z. ,\ n aircraft rccci,'cs the 30 Hz F~f signal together with the 30 li z amplitude modulated (A~I) signal created by the rotation of the transmitter. The F~ r signal phase is the same in all directions but the A~ I signal phase changes in relation to the aircraft's radial position from the beacon. The F~I and AM signals arc arranged to be in phase at the North position so that phase differences rclate to magnetic bearings around the beacon. These pos itio ns arc referred to as ' radials'.
::-..,\
~.~... ~\~ l.'
The VOR information can be displayed in the aircraft either as the position of a needle on a radio magnetic indicator (R.\1I) . or by a deviation bar, oftcn coloured orangc, on the compass display, which has a full-deflection scale of ± 10o. In the lat1Ji'c ase, when tbe aircraft is positioned on the centreline of an air:v&r..."rt.lilP~~viation bar lies central in the compass gi"ing a clea r indication of flte Imcntft's position in relation to a selected radial. An R.Ml necdle, o n the ~ ~).t other hand, always points directly to\va.rds the beacon. VOR beacon has range of about 200 m.utical miles. \'\11en the aircraft is outside of this range, a flag appears, usually red or red and white striped, in the aircraft display indicating dmt any VOR dara displayed is not valid. _\
Tests The following tests would be carried out on the aircraft: •
~Iarker
•
VOR opemtion
•
Locali se r operation
operation
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•
Glideslope opcration
•
Combined LocaJiser and Glideslope opcration
•
VI-IF CommU!1lcations receiver and transmitter tests
NAV /COMM Test Set The navigational communications test set is used to tes t the operation o f ILS, VOR receit'ers and vllr Transmitter/Receive rs. The tes t set provides accurate measurement of fre quency, output power, Al\f/FM modulation and reCelyer sensiti\·ity.
VHF Communications Testing B2 Engineers will be expected to use N 1\ V/C01\.'I1V[ test equipment to a greater depth [han B I Engineers . f',rost ramp test equipment for VH F Communication transmitter/ recei\-ers ha\·c a fn:ed 2 to 3 fOOl whip antenna and will allow you to look fit the peak and/ or a\-erage output power of the uansminer. These meters usually ha\-e (wo settings; 0 to 10 \'Cans for light aircraft and 0 [0 100 \X 'ans for air transport systems. \Xlhen tcsting the ai.rcraft's receiver, mas! combined N av/Comm test Sets usc the Marker Beacon audio frequencies of 400 l iz, 1300 I-I z and 3000 Hz to modulate the test signal or the 1020 liz tone used to simulate ground station idem. A ramp test set will always haye a 'CO~I Fixed' frequency, usually 118.000 i\U-lz, for a stable channel centre frequency. t.iost modern test sets \...·iIl then allow ,ariable frequcng' outputS from 118.000 MJ-Jz up to 156.0001[J-fz ,\)\.i(121}-"icl~lz and 8.33 kHz steps. , \\c\!..c"
. l ...."\\} -
«'•
'- , ....7<
,\'"\. -'"1.'-';'; (.0'\'
VORT estmg ' For \-OR and ILS operation, except 1[arker Beacon, most ramp test sets hat""c a fixed, extendable antenna that will allow you to carry out the majority of ~ checks with the test 5Cl standing vertically in tlte cockpit. rn this way, you ca.R sct the tcst parameters and look at the results o n the approp riate instrume n~ without Jea\'ing the flight deck. ,. . \'\11en testing VOR lhe ramp test \Vlll allow adjustment of the 30 Hz variable phase signal., but nOl the 30 Hz reference phase as this is used as ~~ benchmark for all VOR tests. T he VOR ramp tester will have a
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~[OSt ramp testers work on a fi...... ed frequency of 108.00 Mf iz, but there are some wirh a 'HI/LO' switch chat allow resting at 112.80 ~[Hz and 108.00 l>. fHz respectively.
\'(/hen testing aircraft VOR systems you should be checking for: •
Receiver sensiti,"ity
•
Radial accuracy
•
TO/FR01\·f Opera tion
ILS Testing With mosr ramp test sets, the Localiser, Glideslope and i>.larker operations are tested together. As 1 am sure you are aware, if not look at ~[odu1e 13 Chapter ..J , Localiser and Glideslope receiyers work off VHF and UHF carrier wa'"e frequencies respecti,"e1y, modulated with a 90 Hz and 150 Hz conriouous tones. These a.re not decoded for aural purposes, bur the receiycrs monitor their Diffe ren ces in D epth of AloduJatioo (DDA1), which will be equal when lhe aircra ft is on the runway centreline for the Localiser and on the centre of the Glidepath for the Glideslope. ~ I osl test sets aUow stepped control of the DD~[ to allow specitic and precise Localiser and Glidcslope deviation indications 00 the flight deck displays. TIle 90 Hz signal is used for indicacing left of the ruoway and above the glidepath, while the I SO Hz signal indicates the opposite. As with VO~ tq..!itill g, [he output RF attenuatar allows you to test for receiver sensirixtP~~'as requircd. L"OW-''
,
. ~\"\\
~f,\r
("~
,, \ ....').•.5.
Fuel Capacitance / Quantity Test Set Before we examine what the test set docs, we need about a modcrn fucl contents gauging system
La
learn a few simple facts
'0
""" ,.
f\·lodern aircraft fuel conteots gauging systems arc based on the measurement of capacitance. The principlc is that the "alue of a capaciwr having fixed dimensions will bc dependent upon dIe dielectric constant of the matler bCI:wcen the plates. The space between the pla tes in a fuel contents capaciL'lncc= unit will be filled with fuel, air or a combination of both. Air has a dielectric constant (k) of I and fuel has a dielectric constant (k) of approximately 2. The total capacitance of a tank contents unit will thercfore bc proportional to the amount of fuel in the tank.
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,
I
-
d
- '
d:::- D istance Bet\vcen Tubes a :::- \,'all A rea of Tubes k :::- D ie! ectri e Constant
,.\; ,. (k = I)
Fucl (k
= 2)
Cap aei ta nee ::: k x a d
-
Fig ure 4.10 - Simple F\le l Tank Capacita nce Unit
If you examine the illustration, you will see that fu el co ntent is pro portional to capacitance. "'lIen a tank is being filled , the space between the capacitance unit tubes will progressively fill \\~th fuel thus increasing the cap acitance value of the unit. A number of tank units connected in pa rallel are usually installed in a fuel tank, T his compensates fo r changes in aircra ft attitude or surging of fuel in the tank, F or example, if the aircraft banks and fuel runs tmvards one end of the ra nk, some units will increase in capacitance whilst the capacitance of others will reduce, the o\'erall capacitance remains unaltered .
." capacitance uo- .
fuel gaugtng " system Incorporates a re ference or compcnsanng . _O ...W:.~c'" capacitance unit. This unit is fully immersed in fuel and is used to compensate 1\1g~
~\,',I.<{f': c
fo r changes in pennittivity when diffe rent fue l specifica tions are used.
,~
\,k'
,,<{ .,..
Reference Capacitor
U'~
•
I
~
-
'"
~.
~
0'
r
Tank Capacito r Fig m e 4.11- Simplified F uel Co nte nts Ga ug ing System
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The total capacitance value from the tank unitS is fed [0 a bridge [ect.ifie[ ci.rcuit where its amplitude is compared with mat of the reference or compensating circuir. Where the bridge is unbalanced, the errOl" signal is amplified and used to drive a moving coil instrument calibrated to read fuel content in lbs or kg. At the same time, a wiper arm is moved across a potentiometer in the reference or compensa t.ing circuit to rebalance the bridge when the instrument pointer indicates the correct fuel quantity. i\ lodern fuel quant.ity systems still operate on the capacitance principle but the signals lire fed to a fuel quantity processor where mey arc: corrected for density, converted to lb or kg and displayed on a digital fuel quantity indica (Or for cach tank and on a total fuel quantity indicator. A further advamage of a capacitance system is that the fuel content is measured as mass, nOl volume. If the fucl temperature changes, the fuel will alter in volume but trus changes its dielecuic constant. As fuel exp:wds, the dielectric constant reduces. The resulr is, the mass measurement is not ahered by changes in fuel temperature. Because capacitance is used as a means for measurement, it is important that the capacitance of rhe connecting cables in the system is kept at a constant value. Coaxial cables having fixed, low capacitance are used in these systems.
Test Sec The test set may be called a Fuel Capacitance Test Set or a Fuel Quantit), Test Sec. The test set incorporates a bridge rectifier tcSt circuit and capacirors that arc used to carry our several tests that include: •
}'lcasuri9tvhe capacitance yalues of single tank units "'I~6
...
. (.~\,,;~ i~asuring the roral capacitance value of several [ank units ... \):>"t'"
...
\. ,
\~' .
:-;'
c. co'\!.
•
Checking compensating units
•
Measuring the total capacitance of units and wiring
•
Simulating capacitance output from one or morc units to calibrate fuel coments gauges
•
Measuring the insulation resistance of tank units and wiring
•
Conduct.ing fault diagnosis and isolating fuel gauging system problems.
1n :1ddition to ca rrying out the above tests on capacitive fuel gauging systems, the set C:1n be lIsed to test any AC capacicive fl uid gauging system, including oil and water sysrems. A tcs t set is available that can test both AC :1 nd DC sys tems. The sets are portable and have a rugged design making them suitable for use on the ramp as well as in a repair workshop.
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Calibration t\ final wo rd should be said regarding the calibra tion of test equipmen t. Each
manufacturer will stipulate the calibration period for thci r c(luipmenL j\ n /\pp rm'ed j\ laintenance O rganisation is obliged 10 ha\'c a sys tem for c01llrolling the test eCjuipmcm it uses. \'(Ie discuss calibra tion in other PflrtS of this module. I lowc\"cr, the equipment should carry proof o f calibratio n and info rmation on when the next calibration is d ue. You must not use the equipment if this informacion is not :lyaiLtble. In a simila.r vein, if an item of equipment proyes to be defective in usc. you must \\·i thdra\\.o" it fro m usc, quarantine it, and clearly identify it as being de fecth-c.
Conclusion \'\ "c hayc coYercd a number of itcms of test equipment that might be descri bed being 'in general use'" \\nat you would undcrHand to be 'in general use' docs depend o n the type of aircra ft you work with. For example. J ha\'e not included GPS, l\ [icrowa\"c Landing Systems. ARlNC 429 DAT!\BLS cte" A geneml usc item of cquipment, such as the Pitot/ Static test set, lends to be used on virtually all aircraft and that is the type of equipment wc ha\"c cO\'ered in this chapte r. I3efore you take a well-earned break, there are a few multichoice answer questions fo r you to try . :IS
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Revision Avionic General Test Equipment Questions 1. T he pressure in a Pitot / Static tes t set should be re turn ed a tmosphe ric press ure foU owing a test: a)
to
ground
rapidly by opening the Vent Valve
b) gradually by slowly opening the Release Valve c)
br disconnecting the hose from the outlet pon
2. When carrying out an Altitude leak tesl with a Pi lot/ Static test se l, the pum p output sho uld b e selected to de liver: a)
ncgaavc pressure
b) positive pressure
c) positive pressure up to 1500feer and negative thereafter ')\.)0'":
"Iauu ...
e'
GO\\"''?: . ~,,'(.'S'. When preparing to conduct an Airspeed leak test with a Pitot/ Static ,\"\\. l-) test set, the output hose should b e connected to the:
·.,s~
(. ,,-v.. .
a) Static vent on the aircraft b) airspeed indicator on the £lighl instrument panel c)
Pilot head on the aircraft
4. During a n Altitude leak tes t, a n air leak in the sys te m will be revealed by a : a)
steady reduction in indicated altitude over the test period
b) steady increase in indicated altitude over lhe rest period
c) fluctuation in the configured altitude setting over the test period
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5. \"Vhen testing the operation of an Altitude s ..vitch, you should check the altitude at which the switch:
a) closes during ascen t only b) opens during descent only c)
closes durlng ascent and opens during descent
6. The operation of an installed Mach switch: a) cannot be c hecked in sim
b) is checked using an.Air Data Test Set c)
can only be checked in iligbt
7. When verifying an SSR Transponder identification code with a test set, the code must be firs t dialled into the:
a)
aircraft Transponder
b) [cst set c) ATC radar
i:
. ~~S1 . ,\,<'" " (\'f
- 0\! l'
"
,
( o\\<.:'b '
8. When verifying the encoded altitude of an SSR Transponder, the (\,(16 - 1l'fHiude code is first obtained ftom: 1.\./
a) Air Traffic Control b) the test set c)
the Trans ponder
s---
9. When carrying out Marker tests, the codes, tones and ligbts are.: cbecked on tbe :
a) tcst set
b) aircraft equipment c)
I nner, I\liddle and Ourer Markers
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10. TCAS tests may be carri ed out on equipm ent incorporating a: a)
il.lodc 5 Tmnsponder
b) Audio Signal Generator c) :\i\[jF?-.[ modulator
11. When the DME is selected to the ILS channel, it should indicate distance to:
a) a selected \lOR beacon b) touchdown c) outer marker
12. When a Ttanspo nd et/DME test se t is used in a repair workshop, the test signals arc con nec ted to the equ ipm ent under test through a: a)
remote antenna
b) imegral antenna
c) coaxial cable
13. A Fuel Quantity Test Set used to calibrate fuel gauges simulates value~--R.fi ,~,~i;. ,...
_
. ~~<;0
,
""
",.(\~
... l~'o
(,\.-'\ '--
...,0\.' ~
.
a) capaClrance b) reslStanCe c)
r,
current
o
." .<
o
14. A Fuel Quantity Test Set measures the output from tank units us ing
a)
ratiometer
b) bridge rectifier circuit
c) ohmmeter
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15. Fue l tank quantity indicating sys tem s:
a) arc matched
[0
the density of one type of fuel
b) hayc a fuel density selector
c) are unaffected by changes in fuel density
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Revision Avionic General Test Equipment Answers I.
B
2.
A
3.
e
4.
A
5.
e
6. B
7. A
e
8.
9. B
10. A 11. B
'),l)l\1
'1i..'\\,)u ~f Co\\c. ~ -
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Engineering Drawings, Diagrams & Standards I n trod uc don .\n engineering drawing is a comprehensiye, documclHcd record of a dcsigncr's requirements that is produced by draughtsmen in accordance \\';th an approved standard that can be understood by a !..rained pcrson using the d r:'I\\;ng, Dr~l\\;ngs may be used to conyey the requiremems for one or more task proccsses that include, for example, production planning, purchasing. manufacture, assembly, testing and inspection of an item, It is important that a dra\\~ng contains sufficient information co enable the pcrson using it co ca rry out the (e(luiced task. A dra'\i-ing must conL'lin all the required dimensions. limits, fits, material specifications and processing information so that the person us ing it docs not ha\'c to refer back to the design department, To avoid any chance of misinterpretation, the person who produces the drawing and the p ct~on \vho uses it must be familia r with the terms, symbols, abbre\'iations and present:u:ion methods set out in the approved an d agreed standflrd. \X1hen you consider the myriad of different manufactured products and sys tems that exist, it would be virtually impossible to have one uniyersal set of inflexible rules that could be rigidly applied to all cases. 'nlC stfllldard th:u is useckilS" the general basis for engineering drawing practice is British St:md;Jrd •. 7..\\\.'fBS) 308. W'ithin the aerospace indusrry, there are particular industry related ,(.'l\\":~'" drawing requirements that are published under the Society of B ritish ., \v((' Aerospace Companies' (SBAC) Technical Specific.'uion (TS) 88, ,\~. . \.. -' Appro\'ed Design Organisations then amcnd the B5 and SBi\C drawing U"''\.':'\ " practices to suit their indi\;duaJ company relatcd requirements and produce their own Drawing Office Standards, "
I n current drawing practice, the International Organisau'on ias S'imd
p;= :;
The aircra ft maintenance engineer \"ill encounter drawings and diagmtA'S produced to sen'c man)' different purposes. Aircraft and aircra ft compon~t manufacturers work to detailed working drawings of items to be manufact¥ed and assembled. Aircraft Structural Repair 'Manuals and Aircraft StruG.t'llral Repair Schemes also contain detailed workiA g drawings o f items ~ be manufactured and assembled in support of autho rised structural repa i ~""
",
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In:l completely different vcin, Aircraft Mainten:lnce Manuals contain hundreds of illustrations of sys tem and structural components. Many of these :Ire produced solely for :lssembly, instalianon, inspection and servici ng purposes :lnd :Ire nor intended to be precise working drawings. Installation drn\\~ngs show how p:'lrts arc to be assembled, secured, locked and adjusted in the :lircraft. These may be in rhe form of pictorial views :lnd they may consist of single parts or exploded views of assemblies of p:lrts. There arc Rlso schcmauc system diagrams, wiring di:lgrams and servicing di:lgrams. We will be dealing with diagrams later in this chapter. For now, we will be conecntraung on engineering drawing praeuee.
Drawing Sheet Sizes \Ve may as well start with the most basic item and that is the p:lper wc draw on. There is no hard and fast rule on what size of paper you should usc except th:lt the intended drawing. at the scale you choose, should preferably fit on it! To gi\'e you an idea o f size, the British Standards Tnsurucion have produced an 1\ series of recommended paper sizes. The ,,~dth and length of each size arc in the rauo of 1:I.41-t(root of 2). For example, 1\4 in [he series is the paper size of this book.
AO
il l
t\4 210mm x 297mm Figure 5. 1 • Dra\ving Sheets
D rawing Formats The next basic decision is how the drawing will be presented on the chosen sheet. There arc two possibilities: the Portrait fonnat where the sheet is to be viewed with the longest side "eruca! and the Landscape format where the sheet is to be viewed with the longest side horizontal.
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Landscapc
Po rtr:l.it
Figure 5.2 · Drawing
FO Dll~l
Pictorial Views PiClOrial \·jews are frequently added to engineering drawings to supplement them by illustrating the appearance of items. The? are not suited for accurntely pomaying complex proftles or shapes or for com'eying detailed technical infonnation. They are used extensively in maintenance ma nuals. overhaul manuals, and in pans catalogues . There arc three methods of producing a pictorial view of an object: the Perspeco,'c Projection, the Obliclue Projection and the Isometric Projection. Perspective Projection \\1hen we set about drawing a ,'iew o f any object in the Natural \'( 'orld, we arc faced With perspective, things look smaller the further away they arc rrom YOll. I f we were to draw an object as we see it, parallel lines would appear to converge in relation to their increasing distance from us . I f we wcre to exrend these receding lines, they would ultimately converge to intersect at a point \\le call ths.'vanishing point'. There are sC\'eral ways of drawing perspective views. ~f6epeWding on from which angle you view the object. i\ popular method is to • ~\c'?-(. l' draw one true face, as if it ·were across your line o f sight, so that all the ~~-..; ell remaining faces recede towards a single vanishing point.
.. ,;........ ~
., L
\.... ).
-
Vanishing Point
'0"\ c·
~.
'0
"" j
~ -'" L
,"'"
FroOl
.~ ,~
0'
,~
Fig ure 5.3 · Perspective View
'§
'v £'
T hough perspectivc views look ,"cry natural, they are not bes t suit&! , for displaying technical information. Parallel lines do not look parallel a@"' similar dimensions [rom froOl to rear do not look simil1. The perspective \ricw would be sui led to portraying the general appearance of an item in a sketch.
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Obliqu e P ro jecti o n This type of \ricw also has ooe true face parallel (0 tbe plane of projection, or across the line of sight. The difference is that the parallel receding lines remain parallel and there is no va nishing point. The no rmal method of producing an o blique projection is to draw the true fro nt face af full-scale and the receding lines to half-scale at 45° to the ho rizontals on the front face. Less used variations of the oblique projection include dra\\>;ng the receding lines either at 30° or at 60° ro the horizontal where this is thought [Q produce a clearer view of particular features. Oblique projections are a useful way of illusmning the general appearance of an item, possibly to show you the final shape of a mo re complicated drawing, or simply [Q indicate where an item is positioned in relation to The :tircrnft stmcnlre. There is only one true face in the oblique projection.
Oblique
Isometric
Fig ure 5.4 - Isomelrie & Oblique Projections
Isometri c Pro jecti o n T his is the \t!oS\l..'\~dely used fonn of pictorial new. Tt is frequently used to SUP~'P~rt~ views on working drawings and to produce illustrations in ({Dhilitenance manuals, overhaul manuals and parts catalogues. The receding 't ¢ ,,';1. lines remain pa.rallel and there is no \'anishing point. AU receding lines arc . ,.;.\\J),' projected at 30° to the horizontal and all sides arc drawn to the same scale. 1.. 1.'- '\ This feature gives this type of projection its name in that 'iso' means the 'same' and 'metric' means 'measure'.
Orthographic Projection \'\!hen it is necessary lO show the exact size and shape of a complex item there will be a need to present it as a number of different " iews. A cube has SL": sides and each of these can be drawn as a separa te view. These would be rhe: front, rcar, lefr side and right side elevations and the top and bottom views . \Y/e used a cube as a simple example bur all objects can be considered as having a similar number of views .
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,,
,
"
,, , , I
,1
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, 1-
I I II
I
I
......
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'@1"" ?f\ ,W , --G',~,0--
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I
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------I - ----~------,-----Re Left Side 5;d '
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-
-
-
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I I I
" l
I I I
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.-----~-----~-----~----- -
, ~
,' ,
'
,'
Bottom I ------
Figure 5.5· Orthographic Projection
It is not necessary to draw all
SL'\: news to fully show an item. Normally, only tbe 'dews that are required to clearly show the required feature s and dimensions are drawn. The most commonly encountered drawings consis t of one, two or three vicws. Occasionally, it may be necessary to produce an additional view that may for example be projected from a sloped surface in order to 5bO\v irs [me shape. In rhis case, an auxilialJ \-icw is drawn.
The "iews are arranged Oil a dra\"'v-i.ng in thcir celation to the fro nt elevation. There are t\vo arrangemcnt systems, each of which depends 011 tbe type of orthographic projection used. TIIere are two types of orthographic pro jection, Fir~u&ngle Projection (European) and Third Angle Projection (American). '!.Q\1S·Oth systems of projection are commonly used in general engineering bur . c.o\\C~c drawtngs produced in the aircraft industry now conform to "Ibird Angle • 'O"..~("'': Projection. However, there are early drawings still in circulation that may still df'- !o.. conform to First Angle Projection . Both projection systems will show th e -c'x'<""'actual size and shape of an item when 'viewed in the horizont.al and vertical l) planes. Occasionally, for convenience, the drawing rna}' be scaled up or down
C;'
The system of projection llsed will be indicated by a symbol in the form of t5.£ fr ustum of a cone that is included In the infonnation block on the drawing. ~ ?
"
CJ @ @CJ Fi rst .A ngle Projection
Third Angle Projection v
Figure 5.6 - Symbols Indica ting prl jection Me1hod
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CHAPTER FIVE ENGINEERIN G DRAWIN G
First Angle Projection T he principle used in fi rst angle projecrion is {Q place the drawing of (he sidc you arc "icwing on the opposite side of lhe object. It is as though you arc projecring the side th rough the object onto a screen on the oppositc side. Side Vicw
From Vicw
[3
1
'\
0
Plan \'jew Figure 5.7 - FUsI Angle Projection (Principle)
For example, the lOp, or plan view, will be posirioned undernc:nh the fro m eleva tion. The left side elevation will be positioned to d lC right of the front elevauon and vice versa.
r im J\nglc
From View
Side Vicw
Plan View Fig ure 5.8 - Firs t Angle Projection (Arrangement)
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Third Angle Projection The principle used in third angle projection you are viewing adjacent to that side.
1S [Q
place the drawing of the side
j
"plan View
Front View
~cVicw
Figure 5.9 - Third Angle Pro jection (Principle)
For example, the rap, or plan view, will be positioned over the front eievanon. The left side clention will be positioned to the left of the front cle\"ation and vice versa .
Third Angle
Snnboi
® -E3-
0
Plan View
0
[]]
Side View
Front View
Fig ure 5.10 - T hird Angle P ro jection (Arrangement)
,
~ ,
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Views The general practice when producing views of aircraft parrs is to drilw them looking at the left side of the aircraft. This means that when there is a handed part, the left hnnd part would be drawn. \V'hen a different view is dr:nvn, it will be clearly annotared to identify it. for example, "'iew looking forwa rd :n station XV'. The number of differem ,riews of an item that will be shown on a drawing will depend on its complexity. Simple items may be fully described in twO views whcrcils a more complex item may require three or even four views to show all the details. Auxiliary Vi ews When an item has a sloping face, neither the plan ,;e\v nor any other clenuon will show the true shape of the inclined face. The true shape is drawn using an auxiliary "iew Ihal is projected on a plane that is parallel to the inclined surface.
CD View in Direction of Arrow
Figure 5.U - Auxiliary View
EJ
Secti onal Vi ews It is somerimes necessary [0 show an item \vith all or part of a view cur away to reveal hidden fealUres. A sectional view may consisl of a plan or elevation that has been completely cut through along one of tbe centre-lines. Alternatively, a part or half secoonal view may be considered more suitable. Siaggercd sectioning may be used [0 rake in particular features. I-Iarching lines arc normally drawn to indicate exposed cut-awny surfaces. These lines arc drawn at 45° to the axis of the section. Adjacent sccooncd surfaces arc hatched in different dirccOons. Bolts, screws, rivets, pins, shafrs and ribs arc not normally secooncd along their longitudinill axis.
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SeeciDn A-A
Part Sectio n
I
Half Section
Staggered Section Figure 5.12 - Sectioned Views
Types of Drawing BS 308 recommends four types of drawing: Single Part Drawing A single part drawing is used to show a single item or assembly and is a detail dra\ving that contains all dIe data required to completely describe the item to be manufactured. This includes the: fonn, dimensions, tolerances, material, special processes and finish. The final drawing mal' consist of one or more sheets.
-
y -y
I Figure 5.13 - Sing le Pan Dc:""ing
<;:.
""--:;-
Collective Drawing
-
)Q
7
A collective drawing is used when a single part is produced to a series o f. different specifications, each having different features but based on a simil~ shape. The differing features may simply be in the material or finish etc . .tp other examples, the differences may be in length or thickness. Because t:§e items are all similarly shaped, they are defined by a single drawing th3f'is accompanied by an identification table and a code for each item . You may,Ji"ear these type drawings referred to as tabular drawings fo r that reason. -\~ood example would be that of a special bolt that is supplied in different q1aterials and protecti\'e finis hes or in different diameters and length s. &
•
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l B'I -Hr~--E-B I
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A
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AI2
Finish T hread ,\ B I-ITS CAD Unified? CRS ~AT Unified 2A HTS CAD Unified 3A ALA Green Umfled 2A
~i:tteria
()
Figure 5.14 - Collective Drawing
Constructional Drawing A constnlcDonal drawing is an assembly drawing that contains sufficient dimensions and informacion to describe how each component part of the conSLnlc[ion is brought together. Each component is identified on the drawing in relation to a pans lisf that is included as a part of the ride block.
\\'a sher Steel 2 Nut 1/] in .\ 2 Pul er 2 rae -cr I Item Descri Don 1"o.Ofl 4 3 2
Figure 5_15 - Simple Assembly Drawing
E
3 Combin ed Drawing A combined drr\wing contains an assembly drawing and a parts list together with separate drawings fo r all me individual component parts on rhe one drawing.
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Five
DR AW ING
Parts
Assembly
~-$- ~I!~II Figure 5.16 - Com bined
Dra~",i ng
Aircraft Drawings A complete set of dra\vmgs relating to a particular aircraft, together with the specifications and documents assoClated 'with the drawings, provide the record of the information that is required to manufacture and consrruct the aircraft. These drawmgs also form an essential part of the inspection records. A sct of drawings is configured so that cvery component, dimension , process or operation is traceable. Tn this sense, the drawing types bear a relationship with those wc hayc already discussed but you may encountcr slightly different terminology.
General Arrangement (GA) Drawings Thi10rrPe of drawing is used to illustrate the I 'general arrangement' of the ;..Gl1UrCiaft, main assemblies and systems within the aircraft. These drawings only , C<."l\\c.'be. illustrate general profiles . For example, a whole assembly or system will be • \\~..<': shown on the drawing and the location of the component parts will be . \~,\. b identified along with references to their associated drawings. These drawings (P'\...... will also proyidc a guide to the identification of drawings used by the design organtsanon. It is a bit like looking at a map of the whole London underground rail network to identify the whereabouts of a particular line a€1 then a station. You are able to relate these to thq overall profile of the netw~ and recognise the sign~ used but must seek further illustrations to see whar--t particular station looks like. ;
Main Assembly Drawings
'-
'"'
Tl1cse drawings may also just contain a general profile and only giyC su ffici# t infoml ation for the assembly of parts and ~ub-assemblies into the ~n assembly. The detailed information relating to the individual component..{parts or sub-assemblies is not repeated on a main assembly drawing?l" This information will be shown on separate sub-assembly and single part d~",vings. ,'5 v
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You will also encounte r assembly drawings in Aircraft Pam ~ranuals where they are often presented to you as exploded pictorial views. The assembly will carry a reference number and each parr will carry a unique part number. The single parr drawing number relating to each item is included in a mblc adjacem to the drawing.
ltrm Descri lcon Dr.vg
~o.
,
'
4--
Figure 5.17 • Exploded View
drawings provide sufficient infoml<1tion to enable component single parts to be brought together into a sub-assembly. \,('hen special processes such as welding, shrinking, pressing etc. are involved; the drawing will give details of any heat treatments or anti~corrosivc treatments required. Any information related (0 special tools, jigs or fixtures would also be included . .,01..1< los lalla li
"i11CSC
,-
CO\\I;'tl ~ ",~:tq:h ese ~,,\
. ,-~<;;'.'t\"'" l. .
'-
drawings are used to show how parts are assembled inca the aircraft in rcl:\uon [0 other components and assemblies. They illustrate the positioning, clearances, settings, connections, adaptors and methods of locking used to successfully install the parts. You will also encounter this type of drawing in the Aircraft r..laimcilance i\[anual when you remove and install equipment. In [his laner casc, the drawings will often be presented to you as pictorial \Tiews.
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Figure 5.18 • Simple InSlallacioD D rawing
Individual Part Drawings These are single part drawings. They contain all the detailed information required to m."\Oufacrure a pan to the design requirements. T his information will include material specificauons, dimensions, tolerances, machining processes, surface finish and anr special processes required .
Repair Drawings Wie should mention these here, as they arc a special case. Standard repairs that may be applied to all aircraft of a particular type are contained in the rclated Aircraft Stmcmral Repair ~[anual. These repair schemes apply to one pa rticular aircraft type only and include information on the classification of damage, the repair limits, methods for dressing out damage and crack detection, repair practices and the details of repair materials and ff!.steners etc.
"\l!j.,rh~~tfd1ere
is no suitable repair scheme in the Aircraft Structural Repair _Cp\\C~c-'" Manual, the Design Authority may be requested ,to provide one. The scheme is . '0~~~: normally issued in the form of a repair drawing. This drawing can only be used ",'0\ bfor the repair for which it was requested on a given registered aircraft. If a need -0'\'-;~\::: arises to conduct a similar repair to another aircraft, this can only be done if L,: the Design Authority authorises its reusc. \'?hen it is foreseen that there \vill be a frequent need for the repair, thc Design Authority rna}' incorporate the rep:Y:'r into the Aircraft Structural Repair Manual. ,
3:
'.F-i'
We will be discussing the authonsation, control and amendment of airera l{ drawings later in this chapter. For now, we will look at more of the drawing; practices that are used on aircraft drawings.
!
,~
p
Scale
""
:!? r~
A'
d;'
]\ drawing must be produced to a uni fom1 scale. In many cases, the6drawing will be to full size but there ""rill be occasions when a drawing may h~vc to be ",J,d up 0' dowo. The ,c,Je o[ , d",,"og i, T ted " a "cio, the tetro' '[ull
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size', 'half-size' etc. are NOT used. The correct method of identifying 'full size' is 1: 1. On drawings smaller than full size, we usc reduction ratios. For example, half size would be 1:2 and qwmer size would be 1:4. \X1hen drawings arc enla rged to morc than fuj I size, we use enlargement rauos. For example, twice the full size would be 2: 1 and five urnes the fu ll size would be 5: I. T he original scale used on a drawing is stated in the title block. For ex:nTIple: ORIG INAL SCAL E 1:2. \Vhen the abbreviation NTS is marked on a drawing it indicates 'Not to Scale'. No drawing should ever be measured to establish an unknown dimension. Jf a dimension is required, and it is nor shown on rhe drawing, you must refer the query to the Design .Amhoriry.
Lines BS 308 includes recommendations on the types and thickness of the lines used in drawings. All lines should be drawn uniformly black and there should not be a mix of pencil and ink on the same drawing. T he acrua! line thickness in copies of drawings may vary from rhe original drawing bec
t;.
"".,-.
%-
Figllre 5.19 - E xamples of Types of Lines
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Line
Description Continuous Thick (O.7mm) Continuous Thin (03mm)
r\pplicacion Visible: Outlines Visible Edges Dimension ! ,illes Pru jccdon Lin es Leader ! ,i nes Il:ttching Rcyoh'cd Scccion Outlines Tmagina rr Intersections STu)!"t Cemre Lines Outlines of Adj~cem Pans Fictitious Outlines and Edges
-------
Shorr Dashed T hick (O.- mm)
Hidden Ourlincs I IiJden Edges
Dashed Note: Thin d~shed is more - - ----- TShort hin (O.3mm) commonl" used in the UK
_._._.
~
~
-----r--I
I I
..J
_._._.
_
..
_ .. -
Cenm: Lines lines of Snlllnctn' Tra jectories of ".\ IO\·ing" Pans Loci ' Pitch Lines and Pitch Ci rcles limits of parci~ l \'iews or Continuous interrupted \·jews and Irregular seccions when the limit (O.3mm) is not on :m ~X I ~ Limits of partial \·iews or Continuous interrupted views and T hin Zig Zag sections when th e li mit tS nor on an aXIs (suited fur drawinj;s produced by Ill:lCh lne) Chain Thin (O.3mm)
Cutting Pl anes
Chain T hin T hick at ends and at changes of direction ~.7mm Thick .3mm Thin) Chain Thick (O.-mm)
Chain T hin \\ith Double Dash (O..lmm)
Indic:Hes lines or surf:Ices a special requirement aKphes ( u\\"n adjaccnt to surf:lce)
whe~e
~.
Outlines and edges of ad jacent pahs Extreme b:0sltions of mova Ie parts Bend tines and initial oudines prior formi ng
~,
-.
%
'"
(:
~
Table 5.1- Types of Lines
Das h ed Lines
I
.Ai'"'
"
'-'--
§
Dashed lin es should be drawn so that tile dashes arc of eq ual lc!2£th and spacing. The line should always start and end with a dash and shotJil always meet with dashes at tangent points and corners.
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Chain Lines AU chain lines should start and end u>1rh ;! long d;!sh ..<\ thick dash should be used at the ends of cutting plane lines and al changes in direction. \Vhen cenrre lines arc drawn to indicate centres, me lines should always inrersect at long dashes. Centre lines should only be exrended a shorr distance beyond a view but should not Start or terminate at another line on the drawing. I f a cenrre line is requiIed for dimensioning, it should be extended as a profection line. Cenrre lines should not be extended across adjacent views to form a common centre line. \,\'hen a chain line cbanges direction, it should do so at a long dash and when it meets another chain line, at a corncr for example, long das hes should meet. Leader Lines Leader lines arc used to provide margins for dimensions or to indicate where symbols or annotations apply. Arrowheads on leader lines always touch and StOP on a line. Dots on leader lines are always placed within the outline of an item. When an arrowed leader line is used (0 indicate a radius, it should be in line with the centre of the curve or circle. Leader lines should not be too long or intersect with oilier lines. This can be difficult [0 avoid but the lise of alternative dimensions and symbols may overcome the problem.
Dimensioning Funet'ional Dimensions Funccional dimensions are those that direcdy affect the function o f a part and these must be )~~n on the drawing. An example would be the lengdl of the . .') I\II\) pla.l~-£~1l0n of a stepped bolt. <" l .
\.;s-.. .
" . \.>~'\
..: \'\~'(Non~functiona l dimensions Non~functional dimensions do not have any effecr on the function of a part and are provided to suit production or inspection. The depth of a bolt head would be an example of this.
Auxiliary dimensions Auxiliary dimensions are sometimes provided where they might be considered to provide useful supplementary information. TIley are nor pro\·ided with tolerances.
Dimensions All dimensions on a drawing should be displayed in such a way th:lt yOll do nor have to deduce a dimension from adler dimensions. Dimensions are always
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CHAPTER FI V E ENGINEERING DRAWING
positioned so that they cao be read from the bonom or the right hand side of the drawing. Dimensions are nonnally only giyen once and arc not repeated in all views. There should be no more dimensions than arc required to define the pan. The units llsed are stated in the infom1arion in the tide block and arc not further identified in the drawing unless a unit fo r a particular dimension differs from this. I n this case, the unit \"ill be annotated alongside the dimension. The values atuibuted to dimensions should be given to the least number of significan t figures for example, 22 and not 22.0 or 22.5 and not 22.50. I-I O\\'c\'er, values less than onc should be preceded by a zero. i.e. 0.5. Drawing practice in some countries replaces the decimal point with a comma i.e. 22,5. Centre lines. extensions of centre lines and outlines should not be used directh' as dimcnsion lines; they can howCl-er be used as projection lines for them. A dimension line should be drawn between I."WO projection lines that start just dear of the item and extend JUSt beyond the ends of the dimension line. \X'here possible. the crossing of projection and dimension lines should be a\·oided. \X'here tlus is not possible, me dimension line must not be broken at the 1!1tersecuon. Projection lines on the other hand may be broken where tlus aids darin'.
Dimensioning from a Datum This is the most common method of dimensioning used in drawing~ . AU dimensions drawn along it horizontal or vertical plane arc measured frolll it common datum edge or line, In the illustration, there is one vertical damm edge and one horizontal damm edge. The choice of which edge or line to use as a datum i~ often important to the set up of automatic. numericallv controlled maduoes. Additionally, this fonn of dimensioning flvoids th~ pro9~!Tl where the cumulatiye effects of tolerancing separate dimensio n~ can ,i..\\N!su1t in out of limits work .
. '\cC.c . l,'" . \~'\\
,;5
-o~
L
,
\\';1.'('(':
140
95
..
~
40
~ffi=-- Elz iJ!!
65
0
,,-
~l "' ~
·c
2 Holes 15 Dia
All DimenSions 1n i\fillime[!"(~s
Figure 5.20 - Dimcns ioning from a Datum (recom mc nded)
293 Copyright (,") BanJ' Co/l~ 2006 . 200- ARrighlI rtifnYd
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CHAPTER FIVE ENGINEERING D RA WIN G
Chain dimensioning, between individual faces or hole ceorres, is not normally used. The problem with chain dimensioning is that you would have to add each dimension up as you went along a face to establish an overall dimension . .r\ more serious problem occurs because the cumulative effect of all the individual tolerances in a chain of dimensions may exceed the overall permissible limit. Trus will result in the production of unacceptable parts and can create problems when assembling parts rogether.
40
55
45
-
~
~
-8-
-0,
~
20
~
~S
0> N
.I
Figure 5.21- Example of C hain Dime nsioning (not recommended)
An alternative mccilOd that is often employed
00 riveted joints is (0 locate the end holes and add an annotation such as '22 rivets equally spaced'. Tlus method is particularly useful when dealing with curved profiles.
Dimensioning Curves
=
eo'
y
'?:/~{
Symmetrical curves are ideally dimensioned by using radii. \'(' hen a radius IS 50 large that its ct;.~"" cannot be shown on rhe drawing sheet, a zigzag radius line can h~,-drl\t-~. The portion of the line that tOllches rhe arc would be drawn in - ~\;..", e-ligHmenr with the true centre of the curve.
'\\~
_\...~"l
c·
c-. ,
""-." "" "
Using Radius U smg Ordinates Fig ure 5.22 - Dime nsioning Cu rved Profiles
294 Copyighl © Barry Coll~gt 2006 - 2007
All rigbts rmrved
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CHAPTER FIVE ENGINEERING DRAWING
\\lhere a CU1TCd profile is not symmetrical, a method using ordinates can be employed. \\7hen it is possible to use it, the radii method is more accurate in that it is not affected by errors that may be inad\-enently introduced when drawing ordinates.
Dimensioning Tapers T here are a number of methods used to dimen9ion a tapered item. These are self-explanatory from thc illustrations . Do note the use of the international symbol for taper that appears in one of them.
§[EJm I.
40
.1
r--C 40
Efi
. - 7.f -.lli
I.
40
.1
~
--
18
§[ t==t-I.
501 ncl UdCd Angle
-- --
'0
N
&
~ 40
Use of the Taper Symbol Figure 5.23 - Dim ensioning Tapers
,~ol~I'ances t 'Q.~«, .,:...~,... (i-'\(......
't~ c.,3\\e.'~" .
-
As we discussed earlier, for current projects, thc I nternational Organisation for Standardisation (ISO) system for the dimensioning and tolerancing of drawings is used. HOW(Ter, older drawings will still contain earlier system s for Imperial and i\letric terms and tolerances. Most linear and angular functional dimensions will be subject to a tolcrance.g\ general tolerance is often given in the title block and this is intended for usc ~ all dimensions. However, some dimensions may require individual tolcranc~ that differ from the general rolerance. In these cases, the tolerance will b; annotated together with the relaxed dimension on the drawing. 2
,
A tolerance may be unilateral or bilateral. A unilareral rolerance is one wh~ the pennirtcd dimensional variation is in one direction only. Tlus may be ei~r above or below the basic size. A bilateral toler.wee on the other handi1s a permitted dimensional variation that is both above and below the basi§size. The tolerance may be written by showing the upper and lower limits,'bf the dimension. In this case, the upper limit is located above the lo~r limit. Alternatively, the nominal dimension may be giv~n followed by the p'errnissible
295 Copyight © Barr] Col/(gt 2006 - 2007 All rigbts remwd
CHAPTER FIVE ENGINEERING DRAWING
deviations above and belmv mat value prefixed by a plus and a minus respectively.
Sign
A,
~300 20'
~9° 40'~oO ± 40'
R3±.:); I--- 80,S ----->I 79,5
Sphere R?5
ll-+
0 80 - 0,5
I-
+2
grp 0 29 0 19
-I
I-
+ 0,5
80
-I
Figure 5.24 - Dime nsinnal Tolerances
Angular tolerances arc cxp ressed in a similar way, either by showing the upper and lowcr limits in degrees and minutes, or by shO\ving the permissible deviations from the basic angular dimension,
Geometric Tolerances Other dimensional feanues that relate to the shape of an item often require dIe application of a tolerance. \lie have discussed how tolerances are applied to linear and angular dimensions but these do not control features such as, for example, parallelism, squareness or roundness. Early drawing practices involyed annotating the feature \"ith the abbreviation POSN TOL and then adding notes in t1~~ drawing to specify the tolerance. This method was both space and,Jjmc ~6nsuruing and has OO\\' been replaced by an internationally a.!?C..~'(i:.~~ };Stem of geometric tolerances dlat uses symbols that relate to \'J'J.~ lhrtieular features. lC . 1\,.... -
, ~,,~
Single Features Form: Su:ai hrness Flatness Roundness Cyliodrimv ProfIle of a Surface 0 Profile of a Line Additional Informacion )'faximum Material Condition Dimension \'';/hich o fines a T me POSitiOI'P--
( 9~--'
o o
Related Features AttitudeParallelism II ...L Squareness .L. Angularity Location· Posirion ~ @ Coneemriciry Symmetry Composite:
t::: IRunout
I /" I
Figure 5.25 • Geometrical Tolerance Symbols
296 Copyighf 0 Barry Collt,? 2006 . 2007 All rights !"ese/lied
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CHAPTER E NG I N EER I NG
FIVE
DRAWI N G
All me information relating to a particular geometric tOlerance is inserted intO a rectangular frame and an arrowed leader line is then drawn fro m the frame to indicate the location of the related feature . S\·mbolised when Tulerance Applies to ?\fl\fC of the Feature
Symbol for Type Tm:tl Tolerance
of Tolerance
j
j
j
A t
f
Symbo l used when Tolerance is Circular or
Datum
Cdmdrical Figurc 5.26 - Geometric T o lerance Frame
Tf the tolerance IS related to a darum, then a leader line is drawn to connect the frame to that darum. A small, solid equilateral triangle is drawn at me point where the leader line meets the datum line. ~AJternativ cly, the darum line or particular darum features mar be identified by a letter or letters inserted into the frame.
a) Squareness
b) Symmetry Jl' 2 \
c) Position 'l.:;. A
~
""
" d) Parallelism
e) Concentricity
f) Angularity
< ~. ~
o
~
Fig m c 5.27 - Exam ples of Geometric Tolerancing
,
A few examples o f geometric tolcrancing have been illustrated for you and ~ can examine a selection of these to help you to interpret their meanings, anqgo see how they save writing lengthy notes on a drawing. The fIrst exampl~(a) refers to th e squarcness of the part in relation to a darum face that is indJtated by a leader line and a solid triangle. The frame shows the symg§1 for squareness in the first box and indicates that a squareness tolerance ~~O.4mm is required. )'
297 COP)·n"ghf © Barl)' Collegt 2006 - 2007 All nght, /UtlWd
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CHAPTeR Five eNGINEeRING DRAWI NG
The second example (b) is a little mo re complicated and refers to the symmetry of the part. T he arrow from rhe frame points to the hole and the letters in the frame indicate thar this is 10 relario n to rhe datum feanues A-B. These damm features are identified both on the drawing and in the last box in the frame. The frame shows the symbol for symmetry in the first box and indicates that a symmetry tolerance of O.2mm is required with respect to the datum features :\ and B. This means that the axis of the hole must lie between tWO parallel planes O.2mm apart that are symmetrically arranged about the common plane of tbe sims in the ends of the part. Now look at the fifth example (e). The symbol in the first box in the frame indicates concentricity. The second box shows tbe symbol indica ring that the tolerance of 0.2mm is circular in [elarion to the datum circle A. T his means that the outer circle must be within a concentricity of 0.2mm with the datum circle :\. Finally, look at the last example (£). The symbol indicates angularity. The tolerance is 0.5 minute in relarion ro datum A. The final symbol M indicates tint the rolera nce applies only to the maximum material condirion of the dimension or datum feature and may be greater at tile actual finished size.
Machining & Surface Texture Symbols The basic machining symbol is an open tick th at has legs drawn at 60u to a line representing the surface. When the rick bas a crossbar, il indicates that the surface requires machining. If the rick contains a circle, it indicates thar removal of material is not permitted. The symbols are applied normal to the relevant surfaces or nonnal to projecrion lines drawn from tlle surfaces. '"IUll-
-
Surface Te."{ture Equivalents
~
'o rllina [jerome'f 0.02.5 (l O.5 0.1 0.2 0.-1 0.' 1.6 3.2 6.3 12.:; Val ues Microinch 2000 I 2 4 S 16 J2 63 1' - 25 ; 00 I ROLlghn es~ t-:Llmher NI ;..;:: N3 N-I [\:.5 N6 1':7 NB ~ 9 1': 10 :;"';11 N I2
.,
~
~Jl O'" #2
T)Vical r.lachining Symbols 4
P
.1 Vt
No Material to be Removed
ft'
'--='---_
:::r~
Figure 5.28· Mac hining a nd Surface T exture
\'(lhen an Item IS to be machined all over, the machining symbol will be annotated 'ALL OVER'. If a parricular machining process is required, the
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CHAPTER FIVE ENGINEERING DRAWING
symbol will be annotated to reflect this. There arc different systems for expressing surface roughness. It may be expressed in micro-metres. in microinches, or as a roughness index number. The appropriate surface lCX[U[C values nc annotated alongside the machining symbol. \Vhere there are maximum and minimum values, the maximum value is located above lhe minimum value. \Vh cn only one. value is annotated, It represents the maximum permissible ,'atue o f roughness for that surface.
Conventional Representations ~
! i External Thread Intern:ll Thread Thread Insert . \sscmblcd Screw
l
Spring
Bearing
Figure 5.29 - COllvenlional Re presentations
[\ n'l\Tlber of common features may appear in drawings. Examples of these are ;"1.'\.I~lternal and external threads, splines, springs, squared cnds, knurled pin heads {' C\l\\C'b~ ~nd bearings. To avoid complexity, .and to sa'-e time, these items arc not drawn '- \"")5 . tn full but ;'lre illustrated by con,eononal representation. ,\-,\ -
~:.5\':"
-C'o;;:
L-
-~ External Thread
Mv-,
-$--${l) Lmernal Thread
T hread Insert
S 1- _. Sh . P IOCu, atl
+++++++
+ + + +
Holes in Circular PilCh
Figure 5.30 - Conventional Reprcscnlarions
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CH A PTER FIVE EN G INEERIN G DR A WIN G
Break Li nes
\,(!hen a drawing contains a long component of uniform section, ir is sometimes nOI practicable to draw il in full. Break lines can be applied to the section.
[3-l-+ +--, ,--t DD Solid Shaf[
Iiollow Sha ft
G eneric Break Li nes
Break Lines (\'("ood)
00 Figure 5.3 1 - I nlerrupted Views (Break Lines)
.
'0
S,
300 CopJnjhf -0 Barry Colltgr 2006 . 200 -
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CHAPTER FI VE ENGINEERING DRAWING
Abbreviations & Symbols T o awid cl uuering a d nl\\ing up \\~th Jcnglhy narrati\-es and an notations, (here arc: u niver ~ all y accepled abbrc:dations and symbols. l'. lan)' of these are selfexplanator), but we need to see a selection o f them before we can make a judgement on {hal. T e rm
\~~c!llbh-
Ilnmh
T erm
Abb.
\cro~s tl~IS
S'~ndlrd
.'F
Puch CIrcle
,\5SY
Pnellm~uc
H~diu~
eHS
CL
Cemrc line
P:\EL
n /\D or It
Reference
RIF REQD
Uumfcrcd
CH.\\!
Rcqlllred
(:hc:e~c
CIIIID
RC\,(lluuun~
(oulllcr borc
CBORE
RIght hand
(uumersunk
CSK
Rountl
evlindrie:1I "' 0ct:rec of :tngle
CYL
Screw ultends:
hc:ld
Cylinder
Dlameu::r in
nou~
D,:tmeter on dtnl<;nSlOn
I.B
Pound weighl
"S
Cemres
Abb .
I>(:D
di!lmcu~~r
•
RP~I
per minute
hc:~d
RII
ItO liD
Bnrish \ssoci:lrinn
U'
DI\
Bmish St\muard Fine
BSF
"
Hnush St."lndard Pipe
nsp
hf,'Ure
FIG
Bmi
135\,'
Full inthCaled mtwcmem
FJ\!
Lmfied
CU:lr~e
l',\(
Brmdl h.udness
HU
L.:mficd fine
Ruckwdl h:udncss
I-IR
L' nuiui
L::"S
HY
I k'(.2gon
HEX
\',ckel"i
h;J.rdnc~s
I-kugon 11c'(~gon
Ilydraulic I n~ull,ccl
..
:"ccond nf angle
HE" head
L',\f
III-X liD
Sheet
I !YD
Sketch
!i\SCL
SH Sf.: SPI-.(
5pccificaiion Sphene.1I diameter
SPI!ERE 0
Left hand
t D Lit
Sphmc.u r;!d!us
SPIIERE
[.lJll!,(
LG
Spot hee
[mern.ll ciJameter
~I
\behlnc
'C
S'F -\cE
SQ
Scju:ne
It-: o r in'
\l:tdllncd
~\ I C.H
Squ:lre ineh
\\:tlcri:!)
~I ·\TL
Standard
5TO
Slandard wire gauge
!)\'\ 'G
\l:t'(IIllU1l1
\1.\:\
\I.lx material condition
\[!lhmctre '01 to
5cale
'umlxr Outsidc dJamCler P~lIern
number
\I~[C or
6:D
R
SQ
~
Taper
~f) 1
Thread~
"TS
l'ndcreut
'0 OlD
\'(IIUIll('
\'01.
\\ei~ht
\",.
rer Inch
P\TI~O
TPI
L"CLT
.
,
Table 5.2· Abbreviatio ns a nd Symbols
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CHAPTER FIVE ENGINEERING DRAW I NG
Process & Identification Markings lr is ofren necessary
[Q apply ao identification m!uk, such as a specification number, part number or drawing number, to an item. The drawing will specify the mark to bc applicd, its location and the method to be used [0 apply the marking. Additionally, componenrs may also need marking to show thar a p:lCticular process has been carried out. These marks are in the form of symbols and they arc shown on the drawing. \Ve should look at a few of the mote commonly used symbols.
0
0 0
e 0
~V
@
0D
Annealed Normalised Steel Parts Solutio n Treated and not Requiring Precipiration Solutio n Treated and Rcquiring Precipitation
I~ IC 5
®
Pressure Test Sah'agcd
\'\ 'clding
F
Dye Penetr:uH Chcck
Solution Treated and Prccipi [JI.ted
F
L"ltra-SoOlc Test
Hardened and Tempered
@
,\ nodic Flaw Detected X-Ray f law Detected
;\lcchanical Test
XR
SR
Stress Relie\-ed
F
®
Proof Loaded
CD
,-
(pi pe~)
Prccipitation
M
®
Cleaned
~ I agneric
Flaw D etec[ed
Etch Inspection of Steel
Repaired and Reconditioned 1\"\(Y-; Tab le 53· Pwcess & T reatm ent Symbols
11,)\)b - ..
, (p\\t:~ .S,)~(·':
.i,,\.f;"'" 0.." \:.,c""
Drawing Authority All UK civil aircraft ate constructed from pans and components that arc manurncrured to approved dra.\yi.ogs ..All llrcraf( design drawings and their related documentation are produced by an organisation thar has been approved by the Civil Aviation Authority (CAA) in accordance with Brirish Civil Airworthiness Requirements (BCARs). This means that there will be an appro\"Cd Design Authority for each aircraft type. The rCCJuirements state tbat all calculations that affect the airworthiness of an aircraft must be subjected to an independent check_ An example of dlis would be the stress calculations carried out when producing drawings of strucrural parts. This means that all drawings are subjected to a system of inspection and certification that is similar to that applied to the parts that are manufacrured from them.
302 C,PJ 'righl 0 Barry Co//tgt 2006 - 2007
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CH A P T ER FIVE E NGI N EERING D R AW IN G
Drawings arc used by personnel engaged in: purchasing, planni ng, production, m anufacmring, assembling and inspection. This means that a drawing m ust contain all the infonn ation required by the departments engaged in these tasks . The information would include: dimensions, limits, classes o f fi t, material specifications ete. Su fficient infonnation must be contain ed in a drawing to allow people [Q achieve their tasks without ha\Ting to continually refer back to the Design Authority. Authorised notes made on a drawing by the Drawing Office are made so that they can be read in the same orientation as the drawing title block. No unauthorised notes, calculations or sketches should ever be made on a drawing by an: person for any reason. I f a need arises to deviate from an approved drawing, or its related documentarian, during manufacrure or repair, this can o nly be done with the prior app roval of the CAA through the appro,;ed Design Authority. You cannot deviate from a drawing "\vithout tllls approvaL Equally, when overhaul, maintenance and repair tasks afe being carried out, the Certifying Engineer mus t ensure [hal all repairs and replacement parts comply with app roved dra\vings and documentation. Botish Civil .Airworthiness Requirements (BCARs) Part A states that every d rawing m ust comatn a descriptive title, a drawing number, an issue number and the date of issue. It is a requirement that all alterations made to drawings must be made in accordance ,,;th an authorised drawing amendment system that \"ill also ens ute that the design records are amended . O ther than the correction of minor clerical errors, when a change is made it will include th e allocation of a new issue number and dare. T here is a reqUlIemem for Design Authorities to periodically reVlCW and amend the total drawing lists for a product such as an aircraft to reflect the curren,! d rawing issue numbers and dates. Each aircraft variant and its 1ll..1J ' .,,,onodlllcation state must be clearly identifiable in relation to an appropriate list o)\\<::;;.c. ... o f drawings . You need to be careful when obtaining aircraft drawings that YOll .• . \':I).({": are using th e approp riate issue number. This can be ch ecked by reference to .\~\. hthe updated drawlOg li sts that are periodically issued by the Design Authority . (P~"':"\ ~ Some variants, such as prOtOtypes or pre-modification aircraft for example, m ay confonn to earlier issue state drawings. It is essential that Approved Maintenance Orgaillsations have a drawing issue control system mat ensur"?s that only the correct drawings are issucd.
-?.•
•
:rS-
\,\lidl the foregoing information in mind, we can now exam.lOe how it 1S" ~ presented in a drawing.
303 COP.J·righf © Barry
CO!!fg~
2006 - 2007 All right.f rmrn!d
1l'lVlv.pm166. com
CHAPTER F I VE ENGINEERING ORA.W I NG
Identifying Title Block Information
£z 9vll£V
Copyng . h t K' ""'''''j!!U If!,I PrOlceUon ' oncel n;~~~,~~~1
D rowmg Changes ,~
,"" , '"
~
.\h""",,,,, T 111112-lmm
I
I
I
I
,.!l.
I
I'n.>ee, .... lknwk,
-$-
~- I
4
(;r."n.
h sue nlle 15 11/07
,
Ijmnl
hrush
\PPR
\X,,,!,.n,
"
,
I
,
, OF ,
hem l'ln;»n "m
U
Company Name
~blcrul
Chro
,
3 2
C",dO
SI!"C.' !
F
SeeDon .\ -. \
Do Nor Scale 0 ....",.
CJ
m
, ., , . , ,I ' ,I ' ,i , ,i,
~m)
@
Title
DmwingNo. tl3R 46 23
B
I .. r IShee,,"
\
Shc~'
,
,
C
,
Figure 5.32 - LayOUt oran ,\ircraft Drawing
Drawing N umb er All drawings must each have a unique drawing number. No tWO drawings should e\'er bear the same number. The Design Authority must maintain a complete register of all the drawings thal it issues. There are three features contained in a drawing number and we can use the number on the illustratcd dmwing t~J\'f:,apYtlf these. ,e fAct.l'tt~t part of the number shown in the example is .A3R. The 1\3 is the ~) ~~~ projecl identity' code, which in this case indicates the type of aircraft. The '«\'~\. letter R indicates tbar this is a repair drawing. The second pan of the number is (y~"" -1-6 and that is the 'group breakdown', which in this case indicates a location on the aircraft. The final pan is 23 and that is the regisrer number that has been allocated in the related group of drawings. Apan from repair drawings, the drawing number is normally used as me part number of the item. Shee t N umbers There are occasions when a complete drawing cannot be produced on a single sheet. In these cases, continuation sheets arc used. TIle first sheet will be identified as being 'SHEET 1 OF X SHEETS' and each continuation sheet will be identified by its appropriate sheet number, i.e. 'SHEET 2 OF X SHEETS'. \\fhere a schedule of parts is included that is applicable [Q all the sheets, it will appear on Sheet 1 of the set.
304 COP)"n"ghf 0 Bmry Gill&, 2006 - 2007 A ll rights rmrod
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CH A PTER F I V E ENG I NEER I NG DR AW I NG
O"~m.1
Sc:tk
1:1
D ra\\'n
It:..-
Sm~~s
) M.J;;Z
Limits +1),5 R:!dii ~(I,2 Problems
Titl ~ D r.l\\; ng N o. Aircraft Re pair to Cnmpa.,: Fr:un c SIn :\3 46 23 511 Snho 200G-A BCD 51" I "r I 5hL'
Check NJ;U;u Iss ue 1/1/0 " PPR N D:nc
F igure 5.33 - Title Block
Authorisation Note tha t tbe signature block identifies the draughtsman, the p erson who carried o ut the stress calculations, the person who checked the dra\l,.
Drawing Changes Any cha nge made to a design dra\ving, other than the correction o f minor clerical errors, must be accompanied by a new issue number and date. Changes may affect the part number referencing of parts on the drawing. All parts that are added to the drawing because of the change, or parts that have been altered by the change, assume the part number identity d f the new issue number. Parts on ~~o dra\ving that have n ot been altered by tbe cha nge retain the original ,C\Ij~ue number as a part number. However, in all cases \vhere the . G~)\\e't-c - interchangeability o f an item is affected, a new drawing number and part "_ ~~J~' number must be allocated.
, .", '" (P~· " '\~
D rawing Changes D a[e
Issue
Aheration
1/ 4/ 07
2
( I) D imenSion 25mm (DS) was 24mm
4/ 7/08
3
(1) Radi i Limit 0,2 \\"as 0,5
~
-
Figure 5 .34 - Drawing Cha nges
305 CoP)"rig "! © BtI!1J' Collt~ 2006 . 2007 AU rig"'! rm nJed
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CHAPTER FIVE ENGINEERING DRAWING
Details of changes are written into rhe drnwing changes column on the drawing. Alternatively. the changes may be recorded on an 'Alteration Sheet' that is cross-referenced with the drawing. Each change is related to a change number annotated in the 'issue' column on the d.rawing. The derails of [he change are often accompanied by a drawing grid reference to help you [Q locate the alterations. You will sometimes see a letter used instead of an issue number. It is common practice to use letters to register drawing changes on prototype aitcraft and then to use numbers on production aircraft. This means thal all aircraft drawings become 'issue l' as soon as production Sta.rts. i\ change to a single part drawing may necessitate changes to drawings of associated pa rts. For example, changes may be required where the alLered single parr has to be assembled with other single parts. This can directly affect manufacruring tasks in progress. The drawing change system would normally carer for this by issuing an instruction that details the effects that a change has on other drawings, on parts already in produccion and on partS held in srock. The organisation also issues Drawing Master Reference Lists rhat derail all the current issues of all drawings related to a particular component or assembly.
Drawing Queri es
It is common for queries to arise as the result of inadyenent drawing errors, ambiguous instructions or difficulties encountered in purchasing, manufacruring or assembling the partS as drawn. The design drawing office should have procedures that enable a query to be raised by a client and for supplying a satisfactory ans·wer. A drawing query is initially raised on a Drawing Query Form that is sent to rhe Design Authority Drawing Office. The answer may be framed in one of three forms. An immnwate, provisional answer may be written on rhe Drawing Q uery F't~nl th-ht is rhen rerurned to the originator. A temporary, fully aREt~v
U,'X'
.
Drawing Office Query Fonm and Drawing Office Instructions must be identified and cross-referenced to the amended drawing. The answer to a query must include details of the effects on other drawings, parts in srock and parts that arc in work. TIlere is an upper limit on the number of Drawing Query Forms and Drawing Office Instructions permitted on a drawing. \'(Then this limit is reached, the drawing must be either re-issued or replaced by a new drawing. Concess ions A 'concession' is an authorisation to accept a limited quantity of materials, parts, assemblies and equipment, which may nor be strictly in accordance with the relevant drawings, specifications or other documents that define the design.
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Concessions are applied for by the user and must be authorised by the Responsible Design Organisation. A copy of each concession note is retained by rhe authorising Responsible Design Organisation. The Civil Ayiation Authority does not approve concessions but they do act as an advisory service when required and they do re\·iew the records of concessions periodically and when issuing Certificates of Airworthiness. A concession may not require a change to a dra\\-;ng issue number, the Responsible Design Organisation decides which concessions should be recorded in the inspection records of particular components , assemblies or parts. A document that is similar to a concession may sometimes be encountered. T~ is known as a 'production permit' and it may be applied for by the user and authorised by the Design Office before production starts rather than during or after it. A typical example would be permission to produce a part in an altematiyc material. It is really just the same as a concession. Part Referencing Except for repair drawings, the part number o f an item is normally its related drawing number. ~~ drawing of an assembly may cons ist of individual parts where each has a separate single pan drawing. These parts will have part numbers that relate to their related dra\ving numbers. Parts th,l(" appear on a drawing that have no separate single part drawing \vill normally take on that dra\ving number as the part number that IS then suffixed by an item number.
,nHe,d P Outside 3
I -
2
I
-
~
I
~
Pan Hea d Omside
l 4
--
1
I'D
,
~
,
~
( hannel
I
.\346234 :\"D
Channci
I
,
SPS4/452
Pan liD Rin:1
SP86/ 543
SNP HDRh·et
2U
I
It) 46 .l;
I
Angle
I
hem
Pan No.
Item ISS
D~scripti o n
5
1\.) 4311
4
3
LiZ lclS \\;·G 1.72 18 S\v·G
"
t\SDN PH uanrit\"
L72 18 SWiG \Iaterial
Figure 5.35 - Pan Refe[encing
,-,
0~
_0
Parts shown on a drawing are often identified by an item munber th>\f is contained in a small cirde or 'balloon' with a leader line to show the loca ~ of tbe related part. These item numbers arc listed in a schedule o f parts 8 n the drawing where each item number is shown against its related descriR:#bn, pan number, guanrity and, \vhere applicable, the material specification. B:ought in standard parts are listed against their BS or SBAC specification numbers . There
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would be no drawing proY"ided for these parts. Consumabl~ items such as shimming and gasket materials \v-ilJ be listed by their specification numbers in the part number column and the quanriry will be shown as 'As Required ' or 'tl/R'. Brief manufflcruring notes may be annOlated adjacenr (0 thc itcm numbcr 'balloons' on the drawing. \\nen [he abbreviation 'NO' appears by an item number it indicales that there is no separate drawing provided for that item. In these cases, the part number of the item will nomlally consiS[ of Ihe drawing number followed by the item number, i.e.A3 46 23 4 NO. The information rcquircd for the m:mufacl'urc of an N D item will be enrered in rhc description and material columns on the parts lisL I\n :dternati,"c mcthod of pafts referencing is to show the parr number of an ilem in the 'balloon' and to list the all items in tbe pam list against thcir marginal grid references on the OO\\;ng. Hand ed Pa rts J landed parts arc those where there are left band (port) and right hand (starboard) parts, or upper and lower parts, or inner and outcr parts, or forward and rear pans. Drawings containing handed pafts will show the left hand, upper, inner or forward pan d.rawn. This pan will have an odd p."1r1 flumb er and the opposite hand will have the consecuci\"e t:1"t: n p:lrr number. Parts that arc not handcd ha,·c an odd drawing number. -nlC parrs schedule on a drawing contains two item quanrity columns, one for 'AS ORA \'\tN' and the other for 'OPP llANO'.
Microfilm, Microfiche & Computerised Presentations Microfil m b "1\\\1
'!.I",r
. ~\C.-;..c."
..~~J~&Hnical informacion is carried on d.ra\Ving~, maimenance, onrhaul and repair \. .., \-..,. manuals and parts catalogues. The aircraft maintenance engineer is constantly 1\""~"1. reminded to consult this material and not try to commit information to ( . .' memory. That is, of course, excellent adnce because memory is a vcry unreliable ,"ehicle. The problem is that the number and thickness of manuals seems to increase O\'er time at an exponential rate and there would never be sufficient copies to serve every individual in the workplace. The: answer to mat problem came some years back in the form of microfilming. The advantage of microftlming is that a few 35mm reels, representing hundreds of pages, or entire sets of drawings, require vcry little storage space. A further advantage is tllat, instead of carrying bulky manuals around, the cngineer could pick up a light cassette from the Technical Library holding a small reel of microftlm that might contain the entire contenrs of a maintenance m:UlUal. The film quality was good and intricate details were nor lost in the process o f reproducing the manuals or drawings. Another advantage is that manuals and drawings become creased and soiled with use whereas the microfilm reel is protected in its cassette.
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Special JSmm microfilm viewers were provided adjacent to each workstation that enabled the operator to select and view any selected part of a manual or a dra\\~ng as Ii stiU frame projected onto a ,ie\\~llg screen. In many cases, the projected still image could be enlarged to reveal Intricate details. 70mm microftlms can be used to store very large drawings and these can be read with the naked eye if required. Some viewers were eguipped with printers that enabled the operator to obtain working copies of selected informacion. If this all sounds like the answer to a ll1..1..iden's prayer, there were drawbacks, the chief one being the control of infonnarion change or in simpler terms, amendment controls . \\Then amendments are issued, a reel of microfilm is nor the easiest thing in the world to alter. Firstly, the Technical Library mu~t be able to locate all the reels in circulation. Frames could be cut out and ncw ones spliced in but this would be time consuming and ci1e results would not be practical. Replacing the whole reel would seem a better solution but this would invoke fliming all the material again. One answer was to replace microfilm reels with a series of single microfiche slides .
Microfiche If you imagine the space that a whole set of aircraft dra\\·1ngs would take up, you \vill appreciate the value of having them stored on microfiche. ~-\ microfiche is a single microfihn frame that might contain, for example. a single drawing. Each microfiche is stored in a prmective, transparent envelope. ff you need to obtain a copy of the drawing, you can print it o ut usmg the microfiche copy as a negative. T housands of drawings can be stored in microfiche form and the), are yery easy to file in sequence and tak.,e up yery little space. i\h nuals can also be recorded onto microfiche where a single frame might contain one or rmifc pages. Amendment control now becorhes simple. If a drawing or a ;..\\~ anual is amended, the single microfiche frame that is affected can be easily . , ~ . (,0\ e. replaced. _ . ~\....\. .\\.~
(,o\:"
'9~{(.'l
'" Computerised Presentation
Ja
T he advances in information technology bave overtaken the microfilm microfiche systems of presenting information for many tasks . IVfanuals anii~ drawings can now be accurately stored In CD forma t and on mainframe har4drives. Computer temlinals can be used to access any desired part of the store ~ information wimout actually handling or transporting it in hard copy fo~ Change control can be exercised from the main ·terminal so that the users will always receive the latest version. One consideration mat bas to be made is .~t malmenance personnel need to have a sufficient knowledge of com~ter operauon to be able to successfully access and locate the infonnatiOl~'they reqlllre.
'"
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However, the problem of hard copying still exists. "'hen printS are taken, they are no longer under the amendment conrrol system. It is imponanr that prints are nor rerained for convenience and passed on aher they have served [heir initial purpose. h is a legal requirement that a Certifying Engineer ensures that all work is done in accordance with approved manuals and drawings and, in thai respect, ani}' the latesr issues will qualify as being approved. Looking (Qwards the future, information processing is becoming ever more miniarurised and it is now feasible thar an engineer can access any informacion at any place at any time. Infonnation in narratiYe, photographic and diagrammatic forms can be transmitted to computer screens anywhere in the World. Video links can display acrual work in progress and specialis t advice can be obtained with the specialist being able to remotely view the work in hand. There is need for caution. The more forms that information appears in, the more is the risk that it will become corrupted and be misinterpreted . The Inlernet is a good example; the sheer volwne of information :wailable is often the reason why people find it difficult to locale what they want or be certain of its accuracy.
Specification 100 of the Air Transport Association (ATA) of America Introduction ATA Spec 100 conrains the format and contents gujdelines for technical manuals written by aviation manufacturers and suppliers and is used by airlines and other parts of the aircraft maintenance industry. It is often referred to as the ATA system of chapter numbers. The specification pro,Tides the industry wide standard for aircraft system and chapter numbering. The advantage of this system is ~~n.f infonnacion on systems and equipment wi.U always be 10cate~ ,~Ji:fl~' sa"i"ne place in the manuals regardless ohhe aircraft or equipment )=:yp.ac~\\\~A Spec 100 also includes a method for coding electrical cables that are l.~»'Lsed in aircraft circuits and shown in wiring diagrams. \'il e wi.1l deal with this ..\-.b-.. . . aspect later in trus chapter when we examine \'('iring Diagrams.
C""I'\" .
You should note mar in the year 2000, ATA Specification 100 was incorporated inro ATA Specification 2200 • Inform..1tio n Standards for Aviation ;"[ainrenance. It may be some time before the syUabus catches up with rhis title change but at least you know aboU[ it. The basic format is unchanged so we are safe to continue.
"",,
Manuals
.~
The manuals that we encoumer most frequently arc: •
Aircraft l\[aintenancc Manuals
•
Componenr Overhaul !>.Ianuals
•
Illustrated Parts Catalogues
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~
"
C H A PT E R F IV E E NGINEERING DR A W I N G
•
Wiring Diagram Manuals
•
Structural Repair Manuals
These manuals an: prepared, issued and updated by the manuf
71 - 11 - 02 Chapter (Sys tem)
Subject (01 to 99)
Section (Sub-System) Figure 5.36 - AT A Specificatioo 100 (AT A Spec 22(0) 5ub jecI Reference 1.\')\\-:
The first number in the three-pan subject number given tn the example is the chapter number and tlus sen'cs ro identify the related system. The middle part of the number is tbe sectlon number and this identifies the related sub-system. The final part of the number is the subject number and this identifies the location of all the infonnation related to the subject, wluch can be a speci~c component, minor assembly or a simple system or circuit. \,{'hen information is related to the svstem as a whole, the refcrence numbcr for oUL example would be 71-00-00. . ~
tJ%
Groups .~
t\ typical Aircraft J\[aintenance Manual is divided in to the following groups:
R.:
""'
>;
Chapters
I to
Clulptcrs
5 to [2
Aircraft General group
Chapters 20 to 49
Airframe system group
Co/!)rij hr
4
Reservcd for individual operator's use.
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CHAPTER F I VE ENGLNEER I N G DRA W ING
Chapters 51 to 57
Structure group
Chapters 60 to 62
PropcUcr/ rotors group
Chapters 70 to 83
Power plant group
91
Chapter
Charts
Chapters The chapters contained in each group are:
Airc raft General Group Chapter 5
Time limits/maintenance checks
Chapter 6
Dimensions and areas
Chapter 7
Lifting and shoring
Chapter 8
Levelling and \veighing
Chapter 9
Towing and taxiing
Chapter
to
Parking and mooring
Chapter 11
Required placards
Chapter 12
Servicing
Airframe Sys tem Group Chapter 20
Standard practices
~ ?.l)l'
~I)O' . con di"UOUlng I Air C1lanrol;""~
. L..6"Y' \:{I.~<' I.
,~"\\
•
Chapter 22
Autoflight
Chapter 23
Communications
Chapter 2-l
Electrical power
<' .
. ",'X"'"
c'
~.
'c
'", ~
Chapter 25
Equipment furnishings
T<
Chapter 26
Fire protection
Chapter 27
Flighr conerols
-
Chapter 28
Fuel
~
" "
-
I?
CO
p
'""'" ,;
'0
Chapter 29
Hydraulic power
Chapter 30
1ce and rain protection
~
'v oS'
•
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Chapeer 31
Instruments
Cbapter 32
Landing gear
Cbapeer 33
LI.ghts
Chapter 34
.:-.ia\-iganon
Chapter 35
Oxygen
Chapter 36
Pneumatic
Chapter 38
\, 'ater and waste
Chapter 49
Airborne auxiliary power
F I VE
DRAW I NG
Struc tures Group Chapter 51
Structures
Chapter 52
Doors
Chapter 53
Fuselage
Chaplet 54
~acelles / p ylons
Chaprer 55
Stabilisers
Chapter 56
\\-indows
Chapter 57
\'Ciogs
PropcUcrs/Rotors Group
II
1()Cl ., \\.\ ) .c.'} ~ hapter 60
. (},\\I.'_
~- ,,:-.~(\
\J)""" ~:.5 . (P~
Standard practices propellers/ rotors
Chapter 61
Propellers
Chapter 62
RO[Qrs
Power Plant Group Chapter 70
Srandard practices engines
Chapter 71
Power plam
Chapter 72
Engine
Chapter 73
Engine fuel and control
Cbapter 74
Ignition
Chapter 75
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CHAPTER FIVE ENGINEERING DR A WING
Chapter 76
Engine controls
Chapter 77
Engine indicating
Chapter 78
Exhaust
Chapter 79
Oil
Chapter 80
Scarting
Chapter 81
Turbines
C hapter 82
\'(later [njection
Chapler 83
Accessory Gearboxes
Chapter 9}
Chnrcs
Section s Each chapter is divided into sections. \'(te would be filling pages with these if all were to be shown here. An example will suffice as follows: Chapter 27
Sub-system
00 Flight controls 10 Aileron and tab
20 Ruddcr and tab 30 Elevator and tab -lO Horizontal stabiliser control
50 Flaps 60 Spoilcr and drag control 70 Gust lad and damper 80 Lft augmentation
r .
E ach section is divided into subjects tha t are numbered from 01 to 99. Each subject is laid out as a sequence of topics, each topic being contained within an allocated block of page Dumbers. The Aircraft r..ra.inrenance !-.'lanual deals with the maintenance of the related aircraft and all its installed equipment. In an Aircraft Maintenance ~[anua1, subject topics are as follows:
me
Pages 001 to 99
Description and operation
Pages
T roub!eshooting
101
[0
199
Pages 201 to 299
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Subjects
Maintenance practices
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Pages 301 to 399
Sen;cing
Pages 401 to 499
Remonl/insrallation
Pages 501 to 599
Adjustment/test
Pages 601 to 699
Inspection/ check
Pages 701 to 799
Cleaning/ painting
Pages 801 to 899
Apprm-ed repairs
Component ~ranuals deal with the maintenance, oyerhaul and repair of n on ~ U1s talled equipment. In a component manual, tht; subject toptcs arc adapted to the type o f component. \Ve will use ao example. An Engine 1hnual deals with all engine related matters when the engllle is removed from the aircraft. T his mallual does not contain information rela ting to the ma.intenance of the engine when it is Installed in an aircraft. That infonnation would be found in Lhe Aircraft J\1alntenance Manual. The Engine Manual is laid out with the power plant group chapter numbers but the subject topics are slighdy different as follows: Pages 001 to
99
Pages 101 to
199
r.loduJe identi fi cation Fault isolation
Pages 201 to 299
Special procedures
Pages 301 to 399
Remoyal
Pages -1-01 to 499
Installation
599
Disassembly
699
Clearung
Pages 801 to 899
Inspection/ check
Pages 901 to 999
Repair
Pages 1001
1099
Assembly
Pages 1101 to 1199
Servicing
Pages 1201 to 1299
Storage
('. to
o ""9,
i
~
r
IG
Pages 1301 to 1399
Testing
l
!? ,
•.:J
2 0-
Wie can look at an e:-;ample of subJ·ect idenrifica 'on in this manual. 71 - ~1 . '" 02 would be c...... plained as follows: 71 is the chapter nwnber for the ~wer plant. 11 is the section number for the Engine Cowlings. 02 1S the .~bjecr 'v number for the Fan Cowl. All we nm",. need is the relevant page num!?,cr block ~~~ _ _ to_.c v
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E ffectivity Aircraft and aircraft components rend to end up with a number of variants. Sometimes, these variants are cleady identified by eype numbers such as 737 300 or -U)() etc. In other cases, the aircraft may ha,'c becn modified during a production run so that for example, aircraft POSt production number ),."YZ would be different in some way to earlier numbers. \X 'irmn fleers, there may be some registration numbered aircraft thar differ from others. For example, GABeD is different to G-D CBA. The poill[ we are making here is thar some subject topics will refer to practices that are relevant only to specific aircraft and we need a way of alerting people to this.
EqUl pmene Applicability
Flight Controls Topic Flaps Servicing
27-50-02 )'
Effectivity: ASOO/ALL
Page 336Jun. 04/98 Subject Page Dare of Issue Figure 5.37 - Example of Maoual Page Foot loformatioo
.:.~\~\ U'\'
;:.
Each aircraft is maintained to an authorised Aircraft ~[aill[enance Manual. The pages in the manual bear the ATA Spec 100 (ATA Spec 2200) subject reference on the bottom right of the page, together with the page number and its issue date. On the le ft hand foot of the page is an 'Effecth¥iry' sta tement. Tf the topic on 6tbl1\~~ge applies to all aircraft of the type, the sraremenr will say 'A)~Ji.(.:.'toI:f~~b topic applies to a restricted number of aircraft, the sta tement will ~ ~h
statements also appear on the pages in component overhaul manuals. Engines, for example, have a number of variants in one type. In this case, the 'effectivity' statement will indicate the relevant variant or variants. For example,
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ATA Spec 100 Zoning System ATi\ Spec 100 contains a 'zomng' system for lo cating components on large aircraft. The system d ivides the aircraft up into ma jor zones, m ajor sub-wnes and zone~ .
Ma jor Zones Ma jor zo nes are defined by a standard series of numbers as follows: ['"lajor Zone No -
~.-\ rea
100 - Lower half of fuselage below mal11 cabin floo r to the rear press ure bulkhead 200 - Upper half of fuselage
to
the rear pressure bulkhead
300 - Tail empennage and fuselage belund rear pressure bulkhead
-too - Powerplams, struts and pylons sao - Port wtng GOO - Starboard \\·ing 700 - Landing gear and landing gear doors 800 - Doors
Note: A Special 900 Series of numbers are used features specific to variants of an aircraft type.
[0
identify unconmlOn
F igure 5.38 - ZOlling
Major Sub-Zones , T he major zones are subdivided inLO major sub-zones. To avoid ftllini 'Another book \virh these, an example of the sub-zones related to major zone 300 is shown below:
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t\[ajor Sub-Zooe No - Area 310 - Fuselage behind rear pressure bulkhead. 320 - Vertical stabiliser and rudder. 330 - POrt horizomal stabiliser and port elevator. 340 - Starboard horizontal stabiliser and starboard elevator.
Z on es Each major sub-zonc is sub-diyided intO individual zones. Again, to avoid filling another book, a sample of zones related to major sub-zone 320, the vertical stabiliser, is shown below. Zone No. - Area 321 - Leading edge of vertical stabiliser
322 - Vertical stabilise! auxiliary spar to front spar 323 - Front spar to rear spar 324 - Rear spar to trailing edge
325 - Lower rudder 326 - Upper rudder
327 - Tip of vertic",l stabiliser
,"" - -
')\~1'
• 'l.\J'" . (... )\\e.'t>....
s<' t. ~ ~~ The zoning system cUvides the whole aircraft up into zones so that any area can
"f\"'~\.f:"'" ( ."
be clearly identified. Zone numbers arc used in maintenance manual servicing schedules, inspection schedules and repair schemes to identify specific areas of the aircraft.
Station Numbering Poims along the fuselage and the wings can be located using a system of station numbers. Although not an ATA Spec 100 system, it is worth noting it here in case it comes up.
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FIVE
DRAWING
fuselage Smtions (FS)
Lin,,·=-gl~ .~
\Y1... Station 0
o
ws
, 0
\'"ing Srations (WS) (Butt Lines)
61 Figure 5.39 - Slation N umbe rin g
Fuselage Stations (FS) These are ti..ed points along the longitudinal axis of the fuselage that are measured from a 'Datum' posicion. The darum is nonnally a nononal point located towards the front of the fuselage, in many cases this is at the nose of the aircraft or, In some examples, at a position in space forward of the nose. The stations are measured in inches from dle darum. For example, fuselage station (FS) 100 would be located "100 inches to the rear of the darum. i\ Icasurements in front of the datum arc always negauve (-) and those to [he rear of the datum are always posiu\·e (+) so FS - [0 would indicate a station 10 inches in front of the datum. The actual station numbers shown in the l'vIaimetlance Manuals are usually drawn to coincide with bulkhead, frame or "I\\\.)fib positions" T he danun position (FS 0.00) is fi~ed in the design speci fication s . (,ll\\c,?e" for the aircraft.
...
.• \J;\~{" ~\ ~
,~ \ -
(P~·
Wing Stations ('""tiS) These arc measured left and right from dle fuselage centre-line (\'{fS 0.00) . This line is also referred to as being thc 13utt Line (BL)'. Each ·wing has identiQ1 station numbers. For example. \","·5 130 Left is a position on the port wing ~ is 130 inches to the left of the Butt Line. ~ Water Line (\VL) This line is used to aCl as a darum for measurements along the nonnal axis 'Of the aircraft. The position of the water line danlm is ti'(ed in tbe design specifications. In some cases, the water line may be ti'(ed at a position b~w the landing gear full- extended position. Vertical rrteasurements above the iJater line arc normally positive (+) and measurements below the water 1i& are normally negati'""~ (-). For example, \'{fl.. 96 could relate to a positio~on the vertical stabiliscr that is 96 inches above the wacer line datum. .,~ ~
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Component Stations Components such as ailerons, elevators and rudders may have thcir own station numbeting system. J n these cases, the dahlm may be set at the inboard end of the control surface. For example, Aileron Station AS 48 port would indicate a position 48 inches from me inboatd edge of the port aileron. You can deduce from this, that it is possible to specify the exact position of any area on an aircraft by stating its station numbers. For example, a component may be located as FS 120, WL 63, BL 23. It is worth noting that the wing stations (\""\15) may be alternatively expressed as bun lines (BL)
Aeronautical & Other Applicable Standards Introduction A standard is defined as a 'Tecbnical specification appro,ed by a recognised national, international or company standardising body for repeated application'. A specification is defined as '1\ statement of the attributes of a product, process or service'. The advantage of standardisation in design is that it saves time in development, production and training. The standardisacion of limits and fits for example enables lhe interchangeability of pans, an advantage that we seem to take for granted these days. The standardisation of materials and their procesSlng gives confidence tbat structures and parts will withstand the normal design operating stresses and strains that lay within the known properties of tbe materials they are made from . The srandardisation of electrical cables and parts ensures that equipment can be rared correctly, Imagine the situation where there was no interchangeability of screw threads, parts or materials and you have the problems mat existed less than a cenUlry ago,
'(, • '1'0F\,'
Wb~'#e"!.~amine
the requirements for aviation safety, we will recognise mat
.,.,(\.'{tb.M~ is also a need for commonly agreed standards in civil aircraft operations. ;'\\. \d '{ . The problem with standardisation is that everybody wants his or her own
_.,<:;",i..\~"\ \,. ,'
version to be adopted, the abundance of languages and belief systems in our World being a perfect example. \",,\'e have the Continental Europea ns with their metric syslcm, the Americans who do not want to use it, the former Commonwealth countries reluctantly abandoning me old imperial system whilst the Americans have developed their own, some say more logical, version of it. Wle ba\re had several different versions of metric, American and Bri tish screw threads. W\'(/2 brought this to a head when designers started intcnni.xing British and American engines and aircraft. Spanners that fitted aircraft bits would not fit engine bits and that was before tIle Continentals gOt into the act. Standardisation has not had an easy path but tIle major industrial countrics are gradually being pressured to adopt a universal standard in a global marketplace. One problem is that aircraft tend to have very long service lives so the eCJuipment standards that were used in tllClr initial design will endure long past
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tbe introduction of ne\ver standards. It is for that reason we must still look at the earlier standards. In dus section, we will initially look at tbe standards for the design, operation and maintenan ce of civil aircraft before examirung the various standards relating to actual parts.
Aviation Standards In the United Kingdom, an Act of Parliament 10 the form of the Air Navigacion Order (ANO) is the principal stannary instrument containing the legislacion for the regulation of civil aircraft operations. An infringement of an Act of Parliament is a criminal offence in the eyes o f the law. Although the ANO establishes in la\v the basis for the regulation of Clvil aviation in rhe UK, it does not prmide specific details of technical requirements . T lus information, including the administrati\-e procedures, is contained in the British Civil Ait\vorthiness Requirements (BeAR) that incorporates, and is gradually being replaced by, the European Joint Ait\vorthiness Requirements U~A.R) . These 1.11 turn are being reissued as European .!h-ianon Safety Agency (EASA) requirements. Just to summarise, BCA.Rs are issued by the UK Civil ,-\viation Authority, JARs arc published by the Joint Aviation Authorities and the EASA requirements arc published by the European AVlation Safety Agency. The £ASA requirements arc valid throughout the EU and in countnes still affiliated with the Joint Anation Authorities OAA) of which the UK is one. BCAR, JAR and now the E,-\SA requirements comprise minimum technical requirements and adminisuatiye procedures that fo rm the basis for: the construction of aircraft; the approval of equipment; the approval of design, manufacturing and maintenance organisations; the approval of personnel; cerqt\&ation and continued airworthiness procedures . "006~
...
. Cp\\C~c'" The members of the Joint Aviation Authorities Committee are representatives ~~{\':
,,(\'- r; . 0':;;'~"''C 1.....
of the Civil Aviation Authorities of all the countries that ha\-e signed up to the Joint Aviation Authorities Requirements OAR). "lbcse form a common airworthines s code accepted by the ' -arlOUS European Airworthiness Authorities and are designed to prevent a proliferation of differing airworthiness codes o f practice within Europe. ;. -oj
The requirements related to the de;;ign of large aircraft and aircraft systems a~ contained in EAS1\ Part 25. There are other requirements dealing with specifi{ equipment design such as engines OAR-E), propellers OAR-P) and auxiliar£' power units OAR-APlJ) . JAR Ops deals with the requirements for aircraf operations. Other publications include EASA Part 145 that contains tp:e requirements to be met by Approved Maintenance Organisations. Er\SA ~.:!it 66 contains the qualification requirements for approving Certifyi.ng Engin ~~rs. EASA Part 147 contains the requirements to be met by organis~ons providing basic licence training and type trainin¥. Because these requirpibents are agreed by all member nations, the\' form the basis for th~~viation standards in Europe. .~
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Also worth a men non are the Civil Aircra ft Airworthiness Informacion and Procedures (CA.1\1P). 111ese are published by the UK Civil Aviation Authority in me fonn of leaflets that comain info rma tion of a general nfl/lire on overhaul, rep:ur, maintenance, operation and procedures. T11e information is advisory only and is imended to enha nce the knowledge of readers on topics not readily found in textbooks. These leaflets do not include infonnation on specific types of aircraft, engines or components. The appro"cd Aircraft and Component Maintenance manuals published by the manufacturers arc the sale authority for this and must al\Y"ays be consulted for detailed informacion. The American Federal Aviation Administration (FAA) administers rhe US legislation in regard to civil aircraft operations. p,[any of the E..t\SA requirements, for example EASA Part 145, are based on the framework and appropriate content of tbe corresponding FAA requirements (PARs). The American and European administrations are merging their requirements as far as this is practical
Standards for Materials & Components United Kingdom 1n me United Kingdom the specificacions for materials and parts for sole use in aircraft manufacture and repair come under one of the following headings: 1. The British Standards (BS) _-\erospace 'A' Series of Aircraft ~'laterials
and Components.
2. The Society of British Aerospace Companies (SBAC) 'AS' (Aircraft Standards) series specifications for items nor listed in British Standards.
3. ~~Old1.Bhectorate of Technical Development (DID) specifications . C..,\\e.'?tTIay still be found in use. \y;l.~{'\
;.,.. . . . ..,
... .""'>!.: ..
4. The old Aircraft General Smndards (AGS) may still be found in use .
,~
(, ('\
The specification codes and identification methods for aircraft materials and components were dealt with in Module 6. T he British Standards (BS) .r\ codes are in general use for pans and materials used on British manufacrured aircraft, supplemented by the SBAC .Aircraft Standard (AS) codes where there arc parts not listed in the British Standards. The list of codes increased in size with the introduction of metric and ISO threaded fasteners. As we move towards international standardisation, many of the European EN standards have been accepted by the British Standards Institute (EST) and you will find many BS EN numbered specifications in usc. America There are a number of American Government, Federal, l\1ilitary and Chril , standards organisations. There are rhe commonly used Air Force/Navy (A.N) .§v , series of specifications. There are the National Aerospace Standards (NAS) and the Military Standards (MIL and l\IS). In addition to these, there are Society of
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AutOmotive Engineers (S.r\E) spccificaoons and Aeronautical .Ma terials D ivision o f SAE Speci fi cations (Ai'vfS). You may also come across terms such as Nayal Aircraft Factory (NAtl ; Air Force/ N avy Design (AND); Aeronautical Standard (AS); American Standards Assooation (ASA); American Society for Testing and Materials (AST?\1) . You can become confused by tbe term AN when dealing with American threaded fasteners. AN stands for Air Force/ Navy standard so a bolt ~-\_N 5 - 22 refers to an Air Force/Nay)" 5/ 16 in diameter bolt. However, the series of threads used on bolts include American National Fine (NF) and American Nanonal Coarse (NC) . The term American National used In this context is not the standard of the bolt it is the thread series . In the same series, we have American Standard Unified Coarse (UN C) and American Standard Unified Fine (UNF) . In a similar way, you \",ill find that British Standard faste ners are to BS standards but the threads used can be B. . \, BSF, BSP or Uni fied .
Europe T he European CommIttee for Standardisation (CEN) issue the European EN Standards . The International Organisation for Standardisation (ISO) sta.ndards are gradually being accepted as equivalents to the European EN standards. As we move rowards international standardisation, you find that the EN standards arc gradually being replaced by the ISO standards. International
The International Organisation for Standardisation (ISO) Issues the internationally agreed standards. The vatious national standardisation bodies hayc gradually drawn their national standards Into line with the ISO standards . In Europe, the national organisations first aligned with the European EN stand :b~s that were in turn aligned wirh rhe ISO standards. The American '1\.:ijUadd!trds bodies are also aligning w1th these international standards where this . C.0~e.r;.:- -- is practicaL. The process is a grad~al o~e and you have to ~eme mber that \~' h c n (-,:\'i.'i.'i a standard IS adopted for new projects It does not automancally back date ItSelf ,\"\'-~. ~ . to cover all e..xisting aircraft. You will still need to know the standards for parts . o
As an example of rhe dual standards in use, we can look at the British Standa~ fo r limits and fits. BS 4500 is the current metric standard comprising ~ EN2028G· l and BS EN20285-2. However, we have BS1916, which is th~ " imperial version. There are still UK manufacrured aircra ft around that use rhi' imperial system and most American manufacrured aircraft will continue to us ~ ie r
still
"
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Wiring Diagrams & Schematic Diagrams Introduction In order that we may assemble and maintain an electrical system we need to be able to ha,c an unders tanding of the system opera non, be able to locate components in tbe aircra[[ and know the specificanons of components and their interconnecting cables and conneCtors. To achieve these aims, an electrical circuit is often presented in three diagrammatic fonns: as a circuit or scbemau"c diagram, as a location diagram and as a tn ·ring ditlgl'llm . Each diagcammaric form serves a different purpose.
Circuit Diagrams A circuit diagram consis ts of a ,ery simple layout controlling symbols to illustrate components and their imerconnections. Circuit diagrams arc used to show operating principles and do nor include any infonnacion on the practical construction of a circuit. Interconnecting conductors are represented by either horizontal or vertical lines that do not cross ove.r each other. Components such as s\\~ tches tbat are essential to the illustration of operating principles are shown whilst those such as connectors that do not directly affect operation will not be shown.
Schematic Diagrams Schematic diagrams are well suited to training and faultfinding tasks. They are lIsed to show the functioning of many o f the aircraft and engine mechanical ~nd electrical sYi ffiflls. A schematic diagra ~n is really a circui~ diagram in that it IS pure~r. ~~'-gn ed. to demonstrate opera non and although It uses symbols to id ~n N.ft'-:'\nd show the functional location of components in relation to each \\';\(S ther, it does not show their acttL"lllocation in the aircraft system. Similar [Q a circuit diagram, interconnecting cables are drawn horizontally and vertically and are not representative of practical construction.
15\· -\C
LBt.:S ~
Figllte 5.40 - Example of a Schematic Diagram
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Location Diagrams A derailed diagram of an electrical circuit can be confusing if you cannot lo cate the actual components to the aircraft. ivlodern aircraft contain many electrical and mechanical systems that are closely packed together and finding specific components can become a challenge, particularly if you do not know in \vhat region of the aircraft to start your search in. In this respect, a simple location diagram is just as essential as a detaued wiring diagram.
Sensor Panel AC
Figure 5.41- Example of a Location Diagram
A location diagram is provided either as a plan or cmaway pictorial vie\v of the aircraft. The acmal location of each component is shown in relation to the aircraft. l\hjor components may be shown using silhoue ttes or d rawings of tbe actual com ponents wh ilsr smaller components may be identified by a legend code such as numbers, letters or symbols. The sole purpose of the loca tion diagram is to let you find a system component quickly. The use of zone .I~ [eceHtes would be a useful means of indicating their locations.
"QUI:
.,
- ,\,>;'I.~' . . ,\',\ ~
,;5\~
(.o~ .
\\ e.\!.c GO'\ ,
Wiring Diagrams Electrical wiring diagrams use standard symbols to represent components a!)p their interconnections but they contain a lot more infomla tion, such ~ component part numbers, cable sizes and identi fi cation codes for each o f cables and conllecrors in a circuit. \'{!iring diagrams show all connections, t~ routing of conductors and the poims where they pass in the structure. Th ~ aircraft configuration used for wiring diagrams is that of the aircraft on tlWJ ground with power off unless this is otherur"ise stated on the diagram. 3
tt%.
,~
c'
l\hny wiring diagrams are drawn as 'routing diagrams' in that conduc)~")-s, connections and components are arranged in their related rurcraft loca~ns, these being shown as columnar regions on the draWIng. In fonnatijO is supplied us ing symbols that are internationally recognised and trus red?,~s the complexity of tbe diagram. .Additional infqnuation is provi~ using abbrenations and codes. The symbology and the coded fonna t p rovide
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CHAPTER F I VE ENGINEERING DRAWING
comprehensive infonnation in a 'shorthand' presentation that is designed for easy interpretation by uained personnel. DisU1bution Panel 4R 28V DC
\li
:c\
5\ 5.\ 5.\
Irliglu Deck P:lnd 6P I ]~WC~~;~~~ I I I "--!' 0
< < 'n
I
~,
6:\
j j ~" Xl
1:\
~ J\
.\
8B
G~ I
Be
,\11 obi", """, L"runn'n XI
I
I I I , eD 60 1. 1 6E = [ ] r-7A ~ 9B _A I
"--!'
I BA
n :GA I
I I I
'
I TB I GO
ala
urn,-,,,_lothl:rv,,:.,, <",{<:d,
Relay I 40XYI
"--!'
PR I Be
58
I I
I I
GAI BA
Engine Bay Zone 1
7 D~ I" - F.
=to
I I I I I I I I
:H:lX
9C
I Be
Lse
I PR
5e
5D
I' I I
I I
B A
,I-0-:t ~ Ea~{l;
-
Figure 5.42 - H ypoth e tical Example of a Wiring D iagram
ATA Specification 100 (ATA 100) Cable Code System Each cable used in an aircraft electrical Clfcuit is usually markcd with a code that identifies the type of cable, me cable size, the related circuit and the segment of that circuit in which it belongs. Manufacturers may use their own codes or they may adopt the ATA 100 system. The cable coding system and the details for its \\Bterpreration are published in the Wiring Diagram l'vlanual C f OG lOr eac h" ~~ t-rype. - ~\e~;;:''''
"' U" -
~:\'I>'{~ ,
\~'\.
,.;~
·o'\; ·
'0 \
-
c'
3
F
Un;t NumbcJ
J
K
B
F 22 e N v
I
L
Suffix D""
L CW!cSize
Circuit FwlCtion L.e tter Circuit Designation LeITer
L ___
C'lble Segment
Cable Number - - - Figure 5.43 - Basic ATA 100 Cable Coding System
If you examine the illustracion, you will see that the code consists of a combination of letters and numbers. There are SL'" groups of numbers and leners. The Unit Number is used to identify II particular circuit where there are a number of identical circuits. The Circuit Function and the Circuit Designation letters identify the circuit function within a parocular circuit. The Cable N umber is used to differentiate between cables that do not have a common tenuinal within the same circuit. A different number is given to each
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cable, beginning from number one. The Cable Segment letter is used to identify eaeh part of a same numbered cable run. Each part IS defIned as a part that connects between two tenninals in tbe run. Segments are lettered in sequence but the letters I and 0 arc nor used. The Cable Size is written as a Standard \X/lre Gauge number. The final group of Suffn;: Data letters are lIsed to identify the make o f cable and its connection, for example, Unin~"\'in, nonearthed in a single-phase system. Wiring Diagrams may just display the Cable Number and the phase letter of the 5uffL""{ Data. The cable size and make are normally shown as a separate statement 011 the diagram, for example, 'All cables are Univyvin 20 except where otherwise stated'.
Wiring Diagram Manual T he \\·iring Diagram Manual (\\Di'vI) contains the wiring diagrams and the charts that form chapter 91 of tbe ATA specification together with detailed 1l1formation on the cable codes, abbreviations, tenninology , circuits, the standard symbols used and their interpretation. The manual is laid Out using the ATA. Specification 100 format. The \\iiring Diagram lvranual includes an Equipment Lis t . ."1111:; lists every item o f electrical equipment ragether with its parr number. Each item is shown against an aircraft zone and station number to aid location and identi fy the chapter, section, subject reference number of the related system / equipment in the ATA IOO system. Also included is a \\·t[e List that gives infonnation on wire 100m part numbers and wire bundle cable [lUmbers and the types of tenntnation together with cross-references to the wlrtng diagrams. Cha-RW 20 of the \\/DM is concerned with Standard Practices . This chapter
'2.\)Qncld~es in fonnation on all aspects of standard working practices including: . (si\\tC~e _ ,\>.~J{~
crimping and soldering techniques, special repairs, cable protection, cleaning, connection methods and rhe treatment of earth terminals .
'\. !"..
. A""I.
~",\':J
Cp"Q·
All Wiring Diagrams in the \'i/iring Diagram Manual are referenced in accordance with the ATA Spec 100 system. The ATA 100 reference number, diagram reference number and issue number are displayed at the bottom of ttte diagram. For example 24 - SO - 01 would indicare a particular subject relacing w Chapter 24 - Electrical Power, Section SO - Electrical Load Distributio~ \'Vhere a diagram has more than one sheet, each bears a sheet number. Th~ manual contains an index of diagr:un numbers that is divided up into chapteE and section groups. ~ .~
Symbols It would be difficult to produce a clea r diagram with all compon~o.ts and connections drawn in their actual fonn. J t is far easier to represent t~se Items with internaci.onally recognised symbols. Symbols are chosen so that they can
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be easily drawn and their meaning is dear and unambiguouso Symbols do nO[ represent the actllal mechanical construction of the items; they o nly indicate the 'function'. The British Smndards Institution has published a range of symbo ls for lise in wiring diagrams. These symbols arc similar to those adopted by o ther recognised s tandards bodies. The \'{!iring Diagr.lm i\fanual contains a lis t of the symbol s used in the \Xoiring Diagrams together with details of their interpretation. Conductors: ~ rcthods
of
sho\\1ng conductors that cross \\i thom connecting
+,
-,0'
- '(
~i~
Re~istOrs:
General
Bohed -a-o-
Termim.ls
Clip in
\' "indings
Bolted IS\.<>
D irectional Symbol Series \'\"inding
-
Fuses:
-..JWV\r-
Negligible 1Jl.Jl.r Inductance Non- Iomcar ~
-G-
Liquid Rheostats:
Shding
~,\
L
"0,....
"(\~
~- \':"~I-;:C;:C=:-:::--==-----1 Inducroror_=_ Choke Capacitors:
\X.W.J
Genel'al
-II-
Elecrrolrcic
---ij ~
Non·Polari~cd
Electric Cells
o
0
Bell Buzzer
Horn
t\ltcrn:uing
~
Current
Scparabk
0
Contact Non-Scp:u'ablc
Contact
•
L1.mps: Filament Signal DI!.chargc
~
f-:,,,.-'P~o~I~'n~s~c~d,-_II_'_---1 Switches: D irect Current -
Jy-
\X"arning De"ices:
D isconncct plug and recept..'lcle Ground Rectifier
T T
Lfl-
QUIck
Electrolytic ..:!J1 L.
...L...L
Instrument -Q::::QShunts : :
Push Pull Push to Reset Toggle Switched
II I
Circuit Breakers (Double Pole) Rcla\"
Circuit Bre:l.ker:
~
C"bon Pdc
/
Busbars
-Jwv-
Contact
y
Socker '\[o\'eable Conrac[ \'ariabilj[y Symbol Insulated Coupling :\fcchanical Coupling
0'
Connecnons 1 ~ between condllcu~ Links:
•+
Plug
Single Pole Single Throw -0........... Tumbler \ 0Rotan.' I I Double Polc 6J-~1 D ouble 111roW , ,
-B-
.R
Y
--6@
8
,\rc
__ x __
DCShunt \Iotor
~
f®\
DCSCrlcs~ ,\Iotor
~
compouncl~ DC
-
Gcnm''''~o Phase AC Motor
T li ble 5.4 - Exa mples of B5 Elecuical Symbo ls
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There are many symbols and we ha\'e to recognise that there arc European and ;\merican aircraft in se n'ice and European and American textbooks, Examiners, particularly electricians, seem to be drawn to electrical symbols so we will add a few more examples as a precaution, Single pole si ngle throw s\\'Hch normally open Double pole double throw S\\'ltch normal l\' closed at one position Single pole double thro\\" switch off in centre posmon Rela\" - normalhopen - closes ' when ene rgised Rela\' - normally closed - opens' when energtsed
~A-
~ , ~ A0-
\
t\C generator
¢
DC generator
~
Potentiometer
--IN:-
0-
~£
T ------- - - - ----,.,.,.
Coil ur inductor wirh fl ir corc
.JOOOOO'-
Coi l or inductor \\'nh Iron core
...J
0000 0 '-
Vanable coil or inductor
...J
O ~OO
Transformer
0'
---0----
Un shielded conductor ShIelded Conductors
$
AC generator
!
Diude
~
Si licon rcc u ller
~1
7.cner diode
~
TransIstor Tr:lnsis tor
The rmocouple (heated)
'T he rmal switch
] [
Bimetallic thermal e1emem
--y --y ==>[ ---cD-
-"'- F,
Tab le 5.5 - Add itional Electrical Symbols Found in Usc
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Hydraulic System Schematic Dra\"ings As the syllabus refers to schematic di:lgrams in rhe same brcath as \'':/iring Diagrams, we nughr assume that they anI}' apply to elecuical schematic diagrams. However, schematic diagrams are widely used for other systems and, in particular, hydraulic systems. To cover rhe siruation of possible ambiguity, we can e....anUne an example of a hydraulic sys£em schematic diagram. You will see thaI standa rd symbols arc also used to represent components in the system, The layout illustrates the functional location of components in relation [Q each other and the fluid flow but ir does not represent practical assembly, connection or locacion in the aircrnft.
1j
Fig ure 5.44 .. H ydraulic Sys tc m Sche matic Diagram
')W-"
.,\.)\)6 " ..
.
~~ci,c'"
CO'
\l,<;I.'(5l11C symbols used 10 represent hydraulic components in the diagram are taken . ,\,\... from Imemariomll Standard 1501219, A=in, theee are man)' s\,mbols so J·USt II .(I.\:.' / : > " . (. ,'X..... representative few of the more common ones are illustrnted here.
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Flow IJne
Drain IJne :-":on - Rerum Valve
'f7\t
~ I otor ~y
Flow Control \ 'aI"c Row Control Vake - prcs~urc
Aen",,,,
ITI116=
and tcmpcrantrc
¢
cornpcn~a[cd
Direction:1l Control VaI\,C (two p()~icion
Pressure Control V:tkc - nonn.1l1r do~ ,
Pressure Control \ 'ake - normal]", opcn
Filtcr
m:1l1u:1l)
-B<>
Dircc[]oll.ai Comrnl \ "aiw solc.:ntlid opcF.ltcd
TIuec PO~lUon Directional Control \ 'ake
$
rnm
Reserw)IT
Table 5,6 - ISO H ydra ulic Compollent Symbols
Conclusion
r. '0
\,\'c hay'c rcachcd the end of chapt~r five, \'Cc will ha\-c occasion to revisit some o f the topics in further dctail as we progress th rough the remaining chapters ~ this module, in particular the .-\ircraft Maintenance lI. lanual, the Strucrurnt Repair ).{anual, Repair Drawings and \'('iring Diagrams. In the meantime, ther: arc a few multi-choice answer questions prO\;dcd for you [0 try. .~
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CHAPTER FIVE ENG I NEERING DRAW I NG
Revision Engineering Drawing Questions 1.
Whe n a ch ange is m a de to an a ircraft d rawing: a)
a new drawing number must be allocllled
b) the old issue number is retained btl[ a new issue date must be allocated
c) a new issue number and date must be allocated
2.
The type of pictorial drawing th a t shows o ne fa ce in true elevatio n with the receding lin es drawn to half size and kep t pa ra llel a nd ang le d at 45 0 to the ho rizont al is kn own as : a) isometric b) oblique c)
perspective l\)lf\ "l\J~ \6
C'
.,;-......... .,(''f'
,. -
C....,\\c.'f' ,,\,;,-'(S'. In a third ang le p to jecti on, th e plan placed:
,.j."~'"
V ICW
of th e to p s urface is
a) above the front elevation
b) below the front elevation c)
4.
to
the left hand side of [he front elc\racion
The edge of a s urface from w hi ch all dimen sions are m e asured is called th e:
D :P~
a) chain line
"
b) datum line c) reference point
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5.
A drawing that is used to show how parts are assemb led into an ai rc raft in relation to other com ponents and assem b lies is known as :
a) an installation drawing
b) a general arrangement drawing c) collecti\'e drawing
6.
A scale of 1:2 indicates th at a d raw i.ng is : a) two times full size b) hal f of full size c) full size
7.
The thickness of the h ead of a bolt o n a drawing is regarded as :
a) an auxiliarr dimension b) a functional: dimension c) a non-functional dimension
8.
Dim en sions are placed on a drawing so that th ey can be read fr om the: 11)\J
a) bottom of the drawing b) left hand side and the bottom of the drawing c) right hand side and bottom of the dr-l.\\ing r
.,.,
-
"
9.
A D rawing Master Re ference Lis t details the :
.~
a) current issues of all drawings b) specifications of all pans shown on a drawing c) changes made
to
a drawing
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10. Aircraft d e sig n drawings a re p roduced by o rgani sati on s a pproved by th e: a)
British Standards Ins titute (ESI)
b) Society o f Bcinsh Aircraft Constructors (SBAC)
c) Civil Avianon Authority (CAA)
11. When a cutting plane passes throug h a bush and a bolt on a sectio ned view, the : a)
bush and the bolt are each hatched in opposite directions
b) bush is hatched, the bolt is not c)
bush and bolt are nor hatched
12. T he line used to represent hi dden d eta il on a d ra\\'ing is a : a) dotted line b) alternate long and short dashes c) light solid line
13. A schematic drawing shows the : ')\)~,
\)\lb - , \ e.'?j"l" ll~YS1C;u . -, Iocanon . . C<3Y'
0
f componems ill .
:t
system
'"1.<;).~ (':
"V
,~
~\......
.
~0":
\... .'- '.
b) functional arrangement of components in a system
. (\c
c) shape and size of components in the system
14. Break lines are us ed o n a drawing: a)
to indicate where a component will fail
b) to indicate [hat a part is sectioned c) where it is impractical to draw long constant section items
334 Cop)'rig,ht 0 Bony Co!kgr 2006 . 2007 A ll right.; rmrvld
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CHAPTER
E N GINEERING
FIVE
DRAWING
15. A process and treatment symbol HT within a circle indicates:
a) high tension b) hardened and tempered c)
heat treated
16. The letters ND annotated by an item on a drawing indicates: a) no separate drawing eXists
b) not dimensioned c) new drawing
17. The rule for dimensioning a d rawing of a manufacnlred part is that :
a) as many dimensions as possible should be shown b) functional dimensions must be repeated in each v iew c) the nnnllnum number of dimensions required for manufacture are sh own
18. The letters peD on a drawing indicate: - b ruc -ld 'l0'00 • "\lI 3J - pItc e -lamCtcr
_ ,, <-o:..e."
~~,
~-
"\''1. ~p>:f U§~ ·
'0:"» .
(,0" -
b) prototype component draw10g
'
c)
process con tro li e d-Item
19. Zone number 110 refers to a location in the:
a} empennage b) lower part of the fuselage
c) port wing
335 Cr;p]dght © Bi/l7)' Colltg~ 2006 - 2007 AU rights I?set"Vl!d
IJIWIlJpart66.coIJI
CHAPTER FIVE ENGINEERING DRAWING
20. Pages 001 to 199 in a maintenance manual refer to : a) mainrenance pracuces
b) sen"lclng
c) description and operation
21. In th e ATA Spec 100 cahle code 1 EG 8 C 22 NMS V, the 22 represents the:
a) cable size b) cable number
c) unit number
22. The geometric tolerance symbol illustrated below is related to:
a) roundness b) poslUon c) profile
23. The symbol illustrated below is related to: a)
nm,"
0
\1('
a''',\'iCl,,,FIFY 0
. ~\e~c'"
~5-: L-~) machining
,,«'t'",
"
,~
..:) \"\';'
c) a datum face r,
""" ~ '0
24. The geometrical tolerance symbol illustrated be low is related to:
a) flatness
-" ~
/
b) parallelism
;::; "
/
c) squareness
336 Copyri,ghl 0
Bat7) ull~ 2006
. 2007
All right; Tmrwtl
u,wu'parl66.rom
CHAPTER FIVE ENG I NEERING DRAWING
25 . T he tole rance fram e illustra ted b e low ind ica tes :
a) s(luareness rolerance of O.2lmn in relation to d:llllln featu res t\-B b) a tolerance of 0 .2 micrometre in surface A-B finish .
c) a tolerance of O.2mm in the positlon of datum face .A-B
26. The d rawing sym bol illustrated b elow indicates:
, ) an internal thread b) an e:ncrnal thread
0) concCntoClt\'
0
27. T he el ectrical symbol iUustrated be low re presents a :
a) potenuometer b) WIns former c) inductor
1i',\'-:
.,,\\28. 'Th e e lectrical symb ol i.llustrated below represents a: \~ -e.G ... . (.,)\ ....
• \"\~~~";
a) potentiometer
\ ~.
.
~
-C'' !.,,~". L
b) tranSfo mler c) induclOr
~
~
,
./.
329. The electrical sym bol illustrate d b elow represents a:
",
a) push-pull circuit breaker b) push c)
[0
reset circuit breaker
toggle switch circuit breaker
337 Copyright 0 Btl'!) Collt§ 2006 - 2007 All n"ghu rmn'l!d
n V1l'.parl66.rolll
CHAPTER fiVE ENGINEERING DRAWING
30. \Vhen a drawing consists of a number of sheets and il has a schedu le o f parts that is appli cable to aU th e sheets, il will appea r on: a)
the last shect
b) all sbeets
c) Sheet 1
31. All new aircraft drawings arc dimensioned and toleranced to th e:
a) BS system b) ISO system c) SBAC system
32. ATA Spec 100 con tains the form at and guide lines for: a) drawing practices b) technical manuals
c) maintenance o rganisations
33. The
Aviatio~.R e qu jrcm e nts
(BCAR a nd JAR) contain:
" )\)0 • 1.\.)I.:
. \ - ~Q . . ru reqUlIements . . (,~c'ttn ntmum techruc, .
.. (
-0'
\)';I.~ (~
b) stannory legislation
~(\.
"- ~,~ ,
c) advisory information
34. The abbreviation AN used m connectio n wi th th e American AN standard m e ans :
a) American National. b) All Nation c) Air Force/Navy
338 CopJ7ighl
e &rry CoIlf8t 2006 . 200-
rill righli rtlmrn
_1P.pof166.~
CHAPTeR FIve eNGINeeRING DRAWING
35. Draw ing practice in th e U K gen e rally confo rm s to: a)
Aircraft General Standards (AGS)
b) British Stancb rd (BS) 308 c) EASA Part 145
339 CopJrighl () Ba'1J Cofftge 2006 . 200i All nghlI muvrd
1l~I.part66.ro",
CHAPTER FIVE ENGINEERING DRAWING
Revision Engineering Drawing Answers 1.
e
16.
A
31 .
B
2.
B
17.
e
32.
B
3. A
18.
A
33.
A
B
19.
B
34.
e
5. A
20.
e
35.
B
6. B
21.
A
7.
e
22.
e
8.
e
23.
B
9. A
24.
A
e
?-,.
A
26.
B
27.
e
13. B
28.
A
e
29.
A
15. B
30.
e
4.
10.
II. B ,,~)~\'1
1?_. A• ;,\J(16-.c o\\":-;'-
"P;,.(1~
,. \''\,
'
,
....
~\ \.
. <'~
'"
14.
340 Cop)'righl
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All righlJ rewwd
='.p(//166.co",
FIT S
&
CHAPTER S I X CLEARANCE S
Fits & Clearances Introduction \,\'clcome to chapter six. \X'e touched "er:-~ briefly on rhis lopic in Chapter 2 when we examined allowances in re."pecr of shafts and their holes. You may recall that the leon 'allowance' is defined as being the difference bet\veen the high limit of size for a shaft and the Im,-cr limit of size for a hole. The allowance can ha\-e a posiri\-e or negative value. A positive allowance produces a clearance fit whilst a negariye allowance produces an interference fit. \Ve now need to broaden our conceprion to include other parts that may be subject to movement when assembled together. For example, when bolts arc to be used to join a componem flange to a gea rbox. we must establish the diameters o f the clearance holes in the flange and the tapping holes in the gearbox to enable accurate machining to take place. \Vhen considering the l:mer, we need to know what class of fit we wish to obtain when the screw threads are engaged. Taking another example, wd may intend to usc a nut and bolt to secu re two pieces of structure together, where no movement of the structure in relation to the bolt can be pe.rm.itted. In this case, we need to consider the diameter of hole required to create a suitable interference fit along rhe grip length of the bolt. To extend our thinking e\-cn further, we may need 10 ensure that sufficient clearance exists bet\vcen a rotating shaft and it's bcari.9g, to permit adequate lubrication without introducing unwanted c ~\\I:\~br;cion through e....cessi\-e radial movement. . O..,\\.:.
,
.~
We need to consider whether we require a cleaIil.!1Ce fit, a lranSlUon fit or .ao interference fir. \,\ 'e also need to consider dle grade of the fit in terms of l~&v precise an allowance is needed in rclarion tb the intended use of~he components, the ability of a machinist to prfduce it and the deg~~ of interchangeability of parts that is required. An. Ancient Roman Cad~right installing chariot wbeels on a Friday afterno~n may not have gecn tOO bothered about all this, depending on how ruthless the owner was df course! \Ve would need to exercise a lot more care whch for example we arc dealing
341
C H A PTER S I X FIT S & CLE A R A NC E S
with shafls supported in precision bearings in high-speed modern machinery that is SUbjCC1Cd to extreme temperature and load variations. Finally, there is the '1uesrion of wear. \'I/hen objects that arc in contacr move, either as function of their operation or through vibration, they will inevitably wear over time. W,Ie must consider the maximum amount of wear that can be permitted before our initial clearance migrates beyond a "alue that maintains the required fir. In other words, we need [Q establish permissible wear limits. Looking back on what we have discussed so far, we can see that this subject is o f critical importance to not just the designers and manufacturers but to those whose task il is to inspect, maintain and overhaul aircraft equipment. The syllabus for this chapter commences with the drill sizes for boltholes and the classes of fit before leading us on inro the British Standards 854500 and BS1916 syslems used for metric and imperial fits and clearances respectively.
Drill Sizes fo r Bolt Holes Tables of drill sizes are provided for clearance holes and tapping holes for the ranges of B5\'(/, BA, BSF, metric, Unified, AN, and ISO metric bolt sizes. Before we examine any examples, we need to recall that drills arc provided in lenered, numbered and fractional sizes with decimal inch and metric mm equivalents. /\s you may guess, there will be small variations in diamerer between metric and imperial drill sizes . For example, Ii number 10 drill is 0. 1935in but the nearest metric equivalent is 4.9mm, which is 0. 1929in, 0.0006il1 smaller. Similarly, the nearest numbered drill size to a 2.0 mm drill is No. 47, which is O.lmm smaller. This underlines rhe importance of using a tap drill size chan when drilling boltholes rather than relying on guesswork.
.
The syllabus in~es drill sizes so there is little alternative bur to give you tables foc'5"'-1I"tn . ibis docs not mean you have to memorise dlcm, they arc very e\!9.o'si'v-~ and arc provided here merely to pennit you to see the ranges and . ~·;I.{ fi'o\V they compare. I will give you one table for lwered and fractional drills ~\"'\. 1,;. and another table for numbered and fractional drills .
."w.
(~,\'!'
Note that drill size E has a fractional (1/ 4m) equivalent as well as decimal and mm equivalents. It is the only lettered drill to ban all these. The lettered drill size range is i\ to Z ,."ith A being the smallest diameter (0.2340) and Z be.ing rhe largest diameter (O...\130). The lettered sizes arc larger than the numbered sizes. 'fllC fractional drill size range increases from 1/64,h in diameter in steps of 1/64,h in. up to 1/Zin diameter. We will not show all of these.
342 Cop)"Tighl 0 Barry Gi/ltgt J006 .. z oo -
All righu reImtd
/J1/V/J,.part66. rom
F IT S
Lefler
,
Fr.lcunn
moo
,1;4-1
5.95
.JIR9
.2,162
6.0
.322R
.2401
p
"'2
.32.;0
6.1
.32-18
8.25
.3268
8.3
.3281
S..U
.3307
8.4
.14-11
6.1 6.25
.2480
6.3
.2_~1k1
6.35
.3346
8.5
.252U
6.4
.3386
8.6
.25S9
6.5
1/4
11 '64
Q
.3320
R
.3390
.2;-0 11 '32-
0.6
_2(,!U 1-'(,.1
"
.16:;8
6.-
26S-
6.-S
.26--
., 0.8
T
.2-56
-.0
ZJ/64
.2770
J
_2-9S
- .1
.2811
K 'J/_ll
11JI.f
. ~\c'!..:.-(' l-~-' -
.21S11
".14
.21B5
7.2
. 2~S4
- -,,.
.2If:4
-:',3
.1'X(I
L
.2913
l·
,.
191M
,
3/8
.3445
8.iS
.3465
8.'
.3504
8.9
.3543
9.ll
.3583
9.1
.35'>4
'1.13
.3622
9.2
.3641
9.25
.3661
9.3
.3701
9.4
.3740
').S
.3750
9.S3
.r-o
'.4
..r80
9.6
.3811)
9.-
.2953
-:'.5
.3838
9.-S
.2%8
-,5-1
.3858
9.'
.m2
-.6
.3860
-.~!~l
5/ 1(,
B.-3
.361:10
,:!9SI1
"
8."
.3580
.2-20
1
.3425 .3·0-
."''''
5
.1661.1
1-16
(~--."
.)1(..0
.146\1
G
<'I,. -. . ~~~
mm
D
_2598
.
Dec Ell
.141.1
r
\\~\
FrncuOll
C
E
'"l\,\,b
Letter
0
.2..;&1
B
C HAPTER S I X C L EAR A NC E S
Dcc Eq
.2'\40 151(.4
&
"
.-'031
--
:\1151
- -;
.:\lJ-;'J
- .8
X
'\ 1 ](\
-.9
Y
.3\25
-.94
_HSti
8.0
25/64
., -'S .-.
,
~
.3898
9.9
.)906
9.92
,::-
.39r
IIU~I
~
-
~
o·
.39-0
'" '" .$
.4U-KI
13/32
7
r
.4f!62
0'
It1.31
.41.10
Table 6.1· Lcncrc d and Fractional Drill Si'Zes & E q u iva lc llls
...'"'"
'j
343 G'P)'righl 0 &'!) Colkgr 2006 200 - AU rit.bll rntnYli
U'/~I.part66. rom
C H APTER S I X F I TS & CLEARANCES
,~
IMI
'"
,,-, I'"
,- "1"
Table 6.2 - N umbered and Fr:actional Drill Sues & Equivalenl8
TIlC numbcrcd drill sizc range is from No.1 [0 No.80 where No.1 is rhe largest diameter (0.2280) and No.80 is the smallest diameter (0.0135).
344 COJ!)ri&N C &17)' CAIIt§' :!006 - ~()() -
AD njlm mmvd
F I TS
&
CHAPTER S I X CLEARANCES
Tap Drill Size Tables \X'hen preparing to drill a hole in preparation for cutting an imemal thread, you need to consu1t the appropriate tap drill size re ference table. These reference tables give the diameters of the tap drill and the clearance drill for each size of threaded fastener. They arc often supplied in combination with a drill size gauge. The drilled hole shou1d be close to, or slighdy larger than the minor diameter of the internal thread. It 6 \'ery important mat you sclect the correct diameter tap drill. I f it is tOO small. yOlI arc likely to break the tap. 1f it is too large, you will not achieve the required depth of thread. There is no need to give you every rap drill reference table for e,"ery class and size of bolt in this module. [ will just show you a couple of ex tract's from an ISO metric tbread tabl e :lI1d from an American National Fine thread table. Note that the tables show the tap size, the diameter of rhe clearance hole and the diameter o f the tap drill. Tap Size
Tap Drill Size (mm) Clcar:mce Hole (mm)
~ I -I
3.3
-1.5
.\15
4.2
5.5
Table 6.3 - Extract fro m ISO melnc thread lable
Tap Size
Tap Drill Size (No.)
Clearance I iole (No.)
-1-18
~2
33
5-4-\
r
29
Table 6.4 - Ex ml ci (rom Ame ric an Na tional Fine thteHd Ilible
Classes of Fits (threaded fasteners)
\vmm:
The module syllabus links 'classes of 6ts' widl 'drill sizes for bolLholcs'. it maS[ probably intends this to mean the definitions for clcarance, transitio~ and imcrfcrence fits, there are, however, other definitions of 'classes of fit' that relate specifically to thc production of the different thread systems. Boltholet. arc often threaded so to overcome any chance of misinterpretation on the paff of cxaminers, J \\-ill include a brief description these classes here before mo\"c on to deal ...\-ith the more standard definitions. This may seem like <~ It and braces' but it is beuer to be
*
,
345 ~~"Jl,.pnrl66. rom
CHAPTER SIX F I TS & CLEARANCES
British Standa rd W hitworth (BS\V) T h reads
\\fhinvorth form threads are used on British Standard \,!hinvorth (BS\X0. Bmish Standard Ene (BSF) and British Standard Pipe (BSP) threads. 'fhere are three classes of fit for external threads and two classes for internal threads. 1.
Close Class External Threads: This class applies to threads where a good snug fil is required. It is achieved by the use of high quality production equipmem and an accurate system of inspection and gauging. It is used for speciaJ applications where close accuracy of pitch and thread fonn are required.
2.
wl edium CI."Iss Internal and Extcrn:1i Threads: This ci:Jss of fit applies to the better class of ordinary interchangeable screw thrcads.
3.
Free Class E::\'1eroal Threads: This class applies ordinary commercial quality bohs.
4.
Nom Jrl/ Cluss IJJteroal Thre:Jds: This class applies co the ordinary commercial quality nurs thac are intended for use with medium Ot ftee class bolts.
to
rhe majority of
British Associ a rion (BA) T hreads This is a metric sizcd system used on some small diameter British fastcners. There is only one class of fit for internal threads sizes OBA to 1GSA. There are nvo classes of fit for bolts:
1.
Close: This class applies to sizes OBA to 10BA and have similar requirements to the 'close fit' definicion for W'hitworth bolts.
2.
Normal: This class is reserved for commercial grade bolts used
10
gen~~l llRt6nee[i.ng. e'2-\)\:
c
_\\~. . \.. -
(. ~...~....
~;I.{
Piiffl':;'[i T h .. ad , There are three tolerance ranges for Unified rhreads of 0.025io diameter and above. Numbered sizes arc limited to Class 2 and UN] threads to Class 3. 1.
Class 1A External and Class 1B 1l1lem:u Threads: These classes apply to the majority of commercial grade faHeners.
2. Class 2A E .v temal and Class 2B I lllernai Th reads : Tbese classes apply to the majority of fasteners used in the aircraft industry nnd equate to the 'medium class' shown above for \\'hitworth threads. 3.
Class 3A External and 3D I ntern.?1 Tbreads: These classes apply to threads that require a snug fit and equate ro the 'close class' for Whinvorth threads. They are being used in increasing quantities in the aircraft industry, particularly on engines.
346 up)Tighl O
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CoIkgt 2006 . 200- A1lrighli rnmwl
F ITS
&
C H AP T ER S I X CLE A R A NC ES
ISO Metri c Threads There arc three classes of fit applied to ISO l\ letric dm:ads. You will notice alphmlUmcric codes in brackets agains t each class . These specify the precise fit of the external and internal thread. W' e will be examining the meaning of these codes when we deal with the British Standards system of fits and clearan ces la ter in this chapter.
I.
Close Fit (H5/h4): This class includes bolts that require a snug fit with good pitch and thread form accuracy. It is produced using high quali!."}" production equipment and ,"er)' accurate inspection and gauging processes.
2.
flt/edilln1 Fit (I-l61g6): This class includes bolts used for general engmeenng. The clearance permits free assembl\" withom risk of seIzure.
3.
Free Fir (E/7Ig8): This class is for general commercial use where quick assembly is judgcd more important than the closeness of fiL
Thread Inserts T hread inserts arc hel ical stee! wire thread li ni ngs that arc often used in componen ts made of soft marerials and as a repair scheme for holes with damaged [h reads. Thread insens arc manufactured for the "a nous types and sizes of thread. Tapping hole drill size tables arc used and the thread insert manuElcrurers supply the taps for ever)' size of insert. The insert is screwed into the tapped hole with a special inserting key or inserting tool supplied by the 1min manufacturers. The tool engages widl an inward fadng dridng tang ,I; J.I."-"?fl 't1~e boltom of the insert that is broken off after the insert is fined. Inserts . . ; (f'WS': are normally a permanent lir but special extractor tools with hardened and ~ \"\
Fastener Clearance Holes The accuracy and finish of the clearance bole through which a fastener passei has a bearing on the strength and fatigue resistance of a joint. 1\ drilled hole ffi seldom very accurate. The bole may not be truly round; it may be slighdy la r~ than the nominal diameter of the drill and have la scored surface. \Vhere l1ffih accuracy is required, the bole is first drilled and is then carefully reameti' to achieve the correct dimension and a smooth fi nish. T h e amOllnl of s~plus material thal is left after drilling to allow for reaming should be rhe mi!'Umum , 'v amount that is necessary to produce a smoofll and accurate h't.~. Large diameter holes may require re1arive1y more surplus material fo r' reaming because the degree of roughness and ovality may be greater. Typically. O.003in
347 C"pyighl
C BII'!) C(Jllt§ 2006 . 200- AU ril,hfS rt/='fd
CHAPTER SI X FITS & CLEARANCE S
would be left for holes up to 1/ 8in diameter with an increase of O.OO l in for each 1/8in step up to 3/ 4in . Holes larger than this would cypicaJ!y be drilled around 1/64io smaller thao the size of the reamer.
Tn many primary and secondary aircraft structural assemblies, both the fasteners and their holes are often produced to 'dose tolerance' . Fasteners used in low load bea ring applications would nor necessarily require rhe same degree of accuracy; the expense alone would prohibit this. For mat reason, there are four general classifications given [Q fastener holes. Class
Applic ati on
Close ream
Close Tolerance bolts,l li-locks and Lockbolts. (interference type fits).
Chss 1
Standard bolrs and Close Tolerance bolts, dependent on type. (interference t)pe fits).
Class 11
Bolts used to annch remO\'able panels.
Class 111
Bolts used to auach rell1O\-ablc panels nor subjected to strucrurnlln:lds. Table 6.5 - Fas te ne r Hole Classifi cations
Classes of Fit \'(le discussed these in Chapter 2. The classes of fit between a shaft and a hole
are: C learance,.\I1h
lorf
<>
co\\.;~
~\,\. (
~.:•."y;.
(~..,~ .
'el,,1?; clearance fi t exisls when there is a posicive allowance between the minimum size of the hole and the maximum size of the shaft. TIle shaft is smaller than th e hoe. I r,
Interference Fit
An interference fir exis ts when there is a ocgacive allowance between the minimum size of the hole and the maximum size of the shaft. T he shaft is larger than the hole. T ransition F it A transition fit exists when the tolerance zones of me shaft and the hole overlap. The fit can be biased towards a clearance fit or inrerference fit dependtng on the actual sizes of the bole and the shaft. The shaft and thc hole arc approximately the same size.
348 Coppight C Barry ColHgt 2006 - 2()()7 All righlJ nltlwd
UlUIJl'.part66.rom
~ ".
~.
~
":r:;
If -
FITS
~O[inimU111
\hrumu lll
Clearance
Interferen ce
&
CHAPTER SI X C L EARANCES
I
o!U II U D-LJ JU
n
Clearance
n
Interference
n
Tr:msirioll
Fig ure 6. 1 - Classes of Fit
From the illustration, yOll can see that the dimensIOns of the shaft and the hole each have upper and lower limits forming tolerance zones. The grade o f fit \\~thin each class depends on where the acrna! dimensions lie \\~thin these tolerance zones. It is essential that component parts be produced \\~th the fit that is appropriate to their imended lise. A rotating shaft for example \viil regUlIe bearing surfaces with clearances that permit lubrication and free running within a range of thennal expansion. In this example, the magnitude of the limits applied will be influenced by the diame ter of the shaft and hole. To take other examples, the security of a run on a wheel mar depend on the correct grade of interference fit and the security of a replaceable bearing bush in a housing will be similarly reliant o n the choice of fit. \'(fhen we translate this to the cleamnce holes for bolts we may see that some bolts may require a clearance fit whilst the holes for close tolerance bolts wOl.~\5\yproduce interference. This is where an extensive range of drill sizes . ?.(\C&ecomes useful. Taking a purely hypothetical example for a 1/4in diameter _c.o\\e~1.:. bolt, we can produce a particular clearance fir or :1n interference fit as follows. ~S,...~1.':
. ,\'.\
:>.,...\'i"
- o'? ~.
'"
For bolt diameter 1/4in:
•
•
Clearance hole diameter limits (clearance tit) o
Maximum diameter 0.2520in (6.lmm)
o
Minimum diameter 0.2500in (6.35mm)
.1"
,f'
Clearance hole diameter limits (interference fit)
o
Maximum diameter 0.2460 (6.25mm)
o
:Minimum diameter 0.2441in (6.2mm)
'"
.~
0'
r?:::::
f!
'"
I stress th:1t these are hypothetical figures; ~fOU must comply )Xjth the specifications for the particular task in hand. Ndte that the drill size,.,~bles do
349 Cop]nghl © Bm7J' College 2006 . 2007 All right! rmn!(d
wlll1V.pmt66.rom
C H APTER S I X F IT S & CLEARANCE S
include all these sizes. Il owe\'er, you may have reaming is rC(luircd.
[0
alter your choice when
Common System of Fits & Clearances Various limit systems have been used [0 specify various types of fit. or these, the Newall system of fits was used extenSi\Tc1y in genernJ engineering. This ha s now been largely replaced by Brirish Standards systems. BS ~SOO (BS EN20286 1993) 'ISO Limits and Fits' is the latest metric standard. BS 19 16 'Limits and Fits for Engineering' is the imperial ,Tersion. Before we deal ,\o;Eh the principles of the Brirish Standards systems, \ye should take a quick look :H the Newall system, bearing in mind that it has been superseded.
Newall System of Limits & Fits The Newa.ll system is a ' hole-basis' system. This means that the hole \"nIl be produced as ncar [0 the basic size as possible and dle desired fit will be achievcd by varying the size of me shaft. There are twO grades of hole specified in thc system, Class :\ and Class B. Class A holes are machined to fin er limits rhrlll Class B holes. There are .LX shaft sizes associatcd with cach nominal size and class o f hole giving six possibl e fits. We only need to look at a simple example of a table for these. Type o f Fit Class of Fit
Shafr
Application
Force
F
Mechanical pr.:,~ur(' r('
Drh'ing
D
Less t.i~ht than :I force fit where p:\ft$ can be dr;\'cn togcther.
Transioun
Push
P
P.lrtS can lx ;15~embkd :md manual prc~~urC'.
Clearance
Running
"
Su;tcd [0 mOlln)! partS.
Interierence
y
Su;l.:d
to
mOl1ng parts.
Z
Su:ted
to
mOI'ing parts.
Table 6.6 - Newall S)'s lcm
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Shrink & Contraction Fits Before we moye on to discuss (he current British Standards system, we can take the opporrunity to examine two particular types of interference fit that are not normally mentioned. Shrink Fit This type fit would require the casing containing the hole to be expanded through heating before the shaft could be assembled into it. "111e shaft could be removed again by re-expanding the hole. ?v!etal wheel oms would be an example. Contraction Fit
This type of fit requires the shaft to be frozen to reduce its diameter and peront assembly into the hole. It would be used in circumsta nces where the application of heat would affect material properties. The Installation of cams onto shafts or gear ri ngs ontO wheels would be example.
British Standards System haye already identified BS +500 (BS EN20286 1993) as the ISO metric Limits and Fits system. BS1916 is the imperial version. The principles are similar for both. \\"ie will use cylindrical shafts and holes as our example but the system is equally applicable to mating parts tha~ do not ha\Ce a circular erossseCOOD.
\'{/e
T ole0"anccs '1~6-'2: . O~\e(tr'""- Before we moye on, we should app reciate that a 'tolerance' is the difference • -v.¢(" ~\..-b
,~,;.;'\Y
~,
between two limits, for example an upper and a lower limit. A shaft will have a maximlun diameter and a minirnwn d iameter and rhus has a 'tolerance zooe'. A hole will also haye a maximum diameter and a minimum diameter and thus a 'tolerance zone'. How fine these tolerance z
-:1
Standard Tolerance Grades
"'"
I
13S 1916 specifies 16 grades of fundamental tolerance (ITI to IT1 6) fo r eaci" basic size range up to 19.69in and 11 grades of tolerance (1T6 to TT16) fcir sizes above this up to 124io. 8S4500 on the other hand has 20 grades £ f standard tolerances (1T01, ITO and ITt to IT18) covering the same size r~e. You need not memorise these figures but you si0uld know their applicati8:ri..
""
Consider the standard tolerance grades ITt to IIrt6 as an example. The lower the numbet, the more precise the manufacrurirlg process and the slj1'1;'ner the "pccified ralemoce will be. Let', cake '0 examPll of, "h,ft that h" a b,"ic ,izc
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within me range SOmm to 80mm, say 6Smm. The smndard tolerance grade ITl for this shaft would be just 2.5 microns but at tolerance grade ITI6, it would be over 1200 microns! Fi ts \'('hen we consider a shaft and a hole, barh will have tolerance zones. It would be difficult to obtain a specified fit by adjusting both zones. To derennine an accurate fit, it is easier to specify a fi..'>:ed tolerance for one parr and then obtain tbe recp.nred fit by altering the tolerance of the otber. For example, if \\'e specify a single tolerance for the bole, we can select a tolerance from a range of standard tolerances fo r the sbaft that will produce the required fit. This would be a 'bole-basis ' system. Conversely, we could specify a single tolerance for the shaft and vary the tolerance of the hole. That would be a ' sh.-di-basis ' system. In most cases, the 'hole basis' system is preferred. Fu ndamental D evia tion
A fu ndamental deviation is the position of the boundary of a [oler:lnce zone in relation to a 'zero' or reference line, whicb is normally the basic size of a component. Fundament:ll Deviation Zero Line Fundamental DC\'iation of Shaft +
D
f .1~<\6
Fundament:tl De\-iation of Hole
.-l
B:lsic Size
D
- '100' -
(p\\c'l-' ~ \',-:o.(t'i ,,!--- \.
,.
,\'
Figure 6.2 - Fundamc[Jlal Deviation
:'.~\o,;.;
\,~ .
In the illustration, we have shown tbe 'zero' line passing through tbe ccnuc of the clearance or 'allowance'. Ths means that botb the hole and the shaft have a fundamental de,riation from this line. The hole deviation is positive, being above the 'zero' line. The shaft deviation is negative, being below the zero line. YOll can sec that this has produced a clearance fit. However, the same result could have been achieved by fi......mg the fundamental de,-iation of the hole at zero and then adjllsring the fundamental deviation of the shaft to obtain the required fit. Before we move on, let me remind you of the definitions for bilateral and unilateral limits.
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Bilateral Limi ts
T hese limits are displaced above and below (he basic size. For example, if the basic size of a hole i~ 2in plus or minus 0.004io , then the high limit i~ 2.004in and the low limit IS 1.996in. The tolerance is 0.00~1O. Unilateral Limits
1bese limits are displaced in one direction only, either above or below the bask size. For example, our 2in bask size hole may be described a being 2in plus 0.008in. ' nlC high limil in this case is 2.00Sio and the low limit is 2.o00in. Com-ersel}" the hole may be specified as being lin minus 0.008in. In this case, the high limit is 2.000il1 and the low limil is 1.992in. In each case, the tolerance is O.OOSin.
1T'Ion
Interfe rence
Clea rance
I
+
1D
=::::---'---"~L-S"ldn=h"T ±--1°1 H ole Tolerance
Tolerance
Basic Size
Fundamental De\' iado n Zero Lin e Figure 6.3 · H o le Basis Sys tem
Imagine that we now fix the fundame ntal deviation o f the hole at a specified ~al®."'\Gfor e.xampie on the 'zero' line. This rrieans that the hole will have ;-.,\.:.,?-c '!.\I~njJateral limits ,",,;th its lower limit being the basic size. Now we choose a (' (P positive fundamental deviation for the shaft that places it just above the 'zero' h.."'''~ line. ' nlC hole and shaft tolerance zones are no\J oyerlapping and we will have ,.:,\\~,," a transition fit. Quite what the transition fit will be will depend on the value of Cp'Q fundamental denation we ga,e to the shaft. Remember, the hole deviation was fD,ed. Now imagine we increase the fundamenta l deviation of the shaft until).ts tolerance zone lies above tbat of the hole. \'(Ie now have an interference ~. AgaIn, the fit will depend on the value of the fundamental deviation we gave to .
.
~.~
~
'"
From the above description, it seems that \ve need to have some standar&' fundamental deviations that we can apply first the hole and then selectiv~ to the shaft. That is what the British Standards I system does. 1n BS 1916, t§e fundamental deVIations for holes are indicated by 21 upper case letters and .ine fundamental deviations for shafts are indicated bv 21 lower case letters. 1i
tb
,
Holes:
_,; Be 0 E FG I-IJ KMN P RST'U VX Y Z
Shafts:
abc de f g h j k
fi n
&
R
p r stu v x y z
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A to II and k to z indicate tolerance zones tbat lie abO\-e the 'zero' line. K to Z and a to h indicate rolerance zones that lie below the 'zero; line. This means that J and j lie on rhe 'zero' line. For mosl generaJ engineering purposes, rhe fundamemal deviation alloned [0 the hole will be 1-1. The required fit is then achieved by selecting an appropriate fundamental det~acion for thc shaft. BS 4500 uses rhe same codes as BS 1916 except that there arc seven cxtrn leller codes, ZA, ZB, ZC eec. The imperial and metric systems arc const.ructed TO be identical so thar the same combination of leners for a hole and a shaft will gi\'c the same dimensions in both systems, albeit BS 1916 will be imperial and BS 4500 the exact metric equivalent. Class ifi cation of Fits W'e can now specify a fundamental deviation and a tolerance grade for the hole and the shafl. [n the hole basis system, we will fix the hole and select options for the shaft. For example: Shaft
J lolc
Clearance
Transition
Interference
116
e6 fS g6
h6
is
,4 15 u4
118
c9 cS f8
j7 k7 1116
k6
-
I
S
- U
Table 6.7 - Examples of Engineering Fils
.
,. CI'"
,
These arc JUSt a few purely hypothetical examples and rou would need [0 refer 10 the British ~1tl dards Data Sheets to obtain selected fits. These arc very extcnsi"~ "1\n\!l1 ic\vould not be appropriate to include them here. However, the ~'i?~9.\~f fit ehC}' produce arc worch examining and examples of these are \l)'tincluded.
~,\.
<'"
Clearance Fits
Transition Fits
Inte rference Fits
\'cry loose running
Push
Light press
Loose running
E:lsy keying
:>.tedium dm'C
Eas), runntng
Tight kcying
Il ea\)' drit·c
Running
Ught dri\·c
Heavy
pres~
or shrink
Table 6.8 - Types of FilS
You should now appreciate that for each combination of fundamental deviation and tolerance grade of hole there is a rnnge of shaft fundamental devia tion and tolerance grade combinations available from which lO choose. For example, a lypical classification fO( a loose running fit would be I !7/d8. A
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typical example for a push fit would be J-I6/j5 and an example o f a heavy dnve fit would be H8/ s7. T here are numerous combinations but do remember, the upper-case letter refers to the hole and (he lower-case leHcr refers fO [he shaft. Also, remember that when the hole classification is fixed and the fits are obtained by varying the shaft classification, we are using a 'hole basis' system.
Schedule of Fits & Clearances for Aircraft & Engines Aircraft and Aircraft Component l\'lamtenance and Repair Manuals lnclude a schedule o f fits and clearances for all mechanical parts . Common features included in these schedules are: the figure/reference identiry of the component, the initial manufacmring dimensions and limits, the assembly clearances and the in-service wear limits. Some schedules may describe tbe initial manufacturing dimensions and limits as the 'permissible dimensions' and the assembly clearances as the 'pennis sible clearances', The information is the same, only the title is different. The schedule is normally cross-referenced to a Figure (drawing) where the components are identified by a reference letter. It would be a good idea to look at a coupic of hypothetical examples of these schedules. Table for i-igu re 123 Manufacmring Limits Parts List Fig/ Ref ItcmNo. Fig 123
A
21-210
10
Dimension (mm) i'v[jn
),lax
6.235
6.236
G
Ass), clearancel (mm) 11in
0.009 20-.')5
OD B
21 -305 10
OD
Max i
In-Sen'icc Wear Limits Dimension limits (mm) i\fin
Max
6.235
6.244
Allowable Clearance lfax
0.011
0.029
6.225
6.226
6.215
6.226
2.554
2.559
2.554
2.620
0.002 2-90
I
0.009
0.08
2.550 2.552
2.540
5~
) --) _.JJ _
~
Table 6.9 - Example Extract from a Fits & Clearances SchcduJe 1
That is enough of an extract to show you the content of this typc of sched~~. T he parts are identified as having internal (lD ) and outside diameters (
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The 'assembly clearances' specify the maximum and minimum clearances that should exist afrer assembly. If we have a component with a ma..ximum internal diameter of 6.236mm assembled onto a shaft with a minimum outside diameter of 6.225mm, the clearance would be 0.011mm. Conversc!y, if the ourside diameter werc at the minimum value o f 6.235mm and a shaft at the maximum value of 6.226mm, the clearance would be O.OO9rnm. T his gives us maximum and minimum assembled clearances of 0.011mm and 0.OO9mm respecri.ely. The 'in-service wear limits' will produce the 'maximum allowable clearance' for LIS. If the maximum i.nternal worn dimension is 6.2+-I-mm and the shaft minimum permissible outside diameter is 6.21Smm, the maximum in-service clearance is 0.029mm. We can now look ar mOlher hypothetical example of an extract from a schedule for the overbaul of nn engine.
Item
Permissible Dimensions (mm) i\ lax
~Ijn
Crankpin dia standard
·W.238
40.225
1st undcrsize (O. 127mlll)
40. 11 1
40.098
2nd undersize (O.2S4mm)
39.984
39.n
Big end bearing standa rd bore size
40.403
40.390
Crankpin running deamnce Table 6.10 - Example of a FilS & l')\f;J~
The
C le ~rances
Permi ss ible Ciearnnccs (mm) ~ I a......
Min
0.178
0.152
Schedule (Engine)
'f,:~~~bi;dimensions' specify the O1flximum and minimum limits o f size
[Qr-.oX~~ manufacnlred parts. The difference between these limits is the
'i.
" .\J:;.... .
( ..\'.
r...
\\.~"\. manufacruring [oic.rance. The 'permissible c1enmnces' specify the maximum
and minimum clearances that should exist when partS are assembled. This is similar to our previous example. The maximum size of a standa rd crankpin is 40.238mm. When this crankpin is assembled imo a bearing with a bore at the minimum limit of -IO.390mm, the clearance would be O.lS2mm. Conversciy, if a cranh.-pin at the lower size limit of -«>.225 were assembled into a bearing with a bore at the maximum size limit of 40A03mm, the clearance would be O.1 78mm. T hus, the maximum perrnissible assembled dearance is O.178mm and the minimum is 0.lS2mm. I will not bother with the oversize entries in rhe example as 1 think the point is already made. The schedules we have discussed are used br engineers to compare acrnal dimensions and clearances with those specified for new partS. Care has [0 be exercised when a clearance is viewed as having reached or exceeded tbe maximum permicted value. .l\'i ost of the wear may ha"f"e taken place on one of the components and not the other. For example, a bush may be worn outside of the maximum limit whilst the shaft may be well within limits. T his means thal the correct clearance could be restOred by replacing tbe bush. You can
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only arove at that conclusion using the dam contained the schedule of fits and clearances for the parts. In many cases, the maintenance manual may require you to renew both parts when one reaches the limit. This is normally based on the philosoph\' that yOLI should only refit parts that h:wc a rCflsollabk life cxpectancy. There is little point in refitting a half-\vorn part fhar is likely to becOl:lc f~ll y ,,:orn long before it:; mating part Idocs ..The message here is to cxcrclse dlscretlon; and comply \nth the approved malntcnancc procedu res of course.
Limits for Bow, Twist & Wear Bowing Limits I f it is suspected that a structurnl member, a strut for example, is bowed, the method used to check for this will depend on rhe design. I f the stnll is free from protruding parts and a straight edge can be applied along its entire length, feeler gauges can be inserted at the point of maximum cleamnce. The amount of bow presem can be calculated as follows: Clearance measured br feele r gauges Lengrh of member
Bo\\'
For eXflmple, if the member is 4ft long and the clearance is ascertained to be 0.080in then the bow is:
_l,W.:~e
... \-..,;}{{..: l.
...\.
~
",'\\.~
')l~'6
Bow::: 0.080 ::; -.!..
1
48
600
4800
or 1 in 600
'l(\~(
-' A maximum bow of 1:600 is normally considered acceptable unless the Repair i\lanual states otherwise. The manual may allow a greater 1Olerance.
c
.-
-o~· C
Depth Gauge _ TraTmel
"'-
-'-
•
~
Straight Edge
j L
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Figure 6.4 - Checking fo r Bow
When the member has protruding parrs, collars for example, it may not be possible to apply a straight edge. The problem may be o,'crcomc by using a trammel clJuipped with three pointers. nle pointers should be first aligned on a surface table. The rrammei may then be offered up to the member and positioned so that the ou ler poimers contact each end. Feeler gauges ma)' then be used in con junction witb the centre poimer to establish the clearance. :\ more accur:tte measurement can be obtained by replacing the centre pointer widl a depth gauge o r a dial test indicator. The depth gauge should first be aligned with the protrusion of the outer pointers using a surface table and the reading noted. Allernati\'ely, if feeler gauges cannot be used in siN, the protnlsion of the centre pointer ma), be adjusted to contact the member. Jf the latter method is employed, the lCammel can be placed on a surface table so that the centre pointer and one end pointer arc in contact with the table. Feeler gauges may then be used to establish the clearance of the opposite end pointer from the table. \'rhen using trus method, you must remember that the clearance you arc measuring will acrually be twice the .alue of the bow.
!
,
Trammel
j
Surface Table
1 1x
I3{)W~
Figure 6.5 - Simple Bow C heck
Twist The course of action you would adopt in order to check for twist would depend on what mechanical strucrure you were dealing with. On the large scale, an aircraft strucrure may be defolTIled after a heavy landing or overstress in night. Evidence of possible distomon ffi.'ly be as simple as a visual indication of wrinkling of the skin. Less obvious evidence may be revealed during symmetry and geometry checks on the struCtUre. These parnclll:!r checks arc dealt with in Chapter 19 so we will not pursue them any further at this point.
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Suffice to say that the limits for errors reyealed dunng such checks will be stated in the Aircraft Maintenance Manual and the Repair Manual. On a relatively smaller scale, shafts, strutS, tie rods etc. may suffer twist if they have been subjected to excessive torSion loads. Again, in extreme cases, the evidence mar be as simple as a visual indication of distortion . HO\vcver, an accurate measurement of small yalues of twisl would require you to establish radial datum lines along tbe torsional a.......is, ideally at each end of the component. . <'\' good illustration of this would be a check on a reciprocating engine connecting rod.
!\ Iandrcl
""''--j\[andrcl
Figure 6.6 - Check for T\\>ist
By using a mandrel, the axis of tbe bore of the big end bearing can be accurately aligned to be parallel with a surface table. A second mandrel may then be inserted through the small end and its parallelism rclat1\T to the surface table flL1.y be checked with a dial tcst indicator. This check can be extended by '1\.""\l.t1l.9ra~g the connecting rod into the vcrtical position and checking the Co\\~~e ... parallelism of the. mandrel at the small end in rclation. to the ~andrel at t~e big . ~';l.{{' end. 1\ mandrel IS an accurately ground bar that WIll fit \V1thout play Into a :\'\. ,\'" bore. It would be classified as a piece of special equipment that is provided • ,~::..-i.\'?! specifically for the inspection of a given part.
i...,'-
T must emphasise that the example given is for a check for [wist on a particuJ:a,r component. However, the principle could be applied to shafts with end fhng&; or fittings where one end can be accurately aligned in a fitting, not necessarily~ . mandrel, allowing you to check the alignment 9f the other end relative to i ~ The limits for twist \vould be unique to the particular component and woul&be stated in the relevant component manual. ff
~ o· ,~
"0
Wea r
J?
o
"
cS;5
'"
Again, the course of action you would adopt to check for wear wouid"depend the component and tbe pennissible limits woUld be specifically [~ed to it. A good illumacion would be a check on Chi ckarnnee in a bill m collet 011
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bearing. Excessive wear will result in large internal clearances. A rough check might be conducted by holding the inner ring and moving the OUlcr ring radially [0 'fccl' for mm-cmcO[. This would be vcry subjccth-e because bearings do han manufactured clearances. A more objecti\'e guide could be obtained by mounting lhe inner ring on a shaft or mandrel and establishing the a\-erage radial movement of the OUler ring by taking readings at different angular positions using a dial test indicator. Again, you would need acccss to the perm.issible wear limitS in the relevant component manual before a judgemcnt on acceptability could be made. The limits, and the conduct of the check, are not personal choices; the approved manual procedures must be applied.
~_DiaI Tcst
W
Indicator
I
2
8
7
-++++*-+--1+-1- 3 6
4
, Figure 6.7 - Bearing Cleara nce Check
I have been usi~\gsillustrations to show some compone~r610 rhe next section, we will
geneml principlcs of measuring look at some furthcr general tc'(,i)n\'q"ties that are employed to check various components.
Tn
~;..~f"l
,'0'
Q
<,~
_ ~,''l \....~.'
r
Standard Methods for Checking Shafts, Bearings & Other Parts In Chapter 3, we discussed .acious measuring instruments and their uses in checking component dimcnsions. This infonnacion will apply in this section. For example, the diameter, roundness and parallelism of shafts and round bars may be roughly checked with the usc of external calipers. Conversely, similar checks can be conducted an bores using internal calipers. If we wish to obtain accurate measurements, we can employ outside and inside micrometers or vermer calipers.
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8 Figure 6.8 • Simple Cbecks for Ovality & Parallelism
\'(Thcn we wish to conduct checks to establish if ~ hafts or bores are within their prescribed tolerances, we mar employ the system of limit gauging.
Limit Gauging Limit gauging is often used as an altcrnacive to direct measurement to indicate whether a component is inside or outside its ] specified tolerance. Another advantage is that it can be used by semi-skilled personnel.
UNnr(i
O
Plug r."~C
Figure 6.9 - Limit Gaugi ng ~
~
mo~
A limit gauge has two gauging elements, GJ and NOT GO. The
common limit gauges are 'plug' gauges for asses~ing holes and 'ring' gauges for I R assessing shafts. Taking a 'plug' gauge as an example, the GO end of the gaug~ ensures that the related hole is nOI below the low or ' nJaximunl metal ' lirrll'f of size and the NOT GO end ensures that the hole is not above its high B,r 'minimum metal' limit. If the GO cnd enters the hole and the NOT GO d:!a does not, the hole must be within its size limits br tolerance. T his is kno\v,¥"""""as Taylor'S principle of gauging. ~ ~
OJ
%0
Wle can now interpret this for the function of a 'ring gauge'. 'nle 'ring' ensures that the shaft is not above the high limit or 'maxinui"m metal
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condiu·on ' and rhe NOT GO ring ensures that it is not below the low limit or 'minimunl m e tal condition '. In other words, the shaft is within its tolerance.
Alignm ent Checks TIlerc may be occasions when we need to check the alignment of two bearin~. A good illustration would be a shaft that has to pass through twO separate bushes or bearings. If the bores of these bearings are not axially aligned, the shaft will be at risk of seizing or creating unequal bearing wear. Dial Gauge
Mandrcl - -/ ' Y-::>I.
Surface Table
Figure 6.10 - Check for Alignment
J Jere we ha\(f :I$6'thcr requirement for a mandrel that is passed through the [wo R9cih~. I f there is any significant misalignment, the chances are that the d uh·J11i el will not fit. However, providing we can sec the component parallel to \."':J. a surface table so rhat the axis of the bores are also parallel to it, we can use a dial test indicator to check the alignment of the mandrel. Run-Ou t Checks 1\ rorating assembly that is not concentric with its a.'Cis of rorarion will create problems. For example, when a shaft is carrying componems that revoke within casings, the reduction in running clearnnces can lead to fouling and chafing in addition to vibration. Run-out checks are designed [Q detect radial variations in [be angular path of points on the circumference of a revolring shaft and detect the track of points on the axial faces of components that are being dri\-en by the shaft. Errors may occur on a manufactured assembly simply because the parts of the rorating assembly are not within their individual or assembled rolcrances ../\ good example would be a shaft carrying a couple of turbine wheels . All parts of the assembly should run concentric with the axis and rhe rurbine discs should rOtate true in the radial plane. Variations would ca Llse dynamic imbalance and vibrarioo leading to accelerated rates of wear and fatigue failu re.
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2 Fm 1
7
3
6
5
4
Figure 6. 11- Run-oUl Mcasurcmcnl
illustration shows a hypothetical rotaring Jsembly and positions where a manufacturer has specified that run-out is [0 be measured . The wtal indicated run-out (TIR) for each position is determined by the biggesl recorded difference in the readings on a dial res t indicalor taken al specified points as the shaft is rurned through one complete revolution. The manu fac rurer has set the limit for the 'total indicated run-out' at each posItio n. Note that the 'total indicated run-out' given hete is the to tal range of DTI deflection occurring at one position during one complete revolution. Note that the unit we selected for use in this particular illustration was the micfon ()l). If you are not familiar \vith this unit, a micron is a micromerre, which is o ne millionth of a metre (0.000 DOlm) o r one thousandth of a millimcl le (O.OOlmm). It was included hem ~lIst to show you H exists. 10.1.'/') - i\\ecre. "C...~ '" \Vhcn a shaft is not runrung concentrtcallr besause 1t 15 bowed or bent, the ;: .\"\,:>\{, error normally occurs either side of the true axis of rotation. 'nlere will be a _,;,\¢'' ' ~ maximum positiye DTl reading at one point, chknging to a maximum negative (,0.;;:,..... reading after 180 degrees revolution. The aClU:ll run-alit in this case will be half (he value o f the full range o f DTl dcflection during one complete revolution . An example would be an aircraft engine propeller shaft. A hea\; landing or a shock load 00 the propeller mJ)' cause the shaft to deflca pcnnanently out of its true rotational axis. This would cause the propeller {ij. run our o f true and would create scycre v-jbratiot~ . ' [11 C
E
co
""
363 Cup)"right © B(lT7j' Col/tgt 2006 - 2007 All rights rmroed
CHAPTER S I X F I T S & CLEARA N CES
Dial Test
Figure 6.12 - Propeller Shafl Run-oul Chtck
'n,e propeller is rcmo\'cd and a ballast weight is anachcd to thc shaft to restrict any backlash in the shaft reducrion gear. A dial test indicator (0'1'1) is then anached lO lhe reduction gear casing and its plungcr is set (0 comact a plain portion ncar the front of the shaft. The shaft is then rmatcd by hand through one full revolution and the full range o f defl ection of the OTI poimcr is recorded. 'lllc run-o ul of the shaft is b"lf (be total of tbe DTI fiu ctll:lIion. For examplc, if thc DTI reading ranged from -O.020in to +O.040in during one compl cte rcvoluuon of the shaft, the tOtal '-:trianon is O.060in giving a run-out O.030in. Thi s value is then compared with the specified tolerance in the Aircraft t-. laimenans::c Manual. TI,e check is normally repcated using a diffcrcnt locaoon 0l~\OC pl}Un poroon of the sha ft .
..
. ;..\~ ....~ ~~ rl.aClround tubc or a ba r is suspected of being bowed, it can be chccked for \. 1-. \\:>
,. . . .5\~' "-..,..
run-our by supporting it on V blocks on a surface table and then using a DTI to check for variaoon whilst rolanag the item through one rcyolmion. Again, the run-out would be half the yalue of the total DTl de£lecuon during one re"olution. Ch ecking Surface Deprcssions A depression may form on a flat or cUfyed surface as the result of impact, wcar o r corrosion. The surfacc may for example be on a propeller blade face or on a structural member such as a leading edge or a tubular strut or rod. Dents in the middle third region of load-bearing hollow rubes and push·pull rods arc particularly hazardous and would not nonnally be acccptable. A method used to measure rhe depth of a depression is to mount a dial test indica tor on an adapt or block that has a straight edge. TIle DTI plunger is set at right anglcs to the edge Readings are then taken at twO points on the unaffected surface adjacenr 10 the dcpression, for example Y and Z in the illustration. A reflding is then taken fir rhe maximum depth X in the depression.
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..{U U
Dial TeSt _ _ Indicator
'H"
Adap tOl~ Block
L
I
I
-I
z
y
Figure 6.13 - Meas uri ng a Dt:pressioD
The true depth o f the depression is o b tained by subtracting the average of the two readings Y and Z from reading X.
Depth::::X- Y + Z
2 Oth er MeasllJ'ement Tech niques I am including tillS secoon merely to show :ou that measuring capability has been advancing with technology and there are instruments capable of extremely fine measurement. Automated ad,anced measuring machines now exist that are capable of spatially checking and storing literally hundreds of work-piece datum positions in computer software form so that precise information on shape, size and location can be retrieycd. These arc beyond the uGlc ejWOof this module bur we can examine a couple of instruments lhat ha\-e . l' b een aroun d.lor some orne " " ulat ' illustratc wIlat"IS 1pOSS "bl ( o\\'-'?'I e. c_\V'i.'i.' ' Comparators ;,.,'\,---' "
,
::;"'\\"?'
(,0'\ .
\'(1hen there is a requirement to measure particularly small variaoons, a comparator is often the most effective instrument. The dial test indicator,..is often used as a foml of comparator but actual cOfTIparators work on a differeiy principle. \\le did not deal with these in Chapter 3 because they do not fiB under the beading of commonly used ins&illnents. However, a brik description would not go amiss here as they are used in inspectiorD departments.
I
.f
T he principle of a comparator is that it accurately compares a measur§:l dimension against a known standard. They can b~ used to measure angularyitid linear dimensions and smface finishes. There are mechanical, optical, clec,@cal and pneumatic comparators but a feature that tlJey all have in commondfthat they magnify m ovement of a stylus and so enab~e accurate comp arisop~ of size with a standard such as a slip gauge. For illustration, we can look a~ simple optical comparator.
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Graduated Scale
1..1g ht • ,,-~ ProjcClor I
j}r.
-
,
Projected lmag,_c___./I~ of Datum Line
•
Stylus
Figure 6. 1-4 - Optkal Comparato r
t-. ragnifieanon of movement in the simple optical comparator ill ustmted is achieved in the ratio of the lengths of the lever arms between thc stylus and the mi rror and the amplified movement of a reflected ray in a tilting mi rror. A very sm:ill deflection of the stylus is multiplied at the mirror. A light sourcc projects the image of a datum line Onto dle mirror where it is then reflccted omo a graduated scale. As the stylus deflects, the mirror rotates and the datum line moves across the scale. These instruments are extremely sensiti,"c and art" capable of magnifying sm:ill variations seYt"ral rimes. This magnification of mon:ment is actually quite low when compared to that achie,"cd by clectrical comparators that can magnify mO\"ement up to 30,()(X)X! AUlocoUim ators t\lIlocollimators are primari ly used to detect angular variations but they can be acl:-tpted (0 linear measurement. The principle is that a be:un of light striking a reflecting surface that is at riglH anglcs to the beam will be reflected su-aigbt back along its path. I f the beam is directed through a lens, it will form an image at the focal poJ,~(\"' l-f owe"er, if the mirror is tiited, the light beam will be reflected. !.~h· angle that is twice that of the angle of tilt and the image will _\\
.I!,~
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t :- -_::-~D;'Phccmc'"
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,\Iicromeler Drum
~
~, Serting
.!
unts'
,.-
.;1
Eyepiece
Reflected I mage Fig ure 6. 15 - AUlOcoHima lOr
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The autocollimator has two pans; one part cOll,1prises fl micrometer dnlll1, a light soucce and a lens with an illuminated eyepiece that reveals the reflected ill13gC of a datum line and a pair of sighting lines . The second parr comp nses a mirror mounted on an adaptor block that has a s~raight edge. You do not need to remember this next bit because it is included for ill ustration only. T he micromcrcr drum is graduated in seconds of an arc where one revolution equals thirry seconds. A scale adjacent to the eyepicce is grad uated into ten minutes of an arc whete one divis ion is equal to o ne rcvolution o f the micrometer drum. Any moyement that tilts the mirror will shi fl the da tum linc image in the eyepiece. The micrometer drum is then rotated to bring the datum within the sight lines and a reading of deflection is thus achic\'cd. Thesc instruments arc also extremely seositiye and are often used to test surface flatness. Suitably fldapted they haye man~- flpplications.
Conclusion This chapter bas provided you \\;th an introduction into the extensive subject of tits and cleamnces. Space would prohibit the inclusion of c\'ef)' check on eycry typc of flircrnft component. TIle informauon contained here, when taken in conjunction \\·; th the information on dimensions, allowances find tolcrn!1ces fine! precision meflsuring tools contained in earlier chflpters, should enable you to be farniliflr with any question posed and to recognise the flppro priflte answer. Before we mm-e onto someth ing completely dirferent, takc fI breflk and hfl\'e a go at the questions that foliow.
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Revision Fits & Clearances Q ues tions 1. On a number drill s ize cha rt , the sm aller driUs b ave the: a) lowest numbers
b) highest numbers
c) c,'cn numbers
2. The British Stand a rd used in the U K for ISO fi ts and Limit s is:
'J
B5308
bJ B55750
cJ
B54500
c) clearance
4. Wh en cond ucting a run-o ut check on a p ropcllcr sh aft tha t has been sub jected to a s hock load , th e d ial test indicator defl ects from -O .04in 10 + O.06in during on e comple te revolution of th e shaft. This indi cales a run-oul o f:
a) O.05in b) O.02in c) O.lin
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5. T h e s h aft SIze tha t \yo uld p rodu ce a cle arance fit di am e te r is:
CHAPTER SIX CLEARANCES
In
a ho le O.8Sin
:I) 0.852
b) 0.85
c) 0.848
6. A numbe r driU s ize tab le h as a ra n ge o f s izes fro m :
a) I roSO b) 1 to 80 c)
I to 26
7. In th e absen ce of oth er limits, the m ax imum permissib le bow limit IS:
a)
1,600
b) 1200 c)
10300
8. On.. a le ttered drill size table, A is: ll.)IJ
the largest size
b) the smallest size c) leflour
9. A typical reaming a lJowan ce fo r h o les up to 3/ 4in diameter
i%-
between:
:I) 0.018in to 0.023in b) O.ODin to O.0 18in c) 0.OO3in to 0.008in
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10. A limit system where the hole d iam ete r is fi xed and th e fit is achi eved by se lecting an a pp ropriate shaft d iame te r is known as a: a) shaft basis system
b) hole basis system c) shaft and hole basis system
11. Whe n a shaft and a hole a re at their ' m aximum m e ta l' conditi on, it m e ans:
a) the largest shaft is fi tted in the smallest hole
b) the smallest shaft is fi tted in the largest hole
c) the largest shaft is fi tted in the largest hole
12. In the system of limits and fi ts, H 7/ h7 m eans: a) Shaft H7 assembled to hole h7 as an interference fit b) Holc 1-1 7 assembled to shaft h 7 as a clearance fit
c)
Hole H7 assembled to shaft h7 as a trnnsirion fit
13. Th e equipm&r t that could be used to check th e run-o ut o fa shaft is :
.,~,b 1-
.\"\\
(
~:\s("
.&..;~, th ree point trammel and a surface table L. b) a pair of V blocks, a OTI and a surface table
,;,.,\"i"
"y\"
c)
eXTernal calipers and a steel rule
14. T h e tap lIsed to prepare a hole fo r a thre ad insert is : a) the same size as the original thread diameter b) supplied by the thread insen manufacmrer c) one size larger than the original thread diameter
370 CopyighlC &rry Co/~ 2006 -2007 All nghtl rmnw
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15. A letter E drill equates a) O.250i.ll dct;imal,
&
CHAPTER S I X CLEARANCE S
to:
1/ -Jin fractional
b) O.625in decima l, S/8in fractio nal c) O.12Sin decimal, 1/ 8 fractional
16. If the limi ts for a shaft are 20.04mm a nd 20.03mm an d for a bole a re 20.06 ml11 a nd 20.05mm, the assembled clearance limits are:
a) 0.0 111111110 O.03mm
b) O.O lmm
to
0.02mm
c) 0.02mm to O.03mm
17. \Vhen th ere is a positive allowance be rween the smaUest poss ible hole a nd the largest possible shaft, the only fit possible is a : a) interfen:ncc fit
b) transition fit c) clearance fit
18. A Dress fit is achieved by: '00 .,\0(:)-• ,:>,..:\!.I!"'1..
\\;\~~~ :0.. \-
-{\~....
(:.§:....
l·\"
-
a) expanding the hole using heal b) contracting the shaft by freezing c) using mechanical force
C;.
-
.,
-0
19. T be depth of a small surface depression resulting from blending corros ion may be accurately m easured using a:
0\11-;:,
"?
a) dialtcst indicator
.~ ,.
b) straight edge and feeler gauges
p5.::
,..
o·
c) th ree poim tranuncJ
,~
~
371 1l'1l'1l'.porl66.tfJ11I
CHAPTER S I X FITS & C LE ARANCES
20. Th e purpose of a re am e r is to:
a) producc OYcrsi7.c holes b) enlargc holcs to obtain an accurate dimension and a smooth fi nish c) drill holes
10
an accurate size
Revision Fits & Clearances Answers B
16.
A
2. C
17.
C
3. A
18.
C
4. A
19.
A
5.
C
20.
B
6.
B
I.
\\~Jb
'l\)\)"":
~\ .\!.~ ..
(P ..
\"\:.tfl-.
"
. (\~'
1\"
I'>'
A
8. B 9. C
r . "
-';
<, -E.
ID. B
I I. A
,
.~
12. C
.~ .~
"
13. B
0
'u
14. B
"'"
'" c§''0 ,
15. A
,
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ELECTRICAL
CABLES
CHAPTER SEVEN & CONNECTORS
Electrical Cables & Connectors Introduction Tills syllabus for this chapter includes : dle installation and testing of electrical cables and CO!U1ectors; bonding techniques and bond testing; crimp tools; connector pin removal and insertion; the testing and installation of co-axial cables and \\>iring protection techruques. Most tests carried out by the maintenance technician will be in support of inspection or faul t diagnOSIS. The testing of circuits should be carried Ollt in the right ordet, wruch is normally: bonding, cOn afllli{y, inslllacion and [hen ;. j functional test. For that reason. we will structure the first part o f tills chapter .in that order by commencing ""i.th bonding techniques and testing before moving on to deal with continuity, insulation and functional testing. Common sense tells us that there must be an assumption tbat you will already be aware of the information on the types and characteristics of cables, connectors and tenninations and o n the operation, function and use of electrical and a\i.onic general test equipment that is contained elsewhere in the syllabus. It does not make sense to repeat all of that related infornlation in its entirety when the syllabus for this particular chapter does not include it. However, tllls chaptet is frequently used .in isolation to support conversion courses. TillS presents us witb a dilemma that we can overcome to some extent by 6WJ.sensing selected extracts from that information where it might have a ~ c 1.a~1trricular bearing on a topic or, more importantly, tempt a 'quizmaster' into " Co\\'"'~ pOSIng you a question on the assumption that 'you should know ie - it does ,.:..\,{":S{. happen. TillS sbould explain why we might occasionally divert to include "s~""'''' . supporting infonnation. The references that follow identify the sources of that (.;:0",,information. ~-\..lso included, are useful referelices from the Civil Aircraft Airworthiness Information and Procedures (CAAIP). Again, relevant in fonnation from these latter references will be included here. "c ~,
~
""?:. ~
"
References Ivfodule 6 (l'vIaterials and Hardware), Chapter 11 (Electrical Cables aiKl 0 ' Connectors): ,~
""'" ...,~
This chapter includes iofonnarion on :
,~
'"
~~
•
Cab le types, construction and characteristics
•
High tension and co-axial cables
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CONNECTORS
•
Crimping
•
Connector types
•
Pins, plugs, sockets, insulators
•
Current and \-olmge rating
•
Coupling
•
Identification codes
b-fodule 7 (J.faintcnance Practices), Chapter 3 (\'(/orkshop Practices): T lus chapter (in tlus module) includes infonnarion on: •
Operation, function and use of electrical general test equipment
Module 7 (Maintenance Practices), Chaprer -I- (Avionic General Test Equipment): This chapter (in this module) includes information on: •
Operation, function and use of avionic general test equipment
Ci\ril Aircraft J\i.f'J?rthiness Information and Procedures (CAAIP): '1(\(16 ...
_C~\~g>art 9, Leaflet 9-1, Bonding and Circuit Testing - ~';I.««' . ~,... >• Part 9, Leaflet 9-3, Cables - Installation and ~laimenance ,~
...~...'l\ -
, '"
•
Part 11, Leaflet 11-5, Aircraft Electrical Cables
Electrical Bonding Techniques T he build-up of static and electro-static induced electrical charges created by fricoon and from lightning strikes bas to be dissipated so dlat the safety of the aU:crafl, crew and passengers is not jeopardised. A failure to do rlus risks structural or component damage. malfunction of equipmem and the creation of high potential differences in the aircraft. Bonding is the electrical interconnection of metallic aircraft parts to enable me safe distribution, normally at earth potential, of electrical charges and currents and to provide a low resistance path for electric current in earth return systems. Bonding reduces the risk of electric shocks from the electrical power supply system and
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the risk of electro-magnetic interference with the operation o f aircraft radio and Ila,igational aids . Tills topic atuacts a number of questions so you will fInd a lot of detail included here. Better La have too much information lhan too little! I
Bonding Conductors A ircraft bonding conductors are classified as: primary conductors that are designed to carry and dissipate lightning strikes, and secondary conductors that senT the remaining bonding functions. The aircraft metal structure and its equipment may also provide an inherent bonding path that is supplemented by the provision of additional conductors . T he most likely question to be asked is why a primary boncling conductor has such a large cross-sectional area. The answer is that it is designed to carry a lightning strike. Notwithstanding this, 1 will give you in formation on me size regulations for primary and secondary conductors, extracted from BCARs - in case some 'quizmaster' is tempted to go down that route. Hopefully not! Primary Conductors These have a large cross-sectional area because they arc designed to carry lightning strikes. Copper primary conductors must have a cross-sectional area not less than 0.0045 sq in (0.25in wide by 26swg). In cases where a slOgle conductor is likely to carry me full discharge from an isolated part of the aircraft, the cross-sectional area must not bc les ~ than 0.009 sq in (0.5in wide by 26swg). Primary conductors made from aluminium must have a relatively larger cross-sectional arca dlat gives a current surge capacity equal to that of a coppc[...ConciuctOr. )(\1.)1
'""),§.:i:-, - . co\\c"C'- - Secondary Conductors <.
,=c'
.\0:' ~o"'X" .
~.
'?I"-{{....
T hese are designed to dissipate charges other than those created by lightning strikes. A braided copper secondary conductOr must have a cross-sectional area not less than 0.001 sq in (+-I- strands of 39swg). Single solid Wl1:e copper secondary earth conductors must not be smaller than 18 SWG. ..-..,
~
~.
Additional Conductors
\\1hen additional copper conductors are used to prO\ride or to supplement th~ primary bonding path through the aircraft str~cture or its equipment, thef must have a cross-sectional area not less than 3 mm2 (0.00..J.5in2). If one of these conductors is likely to carry the. fuU disehar~e from an isolated part of t§l: aircraft, the cross-sectional area mUSt Ilor be less than 6 mm2 (O.OO?ii?). Aluminium Primary conductors must have a telatively larger cross-sect;ig nal area that gives an eqwyalent current surge capacity to that of a s&pper conductor. '
'u
& ,
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CHAPTER SEVEN ELECTRICAL CABLES
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Additional secondary bonding conductors made of braided copper should nor be smaller than 1 mm l (0.00 I5inl). \Then a single copper wire is used, it should nm be less (han 1.2 mm 2 (0.OOI8inl).
Bonding of Met al Aircraft T he skin of an all-metal constructed aircraft will prO\"ide adequate protection against a lighllung discha rge pro'\;ding there is sacisfacrory electrical contact al the skin joints. 111e electrical resistance across a joint should be less than 0.05 o h m for it to be considered as being a satisfactory contact. Ir is worth noting here thaI an odic films are non-conductive so ther do nor provide metal-lO" metal contact. The earth system of a metal aircraft is nonnally considered to consist of the aircraft strucnt rc. There must be a means of aUlomatically connecting the aircraft structure with the ground when the aircraft lands. "I1lls con nection is normally achieved by the prO\;sion of an electrically conducting nose or tail wheel tyre. 11le tyre is impregnated \1,;th a conducting material th:u provides a low resist:\nce path to the ground. Alternatively, an earth strap or discharge wick may be fitted so that it trails from the landing gcar to make contact \1,~th the ground on touchdown.
Bonding of Non-Metallic Aircraft Non-metallic aircraft of either wooden or plastic composite construction incorpora te a ' C::lgC ' made up of a network of thin copper strips att ached to lhe fuselage longerons and the wing spars tbat have a current surge capaciry eClual to that of;., Rl1mary conductOrs. T he 'cage' fomlS a part of the stnlctural configuqlg.CIl"' of the aircra ft and all metal parts such as the landing gear, e"gin'ef~nd control hinges etc. are connected to it by flexible, braided copper ~>{ships. "I1lls 'cage' forms the earth system and all the main Strips are connected . \~............, TOgether in a network that tenninates at the earth point of the radio and at a -1\....... conducting nose or tail wheel tyre or a trailing eanh Strnp or discharge wick I.. attached to the landing gear. The system ensures that no potencial difference will e..xist between metal pam and it \\--ill discharge any static charge to earth on la nding. Glass fibre and many composite materials do not conduct dectricity. A buildup of static electricity on the sucfaces of parts made from these materials is reduced or removed by the application of a conducu-"e paint fin ish during manufacture.
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Bonding - Refuelling/De-Fuelling Operations \Vhen conducong aircraft refuelling/de- fuelling operations, it is imporrant that the fuel tanker and lhe aircraft are at a common eiectric;li potencial. The fuel tanker is earthed to ground through an earth strap permanently attached to the vehicle. The aircraft is earmed to ground through a conducting nose or tail whcel or an approved earth point. Prior to connecting the fuel hose, the alrcraft and the fuel tanker arc bonded together through a bonding lead that connects from me tanker to an approved bonding attachment point on the aircraft. 111C fuel hose is also equipped with a bonding ,vire that mus t be connected to an approved point on the aircraft before connecting the fuel hose to the aircraft fuelling point. The bonding leads remain in place until aft er the fue! hose has been disconnected on completion of the refuel/ de-fuel operauon.
Bonding Connections T h e number and the location of the aircraft bonding connections are very important and you must consult the aircraft drawings if in any doubt. A good example that often draws a question is that of an aircraft engine that is fitted on ro non-conducting mountings. The engine Jhould be equip ped with (\VO primary bonding conductors, one each side of the engine, to connect Jt to the aircraft stnlcrure. It is important that, \vhen making or renewing a bonding connection, the correct conductor with the specified current rntirig is used. Tt must be borne 10 mind that d1e conductor may have been designetl to carry high loads resulting from particular circuit fault conditions . All ~ding connections must be properly secured ro prevent lOterrruttent 1\.)~ntacr that may be caused by "ibration. In bonding systems, intennittent . (.o\\C~e" contact is considercd worse tha~ no contact at all . .,.,~':\.~(': I ,\.....<.. y" \\Then bonding connecnons have to be made across moving parts or parts (.ci-X'.\'\'f where there is relative moYcmenr bccausc of vibration, a flexible 'bonding jlunper ' is used. Care must be taken to fit these connections so that they do not foul or interfere ,..;rith the electrical ~r mechamcal operation 'Bf neighbouring equipment. A good example would be the flexible bonding connections from flying control surfaces . Thesb should be kept as short possible ro a,Toid fouling the control surface mechanism whilst penni tung a ~ and free range of movement. These particular connectors should not
at
I
tinned.
bf =;
.~ .~
The lay o f all primary conduc[Qrs must be as straight as possib le and sh~ bends must bc avoidcd. Primary conductors b ften consist of flat bni.@ed copper wire strips. The cross-sectional areas jof solid bonding strip~l..and braided bonding cords that have been supplied by lTl.'lnufactuters are sp&ified on data sheets. If you have to renew a connJction and cannot ~eh the specification of the original conductor, you must use a replaceme~t that is
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CHAPTER SEVEN ELECTRICAL CABLES
&
CONNECTOR S
made of the same material as the original but with a larger cross-seceional area. Bonding Jumpers consist of cormgated flexible Strips or braided copper or aluminium cords that have a tag or lug at each end. The tags or lugs are crimped onto the braided cord and it is essential that the correct crimping (Dol be used when fitting these. \'\'hen crimping tags or lugs to aluminium braided cord, the connections should be first treated with anci-oxidanc crimping compound that comprises 50% by weight zinc oxide in white petroleum jelly. When installing 'Bonding jumpers', care must be taken to ensure tha I tbey are not roo taut or too slack and, as ahead)' stated, they must not interfere with the electrical or mechanical operation of equipment. I f the fitting instnlctions call for the t.1.gs or lugs to be soldered onto a copper braided cord, pa rticular care is required because overheating and subsequent cooling \vill cause britdeness and a loss of flexibility in the cord up to one inch from the tag or Jug. This is caused by the capillary action of the braid that 'draws' the molten solder along the cord. Soldering sbould be carried out using a resin flm;.
A number of types of joint on an aircraft are self-bonding. Ri\-eted or bolted metal-to-metal jointS, clamped or bolted fitrings, metal panel hinges and cowlings joined by metal fasteners and latches fit this description providing they are not separated by anodic films or non-conductive compounds or gaskets. Control surface hinge bearings may also provide a satisfactory bonding path prO\;ding the bearing is metal to metal or is packed with conducting grease. High-pressure flexible pipes are normally self-bonding through their reinforcing layer of wire btaiding. A bonding test rna}' be carried out between the pipe end couplings and between an end coupling and a component connection. Rcs~~f3T1ce should not exceed O.050hm. "f)6 - ~
>1.~
S ~~~!ectrical
cables are screened with metal braiding
to
reduce or prevent
~.}.{rhcm causing or being affected by electro-magnetic interference. One end of
. ~,\. .... (''''~'"";<'
the screening only is nonnally single point bonded to earth so that the metal sheath cannOl provide a path for a circulating current or ' carch loop ' under normal or earth fault conditions. Confwnation that an acceptable bonding patb exists through the joints we have discussed may be achieved by carrying Out a bonding test.
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ELE C TRI CAL
CA BLE S
CH A PTER SEVEN & CONN E CTO RS
Flexible Bonding Connec tors \\le have already discussed 'Bonding J umpers', \vhich are the flexible bonding connectors used as a bridge benveen munng bomponems and the aircraft structure. There is also a need for ~ flexible bonding connec[Or to bridge rigid pipes that are joined by flexible rubber hose connections. Metal clips are poSItioned on the rigid pipes 1( 2in away from each side of the hose connection. A bonding jumper in the fonn of a Icorrugated strip or a braided \vire is then attached to the clips [0 create th e conducting bridge. The connector must not be tucked under the clips; it must be connected to them securely by bolted tags. The metal pipe surface under each clip must be cleaned to ensure good metal-to -metal contact and the area should be Ie-protected after the clip has been fitted. Bonding Strip
,;:::;~ :....b
i='
L
T
~
::;;~
-
-....
i-.= ~
Figure 7. 1 - Example of a Flexible Bonding COll1lcctor
If a flexible bonding coonection breaks, it must not be repaired. No attempt s ho uld ~be
made to repair a bonding strip or cord by soldering it wgether; it dHustl~e replaced with a new conductor. Each rigid pipe run that has been ~\C.,?-c '}.G' bonded in this way must be earthed at each end of the run and particularly '\."t<' C..... within a radius of 8ft of any unscreened radio eguipmcnt or aerials where the , ",. · - f:> t apart. <.. ~ eartI 1 connectlons must not be morc tIlan .,~ I ~<. ':' U ...<;; · When bonding connections arc made between differing m aterials that have the potential to create elecrrolytic corrosiotJ between them, a corrosion washer.-4 often used at the connection points to reduce the ris k. Thes e washers a:b: manufactured from materials that have a reduced electrode potential in re1atiO"a . to the materials they contact. \",\lith this in minU, it is important to use tKt correct specification washers. r,;<
'v §,
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CHAPTER SEVEN ELECTRICAL CABLES
&
CONNECTOR S
E a rth Terminals The earth system on a metal aircraft connects to the strUcrurc. There 'Will normally bc a number of main eanh point connections on the structure. The leads That connect to these terminals are normally insulated cablcs thar ha'·c crimped tcrminal lags that connect onto lugs on the eanh terminals. It is imponanr thaI Ihese terminal assemblies are checked for electrical resistancc between them and the strucnJre. Thcy arc classic corrosion points; think how often the bane!,)' earth connection on a car gives you troublel ·11'e maximum resistance value is specified in aircraft maintenance manual, for example O.0250hm. Additionally, whcn an earth tenninal is required to ca rry a specified live clcClricalload as a part of a circuit opemtion, a milli-volt drop [CSt must bc carried Out across it after connection is made. \'\'here a resistance value is found to exceed the limit, the terminal assembly should bc dismantlcd and the contacring faces should be cleaned with a fine abrash·e such as aluminium wool. Thc lcnninal is then re-made using new corrosion washers and is rcprotcctcd. \'(Ihen earth connections arc initially made ontO the aircraft S(n1cture, it is often necessary to remO\"e non-conducti,-e protective coatings to establish a sound connection. Where this is done, the approved anti-oxidising and scaling treatment should be applied. Protective coatings [hat arc known to be nOIlconductive include priming and finishing paintS, anodic fLims, varnishes, chromic and phosphate coatings. i>.fctallic coatings such as rin and cadmium arc conductive and need not be disturbed. To sum up, when an earth tcrminal has been reassembled or replaced, a bonding {cs t and a milli-l'olt drop test should be carried out. The conduct of a milli-voit drop test requires a current flow and tills is discussed later in this chapter. '1r~,f
'l\'t\.,G
I\{aih E arth System \ ....:\0\'"
In the a.bscnce of a fuse or a circuit breaker, there is likelihood that the main earth s)'stem could fonn part of an unprotected short circuit under an electrical circuit faulr condition. In another case, the syscem may be designed co pass a specified e1eccricalload to eanh under normal operation. In [he first case, the cross-secrional area of the system connectors and any associated connections to them must be capable of carrying the short circuit to earth without the risk of o,"erheating or fire. In the second case, the system must be capable of passing the electrical current withom creating an excessive voltage drop and withom overheating.
Resistance Values The Civil Aviation Authority has laid down maximum resistance values fo r primary and secondary bonding conductors. These are:
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..,o ,
~
~
ELECTR I CAL
CAB L ES
CHAPT E R SEVE N & C ONNECT O R S
Maximum
Classifi c ation
Tes t Conditi o n
Prim:!ry
Bl:lwccn cxt renmies of the fixed portions of :lircraft of non·mct:!llic o r c ~ ll1pos ite construcuon.
--
Be[wCt.'n e'l;trClllIties of the fixed portions of metalhc aircraft
0.050nm
--
Between bonded components and portions of main earth syslems to which they arc conncCted.
0.U5 ohm
Between metallic pans norm:llly in contaCt with flammable liquids and rhc mam earth sr~ rcm. :lnd also oerwccn rhe parts themselves.
I ohm"
Seeondan'
Resistance Estil11:Hed and stated b,- the manufacturer
--
Between all isolated cnncluc(lng pans which rna\' be subject 10 appreciable dectroH.,UC charglllg and the Ill:!in earth Sl"Slem. lpa rtS hanng :111 area wearcr than 3m l and lon~er than 3in)
The lesser ,·:tltle oi 0.5 megohm or 100 kilohrns per sl:! flof su rface area
--
l3clwccn cqwpment supplied from an unearthed system of an\" \'olt:lj.!:c and the rn:lln eanh s\"Stem.
I ohm"
--
Between eqwprnent cont:l.lmng Circuits carrying 5U "olts (rms or de) or more. and the malO earth sy.:;tem.
1 ohm·
Table 7. 1 · Bo nding ResiSlllnee Values (EXlrael from CAAlP LcafleI 9.1)
"U'-f
"l\,,-,b - ...
. "ci!.":'" .., The I ohm "alues shown in the secondary table allow for the maximum _C(w · :\.:\.<;<' resistance of any cable that may be used in these examples. Howe\'er, the ... '" '\:, resistance across a single contact should not exceed 0.050hm. The odd ......<"~:\. quesrion arises on the maximum bonding resistance values, for example, (,~'\ maximum resistance for lhe primary circuit classi fi cation is 0.05 ohm and the maximum value for the secondary circuit classification is genemlly I OIU¥l. However, lhc resistance between a flexible hose and the component % connects to for example should DOt exceed 0.050hm. ''S
3-
Bond Testing Bonding T eSt Set
1\ bODding tester consists of a low range (0 - O. l ohm) ratiometer~'type
ohmmeter incorporating a 1.2V nickel alkaline cell contained in a \",&den carrying case. The test set incorporates twO flexible cables, one of 69,lt length and another of 6ft length, thar are connected to the instrument with'iplug and socket connectors. The 60ft cable has a single spike probe and the 6f[ cable has
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CON N EC T OR S
&
a double spike probe. The length of these twO cables is critical :lnd rheir resistance is accommoda ted for in the calibration of the instrumenf. They must nOI be altered or modified in any way. If a cable is damaged, rhe teSI SCI and rhe cables must be returned to the manufacturer for repair and fe-calibration. T(.:ak Carrying Case
Iuuiometer Type
Figure 7.2 - Bonding Tester
Prep3ration ror Bonding Test The state of charge of the nickel alkaline cell should be checked by connecting I.he twO spikes of the 6ft cable probe together with a suitable shorting link and obscn·ing that the instnunent pointer deflects fUU-SC.1/C. Following this check, the twO spikes o f the 6ft cable probe should be shorted by pbcing the single spike of the 60ft cable probe across them to check (hal the pointer tlligns with the zC£o mIlrkon the instrument scale. 'll'\.\ b
,)\1\.1 -
~~I~~~ct 0 1 Bonding Test
<,
\"\~tThe 60ft cable should be connected first to a main earth poim, called a bond -.~~...... darum point, at the position shown in the Aircraft Maintenance Manual (. ..'-;: . Because the lead is 60ft long there may be a need to identify further bond datum points for successive tests on a large aircraft. I f this is the case, the resistance \"alue between the main earrh points needs to be checked before proceeding [0 check the furthest point. <.,.
Af[er [he 60ft lead has been connected, the twO spikes of the 6ft cable arc then brought into contact with the aircraft structure at selected points specified in the maintenance manual. This enables readings to be raken of (he bonding resistance in ohms of the strucrnre between the probes. TIle probe spikes have [0 penetrn.tc through any non-conducting surface finish [0 make effective contact with the metal strucrnre. Anodic films arc non-conductive so the spikes have to penetrate through them. Corrosion rends to fo rm around bonding and earth system connections and is often the cause of high resistance readings. \'(!here this occurs, the connections should be stripped and cleaned. It is imporL'lnt [Q restore the surface finish at any contact points where it has been penetrated or abrn.ded.
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CABLES
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SEV E N
CONNECT O R S
The bonding tester should be periodically checked for accuracy. This is done by using the tester to measure standard test resistors. 111ere are three standard test resistors, O.02ohm, O.06o hm and O.l obm. The accuracy of the bonding tester readings should be \v;dun 10% o f these values. ~r\ pan from replacing tbe alkaline cell and checking the tester fat accuracy, dlere is no other selvicing pennined on the test set. N o attempt should be made to open the case or rep air or modify the cables. If a fault develops, the complete test set, including the cables, sho uld be returned to the manufacturer for repair and re ~ calibration. There is a requirement to check the resistance between the aircraft main earth sys tem and the ground. This is normally carried out by positioning a metal plate under the conducting nose or tail wheel and 1connecting a 250V or a 500V resistance tester to the aircraft main earth system and to the metal plate . The resistance reading should not exceed 10 megohms. An alternative type o f bond tester consists of a special milli-ohmmeter that bas four terminals and two flexible cables, each havin g a double spiked probe. 'l1us instmment has a self-contained battery that delivers a two amp current. \'(then the probes are brought into contact with the aircra ft stmcture, the instrument measures the voltage drop and compares tbis with the current flowing ro gJ.ve a reading of the bonding resistance of the structure between the probes. Befo re using dus type of test set, the tWO probes should be placed close together across a low resistance conductor to check that the instrumem pointer will read 2cro.
Circuit Testing - General For those of us who are of a purely mechanical persuasion, it might help to temind~oursel\'es a few basic terms before describing the various tests. r")\JI) '1(\ill) - -
\\'!,'Z...c --
,~\....'1>-~~~ , l<' ~'I..
~~\~
(p'X
(0" -,
~eries
and Parallel Circuits
1\ series circuit is a circuit where all the cables and components are connected to fonn a continuous chain so that there is only one possible current flow path across the terminals o f the voltage source . .A p arallel circuit on the other hand, is a circuit where twO or more current flow pa ths e...ust between tJ:l-e terminals o f the voltage source. The more alternatives there are, the l~ resistance there will be to the flow of current. j. r~
==jj
,s"
.~
'"--
'~ Series
Pa rall cl
Figure 7.3 - Circuits
1
P
":
0"
'~
Series Paralli:!
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'u "
'"
,
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383 Copyigbt :0 BrIl"lJ' Col/(U 2006 - 2007 All rights wurvtd
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CHAPTER SEVEN ELECTRICAL CABLES
&
CONNECTORS
O pe n a nd Short Ci rc uits
A short circllit is a term used to describe an abnormally low rcsisrancc path. The path may exist between two adjoining circuits, across a component in a ci rcuit, or it may be between a circuit and earth. \'('hen a short circuit occurs, resistance drops and current flow increases. The increased current now may create o\'crhearing o r sparking and may blow the circuit fuse. An Opl.'n circ uit on the olher hand, is a term used to describe a circuit (har is not complete because of a break or an unintentional disconnection. Current cannot now in such a circui l unless it has an alternati"e route through a parallel branch. D e tecting Short Circ uits When a short circuit occurs, the fall in resistance and the increased current now will oftell blow rhe circuit fuse and open the circuit. Failing rhis, it could bum our another component and cause an open circuit. \'('hen there arc a number of resistors in circuit, these mar prevent the current reaching the level where the fuse will blow and the circuit willscill be li\·e.
Figure 7.4 - Volnnetc r in all Open Cire lli1
I f the circ~{bi s 1!dii live, a voltmeter can be used to detect a componenl that has *,~W'~'il OUt. There is no need to break into a circuit when using a ~~
I
c;,,!L be Isol:ucd
InliSl
Fig ure 7.5 - Ohmmete r in an Open Cireuil
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Affrights mmItd
ELECTRICA L
CABLES
CHAPTER SEVEN & CONNECTORS
If the circlln is not live, an ohmmeter can be used to detect a resistor that has a short circuit across it. Again, there is no need to break into a circuit to use an ohmmeter unless you need (Q disconnect a parallel branch of the circuit An o hmmeter will register zero when placed in parallel across the shorted resistor. If the resis tor is a part of a parallel circuit, it must be isolated from the other paths or the ohmmeter will indicate the resistance of them. Will Read the
__ Reads Zero
r-vV\/VI'--, Break Circuit here to Obtain True Reading
Detecting a Short Problem of Detecting Open Circuit in a Series Circuit Circuit in a Parallel Circuit I
Figure 7.6 - Examples of use of Ohnuneter
The connection of an ammeter does require you to break the circuit in order to connect the instrument in series with it. If an amq:1eter is placed in series with a circuit thar has shorted, it will register zero if the fuse has blown or will give a high reading if the circuit is scilJ li.e. Open Circuit
No Current Flow
Shorr Circuit
l
r L
I
Zero Cading
~
A
Open Cireui t Zero Reading
(FUSI:
Reduced Current Flow
A
Blown) ,,~
!-<
~,
'
Increase Currenr Flow Blows Fuse
Figure 7.7 - Examples of use of Ammeter
,
p
~
"""
W/e \,,;ll can now move on and look at the testing of circuits . Remcmbe~ , the correct sequence for tests is: bondJilg, continuity, insulation and tlien a fUDctionaileSt. \Ve have dealt with bonding tests so we will now m~xC on to . . . ~ conuOluty tesnng. '7
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CHAPTER S EVEN ELECTRICA L C ABLE S
&
C O NNECTOR S
Co n tinu ity Testing A break in a cable or at a connection can be detected using a continuiry tester consisting of a low voltage battery (2.5'V) connected to either a lamp or a low reading voltmeter. If the circuit has a known resismnce then a contin uiry check could also be ca rried out using a low range ohmmeter or a multimctcr. It is imporrant that whatcve r method is used, the voltage of the leSler must be kept low to avoid damaging circuit components such as fuses and lights. It is imporrant that the circuit being tested is disconnected from ItS power source. If a voltmeter or lamp is being used to check continuity in an open circuit that is connected to its own power source, there will be a problem. If the voluneter for example happens to be connected across a break, it will act as a parallel circuit or shunt and register the ,'oitage of the circuit power source. T his is why the continuity tester has its own banery power.
P reparation for Continu ity T est The main electrical powe r supply to the circuit must be disconnected. -nle circuit fuses should be checked to ensure ther are present and intact. 'The circuit should be checked to establish that there are no other disconnections. All switches and circuit breakers should be closed to complete the circuil. Condu ct of Continuity Test A low \'ohage continuity test should be carried ou[ by working progressively through the circuit. TIle testing should commence at the circuit fuse :'Ind progress to the equipmen t. Parallel circuits should be tesced sys tematically keeping the number of disconnections to the minimum required. Remember chat it is pointless attempting to check several low resistance paths in parallel togerher, they need to be disconnected and checked independently. I
I Iow
C I lIi"lIncter '00~ can alb k concnwry . . .m a ClfCUll. · · 1n t1·liS so e use d to cIlec'
r~n$t\u
ca~\'tl~ circuit must again be disconnected from its power source. Remember,
\):>'tShmmerers have their own battery. I f the ohmmeter is connected across a break, it will register infinity. If it is connected across any other good component in the circuit, it "ill gi\'e a resistance indication. To remind YOli. care needs to be taken when an ohmmeter is used to check for continuity in a parallel circuit. If the ohmmeter is pbced across an open circuit in one of the paths. it will read the reSistance of the components in the alternate path. The circuit to the open path needs to be broken to isobte it and then the ohmmeter wiU read infinity, which tells you that there is :'In open circuit of infini te resis tance. Ammeters arc not suitable for continuity testing as they have to be connected in series and they rei), on having the circuit power source connected. I lowever, if an ammete r is placed in an open series circuit, it will register zero current flow. I f it is placed in the intact section of a live pa rallel circuit, if will give an indication despite the fact that the alternate path may be open circuitcd.
386 UP)'1i!/J1 0 Batry u/~ :!006 . ~oo -
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EL EC T R I CAL
CAB LE S
CHAP T ER SEVEN & C ONNECTORS
Millivolt Drop Test Loose o r corroded connections or poorly made swaged connections often c reate high resistance in high current circuits. You may recall that '\ve referred to tlus p ro blem when discussing corrosion at bonding and earth connections. These faults can cause low volragc at the tenninal connections to circuit equipment and the connections may get hot as a result. It might be possible to detect the voltage drop by progressively checking through the circuit with an accurately calibrated voltmeter. However, a millivoltmeter may be connected to each side o f a suspect connection and the "\""oltagb drop acro~s it when a given currem IS passe~ ~ay the? _be c.alculatcd. ~hc dlop shoul~ be. compar.ed with the allowable hmu spcClt1cd Hl the maJntenance publicanon, rypKally 5
miJfkol{s / 10 am p s current flow. For continuously rated circuits, the normal operating current should be flowing in the circuit during the test. For short rated circuits, the current should be reduced using suitable resistors to prevent overheating. If the component is a relay, then the relay has to be closed with the power on in order to conduc t dlC test.
Insulation Resistance Testing The approved aircraft test schedules for the particular aircraft must be consulted and complied with when conducting th ese tes ts. \X' here specified in
the Aircraft ivlaintenance .Manual, eguipmenr insrhlIed in an aircraft is normally tested using a 2S0V insulation tester that has a maximum voltage control that will preyent the test voltage exceeding 300V . Ail systems having ,-oitages of 3DV or over that are related to the safe operation of the aircraft must be tes ted separately. Circuits other than these can be connected in groups for test bur th e number of circuits so joined is to be limited w here the insulation resis tance 1i"J& fJ~d to be less than the minimwn specified value. _
'" ,
'\.b ~;.
, 5'''1'
( ~~ .
,\~C,!;. ~
_lP\\ ~.~,s5'
~
\\lhen an insulation test has been carried out, a functiooal check shouJd be carried out on all the sen1ces in,olved in the test. ShouJd a fault be foun d and rectified, a furthe r insulation test and functional t cst should be carried o ut on the affected circuits .
~.
I
T h e resuJts of insulation resIstaoce tests are of little value unless thev can 6} compared to rcsuJts obtained ducing earlier t~S ts. Insulation resi~rance 1% affected by humidity and temperarure. If the l aircraft has been in dam-g conditions for any length of time before the test, fower than nonnally expecte~ readings may be obtained. The amb ien t temperature and hllmiditf! con diu'ons should always be recorded along insulation resismnce te?'t results. T his enables a comparison to be made when a future test indicate~"1t drop in resistance.
+th
Preparation for Insu la tion Resistance T est
'&""'"
.~
'"
I have extracted and condensed the follo\Vin~ information fromciLAAIPs , because many questioners use these as a referenct;.
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C H APTER SEVEN E L ECTRICAL C ABLES
&
CONNEC T OR S
The system should be disconnected from the aircraft battery and an}' other external power supply. Circuit breakers should be closed. The powel' selection switch should be set to the nonnal in-flight position. Circuit switches and micro-switches should be made. Dimmer switches should be seL to the minimum resistance position. Any ancillary systems that may be connected to the system under test should be disconnected from it. Ths includes generators, acruarors, capacitors, capacitive filters, semi-conductors, voltage regulator coils, instruments, fue excinguishers etc. \\'7here rhe insulation resistance of these items is less than 2 megohms. they may be disconnected at the earth lead leaving the component in circuit. Components such :1 capacitive filte rs should be disconnected to pre\"ent them from being damaged. Cutours and relays that would normally be open should be bridged across their terminals to ensure continuiry. Suppressors should be disconnec[Cd from the circuit :l.Od bridged to ensure continuity. When a suppressor cannot be bridged and it is has plug and socket connections, the circuit capacitors must be discharged before re-connecting them back into the system to avoid arcing at the pins. All disconnected components that are connected in series must be bridged to ensure continuity in rhe circuit. System component controls should be set to the positions specified in the test procedure. Conducting lh e Test \X' hen testing single-pole circuits, the line connection from the tester should be connected to the cable or group of cables and the earth connection should be connected [0 canh. When it is pennined to test a bunch of cables together, the group should be kept small. I f the insulation resistance of a group is below the minimum specified value, the number of cables in the group should be progressivcl), reduced until the defective cables arc identified.
If the system is t'Wo-pole (older aircraft), the test can be conducted by first connecting the ~R'H! [ to each battery lead and measuring the resistance between them a~.~\dllc n c hecking the resistance between each battery lead and earth. T4..Q~'uit fuses would be left in during this test. \.y\.....'
, '.",
"
(\~....
Minimum Insulation Resistance Values There are minimum insulation resistance values that have been issued for guidance anI),. These values are likely to be considerably less than the acrual values obtained on new equipment. I have extracted a table of these values from CAA IPs as the odd 'luestion does rum up that requires you to know them.
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ELECTRICAL
CABLE S
CH A PTER S EVEN & CO NNECTOR S
Wiring (including accessories for jointing :md Icrminatin,l() E!l~illc n~ccl!es, undercJlrri~j!c wheel wclls and other lo(~tions expo$eJ [ 0 \\"e~ther OT extn:ml'S of (emp~ r~ rurc,
Galk y ;md Othl'r nnn-essf'mial :md in
sen'ice~,
lighting, signalling
Other SCf'nees
2 megohms 5 megohms
10 nwguhms
W iring acceliliories .done (terminal blocks, C<)nllcclOr5, plugs and sockets) Between terminals Betwccn tcnninals bunched together and earth,
1()() me).tuhms 2{~1
megohms / terminals
~(),o(
Rot;lt in g lI.bch ine ry ROlminl! m:tch1l1cry
The gfe:ltef uf: rated \'o)t:tgc/ 150 :lnd 0.5 111l:J.!ohms
All olhcr equipment
\11 0I11er equipment (including insrrumems;
:; megohms
T a ble 7.2 - Minimum Resis ta nce Values (E x t:ract from CAAJPs Lcafl cI9- t )
Functional Tests 1\ funcliona l test of a system is required following circuit !cs ting. \Vhe rc\'er
possible, fu nctio nal tests should be carricd out using an exterm li g ro und (':/CCLric;" p o w er supply connected to the aircra fl ground supply connector. Some circuit s, such flS standby lighting, fire powered by thc aircraft battcry and this would be used as the power source for functional testing in thcse cases . , i-~ll 'hWessary precautions in respect of personnel and aircraft safety should be _(.o\\c,,?-c ,J taken. Functional tests must be carried out in accordance with the approved \.....,).~ test schedules in the Aircraft l\{aimenancc p,{anual.
....,\ ,........
,,{ (..0 '\ .
c
Some circuits depend on the inherent resistance value of the circuit for lheir successful operation. \\'here this is the case, tbe resistance of the circuit should be measured with a low reading obmmctcr to Cl;tablish that the resismnce...is within limils.
..,"
.~
Afler the functional test of a system has been carried our, the c.!ectrical circuishould be s\vitched off, the fuse should then be removed or the circuit breaker:: trippcd. The circuit switch should then be selected on again and a check made , to vcrify that the circuit has actually been isolated. .;J .~
o·
Fuses
""
f! o
"-
I-laving mentioned fuscs in the last section, a q uestion sometimcs arisWbn a description given to some cartridge fuses in circllirs. Fuscs that brca~,.:al vcry high fault currents are now described as HBC fuses (high Im::a king f apaClty). T hcse used to be referred to as being HRC fuses (high rupturing capacity).
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CHAPTER SEVEN ELECTRICAL CABLES
&
CONNECTORS
Aircraft Electrical Cable Classifications \Vie arc going to sidetrack from the syllabus for a while to examine rypes of electrical cable and their conductors and some of the faults that occur. T his is an area where questions arc sometimes asked on the 'you arc assumed 10 know it' basis. II docs seem logical that wben faults are revealed during tests, there is usually a cause. \'\fe will commence by looking at the classification of cables. Airframe Cab les Airframe cables arc constructed to be robust enough to meet the requirements of 'Open' airfra me and power plant wiring. In terconnect Cables These cables arc used in prorected areas such as in the inre rconnection of racked equipment. They are usuallr installed within an assembly that is then fined to the aircraft so rhey are Dot subjected to stresses and strains such as being 'pulled through' cable looms etc. They ha\'e trunner insulation than airframe cables and this saves weight and space and makes it easier to rum looms through small radii when routing them [0 connectors. Americans refer to these type cables as 'hook-up' cables.
Equipment '\'Vire' This rype of c:lble is referred to as 'wire' and it is used inside equipmenr. It is very flexible and is re:ldily soldered. Pire Resistant Cables This type of cable is designed to retain a specified level of electrical insulation for a minimum period of {i"e minutes when exposed to a fire. "l)O-
~i~~1~[~8OC&bl~S
... ~~s'fhis type of cable is designed to operate for a minimum perio d of fiftcen ~ minutes in a fire and is used in designated fire zones. Multieore Screened and Jacketed Cables Airframe and imerconnect cables are sometimes supplied in multicore fonn up to four cores. 111e cores are lightly twisted together and are usually jacketed o r sheathed. Some are jacketed and then screened with brnid. The cores are nonnally coloured to assist identification. Data Bus D ata bus cables arc designed to specifica tions set by (he Design AUlhority and may only be repl:lced by cables of the same specificacions.
390 upynghl C Barry C,lk§ :!006 . 20(r All rights mtmd
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ELECTRIC A L
C A BLES
CHA P TER SEVEN & CONNECTORS
Conductor Plating
tb
Conductors uscd in electrical cables are plated improve thei r resistance to corrosion and to aid the making of c~ble tenninations . The plating material also h~s an important bearing on the temperature r~ting of the cable.
P lating
Ma.ximum Con ti n uo us Temperature
Tin plated co pper
135°C
Silver plated copper
20DoC
Nickel plated copper
26DoC
Nickel Clad plated copper (used on fire -rcsistant cable)
260°C
T able 7.3 - Co nd liClor P lating
Aircraft Electrical Cable Failures These are but a few of the possible causes for cable failure. They are included because 'quizmasters' particulacly like to ask you about them. Abrasion Tlus describes the wearing th.rough of cable insulation at points where the cable rubs on a neighbouring cable or on a part of tbe aircraft structure. Tt can occtq,il\,'hen the insulation and flexibility of a cable is less stiff tha n that of its oC l.Q\t~~;acting neighbour. It is particulacly pre-.;-alent in regions of high vibranon .
. O·;\l.C:~ .~ \...,,~s0:. \~"\\. ,;.'- . L:-~'\ .
Glycol Fires If de-icing fluid contallung glycol contacts silver plated electrical conductors it can cause a reaction that \.vil.I start an electrical fire. It is for tlus reason that silver plated conductors are not used in regions where de-icing fluid may €"e -0 present.
-.
r-,. "
.
~
Knuckling Through
co T lus is damage created by an electrical cable corc metal conductor pushi4i; through the insulation cover. It is caused by applying excessive 'pull throu~ ' force when installing the cable. R..' 0-
Red P lague
1'f
&
~
Cables with silver plated conductors will suffer from tills defect if t~plating becomes damaged and is subsequently exposed to moisture. It i~for tillS
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CHAPTER SE V EN ELECTRICAL C A BLES
&
CONNECTORS
reason that silver plated conductors are considered unsuited for usc in pressurised regions of rhe rurcrnft.
Ull-
Wet Arc Tracking
This is the fail ure of cable insulation resistance resulting from 'tracking' when the cable is subjected to fl uid contamination. T he cause has been attributed to insulation damage resulting from h ot stamp prindng informaLion on ie. ''\Ie can now move on TO the next item on the syllabus, which is the usc of crimp TOols.
Use of Crimp Tools: Hand & Hydraulic Crimped Connections The most common type of cable termination is the crimped or 'solder less' conneccion. "nle connection is made by compressing a metal remlination OntO the prepared end of a cable so that there is a firm and close metal-to-metal contacl between the cable conductor aod the termination. A correctly crimped tennination should produce a joint that has the same tensile strength as the cable conductor. A crimp termination consists of two parts, the crimping 'barrel :tlld tongue' and in some cases a pre-insulated copper sleeve that is designed to fit over the ccim ping ba rrel at one end and to grip and support the cable insul:t tion at the other end. T he colour of the pre-insubted sleeve identifies rhe size of the termination and [he crimping tool required . T he ba rrel fits closely over the prepared end of the cable conducto r and when it is compressed in the crimping process, it will grip it in close contact. There are numerous types o f tongue including ring, spade, hook and fo rk lerminations. Th~fng type is the most commonly used because it resists being pulled ouR.~£ tH~ tCIJTIinal and caus.ing a circuit failure when under tension. Prl!.l$.'uilfis applied in the crimping process by a hand or hydraulically operated , ~;.~tb~l that incorporates a die that is speciall)l configured to produce a particular ,\-..\ l". cross-sectional shape in the completed tenninaoon . .....;.'\\"c
(. ~-,\:
[mutauon B:I.I:rcl
Ihrrcl Length
+lr·llm~1
I'
Gnp
Length
'14
~ [[ I
Pre-msulated
r u,~p<,SI~<
t
Crimping HJrrd :lnd Tongue
~ ~II!#;J ~~~=~(I~I Insllimionl ,
Gnp
,I,
,i B3rrc1 Length
F ig ufe 7.8 - Examples o f E nd and In-Line Crimp T e rmina lions
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CHAPTER SEVEN & CONNEC T ORS
It is vcry important that the metals being crimped together arc sim.i.lar to avoid the risk of gah-anic corrosion. T ills means, copper to copper and alumi(llum to aluminium. Aluminium crimp rcnninations are charged with an ant-oxidant compound. Crimp cross-sectional fo rms are, hexagonn l, dinmond or '\'\" shaped. 'fhe pre-insulated diamond form is nonnally found in the i\jvn) (A ircraft and f-.larine Products) range of terminations. lVlany terminations are internally se rrated to increase the te[ls ilc strength ~f the completed crimp.
=======::r:;{ro.: :COO====
==~
(Q:.C1===
Fig urc 7.9 - Complclcd A1vlP T cnnimuions
I n-Line Spikes There arc occasions \vhen two lengths of a cablfj may be joined togcther by a crirqi}"o(:onncction eiciler by the manufacturer or as a temporary measure ",0\.jSt:n
ik
I f in-line splicing is specifically authorised, a number of restrictions related standard codes of practice may be imposed. We should take a brief look ai some of the morc obvious restrictions. Splices in twO or morc cables should b ~ staggered so lhat they do not lie together side by side where there could be ri8k of cracking. Splices made in a single length of cable must be a minimum cff 500mm apart and limited to no more than twO splices in a three-metre run '({,f cable. A cable 100m comaining a splice should be, strop tied within a maxi m~n distance o f SOmm either side o f the splice and single cables should}""bc supported either side of rhe splice by clamps ho more than SOOmm iran. Splices should neve r be used on bends or be poLtioned under a cable <&. Splices should be protected against moisture ~nd lay horizontal ~Wterever practicable. Where a splice bas been installed as tempora ry measure, it must
e;
r
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CHAPTER SEV E N EL E CTRICAL CABL E S
&
C ONNECTOR S
be recorded in the aircraft technical log book together ,,,,;th precise details of the locarion, cable size and type, terminarion type and equipmem used. '111e butt rype of in-line connector has a crimping barrel at each end to accept the cable conductors. PoUowing the crimping process, an insularion sleeve is pulled O\'er the connection.
Prep aring the Cable Before a crimped tennination can be applied [0 a cable, the insulation has to be stripped back to expose the end of the metal conductor. Extreme care is rC(luired when doing tlus to avoid nicking or cutting the wire strands. Particular ca re is required when stripping an aluminium conductor because lhe wires are very easily broken when nicked. Tn addition to lhis, aluminium (Iuiekly forms an oxide when exposed 10 air thal acts as an insulator and increases the resistance at the joint. Aluminium crimp barrels are filled with an anti-oxidant compound [0 prC','em this occurring. In all cases, the recommended stripping tool should be used for remo\;ng the insulation. You would nO[ for example, usc tools such as side-curting pliers for dUs job. If you c..'(amine the illustrarion, rou will see that the insulation is cut back to expose a length of metal conductor equal to the b:lrrel lengch pillS O.Smm. \X'hen this has been achie\'ed, rhe exposed conductor wires should be Iight~v twisted by Imnd to bring them back into the narura1lay and rightness of [he conductor. They must nc\'er be tightly twisted because tlus will reduce the diameter of the conductor.
H and Crimping Tool
, , ,\....'
~0~
( .~~~ .
The most com~t\{:rhand operated crimping tool is the 'Pre-insulated Diamond Grip' (PI~O) tYPe, Some tools are equipped with a selection o f sizes of die to suirdtMitrent sizes and types of tenninanon. Other types arc suited to one size \.... ::shild lype of termination only. The tools have colour coded h:llldlt..·s that rdate (Q the colour codes used to identify termination size ranges. Additionally, the tools are marked widl the. temu'nation size range that they are suited for use on. Colour coded handles
Insulation crimping adjusoncllt pin
o \\
0
.........
Q
.o-=-
Cable size range stamp
R3chet
-
-
Fig ure: 7.tO • Trp ical Hand Crimping Tool
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EL ECTRICAL
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CHAPTER SEVEN & CONNEC T ORS
The tool illustrated has a self-locking ratchet mechanism that prevents the barrel crimping jaws from being re-opened until the crimpmg action is completed. The handles of the tool have to be sCJueezed fully closed before the jaws can be released. This ensures that the crimp has been fully completed. T he tool incorporates two adjustment pins that a~e selectively placed in one of th ree holes in each barrel crimping jaw to sct thc requLred grip on the cable InsuJation during the crimping process . The holes are numbered, 3 (loose), 2 (medium) and 1 (tight). .A test crunp shouJd carried out to establish the correct setting.
bb
Crimping Barrel Abuts Lo catOr
Locator Indem ed \\-'indo\\" Facing Loca ror Figure 7.11 - Locating Crimping T ecminario ns in Too l
The temunation is inserted tongue flrst into thl barrel crimping jaws o f the tool until the barrel rests flush against the rool StOP or locator. An in-line crimp is positioned in the tool with its centre, indented window facing the locator. The prepared cable end is inserted into the terminal barrel. If the termination is not a pre-i.nsuJated type, the cable is pushed i6. until the end of the cable insulation rests flush against the end of the barrel. If the termination is of the pre-in\~Hlated type, the cable is inserted into tbe termination until the end o f the "1('1I...(illbih insulation reaches the top of the insulation support. The handles of the . C0~C~':' - tool are then squeezed fully together to complete the crimp and release the "'(t.... Jaws.
c· '"
,~
,,\",::-.' ~
(.0"'\ ·
"
T h e crimping tOol will indent a 'dot code' mark onto _'"\ .MP termination sleeves d uring the crimping process. 111e presence of this imprint verifies to the operator that the right combination o f too l and termination has been us~ . .A fter the crimped termi nation has been remove? from the jaws, it shouJd ~ Inspected to check tbat it is correctly formed and that the 'dot code' is deaqj: imprinted on tbe sleeve. 2""
e;
AMP Range
?
'" 0'
The form of crimp is determined by factors such as the size and cons~~on of the conductor, the materials aod the form of the termination. It is e ~ntial that the correct type of die and crimping tool be !used. The AMP ran&s is o ne of the most common. In order to ensure that the correct combi.natiiji of tool and tennination is being used, the ..'\1'\'0) range ha.s colour and dot codes for
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CHAPTER SEVEN ELECTRICAL CA BLE S
&
CONNECTORS
insulated terminals used on 10 A \,('G (American \'(·ire Gauge) and smaller cables. Cables larger than 10 AWG have un-insulated terminals and vinyl or heat shrink tubing is pulled over the termination prior [0 crimping.
Ydlo\\'
26-22
One
Rnl
22·1H
( )Ill:
12-10
One
Blue
Yellow
Table 7.4 - AJ\fP Colo ur and DOT Codes
Hydraulic Operated Crimping Tool Hydrnulic crimping tools are normally used to conduct crimping operntions on heavy cables wherc high force may be required. There arc a number of different rypcs. Some are hand operated whilst others arc operatcd by a foOl pedal. The crimping head incorporates a four posicion 'nest die' that can be moved to select the corrcct sized die by releasing a 'nesl lock' and rurning a lillllnb knob. 'n1C two-ha ndled, hand-operated tool is self-contained and is opernted by I:\visting the reservoir handle clod.-wise, inserting the crimp tcnnimHion and then raising the ram using the pump handle until the tenninarion is just held. The prepared cable is then inserted into the b:mc\ and the mm is raiscd fully us ing the pump handle until a sudden decrease in effort is felt. ·111is indicates that the internal hydraulic prt!ssurt! relicr ".'Ilve in the tOol has opene~ \~{ihis is sometimes re ferred to as being the by-pass valve. The opening ",t',Jfthis valve indicates that the crimping process has been fully cQ".tr;lpk;t'i!d. When this occurs, the ram can be released by rwisring the reservoir \\=''< lfundle anti-clochvise. , c-
,.
o
,\'
~«\""
~-x.
The fOOl operated pedal type is similar in its operation except that it has I:\VO partS; a crimping head and a foot pump with a selector \'ah"e in place of the rcsen:oir handle. In both tool types, it is important to ensure that the correct hydraulic fluid is used and that the internal pressure relief valve setting is correct.
Testing of Crimp J oints A badly fonned crimped joint may suffer high resislance and cause a \rohage drop across the connection. A milli-I'ol( drop (est is used to check t'h:tt any voltage drop occurring is within specified limits. When the joi nt is in an installed circuit, an external power supply should be connectcd through the :1ircmft electrical ground supply connector. \"'{'hen crimped joints h:we been made on lll1i1l!Halled cables, a milli-volt drop reS[ rig is nonnally used thar has
396 C,p)'nghl
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ELECTR1CAL
CABLES
CHAPTER SEVEN & CONNECTORS
its own source of electrical power. \Vhen a large number of crimp joints are being made, tcsts may be carried out on sample basis.
Checlcing of Crimping Tools Crimping tools arc prone to wear lUst like any other tool and the accuracy of dies find the compressIOn fo rce needs to b ~ checked periodically. It is importflnt to check that fI tool will consiStently produce a sfltis facto ry crimp. The periods at which calibration is reguired would depend on usage . .:\ tool mfly be used infrequently or it may be in continuous daily use. The calibration period is bflsed on a number of crimping operations, nomlally a th ousan d crimps, father than a calendar period. Typical checks would include the usc of GO and NOT GO gauges to cbeck the wear on dies, milli-\'olt drop tests on test crimps and pull off checks on completed test terminations. Cable insulation stripping tools also need periodic checks to \'erify that they are operating correctly. Tools should be identified so that they each relate to a calibration record. In some organisations, the identity of the crimping tool forms part of the work record and thus offers traceability back to Ihe (001 and its record.
Connector Pin Removal & Insertion There is fI range of electrical plug and socket connecro rs. These werc discussed in Modulc 6 Chapter 11. To remind you, an illustration of a typical plug and socket assembly is repeated here. Socket
r ,
-
C
-0
,
To~ ~
Pill '-
L -_ _ _ _ _ _~ILI
Socket Assembly
______~ PIlig Asservblr
Fig ure 7.12 - Plug and Socke t Assembly
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CHAPTER SEVEN ELE C TRIC A L CABLES
CONNECTORS
&
The svllabus for this section deals solelr with the rcmo\'al and insertion of the pins . Actually, the procedure for the pin and socket contacts is the same so we will refer to them as the 'contacts'. There are different designs of insertion and extraction tools. There are metal tools and disposable plastic tools. They are nOffimlly colour coded to correspond to the size of contact and havc a Part No. Plastic tools for example have different coloured tips at each end to signify whether they arc for insertion or extraction and the colours relate to contact sizes. The extraction end tip is normally white whilst the insertion end colour relates to the contact size. For example: Contact Size
D escription
Part No.
Insertio n Colour Tip
Exttaction Colour Tip
22D
ABCO -22 0-01
113-000
Green
\,("hirc
Table 7.5 · Example Tip Colours
Ftom this hypothetical example, you ca n see that it is very important to check that you arc using dIe correct tool in relation to the con£acr. The tips o f disposablc plastic tools can become disrorted and damaged and will in rum cause dam:lgc [0 inscns and contacts if )'ou continue to re-use thcm in this statc. You mllSt only remove or inscrt one contact at a rime to a\"oid the risk of inad" enenr cross-connection. I f a pin is bent, you must not anempt to straighten it because the resulting work hardening ha s the potential to causc the pin to fracture in use creating a circuit failure.
Figure 7.13 . Examples of Contac t Insertion and EXltac rio n Tools '1';
Removal of Wired Contac ts There are two types of contact relention llsed in plug and socket connectOrs; front release and rear release. Both types are removed from the r ear of the plug or sockct. A different extraction/ insertion tool is used for each type. Before a contact can be removed, thc plug or socket cable srrain relicf clamp should be loosened and the back shell unscrewed and pulled back to expose the rear insert of the plug or socket.
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Front Rele:1se
Rear Release
J=~
eLJ[Jit..
,cc-_I'~]
C:::iim Rele",
~eacRCkase
g.:
,EJREl
Front Release
Figure 7.14· COWlcctor Pin Release Methods
Rear Release '111e extraction tool is located meet the cable thar connects to the rear of the comact being remoycd. Tbe tool is then inserted imo tbe contact bole in the plug or socket until it meetS firm resistance to further movement. 'n1e cable is then held against the tool and both ate pulled to extract the contact out of the rear of me plug or socket. Front Release The T;juaction tool is placed over the contact to be removed. The plunger on
"luO-ihe. 1601 is held back while the tool is pushed into the plug or socket to release
. Co\\c'?,.e'" the contact. The plunger is then depressed to eje~t the contact out of rhe rear of the plug or socket.
" y..~{(.'
c'" '" 0«'
~ C·
. 'f..
Removal & Insertion or U n-Wired Con tacts
r. b
I "" Vacant contact insert boles in plugs and sockets are fitted with proper Si2~~ contacts and sealing plugs. The rear of the plug or socket is exposed as;, before and the sealing plug is removed. An wH.v}red contact extraction tool it then pushed imo the contact insert hole until it Ib utts against the shoulder the contact. The tool is then pulled back to remove the contact from the nil(r of the plug or socket. \'('hen inserting an un-wired contact, the insertion toetts used to push the contact into the rear of the contact insert hole until it ~1: ts fIrm resistance and locks. The tool is then drawn lback out and a sealing!plug is fItted into the rear of the co ntact insert hole. You may sometimes h~ that all unused holes in a cable sealing grommet should be fitted with an :pproved filler plug. We have just discussed this sealing plu I. ,.~
r!
399 Com-right © Bart)· College 2006 - 2007 All ril l}!S mm:~d
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CHAPTER SEV EN ELECTRICAL CABLES
&
CONNECTOR S
Crimping Contacts \Vhcn preparing to fit a new comact OntO a cable by crimping, the cable must be prepared by stripping thc insularion back \\~th the appropri...'ltc cablestripping tool. The length of exposed conductor wire should be sufficient to allow the conductOr to be insened into the crimp barrel of the new contact until it is visible in the inspection hole in the base of the barrel. T he contact is positioned 10 the apptOpriate crimping tool, the prepared cable conductor is inserted into the crimping barrel and the tool is operated to crimp the joint. The contact should then be inspected to check that it has not been distorted. The crimped joint must be checked to ensure that the conductOr is still visible in the barrel inspeecion hole and that the crimp pattern is even and does not overlap onto the cable entry end of the barrel or OntO the shoulder of the contact. Finally, there should be no evidence of cracking around the inspeccion hole or around the cable entry cnd of the barrel.
AF Crimping Tools The AF series of crimping tools arc used to crimp contacts onto cables. They come in £\\'0 sizes, the AF8 and the AFJ\I8. Both tools have a ratchet mechanism that \,,-jJj only release the tool when the crimping process is complete. The AF8 is a high precision hand-crimping tool that incorporates an adjustable turret head that has numerous scrcing opcions for the size and position of the contact. The AF1[8 is similar bur has a different type of turret head.
Insertion
of'\ll'~ed
'1\.'1.10 ...
Contacts
,c; ...
B~\\'tYPes of contact arc inserred from the rear of the plug or socket. The
[1
.<.y~'\. =(. ~,'\... .
~':oSt'able is positioned in the open s.ide of the appropriate insertion rool so that the
LOol rests against the contact shoulder. \'(' hen inserting a contact that is positioned near the edge of a connector insert, the open side of the insertion tool must face towards the edge of (he insert. This ensures that [here will be adequate clearance as lhe contact is inserted, avoiding strain on the insert. The contact is pus bed straight lora the appropriate hole at rhe rear o f the insen in the plug or socket without twisting unci! it meets firm resismnce to movement. A slight click sensation may be fcle This indicates that the contact has seated and locked correctly. The cable may then be released from the tool and rhe tool drawn straight rearwards to remove Ie \Vben there arc unused can met insert holes in a plug or socket, these should be fined with correct sized contacts and scaling plugs as we discussed earlier.
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Testing Contact Sockets \\:then using a test probe to connect a test meter [0 a socket contact, you must use a probe thar has the same size or a size smalle r than the pin that will mate with it. If you attempt. to force an oversize probd into the socket, it will spread it and this will lead to imerminent contact o r an open circuit when the connector is assembled.
Co-axial Cables The importance of taking precautions when installing co-axial cable is best illustrated by examining its characten stics and use. You n1fly be familiar with tills type of cable because iT is used among other things as a link between your televisio n aecial and the set. .r\ cO-fl xifl l cable is a concentric twin wire line. It consists of a central axial solid conductor wire that is encased in dielectric material that is sheathed with a metal braid conductor co,ered b, an outer proteeti\-e jacket. Properly I"uatched co-axial cables ha\"e low characte ristic impedance and providing the: are correctly tenninated this will remain constant regardless of length. A mismatched cable on the other hand will suffer a large power loss in relation to irs length. Coaxial cable is screened in that electro-magnetic fields arc confined in the space between the central conductor and the braiding. Because the braiding is eanhed, little o r no external interference can penetrate into the co-axia l cable circuit. Insulator
Br:J.id
i \'\'ire Conductor Figure 7.15 - Co-Axial Cable
Co-a:-.i. al cables arc widely used for connecting radio and radio navigatiol~ equipment to antennae because of their ability to prevent radio frcqllel1~ interference (R.FI), particularly to reeciying systems. Rigid co-a.ua1 cable is als~ used in telephone communication systems to carl)' signals over significantdistances. The distance between the inner and outer conductors, the diclectri.e: constant and \'olume of the insulating material and the capacitance arc ali critical to the maintenance of low attenuation (energy loss) and interferensc. These will be affected if the cable is distorted by tight bends o r is compre~sed by m-er-tightened clamps. Finally, co-axial cables that link some eX%I"nal aircraft aerials arc exposed to the airflow and thele are speed-rated in te!i:T1s of " tenSI1e strengt 11. thcu t '" ~v
~
<
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CHA P T E R S EVEN E LE CTR I CA L C A. BL ES
&
C O NNECTORS
Byd y
~MII
lsolfltion Washer
Contact
Cap
! tp~D Bmicl SuPpOrt
Fertrule
Figure 7.'1 6 · BNC ConneclOr for Co-A.xial Ca ble
From what we have discussed so far, it is clear that if we do not exercise ca ution when selecting, routing, clamping and terminating co-axial cables we will run the risk of degrading me very characteristics that arc critical [0 the systems in which they arc used.
I nstallation of Co-axial Cable Co-axial cable ruos should be kept as direct and as short as possible to further minimise the risk of electro-magnetic interference. In manr cases, cables are marched [Q the equipment they serve. \'('hen routing the cable nround corners, you must keep in mind the risk of creating kinks lhat will distort the dielectric. As a guide, when insta lling single co-axial cable, you should observe a minimum bend radius of not less than six times the cable diameter to avoid kinking the cable. Some textbooks quote a 'rule of rhumb' limit of no less lhan ten times cable diameter so be alen that you may encounter either figure. TIle cable should be supported at least every 2ft throughout its run. \Xfhen clamping a cable, YOli must ensure that the clamp docs not squeeze tbe cable and compress the di~fttic. This will affect the dielectric constant. You should not route co-a~ crtt:les close to sources of higb dectro-magnecic radiation or in ar~~\\ltfe;e the cable may be trodden on or used as a bandhold . . \,,>':\.;;f~ ,\,<" &; Co-axial
. ,"',\":5\~
....
cable terminations are normally of the BNC type and they arc critical to me maintenance of the characteristic impedance of the cable. Therc are many different configurations for these bur some general rules apply. When stripping a cable to receive a termination, you must ensure that you do nOt cut the braid strands in tbe outer conductor or damage the centre conductor. T he exposed centre wire should be insened in the connector body uncil the dielectric is firmly seated against the dielectric in the connector. The wire braid should make good cont:lct wim the connector ferrule or sleeve. Care must be taken when tightening a connector assembly to avoid damaging the seals or washers.
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Testing Co-axi al Cables FauJts in co-axial cables can increase attenuanon ()r energy loss Ln the cable run and create electrical interference. This is critical to the operation of the equipment that the cable sen-es. i\ simple continuity test 'Will not always be accurate enough to measure these losses or identify their sources. A Tilll c Domain R cOcctometer will do this . This piece of test equipment can measure the attenuation of energy along the cable length whilst comparing it with the attenuation in a theoretically perfect cable of a similar configurarion. T he results are dispbyed o n a cathode ray tube that displays the loss of energy over distance. This accurarciy measures the value and identifies the position of any loss that deviates from the standard. This couJd be from a bad connector, a tcmunatiOll or damaged braiding etc. The instrument can also be used to measure attenuation per urut length be fo re and after a cable run is installed.
Fibre Optic Cable I will include a yery brief note on fibre optic cable because it is reported that a couple of questions on testing it have been encountered. Fibre optic cable IS now in \videspread use. It has a large bandwidth and can carry a multirude of channel frequencies. There are no electrical conductors so the cable is not affected by electro-magnetic interference and can be routed through high , voltage fields. The thin glass fibre core means that the cable is very small in diameter. Fibre optic cable suffers very linle loss of optical power over large distances . A light source such as a laser transmits the light signal at one end of the cable, which is received by a light sensitiye cell at the other. The installation, routing and coupling of fibre optic cables requires special care to avoid introducing backscatter and attenuation pf the light signal. Continuity and)\lf.fenuatioo testing is carried out using a calibrated light generator and an c :J')rt!JpdcaJ Power Meter (OPM) . This instrument ~vill measure the power at the . C0\\C~ receiving end of a cable enabling a comparison to be made with the power e .\"\",st" being transmitted. T here is also an Optical T ime Domain R e fiectometer {\~-..\. ~ (OTDR) that will measure anenuaoon in a cable and compare it with a ())\;" theoretically perfect cable . This latter instrument will measure and identify the position of losses in the cable run.
Wiring Protection Techniques In troduction The installation of electrical cabling in an aircraft conforms to the requiremefrs of the Aircraft i\Iaintenance Manual, the \\firing Diagram .l\{anual and ~ny associated Modification Drawings. Cables must be protected against abr~on, mechanical strain, electro-magnetic i nterfercn~e, excessive hear sdilrces, extreme ambiem temperatures, chemical contamination and mois rur~,Jngress. Humans tend to live in aircraft fo r relarivelyj long p eriods and~his can introduce additional risks in food preparation and toilet areas and in places
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where food and drink is consumed. Hazardous cargo can introduce further risks if the nlles for its carriage arc not strictly obseryed. Un~pressurised regions of the aircraft are exposed to low temperatures at altitude and a variety of ambient conditions at locations around the Wodd. All of this underlines the need ro protect electrical cabling. \,'hen cables arc installed in groups or bundles, it is important thar cables ser...ing main and standby functions in a system are not installed together in the same loom. Common sense would show that rou cannot risk losing both the main and back-up systems because of a single loom failure. Additionally, cables carrying high AC or pulsing DC currents that create electro-magnetic interference fields should not be placed in close proximity with sensitive circuit cables that may be affected by them. This highliglm the need to comply with rhe authorised manuals when installing electrical cables.
Cable Looming & Loom Support T here are three principal methods of wiring installation used in aucr:1 ft. These are: open loom, ducted loom and conduit. Ope n Loom The most convenient and quickest W:1)' to inscall winng is to install it as an open loom. This offers easier access for faultfinding and maintaining individual circuits. Cables that run close to each other along parallel paths arc grouped together into a bundle that is bound together by either continuous lacing cord or by binding or strapping them together at suimble intervals. The bundle is supported by clamping it [0 (he aircraft structure at intervals using metal clips cushioned with non-metallic insulating material. W' hen a sub-group of cables have to branch 'AW from the main loom, they must be arranged so that they form a s"tQd~h curve away from the loom and do nor eross any other cables in thE..~)Utcess. Sometimes, cables arc pre-formed into harnesses or looms by the , 'i!>~{hi.anufacturers and have the correct bends and branches needed to insmll the _ ,\-..\ L. loom as a complete assembly into the aircraft.
-,\~
. +"'1'.
\...).-'\'
Lacing: L1cing consists of continuous PVC covered nylon or waxed linen cord that is bound to the loom with knOtS consisting of a clove hitch finished with a square or reef knot at inren'ais of 3in to 4io along the run. Tn smaller looms, rot-proof waxed linen or braided flax tape may be used instead of cord. Cables in the loom should be held close together and there should not be aoy evidence of individual cables sagging or separating from tlle main bundle, additional knots should be used where necessary to preYCnt this. The disadvamage with lacing is that a break in the lacing will loosen the bundle and its use is not recommended in many areas including the power plant. Binding: i\ bundle of cables may be spot tied tOgether at suitable intervals using pre-tensioned nylon straps. These are sometimes coloured to aid circuit identification. 1\ special binding tool incorporating a reel of strapping may be used to apply tcnsion and lock the straps. Alternatively, Ty-Raps are now a common feature used in strapping. The bundle is then supported by clamping
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it to the aircraft structure at intervals using clamps cushioned \vith non-metallic insulating material . I
Fig ure 7.17 - E xamples of Open & Dueled Wiring
Ducted Loom
A dUeled loom is bound together in a similar way to an open loom except that the bundle is supported in a rectangular-sectioned duct attached to the aircraft strucmre. D ucting may be manufacmred from alurniluum alloy or fibre reinforced plastic material. Some ducts may incorporate channels designed to support and route individual lo oms related to specific systems. Channels may also be used [0 separate looms where there is a risk of electro-magnetic interference between them. Conduit
CQr~Hits are plastic or metal rigid or flexible tubes that are clamped to the ,,\)\) \\ >~r:. ~IC alIcraft strucmre. They fully enclose and protect cable looms when they are -o\\\:.~ , ~..; C routed through areas where they are prone to chafIng or crushing. The inside fJ \"'I'=' diameter of the conduit should be around 25% larger than the outside diameter ,;~\¢-"\.... of the cable bundle . . 0\1 ' L. High-enetgy ignition cables used in gas turbine engwe 19runOI1 systems ~e often routed through nickel alloy rigid conduits ~hat are screened with nick?iplated braid. --::;
%
Cable Routing & Protection
.::? (J
Cable mns should nor be routed in the bottom of lhe fuselage or bilge ~~s where they are prone to contamination from spilt or leaking fluids ~ nd accumulated moismre. Cables should be routed so that they mn down~rds and away from connections with equipment. This is to prevent 9.1~Sture seeping along the cables ioto the equipment. i\Jt~natively, the cable ~ should incorporate a downward loop (drip loop) adjacent to the equi pment connecnon.
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The radius of bends in the run of a bundle of cables should not be Jess than eight times the o uts ide di:lflJ eter o f tiJe bundle. \\lhen cables have to pass though a bend to connect with a terminal block, a minimLUTI bend radius of tiJree times the o utside diam eter o f tbe bundle is acceptable providing the cables are adequately suppo rted at each end of the bend. You may come across different figures for this quoted in various textbooks; the figu res quol cd here are from Civil Aircraft Airwonhiness Informacion and Procedures (C J\J\ IP). \'(' here cables have to connect with moving parts, such as the retractable landing gear, they must have adequate slack ro avoid being placed in tensio n throughout the range o f movement whilst not being exposed to fouling or chafing in the process. Electrical cables should not pass close to unguarded mechanical control cables or rods or imerfcre in any way with their movement. As discussed earlier, cables carrying AC or pulsing DC current will radiate clectro+magnetic fields that can induce stray currents in adjacent cables. Cables must be rou[(~d to avoid interference between electrical circuits. t>.lain and sta ndby circuit cables servi ng the same system should not be PUt together in a loom. Cable runs must be supponed separate from pIpes carrying fluids o r gases, particularly o"-l'gen lines, and be routed to allow the maximum possible clearance from them. They must neyer be anached to them or be allowed to contact them. To avoid the risk of contamination from spilt or !caking fluids, cables should always be routed ab Ol'e fluid pipelines rather than below them. As a guide, opcn cabling should be at least SL"": inches dea r and conduit should be at least twO inches dear and not be attached to the pipes. Cables that arc foured through engine nacelles may have to pass through hot zones and should have high temperature resistant insulation. Cables must n0-fl)Re supported or routed in such a way that they come inlo contact wi.d'() ~llarp edges presented by lightening holes in structure, panel e~~. . fftDngs and fasteners. \\:rhen cables are roured through an aperture in a . ~:l5 Sulkhead, rhe edges of the aperture must be smoothly radiused and fitted with . ~""\\. <.,; an insulated sleeve or grommet. Ducts and conduits must have smooth . ,'~...:..~\surfaces, exits and entrances . \"\ben cables are routed through a pressure bung L· into a pressurised comparunenr, they must be fitted into the correct diameter holes to ensure adequate sealing. Conduits must incorporate provisions to drain away any moisture that accumulates in rhem. For e..xample, a 1/8in diamcter drainage hole should be provided at the lowest poim in each conduit run.
Cable Clamps Cables must be adequately supported by securing them to the aircra ft strucrure using suitably spaced cable clamps lined with non-metallic insulating material. This is necessary to prevent vibration that would otherwise present the risk of fractured conductors and damaged insulation. When clamping cables, the
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objectiyc is to prmride secure support and to prC'Tnt movement, nor to squeeze them to the point they deform. Clamps should be suitably placed so that the cables do not sag or yibrate between them. Cables must ne,'er be installed or clamped so that they arc placed in tension o r in a way where they would be put into tension because of structural flexing created during any aircraft operation. As a guide, you should be abld to flex unsupported sections of cable runs be['..'leen clamps by abOUT half an inch by hand.
Protective Sleeving There are instances where additional cable proteacion 15 required, either to give added support or to protect against abrasion lor heat sources. In another instance, shorr sleeves may be fitted to prm;de insulation at temllnations or for identification purposes. In the latTer case, thl slee\'es are installed onto the cable using special tools. \'\ben small diameter sleeves are fitted , the recommended lubricant should be used together with a thimble jig or needle tool. Three prong fitting pliers are used to stretch and fit larger diamctcr sleCycs. The sleeyc should not be strctched more than 300% of the sleeve internal diameter and care should be taken to slille the slee,-e into position on thc cable whilst not pcrmitting It to roll up. I Spiral PVC wrapping is somecimes used to enclose cable and provide a flexible sleeye that guards against abrasion. TIllS is not classified as conduil so care should be exercised in irs use. Cable bundles are Jometimes enclosed in ilexible PVC tubing. The bundle is pulled through th~ tubing prior to connection, often with rhe help o f a little talcum po\Vde~ to reduce friction. Teflon (PTFE), nylon and fibregbss sleeves and wrapping tapes are also found in use. Cables exposed to heat sources may need ad9itional protection. Nylon is lirn4h~1:0 around 140°C. Teflon (PTFE) has a safe ''lorking temperature up to '1\Jm50 er and is a good, and expensive, electrical insulator. Other materials such c _O )\\c'b '" as asbestos felt and glass fibre would be suited to this temperature range. The r:: ~~~{' short term and continuous ratings for a gi,-en cable type and size determines .\~"\.'" = the protection tbat is required. CA.A J\.mvorthiness Notice 12 Appcndi.'i: 32 C.c-X'i' . u~ed to rder to this . CAAIV Leaflet 11 -5 ~-\ircraft Electrical Cables contains information on approvcd cables and remperarure lranges.
C
Heat Shrink Sleeving
~
I-Ieat shrinkable sleeve consists of a polymer plastic that has enhancecf elastomcric properties. Supplied in its expanded form, it will shrink when bea.2 is applied, moulding itself tightly to tlle contours of the ca ble. Once the slcc"'f has been shrunk, it cannot return to its original form. Heat is applied from'-'a heat gun. This may be a direct heat or an infrar~d lype. The gun is usually to a temperature 10(}oC above the initial heat shrink temperature of the sl~'e . The ~uns can prodLlce [emp~raturcs up to 60(}l~ so cxtrem.e care needs ~ be exerased. The sleeve matenals and the heat guns are typICally prod~~d by Rlychcm Lrd. ~v
fJ.t
'.
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Shielding \'(!e have referred to shielded cables earlier in this chapter. \'{!hen current nows in a conductor, it sets up an electro-magnetic field around it. Merely switching the current off and on causes the field to collapse and reform. \X!hen AC or pulsating DC current is passed, the field is constantly changing. I f this field is allowed to Cllt through another conductor, it will induce a flow of current in il. This could be hazardous if rhe other conductor is an adjoining cable serving a critical function or is a communications cable carrying a radio signal. TIle interference ca n be reduced or even eliminated by shielding the cables. Shielding is simply a means of intercepting the radiated field and passing the induced current to earth. Shielding used to screen cables from producing or receiving dectro-magnetic interference, known as radio frequency interference (RFI), consists of tin, cadmi\.UU or nickel- plated copper braid sheathing that encases the cable. The braid is conneered [Q earth at a single point connection at onc end of the sheath to a'-oid the for:mation of an earth loop where currents circuhne in the braiding.
Conclusion We have now reached the end of this chaprer. .A point worth noting is that different textbooks sometimes tend [0 quote conflicting general guidance figures for things such as minimum bends in co-axial cables and cable looms for example. Tn one example, t'-\'O con flicting figures were quoted in separare places in the same bookl Here we have the Catch 22 problem of which book is used by the examincr and which is used by the candidate and in onc case, which page. T he information contained in this chapter has, as far as is pra cticable, been verified against that contained in current CAA publications . Examiners use ,!hOse but do be on your guard for the odd 'rogue' question fronl. tt\E Ha~k unearthed in a car boot sale, metaphorically speaking of course! AN.C."'I\\~tal point is that you should always use the authorised Aircraft ..: \,>)."tKlaimenance and Wiring ~Ianuals for all infonnation on parciculflr aircraft .<-..f:-'' - . electrical wiring installations. This is a good time to take a break Or a short dash (0'X"" to the pub -strictly for counselling of COur5C!
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Revision
Electrical Cables & Connectors
Questions
1.
The correct sequence for circuit tests is : a)
' . .•.tn~ulanon, funcnona.t,
. .ty COnl1JlUl
an db on 8lng 1
b) bonding, continuity, insulation and function al
c) continuity, insulation. functiona l and bonding
2. When the two spikes of the 6ft lead are shorted by the single spike of the 60ft lead on a bonding tester, the instrument pointer should: a) align
WIth
the zero mark on the scale
b) move to full-scale deflection
c) register in ohms the resistance of the leads
..;gre
3. maximum permissible resistance for bonding between primary c '2.\."IOb - s tructure components and the main earth system to which they are . (.o\\C~ connected is : ~ . \"I',I.~('~
\,,"'.... 1'.5 r' - ('I~.
L·
hi
a) 0.5 ohm
b) 0.05 ohm
c) 0 .005 dun
4 . The maximum permitted resistance between an aircraft main eartif-" system and the ground is: a)
10 ohms
b) 10 kilohms
c) 10 megohms
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5. The main reason why primary bonding connectors have a large cross-sectional area is that (hey must be capable of::
a) carrying circuit fault currents
CO
earth
b) bonding highly stressed strUcmres
c) carrying a lightning discharge
6. An a ircraft engine that has non-conductive mountings must be bonded to the aircraft structure with:
a)
twO primary
conductors, one each side of the engine
b) one primary conductor c) cwo secondary conductors, one above and one below the engine
7. Non-conductive plastic composite panels:
a) do not require bonding b) are treated wirh conduccive paim c} are equipped wid, a bonding jumper
cond1~ting a bonding test on anodised structure: §VO~'':Sth112 prongs must penetrate the anodic ftIm
8. \'Vhen
_ 'C>).t~" ,
(0'1....
...
;\.~ c
\. h'
b) the prongs do not have to penetrate the anodic film c) a 500V resistance tester should be used
9. Following a functiona l test on a circuit:
a) a duplicate check must be carried Ollt .~
b) rhe fuse should be removed to check that it isolates the circuit c) a bonding test must be carried out
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10. The tests carried out to check a n earth tenninal would include: a) insulation resistance and continuir..
b) bonding and continuity c) bonding and milli,'oIIS drop test
11. An instrument that can be used fo r continuity testing is a: a)
low range o hmmeter
b) mcgger c) ammeter
12. Mt er stripping a cabl e in preparati on fo r c rimping, the exposed conduc to r \\;res should be:
a) straightened b) twisted rightlr (a reduce the core diameter c) lightly twisted by hand to restore the lay
L3 . The correct b and-c rimping tool may be identified by:
1W'-:the colour of the handles and tile t.ermination size marking b} the part number c) the colour of the rcmo'-able die
14. The insulation grip exertcd by a ha nd-crimping tool: a) cannot be adjusted
b) is adjusted by the pressure exerted on the handles c) is adjusted b)' altcring the position of the pins.
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15. The jaws of a ha nd-c rimping tool can b e re-o pened:
a) as soon as pressure on the handles is released
b) only after the handles have been full}' closed c) when rhe lock is withdrawn
16. A h ydrau li c crim p ing to o l \"ill have comp leted the crimp w he n the:
a) pressure relief valve opens b) pwnp handle locb
c) reservoir \'ah-e closes
17. The d ots imprinted on the p re-i nsulated sleeve during Crlmpmg indica te th at th e: a) crimp has been completed
b) correct combination of [Qol and tcrminaaon was used c) correct pressure was applied
18. A fo rward rele ase connecto r contact is :
I''"' extracted from the front a) re~~~d1ffia _;)\\c'/:.I:."
...{\ C'b) released at the rear and extracted from the front
",
,,"..;:\~
, ,,~'" .
c) released from the front and extracted from [he rear
- ...0 1.... ''>
19. U nu sed contact ho le s in a connector in sert a re: a) fitted with a con tact and a sealing plug
b) left open c) filled witb porting compound
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20. \Vhen inserting contacts near th e edge of a connector insert the insertion tool should be positioned: a) with the gap facing the centre of the insert b) with the gap facing me OUler edge of the insert c)
:ll
the from of the insert
21. The minimum pemlitted bend radius in a cab le loom approac hing a te rm inal block w h ere both e nds oCthe bend a re suppo rted is : a) eight times the outside diameter of the bundle
b) three times the ourside diameter of the bundle c) six times the inside diameter of the bundle
22. Cab les conn ected to equipment should he routed: a) downwards and away from the equipment b) upwards and away from the equipment c) in a suaiglu line away from the equipment.
23. Wben installing a co-axial cable, th e minimum bend radius is: .,~""G
. ".(.:i!,":"
\. .~"\\ . '\"\:-.-(~'
~ ~
llJ\l
a) eight rimes diameter of cable b) six times diameter of cable
,\....'1.
,5\":(.0'\·
c) three rimes diameter of cable
24. The temperature setting on a heat gun would be set :
a) (SOC aho\·c the initial heat shrink tempernture
c)
100°C above the initial heat shrink temperaturc
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CHAPTER SEVEN ELECTRICAL CABLES
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25. The braided m e ta l sh eath o n a s hielded cable servlllg electro ni c equipme nt is:
a) connected to earth
fit
both ends o f the bmid screening
fit
one end of the braid screening
b) no t earthed
c) connec led to eanh
Revision Electrical Cable s & Connectors Questions B
16.
A
2. A
17.
B
3. B
18.
c
4. C
19.
A
5. C
20.
B
I.
6.
A
B A
23.
B
9. B
24.
c
10. C
-,.
C
?-
r
>
,
-j ~
~
I I. A
-
12. C 13. A
14. C
IS. B
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CHAPTER EIGHT RI V ETING
Riveting Introduction In Module 6-5-4, we dealt with the various types of solid and blind rivets and special fasteners in tcrms of their specifications and identification and the heat treatment of solid rivers. Therc is a degree of correlation between those topics and this chapter in regard to usage and installation. With that in mind, some modified illustrations from Module 6 will be rcpcatcd here where they have a particular bearing on installation processes and tools. Do nOte that the title of this chaptcr does not specify an!" particular type of rivet. \Ve must assume from this that all types, solid, blind and special fas teners, are included. In addition to tlus, it wo\lld be difficult to describe the tools used for rivcting and dimpling without also describing the particular installation processes to which thc), rclate. \Vc will be maki ng a start by discussing the principles underlying riveted joints in order to give some appreciation of why rivet selection, spacing and pitch are so important in aircraft design.
Riveted JDints
,
. ,\"\.
~
U"\
~
~~ I . ;.0\Ri\Tenng pre-dates the screw thread by several centuries as a simple, light and \.. n\\c\!..-= effective means of fastening m.'ueri.lls together; so effective that we place a lot \"\":\~{' of reliance on it today. Howeycr, there are a few rules. For example, when an aircraft is Hying through turbulence, the structure flexes and the I·oimed skins . arc constantly pulling against lines of rivets before springing back into shape under me clasuc tensile, compressive and shlar stresses generated in the metals. \'(!hen we gaze abstractly out of a passengcr window at the \virig s bending, \ VC do not \vish to contemplate the I effect o f rivets shearing ~ altematively, tearing through thin skinned joints. Someone, somewhere, mu ~ surely have considered tlus? ~
The pattern of riyeting used on an aircraft bas been specifically chosen by t!-$ manufacturer after ex tensive stress calculations and structural test programrnf!~ . \'\!hen we arc required to conduct repairs, we do so by complying wit.$ a manufacrurer's approved repair scheme in every detail. This ensures tha @:he repair will meet the same design requLrements and quality of riveting ~at is essential to the maintenance of the original strucrural i.ntegrity. i f we fifi''ci that we need to deviate from a repair schemc, wc must first gain the a p pro.~l of the manufacturer to ensure that the strength of the repair will not be affected. For our part, tbe pracucal drilling and riveting tecHniques that we use must be ~
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CHAPTER EIGHT RIVETIN G
conducted to the highest of standards to ensure that we do nor negligently degrade the integrity of the structure. That should go some wn.}' rowards answe ring the question posed abm·c.
Shear Strength Shear stress occurs in a body when opposing forces tr), to make its material slip in opposite directions either side of a plane surface inside the body. If rhe shear stress exceeds the clastic limit of the material, permanent defonnation will occur. I f the stress exceeds the ultimate load limit for the material, the mat erial will fail across the shear plane. In other words, it will be cur in IWO. The load \'alue where this occurs identifies the shear strength. The limiting value for a rivec would depend on the cross-sectional area of the shank, the sltength of the material and the force applied to it. In a correctly designed ri\'eted ioiot, the ri\'ers react shear forces only and should never be required to react tensile or compressive forces. She:!.r Pbne
Shear Pbnes
~
_.------,
~r===\ Single: Shear
-
=~EiP
Double Shcar
'=~Q:::s~= Figure 8. 1 - Shear Strength
\'(!hen there is JUSt one shear plane, such as that found where jointed, the join~~,described as being subject co ' single sbear '. "~'I..)b
. "'~\!..c ~
l.')\ . \\).1: 0
tWO
skins arc
-
Single Shea r Suess =
Shear Force Cross-sectional Area
There are occasions where a body may have tWO shear planes, such as in a fork-ended joint, or where three skins are joined by a single fastener. TIus is known as ' double shear '. Doubl~
Shear Stress
Shear Force 2 x Cross-sectional Area
In both examples, increasing the cross-sectional area of the shank or reducing the shear force will reduce the \,atue of the shear stress. However, a rivet made of lead will fail at a very low stress niue, whereas a rivet made o f high strength aluminium alloy will fail at a much higher stress value. Selection of (he correct rivet depends on the right balance of material sltength, cross-sectional arca and the predicred working sltess.
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Diamcu'r :md Cruss-seCU'On:IJ Aretl: A point you need to keep in mind is the square law relationship mat exists between diametet and cross-sectional
area. If you double the diameter, you do not double the area - you quadruple it.
So, if you wamed to replace one 1/8in diamet~r ri\'et with 1/16in diameter rivets, you will need four 1/ 16in diameter riyets to achie,e the same crosssectional area as the one 1/ 8in diameter ri'-et. The shear strength of a riyet is proportional to its cross-sectional area not its diameter.
Another consideration to be made is the bearing strength of the material in which the rivets are placed.
Bearing Strength The bearing strength o f a maLerial is its ability to resist the riwt tearing sideways through it under load. You might have rivets with high shear strength but it will serve little purpose if for example, they are llsed to clamp two pieces o f tinfoil together! The fact that the rivets will not shear under load merely means that they \vill rip through the tinfoil instead.
LandI
o
0
f-------
'l\)\}~
'1\,\\)6 - ...
,'c. . .o.:
. Cp\l .
Figure 8.2 - Bearing Strength
...
. \.,.:I.,t' ~
~,'-
,.:5\'?-
L-~'~
~
Shear Strength V s Bearing Strength We could cover all our bets by constmcting a joint where the shear strength pf the rh-ets and the bearing strength of the material of the clamped skins ha~ such high factors of safety that nothing could qer go wrong. The problem 1$ that the forces transferring across this joint would result in the failure o f tiTt surrounding structure unless we made it equally as strong. Once we go dowr£. that path, the overall increase in 'weight would mJke the structure econoffilcaUf umiable to operate. \X7hen we consider the design of a joiot in the light of wh~ we ha,Tc discussed so far, we can appreciate that a joint should not be t§O weak, but neither should it be too strong. The joiht should be strong enollgg-~ o accept the design operating loads with the required factor of safety, but sb.,9uld not have too much additional strength. To a~lle\e tills, we must s4lke a balance between the shear strength of the rivets in me joint and th~~vbearing strength of tlle materials they damp together.
-!f
417 COPJ'right
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CHAPTER E I G HT R I VETING
Incorrect Shear strength high Bearing strength low Incorrect Bl!aring strength high Shear strength low Correct
B
Bearing strength slightly higher than shear strength
Figure 83 - Ri\'e r Selection
The illustration shows three combinations of rh'et diameter and skin thickness where the material strength of the civet and the material it is placed in is similar. In the first example, we hayc a large diameter oyet damping twO thin skins. The shear strength of the ci\Tet is very high compared to the low bearing strength of the skins. The joint can fail at low load resulting in the intact rh'e! ripping sidew:l),s through the skins. In the second example, we haye a small diameter riYet clamping twO thick skins - very low rivet shear strength and high skin bearing strength. f\g:tin, the joint can fail at low load and will result in dte ovct shearing in tWO leaving the skins intact. TIle third example strikes a balance where (he civer diameter is such that its shetlr s trL'ng lh is slightly less than the bcaring strengrh of tIlt., m:llcri:l/ of the skins - boti;\,(\te strong and will support rhe design operating IO:lds. 1\ failure ?f-...ilils) jOint because of a design overload will cause the ril'cl 10 shc:u ra.{ft'a-'1han 111e material to rip. That is very fortunate for us; il is much less ,?>).\£xpensi\'c to replacc a few rivets than to repair tom skins! This balance is generally struck when the ri"et diameter (0) is at least three limes the thickness of the thickesl single skin in the joint and the material strength of the ri"ets is similar 10 that of the material in which they arc used. In the light o f what we haye discussed. it is very imponanr that you comply with the approved slructura1 repair scheme instructions when selecting rivets :Ind sheet material. There are ri,'et rabies that gi,7e the ultimate strengths and single shear strengths for the range of diameters of various rivet material specificatio ns. A fi ctional illustration of a typical table follows:
R,j"ct diameter Smgle shear strength (psi)
1/ 16 3/ 32 I/ B 5/ 32 3/ 16 1/ 4 5/16 3/ S 90
200
360
570
S()()
T able 8.1 - AI Alloy 0 1234 cive ts
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1,440 2,280 3,200
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CHAPTER EIGHT RIVETING
Tile shear strengdl values are d o ubled when the joint involves double shear loads such as those found where three skins are jdined. There are also tables giving dIe bearing strength for each combination of single sheet material thickness and rivet diameter for different sheet material specifications. A fictional extract [rom a table foUows:
Rivet diameter
1/16 3/32 1/8 5/32 3/ 16 1/ 4 5/ 16 3/8
Bearing strength (p';)
260
390
520
650
780 1,040 1,300 1,565
T able 8.2· Al Alloy sheet 1234·T3 O.05in thick
Taking a hypothetical example from the extracts, the diameter of [he rivet should be at least three times the thickness of the sheet. The sheet we have chosen to give as an example is 0.05rn thick. This means we choose a rivet 3 X 0.05 ::: 0.15in diameter. The nearest standard ~ized rivet to this is 5/32in (0.156in). '111C shear strength of our D1234 5/ 32in rivet is S70psi. The bearing strength of the sheet m aterial when used with tHis sized rivet is 6S0psi. Note that the rivet \\o1.l1 shear before the sheet material tears. Common sense tells us that any component will fail at its weakest point and we have shown that dIe rivets in a joint will fail befbre the surrounding material. P utting a riveted joint in a sheet of material therefore \\-ill reduce the original tensile strength of the sheet by anything up to 25 %. Turning this around, the shear strength of the riveted joint would be around 75% of the strength of the original sheet. Repairs have to be as strong as the original structure and the manue'lCturers will produce repair schemes that lachieve this. For our part, it 1 1\.\\l tmpNa sises the need to comply with these repair schemes .
e. CJ'\\<.:t:
,.... \"\;).\\'. ,,\,<'"
. (\.,..
(~~'
When we consider the strength of a riveted joint, we should note that the more boles we drill in it, and the closer dlose holes are to each other, the weaker the sheet material becomes. Howe,er, the shear strength of the rivets we place in the holes will finally detennine the tensile strength of the joint because the rivets should fail first. \'{/c can say, therefore, that the strength of a riveted joipt is determined by the shear strength, number and pitch of the rivets in it.
%
-
Load Transfer \,(:rhen a load transfers across a multiple row riveted joint, the load is not eve6l'y distributed between the rows . The row of rivets nearest to the 1000d will al~7s react the highest part of the load, passrng a propornonate1y lower load on~ [he fo llo\\ring row and so on. ;( ,~
~ ,
419 CQPJ"rigbt © B,,~' Colleg 2006 - 2007 All rights rmrvd
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C H AP T ER E I G H T R I VE TIN G
Lower Lower I.oad
Figure 8.4 _ Dis tribution of Suess in a Mulci-Row Rive t Panc rn
11tis means dUll the first row is at more risk frorn distorted holes and ri\'el shear than subse<:]uenr rows. You can demonstrate this by pinning a length of elastic band to a board with n\'Q or more evenly spaced dress pins and Then pulling the [rcc end of the band to see which pin deflects first. Designers can compensate for this by varying me number, diameter and spacing of the rivets to even our the srress levels. Howel'er, joints mat are subject to load rcversals create a more complex problem. Single row riveted joints are clearly exposed to the full load and are thcrefore potentially weak in anything but low stress levels. For this reason, single row ri\'eted joints are nonnally a\·oided in favour of multiple rows.
Lap Joints T he simple lap joint is only used ill lightly stressed siruations where there is no requirement for a flush surface. The joint is constructed by overlapping two skin panels. It has a disadvantage in tlmt the loads transferring across the joint are nOt truly aligned. Tins means that when a tension load is applied across the joint, the twO ~t~rs will attempt to align with each other and there is a tendenc~ fpo.<:tor light stress.
@
I
I r
..,,, 0
"%
L'lp joint
" f:
I
'(-
@
,
~
.~
I
;;!::;, 0'
'0
,~'"
Figure 8.5 - Lap JointS
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joggled Llp joint
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CHAPTER EIGHT RIVETING
The primary purpose of joggHng a lap joint is to produce a flush sur face. The load transfer across this type of joint is still not lfuiy aligned and ir suffers the same disadvantage as (he simple lap joint.
Butt Joints The bun joim is suited to highlr stressed siruanons where heavier gauge skins are joined O\'er frames and ribs. The skins arc aligned wirh each other in tlus type of joint. eliminating an~' tendency for them to distort under load, thus keeping the rh-ets wholly in shear. The single shear bun joim has a single strap or doubler attached to the underside of the joint that produces a smooth aerodynamic upper surface, making it well suited to higher speed, large allcrafl. \Vhere a repair has to be made to;l, large hole in the skin, and strength has to be restored. a flush insert patch supported by a butt strap could be used. As with all multiple row riveted joints, the load is not evenly dh-ided between the rows.
Single Suap (Single
~hl..-:lr)
gg Double Strap (Double Shc;u) Figu re 8.6 - Butt J oiols "II ,-
:.\.\I~l'he-doub1c si1(:ar butt joint is proportionately stronger and fatigue resistant but
I.."\\\.' -
\ ... ;\«.... . ;\:'I.!ei
. ~~'Y."'" ~~
,
suffers the disadvantage oflosing the flush upper surface. This joint has a strap or doubler on each side of the joint. In addition to providing a stronger joint. tbe ri'-ets arc better supported. However, Ihe load transfcr across the rivet wws is still une\'cnly distribuced.
Fr:tmc-
I
Butt Stmp
Figure 8.7 - ButtJ oiot:at:a Frame
-
p::::
On some large aircraft, there is a need to strengthen a butt joint at a framc;.,:;nd often provide a joint where there is a change of section invoh-log skins f12:..,& ting at an angle. In these cases, rhe bun strap or doublcr may be repIaced,.~vith an extruded metal bat with a cross-section that matcs ,vith the skins ar# clamps them to the frame. strengthening the joint in the process. A
421 CO/')njhl :O 8m!) Co,j,§ J006 - 200- AU riJ,hfl I1'srnvd
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CHAPT E R
EI G H T
RI VE TIN G
commonly used in these situations. The skins may be ril-eled to the inside or the oUlside o f the extruded secrion.
'Y' • Bar
Figure 8.8 - Reinforced JOinl
Flange Joints Flange joints arc used on strllcrural componems such as fanners and ribs where mainly compression loads are felt.
Figure 8.9 - Flange Joints
Sphere ofinOuence The 'sphere of influence' is the area within which a propedy closed rivet will damp the sheets in tight contact wim each other. \'X'hen sheets are clamped together with rivets, the aim is to produce 'faring surfaces' that are tightly fastened in full cont."lC( with each other to fonn a sealed joint. The telm 'faying surfaces' denotes the surfaces that contact each o ther in a joint arc thus hidden frolll view. Normally, these surfaces arc treated with insulating jointing compound prior 1'0 assembly to protect against cleclrolytic corrosion. This is of parrjcular importance when there is a risk of dissimilar metal corrosion. 111e la st thing we need is gaps between me 'faying surfaces' in a rivel'ed joinr that :tllow moism re in to eventually degrade the strength of the joint. I t is for these re:tsons that we need to consider the sphere of influence of a rivet.
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T he sphere of influence of a correctly installed rivet is a circular area that has a diameter approximately five times (SD) that of the shank of the rivet that lies at its centre. This is where the pitch and spacing of rivets in a joint becomes important. If the pitch were set at 4D for example, the spheres of influence of adjoining rivets would overlap, producing tight c'cntact between sheets. If the sp acing between the rivet rows were to be also set at 4D, then the spheres of influence existing between ri,ets tn the adjoining rows would also overlap .
-
SD
o
o
o Figure 8.10 - Sphere of Influ ence
Rivet Layout Terms Spac~¥J
is tlle term used to denote the distance between adjommg aver rows. -,0l.i firt lJ is the term used to deno te the distance Ibcc\vcen the centres o f two (,0\1.C'?(' - ad joining ri,~ets in a row. The 'hmd ' denotes t~e distance between the . \ .....(t": centre of a rivet and the edge ot the sheet. All these dIstances are expres sed as :\.,,\ .... values of the riFct shallk diamcter D .
f:nTI
,,5\';' (C\~
Edge of Sheet ------------
~ - - - - - - - - --- , --~
I
F igure 8. 11 - Rivet Layo ut Terms
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CHAPTER EIGHT RIVETIN G
A pitch or spacing of greater than -lD would not produce the required contact. A pitch and spacing of less than -l0 would increase the overlap but we must be ca reful not to increase the number of rivers in the joint [0 the point where an excessive number weakens it.
Ri vet Pitch & Spaci ng It is vcry important that rivets are placed In a joint so that they win give it maximum strength. Conversely, we must not place them so close together that the holes will weaken the joint. By a similar token, we must be careful nOt to place rows of rivets too close to tbe edge of the joint ,""here they arc likely to tear through, or 100 far away from it where the edge of the sheet willlif( up.
-.-2D
___ ,t~:l, __________ _
--~-~ -~
0
0 0
0 0
0
2D ~~4--4---------7 2D' 3D '
I
I
Figure 8.12 - Minimum Pitc h & Spacing on a Staggered Lap Joint
To avoid weak~J,1'g the joint, the pitch between rivets in a highly stressed joint shC(l!!d' Re\;cr be less than 30 in a staggered row riveted joint or less than -H~ i.i»<.;: fdouble chain riveted joint. The sp:Jcing should be not less than 21/ .. 0 ,. 'i.\~~fn a staggered row joint and not less than -lD in a double chain joint. In all :\J)....... . cases, the Imld, or distance from a ri,er to the edge of a sheet, must be no less ,,>,'\." than 20.
"
'"~,"
Figure 8.13· Minimum Pitch & Spacing o n a D o uble Chain Lap J oint
424 CtJ/!)T7ghl 0 BaTT)' COllf§ 2006 . :!()(r
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CHAPTER EIGHT R I V ETING
We might make the mistake of thinking tha t all riyers o n aircraft run in straight lines. There are many examples of circular river lines on insert repairs and on access pands. The rules for ciycr spacing, pitch and land srill apply in these situn tions. Mnrking out the rivet positions is do n~ using angular measurements but the measurements still coofoan to similar limitations. ~
/
/
/
e
I /
e
/
I I
-----, e e r, e
a
\ \
e
, "-
, e
-
e 0
a
\
a
a
e
\
\
Ie
0
/
e
e
I /
Y / ~
-~--
: : :::: : :; : I : :
2
\ I I /
"
Figure 8.14 - Inserl & Do ubler
The figures we have been using here are those that are generally quoted. \'I/heneyer you are engaged in the repair of aircra~t structures you must always comply nith the figures for pitch, spacing and and stated in the appro ved repair scbeme. 1
Dimpling In order that we may describe the tools, we need to gain some idea of their application. \Ve cannot do that without having knowledge of the vanous riveting processes. The syllabus main heading for this chapter is 'Riveting' and that, by default, seems to indicate we should expect to be questioned on it. \y"'e will cover that eventuality by describing the riveting processes and introdus§: the tools as and when they are needed. \, ' ith thad strategy in mind, we need ~ look at a few basic points. 1;2-
Action of a Rivet
o
"0-
A t tlus point, it would be usefuJ to examine what occurs when a solid ri\~ is installed. The act of forging the second head on the rivet expands the sh~'k to squeeze against and grip the sides of the drilled hole. The expansion 'tf the rivet material against the sides of the drilled holc lstresses and work qfrdens it, increasing its strength, and it also stresses and hardens [he material iri the area surrounding the hole, increasing its bearing strength.
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CHAPTER EIGHT RIVETIN G
The heads on each end of the river secure it and fumly clamp the join! materials together within the 'sphere of influence' as described earlier. The :1mounr by which the dinmetcr of rhe drilled hole exceeds rhe diamClcr of (he rivet shank is c:1l1ed the c1ear:wce. Too small a clearance will destroy the prorecti\·c cO:lIi ng on the rivet during insertion and will create burrs rhar may become trapped under the rivet head. As the rivet anernpts to exp:lnd, it will push up the surrounding material creating gaps between the faying surf.1ces. Too large a hole will not allow the rivet to grip cmrectly when it expands during driving and the sheets may separate. Rivet material mar swell into the gap in the surrounding faying surfaces. It is important that rivets arc installed in a way that allows them to de\'clop maximum strength. The relationship between the relative strengths of the sheets being joined and the riYers is not the only consideration. The choice of rivet length is important to the achievement of a correctly fonned head.
Solid Rivet Dimens ions The diameter (0) of a solid ri,Tet is taken as being the plain shank diameter. The length of protruding head rivets is taken from the bearing surface under the head to the end o f the shank. The length of a countersunk rivet is measured from the flat upper surface of the head [0 the end of lhe shank. Some specifications of countersunk rivets also have a raised portion on the bead. The length of a raised countersunk head rivet is measured from the upper end of lhe coumersunk portion, not including the raised portion of the head, to the end of the shank. The angles of different types of countersunk rivCt include 90°, 100° and 120°. TIle 100° countersunk head is now the most commonly uscd lype.
-- ("'
I
--
~
Shank Diameter Fig ure 8.15 · Solid Ri vel Dimensions
Allowance The length of the plain shank of a solid rivCt that protrudes through the far 'u side of a drilled hole when ir is inserted is called the 'allowance' and it is ,§' , expressed in tenns of rivet diameter '0 '. Ir is important that the correct
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CH A PTER EIGHT RI V E T ING
'allowance' is provided to ensure that there is sufficient material for the rivet head to form properly when drin'n . An 'allowa nce' of 1.SD is sufficient to form a flat head "I jD diameter and O. SD thick during reaction riveting.
rIat Head
F igure 8.16· Allowance
.An allowance of 1.50 will also permit the formation o f a snap head and O.7SD will pennit the formation of a countersunk head. \"'\nen selecting rivets, an appropriate standard length rh-ct should be chosen. Rivets are normally supplied in a variety of manufactured lengths. It is possible to cut a ri\-et to dIe correct lengdl using a rivet curting tool. This is not recommended practice as there is a risk that any tool marks on the tail of thb cur shank will initiate cfflcks in the driven head as it is being fooned .
Drilling Pilot H oles All holes should be drilled cleanlv to the correct size. This means that drill bits should be sharp. It is normal practice to drill pilot holes in repair pieces and then :tpen them up to accept the civets when you arc assembling the pieces . ..,\..,6Fhi~'?tvoids the siruation where you have to use a fully drilled hole as a 'guide' . O,'Ut:"t-C: ~ to drill a second hole in an underlying sheet, with the attendant risk of • \"'>).<'(.... enlarging the first one. e 'I.,,\"\.'I. -
,,~
(,0\: .
"""
\'\!hen marking out the pitch and spacing for rivets, scribers should not be used as they leave sharp indented lines that will act as a stress raiser. \"'\'here thin lines have to be drawn, a graphite pencil no harder than HB rna>' be used b.nr all traces of the marks must be removed as soon as they are no longer neede§.: Graphite is a cathode to aluminium alloy and causes galvanic corrosion . Fcft accurate drilling, and to avoid the possibility bf the drill running off au.f scoring the surrounding surface, a light centre pop should be made at each the marked rivet positions . I
or f
, Hole Lrn::arion Peg
Figure 8.17 - Hole Location Tool
427 Copyighl '0
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CHAPTeR
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H ole Location There are occasions when it may be necessary to locau:! or 'pick-up' a hole in underlying stnlcture and transfer that location ontO a repair piece. -nlis is usually due to a lack of access from the rea r. A simple hole locacion tool can provc effective when faced with doing dus. 'n1e peg is inscrrcd into [he exiscing hole and the position of the new hole is marked by part drilling through the drill guide. Drill StOPS
\X!hen carrying out repairs to aircraft structure, there is a very real risk of creacing very expensi\"c damage if you fail to consider what lies within range of the drill. There have been numerous instances where an o perator has inndvertcntly drilled into underlying primney structure. components or cnblc looms, turning n simple job into a major sttucrural repair - or a career altering disaster! \X'here there is a risk of this occurring, there are a couple of simple tools that can be employed. It may be possible to slide a thin hardened steel back-platc between the back of the repair and the structure at risk. ' nus will block the driB from moving on and will gi,-e you a posicive indication that maybe you should stop drilling. If you employ this method, do remember to remo\"e the piate before you close up the repair!
~~----~) Figure 8.18 • Drill Stop
Another s~p' lc ~o1 is a locally manufacrured drill StOP that consists of a collar that is 8:tffl~1: d OntO [he drill bit to limit its depth of penetracion. This is a ROlre~'-'6pcion than wrapping tape around the drill to indicate the limit of .: \'\:-<;penerraoon because the tape gradually gets pushed back if you arc drilling {\~. . \ man)' holcs. Do check the coUar after you ha,'e finished drilling each hole. It is ( ....~... a bit embarrassing to find that it is missing after drilling a hundred holes! Dri Ui ng Machines ll1ere is a range of hand and power-operated drilling m:l.c1unes and these may be found illustrated in Chapter 3 (Figures 3-39 and 3-40) of this module together with descriptions. Elecrrically dri"en machines should never be used on aircraft where there is a possibility of flammable vapours. The generalpurpose air-operated drills are supplied with 6ther straight o r pistol grips. These arc powered by an air driven impellcr that transmits its drive through gea rs in a gearbox. Angled and offset air-driven drills are supplied [Q carer for areas where access would not be possible with a general-purpose drill. rl11ese drills require a separate chuck collet for each size of drill.
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D e-Bwring
It is ycry important that all swarf and rough edges are removed after drilling. This often invoh'es dismantling the repair after drilling to do this before final assembly and ri\-eting. i\ special de-burring tool is supplied that is similar to the countersinking tool that will be discussed later on in this chapter. -nus tool has a micro-adjustable depth cutter with a concentric scop sleeve and it will remove sharp edges and burrs without creating an unnecessary chamfer on the edge of the hole. Some repair schemes actually call for rndius de·burring to eliminate sbarp edges and this requires a specially shaped cutter. You oftcn hear that a drill that is t\vice the diameter of the hole can be turned by hand in the hole to remoye burrs . This is nOt considered good practice. On no account should emery cloth or files be used to remove burrs.
Hol e Size Wle ha\"c already discussed me importance of drilling the correct size clearance hole for a river. Tbe size of the hole has a direct influence on the strength of the joint. T o remind you, if there is too little clearance, the sheets will buckle as the rh-et expands. [f the clearance i~ too large, the sheets may be forced apart. The choice of hole size is gi,en in tables pro\;ded by the aircraft manufacrurer. The ri\-er hole and countersink sizes vary \\~th the thickness of the materials being joined and the type, material and length of n\"el being used. In very general tenns, the longcr the riyet and the harder the rivet material, the smaller the clearance required. Howcyer, close tolerance and inrerference fits are frequentlr specified.
COIHltersinking "')~,0b
-
, (.~,\\.;~.; '" \Vhcn countcrsunk rivets are specified, there are t\vo methods used for . ~,,~(' adapting drilled holes to accept a countersunk head. Cut countersinking is used on truck gauge materials whilst dimpling is used on sheet materials of 16S\\!G and thinner.
Cut Countersinking \'\/hcn the thickness of the sheet is greater than the depth of the countersunk. portion of the riyct head, cm countersinking is carried out uSing a speci#," countersinking tool that may be fined into hand or power operated drills. 'TIj(s tool can be set to adjust the depth of cOllntersink. There is a [able for guidaa?;:-e on the minimum sheet thickness for counrersinking particular rivet diam~-;s. For example: 6"
§;
429 CIJP)'righl 0 Btlf!)' Colkge 2006 - 2007 AU righl! rtu rvtd
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C H APTER E I G HT RIVETING
Rh'ct diameter
1/ 8
5/32
3/ 16
1/ 4
Mimmum Shcct Thickness (SWG)
18
16
14
12
Tab le 8.3 - Minimum Shee t Thickn ess
Dcpth of Cu t Tube
Holder
D epth or Cut Adjustment
Grub Screw
Spigot Currer Figure 8.19 - Countersinking T ool
.,(\~\"";
T he rool hdN.lcr is designed to accept one of a pair of cutters provided for cach 9n..op 'ie:6'o and 120 Q countersink angle. Each cuner has an a.xial bo re designed to , \'\~~~c:cept an interchangc:lblc centralising pilol spigot to locate it and to enable it .\{:"\ .. to cut a conccntric countersink in a drilled hole. The interchangeable cuttcr is (. ~...~,,, threaded and screws into the tool holder, the pilot tip is locked in the cuner bore with an Allen grub screw. Olle cutter in each pair can acccpt 2...tmm and 3.2mm di!l.mcter pilot spigots and the orner will !l.cccpt -Imm and 4.8mm spigOts. holdcr can be rotated to adjust the position of the cutter in rdarion to the scop tube. This is done by pulling back the spring loaded lock slec\'c to disengage it from the serrations and then scre'\\ting the holdcr in or out. Adjustment may be made in steps of O.DO l in using the graduatcd scale on thc holder.
-nlC
-
,
.~
;-
'"'" 0'
'v
~
't
430 GIP)77"M C Bm!)' Colk,gt :!006 . 200 -
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CHA P T ER EI GHT RI VET I NG
Head Stands Proud 0.U01 in
1
A I \
AllO\vance
1
\.
Ed J . ge., }.fust Not Create 'Feather'
Figure 8.20 - C ut- Coulltersink
The depth of cut should be adjusted so that it dais not create a feathered edge at the bottom of the countersink and all O\VS the head of an inserted rivet to stand proud by approximately 0.001 in. It is not l cceptable for the head to lie below the surface of the sheet or to project toO much above it. The countersunk recess must be free from scores and thatter marks.
Dimpling When the thickness of the sheet is dose to, o~ less tban, the depth of the countersunk rivet head, dimpling is reqUlred. T his is a process where sheet of 16S\\7G and thinner is indented around a drilled hole to accept a countersunk ovet head. There is a choice of three methods, punch dimpling, spin dimpling and hot dimpling. The method chosen depends on the ductility of the sheet material. Soft., ductile aluminium alloys are suited to punch or spin dimpling at room temperarure. Less ductile aluminium alloys and titanium would become wQt~futrdened and crack and these require hot dimpling.
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\,\lhere doubt exists as to the suitability of a material for dimpling, a controlled test should be carried out. It is unlikely that you will be called upon to do this test because the manufacturers \...1.1..1 have alread), cleared the material and method prior to issuing the approved aircraft repair scheme. However, someone might pick on it as a suitable question so we had best look at it. T \!e tes t im-olves producing a test piece from the material approximately 8in lo ~ by 1in wide. The test piece is then drilled and dimpled along its centre-hilS. using the same pitch that will be used in the reRair scheme. The test piece (:f then bent sharply across each of the dimples in turn. Apart from an expectect width-wise crack, there should be no eyidence of other circwnferential qr radial cracks. Pun eh DimpUng This is the most straightforward method and is suited to soft aluminium .4lloys and some sheet steels. The tools include a I71a19 die that incorporates 'ii'pilot spigot that engages with a femalc dic. A hole marching the diame# of the spigot is drilled in the sheet and the punch is then placed and driven t6 squeeze the metal loto the female die thus ctea ti.ng the dimple.
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Punch
Pilot Tip
Figme 8.21 - Punch Dimpliog Tools
Dimpling increases the size of the drilled o"et hole. Because of rhis, the hole is normally drilled undersize and is reamed to fit the pilot spigot. After dimpling. there is normally enough surplus mateoal left to permit a final smomh hole size to be achieved by a reamer or clearance drill. The hole is then dc-burred. Spin Dimpling
T he tools used for spin dimpling are very similar to those used for punch dimpling. ·I11e difference is that the male die is spun and pressed into the fem:tle die. The material around the hole stretches and spreads around the com-our of the die. \,({hen clad aluminium alloys are dimpled in (his way, the cbdding is also spread by the spinning action. Hot Dimpling
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_ ."!\\Ci ; The'\~o.:. i~ A~\ll~;oi~e of two methods. The first method, referred to as 'coin~ fljfnp1.'irig', employs electrically heated dies. These are nonnaU), incorporated \'l:-' in ro a portable har squeeze-dimpling tool whose jaws may be adiuS[ed [0 accommodate different material thickness. The second method invoh-es passing a specified e.lectrical current through the material to resistance heat it whilst pressing it between the rues to foon me dimple.
Sheet Gripping Pins It is necessary to hold repair pieces in correct alignment during drilling and hold them finnly together in close coomct during rivering. Sheet gripping pins can be inserted inro a suitable number of pre-drilled holes and tightened to draw the pa rts tOgether so that there are no gaps. A gripping pin is a close fir and it must be Llsed in the correct sized hole. They must never be fo rced or used 1"0 lever holes into alignment!
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Unlock ed
Un locked
Locked Figure 8.22 - Gri p Pins
Two types of grip pin are illustrated; the circular type incorporates a plastic end cap that protects the surface of the sheet from contact with the metal pin. The plastic caps are colour coded to indicate pin diameter.
Spring Loaded Plunger
1
Fig ure 8.23 - Cicco Fastener
The Cleco fastener is another type of gripping de.ice in common use. It is installed using special pliers thaI grip the fastener and compress the locking plunger, allowing the fastener to be inserted into the hole. When the pliers are released, the plunger springs up locking the fastener securely in the hole wh.ilst dra\ving the sheets together. These fasteners are also colour coded to indiq.te the size of hole the)' fit. .~
-"
-.
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Jointing Compounds & Sealants T he faying surfaces are normally coated with a jointing compound prior Jb final assembly. This is re fe rred to as 'wet assembly'. 111e primary objective i#to ~ inhibit electrolytic action between the sheet materials and to preyen'~ the ingress of moisture. This is particularly important where materials of diw rent electrical potential are to be joined. Yellow chromate jointing compound is commonly used, zinc chromate and barium chromate being exam~~s . The compound should be applied as a thin even film.
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Sealam is used [0 seal ri"cted joints in pressuriscd cabins and in fuel tanks. " is normally applied in a O.030in thick layer so that it exudes from the joint as it is riveted.
Jigs, Fixtures & Jury Struts This topic is often overlooked when discussing st:rUcrurai repairs so we will give it a special mention here. The structure of an aircraft may require supporting during repair work that involves dismantling primary IO:ld bearing components. The structu re may have to be supported on adjust:lb!e tresues and fonners or be held in shape with special alignment fi."cures :lncl jigs. The approved rep:lir scheme will specify what is required. Ilowcver, once the structure has been correctly supported and braced, you must ensure that the e'luipment will not be inadvenendy interfered with during the course of the repair. This may require the locking of adjustable tresdes etc. and the pro\-ision of clear waming notices. You would not be best pleased ro arrive at work to find that someone had 'borrowed' a tresde during the night. It has happened. Jigs and alignment fL"rures are also used to locate parts in their correct locations while they are being ri\·eted and are often used to locate and guide tools. For example, when holes have to be drilled within \·ery close tolerances, a drill fixture may be used to hold the drill accurately in position while a hole is being drilled. \,(/here jigs and fi."rures have a direct bearing on the accuracy of a repair rhe), must be subjected to periodic checks to ensurc that they still confonn to specification. I\ lot of this equipment is nOI in frequent lise and a decision needs ro be made whether to rely on a pre-use check, with the obvious time penalty if il fails, or to periodically check the equipmcnt in srorc. Jury strurs :lre (cmpor:uy load s upport devices that are fiXed into position to fulfil the role "R-f" stressed components when they arc being rcmo\fed, disasscmb...ldd")or repaired. Again, strict precaucions must be taken to prevent th<:.~e'b~~ng imerfcred with during a repair.
,• \"\)10: Installing Solid Rivets Up to this point, we havc co\·ered the poslDoning, drilling, countersinking, dimpling, and de-burring aspects of the ri,eting process. We havc also discussed the requircment [0 remove swarf and other debris before applying jointing compound and bringing the sheets to be joined together in close contact and securing them with grip pins so that there are no gaps. We can now look at the process of installing and closing the Dvets. \'{Ihen we arc preparing to install a rivet, we must ensure that wc h:wc the correct 'allowance'. As you already know, rhe 'allowance' is the amount of material that is rcquired ro fonn a rivet head and is cxpressed as the lengrh of the pan of a rivet shank that protrudes through the work. To remind you, for a solid rivet requiring a flat or snap fanned head, an allowance of 1.5D is sufficient. For a coumcrsunk fonned head, 0.75D would be sufficient. Rivet s
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may be closed using hand or pneumatic hammering. Hand hammering is rarely employed on modern aircraft but we do nced to dxamine it. Before we do, we should remind ourselves of the heat trcatrnent of ovets .
Heat Treatment Hear treatable, aluminium alloy rivets ha,e to be hard and strong in order to fulftl their role. However, they cannot be dri,en in that state because they would crack. Prior to installing these rivets, ther are 'solution' treated to rcnder them temporarily malleable. As a rule, the rivets must be uscd \vithin 0"0 hours of thi s treatment or they \'oill have 'age hardened' sufficiently ro prevent their usc. They may be re-treated twice more because there is a maximum limit imposed of three 'sollltion ' treatments in total. Repeat treatments beyond this number will embrittle tbe ri'ets. The time between a 'solution' treaun ent and lOstallation can be extended by refrigerating the rivets. Age hardening can be delayed by refrigerating the rivets immediately after quenching. \'('hen stored at a temperature ofOoe to -SoC the , rivets will remain in a usable cOfldition for 45 h o urs. \\Then stored at a temperature of _15°e to _20 oe the ovets \vill r~main usable for 150 hours. HO\ve\'er, the ri,'ets must be med within two hours of rcmo\'al from refrigeration. You m ust comply with th e particular rivet specifi cation sheets because there are exceptions. For exam ple, 2024 ri,"ets must be used wi thin twenty minutes of heat treatment or refrigeration .
Hand Riveting \\lh,~,the
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rivet has been inserted into the hole, the
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head must be
2.f\~§friTil.Y supported in a dolly that has a matching profiled indentation. The dolly
should incorporate a heavy bolder or failing tlliJ, it should be backed with a heavy block. To give you an idea, a 1/8in diameter rivet would idcally require a block weighing approximately one to o ne and a half pounds .
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Hand Set
Dollr Figure 8.24 - H and Riveting
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A rool called a draw set is slipped o,er the rivet shank and is tapped lightly with a hammer to ensure that the sheets are drawn fmnly togedler. \,\'hen forming a flat head, a hand set is placed sqlmrely on tOp of the river shank and is struck sharply with a hand hammer to compress the shank to a thickness of 0.50. You must ensure that the dolly is flnl11y backed to preven t Ihe rivet moving and locally distorting the work. As few hammer blows as possible should be employed. A large number of light blows will work·harden the riYCt material making it more difficuil to Conn the head and will more than likely crack it. You sometimes come across descriptions of hand riveting that say that rhe rivet shank may be struck directly 'with a hammer to form a flat head. This is not good praccice because hammer faccs afe often scarred and misshapen and hammers frequently hit more than they are aimed at! 'ne hand set has a smooth polished face specifically designed for the job and transmits the force more squarely, so why not use it? I f you arc required to fonn a snap head, a tool called a snap is used instead of a hand set. You should partially, and carefully, pre-form the shank using the ball of a ball·pein hammer and tllen finish it with the correct shaped snap.
Pneumatic lliveting
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Most of the rivet,s installed in modern aircraft are closed by a reciprocating air driven hammer, more commonly known as a rivet gun. BecaLlse the pre· fomled heads of the rivets are usually on the outside of dlC aircraft skin, and access to the rear is often restricted, the Type of riveting employcd is different to that we have just discLlssed. to this type of riveting, the shank of the ri'-ct is supported by ~i,\~oll y or reaction block whilst the pre-formed head is hammercd.1t¥1rough thc snap. Americans often refer to the snap as a 'scr' and the ...&'IlCfi~n block as the 'bucking bar'. The rivet shank forms by reacting to ~).1.this hammering against the block or 'bucking bar'. This is called rt.' actio n ril'cting and lhe proccss requires t\\'o people, onc (0 operate the riveting hammer and a second person to hold the reaction block against the [ail of thc riveL Before we look at the process, ,ve need to examine the riveting hammcr.
Snap
-
Hcactiun Block (Dolly) Fig ure 8.25 . Reaction Rive ting
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Pneumatic Riveting Hammers Pneumatic riveting hammers are used for reaction riyeting solid ri'-ets. There are two main types. light, fast hammering, shorr stroke guns delivcr up to 2,000 light blows per minute suited to driving 3/32in to 1/8in diameter rivets and 1,100 blows pcr minute suited to driving up to 1/ 4in diameter light alloy rivets. Slow hammering, long stroke guns deliver heavier blows suited to driving larger diameter rivets; some of these deliver a single blow each rime tbe trigger is pulled. Pi smn lnrerchangeable Sn ap
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Bunon Snap Retainer <>:--- Spring
[I H,unmcr Figure 8.26 • Pneumatic Rive ting Hammers
Riveting hammers are available \vith pistol grip, straight or offset handles . All types opcrate on a similar principle. Thc air pressure supply is controlled by a throtde button or trigger and it causes the pistorl in the hammer to oscillate back and fo rth in its barrel. The piston impacts with an interchangeable snap that is loosely held in position in the gun by a snap retainer 'beehive' spring thasj\\.fcrewed onto the end of the barrel. I\n air regulator screw on the handle 1\'\Wla{be adjustcd to reduce the maximum rate and force of the blows . The gun c.. -.,W.:.'f.~ . - is operatcd in short bursts. ,
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j\ riveting hammer can be badly damaged if it is operated with no snap fi tted or if the snap is not in right contact with a rivet or other solid object. A far
more dangerous situation will bc created if the trigger is operated with a snap fitted without the retaining spring screwed on the barrel. The hammer can tuw, dIe snap into a pro jectile that can be lethal at close range so, whether ~ spring is in place or not, you must never point a riveting hamnler di anyone when it is connected to an air supplyl :
Using a Riveting Hammer
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The first job is to go and get yo ur ear defenders! The next job is to sc1c~"thc correct hammer for the sizc and material of the fivets you \"1s h to piaai' The correct size snap should be fined into the barrel and secured by scr~ng on the retaining spring before the :111' supply is connected For dafue head rivets, the snap should have an indentation diamc1ter that is slightly larger than
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the rivet head so that the force will be coocemrared ar the centre of the head rather th:1n :u the sides. Howeyer. too large a snap will create an indentation on the skin around the head and too small a snap will indenr a ring on rhe head. Because there arc different shaped dome head rh'cts, it is essen tial that the co rrect snap be used. A snap designed for closing a counrersunk rivet ha s a smooth, slightly convex face to avoid damage to the surrounding skin. I f you wish to adjust the air regulator to deliyer a desired rate and force of hammering, you should ensure that the snap is finnly pressed against a block of wood before o perating the gun to check }'our adjuscmem.
Reacrion Riveting \X' hen the rivet is insened into the drilled hole, another person musr be on the o ther side of the work to firmly apply the dolly or reacoon block to rhe tail of the rivet. The dolly is a hardened Heel block or bar with a smooth polished reaction face that prC\·ents damage to the rivet tail. The river tail is spread by the rcacoon block so the position in which it is held in relation to the ri,·et shank is critical to the correct formation of the head. If the bar is angled, the river shank will be pushed over and it will nor fonn a concentric head. 1f the bar is pressed against the shank tOO hard or the bar is tOO heavy, the head may be driven toO flat and thin. If the bar is not held tightl)' against the shank or it is roo light, the riveting ha.nuner will locall), distorr the skin around the rivel. The aim is to balance the pressure placed on the rivet shank with the pressure being applied by the riveting hammer so that tbe pre-fanned head is held fi rmly against ("he skin. It req uires a good sense of feel [0 achieve this. Because the person holding tbe reaction block has [0 gel it right, there is a large range of different shaped reaction blocks and ban designed [0 give access in difficult locarions.
Figure 8.27 - Examples of Reaction Blocks
When you commence riveting, rhe soap (set) must be placed square onto the rivet head, the trigger of the rivering hammer should be o pe.r:ued gradually and you should restrict the o peration to short bursts. This makes il easier for the person holding the reaction block (bucking bar) to control the exp:l1lsion of the rivet. The rivet should be fanned using the fewest blows possible to avoid work ha.rdening it.
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Figure 8.28 -lns lalling So lid RivclS
A question for you, if the hidden person is in control, how does (he ri\'ering hammer opemtor know when to hammer and whcn to Stop? They usually ha\ye an agreed code of taps . Shouong 'StOp' above a riycring hammer when both of you arc wearing car defenders is not the best idca of the day. j \ 5 the o perator of the hammer, you need to know that the reactio n bar is in place and when thc othc r chap wants yo u to hammer. Yo u need to know exactly when to stop, and you need 1O know whether the rivet has been success fully closed - because you will hav e 1O remove it if it is not!
Squeeze Riveting . ~\\.;.-:"c '\~~' l,', ~ \",-. ,'0:0;:.... -;>.... (.0
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ll ammcring is not ' tcry conduciye to riveting non-metallic and bonded laminated materials together that may suffer through the percussion and vibmtio n. Some thin gauge metal sheets may suffer distortion fo r similar reaso ns. Squeeze o r compression rivering does not employ hammering but it is only suitable fo r ri,-eting small assemblies where there is access to both sides of the work. The rh·ets arc squeezed using equipment that is powered by either hydmulic or pneumatic pressure. ~ '-,
·s
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o
Figure 8.29 - Squeeze Ri veters
T he squeeze ciycting machines in the illustration have one stationrtry jaw and a moving jaw thrtt is moved by a piston in an air cylinder. For dome head rivers, a suitably shaped dolly is fined onto the stationary jaw to support [he preformed head whilst a flat set is fitted OntO the moving jaw. Shims are fined between the jaws and these anachrnents to adjust their separation and (hus the height of the formed head. Before riveting with these tools, the working pressure must be adjusted for the material, thickness and nyet size of the work being carried out. If the tools are required for usc 00 other work where there are changes to these specifications, the working pressure must be re-adjusted to suit. Adjustm'¥i'-ts are done by trial and error using a test piece. ,,()I.)o
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llivet Milling
\'{'hen countersunk rivets have been dosed, the pre-fonned countersunk head usually stands slightly proud. The protrusion is very small but a repair scheme may call for milling of the heads to obtain a specified degree of flush surface. Avdcl civet mandrels also need milling down after Lhey have been cropped. \Yie will discuss the installation of A\Tdel rivets brer i.n th.is chapter. T he Rivet MiI1er is an air-operated tool that incorporates an interchangeable romry cutter housed in a nosepiece and a micrometer adjuster thar moves the nosepiece in or au[ relative to the cuner ro adjust the depth of cU[. Adjusunems cao be made in increments of O.OOO2Sin. You may hear this rool described as a 'Micro-shaver '.
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i'\[icromcter Aclju"'f
o
Lock Sleeyc
Rubber P:td F igure 8.30 - Rivel l\.liller
The rh-et miller illustrated is stabilised by two spring loaded telescopic legs thaI have rubber pads designed to gh-e :l. fcicrion grip on a surface. Prior to usc, the correCt sized cuner and nosepiece sbould be fitted. There are twO sizes, 5/16in and 3/8in. The tool is adjusted by pulling back the sp ring loaded lock sleeve and rotating the micrometer sleeve. The miller is inirially set up by adjusting it so that the cutter just contacts a straight edge placed across the nosepiece. Further ad justment would require trial cutting on a test piece. T he miUer must be held firmly when it is bdng opcrnted. 1t ope rates al around 18,OOORPM and can easily throw itself away from the work area . Care must be taken not to mill into the skin or into the beads of Avdel rivets. One poim worth making here is tbat you are not allowed to mill down the heads of coult\~ofs unk rivets that pwtnJde due to incorrect cut countersinking or ~I.: ;.\\Icllmpling!
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Removing Solid Rivets YOLI may need 10 remove ri\-ets as part of a repair scheme, or because th'€y haye nUl closed correcrly. \\"'hen conducting reaction riveting on large numb ~. of D\-ets, you necd to identify those that require remo\-al if you are not going r~ remo\-c them at the rime. You do not need a thin line to mark thcse, a waX. cmron, not black, may be used to idellrify them. Graphite pencils and crayon§: containing carbon should not be used for this task because they form powerfUl cathodes when left in contaCt \vith light alloys and can calise subseclueRt galvanic corrosion and make stressed stmcture prone to cracking. To pre\~lt enlarging the hole, the removal process is normally conducted on the ~ re formed hcad because this is more symmetrical than the forn1ed head ~d ts oflcn easier to access. ~
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Removing D ome H ead Rive ts File a small flat on the pre-formed head of rhe river. Make a centre punch indentation on the flat at the estimated centre position of the river using a single light blow on rhe punch. Using a drill sHghf~" s n w J/er fh :m the rivet sh:mk, drill down to the depth of the head. Keep rhe force on the drill light to avoid the risk of the rivet spinning around with rhe drill in the hole. If the head docs not come away at this stage, tap it off with a flat chisel in the direction of the rivet row. Support the underside of the rivet with a hollow dolly and usc a par:1l1el pin punch the same diameter as the rivet shank to tap Out the remaining shank into the dollr. If the rivet is installed in bonded, laminated structure, ca re should be taken not to cause de-lamination of the layers when doing this.
~
Knock Head Off
Dnll i lcad
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Et:P Suppon
Drin: OUt Shan k
"'D
IT "' " 1fl Sk;n~
Fig ure 8.3 1 • Removing a Dom e H ead Rivel
. C,"'RemOVlm}' ountersun k Ri vets \\ .. >!...: ...
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\~~\Make a centre punch indcntarion at the estimated centre position of the head ' ~\. . ,.
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\. ~,'\
of the rivet using a single light blow on the punch. Using a drill slighrJ}' smaller than the rivet shank, lighcly drill down keeping the drill concentric with the head until the head either comes away or it can be prized off with a pin punch. SuppOrt the underside of the ri,et in a hollow dolly and tap our the shank with a parallel pin punch the same diameter as the rivet shank. It is possible to drill right down through the shank and push the shank through with the drill. l1Us tactic does carry the risk of enlarging or distorting the hole so extreme care is required if it has to be used.
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Centre Pop
Figure 8.32 - Removing a Countersunk Rivet
Blind Rive ting Blind rivets are used when anI\" one side of
the
rivet hole can be accessed.
They may only be used when specified in a repair scheme. Blind expansion rivets are hoUow and are dosed by pulling a mandrel through the bore of the rivet in an action called broaching. T1H~ fTl.,1.odre1s, often have grooved stems to increase the grip of the Lool mal is llsed to pull the mandrel into the bore to fo rm the blind head and expand the rivet. In some types, the mandrel remains in the bore of the rivet to act as a plug and La increase the shear strength. Friction lock types rely on the friction between the stem and the body to hold the stem In place, whereas mechanical lock types Incorporate a locking collar that is s\vaged into a groove on the Slem as the rivet is broached. In another type, a separate sealing pin has to be d riven in to the bore to plug it. i'v lodule 6 Chapter 5-4 deals with the detailed descriptions and identification of the different types of river. For lhe purposes of trus chapter, we are only interested in the tools used to install them. The blind rivets we shall examine are-")\.'l\f
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Grip
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Because a blind riv et is inserted and dosed from one side of the work only~is not always possible to view the pari of the riyet tbat projects 00 the othe[/~ide. Tt is important that the correct length of rivet is selected in relation ,# thc thickness of the material to ensure that the blind head is able form s;;o rrectly and that the sheets will be properly fastened. The length of the posh~o of a blind rivet that is inside a hole is called the The 'grip' of a countersunk
< grill
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CHAP T ER EIGHT RIVETIN G
head rivet includes the depth of its countersunk portion. Rivets are produced in a variety of di fferent grip lengths, each baving a Grip Range that identifies the material thickness they will fasten.
Dimple Countersunk
Unin:rsalllcad
Cut
COllnter~unk
Figure 8.33 - Grip
Installing Avdel Rivets Avdelrivets are produced with snap or countersunk heads. They have a similar strength to that of solid rivets. Each rivet incorporates a body and a bulbed stem (mandrel).
Avdel
lIT lIT
Chobert
Chobert
Figure 8.34 - Avdel & Chobe n Rivets
The rivet is pre-lubricated at manufacture to give consistent results and it must nor be washed in solvent before being installed. TIle ri,et is inserted into hand riveting pliers or a riveting gun and is then inserted into drilled hole. A steady pressure is applied to push o n the rivet as the tool is operated.
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CHAPTER EIGHT RIVETING
S,\:m puUcd br Too !
~
Stem
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Brcak~
Srem
Stem i'.1i!!cd Rush
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fL--'
Figure 8.35· Avdcl Rh'et
T he tool grips the stem and pulls on it, causing the bulbed end to enter the lower pan of the body to expand and fonn the blind head . This action is caUed 'broaching '. At a SCt load \'allle, [he stem fractures at a break point jus t abO\'c thc manufactured ri\'et head. TIus action Ica\'Cs pan of the stem in the bore of the body to act as a pl ug. TIle surplus stem is snipped o ff with a pneum:uicaUy opernted mandrel cropping tool and is then milled flush with the head o f the rivet. Care needs to be taken when using the ri\'et miller to a\'oid cutting into the head of the n\·cl. Whl10- ,1he installation is complete, rhe tighwcss of the stem or pin in lhe rivet bore is checked using an Al'del Pin TeSler. T his [001 has a (l"\.:--:..C. ... retractable spring.loaded pin that is pushed against the stem. The stem should not moye when exposed to the full compression of the tool. ~\,\ I~bUo\V
o
Figure 8.36 · Blind Ri\'ct Pliers
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CH APTE R EIG H T RI VE T I N G
Riveting pliers arc used when only a few rivets ha,'e to be placed or where access prohibits the use of a ril""ecing gun. Avdel ri\Teting pliers arc designed for one S[rokc operation and they have a ratcher operation that enables them to be used in confi ned spaces where ani), small. repeated handle movemenrs can be achieved. There is also a long handled version dlal enables an oper:ltor to reach a remme location. Oil Reservoir
Intensifier
,, , , ,, , , ,, , , ,,
,\IT Supply
Figure 8.37 • Avdel Riveting Kil
When a large number of rivets are to be fined, a rivering gun can be employed. The i\vdcl riveting kit consists of a hydraulically acruated rive ring gun lim is connected through tWO flexible pipelines to an air.opernted intensifier. 'n1e air supply to the intensifier is regulated at 60psi to SOpsi and acts on one side of a piston to create a high hydraulic pressure on the other. One flexible pipeline carries oil under pressure [Q the riveter whilst the other carries compressed air to the conlrol button. \Vhen the control button on the riveler is oper:ued, it opens an exh au!.~\r>ort that causes the air pressure to fall in the pipeline. This opc'1? A!}~tYl~upply shuttle yalve to the intensifier and the hydraulic pressure F9cll't~ropera te lhe riveting gun. Releasing the control burton shuts off the air ... \':-'\ upply, exhausts the air pressure in the intensifier and releases the hydraulic pressure. The intensifier must ahvays be kept horizontal during ri\·eting. ,\{-'\. -. >" co,
TIle riveting kit contains three front cones that arc clearly marked with the rivet sizes to which each relates. The same cone is used for placing both dome head and countersunk rivets. W1h.en preparing ro inStall ri\'ets, the correct from cone should be fined. The intensifier resen'oir sight glass should be checked to ensure the correct oil level. The air supply should then be connected and the gun operated to check that the barrel extends fully and then retracts when the control button is released . If this tCSt pmves unsatisfactory, it indicates that dle riveter needs priming/bleeding. You must neyer operate the riveter with a ri,-et fined that is not placed in a drilled hole. If you inad\'enently do this, the mandrel will be drawn right through tbe ri\'ct body and into the gun where it will become wedged.
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Installing Chobert Rivets Chobcrt rivcts are simihr to but are not as strong as Avdcl rivets of the same material fi nd size. They arc produced with snap or coun tcrsu nk heads. They consist of a body only and these are nomlally supplied in tubes that facilita te their asscmbly onto a reusable steel mandrel that is part of the installation tool. 'l1,e lower pan of the o"et bore is tapered and the upper part is pa rallel The 'broaching' of [he mandrel through the tapered section fonns the hcad of the rh·c(. As thc mandrel is dmwn completely through the bore, the rivet body expands in the drilled hole. The hoUow bore of tbe placed rivet is then sealed by dri"ing an interference fit pin into iL This inc reases the shea r strength and excludes moisture.
1--- Re-L' sable .\Iandrel
Scaling Pin
Figure 8.38 - Chobe n Ri vet
JYh~1'1'ji.lsl a few Chobert ri,'ets are to be fined, riveting pliers can be used . The ,.~..:. ~\\\ehobert riveting pliers are specific to Chobcn rivets and cannot be used with , ,'\\0... I .
,,<-~.. \.. ~~'\
'" \""~
<' \...'
or ler type nvets. They ha'\"'c an interchangeable jaw and mandrel and the appropriate size must be fined for the size of riYer. TIle mandrel needs light lubrication prior to threading a oyer 00(0 it. The nYet body is threaded Onto the mandrel lIntil it abuts the tool nosepiece. The ri"er is thcn pushed into the drilled hole and a stead~' pressure is applied [0 il as the pliers arc closed ,to completc the broaching acnon. After broaching, the correct sized scaling pinj$ do,'en inlO the bore. ~
:G..
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CHAPTER EI G HT RI VE TIN G
Hydraulic Riveter
PneUm3t.lc RepClluon RI\'ctcl'
Figure 8.39 - Cbobect Repetition Rivelerli
\'{'hen a Iar~ number of ri,"ets have to be fitted, it is advisable to use a reperirion riveter. Repetition riveters mar be eithcr pncumatically or hydraulically operated rypcs. 111e ri\'eter has a mandrel that can acccpt up to thirry ri,'er bodies in a tubc up to 12in long in one loading. Thc rints are :lutomaocally positioned by a spring feed as each river is closed and high rates of closing can be maintaincd. The front jaw assembly, the mandrel and the mandrel spring are removable and a range of sizes is a,-ailable ro suit the rivet size range. Both dome head and countersunk ri\'cts can be placed with the same jaw and mandrel combination. The
cond~Rea- .ol;'l1e reusable mandrel is critical to thc succcssful operation of
lh.isll~h The mandrel should bc checked periodically with a m :mdreJ b ead
,\,\
(~"
,,~\Y.
.
:>,).{
T herc are [wo rypes of rcpetition rh'eter illustrated, one hydraulic and tbe other pneumatic, The hydraulic riveter is larger and has a longer mandrel and bigger diametcr nosepieccs. It is designed ro close larger diameter rivets and the hydraulic pressure is supplied from an air/ oil pressure intensifier. \'I.le discussed the operation of an intensifier when examining the A"del rive[er, I f the ri\'erer fails to broach a rivet cou ecLly, it rna)' indicate that there is air in the hydraulic system. There is a bleed screw on the riveter, \Vhen bleeding the gun, it must be held below the level of the intensifier.
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Chcrrylock
Chen-vlock
Huck
Huck
Fig ure 8.40 - Cherryiock & Huc k Rive ts
Installing Cherrylock Rivets C herrylock riyets arc similar to Avdel rivets except that the stem of the rivet is mechanically locked into the bore wi th a lock. collar during the broaching operation . There is another yersion of this rivet called a 'Cherrymax' that is similar in its operation. In both lypes, the rivet stem has pulling grooves on it and installation requires a special pulling tool. fn the final stage of pulling the stem through the bore, the locking collar is pressed into a recess formed between the head and a groo\'e in the stem. After dus, the stem breaks off slightl! proud of the rivet head. The protruding parts of the broken stem and locking collar are then milled with a rivet miller to produce a flush surface. Care must be taken not to cut into the head of the rivet when milling.
Grooves
Stem l ' OI\'ersal Break
Shear Ring
~
Figure 8.41 • C ben)'lock fvct
,~
Cherry rivets are installed with hand or pneumatically powered tools that §ie fitted with the correct type o f head fo r the size and type of rivet. The riv~ is first inserted into the drilled hole and the pulling tool is then pressed ov ~ the stem until a finn resistance is felt. "Maintaining p~essure on the rivet, tht;~ol is operated to broach the rivet. A pin tester similar to that used for Avdti'£ivets is th en applied to the sheared end of the locked slt em to check that t~re is no movement \vhen the tester is fully compressed.
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CHAPTER EIGHT RIVETING
Installing Huck Rivets I-luck ri,rcts :1.tC vcr}' simil:1.r to Cherry!oc k rivets in their design and operation. They arc mechanically locked in that they have a lock collar that is pressed into place by an anvil on the pulling tool in the fi nal stage of installation.
, Grooves
Ring
)
,J
Figure 8.42 - Huck Rivel
Tucker Pop & Imcx Rivets Tucker Pop and Imex rivets are known as blind expansion, friction lock rivets. They are produced with domed or countersunk heads. A Tucker Pop rivet is mounted o n an.,i~¢ividual sleel mandrel that has a bulbed head that is pulled into the ritcil body to expand the blind head during broaching. There are two stt.l1..dR-.rd" tYpes; the 'break head' type bas a mandrel tbat is waisted near the ~;..~ S!ind head, the 'break-stem' type is waisted part way up the mand rel. In the .\"\\ ... former type, the bulbed head shears and falls away whilst the mandrel is (: ,...'\....,\'v-o completel)' removed leaving a hollow bore. in rhe latter type, the mandrel shears part way up the stem, leaving the lower portion held in the bore to seal it. The break head type is llO[ often used on aircraft due to the problem of locaring and remo\·ing the broken mandrel heads.
Short Rrcak
Break Head
Long Break
li\rEX
ThrEX
Break Stem
Tucker 'Pop' Tucker 'Pop'
Figure 8 .43 - Tucke r Pop & Imex Rivels
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CHAPTER EIGHT RIVETIN G
The Imex rivet is similar to tbe Tucker Pop type except thal both (he Imex types arc sealed in that they lea\~e a part of the mandrel held in the bore to seal
it. The nvo types of lmex rh·er mandrel are the 's hort break' and lhc 'long break'. The 'shan break' mandrel breaks off inside the body after broachmg, rhe 'long break' mandrel breaks off proud of the head. The Imex rivel has a larger and more rapid expansion than the Tucker Pop ri\fc r and is not to be L1sed as a subsomte for the latter without repair scheme authorisation.
Installjng Tucker Pop & Imex Rivets The same riveting lOoIs are used to install Tucker Pop and Imex Rh·ets. \X'hen a small number of rivets are to be placed, ri\·eting pliers may be found more suitable. The pliers ha\-e interchangeable nosepieces and mandrel-gripping jaws [0 match [i\·et sizes. \X·hen installing a ri\'et, dle pliers should be held so that the rivet is held squa re in the drilled hole rhroughotlt the broaching operation. ;\f(er broaching, the pliers han' to be fully opened to release the sheared portion of the mandrel stem. You have to do tlus before attempting to insert another riyct. Particular care needs to be exercised not to let these surplus stems become loose articles.
I
I
Bre:1.k Head ~bndrcl
/
Break Stem - - ; J
~~I"~ Head She3fs Off
6
I-lead Retained in Rivet
Figure 8.4. -1-· Tucker Pop Rivel
.§The long break Imex ri,-et Iea,es a portion of [he mandrel projecting abO\'e th~ pre-fomled head after broaching. The surplus material has to be cropped ani may have to be ground flush. .~ 0 "
'""'" ac'" '0
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CHAPTER EIGHT RIVETING
Shon~ Break ~\landrcl
Long Break_ f\landrd
0
Figwe 8.45 - Imex Rive!
TIle rivcts rna)' also be installed using a rool of the 'lazy tong' type. The mandrel of the rivet is inscrted into the tool while it is extended, the rivct is (hen pushed squarely into the drilled hole and the tool is operated by pushing it down finnly until the mandrel shears. The tool is prO\;ded with a range of interchangeable heads to overcome access problems where a direct, srraiglu approach is n OI possible. The offset head is used where rivets have to be installed close to a flange. The corner head is used for a similar purpose bUI i. offers a differem angle of approach. The right angle head is used when rivets have to be installed inside the confines of a channel.
Off-Sct I lead
Corner Head
Right Angle Head
Coupler Figure 8.46 - Lazy Tong Rivelu
There is a hydraulically operated riveter, used in conjuncuo n with an intensifier, which operates from an air supply pressure of 70psi to SOpsi and it is designed for the installation of Tucker Pop and Imex Rivets. We have discussed a similar arrangement for installing both Avdel and Chobcrt ri\'cls, rh e operating principle is the same. The riveting gun has imerchangeable nosepieces that have an identification iener code that is related to rivet type :md size.
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CHAPTER EIGHT RI V ETING
O il ResermLr (nrensi fier
l(
/~l~
1 /
~I/ I
Air Supply
F igure 8.47 - Tucker Pop & Imex Riveting Kit
The oil level on the intensifier reservoir mUSt be checked prior to use. Tbe correct nosepiece should be fined. After each broaching operation, the gun should be tilted to discard the sheared rivet mandrel before a further nvet is loaded. If the gun fails to broach a rivet in one operation of the gun, the gun should be bled to remove au- from the oil. If the gun still fails to broach a rivet after bleeding, it is likely that the jaws are slipping on the mandrel . The jaws do tend to become clogged with mandrel surface material over time and they need removing and cleaning.
Removing Blind Expansion Rivets
\\ -i!-c
Blind rivets may be rcmm-ed by drilling off the head with a drill the same size as the hole diameter and then punching out the sJ;1ank of the rivet. Because it is not usually possible to support the blind side, blind expansion rivets may have to be removed by drilling them out with a drill of nominal rivet diameter. A hand drill brace should be used and the drilling force should be kept as light as possible to avoid forcing the rh-et to spin in the, hole. Consideration must be givCJt\\!P the removal of ri\Tet swarf and debris.lf 1allowed to remain, rills debris 1..1. 'IW . taycause galvaillc corrosion. I
_ (,,1'>-'1.. "
,.
\, .• ..;<;:'
"!\.,'t -
(.,--.~'
~\".;..
-
Removing Mechanical Lock Rivets These rivets can be removed by filing a small flat on the stem before punchifJg it out from the locked end. The head may then Be drilled off before punchiil"g out the shank with a parallel pin punch, However when the rivets are placed l thin material, the locking collar should be removed first by drilling a pilot hol ~ in the stem that is then opened out to re,eal the lock collar. The collar ma£:: then be drilled or levered off. \\fith the lock c6llar removed, the remai~ stem may be tapped out. After drilling to the depth of the head, the rem~ shank may be tapped out, ji:"
i4-
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'0 ;,
"
OJ
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C H APTER E I G H T RI V E TI N G
Special Purpose Fasteners Because the syllabus is non-specific about river types, it would be prudent for us to cover dlC installation of the more common special fasteners - bener to be safe than sorry! Details of the identification of these fasteners may be found in r-. fodule 6 Chapter 5-4. These type fasteners do not expand in the drilled hole. Many arc interference fits and tlns suesses the material surrounding the hole and strengthens it. Apart from the Jo-boh and the Rivnur, these fasfeners re'luire access to both sides of the work. The special fasteners we will examine in this regard arc:
•
lii- shear civets
•
Hi-Lok bolls
•
Ja-bolts
•
Huck Bolts and Avde.lok Fasteners
•
Ri\'mlts
Installing H i-Shear Rivets The hi-shear civet consists of a hi.gh shear strength pin that incorporalcs a lock collar o f so fter material that is pressed into a groove during the riveting process. The pins arc produced with standard or close tolerance shanks. There is also an oversize range for repair work.
'(\~
",
,
The pin is pushed into a pre-drilled hole and the head is supported on a block. A special hi-shelJMivct 'sct' is used to drive and form the lock collar in the pin groove, Yhe1lactlon of the set also trims the lock collar to the [e'luired length. TQ.Q~'itar is impregnated with a special lubricant during manuf.'lCl11fe to assist \,\).(lrs closure and to provide corrosion protection. It should not be washed in solvent or lubricated prior to installation.
.... ~""
o;n-~;::,
~
D
(ollar Furrow
a
=-c=Jl:;=
Fig ure 8.48 - Hi-Shear Rivet
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CHAPTER EIGHT RI VE TIN G
Installing H i-Lok Bolts !\ J{i-Lok boll consists of a pin that is threaded at onc end to accept a hexagon headed retaining colltl.r. The collar is pre-lubriGlled during manufacture and should not be washed in soh-em or lubricated pn o r LO installation. The pin is d riven into a pre-drilled interference tit hole a,~d the retaining collar is then screwed on . A special wrench 1001 is used to tigh ten the collar uoril, at a pre-set torque, the hexagon bead shears off leaying the remainder of the coliar firmly in place. The relaining collars are colour coded to indic:ue the pre-set tension that is imparted to the bolt when they are installed.
Shear NLH
I
~ She:tr POint
! )
tW-Rcta,rung Collar
~~~r
I -
\
I
1---
Bolt Figure 8.49 - Hi-Lok Bolts
Installing Jo-Bolts A Jo-b91t is a blind type of fastener that is used in lieu of a bolt where access is tifuilea to onc side of the work. It is a high strength, dose tolerance bolt. The . :\\.:.\!...:. 2.~\\f~stener consists of a threaded bolt with a sleeve and a round head that is ~{' \. I" screwed into a hexagon or countersunk headed nut. TIle bolt incorporates ... \...... drh1.ng flats at the end of lhe threaded stem. The fastener is inserted bolt head ,\\'. :-,.....<.y':' first through the accessible side of a pre-drilled hole. Jo-bolts are supplied preU,,\ lubricated and should not be degreased or lubricalcd befo re fitment. r.
r="f
Bolt
i l"'d
Dri\'ing
.~
I
Slee\"c
i~e"goo Fbi
"""ii.-
Blind Head
Closing
"
Bolt Stem Breaks Off
Figure 8.50 - Jo-Boh
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R I VET I NG
A special tool is used to hold the nut and to grip and rotate the driving flats on the bolt stem. Different adaptors must be fined to the tOol to accommodate different sized hexagon heads or cmcifonn slots. As [he handles of the (001 are closed, the bolt screws through the nut until the head contacts the sleeve and forces it over the tapered section of t.he nut to form a blind hefld whilst clflmping the joint' together. At a pre-set torque, the driving flats shear off the stem jusl inside the nut. The remaining threaded bolt stem, together with the nut, acts like a solid ri,tet in the hole.
Fig ure 8.5 1 - J o-B01l Im lallalio n T ool
The special tool is pro,rided with adaptors tha t are designed to fit a hexagon head or the crucifonn recesses on a countcrsunk head.
Ins talling Huck Bolts & Avdelok Fastene rs Huck bolts and Avedelok fasteners are similar in design and installarion. For the purposes of this sccrion, we will describe the Huck bolt. It is supplied in either ' pull' type W 'stump' type form. ll1c 'pull' type has a stem that has pulling g~Q.n:s 1~d it is plflced with a power-operated pulling tool, the 'stump' ty~~'Jsl.{1nven into a drilled bole. The choice of type depends on access. The . \,~~{~ tump' type is used in cases where there is no access for a pulling tool. '\.~
,\'
..;,5\'y-
,~ C'.
Break Neck
/
Bolt Stem
Sheared Off
Collar Swaged
Figure 8.52 - H uck Bo h
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CHAPTER EIGHT R I VET I NG
The 'pull' type bolt is inserted imo the hole and the lock colla r is pushed on. The collar is pre-lubricated during manufacrure to facilitate swaging and should not be washed in solvent or lubricated prior to installation . A power-operated pulling tool is attacbed to the grooved stem and is opem ted to pull the joint together and swage the collar onto rue grooves. l}t a pre-set pull fo rce, the bolt stem fmcrures at a break point. The 'srump' type is inserted into the bole and rhe collar is pushed o n. A support block is placed under the head and the collar is swaged Onto the groO\'cs with a punch.
Installjng llivn uts The Rivnut is a blind fastener tbat incorporates an anchor nut \\~th a locating key to prevent it rotating in its hole when fitted. TIley are used to provide anchor nuts in areas where there is only access to one side of the work. In tills sense, tbey arc not rivets in the true sense that they are llsed to faste n sheets together. [-I owe,cr, the process is similar to installing blind rivets and we do not want to take the chance that someone will decide that they consti tute a 'fair' question.
Closing Tool Figure 8.53·
RiVnlll
A Ri\"llUt is installed with a special tool that incorporates a threaded adaptqf that is screwed into the projecting portion of the nut and pulls TO collapse -it and form a blind bead. The drilled hole has (0 have a recess cut in its periph~' to accept the locating key. A special tool is provided by the manufacturer r~.zao ~s .
~
'"'"
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CHAPTER EIGHT RIVETING
Removing Special Fasteners Special fasteners that are locked in place by coUars may be removed by splitting the collar with a chisel and removing it before tapping OUt the pin using a hollow doUy to support the strucnlre. Alternatively. the collar may be milled off \\~Ih a hoUow mill before tapping out dle pin, using a hollow dolly [Q support the strucnlre. Other types of special fastener may be removed by drilling off the head before punching oUl the remaining coUar and the pin.
Insp ection Inspection of Ri veted Joints Ag:tin, wc nced to take note of the wording in rhe syUabus. It docs not specify whether the inspcction is to check joints jusr aftcr they ha\'e been made or whethcr we are inspccting joints as a part of a scheduled check. Neither does it specify the £}-pe of rivCts in thosc joints. It is no usc tr);ng to 'mind read'; you havc to 'assume' char it means all types in all sinlations because there is nothing stated to contradict (hat view. \X'e will cover this, starting with joints that arc being made.
Inspecting New Joints 1t is no good relying solely on a final inspection to detect failure. If something bas gone wrong during the riveting proccss, and it is allowcd to go undetected until completion, you risk being faced with a complete waste of effon and expense. Inspecrions should be carried out at certain crirical stages during the making of a rive~88~joim . We will examine these.
,,\,,-,b . .
Aft~\lll..'u king OUt,
a check should be made to verify that it conforms
to
the
~)~t'e~uirements of the rcpair scheme. Aftcr drilling, a check should be m'lde ,,\,\ '-. ".,.:..\\'C
I. ~~~
to
confinn the correct location of the holes and that the holc sizes :tnd condition arc suitable for the type of rivets to be installed. If countcrsinking or dimpling h:ts been carried out, the maring of the skins, the condition of the holes and thc flushness of the rivet heads should be checked. Prior to final assembly, thc fit of the parts should be confirmed and they should be checked to ensure they arc frcc of swarf and that all holes are free of burrs . .Additionally, a check should be made to cnsure that any required prorecti"e treatmcnts and jointing compounds have bccn cOt:rectiy applied. After rinting, thc joint must be inspected to ensure that it has been correctly made. 111cre should be no distortion or gaps and the ri'·ets should be tight and properly formed. rlllC p re~fonned heads should bc undamaged and in finn cont:tct with the skin. If you can insert a small feeler gauge between the head and rhe skin, the river should be rejected. Each fcnned river head should be inspected to ensure that it has not been deformed or cracked. rnle metal surrounding each ri,'et should be inspccted for evidence of damage from
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CHAPTER EIGHT RIVETING
rivenng tools. Rivets that are obviously defective should be removed and replaced. You need to exercise a bit of discretion 'when making judgements on what is acceptable or not. Removing a rivet that has a minor defect may create more problems than ir IS worth, particularly in thin materials. You should always consider th e strength requirement of a particular joint and the effectiveness of the rivets in question. \'{ihere a flush surface has been specified and milling has taken place, you should check that there is no damage to the surface of the surrounding skin and that the protective finish has been restored where required. Reflecting on what we have discussed up to this point, there is a lot more to inspecting a ri,eted joint than juSt doing a cursory check after it has been made. The conduct of successful tnspccllon requues an lOspector to know whar faults can occur and why.
CorreClh· Formed
lnsu fficienr Hammering
D oll\" nO I
Snap too Large
Held ~ quare
(Ringed Metal)
Snap too Small (Ringed Rivet Head)
Figure 8.54 - Riveting Faul[s (1)
~
0"<:
,..; C<)\C,,-
' },.{,
. ;:......1....,
( .0'\":~.
\ ..,.
The first illustration shows some of the more common riveting faults. A flat head formed on a solid rivet tail should be approximately O.SO deep by 1.5D diat1].erer and be concentric with the ovct shank. With that in mind. it is easier -identify ovets that have been over or under doyen. Equally obvious is a rivet that has been given an inco rrect allowance. Tf the allowance was too large or too small, the head cannot fonn correctly. Excesslye hammering may have
:li't'b
--~~~~~--=~~~ You need to be morc obselTant to recognise the use of an incorrect sized sn~. A snap that is too large will leave a ring mark in the skin around the snap he~, o ne that is too smaU will ring the rivet head dr even create a 'flashing' ~ squeezed our material around the head that will in turn make the bead appel"E-" small. "5 ;; A n interesting situation once arose when imperial and metric rivets we~ inadvertently mixed together in the same storage tray because of a confusion~ part numbering. To make things worse, the relate~ imperial and metric rive.~g snaps had also been mistakenly allocated the ~ame part number. Th,Oi1g h similar in appearance, the snap head profiles arc not the same. Depend ~g on the metric/imperial and imperial/ metric combinations of oyet and snrP; rings started to appear sporadically on the heads dr on the surrounc@'i'g skin, unmarked rivets only occurring where the correct combination of "'s nap and rivet coincided ch,ncc. You would need to ,len to deduce the unu,u,l
by
br
459 CapJdght 0 &17)' Co/flge 2006 - 2007 All righl. rmrvtd
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C H APTER EIGHT R I V E TIN G
cfluse of that problem but it does show whflt can happen. It also shows the folly of unpacking vast quantities o f riYers and thro\ving them imo a bulk storage tray with others. Once the identity is in doubt. rou ha\'c little choice but discard the lot! The same ring effects may be produced if a snap head is being fanned with an incorrect ri,-et allowance_ If the ri"et tail is toO long, 'flashing' of excess material will appear flround the head, too little allowance will result in a ring mark on the surrounding metal where the snap has indented it.
Rivct
Ri\-cr Long
telt)
toO
Hole Small
Short
Hole [no
[00
Big
Snap nOt Squarc (Ringed on one Side)
Dolly nm Ilcld 'Firm
Figure 8.55 • Riveti ng Fauhs (2)
,\"\\ '0.
,~\'C
, "~
Our next illustration shows the effects of trying to form a flat head on a ril'"et [ail that is either toO long or tOO shan. In both cases, the required dimensions cannOt be achieved. Notice the effect of having a hole drilled undcrsize. The expansion of the river in the hole has forced the surrounding material outwards, creating a gap between the skins as they buckle to accommodate the movement. Convcrsely, a bole that has been drilled ovcrsize will not allow the expanding river tQ grip properly. The skins m:ly be forced apart during hammerin~\..;w.d1ffi~terial may be extruded between them to fo nn a gap_ The imPWJ.anC"c of holding the snap and the reaction block squa re is illustrated ..,"'\.{wfielre a snap has been allowed to dig intO the skin on one side of the head. ~. Finally, if thc reaction block is tOO light, or is nor held firmly in contact with (he rivet rail, [he hammering on the snap may push the surrounding skin in.
E:::U==J
~ ~
He.ld nor Flush (Rrused)
Gap
E::f::F
@
@
Head nor Flush (Sunkcn)
Cmcks on Face of Formed Head
Piece Broken Ollr of Formed Head
RCflcoon Block Pushed too Hard
Figure 8.56 - Riveting Fa uhs (3)
460 COPJright C &17)' Col},gr
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CHAP T ER EIGHT RI VE TING
The final illustration shows some of the faults related to countersunk rivets, In one example, rhe countersink has nor been cut to sufficient depth and the rivet head is lefr standing excessively proud , You must not attempt to miH o ff the excess to correct this problem. In another example, the countersink has been cut l aO deep and the top of the o,et head has ended up beneath the sur face of the skin with possible damage to the surrounding material. Alternativdr, there may be a gap around the head. When rivet heads have been driyen exccssi,~c!y . or they ha\'e been subjected to excessi\'e hanunering, they rna: crack because of work hardening. Even when a rivet has been correctly fanned, there may be signs of superficial vertical or diagonal cracking in lhe material on the sides of a flat head. This can be acceptable prm-iding they do not intersect, or look as if they might, because there is a ask that parr of the head could break off. Any crack that has propagated across lhe flat head into the nciniry of the shank diameter is not acceptable. Neither is it acceptable when anr pan of a head has broken off. If successh'e awts are displaying evidence of cracking, the riveting process may not be the cause. For example, many light alloy [iyets require solution treatment prior to use and ther must be used \....ithin a specified time. This time can often be extended by cold storage, Tbe cracking may be due to incorrect heat ucanncnt, incorrect cold storage temperatures or an operator exceeding the prescribcd time limit. I f any of these are confirmed as the cause, all rivers in the batch will be suspect.
I nspectin g Bli nd Rivets The blind side of this type of ri,et is not no~mallr accessible for a visual inspection. In this case, the inspection is limited to checking that the head is progwy seated on the skin surface and tbat it is rsquarc. T he surrounding skin . ?.\\~~ho@d be inspected to ensure th.:u there is no evidence of buckling or damage (.l~\\";.z..\;. and that countersunk rivets are lying flush with the surface, Depending on the \\1'.\{.... ri\-ct type, sealing pins should be checked for security and thc lock collars on , {\,~,\ Cherry lock ri"ct5 should be checked to ensure that they are propcrly engaged (\.~'\' and fom1cd. Some repair schemes may specify more extcnsi\'e inspection and this may involve a radiological technique,
Inspecting Special Fasteners
.-
1\ general inspection would include checks to ensure that the fastene rs a ~ s'luare, thar the heads are correctly formed and not cracked, and thal any 10q... collars arc properly engaged and 'fonned. The c610uI coding of the heads~f H i-Lok fasreners should match the specifications in the repair scheme. ?f
"
~
~
461 1l'Wll1,pllrt66.rolR
CH A PTER EIGH T RIVETIN G
Post Repair Tests These will be specified in the repair scheme. They may include a pressurisacion proof test when the repair is [Q a pressurised hull, a leakage test where the repair is to integral fuel tank strucrure or a mass balance teS( where fl ying control surfaces have been repaired.
Scheduled Inspection of Riveted Joints TIlis type of inspection relics on your knowledge of aircraft structures and operations and the long-term effects of those operations on strucmres. \'('e will be dealing with this in more detail in Chapter 18 so we will restrict this section to a brief overview. The Aircraft J\laintenance J\Ianual will specify the particular areas to be inspected. These may range from sections of the aircraft skin to areas of boxed in strucrure. In Module 6, we dealt with Ihe corrosion risks associated \\~th different regions in the structure and the effects of different environments. Later in this module, we will be discussing the effects of abnonnal occurrences such as heavy landings and flight through severe turbulence. In addition to nonnal wear and tcar, all these factors can affect a.ctcd joints. An inspector should always be aware of the general signs that might indicate problems. When a stnlclurnl joint loosens in service, there is a strong possibility that 'fretting' corrosion will occur. Fretting corrosion is a combination of mechanical abrasion and chemical corrosion. Fretting in a rivelcd joint is revealed b)t black or brown powder or paste exuding from under the heads of the rivets. Aluminium alloy exudes black powder whilst steel components exude da.rk brown 'cocoa' powder. On aluminium alloy structure, lhis is appears as bl:!ckencd 'smoke trails' leading away from the rivet heads in the direction of the airflow, Once you have confinned that it is powde r or paste, not migrncin~ o~ "pillage, you ha\'e strong grounds for suspecting loose rivers in rhe j~~)\'
. ,\\...~
. U ..
\.... ;\,('
Direction of Airllow
1
Figure 8.57 - Fretting Corrosion in a Ri vet Line
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CH A PTER EIGHT R I V ET IN G
The inspection should Include checks for any signs of distortion or movement in joints and for loose or sheared fasteners. Sheared fasteners often remain in place so YOLl need to check dlcm fo r signs of movement. You should look for evidence of pulled rivets, elongated holes or wrinkles in the skin Ih;; 1 may lndic;;te ;; previous ovcrStress in a joint. StructurJs often try \0 rClurn to shape after an o \'erstrcss and this may be the only evidence le ft that it b;;s occu rred. Fatigue in ;; ri\"etcd joint will always commencl \'lith a smaU crack that will gradu;;Uy progress over time. As the crack mO\'es ;;cross the holes, the joint will become progressi\"cly \veaker and it will then be at risk of a sudden 'zip fastener' failure. Bearing in mind that most aircraft are painted and that paint accumulates and often cracks at joint seams, you need to exercise particular c;;re not to confuse this ""ith an actual crack. \\"11t::re there is doubt, the surface finish must be removed. The Aircraft Maintenance Manual may specify this. Periodic Eddy Current Testing may also be specified in some areas. The edges of the faying surfaces of Joints should be examined for lifting or blistering of the surface finish that would indicate interfay corrosion. Slight undulations or bumps in the skins between rivets or 'quilting' in the areas of overlapping skins can be an indicanon of adyanced corrosion. \Vonn-like trails of lifted pa.int travelling away from rivet heads are an indication of filiform corrosion occurring on the skin surface under the paint. Polyurethane pamt finishes are particularly prone to this.
Conclusion We have fi nally arrived at the end o f this chapter. This has been a subject where practical experience, skill and professionalism counts. Nobody should be permitted to carry out strucrural repairs to primary or secondary loadbearf\'ilg aircraft strucrure unless their competence has been thoroughly tested . :..\~\){hd - proved. Book knowledge and 'tick tests' ha\'e their place but arc nor (,,\\..:-;"0.:. substitutes for practical ability. All repair and moclification work must be done ~:o.sf' strictly in accordance with tbe manufacrurer's approved instructions. No one .. \~:. ~ should ever attempt to substirute fo r these by. using personal experience or (\-..: ,-,'\.' book knowledge alone. Si.milacly, nobody should attempt to engage in strucrural repair work when not properly authorised to do so. In answer to our introductory rhetorical question, me ri,"crs will not shear or pull through t,!:e skins as long as e\'eryone follows the rules. ~. .
~
There is a selection of multi-chOlcc questions im;:.luded fo r rou to Lry. Do no! just rely on answering these, try making up sqme of your own for fut1.lt!:: reference. In rhe meancime, you should consider taking a wen-earned bre~ before tackling the next chapter. ..:: o
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CO/!)7J'j,hl
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CHAPTER EIGHT R I VETING
Revision Riveting Question s 1.
If a riveted jo in t is to b e m ade between two s heets of m etal, each 1/ 16in thi ck, the co rrect rivet diame ter woul d be:
a) 1/ 16io b) 3/ 16in
c) 3/ 8to
2. Th e numbe r of 1/16in diam ete r ri ve ts requi red to match the s hea r strength o f a 1/ 8in di am eter rive t m ad e of th e sam e materi a l is:
a) 4 b) 3
c) 2
'10li'
3. The,gjl~Ri streng th of a riveted join t sho uld be: ,\ -\!.,"
_(.0'\'-.
, ..
.\,\
.~\""
~'
, \\~~{':
a) greater than rhe bearing strength of the skin b) the same as (he bearing strength of the skin
-
c) less than the bearing strength of the skin
." ,
~
Y;24. In a riveted jo int, the distan ce from a rivet cenue to the ed ge o f a sheet sho uld b e no less than :
a) 4D b) 3D c) 2D
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CHAPTER EIGHT RI V ET I NG
5. A lap joint is joggled to: a)
increase its s trength
b) provide a flush surface c)
remove tension from the civets
6. The length of a countersunk rivet is meas ured from th e end of rivet tail to the:
a)
top of dIe head
b) top of the shank c) underside of the head
7. The grip range of a blind rivet refers to th
a) expansion o f the riycr in a clearance hole b) thickness of material that can be fastened c) clamping force applied to the joim
8. ..,~\\,6
" c..:.,..:...
. C~"~ .
When the clearance IS too large, the action of closing a solid rivet may: ""l\)ll ~ ~ . . a) Illdellt the marcnal around the hole
:'-.-:\.~('
. ;-.....,.\ ;:. \ . :;..0(,'-':--
... '- ,-,,, .
b) buckle the material around the hole c) separate th e s heets
9. The term 'raying surfaces' in a riveted joint relates to th e: a)
-.
upper and lower surfaces of the joim
b) COlltaCting surfaces c) the surfaces exposed to the airflow
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C H A P TER EIGHT R I VE T ING
10. The primary purpose o f a p plying jo inti ng compound is to:
a) inhibit electrolytic corrosion b) make a joint leak-proof c) provide electrical bonding
11. Wben rem oving a solid rivet, th e head is d riUed us ing a drill size:
a) slightly larger than tbe riyec shank b) the same as the rivet shank c) slighdy smaller than the rint shank
12. The minimum rive t pitch on a s taggered row lap joint is :
,J
2D
b) 3D c)
m
13. The minimum.rivet sp acing o n a d o uble cham lap joint is : ,,,.'II.)
, ~~~&
,
,,<\~.... ((I\: ·
«,. '-' ..:~~· b) .
IIlb -
3D
c) -ID
14. The 'sphe re of influ enc e' o f a rivel is approxi m a tely:
a) 5D b)
,E
o
m
~
o· ,::;
'"
c) 3D
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466 Copyright C &try C(J/~ 2006 ·200i All rightJ rtItr7td
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CHAPTER E I GHT RI V ETING
15. The term 'allowance' used in relation to a solid rivet refe rs to the : a)
distance between a rivet and [he edge of a sheet
b) difference between the hole and Clvet sha nk diameters c) amount of material rCCJuired to form the head
16. The joint most su ited for use in a high stress area is a : a)
lap joint
b) butt joint
c) fla nge joint
17. A fastener that can be used where there is access
to only one side of
a joint is a : a) Hi-shear fastener b) Avdelok fastener c) Jo-bolt
a) broaching b) reacnon I1vetmg c) sCjueezlIlg '0
'"1-::', '", ~.
19. The mandrel used to install a Chobert rivet:
a) remains in the rivet after insmllauon
I
o
' -;
b) remains in the riveting tool after installarion c) breaks off and is discarded
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CHAPTER EIGHT RIVETING
20. A hole is punch dim p led using a: a) countersinking rool b) male die
c) male and a female die
2 1. T he le ngth of solid rivet shan k p rotrudin g through the work that is
requi red to s llccess full y foml a flat head is:
a) l.SD b) 0.75D
c) 0.5D
22. A solid rive t shou ld be closed using:
a) a large number of light hammer blows b) o ne heavy hammer blow c) a small number of sharp hammer bbws
23. M l er "
,
,
~<-.¢'. . ~
.;\\..:~.:;
i n~~alr~11~n ,
1\'1"
th e stem of an Avd e1 rivet is:
...
~;\1:0. I... a) cropped and milled flush b) left in the 1001
.........""1
c) disc:udcd
24. An Avdel Pi n Tester is used to check the:
a} rightness of the civet in the hole b) tightness of the sealing pin in the riyct bore c) grip length of the pin
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CHAPTER EIGHT RIVET I NG
25. Prior to installation, th e collar of a Hi -s hear rivet should: a)
not be washed or lubricated
b) be soaked
in
light oil
c) be washed with solvent
26. ~lben placing countersunk rivets in 18SWG titanium s heet, the holes should fust be:
a) Cut-countersunk b) hor dimpled c) punch dimpled
27. The indentation diam eter of a riveting snap for a so lid dome- head rivet should he: a)
slightly larger than the head diameter
b) slightly smaUer than the head diameter c) the same as the h ead diameter
-,o\f
't\06-~
(~)\)..:'t-c."'1. 28. During reaction riveting u sing a pneumatic riveting gun, the h e ad is ,. ,,~"\\ -
,,\ (0\·
~
'\.{\' '
\'>.
formed by holding the: a) snap in the gun against the rivet tail b) reaction block against the rivet tail c)
reaction block against the manufacmred head
29 . The most likely cau se of an indented ring on a solid dome head riv:tfi IS:
a) use of an overs ized snap
b) insufficient allowance c)
use of an undersized snap
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CHAPTER E I GHT R I VE TI NG
30. The s nap in a pneumatic riveting gun is held in the gun by a:
a) retaming sp.cing
b) threaded sleeve c) a snap ring
Revision Riveting Answers B
16.
B
2. A
17.
C
3.
C
18.
A
4.
C
19.
B
5. B
20.
C
2'1.
A
7. B
22.
C
8. C
23.
A
9. B
24.
B
10. A
," -,.
A
I I. C
26.
B
12. B
27.
A
J.
6. A,O\.'b ..< -
l~'l\~
. (0\\":'
. ~\."'\.
__..,,,,"l. ~",'f c
,~)...<': .
~
~.
..,"
-, " :::: r-
.~ ~
13. C
28.
B
14. A
29 .
C
~.
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~
~
15. C
30.
'0
A
,,~
470 CopJ7i!"hI 0 Bony GJlk,ge 2006 . 2007 Ail ri!"hls rmrml
WIIIIJI·Ptl/166.rom
CHAPTER NLNE P I PE S & HO S ES
Pipes & Hoses In( ro du crio n In 1\loclule 6, Chapter 6 we dealt 'W1tb tbe identification and description of the types of rigid and flexible pipes and their associated connec[Qrs and unions. The syllabus coment for this chapter is confincd to the bending, belling and flaring of aircraft rigid pipes, and the inspection. testing_ installation and clamping of pipes and hoses. There would be ad\-anrages in srudying the tVlodule 6 coment whilst working on this chapter. Howe\'cr, I will include infonnation that is pertinent. Pipes used in aircraft are normally receiycd as full~" pre-fabricated assemblies and no anempt should be made to re-bend them. Pipes arc manufacUlred in accordance with approyed dra\\rll1gs. This is critical to their inrerchangeabiliry and is designed to prevent Stress on the end fittings and chafing when they arc installed. The manufacturing process depends on the material being worked and the aircraft system in which the pipe will be installed. T"he characteris tics of the pipe material and its heat treatment state must be known be fo re any bending or flaring process is carried out. .All ri~d pipes \vill have at lenst one bend in them to absorb vibration and ro allow for expansion and contraction duc ro tcmpcraUlre changes.
\"\:,-{f"
i\gain,n here arc numerous textbooks, moscly American, covcring this topic, . '\'\~?tch having their own particular slant. The information in this chapter has , 'C l.','\ .... ,... been \'crificd with that contained in the CK Ci\"jJ _\viation publications and these arc included below as useful references.
Refe rences Ci\"il Aircraft Airwonhincss Tnfonnation and Procedures (C:\ P 562) Leafler 5 - 51-lose and Hose Assemblies Leaflet 5 - 6 1nstallation and Maintenance of Rigid Pipes
' -f
Civil r\irc(:lft lnspection Procedures BL/6 - 15 Manufacture of Rigid Pipes
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C H APTER NIN E P I PES & HOSES
Rigid Pipe Materials t."lodulc 6 inclllC!cd information on rigid pipe materials. In the evenr that YOll have not encountered that module, we will cover the salienr details he re. Copper Copper was used eXlcnsivdy in the past but rhe yibrarion that is encountered in aircraft syHellls caused it to work harden and become briule and it required frequent inspection for signs of cracking. Its use was confined ro low pressure and low temperature air and oxygen systems. End fittings can be brazed or silver soldered OntO the pipes. Copper robing is supplied in its fully annealed condicion. The slight work hardening that occurs when forming dle pipe is nOI harmful but copper pipe must never be re-bent whilst it is in sen"tce. Aluminium allo), and corrosion resistant steel pipes are no\"\' used Tungum Tungum is an alloy of aluminium. nickel, silicon and brass that was used in the low to medium pressure hydraulic systems of earlier aircraft. Tubing was supplied in the annealed condition and could be worked at room temperature followed by furrher annealing. The pipe can be readily Oared or have end filtings brazed or sil\'er soldered on. The alloy has a problem in lhat it is very reactive when in Contact with aluminium alloys causing corrosion in them. The pipe securing clips had to be packed with 80 mesh tinned brass gauze, not mbber. Tungum has now been replaced with aluminium alloy and corrosion rcsismnt steel.
High Nicke l Copper Alloy T his pipe material is strong and is suited to use in high, medium and lo\\'pressure systems. 'Che pipes can be easily flared. The presence of copper docs present a ~~~w;osWH risk to aluminium alloys c'\\-;'-,!-: -
~ .(\tumln1Um
\.... ).
,
Pure aJuminium (1100) is very light but is not strong enough to \\~thst:\Od anything more than very low pressures. Aluminiwn tubing is anodised during manufacture and pipes Icss than 3/8in diameter that are made from it arc usually gi\"en a double flare to make them marc resistaQ[ to shear rorces. \,\'e will examine the process or double flaring later in this chapter.
r
"
'",
.~
';:;-
Aluminium Alloy The heat treatment and working of aJuminium alloy tubing varies according to the specification and condition of the alloy. Non-heat treatable specifications 5052 and BS 1..56 arc frequendy used ror low-pressure (250psi) ruel and oil systems and for medium pressurc (1,500ps~ hydraulic and pneumatic systcms. These are bem and flared in the 'as supplied' condition. The stro nger, heat treatable alloy 2024 is also used but it is harder to bend and flare. Duralumin is a heat treatable light aHoy that is used ror piping in low ro medium pressure systems. "nlC heat treatable alloys have to be heat treated be rare and after working. All aluminium alloys are prone to electrolytic corrosion when in
472 upJ1it,hl C &1'1) ulkgt :006 " :00 -
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CHAP T ER N I NE PIPES & HOSES
contact with steel, copper alloys and many non-metals, particularly rubber and graphite compounds . Corrosion Resi.stant Steel Corroslon resistant or stainless steel is used in an annealed or semi-annealed state in high-pressure (over 1500psi) fud, oil, hydraulic, pnewnatic and o:\;gen systems. Stainless sreel was originally confined to use in high-pressure systems but this use has now been extended to all systems where me operating conditions include exposure to corrosive products . Because stainless steel is very strong, the thickness of the U1bing walls can be reduced, making it a l \veight fat weight substiUlte for aluminium alloy. The adyantage gained is a resistance to corrosion and high temperaUire with no weight or strength penalties . .Many modern :urcraft now haye dlelr entire rigid pipe nenvorks made from stainless steel. T hough described as corrosion resistant, stainless steel will react and corrode when In contact with the poly-chloroprene rubber that is often used as a packing and slee"e material on aircraft. ObYiou~ly, it is not suited for used on stainless steel pipes,
Size of Rigid Tubing The size of rigid tubing is determined by its outside diameter and this is graduated in 1/ 16in. steps. Number 8 tubing \vould be, for example, be 8/16 or 1/ 2in. The outside diameter of a tube is stated in conjunction with its \vall thickness graduated Ln thousandths of an illch . The inside diameter of a rigid tube is not stated but if you know the outside diameter and wall thickness it ma\' be calculated . Tbe outside diameter and wall thickness arc normally markeri. on the mbing. "1\)1.\ ' l\..~lJb
_.
_ \\,:~c.-
_L')'\ -o
\""'~~~
Rigid Pipe Bending When manufacturing pipes for use in aircraft systems, bending should be carried o ut wi th the aid of a tube-bending machine. Some pipe materi:!)s require heat treatment to so ften them prior to b ending follo\vcd by furth9," heat treatment after bending to relieve stress and restore their properties . Priqt to conducting any heat treatment, the pipe should be thoroughly cleaned. Th~ presence of carbon in oils, greases and degreasing agents can lead to seriou; weld decay problems in some marerials . 1-ligh-pressure paraffin sprays may 1$ used for initial cleaning bur this should be followed by degreasing with ~ appropriate soh-ent such as trichloroethylene and blow-drying in warm air.
i
;;
"
Tubing normally requires some form of internal support during the be ~ing process to prevent it from tlattening in the bend area . There are limitscto the bend radius that can be formed in given sized d bing and these may d~cide the type of bending equipment to be used. Y
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C HAPTER PJPE S
&
N I NE HO SES
We need to examine the various methods used to provide internal SUppOrl during bending. Before we do this, it would be advantageous to look at the bending machines.
Pipe Bending Machines There are twO types of bending machine, the draw bending machine and the compression bending machine. There are hand operated and power operated models of each type, some being fully automated. Compression Bendi ng
Draw Bending Machine
~Iachine
,, ,
,
o
Fixed Former with Full Depth Profile
:'(0 ,
o
)'fm-ing f-ormer with Half Depth Protile _ _
Fig ure 9.1 - Bending Machines
Draw Bending Machine The draw bend ~!fmachine, sometimes referred to as a m3nd rel bender, is used fOkb):W&illg large outside diameter uJbing up to ..J.1 nun (15/8in) . It is larger than"\'11e compression bending machine and is usually installed on a floorb: \\).~~ounted frame. Because of its size, it is nonnally only found in main ~\.~. .\. production areas where there is likely to be repetitive work TIle machine l. ~.,,\!-incorporates a set of plain and articulated metal mandrcis mat are selectively used as a rube filler to provide support to prevent me rube collapsing during bending operations.
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474 CIJP)Tight 0 Barry Co/bgt 2006 - 2007
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CHAPTER NI NE P IPE S & HO SES
Tube Guide
Tube Stop
o
i\landrd Extractor
Tube
Former Bend r.cn'r
)
~
o
\CO
Bend ;\[mdrels
Figure 9.2 - Draw Bending Machine
The machine consists of an interchangeable fonner that is rotated about a pivot by II bending leH'L 111e periphery of the fonner incorporates a proft!ed semi-circular groove that matches the outside diameter of the pipe to be bent. The pror.Je depth is half me mbe diameter. '111e tube is inserted into the machine until it abuts an adjustable rube stop. It is then clamped onto the former rim with a profiled grip. The appropriate mbe bcmhng mandrel is scrc\vcd onto the connecting rod and is then inserted round end first into the tube. The connecting rod is adjusted to position the mandrel so th:lt the end of the str:ught portion before me round end is about a third of a rube diameter beyond the proposed start of the bend.
(~,,\'"
The rormer is rotated by pulling on the bending leYe[' This action pulls the ,\iil .,\\I,rubc- away from the stop and draws it around the former. t\ roller moumed l.,lW--!.!,; - rube guide holds the rube firmly in conmct with the former as the bend IS \"\:-.\\': fann ed. The mandrel mechanism, the centre pivot and the tube guide arc all \,;;,,<. . ,. interlinked to ensure that me mandrel ahvays occupies the parr of the rube where the bend is being fanned and that the guide is kept at me correct distancc from the fonner. The machine incorporates an adjustable angle stop to limit the rotation of the fanner to achieve the desired angle of bel$. Different materials produce yarying degrees of 'spring-back' when dlC rubc~ released from [he machine so setting the angle stop will be a matter o~ expenence. The draw bending machine is equipped ,\y;th and mandrels to accept various sizes o f mbe.
fl
range of formers, nlbe guid~ :~
,. "-
'"
~.
'v
""
~
475 Copyright
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&17)' Colkgt 2006 - 200 -
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Compression Bending Machine The compression bending machine is more commonly used for small quanri~' work. It is smaller than a draw-bending machine and is usually clamped in a bench \"icc. It is suitable for bending thin-walled mbes and when properly adjusted, it can produce bends of up [0 18().l withom fracruring. rippling or kinking the rube. There are 1'.\'0 sizes of compression bending machine. The smaller type can accept pipes up to 12mm (I / 2in) outside diameter. The larger type can accept pipes from 12mm [0 25mm (lin) outside diameter. The compression bcndi!lg machine docs not employ mandrels so another method of lube ruling has to be used. \,\/e witl look at rube fillers as a separate issue later o n.
Pipe P rc~su re
Indicator
S,,,i,,. Adjuste r
o L--!I,.-'
Guide Bending Le ver Fixed rormer Fig ure 9.3 - Compression Be nding Machine
.,~~\b
"!.(Vo.f'
T~c. :fen:ner in this rype of machine docs nor rotate. The hand bending le,"cr \\;\qbrces a roller along the back of a guide bar [0 gradually fonn the bend by pressing the rube into the profiled groove of the axed circular fortner. The profiled groove has a depth equal [0 the rube outside diameter. The machine is equipped with matched pairs of fonners and guide bars, each pair being marked with the related rube diameter. Different materials and tube wall thickness do not all bend at the same pressure. Scrong, heavy gauge tubing requires a lower pressure setting than liglll gauge tubing. The machine incoq)Qrates a pressure-setting indicator that may be adjusted to suit the specification of rube being bent. Tlus may require a couple of trial bends to get the setting right. I f a tube ripples on the inside of a bend and flattens on the outside, ir usually indicates that the pressure setting is toO low. Too high a pressure on the other hand can cause narrowing or 'th ro:Hing' on the inside of the bend.
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CHAPTER NINE PIPES & HO S E S
Good
Flattt:m:d
I....:.inked
Figure 9.4 - Bend Fauhs
The mbc is inserted lnto the machine until It abuts the tube StOp. It is then pushed imo the former recess and the guide bar is fitted O\"er it with its edges inserted into the fonn er recess. The bending le,er is pulled until rhe roller is firmly pressed OntO the back of the guide bar. "n le prcsliu re-sc u ing indicator is adjusted for the speci fi cation of tube being bent and rhe setting sc rew is locked . The hand-bending le,er is then steadily pulled until the bending is complete. The machin e incorporates an adjustable bend degree SLOp that will p revent over-bending.
Simple Hand Bending Tool ,b
'"II II \
...
. ,\\. .:::..':' ~~"'This hand held rool is suited to small diameter, thin-walled mbing. -nle tool is • \\(' I. S slippcd oycr the rube at the point \,here a bend is required. The tube is . : : ,,,. I)ositioned into the recess in the famler and a clip i, pulled over it to hold it in .\....... .;...-place. In this position, a mark on the bending handle aligns with the zero mark " ~~'\' ... on a degree scale on rbe former. T he bending le\'er is then pulled with one h:md whilst thc tool is supported by the other until the required bend anglc..i.s achic\·cd. ~ /
,
~
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Figu re 9.5 • Simple H and Bending Tool
Bend Radius & Method Draw bending machines are the preferred method of bending because they arc supplied with mandrels, However, depending upon the specification o f the tubing, large radius bends can be made by hand in mbing up to 112 in olltside diameter. \,(/here the outside diameter exceeds 1/2 in or the pipe material, wall thickness or radius of bend requires it; the pipe should be filled wil h fusible alloy flller before bending the pipe by h3nd or in a compression bending machine. \'(!hen the mean radius of the bend is greater than four times the outside diameter of the pi~_e, the pipe can be bent in a compression bending machine without th{(,l]leeu" for filler. In some cases, a dose-fitting helical spring is inse[tw.-in~o cl,e mbe to provide support.
u_W'
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Fusible Alloys Low melting point fusible alloy is used to fill pipes prior to bending in order to provide supporr when full-bore bends are being formed. This reduces the risk of fracrure, wdnkling and kinking. It also provides a simple means of remo\'ing (he filler from the bore after bending without affecting the hear treatment Sta tC of the pipe material. T here are a number of proprietary brands of filler available, 'Cerrobend' being but one example. 111e requirements fo r a suitable fusible alloy arc th:u it should be very ductile, ha\'e a low melting point and should expand on solidifying. The e::>..-pansion of the alloy on solidifying docs cause the pipe to become slightly ovcrsize, roughly O.002in per inch di:Hllerer.
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Bending machines are equipped with fonners and guides that are specially designed to accept tubes loaded with fusible alloy. f'usible alloys usually COntain lead, tin, bismuth and cadmium. They have a melting point tbat is below lOO°C and can be melted by boiling water. However, the alloy must not be exposed to temperarures in excess of lOQoC so ther are normally melted in a separate contalll1f that is surrounded by water that is controlled at a temperarure of 85° to 95°C. Flame heating must never be used on thi s type of aUor. A type o f lead/ rin fusible allor that is sometin1es used for low tensile steel pipes has a higher melting point. This alloy must not be allowed to exceed 400°C. The temperature restrictions are intend~d to protect the condition of the pipe materials. Any temperature exceeding 100° will start to affect the condition of heat treatable aluminium alloys. Temperatures in excess of 400°C will affect the condition of carbon steels. Fusible alloys can have a detrimental affect on high tensile steels and direct contact wilt h them must be avoided. Wlhen heat treatable almninium alloy tubing has been solunon treated to soften it prior to bending, the bending operation must be carried out within two hours of the solution treaunent. If yOll recall, the same rule applies to rivets that have been solution treated . After two hours, the alloy becomes too hard to \vork due to age hardening and it \v-ill crack.
Filling Prior to ftlling or 'loading' as it is sometimes referred to, the pipe must be thoroughly cleaned with soh'ent and thcn dried in warm dry air. Cleaning is essential, particularly when the pipe has been heat treated because thc salts and any l H\,\ces of carbon arc highly corrosive. Folk~\Ving clcaning and drying, th e l~Hllipe\Jshould be coated with a film of clean l~bricating oil or a mixture o f (p\\C~e -' lubricaring oil and paraffin. This should be ca~ried out by either completely ~ \">-;\~("~ filling the pipe or imme rsing it in an oil bath. The use of an oil-soaked fabric ,\,'. '= 'pull-through' does not saris£;" this requirement and it should not be used. T he . '-~'\' , . ...,.. precise nature of the lubrica~t to be used is SP1Clficd by the m an ufacturers of ,,-. the fus ible alloy. for example, the llse of engine lubricating oils is not permitted because the detergent additives in these causes the fusible alloy 'fo stick to the bore of the pipe making it difficult to remove. 3.
-.
~
As an alternative to oil, thin plastic sbeathin9 is sometimes recommclldea;. where it is important to prc\"eot the alloy contacting with the pipe materiab Only the specified sheathing should be used becausc sheadung that is toO thi,* will affect the cooling rate of tbe fusible alloy. '....'
0'
'"
One end of the lubricated pipe is plugged and it is immersed in hot wat~ to within a few inches o f the open end . Wlh en the pipe has stabilised at the &ater temperature, the molten fusible rulo), is poured Unto the open end. Care-~ eeds to be taken not to create air pockets or break fhe lubricanon film d!'i.cing the loading operation. I
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As soon as the pipe has been loaded \\;th fusible aUoy, it should be progressively immersed in cold water, starring with the plugged end, so that the alloy can solidify without the risk of cavities fanning. However, the ductility of lhe alia), does depcnd on rapid quenching so, }'OU should not prolong ir. i\ frcr loading, the pipe should be allowed to stabilise to room temperature before commencing the bending operation.
Removing the Fi ner Following the bending process, the fusible filler alloy is unloaded by immersing the pipe in boiling water. TIle pipe must nOt be agitated in the water bccause this will break the oil film and cause the alloy to stick on to tbe sides of the bore. In extreme cases, the alloy may 'on' tbe inside of the bore and be impossible to remO\·e. The molten filler should pour out of the bore. To avoid the risk of 'onning', any beads of filler remaining should be allowed to solidify before attempting [0 remO\"e them. If plastic sheathing has bcen used, the sheathing must be examined following remonJ to ensure that there has been no leakage of filler ooro the inside waU of the pipe. Final cleaning methods include a stcam jet, nor superheated, or tight fitting felt 'pull-through'. There arc sriff rotary brushes designed for this task but great care must be taken not La abrade or score the pipe bore when us ing these. It is nor possible to visually inspect the bore of a pipe that has a bend in it so
care needs to be taken when unloading fille r and cleaning. If conraminaoon is suspected, fus ible aUoy will register on a mdiograph. Failing this, the manufacturer of the alloy should be consulted for ad\"ice.
Altern ative P..iJIe Bending Fillers 1\'I\\b
...
TQ.e.&~If be occasions when the use of a fusible aUoy may nOf be permitted \"\~qjccause of the intended usc of the pipe. For example, oxygen system pipcs must not be lubricated or COntain any sediment that could spontaneously react
with oxygen. A draw bending machine urilises plain or articulated metal mandrels. Alternao\"e1y, special, close-fin::ing springs that have a high standard of finish are available for use in hand bending or compression bending machines. Prior to insercing a spring, you must ensure that it is the correct size and fonn and that it is not damaged or defonned. \\"hen a m:lndrcl or a spring is exrracted aftcr a bend ha s been formed, it must be inspected to ensure lhat there is no evidence of pick-up or flaking of thc bore of the pipe. Dry sand is also used as a filling medium when bending oxygen s},s[Cm pipes. Care must be taken to plug both ends of [he pipe [0 contain the sand during bending. After bending, the pipe muse be thoroughly cleaned to ensllre [hat no sand particles remain.
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CHAPTER NINE P IPES & HOSE S
Positioning Bends
rn most cases, the positioning of a bend in a pipe in relation to its end connections, or a pipe cnd, or another bend, i~ critical. It is not difficult to achieve correct positioning when us ing a compression bending machine. You need to decide whether to position the bend from a given point in relation to the Inside or to the outs ide of the bend.
c::::
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,
I I
.1
0
~- ---J
~-- --J
-
.\leaSliring Distan ce from Oursid o f Bend to End o f Pipe L
, :
MeaSUring Di stance from Inside of Bend to End of Pipe
Figure 9.6 - Bending to Qmside & Ins ide Measurements
T o create a bend that is at a giyen distance from a point, such as a tube end, you measure off the required distance from the end and mark the tube. You then insert the tube into the machine until it abuts the tube stop, place a square on me tube and align it \Vim the mark. You then move the rube along until the square abuts the outside rim of the foml er. TIle tube may now be bent and the outside of the bend will be at the measured distance fro m the end o f the tube. To create a bend that is at a measured distance from the inside of a bend. you move t;!:le rube until the square abuts the bonom lof the recess in the form~r.
I
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. :-.\.:,:.!,o.:. 7..\\~\~ll;n using a draw bending machine, you must remember that the former
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recess on this type of machine IS o nly a half tube diameter deep. \\i'hen the square abuts the edge of the fonner, this places IYour m ark at the centre of the tube diameter so you need to make an allowance of hal f a tube diameter when measuring and marking the tube if you \v;sh to use the inside or outside of the bend as a reference POIOt. Alterna tively. )'ou may wish to just use the centre. ,....
be~ds
Sligh~
The positioning of offset bends and that Jee in different planes is more complicated and is beyond the requirements of this syllabus. Though IF would be interesting to discuss these, there is litde scope for a multi-choicG: type question on the topic and you already have ,bnough to do.
'-' Flaring Rigid Pipes
,~
;;;
The flaring of the ends of a pipe is done to prepare it for metal-lo~~etal connections. A Hared connector consists o~ a union nut and ~I,) collar. Americans often refer to the collar as a 'sleevej so be prepared to hear either description. The flared end of the pipe is designed to seat on the coned face of
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an adaptor nipple or an e..xternally coned adaptor. As the union nut IS tightened, the collar mates with the back of the pipe flare and draws It mto fum contact with the face of the conical seating of the adaptor (0 form a pressure-right, leak-proof connection. The flare carries tbe load in a connector, not the union nUl.
Figure 9.7 - Pipe 10 Component Connectio n
The end of the pipe is cur square and is then expanded \vith a special flaring tool to form a conical flare. The subject of flare angles often comes up in examinations and ),Oll need to be careful to read the question. The flare angle rna)' be expressed as an included angle or just as a flare angle. However, there are British and American pipe fla.res and their angles are significantly different. 'l\~\b
.\.....\
,
'\'~'- \
'1\\\1~
- Flare Angle
,,(s' '-.
\..... ~~~ .
/
[,I,c< Angle /
.....;, ,\"i (: ('):
Single Flare
Double Aare
British Standard Flare 32° American AN Flare 74 ° Figure 9.8 - Pipe Flares
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The British Standard (l3S), ~.urcraft General Standard (AGS) pipe flare has an included angle o f 32°. The half angle or angle of the fla re is therefo re 16°. The Amcrican Air Force/Navy (AN), r.1ilitary Standard (p,lS), UNF pipe Aflre has an included angle of 74°. The half angle or anglc of the fl are in this case is 37°. The Americfln AN pipe flare is currently the o nc most commonly referred to but yOll do come across questions that fail to specify d1C type of flarc. I f in doubt, the .r\merican AN flare angle is the mor:e likely answcr - better than a crystal ball! Co lb.r 1ncluded .\nglc
Cone
/ \O---L?_O
-:j)' Figure 9.9 - UNF and American AN Flare Ang le
In preparation for flaring, the end of the pipe must be cut square. It is most important that the end is cut at right angles to the a.."Xis of the pipe. The end must be smoothly finished and not bave any ro ugh or burred edges rhat could lnitiate splits i.n the plpe when it is subsequently flared. Special cutting tools ha \"e been designed for cutting tube accurately. Alternatively, a locally ll1:\nuf:\ctured jig may be used [Q provide a guide for cutting and ruing. Roller:-
Size and Cm Adjustment
/
Figure 9.10 - Light Tube Cutler
\\fhen the end of the tube has been cut to length and de-burred, it should lie thoroughl), cleaned internally to ensure that there is no swarf remaining in t1~ bore. There are a number of different flaring tools available. Care must ;pc -' taken to ensure thar the correct type is chosen in regard to BriLish ~nd American pipe flares - rhis time, you must not guess! For the purposes # this section, we will examine two types of flaring tool. Both types incorporate intcrchangeable half bushes mat are size rehted to the outside diamc,#r of the pipe being flared. '~
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Expander Cone Flaring Tool The expander cone flaring tOol can be mouDled in a bench \~ce. It consists of a pressure screw assembly and an opposing die holder that contains a pair of interchangeable half bushes. The expander cone can be rotated independent of the pressure lead screw. TIlls means that the openllor can control the adnnce of the pressure screw as the flare de\TeJops, avoiding excessive pressure that could damage the flare and crack the rube. L.!~.J:;--~ L'l.tch Fitting
f--.J-,-
/-Ialf Bushes
Cone Pressure Scre\\' Expander Turning Sc: ~,,~···:,,:'·~--I.u L'l.tch Retaining Screw Fig ure 9.11 - Ex-pande r Cone Flarin g T ool
\'('hen the pipe end has been cut and cleaned, the union nut and (he collar arc threaded ontO the e,ipe. [f the other end of the pipe has already been flared, do ensure that~e c811ar and union nut beLongmg to that end are also fined. It is a bit ~lbn1hssing if tbey arc not and )'ou make the second flare! The ,,{appropri:ue hal f bushes are then assembled into the tool and the pipe is then \.... placed between them and clamped with its end aligned with the half bush faces nearest the pressure screw. The half bushes must close together fully. If there is a gap, this will result in 'flash lines' appearing on either side of thc pipe flare thal will create a potential source of failure. The bore of the pipe is then oiled and the expander cone is advanced until it just stariS to expand the end of the pipe. The cone is then continuously rotated as pressure is slowly applied through the lead screw until the flarc reaches the limits of the countersunk hnlf bushes. You will learn through experience what rare of feed to apply. If tOO much pressure is applied, the flare will crack. It is most tmportam that rou keep the expander cone rotating as pressure is applied. T he expander cone flaring tool is never to be used to flare stainless stcel tubing because it will pick-up and score the flarc. A single operating tool is used for stainless sleel together with a special buffer lubricant.
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Rolle r Flaring Too l The rolle r flaring tool can also be mounted in a bench vice. The flaring cone in this case is mounted in an off centre ball race a holder that is driven by a turning handle. This arrangement permits the cone to rora te freely in its holder. J\ s the holder is rotated, the apex of the cone moves in a circular path whilst the cone itself rotates around its own axis.
fn
Half Bushes Off-Centre Fbring (nne
I.cad Screw Figu re 9.U - Roller Fla rin g T ool
" .,,:,":
(,,)11'-,
Again, rhe union nut and collar are threaded on to the pipe and it is clamped inro the tool between a pair o f half bushes. Do ensure that the bushes close up full y. The lead screw IS then rotated to bring the pipe o n La the flaring Calle. The holder is then rotated and the off centre cone roUs around the inside wall of the pi pe, pressing it outwards into the fla re pro file. T he pressure lead screw is turnSd slowly to feed the cone into the flare until increased resistance indi'l.';\rcs that it has been completely. foaned. ,,,\\~,
-
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c
Double F laring Soft aluminium and thin-walled aluminium alloy pipes arc weak and may ne>--d reinforcing where the pipe passes into the connector. Tn cases where the pi~ diameter is less than 3/8in and the material of the pipe is soft enough, and c1~ enough, the end of the pipe may be folded in to form a double flare. Thft improves the seal and makes the flare more resistant to shear fo rces that may-' occur between tbe union nut and the pipe. D ouble flaring is not necesS:lry oif thin-w:lJled steel tubing because this material has sufficient strength. =i
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St.'lrting Pin Clamp Block
1=
-
I--
GUIde Pin Fig ure 9.13 - Do uble Flaring Alumini um T ube
The double fla re is formed by clamping the pipe in a special clamp block \\;th the cut end abutting a guide pin. The guide pin is remm-ed and the sra ning pin is then inserted and struck sharpl, with a hammer [0 close it down onlO the shoulder of the clamp block. A finishing pin is men inserted and stnlck to d ose it ooro the shoulder of tbe clamp block. The tool has a fla re pro rLie to march a pa rticular size of mbing and it will automatically fo m1 the correct double nare when it is properly dosed.
Insp ecti ng a Co mpl e ted Flare i\ properly fo rmed Oare will be concentric and be free from cracks, splits and
thi nning. The flare diameter should be almost equal to the mreaded bore of the union nut with 4~ft\ fla re standing just proud of the collar. \'7hen assembled, the fl are should 'fJass through the mreaded portion of the umon nut with nor mo re lhl!o~\1 ;Min c1ea mnce. To check this, the collar should be pushed finnly ~).{'h'gainst the back of the flare and me projection of the pipe beyond the collar face should be measured. The permined tolerances are: Tube Outside Diameter (in) Projection Tolerance (in)
o to
3, 16
to
1 '4
5/ 16
to
3/8
1/ 6-1
to
1/32
7/ 16
LO 1 ['2
1/32
to
1 16
0.0 10
·3
T ab le 9. 1 - Fla re Pro jeclio n Tole rances
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C H A PT ER NINE PI PES & H O SE S
"
I I D;'L I
Bore f Ku t
-
Colla!"
Flare
~
Figure
9.1~
-
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_
Fhrc Projection Tolerance
- Flare P rojection T o lernncc
Pipe Beading T hough not specified in the syllabus for this chapter, it would not be a bad idea to look at pipe beading. Pipe beading is used to pro\"idc a means of coupling rigid pipes with a length of flexi ble hose. The beading is applied by a beading tool that incorporates interchangeable rollers that [mllch the size of beading required.
Tubl:
Profiled Rollers Pressure
-.
. \djustment Screw
~
Figure 9.15 - H and Beading Tool
,. .~
The pipe is normally expanded to form a bead 9iameter 1/8in greater thn~~e outside diameter of the pipe at a distance of 1/4in from the tube end. fi
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C H APTER N IN E P I PE S & HO S E S
Bondin!:?: Stnp j -
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Fig ure 9.16 · H ose
'---
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and Be ad Connector
Inspection & Testing of Aircraft Pipes & Hoses There are a number of similarities in the inspection and testing of rigid pipes and flexible hoses but there are just as maoy differences. For that reason, we shall deal with them separately.
Inspection of Locally Manufactured Rigid Pipes \X!hen a rigid pipe assembly has been through all [he required srages of manufacnlre, il must be subjected to a fi nal inspection. Howeve r, we should not rely solely on final inspection to son good from bad. I f a pipe has been successfully inspected fo llowing each stage of manufacture, be fore being passed on to the next stage, tbe final inspection should not reveal anything untoward. Stage inspections give us the opporruniry to detect defects soon aher they appca!\nnd this saves a lot of wasted rime and effort in conti nuing to work
The final inspcction is still an important feature and we can now look at the points that need to be verified before signing a locally manufacturcd part off fo r lise. •
-nle malerial lIsed and me dimensions achieved should be checked to ensure that they conform fully to the approved drawing speci fi cations.
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CHAPTER NINE PIPES & HOSES
•
The available records should be checked to yerify that any material heat treatment processes thal \vere required before and after manufacture were successfully carried out. This is an important point because you cannot judge whether an item has been heat treated or not merely by looking at it, never mind whether it was successful or not. There must be evidence in the form of a record that relates unambiguously with the actual item you are inspecting.
•
If a fusible alloy has been used during bending, a check must be made to ensure that it has been completely removed and that there is no residue i.n the bore.
•
The end fittings should be checked to ensure that they are of the specified type and that the union nuts c·a n be withdrawn properly and are not obstructed by the close proximity of bends or marker tapes .
•
The pipe and the pipe bends should be inspected to cnsure that there are no ripples, signs of ovality, flaking, kinking, bulging, splitting, scoring or thinning.
•
The pipe flares should be checked for concentricity and for freedom from cracks, splits or thinning. The flare projections should be checked against the specified tolerance.
•
Recorded evidence should be checked to ensure that the pipe has successfully passed any specified pressGre or bore tests. Again, you should vcri~' that the record does acru·ally rdate to the pipe in your hand and that the pipe bears any specified markings to indicate that it has passed the prescribed test.
•
~The pipe should be inspected
ll,)n
to ensurd that it is dean internally and externally and that It has been given the correct protective treatment and is properly blanked io an approved manner. Finally, you should verify that the pipe has been allotted th~ correct pan number and that any specified system code markings, part number marklllgs and inspection marks hayc been properly applied. The system code m"kings m dctffikd in BS M23. Thm "p,""d foc you below. ::;
r
-0 .~
-.
Identification of Aircraft Pipes
ii.
The symbols and colours used to identify pipes used in aircraft comply witf British Standard BS M23 - Tdcnti fication Sch:e me fo r Pipelines. All systeln pipelines in a.n aircraft are marked at intervals along their length and at p o~ts where they pass through bulkheads or other sJucrural members. This is q9he to help you trace a pipe run in a system \,,;th~)Ut confusing it \\';th pip~ in other systems and to draw your attention to any special hazards associatc_f""';.\';th the substance it carries. The specifications for a manufactured RiPe will normally state what ma.rkings should be applied .I 9. . .
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CHA.PTER NIN E PIPES & HOSE S
The symbols, together with the related colour codes, are primed on one-inch wide marker tape and arc applied as a decal to each end of the pipe and then at app roximately two-foot intervals along the pipe length and adjacent to each service point or inspection panel. The marker tapes are also applied to the pipe immediately adjacent to every in-line connected componen t such as valves. regulators and filters. The markers must be affi..xed sufficiently far from the pipe end fittings to prevent tbem from interfering with the union nut or being picked up when the union is assembled. A distance of nor less than nine inches is recommended. \'\/hen a pipe is less than two feet in length, one marker tapc is all that is needed. If a pipe-run can be viewed easily, then [wo-foot imen'als will suffice. In areas where access and viewing are difficuJc, then a shorrer interval may be necessary. There must be at least one marker tape applied to a pipe in each compartment it passes through. On pipes ha\'ing diameters of four inches or more, or pipes that are exposed to harsh em·ironmems, identification tags or paim mar be used in place of marker tape. In addition to the standard identification marker tapes, supplememary information relating to the substance carried in the pipe may be added in the form o f half-inch \\~de market tape displaying black letters, symbols or figures on a white background. These tapes are nor.mal1r positioned adjacenr to the left hand edge of the sta ndard identification marker. These addition:!l lTI:!rkcrs may be used to indicale pipes that carry flammable li'1wds (FL\l\'Q or as a warning (Skull :!nd Crossbones). Additional marker tapes Olay be used to indicate the function of a pipeline. These would include: PRESSURE, VENT, RETURN and DRAIN. Further ma.rkers may be applied to indicale t"he name of the particular system the pipe serves and the direction of flow.
"
fUEL FUEL
of \~-"mi".
Symbol
Fig ure 9.17 - Examples of Aircrafl Pipeline Identification
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CHAPTER NINE PIPES & HOSES
[ will pro,-ide you with illustrations of the more common system identification markers and tables of the symbols and colour codes used. TIle pipe idencificatioo scheme is international and is also used by the military so J "ill give rou a furthe r table sho\\ing some additional markers that may be more commonly identified with milita ry rather than commercial aircraft systems.
~
I IIi! ,I .
"
"Wit. "IW '
""
·n,,,.. 'n"...
,,-
,n,.,",
" ,I>" ". ""...
•• •• •• •
'. ;~
"
• '
".... ,
Figure 9.18 - Aircraft Pipeline Ide nlific:Hion Markets ( I)
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•.
IIR! \J 1II;>;l< -
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£ D[·lC1'\:Ci £ DF·IU'\:L
DI',IU,\:G £
DEIC"{;
.,\\i\b
DEKi:\<.;
£ £ \'\"arning
. "0.:",": -
l.'~\\
,
.
Snnbol
\"\).~~"
Fig ure 9. 19 - Aircraft Pipeline Ide ntification f\.'larkers (2)
"u"
" " " , " "" " 11" ,
'
Direction of Flo\\" t!\TnMY \cTIV \TOK
II \1"ILRY \( T rv nnR n.\TnR\ \{, rlJ \T
If If If
",0 ",0 DRINK''', ",0 W TI'R ",0 [b~1 K''-'' ",0 W nlR DRfl't;}';J'\:l :nR
•
\\
V,\r.t t -\I
\fAU l "! \'\Cl -\1 V,\( l l \! V\( l '\1
\ :>~t
\\ \11 R
\\ ' S IT\\ \ffR \\ \ 511;. \\ \"Il:R
Figure 9.20 - Airc r'.tft Pipcline Identification Markets (3)
491 ~'1VW.part66.ro,"
CHAPTER N I NE P I PES & HOSE S
Symbol
SYSIClll
Bac kgro un d Colo u r
'\If Condllionin~
DOl pallt.:m (5nowflakt:~)
Ihuerr ,\ell\';1\(lf
F ltipscs ....~rh TadiJong lines Yellow :lnd
Urc:uhing
Bru\,\:n 3nd
~re\" b~nJ,
~rc\'
Greco
C()IIlPrl'~H~(1 G3..~
D la.~nn;IJ ~rnpcs
Ur.l.nge
("ohm
Bruad undularinA band ~
Blue
Dc leJllg
Tn anl!lcs
GTC\'
Dnnking \\:llcr
11 .0
Blue :md ~rt:m
Ficc\Tlc;li Ccmdull LIAhlmn)!: tlashes
bands
b~nds
Brnvm and HunAc.: b.md,
Fire Prolcclinn
I-Inll"/..omai diamonds
Urn"'n
!'uel
rour point st:lrs
R,d
Ill"dmuhe
Circles
Blue 3nJ \'cltow band.
Zig-zag lme-
OTln1,!c and grc\' b.lmh
I,ubne:lut>n
SqU:l.n:.~
Yellow
Ram Rqx:llcm
Raindrops
Blue and Art..,· oondi
Pneumatic
Linked 'X".s
Or:lO~e
Sokent
I lori7.()nt.al striIXs
Blue :lOd brown bamb
'·aeulIlll
VcruC31 w:Jv)" Iinl."
Gre\ lnd mange b:.l.nc\'
'\'l
Veroa) ch:tio
Red and blue
\\",lIer Injccuon
ChcHons
Hed. !-lTC\' :md red h'\I1II,
Imlrument
AIT
and blue bands
band~
Table 9.2· Common Pipelinc Identification Ma rkers (DS 1\123)
492 upJrit,hI C &'!J' GJ/k§ JOO6 . 200 -
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CHAPTER NINE PIPES & HOSE S
Symbol
Meaning
Skull and Crossbones
"raming Symbol
Il on/om:11 arro\n;
Flow dm;cuon
FL,,~r
Flammable tl(lld
PHD.''''
Physically dangerou!'
PRESSl'RE
Fluid prcs!'u l'C supply side of system
RI.TL·R\:
Fluid rerum side of system
DR.\ I t<
Flwd drain
VE\:T
Pan of a
ST\nC
Static pressure
PITOT
Static :md dynamIC pressure
T O); IC
POl sonuus or carcinogenic substance
11\,1) PO"TR
Pipc
J~ ;1
~yl;t(;m
\'COt
system
part of that namcd system
Table 9.3 - Examples of Add itional Pipeline Ide,uilication
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\\,~ <'
l'" '
-
Note: TIlis lis t is nOl exhaustive, JUSt an illustration of the kind of informatio n that may be displaycd, Do note dlal the 'skull and crossbones' is a general warning symbol; it is usually accompanied by a second marker tape gh-ing s(le9flt infonnation on the hazard,
~\'\l'~
Sys lem
Symbol
Filtered \"
,Be.:
Jncrun~
I3rn~d
B:lck/Zroulld Colour (,reell ;l1Id red
crosse5
b:lnd~
(hom).;c .md J!fccn bands ur:m~c b;md~
;\IlIno-pmpdbnl Uroad'Ts
Ycllow and
Rocku
Ydlow :md ~fCCfl
c:.uah·~!
Three ycrticll Strip(;$
Rnck.,r Fuel
rour lX'im StaT and crc:,C(;nL R.,J and we\'
Rockel t hali
Lrc
band~
band~
lirecn and gfer Land.
Table 9.4 - Less Common Pipe line Idenlifica tion Markers
~
,
:P~ 0'
~v
The.re is quite a lot of symbology used in pipeline. identification. It sccmii.-\fu aCl like a magnct ro examiners so it is worth familiarising yourself with ~e morc common exampleso
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CH A PTER N I NE PIP ES & HO SES
Testing Rigid Pipes following the manufacture of a rigid pipe assembly, there will normally be a recluircmcl1c for specific tests before the part is signed off Fluid and gaseolls system pipcs h:lVe speci fied flow rates and pressurc racings and rests arc designed to ensure that these specifications will be met.
Bore Test The bore of a pipe should be dimensionally correct and dear. A simple bore test may be specified where a steel ball with a diameter that is 80% of the inl ema l d iametcr of Ihe pipe must be able to pass freely through thc pipe in both direcoons. 'nus test is not aJways practicable due to the design of the pipe or its connections. Furthermore, the specificarioo may call for a more precise form of flow tCSt. Flow tests are conducted 00 a hydraulic tCSt rig to \'eril)' that the pipe is capable of passing a specific volume of fluid in a gi\'en lime under specified test conditions.
Hydraulic Pressure Test Hydraulic pressure testing is carried out on a specially designed hydraulic st:ltic test rig. The rig is used to check flow rates in addicion to pressure testing. The fluid used in the rig must be idencical with that used in the system the pipe is to be installcd in. One end of the pipe assembly is connected to the pressure delivery point on the rig and it is checked for full-bore flow by pumping a specified amoum of fluid through it whilst checking flow rate at the open end. If the flow ratc,\flfSt proves successful, the open end of the pipc is loosely blanked ,opt"hd~a fluid flow is produced ro bleed the pipe of air. The blank is [h~I1;-..'tigt;tened and the fluid pressure is raised in stages, checking fo r leaks at \'\;\.~~'ach stage. TIle pressure is finally raised [a the value specified b)t the rest. '111e \,\~ ~ nonnal proof lest pressurc for a pipe is one and a half urnes lhe maximum \. ..~,~ working pressure of the pipe in se rvice. The proof test pressure should be maintained for the rime specified in the test, typically about three minutes. During this time, the pressure should hold without am' funher need for pumping and there should be no signs of leaks.
Hydraulic Test Rig TIle hydraulic test rig is used for testing aircraft hydraulic system components. A typical rig consists of a steel work table that is supplied with fluid from two single plunger hand pumps, a rotary hand pump, a pressure intensifier, a fluid reservoir, twO filters , two reliefyahtes, two release valves, a stop valve, pressure gauges and a range of connectors and hoses for connecting components into the rig.
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CHAPTER NINE PIPES & HOSES
Hydraulic test rig instruments arc subject to periodic calibration and the calibration record should be checked by the operator before testing commences. The rig fluid must be clean and be of the type used by the system components it tests. The rig filters are of the self-cleaning type but there is a requirement to turn the band-wheels frequently. I f the filters are removed for cleaning, tlle), must be cleaned in the same type of fluid that is used in the rig. I-loses tbat are used to connect components to the rig pressure and rerum points m ust be regularly inspected, a few nasty instances have occurred where a rig hose has burst during a test. The rig is capable of intensifying fluid pressures up to 6,OOOpsi so extreme care is required when disconnecting components and when checking for leaks. The opera LOr should be wearing an apron, gauntlets and a full-face shield when conducting any rests on trus type of rig.
Pressure Testing Pnewnatic &
O:~.-)'gen
System Pipes
Pneumatic and oxygen system pipes are normally subjected to two types of p ressure rest, h~'draulic and pneumatic The hydraulic pressure test carned out using water as the test fluid. "nle second pressure test is carried out using compressed air at maxim u m sys lem wooong pressure whilst the pipe is fully submerged in a \vater tank. 111e pneumatic test rig incorporates a p ressure regulator, a relief yalve, an oil and water trap, pressure gauges and filters . \"'\!hen conductin g the pneumatic pressure test on a pi~e that is connected to the rig, the regulator and relief "alve should be set to the required test pressure and the air pressure should be slowly raised in the pipe. The test pressure should be held for five minutes while the pipe is examined for leaks. Leaks will be indicated by bubbles.
Pipes must be thoroughly cleaned after testing. The approved manufacturing instructions ,vill normally include details of the cleaning process. Typically, pneumatic system pipes are flushed with white spirit or a similar solvent and then dried with a jet of clean, dry air. Pneumatic and oxygen system pipes require particular care in that they must be fre~ of any trace of oil or grea ~. Oxygen pipes are typically cleaned by a vapour degreasing process followed b): washing through in boiling water foll owed by flushing with demineralise<;. watcr. -me pipes arc finally purged and dried ,\.rith clean dry nitrogen, \vater~ pumped air or breathing m''"Ygeo before being blanked. Alternatively, the pipi may be flush ed ,\~th trichloroethylene and blow dried in double filtered dry a~ Any additional external cleaning or drying required during the process sho~ be done with lint· free cotton cloth . If the pipe is not to be used immedia~y I aftcr cleaning, it should be blanked off and th!,!n sealed in a clean polr~~ne ~" bag. ~
"
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CHAPTER N I N E PIPE S & HO SES
Inspection of Installed Rigid Pipes 1\ number of problems can affect installed rigid pipes because of their location
and oper:uing environment. Pipes in exposed positions such as wheel wells may become damaged by regular contact with flying srones, grir, mud, water spray and icc. [n other locations, they may become damaged by carelessly performed and often unrelated servicing activities. Pipes can be stood on, levered aside, used as climbing frames and be otherwise generally abused in often quite unique ways during their sen>ice life. Particular care should be taken when inspecting pipes in exposed locations to check for distortion, chafing, cracks, kinks, corrosion and leaks that might be attributable to these causes. I nspection should also include a check on the condition of the required pipe identification markings Cha fing can occur out of sight under pipe clamps and clips because of constant ,-ibration.l\loisrure and spilt fluids can become trapped in the packing and this can produce e.'Hensi'-e corrosion oyer time. Chafing rna)' also occur between pipes and the aircraft strucrure. Properlr routed and supported pipe runs should not be at any risk from chafing but loose and fracrured pipe clips may quickly alter that situation. Fatigue aho becomes a fearure where pipes become free to vibrate. Sharp bends in pipes that e).-penence high-pressure pulsations, such as hydraulic fluid lines from pumps, can slowly develop an oval cross-section on lhe bends that may eventually lead ro fraCnire. Installed pipes will have been wire locked. The integrity of the locking should be checked. Pi peline connections are often norociousl), hard to reach and require skill, and patience, to lock. Locking wire mar break around the sharp edges o f drilled locking wire boles when too much tension has been applied, or where people have subsequently trodden on it! Com-ersely, wire may have sli pped on large unions and loosened. In either case, the secu ril), of the connections must be verified before re-Iocking. '1(\\,r
Leaks . ~p ;.\ftn/d -systems may be revealed by the presence of fluid or an ~ccliYkUlation of dirt around a pipe connection. Minor leaks in a gas sYSlem will \\).<'bften cause a noticeable loss in system pressure. A major leak can often be heard but detecting the source of a small gas leak is a little more difficult. nle application of a soap solution will often rc,-eal small leaks as bubbles. Care must be taken when checking for either fluid or gas leakage in high-pressure systems because the force of an invisible pinhole stream of fluid or gas C:1n easily penetr.Hc the skin and cause serious injury. 1n a similar vein, care must be taken when checking high temperature fluid or pneumatic lines and lines carrying toxic substances. Leaks in oxygen lines are particularly hazardous where they may create high concentrations of the gas in comparonents. 1fleakage is defCCted from a pipe connection, it may only be tightened where if is suspected [0 be loose but it must never be O\·e r-tig h ten ed in an altemp! to StOP a leak_ Loosening and then re-tightening the connection may cure the leak but if this fails, the connection must be stripped and the cause ascerL'lined_ Leaks arc often the result of solid particles trapped or embedded on the Oared connection faces and tightening down on these just creates the potential for further damage and rarely stOps the leak.
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CHAPTER NINE PIPES & HOSES
\\!hen any damage IS discm'ered, reference must be made to the approved Aircraft Maintenance Manual to check the limits of acceptability. \'('-hen corrosion is disco\"cred. the corrosive product~ must be removed before the depth of any corrosion pits are be measured. I
Testing Installed Pjpes At first sight, this section may seem to be in excess of the syllabus requirement. However, the syllabus merely states 'inspection and testing of aircraft pipes and hoses'. It does not specify at what stage the testing takes place. \'\Ie could make the mistake of believing that it only refers to parts that ha\"e been removed or that have been locally made up - it does not sa)' that! It is jus t as unportant to 'read' the syllabus, as it is to 'read' the question in an exami nation, The real meaning can often lie in what is not stated rather than \vhat is. Again, we \,rill play safe. Hydrauli c Sys tem Aircraft hydraulic systems are often subjected to flow and leak rate tests after components have been disturbed. The Alrcr~ft Mai ntenance r.!anual \ Vlll specify when these tests are required. Installed pipe-\vork is a part o f the system and disturbing it may trigger a requirement for testing. Regardless of this, whenever a hydraulic system pipe has been disturbed, it should be checked for leaks whilst at m aximum syst e m press ure Flow tests are usually conducted using an exterpal tes t rig connected into the system. The hydraulic semces are systematically operated and the hydraulic flow rates arc recorded from the cig flow meter. The leak rate check involves pressucising the system to normal operating prbssllre, rurning o ff the test rig, ~nd,,8becking .the time t,aken for the s)'st~m ptess.ure to drop b)~ a specifi~d ,,\\(}\lTicfunt. All disturbed pipe-work connecuons are IOspected for SlgnS of flwd _co\\C~c. -' leaks during this period. \'{'here a problem is isolated to a single source, the \"';)'~~' part should be removed for inspection. c
P neu matic System High-pressure pneumauc systems are a racity these days on large aircraft btlt may be encountered on light aircraft. These "rill be subjected to leak rate te~ when com ponents have been disturbed . \Vhegever a system pipe has be ~ disturbed, it showd be checked for leaks whilst at maximum systenf pressure . Leaks can be detected by brushing connections with an acid fre£: soap solution. The time taken for the ma..ximum system pressure to drop to specified value is an indication of the leakage raie. Some aircraft have hydrau& accumwators that are charged \\·i th nitrogen. Again, the time taken for acnunulator pressure to fall to a specified ,alue is an indication of the integilty of any seals and charging connections . I ~.;
!if
tBe
0\IV10st large aircraft h""e low-pressure pneumatic systems that rely on :4Jtanium alloy and stainless steel pipes to duct high temperature air from the ~gines to the "arious air operated sen-tces. Grea t care must be taken when checking
497 Copyight © BaFry' College 2006 . 200i All rigl!ts mm'td
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CHAPTER NIN E PIPES & HOSES
these pipes for leaks because the air temperatures are high enough [0 cause painful burns [0 unprotected skin. Again, these systems are subiected to leak rate tests afrer components have been dis turbed. O "J'ge n Sys te m \'i:rhcnever an oxygen system pipe is disturbed, it must be tested for leakage. The leak rest must be carried our with the system at its Donnal maximum value. Leaks are detected using a leak detecting solution that is free of any combustible sllbstances. The solution is applied with a soft brush whilst the connections arc scanned for signs of frothing o r the formation of bubbles. AU traces of the solution must be remm-ed after the test br rinsing it off with dean warer and then drying with a clean. lint free cotton doth. O"-ygen systems are subjected to £low rate tests a.nd special m"1'gen flow meters arc supplied for tlus purpose. Special leak rate testing equipment is also prO\~ided for checking the leakage cates through yalves etc. The predominanl leak calC test is howeyer, the time taken for tbe system pressure to fa.U from maximum to a given \·alue. Great care needs to be taken because an oxygen system that has dropped in pressure below a specified "alue will become contaminated with moisture and it will require purging. Fuel System
,
.'("
\\'?henever an aircraft fuel system component or pipe has been disturbed, the sys tem requires pressure and £low testing. The pressure test is nonnally conducted by opening the related fuel tank low~pressure fuel cock and switclung on the fuel tank booster pumps in the rank concerned. All pipes and connections arc then inspected \"isually for signs of leakJI.ge under pressure. Flow testing is carried out in accordance with the Aircraft ~fa i nlenance ~ranual. TIle tes t is designed to ensure that the system will provide a fuel flow to each engine Il~~ is in excess of the normal requirement when operating at maximum\, pb\V~~. All flow testing is conducted with the aircr:lft levelled larc{iilJf and longitudinally and with the fuel tanks containing the minimum \\:-.o\fbel qu ant ity, which is the un u sable conte nt p lus the am ount of fu el required to conduc t th e test .
.-;.\''c'
Full £low testing is required after a major component change or a major breakdown of the fuel system. .A fuel flow rest rig is required for this [CSt. The rig measures the gra\·jr)' feed and pump pressure fuel £low rates from each lank to each engine.
Aircraft Hoses Hose assemblies for use in high-pressure fluid systems are nonn:illy supplied as completely made up assemblies. There are some l)'pes of hose where the cnd fittings can be replaced. The syllabus for this chapter docs not require you to explain how to construct a hose. However, to inspect a hose, it is useful to know something about it.
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CH A PTE R NINE PIPES & HO S ES
The size o f a hose is given as the internal diameter of its bore. You may recall that the size of a rigid pipe is a meas ure of its outside diameter so there is an important difference. A 1/ 2in hose means that its bore is 1/ 2in diameter, not its outside diameter. 1\ hose as sembly is nonnally classified by it~ maxim l illl design operating pressure. The maximum operating pressure of d hose is inversely proportional [0 the diameter of its bore. The larger the diameter, th e lo\ver the maximum operating pressure. As a guide, they are classified as follows: low pressure up to 2S0psi, medium pressure up to 1S00psi and high pressure above lS00psi. Inn er
I ligh Pressu re I-lose
TmpTcgn::ttcd Cotton Braid Inncr [jner
Braid ~[ e d.iu m
Pressure H ose
Co,er LO\v Pressure II ase
Cotton Braid rnner Tube Figur e 9 .21 - H ose Asse mblies
d6mV}.p\essure hoses are constructed from thin walled flexible mbing that has a
"i\\C.":"':'
~~'(.' (.\.) ;-.,.,,- \.- "\"'.
,..:;~'~
(.\.~"
"2.~' seamless inner liner that is covered with a reinforcing layer of braided cottOil. T he hose would normally ha\·e a protective buter cover made of smooth rubber. Another type has a corrugated constrLlction that incorporates a spiral spring steel wire embedded in the corrugatiolls. Low-pressure hose would be typically used in instrument systems where negative pres sures exist.. Ivfedium pressure hoses have a seamless inner liner that is covered by a layer &J syn~heric rubber impregnated with braided c6tton and a further layer C}f braided steel. ;:: High-pressure hoses have a seamless inner liner covered in two or more layer,f of braided steel that are separated by layers of synthetic rubber impregnatcil. braided cotton that protects them and assists in bonding them in place. Tlili type of hose is typically used on tbe pressure sid~ of hydraulic systems. ..f:
,"'"
The length of a hose assembly that has straight end fittings IS a measureR' the dis tance between the extreme ends of the conn1e ctor nipples. \'Vhen ap angled v connector is fitted, the length measurement is taken from the cent.te of the bore at the end of the nipple in the angled connector. ." I
499 CIl/J)"ri,ghl <0 Bm')· College 2006 - 2007 All righl! remwd
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CHAPTER NINE PIPES & HO S E S
l.cnglh
fOOjl====too1 Length
Length
Fig ure 9.22 · Measuring Hose Length
I Jose assemblies arc designed for specific purposes and it is essentia l lhat the correct hose be fitted. The materials used are specifically designed to resist attack by the fluid used in a system. HowC1'er, these materials are ofren readily atracked by other rypes of fluid. Some hose materials such as PTFE arc vcry susceptible [0 damage from careless handling. Tn addition (0 [he pressure and temperature that a hose is exposed [0, it is also subjected to vibrauon and. in some cases, large angles of flexing.
I nspectjon) . o~t~staJ.l ed Hose Assemblies c.. .,\\<.:-;:; . ,,,;
\\>\1"here is ~ui[e a 10l [a check when inspecting hoses. I ha,-e highligh ted a key word in each paragraph [0 enable you to list the cbecks in rour mind. Aircraft £lex.ible hoses often haH a sen;ce life beyond which they have [0 be replaced. \Xnen inspecting a hose assembly, the da te of manufactu re should be checked to ensure that there is ade~uale life remaining [Q cover rhe next inspection period. The date of manufacture is marked on the hose. It may be displayed as a stencil on the outer cover or be impressed on a band auached to the hose. In some cases, the date is sho\\'n as a colour coded series of woven cotton strands to indic:ue the month and yea r of manufacture. The g enera l cond itio n of a hose may be recognised by the stare of the outer co\'er. This may be flaked, hardened or discoloured and it rna)' displa), circumferential cracks or crazing. These defects are usually acceptable providing the cracks do nO[ extend down to the underlying braiding. If the braiding is exposed, the hose is rejected. If there arc any signs of separation of the cover or braiding from the inner liner, the hose must be rejected.
500 COP)Tlghf 0 Barry ulkgt :!()()6 • ~()(r
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CH A PTER NINE PIPES & HOSE S
Excessively deep and pennanent indentations caused by tight clamping are another cause for rejection. Some hoses that do not flex 1Il service, particularly those used in hot zones such as in engine compartments, often take up a perm anent set. Teflon hoses arc yery prone to this in most locations. Other hoses are p re-fanned into a required shape. You should never attempt to straighten any of these hoses if they are remo,ed for any reason. The condition is not in itself a cause for rejecoon but if you have grounds to suspect they have bee n damaged, you should replace them . Light chafing and cuts 1n the outer coyer ar~ also acceptable providing the braiding is not exposed. !-Iowe\-er, the cause of the damage should be located and put right. \,bere hoses do not have an oueer, protective cover, any damage to the braiding is a cause fo r rejection. Care needs to be taken to check for chafing under securing clips. \'{'here this is eddJnr, both the hose and the clips must be n:placed. Kinks in hoses are primarily caused by incorrect installation or mishandling. A kink is recognised by a sharp increase in radius at one point along a bend . This may be concealed by an outer cover so you should check by using ftnger pressure to detect any discontinuity under the cover. A ny kink in a hose is a cause to re ject it. The ins taUati on o f a hose should be checked to ensure that it is nOt twisted around ils axis, stressed or bent through too ~harp an angle. Hoses have a longitudinal 'lay-line' on their oU[er cover and dus is prm-ided to indicate any twist in the hose. Hoses that connect fr91TI the strucrure to moving components should be examined to ensure that they have no signs o f having chafed against anything during their travel or have been strained or kinked as a result o f incorrect fitment. All supporting clips should be checked to see that the~8be correctly fttted and that there is no sign of chafing or stress. 1\.\\)6 - "
Cp\\<-,?-C'" TIle hose _ \'>;\~«'
~,\ ... (
,-.,5'.-~~,
should be inspected to ensure that there are no signs of contamination on the outer cover. Tins normally shows up as swelling, sponginess, hardening or a break~up of the surface of the hose. Any bose that shows anI.' of these signs of contamination must be re,·ected.
The hose should be checked for evidence of blistering on the outer cov~ . Blisters do not always constitute a cause fo r I rejection. However, the h~ should be removed for further e:xamination. l11C examination will depend on the sys tem in which the hose is fttted . T he checks are as follows: ~
Tf the hose is in a pneumatic system, you shoul1 insert a chisel-pointed neei into the blister parallel \v-idl the outer coyer so that the needle penetrates tl;:e outer cover only. If the blister collapses, the hose should be subjected t<;t~ pressure test at one and a half time the maximum working pressure of.~lEe o system. The test is conducted on a pneumatic test rig with the hose subm~ged in water. WThen the hose is pressuriscd, the air supply should be rurned Gtfr. Air bubbles wi.U be emitted from the air t.rapped un1der the cm-er. If thescxirail o ff and stop , the hose is acceptable. [f a constant stream of bubbles is ~seryed, this indicates a leak and the hose must be rejected.
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CHAPTER NINE PIPES & HO SES
If thc hosc is part of a hydraulic systcm, the blistcr should be puncrured as before. If fluid is emitted from thc puncture hole, the hose must be rejected. If only air is emiued, thc hose should be subjected to a pressure (CSt with hrdra ulic fluid at one and a hal f rimes the maximum system working pressu re for a pcriod of tWO minutes. I f no fluid leaks from rhe hose in rhis time, the hosc is acceptable. ,\ hose asscmbly should be checkcd for signs of corrosIOn on the wire braiding or on rhe end connectors. Anything more than \'ery light, localised corrosion is a rca son to reject the hose assembly. The signs of overheating are scaling, crazing or discolourarion on a rubber outer cover. Thcse arc not acceptable. \'\lhen the outer cover consists of wire braid, this rna)' rum an overall golden brown when exposed to hear. This overall discolourncion is acceptablc but if it displays patches of discolouration, the hose must be rejected. Finally, a hose should be checked for leaks when the systcm is pressurised. Fluid leaks under the outer conr may appear at the cnds of lhc hose or m.1y stain the cm"cr. This may nor occur when the leak is slight and thc outcr co,'cr may havc to be squcczed to re,·cal the leakage. Pneumaric leaks may be detected with the use of brushed on soap solution. If there is any doubt ahout a hose asscmbly, it must be remol"ed for a pres sLlre check. Any leakage from a hose is a cause for rejection.
Inspection of New H ose with Reusable End Fittings Some hoses have re-usable end fittings co save the COSt of renewing :I hose when only the hose section is rejected. Sockets and nipples that have been removed from ~ynserviceabJe hose must be inspected for damage, corrosion and evidell€'C1or parts of the old hose adbecing to the threaded portion. All *?Lc,B~.:or the old hose must be removed. TIle union nuts and the cone faces \\).~ ~hould be checked for damage. ;\frcr the new hose assembly has been made up, the umon outS should be checked for freedom of movement and the hose should be inspected for signs of damage :'11 the point of entry inro the conneclors. Signs of internal damage such as bulges and flaps should be checked by looking mrough rhe hose with a light held at the opposite end. \'{'bere this is nO{ practical, a ball test or a flow tcst should be carried Out. After the new hose has been inspected, it should be thoroughly cleaned and dried and prcssure tested to one and a half times maximum system pressure.
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CHAPTER NINE PIP E S & HOSES
Testing of Aircraft Hoses Ball Test The ball rest is designed to verif" that the bore of a hose is dimensionally correct and is clear. 1\ steel ball ha~ing a diametJr that is approximately 90% o·f the diam eter of t h e bore of the en d fittings should pass freely through the bore under its own weight whilst the hose is suspended from one end. The rest is repeated by passing the ball through the hose in the opposite direction. If the ball becomes lodged in the bore, the hose should be cut open to remove it. This avoids the risk of the hose being inadvertenciy issued for use. The test can be carried out on small bore hoses using a rod dst~ad of a ball.
Flow Test The ball test is not always accepted as sufficient proof that the flow rate is correct. A flow test involves measuring the ~olume of fluid passed through the hose in a specified time under rest conditions.
Pressure Test Removed fluid system hoses are pressure tested 00 a hydraulic test rig similar to that already described for the testing of rigid pipes. T he tes t fluid should be similar to that used in the system the hose will be fitted to. \'\'hen pressure testing a hose that has been removed from an aircraft, tbe operator of the rig should be protected from tbe effects of a possible hose burst by a heavy plastic screen. Polycarbona te screens are often employed for this purpose. \'Vhen com~fI~ssed air is used as a pressure test medium, the hose should be ",\\\"Subfi1erged in water during the test.
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The hose is fitted onto the ng and should be restrained to the approximate shape that it would have in service. Unless otheI"\vise stated, the proof tes t pressure is one and a h alf times the maximum working pressure of the h ose . The test pressure should be maintained in the hose for at least two nunutes. If the hose does nor flex in seI"\Tice, it should be fle xed approximately 15° from its normal shape sevCfal times each \vay during t~ pressure test. However, hoses that do not flex in service that are used in bigti temperature regions should not be flexed during the test because they may b it heat affected and could be damaged by flexing.
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I-loses that are subjected to fleXIng In seI"\'1ce should be flexed through th ~ !lonnal operaung range plus ISO each \vay. Thtre should be no leakage fr~:n the hose assembly during this rest. }
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There may be occasions where a fluid system h~se has to be tested in sin-ifOne end of the hose is blanked off whilst the other end is connected to aAiressure rig through a universal inflation adaptor. Installed hoses can also b(.~rHessure
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tested to check for leaks by subjecting chern to maxunum system pressure usi ng an exte rnal tCSt rig or the system pumps. G asoline fuel hoses tha t have been remO\red from the aircraft should be pressure tested using pam ffin as the {cst fluid. Removed pneumatic and oxygen s),stem hoses afC nonnally proof pressure tesred using water and arc then dried in a warm air blast and re- tested with dry air at maximum system pressure. Oxygen hoses must never be connected to a compressor for a pressure test because of the risk of oil contamination.
Bonding Test I-lose assemblies that havc met:t1 wire braiding, spiral wire reinforcemenl or any other fonn of metal construction must be subjected w ao electrical bonding lest. The bonding resistance should not be more than the greater of 0.05 ohm or 0.025 ohm per foot length.
Storage of Flexible Hoses l lose and hose assemblies should be srored away from sunlighr, strong lights and elcctrical machinery and switchgear. Unpacked hoses must be Hored so that air can circulate freely around them. The swrage temperature should be maintained bcl\veen lOoe and 26°C. The storage life of hose assemblies is calculated from the date of manufacrure or assembly. TIle li fe, storage requirements and the inspection and testing of hose assemblies whilst in storage and prior [Q instillation are contained in the releyant r-. la intcnance l\·lanual or manufacturer's instructions.
Pre-fo[mG~l'ho~"~sscmblies and Teflon (PTFE) hose assemblies that arc st,?:,cll-":a'fter remo\'al from an aircraft should always be stored so that their \.... ).~ ~'hape is maintained. No attempt should be made to straighten them or bend them further. The correct blanks should be fined [0 all hose assemblies whilst 10 storage. Installation & Clamping of Pipes Here we run across a dilemma. I\ Iany people carelessly usc the term ai rcraft pipes when whaT they actually mean are rigid pipes and flexible hoses. ·nle previous section was specified as being the inspection and testing of aircraft 'pipes and hoscs' whereas this section merely refers to 'pipes'. Do we lake the risk that this docs nor include the instaUation of flexible hoses? It is even odds dlal someone, somewhere is going to include them and witi justify the decision by linking thei r installation to the earlier inspection requirement. On thal premise, , would say that we are justified in including their installa tion here! We will start with the instaUation of 'rigid pipes'.
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Ins tallation of Rigid Pipes Before you attempt to fil a manufactured rigid pipe into an aircraft, you must check that it bas the correct part number and that it has been inspected and tested. The inspector's stamp is usually found near to the part number marking. The part should be accompanied by a <;:ertificate of Release to Service and this should be checked. The pipe should be inspected to ensure that there arc no signs of corrosion or damage to the pipe, its end fittings and the flated ends. Tf you suspect that the pipe has been damaged or deformed, it should be returned for further flow and pressure testing. The pipe should be thoroughly cleaned to remove any protective compounds and then blown through willi clean dry air. rt IS recommended that fluid pipes be Hushed out with clean, fihered fluid of the type used in the system. Oxygen pipes are subject to a special degreasing and drying process that ensures that they arc not contammated by oil at grease deposits that could cause an explosion. If the pipe is nor gOlng to be fined immediately, it must be properly blanked off ",,;rh approved blanks. Care should be taken You do not want to trodden on or kicked are ready to lnstall the
nOt to damage the pipe when carrying it to the aircraft. lean it lying around o n the serv;cing platfonn to be over the side! Blanks should not be removed until you pipe - beetles are fascinated by open pipes!
The pipe should be loosely placed into positio? in its supporting clamps and adjusted to align tl1t~ end connections. The unkm nuts should be checked to ensure thal lhey are free [Q rotate and can be pulled back without being obstructed by bends or marker tapes. If the pipe is of the flared end type, you should check that any required nipple adaptors are properly fitted . You might be surprised by rhe number of times a pipe has been fitted \,,;th the nipples left in a plastic bag tied to a component!
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connectors should align \\11th the mating connections - pipes must never be forc ed or levered into alignment becduse tillS introduces stress that could potentially lead to leakage or fatigue failure . This is not always that easy to achieve when YO ll are dealing with stainless steel pipes and you need to exercise a great deal of patience and restraint at rimes. You should also ensure that the pipe is not being pulled into position by the union nuts as they are tightened. TillS puts a direct strain on the flared sections or the pre-Ioad9 sleeyes in a flareless connection. 3-,
15.If the procedure calls for the pipe threads to be lubricated, this must be carrieq: out using the specified lubricant, taking care not to allow it to enter the bore of the pipe. Oxygen pipe connectors must never be lubricated with oil or greasi TI1ere are special dry film lubricants that are sometimes specified for this. ,::.,
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T wo spann ers must always be used when righ~ening, or disconnecting, a w pe connection. \"i"?here a pipe is to be connected o nto a component union ad~ tor, one spanner is used LO hold the component union adaptor whilst the sec~nd is llsed to turn and tighten the pipe union nut onto it. \,!hen a pip_4~to- pipe connection is being made, one spanner is used to hold the eXlernallr9threaded Ulllon adaptor while the internally threaded pipe union nut is turned and
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C H APTER N INE P I PE S & HO S E S
ciglllcncd on to it. I f there is an adap lor positioned berween the pipe connectors, the adaptor is held with one , panner while each pipe L1nion is tightened on to it using a second spanner. Union nuts must nOI be overtightened because this merely places strain on the threads. In rhe case of aluminium alloys, this can lead to fractured threads. Nor all pipes have flared connections. Pipe ends that han compressed rubber gland connectors must be presscd hard against thc shoulder of the recess in the union adaptor before the union nU[ is tightened. Rareless connectors rely either on the compressive force of a ridged metal gland or, a degree of 'bite' exerred by the cutting edge of a compression slee,·e. OYer or under-tightening these connectors when they arc being installed can render rnem ineffective. If this type of connector leaks, it must never be rightened further. TIle connector mUSl be dismantled and inspectcd. The details on these connectors and the methods of pre-setting them may be found in i\[odule 6 Chapter 6 under the heading of 'Standard Rarcless Connectors'. I would remind you of the importance or wire locking pipe connectors 10 unions correctly. Details on this may be found in i\lodule 6, Chaptcr 5 under the heading of ' Locking \'fire'. For example, when a hose union nut is connected to a component through a sc rewed adaptor, there is little point in JUSt wire locking the pipe union nut to the adaptor because the adaptor loosens in rhe same direction as che union. The component adaptor mUSl be locked separalely to a tixcd point on the component. Alternatively, the pipe union nUl may be locked di rectly to a fixed point on the component. G",~:d
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Figurt 9.23 - Locking Unions
\,\' hen flexible hose is used to connect rigid pipes with beaded ends, the correct type of hose that is compatible with the system fluid must be used. t\ gap of 0.25in ro 0.5in (6mm to 12mm) should exist berween dle pipes to pro\'ide clearance should the coupling flex. The hose clips should be fitted on the far side of the beadings and correcrly tightened. i\ 'bonding jumper' should be fitted across the connection 10 ensure electrical continuity.
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Supporting Rigid Pipes Support clamps are used to secure pipes to the aircraft structure, or to components such as the pO\ver plants. The clamps support the pipes, avoid chafing and reduce stress. The [\Va most commonly used clamps are the rubber cushioned clamp and the plain clamp. The rubber-cuslu oned clamp 1S used in areas where "ibration may create a problem with chafing. \,:there there is a group of pipes that run adjacent to each other, multiple pipe clamps may be used. T hese clamps are frequently made of red fibre composite material, alurruruU!11 allor, nylon or moulded mbber. The halves of the clamps are usually bolted together and secured to the aircraft strucnm;. The recetises in these clamps must match the size of the pipes they will support and any cushioning that is req uired. Pipes that run parallel to each other and connect to furrher ppe runs are frequently designed so they all make [heir pipe-to-pipe connections through a fLxed bracket that is attached to the structure or to a component such as a landing gear leg.
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Some support clamps are electrically self- bonded (0 the pipe. Other clips ~, require metal gauze packing to achieve a bonded contact. 'Bonding jumgi'''i=s' are fre(luenciy used to bridge pipe connections and these must be of th ~iype specified in the associated aircraft manual or drawing, .$:( (,
ivIinimum clearances must be maintained to avoid chafing between Pipes and fD::ed structure, control rods and other rigid moving parts and control cables .
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The nurumum clcarnnces are: O.25in (6tnm) from fixed Slrucrurc; O.75in (18mm) from control rods and rigid moving parts; l.Oin (25mm) from comrol cables. These clearances apply throughout the fujI rangc of movcment of parts and the vibrat.ion, movement, expansion and contraction of partS induccd by the aircraft opcration. All fluid pipes are roured beneath clectrica llooll1s, never above them, :'Ind they should clear them by a specified amount.
Fig ure 9.25 • Example of a Rigid Pipe lnslallalion
The distance betwccn pipc support clamps will be specified in the rcl:ucd Aircraft ~1:'Iinrcn:lIlce ~lanua1. Distances range between nine inches for smill diamctcr pipcs and forty inchcs for \'cry large diameter pipes. Smaller di:'lmclcr pipes require morc support than larger diameter pipes. "1.)\(
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p ~,~ .J Rstall ati o n Tes ts «' \"\~ All rigid pipes will have been pressure tested after manufacrure. There is, howe\'er, a requirement [0 carry out a flow and a pressure leSt aftcr the), ha\"c been installed. This is necessary to ensure that there are no leaks from the pipc or its connections [hroughom the pressure range of the system and that the required flow rate for the correct operacion of the system is achie\·ed.
Dcpending upon rhe system, the powcr for carrying out the rests is obtained from either :'In extcrnal tcst rig or the engine driven pumps. All insmlled testing must be carried our in accordance with thc rdevaI1t Aircraft ~laintenance ~fanua1. \'('hile the system services are being operated. the pipe should be checkcd for signs of flexing or movement that would reduce the stipul:'lted clearances. The system should be allowed to denlop its maximum working pressure, which should be maintained for :'I sufficient time [0 permit a leak check to be c:'lrried out. \¥Je ha\'e already discussed the general nature of the chccks that arc nonn:'ll1y associated with parCcll}:'Ir aircraft systems so there is no need to repeat them here. Once the pipe has been installed and tested successfully, if should be locked in the required manner.
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CHAPTE R NINE PIPES & HOSES
Ins talling Flexible Hoses Before a hose assembly is installed, it must be inspected to check that it is not damaged or corroded. Its part numbet, date of manufacture and the date of the last test should be checked. In some cases, the maintenance p rocedure will insist on a further pressure test before installation. T he hose bore should be examined as far as is practicable to ensure that is clear and undamaged. Straight hoses may be checked using a light source but hoses with angled connectors will require a ball tes t. The bore of the hose should be blown through with clean, dry air and, where reconunended by the manufacturer, it should be flushed with the type o f fluid used by the system the hose assembly will be fitted to.
General Ins tallation Points j\s pressure increases, a hose will expand radially and this will shorten it. For this reason, a hose must ne,er be under any tension. There must always be slack in the run of a flexible hose to allm" for this sh rinkage under pressure and to allow for vibratio n and whipping. The minimum amount of slack req uired in all straight hose assemblies is at least 3% of the distance between the fittings to which they are connected. Simply, the hose assembly should be at least 3% longer than the distance between the connection points.
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If a hose runs from one fixed point to another fixed point that is at an angIf~ o the fust, the use of a hose with straight end connections is not accep talf2e. A sharp bend in a hose close to a connection creates a lot of strain. To av?-hl this situation, one end of the hose should be fitted with an angled con!ic·e ror to rcmove any sharp bend in the hose and lhus p revent the hose 'kinkini .
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CHAPTER NINE PIPES & HOSES
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Figure 9.27 .. H ose Inslalla rion
\Vben a hose runs from a fixed point on the aircraft structure to a componem that moyes, rhe hose must be installed so that it will not foul anything throughout the full range of its movement. Additionally. the hose mUSt not be kinked or placed under tension at the extreme limits of its movemenr.
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Figure 9.28 · M ovem e m of Hose Asse mblies
The life of a hose assembly is affected by the amount of bending it experiences. A check must be carried our to ensure that the bend radius is not less than that stipulated by the manufacturer. The.re. are two recommended bend radii for each hose diameter. The minimum radii for hose used in locations where there is no relaci"e movemenr is smaller than the radii recommended for locations where there is relative movement between the end fittings. For example. rhe radii for a hose that connects to the moving part of a landing gear would be larger than a hose that is connected between two fLxed points. The 'flex' radius is approx-imately twice that of the bend radius of a fixed posicion hose, The radii recommended fo r Teflon (PTFE) hoses is different to that given for rubber hose due to the stiffer properties of Teflon. The bend radius of a hose assembly fined to a moving part musr never be less rhan the recommended minimum rhroughour the full range of movemenr o f the part.
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M~ Correct - Sbck and NO[ Tw]srcd Figure 9.29 - H ose 'Lay Line'
A flexible hose must ne,-er be twisted around its longimdinal axis during installation. TIlere is a continuous coloured stripe nmning along the omer cm'er of a flexible hose that is referred to as the 'Iar-line'. This line is prO\-ided to give you an indication that tbe hose is straight. I f the hose t\vists during insrall:Hion, the 'lay-line' stripe will also appear t'.visted. \,\'hen the outer cm-er of the hose consists of wire braiding, there may not be a 'lay-line' marked on it. In this case, the correct lay of the braiding is taken as an indication that the hose is straight.
1\ flexible hose assembly will £lex when internal pressure is applied and it can 'whip' because of surges in pressure. TillS will reduce the clearances bet\veen the hose and neighbouring parts with the risk of contact and chafing. \'{Then installing hose assemblies, there must be adequate clea rance bc::twcen the hose and other parts o f the aircraft structure. \\'Th en there arc a number o f hose assemblies in the sam e "lClnItY, thought must be given 10 correctly routing them 1O a\'oid these problems. The Aircraft Maintenance Manual \viU illustrate the correct routing for you.
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Figure 9.30 - Hose Installation
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Ins tallation When the threads of the hose connectors require lubricarion, this will be specified by the manufacturer together with the rype of lubricant. Lubricanr should be applied to me exteITlal threads of the mating adapror rather than the inrernal threads of the union nut. This avoids the risk of lubricant entering the bore of the hose. Oxygen hose unions muS( not be lubricated with oil or grease. Special lubricants ate a\-ai1able for oxygen connecrions but these must only be applied where authorised and in the manner prescribed in the releyam manual. The hose should be offered up to its connections and the union nuts turned up to fi nger rightness only. The hose should then be held to prevent it t\visting while the union nuts are tightened up with a spanner. Many hose end fittings incorporate additional flats on the hose end socket chat can be held with a spanner to prevent the hose twisring while the union nut is rightened with a second spanner. You must neyer use these hose socket flats in an attempt ro righten the union, they ate not designed for TUrning and you \"ill t\vist [he hose. The hose 'lay line' should be checked to ensure [hat the hose is nOt twisted. \,{'hen a hose is connected [0 a moving part, you must check that it can only move in the specified plane. \Vhen a hose moves in more man one plane, a torsional load is created thar can strain wire braided covers and put a strain on end fi ttings, potentially loosening chern. Where a hose is required [0 move in more than ooe plane. wire braided hoses are nor used and special anenrion is given ro the locking of connectors.
Supporting Flexible Hoses When there is qgllfclativc movement of the hose, it is clipped at intenTals of about t\\!ep~four inches to give support. ,'('hen fitting and rightening clips on fl~'tin'te~ose assemblies, you must not avec-tighten mem or distort the hose ... \,\;I.\,tbVCC. If a hose displays excessively deep and permanent indentations caused \~,\ by clamping, it should be rejected. The position of me clips should nor put a (~...~. . .~ . strain on the hose and distort it. Hoses that are designed to flex in one plane are often fitted into a large U sectioned support guide that guides the movement of the bose. Ollie guide must be correctly posirioned in relation to the movement of the hose to avoid the possibility of [he hose movement being restricted at the limits of travel. Anti-chafing cape is sometimes wrapped around secrions of a hose where there is a risk of it chafing agains t n6ghbouring parts. TIus must be strictly limited because it reduces the flexibility of rhe hose. Leather must nor be llsed to provide chafe protection because it contains acid that will corrode metal parts that it contacts. l'v letal braided hose is normally self-bonding. Other hoses may require 'bonding jumpers' to bridge the connections. T he Ai rcraft Maintenance Manual will specify the bondi ng measure to be taken and any required bonding tes ts.
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Post Installation Tests Wlhen a hose assembly has been 1nstalled, the assOClated system must be tested to ensure that the flow, pressure and electrical bonding are correct. \\Then the system services are operated, the hose should be checked for leaks, chafing and excessive flexing under pressure, If it 1S connected 10 a moving part, the hose movement must be checked over the full range of travel to ensure that it is not kinked, tensioned or fouled,
Installing Flexible Metallic Hoses This type of flexible hose consisrs of an annular or spirally convoluted, seamless, flexible inner lining covered in stainless steel wire braid, \\"'e will give this type of hose a special mention because there are a couple of important points to remembet when installing it, Flexible metallic hose assemblies are thin \valled, highly stressed parts and it is very important that they arc not stressed beyond theu elastic limit. Great care is required when installing the hose, Unions should be initially hand tightened with the hose lying in its natural position . The construction of this type of hose means that it \,,;11 always try to adopt a nanual posnion so It is important that you do nOt t\vist it or place it in any tension "",hen the connections are finally tightened . Some of these hoses haye end firtings with Vee band flanges. Care needs to be taken that the correct 0 ring seals are fitted and that the flanges fit correctly into the mating spigots before tightening down the Vee band clamps.
This has been a brief excursion imo the world of high-tech 'plumbing'. Module 6, Chapter 6 contains a lot more detail on the varioLis pipes and unions that are associated with this. The network of aircraft pipes and hoses is there to transport fluids from where they are stored to where they are needed and back again if so required. We hope that it \vill continue do this with some assuran'fe that the fluid s will actually stay within the sys tems because we soon feel tlfS effects if they do not. Anyone who has had the dubious pleasure of chasinit 'phantom' leaks through a labyrinth of under-floor compartments understand this - and how long cheap torch batteries last! It is an importal15: topic and exarruncrs will know this, T haye included a few multi-choice answdJ" questions for you to try before we moye on to a completely different topi:~ , Meanwhile - the coffee pot beckons! 2 y:::::
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CHAPTER NINE PIPES & HOSE S
Revision Pipes & Hoses Questions 1. Th e Am eri can AN flare angle is:
a) -l5° or 90° inclusive angle b) 37° o r 74° inclusive angle
2. A rig id pipe is proof press urc tes ted
31 :
a) maximum working pressure b) 2 times maximum working pressure
c)
1112 times maximum working pressure
3. To absorb
vibration and expansion a nd contractio n tc mp e [atur.~\~ rig i d pipe asscmbly shou ld luwc:
du e to
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c} a sliding nipple adaptor at one end
4. Th e size of rig id mel a l tubing is a m eas ure of its:
a) internal bo re diameter b) e:ucrnal diameter c) wall thickness
514 Gip)'righr 0 Bar!)' C(lIH.gt 2006 . 200- All rights rmrvrd
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CHAPTER NINE PIPES & HOSES
5. Low melting point, fusible alloy is us ed to :
a) provide internal support \vhen bending tubing b) attach nipple adaptors into flares c)
tin the bore of rigid pipes
6. A draw bending machine is equipped with a set of:
a) spnngs
b) fixed formers c) mandrels
7. If a tube ripple!,; on the inside and flattens on the outside 'when being bent in a compression bending machine, the probable cause is that: a)
the pressure
~e tt:ing
was roo high
b) the pressure setting was roo low c) the mandrel had not been fully inserted
8. The most probable cause of diametrically opposed flash lines on a pipe flare is:
'!W:I
the half bushes in the flaring tool were not fully closed up b) exccssi,e pressure was applied during flaring c)
the pipe end \vas not cut square ~
's'
."
-. fillet!:~.
9. When bending an o>..]'gen sys tem pip e assembly it should be ,,;th : a) low melting point, fusible alloy
-
b) mineral oil c) dry sand
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CHAPTER NINE P I PE S & HOSES
10. An e>.:panding cone Oaring tool s hould nor b e u sed to produce flare s m:
a)
heat treatable aluminium alloy rube
b) stainles s steel rube c) copper tube
11. I n a correctly formed fla re, th e end of the pipe s h o uld: a)
project slightly beyond the face of the collar
b) be slightly recessed inside the collar c) align wi th tbe face of the collar
12. A bore test o n a rigid pipe is carried out u sing a s tee l ball 'w ith a dia m e ter: a)
80% o f the imernal diameter of the pipe
b) 80% o f the external diameter of the pipe c) 90% o f the internal diameter of the end connector
13. \'(/ben a pine lis s ubjected to a press ure tes t u sing compressed a ir it ~ O\.' I m u s trRlf;a.t1 v ( .L1W:.'f: {(" a) filled with water and tested at maximum s\,stem pressure , '" lo-
-
,\...\
,.,---.\\~ \..
'"
b) tested d ry at 1 V2
. tlIlles maximum system
'
pressure
'''\
c)
fully submerged in water and tested at maximum system pressure
14. Th e w arning s ymbol u sed in th e B S M 23 Iden tific ation Scheme for pipelines is: a) a red triangle on a white background b) a black sh.llil and crossbones on a white background c)
the black letters HAZ on a yellow background
516 CopJnght © &17)' Colle!! 2006·2007 All right! rmrvrd
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CHAPTER NINE PIPES & HOSES
15. \Vben carrying o ut a p ressu re tes t on a hose ass embly that d oes not fl ex in se rvice, th e hose s h ould :
a) bc flexed 15° cither side of its normal [cst position during tbe test b) not be flexed during the test c) be bent at 90° before carrying out the tcs t
16. T he miOimum le ngth o f a s traig ht hose tha t is fitted b e tween two fixe d con necti o n s 100cm ap art s h o uld be:
a) 100cm b) 103cm
c) 120cm
17. A comin uou s co lo ured line running a lo ng th e len gth o f a h ose outer cover is intend ed to indi cate : a) stretch
b)
t\Vl st
c) flex
~i.\\l8. A\'[ut o n
the o ute r cover of a h ose that exp oses the braiding is :
_ ~\\L":"":' ...
\
'
,~(" l-'
.
a) acceptable subject to a srstem pressure ('est b) acceptable providing the braiding is not cut c)
not accep table
.""c-:S.. 19. When ti g htening a h ose conn ec tio n that has additional hexagon fl at; on th e hose soc ke t, you s hould lI SC two spanne rs to :
a)
--
hold the hose socket and turn the union nut
b) hold the union nut and rum the hose socket
c) hold the mating component adaptOr and turn the bose socket
517 CopJrighl C Btm) Co/legt 2006 - 2OQ- AD righls rrsm vd
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CHA P T E R NIN E PI PES & HO SES
20 . If yo u discover fr etting corrosio n on th e w ire braiding o f a hose und er a suppo rt clip you s h o uld :
a) clean off lhe corrosion and tighten the clip b) clean o ff lhe corrosion and replace the clip c) replace the hose and the clip
Revision Pipes & Hoses Answers B
16.
B
2. C
17.
B
3. A
IS.
C
4.
19.
A
' 8\'1.
c
I.
B
5. A
,
"'I.)\'\u 6. 1.. ~';';"''; ~
~ .\('
, ," '"
.'
7.
B
~
S. A
9. C 10. B II. A 12. A
13. C 14. B IS. A
518 COPJT1ghl C &ny GJ/kgr :!006 . 200 - All right! mm'td
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