Mechanical Vibrations Solution

MECHANICAL VIBRATION

10ME72

VTU Question Paper Solutions CHAPTER-1

INTRODUCTION

1.

a) Define the following terms : i) Periodic motion

ii) Degree of freedom

iii) Resonance

iv) Phase difference.

JUNE/JULY 2016

Ans:

i)

Periodic motion, in physics, motion repeated in equal intervals of time. Periodic motion is performed, for example, by a rocking chair, a bouncing ball, a vibrating tuning fork, a swing in motion, the Earth in its orbit around the Sun, and a water wave. In wave. In each case the interval of time for a repetition, or cycle, of the m otion is called a period. a period.

ii)

Mathematical modeling of a physical system requires the selection of a set of variables that describes the behavior of the system. The number of degrees of freedom for a system is the number of kinematically independent variables necessary to completely de scribe the motion of every particle in the system DOF=1Single degree of freedom (SDOF)

iii)

DOF=2 Multi degree of freedom (MDOF)

Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies or resonance frequencies. At resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations. This is because the system stores vibration stores vibration energy.

DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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MECHANICAL VIBRATION iv)

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Phase difference is the difference, d ifference, expressed in degrees or time, between two waves w aves having the same frequency and referenced to the same point in time. time.

[2]

Two oscillators

that have the same frequency and no phase difference are said to be in phase. Two oscillators that have the same frequency and different phases have a phase difference, and the oscillators are said to be out of phase with each other.

1.b. Add the following motion analytically and check the solution graphically.

0

x1 = sin (8t+30 ),

0

x2= 2cos (8t-15 ).

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Ans:


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c) Represent the periodic motion given by following Fig by harmonic series. JUNE/JULY 2016


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1.

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a) Find out the natural frequency of the system shown in Fig by using (i) Newton’s method (ii) Energy method. JUNE/JULY 2016

Ans:


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b) Determine the natural frequency of spring mass system taking the mass of the spring into account.

JUNE/JULY

2016

Ans: Mass on a Spring

Consider a compact mass that slides over a frictionless horizontal surface. Suppose that the mass is attached to one end of a light horizontal spring whose other end is anchored in an immovable wall. (At time , let be the extension of the spring: that is, the difference between the spring's actual length and its unstretched length can also be used as a coordinate to determine the instantaneous horizontal displacement of the mass.

Fig. Mass on a spring. The equilibrium state of the system corresponds to the situation in which the mass is at rest, and the spring is unexpended In this state, zero horizontal force acts on the mass, and so there is no reason for it to start to move. However, if the system is perturbed from its equilibrium state (i.e., if the mass is displaced sideways, such that the spring becomes extended) then the mass experiences a horizontal force given by Hooke's law,

Here, is the so-called force constant of the spring. The negative sign in the preceding expression indicates that is a restoring force (i.e., if the displacement is positive then the force is negative, and vice versa). The magnitude of this restoring force is directly proportional to the displacement of the mass from its equilibrium position .Hooke's law only holds for relatively small spring DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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extensions. Hence, the displacement of the mass from its equilibrium position cannot be made too large, otherwise ceases to be valid. Incidentally, the motion of this particular dynamical system is representative of the motion of a wide variety of different mechanical systems when they are slightly disturbed from a stable equilibrium state. Newton's second law of motion leads to the following time evolution equation for the system,

where . This differential equation is known as the simple harmonic oscillator equation, and its solution has been known for centuries. The solution is

We can demonstrate that Equation is indeed a solution of Equation by direct substitution. Plugging the right-hand side of Equation and recalling from standard calculus that

,

we obtain

the follows that Equation is the correct solution provided


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Figure 2: Simple harmonic oscillation.

Figure 2 shows a graph of

versus

derived from Equation (3). The type of motion displayed

here is called simple harmonic oscillation. It can be seen that the displacement

oscillates

between and . Here, is termed the amplitude of the oscillation. Moreover, the motion is repetitive in time (i.e., it repeats exactl y after a certain time period has elapsed). The repetition period is

(7)


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This result can be obtained from Equation (3) by noting that period

: that is,

is a periodic function of

with

. It follows that the motion repeats each time

increases by . In other words, each time increases by motion (i.e., the number of oscillations completed per second) is

. The frequency of the

(8)

It is apparent that

is the motion's angular frequency: that is, the frequency

radians per second. (The units of the units of

are hertz--otherwise known as cycles per second--whereas

are radians per second. One cycle per second is equivalent to

second.) Finally, the phase angle, maximum displacement, where

converted into

radians per

, determines the times at which the oscillation attains its

. In fact, because the maxima of

occur at

,

is an arbitrary integer, the times of maximum displacement are

(9)

Varying the phase angle shifts the pattern of oscillation backward and forward in time. (See Figure 3.)


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Figure 3: Simple harmonic oscillation. The solid, short-dashed, and long dashed-curves

correspond to

,

, and

, respectively.

Table 1: Simple harmonic oscillation.

0

0

0

0

0

0


0

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Table 1 lists the displacement, velocity, and acceleration of the mass at various key points on the simple harmonic oscillation cycle. The information contained in this table is derived from Equation (3). All of the non-zero values shown in the table represent either the maximum or the minimum value taken by the quantity in question during the oscillation cycle. As we have seen, when a mass on a spring is disturbed it executes simple harmonic oscillation about its equilibrium position. In physical terms, if the mass's initial displacement is positive ( ) then the restoring force is negative, and pulls the mass toward the equilibrium point ( ). However, when the mass reaches this point it is moving, and its inertia thus carries it onward, so that it acquires a negative displacement ( ). The restoring force then becomes positive, and pulls the mass toward the equilibrium point. However, inertia again carries it past this point, and the mass acquires a positive displacement. The motion subsequently repeats itself ad infinitum. The angular frequency of the oscillation is determined by the spring constant,

,

and the system inertia, , via Equation (6). On the other hand, the amplitude and phase angle of the oscillation are determined by the initial con ditions. To be more exact, suppose that the instantaneous displacement and velocity of the mass at follows from Equation (3) that

are

and

, respectively. It

(10) (11)

Here, use has been made of the trigonometric identities and

. (See Appendix B.) Hence, we deduce that

(12)


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and

(13)

because

and

The kinetic energy of the system, which is the same as the kinetic energy of the mass, is written

(14)

The potential energy of the system, which is the same as the potential energ y of the spring, takes the form (Fitzpatrick 2012)

(15)

Hence, the total energy is

(16)

because and . According to the previous expression, the total energy is a constant of the motion, and is proportional to the amplitude squared of the oscillation. Hence, we deduce that the simple harmonic oscillation of a mass on a spring is characterized by DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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a continual back and forth flow of energy between kinetic and potential components. The kinetic energy attains its maximum value, and the potential energy its minimum value, when the displacement is zero (i.e., when ). Likewise, the potential energy attains its maximum value, and the kinetic energy its minimum value, when the displacement is max imal (i.e., when ). The minimum value of rest when the displacement is maxim

is zero, because the system is instantaneously at

a) Set up differential equation for a spring mass damper system and obtain the complete solution for the under damped condition.

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Spring-mass-damper system:

Fig. shows the schematic of a simple spring-mass-damper system, where, m is the mass of the system, k is the stiffness of the system and c is the damping coefficient. If x is the displacement of the system, from Newton’s second law of motion, it can be written & && mx −cx − kx & & Ie mx  cx  kx  0 1 This is a linear differential equation of the second order and its solution can be written as x  e st (2) s t & dx Differentiating (2),

 x  se dt 2

2 st

d x

dt

2

&&  x  s

e


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st

ms e  cse

Substituting in (1),

2

ms  cs  k e

st

st

ke

st

 0

0

2

Or ms  cs  k  0 (3) Equation (3) is called the characteristic equation of the system, which is quadratic in s. The two values of s are given by c2

c

k

s

1, 2 −

 2m

− 2m m

(4)

The general solution for (1) may be written as s t s t (5) x  C e  C e 2 1 2 Where, C1 and C2 are arbitrary constants, which can be determined from the initial conditions. 2 c k In e uation 4 , the values of s1

= s2, when

 m

2m c Or,

k 

ω  n

(6)

m 2m Or c  2mωn , which is the property of the system and is called critical damping coefficient and is represented by cc. Ie, critical damping coefficient = cc  2mωn The ratio of actual damping coefficient c and critical damping coefficient cc is called damping factor or damping ratio and is represented by ζ. c

Ie, ζ 

(7) cc

c

c

In equation (4), 2m

Therefore, 

s1, 2



can be written as 2m

cc

c

 ζ . ωn

 cc

2m

 −ζ . ωn 2 2  − ζ ζ .ωn  − ω n  

2 ζ − 1 ω 

n

(8)

The system can be analyzed for three conditions. (i) ζ > 1, ie, c > cc, which is called over damped system. (ii) ζ = 1. ie, c = cc, which is called critically damped system. (iii) ζ < 1, ie, c < cc, which is called under damped system. DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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Depending upon the value of ζ, value of s in equation (8), will be real and unequal, real and equal and complex conjugate respectively.

b) A mass of 2kg is supported on an isolator having a spring scale of 2940 N/m and viscous damping. If the amplitude of free vibration of the mass falls to one half its original value in 1.5 seconds, determine the damping coefficient of the isolator.

Solution:

2940  38.34r /

k Undamped natural frequency = ωn 



s

2

m

Critical damping coefficient = cc  2 m  ωn  2  2  38.34  153.4 N − sec/ m

Response equation of under damped system =

x

 A1e

−ζωnt

  sin 1 − ζ 2 ωn t  φ1  

−ζωnt

Here, amplitude of vibration = A1e X 0

If amplitude = X0 at t = 0, then, at t = 1.5 sec, amplitude = 2

Damping coefficient C  ζ  C C  0.012  153.4  1.84 N DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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a) Define the term “Transmissibility”, and derive the expression for transmissibility ratio due to harmonic excitation.

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ANS:

Transmissibility is defined as the ratio of the force transmitted to the foundation to that impressed upon the system. Assuming that the forcing function is harmonic in nature, we shall consider two cases of vibration transmission - one in which force is transmitted to the suppo rting structure, and one in which the motion of the supporting structure is transmitted to the machine. Consider the system shown in Figure 1, where f(t) is the harmonic force acting on the system and fT(t) is the force transmitted to the supporting structure or base. The force transmitted through the spring and damper to the supporting structure is :

The magnitude of this force as a function of frequency is :

The magnitude of this force as a function of frequency is :

(2) DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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The oscillation magnitude as a function of frequency is :

(3) Substituting equation (3) into (2) :

T is defined as the transmissibility and represents the ratio of the amplitude of the force transmitted to the supporting structure to that of the exciting force. 5.6

3

b) A 100-kg machine is supported on an isolator of stiffness 700  10 N/m. The machine causes a vertical disturbance force of 350 N at a revolution of 3000 rpm. The damping ratio of the isolator is ζ = 0.2. Calculate (a) the amplitude of motion caused by the unbalanced force, (b) the transmissibility ratio, and (c) the magnitude of the force transmitted to ground through the isolator. JUNE/JULY 2016 Solution:

(a) From Window 5.2, the amplitude at steady-state is F o / m

2 X   n

  2 

= 83.67 rad/s and



1/2

2

k

Since

n 2

2 2

= 314.2 rad/s,

 300

n m

60

(b) From equation (5.7), the transmissibility ratio is DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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2

1 2 r



F T

 2

2

2

F 0

1 r  Since r 

  2 r 

= 3.755, this becomes n FT

 0.1368

F 0 (c) The magnitude is F T

F  0.1368  350 0

F  T

  4 7. 

F

0


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Plot the T.R. of Problem 5.6 for the cases ζ = 0.001, ζ = 0.025, and ζ = 1.1. Solution:

T.R.=

2 2  1r 2      2r 

1 2 r

2

A plot of this is given for ζ = 0.001, ζ = 0.025, and ζ = 1.1. The plot is given here from Mathcad:


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5. a)Discuss the principle of operation of a vibrometer and an accelerometer. Draw the relevant frequency response curve JUNE/JULY 2016

Need for vibration measuring instruments 1.To detect shifts in the natural frequencies – could indicate a possible failure or need for maintenance 2.To select operational speeds to avoid resonance 3.Theoretically estimated values may be different from the actual values due to assumptions. 4.To design active vibration isolation systems 5.To validate the approximate model 6.To identify the system in terms of mass, stiffness and damper. When a transducer is used in conjunction with another device to measure vibrations it is called vibration pickup. Commonly used vibration pickups are seismic instruments. If the seismic instrument gives displacement of the vibrating body – It is known as VIBROMETER. If the seismic instrument gives velocity of the vibrating body – It is known as VELOMETER. If the seismic instrument gives acceleration of the vibrating body – It is known as ACCELEROMETER. SEISMIC INSTRUMENT

Vibrating body is assumed to have a Harmonic Motion given by y Ysinωt − − − 1 Eq. of motion for the mass m can be written as mx&&  c x& − y&   k x − y   0 − − − 2

Defining relative displacement as z  x − y − −3


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or, x  y  z

Substituting this value of x in equation 2 mz&&  cz&  kz  −my&& mz&&  cz&  kz  mω Ysinωt - - - 4

Steady state solution of eq. 4 is given by z  Zsin ωt − φ 

The frequency response plot is shown in the fig. The type of instrument is determined b y the useful range of frequency 4 ζ = 0 . 0 ζ = 0 . 1 3 ζ = 0 . 2 = 0 . 3 2 l e m c o r c r t e A e e

Y / Z 1

r e t e m o r d b i n a V

ζ = 0 . 4 ζ = 0 . 5

=0.70 7

r e t e m o l e V

ζ = 1 0 0

1

2

3

4

ω /ω n

(r)

The phase angle plot shown below indicates the phase lag of the seismic mass with respect to vibrating base of machine


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Z

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r

Y  1 − r   2ξ r 

Vibrometers Used for measurement of displacement of vibrating bod y. It means when Z / Y ~ 1, the observed reading on the scale directly gives the displacement of th e vibrating body. For this to happen r = ω / ωn ≥ 3. r

2

Z

z  Zsin ωt − φ 

1 2

2

1 − r   2ξ

2 r

Y



y Ysinωt Thus when r = ω / ωn ≥ 3, Z / Y ~ 1, but there is a phase lag. Z lags behind Y by an angle φ or by time lag of t = φ / ω. This time lag is not important if the input consists of single harmonic component. Thus for vibrometers the range of frequency lies on the right hand side of frequency response plot. It can also be seen from the plot a better approximation can be obtained if ξ is less than 0.707. Accelerometer Accelerometer measures the acceleration of a vibrating body.

They are widely used for measuring acceleration of vibrating bodies and earthquakes. Integration of acceleration record provides displacement and velocity. Z

r

Y  1 − r   2ξ r 


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b) A shaft is 30 mm diameter and 4 m long and may be regarded as simply supported. The density is 7 830 kg/m3. E = 205 GPa. Calculate the first three critical frequencies. SOLUTION 3

The essential information is d = 0.03 m L = 4 m ρ = 7830 kg/m E = 205 GPa First calculate the distributed weight w by calculating the weight of 1 m length.

A = πd2 = π x 0.032 = 706.9 x 10-6 m2 4 4

Volume = A x 1 = 706.9 x 10 -6 m 3

Weight = ρ x A x g = 7830 x 706.9 x 10 -6 x 9.81 = 54.3 N/m DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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4

I = πd 64

=

f n = 1.572

4

π x 0.03

= 39.76 x 10 -9 m 4 Now calculate the funamental frequency.

64 gEI 4

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9.81 x 205 x 10 x 39.76 x 10 -9 =1.572

4

= 3.77 rev/s

wL 54.3 x 3 2 If the shaft took up the second mode the frequncy would be 3.77 x 2 = 15.1 2

rev/s If the shaft took up the third mode the frequncy would be 3.77 x 3 = 33.9 rev/s

6.

a) What is dynamic vibration absorber ? Explain briefly the dynamic vibration absorber with diagram and equations.

Dynamic Vibration Absorbers Basic Concept

Dynamic Vibration Absorbers (DVA) are based on the concept of attaching a secondary mass to a primary vibrating system such that the secondary mass dissipates the energy and thus reduce the amplitude of vibration of the primary system.


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There are many application of DVA, A few are noted below: vibration control of transmission cables control of torsional oscillation of crankshaft control of rolling motion of ships chatter control of cutting tools control of noise in aircraft cabin vibration control of hand held devices DVAs are generally of three types Vibration Neutralizer : Here, a secondary mass is connected to the primary using a

spring element. Auxiliary Mass Damper : Here the secondary mass is connected to the primary by a

damper/dashpot. Dynamic Vibration Absorber : A general case where both spring and damper are

used to connect the secondary mass, with the primary system.


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b) Find the natural frequencies of the system shown in Fig. Also draw the mode shapes and locate the node.


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a) Determine the natural frequency of the system shown in Fig, by using Holzer’s method. Assume K = 1N/m, m = 1kg.


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MECHANICAL VIBRATION 8.

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write a short note on any FOUR a. b. c. d. e.

Dynamic testing of Machines Machine condition monitoring Orthogonality of Principle modes Machine vibrating monitoring Experimental modal analysis

Dynamic Testing Machines Dynamic Testing Machines-Servo Electric:

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We offer Dynamic Universal Testing machines with the following load ranges: 1k N to 2.5 kN double column, table top Dynamic Testing Machine. 1 to kN to10 kN, Dual column dynamic testing machines. 10 to 63 kN, Dual column dynamic testing machines. 63 kN to 600 kN Dual column dynamic testing machines. 600 kN to 3000 kN and higher, four column dynamic testing machines.

Construction:

The machines come with an actuator which can be integrated on the upper cross head or the lower cross head. High stiffness construction of load frame which are aligned to the highest precision. The columns are induction hardened and chromium plated. Movable upper crosshead for facilitate quick, easy, and accurate positioning by two long-stroke actuators. A passive clamping system and a T slot table in the lower cross head for clamping compo nents. Double ended, equal area linear actuator with hydrostatic bearings for the best friction free static and dynamic performance, allows high side-loads and emergency running, integrated in machine’s DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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base to shorten the force train. The LVDT displacement transducer is integrated to provide a high stroke accuracy and control. Servo valve with manifold mounted direct on the actuator for the highest possible response and most accurate test control Close coupled accumulators to minimize hydraulic pressure fluctuations and filter is mounted direct at the actuator High precision fatigue rated flat load cell for static and fatigue tests fixed on upper or crosshead in machines where the actuator is located in the base or to the end of the piston rod in machines where the actuator is located on the upper cross head. The hydraulic power pack can be i. integrated in the machine base ii. separate free standing power pack iii. integrated in 19" control console

Machine Condition Monitoring Machine condition monitoring is the process of monitoring the condition of a machine with the intent to predict mechanical wear and failure. Vibration, noise, and temperature measurements are often used as key indicators of the state of the machine. Trends in the data provide health information about the machine and help detect machine faults early, which prevents unexpected failure and costly repair.

Machine condition monitoring is important because it provides information about the health of a machine. You can use this information to detect warning signs early and he lp your organization stop unscheduled outages, optimize machine performance, and reduce repair time and maintenance costs. Figure 1 shows a typical machine failure example and the warning signs. Types of Machine Condition Monitoring Each of the five main varieties of machine condition monitoring serves a different role. 

Route-Based Monitoring Route-based monitoring involves a technician recording dat a intermittently with a handheld instrument. This data is then used for trending to d etermine if more advanced analysis is needed.



Portable Machine Diagnostics Portable machine diagnostics is the process of using portable eq uipment to monitor the health of machinery. Sensors are typically permanently attached to a machine and portable data acquisition equipment is used to read the data.


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

Factory Assurance Test Factory assurance test is used to verify that a finished product meets its design specifications and to determine possible failure modes of the de vice.



Online Machine Monitoring Online machine monitoring is the process of monitoring equipment a s it runs. Data is acquired by an embedded device and transmitted to a main server for data analysis and maintenance scheduling.



Online Machine Protection Online machine protection is the process of actively mon itoring equipment as it runs. Data is acquired and analyzed by an embedded device. Limit settings can then be used to control turning on and off machinery

c. Orthogonality of Principle modes

For a system with three-degree of freedom the orthogonalit y principle may be written as m1A1A2 + m2B1B2 +m3C1C2=0 m1A2A3 + m2B2B3 +m3C2C3=0 m1A1A3 + m2B1B3 +m3C1C3=0 Where m1, m2, m3 are masses. A1, A2, A3, B1, B2, B3, C1, C2, C3 are the amplitude of vibration of the system. We will make use of the equation in matrix iteration method to find the natural frequencies and mode shapes of the system.

Machine vibrating monitoring Vibration measurement and analysis Br oad band vibr ation measur ement is the most wi dely used and cost-eff i cient method f or the diagnosis of general m achi ne condi ti on.

Machine condition is generally diagnosed on the basis of broad band vibration measurements returning an RMS value. ISO 10816 keeps the lower frequency range flexible between 2 and 10 Hz, depending on the machine type. The upper frequency is 1000 Hz. ISO 10816 operates with the term vibration magnitude, which, depending on the machine type, can be an RMS value of vibration velocity, acceleration or displacement. If two or more of these parameters are measured, vibration severity is the one returning the relative highest RMS value. For certain machines, ISO 10816 also recognises peak-to-peak values as condition criteria. The standard consists of several parts, each treating a certain t ype of machines, with tables of limit values differentiating between acceptable vibration (green range), unsatisfactory vibration (yellow range), and vibration that will cause damage unless reduced (red range).


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Modal analysis Modal analysis is the study of the dynamic properties of structures under vibrational excitation.

Modal analysis is the field of measuring and anal ysing the dynamic response of structures and or fluids during excitation. Examples would include measuring the vibration of a car's body when it is attached to an electromagnetic shaker, or the noise pattern in a room when excited by a loudspeaker. Modern day modal analysis systems are composed of 1)sensors such as transducers (typically accelerometers,load cells), or non contact via a Laser vibrometer , or stereophotogrammetric cameras 2) data acquisition system and an analog-to-digital converter frontend (to digitize analog instrumentation signals) and 3) host PC ( personal computer ) to view the data and analyze it. Classically this was done with a SIMO (single-input, multiple-output) approach, that is, one excitation point, and then the response is measured at many other points. In the past a hammer survey, using a fixed accelerometer and a roving hammer as excitation, gave a MISO (multipleinput, single-output) analysis, which is mathematically identical to SIMO, due to the principle of reciprocity. In recent years MIMO (multi-input, multiple-output) have be come more practical, where partial coherence analysis identifies which part of the response comes from which excitation source. Using multiple shakers leads to a u niform distribution of the energy over the entire structure and a better coherence in the measurement. A single shaker may not effectively excite all the modes of a structure. Typical excitation signals can be classed as impulse, broadband, swept sine, chirp, and possibly others. Each has its own advantages and disadvantages. The analysis of the signals typically relies on Fourier analysis. The resulting transfer function will show one or more resonances, whose characteristic mass, frequency and damping can be estimated from the measurements. The animated display of the mode shape is very useful to NVH (noise, vibration, and harshness) engineers. The results can also be used to correlate with finite element analysis normal mode solutions.


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VTU Question Paper Solutions Unit-1 Introduction

1. Define Automation? (Jan 15, June 13)

Ans: Automation can be defined as the technology that incorporates and integrates all the traditional fields of electrical/mechanical/electronics with the modern computer technology to operate and control the manufacturing process.

2. Describe the three basic types of automated manufacturing systems? (June/July 13)

Ans: The automation systems can be conveniently classified into three types viz.,

a. Fixed automation b. Programmable automation c. Flexible automation


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MECHANICAL VIBRATION a)

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Fixed automation: Fixed automation is a system in which the sequence of processing

(or assembly) operation is fixed by the equipment configuration. Each operation in the sequence is usually simple, involving per phase a plain linear or rotational spindle. It is the international and coordination of many such operations into one piece of equipment that makes the system complex. Ex- Mechanized assembly lines, Machining transfer lines. Features:- 1) High initial investment for custom engineered equipment, 2) High production rates and 3) Relative inflexibility of the equipment to accommodate product variety.

b)

Programmable automation: In programmable automation, the production equipment

is designed with the capacity to change the sequence of operations to accommodate different product configurations. The operation sequence is controlled by a program, which is a set of instruction coded so that they can be read and interpreted by the system. New programs can be prepared and entered into the equipment to produce new products. Ex- NC machine tools, Industrial Robots Features:- 1) High investment in general purpose equipment 2) Lower production rates than fixed automation 3) Flexibility to deal with variations and changes in product configuration and 4) High suitability for batch production.

5) Programmable automated production systems are used in low-and medium volume production. c)

Flexible Automation: Flexible automation is an extension of programmable

automation. A flexible automated system is capable of producing a variety of parts (or products) with virtually no time lost for changeovers from one part style to the next. There is no lost production time while reprogramming the system and altering the physical setup


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(tooling, Fixtures, machine setting) accordingly the system can produce various mixes and schedules of parts or products instead of requiring that they be made in parts processed by the system are not significant. Features:- 1) High investment for a custom- engineered system 2) Continuous production of variable mixtures of products 3) Medium production rates, and 4) Flexibility to deal with product design variations.

3) Explain ten strategies for automation and process improvement? (Jan 15, June 13)

Ans: Following ten strategies can be adopted for improvements for automation and process improvement. 1.

Specialization of operations: The first strategy involves the use of special-purpose

equipment designed to one operation with the greatest possible efficiency. This is analogous to the specialization of labor, which is employed to improve labor productivity. 2.

Combined operations: Production occurs as a sequence of operations. Complex parts

may require dozens or even hundreds of processing steps. The strategy of combined operations involves reducing the number of distinct productions machines or workstations through which the part must be routed. This is accomplished by per forming more than one operation at a given machine, thereby reducing the number of separate machines needed. Since each machine typically involves a setup, setup time, waiting time and manufacturing lead time are all reduce. 3.

Simultaneous operations: A logical extension of the combined operations strategy is

to simultaneously perform the operations that are combined at one workstation. In effect, two or more processing (or assembly) operations are being performed simultaneously on the same work part, thus reducing total processing time. 4.

Integration of operations: This strategy involves linking several workstations

together into a single integrated mechanism, using automated work handling devices to DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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transfer parts between stations. In effect, this reduces the number of separate work centers through which the product must be scheduled. With more than one workstation. several parts can be processed simultaneously, thereby increasing the overall output of the system. 5.

Increased flexibility: This strategy attempts to achieve maximum utilization of

equipment for job shop and medium-volume situation by using the same equipment for a variety of part or products. It involves the use of flexible automation concepts. Prime objectives are to reduce setup time and programming time for the production machine. 6.

Improved material handling and storage: A great opportunity for reducing

nonproduction time exists in the use of automated material handling and storage systems. Typical benefits include reduce work-in process and shorter manufacturing lead times. 7.

On-line inspection: Inspection for quality of work is traditionally performed after the

process is completed. This means that any poor quality product has already been produced b y the time it is inspected. Inspection into the manufacturing process permits corrections to the process as the product is being made. This reduces scrap and brings the overall quality of the product closer to the nominal specifications intended by the designer. 8.

Process control and optimization: This includes wide range of control schemes

intended to operate the individual processes and associated equipment more efficiently. By this strategy, the individual process times can be reduced and p roduct quality can be improved. 9.

Plant operations control: Whereas the previous was concerned with the control of the

individual manufacturing process. This strategy is concerned with control at the plant more efficiently. Its implementation usually involves a high level of computer networking within the factory. 10.

Computer-integrated manufacturing (CIM): Talking the previous strategy one level

higher. We have the integration of factory operations with engineering design and the business functions of the firm. CIM involves extensive use of computer applications, Computer data bases and computer networking through the enterprise.


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4) Briefly explain the role of manufacturing support systems with a sketch? (Jan 13)

Ans: A production system is a collection of people, equipment, and procedures organized to perform the manufacturing operations of a company (or other organization) Production systems can be divided into two categories or levels as indicated in Figure below.

Production Facilities:

The facilities of the production system consist of the factory, products machines and tooling the equipment in the factory, and the way the equipment is organized. Manufacturing support systems:

This is the set of procedures used by the company to manage production and to solve the technical and logistics problems encountered in the company. Ordering materials, moving the work through the factory, and ensuring that products meet quality standards. Product design and certain business functions are included among the Manufacturing support systems. In modern Manufacturing operations, portions of the production system are automated and/or computerized. However, production system include people, People make these system work. The facilities in the production system are the factory, production machines and tooling, material handling equipment, inspection equipment, and computer systems that control the Manufacturing operations. Facilities also include the plant layout. Which refers to the way the equipment is physically arranged in the factory. The equipment is usually organized into logical groupings, and we refer to these equipment arrangements and the workers who operate DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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them as the Manufacturing systems in the factory.

5) Discuss some of the reasons used to justify automation system. (Dec 14)

Ans: The correct incentive for applying automation is to increase productivity, and/or quality beyond that possible with current human labor levels so as to realize economies of scale, and/or realize predictable quality levels. Some of the reasons which justifies automation system are as follows,



To increase labor productivity: Automating a manufacturing operation usually

increases production rate labor productivity. This means greater output per hour of labor input. 

To reduce labor cost: Ever-increasing cost has been and continues to be the trend in

the world’s industrialized societies. Consequently, higher investment, machines are increasingly being substituted for human stimulated for human labor to reduce unit product cost. 

To mitigate the effects of labor shortages: There is a general shortage of labor in

many advanced nations and this has stimulated the development of automated operations as a substitute for labor. 

To reduce or eliminate routine manual and clerical tasks: An argument can be put

forth that there is social value in automating operations that are routine. Boring, fatiguing and possibly irksome. Automating such tasks improve the general level of working conditions 

To improve worker safety: Automation not only result production rates than Manual

operation and transferring the worker form active participation in the process to a monitoring role. Or removing the worker from the operation altogether, makes the work safer. 

To improve product quality: Automation not only result in higher production rates

than manual operation. It also performs the manufacturing process with greater DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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uniformity and conformity to quality specifications. 

To reduce manufacturing lead time: Automation helps reduce the elapsed time

between customer order and product delivery, providing a competitive advantage to the manufacturer for future order. By reducing manufacturing lead time. The manufacturer also reduce work-in-process inventory. 

To accomplish that cannot be done manually: Certain operations cannot be

accomplished without the aid of a machine. These processes require precision. Miniaturization or complexity of geometry that cannot be achieved manually. Examples include certain integrated circuit fabrication operations, rapid prototyping processes based on computer graphic (CAD) models, and the machining of complex, mathematically defined surfaces using computer numerical control. These processes can only be realized by computer controlled systems. 

To avoid the high cost of not automating: There is a significant competitive

advantage gained in automating a manufacturing plant. The adv antage cannot easily be demonstrated on a company’s project authorization from. The benefits of automation often show up in unexpected and intangible ways such as in improved quality higher sales, better labor relations, and better company image. Companies with their customers, their employees, and the general public. 6) With a neat sketch, explain various types of plant layouts. (Jan 15)

Ans: Plant layout is a physical arrangement of machinery and other facilities in a planned manner within the factory premises. For manufacturing units the plant layout is divided into four types. i.

Fixed Position Layout

ii.

Process Layout

iii.

Combined or Group Layout

iv.

Product Layout

Fixed Position Layout:


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If the product is large and heavy, and therefore difficult to move in the factory, it typically remains in a fixed location. Workers and processing equipments are brought to the product, rather than moving the product to equipment. This type of layout is referred to as a fixed position layout.

Process Layout:

The individual parts that comprise the large products are often made in factories that have a process layout, in which the equipment is arranged according to function or type. Different parts, each requiring a different operation sequence, are routed through the departments in particular order.


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Group Layout:

The term cellular manufacturing is often associated with production involving processing or assembly of different parts or products accomplished in cells consisting of several workstations or machines. Each cell is designed to produce a limited variety of part configuration, that is, the cell specialize in the production of a given set of similar parts or products, according to the principles of group technology. Product Layout:

In this the workstations consists of production machines and workers equipped with specialized tools. The collection of stations is designed specifically for the product to maximize efficiency. The work is usually moved between stations by powered conveyors.

1.

a) Define the following terms : i) Periodic motion

ii) Degree of freedom

iii) Resonance

iv) Phase difference.

JUNE/JULY 2016

Ans:

i)

Periodic motion, in physics, motion repeated in equal intervals of time. Periodic motion is performed, for example, by a rocking chair, a bouncing ball, a vibrating tuning fork, a swing in motion, the Earth in its orbit around the Sun, and a water wave. In each case the interval of time for a repetition, or cycle, of the m otion is called a period.

ii)

Mathematical modeling of a physical system requires the selection of a set of variables that describes the behavior of the system.


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The number of degrees of freedom for a system is the number of kinematically independent variables necessary to completely de scribe the motion of every particle in the system DOF=1Single degree of freedom (SDOF)

iii)

DOF=2 Multi degree of freedom (MDOF)

Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies or resonance frequencies. At resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations. This is because the system stores vibration stores vibration energy.

iv)

Phase difference is the difference, expressed ex pressed in degrees or time, between two waves having the same frequency and referenced to the same point in time. time.

[2]

Two oscillators

that have the same frequency frequenc y and no phase difference are a re said to be in phase. Two oscillators that have the same frequency and different phases have a phase difference, and the oscillators are said to be out of phase with each other.

1.b. Add the following motion analytically and check the solution graphically. 0

x1 = sin (8t+30 ),

0

x2= 2cos (8t-15 ).

JUNE/JULY 2016

Ans:


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c) Represent the periodic motion given by following Fig by harmonic series. JUNE/JULY 2016


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CHAPTER-02 UNDAMPED (SINGLE D.O.F) FREE VIBRATIONS 1) Explain different types of manufacturing operations? (June/July 13)

Ans: There are certain basic activities that must be carried out in a factory to convert raw materials into finished products. Factory activities are i.

Processing and Assembly Operations

ii.

Material handling

iii.

Inspection and test and

iv.

Co-ordination and control.

i. Processing and Assembly Operations:

Manufacturing processing can be divided into two basic types 1) processing operations and 2) assembly operations. A processing operation transforms a work material from one state of completion to a more advanced state that is closer to the final desired part or product. It adds value by changing the geometry, properties, or appearance of the starting material. In general, processing operations are performed on discrete work parts. A processing operation uses energy to alter a work parts shape, physical properties, or appearance to add value to the material. The forms of energy include mechanical, thermal, electrical, and chemical. The energy is applied in a controlled way by means of machinery and tooling. Human energy may also be required but human workers are generally employed to control the machines, to oversee the operations, and to load and unload parts before and after each cycle operation. A desirable objective in Manufacturing is to reduce waste in either of these forms.


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More than one processing operation is usually required to transform the starting material into final from. The operations are performed in the particular sequence to achieve the geometry and/or condition defined by the design specification.

ii. Material handling and storage

A means of moving and storing materials between the processing and assembly operations must be provided. In most manufacturing plants, materials spend more time being moved and stored than being processed. In some cases, the majority of the labor cost in the factory is consumed in handling, moving, and storing materials. It is important that this function be carried out as efficiently as possible.

iii. Inspection and testing:

Inspection and testing are generally considered part of quality control. The purpose of inspection is to determine whether the manufactured product meets the established design standards and specifications. For example, inspection examines whether the actual dimensions of a mechanical part are within the tolerances indicated on the engineering drawing for the part and testing is generally concerned with the functional specifications of the final product rather than the individual parts that go into the product.

iv. Control

The control function in manufacturing includes both the regulation of individual processing and assembly operations, and the management of plant-level activities. Control at the process level involves the achievement of certain performance objectives by proper manipulation of the inputs to the process. Control at the plant level includes effective use of labor, maintenance of the equipment, moving materials in the factory, shipping products of good quality on schedule, and keeping plant operating costs at the minimum level possible


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2) Briefly explain the following terms with mathematical model: (Jan 14, June 13)


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i.

Production rate

ii.

Production capacity

iii.

Utilization and availability iv. MLT

v.

WIP

Ans:

i. Production rate : Production rate for an individual processing or assembly operation is

usually expressed as an hourly rate, i.e, work units completed per hour (pc/hr). For a batch production, the batch time per machine is given by,

Tb = Tsu+ Q Tc

Where,


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Tsu- setup time to prepare for the batch (min) Q- batch quantity(pc) Tc- cycle time per work unit (min/cycle)

The average production time per unit for a given machine is

Tp = Tb/Q

Then the average production rate for a machine is given by

R p = 1/Tp = Q/Tb

ii. Production capacity : It can be defined as the maximum rate of output a plant can produce

with the available resources and operating conditions. It is given by,

PC = n Sw Hsh R p

where, n= number of work centers working Sw= number of shifts per period (shift/wk) Hsh= hours/shift

iii. Utilization and availability : Utilization gives a measure of how best the production

resources are being utilized, when they are effectively available.


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It can be defined as the amount of production relative to its available capacity.

U = Output/Capacity Availability is also used as a measure of the reliability of a machine. For defining availability, the reliability terms like Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR) are used. MTBF indicates the average length of time

between breakdowns of the machine, while MTTR indicates the average time required to service the machine, when a breakdown occurs and make it operational.

A = (MTBF-MTTR) / MTBF

iv. Manufacturing Lead Time: It can be defined as the total time taken to process a product

through the plant. MLT expresses the total time required to produce and deliver a product, starting from raw material stage to the customer. It is given by,

MLT = no (Tsu + QTc + Tno) Where,


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no= number of separate operations Tno= non operation time associated with the same machine or workstation

v. Work in Process: It is the quantity of parts or products currently located in the factory

that is either being processed or between processing operations. WIP is the inventory that is in the state of being transformed from raw material to finished product.

WIP = [A.U. (PC). (MLT)] / [Sw.Hsh]

3) Define production quantity and product variety. Express them mathematically. (Jan 13)

Ans: Production quantity refers to the number of units of a given part or product produced annually by the plant.

Product variety refers to the different product designs or types that are produced in a plant.

Let us identify each part or product style by using the subscript j, so that Q j = annual quantity of style j. then let Qf = total quantity of all parts or products made in the factory. Q j and Qf are related as �

�� ∑ �� =

�=1


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Unit-3 Industrial Control System 1) With a block diagram, explain the levels of automation. (Jan15, July 13)

Ans: The concept of automated system can be applied to various levels of factory operations.

(1) Device Level: This is the lowest level in automation hierarchy. It includes the actuators, sensors and other hardware components that comprise the machine level. (2) Machine Level: Hardware at the desired level is assembled into the individual machine. Control function at this level include, performing the sequen ce of steps in the program of instructions in the correct order of making sure that each step is properly executed. Ex: Industrial Robots, Power conveyors. (3) Cell or system Level: It operates under instructions from the plant level. It is a group DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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of machines connected and supported by a material handling system, computer and other equipment appropriate to manufacturing process.


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(4) Plant Level: This is a factory or production system level. It receives instructions from the corporate information system and translate them into operational plans for production. (5) Enterprise Level: This is the highest level, consisting of the corporate information system. It is concerned with all the functions necessary to manage the company. 2) Differentiate between Continuous Control and Discrete Control. (Dec 14, July 13) Ans:

1

Comparison of

Continuous Control in Pressure

Discrete Control in Discrete

Factors

Industry

Manufacturing Industry

Product Output

Weight measure, Volume of

Number of parts or products

Liquids etc., 2

Quality

Consistency, Concentration of solution, Conformance to specification

3

4

Dimension, Surface finish, Appearance, Product Reliability

Parameters

Temperature, Volume, Flow rate etc.,

Position, Velocity, Acceleration, Force

Sensors

Flow meters, Thermocouple, Pressure sensors

Limiting switches, Photo elastic sensors, strain gauges

Variables and

5

Actuators

Valves, Heat pumps

Switches, Motors

6

Time constant

Measured in terms of seconds, minutes, hours

It is less than a second or fraction of a second

3) Write a brief note on advanced automation functions. (July 14 Jan 13)


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Ans: Advance automation functions include Safety monitoring, Maintenance and Error detection & recovery. Advance automation functions are made possible by special subroutine included in the program of instructions. Safety Monitoring: Automating a manufacturing operation is to remove worker from a

hazardous working environment. Automated system maintain safety of workers in the vicinity of the system and protect the equipment with the system. It uses sensors to track the system operation and identify conditions and events that are unsafe. The types of responses include the following,

•

Completely stopping the automated system

•

Sending an alarm

•

Reducing the operation speed of the process

•

Taking corrective actions to recover from the safety violations

Examples: Limiting switches, Photo elastic sensors, Temperature sensors, Heat and smoke detection, Pressure detection system etc.,

Maintenance and Repair diagnostics: Modern automated production systems are complex

and sophisticated complicating the problem of maintenance and repairing them. Three modes of operation are typical of a modern maintenance and repair diagnostics system,

•

Status monitoring

•

Failure diagnostics

•

Recommendation of repair procedure

Error detection: The error detection step uses the automation system to determine when a

deviation or malfunction has occurred. The sensors available will interrupt and send the error signal. Classifications of errors are as follows,


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MECHANICAL VIBRATION •

Random errors

•

Systematic errors

•

Aberrations


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Random errors occur when the process is in statistical control. Ex: Large variation in part dimensions when the process is in statistical control.

Systematic errors are due to assignable causes such as raw material properties or drift in an equipment setting.

Aberration is the error due to human mistake or equipment failure. Ex: Errors in control program, Improper fixtures, Shear pin failure, Rupture of pressure vessels etc., Error recovery: It is concerned with necessary corrective action to overcome the error and

bring the system back to normal mode or working condition. The type of strategies for error recovery,

•

Make adjustment at the end of current work cycle

•

Make adjustments during the current cycle

•

Stop the process and take corrective actions

Error recovery requires an interrupt system, measures have to be taken to correct the program or appropriate sub routine to recover the error occurred.

4) Briefly explain the open and closed loop control system. (June 13)

Ans: Control system executes the program of instructions. It causes the process to accomplish its defined function to carry out manufacturing operations. Control systems can be either closed loop or open loop control system. DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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A closed loop control system is also known as feed back control system in which output variables is compared with an input parameter and any different between the two is used to drive the output into agreement with the input. It consists of six basic elements,

i.

Input iv.

Feed back sensor

ii.

Process

v.

Controller iii. Output variable

vi.

Actuator

•

• •

The input parameter represents the desired value of the output. Process is the operation or function being controlled. Output variable is some process variable, perhaps a critical performance measure in the process.

•

Sensor is used to measure the output variable and close the loop between input and output.

•

Controller compares the output with the input and makes the required adjustment in the process to reduce the difference between them.

•

Actuators are the hardware devices that physically carry out the control action.


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In open loop control system there is no comparison between actual value of the output and the desired input parameter.

The controller relies on an accurate model of the effect of its actuator on the process variable. With an open loop system, there is always the risk the=at the actuator will not have the intended effect on the process, and that is the disadvantage of an open loop system.

5) Explain Direct Digital Control (DDC) system, with a block diagram. (Dec 14)

Ans: Direct Digital Control is a computer process control system in which certain components in a conventional analog control system are replaced by the digital computer. The regulation of the process is accomplished by digital computer on a time shared, sampleddata basis rather than by many individual analog components working in a dedicated continuous manner. With Direct Digital Control the computer calculates the desired values of the input parameters and set points and these values are applied through a direct link to the process, hence the name Direct Digital Control.

DDC was originally conceived as a more efficient means of performing the same kinds of control actions as the analog components it replaced.

In Direct Digital Control, some of the control loop components remain unchanged, including the sensors and transducer as well as amplifier and actuators. Components likely to be replaced in Direct Digital Control include the analog controller, recording & display instruments and comparator. DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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CHAPTER-03 DAMPED FREE VIBRATIONS (1 DOF) 1) What are the different components of a manufacturing system? Explain in brief. (Dec 14, June 13)

Ans: A manufacturing system is a collection of integrated equipment and human resources, whose function is to perform one or more processing and/or assembly operations on a starting raw material, part, or set of parts. A manufacturing system consists of the following components: i.

Production machines (plus tools, fixtures, and other related hardware)

ii.

A material handling system

iii.

A computer system to co-ordinate and/or control the preceding components and

iv.

Human workers to operate and manage the system.

i.

Production machines:

Most manufacturing in modern-day manufacturing systems is done by machines of one form or another. Machines can be classified according to worker participation in the task, as: manuallyoperated; semi-automated; or fully automated.

•

Manually operated machines

Manually operated machines are controlled or supervised by a worker or operator, there is a clear division of labor, whereby the machine provides the power for the operation and the worker DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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provides the control. Conventional machine tools (such as lathes, milling machines, drill presses etc.) fit this category. The worker must attend the machine continuously during the work cycle.

\

•

Semi-automated machines

A semi-automated machine performs a portion of the work cycle under programme control, and then a worker assumes control for the remainder of the cycle. An example of a machine in this capacity is a CNC lathe, where the CNC machine performs its processing operation as per the programme, and then the worker unloads and reloads the machine for the next work cycle. The worker must attend the machine every cycle, but need not be continuously present.


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Fully automated machines

This has the capability to operate with no human attention for periods of time that are longer than one work cycle. Some form of machine tending will be required periodically. For example, to replenish the machines with raw materials etc.

ii.

A material handling system:

For most processing and assembly operations the following material handling actions can be distinguished: loading work units at each station; positioning work units at the station; unloading work units from the station after processing; transporting work units between stations; and performing temporary storage, if necessary, also. Some of these actions are linked so that the same machinery may be used to perform the actions (for example, many load and unload actions); whereas other actions are specialized and require their own equipment. Loading, positioning, and unloading work units are a group of actions that are regularly performed together at individual workstations. These groups of actions may be manual, semiautomated or fully-automated. In loading the workstation is supplied with the correct type and amount of work units so that the processing operation can be performed; positioning requires the work unit to be oriented or located correctly within the processing machine, so that the processing action can be performed upon the work unit accurately; while unloading involves the removal of the processed work units from the workstation, often for further material handling DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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processes to occur. There are, in general, two types of work transport: fixed routing, and variable routing Fixed routing uses the same sequence of workstations to process identical work units as they passed through the system; whereas, with variable routing, work units are transported through a variety of different station sequences to allow for variable processing to be performed on transported work units. Both work types emphasize different types of

automated material handling equipment. Fixed routing typically deploys conveyors of powered roller, belt, drag chain, or overhead trolley type, and can use rotary index mechanisms, and walking beam transfer equipment. For variable routing an automated guided vehicle system is favored, together with power-and-free overhead conveyors, or cart-on-track conveyors, or monorail systems.

Figure: Fixed (a) and Variable (b) routing of work units through a work sequence

Pallets may also be used in the material transport system, if it is designed in such a way that it can accommodate a pallet fixture- a specialized work-holder explicitly designed for positioning DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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and clamping pallets to the material transport system. Direct transport may also be used. This involves designing the transport system to move the work unit itself. This has obvious advantages over work carriers and pallet fixtures, which have to be specially- designed and are expensive, and it is an option that is available both for manual and automated material handling systems. iii. A computer system:

Computer systems are an integral part of automated manufacturing, as they are required to control fully-automated and semi-automated equipment and participate in overall co-ordination and management of the manufacturing system. •

Typical computer functions in a manufacturing system: Communicate instructions to workers (receive processing or assembly –

instructions for the specific work unit) –

Download part programs to computer-controlled machines

–

Control material handling system

–

Schedule production

–

Failure diagnosis when malfunctions occur and preventive maintenance

–

Safety monitoring (protect both the human worker and equipment)

–

Quality control (detect and reject defective work units produced by the system)

–

Operations management (manage overall operations)

iv. Human Resources:

Humans also have a role to play, even if it is only in a supervisory capacity. In cases where humans perform some value-added work on work units, the work done is called direct labor — that is, physical labor that results in an increase in value o f the processed work unit. This generally includes direct work done on work units or work done to control the machines that are processing the workpart. Human workers are also required to: manage and support the system as computer programmers; operate and direct computer activities; maintain and repair the automated manufacturing system, as required; and the performance of other, similar, indirect DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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labor roles. 2) Discuss the different categories of manufacturing system. (Jan 13, July 14)

Ans: Various types of manufacturing systems can be created from a consideration of different entities that have an impact upon manufacturing system design. These considerations include:

i.

Types of operations performed

ii.

Number of workstations

iii.

System layout

iv. v.

Automation and manning level and Product or part variety

i.

Types of operations performed:

At the highest level, operational types include processing operations on individual work units, and assembly operations to combine individual work parts into subassemblies, or full assemblies. Additional parameters include: Size or weight of the part or product — this has a significant influence on the type, scope, and scale of manufacturing equipment chosen to process the item Part or product complexity — part complexity correlates with the number of processing operations required, while product complexity refers to the number of components that must be assembled Part geometry — machined parts are rotational or non-rotational, which has a significant effect on the processing machine operations that can be performed on the parts, plus the material handling system must be designed in an appropriate fashion for rotational and non-rotational parts. DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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Number of workstations:

The number of workstations in a manufacturing system exerts a strong influence on the performance of the manufacturing system, in terms of its workload capacity, production rate, and reliability. The number of workstations is a good measure of the size of the manufacturing system; the more workstations it has, the bigger it is generally found to be. As the number of workstations increases, the more work can be performed by the system, which may translate into a higher production rate than if a number of single workstation systems were deployed concurrently. The use of

multiple

workstations can also produce a synergistic benefit, when compared against single workstation systems, as the total amount of work performed on the part or product is too complex to accomplish at a single workstation; instead the task is divided among a multiple of stations, thus simplifying the complexity of the task into simpler work elements.

iii.

System layout:

System configuration, or the layout of the manufacturing system’s workstations, is also an important factor. This applies mainly, of course, to systems with multiple workstations. Workstation layouts for fixed routing are usually arranged linearly, as in a production line, while variable routing layouts can have multiple configurations. System layout is an important factor for the design of the material handling system. If we consider the number of workstations (where number of workstations may be depicted by n) against system layout, we can determine the workload of the system, which is defined as the amount of processing or assembly work accomplished by the system expressed in terms of the time required to perform the work. It is the sum of the cycle times of all the work units completed by the system in a given period of interest. The workload capacity of a manufacturing system increases in proportion to the number of workstations in it. iv.

Automation and manning level:

The level of automation deployed is an important characteristic of the manufacturing system. Workstation machines may be manually-operated, emiautomated, or fully-automated. This factor allows us to define the amount of time that a human operator is required to be in attendance at a workstation as the manning level DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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( M i) of the workstation (where i denotes a particular workstation in the system). The average manning level of a multi-station manufacturing system is given by:

where M is the average manning level for the system; wu is the number of utility workers assigned to the system; wi is the number of workers assigned specifically to station i, for i = 1, 2, 3, …, n; and w is the total number of workers assigned to the system. By including the manning level scenario into the classification, we can see that there are two levels for single station systems (manned and fully automated), and three levels for multi-station systems (manned, fully automated, and hybrid — that is where some stations are manned and others are fully automated).

v.

Product or part variety:

This factor examines the manufacturing system’s flexibility for dealing with variations in the parts or products it produces. Part or product variations that could

occur in

manufacturing systems include: variations in type or colour of plastic or moulded parts; variations in electronic components placed on circuit boards; variations in the size of printed circuit boards handled; variations in part geometry; and variations in parts and options in an assembled product. The cases of part or product variety in manufacturing systems are single model, batch model, or mixed model — the details of which are outlined in Figure


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Cases of part or product variety in manufacturing systems: (a) single model, (b) batch model and (c) mixed model

3) Differentiate clearly between fixed routing and variable routing with a sketch. (Jan 15)

Ans: Fixed routing uses the same sequence of workstations to process identical work units as they passed through the system; whereas, with variable routing, work units are transported through a variety of different station sequences to allow for variable processing to be performed on transported work units. Both work types emphasize different types of automated material handling equipment. Fixed routing typically deploys conveyors of powered roller, belt, drag chain, or overhead trolley type, and can use rotary index mechanisms, and walking beam transfer equipment. For variable routing an automated guided vehicle system is favored, together with power-and-free overhead conveyors, or carton-track conveyors, or monorail systems. 4) Explain briefly, single station manned work stations and single station automated cells. (Jan 13, June 13) Ans:

Single station manned work stations


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The most common type of single-station manufacturing cell is the single-station manned cell, consisting of one worker tending one machine; it is found throughout job shop and batch production environments, and is found even in high production scenarios. In the single-station manufacturing cell the machine is usually manually or semi-operated. In the manually operated station the operator controls the machine, loads and unloads the work, and monitors the work cycle either continuously or for most of the cycle time. It may also require the operator to use a variety of work tools, such as screwdrivers, wrenches, or portable powered tools etc., to perform additional processes in the cell. All work tasks are performed at one station (one location) in the factory. In the semi-automated station the machine is controlled by a part program, leaving the operator free to perform additional tasks, such as loading and unloading the machine, performing tool maintenance, and controlling changeovers. Typicallyoperators’ attention would be required at the end of every work cycle, and not necessarily on a continual basis. Single station automated cells

The single-station automated cell consists of a fully automated machine that can operate unattended for a time period longer than one machine cycle. The operator must load and unload the machine, and otherwise tend it, but is not required to be at the machine except periodically. Advantages of single-station automated cells include: •

Labor cost is reduced compared to the single-station manned cell

•

It is the easiest and least expensive system to implement

•

Production rates are generally higher than for a comparable man ned machine

5) Briefly describe multi station system with respect to manufacturing. (Jan 14) Multi-station Systems with Fixed Routing

Ans:

A multi-station system with fixed routing is essentially a production line, which consists of a series of workstations laid-out so that the part/product moves from one station to the next, while a value-adding work element is performed at each workstation along the way. Material


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transference between workstations is usually accomplished by means of a conveyor system, or other mechanical transport system, although there are cases where material may be transferred by hand. Conditions that favor the use of production lines include: •

Part/product quantity is very high (up to millions of units)

•

Work units are identical, or very similar

•

Total work content can easily be divided into work tasks that may be performed at separate workstations

Figure: Multi-station System with Fixed Routing

Multi-station Systems with Variable Routing

A multiple-station system with variable routing is a group of workstations organized to achieve some special purpose. It usually handles medium-sized production quantities, although it has been used for production quantities beyond this, in certain situations, such as: •

Assembly of a family of parts having similar assembly requirements

•

Production of a complete set of components that are used in the assembly of one unit of final product

Machines may be manual, semi-automated, or fully-automated. The first two types i.e. manually-operated and semi-automated may be arranged into machine groups called DEPARTMENT OF MECHANICAL ENGINEERING, SJBIT

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machine cells; and it is the emergence of numbers of these cells that gives rise to cellular manufacturing. Fully-automated machines, on the other hand, may be arranged into flexible manufacturing systems. See Figure

Figure: Multi-station System with Variable Routing


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CHAPTER-05 VIBRATION MEASURING INSTRUMENTS 1) Define Group Technology. What are its objectives? Explain the appropriate conditions at which GT is applied. (Jan 13)

Ans: Group Technology (GT) is a manufacturing philosophy in which similar parts are identified and grouped together to make advantage of their similarities in design and production.

Similar parts bare arranged into part families, where each part family possesses similar design and/or manufacturing characteristics. Objectives: •

Standardization of tooling, fixtures and setups is encouraged

•

Material handling is reduced

•

Process planning and production scheduling are simplified

•

Work-in-process and Manufacturing Lead Time are reduced

•

Improved worker satisfaction in a GT cell

•

Higher quality of work

Conditions:

1. The plant currently uses traditional batch production and a process type layout

◦

This results in much material handling effort, high in-process inventory, and long manufacturing lead times

2. The parts can be grouped into part families

◦

A necessary condition to apply group technology


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◦

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Each machine cell is designed to produce a given part family, or a limited collection of part families, so it must be possible to group parts made in the plant into families

2) Define Cellular Manufacturing. What are its objectives? (Jan 13, July 13)

Ans: Grouping the production equipment into machine cells, where each cell specializes in the production of a part family, is called cellular manufacturing. Objectives:

•

To shorten Manufacturing Lead Times by reducing setup, work part handling and

batch sizes. •

To reduce Work-in-process inventory by adopting smaller batch sizes and shorter

lead times. •

To improve quality by allowing each cell to specialize in producing a smaller number

of different parts. This reduces process variability. •

To simplify production scheduling: similarity among parts in the family reduces the

complexity of production scheduling. •

To reduce set up times: this is accomplished by using group tooling.

3) Explain the three categories of parts classification and coding system. (Dec 10)

Ans: A part coding system consists of a sequence of symbols that identify the part’s design and/or manufacturing attributes. The symbols are usually alphanumeric, although most systems use only numbers. The three basic coding structures are:

i.

Chain-type structure or polycode

ii.

Hierarchical structure or monocode


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Hybrid structure

i. Chain-type structure or polycode

Chain-type structure, also known as a polycode, in which the interpretation of each symbol in the sequence is always the same, it does not depend on the value of the preceding symbols.

ii. Hierarchical structure or monocode

Hierarchical structure, also known as a monocode, is one in which the interpretation of each successive symbol depends on the value of the preceding symbols.


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iii. Hybrid structure

Hybrid structure is a combination of hierarchical and chain-type structures. 4) What do you mean by PFA? Explain the basic steps involved in PFA. (Dec 14, June 13)

Ans: Production flow analysis (PFA) is a method for identifying part families and associated machine groupings that uses the information contained on process plans rather than on part drawings.

Work

parts identical similar

with or process

plans are classified into part families. These families can then be used to form logical machine cells in a group technology layout.

The procedure in production flow analysis must begin b y defining the scope of the study, which means deciding on the population of parts to be analyzed.


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The procedure of Production flow analysis (PFA) consists of the following steps:

a) Data Collection: The minimum data needed in the analysis are the part number and operation sequence, which is obtained from process plans. b) Sortation of process plans: A sortation procedure is used to group parts with identical process plans. c) PFA Chart: The processes used for each group are then displayed in a PFA chart as shown below.


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d) Clustering Analysis: From the pattern of data in the PFA chart, related groupings are identified and rearranged into a new pattern that brings together groups with similar machine sequences.


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5) Describe the benefits of FMS. (June 13) Ans:

FMSs achieve a higher average utilization than stand-alone machines in a conventional machine shop. Reasons include: (1) 24 hour per day operation, (2) automatic tool changing at machine tools, (3) automatic pallet changing at workstations, (4) queues of parts at stations, and (5) dynamic scheduling of production that takes into account irregularities from normal operations. It should be possible to approach 80% to 90% asset utilization. I.

Increased machine utilization:

II. Fewer machines required: Because of higher machine utilization. III. Reduction in factory floor space required: Compared to a job shop of equivalent capacity, a FMS generally requires less floor area. Reductions in floor space requirements = 40% to 50%. IV. Greater responsiveness to change: A FMS improves response capability to part design changes, introduction of new parts, changes in production schedule and product mix, machine breakdowns, and tool failures. Adjustments can be made in the production schedule from one day to the next to respond to rush orders and special customer requests. V. Reduced inventory requirements: Because different parts are processed together rather than separately in batches, WIP is less than in batch production. Inventories of starting and finished parts reduced also. Reductions = 60% to 80%. VI. Lower manufacturing lead times: Closely correlated with lower WIP is MLT. This means faster customer deliveries. VII. Reduced direct labor requirements and higher labor productivity: Savings = 30% to 50% VIII.

Opportunity for unattended production .

6) Define FMS. Briefly explain the types of flexibility in manufacturing. (Jan 13)


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Ans: A Flexible Manufacturing System (FMS) is a set of numerically controlled machine tools and supporting workstations connected by an automated material handling system and controlled by a central computer.

There are various approaches to the term flexibility of manufacturing systems. The most frequent meaning of this term is described as follows: •

Possibility of production program change without any significant alteration of machinery

(new NC program, eventual tool change),

•

Speed of production program change from previous product line to new products,

•

Possibility to change production program at level o f individual products.

Manufacturing Flexibility can be generally classified into following types, •

Basic – Machine- Used for variety of operations – Material handling- part mobility and placemen – Operation- variety of operations producing same part features

•

System – Process- variety of parts can be produced with same setup – Routing - ability to use different machines under same setup – Product- changeover of products can be done – Volume- production level can be increased – Expansion - added capacity

•

Aggregated – Program- unattended running of machine is an advantage – Production - ranges of parts, products, processes, volume, expansion – Market- combination of product, process, volume and expansion satisfies


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market demand 7) What are the various types of FMS layouts? (Dec 14)

Ans: The material handling function in a FMS is often shared between two systems:

1. Primary handling system - establishes the basic layout of the FMS and is responsible for moving work parts between stations in the system. 2. Secondary handling system - consists of transfer devices, automatic pallet changers, and similar mechanisms located at the workstations in the FMS. Types of FMS Layouts are as follows, 1. In-line layout 2. Loop layout 3. Ladder layout 4. Open field layout 5. Robot-centered cell

1.

In-line layout: This is most convenient and suitable configuration. In this c onfiguration,

the processing machines/operational sequences are arranged linearly in a straight line fashion. A typical in line configuration is shown in fig. In-line FMS layout: (a) basic in-line configuration with one directional work flow; (b) inline with transfer at workstations to allow back flow on primary handling system.


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2.

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Loop Layout: This configuration has workstations arranged in a loop that si served by

a parts handling system in the same shape, as shown in fig. parts usually flow in one direction around the loop with the capability to stop and be transferred to any station. An alternative form of loop layout is the rectangular layout; this might be used to return pallets to the starting position in a straight line machine arrangement.


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(a) FMS loop layout with secondary part handling system at each station to allow unobstructed flow on loop and (b) rectangular layout for recirculation of pallets to the first workstation in the sequence.

3. Ladder Layout:

The ladder layout consists of a loop with rungs between the straight sections of the loop as shown in fig. the rungs increases the number of possible ways of getting from one machine to the next and reduces average travel distance and minimizes congestion in the handling system, thereby reducing transport time between stations.


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4. Open field Layout:

It consists of multiple loops and ladders. This type is generally appropriate for processing a large family of parts. The number of different machine types may be limited and parts are routed to different workstations depending on which one becomes available first.

5. Robot centered layout: This uses one or more robots as the material handling system. Industrial robots can be equipped with grippers that make them well suited for the handling of rotational parts, and they are often used to process cylindrical or disk shaped part


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SYSTEM WITH TWO DEGREE’S OF FREEDOM traditional and modern quality control methods. (Jan 14, June 13) 1) Explain the traditional

Ans:

Traditional Quality Control

Traditional QC focused on inspection. In many factories, the only department responsible responsible for QC was the inspection department. Much attention was given to sampling and statistical methods, which were termed statistical quality control. Widespread use of statistical quality control (SQC), in which which inferences are made about the quality of the population of manufactured parts parts and products based on a sample taken from from population.

Two principal sampling methods in SQC are,

i.

Control charts

ii.

Acceptance sampling

Control chart is a graphical technique used to track measured variable of interest over time. The chart has a centre line that indicates indicates the value of the process mean under normal operation. Abnormal process behavior is identified identified when the process parameter parameter strays significantly from the process process mean.

Acceptance sampling is a technique in which a sample drawn from a batch of parts is inspected, and decision is made whether to accept or reject the batch on the basis of quality of the sample.


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Typical management principles and practices:

•

Customers are external to the organization

The sales and marketing department are responsible for customers

•

Company is organized by functional departments Inspection department is responsible for quality q uality

•

Inspection follows production

•

Knowledge of SQC techniques resides only in the minds of the QC experts in the

•

organization Modern Quality Control

High quality is achieved by a combination of Good management and Good technology. The management factor is captured in the frequently used term Total Quality Management. The technology factor includes traditional statistical tools combined with modern measurement and inspection technologies.

Three objectives of ―total quality ―total quality management‖:

1. Achieving customer satisfaction satisfaction

2. Continuous improvement

3. Encouraging involvement of entire work force


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Typical management principles and practices: p ractices:

•

Quality is focused on customer satisfaction satisfaction

•

Quality goals are driven by b y top management

•

Quality control is pervasive in the organization

•

Quality must be built into the product, not inspected in afterward

–

•

Production workers must inspect their own work

Continuous improvement

–

A never ending chase to design and produce better products

CHAPTER-07 NUMERICAL METHOD FOR MULTI-DEGREE OF FREEDOM SYSTEM

1) List the characteristics of traditional approach to quality management. (Dec 14)

Ans: The management principles principles and practices that that characterized traditional traditional quality control include the following: •

Customers are external to the organization

–

•

The sales and marketing department are responsible for customers

Company is organized by functional departments


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•

Inspection department is responsible for quality

•

Inspection follows production

•

Knowledge of SQC techniques resides only in the minds of the QC experts in the organization

2) With a block diagram, explain Taguchi’s off-line and on-line quality control system. (Jan 15,Dec 14, Jan 13, )

Ans: Off line quality control: This function is concerned with design issues, both product and process design. It is applicable to production and shipment of the product. Quality and cost control activities conducted at the product and process design stages to improve product manufacturability and reliability, and to reduce product development and lifetime costs.

Taguchi realized that the best opportunity to eliminate variation is during the design of a product and its manufacturing process. Consequently, he developed a strategy for quality engineering that can be used in both contexts. The process has three stages:

•

System design

•

Parameter (measure) design

Tolerance design System design: this may involve the development of a prototype design and will determine the •

materials, parts, and assembly system to be used. The manufacturing process has also to be considered.

Parameter design: At the product design stage the goal of parameter design is to identify

settings of product design characteristics that make the product's performance less sensitive to the effects of environmental variables, deterioration, and manufacturing variations.


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Tolerance design: With a successfully completed parameter design, and an understanding of

the effect that the various parameters have on performance, resources can be focused on reducing

and

controlling

variation

in

the


critical

few

dimensions.

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On line quality control: This is concerned with production operations and customer relations

after shipment. Its objective is to manufacture products within the specifications defined in product design, utilizing the technologies and methods developed in design.

On line quality control includes

•

Process diagnostics and adjustment:

•

Process Prediction and Correction

•

Process Measurement and action

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CHAPTER-08 MODAL ANALYSIS AND CONDITION MONITIRING

1) Explain the following Taguchi methods: (June 13)

i. Robust design ii. Taguchi loss function Ans:

i.

Robust design:

Robust design is defined as a design in which the function and performance of the product or process are relatively insensitive to variations. Taguchi calls the variations as noise factors. Noise factor is a source of variation that are impossible or difficult to control and that affect the functional characteristics of the product.

Types of noise factors:

•

Unit-to-unit noise factors – random variations caused by variability in materials, machinery, or human participation

•

Internal noise factors – (1) time-dependent factors such as wear, spoilage, (2) operational errors such as improper machine settings

•

External noise factors – outside temperature, humidity, input voltage

In product design , robustness means that the product performs consistently despite disturbances and variations in operating environment. Examples of Robust Product Design DEPT OF MECHANICAL ENGINEERING., SJBIT

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•

An airplane that flies in stormy weather as well as in clear weather

•

A tennis racket that returns a ball just as well when hit near the rim as when hit dead center

•

A hospital operating room that maintains lighting and life support systems when electric power to hospital is interrupted

In process design , robustness means that the process continues to produce good product despite disturbances and variations in operating environment.

Examples of Robust Process Design

•

A machining operation that produces good surface finish throughout a wide range of cutting speeds

•

A plastic molding operation that molds good parts despite variations in ambient temperature and humidity in the plant

•

A metal forging operation that presses good parts despite variations in starting temperature of the raw billet

ii.

Taguchi loss function:

According to Taguchi, quality is ―the loss a product costs society from the time the product is released for shipment‖ Loss includes:

•

Costs to operate

DEPT OF MECHANICAL ENGINEERING., SJBIT

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