UNIT - I 1. What is meant by Dynamics of Machinery? MECH/III YEAR/Dy… · · 2017-09-21What is...

MET51 DYNAMICS OF MACHINERY

UNIT - I 1. What is meant by Dynamics of Machinery?

Dynamics of Machinery is the branch of mechanics of machines which deals with the forces

and their effects while acting upon the parts of machinery which are in motion. i.e., system changes

with time 2. Define inertia force?

The inertia force is an imaginary force, which when acts upon a rigid body, brings it in an

equilibrium position. Inertia force = - Acceleration force F= - m.a 3. State D’Alembert’s principle?

D‟Alembert‟s principle states that the resultant force acting on a body together with

the reversed effective force (or inertia force), are in equilibrium.

F + FI = 0 4. What are the principles used in static fore analysis?

1. Principle of Superposition 2. Principal of Virtual work

5. State the principle of superposition?

The principle of superposition states that for linear systems the individual responses to

several disturbances or driving functions can be superposed on each other to obtain the total response

of the system. 6. What is the principle of virtual work?

Its state that if a system is in equilibrium and undergoes an infinitesimal displacement from a

equilibrium position without any lapse of time, the network done is equal to zero 7. Define piston effort?

Piston effort is defined as the net or effective force applied on the piston, along the line

of stroke. It is also known as effective driving force (or) net load on the gudgeon pin. 8. Define crank effort and crank pin effort?

Crank effort gives the required net effort (force) applied at the crank pin perpendicular to

the crank, which gives the required turning moment on the crankshaft. The component of force acting along the connecting rod (FQ) perpendicular to the crank

is known as crank pin effort. 9. Define Inertia Torque.

It is an imaginary torque, which when acts upon a rigid body, brings it in an equilibrium

position. It is equal to the accelerating couple in magnitude but opposite in direction. 10. What do you mean by correction couple or error in torque?

The error in torque (Tc) is given by Tc= m l1(l-L)∞ This couple must be applied, when

the masses are placed arbitrarily to make the system dynamically equivalent A

AMET51 DYNAMICS OF MACHINERY

11. What is meant by turning moment diagram or crank effort diagram?

It is the graphical representation of the turning moment or crank effort for various position

of the crank. In turning moment diagram, the turning moment is taken as the ordinate (Y-axis) and

crank angle as abscissa (X-axis) 12. Define coefficient of fluctuation of energy.

It is the ratio of maximum fluctuation of energy to the work done per cycle

CE= Maximum fluctuation of energy (E) / Workdone per cycle 13. What is meant by fluctuation of speed?

The difference between the maximum and minimum speeds during a cycle is called

maximum fluctuation of speed. 14. Define coefficient of fluctuation of speed?

The ratio of the maximum fluctuation of speed to the mean speed is called the coefficient of

fluctuation of speed. (Cs)

Cs= N1- N2

= 2 (N1- N2)/ N1+ N2

Where N1 = Maximum speed N2 = Minimum speed-, and A

N = Mean speed = (N1+ N2)/2

15. Define coefficient of steadiness?

The reciprocal of the coefficient of fluctuation of speed is -known as coefficient of steadiness

M = 1/ Cs = N / (N1-N2)

16. List our few machines in which fly wheel is used?

Fly wheel is used in: a) punching machines,

b) Shearing machines, c) Riveting machines, and d) Crushing machines.

17. Why flywheels are needed in forging and pressing operations?

In both forging and pressing operations, flywheels are required to control the variations speed during each cycle of an engine.

18. Differentiate static force analysis and dynamic force analysis.

If components of a machine accelerate, inertia is produced due their masses. However the

magnitudes forces are small compared lo externally applied loads. Hence the inertia effects due to

masses are neglected. Such an analysis is known as static force analysis. If the inertia effect due to

masses of the component is also considered it is called as dynamic force analysis.

A

Dr. SJS PAUL MCET MET51 DYNAMICS OF MACHINERY

19. State the difference between Flywheel and Governor.

Sl.

Governor Flywheel No.

1 The function of governor is to regulate the The function of a flywheel is to reduce the mean speed of an engine when there are fluctuations of speed caused by the engine turning

variations in the load. moment during each cycle of operation. 2 It is provided or prime movers such as It is provided on engine and fabricating machines

engines and turbines. via, rolling mills, punching machines, shear

machines, presses, etc.

3 It works intermittently, i.e. only when It works continuously from cycle to cycle.

there is change in load.

4 It has no influence over cyclic speed It has no influence on mean speed of the prime

fluctuation. mover.

20. Define turning moment diagram.

It is the graphical representation of the turning moment or crank effort for various position of

the crank. The turning moment is taken in Y-Axis and crank angle in X-Axis for plotting the turning

moment diagram.

21. Write the functions of Flywheel/ List down the functions of Flywheel. The function of flywheel is used in machines as a reservoir.

It stores the energy when the supply is more and supplies energy when the supply is

lower. It controls the speed variations caused by the fluctuation of the engine power.

A


UNIT II & III

1. What are the causes and effects of vibrations?

The causes of vibration are unbalanced forces, elastic name of the system, self excitations,

winds and earthquakes. The existence of vibrating elements in any mechanical system produces unwanted noise, high

stresses, poor reliability and premature failure of one or more of the parts. 2. What are the different types of vibrations?

Free vibrations, forced vibrations and damped vibrations.

3. State different methods of finding natural frequency of a system.

1. Equilibrium (or 'Newton‟s) method. 2. Energy method and - i 3. Rayleigh-method.

4. What is meant by free and forced vibrations? Or Define free and forced vibrations.

Free vibrations: when no external force acts on the body, after giving it an initial

displacement, then the body is said to be in free vibration. Forced vibration: When the body vibrates under the influence of external force, then the body

is said to be in forced vibrations. 5. What do you mean by damping-and damped vibrations?

Damping: The resistance against vibration is called damping. Damped vibration: When there is. a reduction in amplitude over every cycle of vibration, then

the motion is said to be damped vibration. 6. What are the various types of damping?

1. Viscous damping 2. Coulomb or dry friction damping 3. Solid or structural damping and 4. Slip or interfacial damping.

7. Define resonance.

When the frequency of external force is equal to the natural frequency of a vibrating body,

the amplitude of vibration becomes excessively large. This phenomenon is called resonance. 8. What is meant by degrees of freedom in a vibration system?

The number of independent coordinates required to completely define the motion of a system

is known as degree of freedom of the system. 9. Define steady state and transient vibrations.

In ideal systems, the free vibration continues indefinitely as there is no damping. Such

vibration is termed as steady state vibrations.

A


In real systems, the amplitude of vibration decays continuously because of natural damping and

vanishes finally. Such vibration real system is called transient vibration.

10. What is the of Rayleigh’s method of finding natural frequency of vibrations?

The principle of Rayleigh‟s method is that the maximum kinetic energy at mean position

equal to the maximum energy at the extreme position.

11. What is meant by harmonic forcing?

The term harmonic forcing refers to a spring-mass system with viscous damping, excited by a

sinusoidal harmonic force.

F = Fo Sin wt 12. Define transmissibility.

When a machine is supported by a spring, the spring transmits the force applied on the

machine to the fixed support or foundation. This is called as transmissibility.

13. Briefly explain elastic suspension.

When machine components are suspended from elastic members, the vibration force

produced by the machine components will not be transmitted to the foundation. This is called as

elastic foundation.

14. Specify any two industrial applications where the transmissibility effects of vibration, are

important. All machine tools and all turbo machines.

15. What is vibration isolation? The term vibration isolation refers to the prevention and minimization of vibrations and their

transmission due to unbalanced machines.

16. Define period and cycle of vibration.

Period: it is the time interval after which the motion is repeated itself

Cycle: It is defined as the motion completed during one time period

l7. Define frequency of vibration.

It is the number of cycles described in one second.

l8. Explain the Dunkerley’s method used in natural transverse vibration?

The natural frequency of transverse vibration for a shaft carrying a number of point loads and

uniformly distributed load is obtained by Dunkerley„s formula. 19. State Dunkerley’s formula :

1/ ( ) = 1/ (fn1)2 + 1/ (fn2)

2 + ………….+ 1/ (fns)

2

20. Define critical or whirling or whipping speed of shaft?

The speed at which resonance occurs is called critical speed of the shaft In other words, the

speed which the shaft nut so that the additional deflection of shaft from the axis of rotation becomes

infinite is known as critical speed.

A


21. What are the factors that affect the critical speed of a shaft?

The critical speed essentially depends on

The eccentricity of the CG of the rotating masses from the axis of rotation of the

shaft. Diameter of the disc,

Span of the shaft, and Type of supports connections at its ends.

22. Critical speed of shaft is the some as the natural frequency of transverse vibration. Justify?

We know that critical or whirling speed,

Wcr = Wn = = Hz

If, NC is the critical speed in rps, 2 π Ncr = ⇒ Ncr = (1/ 2 π ) x ( ) = 0.4985/ √

23. What are the causes of critical speed? Or why is critical speed encountered?

The critical speed may occur due to one or more of the following reasons: Bending of the shaft due to self-weight Non-uniform distribution of rotor material, etc Eccentric mountings like gears, flywheels, pulleys, etc.,

24. Define torsional vibration?

When the particles of a shaft or disc move in a circle about the -axis of the shaft, then the

vibrations are known as torsional vibrations. 25. Differentiate between transverse and torsional vibration?

In transverse vibrations; the particles of the shaft move approximately to the axis of the shaft.

But in torsional vibrations, the particles of the shaft move in a circle about the axis of the shaft Due

to transverse vibrations, tensile and compressive stresses are induced. Due to torsional vibrations,

torsional shear stresses are induced in the shaft 26. Define torsional equivalent shaft?

A shaft having diameter for different lengths cab be theoretically replaced by an equivalent

shaft of uniform diameter such that they have the same total angle of twist when equal opposing

torques are applied at their ends. Such a theoretically replaced shaft is known as torsion ally

equivalent shaft.

A


27. State natural frequency of torsional vibration of a simple

system Natural frequency of torsional vibration,

fn=

Where C = Rigidity modulus of shaft,

I= Mass M.I. of rotor, J = Polar of shaft, and

I = Length of node from rotor

28. What are the conditions to be satisfied for an equivalent system to that of geared system in

torsional vibrations?

Two conditions are:

The kinetic energy of the equivalent system must be equal to the kinetic energy of the original

system.

The strain energy of the equivalent system must be equal to the strain energy of the original system.

29. Define logarithmic decrement?

Logarithmic decrement defined as the natural logarithm of the amplitude factor. The

amplitude reduction factor is the ration or any two successive amplitudes on the same side of the

mean position.

A


UNIT – IV

1. What is the function of governor?

The function of a governor is to maintain the speed of an engine within specified limits

whenever there is a variation of load. Governors control the throttle valve and hence the fuels supply

to cater the load variation on engines.

2. How governors are classified?

(a). Centrifugal governors.

Pendulum type: Example: Watt governor. Gravity controlled type. Example: Porter and Proell governors. Spring controlled type. Example: Hartnell and Hartung governors.

(b) Inertia governors. 3. What do you mean by governor effort?

The mean force acting on the sleeve for a given percentage change of speed for lift of

the sleeve is known as the governor effort 4. Define power of a governor?

The power of a governor is the work done at the sleeve for a given percentage change of

speeds It is the product of the mean value of the effort and the distance through which the

sleeve moves; Power = Mean effort x Lift of sleeve

5. What is mean by sensitiveness of a governor?

The sensitiveness is defined as the ratio of the mean speed to the difference between

the maximum and minimum speeds. A governor is said to be sensitive, when it really responds to a small change of speed.

6. Define coefficient of sensitiveness?

It is the ratio between ranges of speed and mean speed.

Coefficient of sensitiveness = (N1-N2) / N

7. What is meant by hunting?

The phenomenon of continuous fluctuation of the engine speed above and below the

mean speed is termed hunting. These occurs over-sensitive governors. 8. Explain the term stability of governor?

A governor is said to be stable if there is only one radius of rotation for all equilibrium

speeds of the balls within the working range.-If the equilibrium speed increases the radius of

governor ball must also increase.

9. What is meant by isochronous condition in governors?

A


A governor with zero range of speed is known as an isochronous governor. Actually the

isochronisms is the stage of infinite sensitivity, when the equilibrium speed is constant for all radii of

rotation of rotation of the balls within the working range, the governor is said to be in isochronism

10. Can a Porter governor be isochronous?

For a Porter governor, Maximum speed N1 and minimum speed N2 are given

by (N1)2 = m + (m/2) x (1+q) x 895 /h1

(N2)2 = m + (m/2) x (1+q) x 895 /h2 For isochronism, range of speed = ∞

N1 - N2 = 0 ⇒ h1 = h2 This is highly impossible. Hence porter governor cannot be isochronous.

11. Give the application of gyroscopic principle?

It is used:

In instrument or troy known as gyroscope,

In ships in order to minimize the rolling and pitching effects of waves, and

In aeroplane, monorail cars, gyrocompasses etc

12. What is gyroscopic torque?

Whenever a rotating body changes its axis of rotation, a torque is applied on the

rotating body. This torque is known as gyroscopic torques.

13. Define steering, pitching and rolling?

Steering is the turning of a complete ship in a curve towards let or right while it moves forward

Pitching the movement of a complete ship up and down in a vertical plane about transverse axis Rolling is the movement of a ship in a linear motion.

14. What is the effect of gyroscopic couple on rolling of ship? Why?

We know that; for the effect of gyroscopic couple to occur, the axis of precession should

always be perpendicular to the axis of spin. In case of rolling of a ship, the axis of precession is

always parallel to the axis of spin for all positions. Hence there is no effect of the gyroscopic couple

acting on the body of ship during rolling. 15. Discuss the effect of the gyroscopic couple on a two wheeled vehicle when taking a turn?

The gyroscopic couple will act over the vehicle outwards. The tendency of this couple is

to overturn the vehicle in outward direction

l6. The engine of an aeroplane rotates in clockwise direction when seen from the tail end

and the aeroplane takes a turn to the left. What will be the effect of the gyroscopic couple on

the aeroplane?

The effect of gyroscopic couple will be to raise the nose and dip the tail.

A


17. Define gyroscopic couple?

If a body having moment of inertia I and rotating about its own axis at ω rad/sec is also

caused to turn at ω rad/sec about an axis perpendicular to axis of spin, then it experiences a

gyroscopic couple of magnitude (I * ω * ωP) in an axis which is perpendicular to both the axis of

spin end axis of precession.

18. Write the expression for gyroscopic couple?

Gyroscopic couple, C = I * ω * ωP

Where I = Moment of inertia of the disc,

ω = Angular velocity of the engine, and

ωP = Angular velocity if precession.

A


UNIT - V

1. Write the importance of balancing?

If the moving part of a machine are not balanced completely then the inertia forces are set up

which may cause excessive noise, vibration, wear and tear of the system. So balancing of machine is

necessary.

2. Write different types of balancing?

l. Balancing of rotating masses a)

Static balancing

b) Dynamic balancing

2. Balancing of reciprocating masses. 3. Define static balancing?

A system of rotating masses is said to be in static balance if the combined mass centre of the

system lies on the axis of rotation.

4. What is dynamic balancing?

A system of rotating masses is said to be in static balance when there does not exist any

resultant centrifugal force as well as resultant couple.

5. What is meant by balancing of rotating masses?

The process of providing second mass in order to counteract the effect of the centrifugal force

of the unbalanced first mass is called balancing of rotating masses.

6. Why rotating masses are to be dynamically balanced?

If the rotating masses are not dynamically balanced, the unbalanced dynamic forces will

cause worse effects-such as wear and tear on bearings and excessive vibrations on machines. It is

very common in camshafts, steam turbine rotors, and engine crank shafts and centrifugal pumps, etc.

7. State the condition for static balancing?

The net dynamic force acting on the shaft is equal to zero. This requires that the line of action

of their centrifugal force must be same.

8. State the conditions for complete balance of several masses revolving in different planes of

a shaft?

l. The resultant centrifugal force must be zero, and

2. The resultant couple must be zero. 9. How do you ensure dynamic balancing of rotating

masses? Conditions of dynamic balancing are

A


l. The net dynamic force acting on the shaft is zero.

The net couple due to dynamic forces acting on the shaft is zero.

10. Differentiate static and dynamic balancing?

Sl

Static Balancing Dynamic Balancing No

1 The dynamic forces as a result of the unbalanced The arrangement made in static balancing masses are balanced by introducing balancing in the gives rise to a couple which tends to rock

plane of rotation or different planes. the shaft in bearing.

The net dynamic force acting on the shaft is made Dynamic balancing considers the net

zero. couple as well as net dynamic force to do

complete balancing.

2 It deals only with balancing of dynamic forces. It deals only with balancing of dynamic force and balancing of couple due to

dynamic force.

11. Define Dalby’s method of balancing masses?

Dalby‟s method is used for balancing several masses rotating in different in planes. In this

method several forces acting on several planes are transferred to a single reference plane.

12. Write phenomenon of transferring forces from one plane to other?

Transferring a force (F) from one plane to another plane having distance „l‟ is equivalent to

transfer of the same force „F‟ „in magnitude and direction in the reference plane is accompanied by a

couple of magnitude „Fl‟. „

13. Why only a part of unbalanced force due to reciprocating masses balanced by

revolving mass? (Or) Why complete balancing is not possible in reciprocating engine?

Balancing of reciprocating masses is done by introducing the balancing mass opposite to the

crank. The vertical component of the dynamic force of this balancing mass gives rise to “Hammer

blow”. In order to reduce the Hammer blow a part of the reciprocating mass is balanced. Hence

complete balancing is not possible in reciprocating engines.

14. Can single cylinder-engine be fully balanced? Why?

No. A single cylinder engine cannot be fully balanced. Because the unbalanced forces due to

reciprocating masses (mω2rcosθ and mω

2rcos2θ) remains constant in direction but varies in

magnitude (because of variation in θ).

15. List the effects of partial balancing of locomotives

l. Variation in tractive force along the line of stroke. 2. Swaying couple, and 3. Hammer blow (i.e., variation of pressure on the rails perpendicular to the line of stroke).

l7. Why are the cranks of locomotive, with two cylinders, placed at 90° to each other?

A


In order to facilitate the starting of locomotive in any position (i.e., in order to have

uniformity in turning moment) the cranks of a locomotive are generally at 90° to one another.

18. Define tractive force?

The resultant unbalanced force due to the two cylinders along the line of stroke, is known as

tractive force.

19. Define swaying couple?

The unbalanced force acting at a distance between the lines of stroke of two cylinders,

constitute a couple in the horizontal direction. This couple is known as swaying couple.

20. Define hammer blow?

The maximum magnitude of the imbalanced force along the perpendicular to the line of

stroke is known as hammer blow.

21. What are the effects of hammer blow and swaying couple?

The effect of hammer blow is cause the vibration in pressure between the wheel and the rail,

such that vehicle vibrates vigorously.

The effect of swaying couple is to make the leading wheels sway from side to side.

22. What are in line engines?

Multi-cylinder engines with the cylinder centre lines in the same plane and on the same side

of the centre line of the crankshaft are known as in-line engine.

23. What are the conditions to be satisfied for balancing of in line engines?

(i) The algebraic sum of the primary and secondary forces must be zero, and (ii) The algebraic sum of the couples due to primary and secondary forces must be zero.

24. Why radial engines are preferred?

In radial engines the connecting rod are connected to a common crank and hence the plane of

rotation of the various cranks-is same, therefore there are no unbalanced primary or secondary

couples. Hence radial engines are preferred.

25 What for the balancing machines are used?

Balancing machines are used to: (i) Determine whether the rotating parts of a machine is completely balanced or not;

(ii) Check the static and. dynamic balancing of rotating parts. (iii) Determine the extent to which the balancing is done (In case of unbalanced system).

26. What are different types of balancing machines?

(i) Static balancing machines, (ii) Dynamic balancing machines

(m) Universal balancing machines.

27. Define hammer blow with respect to locomotives?

A


The maximum magnitude of the unbalanced force along the perpendicular to the line of

stroke is known as hammer blow.

28. What are different types of balancing machines?

Static balancing machines. Dynamic balancing machines, and

Universal balancing machines.

A

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UNIT - I 1. What is meant by Dynamics of Machinery? MECH/III YEAR/Dy… · · 2017-09-21What is...

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