BPUT SAMPLE PAPER FOR MACHINE DYNAMICS

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BPUT SAMPLE PAPER FOR MACHINE DYNAMICS

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time=3 hours

full mark=70

1. EACH QUESTION CARRIES 2 MARKS. ALL ARE COMPULSORY.(2*10)

(a)What is link?Mention the links consists to slider-crank mechanism. 

ans:-A link is defined as a member or a combination of members,connecting other members and having motion relative to them. A slide-crank mechanism consists of following four links a.frame b.crank c.connecting rod slider.

(b)What is a kinematic chain? 

ans:-A kinematic chain is defined as the combination of kinematic pairs, joined in such a way that each link forms a part of two pairs and the motion of each relative to other is definite.

(c)Why rotating masses are to be dynamically balanced?

ans:- If the rotating masses are not dynamically balanced, the unbalanced dynamic force will cause worse effect such as wear and tear on bearing and excessive vibrations on the machines. It is very common in cam shafts, steam turbine rotors, engine crank shafts, and centrifugal pumps, etc.

(d)What are the causes and effect of vibration?

ans:-The causes of vibration are unbalanced forces, elastic nature of the system, self excitation, wind and earthquakes.The existence of vibration elements in any mechanical system produces unwanted noise, high stress, poor reliability and premature failure of one or more of the parts.

(e)Define frequency, cycle, period and free vibration.

ans:-Frequency: It is the number of cycles described in one second Cycle: It is defined as the motion completed during one time period Period: It is the time interval after which the motion the motion is repeat itself.Free vibrations: if the periodic motion continues after the causes of original disturbance is removed, then the body is said to be under free vibrations.

(f)What is the principle of working of centrifugal governor?

ans:-The centrifugal governor is based on balancing of centrifugal force on the rotating balls by an equal and opposite radial forces.

(g)Give the applications of gyroscopic principle.

ans:-t is used: (i) In instrument or toy known as gyroscope. (ii) In ships in order to minimize the rolling and pitching effects of waves. (iii) In aeroplanes, Monorail cars, gyrocompasses, etc

(h)What so you mean by governor effort?

ans:-The mean force acting on the sleeve for a given percentage change of speed for lift of the sleeve is known as the governor effect.

(i)What are the different types of damping?

Viscous damping

Coulomb or dry friction damping

Solid or structural damping
Slip or interfacial damping
(j)What are the causes and effect of vibration?
ans:-The causes of vibration are unbalanced forces, elastic nature of the
system, self excitation, wind and earthquakes.The existence of vibration
elements in any mechanical system produces unwanted noise, high stress,
poor reliability and premature failure of one or more of the parts.
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LONG TYPE QUESTIONS
2.What is meant by Equilibrium speeds in case of governors?Define and explain Governor Effort.What is meant by controlling force in case of governors?
ans:-The speeds at which the governor balls, the arms etc. are in complete equilibrium and the
sleeve does not tend to move upward or downward
are called the equilibrium speeds. The speed at the mean position of the balls
or the sleeve is the mean equilibrium speed and at the maximum and minimum
radius of rotation of the balls without tending to move either way are termed
as maximum and minimum equilibrium speeds respectively. There can be
many equilibrium speeds between the mean and maximum and the mean and
 the minimum equilibrium speeds.
A governor running at a constant speed is in equilibrium and the
 resultant force acting on the sleeve is zero. If the speed of the governor
increases there is a force on the sleeve which tends to lift it. This force will
gradually go on decreasing till the governor starts rotating in equilibrium at the
new position of rotation. The mean force acting on the sleeve for a given
change of speed or lift of the sleeve is known as the governor effort.
The force acting radially upon the rotating balls to counteract its
centrifugal force is called the controlling force. It is provided by
weight of the sleeve, centralload
on the sleeve, compressed spring and the weight of the balls.
3.Explain the terms ralating to governor
1)Isochronism
2)Stability
3)Hunting
 
4)Sensitiveness
ans:-
Isochronism:
This is an extreme case of sensitiveness. When the equilibrium speed is
constant for all radii of rotation of the balls within the working range,
the governor is said to be in isochronism. This means that the difference
between themaximum and minimum equilibrium speeds is zero and the
sensitiveness shall be infinite.
Stability:
Stability is the ability to maintain a desired engine speed without fluctuating.
Instability results in hunting or oscillating due to over correction.Excessive
stability results in a dead-beat governor or one that does not
correctsufficiently for load changes
Hunting:
The phenomenon of
continuous fluctuation of the engine speed
aboveand below the mean speed is termed as hunting. This occurs in
over-sensitive
or isochronous governors.Suppose an isochronous governor is fitted to an
engine running at a steady load.With a slight increase of load, the speed will
fall and the sleeve will immediatelyfall to its lowest position. This shall open
the control valve wide and excess supplyof energy will be given, with the
result that the speed will rapidly increase and thesleeve will rise to its higher
position. As a result of this movement of the sleeve,the control valve will be
cut off; the supply to the engine and the speed will againfall, the cycle being
repeated indefinitely. Such a governor would admit either more or less
amount of fuel and so effect would be that the engine would hunt.
Sensitiveness:
 A governor is said to be sensitive, if its change of speed s from noload to full
load may be as small a fraction of the mean equilibrium speed as possible
and the corresponding sleeve lift may be as large as possible.
4.   What is the difference between static and dynamic balancing for a rotating shaft with several mounting on it.ans:-static balancing:

When a motor rotates with mass center away from the exis of rotation, it experiences centripetal acceleration which generates a radially inward force.an equal and opposite force called centrifugal force acts radially outward which is main source of unbalancing of the mass system The magnitue of unbalanced force remains constant but its direction changes with rotation, so for balancing of masses in same plane. mω^2r=m_bωr_b where m=total mass acting at mass center and m_b=balancing mass. Therefore several masses in rotating shaft which are in same plane can be balanced if we bring the mass center to the axis of rotation by attaching balancing mass. In a system of multiple rotating masses,if combined mass center of the system lies on the axis of rotation, then it is called a statically balanced system.In other words,the vector sum of various unbalanced forces acting in a system must be equal to zero.

Dynamic balancing:

When several masses rotates in a single plane, the system is said to be statically balanced if the vector sum of all forces is zero.However ,when several masses rotate in different planes,the system is subjected to two type of unbalanced centrifugal force and couple. thus a system having neither any unbalanced centrifugal force nor any resultant couple is termed as balanced system.such type of balancing is often called dynamic balancing.In other words, a system consisting of several rotating masses in different planes is said to be dynamically balanced,if the vector sum of centrifugal forces and couples are zero.

5.Explain whirling of shaft.How it affects the speed of the shaft?

ans:- When the speed of a rotating shaft is increased to such a value that it starts vibrating violently in the transverse direction,that speed of this shaft is called whirling speed or critical speed.in other words,critical speed of the shaft is that speed which is equal to its natural frequency of lateral vibration of the shaft.this phenomenon occurs due to many reasons namely unbalanced rotating mass,gyroscopic couple effect,and so forth.The operation of shaft at its critical speed is very dangerous as the amplitude of vibration is maximum which may cause high fatigue stresses and  may lead to failure. When the natural frequency of the system coincides with the external forcing frequency,it is called resonance.The speed at which resonance occurs are known as the critical speeds.These speeds are also called whirling speeds.   At these speeds,the amplitude of vibration of rotors is excessively large amount of force is transmitted to the foundations of bearings.In the region of critical speeds the system may fail because of violent nature of vibrations in the transverse direction.Therefore it is very important to find the natural frequency of the shaft to avoid the occurrence of critical speeds which may result in excessive noise and its breakage into pieces.

6.(a)What is meant by static and dynamic equivalance in machinery?

ans:-D Alembert’s principle states that the resultant force acting on a body together with the reserved effective force(or inertia force) are in equillibrium. Thus principle is used to reduce a dynamic problem into an equivalent static problem. A continuous body may be replaced by a body of two masses assumed to be concentrated at two points and connected rigidly together.Such a system of two masses is termed as equivalent dynamical system.The conditions to be satisfied by an equivalent dynamical system are:

1.The total mass must be equal to that of the rigid body.

2.The center of gravity must coincide with that of the rigid body.

3.The total moment of inertia about an axis through center of gravity must be equal to that of rigid body.

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(b)What is free and forced vibrations?

ans:-

Free vibration:

When no external forces acts on the body,after giving it an initial displacement,then the body is said to be under free or natural vibrations.The frequency of the free vibration is called free or natural frequency.

Forced vibration:

When the body vibrates under the influence of external force,then the body is said to be under forced vibrations.The external force is periodic disturbance force created by unbalance.The vibration have the same frequency as the applied force.

7.(a)What is inertia force?What is its importance?

In the mechanism of a machine,when a link moves with zero acceleration,from newton’s second law ,it is said that the resultant of forces acting on the link is zero and the link is in the static of static equilibrium.However,when a link moves with certain acceleration,the moving mass of the link experiences an additional force called inertia force.Inertia is a property of matter by virtue of which a rigid body resists any change in velocity.

The problem of dynamic force analysis can be converted into static force analysis using D Alembert’s principle.According to it,the vector sum of all external forces and inertia force acting on a rigid body is zero and that the vector sum of all external torques/moments and inertia couple acting on rigid body is also separately equal to zero.A rigid body following above principle is said to be in dynamic equilibrium.Thus we can write the equation of equilibrium just similar to that of static equilibrium as follows.

ΣF+Fi=0

ΣT+Ci=0

The inertia force Fi is the product of mass and linear acceleration.It acts in the direction opposite to that of acceleration.

Fi=-mf

where m=mass of the rigid body

and f=acceleration of the mass of the body

(b)state the condition to have a complete balance of several masses in different planes.

 

ans:-In order to have a complete balance of the several revolving masses in different planes,the following two conditions must be satisfied.

1.The forces in the reference plane must balance i.e the resultant force must be zero.

2.The couples about the reference plane must balance i.e the resultant couple must be zero.

When several masses revolve in different planes,they may be transferred to a reference plane which may be defined as the plane passing through a point on the axis of rotation and perpendicular to it.The effect of transferring a revolving mass (inn one plane)

to a reference plane is to cause a force of the revolving mass to act in the reference plane,together with a couple of magnitude equal to the product of the force and the distance between the plane

of rotation and the reference plane.

 

 

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