BPUT SAMPLE PAPER FOR MACHINE DYNAMICS
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?
Coulomb or dry friction damping
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.
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.
(b)What is free and forced vibrations?
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.
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.
The inertia force Fi is the product of mass and linear acceleration.It acts in the direction opposite to that of acceleration.
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.