Dynamics of Machines Solved MCQs Mechanical

Multiple Choice Questions 22 Pages

Anna University

Mechanical Engineering

Contributed by

Arvind Ramson

V SEMESTER - MECHANICAL ENGINEERING
DYNAMICS OF MACHINERY
ANNA UNIVERSITY :: REGULATIONS - 2017
1. Under logarithmic decrement, the amplitude of successive vibrations are
(a) constant (b) in arithmetic progression
(c) in geometric progression (d) in logarithmic progression.
2. A certain machine requires a torque of (500 + 50 sin 2Q) kN m to drive it, where Q is the angle of rotation of shaft
measured from certain datum. The machine is directly coupled to an engine which produces a torque (500 + 50 sin Q) kN
m in a cycle. How many times the value of torque of machine and engine will be identical?
(a) 1 (b) 2 (c) 4 (d) 8.
3.In which of the following case, the turning moment diagram will have least variations:
(a) Double acting steam engine (b) Four stroke single cylinder petrol engine
(c) 8 cylinder, 4 stroke diesel engine (d) Pelton wheel.
4. If the rotating mass of a rim type fly wheel is distributed on another rim type flywheel whose mean radius is half mean
radius of the former, then energy stored in the latter at the same speed will be
(a) four times the first one (b) same as the first one
(c) one-fourth of the first one (d) one and a half times the first one.
5.A Hartnell governor has its controlling force F given by F =p+qr, Where r is the radius of the balls and p and q are
constants.
The governor becomes isochronous when
(a) p = 0 and q is positive (b) p is positive and q =0
(c) p is negative and q is positive (d) p is positive and q is also positive
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6.A system in dynamic balance implies that
(a) the system is critically damped (b) there is on critical speed in the system
(c) the system is also statically balanced (d) there will be absolutely no wear of bearings
7.Which one of the following can completely balance several masses revolving in different planes on a shaft?
(a) A single mass in one of the planes of the revolving masses
(b) A single mass in a different plane
(c) Two masses in any two planes
(d) Two equal masses in any two planes
8.A simple spring mass vibrating system has a natural frequency of N. If the spring stiffness is halfed and the mass is
doubled, then the natural frequency will become
(a) N/2 (b) 2N (c) 4N (d) 8N
9.Rotating shafts tend to vibrate violently at whirling speeds because
(a) the shafts are rotating at very high speeds
(b) bearing centre line coincides with the shaft axis
(c) the system in unbalanced
(d) resonance is caused due to the heavy weight of the rotor.
10.Critical speed of a shaft with a disc supported in between is equal to the natural frequency of the system in
(a) transverse vibrations (b) torsional vibrations
(c) longitudinal vibrations (d) longitudinal vibrations provided the shaft is vertical.
11.A spring controlled governor is found unstable. It can be made stable by
(a) increasing the spring stiffness (b) decreasing the spring stiffness
(c) increasing the ball weight (d) decreasing the ball weight
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12.A shaft has two heavy rotors mound on it The transverse natural frequencies, considering each of the rotor separately,
are 100 cycles/sec and 200 cycles/sec respectively. The lowest critical speed is
(a) 5367 rpm (b) 6000 rpm (c) 9360 rpm (d) 12000 rpm.
13.A shaft has an attached disc at the centre of its length. The disc has its centre of gravity located at a distance of 2 mm
from the axis of the shaft. When the shaft is allowed to vibrate in its natural bow-shaped mode, its has a frequency of
vibration of 10 radians/second. When the shaft is rotated a 300 revolutions per minute, it will whirl with a radius of
(a) 2 mm (b) 2.25 mm (c) 2.50 mm (d) 3.00 mm.
14.For a spring controlled governor to be stable, the controlling force (F) is related to the radius (r) by the equation.
(a) F =ar – b (b) F = ar+ b (c) F = ar (d) F = a/r + b
15.Which of the following methods can be used to determine the damping of machine element?
1. Logarithmic method 2. Band-width method
3. Rayleigh method 4. Holzer method.
Select the correct answer using the codes given below: Codes:
(a) 1 and 3 (b) 1 and 2 (c) 3 and 4 (d) 1, 3, and 4.
16. If  /n=√ 2 , where W is the frequency of excitation and Wn is the natural frequency of vibrations, then the
transmissibility of vibrations will be
(a) 0.5 (b) 1.0 (c) 1.5 (d) 2.0
17. A slender shaft supported on two bearing at its ends carries a disc with an eccentricity e from the axis of rotation. The
critical speed of the shaft is N. If the disc is replaced by a second one of same weight but mounted with an eccentricity 2e,
critical speed of the shaft in the second case is
(a) 1/2 N (b) 1/√ 2 N (c) N (d) 2 N
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18.A system of masses rotating in different parallel planes is in dynamic balance if the resultant
(a) force is equal to zero
(b) couple is equal to zero
(c) force and the resultant couple are both equal to zero
(d) force is numerically equal to the resultant couple, but neither of them need necessarily be zero.
19.When shocking force is transmitted through the spring, damping becomes detrimental when the ratio of its frequency
to the natural frequency its greater than
(a) 0.25 (b) 0.50 (c) 1.00 (d) sqrt 2.
20.When the mass of a critically damped single degree of freedom system is deflected from its equilibrium position and
released, it will
(a) return to equilibrium position without oscillation
(b) oscillate with increasing time period
(c) oscillate with decreasing amplitude
(d) oscillate with constant amplitude.
21.The critical speed of a rotating shaft depends upon
(a) mass (b) stiffness (c) mass and stiffness (d) mass, stiffness and eccentricity.
22.What is the correct sequence of the following steps in engine analysis?
1. Vibration analysis 2. Inertia force analysis
3. Balancing analysis 4. Velocity and Acceleration analysis
Select the correct answer using the codes given below:
(a) 2, 4, 1, 3 (b) 2, 4, 3, 1 (c) 4, 2, 1, 3 (d) 4, 2, 3, 1
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23.The sensitivity of an isochronous governor is
(a) zero (b) one (c) two (d) infinity.
24.An axial flow fan balanced at one speed often exhibits substantial vibrational effects when operated at other speeds,
mainly due to
(a) primary critical speed effect (b) secondary critical speed effect
(c) unbalanced parts of the fan (d) aerodynamic unbalance.
25.For a given fractional change of speed, if the displacement of the sleeve is high, then the governor is said to be
(a) Hunting (b) Isochronous (c) Sensitive (d) Stable.
26.The critical speed of a uniform shaft with a rotor at the centre of the span can be reduced by
(a) Reducing the shaft length (b) Reducing the rotor mass
(c) Increasing the rotor mass (d) Increasing the shaft diameter
27.If a two-mass system is dynamically equivalent to a rigid body, then the system will not satisfy the condition that the
(a) Sum of the two masses must be equal to that of the rigid body
(b) Polar moment of inertia of the system should be equal to that of the rigid body
(c) Centre of gravity (c.g.) of the system should coincide with that of the rigid body
(d) Total moment of inertia about the axis through c.g. must be equal to that of the rigid body.
28.If the ratio of the length of connecting rod to the crank radius increases, then
(a) Primary unbalanced forces will increase
(b) Primary unbalanced forces will decrease
(c) Secondary unbalanced forces will increase
(d) Secondary unbalanced forces will decrease.
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29. If a spring-mass-dashpot system is subjected to excitation by a constant harmonic force, then at resonance, its
amplitude of vibration will be
(a) Infinity (b) Inversely proportional to damping
(c) Directly proportional to damping (d) Decreasing exponentially with time.
30.In a forced vibration with viscous damping, maximum amplitude occurs when forced frequency is
a) Equal to natural frequency (b) Slightly less than natural frequency
(c) Slightly greater than natural frequency (d) Zero.
31. The value of the natural frequency obtained by Rayleigh’s method
(a) Is always greater than the actual fundamental frequency
(b) Is always less than the actual fundamental frequency
(c) Depends upon the initial deflection curve chosen and may be greater than or less than the actual fundamental
frequency
(d) Is independent of the initial deflection curve chosen.
32. In a multi-rotor system of torsional vibration maximum number of nodes that can occur is
(a) Two (b) Equal to the number of rotor plus one
(c) Equal to the number of rotors (d) Equal to the number of rotors minus one.
33.A rotating shaft carries a flywheel which overhangs on the bearing as a cantilever. If this flywheel weight is reduced to
half of its original weight, the whirling speed will
(a) Be double (b) Increase by√ 2 times
(c) Decrease by √2 times (d) Be half.
34.A mass is suspended at the bottom of two springs in series having stiffness 10 N/mm and 5 N/mm. The equivalent
spring stiffness of the two springs is nearly.
(a) 0.3 N/mm (b) 3.3 N/mm (c) 5 N/mm (d) 15 N/mm.
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35.The critical speed of a shaft is affected by the
(a) diameter and the eccentricity of the shaft (b) span and the eccentricity of the shaft
(c) diameter and the span of the shaft (d) span of the shaft.
36. During torsional vibration of a shaft, the node is characterized by the
(a) maximum angular velocity (b) maximum angular displacement
(c) maximum angular acceleration (d) zero angular displacement.
37. If the rotating mass of a rim type flywheel is distributed on another rim type fly wheel whose mean radius is half the
mean radius of the former, then energy stored in the latter at the same speed will be
(a) four times the first one (b) same as the first one
(c) one-fourth of the first one (d) two times the first one
38. In a Hartnell governor, the mass of each ball is 2.5 kg. Maximum and minimum speeds of rotation are 10 rad/s and 8
rad/s respectively. Maximum and minimum radii of rotation are 20 cm and 14 cm respectively. The lengths of horizontal
and vertical arms of bell crank levers are 10 cm and 20 cm respectively. Neglecting obliquity and gravitational effects, the
lift of the sleeve is
(a) 1.5 cm (b) 3.0 cm (c) 6.0 cm (d) 12.0 cm.
39. The mass moment of inertia of the two rotors in a two rotor system are 100 kg m2 and 10kg m2 . The length of the
shaft of uniform diameter between the rotors is 110 cm. The distance of node from the rotor of lower moment of inertia
(a) 80 cm (b) 90 cm (c) 100 cm (d) 110 c
40. . A shaft of 50 mm diameter and 1 m length carries a disc which has mass eccentricity equal to 190 microns. The
displacement of the shaft at a speed which is 90% of critical speed in microns
(a) 810 (b) 900 (c) 800 (d) 82
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41. For minimizing speed fluctuations of an engine as a prime mover, it must have
(a) Only a flywheel fitted to the crankshaft (b) A governor provided in the system
(c) Both a flywheel and a governor provided in the system (d) Neither a flywheel nor a governor
42. Effect of friction, at the sleeve of a centrifugal governor is to make
(a) More sensitive (b) More stable
(c) Insensitive over a small range of speed (d) Unstable
43. Which one of the following statements is correct?
A governor will be stable if the radius of rotation of the balls
(a) increases as the equilibrium speed decreases
(b) decreases as the equilibrium speed increases
(c) increases as the equilibrium speed increases
(d) remains unaltered with the change in equilibrium speed.
44. A single cylinder, four-stroke I.C. engine rotating at 900 rpm has a crank length of 50 mm and a connecting rod length
of 200 mm. If the effective reciprocating mass of the engine is 1.2 kg, what is the approximate magnitude of the maximum
‘shaking force’ created by the engine?
(a) 533 N (b) 666 N (c) 133 N (d) None of the above.
45. The turning moment diagram for a single cylinder double acting steam engine consists of
ve and –ve loops above and below the average torque line. For the
ve loop, the ratio of the speeds of the flywheel at the beginning and the end is which one of the following?
(a) Less than unity (b) Equal to unity (c) Greater than unity (d) Zero.
46. For a governor running at constant speed, what is the value of the force acting on the sleeve?
(a) Zero (b) Variable depending upon the load (c) Maximum (d) Minimum.
47. A motion is aperiodic at what value of the damping factor?
(a) 1.0 or above (b) 0.5 (c) 0.3 (d) 0.866.
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48. Under service conditions involving jurking, vibration and pulsation of the working load, the bolt of choice would be a
(a) short bolt with high rigidity (b) long bolt with increased elasticity
(c) bolt with a dished washer (d) bolt with castle nut.
49. A uniform cantilever beam undergoes transverse vibrations. The number of natural frequencies associated with the
beam is
(a) 1 (b) 10 (c) 100 (d) infinite.
50. In a reciprocating engine mechanism, the crank and connecting rod of same length r metres are at right angles to each-
other at a given instant, when the crank makes an angle of 45° with IDC. If the crank rotates with a uniform velocity of W
rad/s, the angular acceleration of the connecting rod will be
(a) 2W
2
r (b) W
2
r (c) 
2
r (d) zero.
51. The height of a simple Watt governor running at a speed ‘N’ is proportional to
(a) N (b) 1/ N (c) N
2
(d) 1 / N
2
.
52. The controlling force curve of spring-loaded governor is given by the equation F =ar – c, (where r is the radius of
rotation of the governor balls and a, c are constants). The governor is
(a) stable (b) unstable (c) isochronous (d) insensitive.
53. Two rotors are mounted on a shaft. If the unbalanced force due to one rotor is equal in magnitude to the unbalanced
force due to the other rotor, but positioned exactly 180° apart, then the system will be balanced
(a) statically (b) dynamically
(c) statically as well as dynamically (d) neither statically nor dynamically.
54. In a forced vibrations with viscous damping, maximum amplitude occurs when the forced frequency is
(a) equal to natural frequency (b) slightly less than natural frequency
(c) slightly greater than natural frequency (d) zero.
55. The rotor of a turbine is generally rotated at
(a) the critical speed (b) a speed much below the critical speed
(c) a speed much above the critical speed (d) a speed having no relation to critical speed.
56. The amount of energy absorbed by a flywheel is determined from the
(a) torque-crank angle diagram (b) acceleration-crank angle diagram
(c) speed-space diagram (d) speed-energy diagram.
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57. Sensitiveness of a governor is defined as the ratio of the
(a) maximum equilibrium speed to the minimum equilibrium speed
(b) difference between maximum and minimum equilibrium speeds to the mean equilibrium speed
(c) difference between maximum and minimum equilibrium speeds to the maximum equilibrium speed
(d) maximum difference in speeds to the minimum equilibrium speed
58. A viscous damping system with free vibrations will be critically damped if the damping factor is
(a) zero (b) less than one (c) equal to one (d) greater than one.
59. The balancing weights are introduced in planes parallel to the plane of rotation of the disturbing mass. To obtain
complete dynamic balance, the minimum number of balancing weights to be introduced in different planes is
(a) 1 (b) 2 (c) 3 (d) 4
60. Consider the following statements:
1. In forced vibrations, the body vibrates under the influence of an applied force
2. In damped vibrations, amplitude reduces over every cycle of vibration
3. In torsional vibrations, the disc moves parallel to the axis of shaft
4. In transvers vibrations, the particles of the shaft moves approximately perpendicular to the axis of the shaft
Which of these statements are correct?
(a) 1, 2, and 3 (b) 1, 3, and 4 (c) 2, 3, and 4 (d) 1, 2, and 4.
61. Hammer blow
(a) is the maximum horizontal unbalanced force caused by the mass provided to balance the reciprocating masses
(b) is the maximum vertical unbalanced force caused by the mass added to balance the reciprocating masses
(c) varies as the square root of the speed
(d) varies inversely with the square of the speed
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Dynamics of Machines Solved MCQs Mechanical

Dynamics of Machines Solved MCQs Mechanical

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