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# Machine Dynamics MCQ – Coefficient of Fluctuation of Speed

Coefficient of fluctuation is the ratio of ______

a) The maximum fluctuation of speed to the mean speed

b) The minimum fluctuation of speed to the mean speed

c) The maximum speed to the mean speed

d) The minimum speed to the maximum speed

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Answer: a

Explanation: Maximum fluctuation of speed is defined as the difference between the maximum and minimum speeds during a cycle. The ratio of the maximum fluctuation of speed to the mean speed is known as the coefficient of fluctuation of speed.

Coefficient of steadiness is reciprocal of coefficient of fluctuation of speed.

a) True

b) False

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Answer: a

Explanation: Coefficient of steadiness is defined as the ratio of mean speed to the ratio of maximum fluctuation of speed, hence it is the reciprocal of coefficient of fluctuation of speed.

If the minimum speed of the engine is half the maximum speed, then coefficient of fluctuation is _____

a) 0.5

b) 1.5

c) 2

d) 0.66

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Answer: d

Explanation: Let N be the maximum speed, then minimum speed is N/2

mean speed = (N+N/2)/2 = 3N/4

Maximum fluctuation = N – N/2 = N/2

therefore coefficient of fluctuation = 2/3 = 0.66.

If the mean speed is 3/4th of the maximum speed, the coefficient of steadiness is ______

a) 4/3

b) 3/2

c) 3/4

d) 1

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Answer: b

Explanation: Let N be the mean speed and N1 and N2 be the maximum and minimum speed respectively,

given N=3N1/4 , N1 = 4N/3

also N = (N1+N2)/2

N = 2N/3 + N2/2 : N2 = 2N/3

maximum fluctuation = N1– N2 = 4N/3 – 2N/3 = 2N/3

mean speed = N

therefore coefficient of steadiness = N/(2N/3)

= 3/2.

The moment of inertia of a flywheel is 6500 kg-m2, from the turning moment diagram, it is found that fluctuation of energy is 56000 N-m, if mean speed is 120 rpm, then find maximum speed.

a) 121

b) 119

c) 122

d)118

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Answer: a

Explanation: We know that ΔE= I ω2. Cs

= I ω. (ω1-ω2)

= 6500. 120x(2π/60)2( N1– N2)

N1– N2 = 2

N=120

Therefore, N1=121.

The moment of inertia of a flywheel is 6500 kg-m2, from the turning moment diagram, it is found that fluctuation of energy is 56000 N-m, if mean speed is 120 rpm, find the minimum speed.

a) 121

b) 119

c) 122

d) 118

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Answer: b

Explanation: We know that ΔE = I ω2. Cs

= I ω. (ω1-ω2)

= 6500. 120x(2π/60)2(N1– N2)

N1– N2 = 2

N=120

Therefore, N2=119.

In order to reduce the heavy weight of the flywheel which of the following mechanical component is used?

a) Bearings

b) Shaft

c) Gears

d) Engine

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Answer: c

Explanation: The mass of the flywheel depends on the velocity at which it rotates, the velocity can be moderated by installing a set of gears. This results in reduction of mass of flywheel.

The main function of a flywheel is to cause variations in the speed of a shaft caused by torque fluctuations.

a) True

b) False

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Answer: b

Explanation: The primary function of a fly wheel is to smoothen out variations in the speed of a shaft caused by torque fluctuations, this is basically to lessen the fluctuations of the torque. If the source of the driving torque or load torque is fluctuating in nature, then a flywheel is used.

When the flywheel absorbs energy, its speed ________

a) Remains unaffected

b) Increases

c) Decreases

d) Goes down to 0

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Answer: b

Explanation: When a flywheel absorbs energy, its speed increases and when it gives up energy, its speed decreases. This effect is reflected in the turning moment diagram of the flywheel.