# Machine Dynamics Questions and Answers – Free Torsional Vibrations of a Two Rotor System

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This set of Machine Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Free Torsional Vibrations of a Two Rotor System”.

1. Consider P and Q as the shaft having two rotors at the end of it, what is the point N known as in the given figure? a) Node
b) Elastic point
c) Inelastic point
d) Breaking point

Explanation: The torsional vibrations occur in a two-rotor system only if the rotors move in the opposite direction, at point N there is no vibration hence it is known as node.

2. In a two rotor system, torsional vibration occurs only if the rotors are moving in the same direction.
a) True
b) False

Explanation: For a two rotor system, the torsional vibration will occur only if the rotors are moving in the direction opposite to each other.

3. In the given figure representing a two rotor system, what is the line LNM called? a) Node line
b) Elastic line
c) Inelastic line
d) Breaking line

Explanation: The given figure represents the shaft PQ with rotors attached to the end of it making it a two rotor system, in the figure, the line LNM is known as the elastic line of the shaft.

4. For occurrence of free torsional vibration which of the condition is necessary?
a) Rotors moving in same direction
b) Rotors having same frequency
c) Rotors having different frequency
d) Rotors rotate in the same sense

Explanation: In a two rotor torsional vibration system, the system will rotate if the rotors are rotating in opposite direction with the same frequency.

5. In the given figure, considering PN as one shaft, If the mass moment of inertia is increased to four times, then what will be the effect on free torsional vibrations of a rotor at P? a) Increases 4 times
b) Increases 2 times
c) Decreases 4 times
d) Decreases 2 times

Explanation: If N is the node then PN can be considered as a shaft of single rotor system, since the free torsional vibrations of a single motor system depends on the inverse of square root of the mass moment of inertia of the system, increasing 4 times will lead to decrease in two times the initial vibration.

6. In the given figure if N is the node then NQ acts as which of the following system? a) Single rotor system
b) Two rotor system
c) Three rotor system
d) Four rotor system

Explanation: If N is the node in the given figure, then N can act as a fixed end of the shaft NQ where the rotor is connected at Q, hence it acts like a single rotor system.

7. Keeping the mass moment of inertia of both the shafts in a two rotor system same, if the length of one shaft is doubled what should be the effect on the length of other shaft?
a) Doubled
b) Halved
c) Constant
d) Increased to 4 times

Explanation: The vibration will occur in a two rotor system only if the frequencies of both the rotors are same, hence L(a)I(a) = L(b)I(b). Therefore the relation is directly proportional.

8. For a two rotor system, the mass moment of inertia of one shaft(A) is twice the other(B), then what is the relation between the length of the shafts.
a) 2L(A) = L(B)
b) L(A) = 2L(B)
c) L(A) = L(B)
d) 2L(A) = 3L(B)

Explanation: For a two rotor system, we have L(A)I(A) = L(B)I(B)
Now I(A)=2I(B)
Therefore, 2L(A) = L(B).

9. For a two rotor system, the length of one shaft(A) is twice the other(B), then what is the relation between the Mass moment of inertia of the shafts.
a) 2I(A) = I(B)
b) I(A) = 2I(B)
c) I(A) = I(B)
d) 2I(A) = 3I(B)

Explanation: For a two rotor system, we have L(A)I(A) = L(B)I(B)
Now L(A)=2L(B)
Therefore, 2I(A) = I(B). 