# Machine Dynamics Questions and Answers – Balancing of Rotating Masses

This set of Machine Dynamics Multiple Choice Questions & Answers (MCQs) focuses on “Balancing of Rotating Masses”.

1. Often an unbalance of forces is produced in rotary or reciprocating machinery due to the ______
a) Centripetal forces
b) Centrifugal forces
c) Friction forces
d) Inertia forces

Explanation: Often an unbalance of forces is produced in rotary or reciprocating machinery due to the inertia forces associated with the moving masses.

2. Balancing of single rotating mass by balancing masses in same plane and in different planes cannot take place.
a) True
b) False

Explanation: Balancing of single rotating mass by balancing masses in same plane and in different planes can be done with the help of both static balancing and dynamic balancing.

3. Which of the following is true for centrifugal force causing unbalance?
a) Direction changes with rotation
b) Magnitude changes with rotation
c) Direction and magnitude both change with rotation
d) Direction and magnitude both remain unchanged with rotation

Explanation: Force acting radially outwards acts on the axis of rotation and is known as centrifugal force. This is a disturbing force on the axis of rotation, the magnitude of which is constant but the direction changes with the rotation of the mass.

4. In a revolving rotor, the centrifugal force remains balanced as long as the centre of the mass of the rotor lies ___________
a) Below the axis of shaft
b) On the axis of the shaft
c) Above the axis of shaft
d) Away from the axis of shaft

Explanation: In a revolving rotor, the centrifugal force remains balanced as long as the centre of the mass of the rotor lies on the axis of shaft. In other cases, the system will be unbalanced.

5. If the unbalanced system is not set right then.
a) Static forces develop
b) Dynamic forces develop
c) Tangential forces develop

Explanation: It is very essential that all the rotating and reciprocating parts should be completely balanced as far as possible, otherwise the dynamic forces will be set up.
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6. What is not the effect of unbalanced forces?
b) Dangerous vibrations
c) Stresses in various members
d) Violation of conservation of mass principle

Explanation: The unbalanced forces increase the loads on bearings and stresses in the various members, as well as it produces unpleasant and even dangerous vibrations.

7. What is the effect of a rotating mass of a shaft on a system?
a) Bend the shaft
b) Twist the shaft
c) Extend the shaft
d) Compress the shaft

Explanation: Whenever an object containing mass is attached to a rotating shaft, it exerts some centrifugal force. This centrifugal force’s effect is to bend the shaft and to produce vibrations in it.

8. Balancing of reciprocating masses is the process of providing the second mass in order to counteract the effect of the centrifugal force of the first mass.
a) True
b) False

Explanation: The process of providing the second mass in order to counteract the effect of the centrifugal force of the first mass, is called balancing of rotating masses. The main objective of balancing is to minimize the unbalanced forces.

Sanfoundry Global Education & Learning Series – Machine Dynamics.