# Machine Dynamics Questions and Answers – Balancing of Several Masses Rotating in Different Planes

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

1. Which of the following statements are associated with complete dynamic balancing of rotating systems?
1. Resultant couple due to all inertia forces is zero.
2. Support reactions due to forces are zero but not due to couples.
3. The system is automatically statically balanced.
4. Centre of masses of the system lies on the axis of rotation.
a) 1, 2, 3 and 4
b) 1, 2, and 3 only
c) 2, 3 and 4 only
d) 1, 3 and 4 only

Explanation: Support reactions due to forces & couple both need to be zero.

2. Which of the following statements is correct about the balancing of a mechanical system?
a) If it is under static balance, then there will be dynamic balance also
b) If it is under dynamic balance, then there will be static balance also
c) Both static as well as dynamic balance have to be achieved separately
d) None of the mentioned

Explanation: Complete balancing means both static & dynamic should be balanced.

3. The magnitude of swaying couple due to partial balance of the primary unbalancing force in locomotive is
a) inversely proportional to the reciprocating mass
b) directly proportional to the square of the distance between the centre lines of the two cylinders
c) inversely proportional to the distance between the centerlines of the two cylinders
d) directly proportional to the distance between the centerlines of the two cylinders

Swaying couple a b

4. In a locomotive, the ratio of the connecting rod length to the crank radius is kept very large in order to
a) minimize the effect of primary forces
b) minimize the effect of secondary forces
c) have perfect balancing
d) start the locomotive conveniently

Explanation: As l increases FS decreases
Hence the ratio is kept large to avoid secondary forces for each balancing.

5. In case of partial balancing of locomotives, the maximum magnitude of the unbalanced force perpendicular to the line of stroke is called hammer blow and this has to be limited by proper choice of the balancing mass and its radial position.
a) True
b) False

Explanation: The effect of hammer blow is to cause variation in pressure between the wheel and the rail, and it may sometimes cause the lifting of wheels from the rails.
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6. Multi-cylinder engines are desirable because
a) only balancing problems are reduced
b) only flywheel size is reduced
c) both (a) and (b)
d) none of the mentioned

Explanation: None.

7. When the primary direct crank of a reciprocating engine makes an angle θ with the line of stroke, then the secondary direct crank will make an angle of . . . . . with the line of stroke.
a) θ /2
b) θ
c) 2 θ
d) 4 θ

Explanation: The secondary direct crank angle is always twice of primary direct crank angle.

8. Secondary forces in reciprocating mass on engine frame are
a) of same frequency as of primary forces
b) twice the frequency as of primary forces
c) four times the frequency as of primary forces
d) none of the mentioned

Explanation: None.

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