# Machine Dynamics Questions and Answers – Balancing of Several Masses Rotating in the Same Plane

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

1. From the given data, find the balancing mass in Kg if r=0.2m required in the same plane.
Masses = 200kg, 300kg, 240 kg, 260Kg, corresponding radii = 0.2m, 0.15m, 0.25m and 0.3m.
Angles between consecutive masses = 45, 75 and 135 degrees.
a) 116
b) 58
c) 232
d) 140

Explanation: To make the calculation simple, we will calculate centrifugal force as m.r
hence
horizontal forces = mrcosθ = 21.6 Kg-m
vertical forces = mrsinθ = 8.5 Kg-m
Resultant = 23.2 Kg-m
R = 0.2 m
therefore, M = 116Kg

2. Graphical method gives the best results.
a) True
b) False

Explanation: In order to find the balancing mass, analytical and graphical approach can be used, analytical method gives the accurate results as there is less scope for error.

3. From the given data, find the balancing mass’s inclination in degrees if r=0.2m required in the same plane.
Masses = 200kg, 300kg, 240 kg, 260Kg, corresponding radii = 0.2m, 0.15m, 0.25m and 0.3m.
Angles between consecutive masses = 45, 75 and 135 degrees.
a) 201.48
b) 200.32
c) 210.34
d) 202.88

Explanation: horizontal forces = mrcosθ = 21.6 Kg-m
vertical forces = mrsinθ = 8.5 Kg-m
Resultant = 23.2 Kg-m
R = 0.2 m
therefore, M =116Kg
theta = vertical / horizontal
= 180 + 21.48 = 201.48.

4. If all the masses are in one plane, then what is the maximum no. of masses which can be placed in the same plane?
a) 3
b) 4
c) 6
d) No limitation

Explanation: while balancing in one plane, any number of masses can be placed, the net result will only depend on the sum of vertical and horizontal components and the resultant should be equal to the unbalance.

5. If the rotation speed of the shaft increases then the balancing mass must also increase.
a) True
b) False

Explanation: The balancing mass to be placed in the same plane as the other unbalanced masses, is independent of the speed at which the shaft rotates.

6. If all the masses are in one plane, then in which of the following condition is possible?
a) Resultant only in horizontal
b) Resultant only in vertical
c) System remains unbalanced
d) No limitation

Explanation: While balancing in one plane, there is no restriction of placing masses, the net result will only depend on the vector sum of vertical and horizontal components and the resultant should be equal to the unbalance.

7. The secondary unbalanced force produced by the reciprocating parts of a certain cylinder of a given engine with crank radius r and connecting rod length l can be considered as equal to primary unbalanced force produced by the same weight having
a) an equivalent crank radius r2/4l and rotating at twice the speed of the engine
b) r2/4l as equivalent crank radius and rotating at engine speed
c) equivalent crank length of r2/4l and rotating at engine speed
d) none of the mentioned

Explanation: We know that secondary force = primary force
FS = FP
Therefore, to balance the force the primary force should contain an equivalent crank radius r2/4l and rotating at twice the speed of the engine.

8. The effect of hammer blow in a locomotive can be reduced by ______________
a) decreasing the speed
b) using two or three pairs of wheels coupled together
c) balancing whole of the reciprocating parts
d) decreasing the speed and using two or three pairs of wheels coupled together 