Engineering Questions with Answers - Multiple Choice Questions

# Machine Dynamics MCQ – Critical or Whirling Speed of a Shaft

1 - Question

Assuming the shaft to be freely supported. Calculate the whirling speed of the shaft : 2 cm diameter and 60 cm long carrying a mass of 1 kg at its mid-point. The density of the shaft material is 40 Mg/m3, and Young’s modulus is 200 GN/m2.
a) 2598
b) 2434
c) 2756
d) 2634

Explanation: I = πd4/64 = 7.855×10-9 m4
d due to 1 kg mass = 28×10-6 m
d due to mass of the shaft = 0.133×10-3m
substituting these values into the frequency relation of transverse vibrations
we get
f = 43.3 Hz
Therefore Nc = 43.3×60 = 2598 rpm.

2 - Question

When the centre of gravity of the rotor lies between the centre line of the shaft and the centre line of the bearing, e is taken positive.
a) True
b) False

Explanation: If the centre of gravity(G) of the rotor does not lie between the centre line of the shaft and the centre line of the bearing, then the value of e is taken positive.

3 - Question

The speed at which the shaft runs so that the additional deflection from the axis of rotation of the shaft becomes infinite, is known as _________
a) Whirling speed
b) Rotational speed
c) Stabilizing speed
d) Reciprocating speed

Explanation: The rotational speed at which the shaft runs so that the additional deflection of the shaft from the axis of rotation becomes infinite, is known as critical or whirling speed.

4 - Question

From the following data, calculate the critical speed of the shaft in rps.
Shaft diameter = 5mm
length = 200mm
Mass of disc = 50Kg at centre of shaft
E = 200GN/m2
Centre of disc at 0.25m away from centre of axis of shaft.
a) 8.64
b) 9.64
c) 10.64
d) 11.64

Explanation: I = πd4/64 = 30.7×10-12 m4
d = 3.33×10-3m
substituting these values into the frequency relation of transverse vibrations
we get
Nc = 8.64 rps.

5 - Question

From the following data, calculate the static deflection in mm.
Critical speed = 8.64 rps
Mass of disc = 100Kg at centre of shaft
E = 100GN/m2
Centre of disc at 0.25m away from centre of axis of shaft.
a) 3.32
b) 9.64
c) 10.64
d) 11.64

Explanation: I = πd4/64 = 30.7×10-12 m4
Nc = 8.64 rps
substituting these values into the frequency relation of transverse vibrations
we get
d = 3.32mm.

6 - Question

If the static deflection is 1.665×10-3m, calculate the critical speed of the shaft in rps.
Centre of disc at 0.25m away from centre of axis of shaft.
a) 8.64
b) 9.64
c) 10.64
d) 12.2

Explanation: d = 1.665×10-3m
substituting these values into the frequency relation of transverse vibrations
we get
Nc = 12.2 rps.

7 - Question

A shaft supported in ball bearings is assumed to be a simply supported.
a) True
b) False

Explanation: A shaft supported in short bearings is assumed to be a simply supported shaft while the shaft supported in journal bearings is assumed to have both ends fixed.

8 - Question

From the following data, calculate the static deflection in mm.
Shaft diameter = 5mm
length = 200mm
Mass of disc = 100Kg at centre of shaft
E = 100GN/m2
Centre of disc at 0.25m away from centre of axis of shaft.
a) 4.32
b) 9.64
c) 10.64
d) 11.64