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A single cylinder oil engine works with a three rotor system, the shaft length is 2.5m and 70mm in diameter, the middle rotor is at a distance 1.5m from one end. Calculate the free torsional vibration frequency for a single node system in Hz if the mass moment of inertia of rotors in Kg-m^2 are: 0.15, 0.3 and 0.09. C=84 kN/mm^2

(a) 171

(b) 181

(c) 191

(d) 201

The question was asked in a job interview.

This intriguing question originated from Torsionally Equivalent Shaft topic in portion Torsional Vibrations of Machine Dynamics

1 Answer

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by (38.6k points)
Correct option is (a) 171

To explain: For a single node system, the node occurs at a distance 1.146m from left end.

The polar moment of inertia = 2.36×10^6 m^4

La = 0.4356m (from same frequency relation)

Substituting these values into the frequency relation we get

f = 171 Hz.

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