Question

A machine that contains an internal rotor is supported on four steel springs that possess negligible damping. The natural frequency of the machine-spring system under vertical vibration is 150 cycles per minute. When the machine is switched on, the rotor generates a vertical harmonic force. The amplitude of steady-state vertical displacement of the machine under this load is 0.509 in when the rotor is operating at 20 rpm (rotations per minute); 1.136 in at 180 rpm; and 0.0333 in at 600 rpm. Calculate the amplitude of steady state vertical displacement of the machine if the steel springs are replaced by four rubber isolators that provide the same stiffness but introduce damping equivalent to ζ = 20 % for the entire system. Comment on the effectiveness of the isolators at various rotor speeds.

Answer 1