Torsional vibration is oscillatory angular motion that causes twisting in shaft sections and couplings of machinery. These fluctuations are superimposed on the steady running speed. High torsional vibration can be destructive and will not usually be detected by accelerometers and proximity probes commonly used to measure lateral vibration.
Severe torsional vibration can occur with the only obvious indication of a problem being gear noise or coupling wear. Often there is no indication of a torsional resonance until a shaft fails. Common problems associated with torsional vibration problems are:
Several measurement techniques are available to measure the torsional oscillations. A torsiograph is a special instrument which can be attached to the end of a shaft to measure the oscillation. It generates a signal proportional to the velocity of the shaft which can be converted to angular displacement. An encoder, attached to the end of the shaft or to a coupling, can provide instantaneous shaft velocity. Demodulation of the pulse signal will provide angular displacement at the location of the encoder. Strain gages can be attached to the shaft to obtain a direct torque measurement. A rotating telemetry system sends the signal to a stationary receiver. EDI maintains these instruments which are used often for diagnosis of torsional related failures.