Torsional Testing
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:
- Shaft Fatigue Failures (see photo above)
- High Gear Noise
- Accelerated Gear Wear
- Gear Tooth Failures
- Damaged Couplings
- Key Deformation
- Slippage of Coupling Hubs
- Motor Winding Loosening
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.
Situations Requiring Torsional Measurements:
- Systems that have experienced previous torsional failures - Measurements can be used to diagnose the cause of the problems.
- Systems with high chance of variability - This could include systems with torsionally soft couplings and/or wide speed ranges or operating conditions.
- Used systems - Trains that have been modified or put into a different service such as re-staging and changing operating conditions should be re-analyzed or tested.
- Critical Systems - A critical system could be defined as one that poses unusually high risks to life, other machinery, or plant processes. This may be a prototype machine or an existing model operating at higher speeds or pressures than previously designed.
- Product development - Newly designed systems that will be mass produced should be tested at the factory.
