Torsional Testing

 

Gas turbine analysis Reciprocating compressor maintenance

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.

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