Torsional Vibration Analysis

Paper No. 65

Torsional Vibration Analysis J. C. Wachel/F. R. Szenasi, 22nd Turbomachinery Symposium, Texas A&M University, September 13-16, 1993.

All rotating machinery systems experience torsional oscillations to some degree during startup, shutdown, and continuous operation. Consequently, the torsional response characteristics of rotating and reciprocating equipment should be analyzed and evaluated to ensure the system’s reliability. Severe torsional vibrations often occur with the only indication of a problem being gear noise or coupling wear. Excessive torsional vibrations can result in gear wear, gear tooth failures, key failures, shrink fit slippage, and broken shafts in severe cases. The torsional vibration response of rotating machinery components is an important consideration in defining the operational reliability of a rotary equipment train. Accurate response prediction requires analysis techniques which consider all forcing functions in the system in addition to the mass-elastic properties of the shafts, couplings, gears, impellers, etc.

The severity of the torsional oscillations and stresses depends upon the relationship between the operating speed and excitation frequencies of unsteady torques and the torsional natural frequencies and mode shapes of the shaft system (critical speeds). The difference between these frequencies is referred to as the separation margin. The magnitude of the stress also depends upon the amplification factor on resonance (damping) and the stress concentration factors.

The API Codes for Turbines, Compressors, and Pumps (611, 617, 610) specify that the torsional modes of the complete unit should be at least 10 percent below any operating speed or at least 10 percent above the trip speed. In some systems, multiples of operating speed and blade passing frequencies can excite torsional natural frequencies. These potential problems should be addressed in purchase specifications to avoid conflicts. An additional margin of at least five percent should be allowed for calculation inaccuracy in torsional analyses. Variable frequency drive motors typically operate from 20 percent to 100 percent of the motor design speed, making it virtually impossible to meet the API specification. Other types of drivers with wide speed ranges may have similar problems. In these cases where detuning is not practical, more detail analyses with particular emphasis on accurate stress prediction is essential to determine reliability.

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