CONTACT:
Harold Simmons, P.E.
Machinery Vibration
  Services
(210) 522-2557
E-mail: hsimmons@swri.org


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KEY TERMS:
 torsional vibration
 vibration problems
 rotor train problems
 process trains 
torsional motion
 high cycle fatigue
 speed clutch
instability

Machinery Vibration Services
Machinery Torsional Dynamics

 

Natural frequencies created by the distribution of train component flexibilities and inertias cause damage when torsional critical speeds coincide with strong excitation frequencies. Process trains driven by synchronous motors or with some types of variable speed couplings can be especially sensitive to damage at resonance because of excessive excitation.

image of installed strain gage transmitter at the coupling
SwRI engineers installed a strain gage transmitter at the coupling to identify damaging critical speed in a rotor train.

Symptoms might not be obvious from normal vibration measurements, but damage can occur by fretting wear at gear teeth and couplings or by high-cycle fatigue of the shafting.

Torsional Measurements

Southwest Research Institute (SwRI) uses torsional measurements to identify torsional vibration problems and damaging critical speeds in rotor trains. If torsional motion problems are suspected, SwRI engineers can identify:

  • Speeds with maximum vibration

  • Frequency of vibration

  • Dynamic stresses and torques

  • Load vibration

  • Backlash

  • Torsional mode shapes

  • Effects of operating conditions

graphic of coupling strain signal
The coupling strain signal shows resonance and backlash as a rotor train starts up.

With proper signal conditioning, the torsional vibration effect can sometimes be detected by case-mounted accelerometers or by sensing the pinion motion in a gear box. Reliable diagnoses, however, require direct shaft measurement. SwRI offers special instrumentation techniques to measure instantaneous velocity or transmitted torque in a rotating shaft.

Torsional dynamic simulations provide enhanced diagnosis reliability and a means for evaluating design changes to resolve problems.

Effects of Torsional Critical Speed

Torsional critical speeds are commonly found below operating speed in long rotor trains, particularly those with gearboxes. If the critical speed is within the operating speed range during startup, damage can accumulate quickly. Using telemetry, SwRI engineers gather reliable data about the severity of the torsional critical speed from shaft strain instrumentation installed on the couplings. These data are used to evaluate both steady and oscillatory torque. SwRI further evaluates torsional vibration using instrumentation and techniques such as:

  • Torsiograph–a packaged torsional velocity transducer for shaft end mount

  • Torsional accelerometer–a piezoelastic or piezoresistive sensor for shaft end mount

  • Watt transducer–an instantaneous reactive power measurement tool for motors or generators

  • Casing acceleration–a tool to relate transmitted reactions at driving or driven components

  • Shaft encoder–a pulse signal demodulation technique for measuring instantaneous velocity at shaft ends or couplings

  • Gear tooth pulse sensor–a technique to measure instantaneous gear velocity by demodulation of gear tooth pulse signals

  • Pinion motion–a technique to relate torsional to lateral interaction at the gearbox

All instruments have drawbacks that limit their precision or practicality for some applications. SwRI provides a combination of sensors to improve the reliability of a diagnosis.

Variable-Speed Clutch Instability

graphic of telemetry data
Telemetry data show extreme coupling backlash and overload during stick slip instability.

Friction drive clutches can develop stick slip instability at partial speed that creates excessive torsional responses leading to clutch and coupling damage. SwRI uses torque telemetry instrumentation to correct problems and to provide precise, robust signals for a high reliability diagnosis.

For more information about machinery vibration services or using torsional measurements to identify damaging critical speeds capabilities at SwRI, or how you can contract with SwRI, please contact  Harold Simmons, P.E. at hsimmons@swri.org or (210) 522-2557.


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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 11 technical divisions.