Torsional Vibration Analysis

Vibrations analysis of rotating machinery offers abundant information about failure root-causes and asset condition, as well as aiding early failure detection and prognosis. Vibration based condition monitoring normally focuses on lateral vibrations, as measurement techniques and technologies are highly developed and standardized, in comparison with torsional vibration measurement. However, the later describes the effects of the main forces acting on rotating machines and the primary shaft function, i.e. transmitting torque.

Torsional vibration measurements allow engineers to better understand the dynamics of their rotating equipment, enabling them to troubleshoot and/or validate performance.

With a better understanding of the reasons and methods to measure torsional vibration, the right instrumentation, and a helpful partner, you will become better positioned to make the decisions you need to decrease downtime and increase productivity.

What is torsional vibration?

Torsional vibration is angular vibration that occurs about the axis of a shaft. It is different than lateral vibration (which occurs in the radial direction) and axial vibration (which occurs along the shaft length). Torsional vibration involves speed fluctuations of various components and the twisting of shaft sections while the machinery is rotating.

Torsional vibrations are quite often a source of issues and faults on the rotating shafts.

Torsional vibrations are evaluated as the variation of rotational speed within a rotation cycle. RPM variations are typically induced by a rough driving torque or a varying load.

Torsional vibration will vary depending on the system’s characteristics and the specific operating conditions (torque effort curve) and any changes to these factors can result in excessive torsional vibration.

Torsional vibrations issues

Excessive torsional vibration and resonance can lead to failures of crankshafts, couplings, fans, gears, engine dampers, compressor oil pumps, auxiliary equipment, and more. These failures typically occur at a 45-degree angle to the shaft axis. Unfortunately, torsional vibration problems may not be apparent until after a failure occurs. The costs to repair such equipment can easily exceed $1,000,000 in parts, labour, and downtime.

Think that over 80% of all torsional systems require system modification during the design stage to reduce the risk of failures.

A cracked coupling due to excessive torsional vibration on VFD motor – ID fan [Photo credit: Engineering Dynamics Inc (EDI)]

What is torsional vibration analysis (TVA)?

Some people use the term torsional vibration analysis (TVA) to mean taking measurements. However, TVA more commonly refers to calculations performed with a computer program. Some industries such as oil and gas require TVA of new equipment per American Petroleum Institute (API).

All rotating shaft systems have torsional vibrations to some degree. Operation on a torsional natural frequency can cause shaft failures without noticeable noise or an obvious increase in the lateral vibrations. In geared systems, however, gear noise may occur that can be a warning of large torsional oscillations. Therefore, it is important to ensure that all torsional natural frequencies are sufficiently removed from excitation frequencies.

A torsional analysis should include the following:

• Calculation of the torsional natural frequencies and associated mode shapes.

• Development of an interference diagram that shows the torsional natural frequencies and the excitation components as a function of speed.

• Calculation of the coupling torques to ensure that the coupling can handle the dynamic loads.

• Calculation of shaft stresses, even if allowable margins are satisfied.

• Calculation of transient torsional stresses and allowable number of starts for synchronous motor drives.

Torsional natural frequencies are a function of the torsional mass inertia and the torsional stiffness between the masses. The natural frequencies and mode shapes are generally calculated by the Holzer method or by eigenvalue-eigenvector procedures. Either of the methods can give accurate results. A good design practice would be to locate the torsional natural frequencies a minimum margin of 10% from all potential excitation frequencies.

For example, mass-elastic data are used to calculate torsional natural frequencies (TNFs) and mode shapes, interference diagram, and forced response. Other industries such as municipalities may specify analysis in the design stage and testing during commissioning of water pumps.

Torsional Vibration - illustration

Why is torsional vibration analysis & torsional vibration testing important?

Torsional vibration and testing may be necessary to determine separation margin from torsional natural frequencies (TNFs) or to verify previous calculations. If a torsional problem is found, the system may need to be modified. Once the mass-elastic model has been normalized to match the measurements, the torsional vibration analysis software can be used to evaluate any proposed solutions. To avoid torsional resonance, this may require changing the torsional stiffness of the coupling, the inertia of a flywheel, etc.

Torsional vibration measurements help engineers increase the reliability and safety of rotating equipment.

What equipment is needed for torsional vibration testing?

An accelerometer is used to measure lateral vibration, but special equipment is needed to measure torsional vibration. Torsional vibration can be measured using a torsiograph, encoder, or laser vibrometer. These devices will determine angular oscillation and/or angular velocity. Some people have even detected high torsional of a disc pack coupling vibration using a simple strobe light to view warping or “pop canning” of the elements during operation.

What are common applications that require torsional vibration analysis & torsional vibration testing?

Situations where torsional testing may be required could include the following:

• If a failure of a component occurs, testing of the repaired system is recommended to investigate the cause(s).

• If a system poses unusually high risks to life, other machinery, or plant processes, testing should be performed to ensure reliable operation. This could include a prototype machine or an existing model operating at higher speeds or pressures than before.

• This could include systems with torsionally soft rubber couplings and/or wide speed ranges or operating. If many assumptions had to be made in the torsional analysis phase due to lack of drawings and technical information, testing should be used to confirm the results.

• Newly designed systems that will be mass produced should be tested. It is much easier to correct a problem with an initial unit at the factory than it is to retrofit many units that have already been shipped to customers.

• Systems that have been modified or put into a different service, such as re- staging and/or changing operating conditions, should be re-analyzed.

• Many municipalities have specifications that require torsional vibration testing of new units by a professional.

In industrial applications, the most common equipment where torsional vibration testing is important is with variable frequency drive (VFD) motors driving large inertia fans and reciprocating engines/compressors. For VFD’s, problems can occur due to tuning of the drive. Reciprocating engines and compressors can have much higher excitation than rotating machinery. Having a wide operating speed range will be more likely to encounter a torsional resonance.

In marine applications, torsional vibration testing is often required on propellar shafts to troubleshoot propulsion problems or to quantify excessive vibrations predicted in a computer torsional vibration model. Torsional vibration measurements are often required to meet meet certain classifications for new build or re-powered ships. Section 7.9 of the ABS Guidance Notes on Vessel Vibration highlights: “If measurement is conducted as per 4-3-2/7.5.8 of the ABS Steel Vessel Rules, torsional vibration measurements are to be taken either at the free end of the propulsion machinery, using a suitable torsional vibration transducer, and/or on the main shafting, using strain gauges. Alternatively, depending on the system characteristics, a mechanical torsiograph, driven from a suitable position along the shafting or free end, may be used for this purpose. “

How is torsional vibration testing performed?

Testing should be performed during startup, shutdown and over the range of operating conditions. Time wave forms can be helpful to determine transmitted torque and overall alternating torque. Time wave forms can also be used to capture peak torque during a transient event such as synchronous motor startup or emergency shutdown (ESD) of a reciprocating compressor. Appropriate sampling rate needs to be used to capture the data. For example, if a telemetry system is set to 0-500 Hz range, then the data is typically sampled at 5000 Hz with the data acquisition (DAQ) system. Fast Fourier transform (FFT) is needed to determine the frequency content of the signal. By varying the operating speed and making a waterfall plot, the TNFs can be determined. A waterfall plot can also be made during a slow startup or unloaded coastdown.

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