When analyzing rotating shafts some terms are often confused. This post will attempt to explain the differences. So, what are vibration, torsional vibration and twist?
What is Vibration?
We measure at least one sensor on one channel of our measurement device to get a measure of vibration in one direction.
This one channel will measure the movement of a mechanical structure and provide the data as either displacement, velocity or acceleration. The type of sensor will dictate which of these three characteristics is measured. The simplest and most convenient sensor is an accelerometer. As the name suggests this measures acceleration.
Vibration may or may not be caused by the rotating of mechanical parts.
Vibration is usually studied in the time or frequency domain.
What is Torsional Vibration?
Torsional vibration will also usually require one measurement channel. Generally this would be a measurement from a shaft encoder or a toothed wheel with a high number of teeth. This will produce a high number of pulses for each revolution.
Torsional vibration (also known as angular vibration, transmission error (TE) or jitter) is the analysis of the torsional dynamic behavior of a rotating shaft.
Torsional vibration is different from the ‘normal’ vibration discussed above. It is the change in rotational velocity through a revolution. However, it can be expressed in displacement, velocity or acceleration.
As an example, consider a torsional system composed of a compressor, driver and coupling. This system can be modelled as a mass-elastic system (inertia and stiffness) to predict stresses in each component. The mass-elastic properties of the system can be changed by
- adding a flywheel (additional inertia)
- using a soft coupling (change in stiffness)
- viscous damping (absorb natural frequency stimulation)
The torsional vibration can be studied against time, in the angle (synchronous) domain and as orders.
What is shaft Twist?
Twist requires the measurement of at least two positions. For example, either end of a crank shaft on an automotive engine. Usually, but not always, the measurement is from a shaft encoder or a toothed wheel with a high number of teeth.
The analysis is effectively the difference between the two measurement positions. So we are looking to see if one end leads or lags the other. These are usually analysed in the time domain, often over one or two revolutions. As in the above examples this phenomenon is often analysed as orders. However, this analysis is a magnitude and not a displacement, velocity or acceleration.
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