What Are Vibration, Torsional Vibration & Shaft Twist?

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.

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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.

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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.

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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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James Wren

Solutions Engineer and Sales & Marketing Manager at Prosig
James Wren is a Solutions Engineer and the Sales & Marketing Manager for Prosig Ltd. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with special interest in data acquisition. James is a founder member of the Dalmeny Racing team.

3 thoughts on “What Are Vibration, Torsional Vibration & Shaft Twist?

  1. Kishor Katare

    Hello team.
    i want measure torsional vibration of compressor shaft,
    i did measured pulses per revolution(PPR) on shaft,I calculated radian of a shaft, as 360/no of pulses in on revolution,now I am stuck what to next.

    1. James Wren Post author

      Hello Katare,

      Thank you for posting a message on our blog.

      There are some building blocks required for performing Torsional Analysis. The first of which is to understand the number of pulses per revolution and the impact of more or less pulses per revolution.

      Depending on which if any orders you are keen to analyse the following article might be relevant
      http://blog.prosig.com/2016/10/07/rotational-order-analysis/

      Once you have captured the data from the shaft using the relevant sensors, it is then possible to process the data to find the Torsional Vibration.
      To process the data one would usually use a signal processing product, like the Prosig Rotating Machinery Software Suite. This software or similar, counts the time between the pulses and how the time between pulses varies from one pair of pulses to the next, this processing pattern repeats over the complete rotary cycle, when done this variation in time can be expressed as the torsional vibration, in for example, acceleration or displacement.

      Feel free to ask if you have further questions.

  2. Matheo

    Some really helpful information in there, thank you for sharing.
    I personally use this device Electronic dynamic balancer.
    The importance of proper dynamic balancing is often omitted.
    Lifetime and energy consumption of machinery can be optimized.
    Keep up the good work.

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