Measurement of torsional vibration

When access is good, measuring torsional vibration can be an apparently trivial task with a twin-beam laser providing almost instant results. ISVR Consulting specialises in the more difficult measurements, such as those where on-road phase locked transient torsion is required from a number of components, or where access is poor. This usually involves fitting some sort of optical or even inductive probe to the required shafts, and we have a range of optical sensors available. In-house software is used to rapidly produce disposable encoder disks for our high speed laser driven fibre optic reflective sensor which was also developed in house. In some cases, low cost non-invasive testing can be carried out on a client’s vehicle within a working day.

The use of precision 1000 slot optical encoders enables the minute motions associated with the polygonal effect of a chain sprocket to be easily resolved.

An often overlooked issue is that many rotating components have bending motions that can be as great as those due to torsion.  These two components may be separated by using two or more detectors simultaneously, and the time history or spectra of torsion and bending movement may therefore be quantified individually.

Elaborate investigations may involve phase-locked measurement of bending and torsion from a number of shafts during an on-road transient.

Clock signals from the sensors can be captured using a high-speed data acquisition system and the torsional motion is then derived entirely by software. An “Instantaneous speed detector” has been developed for use when higher speed and high resolution is required.  This detector produces a real-time output corresponding to instantaneous velocity. A great strength of this system is that standard data acquisition systems can be used to capture this signal together with any associated and resulting noise and vibration measurements. This is particularly suitable for quantifying the effect of torsional impacts.

Graph: wind up of belt system during engine start up

Measured "wind up" of part of a Front End
Accessory Drive (FEAD) belt system during
an engine start up

Photo: torsional measurements

Non-invasive torsional measurements