There is a developing interest in torsional modal analysis of rotating machines. Advances in torsional vibration measurement include the availability of laser torsional vibrometers, that allow accurate non-contact measurements to be made on working machinery. Low cost rotating torsional exciters can be used to excite torsional vibrations under working conditions. In combination, these techniques are readily used to measure system torsional frequency response functions, operating deflected shapes and torsional damping levels. This paper is concerned with the torsional characteristics of Vee-belts and pulleys. To allow torsional vibration analyses to be made, it is important to be able to model the belt characteristics and to experimentally verify the model. A back-to-back vee-belt rig is described and experimental and modelled torsional results are presented for the rig. The arrangement used allows for the independent adjustment of belt tension, locked-in-torque and running speed. Four system torsional natural frequencies (below 400 Hz) were identified from the frequency response data and the measured FRF's and torsional operating deflected shapes are presented. Results from a torsional receptance model of the system are also presented for comparison. Good agreement was obtained between the measured and predicted results. The level of damping in the belt may be a significant contributor to the overall torsional vibration damping of a rotating system.
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