The occurrence of spiral vibrations in rotating machines is a well-known but not very common phenomenon. However, this kind of shaft vibration, usually caused by light rubs between rotating and stationary parts, may give rise to a slow considerable increase of the amplitude of the synchronous (1X) vibration. Owing to the rubs, the normal contact forces cause a friction-induced thermal bow, which in turn determines rather slow changes in amplitude and phase of the 1X vibration vector. The curve described in a polar plot by the tip of the vibration vector is a spiral. The occurrence of expansive vibrations may cause serious damage.

Many studies about this malfunction are focused on the stability analysis of spiral vibrations. Simplified or rather rigorous thermal models can be used to evaluate the friction-induced thermal bow of the shaft and the slow continuous migration of the hot spot generated on the external surface of the rotor. However, owing to the complexity of the problem, some basic parameters of the thermal models can be affected by a significant uncertainty.

This paper shows some unconventional techniques that can provide useful information for optimizing the rotor-to-stator contact modelling as well as for tuning some critical parameters of the thermal models that affect the velocity with which the hot spot moves around the circumferential surface of the shaft. The effectiveness of these techniques is shown by means of the analysis of the experimental spiral vibrations detected in a steam turbine power unit.

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