Analysis of the Shaft Thermal Bow Induced by Rotor-to-Stator Rubs

Abstract:- The friction forces due to light rotor-to-stator rubs in rotating machines can cause a time-varying thermal bow of the shafts. Owing to this, stable partial arc rubs and full-annular rubs can induce mainly progressive changes in the synchronous vibrations of the rotor-trains. Sometimes, stable or unstable synchronous spiral vibrations can occur. The equivalent bending moments that cause the shaft bow can be estimated with model based identification methods developed in the frequency domain: these techniques minimise the error between the machine experimental vibrations and the system response provided by the model. In this paper, the results obtained by the analysis of the experimental response of a turbine-generator unit that was subjected to rotor-to-stator contacts are shown together with the strategy used to identify the causes of the rubs. The rate of the increase of the time dependent bending moments induced by the rubs, estimated in this investigation with an identification technique, provided useful information about the thermal characteristics of the system which should be modelled by means of equivalent parameters in more complex and accurate methods in which the motion and thermal equations of the fully assembled machine are integrated in the time domain. Key-Words: rotating machine vibrations, rotor-to-stator rubs, spiral vibrations, diagnostics, fault identification