Investigation of non-linear phenomena in rotor-damper assemblies

Abstract

The research, sponsored by Rolls Royce plc and the SERC, has involved a three bearing rigid motor assembly incorporating flexible, spring supports and unsealed squeeze-film dampers. A range of practical assemblies involving, at first, one and then two squeeze-film dampers with different static misalignments were studied. Non-linear jump phenomena and subharmonic resonances have been demonstrated both experimentally and theoretically for a range of rotor unbalance. The effects of varying oil supply have been demonstrated. Theoretical investigation has modelled the unsealed squeeze-film damper with the Short Bearing approximation, providing a variable film extent with, for the most part, an absolute zero cavitation level. The equations of motion were solved by applying a Runge-Kutta method and resulting discrete time series have been used for frequency analysis. The practical use of analytical linear approximation has been assessed. Frequency analysis of the experimental and theoretical vibration signals has led to the construction of waterfall diagrams which present the variations in the spectral content of the vibration throughout the speed range. Spectral analysis has indicated the effects of misalignment on the nature and severity of subsynchronous resonances and jump phenomena. In particular, the development of strong subharmonic resonance due to the introduction of static eccentricity to a squeeze-film damper is demonstrated. Also, the distribution of rotor weight between squeeze-film dampers with differing supports has been shown to have a marked effect on the assembly's vibration response, particularly with respect to any jump phenomena that are excited. The research has demonstrated that it is possible for the vibration signatures of similar rotor-bearing assemblies to be strongly contrasting due to different accumulations of tolerances during manufacture, fitting and operation. The practical implications are discussed. Methods of linearisation are discussed so that the non-linear characteristics of the squeeze-film damper might be rapidly estimated with reasonable accuracy. Such techniques should prove to be valuable in extending the capabilities of existing design analysis software which is currently employed to predict the vibration characteristics of complex turbomachinery assemblies.

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