Non-linear vibration transmission through rolling-element bearings in rotating machines

Abstract

Modern aircraft engines are pushing established design methodologies to the limits, in order to satisfy current environmental and economic constraints. This calls existing assumptions into question, such as modelling rolling-element bearings (REB) with linear springs, which cannot accurately predict the transmission of vibrations from one rotor to another, known as ``cross-shaft coupling''. This is further complicated by the inherent non-linearities present in REBs. An improved understanding of the mechanisms governing this phenomenon is required, in order to be able to better diagnose such vibration problems and develop mitigating strategies.

An in-depth numerical study was first carried out, analysing the nonlinear dynamic response of a Jeffcott rotor supported by an REB. A physical bearing model was developed based on the existing literature, which could capture the effect of a wide range of parameters, and the response was computed using a novel application of the Harmonic Balance Method. A strongly nonlinear response was observed due to the bearing, with large shifts in the resonance frequency with excitation amplitude. An extensive parameter study was then carried out, with a particular emphasis on providing physical explantations for the observed behaviour.

These results were then investigated using an existing rotordynamic rig. A well-correlated model of the rig was first developed and parameterised with extensive component-level testing. A separate bearing test rig was designed and built, which was used to validate the bearing model and to identify some key parameters. The focus then moved onto the unbalance response of the rotor rig. The bearing model was found to accurately predict both the vibration transmission to ground, as well as some of the non-linear behaviour, thereby validating the modelling approach in a rotordynamic setting.

The main outcome of this thesis was a fully validated bearing model, which is capable of accurately predicting the vibration transmission, as well as the associated simulation methods. These tools can now be applied in industry, in order to better predict cross-shaft coupling in the future.