Fundamental study into the governing conditions of rotor thermal bows in hydrodynamic bearings

This thesis presents an experimental investigation into the conditions generated within a high speed bearing system where thermally generated shaft bows can arise from differential journal heating, and under certain condtions this effect has been observed to result in unstable shaft vibration - the ‘Morton Effect’. This thesis documents the development of a simplified analytical procedure for evaluating the thermal activity within an orbiting journal when running in hydrodynamic bearings. The aim of this work was to generate controlled experimental data regarding journal differential heating effects to support the understanding and development of appropriate modelling and predictive techniques. A high speed rotor test rig, running in 50mm diameter bearings of fixed and variable geometry configuration, was used to obtain directly measured temperature distributions within the rotor when running under varying speed and unbalance response conditions. Two separate rotor designs were used. The first is designed as a rigid rotor where no structurally influenced rotor dynamic phenomena are present within the running ranges. The second is a flexible rotor designed to operate in a super critical condition where conditions are replicated to provide a rotor that is sensitive to the variables required for thermal bow development and the rotordyanmic conditions promotional of associated instabilities. Existing theoretical models, in combination with operationally observed characteristics, were used to develop a design predicted to become unstable within the test running range. Journal temperature measurements were obtained for rigid and flexible rotors over a range of speed conditions in intentionally introduced mechanical unbalance conditions. Journal temperature differentials were obtained with a clear correlation between journal orbit size and journal temperature differential. The flexible rotor was operated for prolonged periods of time in the predicted unstable region but instability was never initiated for any test condition. Peak journal temperature differentials were measured as 1.7 oC. A new analytical model for the bearing oil film and journal thermal developments is presented which has reasonable correlation to other published literature.