Unbalance Response of Statically Indeterminate Rotors Supported on Hydrodynamic Journal Bearings: Use of the 32 Dynamic Coefficients Bearing Model

摘要

Linear vibration analyses of statically indeterminate rotors supported on hydrodynamic journal bearings demand large amounts of numerical work in order to calculate the static equilibrium positions of the journals relative to the respective bearings at different values of rotating speed and the corresponding stiffness and damping dynamic coefficients. As shown by the author in a previous paper, all that work might be useless if the wrong bearing model is adopted in the unbalance response calculation. In that paper, it was verified that the traditional 8-coefficient bearing model is inadequate for the unbalance response calculation of single spam rotor systems supported on hydrodynamic journal bearings placed close to nodal points of excited modes of vibration. In such situations, one cannot neglect the time varying tilt angle between journals and bearings, the consideration of which leads to the adoption of a 32-coefficient bearing model. A similar analysis is conducted in this work, focusing on statically indeterminate rotors. Numerical results, related to a large turbo set, indicate that the differences between vibration amplitudes calculated using both bearing models can be greater than 80%, while discrepancies in the predicted stability thresholds are small. The conclusions of the study are coherent with previously published theoretical and experimental results.