Nonlinear Analyses in Rotordynamic Engineering

Abstract

Linear rotor-dynamic analyses such as Campbell diagrams of damped eigenvalues and unbalance response analyses are well established for the practical design layout of rotors. They are also required according to many standards such as API. Nonlinear analyses are widely avoided because of their complexity, even if they would be necessary for relevant practical answers. Sometimes questionable substitute linear analyses are carried out in such cases. In this paper four cases requiring nonlinear analyses are described: A vertical pump with water lubricated bearings, a turbocharger with semi-floating oil lubricated bearings, an electric motor with rolling element bearings running through a resonance and a Pelton turbine on tilting pad bearings losing two buckets. The vertical pump is linearly unstable, because of the unloaded bearings. The nonlinear analyses are necessary to receive the limit cycles of the unstable system. In case of the turbocharger the outer oil film of the semi-floating ring bearing is highly nonlinear and cannot be correctly described linearly. In case of the motor running through a resonance the dynamic bearing loads are very high, because the rolling element bearings are not able to provide much damping. The behavior then becomes nonlinear. Moreover, the bearing clearance can lead to nonlinear behavior, if the bearings are not preloaded. The blade loss for the Pelton turbine leads to nonlinear behavior due to the high dynamic bearing load.