Influence of Unbalance Levels on Nonlinear Dynamics of a Rotor-Backup Rolling Bearing System

Rotor drops in magnetic bearing and unbalance in rotors have been objective of study for many years. The com- bination of these two well-known phenomena led to an interesting chaotic response, when the rotor touches the inner race of the back-up bearing. The present work explores the nonlinear rotor-backup-bearing dynamics both theoretically and experimentally using a fully instrumented test rig, where the position of shaft, its angular velocity and the contact forces between the shaft and the backup bearing are sampled at 25 kHz. The test rig is built by a removable passive magnetic bearing, which allows for simulation of magnetic bearing failure (loose of carrying capacity and rotor fall). A theoretical approach is given beforehand and supplies the basis of the study. Finally the presented results are commented on the point of view of nonlinear dynamics applied to the practical use. The theoretical and numerical analyzes are shown through Poincaré maps and double sided spectrum. The latter is important to characterize the condition at different levels of unbalance between forward of backwards whirl. Our preliminary results indicate that for small levels of unbalance the rotor oscillates at the bottom of the backup bear- ing. When the levels of unbalance increase, the dynamical behaviour of the rotor changes, leading to extremely harmful conditions, since the rotor can be lifted from the bottom of the bearing (contact state) and return, starting to impact on the backup inner race innumerable times without reaching a steady state.