An Improved Catcher Bearing Model and an Explanation of the Forward Whirl/Whip Phenomenon Observed in Active Magnetic Bearing Transient Drop Experiments

Abstract

Though many approaches have been proposed in the literature to model the reaction forces in a catcher bearing (CB), there are still phenomena observed in experimental tests that cannot be explained by existing models. The following paper presents a novel approach to model a CB system. Some of the elements in the model have been previously introduced in the literature; however, there are other elements in the proposed model that are new, providing an explanation for the forward whirling phenomena that has been observed repeatedly in the literature. The proposed CB model is implemented in a finite-element rotordynamic package, and nonlinear time-transient simulations are performed to predict published experimental results of a high-speed vertical subscale compressor; with no other forces present in the model, the agreement between simulations and experimental data is favorable. The results presented herein show that friction between the journal and axial face of the catcher bearing results in a forward cross-coupled force that pushes the rotor in the direction of rotation. This force is proportional to the coefficient of friction between the axial face of the rotor and catcher bearing and the axial thrust on the rotor. This force results in synchronous whirl when the running speed is below a combined natural frequency of the rotor-stator system and constant frequency whip when the speed is above a whip frequency.