Rolling contact whirling motion of a rotating shaft supported by a repulsive magnetic bearing and a rotatable backup bearing (Influences of friction coefficient and damping)

Abstract

A rotating shaft supported by a repulsive magnetic bearing may contact with a backup bearing during the passage of the critical speed, as a damping coefficient of the repulsive magnetic bearing is small. Various contacting vibrations between the shaft and the backup bearing due to the rotation of the inner ring of the backup bearing were observed in the experiment. In this paper, a theoretical model of a rotating shaft system is developed considering the freedom of rotation of the inner ring of the backup bearing. The characteristics of these contacting vibrations are clarified. This rotor model considers both slipping and rolling conditions in the contact at the backup bearing, and evaluates the transition phenomenon between them. The followings are clarified. The slipping forward whirling motion occurs when the friction coefficient is small. The rolling whirling motion occurs when the friction coefficient is large. In this case, the forward rolling motion occurs if the damping of the inner ring rotation is small, on the other hand, the backward rolling motion occurs with high whirling speed if the damping of the inner ring rotation is large. Then, the characteristics of the escapement from these contacting vibrations are also clarified and explained. Both the forward and backward rolling motions keep occurring even if the rotational speed increases to the range much larger than the critical speed. Furthermore, only the backward rolling motion keep occurring even if the rotational speed decreases to the range much smaller than the critical speed. These results were observed and confirmed experimentally. As a result, it is clarified that the friction in the contact should be low in order to escape from contacting situation rapidly and naturally after passing the critical speed.