Nonlinear restoring force characteristic and vibration of a rotating shaft supported by a repulsive magnetic bearing (Influences of both the axial displacement and the magnetic length ratio)

A restoring force of a repulsive-type passive magnetic bearing is essentially nonlinear. It may cause various kinds of vibrations in the rotating shaft supported by the repulsive magnetic bearing because the damping coefficient of the repulsive magnetic bearing is small. In this paper, the influences of both the length ratio and the axial displacement of the magnets on the restoring force characteristic of the repulsive magnetic bearing are investigated. As the result, followings are clarified: In the case that the length-difference of the inner and outer magnets is small, the system has the hard-spring nonlinear characteristic when the axial displacement is small, and it changes to the soft-spring nonlinear characteristic when the axial displacement increases. On the other hand, in the case that the length-difference of the inner and outer magnets is large, the system has the soft-spring nonlinear characteristic when the axial displacement is small, and it changes to the hard-spring nonlinear characteristic when the axial displacement increases. These results were validated experimentally. As a result, this paper clarified that the characteristics of both the linear and the nonlinear stiffness can be designed, and particulary the linear system can be realized, by adjusting both the axial displacement and the magnet lengths of inner and outer rings. © 2011 The Japan Society of Mechanical Engineers.