TRANSIENT IMPACT DYNAMICS OF ROTOR DROP ON ROLLING-ELEMENT BACKUP BEARINGS IN A FLEXIBLE ROTOR SUPPORTED ON ACTIVE MAGNETIC BEARINGS

Transient impact dynamics of a rotor dropping on the rolling-element backup bearing with a fixed radial clearance after active magnetic bearings failure is theoretically analyzed in a flexible rotor system supported on active magnetic bearings, the effects of the backup bearing's parameters, such as support damping, support stiffness, radial clearance and impact stiffness, on the transient response are dealt with. It is shown that there are three different types of motion during rotor drop, which are oscillation motion back and forth with bounces in the bottom half of the clearance circle, full clearance whirling motion with impact and full clearance whirling motion without impact. The full clearance whirling motion with impact is caused by the partially annular dry friction during impact between the journal and the inner-race of rolling-element backup bearing, and the full clearance whirling motion without impact is produced by fully annular rubbing between the journal and the inner-race. Either the full clearance whirling motion with impact or the full clearance whirling motion without impact characterized by larger journal vibration and high contact force has a great effect on the reliability of the backup bearings and should be avoided. The rolling-element backup bearing with additional flexibility and damping can greatly improve the dynamic behavior of a rotor dropping on the rolling-element backup bearings and suppress the full clearance whirling motions with or without impact. There is an optimal region of the backup bearing's damping for suppressing the full clearance whirling motions, which depends on the rotational speed and other parameters. The higher the backup bearing's damping, or the smaller the backup bearing's stiffness, or the larger the backup bearing's clearance is, the less the possibility of occurring the full clearance whirling motions for a long time period will be.