Abstract: Transient response of rotor dropping on a rolling element backup bearing with a fixed radial clearance in a flexible rotor supported on active magnetic bearings after active magnetic bearings failure is theoretically analyzed, the effects of the rolling element backup bearing’s parameters, such as support damping, support stiffness, radial clearance of the backup bearing and impact stiffness, on the transient response are dealt with. It is found that there exist three different types of motions during rotor drop, which are oscillation motion back and forth with bounces in the bottom half of the clearance circle, full clearance whirl motion with impact and full clearance whirl motion without impact. The full clearance whirl motions has a great effect on the reliability of the backup bearings. The full clearance whirl motion with impact is caused by the partial annular dry friction during impact between the journal and the inner race of the rolling element backup bearing, but the full clearance whirl motion without impact is produced by full annular rubbing between the journal and the inner race of the rolling element backup bearing. There is an optimal region of support damping for suppressing the full clearance whirl motion, which depends on the rotational speed and other parameters.

Key words: rotordynamics, backup bearing, transient response, active magnetic bearing, stability