Effect of Unbalance on a Journal Bearing Undergoing Oil Whirl

Abstract

SYNOPSIS A time-transient nonlinear analysis has been developed to examine the dynamical limit cycle motion of a plain journal bearing. The effect of unbalance and ambient pressure on the journal limit cycle motion below and above the linearized stability threshold speed is examined. The short bearing approximation is used, and the assumption of a 180° oil film has been relaxed. An arbitrary film extent based upon the instantaneous values of journal displacement, velocity, and ambient pressure is used. It has been found possible to optimize the unbalance to minimize the amplitude and the force transmitted for limit cycle operation above the stability threshold speed. Below this optimum value the whirl is fractional frequency and above this value it is synchronous. It has been found that when the journal is operating above the stability threshold speed, rotor unbalance may result in a smaller limit cycle than the balanced rotor. The effect of a non-zero ambient pressure at the optimum value of unbalance is to increase the size of the limit cycle and make the motion nonsynchronous.