Linear stability analysis of an unbalanced rigid rotor supported by nonlinear journal bearings is undertaken using the Floquet theory, and verified using the numerical integration method. For small magnitude of rotor unbalance and operation of journal bearing in the lightly to moderately loaded regime, the threshold of instability was found to be almost similar to that predicted for the balanced rotor, whilst for moderate to large values of rotor unbalance, the instability threshold values were observed to be much higher than that predicted for the balanced rotor. For journal bearings operation in the highly loaded regime, the instability threshold values of the unbalanced rotor were observed to be lower than that of the balanced rotor. The rotor unbalance magnitude required to suppress journal bearing instability is, however, considerably large, exceeding the permissible unbalance level for rigid rotor. 

RTC 034