Stator Whirl With Rotors in Bearing Clearance

Abstract

In order to identify and assess the effects of stator dynamics on vibration of high speed rotating machinery, a physical model with two degrees of freedom (one for the rotor and one for the stator) has been defined. Assuming that vibratory motion, as excited by rotor unbalance, is synchronous and axisymmetric, the analysis is linear and straightforward. The calculated vibration response reveals and quantifies some new and important phenomena which are a function of the level of bearing clearance (as related to unbalance level) and of the relative values of the rotor and stator natural frequencies. In particular, the following phenomena are noted: 1. As in simpler models, bearing clearance results in a reduction in system critical speed and can result in a “rightward leaning” critical peak causing hysteresis in response on acceleration and deceleration. 2. In certain ranges of parameters, two vibration modes are possible at high supercritical rotational speeds. One, termed stator whirl, results in stator amplitudes proportional to bearing clearance which are much larger than would be expected with very tight or very loose bearings. 3. Amplitude jump phenomena are very often experienced at a rotational speed equal to stator natural frequency. 4. Hysteresis phenomena, opposite in direction to that associated with a right-leaning critical peak, can be experienced on the subcritical side of the critical peak. 5. Disconnected from the main vibration response curve, branches of stable operation are evident and are hypothesized to be realizable if the system is subjected to momentary disturbances. The results have been verified by an analog computer simulation of the same system, and the effects of component damping and friction at the bearing contact point have been evaluated. Some regimes of asynchronous nonaxisymmetric motion in the analog computer model have been sensed, even though the physical model is axisymmetric.