The Dynamic Stability of Rotor/Stator Radial Rubs in Rotating Machinery

The classic phenomenon of “dry friction whip,” generally associated with unlubricated journal bearings, is here reconsidered as playing an important role in the dynamic stability, and consequent integrity of radial rubs in all close clearance rotating machinery, particularly in turbomachinery elements such as labyrinth seals and blade tips. A simple analysis is completed for large (runaway) amplitudes of whipping on an analytic model which includes the stator as an independent dynamic system. Whirl frequencies are computed as a function of rotor and stator natural frequencies and damping. A stability criterion is developed as a function of these same variables. Testing on a simple experimental model gives general qualitative agreement to predicted trends, but is not conclusive quantitatively, probably because of the difficulty in simulating pure Coulomb friction at the rubbing interface. The simple generality that can be inferred from the set of derived stability criteria is that the broadest band of whip-free rubbing is achieved if rotor and stator dampings are made close to one another, and if the rotor and stator natural frequencies are kept dissimilar. Systems with identical rotor and stator natural frequencies are always unstable, and will whip at that same natural frequency. Systems with large stator damping will whip at rotor natural frequency. Systems with large rotor damping will whip at stator natural frequency.