Forced nonlinear response of gas foil bearing supported rotors

Microturbomachinery implements gas foil bearings (GFBs) in oil-free compact units with reduced maintenance and lower life cycle costs. Challenges for GFBs include intermittent contact and wear at startup and shutdown, and potential for large amplitude rotor whirl at high-speed operation. Subsynchronous motions are common in FBs, though hastily attributed to hydrodynamic bearing instability. In actuality, an FB load capacity depends mainly on its support structure, which shows a strong hardening effect. Presently, an FB force is modeled as a third-order structural element with nonlinear stiffnesses derived from measurements. Predictions of the performance of a rigid rotor supported on bump-type FBs and comparisons to rotor response measurements follow. The predictions evidence a Duffing oscillator dynamic behavior with multiple frequency responses, sub- and super-harmonic, within certain ranges of rotor speed. Predicted rotor amplitudes replicate accurately the measured responses, with a main whirl frequency locked at the system natural frequency. The predictions and measurements validate the simple FB model, i.e. a minute gas film with effective infinite stiffness, with applicability to large amplitude rotordynamic motions. For the first time in the open literature, a simple physical model reproduces the richness and complexity of measured rotor-GFB motions.