Effects of a Mechanical Preload on the Dynamic Force Response of Gas Foil Bearings: Measurements and Model Predictions

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Abstract
Gas foil bearings (GFBs) enable efficient, reliable, and maintenance-free operation of high-power-density microturbomachinery (30 krpm. Rotors supported on bump-type GFBs, however, often show large amplitude subsynchronous whirl motions, albeit reaching limit cycles. Presently, commercial GFBs are modified to add a mechanical preload, which induces a hydrodynamic wedge to generate more pressure supporting larger loads. Three metal shims inserted under the bump-strip layers and in contact with the bearing housing create a multiple lobe clearance profile at a very low cost. Shaft motion measurements of a test rotor supported on the GFBs, original and shimmed, are conducted while the rotor accelerates/decelerates to/from 50 krpm and for increasing magnitudes of feed pressure into the air bearings. The shimmed GFBs produce an increase in the rotor–bearing system natural frequency, as expected, and also act to delay the onset speed of large amplitude subsynchronous whirl motions. The bearing feed pressure also aids to reduce the amplitude of rotor whirl motions appearing at speeds above the system critical speed. Most importantly, the nonlinear response induced by the GFBs—i.e., large amplitude rotor subsynchronous whirl—is a forced nonlinearity exacerbated by increasing rotor imbalances. For rotor speeds within the linear operation range (<25 krpm), predictions of rotor imbalance response implementing linearized bearing force coefficients are in good agreement with measured amplitudes for both GFB configurations, original and modified. The predictions and measurements demonstrate the beneficial effect of a mechanical preload on the rotordynamic performance of GFBs.

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