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Plain journal bearings, both with oil and air lubrication, are known to become unstable above a certain speed ChildsD. Such operation is accompanied with rotor-bearing instabilty such as Oil whip. Other bearing designs with non-circular geometries such as spiral grooves, multiple lobes, compliant foil or tilting pads have been proposed to overcome this issueTimothyReview::2011. Oil lubricated tilting-pad bearings (TPB) are the “state-of-the-art” technology, extensively used for the support of compressors, pumps and turbines because they are reliable and provide a greater stability in high speed applications both for steady state and transient operating conditions, due to their possibility to follow the rotor motion ChildsD. Each pad of a TPB is able to direct its load component through the journal center by rotating about a pivot thus attaining its own equilibrium position. The destabilizing forces which cause the whirl instability are greatly reduced or even completely eliminated, if the effects of pad inertia and pivot friction are neglected Lund1964. If the bearing pads are symmetrically arranged, the bearing practically has zero cross-coupling stiffness, and thus is inherently stable IAGT::2005. Tilting-pad bearings produce very low cross coupling stiffnesses than the direct stiffnesses, thus increasing the rotor bearing critical speeds and delaying the onset of whirl. They render a possibility to generate a converging gap profile even for the journal concentric position by applying the so called preload (cf section subsec:Bearing-Preload) TimothyReview::2011. The number of pads, preload, pivot offset and load direction can be varied to achieve the desired performance. The transfer of this technology to gas lubricated systems is the subject of current research and scientific discussion.

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