Effects of Mechanical Preload and Bearing Clearance on Rotordynamic Performance of Lobed Gas Foil Bearings for Oil-Free Turbochargers

Abstract

This article presents the results of experimental studies on the effects of mechanical preload and bearing clearance on the rotordynamic performance of lobed gas foil bearings (GFBs) for oil-free turbochargers (TCs). The lobed GFBs have a first-generation bump-type configuration within a lobe-like inner surface bearing housing. Five test GFBs were fabricated and used for experiments: three circular GFBs and two lobed GFBs. The GFB clearance was measured via GFB displacement versus static load testing. The TC housing vibrations were identified via impact tests and engine driving tests, demonstrating that the TC housing was subjected to engine-induced excitations and TC housing resonances. The second-order harmonic of the engine speed appeared to be most noticeable in the rotor motions due to the engine-induced excitations. Measurements of the rotordynamic performance of the test GFBs were conducted during a series of TC rotor speed-up and slow-down tests for an oil-free test TC driven by the diesel vehicle engine. An increase in the bearing clearance of the circular GFBs led to significant reductions in the onset speed of subsynchronous (OSS) motion. In contrast, the lobed GFBs with larger preloads and for an identical minimum assembly clearance value resulted in a delay in OSS with increasing whirl frequencies. Furthermore, the lobed GFBs increased the TC speeds at engine idle, implying a reduction in the bearing friction. Consequently, the test results suggested that lobed GFBs with the proper combination of mechanical preload and bearing clearance are able to simultaneously improve the rotordynamic performance and reduce the friction losses.