Analytical and experimental investigations of hybrid air foil bearings

Air foil bearings offer several advantages over oil-lubricated bearings in high speed micro-turbomachinery. With no contact between the rotor and bearings, the air foil bearings have higher service life and consequently lesser standstills between operations. However, the foil bearings have reliability issues that come from dry rubbing during start-up/shutdown and limited heat dissipation capability. Regardless of lubricating media, the hydrodynamic pressure generated provides only load support but no dissipation of parasitic energy generated by viscous drag and the heat conducted from other parts of the machine through the rotor. The present study is a continuation of the work on hybrid air foil bearings (HAFB) developed by Kim and Park, where they present a new concept of air foil bearing combining hydrodynamic air foil bearing with hydrostatic lift. Their experimental studies show that HAFB has superior performance compared to its hydrodynamic counterpart in load capacity and cooling performance. In this article, the bearing stiffness and damping coefficients of HAFB are calculated using a linear perturbation method developed for HAFB. The study focuses on circular HAFB with a single continuous top foil supported by bump foil. The research also includes a parametric study which outlines the dependence of the stiffness and damping coefficients on various design parameters like supply pressure ( P s ), feed parameter ( ? s ), excitation frequency (v), and bearing number (?). Furthermore the present research also includes experimental investigation of HAFB with bump foil as compliant structure. In the first phase of the experimental research a high speed test facility was designed and fabricated. The facility has the capability of running up to 90,000 RPM and has an electric motor drive. This article gives detailed description of this test rig and also includes data acquired during the commissioning phase of the test rig. The test rig was then used to measure the load capacity of HAFB.