Experimental Identification of Force Coefficients of Large Hybrid Air Foil Bearings

Foil bearing technology using air or gas as a lubricant has been around since the mid-1960s, and it made significant progress in its reliability, performance, and applications. Even if significant progress has been made to the technology, the commercial applications to relatively large machines with journal shaft diameter bigger than 100 mm was not reported. This paper presents dynamic characteristics of a hybrid (hydrodynamic + hydrostatic) air foil bearing (HAFB) with a diameter of 101.6 mm and a length of 82.6 mm. The test rig configuration in this work is a floating HAFB on a rotating shaft driven by electric motor, and the HAFB is under external load. HAFB stiffness coefficients were measured using both (1) time-domain quasi-static load-deflection curves and (2) frequency-domain impulse responses, and HAFB damping coefficients were measured using only impulse responses. The HAFB direct stiffness coefficients measured from both methods are close to each other in the range of 4∼7 MN/m depending on speed, load, and supply pressure, but frequency domain method shows larger scatter in the identified coefficients. HAFB coefficients simulated with the linear perturbation method using a bump stiffness matched to the load-deflection characteristics at 18,000 rpm show reasonably good agreements with experimentally measured values.