Theoretical Study of Top Foil Sagging Effect on the Performance of Air Thrust Foil Bearing

Abstract

Air thrust foil bearings are used in small turbomachinery to support axial load. Typically, thrust bearings are designed with certain amount of taper from the leading edge until flat land area with uniform clearance. Therefore, the bearing performance is affected by many factors such as taper ratio, taper height, configuration of structural support, top foil thickness, etc. The most popular form of structural support is a corrugated array of bumps in either circumferential or radial direction, and many thrust foil bearings are manufactured with the bump foils in the land area only. Because the taper region does not have the bump foils and hydrodynamic pressure begins to build from the taper region, certain amount of top foil sagging in the taper area is inevitable. This paper studies the effect of the top foil sagging in the taper region on the static performance of the thrust foil bearings. The top foil is modeled as a 2D plate, and finite element method is used to predict the sagging effect of the top foil and coupled with finite difference method to solve Reynolds equation. Hydrodynamic pressure, top foil deflection, minimum film thickness, and power loss with different top foil thicknesses are calculated. Simulations show that under the identical external load, thin top foil allows very large sagging in the taper area resulting in abrupt change of film thickness around the beginning of land area accompanied by larger peak pressure and power loss and smaller minimum film thickness compared to the case of thicker top foils. Further studies with various top foil thicknesses and full bump supports in the taper region give insight to the design principle of thrust foil bearings with various sizes.