Prediction of the Structural Stiffness of Bump-Type Gas Foil Bearings

Abstract

Gas bearings are preferable for contamination free high-speed application such as turbo-expanders, air cycle machine, turbo-compressors, etc. The major issue with gas bearings is its low dynamic coefficients such as stiffness and damping due to the low viscosity of the bearings gases. Low stiffness and damping are the major reasons for rotor instability at high-speed operation. The stiffness of a gas bearing can be tailored by use of spring-like foil structure as in gas foil bearings (GFBs). In comparison with other GFBs such as tape and leaf type, bump type GFBs are more popular among tribologists due to its ease of fabrication and assembly. The stiffness of bump type GFBs is controlled by the stiffness of smooth top foil, pressurized gas, and the bump foils. The bump stiffness predominates among three for the overall stiffness of a gas foil bearing. In gas foil bearings, the foils are very thin of the order 0.1 mm, so prediction of structural stiffness is a challenging task for researchers. Current paper explains about a detailed prediction methodology for structural stiffness of a single bump considering friction with its adjacent structures such as top foil and bearing base. The strain energy of simply supported single bump foil is determined and castigliano’s theorem is used to predict the stiffness. The behaviors of structural stiffness are further studied with various geometrical parameters of the bump foil and compared with models from literature.