Prediction of Stiffness and Damping of Gas Foil Journal Bearing for High- Speed Rotor

Abstract

Contamination-free bearings such as gas bearings are an alternate solution in various high-speed turbomachinery such as turboexpander and turbocharger. The major limitation of gas bearing is its low dynamic characteristics such as stiffness and damping. Gas foil bearings (GFBs) are the bearings with elastic spring-like structure in between the runner and the bearing base. These elastic structures in GFB help to tailor the dynamic coefficients of bearing by modifying foil material and its geometry. A bump type GFB is quite popular among tribologists due to its ease of fabrication and assembly compared to other types of GFB. In a bump type GFBs, the overall bearing stiffness is due to stiffness the bump foil, top foil, and the pressurized bearing gas. The overall damping inside the bearing is due to coulombs friction between (i) top and bump foil and (ii) bump foil and the bearing base. A number of numerical techniques have been successfully developed to predict the static characteristic, unlike the dynamic characteristics. This paper discusses a perturbation technique to predict the dynamic characteristic such as overall stiffness and damping of a bump type gas foil bearing. Finite difference formulation is used to determine four stiffness and four damping coefficients. The four coefficients include two direct and two cross-coupled coefficients. The influence of foil material and its geometry over the dynamic characteristics are also discussed in the paper.