Finite Element Analysis of Bump Forming Process on Thin Phosphorus Bronze Foils Used in Complaint Gas Foil Bearings

Abstract

Gas bearings are used in high-speed turbomachines such as turboexpander to avoid contamination of process gas. The low viscosity of gases uses as a lubricant in gas bearings lowers the rotor dynamic properties such as bearing stiffness and damping. Use of compliant gas foil bearing is a solution to overcome above issue by use of thin spring-like corrugated bump shaped foils in the gas bearings. These compliant gas foil bearings are considered as precision components of high-speed turbomachinery as the radial clearance of these bearings are within 20 to 40 m. So precise fabrications of the corrugated thin bump foil are an essential part in the manufacturing stage of gas foil bearings. The thin bump foils are nearly 100 m thick and with a bump height of 500 m. The bumps are fabricated by forming operation using metal dies. The major problem encountered during the fabrication of thin bump foils using a forming process is the spring back and damage. The spring-back of the thin foils during forming operation destroys the preciseness of the bearings as it affects overall bearing clearance. Current work explains the simulation of forming process using finite element methods to predict the stress distribution, total deformation, damage and spring back. The foil material uses in the investigation is phosphor bronze and metal dies are SS302. The simulation is performed for conditions such as by varying top die loading and unloading speed. The parameter which guides the deformation process and controls the spring back majorly is the stroke speed of top die. The accurate speed for the forming operation is found to be 4.8 mm/s for the current application. The forming process above this speed results lesser spring-back but possibility of increasing damage and thinning of foils. The current work also proposes suitable conditions to design metal dies, where the spring back effect can be minimized.