Development of Foil Bearings for Small Rotors

Lubricant free high speed turbo-machineries are one of the emerging fields in the gas turbine technology. Foil bearings are the major contenders in the lubricant free bearings due to their ability to support significant loads at very high speeds. The paper deals with the various stages in the development of discrete and continuous bump foil bearings and testing of the same for designed speeds and loads. Development of bumps involves determination of bump geometry for the desired load capacity, design of special purpose dies for the fabrication of corrugated sheets, identification of suitable bump material and evolution of heat treatment process. Here Beryllium–Copper (Be-Cu) is used as a bump material because of its self-lubricating property and good mechanical strength. The clearance between the shaft and top foil can be adjusted by providing the back-up foils between the encircling foil and bump foil. The rotor system simulating the weight of a typical micro gas turbine is designed and fabricated. The foil bearings developed are tested under this simulated load conditions at speeds above 50,000 rpm. The results obtained show that the rotor is completely airborne at speed slightly above 9000 rpm and at higher speeds the rotor is stable.