1990-07-01

Foil Gas Bearings for Turbomachinery 901236

Hamilton Standard has developed and patented a new generation of foil journal and thrust bearings for use in high speed turbomachinery. These bearings are presently being used and are in production for air cycle machines used in environmental control systems on many civil and military aircraft. On the Boeing 747 aircraft, the foil bearing air cycle machines have accumulated over one million flight hours. The latest model 747 foil bearing air cycle machine has demonstrated an MTBF (mean time between failure) in excess of 100,000 hours. The present paper describes the Hamilton Standard journal and thrust foil bearing concept, its advantages, and its applications. Planned work to be pursued in the near future is also described.THE ENVIRONMENTAL CONTROL SYSTEM (ECS) on the Boeing 747-400 airplane, in addition to heating and cooling the passenger compartment, pressurizes the cabin and, above all, keeps the passengers comfortable. The heart of the ECS is a compact high speed turbomachine called the air cycle machine (ACM) that circulates fresh air into the cabin through various heat exchangers and valves. This ACM has a turbine, a compressor, and a fan all mounted on a single shaft which usually spins at about 40,000 rpm. The shaft, however, is supported only by a thin film of air circulating between the shaft and a set of thin metal foils. There are no ball bearings, no oil pump, and no externally pressurized air. The film of air that supports the shaft is created only by the rotation of the shaft itself. There are two foil journal bearings to prevent radial displacements and two thrust bearings to limit axial motion. The journal bearings have three layers of thin foils that impart a small preload on the shaft when the machine is not rotating. During starting, the foils get separated from the shaft due to the self-generated pressure, and the shaft becomes fully supported on a cushion of air at about 1500 rpm. The thrust bearings have a pair of washer-shaped bearing plates on which thin sector shaped foils are welded. These foils form an air wedge between the bearing and the shaft disk. When the shaft is stationary, the foils are touching the shaft disk, but as soon as the shaft starts turning, the foils lift-off and the shaft becomes airborne. This is the simple principle behind Hamilton Standard foil bearings, even though the implementation may be complex.

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