An enlarged, high-temperature -compliant foil bearing has been built and tested to demonstrate the feasibility of such bearings for use in aircraft gas turbine engines. At 150 mm in diameter, this is the largest foil bearing known to date. This bearing is a scaled-up version of a patented 100-mm-diameter foil bearing, augmented by coating the foil with a proprietary high-temperature material. In a companion development, a foil bearing as described above has been combined with a 150-mm-diameter active magnetic bearing to make a hybrid foil magnetic bearing.
Foil bearings are attractive for use in some machines in which (1) speeds of rotation, temperatures, or both exceed maximum allowable values for rollingelement bearings; (2) conventional lubricants decompose at high operating temperatures; and/or (3) it is necessary or desirable not to rely on conventional lubrication systems. In a foil bearing, the lubricant is the working fluid (e.g., air or a mixture of combustion gases) in the space between the journal and the shaft in the machine in which the bearing is installed. At no or low speed, the shaft is supported at by a spring-loaded foil journal lining. Once the shaft is rotating rapidly enough, the hydrodynamic and viscous forces exerted by the flow of working fluid between the foil and the shaft force the foil away from the shaft, so that the shaft becomes supported by a film of the working fluid.
The present enlarged, high-temperature foil bearing has been tested at speeds up to 27,000 rpm (at 150 mm diameter, corresponding to a surface speed of 212 m/s) and at temperatures in excess of 1,200 °F (>649 °C). These speed and temperature limits exceed those of rolling-element bearings by several fold.
The hybrid foil magnetic bearing was conceived to take advantage of the strengths of the foil and the active magnetic bearing while utilizing each bearing to compensate for the weakness of the other, for the overall purpose of obtaining high load capacity at all speeds and temperatures (see figure). The active magnetic bearing exhibits excellent performance at low speed, where the surface coating on the foil bearing has limited load capacity. The foil bearing exhibits excellent performance at high speed, where the active magnetic bearing can fail in response to shocks and other transient disturbances.
Unlike a conventional active magnetic bearing, the hybrid foil magnetic bearing can operate without need for a separate protective auxiliary/backup bearing. In case of failure of the active magnetic bearing in the hybrid foil magnetic bearing, the foil bearing plays the role of the backup bearing, so that a rotor can continue to run on the foil bearing alone and then come down to a safe stop. The hybrid foil magnetic bearing exhibits both the high load capacity of the foil bearing and the high static stiffness and control versatility of the active magnetic bearing. Hence, the hybrid foil magnetic bearing system implements a significant advance in range of operation and reliability.
This work was done by Hooshang Heshmat of Mohawk Innovative Technology, Inc. for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Mechanics category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW- 17643-1.