The torque-limited touchdown bearing system (TLTBS) is a backup mechanical- bearing system for a high-speed rotary machine in which the rotor shaft is supported by magnetic bearings in steady-state normal operation. The TLTBS provides ballbearing support to augment or supplant the magnetic bearings during startup, shutdown, or failure of the magnetic bearings. The TLTBS also provides support in the presence of conditions (in particular, rotational acceleration) that make it difficult or impossible to control the magnetic bearings or in which the magnetic bearings are not strong enough (e.g., when the side load against the rotor exceeds the available lateral magnetic force).

The Portion of the TLTBS at One End of a Rotor Shaft includes a ball-bearing assembly and drive cones that can be either mated or separated by asmall clearance as shown here.

The TLTBS includes two similar or identical subsystems, each located at one end of the rotor shaft (see figure). Mounted inside each female cone is a specially designed high-speed bearing with a built-in lubrication system. Mounted on the stator is (1) a non-rotating drive cone capable of mating with the drive cone on the rotor and (2) an electromagnetically actuated inserter mechanism that moves the stator drive cone axially between two extreme positions as described in more detail in the next paragraph. The inserter mechanism contains two electromagnets: one that withdraws the spring-loaded plunger, the second to withdraw the latch when insertion is required. Electric power is applied to the inserter mechanism only during the fraction of a second needed for the axial motion: no power is needed to keep the stator drive cone latched at either extreme position.

In one extreme axial position, denoted the inserted position, the stator drive cone is in contact with the outerbearing- race drive cone under spring load. In this position, the ball-bearing assembly carries the full bearing load.

The other extreme axial position (the one shown in the figure) is denoted the retracted position. In this position, the stator drive cone is withdrawn from the rotor drive cone to an axial clearance of 0.010 in. (≈0.25 mm). During normal operation in the retracted position, the shaft is fully supported by the magnetic bearing. The clearance between drive cones is small enough that if the magnetic bearing fails or if an excessive side load occurs, the ball-bearing assembly can provide full support, preventing damage to the magnetic suspension and making it possible to continue (at least temporarily) operation of the machine.

Because the stator drive cone does not rotate and the rotor drive cone rotates at high speed during normal operation of the magnetic bearing, it is necessary to accommodate, and prevent frictional damage by, the slip that occurs between the two drive cones at the time of initial contact. For this purpose, the mating surfaces of the drive cones are coated with dry lubricant films. In addition, the ball-bearing assembly contains a reservoir designed to dispense lubricant to support an elastohydrodynamic film for the specified lifetime of the system.

This work was done by Edward P. Kingsbury, Robert Price, Erik Gelotte, and Herbert B. Singer of The Bearing Consultants, LLP for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Ohio 44135.

Refer to LEW-17282.

Motion Control Tech Briefs Magazine

This article first appeared in the April, 2003 issue of Motion Control Tech Briefs Magazine.

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