Permanent-magnet rotary bearings with ferrofluid stabilization in the axial degree of freedom are undergoing development. These bearings are totally passive, yet stable in all degrees of freedom. In contrast, previously developed electromagnet and permanent-magnet bearings all exhibit instability in at least one degree of freedom, giving rise to the need for active electronic feedback and power-control circuitry. By making active control unnecessary, ferrofluid stabilization can enable reductions in the overall sizes, weights, and power consumptions of machines that contain permanent-magnet bearings. Unlike passive magnetic bearings based on superconductivity, which are restricted to operating temperatures far below room temperature, the developmental bearings are designed to function at temperatures from 0 to 50 °C.
Figure 1 contains a simplified drawing of a shaft with typical conventional permanent magnet bearings near both ends. The repulsion between like magnetic poles of the rotor and stator portions of each bearing give rise to a magnetic levitation that is stable in the radial degree of freedom but unstable in the axial degree of freedom.
Figure 1 also depicts an elementary ferrofluid stabilizer comprising a permanent magnet immersed in ferrofluid in a container. In the absence of externally applied force, the magnet seeks an equilibrium position that is approximately central in the body of ferrofluid. When the magnet is displaced from the equilibrium position, there arises a restoring force that, if not resisted, pushes the magnet back toward the equilibrium position; that is, the levitation is stable. However, in the configuration depicted here, the restoring force, and thus the load capacity, is too small to be useful. To obtain enough restoring force and load capacity for use in stabilizing a magnetic bearing along a specified axis, it is necessary to confine the ferrofluid in a restricted cavity and optimize the permanent-magnet geometry to increase the restoring force along that axis.
Figure 2 illustrates one of many possible configurations of a shaft equipped with magnetic bearings and with ferrofluid stabilizers designed to generate enough axial restoring force to counteract the axial instability of the magnetic bearings. A prototype of this configuration has been tested and found to be stable. The tests also revealed that magnetic bearings may be useful, to some extent, as vibration isolators.
This work was done by Eliseo DiRusso of Lewis Research Center and Ralph Jansen of Ohio Aerospace Institute. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category, or circle no. 189 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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