BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to passive magnet bearings and, more particularly, to using such bearings to support a rotating shaft having a horizontal axis of rotation.

Description of Related Art Motor and generator armatures, flywheel rotors, and other rotatable components have conventionally been supported and constrained against transverse and axial movement by mechanical bearings, such as journal bearings, ball bearings, and roller bearings. Such bearings necessarily involve mechanical contact between the rotating element and the bearing components, leading to well-known problems associated with friction and wear. Even non-contacting bearings, such as air bearings, involve frictional losses that can be appreciable, and are also sensitive to dust particles. In addition, mechanical bearings, and especially air bearings, are poorly adapted for use in a vacuum.

The use of magnetic forces to provide a non-contact, low-friction bearing avoids the drawbacks attendant to mechanical and air bearings, and thus provides an attractive alternative. However, magnetic bearings

and suspension elements are subject to the constraints of Earnshaw's Theorem which, as applied to magnetic apparatus, holds that any magnetic suspension element, such as a magnetic bearing that utilizes static magnetic forces between a stationary and a rotating component, cannot exist in a state of equilibrium against external forces, e. g., gravity. In other words if such a bearing element is designed to be stable against transverse displacements, it will be unstable against axially directed displacements, and vice versa. The assumptions implicit in the derivation of Earnshaw's Theorem are that the magnetic fields are static in nature, i. e. , that they arise from either fixed currents or objects of fixed magnetization, and that diamagnetic bodies are excluded.

As a consequence, magnetic bearings are designed to be stable along at least one axis, for example, their axis of symmetry, and then external stabilizing means are used to ensure their stability along the remaining axes. The stabilizing means referred to could either be mechanical, i. e. , ball bearings, or, more commonly, electromagnetic. The latter approach uses position sensors to detect incipient unstable motion of the rotating element and magnetic coils in conjunction with electronic servo amplifiers to provide stabilizing forces to restore the element to its (otherwise unstable) position of force equilibrium. The foregoing is usually designated as an"active" magnetic bearing, in reference to the active involvement of electronic feedback circuitry in maintaining stability Less common than the servo-controlled magnetic bearings just described are magnetic bearings that use superconductors to provide a repelling force acting against a permanent magnet element in such a way as to levitate that magnet. These bearing types utilize the flux-excluding

property of superconductors to attain a stable state by properly shaping the superconductor and the magnet in order to provide restoring forces for displacements in any direction from the position of force equilibrium.

Needless to say, magnetic bearings that employ superconductors are subject to the limitations imposed by the need to maintain the superconductor at cryogenic temperatures, as well as limitations on the magnitude of the forces that they can exert.

As may be seen from the foregoing, there presently exists a need in the art for a bearing that is magnetic, yet overcomes the limitations of Earnshaw's Theorem without the drawbacks and limitations attendant to active or superconducting magnetic bearings. The present invention fulfills this need in the art.

SUMMARY OF THE INVENTION Briefly, the present invention is a passive magnetic bearing composed of two elements, one to levitate a horizontal shaft, and the other to restore the shaft to its equilibrium position if it is displaced transverse to its axial axis of rotation. The levitation element is composed of a pair of arcuate segments composed of ferromagnetic material located within an annular radial-field magnet array. The magnet array is attached to the shaft's inner circumference and rotates with the shaft. The arcuate segments remain stationary with respect to the shaft. The magnetic field of the radial-field magnet array generates an attractive force between the arcuate ferromagnetic segments and the magnet array. The arcuate segments are positioned so that this attractive force is directed vertically to levitate the shaft, and also in a horizontal transverse direction to center the shaft.

The restorative element is composed of an annular Halbach array and an annular circuit array located concentrically within the Halbach array.

The Halbach array is attached to the shaft's inner circumference, and rotates with the shaft. The circuit array remains stationary relative to the shaft. There is a repulsive force between the Halbach array and the circuit array that is induced when the Halbach array rotates relative to the circuit array. The repulsive force increases exponentially with a decrease in the radial space between the Halbach array and the circuit array, and thus acts to restore the shaft to its equilibrium axis of rotation whenever the shaft is transversely displaced therefrom.

In summary, one element of the bearing levitates and centers the shaft, while the other element restores the shaft to its equilibrium axis of rotation in the event it is displaced transversely therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross section side view of a rotatable shaft supported at each end by a passive magnetic bearing of the present invention.

Figure 2 is a side view of the passive magnetic bearing of the present invention.

Figure 3 is a cross section view taken along line 3-3 of Figure 2, that shows the restoring bearing element comprised of a rotatable annular Halbach magnet array and a concentric array of stationary close-packed shorted circuits.

Figure 4 is a cross section view taken along line 4-4 of Figure 2, that shows the levitating bearing element comprised of a rotatable annular radial-field magnet array and a stationary concentric pair of arcuate segments composed of ferromagnetic material.

Figure 5 shows an alternative embodiment of the levitating bearing element comprised of a rotatable cylinder composed of ferromagnetic

material and a stationary concentric pair of arcuate segments composed of radial-field magnets.

DETAILED DESCRIPTION OF THE INVENTION Turning to the drawings, Figure 1 shows passive magnetic bearings 11 and 13 of the present invention, supporting rotatable hollow shaft 15 at shaft ends 17 and 19, respectively. Bearings 11 and 13 provide shaft 15 with freedom of rotation about axial axis of rotation 21 of shaft 15. Shaft 15 is a simplification used herein because the primary purpose of this discussion is to explain the structure and operation of the passive magnetic bearing of the present invention. In actuality, shaft 15 is representative of most any structure which requires a rotational degree of freedom about a horizontal axis, e. g., a flywheel rotor or generator armature.

Figure 2 shows a side view of bearing 11 and shaft end 17.

Bearing 11 includes bearing elements 23 and 25, which lie inside of shaft 15 and are adjacent to each other. Bearing 13 is identical to bearing 11, and the structural relationship between bearing 13 and shaft end 19 is the same as that between bearing 11 and shaft end 17, as hereinafter described.

Figure 3 provides a cross section view of element 23, taken along line 3-3 of Figure 2. Element 23 is comprised of Halbach magnet array 27 and close-packed array of shorted circuits 29. Halbach array 27 is an annular array of permanent magnets concentric within shaft 15. The outer circumference of Halbach array 27 is attached to the inner circumference of shaft end 17, and Halbach array 27 thus rotates with shaft 15 about axial axis 21. Halbach array 27 is composed of individual permanent magnets, each of which is particularly oriented with respect to axial axis 21 so that the array, as a whole, generates a reinforced multiple-pole magnetic field within the volume circumscribed by the

array, yet cancels out most of the magnetic field that might otherwise occur outside of the array. A discussion of Halbach magnet arrays is provided in K. Halbach, "Application of Permanent Magnets in Accelerators and Electron Storage Rings,"Journal of Applied Physics, Vol.

57, April 15,1985, pp. 3605-3608, which is hereby incorporated by reference.

Circuit array 29 is a close-packed array of shorted, electrically conductive circuits attached to a rigid support structure (not shown) through the open end of shaft end 17. Circuit array 29 is symmetrically disposed about the axis of equilibrium for shaft 15; that is, when shaft 15 is rotating in its equilibrium position, axial centerline 21 is collinear with the axis of equilibrium. This is the position shown in the drawings.

Circuit array 29 lies within Halbach array 27. Shaft 15 and Halbach array 27 rotate with respect to circuit array 29. Variable separation distance 31 separates Halbach array 27 and circuit array 29. Shaft 15 can be vertically displaced from its equilibrium axis and, when that occurs, Halbach array 27 and axial axis 21 will translate with respect to circuit array 29, resulting in an asymmetrical separation distance 31.

As described in U. S. Patent No. 5,495, 221 issued to Richard F.

Post, the present inventor, and hereby incorporated by reference, rotation of Halbach array 27 relative to circuit array 29 provides a stabilizing centering force on rotating shaft 15. More particularly, the relative rotation of Halbach array 27 about circuit array 29 induces a current in circuit array 29 that ultimately generates a repulsive magnetic force acting across separation distance 31. This repulsive force increases as distance 31 decreases, and lessens as distance 31 increases, e. g., diametrically opposite the minimum value for distance 31. The effect is to restore axial axis 21 of rotating shaft 15 to its undisturbed, equilibrium position whenever a force acts on shaft 15 transverse to axial axis 21.

Figure 4 shows a section view of bearing element 25 of bearing 11, taken along line 4-4 of Figure 2. Bearing element 25 includes radial- field magnet array 33 and a pair of arcuate segments 35. Array 33 is an annular array of permanent magnetic bars that is attached to the inner circumference of shaft end 17 and thus rotates about axis 21 with shaft 15. The magnets composing array 33 are oriented so that their magnetic fields point radially inward towards axial axis 21.

Arcuate segments 35 lie within array 33. Segments 35 are comprised of ferromagnetic material, e. g., ferrite material or "transformer iron"laminated to reduce eddy currents. Alternatively, segments 35 may be comprised of permanent magnet bars oriented so as to produce a radial magnetic field of polarity such as to be attracted to radial-field magnet array 33. Segments 35 are separated by the angle 8, and oriented to attract magnet array 33. Segments 35 are attached to a rigid support structure (not shown) through the open end of shaft end 17, and thus remain fixed relative to the axis of equilibrium for shaft 15, i. e. , axial axis 21 and magnet array 33 can translate with respect to segments 35, and magnet array 33 can also rotate with respect to segments 35.

The attraction between segments 35 and the proximate magnets of array 33 provides both a levitating force and a horizontal centering force acting on shaft 15. By adjusting the angle 0, the levitating, i. e. , vertical, component of the attractive force can be adjusted to approximate 1/2 the combined weight of shaft 15, Halbach array 27, magnet array 33, and any additional load associated with the use of bearing 11. The levitating force generated by bearing 13 can be similarly adjusted, so that bearing elements 11 and 13 combine to levitate the weight of shaft 15 in addition to the weights of Halbach arrays 27, magnet arrays 33 for bearings 11 and 13, respectively, and any additional load associated with the use of bearings 11 and 13.

Figure 5 shows an alternative embodiment for bearing element 25, wherein rotatable cylinder 37 is composed of a ferromagnetic material and concentric stationary arcuate segments 39 are composed of a pair of arcuate radial-field magnets. The attraction between cylinder 37 and arcuate segments 39 provides both a levitating force and a horizontal centering force acting on shaft end 17. By adjusting the angle 0 separating arcuate magnets 39, the vertical component of the attractive force can be appropriately adjusted, as previously discussed in conjunction with the aforementioned embodiment of bearing element 25.

The support system composed of passive magnetic bearings 11 and 13 thus acts to restore axial axis 21 of rotating shaft 15 to its undisturbed, equilibrium position whenever a force acts on shaft 15 transverse to axial axis 21, in addition to levitating and centering shaft 15. Bearings 11 and 13 thus allow shaft 15 to rotate without mechanical friction about axial axis 21.

It should be noted that while the levitating and centering force of bearing element 25 is present even when shaft 15 is not rotating, the restorative centering force of bearing element 23 will be generated only when Halbach array 27 is rotating relative to circuit array 29 at or above a critical angular velocity. Thus a conventional touchdown bearing (not shown) to support shaft 15 is necessary if it is anticipated or possible that a transverse force might act on shaft 15 while it is not rotating, or rotating at less than the critical angular velocity.

It is to be understood, of course, that the foregoing description relates only to embodiments of the invention, and that modification to these embodiments may be made without departing from the spirit and scope of the invention as set forth in the following claims.