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United States Patent  [ii] Patent Number: 5,457,086
Rigney, II  Date of Patent: Oct. 10, 1995
 SUPERCONDUCTING COMPOSITE FOR MAGNETIC BEARINGS
 Inventor: Thomas K. Rigney, n, Torrance, Calif.
 Assignee: Allied-Signal, Inc., Morristown, N.J.
 Appl. No.: 96,119
 Filed: Jul. 22,1993
Related U.S. Application Data
 Division of Ser. No. 778,540, Oct. 17, 1991, Pat. No. 5,270,601.
 Int. CI.6 H02K 7/09; H01F 7/00;
 U.S. CI 505/122; 505/785; 505/171;
505/166; 505/879; 505/400; 505/450; 505/490;
 Field of Search 252/62.54, 62.63;
505/700, 785, 803, 171, 166, 879, 876
 References Cited
U.S. PATENT DOCUMENTS
3,448,062 6/1969 Alden et al 252/62.63
3,614,181 10/1971 Meeks 310/90.5
4,072,370 2/1978 Wasson 310/90.5
4,624,798 11/1986 Gindrup et al 252/62.54
4,797,386 1/1989 Gyorgy et al 505/1
4,892,863 1/1990 Agarwala et al 505/1
4,939,120 12/1989 Moon 505/1
4,954,481 9/1990 DeReggi 505/1
5,177,056 1/1993 Hilti et al 505/785
5,213,703 5/1993 Furuyama et al 252/62.54
FOREIGN PATENT DOCUMENTS
63-174214 7/1988 Japan .
Primary Examiner—Mark L. Bell
Assistant Examiner—C. M. Bonner
Attorney, Agent, or Firm—John R. Rafter
A composite includes granules of Type II superconducting material and granules of rare-earth permanent magnets that are distributed in a binder. The composite is a two-phase structure that combines the properties of the superconductor and magnets with the flexibility and toughness of a polymeric material. A bearing made from this composite has the load capacity and stiffness of a permanent magnet bearing with added stability from a Type II superconducting material.
17 Claims, 2 Drawing Sheets
U.S. Patent Oct. 10,1995 sheet 2 of 2 5,457,086
SUPERCONDUCTING COMPOSITE FOR
This invention was made with Government support under contract No. N00014-88-C-0668, awarded by 5 DARPA/ONR. The Government has certain rights to this invention.
This is a division of application Ser. No. 07/778,540 filed Oct. 17, 1991, now U.S. Pat. No. 5,270,601.
FIELD OF THE INVENTION
The present invention relates in general to superconducting composites and in particular to superconducting composites for passive magnetic bearings. 15
BACKGROUND OF THE INVENTION
Bearing performance is measured by bearing load capacity and stiffness. Load capacity denotes rotor support limit 20 during operation. Stiffness denotes the restoring force imparted to a shaft by the bearing when the shaft is deflected from its geometric axis. High bearing stiffness is desirable in order to maintain accurate shaft positioning as loads are applied to the shaft. 25
Conventional passive magnetic bearings are noted for their high load capacity and stiffness. Such bearings are characterized by two sets of permanent magnets. One set of magnets is employed in the rotor, and the other set of magnets is employed in the stator. Repulsive forces between 30 the two sets of magnets cause the shaft to be suspended. The shaft can be rotated by a minimal amount of force. See, e.g., Meeks U.S. Pat. No. 3,614,181 and Wasson U.S. Pat. No. 4,072,370.
According to Earnshaws Theorem, however, total perma- 35 nent magnet levitation is inherently unstable since the force is related to the inverse square of the distance. As a consequence of this instability, conventional passive magnetic bearings are not practical for use in bearing systems.
High-temperature superconducting bearings are noted for long life, reliability and low parasitic bearing power loss. These passive bearings can be made of Type I and Type II superconducting materials. Type I superconductors have the ability to screen out all or some of the magnetic flux applied 45 by an external source. When cooled below a critical temperature Tc, Type I superconductors exhibit total flux expulsion for applied magnetic fields less than some critical field Hc. This phenomenon is known as the "Meissner Effect." When expelled, the flux flows around the superconductor, 5Q providing a lifting force. This lifting force causes a magnet to be levitated above a Type I superconductor that is held stationary.
However, bearings made of Type I superconducting materials are thought to experience rotor stability problems. As 55 with conventional passive magnetic bearings, bearings made of Type I superconducting materials are not practical for use in bearing systems.
Type II superconducting materials are more commonly used for rotating bearings. Type II superconductors also 60 exhibit total flux expulsion for applied magnetic fields less than a first critical field Hcl. For applied magnetic fields in excess of a second critical field Hc2, the superconductivity is lost. In between critical fields Hcl and Hc2, however, Type II superconductors exhibit partial flux exclusion. Partial flux 65 exclusion is believed to be caused by inhomogeneities (e.g., pores, inclusions, grain boundaries) inside the Type II super
conductor. When the magnetic field is being induced into the superconductor, the superconductor offers resistance to change or displacement of this induced magnetic field. Some of the magnetic flux lines become "pinned" within the superconducting material. This phenomenon is known as "flux-pinning." The remaining flux lines are repelled by the flux lines pinned in the superconductor. This repulsion causes levitation. Thus, levitation does not arise from the Meissner effect. Instead, levitation occurs because the superconductor behaves more like a perfect conductor than a Meissner conductor.
Due to its flux-pinning properties, the Type II superconducting material gives superconducting bearings a measure of stability. Thrust bearings can be created by levitating a magnet above a disk made of a Type II superconductor. See, e.g., Agarwala U.S. Pat. No. 4,892,863. Journal bearings can be created by levitating a cylindrical magnet inside a hollow cylinder made of Type II superconducting material. See, e.g., Gyorgy et al. U.S. Pat. No. 4,797,386.
High-temperature superconducting bearings are ideal for use in aerospace turbomachinery applications where long life, reliability, and low parasitic bearing power loss are required. However, bearings made of Type II superconducting material have only the levitation force (load capacity) and rotor equilibrium restoration force (stiffness) for applications requiring very low load capacity and stiffness.
Therefore, it is an object of the present invention to provide a passive bearing that has the strength and machinability of a composite material, the load capacity and stiffness of a permanent magnet and the stability of a Type U superconducting material.
SUMMARY OF THE INVENTION
Granules of Type II superconducting material and granules of permanent magnets are distributed in a binder. The resulting composite is strong, yet can be machined easily into a bearing structure. Journal bearings can be made by placing a cylindrical rare-earth permanent magnet inside a hollow cylinder made of the composite. Thrust bearings can be made by disposing a rare-earth permanent magnet opposite a disk made of the composite. For both the thrust and journal bearings, grains of the permanent magnets must be aligned in the same direction as the magnetic granules of the composite. These journal and thrust bearings have the load capacity and stiffness of a permanent magnet bearing and the stability of a Type JJ superconducting bearing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flowchart of a process for making a superconducting composite according to the present invention;
FIG. 2 is a schematic diagram of superconducting bearing according to the present invention;
FIGS. 3 and 4 depict various embodiments of journal bearings according to the present invention; and
FIGS. 5-7 depict various embodiments of thrust bearings according to the present invention.
DETAILED DESCRIPTION OF THE
Referring to FIG. 1, a composite is formed from granules of a Type II superconducting material, granules of permanent magnets and a binder. The superconducting granules are prepared from an "extreme" Type II superconductor (step 100). Extreme Type II superconductors are noted for a high critical temperature Tc, strong anisotropy of the mag