SUPERCONDUCTIVE BEARING DEVICE

Abstract: A superconductive bearing device includes a ring-like permanent magnet portion (2) disposed concentrically on a vertical axial rotor (1), and a superconductor (8) disposed below the permanent magnet portion (2) in such a manner as to face the lower surface of the former (2) with a vertical gap between them. The permanent magnet portion (2) comprises a disc (4) fixedly disposed on the rotor (1) and a plurality of ring-like permanent magnets (6a, 6b) so disposed on the disc (4) as to be spaced apart from one another in the radial direction of the rotor (1). The upper and lower end portions of each permanent magnet (6a, 6b) have mutually opposite magnetic polarities, and the corresponding end portions, in the vertical direction, of adjacent permanent magnets (6a, 6b) have opposite magnetic polarities. The gap of the permanent magnets (6a, 6b) in the radial direction is optimized on the basis of the dimension (width) of the magnets (6a, 6b) in the radial direction. Therefore, a load capacity and rigidity can be improved, shaft vibration of the rotor can be prevented and the rotor (1) can be supported stably in the non-contact state.

Description: The present invention relates to superconducting bearing devices, for example, for use in hydraulic machines and machine tools which require high-speed rotation, power storage apparatus for storing excessive electric power as converted to kinetic energy of a flywheel, or gyroscopes according to the preamble of claim 1. The JP-A 1 295 019 discloses such a superconducting bearing which is provided with annular permanent magnets and disk-like superconductors. In this case the superconductors are fixed to a rotary shaft. The annular permanent magnets are disposed at both sides of the superconductors in axial direction of the rotary shaft. The magnets are fixed in the housing of the bearing. A superconducting bearing is also known from the WO-A 90/03524. The bearing according to this document comprises an annular permanent magnet portion having concentric magnets mounted thereon. The permanent magnet portion is connected to a rotary member of the bearing. The permanent magnets are arranged immediately side by side and with respect to their polarity such that adjacent permanent magnets have the opposite polarity. The document discusses the difference between Type I and Type II superconducting material. It is pointed out that the so called pinning phenomenon will occur, when a Type II material is used for the superconductor, which pinning phenomenon will contribute to stabilize the radial position of the rotor of the bearings in question. In recent years, superconducting bearing devices capable of supporting a rotary body in a noncontact state have been developed as bearing devices permitting high-speed rotation of the rotary body and having high rigidity. It is thought that such superconducting bearing devices comprise, for example, an annular permanent magnet disposed concentrically with a rotary body and having axially opposite ends magnetized to polarities opposite to each other, and an annular superconductor opposed to the end face of the magnet and spaced apart therefrom axially of the rotary body. However, the superconducting bearing device has the problem of being insufficient in rigidity with respect to the direction of the rotation axis and in load capacity. Another problem encountered is that the device is unable to support the rotary body in a noncontact state with good stability because the axis of the rotary body deflects owing to insufficient rigidity. These problems are thought attributable to the following reason. The magnetic field strength H and magnetic flux density B of the permanent magnet are in inverse proportion to the distance from the magnet and decrease with an increase in the distance. Suppose the distance between the superconductor and the permanet magnet is Z, the apparent magnetic susceptibility of the superconductor is M, the field strength of the permanent magnet is H and the flux density of the magnet is B. The force of magnetic repulsion between the superconductor and the permanent magnet is in proportion to the product of the susceptibility M and the gradient of field strength dH/dZ or to the product of the susceptibility M and the gradient of flux density dB/dZ. The rigidity is proportional to the product of the susceptibility M and d 2 B/dZ 2 . However, the gradient of field strength dH/dZ or the gradient of flux density dB/dZ of the annular permanent magnet is not great sufficiently, hence insufficient rigidity and load capacity. An object of the present invention is to overcome the foregoing problems and to provide a superconducting bearing device capable of supporting a rotary body in a noncontact state with good stability by preventing the deflection of the axis of the rotary body. To comply with this object, the superconducting bearing device of the the invention is characterized by the features of claim 1. Preferably, the spacing between the annular permanent magnets is 0.2 to 1.0 times the width of the magnets as measured radially thereof. In the bearing device of the invention a magnetic flux produced from the positive pole of one magnet and directed toward the negative pole of the same magnet upon reversion is added to a magnetic flux produced from the positive pole of another pole. This gives a strengthened magnetic flux and increases the gradient of flux density dB/dZ and d 2 B/d 2 Z, consequently increasing the load capacity and the rigidity. Therefore, the deflection of the axis of the rotary body can be prevented, enabling the device to support the rotary body in a noncontact state with good stability. Experiments have shown that the reversion of the magnetic flux starts to become pronounced in a space at a distance of 0.3 times the magnet width from the magnetic pole in the axial direction and at least 0.2 times the width radially away therefrom. Accordingly when two magnets are arranged too closely, the magnetic flux produced from the positive pole of one of the magnets and the magnetic flux entering the nagative pole of the other magnet will interfere with each other to produce an adverse effect. Further with a single magnet, the space region where the flux density is great is limited to a distance corresponding to the width of the magnet from the center of the pole, so that the above effect of addition will disappear when the plurality of magnets are spaced apart by too large a distance. With the magnetic portion comprising a plurality of annular permanent magnets in a concentric combination, the space region in which the gradient of flux density is great, for example, the region retaining 80% of the maximum value thereof, has an axial dimension corresponding to the width of the magnets. Further the radial dimension of the space region in which the gradient of magnetic flux is great is proportional to the spacing between the magnets, whereas the gradient of flux density decreases to a value nearly equal to that of the single magnet if the spacing between the magnet is not smaller than twice the width of the magnets. Since a great magnetic force is available when the superconductor is disposed in this space region of great flux density gradient, it is then possible to optimize the width of the magnets and the spacing therebetween in proportion to the clearance between the permanent magnet portion and the superconductor, whereby an increased load capacity and enhanced rigidity are obtained. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram in vertical section showing the main portion of a superconducting bearing device as a first embodiment of the invention; FIG. 2 is a graph showing the result of an experiment conducted with use of the device of the first embodiment; FIG. 3 is a graph showing the result of an experiment conducted for comparison; FIG. 4 is a graph showing the result obtained by checking the relationship between the diameter of bearings and the rigidity thereof; FIG. 5 is a graph showing the result obtained by checking the effect of the spacing between magnets on the same plane. BEST MODE OF CARRYING OUT THE INVENTION The present invention will be described below in greater detail with reference to the accompanying drawings. In the following description, like parts are designated by like reference numerals. FIG. 1 schematically shows the main portion of a first embodiment, i.e., a superconducting bearing device. With reference to FIG. 1, the superconducting bearing device has a rotary body 1 in the form of a vertical shaft. A permanent magnet portion 2 in the form of a horizontal disk is mounted on the rotary body 1 concentrically therewith. An annular superconductor portion 3 is disposed as opposed to the lower end face of the permanent magnet portion 2 and spaced apart therefrom in the direction of axis of rotation of the rotary body 1. The permanent magnet portion 2 comprises a horizontal disk 4 made, for example, of copper and fixedly mounted on the rotary body 1. The disk 4 is formed in its bottom surface with a plurality of, for example two, annular grooves 5a, 5b concentrically with the rotary body 1. Annular permanent magnets 6a, 6b are fixedly fitted in the grooves 5a, 5b, respectively. The permanent magnets 6a, 6b used are those having a great energy product, such as samarium-cobalt magnets or neodymium-iron-boron magnets. Each of the permanent magnets 6a, 6b has upper and lower ends which are magnetized to polarities opposite to each other. The upper ends of the adjacent magnets 6a, 6b, as well as the lower ends thereof, are magnetized to opposite polarities. For example, the outer magnet 6a has an S pole at the upper end and an N pole at the lower end, and the inner magnet 6b has an N pole at the upper end and an S pole at the lower end. The magnetic flux distribution around the axis of rotation is so designed as to remain unaltered by rotation. Assuming that the radial dimension of the permanent magnets 6a, 6b is the width thereof, the spacing between the magnets 6a, 6b is preferably 0.2 to 1.0 times the width. The superconductor portion 3 comprises a horizontal disk 7 having a bore 7a and made, for exampple, of copper, and a plurality of disklike type II superconductors 8 embedded in the annular portion of the disk 7 around its bore, arranged close to one another circumferentially of the disk and opposed to the permanent magnets 6a, 6b. All the disklike superconductors 8 are equal in volume. The superconductors 8 comprise a substrate made of yttrium high-temperature superconducting material such as YBa 2 Cu 3 O x and incorporating normally conducting particles (Y 2 Ba 1 Cu 1 ) as uniformly mixed with the material, and have properties to restrain the magnetic flux emitted by the permanent magnet portion 2 and penetrating thereinto. The superconductors 8 are arranged at a position spaced apart from the permanent magnet portion 2 and permitting a specified quantity of magnetic flux of the magnet portion 2 to penetrate thereinto, the position being such that the penetrating flux distribution will not be altered by the rotation of the rotary body 1. The superconducting bearing device has a housing (not shown), which has fixedly provided therein a cooling case 11. This case 11 is cooled by a refrigerator 9 or the like by way of a temperature control unit 10, and has the superconductor portion 3 fixed thereto. When the superconducting bearing device is operated, the superconductors 8 are cooled and maintained in a superconducting state with a refrigerant circulated through the cooling case 11. Accordingly, many of the magnetic fluxes emitted from the permanent magnet portion 2 on the rotary body 1 penetrate into the superconductors 8 and are thereby restrained (pinning phenomenon). With the particulate normal conductor uniformly present in the superconductors 8, the distribution of fluxes penetrating into the superconductors 8 is held constant, with the result that the rotary body 1 is arrested by the superconductors 8 along with the permanent magnet portion 2 as if the magnet portion 2 on the rotary body 1 is pierced with phantom pins extending upright from the superconductors 8. For this reason, the rotary body 1 is supported with respect to the radial and axial directions, as held levitated with high stability. The magnetic fluxes of each of the adjacent permanent magnets 6a, 6b at the upper and lower ends of the magnet portion 2 are strengthened by those emitted by the other magnet and reversed to make the gradient of flux density dB/dZ and d 2 B/d 2 Z greater than when the permanent magnet portion comprises a single permanent magnet. This increases the force of magnetic repulsion between the magnet portion 2 and the superconductors 8. Moreover, a great force of magnetic attraction acts therebetween merely when the spacing between the magnet portion 2 and the superconductors 8 slightly increases toward the direction of axis of rotation from the distance at which the force of magnetic repulsion is in balance with the pinning force. Conversely, a great force of magnetic repulsion acts therebetween merely when the spacing slightly decreases from the distance of balance. Accordingly, a greater load capacity and improved rigidity are available. Specific Experimental Example Experiments were conducted with use of the device of FIG. 1. The disk 4 of the permanent magnet portion 2 used was one made of copper. The outer annular permanent magnet 6a used was an annular rare earth magnet which was 90 mm in outside diameter, 70 mm in inside diameter, 10 mm in width (radial dimension), 10 mm in thickness (axial dimension) and 4000 gauss in surface flux. The inner annular permanent magnet 6b used was an annular rare earth magnet which was 60 mm in outside diameter, 40 mm in inside diameter, 10 mm in width, 10 mm in thickness and 4000 gauss in surface magnetic flux. The two magnets 6a, 6b were used in a heteropolar combination which means that the upper ends thereof, as well as the lower ends thereof, had poles of opposite polarities. The superconductors 8 used were those measuring 35 mm in diameter and 12 mm in thickness. These superconductors were embedded in the disk 7 with the center of each positioned on the circumference of a circle with a diameter of 75 mm. The permanent magnet portion 2 and the superconductor portion 3 were positioned in place relative to each other, and the superconductors 8 were thereafter cooled and thereby maintained in a superconducting state. The distance Z between the permanent magnet portion 2 and the superconductor portion 3 was 3 mm at this time. Using a tensile compression tester, the permanent magnet portion 2 and the superconductor portion 3 were moved toward or away from each other to measure the loads required. FIG. 2 shows the result. When the distance Z was in the range of 2 to 4 mm, the rigidity was 3.9 kgf/mm. For comparison, a permanent magnet portion comprising a single magnet, i.e., the outer magnet only, and the superconductor portion were moved toward or away from each other to measure the loads required. FIG. 3 shows the result. When the distance Z was in the range of 2 to 4 mm, the rigidity was 2.9 kgf/mm. Another experiment was also conducted in which the outer magnet and the inner magnet were used in a homopolar combination in which the corresponding ends thereof were made to have poles of the same polarity. Although not illustrated, the rigidity obtained in this case was 2.4 kgf/mm, and was lower than in the case of the single magnet. To explore the optimum conditions for the arrangment of magnets, a superconductor portion with a diameter of 100 mm was prepared which was fully covered with superconductors, and magnets measuring 25 mm, 48 mm, 60 mm, 90 mm and 100 mm in outside diameter were used singly or in combinations as arranged on the same plane to measure the rigidity. Table 1 shows data as to the bearings. Table 2 shows the measurements of rigidity obtained. Type Magnet Bearing O.D. (mm) Net bearing area (cm 2 ) Single magnet 90S 90 50 60S 60 26 48S 48 30 25S 25 8 100S 100 62 Heteropolar combination 90N+60S 90 63 90N+48S 90 75 90N+25S 90 79 100N+60S 100 70 Homopolar combination 90N+60N 90 63 Bearing data Rigidity (kgf/mm) Plane rigidity (kgf/mm·cm 2 ) Type Magnet Z=2 mm Z = 3 mm Z=6 mm Z = 3 mm Single magnet 90S 2.9(1.8) 1.1 0.035 60S (0.65) 0.023 48S 25S (0.38) 0.035 100S 4.0 Heteropolar combination 90N+60S 3.9(2.7) 1.5 0.043 (2.4) 0.032 90N+48S 90N+25S (2.2) 0.028 100N+60S 8.0 Homopolar combination 90N+60N 2.4 0.9 0.024 Each numerical value given in the columns of magnet in these tables indicates the outside diameter of the magnet, and S or N following the value stands for the polarity of its lower end. The disklike magnet with an outside diameter of 25 mm is 10 mm in thickness, the annular magnet with an outside diameter of 48 mm is 14 mm in width and 10 mm in thickness, the magnet with an outside diameter of 60 mm is 8 mm in width and 9 mm in thickness, the magnet with an outside diameter of 90 mm is 10 mm in width and 12 mm in thickness, and the magnet with an outside diameter of 100 mm is 10 mm in width and 10 mm in thickness. To expedite the experiment, the magnet or magnets were brought closer to the superconductors cooled in advance and moved upward and downward several times by 1 mm each time at Z = 3 mm to measure the rigidity. Since the value of rigidity then obtained is somewhat different from the corresponding value obtained in the case where the parts are first set to Z = 3 mm, followed by cooling, the measurement is given in the parentheses. Regarding the outside diameter of the outer magnet as the diameter of the bearing, the bearing rigidity values are typically shown in FIG. 4, in which the single magnet is represented by a blank circle mark, the heteropolar combination by a dot-and-circle mark, and the homopolar combination by a double circle mark. The graph shows that with the single magnets, the rigidity does not increase so greatly as to be proportional to the bearing area, hence the need for the heteropolar combination. However, the homopolar combination exhibits lower rigidity than the single magnet, hence an adverse effect. FIG. 5 shows the relationship between the plane rigidity (rigidity per unit area) and the spacing between the magnets as established for three different heteropolar combinations. When it is required that the plane rigidity in this case be manifestly above the plane rigidity of 0.035 in the case of the single magnet, the effective magnet spacing is in the range of 0.2 to 1.0 times the width. The lower limit was determined in view of the phenomenon that one of two magnets too closely arranged in heteropolar combination starts to offset the flux of the other magnet. INDUSTRIAL APPLICABILITY The superconducting bearing device embodying the present invention is suitable for use in hydraulic machines and machine tools which require high-speed rotation, power storage apparatus for storing excessive electric power as converted to kinetic energy of a flywheel, or gyroscopes.

Claims: Superconducting bearing device comprising an annular permanent magnet portion (2) disposed concentrically with a rotary body, (1), and a superconductor portion (3) opposed to an end face of the permanent magnet portion (2) and spaced apart therefrom axially of the rotary body (1), the permanent magnet portion (2) comprising a plurality of annular permanent magnets (6a,6b) having axially opposite ends magnetized to polarities opposite to each other, the permanent magnets (6a,6b) adjacent to each other being magnetized to polarities opposite to each other at their ends positioned toward the same axial direction, characterized in that the superconductor portion (3) comprises a Type II superconductor (8); the annular permanent magnets (6a,6b) are arranged with a radial spacing between them on a disk (4) of non-magnetic material fixedly mounted to the rotary body (1); the permanent magnets (6a,6b) are embedded in the disk (4) and have one axial end face flush with an end face of the disk (4); each of the permanent magnets having its flux permitted to effectively penetrate into the Type II superconductor and being pinned thereto; the rotary body (1) being rotatable without varying the distribution of the penetrating magnet flux. Superconducting bearing device as defined in claim 1, characterized in that the spacing between the annular permanent magnets (6a,6b) is 0,2 to 10 times the width of the magnets as measured radially thereof. Superconducting bearing device as defined in claim 1, characterized in that the disk (4) is formed on its axial end face with a plurality of annular grooves (5a,5b) disposed concentrically with the rotary body and radially spaced apart from the rotary body, the annular permanent magnets (6a,6b) being fixedly fitted respectively in the grooves.

Inventors: SHIBAYAMA MOTOAKI, TAKAHATA RYOICHI, TAKAICHI HIROSHI, SHIBAYAMA, MOTOAKI, TAKAHATA, RYOICHI, TAKAICHI, HIROSHI
Applicants: KOYO SEIKO CO, SHIKOKU RESEARCH INST INC, KOYO SEIKO CO., LTD., SHIKOKU RESEARCH INSTITUTE INCORPORATED
Application year: 1992
Main classification: F16C32
IPC codes: F16C32, F16C39, H02J15