Magnetic bearing device

Abstract: This invention discloses a magnetic bearing device comprising a rotary member (4) rotatably supported on a supporting member (21) through a magnetic bearing (23), a free oscillating member (24) which is oscillatable almost coaxially with the rotary member (4) and disposed at a position not being in contact with both supporting member (21) and rotary member (4), magnetic coupling member (28, 29) for oscillating the free oscillating member (24) and the rotary member (4) together, and a damping member (27) disposed between the free oscillating member (24) and the supporting member (21).

Description: This invention relates to a magnetic bearing device and, more particularly, to a single shaft control type magnetic bearing device being formed into an active magnetic bearing in the direction of a rotary shaft, while being formed into a passive magnetic bearing in radial direction. (Prior art) Generally, in magnetic bearings of the single shaft control type, two opposite sides of a rotor are supported by a passive magnetic bearing in radial direction. However, damping characteristics of this passive magnetic bearing in radial direction are not always preferable and the amplitude magnification thereof at the resonance point is large. To prevent accidents at the time of passing the resonance point, a damper making use of eddy currents or a damper whose stator section is movable by a metal spring is adopted as countermeasure. However, the damper making use of eddy currents has a problem of poor damping characteristics, and the damper having a movable stator section has a problem of being easily affected by disturbance and is troublesome in assembly thereof. DE-A-24 57 783 discloses a magnetic bearing device according to the preamble of the independent claims. Damping means are provided as the stiffness of the bearing device is low in radial direction, said damping means comprising a coil, an inductive sensor and an amplifier. It is object of the present invention to provide a single shaft control type magnetic bearing device rotatable at high speed in which damping characteristics are superior and amplitude magnification at the resonance point is reduced, which is not easily effected by disturbance and is easy to assemble. This object is met by the invention as set out in independent claims 1, 4 and 5. Preferred embodiments of the invention are covered by the dependent claims. In the magnetic bearing device of above construction, damping characteristics are superior, amplitude magnification at resonace point can be reduced, resonance point can be safely passed through at the time of both starting and stopping. Furthermore, the device is easy to be built up and not easily affected by disturbance, and therefore when incorporating the device into such an apparatus as turbo-molecular pump, working stability of the pump can be improved. Brief Description of the Drawings Figure 1 is a longitudinal sectional view of an embodiment of the magnetic bearing device according to this invention; Figure 2 is an equivalent circuit diagram thereof; Figure 3 is a longitudinal sectional view of another embodiment; Figure 4 is an equivalent circuit diagram thereof; Figure 5 is a longitudinal sectional view of an example of a turbo-molecular pump using said magnetic bearing device; Figure 6 is a longitudinal sectional view of a further embodiment; and Figure 7 is on explanatory graph showing an example of displacement-returning force characteristic of an O-ring. Several embodiments according to this invention are now described in detail hereinafter with reference to the accompanying drawings. Referring to Figure 5 showing an embodiment in which the invention is applied to a turbo-molecular pump, the pump 1 drives a rotor shaft 4 by a motor 3 disposed in a casing 2, whereby turbo-blades 5 are rotated to discharge an air intaken from an inlet to an outlet. The mentioned rotor shaft 4 is radially supported by an upper magnetic bearing device 8 and a lower magnetic bearing device 9 respectively at upper and lower sections. The rotor shaft 4 is further supported in the axial direction of the shaft by a magnetic bearing 10. Numerals 11, 12 indicate anti-friction bearings for safety which regulate maximum amount of displacement of the rotor shaft 4. A specific construction of the lower bearing device 9 is now described with reference to Figures 1 and 2. In Figure 1, numeral 21 indicates a supporting member fixed to the casing 2. This supporting member 21 is freely inserted in a concavity 22 provided at the shaft center portion in the lower end of the rotor shaft 4, and is supported radially by a magnetic bearing 23 disposed at a position where the outer periphery of the supporting member 21 and inner periphery of the concavity 22 face to each other. The magnetic bearing 23 comprises permanent magnets arranged in such a manner that identical poles of them face to each other. On the other hand, an annular free oscillating member 24 is disposed oscillatably and coaxially with the rotor shaft 4 around the mentioned supporting member 21 at a position below the rotor shaft 4. The upper position of the free oscillating member 24 is limited by a position regulating member 25 and a ball 26, while the lower position thereof is limited by the bottom side of the casing 2. A damping member 27 of rubber, magnetic fluid, vacuum grease or the like is disposed among the mentioned free oscillating member 24, the position regulating member 25 and the bottom side of the casing 2. Since the upper position of the free oscillating member 24 is limited as mentioned above, the oscillating member 24 is kept so as not to be in contact with the end face of the rotor shaft 4. A pair of permanent magnets 28, 29 (serving as magnetic coupling means) are disposed respectively at portions of the mentioned oscillating member 24 and the rotor shaft 4 facing to each other so that they are magnetically coupled by magnetic force of the permanet magnets 28, 29, whereby the free oscillating member 24 is oscillated following the oscillation of the rotor shaft 4. The supporting member 21 is also provided with a further permanent magnet 30 which is magnetically repellent in relation to the permanent magnet 29. That is, the permanent magnet 30 serves as a spring for radially supporting the free oscillating member 24, and therefore instead of the mentioned magnetic spring, any other spring member including metal spring, rubber O-ring can be used. It is to be noted that what is oscillated by the oscillation of the rotor shaft 4 is not the permanent magnet on the stationary side but the free oscillating member 24. As a result of this, the assembly of the magnetic bearing is not negatively affected, and the assembled construction is not easily affected by disturbance. In addition, numeral 31 indicates a spacer. Figure 2 shows an equivalent circuit of the lower bearing device 9 described, and in which reference numeral K1 indicates the spring constant of magnet bearing 23, numeral K2 indicates the spring constant between the permanent magnets 28, 29, numeral K3 indicates the spring constant between the permanent magnets 29, 30, numeral C indicates the damping function due to the damping member 27, numeral m indicates the mass of the free oscillating member 24, respectively. It is obvious that the amplitude at the resonance point can be reduced as compared with the prior arts by appropriately selecting each of these constants. Figures 3 and 4 shows a specific construction of the upper bearing device 8 as another embodiment, and in which numeral 41 indicates an upper wall of the casing 2, and numeral 42 indicates an intermediate member, respectively. In this construction, the intermediate member 42 is oscillatably mounted on the upper wall through the damping member 43, and the intermediate member 42 and the upper wall 41 are magnetically coupled by a pair of permanent magnets 44, 45 (magnetic coupling means). A cylindrical section 46 mounted on the intermediate member 42 is freely inserted in a concavity 47 provided on the upper end center part of the rotor shaft 4 serving as a rotary member, and a magnetic bearing 48 comprising permanent magnets is disposed between the section 46 and the concave 47. The upper bearing device 8 of which the equivalent circuit is shown in Figure 4 also exhibits satisfactory damping performance in the same manner as the foregoing embodiment. Figure 6 shows a modification of the upper bearing device 8, and in which numeral 51 indicates a supporting member fixed to the casing side, and numeral 52 indicates an intermediate member, respectively. In this construction, the intermediate member 52 is oscillatably supported on the supporting member 51 through the damping member 53. Annular silicon gel material is preferably used as the damping member 53 of this kind by the following reasons. First, the silicon gel is easy to handle as compared with liquid, and working attitude of the device can be freely selected. Second, gel is a material of relatively small outgassing and suitable for use in vacuum device. Third, energy absorption coefficient and damping capacity are large as compared with rubber and the like. The supporting member 51 and the intermediate member 52 are coupled to each other by an O-ring 54. As shown in Figure 7, this O-ring 54 has a nonlinear spring characteristic in which returning force/displacement is larger in large displacement range side located on right side from the point A than in small displacement range side located on left side therefrom. In other words, in the displacement range from point 0 to point A, optimum value for damping the rotary member is quite stably kept, while in the displacement range over the point A, displacement of the rotary shaft due to disturbance can be reduced by giving high rigidity. It also is preferable to use any other member such as a hollow O-ring so far as it is a nonlinear type spring having similar spring characteristics. As shown in Figure 6, in this modification, a rotor 4 with turbo blades 5 is rotatably supported with respect to the intermediate member 52 by a magnetic bearing 58 comprising permanent magnets. In addition, numeral 57 indicates a ball for radially supporting the intermediate member 52. The equivalent circuit of the upper bearing section 8 of this modification is substantially the same as that shown in Figure 4, and accordingly this modification also exhibits a satisfiable damping performance. In the turbo-molecular pump provided with the upper and lower magnetic bearing devices 8, 9 described above, satisfactory damping effects can be given to vibration of the rotor shaft 4 and the amplitude magnification at resonance point can be reduced. As a result, advantages of this invention are such that the resonance point can be passed through safely at the time of both starting and stopping the rotor, and that the device is easy to be assembled and not easily affected by disturbance, eventually resulting in improvement of reliability.

Claims: A magnetic bearing device comprising: a rotary member (4) rotatably supported on a supporting member (21) through a radial magnetic bearing (8, 9), damping means (27) arranged between said rotary member (4) and said supporting member (21), characterized by a free oscillating member (24) which is disposed coaxially with said rotary member (4) and is oscillatable therewith, said oscillating member (24) not being in contact with either said supporting member (21) or said rotary member (4), magnetic coupling means (28, 29) axially acting for coupling said rotary member (4) and said free oscillating member (24), said damping means being a damping member (27) disposed between said free oscillating member (24) and said supporting member (21). The magnetic bearing device of claim 1, wherein said free oscillating member (24) is disposed below said rotary member (4). The magnetic bearing device of claim 1, wherein said oscillating member (24) is further radially coupled to said support member (21) by a pair of magnetic members (29, 30) acting as a spring. A magnetic bearing device comprising: a rotary member (4) rotatably supported on a supporting member (2, 41) through a radial magnetic bearing (48), damping means (43) arranged between said rotary member (4) and said supporting member (2, 41), characterized by an intermediate member (42) oscillatably supported on said supporting member (2, 41) through a damping member (43) and rotatably supporting said rotary member (4) through said radial magnetic bearing (48), and a magnetic coupling means (44, 45) magnetically coupling said intermediate member (42) and said supporting member (2, 41). A magnetic bearing device comprising: a rotary member (4) rotatably supported on a supporting member (51) through a radial magnetic bearing (58), damping means (53) arranged between said rotary member (4) and said supporting member (51), characterized by an intermediate member (52) oscillatably supported on said supporting member (51) through a damping member (53) and rotatably supporting said rotary member (4, 5) through said radial magnetic bearing (58), and a non-linear spring member (54) for coupling said intermediate member (52) and said supporting member (51), the returning force/displacement of said spring member (51) being non-linearly larger at large displacement than at small displacement. The magnetic bearing device of claim 5, wherein said spring member (54) is an O-ring, preferably a hollow O-ring. The magnetic bearing device of claim 5, wherein said spring member (54) has a first proportional relationship between the returning force and the displacement up to a certain value of displacement (A), and a second proportional relationship between the returning force and displacement. The magnetic bearing device of any of the preceding claims, wherein said damping member (27, 43, 53) is of a silicon gel material.

Inventors: YAMAMURA AKIRA C O NIPPON FERR, YAMAMURA, AKIRA, C/O NIPPON FERROFLUIDICS CORP.
Applicants: NIPPON FERROFLUIDICS KK, NIPPON FERROFLUIDICS CORPORATION
Application year: 1990
Main classification: F16C39
IPC codes: F16C39