Abstract:
A magnetic bearing for rapidly rotating machines ( 1 ), in particular, for turbo compressors, drag vacuum pumps, or similar, includes two bearings ( 3, 4 ), each with a stator magnetic-ring set ( 5, 7 ) and a rotor magnetic-ring set ( 6, 8 ). Movement of the rotor is damped by magnetic-ring sets ( 5, 6; 7, 8 ) arranged concentrically to each other. The stator magnetic-ring set ( 5  or  7 ) is arranged externally and the rotor magnetic-ring set ( 6  or  8 ) is arranged internally. Non-magnetic elements with good electrical conducting properties ( 32, 54, 55 ) are connected to the outer circumference surfaces of at least one part of the magnetic ring of each of the stator-side magnetic-ring sets ( 5, 7 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetic bearing for rapidly rotating machines, in particular, for turbo compressors, drag vacuum pumps, or similar, comprising two bearings, each with a stator magnetic-ring set and a rotor magnetic-ring set and means for damping the rotor movement. 
     U.S. Pat. No. 5,059,092 shows a magnetic bearing of this kind consisting of two bearings. The first, passively designed bearing has engaging stator and rotor magnetic rings. The second, actively designed bearing is equipped with two rotor magnetic rings spaced axially apart. In the ring chamber formed by these rings, a ring disk made of non-magnetizable material with good electrical conducting properties, effects the desired damping, in particular for the radially directed movements of the rotor. This damping is based on the induction of eddy currents by changing the magnetic flux in the electrically well conducting material. The eddy currents generated by the disk by perpendicularly penetrating magnetic fields generate electromagnetic counteracting forces opposing the radial deflections of the rotor system, thereby damping these movements. 
     One disadvantage of the magnetic bearing of U.S. Pat. No. 5,059,092 is that both the magnetic rings of the passive bearing as well as the magnetic rings and the ring disk located in between, engage. Thus, assembly is complicated. Moreover, changes in the length of the rotor which occur due to temperature loads give rise to bearing problems. Moreover, damping is restricted to the location of the active bearing, meaning that the counteracting forces effecting the damping action are present only locally. 
     It is the task of the present invention to create a magnetic bearing having the characteristics detailed above, in which the damping action is not restricted to one of the two bearings, and in addition to render manufacture and assembly more simple. 
     SUMMARY OF THE INVENTION 
     In that the damping means can be assigned to each of the magnetic rings, preferably assigned at least to a part of the magnetic rings of the magnetic-ring sets each on the stator side, counteracting forces can be generated in both magnetic bearings damping the rotor movement, i.e. not only in the actively controlled bearing. Engaging of rings on the stator and the rotor sides is not required. 
     One advantage is that, compared to the prior art, manufacture and assembly of the bearings is more simple. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     drawing FIGS. 1 and 2 a schematic representation of machines with rotors which are each supported by a magnetic bearing designed in accordance with the present invention, 
     drawing FIG. 3 a turbomolecular/molecular vacuum pump equipped with a bearing in accordance with the present invention, 
     drawing FIGS. 4 to  7  partial sectional views through magnetic bearings in accordance with the present invention with differently designed means for axial control and 
     drawing FIGS. 8 to  10  examples for embodiments of the magnetic bearings with damping means. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the machines depicted schematically in drawing FIGS. 1 and 2, the rotating system  2  is suspended in two magnetic bearings  3 ,  4 . Each magnetic bearing  3 ,  4  consists of two magnetic-ring sets  5 ,  6  (bearing  3 ) and  7 ,  8  (bearing  4 ) respectively. The inner ring set  5 ,  7  in each instance is mounted firmly, the outer ring sets  6 ,  8  which in each instance encompass the respective inner ring set concentrically and without making contact (slot  9 ) are components of the rotating system  2 . The design is in all rotationally symmetric. A drive motor is not depicted. 
     The rotating system  2  is equipped at both face sides with central recesses  11 ,  12 . The walls of these recesses form the receptacles  13 ,  14  for the rotating magnetic-ring sets  6 ,  8 . Receptacle  14  is a pipe-shaped reinforcement made of non-magnetizable material, carbon fibre reinforced plastic, for example, which is fitted preferably by means of a press-fit seat at the rotating system  2 . A section of the reinforcement  14  encompassing the recess  12  carries on its inside the magnetic-ring set  8 . 
     Fixed carriers  15 ,  16  with receptacles  17 ,  18  for the fixed magnetic-ring sets  5 ,  7  project into the recesses  11 ,  12  in such a manner that the outer ring sets  6 ,  8  concentrically encompass the inner sets  5 ,  6 . In the drawing figures each of the lower carriers  16  have a central bore  19  for a shaft end  20  of the rotating system  2 , the face side of said shaft end being assigned to an axial sensor  21 . 
     The axial sensor  21  is part of the means for axially controlling the magnetic bearing  4 . One or several coils  23  each with an U-shaped yoke  24  open in the direction of the ring set  8 , generate the magnetic fields indicated by the dashed lines and arrows  25 . In drawing FIGS. 1 and 2 in each instance two coils  23  are provided encompassing the ring set  8 . Their yoke components  24  are separated by a spacing disk  26  made of non-ferrite material. 
     A controller  27  serves the purpose of controlling the coils resp. the magnetic fields generated by the coils  23  depending on the signal output by the sensor  21 . In the slot  28  located in each instance between the outer rotating rings sets  6 ,  8  and the coils  23 , respectively the face side of the limbs of the yoke components  24 , the magnetic forces serving the purpose of axial control become effective. 
     The ring sets  5  to  8  consist each of rings magnetized in the axial direction arranged with changing poles (as indicated by way of an example for bearing  3  in accordance with drawing FIG. 1) so that the ring sets  5 ,  6  resp.  7 ,  8  of the magnetic bearings  3 ,  4  repel each other. Preferably, so many outer and inner pairs of rings are provided that each of the magnetic-ring sets has at both its ends the same polarity. In the solution in accordance with drawing FIG. 1 the ring sets  5 ,  6  resp.  7 ,  8  each form two cylinders arranged concentrically with respect to each other. The dimensions of the magnetic-ring sets  5 ,  7  resp,  6 ,  8  are preferably identical in each instance. In the solution in accordance with drawing FIG. 2 the diameters of the circumferential surfaces of the rings of both ring sets  5 ,  6  resp.  7 ,  8  of the bearings  3 ,  4  facing each other change in distinct steps (in the same direction), so that also the slot  9  is step-shaped. Also the slot  28  in bearing  4  may (deviating from what is depicted in drawing FIG. 2) also be step-shaped. 
     In the upper bearing  3  the cross section of the rotating magnet can be maintained smaller compared to bearing  4 . This saves costs for the magnetic material. 
     In bearing  4  it is required that the slot  28  between the pole surfaces of the yoke components and the magnets, which are held in place throughout the constant inside diameter of the carbon fibre reinforced plastic tube, be kept small so that the axial bearing can be effective on the magnets. 
     The rings of the magnetic-ring sets  5  to  8  are held firmly in place in their receptacles  13 ,  14 ,  17 ,  18 . Annular spacing discs  31  made of non-ferric materials rest flush against the two face sides of each magnet ring so that the magnetic forces become effective preferably in the slots  9  and  28  respectively. If the material of the annular spacing discs  31  has in addition good electrical conducting properties (copper for example) damping of the rotor movements is already attained by this. However, the damping means will be particularly effective when they are related to the outer circumferential surfaces of the magnet rings of the magnet ring sets  5 ,  7  on the side of the stator. In the embodiment in accordance with drawing FIG. 1 these damping means are formed by a sleeve  32 ,  33  respectively made of a material with good electrical conducting properties, said sleeves encompassing the magnetic-ring sets  5 ,  7 . 
     Moreover, the sleeves have the effect that they encapsulate the magnetic rings of magnetic-ring sets  5 ,  7 . Thus the magnetic materials are protected against aggressive gases (for example, hydrogen in drag pumps). As an example, stepped sleeves  32 ,  33  for the in each instance fixed ring sets  5 ,  7  are depicted in drawing FIG.  2 . At the side of the ring sets they are joined to the related receptacles in a gas-tight manner, welded, for example. Also the rotating magnetic-ring sets  6 ,  8  may be encapsulated in a similar manner. 
     Preferably the inner and outer rings of the ring sets  5 ,  6  resp.  7 ,  8  are arranged in pairs. To the end of improving axial control it may be expedient to add to the outer rotating ring set  8  of the axially active magnetic bearing  4 , further rings. Variants of this kind are depicted in drawing FIGS. 1 and 2. The ring set  8  has two more rings compared to ring set  7 . The two outer rings, designated as  29 , have been added to the set  8 . These may be soft ferric rings; preferably, however, two further magnetic rings are added. 
     In the machine  1 , a turbomolecular/molecular pump depicted in drawing FIG. 3, stator blades  37  are fitted in the casing  35  with the connecting flange  36 . The magnetically suspended rotor  2  carries rotor blades  38  revolving between the stator blades  37 , said rotor blades providing the pumping action for the gases. Pump I is a compound pump. The section equipped with blades is followed by a molecular pumping section  39 . 
     The rotor  2  is suspended in both magnetic bearings  3  and  4 . The magnetic bearing  3  is located at the high-vacuum side. The carrier  15  of the fixed magnetic-ring set  5  with its receptacle  17  is part of a bearing star  41 . 
     The magnetic bearing  4  is located at the fore-vacuum side of the pump  1 . Both bearings have approximately the same stiffness. The centre of gravity of the rotating system  2  is designated as  42 . 
     The pump  1  is equipped with emergency bearings or touchdown bearings  44 ,  45 . The touchdown bearing  44  at the high-vacuum side is located in a recess within the rotor  11 . The touchdown  45  bearing at the fore-vacuum side is located under the magnetic bearing  4  between shaft end  20  and the fixed carrier  16 . 
     As the drive motor  46 , a high-frequency motor with stator  47  and armature  48  is provided. On the side of the stator there is furthermore provided a can  49  which seals off the stator chamber  50  against the fore-vacuum side in a vacuum-tight manner. The can  49  penetrates the slot  28  between the coils  23  with their yoke components  24  and the rotating magnetic-ring set  8 . Said can is therefore expediently made of a non-magnetizable and electrically not well conducting material, carbon fibre reinforced plastic, for example. 
     At the side of the rotor the already detailed pipe-shaped reinforcement  14  is provided. It not only reinforces the ring set  8  but also the motor&#39;s armature  48 . 
     In order to compensate for tolerances, bearing  4  is adjustable via adjustment screws  52  on which the carrier  16  for the fixed ring set  7  rests. Expediently the adjustment is performed such that the rotating system is located axially in the unstable operating point. Axial control can be effected with minimum energy requirements about this operating point. 
     Drawing FIGS. 4 to  7  depict different embodiments for the active magnetic bearing  4 . In the solution in accordance with drawing FIGS. 4 (without magnetic field lines) and  5  (with magnetic field lines) each four magnetic rings form the rings sets  7  and  8 . Only one coil  23  with its U-shaped yoke  24  is provided. The distance between the face sides of the U-limbs of yoke  24  corresponds approximately to the axial dimension of one magnetic ring of ring set  8 . For the purpose of attaining an optimum interaction of the magnetic forces, the face sides of the U-limbs are located at the level of the centres of two neighbouring magnetic rings of ring set  8 , in the embodiment depicted at the level of the centres of the two middle magnetic rings. 
     In the embodiment in accordance with drawing  6 , there is also only provided one coil  23  with its yoke  24 . The distance of the face sides of the limbs of the U-shaped yoke  24  facing the rings of the ring set  8  corresponds approximately to twice the axial dimension of a magnetic ring. Drawing FIG. 7 depicts a solution with five coils  23  and yokes  24 . The ring set  8  has six magnetic rings. The face sides of the, in total six yoke limbs, are located approximately at the level of the centres of the magnetic rings. 
     Between each of the rings of the magnetic-ring sets  7 ,  8  there are located—as already detailed—annular spacing disks  31 , which depending on the material have an influence on the formation of the magnetic field lines and/or the damping effect. 
     Expedient designs for the annular spacing disks  31 , preferably for attaining a damping effect as well as supplementing the coatings of the magnetic rings are explained with reference to the embodiments of bearing  3  depicted in the drawing FIGS. 8 to  10 . 
     If the material for the annular spacing disks  31  consists of a material with good electrical conducting properties expedient for attaining a damping effect, then it may be expedient for the purpose of improving the damping effect to reinforce the circumferences of the annular spacing disks  31  at the point where the magnetic field enters into the slot  9 , for example increasing continuously outwards, and to adapt the shape of the magnetic rings to such circumferences. This embodiment is depicted in drawing FIG.  8 . The reinforced circumference of the middle annular spacing disk  31  close to the slot is designated as  54 . In that the magnetic fields pass through more conducting material, the counterforces generated by the eddy currents and providing the damping effect, increase. 
     In the embodiment in accordance with drawing FIG. 9, for example, the magnetic rings of ring set  5  are coated (coating  55 ) on all sides. At the side they have the function of spacing disks  31 , so that when of sufficient thickness of the coating  55  and suitably selected materials, said spacing disks will influence the magnetic field lines and/or have a damping effect. In addition it is achieved that the magnetic rings are protected against aggressive gases. Such protection may also be attained in that a sleeve  32  is provided, be it step-shaped as already detailed with reference to drawing FIG. 2, or cylindrical as depicted in drawing FIG. 10 (ring set  5 ), for example. 
     The annular spacing disks  31  (resp. coating  55 ) of the magnetic rings need(s) to be sufficiently thick to fulfil their/its purpose, particularly since also the desired stiffness of the bearing depends on the thickness of the spacing disks. In medium size drag pumps a thickness in the range from 0.25 to 1 mm has been found to be practical. 
     Moreover, the employment of spirally wound foil coils  23  has been found to be expedient since their space requirement is relatively small. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.