Abstract:
A vacuum pump ( 50 ) includes a pumping arrangement, a shaft ( 10 ) for driving the pumping arrangement, a motor ( 60 ) for rotating the shaft ( 10 ) and a bearing arrangement supporting the shaft ( 10 ) for rotation, the bearing arrangement having a rolling bearing ( 12 ), supporting a first portion of the shaft ( 10 ), and a thrust bearing ( 30 ). The thrust bearing ( 30 ) houses a plurality of rolling elements ( 36 ) such that they are maintained in bearing contact with an outer race ( 18 ) of the rolling bearing ( 12 ) and a race ( 34 ) of the thrust bearing ( 30 ). In this way axial movement of the rolling bearing ( 12 ) can be resisted whilst allowing radial movement of the rolling bearing ( 12 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a bearing arrangement of a vacuum pump. 
     BACKGROUND OF THE INVENTION 
       FIG. 2  shows a cross-section of a vacuum pump  50  known hereto which comprises a pumping arrangement driven by a single shaft. The arrangement shown comprises a turbomolecular pumping mechanism  52  and a Holweck pumping mechanism  54 , the latter of which is a molecular drag pumping mechanism. The rotors  58  and  59  of the turbomolecular pumping mechanism and the Holweck pumping mechanism, respectively, are arranged to be driven by shaft  56  so that when the shaft is rotated by a motor  60  the shaft drives the pumping arrangement  52 ,  54 . The shaft  56  is supported by a bearing arrangement comprising two bearings which may be positioned either at respective ends of the shaft as shown or alternatively intermediate the ends. In  FIG. 2 , a rolling bearing  64  supports a first portion of the shaft  56  and a magnetic bearing  62  supports a second portion of the shaft  56 . A second rolling bearing may be used as an alternative to the magnetic bearing  62 . When magnetic bearings are used it may also be desirable to incorporate a back-up bearing as well known in the art. As discussed in more detail below in relation to  FIG. 3 , the rolling bearing  64  is provided between the second end portion of the shaft  56  and a housing portion  66  of the pump  50 . 
     With such a pump, it is desirable to allow rolling bearing  64  some movement in the radial direction (radial compliance) but to prevent movement in the axial direction. Any axial movement can lead to clashing between the rotor blades of the turbomolecular pumping mechanism and the stator resulting in pump failure. It is advantageous to allow the radial bearing some radial movement in order to reduce the transfer of vibration from the pump rotor to the pump housing, caused by residual imbalance. 
     The prior art bearing arrangement will be explained with reference to  FIG. 3  which shows an enlarged view of the rolling bearing  64 . The rolling bearing comprises an inner race  68  fixed relative to shaft  56 , an outer race  70 , and a plurality of rolling elements  72 , supported by a cage  73 , for allowing relative rotation of the inner race and the outer race. The rolling bearing  64  is lubricated to reduce wear on its elements and shield elements  74  are provided to resist seepage of lubricant out of the rolling bearing. The shield may be a separate component, held in place by a spring clip, or other fastener, or alternatively may be an integral part of the bearing outer ring. A radial damping ring  75  is positioned radially between the outer race  70  and the housing portion  66  for damping radial movement of the outer race  70 . An axial damping ring  76  is provided between an end face of the outer race  70  and the housing portion  66  which resists axial movement of the outer race but allows radial movement thereof. However, the axial damping ring  76  does not adequately resist axial movement of the outer race because it is, to some extent, compressible in the axial direction and suffers from creep (or compression set) which makes the problem worse over time. 
     Furthermore, even though a lubricant is used in the rolling bearing  64 , due to the potentially high rotation speeds of the pumping arrangement, the rolling bearing increases in temperature during operation. Such an increase in temperature leads to rapid failure of the rolling bearing unless heat can be dissipated from the rolling bearing. A further problem with the prior art arrangement is that the axial damping ring  76  is made from an elastomer which has a low thermal conductivity and is resistant to the passage of heat from the outer race to the housing portion. The housing portion is typically made of Aluminum alloys and can be maintained at a relatively low temperature since such materials have a thermal conductivity in the region of 150 W/mK. 
     It is desirable to provide an improved vacuum pump. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a vacuum pump comprising: a pumping arrangement; a shaft for driving the pumping arrangement; a motor for rotating the shaft; a bearing arrangement supporting the shaft for rotation, the bearing arrangement comprising: a rolling bearing supporting a first portion of the shaft, and a thrust bearing housing a plurality of rolling elements in bearing contact with an outer race of the rolling bearing and a race of the thrust bearing for resisting axial movement of the rolling bearing and allowing radial movement of the rolling bearing. 
     The present invention also provides a vacuum pump comprising: a pumping arrangement; a shaft for driving the pumping arrangement; a motor for rotating the shaft; and a bearing arrangement supporting the shaft for rotation, the bearing arrangement comprising: a rolling bearing supporting a first portion of the shaft, and a thrust bearing having a cage spaced from an outer race of the rolling bearing, the cage housing a plurality of rolling elements in bearing contact with the outer race and a race of the thrust bearing for resisting axial movement of the rolling bearing and allowing radial movement of the rolling bearing. 
     Other preferred aspects of the invention are defined in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is an enlarged cross-section showing a rolling bearing of a vacuum pump according to an embodiment of the invention; 
         FIG. 2  is a cross-section of a prior art vacuum pump; 
         FIG. 3  is an enlarged cross-section showing a rolling bearing of the vacuum pump shown in  FIG. 2 ; 
         FIG. 4  is a cross section showing a rolling bearing of a vacuum pump according to another embodiment of the invention; and 
         FIG. 5  is a cross section showing a rolling bearing of a vacuum pump according to a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments have the general structure shown in  FIG. 2  and differ from the prior art only in the structure and mounting of the rolling bearing. For brevity, therefore, only the rolling bearing arrangement shown in  FIGS. 1 ,  4  and  5  are described in detail hereinafter. 
     Referring to  FIG. 1 , a vacuum pump comprises a shaft  10  supported by a bearing arrangement. The bearing arrangement comprises a rolling bearing  12  which supports a first portion of the shaft  10  and which is positioned between the shaft  10  and a housing portion  14 , in the same way as described with reference to  FIG. 2 . The arrangement shown in  FIG. 1  has rotational symmetry about axis A. The bearing arrangement further comprises a magnetic bearing supporting a second portion of shaft  10  although this is not shown in  FIG. 1 . 
     The rolling bearing  12  comprises an inner race  16  fixed relative to the shaft  10 , an outer race  18 , and a plurality of rolling elements  20 , in a cage  21 , for allowing relative rotation of the inner race and the outer race. The rolling elements  20  are preferably ball bearings made of high strength steel or ceramic. A lubricant  22 , which may for instance be oil or grease, is provided to reduce friction and wear between the moving parts of the bearing  12 . A shield, or flange, portion  24  extends radially inwardly from an axial end of the outer race  18  and is integral with the outer race. Alternatively, shield portion  24  may be a separate part. The shield portion resists the seepage of lubricant  22  out of the rolling bearing. A shield element  25  is also provided to resist the seepage of lubricant. A radial damping ring  26  is accommodated in a circumferential recess  28  in the housing portion  14  to resist excessive radial movement of the rolling bearing  12 . Alternatively, the damping ring  26  may be accommodated in a circumferential recess in the outer race as exemplified in  FIG. 4 . 
     A thrust bearing  30  is positioned between an axial end face of the outer race  18  (including integral shield portion  24 ) and a shoulder  32  of the housing portion  14 . The thrust bearing comprises a race in the form of a disc  34  which is preferably made of a high strength material such as steel and which bears against shoulder  32 . A plurality of rolling elements  36  are provided in contact with the disc  34  and the outer race  18  for resisting axial movement of the outer race but allowing relatively free radial movement thereof. The rolling elements  36  are housed in respective pockets in a cage  38  which is fixed relative to the housing portion  14  and disc  34 , and spaced from the axial end face of the outer race  18 . The rolling elements could alternatively be located within pockets or a circumferential groove formed directly in the outer race  18 . The pockets may be formed as cylindrical recesses, each cylinder having its axis parallel to the pump rotational axis. The housing portion  14  is maintained at a relatively low temperature compared to the rolling bearing, since the housing portion is not a moving part, may be cooled and is typically made of a material with high thermal conductivity. Therefore heat readily passes from the cage  38  which is fixed to the housing portion so that the cage is kept at a lower temperature than the rolling bearing  12  when the pump is in use. The assembly is constructed with a small axial clearance between the cage  38  and the outer race  18 . This clearance may be filled with oil or grease to create a thermal pathway to conduct heat from the rolling bearing  12  to the thrust race  30 . Oil or grease typically has a thermal conductivity in the range 0.10 to 0.16 W/mK. 
     In the prior art, the typical thermal resistance of the axial damping ring is 18 K/W and the radial damping ring is 65 K/W, giving a net thermal resistance of the bearing mounting of around 14 K/W. According to the embodiment, the thermal resistance may be less than 5 K/W, allowing the rolling bearing to be maintained at a cooler temperature. 
     The cage  38  is accurately manufactured to produce a small clearance C between it and the end face of the outer race  18 , and shield portion  24 , to improve the thermal pathway between the rolling bearing  12  and the thrust race  30 . Clearance C is less than 0.5 mm, although preferably it is less than 0.37 mm. More preferably, clearance C is less than 0.10 mm. It will be appreciated that the amount of heat which is able to pass from the rolling bearing  12  to the thrust race  30  is approximately inversely proportional to the size of clearance C and therefore a reduction in clearance C, without risking contact between the axial end face of the outer race  18  and the cage  38 , increases heat transfer. If a lubricant such as oil or grease is disposed in clearance C, the amount of heat which can be dissipated from the rolling bearing  12  to the thrust race  30  is further increased. By way of example, when the clearance C is in the range of 0.05 to 0.1 mm and filled with oil or grease, the thermal resistance of the bearing mounting is in the range 1.3 to 2.6 K/W. 
     The cage  38  is made of a material with high thermal conductivity, such as bronze or bronze alloy to reduce thermal resistance along the thermal pathway from the rolling bearing  12  to the thrust race  30 . For example, the cage may be made from phosphor bronze which has a thermal conductivity in the range of 50 to 80 W/mK. The shape of the cage  38  (i.e. with a large surface area towards the rolling bearing) decreases thermal resistance. Likewise, the shield portion  24 , which may be integral with the outer race  18 , increases the surface area of the outer race facing the cage  38  and thus also decreases thermal resistance. The disc  34  and the bearing outer race  18  are preferably made from a high carbon steel such as AISI 52100 high carbon steel which has a thermal conductivity of 46 W/mK. 
     It will be appreciated from the foregoing that the axial clearance provides greater thermal resistance to the passage of heat away from the rolling bearing than the cage, since the thermal conductivity of oil or grease is about 500 times less than that of phosphor bronze. However, the cage becomes equally influential when it has a thickness of about 5 mm, i.e. about 500 times the thickness of the oil or grease filled clearance. 
     The contact area between rolling elements  36  and the outer race  18  is small meaning that only a negligible amount of heat can be transferred from the rolling bearing to the thrust bearing by this route and cannot dissipate sufficient heat from the rolling bearing on its own. 
     A second embodiment of the invention is shown in  FIG. 4  where it can be seen that the thrust race  85  is located in contact with the outer race  83  rather than spaced therefrom as in the embodiment shown in  FIG. 1 . Clearance C is therefore provided at a greater distance from the outer race  83  than in the previous embodiment. Such a configuration provides enhanced thermal conductivity from the outer race  83  to the thrust race cage  87  but reduces thermal conductivity from there to disc  86 . 
     In order to maintain concentricity between the thrust race  85  and the outer race  83  of rolling bearing  82 , a shoulder  88  is formed on cage  87  which cooperates with recess  89  in outer race  83 . 
     As discussed above the radial damping ring  26  can be accommodated in a circumferential recess  84  in the outer surface of the outer race  83 , such a configuration is illustrated in  FIG. 4 . 
     A third embodiment of the invention is illustrated in  FIG. 5 . Here the cage  97  of thrust race  95  is provided as an integral part of the outer race  93  of rolling bearing  92 . Once again clearance C is provided adjacent ring  96 , this gap can be packed with lubricant or grease as in earlier embodiments to create a thermal pathway between the rolling bearing  92  and the housing portion  14 .