Abstract:
A vacuum pump, in particular a turbomolecular pump or a multi-inlet pump, includes a rotor shaft ( 12 ) that supports at least one rotor device ( 14 ). The rotor shaft ( 12 ) is mounted on the pressure side by way of a bearing arrangement ( 56 ) and on the suction side by way of a bearing arrangement ( 30 ). The suction-side bearing arrangement ( 30 ) is disposed in a high-vacuum area ( 22 ) and includes an electromagnetic bearing. Preferably, a coil ( 32 ) of the electromagnetic bearing is disposed in a recess ( 38 ) of a housing element ( 40 ). The recess ( 38 ) is pressure-encapsulated, in particular by a tubular closure element ( 42 ).

Description:
BACKGROUND 
     The present invention relates to a vacuum pump, particularly a turbomolecular pump or a multi-inlet turbomolecular pump. 
     Turbomolecular pumps comprise at least one rotor including a rotor arrangement with a plurality of rotor disks. Between the rotor disks, stator disks are arranged, being held by stator rings. The rotor arrangement is mounted on a fast-rotating rotor shaft. Turbomolecular pumps have an inlet on the suction side and an outlet on the pressure side. On the suction-side inlet, final pressures of possibly less than 1·10 −10  mbar can be achieved. Frequently, the pressure-side pump connector has additional pre-vacuum pumps connected to it. 
     Multi-inlet pumps comprise at least one intermediate inlet in addition to a main inlet on the suction side. Usually, the rotor arrangement of a multi-inlet pump comprises two pump stages which can be formed e.g. as turbomolecular stages, with said intermediate inlet being arranged between these two pump stages. Often, a further pump stage, such as e.g. a Holweck stage, is provided behind the turbomolecular stages when viewed in the conveying direction. By use of multi-inlet pumps, different pressure levels can be generated at the main inlet and said at least one intermediate inlet. 
     Particularly in fast-rotating vacuum pumps, such as e.g. turbomolecular pumps and multi-inlet pumps, the support of the rotor shaft on the pressure side, i.e. in regions where no low pressures prevail, can be provided by means of electromagnetic bearings. In known vacuum pumps, the electromagnetic bearings provided for bearing support of the rotor shaft are operated in pressure ranges of up to 120 mbar. Further, it is known to use passive magnetic bearings for support of the rotor shaft in the high-vacuum region. 
     Electromagnetic bearings are not customarily used for the bearing support of a vacuum pump on the suction side, which is because of the low pressures in this region due to the circumstance that the coil bodies and sensor devices used therein are components with large surfaces and numerous cavities. Thus, because of the continuous outgassing, achieving the desired final pressure is not possible at all or is possible only with difficulties. Further, it is known to use passive magnetic bearings in the high-vacuum region. 
     For electromagnetic support of the whole rotor shaft, it has been proposed in DE 20 2005 019 644 to arrange the two electromagnetic bearings within a cartridge. Internally of said cartridge, the rotor shaft is arranged together with the bearings and the electric motor. In the direction of the pressure side, the cartridge is substantially open so that, within the cartridge, there will exist the atmospheric pressure or at least a relatively high pressure acting on the pressure side of the pump. The rotor shaft comprises a projection extending from out of the cartridge and carrying the rotor arrangement. Thus, the rotor arrangement is fastened to a cantilevered end of the shaft. Therefore, the constructional length of the pump is restricted. Further, the attachment of the rotor arrangement to the cantilevered shaft end will cause large forces at the bearing sites, entailing the necessity to install correspondingly complex electromagnetic bearings. Further, this constructional design is subject to massive restrictions due to the rotor-dynamic behavior, particularly because of low natural frequencies. 
     From U.S. Pat. No. 5,547,338, it is known to provide bearings of the type with turbulent fluidized bed. These bearings are basically different from electromagnetic bearings because bearings with turbulent fluidized bed are energized by the fields of the opposite permanent magnets. Exactly for the support of rotor shafts in turbomolecular pumps, bearing arrangements of this type are unfit for use because bearings with turbulent fluidized bed are very unstable. Further, if one were to use a bearing arrangement of this type, the ohmic losses in the conductive disks would cause a massive heat-up of the rotor until the magnetic fluxes would be sufficient for achieving the desired bearing effect. Further, this approach would cause a braking effect acting on the rotor and thus entail the need for an increased drive power. 
     It is an object of the invention to provide a vacuum pump, particularly a turbomolecular pump or a multi-inlet turbomolecular pump, wherein the bearing arrangements are improved. 
     The vacuum pump of the invention comprises a rotor shaft carrying a rotor arrangement, wherein the rotor arrangement can include, if required, a plurality of rotors or other suction or pumping devices. The rotor shaft is supported by—usually two—bearing arrangements, notably by a pressure-side bearing arrangement and a suction-side bearing arrangement. According to the invention, the suction-side bearing arrangement is arranged in a high-vacuum region and thus is exposed to low pressures. Further, according to the invention, the suction-side bearing arrangement is an electromagnetic bearing. A high vacuum is to be understood herein as a pressure below 10 −3  bar, preferably less than 10 −5  bar and most preferably less than 10 −10  bar. 
     Particularly if the electromagnetic bearing is arranged in the region of very low pressures, as occurring on the suction side, i.e. in the inlet region of a turbomolecular pump, it is provided according to a particularly preferred embodiment that the coil of the electromagnetic bearing is arranged in a pressure-encapsulated recess. By arranging the coil in the pressure-encapsulated recess, it is safeguarded that the coils itself is not located directly in the high-vacuum region. This precludes the disadvantage that, due to the numerous cavities in the coil, the continuous outgassing will not or only with difficulties allow the final pressure to be reached. By the inventive provision of an electromagnetic bearing in the high-vacuum region, it is possible to support the rotor shaft in its end regions. Particularly, the rotor arrangement can be connected to the rotor shaft between the two bearings. An attachment of the rotor arrangement to a cantilevered arm of the shaft and thus a flying support of the rotor are not required anymore. Further, if also the pressure-side bearing arrangement is formed as an electromagnetic bearing, the resultant fully electromagnetic support of the shaft makes it possible to reach higher rotational speeds because the shaft can be given a very rigid design and the damping and stiffness at the bearing sites can be parametrized by software. 
     SUMMARY 
     According to a particularly preferred embodiment, the suction side is provided exclusively with an electromagnetic bearing. At the most, a safety bearing is installed in addition thereto as a matter of precaution. Herein, the bearing on the suction side can be designed to be simultaneously an electromagnetic radial bearing and an electromagnetic axial bearing, wherein the two bearing directions can be realized by two separate electromagnetic bearings. According to a particularly preferred embodiment, it is provided that the support on the suction side is exclusively a radial support realized by means of a corresponding radial electromagnetic bearing. Particularly, in this embodiment, the axial support is provided on the opposite side of the shaft, i.e. on the pressure side. 
     According to a particularly preferred embodiment, said recess is arranged in a housing element, i.e. preferably in a stationary element connected to the housing. Herein, an opening of the recess is preferably oriented in the direction of the rotor shaft. Preferably, the recess has a circular ring shape and fully surrounds the rotor shaft. Thus, it is possible to arrange an annular coil of an electromagnet within the recess. Preferably, herein, the electric feed lines can be guided into the recess through the housing, and not via the opening of the recess provided in the direction of the rotor shaft. 
     For pressure encapsulation, i.e. for sealing the recess, it would also be possible, for instance, to fill the recess with synthetic resin or the like after arranging the coil in the recess. In case of very low pressures, however, the use of synthetic resin or the like has the disadvantage that e.g. softening agents will outgas in a high vacuum, so that, for instance, the results of the analysis may be adulterated. According to a preferred embodiment of the invention, the opening of the recess is tightly closed by a preferably tubular closure element. Thus, the opening, which preferably is oriented towards the inside in the direction of the rotor shaft, can be closed in a simple manner by a tubular closure element. In case that the recess specifically has the shape of a circular cylinder, the opening of the recess corresponds to the inner circumferential surface of the circular ring-shaped cylinder. The sealing attachment of the closure element to the housing element can be performed preferably via sealing elements such as e.g. O-rings. 
     Depending on the given case, a pressure encapsulation of the electromagnetic bearing can be omitted. For instance, such a pressure encapsulation is not absolutely required if the pressures which are to be achieved are not too low and/or a corresponding outgassing will thus not occur or will have no adverse effects. For instance, in case of an inventive use of electromagnetic bearings in the region of intermediate inlets of multi-inlet pumps, a pressure encapsulation may possibly be omitted. Depending on the given application, e.g. when using multi-inlet pumps in mass spectrometers, a pressure encapsulation will however by advantageous for avoidance of degassing. 
     Preferably, the bearing arrangement on the suction side is designed exclusively as an electromagnetic radial bearing. This design offers the advantage that the encapsulation of the electromagnetic coil can be realized in a simple manner. Further, it is easier under the technical aspect if the axial support of the rotor shaft is realized on the pressure side. In this case, it is possible to use, in a simple manner, an electromagnetic axial support because the existing pressure are distinctly higher and an encapsulation of the electromagnetic coil will thus not be required. Particularly, it is possible to configure the pressure-side bearing as a radial as well as axial bearing wherein, according to a preferred embodiment, there is provided an electromagnetic combinatory bearing so that an electromagnetic bearing will provide both the axial and the radial support of the rotor shaft on the pressure side. 
     Further, it is advantageous if bearing sensors, provided to determine particularly the position of the rotor shaft, are also arranged within the encapsulated recess. With the aid of the bearing sensors, which preferably are connected to corresponding electronics, the control of the electromagnets will be performed. 
     In the inventive configuration of the support of the rotor shaft, the suction-side bearing arrangement in turbomolecular pumps having a sole inlet and one outlet is arranged immediately in the inlet region where a high vacuum prevails. Thus, the support of the rotor shaft can be realized in the end region thereof so that the rotor of the turbomolecular pump is arranged between the two bearing arrangements. 
     When supporting a rotor shaft of a multi-inlet pump and practicing the inventive use of an electromagnetic bearing in the high-vacuum region, the electromagnetic bearing can be arranged in the region of the main inlet where the lowermost pressure prevails. In this case, the electromagnetic bearing is preferably arranged in a pressure-encapsulated recess, as explained above. 
     However, in multi-inlet pumps, it is also possible to provide the suction-side bearing arrangement in the region of the intermediate inlet. In this region, which also is in a high vacuum condition, it is according to the invention possible to arrange an electromagnetic bearing. Since the pressure in the region of the intermediate vacuum inlet is higher than in the region of the main vacuum connector, it is not absolutely required to arrange the coil of the electromagnetic bearing in the pressure-encapsulated region. In this configuration, the electromagnetic bearing is thus arranged between two rotors of the rotor arrangement. This design offers the advantage that only one of the two rotors is arranged on a cantilevered end of the rotor shaft. Then, the second rotor or, if provided, a plurality of rotors or corresponding pump devices, can be arranged between the suction-side bearing and the pressure-side bearing, which preferably are both designed as electromagnetic bearings. Also by the provision of an electromagnetic bearing in the region of the intermediate vacuum connector, the total length of a multi-inlet pump can be increased. Since only one rotor and thus a smaller weight are arranged on the cantilevered end of the rotor shaft, the bearing is subjected to smaller forces than e.g. in case of a cantilevered rotor shaft as described particularly in DE 20 2005 019 644. 
     The invention will be explained in greater detail hereunder by way of preferred embodiments with reference to the accompanying drawings. 
    
    
     
       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 the preferred embodiments and are not to be construed as limiting the invention. 
         FIG. 1  is a schematic sectional view of a turbomolecular pump with a bearing arrangement according to the invention; 
         FIG. 2  is a schematic sectional view of a multi-inlet pump with a bearing arrangement according to the invention; and 
         FIG. 3  is a schematic sectional view of a multi-inlet pump with an alternative bearing arrangement according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A turbomolecular pump as schematically shown in  FIG. 1  comprises a rotor shaft  12  arranged in a housing  10 . Rotor shaft  12  carries a rotor arrangement  14  which in the illustrated embodiment forms one rotor. Rotor arrangement  14  comprises a plurality of rotor disks  16 . Between the rotor disks  16 , stator disks  18  are arranged which are fixed by stator rings  20 . A suction side  22  of the pump forms the high vacuum connector so that a medium will be sucked in the direction indicated by arrow  24 . An outlet  26  of the turbomolecular pump and respectively the pressure side  28  is normally connected to a pre-vacuum pump. 
     According to the invention, a bearing arrangement  30  arranged on the suction side comprises an electromagnetic bearing. Said bearing comprises a coil  32  of an electromagnet and a bearing element  34  arranged for rigid rotation with rotor shaft  12  and provided e.g. in the form of so-called electric sheets. Coil  32  is connected to an electric connector  36  for energy supply. Coil  32  is arranged in a recess  38  of a housing element  40 , said recess in the illustrated embodiment having the shape of a circular cylinder. Recess  38  surrounds rotor shaft  12  in an end region. Recess  38  is closed by a tubular closure element  42  as well as by sealing elements  44  preferably provided as O-rings, so that recess  38  is pressure-encapsulated. Thus, internally of recess  38 , there does not exist the high vacuum prevailing in the region  22 . Thereby, it is prevented that the numerous cavities existing in the coil make it difficult or even impossible to reach the final pressure. 
     For controlling the voltage supplied to coil  32  via line  36 , at least one bearing sensor  46 , schematically represented in interrupted line, is arranged within recess  38  so as to detect particularly the position of shaft  12 . Via a conduit  48 , the signal of bearing sensor  46  is forwarded to an electronics unit  50  which then will control particularly the level of the voltage supplied to coil  32 . 
     Further, the illustrated suction-side bearing arrangement  30  is provided with a mechanical safety bearing  51  formed e.g. as a ball bearing. Safety bearing  51  is arranged in said housing element  40  and has a small distance from a pin  52  of shaft  12 . The safety bearing substantially serves for safeguarding emergency running properties in case of fallout of the electromagnetic bearing. In the illustrated embodiment, housing element  40  is cup-shaped and surrounds a suction-side end portion  54  of rotor shaft  12 . 
     In the illustrated embodiment, the bearing arrangement  56  provided on the pressure side  28  is also formed as an electromagnetic bearing arrangement. Provided for radial support is an electromagnetic coil  58  arranged to cooperate with a bearing element  60  which is arranged on shaft  12  and corresponds to bearing element  34 . For axial support, there is provided, in the illustrated embodiment, a second electromagnetic coil  62  of U-shaped cross section which cooperates with a further bearing element (axial disk)  64  arranged on shaft  12 . Said axial disk  64  extends into the recess of coil  62  so as to form an axial bearing. The two coils  58 , 62  each comprise an electric connection line  66 . Further, the bearing arrangement is provided with position sensors  68 , schematically represented in interrupted lines, for detecting the position of shaft  12  in the axial and respectively radial direction. Also said position sensors are connected, via lines  70 , to said electronics unit  50  which will control the coil voltages in dependence on the sensor signals. 
     Corresponding to the bearing arrangement  30  comprising an electromagnetic bearing, also the bearing arrangement  36  is provided with a safety bearing  69  usually designed as a ball bearing, which surrounds a bearing pin  70  at a distance. 
     Rotor shaft  12  is driven by a drive means  72 , usually comprising an electric motor, which, via a line  74 , can also be connected to said electronics unit  50  for control. 
     The schematic sectional view of  FIG. 2  shows a preferred embodiment of the inventive bearing arrangement in a multi-inlet pump. The bearing arrangement  30  on the suction side and the bearing arrangement  56  on the pressure side are configured identically to those in the turbomolecular pump described with reference to  FIG. 1 . Identical and similar components are designated by the same reference numerals as in the turbomolecular pump ( FIG. 1 ). 
     As used in a multi-inlet pump, the motor shaft  12 , which again is supported in its two end regions by the bearing arrangements  30  and  56 , comprises a rotor arrangement with a plurality of condenser stages  76 , 78 , 80 . In the illustrated embodiment, the first two condenser stages  76 , 80  are turbomolecular pumps which respectively comprise a rotor  14  with rotor disks  16 . Arranged between the rotor disks  16  are stator disks  18  held by stator rings  20 . The two rotors  14  are arranged at a mutual distance on a rotor shaft  12 . Between the two rotors  14 , housing  10  is formed with an inlet opening  82  which is an intermediate inlet. 
     Further, the multi-inlet pump shown in  FIG. 2  comprises a main inlet  84  which is the high vacuum connector. Via the main inlet, the sucked gas will flow in the direction of arrow  24 . Additionally, in the region of the intermediate inlet  82 , medium is sucked in via the intermediate inlet as indicated by arrow  86  and is conveyed to the left in  FIG. 2 . 
     The third condenser stage will then convey the medium in the direction of the pressure side  28  and respectively the outlet  26 , as indicated by arrow  88 . Outlet  26  normally has a pre-vacuum pump connected thereto. 
     The third condenser stage  80  can be formed e.g. by a Holweck stage or the like. Normally, via the rotor  14  depicted on the left-hand side in  FIG. 2 , also the rotating components of the third condenser stage are connected to the rotor shaft  12 , and they are driven in common. 
     By the inventive configuration of the suction-side bearing arrangement  30 , it is made possible to provide the two bearing arrangements  56 , 30  on the shaft ends, thus allowing to realize a maximum distance between the bearings. 
     Also the schematic sectional view of  FIG. 3  shows an embodiment of a multi-inlet pump wherein, in comparison to the suction-side bearing arrangement  30  shown in  FIG. 2 , the suction-side bearing arrangement  90  has been relocated further inwards. Identical and similar components are again designated by the same reference numerals. 
     The suction-side bearing arrangement  90  is arranged in the region of the intermediate vacuum inlet  82 . Although there still exists a high vacuum of e.g. 10 −3  to 10 −5  mbar, the placement of an electromagnetic bearing in this region does not make it necessary to arrange the coil  32  within a pressure-encapsulated recess as is the case for the bearing arrangement  30 . However, it is possible to provide a pressure-encapsulated arrangement also for the coil. 
     In the illustrated embodiment, the coil  32  of suction-side bearing  90  is arranged in a housing element  92 . Again, the coil  32  is arranged opposite to the bearing element  50  which is fixedly connected to shaft  12 . Further, a sensor  46  is provided which is connected to electronics unit  50  via a line  48 . 
     Therefore, in the embodiment of a multi-inlet pump illustrated in  FIG. 3 , the bearing arrangement  90  is arranged intermediate the two rotors  14 . The rotor  14  connected to high-vacuum connector  84  is thus tightly connected to a freely cantilevered shaft projection  94 . 
     The invention has been described with reference to the preferred embodiments. Modifications and alterations may 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.