Patent Publication Number: US-7588417-B2

Title: Turbomolecular pump having multistage stator spacers

Description:
The present invention relates to turbomolecular pumps adapted to pump gases and create a high vacuum in a vacuum enclosure. 
     Turbomolecular pumps generally have a multistage peripheral stator with vanes engaged between the blades of a multistage central rotor. 
     A multistage rotor of a turbomolecular pump comprises an axial series of rotor stages each constituted by a central ring from which the blades of the rotor extend in a substantially radial direction, which blades are regularly distributed around the periphery of the central ring. The assembly is mounted to rotate about an axis of rotation, and is it is driven by motor means. 
     The multistage stator is made up of an axial series of annular stators each forming a stator stage, each stator stage being formed by an annular rim from which inclined vanes extend in a substantially radial direction. 
     The inclines vanes of a stator stage are engaged between the inclined blades of two successive rotor stages. 
     It will thus be understood that the presence of interleaved vanes and blades prevents the rotor from moving axially relative to the stator, whether during assembly of the turbomolecular pump, or during disassembly thereof. 
     Consequently, turbomolecular pumps have means enabling stator elements to be assembled and disassembled radially relative to the rotor. 
     After making a multistage one-piece rotor, the stator is made by progressively assembly around the rotor elements or subassemblies that are to make up the stator. 
     A first known stator structure for a turbomolecular pump is shown in  FIG. 1 . About a one-piece rotor (not shown in the figure) there are fitted progressively a plurality of stator vane sectors, firstly to form annular stator stages, and secondly to form the plurality of annular stator stages that are offset axially in order to make the successive stages of the stator. 
     When considering a rotor having its axis of rotation oriented vertically, a first stator sector having vanes  1  is interposed between two successive bladed rotor stages by the radial movement represented by arrow  2 , and a second stator sector  3  is engaged by being moved radially in the opposite direction along arrow  4  so as to engage both stator sectors  1  and  3  between the same two stages of rotor blades, thereby forming an annular stator stage. 
     Thereafter, an annular spacer  5  is engaged axially by being moved in the direction of arrow  6 , with the spacer  5  coming to bear on the peripheries of the two stator sectors  1  and  3 . The dimensions of the spacer  5  are suitable for holding apart two successive stator stages with the same spacing as the two corresponding successive rotor stages. 
     Thereafter the two following stator sectors  7  and  8  are put into place in similar manner, followed by an annular spacer  9 , and then two stator sectors  10  and  11  and an annular spacer  12 , and so on as a function of the number of stator stages. 
     It will be understood that such a stator is fiddly to assemble, and that it requires components to be provided of dimensions that are determined with sufficient precision to ensure that the accumulated dimensional tolerances, if any, of the components amount to less than the axial dimensional tolerance of the rotor, so as to avoid any contact between the rotor blades and the stator vanes while the pump is in operation. As a result, manufacturing cost is relatively high and it would be advantageous to be able to reduce it. 
       FIG. 2  shows a second prior art turbomolecular pump structure comprising different means for solving the problem of interleaving the stator vanes between the rotor blades. 
     In that prior art structure, there can be seen the rotor  13  mounted to rotate about its axis of rotation I-I as represented by arrow  14 , and comprising a series of rotor stages each comprising inclined blades. There can thus be seen successive rotor stages identified by numerical references  13   a ,  13   b ,  13   c ,  13   d ,  13   e ,  13   f ,  13   g ,  13   h , and  13   i . The stator  15  is made up of by assembling together two half-stators  15   a  and  15   b , each half-stator  15   a  or  15   b  being constituted by a half-shell  16   a  or  16   b  in the form of a half-cylinder, with stator vanes extending radially inwards therefrom such as the vanes  17   a  and  17   b , these vanes being organized as a series of stator stages that are interleaved between the blades of the rotor stages  13   a - 13   i.    
     The presence of two one-piece half-stators  15   a  and  15   b  makes it possible to provide pumps that are insertable, being designed to be housed directly in a casing or a predefined customer system. 
     In this structure, each one-piece half-stator  15   a  or  15   b  is machined from a block of metal so as to provide the vanes  17   a  or  17   b . Because of the configuration and the radially-converging orientation of the vanes, projecting from the concave inside face of the peripheral half-shells  15   a  or  16   b , it is not possible to machine vanes  17   a  or  17   b  of radial length that exceeds some maximum length that is determined by the ability to pass milling tools for working on the vanes. 
     As a result, that prior art turbomolecular pump structure puts a limit on the maximum throughput of the turbomolecular pump, with the throughput limit being determined by the radial length of the vanes. 
     In addition, the half-stators  15   a  and  15   b  are relatively complex and expensive to make by machining, so the advantage of that pump structure is limited. 
     The problem posed by the present invention is to devise a novel turbomolecular pump structure capable of being made at lower cost, while avoiding assembling too great a number of parts and avoiding making parts that are too complex, and also enabling insertable turbomolecular pumps to be made having throughput greater than that obtained by the structure of  FIG. 2 . 
     Another problem is also to avoid needing to make component parts of dimensions that are very precise, since that increases the cost of producing a turbomolecular pump. 
     In order to achieve these and other objects, the invention provides a turbomolecular pump having a multistage peripheral stator of vanes engaged between the blades of a multistage central rotor, in which:
         the stator comprises a plurality of stator vane sectors assembled about the rotor;   the stator vane sectors are distributed in a plurality of axial rows of stator vane sectors;   each axial row of stator vane sectors is secured to a peripheral shell sector, thereby forming a multistage sector for a stator;   each stator vane sector constitutes an individual element;   the stator vane sectors in the same axial row of stator vane sectors are fitted and secured to the concave inside face of the corresponding peripheral shell sector so as to build up the multistage sector for the stator; and   the concave inside face of the peripheral shell sector includes retaining means for individually retaining the stator vane sectors in position.       

     In such a structure, the stator vane sectors constitute individual elements which are easily made by machining metal parts, and the ease of machining enables stators to be made having vanes of radial dimensions that are greater than those which can be achieved in a structure of the kind shown in  FIG. 2 . 
     Simultaneously, the number of parts to be assembled together during assembly of a turbomolecular pump is smaller than that required for the prior art structure of  FIG. 1 . 
     And simultaneously, the component parts of such a turbomolecular pump structure can be made with tolerances that are less strict than can the parts of the  FIG. 1  structure since the peripheral shell sector together with the retaining means itself constitutes a multi-stage spacer which individually positions the vane sectors of the stator, so it is no longer necessary to stack a plurality of parts that are put into place one after another. 
     In an advantageous embodiment, each of the stator vane sectors comprises a single row of vanes, each constituting a single-stage sector for a stator. 
     Nevertheless, without going beyond the ambit of the invention, it is possible to imagine stator vane sectors comprising two or more rows of vanes, each constituting part of a multistage stator, providing that this is possible given the way the vanes are machined. 
     The stator vane sectors can thus comprise at least one annular row of vanes secured to a rim in the form of a sector of a ring. 
     In a first option, the rim may connect together the outside ends of the vanes, in which case it is the rim that then forms the peripheral edge of each stator vane sector and which is fixed to the concave inside face of the corresponding peripheral shell sector in order to form the multistage sector of the stator. 
     Nevertheless, it may be advantageous to provide for the rim to connect together the inside ends of the vanes. Such a structure achieves better effectiveness in vacuum performance, and assembly is made easy by the fact that the vanes are machined from the outside of the rim in the form of a sector of a ring, on vanes that are oriented to diverge radially. It is then the outside ends of the vanes which form the peripheral edge of the stator vane sector and that are fixed to the concave inside face of the corresponding peripheral shell sector in order to make up the multistage sector of the stator. 
     The means for retaining stator vane sectors of the stator on the concave inside face of the peripheral shell sector may be of various kinds. In an advantageous embodiment, the concave inside face of the peripheral shell sector includes a plurality of annular grooves, and each of the stator vane sectors include at least one peripheral edge which is forced radially into one of the annular grooves of the peripheral shell sector. 
     When the stator sectors have a rim interconnecting the inside ends of the vanes, the peripheral edges of the stator vane sectors are constituted by the outside ends of the vanes of said sectors of the stator which are forced radially into the corresponding annular grooves of the peripheral shell sector. 
     When the sectors of the stator have a rim interconnecting the outside ends of the vanes, the peripheral edges of the stator vane sectors are constituted by the rims themselves which are forced into the annular grooves of the peripheral shell sector. 
    
    
     
       Other objects, characteristics, and advantages of the present invention appear from the following description of particular embodiments, made with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view showing a first prior art structure for a turbomolecular pump stator; 
         FIG. 2  is an exploded perspective view showing a second prior art structure for a turbomolecular pump; 
         FIG. 3  is an exploded perspective view showing the general structure of a turbomolecular pump constituting an embodiment of the present invention; 
         FIG. 4  is a perspective view showing how the stator vane sectors are assembled in succession to the corresponding peripheral shell sectors in accordance with the invention; and 
         FIGS. 5 and 6  show two embodiments of stator vane sectors in accordance with the invention. 
     
    
    
     The structures of the prior art turbomolecular pumps of  FIGS. 1 and 2  are described above. 
     Reference is now made to  FIGS. 3 and 4  showing a turbomolecular pump structure constituting an embodiment of the present invention. 
     In  FIG. 3 , the turbomolecular pump of the invention comprises a multistage rotor  18  comprising a series of rotor stages that are axially offset from one another and that are identified by numerical references  18   a ,  18   b ,  18   c ,  18   d ,  18   e ,  18   f ,  18   g ,  18   h ,  18   i , and  18   j . Each rotor stage  18  comprises an inner ring such as the ring  19  from which there project inclined blades such as the blade  18   a  oriented substantially radially outwards. 
     The rotor  18  is mounted to rotate about an axis of rotation I-I as represented by arrow  20 . 
     The stator comprises an assembly of two subassemblies  21  and  22  each forming a multistage sector of a stator, with these sectors being put into place radially around the rotor  18  as represented by respective arrows  23  and  23   a.    
     Each subassembly  21  and  22  comprises an assembly of a respective peripheral shell sector  24  or  25  with an axial row of stator vane sectors  26  or  27 . 
     In the embodiment shown, there can thus be seen a first axial row of stator vane sectors  26  formed by the stator vane sectors  26   a ,  26   b ,  26   c ,  26   d ,  26   e ,  26   f ,  26   g ,  26   h ,  26   i , and  26   j  aligned parallel with the axis of rotation I-I. Similarly, there can be seen a second axial row of stator vane sectors formed by the stator vane sectors  27   a ,  27   b ,  27   c ,  27   d ,  27   e ,  27   f ,  27   g ,  27   h ,  27   i , and  27   j  aligned parallel with the axis of rotation I-I. 
     In the embodiment of  FIG. 3 , the stator vane sectors are of the type having an inside rim, as can be seen more clearly in  FIG. 5 . In this embodiment, the stator vane sector  26   a  comprises an annular row of sectors  126   a  secured to a rim  226   a , itself in the form of a sector of a ring. The rim  226   a  connects together the inside ends of the vanes of the annular row  126   a  of vanes. The peripheral edge  326   a  of the stator vane sector  26   a  is thus constituted by the outside ends of the vanes in the annular row  126   a  of vanes. 
     The invention can also be applied to a second embodiment of a stator vane sector as shown in  FIG. 6 . In this case, there can be seen a rim  226   a  and an annular row  126   a  of vanes. The rim  226   a  interconnects the outside ends of the vanes in the annular row  126   a  of vanes. The peripheral edge  326   a  of the stator vane sector  26   a  is then constituted by the rim  226   a  itself. 
     As can be seen with reference to the peripheral shell sector  24  of  FIG. 3 , the concave inside face of each peripheral shell sector  24  or  25  has a plurality of annular grooves. There can thus be seen annular grooves referenced  24   a ,  24   b ,  24   c ,  24   d ,  24   e ,  24   f ,  24   g ,  24   h ,  24   i , and  24   j.    
     Each annular groove  24   a - 24   j  is disposed and shaped so as to receive the peripheral edge of one of the stator sectors  26   a - 26   j , thereby holding said stator sectors  26   a - 26   j  individually in position. 
     The same applies to the peripheral shell sector  25  which receives and retains the stator vane sectors  27   a - 27   j.    
     During assembly, the stator vane sectors  26   a - 27   j  and  27   a - 27   j  are initially engaged radially so that their peripheral edges (such as the peripheral edge  326   a  of the stator vane sector  26   a ) penetrate into the corresponding annular grooves  24   a - 24   j  of the respective peripheral shell sectors  24  and  25 , as represented, for example, by arrows  28  and  28   a . Two multistage stator sectors  21  and  22  are thus made up which can subsequently be engaged radially around the rotor  18  as represented by arrows  23  and  23   a  in  FIG. 3 . 
     It will also be observed that, where necessary, the peripheral shell sectors such as the peripheral shell sector  25  may include holes such as the hole  29  in order to encourage the space situated at the periphery of the peripheral shell sectors  24  and  25  to be degassed. 
     In the embodiment shown in  FIGS. 3 and 4 , the stator is made by assembling together two multistage half-stators  21  and  22  each occupying a sector of 180°. In which case, the peripheral shell sector  24  or  25  is a half-cylinder and each stator vane sector  26   a - 26   j  and  27   a - 27   j  is a half-ring. 
     In the invention, and depending on the shapes of the vanes, it may be advantageous to provide a stator made by assembling together three multistage stator sectors each occupying 120°, or four multistage stator sectors each occupying 90°, or indeed multistage stator sectors occupying different angles. 
     Nevertheless, the structure shown minimizes the number of parts that need to be assembled together, and that can be advantageous. 
     Likewise, in the embodiment shown in  FIGS. 3 and 4 , the stator is a radial assembly of a single type of multistage stator sectors  21  and  22 . In other words, in this case, each peripheral shell sector  24  or  25  covers the full length of the stator of the turbomolecular pump. 
     Alternatively, the stator may comprise an assembly of a plurality of multistage stator sectors. For example it is possible to make a first stator stage as shown in  FIG. 3  connected axially to a second stator stage of similar structure, possibly with different vane structures and different diameters. The stator is then constituted by an assembly of a plurality of stages, each comprising multiple stages of stator sectors. 
     After making the turbomolecular pump assembly as shown in  FIG. 3 , the assembly needs to be inserted in a casing in order to make a functionally complete turbomolecular pump. A special casing can then be provided together with means for fixing it to an enclosure for evacuating. 
     Nevertheless, in accordance with the invention, it is possible to make a turbomolecular pump that is insertable, i.e. adapted to be inserted in a casing or a client system such as an enclosure for evacuating. The pump is then installed as close as possible to the application, thereby enabling pumping performance to be optimized. The shell sectors  24  and  25  enable the turbomolecular pump elements to be enclosed so as to constitute an assembly, possibly held together by temporary assembly means, which can themselves be removed when the pump is inserted into the client system. 
     In all cases, the shell sectors  24  and  25  should preferably be inserted with little clearance in the casing or the client system so as to ensure that good contact is established between the shell sectors  24  and  25  and the casing all the client system, thereby improving heat exchange. 
     The turbomolecular pump structure of the invention can be used with any possible vane shape in terms of inclination, inside diameter, outside diameter, stage height, and thickness. 
     The system also makes it possible to simplify stacking stages and stators and to be unaffected by the effect of putting the dimensional tolerances of a plurality of elements in series. 
     In addition, the structure improves heat exchange between the rotor and the surroundings via the shell sectors by encouraging thermal contact between the vanes of the stator and the shell sectors, then between the shell sectors and the casing or the peripheral pump housing, and by reducing the number of heat transmission discontinuities due to the one-piece nature of the multistage shell sectors. 
     This structure makes it possible to propose an insertable version for high throughput vacuum pumps. 
     The number of high precision parts is reduced compared with a conventional assembly. The cost of producing the pump is thereby greatly reduced. 
     The present invention is not limited to the embodiments described explicitly, but includes any variant and generalizations that come within the competence of the person skilled in the art.