Abstract:
A multi-stage vacuum pump may include first and second half-shell components defining a plurality of pumping chambers and for assembly together along respective longitudinal extending faces; first and second end stator components for assembly at respective longitudinal seals for sealing between the first and second half-shell stator components when assembled together at the longitudinally extending faces; and annular seals for sealing between the first and second end stator components and the first and second half-shell stator components when assembled; wherein the longitudinal seals have end portions which abut against the annular seals for sealing therebetween and the first and second half-shell stator components have formations for resisting movement of the end portions away from the annular seals when the end portions are compressed between the first and second half-shell stator components.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage entry under 35 U.S.C. §371 of PCT Application No. PCT/GB2013/050087, filed Jan. 17, 2012, which claims the benefit of British Application No. 1104781.8, filed Mar. 22, 2011. The entire contents of PCT Application No. PCT/GB2012/050087 and British Patent Application No. 1104781.8 are incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a vacuum pump, in particular a multi-stage vacuum pump and a stator of such a pump. 
     BACKGROUND 
     A vacuum pump may be formed by positive displacement pumps such as roots or claw pumps, having one or more pumping stages connected in series. Multi-stage pumps are desirable because they involve less manufacturing cost and assembly time compared to multiple single stage pumps in series. 
     Multi-stage roots or claw pumps may be manufactured and assembled in the form of a clamshell. As shown in  FIG. 1 , the stator  100  of such a pump comprises first and second half-shell stator components  102 ,  104  which together define a plurality of pumping chambers  106 ,  108 ,  110 ,  112 ,  114 ,  116 . Each of the half-shells has first and second longitudinally extending faces which mutually engage with the respective longitudinally extending faces of the other half-shell when the half-shells are fitted together. Only the two longitudinally extending faces  118 ,  120  of half-shell  102  are visible in the Figure. During assembly the two half shells are brought together in a generally radial direction shown by the arrows R. 
     The stator  100  further comprises first and second end stator components  122 ,  124 . When the half-shells have been fitted together, the first and second end components are fitted to respective end faces  126 ,  128  of the joined half-shells in a generally axial, or longitudinal, direction shown by arrows L. The inner faces  130 ,  132  of the end components mutually engage with respective end faces  126 ,  128  of the half-shells. 
     Each of the pumping chambers  106 - 116  is formed between transverse walls  134  of the half-shells. Only the transverse walls of half-shell  102  can be seen in  FIG. 1 . When the half-shells are assembled the transverse walls provide axial separation between one pumping chamber and an adjacent pumping chamber, or between the end pumping chambers  106 ,  116  and the end stator components. The present example shows a typical stator arrangement for a roots or claw pump having two longitudinally extending shafts (not shown) which are located in the apertures  136  formed in the transverse walls  134  when the half-shells are fitted together. Prior to assembly, rotors (not shown) are fitted to the shafts so that two rotors are located in each pumping chamber. Although not shown in this simplified drawing, the end components each have two apertures through which the shafts extend. The shafts are supported by bearings in the end components and driven by a motor and gear mechanism. 
     The multi-stage vacuum pump operates at pressures within the pumping chamber less than atmosphere and potentially as low as 10 −3  mbar. Accordingly, there will be a pressure differential between atmosphere and the inside of the pump. Leakage of surrounding gas into the pump must therefore be prevented at the joints between the stator components, which are formed between the longitudinally extending surfaces  118 ,  120  of the half-shells and between the end faces  126 ,  128  of the half-shells and the inner faces  130 ,  132  of the end components. An adhesive is typically used to seal between the half-shells and between the half-shells and the end components, but the adhesive is particularly susceptible to damage by corrosive pumped gases, and is difficult and time consuming to apply consistently. It can also inhibit disassembly and maintenance. 
     A known alternative sealing arrangement is disclosed in US2002155014 providing a one piece sealing member comprising two longitudinal portions and two annular portions. The sealing member is however generally quite intricate to fit in place and expensive to manufacture. 
     SUMMARY 
     The present invention provides an improved seal arrangement for sealing a clam shell pump. 
     The present invention provides a vacuum pump comprising: first and second half-shell stator components defining at least one pumping chamber and for assembly together along respective longitudinally extending faces; first and second end stator components for assembly at respective longitudinal end faces of the first and second half-shell stator components; longitudinal seals for sealing between the first and second half-shell stator components when assembled together at the longitudinally extending faces; and annular seals for sealing between the first and second end stator components and the first and second half-shell stator components when assembled; wherein the longitudinal seals have end portions which abut against the annular seals for sealing therebetween and the first and second half-shell stator components have formations for resisting movement of the end portions away from the annular seals when the end portions are compressed between the first and second half-shell stator components. 
     Other preferred and/or optional features of the invention are defined in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be well understood, some embodiments thereof will now be described in more detail, with reference to the accompanying drawings in which: 
         FIG. 1  shows generally the components of a clam shell stator; 
         FIG. 2  shows a theoretically possible but undesirable sealing arrangement for the half-shell stator components and two stator end components provided for explanatory purposes only; 
         FIG. 3  shows a half-shell having the sealing arrangement of  FIG. 2 ; 
         FIG. 4  shows an end component having the sealing arrangement of  FIG. 2 ; 
         FIG. 5  shows a sealing arrangement for the half-shell stator components of a multi-stage vacuum pump and two stator end components according to an embodiment of the invention; 
         FIG. 6  shows in more detail a portion of the arrangement shown in  FIG. 5 ; 
         FIG. 7  shows in more detail a modified portion of the arrangement shown in  FIGS. 5 ; 
         FIG. 8  shows the sealing arrangement of  FIG. 7  when compressed during assembly; 
         FIG. 9  shows in more detail a further modified portion of the arrangement shown in  FIG. 5 ; 
         FIG. 10  shows a sealing arrangement according to further embodiments of the invention; 
         FIG. 11  shows a half-shell and end stator component according to another sealing arrangement; 
         FIG. 12  shows a longitudinal seal for location in the channel shown in  FIG. 11 ; 
         FIG. 13  shows the longitudinal seal of  FIG. 12  located in position in the half-shell stator component shown in  FIG. 11  together with an annular seal but prior to final assembly and compression; and 
         FIG. 14  shows the seal in  FIG. 11  in use after final assembly and compression. 
     
    
    
     By way of background to the invention, US2002155014 discusses the problem of sealing a clam shell stator. In particular, it indicates that leakage lines exist between a longitudinal gasket providing peripheral radial sealing and O-rings providing axial sealing at the ends which results in unsatisfactory sealing. As a consequence the patent proposes a one-piece sealing member as discussed above. 
     DETAILED DESCRIPTION 
     Looking in more detail now at this problem,  FIG. 2  shows a plan view of the half-shell  102  and sections taken through end components  122 ,  124 .  FIG. 3  shows a view of one end face  126  of the joined half-shells  102 ,  104 .  FIG. 4  shows a view of an inner face  132  of an end component  124 . 
     Referring to  FIGS. 2 to 4 , two longitudinal seal members  138  are located in channels  140  formed in the longitudinally extending faces  118 ,  120  and  142 ,  144  of the first and second half-shells  102 ,  104 . The longitudinal seal members  138  resist leakage of ambient gases into the pump as shown by the arrows G1 over the length of the half-shells. 
     Two generally annular seal members  146  are located in respective generally annular channels  148  of the inner faces  130 ,  132  of the end components  122 ,  124 . The seal members  146  resist leakage of ambient gases into the pump as shown by the arrows G2 over the periphery of the joint between the end components and the half-shells. Accordingly, the leakage of gases through the apertures  150  in the end components or the apertures  134  in the end of the joined half-shells is generally prevented. 
     A problem with this sealing arrangement is that an inconsistent seal is provided between the longitudinal seal members  138  and the annular seal members  146  as indicated by a space S shown in  FIG. 2 . The inconsistent seal allows leakage of gases between the two seal members  138 ,  146 . The longitudinal seal members  138  are configured to be compressed between the two half-shells when they are assembled together to provide a tight fit. However, when compressed there is a tendency for some movement of the seal members  138  in the channels  140  whereby the space S may be created or increased. The longitudinal seal members can be manufactured with a longer length than the length of the channels  140 , however, in this case compression between the half-shells may lead to kinking in the seal members causing leakage. 
     Referring now to a first embodiment of the invention shown in  FIG. 5 , part of a clam shell multi-stage vacuum pump is shown which is generally similar to the clam-shell pump discussed in detail in relation to  FIGS. 1 to 4 , except that the sealing arrangement is different. Accordingly, the general arrangement of the pump will not be described again and like features are given like references. 
     In  FIG. 5 , a section is taken through the end stator components  122 ,  124  and only one half-shell  16  is shown. The stator  10  comprises two longitudinally extending seal members  12  which are located in respective channels  14  of the half-shell stator components  16 ,  18 . The channels  14  are recessed into the longitudinally extending faces  20 ,  22  of the half-shell  16 . Only component  16  is shown in this Figure, although half-shell  18  preferably has a similar arrangement. When fitted together, the half-shells compress the seal members  12  causing slight expansion so that there is a gas tight fit between the seal members and the channels. Each pair of mutually engaging longitudinal faces may have a channel for locating a seal member  12  or alternatively only one such face may have a channel whilst the other face remains generally flat. 
     The longitudinal end portions  24  of the seal members  12  are configured to co-operate with respective end portions  26  of the channels to resist movement of the seal end portions  24  away from the annular seal members  146  when the stator components are assembled and the seal members  12  are compressed. In this way, the end portions  24  are retained in contact with the annular seal members when the pump is assembled and in operation. In the present example, the end portions are enlarged compared to the middle portion  28  of the seal members. The end portions  26  of the channels are likewise enlarged compared to the middle portions  30  of the channels, and are shaped to complement the shape of the seal end portions  24 . More particularly, and as shown in the enlarged drawing of  FIG. 6 , the end portions  24 ,  26  taper outwardly in two lateral dimensions (perpendicular to the longitudinal axis) and are in the form of truncated cones. Of course, there are numerous complementary configurations of the end portions  24 ,  26  which resist movement of the longitudinal seal away the annular seal. For example, the end portions may be trapezoidal having planar tapering sides (i.e. taper outwardly only in one lateral dimension) or may be rectilinear having sides which extend generally laterally to the longitudinal configuration of the seal members and channels. 
     The longitudinal seals  12  may be slightly shorter in length that the length of the channels  14  of the half-shells  16 ,  18  and require slight stretching in order fit in place. A small amount of tension in the middle portion  28  of the seals is generated between the end portions  24 . The tension helps to ensure that the end portions  24  sit tightly against the end portions  26  of the channels so that movement away from the annular seals is resisted immediately upon initial compression. 
     In another arrangement shown in  FIGS. 7 and 8 , the seal end portions  32  are configured so that when the stator is assembled and the seal members are compressed, the longitudinal seal member expands towards the annular seal member. This expansion increases the sealing force between the seal members and preferably as shown in  FIG. 8  also extends the sealing surface which resists the leakage of gas into the pump as the end portion is deformed against the annular seal. 
     In more detail, a longitudinal seal member  32  comprises a middle portion  28  which is generally cylindrical as previously described. The end portion  34  of the seal member has an end configuration which extends towards the annular seal member  146  to a greater extent on either side of the annular seal and is configured to sit proud of the end face of the half-shells. As shown in  FIG. 7 , the end configuration is generally curved. When uncompressed, the end protrusions  35  preferably overlap with the annular seal  146  in the longitudinal direction so that less expansion is required during compression in order to form a good seal between the two seals. The end component  36  in this arrangement comprises a generally annular channel  38  for receiving the annular seal. Additionally a recess  40  is formed in the surface  130  of the end component in the region of the longitudinal seal. As shown in  FIG. 8 , when the seal is compressed the end portion of the channel  42  of the half-shells resists movement of the end portion  34  away from the annular seal and results in the end portion expanding towards the annular seal as shown by the arrows. The provision of the recess  40  in this example allows the end portion  34  to expand around the cross-section of the annular seal member. Accordingly, the sealing force between the seals is increased and the sealing surface  44  is extended adopting an arcuate interface. Although not specifically shown in  FIGS. 7 and 8 , and depending on the material properties of the longitudinal seal and the annular seal, the annular seal may also be deformed by movement of the longitudinal seals towards it during assembly. 
     In an alternative arrangement shown in  FIG. 9 , a longitudinal seal member  46  may have an end portion  48  which is configured like a fledge of an arrow, having an end surface  50  which tapers inwardly, two parallel generally straight sides  52  and a surface  54  which tapers towards the middle portion  28 . The end portion  53  of the channel of the half-shells is configured to complement the shape of the end portion  48  and to resist its movement away from the annular seal. End portions  48  function in a similar way when compressed to the end portions  34  described above in relation to  FIGS. 7 and 8 , such that the sealing force between the seals  46  and  146  is increased and the leakage path is extended. 
     In a further arrangement shown in  FIG. 10 , a longitudinal seal member  47  may have an end portion  49  which is generally trapezoidal with upper and lower surfaces (as orientated in the Figure) that taper outwardly from a generally flat middle portion  51  and side surfaces that do not taper. The channel  53  in the longitudinal sealing surface has an end portion  55  which is shaped to complement the end portion  49  of the seal member  47 . In a modification, a seal member  57  has a generally circular groove  59  for receiving an annular sealing member and for extending the sealing surface between members. 
     A further embodiment of the invention is shown in  FIGS. 11 to 14 .  FIG. 11  shows, in enlarged view, portions of the end component  56  and half-shell  58  without longitudinal or annular sealing members. A longitudinally extending face  60  of the half-shell has countersunk into its surface a longitudinal recess, or channel,  62  for locating the longitudinally extending seal member (shown in  FIG. 12 ). Upstanding generally orthogonally from the recess is a wall  64  having an upper surface which is flush with the face  60 . In another arrangement the wall may extend into the recess of the opposing half-shell. The end face  66  of the half-shell has countersunk therein a generally annular channel  68  for receiving an annular seal member (shown in  FIG. 13 ).  FIG. 11  shows only a cross-section of the annular channel  68  which is generally perpendicular to and formed in the recess  62 . A recessed shoulder  69  is formed for co-operating with a locating shoulder of the longitudinal seal member as described in more detail below. 
     A longitudinal seal member  70  is shown in  FIG. 12  and is shaped to complement the shape of the recess  62 . Seal  70  comprises two elongate portions  72  which fit in the recess  62  and are laterally spaced apart for fitting closely adjacent the upstanding wall  64 . A laterally extending portion  74  of the seal extends between the elongate portions and is configured to be closely adjacent an end  76  of the wall. A claw shaped formation extends from the laterally extending portion  74 , having two protrusions  78  and a generally semi-circular recess  80  similar in size and shape to the cross-section of the annular channel  68 . The end stator component  56  has a generally planar inner face  82  for compressing the annular seal member when it is located in the annular channel  68 . Locating shoulders  71  extend laterally outwardly for co-operating with recessed shoulders  69  of the channel  62 . 
       FIG. 13  shows the annular seal member  146  and the longitudinal seal member  70  fitted in place in the stator half shell but prior to full assembly and compression. It will be seen that in this condition, the locating shoulders  71  of the seal member sit flush against respective recessed shoulders  69  of the channel. In this way, the seal member can easily be fitted in its correct position in the channel. Prior to compression a gap  73  exists between the end surface  76  of the wall and the lateral portion  74  of the seal member. The size of the gap  73  can be controlled within design tolerances to increase or decrease the force applied by the longitudinal seal member to the annular seal member during final assembly and compression. 
     As shown in  FIG. 14  after final compression, the longitudinal seal member  70  and the annular seal member  146  are compressed respectively between half shells  58  on the one hand and between the half-shells  58  and the end component  56  on the other hand, and the lateral portion  74  of the longitudinal seal member expands into the gap  73  and abuts against the wall  76 . The lateral portion also expands towards the annular seal member and the claws  78  expand laterally towards the annular seal member as shown by the arrows. Preferably, the seals deform to some extent to provide a tight fit and a good seal. Whilst the seals are deformed against each other a generally semi-circular sealing surface is formed which resists leakage into the stator. 
     The longitudinal seal member in the embodiments described above may take the form of a gasket having a generally flat configuration in which it has greater extent in two dimensions and less extent in a third dimension. The gaskets may be formed from a relatively hard material such as a metal. In this case, it is important to control the sealing force between the gasket and the annular seal member so that the gasket does not damage the annular seal member when they are compressed together.