Abstract:
The present invention relates to a multi-stage vacuum pump comprising: first and second half-shell stator components and first and second end stator components which when assembled define a plurality of pumping chambers. The half-shell components are assembled together along respective pairs of mutual engaging longitudinal faces and the end stator components are assembled at the ends of the half-shell components at respective pairs of mutual engaging end faces. An annular channel is counter-sunk in at least one end face of each pair of mutual engaging end faces for receiving respective sealing members for sealing between the half-shell components and the end stator components. Recesses are counter-sunk at the end portions of the longitudinal faces for receiving a sealant and supports provide support for the annular sealing members at the recesses when the annular sealing members are received in the annular channels to resist protrusion of the annular sealing members into the recesses when compressed between mutually engaging end faces.

Description:
[0001]    This application is a national stage entry under 35 U.S.C. §371 of International Application No. PCT/GB2015/052068, filed Jul. 17, 2015, the entire content of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a vacuum pump, in particular a multi-stage vacuum pump and a stator of such a pump. 
       BACKGROUND TO THE INVENTION 
       [0003]    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. 
         [0004]    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. 
         [0005]    The stator  100  further comprises first and second end stator components  122 ,  124 , also known as head plates. 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. 
         [0006]    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 . 
         [0007]    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. 
         [0008]    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. 
         [0009]    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 OF THE INVENTION 
       [0010]    The present invention provides in the embodiments an improved seal arrangement for sealing a clam shell pump. 
         [0011]    The present invention provides a multi-stage vacuum pump comprising a stator comprising: first and second half-shell stator components and first and second end stator components which when assembled define a plurality of pumping chambers; the half-shell components being assembled together along respective pairs of mutual engaging longitudinal faces and the end stator components being assembled at the ends of the half-shell components at respective pairs of mutual engaging end faces; a longitudinal channel counter-sunk in at least one longitudinal face of each pair of mutual engaging longitudinal faces for receiving respective longitudinal sealing members for sealing between the half-shell components; an annular channel counter-sunk in at least one end face of each pair of mutual engaging end faces for receiving respective sealing members for sealing between the half-shell components and the end stator components; recesses counter-sunk at the end portions of the longitudinal faces for receiving a sealant for sealing between the longitudinal sealing members and the annular sealing members, supports for supporting the annular sealing members at the recesses when the annular sealing members are received in the annular channels to resist protrusion of the annular sealing members into the recesses when compressed between mutually engaging end faces. 
         [0012]    The present invention also provides multi-stage vacuum pump comprising a stator comprising: first and second half-shell stator components and first and second end stator components which when assembled define a plurality of pumping chambers; the half-shell components being assembled together along respective pairs of mutual engaging longitudinal faces and the end stator components being assembled at the ends of the half-shell components at respective pairs of mutual engaging end faces; a longitudinal channel counter-sunk in at least one longitudinal face of each pair of mutual engaging longitudinal faces for receiving respective longitudinal sealing members for sealing between the half-shell components; an annular channel counter-sunk in at least one end face of each pair of mutual engaging end faces for receiving respective sealing members for sealing between the half-shell components and the end stator components; shallow recesses counter-sunk at the end portions of the longitudinal faces for receiving a sealant for sealing between the longitudinal sealing members and the annular sealing members, wherein the depth of the shallow recesses is less than the depth of the longitudinal channels and counter-sunk into each recess is at least one deep pocket extending from the longitudinal channel for allowing sealant to flow around a longitudinal sealing member received in the longitudinal channel for preventing the formation of a leakage path. Other preferred and/or optional features of the invention are defined in the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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 in more detail, with reference to the accompanying drawings, in which: 
           [0014]      FIG. 1  shows generally the components of a clam shell stator; 
           [0015]      FIG. 2  shows a plan view and section of part of a half shell stator component without adequate sealing and the formation of a leakage path; 
           [0016]      FIG. 3  shows one example of a half shell stator component with adequate sealing; 
           [0017]      FIG. 4  is an enlarged view of an end portion of the half shell stator component shown in  FIG. 3 ; 
           [0018]      FIG. 5  is a section taken through a recess at the end portion shown in  FIG. 4 ; 
           [0019]      FIG. 6  is a plan view of a longitudinal face of another example of a half shell stator component with adequate sealing; 
           [0020]      FIG. 7  is an enlarged view of an end portion of the half shell stator component shown in  FIG. 6 ; and 
           [0021]      FIG. 8  is a section taken through a recess and deep pockets at the end portion shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    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 three-dimensional sealing member as discussed above. This three-dimensional sealing member is expensive to manufacture and intricate to fit in place. 
         [0023]    Previous patent applications of the present applicant have proposed the use of four separate sealing components, namely two longitudinal sealing members, or gaskets, for sealing between the half-shell components  102 ,  104  in  FIG. 1  and two annular sealing members, or O-rings, for sealing between the end faces of the half-shell components and the end stator components  122 ,  124 . Considerable difficulty was encountered when sealing at the interfaces between the longitudinal sealing members and the annular sealing members and the applicant&#39;s previous applications address a number of solutions to the difficulties. The present application differs from the previous applications in that rather than sealing the interfaces between the sealing members the present embodiments apply a sealant (which may be liquid or gel prior to curing to solid) to seal between the longitudinal sealing members and the half-shell components and between the annular sealing members and both the half-shell and end stator components. Therefore, the present embodiments do not have direct interfaces between sealing members. However, even without these interfaces, sealing is problematic particularly given that the differential pressure across the seal can be both positive and negative and vary by several bar. Early experiments conducted by the applicant and the attendant problems which arose are now described with reference to  FIG. 2 .  FIG. 2  shows an end portion of longitudinal face  120  of a half shell component  102  and a portion of the end face  128 . In this early arrangement, a longitudinal sealing member  140  is received in a counter sunk deep channel  142  which extends over the length of the longitudinal face leaving a space between the end of the longitudinal member, or channel, and an annular sealing member  146 . The annular sealing member  146  is received in an annular channel  150  counter sunk in the end face  128 . A shallow recess  144  is counter sunk in the longitudinal face surrounding the end of the deep channel  142  and in the space between this channel and annular channel  150 . Sealant  152  is applied in the shallow recess for sealing between the longitudinal member  140  and the annular member  146 . In this way, there is no direct interface between the longitudinal sealing member and the annular sealing member. 
         [0024]    It was found however as shown in section A-A that the sealant  152  did not penetrate sufficiently into the channel  142  to provide an adequate seal between the longitudinal sealing member  140  and the half shell components. Spaces  154  in channel  142  are formed and as shown in the plan view in  FIG. 2  a leakage path  156  allows the flow of gas from atmosphere along one side of the sealing member around its tip and along the other side of the sealing member into the pump. 
         [0025]    In order to increase penetration of the sealant around the longitudinal sealing member the depth of the recess  144  was increased so that it was approximately equal to the depth of the longitudinal channel  142 . This arrangement provided adequate sealing about the longitudinal sealing member  140  but resulted in less than adequate sealing at the annular sealing member  146 . In this regard, the sealant is fluid when applied until allowed to cure, and when the annular sealing member is compressed between the end faces of the half shell components and the internal face of the end components a kink is formed in the annular sealing member where it protrudes into the deep recess between the half shell components and displaces sealant which in its fluid state cannot provide sufficient resistance to kinking. Whereas the shallow recess shown in  FIG. 2  provided sufficient support for the annular sealing member to avoid significant kinking, by deepening the recess and solving the sealing problem around the longitudinal sealing member it created a different problem around the annular sealing member. 
         [0026]    Embodiments of the invention provide a solution having adequate sealing at both the longitudinal sealing member and the annular sealing member. 
         [0027]    A first embodiment of the invention is described with reference to  FIGS. 3 and 4 .  FIG. 3  shows a half shell stator component  10  similar in general structure to component  102  in  FIG. 1  having two longitudinal faces  12  located on either side of a series of pumping chambers shown generally at  14 .  FIG. 4  is an enlarged view of an end portion of one of the longitudinal faces. In this first embodiment, one or both of the half shell stator components may be structured as shown. In this regard,  FIGS. 3 and 4  show one half shell component and the other half shell component may correspond generally in structure or alternatively the other half shell component may comprise a planar longitudinal face for cooperating with the half shell component shown for sealing the pump. 
         [0028]    Stator component  10  comprises a deep longitudinal channel  16  extending along a length of each of the longitudinal faces  12  for receiving a longitudinal sealing member (not shown in these Figures but see  FIG. 2 ). The ends of the deep channel are separated from the end face  18  of the half shell component by a deep recess  20 . When the half shell components are assembled together they foil  1  an annular channel  22  which extends around the circumference of the pumping chambers  14  for sealing the end faces. 
         [0029]    As previously discussed, a problem with such a deep recess as shown in  FIG. 3  is that it results in kinking of the annular sealing member received in channel  22 . In the present arrangement, the annular sealing member is supported across the deep recess. In this way, sealant applied in the recess sufficiently seals around the longitudinal sealing member and also can seal adequately against the annular sealing member without kinking. 
         [0030]    In more detail, a support  24  upstands from the counter sunk surface of recess  20  at the end face  18  for supporting the annular sealing member. As shown, the support is formed by a wall which is generally in line with the counter sunk surface of the annular channel  22 . The annular channel has a width for receiving and locating the annular sealing member and the wall extends only partially over the width of the annular channel. On at least one side, and preferably on both sides as shown in  FIG. 4 , is a space  26  between the wall and the end face so that sealant can flow and directly contact and seal against the annular sealing member in the annular channel. In this way, the arrangement supports the annular sealing member whilst also permitting adequate sealing between the sealant and the annular sealing member. In another example, the support  24  may extend from the opposing half-shell stator component with its end abutting or closely adjacent the counter-sunk surface of recess  20 . 
         [0031]    During assembly, a longitudinal sealing member is inserted in each of the longitudinal channels  16  shown in  FIG. 3 . In order to locate the sealing member and provide a small tensile force the longitudinal channel has two pinch points  28  referenced in  FIG. 4  for applying pressure at respective end portions of the sealing member. Following location of the longitudinal sealing members in both channels  16 , sealant is applied to the channels and recesses  20  prior to assembling the half shell components together or injected after the components are assembled together. At least one overflow path, or channel,  30  is provided at each end portion to allow sealant to escape either under compression of the half shell components together or following pressure from sealant injection. 
         [0032]    The deep recess  20  is of comparable depth to that of the deep channel  16 . Therefore, the sealant when applied can penetrate around the longitudinal sealing member when it is positioned in the longitudinal channel.  FIG. 5  shows a section through one of the deep recesses  20  counter sunk from longitudinal face  12 . In  FIG. 5 , the sealant  32  is shown penetrating and surrounding the end portion of one longitudinal member  34  thereby providing an effective seal. The depth of the recess allows the sealant to prevent the formation of a leakage path around the longitudinal sealing member. As shown in 
         [0033]      FIG. 4 , the supporting wall  24  supports the annular sealing member to resist kinking whilst allowing sealant to flow on either side of the wall to contact and seal against the annular sealing member. Therefore, the arrangement shown in  FIGS. 3 to 5  provides adequate sealing between the sealant and the longitudinal sealing members and between the sealant and the annular sealing members to provide effective sealing of the pump. 
         [0034]    A second embodiment of the invention is described with reference to  FIGS. 6 to 8 , in which like reference numerals are used to refer to like components of the first embodiment. 
         [0035]    The principal difference between the second embodiment and the first embodiment is that a shallow recess, rather than a deep recess, connects the deep channels of the longitudinal sealing members with the annular channels of the annular sealing members. The reduced depth of the shallow recess reduces kinking of the annular sealing members since there is less space between the half shell stator components into which the annular sealing members can protrude when compressed. However, as previously indicated, a shallow recess around the deep channel of the longitudinal sealing members prevents or reduces penetration of the sealant around the longitudinal sealing members. Therefore, the second embodiment comprises a deep pocket in the shallow recess to permit adequate penetration of sealant. 
         [0036]    In more detail, and referring to  FIGS. 6 to 8 , a shallow recess  36  is positioned between the deep channels  40  of the mutually engaging longitudinal faces  12  of the half shell stator components  102 ,  104  and the annular channels  22  in the end face of the half shell components. The shallow recess has insufficient depth in itself to allow penetration of the sealant  32  into the deep channel and around the end portions of the longitudinal sealing members for effective sealing. However, in this embodiment, deep pockets  38  extend outwardly from the deep channels for receiving a sealant so that it can penetrate more deeply into channels. In the Figures, the deep pockets are located at each of the longitudinal ends of the deep channels and extend transversely on both sides of the channels and generally perpendicularly to the deep channel into the shallow recess  36 . Alternatively, there may be a single deep pocket. 
         [0037]    The shallowness of the recess  36  means that the annular sealing member  42  may not require support across the gap between the half shell stator components  102 ,  104  to prevent significant kinking of the annular sealing member. Nevertheless, a support such as a wall  25  shown schematically may be provided upstanding from the counter sunk surface of the shallow recess to give additional support to the annular sealing members across the space between the half-shell components, similarly to wall  24  of the first embodiment. 
         [0038]    In assembly, the longitudinal sealing members  34  are positioned in the deep channels  40  and secured in tension between the pinch points  28 . The two half shell stator components  182 ,  104  are brought together along their respective mutually engaging longitudinal faces  12  compressing the longitudinal sealing member and providing sealing along the length of the stator. Sealant  32  may be applied prior to assembling the half shell stator components or injected following assembly. If applied prior to assembly the overflow channels  30  allow excess sealant to escape or in the alternative the side channels  26  can be used to inject sealant under pressure into the assembled components. The deep pockets  38  allow sealant to flow from the shallow recesses  36  around the cross-section of the end portions of the longitudinal sealing members to provide adequate sealing as shown in  FIG. 8 . 
         [0039]    In both embodiments, bores  44  are provided in the half shell stator components for receiving fastening members such as bolts for fastening the components together. 
         [0040]    When the half shell stator components are assembled together they define the annular channels  22  at each end of the assembly and following assembly the annular sealing members  42  are positioned in the annular channels. Assembly of the end stator components  122 ,  124  at the end faces of the assembled half shell components compresses the annular sealing members. This compression applies an axial force to the annular sealing members but as the gap between the half shell stator components is reduced by the shallow recess  36  the annular sealing members do not protrude into the recesses to affect adequate sealing, particularly if a supporting wall is provided. 
         [0041]    The present description uses the terms ‘deep’ and ‘shallow’. In the context of this description, ‘deep’ refers to the depth substantially equal to that of the longitudinal channels counter sunk into the end faces  12  for the longitudinal sealing members. The depth is required for receiving the longitudinal sealing members and to allow sealant to seal around the end portions of the longitudinal sealing members to prevent leakage. ‘Shallow’ refers to a depth counter sunk into the end faces  12  which is less than ‘deep’, preferably less than half of the depth and more preferably less than a quarter of the depth, and which in insufficient to allow sealant to penetrate around the longitudinal sealing members. The exact measurements of deep and shallow depend on the overall measurements of the stator and pump, however typically ‘deep’ may be 2 mm or more, and ‘shallow’ may be 1 mm or less or preferably 0.5 mm or less.