Abstract:
The invention concerns a vacuum pump consisting in the assembly of two stator half-shells ( 101, 102 ) and two directly mounted end parts ( 31, 32 ) with an interposed single-piece continuous pressure seal ( 33 ). The pressure seal ( 33 ) comprises two annular end parts ( 34, 35 ) generally parallel to each other and connected by two side-members ( 36, 37 ) which are generally perpendicular thereto. Thus, the number of components to be assembled to produce an multistage dry vacuum pump is reduced, while providing satisfactory impermeability to outside atmosphere.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to multistage dry vacuum pumps such as Roots type multistage pumps, claw type pumps, and combined Roots-and-claw type pumps.  
           [0002]    Such multistage dry vacuum pumps are made up of a plurality of compression stages connected in series.  
           [0003]    [0003]FIGS. 1 and 2 show a multistage Roots type dry pump of the prior art. FIG. 2 is a perspective view in longitudinal section showing the stator of such a Roots pump. In the stator  1 , between a gas inlet  2  and a gas outlet  3 , there can be seen five successive compression chambers respectively referenced  4 ,  5 ,  6 ,  7  and  8 . Adjacent chambers are separated by respective transverse walls  9 ,  10 ,  11 , and  12  each pierced by two holes such as the holes  13  and  14  in the transverse wall  12  for passing the shafts of two parallel rotors, not shown, that are mechanically coupled together, and that carry compression lobes of the Roots or claw type. Adjacent chambers are interconnected via a gas flow duct such as the gas flow duct  15  connecting the delivery outlet of the first compression chamber  4  to the suction inlet of the second compression chamber  5 .  
           [0004]    The rotor lobes that penetrate into the compression chambers  4 - 8  are of a diameter that is greater than that of the rotor shafts that pass through the holes  13  and  14 . It is therefore not possible to engage an entire rotor axially in the stator  1  by mere axial displacement. Nor is it possible to envisage machining a one-piece stator  1  in such a manner as to make the cavities constituting the compression chambers  4 - 8 .  
           [0005]    To make both machining and assembly possible, and also to provide good sealing, the stators of known dry vacuum pumps are generally built up as an axial assembly of a plurality of stator elements, respectively referenced  16 ,  17 ,  18 ,  19  and  20 , which are assembled to one another via their respective front end walls such as the front end wall  21  of stator element  16 , with interposed between the walls respective sealing rings  22 ,  23 ,  24 ,  25  and  26  that become compressed axially so as to isolate each compression chamber  4 - 8  from the outside atmosphere.  
           [0006]    Such a structure for a dry pump of the Roots or claw type requires each stator element  16 - 20  to be machined separately, and then it requires an assembly operation to be performed that is lengthy and difficult, consisting in fitting both rotor shafts in a support frame, in adjusting the positions of the lobes in the last compression chamber  8 , positioning the last stator element  20  together with the sealing ring  26 , fitting the lobes for the last compression chamber but one  7 , bringing the last stator element but one  19  together with the sealing ring  25  into position, and so on to the first stator element  16 . Given that clearance between the rotor lobes and the walls of the stator is very small in order to seal each compression stage of the vacuum pump, it will be understood that such assembly is particularly lengthy and difficult to implement, and it is generally accepted that several hours of labor are required to perform this operation on a five-stage dry vacuum pump.  
           [0007]    Another problem, in such known multistage dry vacuum pumps, is the difficulty of aligning the stator elements with one another, given that errors are liable to accumulate between the first stator element  16  and the last stator element  20 , thus making it difficult to control the clearance between the rotors and the stator in mass production.  
           [0008]    Documents EP 0 476 631 A and JP 03 145594 A describe vacuum pump structures having a stator made up of two half-shells that are assembled together radially with a longitudinal assembly surface generally parallel to the axes of the rotors, the stator being closed in leaktight manner as its ends by two fitted endpieces that are engaged axially. Those documents do not mention the advantage of such a stator structure in the form of two half-shells, and they do not describe means for providing sealing between the stator and the rotor.  
           [0009]    The difficulty lies in the need both to provide peripheral radial sealing in the longitudinal assembly surface between the two half-shells so as to prevent gases passing between the outside atmosphere and the internal cavities of the pump, while simultaneously providing axial sealing at the ends between the half-shells and the fitted endpieces.  
           [0010]    In traditional manner, it might be imagined that axial sealing at the ends could be provided by sealing rings of the kind shown in FIGS. 1 and 2 for the prior art pump, and that radial peripheral sealing could be provided by longitudinal gaskets compressed between the two half-shells. Unfortunately, that solution presents a major drawback stemming from the fact that leakage lines exist between the longitudinal gasket providing peripheral radial sealing and the O-rings providing axial sealing at the ends. Sealing therefore is unsatisfactory.  
         SUMMARY OF THE INVENTION  
         [0011]    The problem posed by the present invention is that of designing a new multistage dry vacuum pump structure that makes it possible to reduce significantly the number of parts to be assembled during assembly, while facilitating assembly and enabling it to be performed more quickly, and while also providing sealing that is satisfactory between the internal cavities of the vacuum pump and the outside atmosphere so as to avoid any risks of the pumped gases being polluted by the outside atmosphere, and any risks of the outside atmosphere being polluted by the pumped gases.  
           [0012]    The solution of the invention consists in providing a continuous one-piece sealing gasket which provides both types of sealing simultaneously in a stator structure in the form of two half-shells.  
           [0013]    Thus, to achieve these objects, and others, the invention provides a multistage dry vacuum pump made up of a plurality of compression stages placed in series, the pump having at least one rotor mounted to rotate in a stator that is closed in leaktight manner at its ends by two fitted end pieces; furthermore:  
           [0014]    the stator is made by radially assembling together two half-shells on a longitudinal assembly surface, each compression stage thus being contained in two corresponding portions of each of the half-shells, the two half-shells, once assembled together, containing all of the compression stages;  
           [0015]    a continuous one-piece gasket serves to provide both peripheral radial sealing in the longitudinal assembly surface of the half-shells, and axial end sealing between the half-shells and the fitted end pieces so as to isolate the compression stages from the outside atmosphere.  
           [0016]    In an advantageous embodiment, the gasket comprises two annular end portions that are generally parallel to each other and that are interconnected by two longitudinally-extending portions that are generally perpendicular thereto.  
           [0017]    With a sealing gasket of this structure, in the assembled state, the longitudinally-extending portions of the gasket are compressed laterally between the two half-shells in the longitudinal assembly surface, while the two annular end portions are compressed between the two half-shells acting together on the one hand and the respective fitted end pieces on the other hand.  
           [0018]    To provide good compression of the first annular end portion of the gasket, it is advantageous to provide a first fitted end piece that has an axial nose shaped to occupy a corresponding axial recess in the first end of the stator body as made up by the two assembled-together half-shells. In this way, the first annular end portion of the gasket is compressed radially by the two half-shells against the axial nose.  
           [0019]    Preferably, the axial nose includes a peripheral annular groove for receiving said first annular end portion of the gasket.  
           [0020]    To facilitate positioning the sealing gasket and to provide good compression of its longitudinally-extending portions, at least one of the half-shells has two longitudinal grooves in its longitudinal assembly surface for receiving the longitudinally-extending portions of the gasket.  
           [0021]    The second annular end portion of the gasket may merely be compressed axially by the second fitted end piece against the end faces of the two half-shells.  
           [0022]    Preferably, the two half-shells comprise respective grooves in their second end faces, which grooves are shaped to receive said second annular end portion of the gasket. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    Other objects, characteristics, and advantages of the present invention appear from the following description of particular embodiments, given with reference to the accompanying figures, in which:  
         [0024]    [0024]FIG. 1 is an exploded perspective view of a prior art structure for a multistage dry vacuum pump stator;  
         [0025]    [0025]FIG. 2 is a perspective view in longitudinal section showing the FIG. 1 pump, after the stator has been assembled;  
         [0026]    [0026]FIG. 3 is a perspective view showing a first stator half-shell and a first end piece for a dry vacuum pump constituting an embodiment of the present invention;  
         [0027]    [0027]FIG. 4 is an exploded perspective view showing the first stator half-shell and the first end piece after they have been assembled together with a sealing gasket of the invention interposed between them, and also showing the second stator half-shell and the second end piece prior to assembly;  
         [0028]    [0028]FIGS. 5 and 6 are perspective views from two different viewpoints showing a stator half-shell together with a sealing gasket for the embodiment of FIGS. 3 and 4;  
         [0029]    [0029]FIG. 7 is a side view showing the inside face of the stator half-shell of the preceding figures, with the sealing gasket in place;  
         [0030]    [0030]FIG. 8 is a cross-section of the half-shell and of the gasket of FIG. 7 on section plane A-A, with the rotors mounted;  
         [0031]    [0031]FIG. 9 is a perspective view of a sealing gasket constituting an embodiment of the present invention; and  
         [0032]    [0032]FIG. 10 is a cross-section view of the FIG. 9 sealing gasket. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    In the embodiment of FIGS.  3  to  10 , the multistage dry vacuum pump of the invention is a five-stage pump in which there can be seen the usual structural elements of the prior art pump as shown in FIGS. 1 and 2, and identified by the same numerical references. Thus, there are the gas inlet  2 , a gas outlet that is not visible in the figures, the successive compression chambers  4 ,  5 ,  6 ,  7  and  8 , the transverse walls  9 ,  10 ,  11  and  12  separating the compression chambers, the holes  13  and  14  for passing the rotor shafts, and the duct  15  for passing gas between two successive compression chambers. In FIG. 8, there can also be seen the rotors  51  and  52 .  
         [0034]    In the invention, the stator is made up of two half-shells respectively referenced  101  and  102 , which shells meet on a longitudinal assembly surface  30 . The longitudinal assembly surface  30  is preferably plane and contains the respective axes I-I and II-II (FIG. 3) of the two coupled-together rotor shafts.  
         [0035]    As a result, after the half-shells  101  and  102  have been assembled together radially, each compression stage of the pump, e.g. the first compression stage constituted by the first compression chamber  4  and the rotor lobes it contains, is contained in two corresponding portions of each of the half-shells  101  and  102 . In other words, once assembled together, the two half-shells  101  and  102  contain all of the compression stages of the pump.  
         [0036]    The main stator body as constituted in this way by the assembled-together half-shells  101  and  102  is closed in leaktight manner at its ends by two fitted end pieces, respectively a first end piece  31  and a second end piece  32 .  
         [0037]    In the invention, sealing between the outside atmosphere and the internal cavities of the vacuum pump is provided by a continuous one-piece sealing gasket  33 . In the embodiment shown in the figures, and most clearly visible in FIGS. 9 and 10, the sealing gasket  33  comprises two annular end portions  34  and  35  that are generally parallel to each other, and that are interconnected by two longitudinally-extending portions  36  and  37  which are generally perpendicular thereto. In this embodiment, which is adapted to the general structure of the above-described stator, the longitudinally-extending portions  36  and  37  of the gasket  33  are generally parallel to each other and they interconnect the two annular end portions  34  and  35  via respective connection zones  38 ,  39 ,  40  and  41  that are diagrammatically opposite in pairs.  
         [0038]    As can be seen in FIG. 9, in this embodiment, the first annular end portion  34  is generally circular and of smaller diameter than the second annular end portion  35  which is itself oblong in shape so as to fit around the space occupied by the coupled-together rotors that are offset vertically relative to each other. The longitudinally-extending portions  36  and  37  are axially connected directly to the top and bottom zones respectively of the second annular end portion  35 , whereas they are connected radially via bends  42  and  43  to the first annular end portion  34 .  
         [0039]    In the embodiment of FIG. 10, the gasket  33  is substantially circular in cross-section, as can be seen where the longitudinally-extending portions  36  and  37  are in section. Nevertheless, it will be possible for the gasket to have a cross-section of some other shape, e.g. square, rectangular, etc. The gasket can be made of elastomer or of any suitable material such as a metal of the copper, aluminum, or indium type.  
         [0040]    With reference more particularly to FIGS. 3 and 4, it can be seen that the first fitted end piece  31  has an axial nose  44  shaped to occupy a corresponding axial recess  45  in the first end of the stator. The axial nose  44  has a peripheral annular groove  46  for receiving the first annular end portion  34  of the gasket  33 . As a result, in the assembled position, the first annular end portion  34  of the gasket  33  is compressed radially by the two half-shells  101  and  102  onto the axial nose  44  of the first fitted end piece  31 . The annular groove  46  can be rectangular in cross-section, and of a depth that is smaller than the diameter of the gasket  33 .  
         [0041]    At least one of the half-shells  101  and  102 , e.g. the half-shell  101 , has two longitudinal grooves  47  and  48  (FIG. 3) in its longitudinal assembly surface  30  for receiving the longitudinally-extending portions  36  and  37  respectively of the gasket  33 , as shown in FIGS.  4  to  6 . As a result, the longitudinally-extending portions  36  and  37  of the gasket  33  are compressed laterally between the two half-shells  101  and  102  in the longitudinal assembly surface  30 . The cross-section of the longitudinal grooves  47  and  48  can be rectangular, with depth smaller than the diameter of the gasket  33 .  
         [0042]    As can be seen in FIG. 4 and in FIG. 7, the second annular end portion  35  of the gasket  33  is compressed axially by the second fitted end piece  32  of the stator against the two half-shells  101  and  102 . In the embodiment shown, the two half-shells  101  and  102  have grooves in their end faces at the second end such as the end face  49  of the first half-shell  101  (FIG. 3), these grooves, such as the groove  50 , being shaped to receive the second annular end portion  35  of the gasket  33 . The section of the grooves such as the groove  50  can be rectangular, being of depth smaller than the diameter of the gasket  33 . The grooves such as the groove  50  connect to each other so as to make up a continuous groove, and they also connect at their connection points with the longitudinal grooves  47  and  48  in the longitudinal assembly surface  30 .  
         [0043]    To assemble the pump of the invention, the first annular end portion  34  of the gasket  33  is engaged in the annular groove  46  of the axial nose  44  of the first fitted end piece  31 , and then the first half-shell  101  is applied sideways against the axial nose  44 . The longitudinally-extending portions  36  and  37  of the gasket  33  are engaged in the longitudinal grooves  47  and  48 , and a first half of the second annular end portion  35  of the gasket  33  is engaged in the groove  50 . It is then possible to mount the rotors and it is easy to position the rotor lobes in the compression chambers  4 - 8 . Thereafter, the second half-shell  102  can be applied laterally both against the axial nose  44  and against the longitudinal assembly surface  30 , with the second half of the second annular end portion  35  of the gasket  33  being inserted in the corresponding front-end groove of the second half-shell  102 . Thereafter the second fitted end piece  32  can be brought axially against the end faces such as the face  49  of the half-shells  101  and  102 .  
         [0044]    Such a pump can be assembled much more quickly than the prior art pumps in general use. Simultaneously, sealing is provided in a manner that is very effective and satisfactory.  
         [0045]    The invention applies in particular to making a multistage primary pump of the Roots type, or of the claw type, or of the Roots-and-claw type.  
         [0046]    The present invention is not limited to the embodiments described explicitly above, but includes the various generalizations and variants that are within the competence of the person skilled in the art.