Vacuum pump and method for assembling same

Abstract

The invention relates to a vacuum pump comprising two half-shells (3, 4) that are assembled together; at least one support (8, 9) in planar contact with one axial end of one of the half-shells (4); at least one closing flange (5, 6) that is separate from the at least one support (8, 9), being arranged facing the support (8, 9) and being mounted so as to be able to move in axial translation between the support (8, 9) and the axial end of the half-shell (4); and at least one pushing element (22; 28) that applies a pushing force to the closing flange (5, 6) in order to push the closing flange (5, 6) against the half-shell (4).

Description

Description Title: Vacuum pump and assembly method Technical field of the invention

[0001] The present invention relates to a dry vacuum pump, in particular a multi-stage dry vacuum pump, such as a Roots or Claw type. The invention also relates to a method for mounting such a vacuum pump. Technical background

[0002] Multistage dry vacuum pumps consist of several pumping stages in series through which a gas to be pumped circulates between a suction and a discharge port. Among the known vacuum pumps, we distinguish between rotary lobe pumps, also known as Roots pumps, and claw pumps. These vacuum pumps are called "dry" because, during operation, the rotors rotate inside a stator without any mechanical contact between themselves or with the stator, thus eliminating the need for oil in the pumping stages.

[0003] To allow both machining and assembly and ensure good sealing, dry vacuum pump stators are for example made up of the axial assembly of several stator slices, assembled along their respective transverse walls, with interposition of sealing elements.

[0004] Other vacuum pump designs feature a stator made of half-shells closed at the axial ends by two end pieces with interposed sealing elements. Assembly of such a pump is much faster, less expensive, and pumping performance can be increased.

[0005] In a known configuration, the end pieces form "bearing supports" because they receive bearings through which the rotor shafts can rotate.

[0006] The end pieces are positioned relative to a half-shell using positioning pins placed between the half-shell and their respective end pieces, at the transverse bearing surfaces of the half-shell and the opposing end piece. The end pieces are then secured to the half-shell with several screws, tightened on either side of each pin. The contact between the end pieces and the half-shell at the flat transverse bearing surfaces ensures a robust and statically fixed assembly.

[0007] However, this design requires sufficient axial clearance on another face of the half-shell, between a transverse face of each end piece and the half-shell, on both the first and last pumping stages, to prevent the system from becoming over-constrained. This axial clearance necessitates sealing the half-shells with transverse walls at their axial ends to create a watertight seal around the pumping stages. These walls are, however, difficult to fabricate without direct access for the tooling.

[0008] Another problem is that, since the positions of the end pieces and shafts are referenced to one of the half-shells, it may be necessary, during the assembly of the other half-shell, to ensure the absence of axial contact between the shafts and this other half-shell, to provide slightly wider pumping stages. These stage enlargements may reduce pumping performance, particularly in thin, high-pressure stages. Summary of the invention

[0009] One aim of the present invention is to provide a vacuum pump exhibiting good pumping performance due to good control of clearances and whose assembly is mechanically robust.

[0010] To this end, the invention relates to a vacuum pump comprising: - two half-shells fitting together, - at least one support in planar contact with an axial end of a half-shell, characterized in that the vacuum pump comprises: - at least one separate closing flange, at least one support, arranged opposite the support and mounted to move axially between the support and the axial end of the half-shell, and - at least one thrust element exerting a thrust force on the closing flange to push the closing flange against the half-shell.

[0011] The closing flange, pushing against the half-shell, prevents the system from becoming over-static while allowing planar contact between several surfaces of the half-shell and different parts of the closing flange-support sub-assembly.

[0012] The assembly provides good mechanical robustness because the flat contact between the half-shell and at least one support allows the at least one support to be fixed to the half-shell by screwing without risk of deformation, guaranteeing control of play.

[0013] Each half-shell is positioned directly on at least one support, therefore relative to the same origin and independently of each other. Consequently, it is not necessary to provide larger stages for either half-shell, thus improving pumping performance.

[0014] The axial clearance between the support and the closing flange also provides thermal decoupling between the two parts, significantly limiting heat exchange between the half-shells and the supports that house the bearings. This allows for further heating of the half-shells if necessary.

[0015] Furthermore, this solution is inexpensive because the number of parts in the vacuum pump is limited.

[0016] The vacuum pump may comprise a single support-flange subassembly according to the invention at one axial end of the vacuum pump or the vacuum pump may comprise two flange-flange subassemblies according to the invention, one subassembly being arranged at each axial end of the vacuum pump.

[0017] The vacuum pump may also include one or more of the features described below, taken alone or in combination.

[0018] At least one thrust element may include an elastic element positioned between the closing flange and the support, configured to apply stress to the closing flange against the half-shell. The function of the elastic element of the thrust element is to elastically stress the closing flange against the half-shell, that is, to push the closing flange while simultaneously exerting a restoring movement on the closing flange.

[0019] At least one thrust element may include a cylindrical pin inserted into an additional housing provided on one side in the support and on the other side in the opposite closing flange, the closing flange being able to slide along at least one cylindrical pin.

[0020] The elastic element may include at least one elastic washer, or at least one spring, or at least one sleeve made of elastomeric material mounted on the cylindrical pin. Several embodiments are possible for the elastic element, the embodiment where the elastic element is mounted on the cylindrical pin being only one among others.

[0021] At least one thrust element may include an adjusting screw tightened in a tapped hole in the support, one end of the adjusting screw bearing against the closing flange.

[0022] The vacuum pump may include a sealing gasket interposed between the closing flange and the support.

[0023] The vacuum pump may include at least one positioning pin interposed between the half-shell and a support for positioning and at least one screw tightening the support onto the half-shell for holding in position.

[0024] The half-shells define, for example, at least two pumping stages arranged axially one behind the other, a closing flange transversely closing a pumping stage located at an axial end of the vacuum pump, by planar contact between flat transverse support walls of the closing flange and the half-shell.

[0025] The supports may carry respective bearings in which the rotor shafts are likely to rotate.

[0026] The invention also relates to a method for mounting a vacuum pump as described above, in which: - at least one support is held in position against the half-shell, using at least one positioning pin, at least one closing flange being mounted to move axially between the support and an axial end of the half-shell, at least one thrust element exerting a thrust force on the closing flange against the half-shell, - then we screw in at least one screw tightening the support onto the half-shell to hold it in position. Brief description of the figures

[0027] Other advantages and features will become apparent upon reading the description of the invention, as well as the accompanying drawings, which show:

[0028] [Fig.1] Figure 1 is a very schematic side view representation of an example of a vacuum pump.

[0029] [Fig.2] Figure 2 shows an exploded view of the components of the vacuum pump in Figure 1.

[0030] [Fig.3] Figure 3 shows elements of Figure 2 with one axial end of the vacuum pump in the assembled state and the other axial end in the exploded state.

[0031] [Fig.4] Figure 4 shows the elements of Figure 3 in the assembled state.

[0032] [Fig.5] Figure 5 shows the elements of Figure 4 with the supports, the closing flanges and the half-shell in transparency.

[0033] [Fig.6] Figure 6 shows a cross-sectional and top view of the elements of Figure 4.

[0034] [Fig.7] Figure 7 shows an enlarged view of a detail circled in Figure 6.

[0035] [Fig.8] Figure 8 shows a view analogous to Figure 6 for another example of the realization of the elastic members of the thrust elements.

[0036] [Fig.9] Figure 9 shows a cross-sectional and top view of the vacuum pump for another example of thrust element embodiment.

[0037] In these figures, identical elements bear the same reference numbers. Detailed description

[0038] The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Simple features of different embodiments can also be combined or interchanged to provide other embodiments, without departing from the scope of the invention as defined by the claims.

[0039] A primary vacuum pump is defined as a positive displacement vacuum pump configured to draw in, transfer, and then discharge a gas at atmospheric pressure or above. The rotors of a primary vacuum pump can be of the Roots or Claw type. The rotors are mounted on two shafts driven by a primary vacuum pump motor. A primary vacuum pump is also configured to be able to operate at atmospheric pressure.

[0040] A Roots vacuum pump (also called a "Blower" in English, Roots compressor, or "Booster" in English) is defined as a positive displacement vacuum pump configured to use two Roots rotors to draw in, transfer, and then discharge a gas to be pumped. The Roots vacuum pump is mounted upstream and in series with a vacuum pump. primary. The rotors are supported by two shafts driven in rotation by a Roots vacuum pump motor.

[0041] An "upstream" element is defined as one that is positioned before another relative to the direction of flow of the pumped gases. Conversely, a "downstream" element is defined as one that is positioned after another relative to the direction of flow of the pumped gases. The arrows F in Figure 1 indicate the direction of flow of the pumped gases.

[0042] The axial direction L is defined as the longitudinal direction of the vacuum pump in which the rotor shafts extend. The transverse direction T is the direction perpendicular to the axial direction L. The transverse plane (T, V) is the plane perpendicular to the longitudinal plane (L, T).

[0043] The terms "above", "below", "high", "low", "superior", "lower" are defined with respect to the vertical or gravity direction, with respect to the orientation of the vacuum pump 1 placed on the ground or on a frame.

[0044] The invention applies to any type of dry vacuum pump, that is to say, having two or more pumping stages, such as having two to ten pumping stages. This vacuum pump may be a primary vacuum pump having a plurality of pumping stages and configured to discharge the pumped gases at atmospheric pressure, or a vacuum pump, known as a Roots pump or Roots compressor, with two to three pumping stages which, in operation, is connected in series and upstream of a primary vacuum pump and whose discharge pressure is that obtained by the primary vacuum pump.

[0045] Figure 1 shows a diagram of such a vacuum pump 1.

[0046] The vacuum pump 1 includes a stator 2 comprising at least a first and a second complementary half-shells 3, 4 which fit together and at least one closing flange 5, 6 for closing the axial ends of the half-shells 3, 4.

[0047] At least one closing flange 5, 6 may be flat or partially flat. In particular, at least one closing flange 5, 6 may have a nose 7 (projecting part) that engages axially in the half-shells 3, 4 at the axial ends of the half-shells 3, 4 to close them, especially in the case where the vacuum pump 1 has a one-piece, three-dimensional seal 18, as will be seen later. The nose 7 is, for example, a flat, oblong structure with shaft passage holes through it.

[0048] The stator half-shells 3 and 4 can define at least two pumping stages T1-T6 arranged axially one behind the other and mounted in series between a suction port 11 and a discharge port 12 of the vacuum pump 1, such that between two and ten pumping stages (six in the illustrative example). The pumping stages T1-T6 are axially separated from each other by a transverse interstage wall 13 of the respective half-shells 3 and 4.

[0049] The vacuum pump 1 also includes two rotor shafts 14 configured to rotate in the pumping stages T1-T6 so that the rotors 14 drive a gas to be pumped between the suction port 11 and the discharge port 12 (Figure 1).

[0050] The rotors, for example, have lobes with identical profiles, such as two or more lobes of the "Roots" type, or the "Claw" type, or another similar positive displacement vacuum pump design. The shafts supporting the rotors are driven by a motor M located, for example, next to the discharge stage T6 at one axial end of the vacuum pump 1.

[0051] Each pumping stage receives two coupled rotors 14, the pumping stages comprising a respective inlet and outlet. When the rotor shafts 14 rotate in opposite directions around their respective axes of rotation, the gas drawn in from the inlet is trapped in the volume generated by the rotors 14 and the stator 2, and then carried by the rotors 14 to the next stage.

[0052] The successive pumping stages T1-T6 are connected in series, one after the other, by respective transfer channels 15, connecting the outlet of the preceding pumping stage to the inlet of the following pumping stage. The transfer channels 15 are, for example, arranged on one side and / or the other of the pumping stage. The openings of the transfer channel halves 15 are visible on half-shell 4 in Figures 2 to 4. This is the lower half-shell 4, in which the outlets of the pumping stages T1-T6 are located, here designated as the second half-shell.

[0053] The inlet of the first pumping stage T1 communicates with the suction port 11. The outlet of the last pumping stage T6 communicates with the discharge port 12. The axial dimensions of the rotors and pumping chambers (and therefore the flow rates generated) are, for example, equal or decreasing with the pumping stages, with the pumping stage T1 located on the side of the suction port 11 receiving the rotors with the largest axial dimension and presenting the largest flow rate generated.

[0054] These vacuum pumps are called "dry" because in operation, the rotors rotate inside the stator 2 without any mechanical contact between them or with the stator 2, which makes it possible not to use oil in the pumping stages.

[0055] The half-shells 3, 4 are assembled together according to an assembly surface 16. The assembly surface 16 passes for example through a median longitudinal plane of the dry vacuum pump 1 which contains for example the axes of rotation of the rotor shafts 14.

[0056] The pumping stages T1-T6, the interstage transverse walls 13 and the transfer channels 15 are partly formed in the first half-shell 3 and partly in the second half-shell 4.

[0057] According to one embodiment, a closing flange 6 transversely closes the last pumping stage T6 located at the axial end of the vacuum pump 1, by planar contact between a planar transverse bearing wall of the closing flange 6 and a planar transverse bearing wall 4a of the half-shell 4, here between a planar transverse bearing wall 7a of a nose 7 of the closing flange 6 and a planar transverse bearing wall 4a of the half-shell 4 (figures 6 and 7).

[0058] According to one embodiment, another closing flange 5 transversely closes the first pumping stage T1 located at the other axial end of the vacuum pump 1, by planar contact between a planar transverse bearing wall of the closing flange 5 and a planar transverse bearing wall 4a of the half-shell 4, for example here between a planar transverse bearing wall 7a of a nose 7 of the closing flange 5 and a planar transverse bearing wall 4a of the half-shell 4 (figure 6).

[0059] The flat contact between the closing flange 5, 6 and the half-shell 4 minimizes leakage and thus ensures good pumping performance, including at the pumping stages T1, T6 located at the axial ends of the half-shell 4, without the need to machine transverse closing walls at the axial ends of the half-shells 3, 4. The pumping stages T1, T6 located at the axial ends can be made by front machining, which is simpler, less expensive and allows for relatively thin pumping stages.

[0060] Sealing grooves 17 can be provided in the half-shells 3, 4 at the level of the assembly surface 16 and in the closing flanges 5, 6 for example here at the level of the noses 7, to receive at least one sealing element, such as a sealing gasket 18.

[0061] The sealing gasket 18 is for example three-dimensional and one-piece. It includes for example two annular end parts mounted on the noses 7 and interposed between the closing flanges 5, 6 and the half-shells 3, 4, as well as two stringers connecting the annular end parts, the stringers being interposed between the half-shells 3, 4 (figure 2).

[0062] In another embodiment, the sealing joint 18 is made of several parts that can be joined (end to end), or the sealing element is made of jointing compound. In particular, in these cases, the closing flanges 5, 6 may not have a nose.

[0063] The stator 2 of the vacuum pump 1 also includes at least one support 8, 9, arranged opposite the closing flange 5, 6. The support 8, 9 can carry respective bearings 19, such as bearings, in which the shafts of the rotors 14 are likely to rotate.

[0064] The support 8, 9 and the closing flange 5, 6 are separate elements. For example, there is an axial play of between 0.15 mm and 0.25 mm, such as 0.2 mm, between the closing flange 5, 6 and the support 8, 9.

[0065] Holes are of course provided in the transverse interstage walls 13 of the half-shells 3, 4 separating the pumping stages T1-T6, in the closing flanges 5, 6 and in the supports 8, 9 for the passage of the rotor shafts 10.

[0066] The vacuum pump 1 may include a sealing gasket 23, for example an annular one, interposed between the closing flange 5, 6 and the support 8, 9, for example received in a groove of the support 8, 9 and surrounding the shaft passage holes and at least one thrust element 22 (which will be described later). The sealing gasket 23 provides a seal between the support 8, 9 and the closing flange 5, 6 to prevent, in particular, the trapping of gases or particles between the parts. It is dimensioned to take into account the axial clearance between the support 8, 9 and the closing flange 5, 6.

[0067] At least one support 8, 9 is in planar contact with an axial end of a half-shell 4, here the second or lower half-shell. More precisely, a planar transverse bearing surface 8a, 9a of the support 8, 9 is in planar contact against a planar transverse bearing surface of an axial end of a half-shell 4. The planar contacting surfaces are shown in Figure 5 with hatching.

[0068] The supports 8, 9 and the axial ends of the half-shell 4 may have flat transverse bearing walls 8a, 9a, 4b extending well beyond the pumping stages in the transverse plane to optimize the contact surfaces.

[0069] For each subassembly, the vacuum pump 1 includes, for example, at least one, such as two, positioning pins 20 interposed between the half-shell 4 and the support 8, 9 for positioning (or MIP) and at least one screw 21 clamping the support 8, 9 on the half-shell 4 for holding in position (or MAP), such as at least one pair of screws 21 clamped on either side of each positioning pin 20 and here two other screws 21 distributed between the pairs of screws.

[0070] The closing flange 5, 6 is mounted movably in axial translation between the support 8, 9 and the axial end of the half-shell 4.

[0071] As more clearly seen in Figures 2, 3 and 6, the vacuum pump 1 further comprises at least one thrust element 22 exerting a thrust force on the closing flange 5, 6 to push the closing flange 5, 6 against the half-shell 4.

[0072] For example, there are two thrust elements 22 per closing flange-support sub-assembly, the thrust elements 22 being interposed between the support 8, 9 and the closing flange 5, 6, for example arranged on either side of the shaft passage holes.

[0073] According to an example of an embodiment more clearly visible in Figure 7, at least one thrust element 22 comprises a cylindrical pin 24 inserted into an additional housing provided on the one hand in the support 8, 9 and on the other hand, in the opposite closing flange 5, 6, to guide the movement of the closing flange 5, 6 in axial translation, the closing flange 5, 6 being able to slide along at least one cylindrical pin 24.

[0074] The housing for the support 8, 9 can be through-hole to facilitate assembly. The housing for the closing flange 5, 6 can be blind, to form a stop for the cylindrical pin 24. The closing flange 5, 6 can thus slide along at least one respective cylindrical pin 24, here two since two thrust elements 22 are interposed between the supports 8, 9 and the closing flanges 5, 6.

[0075] According to one embodiment, at least one thrust element 22 includes an elastic member 25 interposed between the closing flange 5, 6 and the support 8, 9, and configured to stress the closing flange 5, 6 against the half-shell 4.

[0076] According to one embodiment, the elastic element 25 comprises at least one elastic washer mounted on the cylindrical pin 24, in particular convex such as conical, for example metallic such as steel, in particular a Belleville washer. There are, for example, at least two elastic washers stacked in opposition (also called "series stacking") on each cylindrical pin 24, here three elastic washers.

[0077] The closing flange 5, 6 is not fixed to the support 8, 9 but is simply pushed under the constraint of the thrust elements 22 against the half-shell 4, the closing flange 5, 6 being able to slide along the cylindrical pins 24. This makes it possible to press the closing flanges 5, 6 against the half-shell 4 to make them almost independent of the supports 8, 9. The assembly is also mechanically very robust since the contact between the support 8, 9 and the half-shell 4 can be made at the level of flat transverse bearing walls.

[0078] The vacuum pump 1 may comprise a single support subassembly 8 or 9 - closing flange 5 or 6 according to the invention at one axial end of the vacuum pump 1. Alternatively, and as shown in the figures, the vacuum pump 1 may comprise two closing flange - support subassemblies according to the invention, one subassembly being arranged at each axial end of the vacuum pump 1.

[0079] During assembly, for each sub-assembly of support 8 or 9 - closing flange 5 or 6 at an axial end of the vacuum pump 1, at least one support 8, 9 is held in position against the half-shell 4, using the positioning pins 20, at least one closing flange 5, 6 being mounted movably in axial translation between the support 8, 9 and the axial end of the half-shell 4, here the nose 7 of at least one closing flange 5, 6 engaging axially in the half-shells 3, 4 at the axial ends of the half-shells 3, 4, at least one thrust element 22 exerting a thrust force on the closing flange 5, 6 against the half-shell 4.

[0080] Then we screw the screws 21 tightening the support 8, 9 onto the half-shell 4 to hold it in position.

[0081] The closing flange 5, 6, in pressure against the half-shell 4, makes it possible to avoid hyperstaticity of the system while allowing a planar contact between several surfaces of the half-shell 4 and different parts of the closing flange-support sub-assembly.

[0082] The assembly allows good mechanical robustness by being isostatic because the planar contact between the half-shell 4 and at least one support 8, 9 allows the support to be fixed to the half-shell 4 by screwing without risk of deformation, guaranteeing control of the plays.

[0083] The positioning of each half-shell 4 is carried out directly on the supports 8 and 9, therefore relative to the same origin and independently of each other. Consequently, it is not necessary to provide larger stages for the other half-shell 5, which improves pumping performance.

[0084] The axial clearance between the support 8, 9 and the closing flange 5, 6 also provides thermal decoupling between the two parts, significantly limiting heat exchange between the half-shells 3, 4 and the supports 8, 9 that house the bearings. This allows for additional heating of the half-shells 3, 4 if necessary.

[0085] Furthermore, this solution is inexpensive because the number of parts in vacuum pump 1 is limited.

[0086] Although figures 2 to 7 illustrate elastic elements 25 of thrust elements 22 made by elastic washers, other embodiments are of course possible.

[0087] An elastic element 27 according to another example mounted on the cylindrical pin 24 may include at least one spring or at least one sleeve made of elastomeric material as illustrated in figure 8.

[0088] The other characteristics of this example implementation are similar to the previously described examples implementations.

[0089] Figure 9 shows another example of the implementation of thrust elements 28.

[0090] In this example, at least one thrust element 28 exerting a thrust force on the closing flange 5, 6 to push the closing flange 5, 6 against the half-shell 4 is achieved by an adjusting screw (also called a "jack-mounted screw" or "jack screw"). Each thrust element 28 has an adjusting screw tightened in a respective tapped hole in the support 8, 9, and whose end bears against the closing flange 5, 6.

[0091] To facilitate assembly, the holes for the adjustment screws can pass axially through supports 8, 9.

[0092] Tightening the adjustment screws exerts a pushing force on the closing flanges 5, 6, the adjustment screws being put under tension by the threads of the supports 8, 9.

[0093] The other characteristics of this example implementation are similar to the previously described examples implementations.

Claims

DEMANDS [Claim 1] Vacuum pump (1) comprising: - two half-shells (3, 4) fitting together, - at least one support (8, 9) in planar contact with an axial end of a half-shell (4), characterized in that the vacuum pump (1) comprises: - at least one closing flange (5, 6) separate from at least one support (8, 9) arranged opposite the support (8, 9) and being mounted to move axially between the support (8, 9) and the axial end of the half-shell (4), and - at least one thrust element (22; 28) exerting a thrust force on the closing flange (5, 6) to push the closing flange (5, 6) against the half-shell (4), at least one thrust element (22) comprising: - an elastic element (25; 27) interposed between the closing flange (5, 6) and the support (8, 9), configured to exert pressure on the closing flange (5, 6) against the half-shell (4), and - a cylindrical pin (24) inserted into an additional housing provided on one side in the support (8, 9) and on the other side in the opposite closing flange (5, 6), the closing flange (5, 6) being able to slide along at least one cylindrical pin (24). [Claim 2] Vacuum pump (1) according to claim 1, characterized in that the elastic element (25; 27) comprises at least one elastic washer or at least one spring or at least one sleeve made of elastomeric material mounted on the cylindrical pin (24). [Claim 3] Vacuum pump (1) according to any one of the preceding claims, characterized in that it comprises a sealing gasket (23) interposed between the closing flange (5, 6) and the support (8, 9). [Claim 4] Vacuum pump (1) according to any one of the preceding claims, characterized in that it comprises at least one positioning pin (20) interposed between the half-shell (4) and a support (8, 9) for positioning and at least one screw (21) clamping the support (8, 9) on the half-shell (4) for holding in position. [Claim 5] Vacuum pump (1) according to any one of the preceding claims, characterized in that the half-shells (3, 4) define at least two pumping stages (T1-T6) arranged axially one behind the other, a flange of Closure (5, 6) transversely closing a pumping stage (T1, T6) located at an axial end of the vacuum pump (1), by planar contact between planar transverse support walls (7a, 4a) of the closure flange (5, 6) and the half-shell (4). [Claim 6] Vacuum pump (1) according to any one of the preceding claims, characterized in that the supports (8, 9) carry respective bearings (19) in which the rotor shafts (14) are capable of rotating. [Claim 7] Vacuum pump (1) according to any one of the preceding claims, characterized in that at least one closing flange (5, 6) has a nose (7) engaging axially in the half-shells (3, 4) at the axial ends of the half-shells (3, 4). [Claim 8] A method for mounting a vacuum pump (1) according to claim 4, wherein: - at least one support (8, 9) is held in position against the half-shell (4), using at least one positioning pin (20), at least one closing flange (5, 6) being mounted movably in axial translation between the support (8, 9) and an axial end of the half-shell (4), at least one thrust element (22) exerting a thrust force on the closing flange (5, 6) against the half-shell (4), - then we screw at least one screw (21) tightening the support (8, 9) onto the half-shell (4) to hold it in position.

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