Vacuum pump and sealing gasket

Abstract

A vacuum pump (1) comprising a first sealing gasket (13) including a first and a second annular end portion (13a) and two longitudinal members (13b) connecting the annular end portions (13a), the longitudinal members (13b) being interposed between two half-shells (3, 4) of the vacuum pump (1), in sealing grooves (16) formed in at least one of the two half-shells (3, 4), the longitudinal members (13b) further having a respective bulge (17), the bulges (17) having at least one respective recess (18), the bulges (17) conforming to a cross-section of the sealing grooves (16) around the at least one recess (18). Abbreviated figure: Figure 1

Description

Title of the invention: Vacuum pump and sealing gasket Technical field of the invention

[0001] The present invention relates to a vacuum pump, in particular comprising a stator with complementary half-shells. The invention also relates to a vacuum pump seal, in particular a three-dimensional one. Technical background

[0002] Dry vacuum pumps comprise one or more pumping stages in series through which a gas to be pumped circulates. Among known vacuum pumps, a distinction is made between rotary lobe pumps, also known as "Roots" pumps, and claw pumps, also known as "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] However, in certain pumping applications, such as in pumping processes used in the semiconductor, flat panel display (FTD), photovoltaic, and coating industries, the gases used may require maintaining the vacuum pump at particularly high temperatures, such as on the order of 200°C. This is especially true when the gases used are corrosive, such as those used during the cleaning phases of process chambers, or when these gases contain resin vapors, for example. These high temperatures combined with aggressive chemical species necessitate the use of high-performance elastomeric materials such as High Temperature FKM or High Temperature FFKM for the static seals of the pump stator parts.Also, the design of the seal / seal groove assembly must be designed to take into account the cold and hot sealing requirements as well as the intrinsic thermomechanical characteristics of these elastomers.

[0004] Multistage vacuum pumps with a sliced ​​architecture, in which the stators are made up of the axial assembly of several stator elements, easily meet this problem by using O-rings received in seal grooves with rectangular sections, the seals being axially compressed between the stator elements.

[0005] Multistage vacuum pumps with a half-shell architecture are at a disadvantage. This architecture requires the use of either sealant paste, butted gaskets, three-dimensional gaskets, or a combination of these achievements.

[0006] For example, a vacuum pump structure is known from US patent 6,572,351B2, having a stator made of half-shells assembled along a longitudinal assembly surface generally parallel to the rotor axes. The vacuum pump includes a one-piece seal having two annular end sections and two longitudinal members connecting the annular end sections and being perpendicular to them. The two annular end sections are parallel to each other and each is interposed between an end piece and the half-shells. The longitudinal members are interposed between the half-shells. This one-piece seal provides both sealing between the half-shells and sealing between the half-shells and the end pieces to isolate the pumping stages from the outside atmosphere.

[0007] However, gas leaks can propagate along the stringers of the three-dimensional seal and thus pass through the vacuum pump from the high-pressure stages to the low-pressure stages. These gas leaks can negatively impact pumping performance, either in terms of the maximum vacuum pressure reached or the power consumed at this maximum vacuum pressure. Summary of the invention

[0008] One object of the present invention is to resolve at least partially one of the aforementioned drawbacks, in particular by improving the sealing of the stator parts of vacuum pumps with a half-shell structure.

[0009] To this end, the invention relates to a vacuum pump comprising: - a stator comprising at least one first and a second complementary half-shells and a first and a second end pieces, the half-shells and the end pieces assembling together to form at least one pumping chamber of a pumping stage, - two rotor shafts configured to rotate in at least one pumping chamber, - a first sealing gasket comprising: - a first and a second annular end section interposed between a respective end piece and the half-shells, and - two longitudinal members connecting the annular end sections, the longitudinal members being sandwiched between the half-shells in joint grooves formed in at least one of the two half-shells, characterized in that the longerons further have a respective bulge, the bulges having at least one respective recess, the bulges conforming to a cross-section of the joint grooves around at least one recess.

[0010] The bulges formed in each spar of the three-dimensional sealing joint Locally filling the gasket grooves prevents any potential gas leaks that might occur along the gasket grooves of the half-shells by breaking the propagation of this longitudinal leakage flow. The presence of at least one recess reduces the mechanical stresses that can be exerted on the bulges due to the significant differences in thermal expansion between the high-performance, high-temperature elastomer materials and the metallic materials of the half-shells. This is achieved by allowing deformation of the bulge edges within the recess, particularly when the vacuum pump is heated to a high temperature. This ensures homogeneous compression of the bulge, preserving the thermomechanical characteristics of the seal.

[0011] The vacuum pump may further include one or more of the characteristics described below, taken alone or in combination.

[0012] At least one recess can be provided in a flat face of each bulge facing the other half-shell.

[0013] Two recesses can respectively be provided in opposite flat faces of each bulge.

[0014] At least one recess may have an oblong cross-section.

[0015] For example, it is provided that a first cross-section of the bulge at the level of at least one recess is less than or equal to 90% of a second cross-section of the bulge outside of at least one bulge.

[0016] For example, it is provided that a first cross-section of the bulge at the level of at least one recess is greater than or equal to 85% of a second cross-section of the bulge outside of at least one bulge.

[0017] The joint grooves may have a rectangular cross-section and the bulges may have a complementary shape fitting into a block.

[0018] The longerons may have transitional portions on either side of at least one bulge, these transitional portions being interposed between, on the one hand, cylindrical strands of the longerons and, on the other hand, the bulges. The transitional portions have an increasing cross-section, increasing from the cross-section of the cylindrical strand to the cross-section of the bulge.

[0019] The vacuum pump may include at least one second sealing seal of similar shape to the first sealing seal, the at least one first and second sealing seals forming at least two successive sealing barriers for gases.

[0020] The first sealing gasket and / or the second sealing gasket is for example made of fluoroelastomer or perfluoroelastomer material having resistance to high temperature, in particular above 150°C.

[0021] The vacuum pump may include a heating device configured to heat the stator to a temperature above 150°C.

[0022] The stator half-shells can form at least two pumping stages mounted in series between a suction port and a discharge port of the vacuum pump.

[0023] The invention also relates to a sealing gasket for a vacuum pump as described above, comprising a stator having at least a first and a second complementary half-shells and a first and a second end piece, the half-shells and the end pieces assembling together to form at least one pumping chamber of a single pumping stage and two rotor shafts configured to rotate in at least one pumping chamber, the sealing gasket comprising: - a first and a second annular end section interposed between a respective end piece and the half-shells, and - two longitudinal members connecting the annular end sections, the longitudinal members being sandwiched between the half-shells in joint grooves formed in at least one of the two half-shells, characterized in that the longerons further have a respective bulge, the bulges having at least one respective recess, the bulges conforming to a cross-section of the joint grooves around at least one recess. Brief description of the figures

[0024] Other advantages and features will become apparent from the following description of a particular, but by no means limiting, embodiment of the invention, as well as from the accompanying drawings in which:

[0025] [Fig-1] Fig. 1 is a schematic exploded view of elements of an example of vacuum pump.

[0026] [Fig.2] The [Fig.2] is a partial enlarged view of the sealing gaskets assembled in a half-shell of the vacuum pump of the [Fig.1], at the level of the assembly surface.

[0027] [Fig.3] The [Fig.3] is a cross-sectional view of the elements of the [Fig.2] at the level of the recesses of the stringers of the sealing joints.

[0028] [Fig.4] The [Fig.4] is a perspective view of a bulge of a vacuum pump seal spar and of transitional portions on either side of the bulge.

[0029] [Fig.5] The [Fig.5] shows a first cross section AA and a second cross section BB of a sealing joint and a partial top view of the bulge of the [Fig.4] on which the cross sections AA and BB are located.

[0030] In these figures, identical or similar elements bear the same numbers of reference.

[0031] Only the elements necessary for understanding the invention are represented. Detailed description

[0032] 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.

[0033] A primary vacuum pump is defined as a positive displacement vacuum pump configured to draw in, transfer, and then discharge the gas to be pumped at atmospheric pressure using two rotor shafts. The rotor shafts are driven by a primary vacuum pump motor. A primary vacuum pump can be started from atmospheric pressure.

[0034] A Roots-type vacuum pump or Roots compressor (also called a "Roots Blower") is defined as a positive displacement vacuum pump configured to draw in, transfer, and then discharge the gas to be pumped using Roots-type rotors. The Roots-type vacuum pump is mounted upstream and in series with a primary vacuum pump. The rotors are mounted on two shafts driven by a motor of the Roots-type vacuum pump. The Roots-type vacuum pump has one to three pumping stages.

[0035] The term "upstream" refers to an element that is placed before another with respect to the direction of flow of the gas to be pumped. Conversely, the term "downstream" refers to an element placed after another with respect to the direction of flow of the gas to be pumped.

[0036] The axial direction is defined as the longitudinal direction of the pump in which the axes of the rotor shafts extend. The transverse direction is the direction perpendicular to the axial direction. The transverse plane is a plane perpendicular to the longitudinal plane (here generally a horizontal plane).

[0037] Fig. 1 shows elements of an example of a so-called dry vacuum pump 1.

[0038] The vacuum pump 1 includes a stator 2 comprising at least a first and a second complementary half-shells 3, 4 and a first and a second end pieces 5, 6. The half-shells 3, 4 and the end pieces 5, 6, assemble together to form at least one pumping chamber of a pumping stage T1-T6.

[0039] The half-shells 3, 4 of the stator 2 form, for example, at least two pumping stages T1-T6 mounted in series between a suction port 7 and a discharge port 8 of the vacuum pump 1, such that between two and ten pumping stages (six in the illustrative example on [Fig.l]). The vacuum pump 1 can be a primary vacuum pump ([Fig.l]) or a Roots type vacuum pump.

[0040] The vacuum pump 1 also includes two rotor shafts (not shown) configured to rotate in at least one pumping chamber so that the rotors drive a gas to be pumped between the suction port 7 and the discharge port 8.

[0041] The rotors may, for example, have lobes with identical profiles, for example of the "Roots" or "Claw" type, or of another similar positive displacement vacuum pump principle. The shafts carrying the rotors are driven by a motor (not shown) located, for example, at one end of the vacuum pump 1.

[0042] Each pumping chamber receives two coupled rotors, the pumping chambers comprising a respective inlet and outlet. During rotation, the gas drawn in from the inlet is trapped in the volume generated by the rotors and the stator 2, and is then carried by the rotors to the next stage.

[0043] The successive pumping stages T1-T6 are connected in series, one after the other, by respective interstage channels 9 connecting the outlet of the preceding pumping stage to the inlet of the following pumping stage. The inlet of the first pumping stage T1 communicates with the suction port 7. The outlet of the last pumping stage communicates with the discharge port 8. The axial dimensions of the rotors and pumping chambers (and therefore the resulting flow rates) are, for example, equal or decreasing with the number of pumping stages. The pumping stage T1 located on the side of the suction port 7 receives the rotors with the largest axial dimensions and has the highest resulting flow rate.

[0044] 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.

[0045] The vacuum pump 1 may include a heating device 11 configured to heat the stator 2 to a temperature above 150°C.

[0046] The half-shells 3, 4 are assembled together according to an assembly surface 10. The at least one compression chamber and the transfer channels 9, if applicable, are partly formed in the first half-shell 3 and partly in the second half-shell 4.

[0047] The assembly surface 10 is for example a flat assembly surface, passing for example through a median plane of the vacuum pump 1. The flat assembly surface 10 contains for example the axes of the rotor shafts.

[0048] One end of the half-shells 3, 4 is closed by the first end piece 5 and a second end of the half-shells 3, 4 is closed by the second end piece 6. Openings are of course provided in the trans walls reversals of the half-shells 3, 4 separating the pumping chambers where applicable, and in the end pieces 5, 6 for the passage of the rotor shafts.

[0049] The half-shells 3, 4 and the end pieces 5, 6 are assembled together, for example by the complementary assembly of noses 12 carried by the end pieces 5, 6 and recesses formed at the ends of the half-shells 3, 4. The nose 12 protrudes in the axial direction. It has, for example, an oblong transverse shape, for example, a solid shape. The recesses are formed, for example, in the pumping chamber of the first pumping stage T1 and in the pumping chamber of the last pumping stage.

[0050] The vacuum pump 1 further includes a first sealing gasket 13. This gasket 13 is three-dimensional and can be in one piece, i.e. monobloc, or butted, i.e. formed by joining several elastic parts of the sealing gasket end to end.

[0051] The first sealing joint 13 comprises a first and a second annular end parts 13a interposed between a respective end piece 5, 6 and the half-shells 3, 4. The first sealing joint 13 also comprises two stringers 13b connecting the annular end parts 13a, the stringers 13b being interposed between the half-shells 3, 4.

[0052] The first sealing gasket 13 is elastic. It is obtained, for example, by press molding or injection molding. The first sealing gasket 13 is, for example, made of a fluoroelastomer (FKM) or perfluoroelastomer (FFKM) material exhibiting resistance to high temperatures, in particular above 150°C.

[0053] At least one annular groove can be provided in at least one nose 12 and / or in at least one recess to receive an annular end portion 13a of a sealing gasket 13.

[0054] The stringers 13b are interposed between the half-shells 3, 4 in gasket grooves 16 formed in the assembly surface 10 of at least one of the two half-shells 3, 4, for example in a half-shell 3, here the lower one. There are, for example, two gasket grooves 16 formed in the assembly surface 10 of the first half-shell 3 on either side of at least one pumping chamber to receive a respective stringer 13b of the first sealing joint 13.

[0055] The first sealing joint 13 (the stringers 13b and the annular end parts 13a) has, for example, a substantially circular cross-section in the uncompressed state.

[0056] More clearly visible in the enlarged view of [Fig. 2], the longerons 13b further have a respective bulge 17, the bulges 17 having at least one respective recess 18. The bulges 17 conform to the cross-section of the joint grooves 16 around the at least one recess 18, particularly when cold, that is to say, even when The temperature of the stator 2 is below 150°C. The sealing gasket 13 also protrudes from the sealing grooves 16 when cold and in the uncompressed state.

[0057] The bulges 17 formed in each spar 13b of the three-dimensional sealing joint 13 locally filling the joint grooves 16 make it possible to block any gas leaks that may occur along the joint grooves 16 of the half-shells 3, 4 by breaking the propagation of this longitudinal leakage flow.

[0058] Because the bulges 17 fill the sealing grooves 16 when cold, the elastomeric material of the sealing gasket 13 is subjected to significant mechanical stresses when hot. This effect can be exacerbated in certain pumping applications where the gases used may require the vacuum pump 1 to be maintained at particularly high temperatures, such as on the order of 200°C. These high temperatures, combined with aggressive chemical species, necessitate the use of high-performance elastomeric materials such as high-temperature FKM or high-temperature FFKM for the sealing gaskets, and it is observed that the coefficients of expansion are very different between these materials and the metallic materials of the half-shells 3, 4.This can lead to the local exceeding of the mechanical characteristics of the elastomer material at the level of the bulges 17 under stress and therefore to the irreversible degradation of the seal 13 and, ultimately, to the creation of a significant fining which can lead to the loss of sealing associated with the cold return of the vacuum pump 1, which can occur for example for maintenance or during a standby phase of the vacuum pump 1. .

[0059] The at least one recess 18 prevents this by allowing deformation of the edges of the bulge 17 within the recess 18, the material deforming where there is the least resistance, therefore towards the interior of the recess 18, particularly when the vacuum pump 1 is heated to a high temperature. This allows for homogeneous compression of the bulge 17, respecting the thermomechanical characteristics of the sealing gasket 13.

[0060] According to one embodiment, the joint grooves 16 have a rectangular cross-section and the bulges 17 have a complementary shape fitting within a block. The length of the bulge 17 (in the longitudinal direction of the joint groove 16) is, for example, between 10 mm and 14 mm.

[0061] The bulges 17 are for example located in the middle of the stringers 13b, which in the example corresponds to the level of a transverse wall of the stator 2 interposed between the third and second pumping stage T2, T3 ([Fig.1]).

[0062] At least one recess 18 is, for example, formed in a flat face of each bulge 17, facing the other half-shell 4 ([Fig. 2]). The joint grooves 16 are, for example, formed in a first half-shell 3 and the recesses 18 are formed in the flat faces of the bulges 17 facing the second half-shell 3. shell 4.

[0063] The recess 18 is for example made in the center of the flat face of the bulge 17.

[0064] Two recesses 18 may be formed respectively in opposite faces of each bulge 17 (Figures 3 and 5). The recesses 18 may be identical (shapes and volumes). These two recesses 18 are, for example, formed at the top and bottom, that is, one facing the other half-shell 4, the other recess 18 being formed in an opposite flat face of the bulge 17.

[0065] The recesses 18 can extend in a direction parallel to that of the joint grooves 16 and that in which the bulges 17 extend.

[0066] The recesses 18 have for example a longitudinal section (or horizontal, i.e. in a plane parallel to the assembly surface 10) of oblong shape or in other words, having a shape longer than wide and whose angles are rounded.

[0067] Thus, thermal expansion will increase the lateral dimensions of the bulge 17, the latter being able to deform inwards due to the central recesses 18.

[0068] Fig. 5 shows a first and second cross-sections BB, AA of the bulge 17 offset longitudinally along the bulge 17.

[0069] The first cross-section BB corresponds to the areas of the bulge 17 having at least one recess 18, here two.

[0070] The second cross section AA corresponds to the areas of the bulge 17 following the cross section of the joint grooves 16.

[0071] The bulge 17 has for example a thickness without a recess on either side of at least one recess 18 (second cross section AA), of at least 2mm.

[0072] For example, it is provided that the first cross-section BB of the bulge 17 at the level of at least one recess 18 is less than or equal to 90% of the second cross-section AA of the bulge 17 outside of at least one bulge 18.

[0073] For example, it is provided that the first cross-section BB of the bulge 17 at the level of at least one recess 18 is greater than or equal to 85% of the second cross-section AA of the bulge 17 outside of at least one bulge 18.

[0074] These surface ratios make it possible to promote the elastic deformation of the bulge 17 by avoiding its plastic deformation.

[0075] According to one embodiment, the area of ​​the first cross-section BB is between 8 mm² and 11 mm² and the area of ​​the second cross-section AA is between 9 mm² and 13 mm². The difference in cross-sections is, for example, between 0.5 mm² and 3 mm².

[0076] As can be seen in Figures 2 and 4, it can also be foreseen that the longerons 13b have transitional portions 19 on either side of each The bulges 17 are interposed between, on the one hand, the cylindrical strands of the longerons 13b and, on the other hand, the bulges 17. The transitional portions 19 have a progressively increasing cross-section. The cross-section of the transitional portions 19 is increasing, gradually rising from the cross-section of the cylindrical strand to the cross-section of the bulge 17. These transitional portions 19 allow for a gradual increase in the mechanical stress exerted on the bulges 17, thus preventing the formation of weak zones.

[0077] The vacuum pump 1 may further include two sealing rings 13, 20 (figures 1, 2 and 3), the second sealing ring 20 having a shape similar to the first sealing ring 13.

[0078] Like the first sealing gasket 13, the second sealing gasket 20 is three-dimensional and can be one-piece, i.e., monobloc, or butted. It comprises a first and a second annular end portion 20a interposed between a respective end piece 5, 6 and the half-shells 3, 4. The second sealing gasket 20 also comprises two stringers 20b connecting the annular end portions 20a, the stringers 20b being interposed between the half-shells 3, 4.

[0079] Like the first sealing joint 13, the stringers 20b of the second sealing joint 20 may have a respective bulge 17, the bulges 17 having at least one respective recess 18, the bulges 17 conforming to the cross-section of the joint grooves 16 around at least one recess 18.

[0080] All the characteristics of the bulges 17 and the recesses 18 of the second sealing joint 20 can be the same as those of the first sealing joint 13.

[0081] The first sealing gasket 13 and the second sealing gasket 20 are arranged so that the first and at least one second sealing gasket 13, 20 form at least two successive sealing barriers for gases.

[0082] The end annular parts 13a, 20a are parallel to each other and perpendicular to the longitudinal members 13b, 20b.

[0083] The multiplication of sealing barriers ensures good sealing both from the outside in and vice versa, and allows the use of different materials that can offer decreasing resistance to corrosive and / or thermal gases with increasing distance from the pumping chambers. Indeed, it can be anticipated that at least one first sealing gasket 13 inside will be made of a material more resistant, particularly to corrosion, abrasion, and / or high temperatures, than the material of the second sealing gasket 20. The material of the second sealing gasket 20 can thus be more economical than the material of the The first internal seal 13 is acceptable in terms of safety. The second seal 20 is, for example, made of fluorinated elastomer (FKM) material and the first seal 13 is, for example, made of perfluoroelastomer (FFKM) material.

[0084] Although the figures illustrate a vacuum pump 1 in which the end pieces are provided with noses 12 and the half-shells 3, 4 with additional recesses, the invention also applies to a vacuum pump without noses or recesses. The sealing ring(s) 13, 20 are then compressed between the faces of the end pieces and the transverse faces of the ends of the half-shells 3.

Claims

Demands

1. A vacuum pump (1) comprising: - a stator (2) having at least one first and a second complementary half-shells (3, 4) and a first and a second end piece (5, 6), the half-shells (3, 4) and the end pieces (5, 6) being assembled together to form at least one pumping chamber of a pumping stage (T1-T6), - two rotor shafts configured to rotate in the at least one pumping chamber, - a first seal (13) comprising: - a first and a second annular end portion (13a) interposed between a respective end piece (5, 6) and the half-shells (3, 4), and - two stringers (13b) connecting the annular end portions (13a), the stringers (13b) being interposed between the half-shells (3, 4) in seal grooves (16) provided at least in one of the two half-shells (3, 4), characterized in that the longerons (13b) further have a respective bulge (17),the bulges (17) having at least one respective recess (18), the bulges (17) conforming to a cross-section of the joint grooves (16) around the at least one recess (18).

2. Vacuum pump (1) according to claim 1, characterized in that at least one recess (18) is provided in a flat face of each bulge (17) facing the other half-shell (4).

3. Vacuum pump (1) according to any one of the preceding claims, characterized in that two recesses (18) are respectively provided in opposite flat faces of each bulge (17).

4. Vacuum pump (1) according to any one of the preceding claims, characterized in that at least one recess (18) has an oblong cross-section.

5. Vacuum pump (1) according to any one of the preceding claims, characterized in that a first cross-section (BB) of the bulge (17) at the level of at least one recess (18) is less than or equal to 90% of a second cross-section (AA) of the bulge (17) outside of at least one bulge (18).

6. Vacuum pump (1) according to any one of the preceding claims, ca- characterized in that a first cross-section (BB) of the bulge (17) at the level of at least one recess (18) is greater than or equal to 85% of a second cross-section (AA) of the bulge (17) outside of at least one bulge (18).

7. Vacuum pump (1) according to any one of the preceding claims, characterized in that the seal grooves (16) have a rectangular cross-section and the bulges (17) have a complementary shape fitting into a block.

8. Vacuum pump (1) according to any one of the preceding claims, characterized in that it comprises at least one second sealing seal (20) of similar shape to the first sealing seal (13), the at least one first and second sealing seals (13, 20) forming at least two successive sealing barriers for gases.

9. Vacuum pump (1) according to any one of the preceding claims, characterized in that the first sealing seal (13) and / or the second sealing seal (20) is made of fluoroelastomer or per-fluoroelastomer material having resistance to high temperature, in particular above 150°C.

10. Vacuum pump (1) according to any one of the preceding claims, characterized in that it comprises a heating device (11) configured to heat the stator (2) to a temperature above 150°C.

11. Vacuum pump (1) according to any one of the preceding claims, characterized in that the half-shells (3, 4) of the stator (2) form at least two pumping stages (T1-T6) mounted in series between a suction port (7) and a discharge port (8) of the vacuum pump (1).

12. A seal (13, 20) for a vacuum pump (1) according to any one of the preceding claims, comprising a stator (2) having at least one complementary first and second half-shells (3, 4) and a first and second end piece (5, 6), the half-shells (3, 4) and the end pieces (5, 6) assembling with each other to form at least one pumping chamber of a pumping stage (T1-T6) and two rotor shafts configured to rotate in at least one pumping chamber, the seal (13, 20) comprising: - a first and second annular end parts (13a, 20a) interposed between a respective end piece (5, 6) and the half-shells (3, 4), and - two stringers (13b, 20b) connecting the end annular parts (13a, 20a), the stringers (13b, 20b) being intercalated between the half-shells (3, 4) in joint grooves (16) provided in at least one of the two half-shells (3,4), characterized in that the stringers (13b, 20b) further have a respective bulge (17), the bulges (17) having at least one respective recess (18), the bulges (17) conforming to a cross-section of the joint grooves (16) around at least one recess (18).

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