Vacuum pump and sealing joint
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- PFEIFFER VACUUM SAS
- Filing Date
- 2024-05-31
- Publication Date
- 2026-06-03
AI Technical Summary
Multi-stage vacuum pumps with half-shell architecture face challenges in maintaining effective sealing due to gas leaks along the three-dimensional joints, which can impact vacuum performance and efficiency, especially under high temperatures and aggressive chemical exposure.
The design incorporates additional half-shells with bulges in the seal lengths that fit into joint gorges, allowing for localized filling and deformation to prevent gas leaks, while managing thermomechanical stresses through the use of high-temperature-resistant fluoroelastomeric materials and additional sealing barriers.
This configuration effectively blocks gas leaks and reduces mechanical stress on the seals, ensuring reliable sealing performance even at high temperatures, thus maintaining vacuum integrity and extending the lifespan of the pump.
Smart Images
Figure EP2024065002_30012025_PF_FP_ABST
Abstract
Description
Description Title of the invention: Vacuum pump and seal 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 seal for a vacuum pump, in particular a three-dimensional seal. Technical background
[0002] Dry-type vacuum pumps have one or more pumping stages in series through which a gas to be pumped circulates. Known vacuum pumps are those with rotary lobes, also known as "Roots," or those with claws, also known as "Claws." These vacuum pumps are called "dry" because, during operation, the rotors rotate inside a stator without any mechanical contact between them or with the stator, which eliminates the need for oil in the pumping stages.
[0003] However, in certain pumping applications, such as in the pumping processes used in the semiconductor, flat panel display (FIAT Panel Display) and photovoltaic industries and coating processes, the gases used may require the vacuum pump to be maintained at particularly high temperatures, such as around 200°C. This is particularly the case when the gases used are corrosive, such as the gases used during the cleaning phases of process chambers, or when these gases include resin vapors, for example. These high temperatures combined with aggressive chemical species require the use of high-performance elastomer materials such as High Temperature FKM or High Temperature FFKM for the static seals of the stator parts of the pumps.Also, the design of the seal / seal groove pair must be designed to take into account the sealing requirements both cold and hot as well as the intrinsic thermomechanical characteristics of these elastomers.
[0004] Multi-stage vacuum pumps with a sliced architecture, in which the stators are made up of the axial assembly of several stator elements, easily address this problem by using O-rings received in rectangular section seal grooves, the seals being axially compressed between the stator elements.
[0005] Multi-stage vacuum pumps with a half-shell design are at a disadvantage. This design requires the use of either sealing compound, butt-joints, three-dimensional seals, or a combination of these.
[0006] For example, document US6,572,351 B2 discloses a vacuum pump structure having a stator in half-shells assembled along a longitudinal assembly surface generally parallel to the axes of the rotors. The vacuum pump comprises a one-piece seal having two annular end portions and two longitudinal members connecting the annular end portions and which are perpendicular to them. The two annular end portions are parallel to each other and each is inserted between an end piece and the half-shells. The longitudinal members are inserted between the half-shells. This one-piece seal ensures both the seal between the half-shells and the seal between the half-shells and the added end pieces to isolate the pumping stages from the outside atmosphere.
[0007] However, gas leaks can propagate along the longitudinal members 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, whether in terms of the ultimate vacuum pressure reached or the power consumed at this ultimate vacuum pressure. Summary of the invention
[0008] An aim of the present invention is to at least partially resolve one of the aforementioned drawbacks, in particular by improving the sealing of the stator parts of half-shell structure vacuum pumps.
[0009] To this end, the invention relates to a vacuum pump comprising: - a stator comprising at least a 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 the at least one pumping chamber, - a first seal comprising: - a first and a second annular end portion interposed between a respective end piece and the half-shells, and - two side members connecting the annular end parts, the side members being interposed between the half-shells in joint grooves formed at least in one of the two half-shells, characterized in that the side members further have a respective bulge, the bulges having at least one respective recess, the bulges matching a cross section of the joint grooves around the at least one recess.
[0010] The bulges formed in each spar of the three-dimensional seal locally filling the seal grooves make it possible to block any possible leaks of gas that can occur along the seal grooves of the half-shells by breaking the propagation of this longitudinal leakage flow. The at least one recess alleviates the mechanical stresses that can be exerted on the bulges due to the large differences in thermal expansion between the high-performance, high-temperature elastomer materials and the metallic materials of the half-shells, by allowing deformation of the edges of the bulge in the recess, particularly when the vacuum pump is heated to high temperature. This allows for uniform compression of the bulge while respecting the thermomechanical characteristics of the seal.
[0011] The vacuum pump may further include one or more of the features described below, alone or in combination.
[0012] At least one recess may be provided in a flat face of each bulge facing the other half-shell.
[0013] Two recesses may be provided respectively in opposite flat faces of each bulge.
[0014] The at least one recess may have an oblong-shaped section.
[0015] For example, it is provided that a first cross-section of the bulge at the level of the at least one recess is less than or equal to 90% of a second cross-section of the bulge outside the at least one bulge.
[0016] For example, it is provided that a first cross-section of the bulge at the level of the at least one recess is greater than or equal to 85% of a second cross-section of the bulge outside the at least one bulge.
[0017] The joint grooves may have a rectangular cross-section and the bulges may have a complementary shape that fits into a paving stone.
[0018] The side members may have transitional portions on either side of the at least one bulge, these transitional portions being interposed between, on the one hand, cylindrical strands of the side members and, on the other hand, the bulges. The transitional portions have an increasing cross-section, increasing from the section of the cylindrical strand to the section of the bulge.
[0019] The vacuum pump may comprise at least one second seal of similar shape to the first seal, the at least one first and second seal forming at least two successive sealing barriers for the gases.
[0020] The first seal and / or the second seal is, for example, made of a fluoroelastomer or perfluoroelastomer material having resistance to high temperatures, in particular above 150°C.
[0021] The vacuum pump may include a heating device configured to heat the stator to a temperature greater than 150°C.
[0022] The stator half-shells can form at least two pumping stages connected in series between a suction port and a discharge port of the vacuum pump.
[0023] The invention also relates to a seal for a vacuum pump as described above, comprising a stator comprising at least one first and one second complementary half-shell and one first and one second end piece, the half-shells and the end pieces assembling together to form at least one pumping chamber of a pumping stage and two rotor shafts configured to rotate in the at least one pumping chamber, the seal comprising: - a first and a second annular end portion interposed between a respective end piece and the half-shells, and - two side members connecting the annular end parts, the side members being interposed between the half-shells in joint grooves formed at least in one of the two half-shells, characterized in that the side members further have a respective bulge, the bulges having at least one respective recess, the bulges matching a cross section of the joint grooves around the at least one recess. Brief description of the figures
[0024] Other advantages and characteristics will appear on reading the following description of a particular embodiment of the invention, but in no way limiting, as well as the appended drawings in which:
[0025] [Fig.1] Figure 1 is an exploded schematic view of elements of an exemplary vacuum pump.
[0026] [Fig.2] Figure 2 is a partial enlarged view of the seals assembled in a half-shell of the vacuum pump of Figure 1, at the assembly surface.
[0027] [Fig.3] Figure 3 is a cross-sectional view of the elements of Figure 2 at the recesses in the seal stringers.
[0028] [Fig.4] Figure 4 is a perspective view of a bulge in a vacuum pump seal spar and transitional portions on either side of the bulge.
[0029] [Fig.5] Figure 5 shows a first cross-section AA and a second cross-section BB of a seal and a partial top view of the bulge of Figure 4 on which the cross-sections AA and BB are located.
[0030] In these figures, identical or similar elements have the same reference numbers.
[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. Single features of different embodiments may 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 a volumetric vacuum pump that is configured to, using two rotor shafts, suck in, transfer, and then discharge the gas to be pumped at atmospheric pressure. The rotor shafts are rotated 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" in English) is a volumetric vacuum pump configured to, using Roots-type rotors, suck in, transfer and then discharge the gas to be pumped. The Roots-type vacuum pump is mounted upstream and in series with a primary vacuum pump. The rotors are carried by two shafts driven in rotation by a Roots-type vacuum pump motor. The Roots vacuum pump has one to three pumping stages.
[0035] "Upstream" means an element that is placed before another in relation to the direction of flow of the gas to be pumped. Conversely, "downstream" means an element placed after another in relation 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] Figure 1 shows elements of an example of a so-called dry vacuum pump 1.
[0038] The vacuum pump 1 comprises 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 are assembled 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 connected in series between a suction port 7 and a discharge port 8 of the vacuum pump 1, such as between two and ten pumping stages (six in the illustrative example in FIG. 1). The vacuum pump 1 may be a primary vacuum pump (FIG. 1) or a Roots type vacuum pump.
[0040] The vacuum pump 1 also comprises two rotor shafts (not shown) configured to rotate in the 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 have, for example, lobes of identical profiles, for example of the "Roots" type or the "Claw" type or another similar principle of volumetric vacuum pump. 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 mating rotors, the pumping chambers comprising a respective inlet and outlet. During rotation, the gas sucked from the inlet is trapped in the volume generated by the rotors and the stator 2, then is driven by the rotors to the next stage.
[0043] The successive pumping stages T1-T6 are connected in series one after the other by respective inter-stage 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 the pumping chambers (and therefore the flow rates generated) are for example equal or decreasing with the pumping stages. The pumping stage T1 located on the side of the suction port 7 receives the rotors of largest axial dimension and has the largest flow rate generated.
[0044] These vacuum pumps are called "dry" because during operation, the rotors turn inside the stator 2 without any mechanical contact between them or with the stator 2, which means that no oil is used in the pumping stages.
[0045] The vacuum pump 1 may comprise a heating device 11 configured to heat the stator 2 to a temperature greater than 150°C.
[0046] The half-shells 3, 4 are assembled together along 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] A first 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. Orifices are of course provided in the transverse walls of the half-shells 3, 4 separating the pumping chambers where appropriate, 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 provided at the ends of the half-shells 3, 4. The nose 12 projects in the axial direction. It has, for example, an oblong transverse shape, for example solid. The recesses are provided, 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 comprises a first seal 13. This seal 13 is three-dimensional and can be a single piece, i.e. a single block, or joined, i.e. formed by placing several elastic seal parts end to end.
[0051] The first seal 13 comprises a first and a second annular end portion 13a interposed between a respective end piece 5, 6 and the half-shells 3, 4. The first seal 13 also comprises two longitudinal members 13b connecting the annular end portions 13a, the longitudinal members 13b being interposed between the half-shells 3, 4.
[0052] The first seal 13 is elastic. It is for example obtained by press molding or injection molding. The first seal 13 is for example made of fluoroelastomer (FKM) or perfluoroelastomer (FFKM) material having high temperature resistance, in particular greater than 150°C.
[0053] At least one annular groove may be provided in at least one nose 12 and / or in at least one recess to receive an annular end portion 13a of a seal 13.
[0054] The longitudinal members 13b are interposed between the half-shells 3, 4 in seal 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 seal grooves 16 formed in the assembly surface 10 of the first half-shell 3 on either side of the at least one pumping chamber to receive a respective longitudinal member 13b of the first seal 13.
[0055] The first seal 13 (the strands of the side members 13b and the annular end parts 13a) has, for example, a substantially circular section in the uncompressed state.
[0056] Better visible in the enlarged view of Figure 2, the side members 13b further have a respective bulge 17, the bulges 17 having at least one respective recess 18. The bulges 17 match the cross section of the joint grooves 16 around at least one recess 18, in particular when cold, that is to say even when the temperature of the stator 2 is less than 150°C. The seal 13 also protrudes from the seal grooves 16 when cold and in the uncompressed state.
[0057] The bulges 17 formed in each spar 13b of the three-dimensional seal 13 locally filling the seal grooves 16 make it possible to block any gas leaks that may occur along the seal grooves 16 of the half-shells 3, 4 by breaking the propagation of this longitudinal leak flow.
[0058] Because the bulges 17 fill the seal grooves 16 when cold, the elastomer material of the seal 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 around 200°C. These high temperatures associated with aggressive chemical species require the use of high-performance elastomer materials of the high-temperature FKM or high-temperature FFKM types for the seals and it is noted that the expansion coefficients 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 significant creep which can lead to the loss of sealing associated with the return to cold of the vacuum pump 1, which can occur for example for maintenance or during a waiting phase of the vacuum pump 1.
[0059] The at least one recess 18 makes it possible to avoid this by allowing deformation of the edges of the bulge 17 in the recess 18, the material deforming where there is the least resistance, therefore towards the inside of the recess 18, in particular when the vacuum pump 1 is heated to a high temperature. This allows compression of the bulge 17 to be uniform and to respect the thermomechanical characteristics of the seal 13.
[0060] According to an exemplary embodiment, the joint grooves 16 have a rectangular cross-section and the bulges 17 have a complementary shape fitting into 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 side members 13b, which corresponds in the example to the level of a transverse wall of the stator 2 interposed between the third and second pumping stages T2, T3 (figure 1).
[0062] At least one recess 18 is for example provided in a flat face of each bulge 17, facing the other half-shell 4 (figure 2). The seal grooves 16 are for example provided in a first half-shell 3 and the recesses 18 are arranged in the flat faces of the bulges 17 facing the second half-shell 4.
[0063] The recess 18 is for example provided in the center of the flat face of the bulge 17.
[0064] Two recesses 18 may be respectively provided 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 provided at the top and at the bottom, that is to say one facing the other half-shell 4, the other recess 18 being provided in an opposite flat face of the bulge 17.
[0065] The recesses 18 may extend in a direction parallel to that of the seal 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 the angles of which 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] Figure 5 shows 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 matching the cross-section of the seal grooves 16.
[0071] The bulge 17 has, for example, a thickness without recess on either side of the at least one recess 18 (second cross-section AA), of at least 2 mm.
[0072] For example, it is provided that the first cross-section BB of the bulge 17 at the level of the at least one recess 18 is less than or equal to 90% of the second cross-section AA of the bulge 17 outside the 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 the at least one recess 18 is greater than or equal to 85% of the second cross-section AA of the bulge 17 outside the at least one bulge 18.
[0074] These surface ratios make it possible to promote the elastic deformation of the bulge 17 while avoiding its plastic deformation.
[0075] According to an exemplary embodiment, the area of the first cross-section BB is between 8mm 2 and 11mm 2 and the area of the second cross-section AA is between 9mm 2 and 13mm 2 The difference in sections is for example between 0.5mm2 and 3mm 2 .
[0076] As can be seen in Figures 2 and 4, it can further be provided that the side members 13b have transitional portions 19 on either side of each bulge 17, these transitional portions 19 being interposed between, on the one hand, the cylindrical strands of the side members 13b and, on the other hand, the bulges 17. The transitional portions 19 have a respective evolving cross-section. The cross-section of the transitional portions 19 is increasing, it increases progressively from the section of the cylindrical strand to the section of the bulge 17. These transitional portions 19 allow a graduation of the mechanical stress exerted on the bulges 17, which makes it possible to avoid the formation of weak zones.
[0077] The vacuum pump 1 may further comprise two seals 13, 20 (figures 1, 2 and 3), the second seal 20 having a shape similar to the first seal 13.
[0078] Like the first seal 13, the second seal 20 is three-dimensional and may be a single piece, i.e., one-piece, 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 seal 20 also comprises two longitudinal members 20b connecting the annular end portions 20a, the longitudinal members 20b being interposed between the half-shells 3, 4.
[0079] Like the first seal 13, the longitudinal members 20b of the second seal 20 may have a respective bulge 17, the bulges 17 having at least one respective recess 18, the bulges 17 matching the cross-section of the seal grooves 16 around the at least one recess 18.
[0080] All the characteristics of the bulges 17 and the recesses 18 of the second seal 20 may be the same as those of the first seal 13.
[0081] The first seal 13 and the second seal 20 are arranged such that the first and the at least one second seal 13, 20 form at least two successive sealing barriers for the gases.
[0082] The end annular portions 13a, 20a are parallel to each other and perpendicular to the side members 13b, 20b.
[0083] The multiplication of sealing barriers makes it possible to ensure good sealing both from the outside to the inside and vice versa and allows the use of different materials capable of offering performances of resistance to corrosive and / or thermal gases decreasing with the distance from the pumping chambers. Indeed, it can be provided that the at least one first sealing gasket 13 inside is formed of a material more resistant, in particular 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 first internal seal 13 while being acceptable in terms of safety. The second seal 20 is for example made of fluorinated elastomer material (FKM) and the first seal 13 is for example made of perfluoroelastomer material (FFKM).
[0084] Although the figures illustrate a vacuum pump 1 for which the end pieces are provided with noses 12 and the half-shells 3, 4 with complementary recesses, the invention also applies to a vacuum pump without noses or recesses. The sealing gasket(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
CLAIMS
1. Vacuum pump (1) comprising: - a stator (2) comprising at least one first and one second complementary half-shell (3, 4) and one first and one second end pieces (5, 6), the half-shells (3, 4) and the end pieces (5, 6) assembling 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 longitudinal members (13b) connecting the annular end parts (13a), the longitudinal members (13b) being interposed between the half-shells (3, 4) in joint grooves (16) formed at least in one of the two half-shells (3, 4), characterized in that the longitudinal members (13b) further have a respective bulge (17), the bulges (17) having at least one respective recess (18), the bulges (17) matching 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 one of the preceding claims, characterized in that two recesses (18) are respectively formed in opposite flat faces of each bulge (17).
4. Vacuum pump (1) according to one of the preceding claims, characterized in that the at least one recess (18) has an oblong cross-section.
5. Vacuum pump (1) according to one of the preceding claims, characterized in that a first cross-section (BB) of the bulge (17) at the at least one recess (18) is less than or equal to 90% of a second cross-section (AA) of the bulge (17) outside the at least one bulge (18).
6. Vacuum pump (1) according to one of the preceding claims, characterized in that a first cross-section (BB) of the bulge (17) at the at least one recess (18) is greater than or equal to 85% of a second cross-section (AA) of the bulge (17) outside the at least one bulge (18).
7. Vacuum pump (1) according to one of the preceding claims, characterized in that the seal grooves (16) have a cross section rectangular and the bulges (17) have a complementary shape fitting into a paving stone.
8. Vacuum pump (1) according to one of the preceding claims, characterized in that it comprises at least one second seal (20) of similar shape to the first seal (13), the at least one first and second seals (13, 20) forming at least two successive sealing barriers for the gases.
9. Vacuum pump (1) according to one of the preceding claims, characterized in that the first seal (13) and / or the second seal (20) is made of fluoroelastomer or perfluoroelastomer material having resistance to high temperatures, in particular greater than 150°C.
10. Vacuum pump (1) according to one of the preceding claims, characterized in that it comprises a heating device (11) configured to heat the stator (2) to a temperature greater than 150°C.
11. Vacuum pump (1) according to 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) connected 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 one of the preceding claims, comprising a stator (2) comprising at least one first and one second complementary half-shell (3, 4) and one first and one second end piece (5, 6), the half-shells (3, 4) and the end pieces (5, 6) assembling together to form at least one pumping chamber of a pumping stage (T1-T6) and two rotor shafts configured to rotate in the at least one pumping chamber, the seal (13, 20) comprising: - a first and a second annular end portion (13a, 20a) interposed between a respective end piece (5, 6) and the half-shells (3, 4), and - two longitudinal members (13b, 20b) connecting the annular end parts (13a, 20a), the longitudinal members (13b, 20b) being interposed between the half-shells (3, 4) in joint grooves (16) formed at least in one of the two half-shells (3, 4), characterized in that the longitudinal members (13b, 20b) further have a respective bulge (17), the bulges (17) having at least one respective recess (18), the bulges (17) matching a cross-section of the joint grooves (16) around the at least one recess (18).