AU2012351430A1

AU2012351430A1 - Travel track having clip attachment for a variable compression ratio engine

Info

Publication number: AU2012351430A1
Application number: AU2012351430A
Authority: AU
Inventors: Sylvain Bigot; Philippe DURY; Vianney Rabhi
Original assignee: MCE5 Development SA
Current assignee: MCE5 Development SA
Priority date: 2011-12-16
Filing date: 2012-12-07
Publication date: 2014-06-12
Also published as: JP2015500440A; CN104145083B; EP2807339A1; CN104145083A; FR2984399B1; CA2858924A1; KR102064621B1; EP2807339B1; ES2598286T3; KR20140103947A; FR2984399A1; WO2013088032A1; JP5990597B2

Abstract

The invention relates to a travel track (50) having a clip attachment (55) for a variable compression ratio engine (10), including at least one curved segment (51), the outer surface (52) of which engages with a travel track provided on the transmission member (3) or on the control member (7) of the variable compression ratio engine (10) and the inner surface (53) of which is kept in contact with an outer curved surface (54) provided on the toothed wheel (5) of said engine, and at least one clip (55) that is provided on at least one of the ends of the curved segment (51) and is attached to one or the other end of the outer curved surface (54), provided on the toothed wheel (5), by coupling with a hook shape (56) or a projection (56) that comprises said wheel (5) near said end of the curved outer surface (54), said hook shape or projection being complementary to that of the clip (55) such as to be able to engage with said clip.

Description

1 CLIP-FASTENED RACEWAY FOR A VARIABLE COMPRESSION RATIO ENGINE The present invention relates to a clip-fastened raceway for a variable compression ratio engine. 5 International patents WO 98/51911, WO 00/31377, WO 03/008783, and WO 07/085739, belonging to the Applicant, disclose different mechanical devices for a variable volumetric ratio engine and/or a variable cylinder case. It is observed that international patent WO 98/51911 describes a device which serves to improve the 10 overall efficiency of internal combustion piston engines used at variable load and speed by adapting their effective displacement and/or their volumetric ratio during operation. This type of engine is also known to one having ordinary skill in the art under the name "variable compression ratio engine". This term will be used hereinafter. 15 It is observed from international patent WO 00/31377, that the mechanical transmission device for a variable compression ratio engine comprises a piston which is affixed, in its lower portion, to a transmission member cooperating, on the one hand, with a rolling guide device, and on the other hand, with a toothed wheel affixed to a connecting rod, said wheel allowing movement to be transmitted 20 between the piston and said connecting rod, the latter being articulated around a crank comprised by a crankshaft. According to the same patent, it can be observed that the toothed wheel further cooperates with a control member whose adjustable vertical position with respect to the engine block, which the variable 25 compression ratio engine comprises, allows for controlling the compression rate of said engine. It is noted that, according to international patent WO 00/31377, the toothed wheel referred to as a "gear" in said patent comprises segments having a curved profile which are housed in a curved groove housed at mid-width of each toothed sector which said wheel comprises. 30 The term "runway strip" has thus been retained to designate said segments from international patent WO 03/008783, replaced by the term "raceway" in international patent WO 07/085739 relating to improvements or alternatives of the same variable compression ratio engine. 35 It is noted that the raceways comprised by the toothed wheel cooperate with other raceways which comprise, respectively, the transmission member and the control member, the contact point between the raceways and those of said members being positioned in the vicinity of the pitch circle of the gear assembly constituted of said wheel and said members so that said raceways define the center-to-center spacing of said gear, the operating clearance of which they control in the area of the teeth. 40 2 International patent WO 07/085739 also recalls that the minimum operating clearance between the teeth of the racks having large dimensions and those of the toothed wheel is determined by the location of the raceways comprised by said racks and wheel. 5 According to the same patent, during the operating of the variable compression ratio engine, the raceways of the racks and toothed wheel can be maintained in contact by means of a pressing device constituted of pressure means affixed to the cylinder housing of said engine, said means can in particular comprise a spring and/or a piston housed in a cylinder and subjected to a hydraulic pressure. 10 It is noted that, according to patent WO 03/008783, the raceways of the transmission and control members are made by machining the bodies which constitute said members. This particular configuration is made necessary by the need to rigidify said members, their rigidity being significantly reinforced when the raceways are integrated to the constitutive material of said members. 15 However, making said raceways and lateral sides which reinforce the rigidity of said members makes it impossible to use certain usual manufacturing methods for making gear teeth like a hob or a grinding wheel, these circular tools with a large diameter requiring the necessary clearance to operate. Alternatively, it is for example possible to use precision forging stock of the large teeth of said members, 20 then to high-speed machining or precision electro-chemical machining to finish the teeth and the raceways of said members. It is noted that the precision electro-chemical machining makes it possible to finish said teeth of said transmission and control members after having cemented and soaked them with a precision on the 25 order of several microns. It is noted that these different patents do not, however, describe any mode for fastening raceways to the toothed wheel with the exception of international patent WO 00/31377 whose figure 6 shows raceways having dovetailed sections engaged in a groove having a complementary shape housed in the 30 toothed wheel. This solution was never experimented on since its implementation was deemed too complex and potentially would lead to various operational problems. 35 International patent WO 03/008783 indicates that the electron beam welding can be used as a mode for fastening said raceways on said wheel, this method being used to carry out various physical demonstrators of the variable compression ratio engine being the object of different patents cited in reference. 40 The main advantages of electron beam welding are that it is moderately complex to carry out, the cost is acceptable, and a low welding energy which limits the volume of thermal matter that is affected.

3 This low welding energy also limits the permanent set of the toothed wheel caused by welding, said permanent set affecting the geometric integrity and the operational performance of said wheel. In spite of its advantages, the electron beam welding of the raceways on the toothed wheel presents the 5 disadvantage of having insufficient resistance to fatigue, particularly when the variable compression ratio engine is used with heavy loads. Various tests of said engine carried out by the Applicants of the present patent application have led to the partial or total unsoldering of the raceways fastened to the toothed wheel according to this method. 10 The breaks of said raceways subsequent to the unsoldering, accompanied, or not, with the total or partial detachment of said raceways from the toothed wheel, have led to the deterioration or even the destruction of said engine depending on whether said detached raceways or raceway portion are passed, all or in part, into the gear system of said engine. 15 In addition, the electron beam welding requires the raceway to be adequately flattened against the toothed wheel at the time of welding, without which said welding would be too strongly stressed during the operation of the variable compression ratio engine. In addition, electron beam welding requires the portion of the toothed wheel exposed to welding to be masked during the cementation phase, or an 20 overthickness of the material to be removed by machining after cementation and before welding, in the welding zone to be provided. Indeed, the electron beam welding is incompatible with a high carbon ratio of the steel constituting the toothed wheel or that constituting the raceway to be welded. The necessary masking is not well 25 adapted to the industrial production of said wheel since it is too complex and expensive. As an alternative to electron beam welding, the possibility of securing the raceways onto said wheel by laser welding, friction welding, induction welding, brazing, or any other type of welding or fixed connection known to one having ordinary skill in the art, including any type of elastic fastener, was 30 evaluated. These more or less complex implementation methods have various drawbacks, and particularly those connected to the deformation of the pieces welded together or to an insufficient resistance to fatigue of the welding. 35 Other methods have also been envisioned to secure the raceways on the toothed wheel of the variable compression engine and particularly that which consists in making the toothed wheel by forged half wheels, one of said half wheels directly integrating the raceways of said wheel at the time of forging, and the two half wheels being subsequently assembled to one another as claimed in international 40 patent WO 03/008783, the description of which clearly indicates that the assembly of said two half wheels can be carried out, for example, by welding of any type, and particularly by electron beam or by laser, or by any brazing method, and particularly electromagnetic induction brazing.

4 The latter method, consisting in one of the half wheels carrying raceways integrated to its material, provides excellent hold for said raceways on the toothed wheel. 5 In addition, said method excludes any problem of flattening said raceways onto said wheel necessary at the time of welding said raceways onto said wheel since there is no longer any welding, but also during the grinding of said raceways once mounted on said wheel, or when the variable compression ratio engine is operating. 10 To the contrary, the principle of the half wheel carrying the raceways integrated to its material present the drawback of limiting the fabrication methods usable to finish the teeth of the toothed wheel because the presence of said raceways on said wheel very early on the machining range excludes the use of a wheel or milling or grinding cutter to finish said teeth. 15 Alternatively, a cutting by means of a knife and a shaving wheel finishing can be used, these two methods being in theory cheaper and less precise than the more conventional gear machining tools. In addition, the material used for the raceways cannot be different from that used to make the toothed wheel since said raceways are made from the same piece of material than said wheel, which does not 20 allow for optimizing it as a function of the specific needs adapted to said raceways. It is the same for surface treatment and thermal treatment of said raceways which can hardly be different from those of the toothed wheel, said raceways being definitely affixed to said wheel. 25 Another method can consist of decreasing the outer diameter of the raceways of the toothed wheel so that said diameter is substantially less than the diameter at the bases of the teeth comprised by said wheel. This method simultaneously allows the integration of said raceways in the material of the toothed wheel 30 and the use of a cutting or grinding wheel to make the teeth of said wheel. The drawback of this method is that it moves the pitch circle of said gear system away from the contact surface of the different raceways which said system comprises. 35 This moving away introduces an important gliding component in the rolling area operated at the contact point between the different raceways of said system, said component leading to an increase of the energy dissipated by friction by said system, and said increase diminishing the mechanical efficiency of the variable compression ratio engine. 40 It is to solve these problems that the clip-fastened raceway for a variable compression ratio engine according to the invention is provided to: 5 - be mounted on the toothed wheel once the latter has been entirely finished; - not require any welding for its fastening onto the toothed wheel, and, as such, eliminate any risk of thermal deformation or alteration of the mechanical characteristics of said wheel; 5 - render usable any hob and grinding wheel machining process to cut and finish the teeth of the toothed wheel; - be dismountable and remountable if necessary; 10 - be robust and ensure the long lasting operation of the variable compression ratio engine over a period covering at least the lifespan of a vehicle with no risk of separation of the raceway of said toothed wheel; 15 - be, if necessary, made in a different material than that of the toothed wheel, and to receive a surface treatment and a thermal treatment different than those of said wheel; - remain perfectly flattened against the toothed wheel so that its diameter remains as constant as possible no matter the load of the variable compression ratio engine; 20 - ensure a contact point between the different raceways of the gear assembly, positioned as close as possible to the pitch circle of said assembly; - be economically advantageous because of the simplicity of the methods and processes required for its 25 fabrication and mounting. Thus, the clip-fastened raceway for a variable compression ratio engine according to the present invention comprises: 30 - at least one curved segment whose outer surface cooperates with a raceway housed in the transmission member or in the member for controlling the variable compression ratio engine and whose inner surface is maintained in contact with a curved outer surface housed in the toothed wheel of said engine; 35 - at least one clip, housed in at least one of the ends of the curved segment and fastened to one or the other of the ends of the curved outer surface housed in the toothed wheel and hooking onto a hook or protrusion which said wheel comprises in the vicinity of said end of its curved outer surface, said hook or protrusion being complementary to that of the clip so as to be able to cooperate with the latter. 40 The clip-fastened raceway according to the present invention comprises a curved segment which is maintained laterally on the curved outer surface housed in the toothed wheel by at least one lateral stop abutment which prevents said segment from moving parallel to its axis of curvature.

6 The clip-fastened raceway according to the present invention comprises a curved segment whose lateral stop abutment is constituted of at least one edge housed in at least one side of the curved outer surface housed in the toothed wheel. 5 The clip-fastened raceway according to the present invention comprises a curved segment whose edge housed in at least one side of the curved outer surface comprised by the toothed wheel comprises recesses in the area of the bottom of the teeth which the toothed wheel comprises. 10 The clip-fastened raceway according to the present invention comprises a curved segment whose inner surface comprises longitudinal and/or lateral stop pins which cooperate by nesting with pins or grooves having a reverse relief which the curved outer surface housed in the toothed wheel comprises. The clip-fastened raceway according to the present invention comprises a curved segment, at least one 15 of the ends of which is fastened to the toothed wheel by means of a mechanical anchoring connection. The clip-fastened raceway according to the present invention comprises a curved segment whose inner surface and/or curved outer surface housed in the toothed wheel comprises raised and/or recessed geometric patterns which allows the lubricating oil comprised by the casing of the variable compression 20 ratio engine to circulate between said inner surface and said curved outer surface. The clip-fastened raceway according to the invention comprises a curved segment cooperating with a shim between said segment and the curved outer surface housed in the toothed wheel. 25 The clip-fastened raceway according to the invention comprises a curved segment which is longitudinally maintained on the curved outer surface housed in the toothed wheel by at least one longitudinal stop abutment which prevents said segment from rotating about its axis of curvature. The description that follows of the accompanying annexed drawings given by way of non-limiting 30 examples will allow to be better understand the invention, the characteristics which it presents and the advantages which it can provide: Figure 1 shows a variable compression ratio engine which comprises the clip-fastened raceway according to the invention. 35 Figures 2 and 3 are perspective views of the clip-fastened raceway according to the present invention and of the toothed wheel onto which it is mounted, respectively mounted and dismounted, of said toothed wheel which the variable compression ratio engine comprises. 40 Figures 4 and 5 are cross-sectional lateral views of the clip-fastened raceway according to the present invention and of the toothed wheel onto which it is mounted, respectively mounted and dismounted, of said toothed wheel which the variable compression ratio engine comprises.

7 Figures 6 and 7 are exploded and lateral cross-section perspective views, respectively, of the clip fastened raceway according to the present invention and of the toothed wheel onto which it is mounted, when the inner surface of the curved segment comprises longitudinal and/or lateral stop pins 5 which cooperate by nesting with pins or grooves having a reversed relief which the curved outer surface housed in the toothed wheel comprises. Figures 8 and 9 are exploded and lateral cross-section perspective views, respectively, of the clip fastened raceway according to the present invention and of the toothed wheel onto which it is 10 mounted, showing one of the ends of the curved segment which is fastened to the toothed wheel by means of a mechanical anchoring connection. Figures 10 and 11 are exploded and lateral cross-section perspective views, respectively, of the clip fastened raceway according to the present invention and of the toothed wheel according to an 15 alternative which provides for the curved segment to cooperate with a shim sandwiched between said segment and the curved outer surface housed in the toothed wheel. Figure 1 shows an engine block or cylinder case 100 which comprises at least one cylinder 110 in which moves a combustion piston 2 by means of a transmission device 1 and pressure means which allow the 20 main mobile components of a variable compression ratio engine 10 to be maintained in position. The mechanical transmission device 1 comprises in the lower portion of the combustion piston 2 a transmission member 3 affixed to said piston and cooperating, on the one hand with a rolling guide device 4, and on the other hand, with a toothed wheel 5. 25 The toothed wheel 5 cooperates with a connecting rod 6 connected to a crankshaft 9 in order to perform the transmission of the movement between the combustion piston 2 and said connecting rod 6. The toothed wheel 5 cooperates opposite the transmission member or piston gear rack 3 with a control 30 member or rack referred to as control rack 7 whose vertical position relative to the cylinder case 100 is controlled by a control device 12 comprising a control actuator 8 whose actuator piston 13 is guided in a cylinder of an actuator housed in the cylinder case 100 and closed in its upper portion by a common cylinder head 300. 35 The hydraulic double-acting control actuator 8 comprises above the actuator piston 13 an upper chamber of the actuator and below the piston of said actuator 13 a lower chamber of the actuator, said chambers having a displacement different from one another for the same stroke of the actuator 13. The hydraulic double-acting control actuator 8 is constituted of a lower rod of the actuator 16 affixed to 40 the control rack 7 and a tightening screw provided with a closing head housed in the upper chamber of the actuator and allowing for the actuator piston 13 to be fastened to said lower rod of the actuator 16 of the control rack 7.

8 Figures 2 to 5 show a toothed wheel 5 comprising at least one raceway 50 which is constituted of at least one curved segment 51 comprising an outer surface 52 which cooperates with a raceway, not shown, housed in the transmission member 3 or in the control member 7 of the variable compression 5 ratio engine 10. The curved segment 51 comprises an inner surface 53 which is maintained in contact with a curved outer surface 54 housed in the toothed wheel 5 of the variable compression ratio engine 10. 10 The outer 52 and/or inner 53 surface of said curved segment 51 can be flat, arched, or have a complex profile in the direction parallel or perpendicular to the axis of curvature of said segment. The outer 52 and/or inner 53 surface of said curved segment 51 can be cemented, nitrided, or treated according to any method known to one having ordinary skill in the art, and particularly coated with a 15 layer of hard material such as, for example, amorphous carbon of the DLC type (Diamond Like Coating), said treatments and/or coatings intended to increase the mechanical resistance to fatigue and/or wear of said outer 52 and/or inner 53 surfaces. The toothed wheel 5 comprises at least one raceway 50 which is constituted of at least one clip 55, 20 housed in at least one of the ends of the curved segment 51. The clip 55 is fastened to one or the other of the ends of the curved outer surface 54 housed in said toothed wheel 5 by hooking onto a hook or protrusion 56 which said wheel 5 comprises in the vicinity of said end of its curved outer surface 54, said hook shape or protrusion 56 being complementary to that 25 of the clip 55 so as to cooperate with the latter. The raceway 50 comprises a curved segment which is laterally maintained on the curved outer surface 54 housed in the toothed wheel 5 by at least one lateral stop abutment constituted of at least one edge 57 which prevents said segment 51 from moving in parallel to its axis of curvature. 30 Each edge 57 of the lateral stop abutment is housed in at least on side of the curved outer surface 54 which the toothed wheel 5 comprises. Each edge 57 comprises recesses 58 in the area of the bases of the teeth 59 which the toothed wheel 5 35 comprises. The recesses 58 make it possible, in particular, to allow passage of a cutting or grinding wheel during the machining of said teeth 59, or are made following the passage of said cutting or grinding wheel. 40 The inner surface 53 of the curved segment 51 comprises longitudinal and/or lateral stop pins 60 which cooperate by nesting with pins or grooves 61 having a reversed relief which the curved outer surface 54 housed in the toothed wheel 5 comprises (figures 6 and 7).

9 The stop pins 60 which said curved segment 51 comprises and/or the pins or grooves 61 which said curved outer surface 54 comprises make it possible to support said curved segment 51 during the operation of the variable compression ratio engine 10, while preventing any creeping phenomenon 5 which would lead said curved segment 51 to be displaced along its length with respect to said curved outer surface 54. At least one of the ends of the curved segment 51 of the raceway 50 can be fastened to the toothed wheel 5 by means of a mechanical anchoring connector 62 which may be, for example, according to a 10 particular embodiment, a rivet, a screw, a peg, or any other fastening means known to one having ordinary skill in the art (figures 8 and 9). The raceway 50 comprises, on the inner surface 53 of the curved segment 51 and/or on the curved outer surface 54 housed in the toothed wheel 5, geometric patterns, raised or hollow 63 which allow the 15 lubricating oil which the casing of the variable compression ratio engine 10 comprises to circulate between said inner surface 53 and said curved outer surface 54 (figure 3). Figures 10 and 11 show the raceway 50 whose curved segment 51 cooperates with a shim 64 which is sandwiched between said segment and the curved outer surface 54 housed in the toothed wheel 5. 20 Said shim 64 makes it possible, in particular, to adjust the rigidity of said curved segment 51 by adjusting the thickness, the sum of the thicknesses of said shim and of said segment being provided for positioning the contact surface between said segment and the raceway housed in the transmission member 3 or in the control member 7 in the vicinity of the pitch circle of the gear which the toothed 25 wheel 5 and said members 3 and 7 constitute. It is noted that the shim 64 can be constituted of a material, different from that of the raceway 50 or of the toothed wheel 5. According to an alternative embodiment, said shim can also be glued either on the inner surface 53 of the raceway 50 or on the curved outer surface 54 of the toothed wheel 5, or on both. 30 The curved segment 51 of the raceway 50 is maintained longitudinally on the curved outer surface 54 housed in the toothed wheel 5 by at least one longitudinal stop abutment, not shown, which prevents said segment from rotating about its axis of curvature. 35 Said longitudinal stop abutment can be, for example, constituted of a cylindrical stop pin housed, on the one hand, in said curved segment 51, and on the other hand, in the toothed wheel 5. From the aforementioned description, it is to be understood how the raceway 50 is mounted according to the invention on the toothed wheel 5 of the variable compression ratio engine 10, for example in 40 relation to figures 4 and 5.

10 A first clip 55 housed at a first end of the curved segment 51 constituting the raceway 50 according to the invention is hooked onto a first hook 56 which the toothed wheel 5 comprises in the vicinity of a first end of its curved outer surface 54, then said segment 51 is biased in extension in its elasticity until a second clip 55 housed in its second end hooks itself on the opposite hook 56 made in the vicinity of the 5 second end of the curved outer surface 54 of said wheel 5. Once this assembly has been performed, said raceway 50 can be grinned directly on said toothed wheel 5 so the outer diameter of said raceway 50 can be adjusted until the clearance sought between the teeth 59 of said wheel 5 and the transmission 3, 5 and control 7 members with which it cooperates has 10 been obtained. It is noted that it is possible to prepare raceways 50 of different thicknesses that can be matched with the toothed wheel 5 so as to adjust the clearance at the teeth of the gear system constituted of said wheel and transmission 3 and control 7 members without having to grind said raceways after they have 15 been mounted on said wheel, the thickness of the raceways to be matched with the toothed wheel 5 being determined by the measured thickness of the teeth of said wheel. It must be understood that the aforementioned description has been given only by way of example and that it does not limit the field of the invention, the scope of which would not be left by replacing the 20 details of execution described by any other equivalent.

Claims

1. Clip-fastened raceway for a variable compression ratio engine (10), said engine comprising a cylinder 5 case (100) receiving a transmission member (3) provided with a rack, to which are fastened a combustion piston (2), a control member (7) provided with a rack and connected to a hydraulic double acting control actuator (8), a toothed wheel (5) which cooperates, on the one hand with the transmission member (3), and on the other hand with the control member (7) to transmit the mechanical work produced by the combustion on the combustion piston (2) by means of a connecting 10 rod (6) connected to a crankshaft (9), characterized in that it comprises: - at least one curved segment (51) whose outer surface (52) cooperates with a raceway housed in the transmission member (3) or in the control member (7) of the variable compression ratio engine (10) and whose inner surface (53) is maintained in contact with a curved outer surface (54) housed in the toothed 15 wheel (5) of said engine; - at least one clip (55), housed in at least one of the ends of the curved segment (51) and fastened to one or the other of the ends of the curved outer surface (54) housed in the toothed wheel (5) and hooking onto a hook or protrusion (56) which said wheel comprises in the vicinity of said end of its 20 curved outer surface (54), said hook or protrusion being complementary to that of the clip (55) so as to be able to cooperate with the latter.

2. Clip-fastened raceway for a variable compression ratio engine according to claim 1, characterized in that the curved segment (51) is laterally maintained on the curved outer surface (54) housed in the 25 toothed wheel (5) by at least one lateral stop abutment (57) which prevents said segment (51) from moving parallel to its axis of curvature.

3. Clip-fastened raceway for a variable compression ratio engine according to claim 2, characterized in that the lateral stop abutment is constituted of at least one edge (57) housed in at least one side of the 30 curved outer surface (54) of the toothed wheel (5).

4. Clip-fastened raceway for a variable compression ratio engine according to claim 3, characterized in that the edge (57) housed in at least one side of the curved outer surface (54) of the toothed wheel (5) comprises recesses (58) in the area of the bases of the teeth (59) which the toothed wheel comprises. 35

5. Clip-fastened raceway for a variable compression ratio engine according to claim 1, characterized in that the inner surface (53) of the curved segment (51) comprises longitudinal and/or lateral stop pins (61) which cooperate by nesting with pins or grooves (61) having a reverse relief which the curved outer surface (54) housed in the toothed wheel comprises. 40 12

6. Clip-fastened raceway for a variable compression ratio engine according to claim 1, characterized in that at least one of the ends of the curved segment (51) is fastened to the toothed wheel (5) by means of a mechanical anchoring connection (62).

7. Clip-fastened raceway for a variable compression ratio engine according to claim 1, characterized in 5 that the inner surface (53) of the curved segment (51) and/or the curved outer surface (54) housed in the toothed wheel (5) comprises raised or recessed geometric patterns (63) which allow the lubricating oil comprised by the casing of the variable compression ratio engine (10) to circulate between said inner surface (53) and said curved outer surface (54). 10

8. Clip-fastened raceway for a variable compression ratio engine according to claim 1, characterized in that the curved segment (51) cooperates with a shim (64) sandwiched between said segment (51) and the curved outer surface (54) housed in the toothed wheel (5).

9. Clip-fastened raceway for a variable compression ratio engine according to claim 1, characterized in 15 that the curved segment (51) is longitudinally maintained on the curved outer surface (54) housed in the toothed wheel (5) by at least one longitudinal stop abutment which prevents said segment (51) from rotating about its axis of curvature.