Container moulding unit with adjustment spacer

Abstract

The present invention relates to a moulding unit (1) for manufacturing a container from a plastic preform, this moulding unit (1) having at least: a mould (3) comprising a wall formed by two hinged half-moulds (3A, 3B), a mould bottom, the mould bottom being pierced at the centre thereof with an opening, an actuator comprising a piston and a rod at the end of which is mounted an insert capable of sliding through the opening of the mould bottom, between a so-called extended position and a so-called retracted position, and a saddle (14) on which the mould bottom is mounted, the saddle (14) comprising at least a lower stage (14A) and an upper stage (14B) mounted on the upper stage (14A), the upper end of the lower stage (14A) of the saddle (14) carries a so-called wear ring (19) provided with a groove (20) in which, in the closed position of the mould (3), jaws (21) mounted on the half-moulds (3A, 3B) engage, the moulding unit is characterised in that it has at least one spacer (23) inserted removably between the lower stage (14A) and the upper stage (14B) of the saddle (14) so that the spacer (23) extends above the wear ring (19), the spacer (23) being provided with a through-hole forming part of the recess (17) in which the piston (16) of the actuator (15) is mounted, and in that the lower stage (14A) of the saddle (14) is provided with a lower bore forming part of the recess (17) for the piston (16) of the actuator (15).

Description

Container molding unit with adjustment spacer

[0001] The present invention relates to the manufacture of containers, such as bottles, which are produced by stretch-blow molding from plastic preforms (mainly thermoplastic, for example PET or recycled PET known as rPET), and more particularly relates to a molding unit comprising at least one adjustment spacer making it possible to avoid adjustments on the machine after a change in preform format in particular. State of the art

[0002] In the field of packaging, it is well known that a container comprises a wall portion (usually cylindrical in shape), an open neck at an upper end of the wall portion, and a closed bottom at a lower end of the wall portion.

[0003] The manufacture of a container by blow molding generally comprises inserting a preform, also commonly called a blank, into a molding unit having a mold sidewall defining a counter-cavity of the wall portion of the container and a mold bottom defining a counter-cavity of the bottom of the container, said preform having been previously heated to a temperature above the glass transition temperature of the material, and injecting into the hot preform a fluid (such as air) under pressure. The blow molding can be (and generally is) completed by stretching the preform by means of a sliding rod called a stretching rod which is introduced into the preform.

[0004] Thus, blow molding while being stretched in the longitudinal direction by a coaxially provided stretching rod is well known. However, this type of method for manufacturing a container by stretch-blow molding has the disadvantage of causing an offset of the preform and / or the container during blowing, said offset being caused by the lateral oscillation of the preform due to the vibration phenomenon induced by the air pressure blown into the preform, so that this results in an uneven thickness of the walls of the hollow container thus obtained. Furthermore, this type of method also does not allow the parameters to be adjusted to obtain a development speed induced by pre-blowing or blowing greater than that of longitudinal stretching, so that, at the end, the center of the bottom of the preform, also called the injection point, is offset relative to the center of the bottom of the bottle.

[0005] To overcome this drawback, it has already been considered to pinch the bottom of the preform between the stretching rod and a counter-rod extending from the bottom of the mold during blowing. This is the case in particular in documents US4547333 and JP10071641.

[0006] Said documents US4547333 and JP10071641 describe a method for manufacturing containers by stretch-blow molding from plastic preforms, said preforms being previously heated, in which the base of the preform is held by an inner stretching rod, extending into the preform, and an outer counter-rod, extending coaxially with the stretching rod from the mold bottom and bearing on the outer wall of the base of the preform, and the blow molding is carried out while simultaneously stretching the preform in the longitudinal direction by the stretching rod and retracting the counter-rod.By stretching in the longitudinal axis direction and blow molding in a state in which the lower part of the preform is clamped and fixed by a stretching rod and a counter rod, the uneven distribution of wall thickness at the time of container formation is alleviated and, at the same time, the lateral oscillation due to the pressure of the blowing air is eliminated and the injection point remains centered on the axis because it is held between the stretching rod and the counter rod, thereby preventing uneven wall thickness of the containers in the circumferential direction as well.

[0007] Furthermore, as the amount of material required to manufacture each container is continually decreasing over the years for economic and environmental reasons, it is necessary to improve the structural rigidity of the container.

[0008] The improvement of the structural rigidity of the wall portion is achieved in a known manner by means of a series of annular grooves forming stiffeners whose function is to minimize the risk of ovalization of the container (i.e. the deformation of the section of the container from a circular shape to an oval shape).

[0009] Several techniques are also known to ensure the structural rigidity of the container bottom. One technique involves providing the container bottom with stiffening artifacts such as ribs, whose function is to minimize the risk of the bottom collapsing under bending stresses due to the hydrostatic pressure of the contents and, possibly, the weight of stacked containers on the same pallet.

[0010] Another technique is to provide additional stretching of the container bottom material to mechanically increase its crystallinity (and thus its mechanical rigidity). As indicated in published PCT application WO 99 / 52701, this is generally achieved by means of a movable mold bottom slidably mounted relative to the mold sidewall, so that the container material is first blown past the final shape in a retracted position of the mold base and then stamped to the final shape by moving the mold bottom to a raised position in which it completes, together with the mold sidewall, the counter-impression of the container. The molding unit comprises a cylindrical shell and at least a portion of the mold bottom has a cylindrical support slidably received in the cylindrical shell.

[0011] The mold bottom is typically moved by means of a linear actuator such as a hydraulic or pneumatic cylinder, comprising a cylindrical housing and a rod which is slidably mounted within the housing and to which the mold bottom cylinder liner is attached.

[0012] Document US 2013 / 063461 is also known, which describes a mold equipped with a jack provided with a so-called counter-stretching rod, which passes through the mold base and against which the base of the preform is pressed during stretching. For this purpose, the counter-stretching rod is mounted to slide relative to the mold base between an extended position (which the rod occupies at the start of forming), and a retracted position corresponding to the end of forming.

[0013] This architecture is certainly an improvement over older architectures, but it does have a major drawback: it is only suitable for one preform size. However, during production, the preform model may be changed without changing the container model - and therefore without changing the mold. In particular, the length of the preform may vary. If the new preform is longer than the old one, then the counter-stretching rod is too long and risks piercing the bottom of the preform in the deployed position. Conversely, if the new preform is shorter than the old one, then the counter-stretching rod is too short and no longer comes into contact with the bottom of the preform in the deployed position, which makes it ineffective in its function of holding the preform in the mold axis.

[0014] In order to overcome this drawback, a mold has already been designed equipped with an interchangeable counter-stretching rod allowing the mold to be adapted simply and quickly to preforms of different lengths while ensuring that they are held in the mold axis during forming. This is the case in particular for patent application WO207 / 077209 filed by the applicant.

[0015] Said document WO207 / 077209 describes a mold for manufacturing a container from a preform, this mold comprising: a side wall; a mold base pierced in its center with a passage; a jack comprising a body and a counter-stretching rod slidably mounted relative to the body between a deployed position in which the counter-stretching rod projects relative to the mold base and a retracted position; a saddle on which the mold base is mounted, this saddle comprising a lower stage, an upper stage and a spacer interposed removably between the upper stage and the lower stage and partly defining the housing.

[0016] However, these types of molds have the disadvantage of requiring a process of changing the settings of the machine on which they are mounted, these adjustments on the machine can take several hours, leading to a significant loss of production. Disclosure of the invention

[0017] One of the aims of the invention is therefore to overcome these drawbacks by proposing a molding unit of simple and inexpensive design, comprising at least one spacer which allows a change of preform format without having to change the machine settings.

[0018] For this purpose and in accordance with the invention, a molding unit is proposed for manufacturing a container from a plastic preform, this molding unit comprising at least: a mold comprising a wall formed by two articulated half-molds defining an internal cavity distributed around a main axis of the mold and defining at least partially an imprint for a side wall or body and having, at a lower end, a lower opening, a mold base which fits into the lower opening and has an upper surface defining at least a partial imprint of the bottom of the container, the mold base being pierced in its center with an opening (9), a cylinder comprising a piston and a rod at the end of which is mounted an insert capable of sliding through said opening of the mold base,between a so-called deployed position in which a free end of the insert projects relative to the mold base and a so-called retracted position in which the free end of the insert is located in the vicinity of the mold base, and a saddle on which the mold base is mounted, said saddle comprising at least one lower stage and an upper stage mounted on the lower stage, said saddle internally defining a housing in which the piston of the jack is mounted, the upper end of the lower stage of the saddle carrying a so-called wear ring provided with a groove in which, in the closed position of the mold, jaws mounted on the half-molds fit; said molding unit is remarkable in that it comprises at least one spacer interposed in a removable manner between the lower stage and the upper stage of the saddle so that said spacer extends above the wear ring,the spacer being provided with a through hole forming part of the housing in which the piston of the jack is mounted and in that the lower stage of the saddle is provided with a lower bore forming part of the housing for the piston of the jack.,

[0019] Preferably, said spacer rests on the upper face of the wear ring.

[0020] Furthermore, the upper end of the lower stage of the saddle has an annular shoulder on which the wear ring rests.

[0021] The upper end of the wear ring projects from the upper end of the lower stage of the saddle.

[0022] In addition, the spacer comprises a lower annular shoulder such that the upper end of said wear ring bears on said shoulder.

[0023] Furthermore, the jaws which fit into the groove of the wear ring, in the closed position of the mold, are removably mounted at the lower end of the half-molds.

[0024] Preferably, the lower end of each half-mold comprises an internal semi-annular shoulder on which a so-called reference shim rests, which is removably interposed between said semi-annular shoulder and the jaw.

[0025] Another subject of the invention relates to a saddle intended to receive a mold base of a molding unit for manufacturing a container from a plastic preform, said molding unit comprising at least: a mold comprising a wall formed by two articulated half-molds defining an internal cavity distributed around a main axis of the mold and defining at least partially an imprint for a side wall or body and having, at a lower end, a lower opening, a mold base which fits into the lower opening and has an upper surface defining at least a partial imprint of the bottom of the container, the mold base being pierced in its center with an opening, a cylinder comprising a piston and a rod at the end of which is mounted an insert capable of sliding through said opening of the mold base,between a so-called deployed position in which a free end of the insert projects relative to the mold base and a so-called retracted position in which the free end of the insert is located in the vicinity of the mold base, and a saddle on which the mold base is mounted, said saddle comprising at least one lower stage and an upper stage mounted on the lower stage, said saddle internally defining a housing in which the piston of the jack is mounted, the upper end of the lower stage of the saddle carries a so-called wear ring provided with a groove in which, in the closed position of the mold, jaws mounted on the half-molds fit; said saddle is remarkable in that it comprises at least one spacer interposed in a removable manner between the lower stage and the upper stage of the saddle so that said spacer extends above the wear ring,the spacer being provided with a through hole forming part of the housing in which the piston of the jack is mounted and in that the lower stage of the saddle is provided with a lower bore forming part of the housing for the piston of the jack.,

[0026] Preferably, said spacer rests on the upper face of the wear ring.

[0027] Furthermore, the upper end of the lower stage of the saddle has an annular shoulder on which the wear ring rests.

[0028] In addition, the upper end of the wear ring protrudes from the upper end of the lower stage of the saddle.

[0029] Said spacer comprises a lower annular shoulder such that the upper end of said wear ring bears on said shoulder.

[0030] Other advantages and characteristics will emerge more clearly from the following description of several variant embodiments, given as non-limiting examples, of the molding unit according to the invention, with reference to the appended drawings in which:

[0031] is a perspective view of the molding unit according to the invention,

[0032] is a radial sectional view of the molding unit according to the invention shown in the,

[0033] is an exploded perspective view of the saddle of the molding unit according to the invention,

[0034] is a torn side view of the molding unit according to the invention shown in the,

[0035] is a radial sectional view of a detail of the molding unit according to the invention,

[0036] is a partial radial section of the lower end of the molding unit according to the invention,

[0037] are side views of the different stages of changing the spacer of the saddle of the molding unit according to the invention. Method of carrying out the invention

[0038] In the following description of the molding unit according to the invention, the same numerical references designate the same elements. The different views are not necessarily drawn to scale.

[0039] Figures 1 and 2 show a molding unit 1 for manufacturing a container by blow molding (or stretch blow molding) from a blank 2 made of plastic material. In the example illustrated, the blank 2 is a preform, but it could be an intermediate container obtained from a preform having undergone one or more preliminary forming operations. According to a preferred embodiment, the material from which the blank is made is PET and / or rPET (recycled PET).

[0040] Said molding unit 1 comprises, firstly, a mold 3. This mold 3 comprises a wall 4 formed by two articulated half-molds 3A, 3B, defining an internal cavity 5 distributed around a main axis of the mold 3 which, when the containers to be formed are symmetrical in revolution, forms an axis of symmetry of the mold 3.

[0041] The cavity 5 at least partially defines an imprint 6 for a side wall or body of the container. The wall 4 has an opening 7 defining a passage for a mold base 8 provided with a central opening 9 and comprising an insert 10 mounted to move relative to the mold base 8 between a retracted position in which the insert 10 is spaced from the opening 9 (and therefore extends back relative to the cavity 5), and a deployed position in which the insert 10 at least partially projects from the mold base 8 into the cavity 5.

[0042] Said mold base 8 has a molding surface 9 defining an imprint for the relevant part of the container 2. In the deployed position, the insert 10 projects from the mold base 8, thus completing the imprint of the container, against which the material is applied during blowing. The distance separating the retracted position from the deployed position is called the "stroke" of the insert 10. For the sake of clarity, the mold 3 has been shown in a configuration where the stroke of the insert 10 is relatively large compared to the height of the container. This configuration is in no way limiting; on the contrary, the stroke of the insert 10 may be small compared to the height of the container without departing from the scope of the invention.

[0043] As illustrated in the, the blank 2, then the container formed from it, rest on an upper face of the mold 3 by means of a collar 11 of the blank 2 (respectively of the container), which delimits a neck 12 of the blank 2 (respectively of the container), held outside the mold 3. Under the collar 11, the blank 2 (then the container) has a body 13, which extends generally in the axial direction, and a bottom 14, which is initially hemispherical, then, once formed against the mold bottom 8, extends generally in the radial direction from a lower end of the body 13.

[0044] To form the container, the blank 2 is introduced into the mold 3, and a fluid (preferably a gas, for example air) is injected under pressure. Conventionally, the forming can comprise at least two phases, namely: a pre-blowing phase, during which a fluid is injected into the blank 2 at a so-called pre-blowing pressure (the pre-blowing pressure usually being less than or equal to 15 bars, and for example of the order of 7 to 12 bars), and a blowing phase during which a fluid is injected into the blank 2 at a so-called blowing pressure (the blowing pressure being greater than or equal to 15 bars, and for example of the order of 30 to 40 bars).

[0045] Furthermore, the molding unit 1 also comprises a stretching rod, not shown in the figures, capable of extending inside the blank 2, coaxially with the longitudinal axis of said blank 2, and of being moved along said longitudinal axis, in order to stretch said blank 2 during the pre-blowing step, in particular in a manner well known per se.

[0046] The molding unit 1 comprises, secondly, a boxing system called a saddle 14. The term "boxing" is used here to designate an operation of pushing back the material of the container during forming. This saddle 14 comprises, firstly, a jack 15 for controlling the position of the insert 10 of the mold base 8 which is secured to the upper end of said saddle 14.

[0047] Said jack 15 comprises a piston 16 extending in a housing 17 made in the saddle 14 and a rod 18 carrying at its upper end said insert 10. The piston 16 and the rod 18 are integrally movable relative to the jack body formed by the housing 17 made in the saddle between a retracted position, corresponding to the retracted position of the insert 10 of the mold base 8, and a deployed position, corresponding to the deployed position of said insert 10 of the mold base 8.

[0048] Furthermore, with reference to figures 2 and 3, said saddle 14 comprises at least one lower stage 14A and an upper stage 14B mounted on the lower stage, said saddle 14 internally defining a housing 17 in which the piston 16 of the jack 15 is mounted. In this particular embodiment, said saddle 14 also comprises a cooling circuit, not shown in the figures, comprising at least two pipes, an intake pipe and an exhaust pipe, in which a cooling fluid circulates.

[0049] With reference to Figures 3 to 6, the upper end of the lower stage 14A of the saddle 14 carries a so-called wear ring 19 in the form of an annular part provided at its periphery with an annular groove 20 in which, in the closed position of the mold 3, jaws 21 mounted on the half-molds 3A, 3B fit. Each jaw 21 has a substantially semi-annular shape and is removably mounted at the lower end of the half-molds 3A, 3B by means of screws 22.

[0050] Furthermore, the saddle 14 comprises a spacer 23 interposed in a removable manner between the lower stage 14A and the upper stage 14B of the saddle 14 such that said spacer 23 extends above the wear ring 19. Said spacer 23 is in the form of an annular part and bears on the upper face of the wear ring 19. More particularly, said spacer 23 is secured to the wear ring 19 by means of screws not shown in the figures, the upper stage 14B of the saddle being secured to the spacer 23 by other screws.

[0051] Furthermore, the spacer 23 is provided with a through hole 24 forming part of the housing 17 in which the piston 16 of the jack 15 is mounted. In addition, the lower stage 14A of the saddle 14 is provided with a lower bore forming part of the housing 17 for the piston 16 of the jack 15.

[0052] In this particular embodiment, the upper end of the lower stage 14A of the saddle 14 comprises, at its periphery, an annular shoulder 25 on which the wear ring 19 bears and the upper end of the wear ring 19 projects from the upper end of the lower stage 14A of the saddle 14. In addition, the spacer 23 comprises, at its periphery, a lower annular shoulder 26 such that the upper end of said wear ring 19 bears on said shoulder 26, the depth of said shoulder 26 being substantially equal to the height by which the wear ring 19 projects relative to the upper end of the lower stage 14A of the saddle 14.

[0053] Furthermore, the jaws 21 which fit into the groove 20 of the wear ring 19, in the closed position of the mold 3, are removably mounted at the lower end of the half-molds 3A, 3B.

[0054] In this respect, the lower end of each half-mold 3A, 3B comprises an internal semi-annular shoulder 27 on which a so-called reference shim 28 rests, which is interposed in a removable manner between said semi-annular shoulder 27 and the jaw 21. Said shim 28 is in the form of a substantially semi-annular part which is held between the jaw 21 and the shoulder 27 by means of the screws 22.

[0055] The different stages of changing the spacer 23 of the saddle 14 during a change of format of the preforms will now be explained with reference to figures 7 to 19.

[0056] The saddle 14 is previously dismantled, after opening the mold 3, i.e. after the half-molds 3A, 3B are opened and the jaws 21 are released from the groove 20 of the wear ring 19, with reference to the.

[0057] Screws 29 securing the upper stage 14B to the spacer 23 are then unscrewed, and the upper stage 14B including the piston 16 of the jack 15 is disengaged from the lower stage 14A of the saddle 14, then screws 30 securing the spacer 23 to the wear ring are removed, as well as the so-called upper O-rings 31 of the cooling circuit of the saddle 14 and finally the spacer 23 is removed.

[0058] Second so-called lower O-rings 32 of the cooling circuit, corresponding to the lower face of the spacer 23, are also removed and replaced by new lower O-rings 32 before the installation of a new spacer 23et, of height adapted to the dimensions of the new preform, then new upper O-rings 31 are positioned on the upper face of the spacer 23et before the new spacer 23 is secured to the upper face of the wear ring 19 by means of the screws 30et.

[0059] Finally, with reference to the set, the upper stage 14B of the saddle 14 is secured to the spacer 23 by means of the screws 29.

[0060] Furthermore, a new reference shim 28 corresponding to the height of the spacer 23 is obviously put in place simultaneously at the lower end of the half-molds 3A, 3B so that the jaws 21 can fit correctly into the groove 20 of the wear ring 19.

[0061] It is understood that the replacement of the spacer is particularly quick and easy. In addition, such a saddle 14 according to the invention makes it possible to adapt the molds 3 to the format of the preform by eliminating the height adjustment of the wear ring at the machine. Indeed, all the height adjustments are made on the mold 3 before mounting on the machine, which makes it possible not to stop the machine and to continue producing another format of bottle, unlike the devices of the prior art which required adjustments of the machine taking several hours.

[0062] Finally, it is clear that the examples just given are only specific illustrations and are in no way limiting as to the fields of application of the invention.

Claims

Molding unit (1) for manufacturing a container from a plastic preform (2), this molding unit (1) comprising at least: a mold (3) comprising a wall (4) formed by two articulated half-molds (3A, 3B) defining an internal cavity (5) distributed around a main axis of the mold (3) and at least partially defining an imprint (6) for a side wall or body and having, at a lower end, a lower opening (7), a mold base (8) which fits into the lower opening (7) and has an upper surface defining at least a partial imprint of the bottom of the container, the mold base (8) being pierced in its center with an opening (9), a jack (15) comprising a piston (16) and a rod (17) at the end of which is mounted an insert (10) capable of sliding through said opening (9) of the mold base (8),between a so-called deployed position in which a free end of the insert (10) projects relative to the mold base (8) and a so-called retracted position in which the free end of the insert (10) is located in the vicinity of the mold base (10), and a saddle (14) on which the mold base (8) is mounted, said saddle (14) comprising at least one lower stage (14A) and an upper stage (14B) mounted on the lower stage (14A), said saddle (14) internally defining a housing (17) in which the piston (16) of the jack (15) is mounted, the upper end of the lower stage (14A) of the saddle (14) carrying a so-called wear ring (19) provided with a groove (20) in which, in the closed position of the mold (3), jaws (21) mounted on the half-molds fit (3A,3B),characterized in that it comprises at least one spacer (23) interposed in a removable manner between the lower stage (14A) and the upper stage (14B) of the saddle (14) so that said spacer (23) extends above the wear ring (19), said spacer (23) being provided with a through hole forming part of the housing (17) in which the piston (16) of the jack (15) is mounted, and in that the lower stage (14A) of the saddle (14) is provided with a lower bore forming part of the housing (17) for the piston (16) of the jack (15)., Molding unit according to claim 1, characterized in that said spacer (23) bears on the upper face of the wear ring (19). Molding unit according to any one of claims 1 or 2, characterized in that the upper end of the lower stage (14A) of the saddle (14) comprises an annular shoulder (25) on which the wear ring (19) bears. Molding unit according to any one of claims 1 to 3, characterized in that the upper end of the wear ring (19) projects from the upper end of the lower stage (14A) of the saddle (14). Molding unit according to claim 4, characterized in that the spacer (23) comprises a lower annular shoulder (26) such that the upper end of said wear ring (19) bears on said shoulder (26). Molding unit according to any one of claims 1 to 5, characterized in that the jaws (21) which fit into the groove (20) of the wear ring (19), in the closed position of the mold (3), are removably mounted at the lower end of the half-molds (3A, 3B). Molding unit according to claim 6, characterized in that the lower end of each half-mold (3A, 3B) comprises an internal semi-annular shoulder (27) on which rests a so-called reference shim (28) which is interposed in a removable manner between said semi-annular shoulder (27) and the jaw (21). Saddle (14) intended to receive a mold base (8) of a molding unit (1) for manufacturing a container from a plastic preform (2), said molding unit (1) comprising at least: a mold (3) comprising a wall (4) formed by two articulated half-molds (3A, 3B) defining an internal cavity (5) distributed around a main axis of the mold (3) and at least partially defining an imprint (6) for a side wall or body and having, at a lower end, a lower opening (7), a mold base (8) which fits into the lower opening (7) and has an upper surface defining at least a partial imprint of the bottom of the container, the mold base (8) being pierced in its center with an opening (9), a jack (15) comprising a piston (16) and a rod (17) at the end of which is mounted an insert (10) capable of sliding through said opening (9) of the mold base (8),between a so-called deployed position in which a free end of the insert (10) projects relative to the mold base (8) and a so-called retracted position in which the free end of the insert (10) is located in the vicinity of the mold base (10), and a saddle (14) on which the mold base (8) is mounted, said saddle (14) comprising at least one lower stage (14A) and an upper stage (14B) mounted on the upper stage (14A), said saddle (14) internally defining a housing (17) in which the piston (16) of the jack (15) is mounted, the upper end of the lower stage (14A) of the saddle (14) carrying a so-called wear ring (19) provided with a groove (20) in which, in the closed position of the mold (3), jaws (21) mounted on the half-molds fit (3A,3B) characterized in that it comprises at least one spacer (23) interposed in a removable manner between the lower stage (14A) and the upper stage (14B) of the saddle (14) such that said spacer (23) extends above the wear ring (19), the spacer (23) being provided with a through hole forming part of the housing (17) in which the piston (16) of the jack (15) is mounted and in that the lower stage (14A) of the saddle (14) is provided with a lower bore forming part of the housing (17) for the piston (16) of the jack (15)., Saddle according to claim 8, characterized in that said spacer (23) bears on the upper face of the wear ring (19). Saddle according to any one of claims 8 or 9, characterized in that the upper end of the lower stage (14A) of the saddle (14) comprises an annular shoulder (25) on which the wear ring (19) bears. Saddle according to any one of claims 8 to 10, characterized in that the upper end of the wear ring (19) projects from the upper end of the lower stage (14A) of the saddle (14). Saddle according to claim 11, characterized in that the spacer (23) comprises a lower annular shoulder (26) such that the upper end of said wear ring (19) bears on said shoulder (26).

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