EP3472465B1 - Roller piston for a hydraulic machine, integral with a centering element formed to limit friction with a roller - Google Patents

Description

GENERAL TECHNICAL FIELD

The invention relates to the field of pistons and in particular roller pistons.

In particular, the invention advantageously finds use in hydraulic machines with radial pistons.

STATE OF THE ART

The term “roller piston” is used to refer to a piston adapted to slide in a complementary chamber along a longitudinal axis and which carries at one of its ends a roller mounted to rotate about an axis transverse to the longitudinal axis and which rests on an associated part.

Roller pistons are used in particular in hydraulic devices. Some of these devices have a cylinder block comprising a plurality of radially distributed cylinders each comprising a roller piston bearing on a lobed cam. The relative rotation of the cylinder block with respect to the lobed cam induced by a hydraulic pressure applied in the cylinders drives a shaft or vice versa, that is to say that the mechanically induced relative rotation makes it possible to generate a fluid pressure. Such devices are described for example in documents FR 2 651 836 and FR 2 955 903 .

With reference to figures 1a and 1b appended which respectively represent a complete assembled piston and the various parts composing a piston before their assembly, a piston 1 generally comprises a body A110 comprising a cylindrical guide surface A111 centered on a longitudinal sliding axis C-C '. The upper part A110a of the body has the shape of a cradle recess A112 for accommodating a roller 120 intended to roll on a lobed cam. The pistons 1 follow the shape of the cam during a relative rotational movement of the cylinder block with respect to the cam and perform thus back and forth movements inside their cylinder along the sliding axis C-C '.

Various technical additions have made it possible to develop the piston 1. In order to allow the centering of the roller 120 in the piston, on the axis C-C ', the patent FR 2,561,836 offers centering elements A130 placed on the lateral faces of the body A110 and orthogonal to the rolling axis RR 'of the rollers 120. These guide elements A130 accompany the piston 1 in its movement along the sliding axis CC' and prevent the translation of the roller 120 along its rolling axis R-R '. Further, according to one embodiment, a guide groove A131 provided on the outer surface of the centering elements A130 allows a clip (not shown) fixed in the cylinder block to come into engagement in the guide groove A131 and so prevent the piston from rotating along its sliding axis C-C '.

To keep the roller 120 in position in the piston, the document FR 2 899 650 has a bearing A140 of complementary shape to the cradle A112 of the piston 1 and which is housed at the bottom thereof. The A140 bearing is made of one or more friction limiting materials to promote the rolling of the roller 120. The upper edges of the recess A112 and the A140 bearing have stop or retaining surfaces A113, which consist of a protrusion or Advanced draft of the wall from the top edges inward.

Currently, the cradle A112 is produced by a specific machining restart operation in a direction transverse to the axis of the piston. However, this technique, combined with the necessary presence of A130 guide elements to position the roller, involves high complexity and manufacturing costs: elements requiring several machining operations, additional assemblies, robotic and manual deburring, etc.

New piston and roller architectures were subsequently developed. The document WO2012010241 , illustrated by the figure 1c , has a piston comprising an A110 body with an A112 cradle achievable by sintering in particular. For this, the upper part 110a does not extend more than 180 degrees around the cradle recess and forms divergent edges. Otherwise, rework operations would be necessary.

There is nevertheless a need to improve this new piston architecture, some solutions of which are presented in the document EP2015 / 080375 , in particular as regards the centering of the roller within the cradle, support of the bearing, etc.

Pistons close to the invention are also disclosed in DE 10 2006 012458 .

PRESENTATION OF THE INVENTION According to a first aspect

• ∘ un corps ayant une surface de guidage,
• ∘ une partie supérieure, présentant un évidement en berceau selon un axe de roulement, adapté pour recevoir le galet de roulement, dans lequel le berceau est à bord divergent, c'est-à-dire qu'il se referme à moins de 180° selon l'axe de roulement,
• ∘ au moins un élément de centrage formant butée axiale au galet, et configuré pour maintenir le centrage axial du galet dans le berceau selon l'axe de roulement,

dans lequel
le corps, l'au moins un élément de centrage et la partie supérieure sont constitués d'une seule et même pièce venue de matière, et en ce que
l'élément de centrage présente une forme bombée pour venir au contact d'une partie seulement de la surface d'une extrémité axiale du galet, ladite partie étant située au moins sur l'axe de roulement du galet de façon à limiter le couple résistant de frottement.According to the first aspect, the invention relates to a piston with a rolling roller adapted to receive a roller and to slide in a cylinder along a sliding axis, said piston comprising:

• ∘ a body having a guiding surface,
• ∘ an upper part, having a cradle recess along a rolling axis, adapted to receive the rolling roller, in which the cradle has a divergent edge, that is to say that it closes at less than 180 ° according to the rolling axis,
• ∘ at least one centering element forming an axial stop on the roller, and configured to maintain the axial centering of the roller in the cradle along the rolling axis,

in which
the body, the at least one centering element and the upper part are made up of one and the same integral part, and in that
the centering element has a domed shape to come into contact with only part of the surface of an axial end of the roller, said part being located at least on the rolling axis of the roller so as to limit the resistive torque of friction.

• dans toute section orthogonale à l'axe de coulissement, les dimensions de la forme bombée de l'élément de centrage sont identiques,
• dans lequel la forme bombée est concave, préférablement en arc de cercle,
• lequel la forme bombée est convexe préférablement en arc de cercle, faisant ainsi saillie vers l'intérieur de l'évidement,
• la forme bombée s'étend sur toute la largeur de l'élément de centrage,
• la forme bombée fait saillie vers l'intérieur de l'évidement et s'étend sur une partie seulement de ladite largeur de l'élément de centrage, préférablement moins de 50%, encore préférablement moins de 75%,
• l'au moins un élément de centrage comprend dans son prolongement selon l'axe de coulissement une saillie en forme d'oreille,
• la saillie en forme d'oreille, dans une section orthogonale à l'axe de roulement, présente une forme triangulaire ou en arc de cercle,
• la saillie en forme d'oreille, dans une section comprenant l'axe de coulissement et l'axe de roulement, présente une forme triangulaire ou en arc de cercle.

The invention may include the following features, taken alone or in combination:

• in any section orthogonal to the sliding axis, the dimensions of the convex shape of the centering element are identical,
• in which the convex shape is concave, preferably in an arc of a circle,
• in which the domed shape is convex preferably in an arc of a circle, thus projecting inwardly of the recess,
• the convex shape extends over the entire width of the centering element,
• the domed shape protrudes inwardly from the recess and extends over only a part of said width of the centering element, preferably less than 50%, still more preferably less than 75%,
• the at least one centering element comprises in its extension along the sliding axis a projection in the form of an ear,
• the protrusion in the form of an ear, in a section orthogonal to the rolling axis, has a triangular or arcuate shape,
• the protrusion in the form of an ear, in a section comprising the sliding axis and the rolling axis, has a triangular or arcuate shape.

According to the first aspect, the invention also relates to an assembly comprising a piston as described above and a bearing adapted to be positioned at the bottom of the recess.

According to the invention, the pad comprises a shape complementary to the convex shape, so that it is locked in rotation along the axis of rotation by the convex shape of the centering element. Advantageously, the pad has a divergent edge, that is to say that it closes at less than 180 ° along the rolling axis, or with converging edges, that is to say that it closes. strictly more than 180 °. Advantageously, the pad comprises at least one cut angle. According to the first aspect, the invention also relates to a system comprising an assembly as described above and a rolling roller, the bearing being configured to be interposed between the bottom of the recess and said roller, to facilitate the rolling of the roller in the 'obviously.

Advantageously, the roller, orthogonally to the rolling axis, is defined a flat rolling section and a flat end section at the axial end of the roller, said rolling section corresponding to the section of the roller intended to roll on the bearing, and wherein said end section has an area smaller than the rolling section.

Advantageously, the roller comprises at the end a surface comprising a conical part of revolution or a frustoconical part of revolution or a spherical cap or a cylindrical part of revolution.

The invention also relates according to the first aspect to a hydraulic machine comprising a lobed cam and a cylinder block comprising a plurality of radially disposed cylinders and a plurality of systems as described above, each system being housed in a cylinder, the roller possibly coming in contact with the lobed cam.

Advantageously, the roller is in contact with the lobed cam whatever the operating mode of the hydraulic machine.

The invention also relates according to the first aspect to a method of manufacturing a piston as described above, in which the piston is produced by applying a uniaxial compressive force to a material to be formed, no other force being exerted. on matter.

Advantageously, the piston is manufactured by sintering, stamping, die-stamping, or injection molding.

According to the first aspect, the invention also proposes a method for assembling an assembly as described above, using a manufacturing method as described above, comprising a step of lining the cradle with a pad.

Advantageously, the bearing is crimped to the piston.

Advantageously, the crimping is carried out along the centering element, outside the top of the convexity.

Advantageously, the crimping is carried out on the pad at the level of the cut corners.

According to a second aspect

• ∘ un corps ayant une surface de guidage,
• ∘ une partie supérieure, présentant un évidement en berceau selon un axe de roulement, adapté pour recevoir le galet de roulement, dans lequel le berceau est à bord divergent, c'est-à-dire qu'il se referme à moins de 180° selon l'axe de roulement,
• ∘ au moins un élément de centrage formant butée axiale au galet, et configuré pour maintenir le centrage axial du galet dans le berceau selon l'axe de roulement,

le corps, l'au moins un élément de centrage et la partie supérieure sont constitués d'une seule et même pièce venue de matière, et en ce que l'élément de centrage comprend au moins une rainure d'encoche située hors de l'axe de roulement, ladite rainure s'étendant sur toute la hauteur de l'évidement en berceau,
de sorte que le galet vient au contact d'une partie seulement de la surface d'une extrémité axiale du galet, ladite partie étant située au moins sur l'axe de roulement du galet de façon à limiter le couple résistant de frottement.According to the second aspect, there is also disclosed a piston with a rolling roller adapted to receive a roller and to slide in a cylinder along a sliding axis, said piston comprising:

• ∘ a body having a guiding surface,
• ∘ an upper part, having a cradle recess along a rolling axis, adapted to receive the rolling roller, in which the cradle has a divergent edge, that is to say that it closes at less than 180 ° according to the rolling axis,
• ∘ at least one centering element forming an axial stop on the roller, and configured to maintain the axial centering of the roller in the cradle along the rolling axis,

the body, the at least one centering element and the upper part consist of one and the same integral part, and in that the centering element comprises at least one notch groove located outside the rolling axis, said groove extending over the entire height of the cradle recess,
so that the roller comes into contact with only part of the surface of an axial end of the roller, said part being situated at least on the rolling axis of the roller so as to limit the resistive friction torque.

• l'élément de centrage comprend une autre rainure, située sur l'autre élément de centrage en vis-à-vis et dans lequel cet autre élément de centrage comprend une rainure d'encoche située hors de l'axe de roulement,
• chaque élément de centrage comprend deux rainures d'encoche de part et d'autre de l'axe de roulement,
• l'élément de centrage présente une forme bombée pour venir au contact d'une partie seulement de la surface d'une extrémité axiale du galet, ladite partie étant située au moins sur l'axe de roulement du galet de façon à limiter le couple résistant de frottement,
• la surface bombée se trouve entre les deux rainures,
• le piston comprend en outre au moins un perçage traversant l'élément de centrage au niveau du fond d'une rainure d'encoche,
• la rainure comprend à son fond un étagement avec une surface inférieure et une surface supérieure côte à côte,
• l'au moins un élément de centrage comprend dans son prolongement selon l'axe de coulissement une saillie en forme d'oreille,
• la saillie en oreille, dans une section orthogonale à l'axe de roulement, présente une forme triangulaire ou en arc de cercle,
• la saillie en oreille, dans une section comprenant l'axe de coulissement et l'axe de roulement, présente une forme triangulaire ou en arc de cercle.

The piston may include the following features, taken alone or in combination:

• the centering element comprises another groove, located on the other centering element facing each other and in which this other centering element comprises a notch groove located outside the bearing axis,
• each centering element comprises two notch grooves on either side of the rolling axis,
• the centering element has a domed shape to come into contact with only part of the surface of an axial end of the roller, said part being located at least on the rolling axis of the roller so as to limit the resistive torque friction,
• the curved surface is between the two grooves,
• the piston further comprises at least one bore passing through the centering element at the bottom of a notch groove,
• the groove comprises at its bottom a step with a lower surface and an upper surface side by side,
• the at least one centering element comprises in its extension along the sliding axis a projection in the form of an ear,
• the protrusion in the ear, in a section orthogonal to the rolling axis, has a triangular or arcuate shape,
• the lug projection, in a section comprising the sliding axis and the rolling axis, has a triangular or circular arc shape.

According to the second aspect, the invention also relates to an assembly comprising a piston as described above and a bearing adapted to be positioned at the bottom of the recess.

Advantageously, the pad comprises at least one tongue to be housed in a corresponding notch groove. Advantageously, the pad comprises a tongue configured to be housed in the bore.

Advantageously, the pad has a divergent edge, that is to say that it closes at less than 180 ° along the rolling axis, or with converging edges, that is to say that it closes. strictly more than 180 °. Advantageously, which the pad comprises at least one cut angle.

According to the second aspect, the invention also relates to a system comprising an assembly as described above and a running roller, the bearing being configured to be interposed between the base. of the recess and said roller, to facilitate the rolling of the roller in the recess.

Advantageously, the roller, orthogonally to the rolling axis, is defined a flat rolling section and a flat end section at the level of the axial end of the roller, said flat rolling section corresponding to the section of the roller intended for rolling on the bearing, and wherein said end section has an area smaller than the rolling section.

Advantageously, the roller comprises at the end a surface comprising a conical part of revolution or a frustoconical part of revolution or a spherical cap or a cylindrical part of revolution.

The invention also relates according to the second aspect to a hydraulic machine comprising a lobed cam and a cylinder block comprising a plurality of radially disposed cylinders and a plurality of systems as described above, each system being housed in a cylinder, the roller possibly coming in contact with the lobed cam.

Advantageously, the roller is in contact with the lobed cam whatever the operating mode of the hydraulic machine.

According to the second aspect, the invention also provides a method of manufacturing a piston as described above in which the piston is produced by applying a uniaxial compressive force to a material to be formed, no other force being exerted on matter. Advantageously, the piston is manufactured by sintering, stamping, die-stamping, or injection molding.

Advantageously, a step of axial drilling in the groove passing through the centering element is provided.

According to the second aspect, the invention also provides a method of assembling an assembly as described above, using a manufacturing method as described above, comprising a step of lining the cradle with a pad.

Advantageously, the bearing is crimped to the piston.

Advantageously, the crimping is carried out with a reserve of material present at the bottom of the recess, in the groove, in which the reserve of material is preferably on the upper surface of the step described above.

Advantageously, the crimping is carried out on the pad at the level of the cut corners.

According to a third aspect

• ∘ un corps ayant une surface de guidage,
• ∘ une partie supérieure, présentant un évidement en berceau selon un axe de roulement, adapté pour recevoir le galet de roulement, dans lequel le berceau est à bord divergent, c'est-à-dire qu'il se referme à moins de 180° selon l'axe de roulement,
• ∘ au moins un élément de centrage formant butée axiale au galet, et configuré pour maintenir le centrage axial du galet dans le berceau selon l'axe de roulement,

le corps, l'au moins un élément de centrage et la partie supérieure sont constitués d'une seule et même pièce venue de matière, et en ce que l'élément de centrage comprend une rainure et intègre un pion logé dans la rainure, ledit pion comprenant une forme en saillie vers l'intérieur de l'évidement, le long de l'axe de roulement.According to the third aspect, there is disclosed a piston with a rolling roller adapted to receive a roller and to slide in a cylinder along a sliding axis, said piston comprising:

• ∘ a body having a guiding surface,
• ∘ an upper part, having a cradle recess along a rolling axis, adapted to receive the rolling roller, in which the cradle has a divergent edge, that is to say that it closes at less than 180 ° according to the rolling axis,
• ∘ at least one centering element forming an axial stop on the roller, and configured to maintain the axial centering of the roller in the cradle along the rolling axis,

the body, the at least one centering element and the upper part consist of one and the same integral part, and in that the centering element comprises a groove and incorporates a pin housed in the groove, said pin comprising a shape projecting towards the inside of the recess, along the rolling axis.

• le pion comprend un doigt et un élément de frottement, le doigt étant configuré pour être inséré dans la rainure,
• l'élément de frottement s'étend sur toute la largeur de l'évidement
• l'élément de frottement s'étend sur une partie seulement de la largeur de l'évidement, préférablement moins de 50% de la largeur de l'évidement,
• le doigt est configuré pour être inséré dans la rainure, et dans lequel le pion et la rainure forme un assemblage en queue d'aronde,
• l'au moins un élément de centrage comprend dans son prolongement selon l'axe de coulissement une saillie en forme d'oreille.
• la saillie en forme d'oreille, dans une section orthogonale à l'axe de roulement, présente une forme triangulaire ou en arc de cercle,
• la saillie en forme d'oreille, dans une section comprenant l'axe de coulissement et l'axe de roulement, présente une forme triangulaire ou en arc de cercle.

The piston may include the following features, taken alone or in combination:

• the pin comprises a finger and a friction element, the finger being configured to be inserted into the groove,
• the friction element extends over the entire width of the recess
• the friction element extends over only a part of the width of the recess, preferably less than 50% of the width of the recess,
• the finger is configured to be inserted into the groove, and in which the pin and the groove form a dovetail joint,
• the at least one centering element comprises in its extension along the sliding axis a projection in the form of an ear.
• the protrusion in the form of an ear, in a section orthogonal to the rolling axis, has a triangular or arcuate shape,
• the protrusion in the form of an ear, in a section comprising the sliding axis and the rolling axis, has a triangular or arcuate shape.

According to the third aspect, the invention also relates to an assembly comprising a piston as described above and a pad adapted to be positioned at the bottom of the recess.

Advantageously, in which the bearing has a divergent edge, that is to say that it closes at less than 180 ° along the rolling axis, or with converging edges, that is to say that it closes strictly more than 180 °.

Advantageously, the pad comprises at least one cut angle. The invention also relates according to the third aspect to the system comprising an assembly as described above and a rolling roller, the bearing being configured to be interposed between the bottom of the recess and said roller, to facilitate the rolling of the roller in the 'obviously.

Advantageously, the roller, orthogonally to the rolling axis, is defined a flat rolling section and a flat end section at the level of the axial end of the roller, said flat rolling section corresponding to the section of the roller intended for rolling on the bearing, and wherein said end section has an area smaller than the rolling section.

Advantageously, the roller comprises at the end a surface comprising a conical part of revolution or a frustoconical part. of revolution or a spherical cap or a cylindrical part of revolution.

The invention also relates according to the third aspect to a hydraulic machine comprising a lobed cam and a cylinder block comprising a plurality of radially arranged cylinders and a plurality of systems as described above, each system being housed in a cylinder, the roller possibly coming in contact with the lobed cam.

Advantageously, the roller is in contact with the lobed cam whatever the operating mode of the hydraulic machine.

According to the third aspect, the invention also relates to a method of manufacturing a piston as described above, with the exception of the pin in which the piston is produced by applying a uniaxial compressive force to a material to be formed, no other force not being exerted on the matter.

Advantageously, the piston, with the exception of the pin, is manufactured by sintering, stamping, forging, or injection molding.

According to the third aspect, the invention also relates to a method of assembling a pistontel as described above, using a manufacturing method as described above, further comprising a step of inserting the pin into the groove. .

According to the third aspect, the invention also relates to a method of assembling an assembly as described above, using a method for assembling the piston as described above, comprising a step of lining the cradle with a pad.

Advantageously, the bearing is crimped to the piston.

Advantageously, the crimping is carried out along the centering element, outside the top of the convexity.

Advantageously, the crimping is carried out on the pad at the level of the cut corners.

PRESENTATION OF FIGURES

• Les figures la et 1b, déjà présentées, représentent un piston conforme à l'art antérieur respectivement en position assemblée et à l'état éclaté avant assemblage,
• La figure 1c, déjà présentée, illustre un piston déjà connu de l'art antérieur,
• Les figures 2a et 2b illustre une vue tridimensionnelle d'un mode de réalisation (sans que l'élément de centrage ne soit conforme à l'invention),
• Les figures 3a et 3b illustrent une section longitudinale passant par l'axe de coulissement, d'un piston avec coussinet et galet, pour deux variantes différentes de coussinet,
• Les figures 4a à 4e illustrent différentes formes de galet,
• Les figures 5a à 5d illustrent différentes formes d'oreilles de l'élément de centrage,
• Les figures 6a à 6c illustrent plusieurs variantes d'un mode de réalisation de l'élément de centrage,
• La figure 7 illustre une section longitudinale d'un piston avec un galet,
• Les figures 8a à 8d illustre plusieurs variantes d'un autre mode de réalisation de l'élément de centrage,
• Les figures 8e à 8h illustrent un complément apporté aux figures 8a à 8d,
• Les figures 8i à 8k illustrent un autre complément apporté aux figures 8a à 8d,
• Les figures 9a, 9b, 9c illustrent deux variantes d'un autre mode de réalisation d'un élément de centrage,
• La figure 10 illustre une machine hydraulique,
• Les figures 11a et 12b illustrent un mode de réalisation du coussinet et de l'évidement correspondant,
• La figure 12a illustre un mode de réalisation d'un coussinet adapté pour un sertissage au niveau de ses angles,
• La figure 12b illustre une vue en coupe du sertissage de la figure 12a.

Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which should be read with reference to the accompanying drawings, in which:

• Figures la and 1b, already presented, represent a piston according to the prior art respectively in the assembled position and in the exploded state before assembly,
• The figure 1c , already presented, illustrates a piston already known from the prior art,
• The figures 2a and 2b illustrates a three-dimensional view of an embodiment (without the centering element conforming to the invention),
• The figures 3a and 3b illustrate a longitudinal section passing through the sliding axis, of a piston with bearing and roller, for two different bearing variants,
• The figures 4a to 4e illustrate different shapes of pebbles,
• The figures 5a to 5d illustrate different shapes of the centering element ears,
• The figures 6a to 6c illustrate several variants of an embodiment of the centering element,
• The figure 7 illustrates a longitudinal section of a piston with a roller,
• The figures 8a to 8d illustrates several variants of another embodiment of the centering element,
• The figures 8th at 8h illustrate a complement to figures 8a to 8d ,
• The figures 8i to 8k illustrate another complement to figures 8a to 8d ,
• The figures 9a, 9b, 9c illustrate two variants of another embodiment of a centering element,
• The figure 10 illustrates a hydraulic machine,
• The figures 11a and 12b illustrate an embodiment of the pad and the corresponding recess,
• The figure 12a illustrates an embodiment of a pad suitable for crimping at its angles,
• The figure 12b illustrates a sectional view of the crimping of the figure 12a .

DETAILED DESCRIPTION

The figures 2a, 2b , 3a, 3b show different embodiments of the body 110 of a piston 1, some comprising in assembled or exploded view a roller 120 and a bearing 140.

The body 110 comprises a guide surface 111, an upper part 110a having a cradle-shaped recess 112, and a lower part 110b, located opposite the upper part 110a.

The guide surface 111 is cylindrical, centered on a longitudinal sliding axis C-C ', preferably cylindrical of revolution about the axis C-C'. It makes it possible to guide the piston 1 in a complementary cylinder 2 along the sliding axis C-C ', as mentioned previously.

The cradle-shaped recess 112 is adapted to receive a roller 120. For this purpose, the recess 112 preferably has the shape of a semi-cylindrical cavity, of revolution, about a rolling axis RR ′ orthogonal to the longitudinal axis. sliding CC 'and secant thereof. By semi-cylindrical is meant a shape corresponds to one half or less than half of a cylinder

The semi-cylindrical cradle, by definition, closes at less than 180 ° along the rolling axis R-R ', so that it does not close on it: we say that the cradle has divergent edges. The angle δ, less than or equal to 180 °, defines the amplitude of the material of the cradle 112. The free opening is therefore greater than 180 ° depending on the cylinder which can be defined by the semi-cylindrical cradle. The angle β, complementary to the angle δ, defines the angle open to the outside.

The two angles are defined by two straight lines passing through the axis of rotation R-R ', in a section orthogonal to this axis.

In other words, when the roller 120 is put in place, the upper part 110a extends less than 180 ° around the roller.

In order to allow the roller 120 to roll in the cradle-shaped recess 112, a bearing 140 is arranged in said recess 112, between the piston 1 and the roller 120, thus limiting friction.

The roller 120 has a complementary external diameter, except for the required clearance and the bearing 140 near, the internal diameter of the cavity forming the recess 112. It is engaged in this cavity forming the recess 112 and rotates on itself according to l 'R-R bearing axle'. In a variant, illustrated in figure 3a , the piston 1 is designed so that the rolling axis RR 'of the roller 120 is substantially parallel to the upper surface of the upper part 110a. In other words, the cradle 112 has an exactly semi-cylindrical shape and covers exactly 180 ° around the roller 120.

The pad 140 closes itself beyond 180 °.

In another variant, illustrated in figure 3b , the piston 1 is designed so that the rolling axis RR 'is outside the cradle 112. The cradle 112 therefore has a shape which corresponds to only part of a cylinder of revolution, the part being lower or strictly lower than the half of a cylinder.

The pad 140 closes on the figure 3b at 180 ° around the roller 120, and therefore extends outside the cradle. The angle δ 'is defined, which is less than or equal to or greater than 180 °, which corresponds to the amplitude of the material of the bearing 140.

There are constraints relating to the bearing 140, illustrated in particular by the arrow in figure 3b , so that it does not touch the lobed cam. These constraints mainly relate to its dimension (length and thickness). The more the pad extends out of the recess, the more its thickness is constrained so as not to touch the lobed cam.

There is in practice a rolling axis in the cradle 112, defined by the semi-cylindrical shape of the cradle 112, and a rolling axis of the roller 120, attached to the roller 120 and defined by its cylindrical shape of revolution. When the roller is placed in the cradle, these two axes are merged. By abuse, we will speak of a single bearing axis R-R '.

Two general variants can be distinguished. A first variant in which the pad 140 does not hold the roller 120 inside the recess 112. In other words, the pad 140 itself also has a divergent edge and the angle δ 'is less than or equal at 180 °. A second variant in which the pad 140 holds the roller 120 inside the recess 112. In other words, the pad 140 has converging edges and the angle δ 'is strictly greater than 180 °.

Two axial ends 120a, 120b are defined on the roller, each of which comprises a surface S120 (see figures 4a to 4d ). The surface S120 can have different shapes, which will be explained later.

A planar rolling section Sr is also defined orthogonally to the rolling axis R-R ', which has the shape of a disc and which corresponds to the part of the roller 120 necessarily in contact with the bearing 140. It is defined in besides a planar end section Se, which corresponds to a section of the roller at an axial end 120a, 120b and which also has the shape of a disc, since the roller 120 is of revolution for reasons of symmetry. As will be seen later, the end section is not necessarily in contact with the bush 140. The sections are planar surfaces. A planar end section Se does not correspond to the surface S120 which may not be planar.

A rolling zone 140a is similarly defined for the bearing 140a, on which the roller 120 can be caused to roll (this zone depends on the type of roller 120). On the other hand, depending on the shape of the pad, certain zones 140b cannot be in contact with the roller 120.

The pad 140 typically comprises several layers of materials: a first layer of sheet metal of steel or of copper metal, a second sliding layer of a suitable material such as a fluorinated synthetic material, optionally loaded with particles of copper metal. These layers are cut and arched, or rolled, to achieve the desired diameter.

The body 110 further comprises at least one centering element 130 acting as a stop to maintain the centering of the roller 120 in the piston 1, that is to say to prevent the roller 120 from exiting axially by translation along the axis of bearing RR 'of the recess 112, on the cylindrical side surface of the body. The centering elements 130 are located in the upper part 110a, on two locations of the cradle recess 112 diametrically opposed with respect to the sliding axis C-C ', in order to prevent a translation of the roller along the axis. bearing R-R '.

Most preferably, the piston comprises two centering elements 130 facing each other.

To limit friction with the roller 120, the centering element 130 advantageously has a shape to come into contact with only a part of each surface S120 of the axial end of the roller 120. This shape is convex, that is to say- that is, the flat end sections Se all have areas strictly smaller than the flat rolling section Sr.

Preferably, the area of the contact surface is less than 50% of the area of the surface S120 of the end of the roller 120. This part is located on the rolling axis RR 'to limit the frictional resistance torque. .

The term “point support” is understood to mean a support between the inner face of the centering elements 130 and the surface S120 forming a disc whose surface is less than 20% of the surface S120. Preferably, the circle formed by the support has an area less than 10% of the area S120, and preferably less than 5%.

By line support is meant here a support between the internal surface S130 of the centering elements and the surface S120 3 forming a rectangle the area of which is less than 30% of the surface S120.

In other words, the point contact excludes a peripheral edge of the axial end of the roller 120. As indicated previously, preferably, the contact surface (which is an end section Se) is a disc (for reasons of symmetry) whose area is less than 50% of the rolling section Sr, preferably 20%, preferably 10%, preferably 5%. When this area is sufficiently small (disc of almost zero radius), one can thus speak of “point contact”.

Indeed, the tangential speed of the roller is more important as one moves away from the center. Frictions occurring farther from the center are likely to generate a greater resistive torque.

The body 110, the upper part 110a, the at least one centering element 130 consist of one and the same integral part. The outer surface of the centering elements 130 completes the cylindrical shape of the contact surface 110 so that the piston essentially forms a complete cylinder, the generatrices of which are parallel to the sliding axis C-C '. Preferably, the assembly forms a cylinder of circular section.

Various manufacturing processes according to a unidirectional process, in particular sintering, forging, molding, make it possible to obtain such parts and will be detailed below. All the processes have in common a step of striking or removing the part from the mold in a single direction. The cradle recess 112 which has a divergent edge (that is to say that the angle δ is less than or equal to 180 °) allows the use of such methods, when the sliding axis CC 'is oriented in the typing direction. This means that any section of the part is included in the previous one when approaching the exit end of the recess 112. It would otherwise not be possible to unmold the part.

These processes reduce the cost of manufacturing parts and simplify rework and machining operations.

In the following, the method with uniaxial stamping will be referred to as the methods described above.

The upper part 110a and the centering elements 130 may extend along the sliding axis CC 'above the cradle 112, so that the cavity comprises a semi-cylindrical part at the bottom of a cylindrical orifice the shape of which is rectangular in a section orthogonal to the sliding axis C-C '.

The opening at less than 180 ° of the cradle can generate a stress at the level of the rolling axis R-R ', which can be located at the edge of the recess 112, since it is around the axis of RR 'bearing that friction takes place. However, this friction at the edge can damage the part and generate bearing problems for the roller 120 and the resistance of the materials.

To this end, as illustrated in figures 5a to 5d , the at least one centering element 130 may comprise in its extension along the sliding axis CC ′ a projection 132 in the form of an ear. This projection 132 thus extends, along the sliding axis CC-, beyond the end of the recess 112.

Consequently, the rolling axis R-R 'passes through the ears 132 of the centering elements 130.

In the remainder of the description, contact with the centering element 130 may mean contact with the lug-shaped projection 132 of the centering element 130.

The ear-shaped protrusions 132 can have different shapes.

In a section orthogonal to the rolling axis R-R ', the protrusion has a triangular shape, preferably isosceles (see figure 5a ), or a circular arc shape (see figure 5b ). This form has several purposes: the first consists in ensuring that the ear 132 does not extend beyond the roller 132 (otherwise it could touch the lobed cam), the second consists in limiting the addition of material ( for reasons of weight and economy).

In a section comprising the rolling axis RR 'and the sliding axis C-C', the lug protrusions also have a triangular shape (see figure 5c ), with an inclined pan of revolution, or in an arc of a circle of revolution (see figure 5d ).

This is to limit the addition of material by simplifying the manufacturing process as much as possible. This shape can for example be taken up by turning.

For the protrusion in the lug 132 and for the various variants which will be presented below, it is recalled that these shapes must be produced from the material of the body of the piston 110, by a method with uniaxial striking.

Now, several embodiments of the shape of the centering member will be described. On the figures 2a and 2b , these embodiments of the centering element are not illustrated.

First embodiment

In a first embodiment illustrated in figure 6a to 6c , the centering element 130 has a convex shape 131, 133.

More precisely, it is the internal surface S130 which has a domed shape.

To allow realization by a method with unixial strike, the dimensions of the convex shape are the same in any section orthogonal to the sliding axis C-C ', or, at the very least, increase as the we approach the end of the recess 112.

To limit friction, the convex shape is centered on the rolling axis R-R '.

In a first variant illustrated in figure 6a , the convex shape 131 extends over the entire width of the centering element 130 and is concave, that is to say that the internal surface S130 which has a convex shape is oriented towards the outside of the recess 112.

The concavity can be achieved using an arc of a circle, or an approximate shape.

The convex shape 131 can then be a complementary portion of a cylinder of revolution, so that in any section, the dimensions are the same. The recess 112 then has the shape of a portion of a cylinder of revolution at its axial end. Alternatively, the domed shape 131 can be a portion of a hyperboloid, so that between two successive sections in a plane orthogonal to the sliding axis C-C ', the dimensions change, each section being included in the previous one when going towards the outside of the recess 112. The recess 112 then has the shape of a paraboloid at its axial end.

In a second variant illustrated in figure 6b , the domed shape 133 extends over the entire width of the centering element 130 and is convex, that is to say that the internal surface S130 which has a convex shape projecting towards the recess 112.

Convexity can be achieved using an arc of a circle.

The domed shape 133 can then be a portion of a cylinder of revolution, so that in any section, the dimensions are the same. The recess 112 then has the shape of a complementary portion of a cylinder of revolution at its axial end. Alternatively, the domed shape 133 can be a portion of a paraboloid, so that between two sections, the dimensions change. The recess 112 then has the shape of a hyperboloid at its axial end.

Contact with the roller 120 is made at the level of the top of the convexity.

For the sake of clarity, it is specified that if the domed internal surface 130 is convex, then the cradle recess 112 has a concave shape, and vice versa.

In a third variant illustrated in figure 6c , the domed shape 139 extends over only a part of the width of the centering element 130 and is convex, that is to say that the internal surface S130 which has a convex shape 139 of the centering element 130 is oriented towards the recess 112 by protruding therefrom. In this variant, the width of the convex form represents less than 50%, preferably less than 75% of the width of the centering element 130.

The domed shape thus creates a protuberance on which the roller 120 rubs at the level of the rolling axis R-R '.

This third variant can also be combined with the first or the second variant.

Contact with the roller 120 is made at the top of the convexity In this first embodiment, the pad 140 has a particular shape, which is complementary to the convex shape used.

Indeed, the complementary shape of the pad 140 can thus come into abutment against the convex shape 131 of the centering element 130, which prevents it from being rotated by the roller 120. It is thus not necessary to provide specific tabs, as described in the document EP2015 / 080375 .

In the case of the second variant of this embodiment, the zones 140b of non-contact with the roller 120 are located in the corners of the recess, as illustrated in figure 6b . More precisely, seen from above, the contact zone 140a is represented by the largest rectangle inscribed in the cradle recess 112, said rectangle coming from the surface S130 of the centering element 130. The zones 140b are therefore located on the length of the centering element 130, with the exception of the plumb line of the top of the convexity.

In the case of the third variant of this embodiment, the zones 140b are also located in the corners and, seen from above, the contact zone 140a is represented by the largest rectangle inscribed in the cradle recess 112, said rectangle coming from the surface S130 of the centering element 130. The zones 140b are therefore located along the length of the centering element 130, with the exception of the plumb line of the apex of the convexity.

In this first embodiment, the roller 120 advantageously also has a particular shape, as illustrated in figure 7 , and already shown in figures 4a to 4d .

More precisely, to prevent the contact from being linear with the centering element 130, the end section Se of the roller 120 has an area smaller than the rolling section Sr of the roller 120. In other words, the surface S120 end has a convex shape.

To verify this, the surface of the axial end of the roller may include a frustoconical part of revolution (see figures 4a , 4b and 7 ) or a conical part of revolution (see figures 4c ) or includes a spherical cap (see figure 4d ) or part cylindrical of revolution (see figure 4e ). The frustoconical part of revolution or the conical part may form an angle of between 45 and 90 ° with respect to the rolling axis.

The smaller the contact surface, the lower the friction. Nevertheless, other constraints can then intervene (symmetry of the roller, wear, complicated manufacturing, etc.)

The transition between the last rolling section and the first end section can be continuous or else discontinuous (except for the end with a cylindrical part of revolution for example, for which a discontinuity is necessarily necessary so that the area of the section Se, Sr change).

When the domed shape is convex, that is to say it protrudes inside the recess 112 ( figures 6a and 6b ), we understand that there is no particular condition on the angles or radii of curvature. In particular, a standard roller (purely cylindrical in shape) can be used, which generates line contact. Preferably, a roller with a convex S120 end surface will be preferred, in order to generate a point contact.

On the other hand, when the shape is concave, as in the first variant described above ( figure 6a ), it must be ensured that the end section Se has an area small enough not to come into contact with the retaining element 130 elsewhere than in an area around the rolling axis R-R '. In this variant, a point contact is thus made.

For example, in the case of a centering element 130 with a convex shape in an arc of a circle and of a roller 112 whose end is in a spherical cap, the radius of curvature of the spherical cap must be less than that of the convex shape of the centering element 130.

Second embodiment

In a second embodiment illustrated in figure 8a, 8b , 8c and 8d , the centering element 130 comprises at least one notch groove 134 located outside the rolling axis R-R '.

In this way, unlike the grooves already presented in the document EP2015 / 080375 , the friction is reduced since there is less contact on a peripheral ring of the roller 120 around the rolling axis R-R '.

The notch groove 134 extends over the entire height of the centering element 130, to be able to accommodate a tongue 142 provided in the pad 140 (see figure 8b ). This tongue 142 prevents the bearing 140 from being rotated by the roller 120. The groove 134 is not opposite the axis of rotation of the roller R-R '.

In a variant, the groove 134 of the centering element 130 facing one another is located on the other side of the rolling axis.

In another variant, the centering element 130 comprises two notch grooves on either side of the rolling axis R-R '. More grooves 134 can be provided.

The multiplicity of grooves improves the retention of the pad 140 in place.

In another variant, the centering element 130 comprises a convex domed shape 139 as illustrated in the third variant of the first embodiment. On the figures 8c and 8d , the surface S130 which is of convex shape 139 is located between two notch grooves 134 located on a centering element 130.

In another variant, the centering element 130 comprises a convex domed shape as illustrated in the first variant of the first embodiment. The surface S130 of convex shape is therefore located over the entire width of the centering element 130.

In the same way as for the first embodiment, the end section of the roller 120 has an area smaller than the rolling section to reduce friction. We refer to the description already made.

To hold the pad 140 to the bottom of the cradle, crimping techniques can be used. The figures 8th (not to scale and showing only a tongue in a groove), 8f and 8g (magnification of tongue and top surface) illustrate respectively a sectional view, orthogonal to the rolling axis R-R ', and a top view, of the grooves 134. To facilitate crimping, these have at their bottom a step 135, in the direction of sliding C -C ', with a lower surface 135a and an upper surface 135b side by side, so that the bottom of the groove 134 is not flat. The tongue 142 is then positioned on the lower surface 135a and, with a suitable striking tool, the tongue 142 is crimped with the material of the upper surface 135b ( figure 8h and 8g , after hatch crimping). The thickness of the tongue 142 must therefore be less than the difference in level of the step between the two surfaces 135a, 135b.

Except where the tongue is located, the groove 134 therefore has a greater width than the width of the tongue 142.

In terms of assembly, it is either possible to fit the bearing 140, then the roller 120 and crimp by passing the tools through the groove 134, or to fit the bearing 140, crimp it and fit the roller 120. If the pad 140 surrounds the roller 120 at more than 180 ° to maintain it, the roller 120 can be inserted in force thanks to the elasticity. Alternatively, the crimping is carried out by embossing the material of the groove. There is therefore no need for staging.

Another variant for holding the pad 140 is presented in figures 8i, 8j and 8k . An axial bore 137 is made in the centering element 130, inside the grooves 134. The bore is substantially parallel to the axis of rotation RR '(dotted on the figure 8i ) and aims to create a recess inside which the tongue 142 can come to block. For this, the hole 137 is at the bottom of the recess 112.

In this variant, the tongue 142 has a greater length than that of the groove 134 (see figure 8k ). Preferably, a hole 137 is made for each groove 134.

For two facing grooves 134, only one drilling operation can be performed.

As shown in figure 8j , after having made the holes, the pad 140 is inserted, the tongues 142 of which are elastically deformed in the grooves 134, so that once the bottom of the recess has been reached 112 has been reached, the tongues 142 are deployed in the respective hole 137 . If the deformation is not elastic, the tongue 142 can be inserted or folded back by a punch.

Additionally, a hole is drilled at the bottom of the recess, along an axis parallel to the rolling axis R-R '. This hole makes it possible to insert a tab provided on the pad therein and to limit the line contact, in the case of a standard roller, since the hole is located on the top of the convexity.

Third embodiment

In a third embodiment, illustrated in figure 9a , the centering element 130 comprises a groove 136, preferably centered on the rolling axis RR 'and incorporates a pin 138 ( figure 9b ) housed in the groove 136. Alternatively, the groove 136 is not centered.

This pin 138 comprises a finger 138a, able to be inserted into the groove 136, and a friction element 138b projecting towards the inside of the recess 112, along the rolling axis R-R ', in order to come into contact with the roller 120.

To limit friction, the geometry of the friction element 138b and / or of the roller 120 can be adapted. In the case of a roller with an end section Se of area smaller than the rolling section Sr, as described for the first embodiment, a straight pin may be suitable.

Otherwise, the pin may have a surface curved towards the inside of the recess 112. In this case, a standard roller, of cylindrical shape of revolution is suitable.

To promote friction, the pin 138 and more precisely the friction element 138b can incorporate a friction material such as bronze or plastic. This material can be in the form of a ball attached to the pin.

The friction element 138b extends either over the entire width (or substantially) the entire width of the recess 112, or only part of it. By only a part is meant less than 50% of the width of the recess 112.

To improve the holding of the pin 138 in the groove 136, the two can form a dovetail attachment, as illustrated in figures 9c . The finger 138a has the shape of a tenon, which slides in the groove 136 parallel to the sliding axis C-C '.

In addition, to hold the pin 138 along the sliding axis C-C 'in the groove 134, crimping can be carried out after insertion of the pin. For this, the upper surface of the finger 138 is located 1 to 3 mm below a part of the upper surface of the piston at the location of the crimping.

Hydraulic machine without piston retraction

The pistons described above find application in particular in hydraulic machines M0 as described in the introduction and whose roller is in permanent contact with the lobed cam M1, and illustrated in figure 10 . In other words, whether the machine is operating, stopped, disengaged or engaged, the roller 120 touches the lobed cam.

These machines include a cylinder block M3 within which is radially disposed a plurality of cylinders M2 inside which slides a respective piston 1 when the roller 120 rolls on the lobed cam M1. The M3 cylinder block rotates a drive shaft M4.

To maintain the rollers 120 on the cam M1, springs are generally placed under the pistons, resting on the cylinder block and on the lower part 110b of the piston 110.

If necessary, these machines are then convertible, or disengageable, at the level of the drive shaft.

In this embodiment, it is therefore not necessary for the bearing 140 to hold radially, that is to say in the sliding direction C-C ', the roller inside the cradle 112.

Hydraulic machine with piston retraction

The pistons described above also find application in hydraulic machines such as those described in the introduction and whose piston is retractable in its associated cylinder. An M5 system for managing the pressure of the machine casing is then provided. In the absence of piston supply pressure, and with crankcase pressure, the pistons retract into their cylinders, which disconnects and deactivates the hydraulic machine. In an equivalent manner, return springs can be provided to return the pistons to the bottom of their cylinder. The two methods can be used together.

The roller 120 must then be held in the cradle recess 112 so that it does not fall into the machine. There is then a pad 140 in the cradle recess 112 which closes more than 180 ° around the recess. When the roller 120 is disposed in the recess 112, the pad closes more than 180 ° around the roller, so that it is held radially. Shapes of attachment of the bearing 140 are obtained in the piston by metal deformation. The bush enters the piston by elasticity, or alternatively, it is placed before the striking of forms 118. The roller enters the bush by elastic deformation thereof, and then it is held.

Manufacturing process

We refer to the document EP2015 / 080375 and to the parts relating to sintering, stamping, forging, injection molding, the characteristics of which apply to the present invention.

The piston presented here is in fact designed to be able to be manufactured by these different processes.

Thus, a forming apparatus comprising at least two elements movable in translation with respect to one another is used to apply a uniaxial compressive force to a material to be formed. No force or movement in any other direction is exerted on matter.

Assembly process

After the manufacturing process, a step of lining a pad is implemented, that is to say a pad is arranged in the cradle recess.

To fix it, various possibilities are described in the present description, in particular as regards crimping.

Complements

The body 110 comprises a peripheral annular groove or groove 150, preferably located at the level of the lower part 110b (see figures 2a and 2b for example). The groove 150 is adapted to receive a seal or sealing ring (not shown and generally referred to as a “segment”), intended to rest slidably on the internal surface of the associated cylinder and thus isolate the cylinder 2 in two parts when the piston 1 is installed.

The body 110 of the piston 1 may have a constant diameter over its entire length along the sliding axis CC 'outside the cradle 112 and the aforementioned groove 150 (see figure 3a for example ).

It can also have a variable diameter. The diameter of the body 110 at the level of the lower part 110b, typically below the peripheral groove 150, can thus be less than the diameter beyond the peripheral groove 150, that is to say towards the upper part 110a. In this way, the part of the body 110 below the groove 150, more fragile, is not in contact with the cylinder 2.

This narrowing shape can be achieved by the aforementioned methods (see above).

In addition, the shape of the body 110 of the piston 1 can also be narrowed at the axial ends along the sliding axis CC 'of the piston 1, in the upper part 110a and lower part 110b, in order to optimize the pressure stresses in use ( not shown in the figures).

Embodiment of the recess with boss

With reference to figures 11a and 11b , this embodiment has an adaptation of the shape of the recess 112 and of the pad 140. In the four angles formed by the recess 112 when it reaches the plane orthogonal to the sliding axis C-C ', the upper part 110 has at least one boss 118 oriented towards the inside of the recess 112. More concretely, the recess 112 has a semi-cylindrical shape and over a certain axial length (along the axis R-R ') located near at one end, the radius of the semi-cylinder is smaller so that a recess (the boss 118) extends from the upper part 110a towards the interior of the recess 112. Further, it is important that the boss 118 does not project in a balcony above the recess 112, which would otherwise mean that the opening angle is not always greater than 180 °. Therefore, the boss 118 is not provided on the centering elements 130.

This diameter is only reduced in the upper part of the recess 112, that is to say that at the bottom of the recess, the radius is constant over the entire axial length of the recess 112 Preferably, there are four bosses 118, each positioned close to one of the four angles (see figure 11b ). Additionally, in order to be able to be put in place, the pad 140 has near each angle, on two opposite sides, a cutout 144 forming a recess, each cutout allowing the installation of the pad 140 at the level of the bosses 118 (see figure 11b ). Alternatively, it is simply the thickness of the pad which is less at the level of the bosses 118.

Before installation, the pad 140 has a substantially rectangular shape. On each of two opposite faces, the two slits are made, extending in the direction of the other opposite face. For reasons of symmetry, the two cuts 144 are each made at the same distance from the respective nearest edge.

The depth of the bosses 118 is less than the thickness of the pad 140, to avoid there being any risk of friction with the roller 120. The bosses 118 are produced before the pad 140 is fitted. Preferably, they come from of the uniaxial process for obtaining the piston 110.

The piston 1 can include material reserves 117 at the edge of the recess 112. In this way, when the pad 140 is placed, a crimping can be carried out using a punch on each material reserve, which is deformed to cover the end of the pad 140 and block it in the recess 112 ( figure 12b ). This technique is advantageous for a rigid pad 140. It makes it possible to obtain an assembled piston 1, ie comprising the segment and the bearing which do not come apart. Such a piston is then ready for use.

The pad 140 may include at least one cut angle 141, in order to free up space to allow the material displaced by the crimping to come and block the pad 140, without the material reserves 117 extending out of the socket. recess 112. Preferably, to better maintain the pad, the four angles are cut. Preferably, the cut angle 141 for crimping is 1 to 3 mm below a portion of the top surface of the piston at the crimping location. Preferably, the cut angle 141 is located in a plane perpendicular to the axis CC 'and passing substantially at the level of the axis RR'.

This variant can be applied to all the embodiments described in the present description.

In particular, thanks to these cut angles 141 ( figure 12a ), it is possible to set up a pad 140 with converging edges, that is to say that it closes at an angle δ 'to more than 180 ° around the recess 112, while maintaining it to the piston by an easily accessible crimp. Alternatively or additionally, the crimping can be carried out on the edge of the pad 140 which is in contact with the centering element 130. In this case, for operational reasons, preference will be given to crimping in the zones 140b of non- contact of the pad 140, which essentially corresponds to the angles of the pad. If ever the only roller 120 used is sufficiently convex, crimping can take place at any location on the bushing along the centering element 130.

On the figure 12a , for reasons of simplification, the pad 140 shown does not necessarily have a shape complementary to the centering element 130 as has been presented in the previous embodiments.

The figure 12b can then illustrate a sectional view at the cut corners.

Throughout the present description, a planar surface cannot be defined as convex or concave. It should therefore be understood as "strictly convex" or "strictly concave".

Claims (15)

1. An assembly comprising:

   - a piston (1) with a bearing roller (120) adapted to receive a roller (120) and to slide in a cylinder (2) along a sliding axis (C-C'), said piston (1) comprising:

   ∘ a body (110) having a guide surface (111),

   ∘ an upper portion (110a), having a cradle-shaped recess (112) along a rolling axis (R-R'), adapted to receive the bearing roller (120), wherein the cradle (112) has a divergent edge, that is to say it closes at less than 180° along the rolling axis (R-R'),

   ∘ at least one centering element (130) forming an axial abutment at the roller (120), and configured to maintain the axial centering of the roller (120) in the cradle (112) along the rolling axis (R-R'),

   wherein the body (110), the at least one centering element (130) and the upper portion (110a) consist of an integrally formed single piece, and wherein the centering element (130) has a domed shape (131, 133, 139) to come into contact with only portion of the surface (S120) of an axial end (120a, 120b) of the roller (120), said portion being located at least on the rolling axis (R-R') of the roller (120) so as to limit the resisting friction torque,

   - a pad (140) adapted to be positioned at the bottom of the recess (112), the assembly being characterized in that the pad (140) comprises a shape complementary to the domed shape, so that it is blocked in rotation along the axis of rotation (R-R') by the domed shape of the centering element (130).
2. The assembly according to claim 1, wherein, in any section orthogonal to the sliding axis (C-C'), the dimensions of the domed shape of the centering element (130) are identical.
3. The assembly according to claim 1 or 2, wherein the domed shape (131) is concave, preferably arc-shaped.
4. The assembly according to claim 1 or 2, wherein the domed shape is convex (133, 139) preferably arc-shaped, thereby protruding inwardly of the recess (112).
5. The assembly according to any one of claims 1 to 4, wherein the domed shape (133) extends over the entire width of the centering element (130).
6. The assembly according to any one of claims 1, 2 and 4, wherein the domed shape protrudes inwardly of the recess (112) and extends over only part of said width of the centering element (130), preferably less than 50%, more preferably less than 75%.
7. The assembly according to any one of the preceding claims, wherein the at least one centering element (130) comprises in its extension along the sliding axis (C-C') a lug-shaped protrusion.
8. The assembly according to any one of the preceding claims, wherein the pad (140) comprises at least one cut corner (141).
9. A system including an assembly according to any one of the preceding claims and a bearing roller (120), the pad being configured to be interposed between the bottom of the recess (112) and said roller (120), to facilitate the rolling of the bearing roller (120) in the recess (112).
10. The system according to claim 9, wherein there are defined at the roller, orthogonally to the rolling axis (R-R'), a rolling planar section and an end planar section at the axial end of the roller (120),
    said rolling section corresponding to the section of the roller intended to roll on the pad (140),
    and wherein said end section (Se) has an area smaller than the rolling section (Sr).
11. A method for manufacturing a piston of an assembly according to any one of claims 1 to 8, wherein the piston is made by applying a uniaxial compressive force on a material to be formed, no other force being exerted on the material.
12. The manufacturing method according to claim 11, wherein the piston is manufactured by sintering, stamping, die-stamping, or injection molding.
13. A method for assembling an assembly according to any one of claims 1 to 8, using a manufacturing method according to any of claims 11 to 12, comprising a step of lining the cradle with a pad (140), wherein the pad is crimped to the piston.
14. The assembly method according to claim 11, for a piston whose domed shape is convex according to claims 4 to 6, wherein the crimping is carried out along the centering element (130), out the top of the convexity.
15. The assembly method according to claim 14, for an assembly according to claim 8, wherein the crimping is carried out on the pad at the cut corners.

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