EP3258100B1 - Piston for a hydraulic machine with radial pistons having limited friction - Google Patents

Description

Field of the invention

The present invention relates to the field of hydraulic machines with radial pistons, such as pumps or motors.

The present invention relates more particularly to a piston of a radial piston hydraulic machine whose friction created during the operation of said machine is limited.

State of the art

We know the document FR2651836 which describes the structure of a radial piston hydraulic motor for a hydraulic circuit.

A radial piston hydraulic machine, such as a motor or a pump, comprises a lobed cam opposite which are distributed pistons which are each housed in a respective cylinder. The cylinders are generally generally arranged in a star in a cylinder block connected to the shaft of the hydraulic machine. The cam is a cam having eccentricity relative to the shaft, or a multilobed cam.

The pistons each comprise a roller, also called roller, in the form of a cylinder of revolution rotatably mounted in a housing formed at one end of each of said pistons, so that said roller rolls along the lobed cam during the rotation of the piston. cam during engine operation. The axis of rotation of the roller is perpendicular to the axis of the cylinder in which the piston slides.

In order to ensure the axial retention of the roller, along the axis of said roller, in the housing of the piston in which said roller is mounted, inserts are mounted between each end of said roller and the cylinder in which said slide piston.

However, the solution presented in the document FR2651836 has the disadvantage that large friction is generated and that jamming can occur during the rotation of the roller in the piston, thus reducing the life of the piston, decreasing the efficiency of the machine, and imposing to perform regularly maintenance operations.

We also know the documents FR2727471 , DE10205493 and EP1062426 . However, the solutions presented in these documents on the one hand do not reduce the friction generated during the rotation of the roller in the piston housing, and on the other hand are solutions that are difficult and expensive to manufacture.

General presentation of the invention

In this context, the present invention aims to propose a new piston architecture for a radial piston hydraulic machine, such as a pump or a motor.

The above object is achieved according to the invention by means of a piston configured to slide along a cylinder of a radial piston machine, said piston comprising a roller mounted to rotate about a first axis in a housing located at a end of said piston, characterized in that said piston comprises two inserts located on either side of the roller so that each of the inserts form a stop for a translation of the roller along the first axis inside the housing, each of the inserts. comprising a first face bearing on only a portion of the surface of an axial end of the roller, and a second face configured to slide along the cylinder, the roller being movable in translation along the first axis between the two inserts, and the inserts being movable in translation along the first axis inside the housing.

As will be seen later, the invention thus makes it possible to substantially limit the friction created by the rotation of the roller inside the piston housing.

According to another advantageous characteristic, the first face of the inserts is supported on less than 20% of the surface of the axial end of the roller.

According to another advantageous characteristic, the inserts each comprise a main body which comprises a cavity in which is inserted a piece projecting from said cavity towards the interior of the housing to bear against the roller.

According to another advantageous characteristic, the inserts are formed solely of a main body forming both the first face and the second face.

According to another advantageous characteristic, the support of the first face of each of the inserts is centered around a point whose spacing relative to the first axis is between 0% and 30% of the radius of the roller.

According to another advantageous characteristic, the first face of the inserts is concave and the ends of the roller are convex, or the first face of the inserts is convex and the ends of the roller are concave.

According to another advantageous characteristic, the first face of the inserts is flat and the ends of the roller are convex, or the first face of the inserts is convex and the ends of the roller are planar.

According to another advantageous characteristic, the second face of the inserts comprises a groove or rib configured to cooperate with a rib or a complementary groove located on the cylinder.

In addition, the invention also relates to a radial piston hydraulic machine comprising a piston according to any one of the abovementioned characteristics.

In addition, the invention also relates to a vehicle equipped with a radial piston hydraulic machine of the aforementioned type.

Presentation of figures

• la figure 1 représente une vue en coupe simplifiée d'une machine hydraulique conforme à la présente invention ;
• la figure 2 représente une vue en perspective simplifiée d'un piston conforme à la présente invention ;
• la figure 3 représente une vue en coupe axiale d'un piston conforme à la présente invention à l'intérieur d'un cylindre ;
• la figure 4a représente une vue schématique en coupe d'une première variante possible pour l'appui entre le galet du piston et un insert maintenant axialement ledit galet ;
• la figure 4b représente une vue schématique en coupe d'une deuxième variante possible pour l'appui entre le galet et l'insert ;
• la figure 4c représente une vue schématique en coupe d'une troisième variante possible pour l'appui entre le galet et l'insert ;
• la figure 4d représente une vue schématique en coupe d'une quatrième variante possible pour l'appui entre le galet et l'insert ;
• la figure 4e représente une vue schématique en coupe d'une cinquième variante possible pour l'appui entre le galet et l'insert ;
• la figure 4f représente une vue schématique en coupe d'une sixième variante possible pour l'appui entre le galet et l'insert ;
• la figure 5a représente une première variante possible d'insert pour le maintien axial du galet ;
• la figure 5b représente une deuxième variante possible d'insert pour le maintien axial du galet.

Other characteristics, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting example and in which:

• the figure 1 is a simplified sectional view of a hydraulic machine according to the present invention;
• the figure 2 represents a simplified perspective view of a piston according to the present invention;
• the figure 3 represents an axial sectional view of a piston according to the present invention inside a cylinder;
• the figure 4a is a schematic sectional view of a first possible variant for the support between the piston roller and an insert now axially said roller;
• the figure 4b is a schematic sectional view of a second possible variant for the support between the roller and the insert;
• the figure 4c is a schematic sectional view of a third possible variant for the support between the roller and the insert;
• the figure 4d is a schematic sectional view of a fourth possible variant for the support between the roller and the insert;
• the figure 4e is a schematic sectional view of a fifth possible variant for the support between the roller and the insert;
• the figure 4f is a schematic sectional view of a sixth possible variant for the support between the roller and the insert;
• the figure 5a represents a first possible variant of insert for the axial retention of the roller;
• the figure 5b represents a second possible variant of insert for the axial retention of the roller.

Description of the invention

We have shown on figures 1 , 2 and 3 a piston 1 for a hydraulic machine M piston 1 radial.

The piston 1 comprises a cylindrical body 10 and which is adapted to slide in reciprocating movements in a cylinder 2 along a second axis Ω.

The piston 1 is centered around the axis Ω. The piston 1 comprises a first portion with a first diameter which forms a piston body 1, and a second portion having a second diameter which is larger than the first diameter and which thus forms a piston head. According to another possible variant, the piston is formed of an element having only one diameter and which forms both the piston body and the piston head.

The piston 1 comprises a directory sealing element according to the second axis Ω making it possible to guarantee the seal between the piston 1 and the cylinder 2. This sealing element may be formed of a ring or a segment of seal.

The piston 1 comprises a roller 3 which is rotatably mounted in a housing 11 of complementary shape to said roller 3 which is formed at one end of the head of said piston 1. The roller 3 is a cylinder of revolution which comprises as main axis the first axis β.

During operation of the machine, the roller 3 rolls along a cam C lobed M hydraulic machine, thereby causing the rotation of said roller 3 about the first axis β, which is perpendicular to the second axis Ω. The roller 3 allows a relative displacement by rolling between the piston 1 and the cam C limiting the friction.

In order to reduce the friction created by the rotation of the roller 3 inside the housing 11, a bearing 4 whose shape is complementary to the roller 3 is installed at the bottom of the housing 11, so that the roller 3 does not rub against the walls of said housing 11 but against said pad 4.

The pad 4 is generally made of a copper alloy, or metal coated with synthetic material charged with particles of copper alloy, in order to limit the coefficient of friction between the roller 3 and the pad 4.

In order to maintain axial position of the roller 3 with respect to the first axis β inside the housing 11, inserts 5 are arranged on either side of the axial ends 31 and 32 of the roller 3.

These inserts 5 each form an abutment limiting the axial displacement of the roller 3 according to a translation of axis β. The inserts 5 are interposed between on the one hand the wall of the cavity of the cylinder 2 in which the piston 1 is disposed, and the other ends 31, 32 of the roller 3, and thus realize the axial abutment of the roller 3 respectively d one side and the other of the roller 3.

On the variant illustrated on the figure 3 , the spacing between the inserts 5 is greater than the length of the roller 3, so that the roller 3 is movable in translation between the two stops formed by the inserts 5. The roller 3 therefore relies on only one of these two stops, the inserts 5, at a given moment. The translational play of the roller 3 must advantageously prevent jamming of the roller 3, even when the hydraulic machine M heats up. This game must preferably be greater than or equal to 0.1 mm, preferably 0.2 mm, and may be up to 1 mm.

When a gap exists between the inserts 5 and the roller 3, the inserts 5 form an abutment alternatively, the insert 5 which is pressed against the cylinder 2 forming a stop, the other insert 5 being slightly spaced from the cylinder 2 because of the slight difference between the diameter of the piston 1 and the cylinder 2. However, it is possible that the two inserts 5 are in contact against the cylinder 2 simultaneously.

However, according to one possible variant, the inserts 5 can be sufficiently close together to prevent the roller 3 from sliding along the axis β. This choice can be interesting to obtain a silent hydraulic machine M, or if there is no heating.

The inserts 5 are advantageously located on either side of the ends of the pad 4, so as to be in contact with the lateral surface of the pad 4. Thus, the spacing between the inserts 5 is defined by the length of the pad 4. In this way the inserts 5 also define the axial location of the pad 4 in the piston, along the first axis β.

The inserts 5 may be adapted to cooperate with the housing 11 of the piston 1 so as to limit or completely block the rotation of said inserts 5 along the first axis β. Thus, the inserts 5 may comprise a groove which engages in a groove formed inside the housing 11, so as to limit or block the rotation of the inserts 5 along the first axis β. The inserts 5 can also be mounted tight inside the housing 11, thus blocking rotation and translation along the first axis β.

Each insert 5 comprises a first face 51 which is intended to bear against the roller 3, and a second face 52 configured to bear against the cylinder 2 and slide along said cylinder 2 during the movements of the piston back and forth 1 inside said cylinder 2.

Such inserts 5 thus allow a simplified assembly of the piston 1 because they allow on the one hand that the piston 1 is composed of simple elements to manufacture (for example with simple shapes to be machined), and on the other hand they allow to 'soften manufacturing tolerances.

The inserts 5 can be mounted so as to slide along the first axis β inside the housing 11. Alternatively, it is however possible to mount the inserts 5 fixedly inside said housing 11.

The inserts 5 shown on the figure 3 , according to a first possible variant of the inserts 5, comprise a main body 53 which comprises a cavity 54 opening at the first face 51 of said inserts 5. A piece 55 is inserted into the cavity 54 and projects from said cavity 54 so that the support between the inserts 5 and the roller 3 is performed by the piece 55. The second face 52 is formed by the main body 53.

The fact that the inserts 5 comprise a main body 53 and a part 55 makes it easier to manufacture the inserts and to reduce the manufacturing cost.

Indeed, the main body 53 can be in an inexpensive and easy to form material, such as plastic, because said main body is not in contact with the roller 3.

The part 55 may in turn be in a material that is adapted for friction against the roller 3. The part 55 may for example be of a hardness that is similar to the hardness of the roller 3. The part 55 being of a reduced size and having a simple shape, its manufacture is simple and inexpensive.

The roller 3 is generally made of steel, the piece 55 may be steel, nitrided steel, bronze, or polymer such as for example Vespel®. The material of the part 55 may be adapted depending on the material of the roller 3. The part 55 may also have undergone a heat treatment and / or chemical to improve its hardness and / or its coefficient of float such as nitriding.

The part 55 is a part adapted for friction, and can thus be made of copper alloy in order to limit the coefficient of friction between said part 55 and the roller 3.

In order to prevent a rotation of the piston 1 inside the cylinder 2, the inserts 5 may comprise a groove or a rib directed along the axis Ω and located on the second face 52 of said inserts which cooperates with a rib or a rib. complementary groove formed in the cylinder 2 so as to form a slide connection of axis Ω. The complementary rib formed in the cylinder 2 may be formed by an element which is inserted into the radially outer wall of the cylinder block and which projects inside said cylinder. Such a solution is for example described in the document FR2727471 .

In order to reduce the friction between the inserts 5 and the ends 31 and 32 of the roller 3, the first face 51 of each insert 5 is configured to bear against only a portion of the surface of the axial ends 31 and 32 of said roller 3. Otherwise, said first face 51 of each insert 5 does not bear against the entire surface of the axial ends 31 and 32 .

The inserts described in the document FR2651836 abut against the entire surface of the ends of the roller because the face of the inserts bearing against the ends of the roller is a flat circular face and said ends of said roller are flat circular faces of the same diameter as the inserts.

In order to limit the friction surface, the first face 51 of the inserts 5 according to the invention is thus configured not to come into contact with the ends 31 and 32 over their entire surface (thus forming for example a point or line support).

Preferably, the contact surface between the first face 51 of the inserts 5 and the axial ends 31 and 32 of the roller 3 is less than 20% of the area of said ends 31 and 32. Even more preferably, the contact surface between the first face 51 of the inserts 5 and the axial ends 31 and 32 of the roller 3 is less than 10% of the area of said ends 31 and 32. And even more advantageously, the contact surface between the first face 51 of the inserts 5 and the axial ends 31 and 32 of the roller 3 is less than 5% of the area of said ends 31 and 32.

Advantageously, in order to further limit the friction between the first face 51 of the inserts 5 and the ends 31 and 32 of the roller 3, the point support between the inserts 5 and the roller 3 is located near the first axis β, or otherwise on the axis of rotation of the roller 3. The fact that the support between the inserts 5 and the roller 3 is as close as possible to the first axis β is indeed preferable because the radial speed of the roller 3 is lower at the of its axis of rotation. The support between each insert 5 and the roller 3 forms a circular zone. Preferably the support between the first face 51 of the inserts 5 and the axial ends 31 and 32 of the roller 3 is formed in a central zone of the ends 31 and 32, so that the peripheral portion said ends 31 and 32 (radially distant from the first axis β) are not in contact with the inserts 5.

The spacing between the center of the support between the first face 51 of the inserts 5 and the axial ends 31 and 32 of the roller 3 and the first axis β is less than a maximum radius, for example between 0% and 30% of the radius of the roller 3.

Preferably, the support between the first face 51 of the inserts 5 and the axial ends 31 and 32 of the roller 3 is centered on the axis β, so as to minimize friction.

The Figures 4a, 4b, 4c and 4d illustrate four possible variants of shapes of the first face 51 of the inserts 5 and roller 3 to reduce the surface of the support between said first face 51 and the ends of the roller 3. However, the variants given in these figures are not other forms of course being possible in order to provide one-off support.

On the figure 4a on the one hand the first face 51 of the inserts 5 has a convex shape, that is to say that it is convex towards the inside of the housing 11 of the piston 1, and on the other hand the ends 31 and 32 of the roller 3 are flat.

On the figure 4b , which is a variant equivalent to the variant illustrated in figure 4a on the one hand the first face 51 of the inserts 5 is flat, and on the other hand the ends 31 and 32 of the roller 3 are convex, that is to say that they are bulged outwardly of the housing 11 piston 1.

The occasional support of the variants illustrated in Figures 4a and 4b is that of a sphere resting on a plane.

The variants illustrated in Figures 4a and 4b have the advantage of being easy to achieve, one of the two bearing surfaces being flat, thus allowing the use of rollers 3 or existing inserts 5.

On the figure 4c on the one hand the first face 51 of the inserts 5 is concave, that is to say hollowed towards the outside of the housing 11, and on the other hand the ends 31 and 32 of the roller 3 are convex, it is ie, bulging outward of the housing 11. In addition, the radius of curvature the concavity of the first face 51 of the inserts 5 is greater than the radius of curvature of the convexity of the ends 31 and 32, so that the first face 51 of the inserts is in contact only on a portion of the ends 31 and 32. The difference between the two radii of curvature must be such that the contact surface between the inserts 5 and the ends 31 and 32 of the roller 3 is less than 20% of the area of said axial ends 31 and 32 of the roller 3. Such a characteristic can for example, when the radius of curvature of the convex surface is less than 20% of the radius of curvature of the concave surface.

On the figure 4d , which is a variant equivalent to the variant of the figure 3c on the one hand the first face 51 of the inserts 5 is convex, that is to say convex towards the inside of the housing 11, and on the other hand the ends 31 and 32 of the roller 3 are concave, it is In other words, the radius of curvature of the convexity of the first face 51 is smaller than the radius of curvature of the concavity of the ends 31 and 32 of the roller 3, so as to ensure a punctual support and not surface.

The occasional support of the variants illustrated in Figures 4c and 4d is that of a sphere on a curved plane whose radius of curvature is greater than the radius of said sphere.

The variants illustrated in Figures 4c and 4d have the advantage of preventing a radial displacement relative to the first axis β of the roller 3 in the housing 11, the inserts 5 acting as radial abutments to the roller 3. This thus reduces the necessary depth of the housing 11, and thus d on the one hand to reduce the material required for the manufacture of the piston 1, and secondly to ensure more easily that the end of the piston 1 comprising the housing 11 does not come into contact with the lobed cam during operation of the machine. This furthermore makes it possible to maintain the roller 3 with the piston 1, during the assembly of the hydraulic machine M, or when, due to a lack of pressure in the hydraulic machine M, the piston 1 is retracted and the roller 3 is spaced from the cam C. Indeed, in these variants the inserts 5 grip the roller 3 from the moment when the piston 1 assembled is placed in the cylinder 2.

According to another possible variant, the first face 51 of the inserts 5 is convex, that is to say convex towards the interior of the housing 11, and the axial ends 31 and 32 of the roller 3 are also convex, that is, ie bulging outward of the dwelling 11.

In addition, according to a variant illustrated in figure 3e the concavity and / or the convexity of the faces 51 of the inserts 5 and / or the ends 31 and 32 of the roller 3 may be formed by an angle instead of being formed by an arc of a circle, as in the variants illustrated in FIGS. figures 3a-3d . In this case, when the first faces 51 of the inserts 5 are concave and the ends 31, 32 of the roller 3 are convex, or conversely that the first faces 51 of the inserts 5 are convex and that the ends 31, 32 of the roller 3 are concave, the difference in angle between the first faces 51 of the inserts and the ends 31, 32 of the roller 3 to be sufficiently high so that the first face 51 of the inserts 5 is in contact only with a portion of the ends 31 and 32 Thus, the angle difference must be greater than 20 °, preferably greater than 30 °, and still more preferably 40 °.

As illustrated on the figure 3f , it is of course also possible to use inserts 5 having a first angular face 51 with a roller 3 having axial ends 31 and 32 rounded, whether concave or convex. Similarly, the axial ends 31 and 32 may be angular and the first face 51 of the rounded inserts 5, whether concave or convex.

The inserts 5 can be configured to cooperate with the housing 11 of the piston 1 so as to limit the rotation of the inserts 5 along the first axis β. The shape of the inserts 5 can be adapted so that the rotation of the inserts 5 along the first axis β is completely blocked, or so that a movement of the inserts 5 along the first axis β is still possible.

In particular, in the case of mounting the roller 3 with axial play between the inserts 5, the position of the inserts 5 in the housing 11 can be determined by the support on the cylinder 2, while there may be a slight angle between the piston 1 and the cylinder 2, which induces a non-perpendicularity between the roller 3 and the inserts 5.

The position of the inserts 5 in the housing 11 can also be determined by the bearing on the bearing 4, especially in the case of mounting without axial play of the roller 3 between the inserts 5. However, a certain angular tolerance remains.

In all cases, the particular shape of the first face 51 of the inserts 5 and the ends 31 and 32 of the roller 3 makes it possible to maintain the limited support between the roller 3 and the inserts 5, preferably located at the level of the first axis β.

The angular tolerance of the inserts 5 in the housing 11 of the piston 1 may be between 0 ° and 5 °, and said angular tolerance is preferably 0.5 °.

The Figures 5a and 5b represent two possible variants for producing the inserts 5.

In the insert variant 5 illustrated on the figure 5a which conforms to the insert variant illustrated in figure 2 the insert 5 comprises a main body 53 in which a cavity 54 is hollowed out at the first face 51. A part 55 used to come into contact with the roller 3 is inserted into the cavity 54 and projects from said cavity 54 inwardly of the housing 11 of the piston so that the ends 31 and 32 of the roller 3 abut against said part 55 and not against the main body 53.

The piece 55 may for example be a cylindrical pin which is inserted into the cavity 54 of the main body 53 and which comes into contact with the roller 3 at one of its circular faces. The part 55 can also be a ball which is rotatably mounted in the cavity 54, thus making it possible to improve the rotation of the roller 3.

It is thus understood that the shape of the first face 51 of the insert 5 which abuts against the roller 3 may be chosen by inserting a part 55 which has the desired shape.

In order to maintain the position of the workpiece 55 in the cavity 54 of the main body 53, said workpiece 55 can be mounted tightly within said cavity. The part 55 can also be overmolded in the body 53, and be shaped to prevent the rotation of said workpiece 55 relative to the body 53, and prevent its tearing out of said body 53. Thus, the workpiece 55 may comprise for example grooves along of its circumference, a pin, or anti-rotation ribs on the face of the part 55 in plane support on the body 53.

Preferably, the piece 55 is made of a copper alloy, such as bronze for example, in order to limit friction with the roller 3.

In the variant illustrated on the figure 5b the insert 5 is composed solely of its main body 53 which forms both the first face 51 and the second face 52.

According to a possible variant, the main body 53 may comprise a pion-shaped cylindrical end projecting towards the inside of the housing 11 and bearing against the roller 3.

The insert 5 may be of molded plastic material in a material having a molding accuracy, temperature resistance, sufficient hardness. For example a filled thermoplastic, such as a polyamide loaded with beads or glass fibers.

The insert 5 can also be made in the mass in a material capable of friction on steel, such as a cuprous material, for example a bronze. In this case, it can be machined or molded.

The second face 52 of the inserts 5 is of complementary shape to the wall of the cylinder 2 against which said second face 52 comes to rest. Thus, the second face 52 is preferably a surface forming a portion of a cylinder whose diameter is equal to the diameter of the cylinder 2.

According to a possible variant, the second face 52 of the inserts 5 may be formed by an additional piece which is fixed on said inserts 5, which is attached or overmolded, and which can for example be used to limit the friction between the inserts 5 and the cylinder 2. additional piece may have a surface forming a portion of a cylinder whose diameter is equal to the diameter of the cylinder 2 in which the additional piece slides.

In addition, the additional piece may be adapted to not be rotated about the first axis β. The additional piece can thus comprise elements projecting from said additional piece and cooperating with elements of complementary shape dug in the insert 5 in order to block the rotation along the first axis β. Elements projecting from the additional part may also cooperate with elements of complementary shape dug in the cylinder 2, so as to prevent rotation of the additional part along the first axis β with respect to the cylinder 2. These projecting elements may for example for example, ribs formed on the additional piece. Of course, similarly, in place of projecting elements formed on the additional piece and cooperating with elements dug in the insert 5 or the cylinder 2, it is possible to use elements dug in the cooperating additional part. with elements protruding from the insert 5 or cylinder 2.

According to another possible variant each of the axial ends 31 and 32 of the roller 3 comprises a ball rotatably mounted in said roller 3. These balls, which are centered on the first axis β, come into contact with the first face 51 of the inserts 5, and allow to limit the friction. According to another possible variant each of the axial ends 31 and 32 of the roller 3 comprises a cylindrical end in the form of a pin making a contact on a part of the surface of the insert (5).

Naturally, the present invention is not limited to the embodiment which has just been described, but extends to any variant as defined by the respective subject of the appended claims.

Claims (7)

1. A piston (1) configured to slide along a cylinder (2) of a radial-piston hydraulic machine, said piston (1) comprising a roller (3) rotatably mounted about a first axis (β) in a housing (11) located at one end of said piston (1), said piston (1) comprising two inserts (5) located on either side of the roller (3) so that each of the inserts (5) forms a stop for a translation of the roller (3) along the first axis (β) inside the housing (11), each of the inserts (5) comprising a first face (51) bearing on only a portion of the surface of an axial end (31, 32) of the roller (3), and a second face (52) configured to slide along the cylinder (2), the roller (3) being movable in translation along the first axis (β) between the two inserts (5), and characterized in that the inserts (5) are movable in translation along the first axis (β) inside the housing (11), the inserts (5) each comprising a main body (53) which comprises a cavity (54) in which a part (55) is inserted protruding from said cavity (54) inwardly of the housing (11) to bear against the roller (3).
2. The piston (1) according to claim 1, wherein the first face (51) of the inserts (5) is bearing on less than 20% of the surface of the axial end (31, 32) of the roller (3).
3. The piston (1) according to any one of claims 1 or 2, wherein the bearing of the first face (51) of each of the inserts (5) is centered about a point whose a distance from the first axis (β) ranges between 0% and 30% of the radius of the roller (3).
4. The piston (1) according to any one of claims 1 to 3, wherein the first face of the inserts (5) is concave and the ends (31, 32) of the roller (3) are convex, or the first face of the inserts (5) is convex and the ends of the roller (3) are concave.
5. The piston (1) according to any one of claims 1 to 3, wherein the first face (51) of the inserts (5) is flat and the ends (31, 32) of the roller (3) are convex, or the first face (51) of the inserts (5) is convex and the ends (31, 32) of the roller (3) are flat.
6. The piston (1) according to any one of claims 1 to 5, wherein the second face (52) of the inserts (5) comprises a groove or rib configured to cooperate with a complementary groove or rib located on the cylinder (2) .
7. A radial-piston hydraulic machine comprising at least one piston (1) according to any one of claims 1 to 6.

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