EP1971775A1

EP1971775A1 - Hydraulic machine, in particular hydraulic motor, with a reciprocating movement, and differential piston for such a machine

Info

Publication number: EP1971775A1
Application number: EP06841882A
Authority: EP
Inventors: Matthieu Darbois; Sébastien FURET; Philippe Duquennoy; Dominique Berton
Original assignee: Dosatron International SAS
Current assignee: Dosatron International SAS
Priority date: 2006-01-13
Filing date: 2006-12-07
Publication date: 2008-09-24
Anticipated expiration: 2026-12-07
Also published as: FR2896280A1; EP1971775B1; DE602006016868D1; US8136441B2; ATE480711T1; US20100269685A1; ES2352353T3; WO2007080251A1

Abstract

Hydraulic machine, in particular hydraulic motor, comprising a casing, a differential piston (4) having a region of large cross section and a region of smaller cross section, which can slide with a reciprocating movement respectively in the casing and in a housing coaxial to, and of smaller diameter than, the casing, the piston separating two chambers of the casing and being provided at the location of its large cross section and its small cross section with sealing lips (48, 49) which are oriented in opposite directions and which face one another. The sealing lips (48, 49) are provided on detachable circular rings (50, 51), and the piston (4) is molded in one piece.

Description

HYDRAULIC MACHINE, IN PARTICULAR A HYDRAULIC ENGINE, WITH ALTERNATIVE MOTION, AND DIFFERENTIAL PISTON FOR SUCH A MACHINE.

The invention relates to a hydraulic machine, in particular a hydraulic motor, a machine of the type that includes:

- an envelope ;

a differential piston having a zone with a large cross-section and a zone with a smaller cross-section, capable of sliding in reciprocating motion respectively in the envelope and in a housing coaxial with the envelope and of smaller diameter, the piston separating two chambers of the envelope;

- Hydraulic switching means for the supply of liquid and the evacuation of the chambers separated by the piston, these switching means being controlled by the movements of the piston and can take two stable positions;

and triggering means capable of causing, at the end of the piston stroke, a sudden change in the position of the switching means, under the action of an elastic means, for reversing the stroke, the piston being provided with at its large section and its small section of sealing lips oriented in opposite directions and facing one another,

A hydraulic motor of this type is known, for example from EP-BO 255 791, or from US Pat. No. 5,505,224 or from EP-B-1 151 196. This hydraulic motor can be used to drive a device for injecting an additive into a main liquid ensuring the operation of the engine.

In known engines, each sealing lip is molded in one piece with the corresponding region of the piston. The zones of different section of the piston are made separately and then assembled. Although this solution is satisfactory for sealing and operation, it lacks flexibility for manufacturing and maintenance. Practically, when a sealing lip is damaged, it is necessary to change at least the corresponding part of the piston and, generally, the entire piston

The invention aims, above all, to provide a hydraulic machine of the kind defined above, including a hydraulic motor, the manufacture of the piston is made simpler and easier maintenance.

It is also desirable to improve the conditions under which the sealing between the different sections of the piston and the wall of the envelope or housing is achieved. According to the invention, a reciprocating hydraulic machine, in particular a hydraulic motor, of the kind defined above is characterized in that the sealing lips are provided on removable circular rings, and that the piston is molded of a single room. Advantageously, each sealing ring is attached to the piston by a bayonet type assembly. Preferably, radial projections are provided on the inner surface of the ring and corresponding peripheral grooves are provided on the outer surface of the piston, with grooves parallel to the geometric axis of the piston to allow the coming of each projection of the ring to right of a corresponding peripheral groove of the piston.

Preferably, the sealing ring for the large section of the piston has a V profile whose concavity is turned towards the small section, and has on the opposite side a frustoconical sealing lip whose diameter increases towards the cover of the envelope. The circular ring of the small section of the piston has a V-shaped cross section whose concavity is turned towards the large section.

Preferably, a snap of the rings on the piston is provided at the end of rotation of the bayonet assembly. Advantageously, the removable circular rings are made of plastic.

The invention also relates to a differential piston, for a hydraulic machine as defined above, the piston having a large section area and a smaller section area, and being adapted to reciprocally slide respectively in an envelope and in a a barrel coaxial with the casing and smaller diameter, the piston separating two chambers of the casing and being provided at its large section and its small section of sealing lips oriented in opposite directions and turned one towards the other, characterized in that it is molded in one piece and comprises means for assembling with removable circular rings on which the sealing lips are provided.

The assembly means are advantageously of bayonet type and preferably comprise grooves on the outer surface of the piston, with grooves parallel to the geometric axis of the piston to allow radial ribs to come on the inner surface of a piston. sealing ring to the right of a corresponding peripheral groove of the piston.

Advantageously, the differential piston comprises at least one window for latching at least one ring at the end of assembly to bayonet.

The invention consists, apart from the arrangements set out above, in a certain number of other arrangements which will be more explicitly discussed below with reference to an embodiment described with reference to the accompanying drawings, but which is in no way limiting. On these drawings:

Fig. 1 is a simplified schematic representation in vertical section of a hydraulic motor according to the invention.

Fig. 2 is a perspective view, in vertical section, of a hydraulic motor according to the invention.

Fig. 3 is a partial vertical section of the hydraulic motor along line III-III of FIG. 4.

Fig. 4 is a partial vertical section along line IV-IV of FIG. 3.

Fig. 5 is a perspective view of the differential piston with valves, valve holder, elastic means and push in place.

Fig. 6 is an exploded perspective view of the piston of FIG. 5 and different parts disassembled.

Fig. 7 is an exploded perspective view of the piston alone and disassembled seals. Fig. 8 is a bottom view of the seal of the large section of the piston.

Fig. 9 is a section along the line IX-IX of FIG. 8.

Fig. 10 is a vertical section of the seal of the small section of the piston.

Fig.11 is a perspective view of an alternative embodiment of the pusher in two parts during assembly, and

Fig.12 is an exploded perspective view, similar to Fig.7, of an alternative embodiment of the piston and seals.

Referring to the drawings, especially Figs. 1 and 2, we can see a hydraulic machine consisting of a reciprocating differential hydraulic motor.

The motor M comprises a casing 1 consisting of a cylindrical body 2 surmounted by a cover 3 assembled to the body 2 in a removable manner, in particular by screwing.

A differential piston 4 is disposed in the casing 1 to slide in reciprocating motion. The piston 4 comprises, in the upper part, a zone 5 of large section, in the form of a ring, the periphery of which bears tightly against the inner wall of the casing 1. A substantially cylindrical barrel 6, coaxial with the envelope 1 and smaller diameter than the crown 5 is integral with this crown and extends downwards. The lower part of the barrel 6 slides in a sealed manner in a cylindrical housing 7 coaxial with the casing 1. The barrel 6 is closed at the bottom by a bottom 6a, constituting the zone of small section of the piston. The piston 4 separates the interior volume of the envelope 1 into two chambers, respectively 8 located below the crown 5, and 9 located above the crown 5. The chamber 8 is an annular chamber between the envelope 1, the outer surface of the housing 7 and the outer surface of the barrel 6. An inlet manifold 10 opens at the bottom of the chamber 8. The interior volume of the housing 7 located below the bottom 6a of the piston constitutes a third chamber 11, or outlet chamber, to which is connected an outlet pipe 12 axis orthogonal to that of the casing.

A cylindrical sleeve 13 coaxial with the casing 1 extends downwards to allow connection to an injection device J of a liquid additive in the outlet chamber 11. This injection device is actuated by the motor whose piston 4 is connected by a rod 4r to a pumping means of the additive. For more details on this type of engine we can refer to EP 0 255 791 or EP 1 151 196.

Hydraulic switching means C are provided for the supply of liquid and the evacuation of the chambers 8, 9 separated by the piston. These switching means C are controlled by the movements of the piston 4 and comprise a rod 14 acting on a dispensing member 15 which can take two stable positions. In one of the stable positions, the chamber 8 receives the pressurized liquid while the chamber 9 is connected to the outlet 12. In the other position of the dispensing member 15, the chamber 9 receives the liquid under pressure and is isolated from room 11.

In the example shown in the drawings, the distribution member

15 is constituted by a valve holder comprising at least a first valve

16 whose seat is located under the crown 5, the valve 16 having a head located in the chamber 8. The valve 16 closes by moving from bottom to top according to the drawings. The dispensing member 15 is provided with at least one other valve 17 whose head is located in the chamber 9. The seat of each valve 17 is located on the bottom 6a. The valve 17 closes by a downward movement against its seat. The engine further comprises triggering means comprising a pusher 18 capable of causing, at the end of the stroke of the piston, by bearing against a stop, a sudden change in the position of the switching means C under the action of an elastic means E, for reversing the stroke of the piston.

The rod 14 is articulated at one end 14a at a point fixed with respect to the piston 4. The other end 14b of the rod can move in a vertical window 19 of the valve holder and abut against one of the two ends of this window, in one of the two stable positions of the dispensing member 15.

According to the invention as visible in FIGS. 2 and 6, the resilient means E is integral, at each of its ends, with a hinge member 20a, 20b respectively received in a housing 21 provided on the link and 22 on a another moving part of the machine, namely the pusher 18 in the example shown. Each housing 21, 22 is open in a direction substantially opposite to the direction of the force exerted by the resilient means E in the wall of the housing considered. The disassembly of the articulation member 20a, 20b of its housing is effected by a simple translational movement exerting a force contrary to that of the elastic means E. The holding of the articulation members 20a, 20b in their housing is ensured by the effort developed by the elastic means E.

This elastic means E is advantageously constituted by a leaf spring 23 substantially arc-shaped convex curve, in particular substantially in a semicircle. The blade 23 is integral at each end of a cylindrical pin orthogonal to the plane of the curve arc, this pin constituting the articulation member 20a, 20b.

Preferably, the leaf spring 23 is made of plastic material, and is molded in one piece with the cylindrical pins 20a, 20b. The spring blade 23 turns its convexity on the opposite side to the bottom 6a of the piston.

The link 14 is articulated, at its end 14a remote from the housing 21 receiving the pin 20a, in a notch 24 (Fig.6), fixed relative to the piston 4. The rod 14 (Fig.2 and 6) is advantageously formed by two parallel elementary links 25, 26 interconnected by a cross member 27 closer to the end 14b than the other end 14a. The ends of the rods 25 and 26 forming the end 14b, have a cylindrical shape of geometric axis orthogonal to the longitudinal direction of the rods. Each of these cylindrical ends comprises a housing 21 facing the other elementary rod, and open on the opposite side to the end 14a. Each housing 21 receives an end of the peg 20a which protrudes transversely on both sides of the blade 23. Preferably, a snap of the pin 20a in the housing 21 is provided.

Each elementary rod 25, 26 has two parallel branches between which is provided at the end 14a, a hinge pin 28 received in the notch 24 of the piston. An axle 28 is provided for each elementary link 25, 26. Each axis 28 is molded in one piece with the two branches of the elementary link. The two elementary links 25, 26 are molded in one piece with the cross member 27 so that the rod 14 is constituted by a single piece of plastic material. The indentations 24 of articulation on the piston form a housing substantially in a semicircle for free engagement of an axis 28. The notches 24 are provided in walls 29,30 parallel to each other and to the geometric axis of the piston, equidistant from these walls. The spacing between the walls 29, 30 is equal to the spacing between the hinge pins 28 of the two elementary links, while the thickness of the walls 29, 30 is smaller than the distance between the internal faces of the two parallel branches. of the same elementary link. Each wall 29, 30 is framed by two parallel branches of a single elementary rod.

Each notch 24 is open in the opposite direction to the force exerted by the resilient blade 23 on the rod 14, which force pushes and maintains each axis 28 bearing against the bottom of the notch 24. The axle 28 is simply engaged in the notch 24, preferably without snapping. The assembly and disassembly of the link 14, with its two axes 28 respectively in the indentations 24, are particularly simple and fast.

The pusher 18, as visible in FIG. 6, comprises a rod 31, for example of cruciform cross-section, guided in sliding in a well 32 (FIG. 2) of the piston. The rod 31 comprises, substantially mid-length, a groove 33 for receiving an O-ring 34 providing a tight sliding in the well 32. The upper end of the rod 18 is integral with a frame 35 having a transverse base integral with the upper end of the rod 18. The frame 35 also comprises two longitudinal walls 35a, 35b, parallel to the geometric axis of the rod 18, flanking the resilient blade 23 and connected at the top by a crossmember 36. Each wall 35a, 35b has at its base integral with the rod 18 a recess 22 substantially in a semicircle, opening on the opposite side to the rod 14. Each notch 22 receives one end of the hinge pin 20b, preferably snapped. Frame 35 forms a a kind of gantry and the crossbar 36 at the top is the high abutment of the pusher.

The ends of the blade 23 preferably have a reduced width which determines the minimum allowable gap between the facing faces of the inner end walls of the housing 21 or 22.

The pusher 18 with its frame 35 is also made of a single piece of plastic material. The mounting or dismounting of the pin 20b in the housings 22 is particularly simple and rapid, by elastic deformation of the blade 23. The two longitudinal walls 35a, 35b of the gantry 35 have outwardly, on both sides of the zones. provided with indentations 22, projections 37, for example in the form of a truncated pyramid, limited by a planar top face parallel to the geometric axis of the rod 31 and orthogonal to the pin 20b. The projections 37 are adapted to cooperate with walls 38 (FIGS. 5 and 6) in the form of a straight dihedral, integral with the piston, guiding the gantry 35. The walls 38 are connected to the walls 30 along an edge parallel to the piston axis, on the opposite side to the indentations 24.

The valves 16, 17 are mounted on the valve holder 15 (FIG. 6) having substantially the shape of a frame 39, vertical in the operating position of the engine, having on its two vertical sides ribs 40 projecting towards the outside, adapted to cooperate with guide grooves (not visible) provided in the piston 4. The lower horizontal side of the frame 39 has two open housings 41 allowing a coupling, snap-fastening of the cylindrical rod of a valve 17, by a translation perpendicular to the plane of the frame 39. The lower edges of each housing 41 are housed, with a certain clearance in a direction parallel to the axis of the piston, between two flanges or disks 42a, 42b (Fig.4) secured to the rod of the flap. The valve itself is constituted by a disc having at its periphery a groove 43 (Fig.4) to receive a seal. The respective seats of the valves 17 are provided on the bottom 6a of small section of the piston 4. The valves 17 are located on the side of the rod 14 relative to the piston 4.

The upper horizontal side of the frame 39 (FIG. 6) extends beyond the vertical uprights of the frame and comprises, at each end, a laterally open housing 44 with a cross-section greater than a semicircle for receiving with a snap-fastening rod. cylindrical 45 (Fig.4) secured to the valve 16. The rod 45 comprises two flanges radially projecting between which is housed the housing wall 44. Two valves 16 are provided to which correspond two seats 45 on the large section 5 of the piston, symmetrically with respect to a plane passing through the axis of the piston and orthogonal to the axis of pivoting of the rod 14.

The valves 16 are oriented with their rod 45 directed upwards which is engaged through the opening of the seat to be hooked to the housing 44, on the same side of the large section 5 as the rod 14. When the lid 3 is disassembled, it is possible to access the rods 45 of the valves 16 to engage in the housing 44, or to disengage.

The valve holder 15, 39 advantageously comprises, on its upper horizontal side, a stirrup 46 projecting upwardly flanking, by its lateral wings, the elastic blade 23 (FIG. 5). This stirrup 46 is designed to ensure, when it comes into abutment against the lid 3 of the envelope, for example as a result of a failure of a part of the engine, a machine operation in bypass (bypass) , the valves being kept open. The valve holder 15, 39 forms a single piece which can be made of molded plastic.

Columns 47, three for example, having a cross-section with three branches, integral with the large section 5 of the piston, project upwards parallel to the axis of the piston and are distributed at 120 ° to one of the other in the vicinity of the periphery of the piston. The posts 47 constitute mechanical safety stops against the cover 3 of the envelope in the event of breakage of a part or overhead, the height of these posts 47 being provided accordingly.

The piston 4 is provided at its large section and small section, sealing lips 48, 49 (Fig.6 and 7) facing towards each other.

The sealing lips 48, 49 are provided on removable circular sealing rings 50, 51, clearly visible in FIG. The piston 4 can thus be molded in one piece, advantageously made of plastic.

Each sealing ring 50, 51 is advantageously fixed to the piston 4 by a bayonet type assembly having radial projections 52, 53 on the inner surface of the respective rings 50, 51. The projections 52, 53 are adapted to cooperate with corresponding peripheral grooves 54, 55 provided on the outer surface of the large section and the small section of the piston. Clearances 56, 57 generatrices parallel to the axis of the piston are provided on the outer periphery of the large and small sections to allow to bring the radial projections 52, 53 opposite the inlet of the peripheral grooves 54, 55 by a translation movement parallel to the axis of the piston. Then, by a rotation around this axis of the piston, the ribs 52, 53 are engaged in the grooves 54, 55 with locking of the corresponding sealing ring 50, 51. The disassembly of the rings 50, 51 is effected rapidly by a reverse movement.

As can be seen in FIG. 9, the sealing ring 50 provided at the level of the large section of the piston has a V profile whose concavity is turned towards the small section, that is to say downwards according to the representation drawings. This V-shaped profile is limited on the outside by the lip 48 which seals against the wall of the casing and, on the inside, by a slightly frustoconical lip 58 which seals against the piston, the ring 50 comprises , on the side opposite the lips 48,58 another frustoconical sealing lip 59 turned in the opposite direction, the diameter of which increases in the direction of the lid of the envelope. This lip 59 provides protection of the sealing zone between the piston and the envelope, at the level of the lip 48, against a fallout of abrasive particles. The circular ring 51 of the small section of the piston has a V-shaped cross section whose concavity is turned towards the large section, that is to say upwards according to FIG. The outer arm of the V forms the sealing lip 49 between piston and casing.

The outer wall of the piston 4, as shown in Fig.6, comprises two recessed areas such as 4a, extending under the seats of the valves 16 and allowing their vertical reciprocating movements.

The motor M can be mounted in the following manner.

The sealing rings 50, 51, shown in FIG. 7, can be assembled to the piston 4, by the bayonet fixing, before the valves are put in place.

A subassembly (FIG 6) can be constituted by engaging the pin 20b of the resilient blade 23 in the notches 22 of the pusher 18, while the pin 20a is engaged in the housing 21 of the rod 14. This subset can then be put in place in the piston 4 by engaging the rod 31 of the pusher 18 in the guide well 32 and by engaging the pins 28 of the link 14 in the notches 24.

Maintaining the blade 23, by lashing the pin 20a in the housing 21 of the rod and the pin 20b in the notches 22, facilitates the handling of the subassembly (link 14 - elastic blade 23 - push 18) during assembly.

Another subassembly is prepared from the valve holder 15,39 in the lower part of which are snapped the valves 17. The valve holder 15,39 is then engaged in the piston 4 in a downward movement vertically by introducing the ribs 40 into the corresponding guide grooves of the piston.

The valves 16 with their upwardly directed rods are then presented below the crown 5. The stem 45 of each valve 16 is then passed through the opening of the corresponding seat in order to snap the upper end of this rod. in the housing 44 of the valve holder 15,39. The entire piston, the pusher, the connecting rod and the valve holder is made. It is then sufficient to engage the piston in the body 2, then assemble the lid 3 to the body 2 by screwing. The dismantling or replacement of faulty parts are deduced from the previous explanations and are extremely simple and fast.

The operation of the engine is similar to that described in EP1 151 196 and will be briefly recalled with reference to FIG. 1 where the piston 4 is in ascending stroke. The pressurized liquid, generally water, arrives through the inlet 10. The valves 16 are closed while the valves 17 are open, allowing the liquid to flow from the chamber 9 to the chamber 11 and the outlet 12.

At the end of the upward stroke, the pusher 18 comes to bear against an abutment linked to the cover 13, which causes under the effect of the leaf spring 23 the tilting of the link 14 towards the other low stable position, with movement of the door valves 15 to the bottom 6a of the piston. The valves 17 close while the valves 16 open. The pressurized liquid passes into the closed chamber 9 and the movement of the piston is reversed. At the end of the downward stroke, the pusher 18 at its lower end meets an abutment G secured to the casing, which causes a new tilting of the rod 14 towards the raised position and a displacement of the valve holder 15 causing the closing of the valves 16 and the opening of the valves 17. The movement of the piston 5 is reversed again and the piston starts up again.

Fig.11 illustrates an alternative embodiment of the pusher 18, the rod 31a is made in two parts 31b, 31c assembled removably. The upper part 31b is extended downwards by a cylindrical shaft 31 d of outside diameter smaller than that of the upper part of 31b. The lower part 31c is cylindrical tubular to receive the shaft 31 d. A lock is provided between the two parts, in particular by a bayonet system 31e, 31f provided at the lower end of the parts. The pusher seal is advantageously constituted by a ring 34a made of plastic material with a V-shaped section, with a double lip similar to the ring 51, but of smaller diameter. The concavity of the V is preferably turned upwards. The ring 34a is mounted on the lower part 31c with a quarter-turn bayonet fixing system, similar to that described with reference to FIGS. 7 to 9. This arrangement makes it easy to change the ring 34c in the event of wear and tear. a choice of plastic material, for the ring 34a, having a good chemical inertia with respect to the liquids passing through the engine and the metering device.

Fig.12 illustrates an alternative embodiment of the removable sealing rings 50, 51. In this embodiment, a snap rings 50,51 on the piston 4 is provided at the end of rotation of the bayonet assembly. For this, the ring 50 has a radially inward projection 52a provided at one end of a rib 52. A corresponding window 52b is provided in the side wall of the piston so that at the end of rotation of the assembly, the projection 52a enters the window 52b. A locking rotation of the ring 50 is thus achieved and prevents undesirable disassembly of the ring 50 in operation. To remove the ring 50, using a tool is pushed from inside, through the window 52b, the projection 52a outwardly to disengage the window.

A similar arrangement is provided for the lower ring 51 which has an inner radial projection 53a, at one end of a rib 53, to engage a window, not visible in the drawing, of the piston 4.

The hydraulic motor, more generally the hydraulic machine, according to the invention can be made entirely of plastic, including the elastic means E formed by the blade 23, and has a high resistance to chemicals without any metal parts.

The number of component parts of the machine is considerably reduced. Assembly and maintenance are simplified and made easier.

The sealing ring 50 of the large section of the piston incorporates a protective lip 59. The removable sealing rings 50, 51 make it possible to maintain the opposite sealing directions in order to obtain the injection of the additive product entering through the sleeve 13. in the exit chamber 11.

The mechanism for controlling the tilting of the rod 14 and the change of position of the valve holder 15, 39 is particularly simple of the three-point type.

Claims

A differential hydraulic machine, in particular a hydraulic motor, comprising: - an envelope (1);

- a differential piston (4) having a zone (5) with a large section and a zone (6a) with a smaller section, adapted to reciprocably slide respectively in the envelope and in a housing coaxial with the envelope and moreover small diameter, the piston separating two chambers (8,9) from the envelope; - Hydraulic switching means (C) for the supply of liquid and the evacuation of the chambers separated by the piston, these switching means being controlled by the movements of the piston and can take two stable positions;

and triggering means capable of causing, at the end of the piston stroke, a sudden change in the position of the switching means, under the action of an elastic means (E), for the reversal of the stroke, the piston (4) being provided at its large section (5) and its small section (6a) of sealing lips (48,49) oriented in opposite directions and facing each other, characterized in that the sealing lips (48, 49) are provided on removable circular rings (50, 51), and the piston (4) is molded in one piece.

2. Hydraulic machine according to claim 1, characterized in that each sealing ring (50,51) is fixed to the piston by a bayonet type assembly.

Hydraulic machine according to claim 2, characterized in that the bayonet-type assembly has radial projections (52, 53) on the inner surface of the ring and corresponding peripheral grooves (54, 55) on the outer surface. piston, with grooves (56,57) parallel to the geometric axis of the piston to allow the coming of each projection of the ring to the right of a corresponding peripheral groove of the piston.

4. Hydraulic machine according to one of the preceding claims, characterized in that the sealing ring (50) provided at the large section of the piston has a V-shaped profile whose concavity is turned towards the small section, and has on the opposite side a sealing lip frustoconical (59) whose diameter increases towards the cover of the envelope.

5. Hydraulic machine according to any one of the preceding claims, characterized in that the circular ring (51) of the small section of the piston has a V-shaped cross section whose concavity is turned towards the large section.

6. Hydraulic machine according to claim 2 or 3, characterized in that a latching of the rings (50,51) on the piston (4) is provided at the end of rotation of the bayonet assembly.

7. Hydraulic machine according to any one of the preceding claims, characterized in that the removable circular rings (50, 51) are made of plastic.

8. Differential piston for a hydraulic machine according to any one of the preceding claims, having a zone (5) with a large section and a zone (6a) with a smaller section, able to slide in reciprocating motion respectively in an envelope and in a housing coaxial with the casing and smaller diameter, the piston separating two chambers (8,9) of the casing, the piston (4) being provided at its large section (5) and its small section (6a ) of sealing lips (48,49) oriented in opposite directions and facing each other, characterized in that the piston (4) is molded in one piece and comprises means for assembly with removable circular rings (50, 51) on which the sealing lips (48, 49) are provided.

9. Differential piston according to claim 8, characterized in that the connecting means with the removable circular rings (50, 51) are of the bayonet type.

Differential piston according to claim 9, characterized in that the bayonet type connecting means comprise radial projections (52, 53) on the inner surface of the ring and corresponding peripheral grooves (54, 55) on the outer surface of the piston, with grooves (56,57) parallel to the geometric axis of the piston to allow the coming of each projection of the ring to the right of a corresponding peripheral groove of the piston.

11. Differential piston according to claim 9 or 10, characterized in that it comprises at least one window (52b) for latching at least one ring (50) at the end of bayonet assembly.