EP3201466B1 - Hydraulic mechanism provided with means for guiding pistons in translational movement - Google Patents

Description

FIELD OF THE INVENTION

The present description relates to the field of hydraulic machines with radial pistons, and more precisely the means for guiding in translation of the pistons of such hydraulic machines.

STATE OF THE PRIOR ART

Hydraulic machines are known comprising a cylinder block having a plurality of housings in which pistons slide comprising means for guiding the pistons in translation in their housings. The document FR 2,727,471 in the name of the applicant, for example, presents a structure of a hydraulic machine in which the pistons are guided in translation by a U-shaped clip, inserted on the one hand in a recess of the cylinder block, and on the other hand in a radial groove arranged on a wedging piece ensuring the maintenance of the piston roller.

However, such a U-shaped clip which spans a part of the cylinder block is fragile and does not withstand significant mechanical forces, in particular shearing, which can occur under certain circumstances, for example in the event of cavitation or hydraulic shock. In particular, certain hydraulic machines, such as tree cutting machines, experience violent power surges during which the machine brakes and restarts suddenly and frequently: such jolts cause pressure reversals in the block. cylinders, so that the pistons can suddenly take off from the cam against which they are normally in contact. In such occasions, the piston may tend to pivot in its housing, thus generating significant bending / shearing forces in the clip, causing fatigue over time and then breaking of the clip. In addition, such sudden and repeated efforts can wear the groove of the wedging piece, reducing the effectiveness of the locking in rotation of the piston even if the clip is not damaged.

The document GB 2,250,784 describes a radial piston engine in which the pistons are stages. The documents FR 2 943 391 and US 6,443,047 describe for their part hydraulic machines whose pistons are provided with guide means.

There is therefore a need for a hydraulic mechanism provided with guiding means in translation which are devoid, at least in part, of the drawbacks inherent in the aforementioned known systems.

PRESENTATION OF THE INVENTION

The present invention relates to a hydraulic mechanism according to claim 1 appended.

More generally, the present presentation relates to a hydraulic mechanism, motor or pump, comprising a cam; a cylinder block rotatably mounted relative to the cam; at least one cylinder formed in this cylinder block; at least one piston assembly able to cooperate with a cylinder, this piston assembly comprising a piston mounted to slide inside said cylinder; a roller rotatably mounted on the piston and configured to cooperate with the cam, the roller being delimited by two transverse end faces; first and second wedging pieces each disposed between an end face of the roller and the internal face of the cylinder on which said wedging piece is supported so as to maintain the axial position of the roller. The cylinder has a groove formed in its internal face, and the piston assembly has a projection configured to cooperate with the groove of the cylinder so as to maintain the orientation of the roller.

In the present description, the terms “axial”, “radial”, “tangential”, “interior”, “exterior” and their derivatives are defined with respect to the main axis of the hydraulic mechanism.

Thanks to such a configuration, the translational guidance is more robust: it is able in particular to withstand power surges from the hydraulic mechanism and the sudden movements of the piston that the latter can cause. Indeed, the guidance is done here within the cylinder itself thanks to the cooperation of the projection of the piston assembly with the groove of the cylinder. Thus, this projection can be shorter and this cooperation can be done more closely the axis of the piston, which reduces the lever arm and therefore the importance of shear forces when the piston seeks to rotate in the cylinder.

In addition, thanks to this configuration, there is greater design freedom to provide the projection of a size sufficient to ensure its solidity and a significant cooperation interface with the groove, which allows better distribution of the mechanical forces. undergone by the projection. Similarly, in this configuration, the wedging pieces can be devoid of groove, which makes them more massive and therefore more solid.

These blocking means are also easier to manufacture and assemble: in particular, the projection can be an integral part of an existing part of the piston assembly, which reduces the number of parts used.

In certain embodiments, the projection is integral with the piston. The guiding of the piston in translation and its locking in rotation is therefore ensured directly.

In some embodiments, the projection is an end portion of a pin inserted into a bore of the piston. Such a configuration is very easy to achieve since it suffices to provide the piston with such a bore and then to engage the pin. This pin can be fixed in the bore, for example by tight fitting, or slide freely in the bore.

In some embodiments, the pin is metallic.

In other embodiments, the projection is a protrusion forming an integral part of the piston. Such a single piece can be obtained by removing material from a precursor piece to form this protuberance; it can also be obtained by molding or other techniques.

In certain embodiments, the projection extends in a direction which does not intersect the main axis of the piston. In this way, the projection does not interfere with a possible member extending along the main axis of the piston, such as a center hole for example.

In some embodiments, the piston has a center hole, extending along its main axis from its underside, and a bore for receiving a projection pin, the center hole and the bore not communicating with the within the piston.

In certain embodiments, the lateral surface of the piston has a flat, the projection projecting from this flat. Such a flat makes it easier to set up the projection: when the projection is a pin inserted in a bore of the piston, the bore is easier to pierce on the flat surface of the flat; when the projection is a protuberance of the piston produced by removal of material, this flat may be the surface remaining following the removal of material. In addition, such flat allows the passage of hydraulic fluid along this portion of the piston, which is particularly useful in the case of a stepped piston.

In certain embodiments, the projection is offset towards a lateral edge of the flat relative to the center of said flat. This makes it possible to reduce the length of the projection, the distance separating the flat part of the piston and the internal face of the cylinder being all the smaller the closer one gets to the lateral edges of the flat part. In the case where the projection is a pin, one can for example provide a single size of pin adapted for all the pistons without fear that this pin, of reduced size to adapt to the smallest piston, will escape from the groove larger cylinders.

In some examples of the description not part of the invention, the projection is integral with the first wedging piece. In such a case, the piston can be left intact, the function of guiding in translation and locking in rotation being integrated with the function of wedging in the wedging piece. It is thus possible to use existing pistons in new cylinder blocks according to the invention by equipping them with this type of wedging piece.

In certain examples of the description not forming part of the invention, the projection is a boss forming an integral part of the first wedging piece. The wedging piece can thus be obtained in one piece, by molding for example. This gives a more massive and therefore more solid part.

In some embodiments, the first wedging piece is plastic.

In certain embodiments, the groove extends to the outer end of said cylinder and opens in the outer face of the cylinder block. In certain configurations, the projection can therefore possibly leave the groove for a short time when the piston follows the cam: in this way, the piston is not locked in translation, which offers a large amplitude to the piston and therefore improves performance of the hydraulic mechanism.

In certain embodiments, said cylinder comprises an outer section, having a first diameter, defining an outer chamber, and an inner section, having a second diameter smaller than the first diameter, defining an inner chamber; the piston has an outer portion, having a first diameter, and an inner portion, having a second diameter less than the first diameter; the groove is formed in the interior section of the cylinder and the projection projects on the interior portion of the piston. This configuration is suitable for stepped pistons which have a narrower inner section, thus making it possible to have a larger number of pistons around the axis of the cylinder block.

In certain embodiments, the groove extends to the outer end of the inner section of the cylinder and opens into the outer chamber of the cylinder. This facilitates the mounting of the stepped piston and increases the cross-section of the hydraulic fluid.

In certain embodiments, the projection extends perpendicular to the axis of the piston.

In some embodiments, the projection has a tapered inner end. In the present description, the term “tapered” means a shape which tapers towards its end; this end being for example frustoconical with possibly a rounded tip. This shape can be assessed in a radial plane (orthogonal to the main axis of the mechanism) and / or in the axial plane (containing the main axis of the mechanism and the axis of the piston). This allows, in cases where the projection can come out of the groove, to facilitate its reinsertion into the groove and to automatically re-center the piston. The assembly of the mechanism is also facilitated.

In some embodiments, the projection height of the projection decreases towards its inner end. In the present description, "projection height" means the height of an element, at a given point, measured from the surface on which this element projects and perpendicular to this surface. This also makes it easier to insert the projection into the groove and prevent it from catching on the edge of the cylinder.

In certain embodiments, the hydraulic mechanism is devoid of any fixing element mounted in an external or lateral face of the cylinder block and overlapping the periphery of a cylinder. The manufacture of the cylinder block is thus facilitated and the risk of stalling and loss of an element in the casing of the mechanism is reduced.

In certain embodiments, the groove of the cylinder is made in the thickness of the cylinder block.

In some embodiments, the groove extends in a direction parallel to the axis of the cylinder.

In certain embodiments, the piston is provided with a seal ensuring the seal between the piston and the internal face of the cylinder.

In certain embodiments, the groove of the cylinder is provided so that it is never overlapped by the piston seal when the piston is guided by the cam. This prevents hydraulic fluid from escaping from the piston chamber bypassing the seal through the groove.

The aforementioned characteristics and advantages, as well as others, will appear on reading the detailed description which follows, of exemplary embodiments of the hydraulic mechanism proposed. This detailed description refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawings are schematic and aim above all to illustrate the principles of the presentation.

• La FIG 1 est une vue générale en coupe axiale d'un mécanisme hydraulique.
• La FIG 2A est une vue en coupe transversale d'un bloc-cylindres selon un premier exemple ne faisant pas partie de l'invention.
• La FIG 2B est une vue en coupe selon le plan IIB-IIB de la FIG 2A.
• La FIG 3A est une vue de face d'une pièce de calage selon le premier exemple.
• La FIG 3B est une vue en coupe selon le plan IIIB-IIIB de la FIG 3A.
• La FIG 4 est une vue partielle en coupe tangentielle de l'ensemble piston engagé dans le bloc-cylindres de la FIG2A.
• La FIG 5A est une vue en coupe transversale d'un bloc-cylindres selon un deuxième exemple faisant partie de l'invention.
• La FIG 5B est une vue selon la flèche VB de la FIG 5A.
• La FIG 6 est une vue en perspective d'un piston selon le deuxième exemple.
• La FIG 7 est une vue en perspective et en coupe de l'ensemble piston engagé dans le bloc-cylindres de la FIG 5A.
• La FIG 8 est en vue de profil d'une variante de réalisation du piston de la FIG 6.
• La FIG 9 est une vue en coupe transversale d'un bloc-cylindres selon cette variante.

In these drawings, from one figure (FIG) to another, identical elements (or parts of elements) are identified by the same reference signs. In addition, elements (or parts of elements) belonging to different embodiments but having a similar function are identified in the figures by numerical references incremented by 100, 200, etc.

• The FIG 1 is a general view in axial section of a hydraulic mechanism.
• The FIG 2A is a cross-sectional view of a cylinder block according to a first example not forming part of the invention.
• The FIG 2B is a section view according to the plan IIB-IIB of the FIG 2A .
• The FIG 3A is a front view of a wedging piece according to the first example.
• The FIG 3B is a section view according to the IIIB-IIIB plan of the FIG 3A .
• The FIG 4 is a partial view in tangential section of the piston assembly engaged in the cylinder block of the FIG2A .
• The FIG 5A is a cross-sectional view of a cylinder block according to a second example forming part of the invention.
• The FIG 5B is a view according to the arrow VB of the FIG 5A .
• The FIG 6 is a perspective view of a piston according to the second example.
• The FIG 7 is a perspective view in section of the piston assembly engaged in the cylinder block of the FIG 5A .
• The FIG 8 is a side view of an alternative embodiment of the piston of the FIG 6 .
• The FIG 9 is a cross-sectional view of a cylinder block according to this variant.

DETAILED DESCRIPTION OF EMBODIMENT (S)

In order to make the presentation more concrete, examples of hydraulic mechanisms are described in detail below, with reference to the attached drawings. It is recalled that the presentation is not limited to these examples.

• un carter en deux parties 1a, 1b, assemblées par des vis 2;
• une came ondulée 3, solidaire du carter 1a, 1b;
• un arbre de sortie 4, monté à rotation dans le carter, autour d'un axe 5, au moyen de deux roulements à rouleaux 6, et, dont l'extrémité intérieure est munie de cannelures 7;
• un bloc-cylindres 8, qui comporte un évidement central muni de cannelures 9, associées aux cannelures 7 de l'arbre pour solidariser en rotation l'arbre 4 avec le bloc-cylindres 8 et pour centrer ce bloc-cylindres par rapport à cet arbre;
• une pluralité de cylindres 10 disposés radialement en étoile par rapport à l'axe 5, contenant chacun un piston 11, qui y est monté coulissant;
• une face plane 12, dont est muni le bloc-cylindres 8, qui est perpendiculaire à l'axe de rotation 5, et dans laquelle débouchent des conduits 13 reliés aux différents cylindres 10;
• un distributeur 14 de fluide aux divers cylindres 10, qui est muni d'une face plane de distribution 15, perpendiculaire à l'axe de rotation 5 et disposée en regard de et en appui sur la face de communication 12 du bloc-cylindres, et d'une face axiale qui comporte deux gorges circulaires 16, 17 communiquant, respectivement, sélectivement avec une alimentation en fluide sous pression 18 et avec un échappement de fluide 19, cependant qu'un dispositif d'entraînement 20 rend solidaire en rotation ce distributeur de fluide 14 avec la partie la du carter, et que des conduits 21, 22, relient les gorges 16, 17 respectivement, à la face plane 15 et sont susceptibles d'être mis en communication, successivement, pendant la rotation relative du bloc-cylindres 8 par rapport au distributeur 14, avec les conduits 13 du bloc-cylindres 8. En l'espèce, la face axiale du distributeur présente une troisième gorge circulaire 16'. Il s'agit en effet d'un moteur à deux cylindrées de fonctionnement, la gorge 16' pouvant être reliée à la gorge 16 ou à la gorge 17 par un sélecteur de cylindrée (non représenté) pour faire fonctionner le moteur en grande ou en petite cylindrée. Il va de soi que l'exposé s'applique indifféremment à des moteurs à une ou à plusieurs cylindrées.

Engine 1 of the FIG 1 includes:

• a housing in two parts 1a, 1b, assembled by screws 2;
• a corrugated cam 3, integral with the casing 1a, 1b;
• an output shaft 4, mounted for rotation in the casing, about an axis 5, by means of two roller bearings 6, and, the inner end of which is provided with grooves 7;
• a cylinder block 8, which has a central recess provided with grooves 9, associated with the grooves 7 of the shaft to secure in rotation the shaft 4 with the cylinder block 8 and to center this cylinder block with respect to this shaft ;
• a plurality of cylinders 10 arranged radially in a star with respect to the axis 5, each containing a piston 11, which is mounted therein slidingly;
• a flat face 12, which is provided with the cylinder block 8, which is perpendicular to the axis of rotation 5, and into which there are conduits 13 connected to the different cylinders 10;
• a distributor 14 of fluid to the various cylinders 10, which is provided with a flat distribution face 15, perpendicular to the axis of rotation 5 and disposed opposite and in abutment on the communication face 12 of the cylinder block, and an axial face which has two circular grooves 16, 17 communicating, respectively, selectively with a supply of pressurized fluid 18 and with a fluid exhaust 19, while a drive device 20 makes this fluid distributor 14 rotationally integral with the part 1a of the casing, and that conduits 21, 22 connect the grooves 16, 17 respectively, to the flat face 15 and are capable of being placed in communication, successively, during the relative rotation of the cylinder block 8 with respect to the distributor 14, with the conduits 13 of the cylinder block 8. In this case, the axial face of the distributor has a third circular groove 16 '. It is in fact a motor with two operating displacements, the groove 16 'being able to be connected to the groove 16 or to the groove 17 by a displacement selector (not shown) to operate the motor in large or in small displacement. It goes without saying that the description applies equally to engines with one or more displacements.

A cylindrical roller 23 is housed in a bearing 24 formed at the end of each piston 11, is rotatably mounted around a roller axis 25 orthogonal to the piston axis 26, coincident with the cylinder axis 10, and is supported on the cam 3. This roller 23 is capable of penetrating at least partially inside the cylinder, so that, on the side of each transverse face 27 delimiting the roller, a recess 28 is formed in the part of the piston 11 which supports this roller, which makes it possible to form, on either side of said roller, spaces. Each recess corresponds at least to the space between at least the surface of the cylinder 10, the cylindrical surface of the roller 23 and the corresponding transverse face 27 of the roller, said space being moreover open to the external part of the piston, at least in the area where the roller is arranged protruding from the piston.

In this example, the plane perpendicular to the axis 25 of the roller 23 and containing the axis 26 of the piston, is a plane of symmetry for the piston 11, the bearing 24, the roller 23, and the two spaces. It would also be possible, and in accordance with the description, to have an asymmetrical arrangement in which the spaces are not symmetrical to each other with respect to a plane perpendicular to the axis 25 of the roller.

It is furthermore necessary to provide a means of maintaining in an axial position in its bearing 24 the roller 23 mounted on a piston 11 and of maintaining its angular orientation constant relative to the axis 26 of the piston, in order to ensure that this roller 23 is placed facing the cam 3 and correctly oriented relative to this cam to roll on the cam.

To this end, each space delimited between a recess 28 of the piston 11 and the internal wall of the cylinder 10 contains a wedging piece 29a, 29b, of shape corresponding to that of the recess and of cross section substantially shaped as a lunula. These wedging pieces 29a, 29b each have a flat face arranged opposite one of the end faces 27 of the roller 23 and a cylindrical face bearing on the internal face of the cylinder 10. These pieces therefore perform the axial wedging of the roller each of which transverse face 27 is in contact with the opposite face of the part 29a, 29b.

In this first example, as shown in the FIG 2A and 2B , the internal wall of the cylinder 10 has a groove 31 extending radially inwards from the external edge of the cylinder 10 in the axial plane containing the axis 26 of the piston 11 and the axis 25 of the roller 23: this groove 31 therefore opens within one of the recesses 28. The groove 31 extends over a length such that the seal 11 ′ of the piston 11 never reaches it when the piston moves back and forth in the cylinder 10, guided by cam 3; in other words, the position of the seal at the top dead center of the piston is more inside than the inside end 32 of the groove 31. Thus, the length of the groove 31 can be less than half, or a third, of the length of the cylinder 10, depending on the length of the piston 11 stroke.

The groove 31 has a tapered shape. More precisely, it has the shape of a truncated cone cut in a longitudinal plane with a rounded inner end 32: thus, both its tangential dimension and its axial dimension decrease when one approaches its inner end 32.

As shown in the FIG 3A and 3B , the wedging piece 29a intended to fill the recess 28 in which the groove 31 is provided comprises for its part a boss 35 of shape substantially complementary to the groove 31. This boss 35 therefore extends radially inwards from the outer edge of the wedging piece 29a over a length substantially identical, or shorter, than that of the groove 31. It also has a tapered shape, more precisely the shape a truncated cone cut in a longitudinal plane with a rounded inner end 36; the angle of the cone can for example be around 5 °.

As shown in the FIG 4 , such a boss 35 is therefore configured to engage in the groove 31 of the cylinder 10 in order to block the roller 25 and therefore the piston 11 in its entirety in rotation about its axis 26. In this example, at the bottom dead center of the piston, the inner end 36 of the boss 35 rests at the inner end 32 of the groove 31; on the other hand, at top dead center, the inner end 36 of the boss 35 projects beyond the outer edge of the cylinder 10 and therefore leaves the groove 31.

Consequently, when the cam 3 again guides the piston 11 inwards, the tapered shapes of the boss 35 and of the groove 31 allow the automatic re-engagement and re-centering of the boss 35 in the groove 31, therefore the realignment of the roller 25 on its nominal axial direction. Such guiding and refocusing is ensured even in the event of a jerk of the hydraulic machine and detachment of the roller 25 from the cam 3.

In this example, the cylinder 10 has a single groove 31 and only the first wedging piece 29a is provided with such a boss 35. However, it goes without saying that two diametrically opposite grooves 31 can be provided in the cylinder 10; in this case, the second wedging piece 29b can also be provided with a boss 35.

The FIG 5 to 7 illustrate a second example of a hydraulic mechanism generally analogous to the first example except that the cylinder block 108 here comprises stepped cylinders 110 configured to receive stepped pistons 111.

These stepped cylinders 110 comprise an outer section 110e, of a certain diameter, defining an outer chamber, and an inner section 110i, having a smaller diameter, defining an inner chamber. The stepped pistons 111 have an outer portion 111e, having a diameter substantially equal, to within a clearance, to the diameter of the outer section 100e of the cylinder 110, and an inner portion 111i, also called the piston foot, having a diameter at less less than the diameter of the inner section 110i of the cylinder 110.

In such a staged configuration, the cylinder is sealed by a seal provided at the level of the outer portion 111e of the piston which is applied against the surface of the outer section 110e of the cylinder 110. The hydraulic fluid can thus be distributed in the inner chamber bypassing the inner portion 111i of the piston to apply its hydraulic pressure both against the inner surface of the outer portion 111e of the piston 111 and against the inner surface of its inner portion 111i.

The outer portion 111e of the piston also supports, in a similar manner to the previous example, a roller 123 and wedging pieces 129.

The inner portion 111i has two diametrically opposite flats 137, preferably arranged orthogonally to the axis 5 of the engine. A bore 138 is further pierced in the flat 137 opposite the fluid supply conduit 113. A metal pin 135 is inserted and retained in this bore 138 so that its distal end 136 projects orthogonally from the flat 137.

This distal end 136 of the pin 135 is thus configured to engage in a groove 131 formed in the surface of the inner section 110i of the cylinder 110. This groove 131 thus extends radially inward from the outer edge of the inner section 110i of the cylinder 110 over substantially the entire length of this interior section 110i. It therefore opens all along the inner chamber and opens at its outer end into the outer chamber of the cylinder 110. This groove preferably extends in the axial plane containing the axis 26 of the piston 11 and the axis 25 roller 23, opposite the fluid supply conduit 113.

Thus, when the piston 111 comes and goes in the cylinder 110, the pin 135 slides in the groove 131 and thus blocks the piston 111 in rotation about its axis 26.

The FIG 8 and 9 illustrate an alternative embodiment of this second example in which the pin 235 is no longer centered on the flat 237 but offset towards a lateral edge 237a of the flat 237.

We especially on the FIG 8 that the piston 211 has a center hole 239, allowing its machining, extending along its main axis 226 and that the latter is disconnected from, that is to say does not communicate with, the bore 238 in which the pin 235 is inserted.

In the sectional view of the FIG 9 , it is also noted that the groove position 231 is also modified in order to cooperate properly with the pin 235. Thus, in this example in which the flats 237 extending orthogonally to the axis 5 of the motor, the groove 231 s' extends in a direction offset from the axial plane of the engine passing through this piston.

The modes or examples of embodiments described in this presentation are given by way of non-limiting illustration, a person skilled in the art can easily, in the light of this presentation, modify these modes or examples of embodiments, or envisage others, while remaining within the scope of the invention as defined by the appended claims.

In addition, the various characteristics of these embodiments or examples can be used alone or be combined with one another. When combined, these characteristics may be as described above or differently, the invention being defined by the appended claims and not limited to the specific combinations described in the present description. In particular, unless otherwise specified, a characteristic described in relation to an embodiment or exemplary embodiment can be applied in a similar manner to another embodiment or exemplary embodiment.

In particular, in the context of the second example described, the length of the groove 131 is such that the pin can never come out of the groove during the operation of the engine 1. However, in the opposite hypothesis, or in any event , it would be possible to provide the pin 135 and / or the groove 131 with a tapered shape similar to that described with reference to the first example.

In addition, in addition to these guide means implementing a pin 135 and a groove 131, or as a replacement, it would be possible to provide a wedging piece 129 of this stepped piston 111 with a boss similar to that of the first example. , configured to engage in a groove in the outer section 110e of the cylinder 110.

Claims (9)

1. A hydraulic mechanism, a motor or a pump, comprising:

   a cam (3);

   a cylinder block (108) rotatably mounted relative to the cam (3);

   at least one cylinder (110) arranged in the cylinder block (108), having an outer segment (110e) of a first diameter defining an outer chamber, and an inner segment (110i) of a second diameter smaller than the first diameter, defining an inner chamber; and

   at least one piston assembly suitable for cooperating with a cylinder (110), the piston assembly comprising:

   a piston (111) slidably mounted inside said cylinder (110), having an outer portion (111e) of a first diameter and an inner portion (111i) of a second diameter smaller than the first diameter;

   a roller (123) rotatably mounted on the piston (111) and configured to co-operate with the cam (3), the roller (123) being delimited by two transverse end faces (27); and

   wherein the inner segment of the cylinder (110) has a groove (131) arranged in its inside face,

   characterized in that it further comprises first and second shim parts (129), each arranged between a respective end face (27) of the roller (123) and the inside face of the cylinder (110) against which said shim part (129) bears so as to maintain the axial position of the roller (123); and

   in that the piston assembly includes a projection (135) projecting from the inner portion (111i) of the piston (111), and configured to co-operate with the groove (131) in the cylinder (110) so as to maintain the orientation of the roller (123).
2. A mechanism according to claim 1, wherein the projection is an end portion (136) of a pin (135) inserted in a bore (138) of the piston (111).
3. A mechanism according to claim 1, wherein the projection is an excrescence forming an integral portion of the piston.
4. A mechanism according to any one of claims 1 to 3, wherein the projection (235) extends in a direction that does not intersect the main axis (226) of the piston (211) .
5. A mechanism according to any one of claims 1 to 4, wherein the side surface of the piston (111) presents a flat (137), with the projection (136) projecting from the flat (137).
6. A mechanism according to claim 5, wherein the projection (235) is offset towards a side edge (237a) of the flat (237) away from the center of said flat.
7. A mechanism according to any one of claims 1 to 6, wherein the groove (131) extends to the outer end of the inner segment (110i) of the cylinder (110) and leads into the outer chamber of the cylinder (110).
8. A mechanism according to any one of claims 1 to 7, wherein the projection possesses a tapering inside end.
9. A mechanism according to any one of claims 1 to 8, wherein the projecting height of the projection decreases going towards its inside end.

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