Abstract:
The invention concerns a device comprising a shift cylinder with a cylinder body ( 17 ) bearing a first sleeve ( 54 ) for guiding a second sleeve ( 42 ) enclosing an axially mobile piston ( 21 ) with an intervening piston seal between the second sleeve ( 42 ) and the piston ( 21 ) and a secondary seal ( 59 ) between the body ( 17 ) and the second sleeve ( 42 ) penetrating into the hydraulic chamber. The invention is applicable to motor vehicles.

Description:
International patent application Ser. No. PCT/FR99/01355, publication No. WO99/64758 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a device for the hydraulic control of a clutch, especially for motor vehicles, comprising at least one emitter or receiver cylinder having a piston. 
     More particularly, the invention relates to a hydraulic control device for a motor vehicle clutch including at least one control cylinder, of the type in which the cylinder comprises a substantially tubular cylinder body in which there slides axially a piston which, through a front transverse face, bounds a hydraulic chamber, and which is in cooperation through a rear transverse face with a piston rod, wherein an aperture for connection of a pipe or a duct is open into the hydraulic chamber, and wherein the cylinder body is made of mouldable material such as plastics material. 
     Such a device is described for example in the document DE-U-2951 6488. 
     2. Description of the Related Art 
     In the latter, the piston is part of an assembly which includes a metallic piston skirt. 
     More precisely, the piston skirt has at its front end a projecting element on which the piston is formed by applied moulding. The skirt serves as a receptacle for two half shells which are so configured as to receive the head of the rod of the piston. 
     In addition, the piston carries a piston seal which is adapted to cooperate with a metallic guide sleeve mounted in the internal bore of the cylinder body and bounding the hydraulic chamber, while the cylinder body carries a secondary seal which is adapted to cooperate with the piston skirt. The cylinder body is for this purpose stepped internally in diameter, so that it has, at its rear end, an enlarged cross section whereby it can contain the secondary seal within it. A securing skirt is mounted in this larger section so as to retain the secondary seal. 
     Such a solution, which calls for a large number of components, is relatively costly and necessitates the presence of a dynamic piston seal carried by the piston and cooperating with the guide sleeve fixed to the cylinder body. 
     An object of the present invention is to overcome these drawbacks in a simple and inexpensive way. 
     SUMMARY OF THE INVENTION 
     According to the invention, a hydraulic control device of the type described above is characterized in that the piston is so configured as to receive the head of the piston rod, in that the piston is surrounded by the piston skirt, in that the piston seal is interposed operatively between the outer periphery of the piston and the inner periphery of the piston skirt, and in that the piston skirt penetrates into the hydraulic chamber. 
     Thanks to the invention, the guide sleeve of the prior art is eliminated, and the piston skirt penetrates into the hydraulic chamber. The piston and the piston skirt are concentric and coaxial components, and they constitute a unit in the form of a sub-assembly which can be handled and transported. 
     It should be noted that the manufacturing cost of the outer surface of the piston skirt is less than the manufacturing cost of the inner surface of the guide sleeve in the prior art, the said outer surface being in cooperation with the secondary seal of the dynamic type. The immobilizing skirt is elongated axially and is a good guide for the piston skirt. The internal bore of the said skirt extends the internal bore of the cylinder body. 
     The piston seal has a simplified form, and acts statically between the piston and the piston skirt. In one embodiment, the piston seal consists of an O-ring seal. 
     The piston skirt has a simple and inexpensive tubular form. This skirt does not have a base portion. 
     Preferably, the piston is of plastics material such that the seating for the head of the piston rod can easily be formed by moulding. 
     Preferably, the cylinder body has an abutment which serves to centre the spring which acts between the base of the body of the cylinder and the piston. 
     The front face of the piston is arranged to come into contact with this abutment. 
     Thus the piston skirt only undergoes weak forces when the piston is in its retracted position. 
     In one embodiment, the piston has a posterior axial fixing groove into which at least one deformation of the piston skirt penetrates. 
     The deformation of the skirt, which constitutes a projecting element, penetrates, for example by insertion, into the groove in such a way that the piston skirt is fixed axially to the piston. For example the piston skirt has lugs or pegs penetrating into the groove of the piston. 
     The skirt can then be snap-fitted on the piston. 
     In another version, the piston skirt is secured on the piston by applied moulding. 
     In a further version, the skirt is secured on the piston by adhesive bonding. 
     It is of course possible to reverse the structures, with the piston having, for example, projecting elements that penetrate into apertures formed in the piston skirt. 
     In another version, the piston skirt comes into abutment on a shoulder of the front and/or rear face of the piston. 
     It will be noted that the piston seal plays a part in the axial retention of the piston skirt on the piston, and that the constitution of the piston in plastics material facilitates assembly of the piston skirt with the piston. 
     In another version, an additional plastics member is associated with the piston for trapping the piston skirt and holding it in position. 
     This additional member may for example be ultrasonically welded on the piston so as to retain the piston skirt. 
     In a further version, the welding operation is carried out locally by indirect application of heat with the aid of at least one energy source of the laser type. 
     The supplementary member may be attached by conical telescopic engagement on the piston in order to trap the piston skirt in position locally. 
     Thus in these embodiments, the piston skirt is sandwiched locally between the piston, which is preferably of a plastics material, and the additional member which is of plastics material. 
     In the case where the control cylinder is an emitter cylinder, the piston skirt has at least one anterior hole which puts the hydraulic chamber into communication with a reservoir associated with the emitter cylinder. 
     Preferably several holes are provided to ensure a higher flow of fluid, and to enable any orientation to be avoided during assembly, with a view always to having a passage in the upper part of the piston. 
     The position of the passage hole or holes is determined according to the action of the piston skirt which is movable from one side to the other of a secondary seal, such as a lip seal. It will be appreciated that no burr is formed at the outer periphery of the piston skirt, and this facilitates sliding of the latter within the cylinder body. 
     Preferably, the securing skirt has at least two posterior abutments, such as lugs or pegs, which are diametrically opposed to each other so as to cooperate with the piston skirt and limit the displacement of the latter. 
     Thus the tolerance of the dead travel of the piston skirt is determined by the aggregate of the tolerances of three components. 
     It is easily possible to incorporate an integral reservoir with the emitter cylinder. 
     This working cylinder can be formed by means of a bellows, or a sealed membrane which is able to be unrolled, interposed operatively between the cylinder body and the piston rod. 
     The membrane or the bellows has at each end a bead for fastening them on the fixed part (that is to say the cylinder body) and on the movable part (that is to say the piston rod). 
     Fastening members are secured, for example by snap-fitting, on the fixed part and the moving part respectively in order to retain the beads. 
     Thus it is possible to return leaked fluid back into the working reservoir, and the reservoir is defined by a flexible and elastic element consisting of the bellows or membrane, which are typically of elastomeric material such as rubber. 
     In addition, filling of the hydraulic control device for the clutch may not have been carried out before it is fitted on the motor vehicle, filling then being carried out with the aid of the main reservoir. The hydraulic control device can of course be filled beforehand. 
     In another version, the main reservoir may be omitted, and reliance then placed on the above mentioned integral reservoir only. In each case, the secondary seal is submerged in the control fluid so that its useful working life is increased because it is not in contact with the air. 
     In one embodiment, a protective cap is provided and replaces the bellows or the membrane. This cap surrounds the fastening skirt. 
     This cap is of plastics material and it includes a portion which defines an abutment and clipping means, such as lugs or pegs, for clipping the cap on the piston rod. 
     In another version, the cap is supported on the fixed part (that is to say the cylinder body or the skirt) in the same way by clipping. 
     In a further version, the cap retains an elastomeric sealing member, on the piston rod and the fixed part respectively. 
     In another version, the cap constitutes one of the members for fastening the bellows and/or the roll-out membrane. 
     The fastening skirt includes passages for giving communication with a working reservoir and/or a main reservoir. 
     The skirt has at least one circumferential or axial groove for enabling fluid to pass freely. 
     The skirt is attached on the cylinder body for example by ultrasonic welding, laser welding, friction welding or clipping, or by adhesive bonding or screw fastening. 
     Ultrasonic welding or laser type welding enable good sealing to be obtained and enable the number of seals to be reduced. 
     The cylinder body includes an aperture which can be closed by a plug. This aperture gives communication with the main reservoir. 
     In the case where, as mentioned above, an integral reservoir is enough, then the aperture is plugged. The plug is a component of the same nature as the cylinder body, being preferably of plastics material. The plug is for example ultrasonically welded on the cylinder body, the said weld giving sealing. 
     The fastening skirt has external mounting apertures which enable the piston skirt to be centred with respect to the cylinder body during the operation of welding by welding the skirt ultrasonically on the cylinder body. 
     The mounting apertures serve for receiving a fitting tool having projections which penetrate into the mounting apertures, so as to be engaged with the piston skirt and also to grip the latter. 
     These apertures define a space around the piston. The mounting apertures, in one embodiment, consist of blind axial slots which are open at the rear of the fastening and guiding skirt. In another version, they are oblong holes. These apertures are apertures that give access to the piston skirt. Because of the simple tool used, accurate fitting is obtained. The said tool ensures that the various bores for guiding the piston are coaxial and aligned with each other. 
     In another version, of course, instead of welding it is possible to use adhesive bonding. 
     The following description illustrates the invention with respect to the attached drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in axial cross section of an emitter cylinder made in accordance with the features of the invention, in which the piston is shown both in an advanced first position and in a retracted second position; 
     FIGS. 2 to  4  are views as seen in the direction of the arrows  2 ,  3  and  4  respectively in FIG. 1, 
     FIGS. 5 and 6 are partial views of an enlarged scale showing, respectively, the front part and the rear part in FIG. 1; 
     FIG. 7 is a partial view taken from FIG. 6, showing the groove of the piston for fixing the piston skirt of the piston; 
     FIGS. 8 to  10  are Figures which are similar to FIG. 1, but which show further embodiments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In all of the drawings, those elements which are common will be given the same reference signs. 
     In FIGS. 1 and 8 to  10 , a control cylinder is shown in the form of an emitter of a control apparatus for a motor vehicle clutch. 
     Such an apparatus (not shown) includes an emitter cylinder which is connected through a duct to a receiver cylinder, the structure of which is similar to that of the emitter cylinder. 
     Each cylinder, whether it is a receiver or an emitter, comprises a piston which is movable axially within a cylinder body for delimiting a variable volume hydraulic chamber. A connecting orifice, on which the duct is connected, is open into the hydraulic chamber. 
     The emitter cylinder includes a piston rod which is connected for example to a clutch pedal operated by the driver, or an actuator which is operated in accordance with predetermined programmes. 
     The piston of the emitter cylinder is adapted to expel a fluid, such as oil, which is contained in the hydraulic chamber, towards the pipe or duct which connects the emitter cylinder to the receiver cylinder. 
     When the clutch is engaged, the volume of the hydraulic chamber of the emitter cylinder is at a maximum, while the volume of the control chamber of the receiver cylinder is at a minimum. 
     During operation of the clutch, the volume of the hydraulic chamber of the emitter cylinder diminishes, while the volume of the control chamber of the receiver cylinder increases. 
     The piston of the receiver cylinder then acts on a rod which itself acts, for example, on the declutching fork that actuates the clutch release bearing which is part of the friction clutch. 
     Each one of the emitter and receiver cylinders includes a spring which acts between the piston and the base of the body of the cylinder. When the driver releases the clutch pedal, or when the actuator reverts to its initial clutch engaged position, the return spring of the emitter cylinder expands in order to return the piston to its initial position, while the return spring of the emitter cylinder is compressed by the clutch spring, for example a diaphragm, which thus returns the piston of the receiver cylinder to its initial position. 
     In FIGS. 1 and 8 to  10 , the emitter cylinder is shown in its delivery position, and it comprises a cylinder body  17  having an inlet port  18  which is closed by a plug  19 . It is through this inlet port that the duct or pipe which connects the receiver cylinder to the emitter cylinder is open into the hydraulic chamber  20  of the emitter cylinder. 
     The chamber  20  is a variable volume chamber, and it is bounded by the body  17 , while the piston  21  is mounted for axial movement within the body of the cylinder  17 . 
     In the examples shown, the cylinder body  17  and the piston  21  are of mouldable plastics material. 
     This plastics material is reinforced if required with glass fibre, Kevlar and so on, in order to give the required strength to the components  17 ,  21 . 
     The cylinder body  17  is of generally tubular form. The body  17  has an internal bore  12 ,  23  of blind form, which is best seen in FIG.  5 . The body  17  is in the form of an internally stepped tube having an axis of axial symmetry X—X (FIG. 1) with, at the front, a transversely orientated base  24 , into which the inlet port  18  of a connecting portion  25  is open, permitting the above mentioned duct to be connected, for example by clipping, the connecting portion  25  having for this purpose internal passages, not given a reference numeral, through which a wire clip  48  is passed to connect the above mentioned duct to the emitter cylinder. In another version, the connection is of the screw type. The body  17  is open at the back. 
     The connection  25  and the base  24  are part of the front part  26  of the cylinder body  17 , while the open other end of the cylinder body is part of the rear part  27  of the said body. 
     The front part  26  has a diameter which is smaller than that of the rear part  27 , due to the fact that the body  17  is in the form of a stepped tube. The internal bore  22 ,  23  accordingly has a rear portion  23  which is within the rear part  27  and which has a larger diameter than its front portion  22  which is within the front part  26 . 
     Stiffening ribs may be provided on the front part  26  delimited by the base  24 . 
     The piston  21  is arranged to slide within the body  17 , and more precisely in the internal bore  22 ,  23  of the latter, between an advance or clutch engaging position, which is shown in the upper part of FIGS. 1,  5 ,  6  and  8  to  10 , and a retracted or declutching position which is shown in the lower part of FIGS. 1,  5 ,  6  and  8  to  10 . In the retracted position the chamber  20  and the control fluid are under pressure, while in the advance position the chamber  20  and the control fluid are depressurised. 
     It will be noted that a frusto-conical projecting element  28  is formed by moulding on the base  24 . 
     The projecting element  28  extends axially, and its base perimeter serves to centre the return spring  29 , which in this example is a helical spring acting as described earlier herein between the body  17  and the piston  21 , and more precisely, here, between the transverse base  24  and the front end of the piston  21 . 
     The piston  21  has (see FIGS. 5 and 6) a transverse wall  30 , the front face  31  of which delimits the hydraulic chamber  20 , with its rear face  32  being in cooperation with the piston rod  33 , which in this example is of metal. 
     The transverse wall  30  is extended forward by a chimney element  34  which has a frusto-conical external profile and a blind cylindrical central hole  35 . 
     The base of the hole  35 , which is orientated axially, is defined by the front face  31  of the wall  30 . 
     The helical return spring  29  is mounted around the chimney element  34 , the base of which serves to centre the return spring, the rear end of which bears on the front face  31  of the transverse wall  30  of the piston  21 . 
     The spring  29  therefore works axially between the base  24  and the front face  31 . The port  18  does of course extend through the base  24  and is open laterally with respect to the projecting element  28 , which is formed by moulding so as to project from the base  24  and serving by virtue of its base perimeter to centre the front end of the spring  29 . 
     The end face  36  of the projecting element  28 , in accordance with one feature, acts as an abutment for the front face  37  of the chimney element  34 . 
     More precisely, in the retracted position the piston  21  comes into contact, through the front face  37  of the chimney element  34 , with the end face  36  of the projecting element  28 . The body  17  accordingly has an abutment,  28 , which limits the forward displacement of the piston  21 . 
     The chimney element  34 , being arranged to come into contact with the projecting element  28  which constitutes the above mentioned abutment, is rigid. The chimney element  34  stiffens the piston  21 , which is made of plastics material. 
     The transverse wall  30  is extended towards the rear by a sleeve portion  38  which has an outwardly divergent bore  39 . 
     In this example, the bore  39  is frusto-conical, its base being partly defined by the rear face  32  of the transverse wall  30  of the piston  21 . 
     The base of the bore  39  is hemispherical in general form, so as to receive the head  40  of the rod  33  at its front end. The head  40  has the general form of a spherical bore which penetrates into the complementary, generally hemispherical base of the internal bore  39  of the sleeve portion  38 . 
     The said sleeve portion  38  is divided into axial fingers by slots, in accordance with one feature. 
     The external form of the sleeve portion  38  is cylindrical, with an outside diameter greater than that of the chimney element  34 , the head  40  of the rod  33  being arranged to spread the fingers of the sleeve portion  38  by contact with the internal bore, until it comes into contact with the hemispherical portion of the base of the bore  39 . 
     The rod  33  is accordingly mounted by a ball-type coupling in the piston  21 , and more particularly in the base of the sleeve portion  38  which is divided into axial fingers by slots. 
     In accordance with one feature, the transverse wall  30 , and therefore the piston  21 , carries at its outer periphery a piston seal  41 . 
     In this example, the seal  41  is an O-ring seal fitted in a groove, not given a reference numeral, of the transverse wall  30  at its outer periphery. The seal may take other forms. 
     The said seal  41  is arranged to cooperate, in accordance with one feature, with the inner periphery of a metallic piston skirt  42 . The piston skirt  42  surrounds the piston  21 , and in this example it is tubular in form. The piston  21  accordingly penetrates into the interior of the skirt  42 . In a manner to be described later herein, the rear end of the piston projects axially with respect to the rear face of the skirt  42 . 
     The piston skirt  42  is for example of anodised aluminium material, or treated steel. The piston skirt  42  projects axially forward with respect to the chimney element  34 . In the retracted position, the chimney element  34  is in contact with the projecting element  28 , while the skirt  42  is spaced away from the base  24 . 
     The piston chimney element  42  is spaced radially away from the chimney element  34  and return spring  29 . 
     The said skirt  42  is in contact with the transverse wall  30  of the sleeve portion  38 . 
     More precisely, the piston skirt  42  is in intimate contact at its inner periphery with the outer periphery of the seal  41 . 
     The seal  41  therefore immobilises the piston skirt  42  with a gripping action. 
     The said skirt  42  tends to close off the sleeve portion  38 . More precisely, the skirt  42 , by exerting a radial force, forces the fingers of the sleeve portion  38 to become deformed radially inwards. Thus, the sleeve portion  38  is fitted in the skirt  42  in a prestressed condition. 
     Therefore, again, the sleeve portion  38  immobilises the piston sleeve  42  with respect to the piston  21  with a gripping action. 
     The said skirt  42  is thus, in accordance with one feature, fixed axially with respect to the piston  21 . 
     The said skirt is also fixed to the piston  21  for rotation with the latter, by virtue of the seal  41  and sleeve portion  38 . 
     In order to complete this axial immobilisation, and also to complete the immobilisation against rotation of the skirt  42  with respect to the piston  21 , it is proposed that one of the elements consisting of the piston skirt  42  and the piston  21  should have one or more lugs penetrating into one or more recesses formed in the other one of the elements consisting of the piston  21  and piston skirt  42 . 
     For example, the skirt  42  may have at its rear end holes into which there penetrate radial projections in the form of lugs, which project from the outer periphery of the sleeve portion  38  at the rear of the latter. The skirt is therefore force-fitted over the outer periphery of the piston  21 , until the radial projections of the sleeve portion  38  penetrate into the holes of the skirt  42 . 
     Thus the skirt  42  is snap-fitted on its piston  21 . 
     In this example, as can be seen in FIG. 7, the rear end of the sleeve portion  38  has at its outer periphery a fastening groove  43 , while the rear end of the piston skirt  42  is deformed locally, radially inwards, so as to form lugs  44  which come into engagement with the rear flank and the base of the fastening groove  43 . 
     In another version, the rear end of the skirt  42  is formed with local teeth or has fingers to penetrate into the fastening groove  43 . 
     In a further version, the rear end of the skirt is secured on the piston  21  by in situ moulding. 
     Regardless of all this, in all cases a sub-assembly is formed consisting of the piston  21  and the piston skirt  42 , to constitute a unit which is movable axially in the body  17 . The skirt  42  and the piston  21  are components which are concentric and coaxial with each other. 
     The said piston skirt  42  prevents the fingers of the sleeve portion  38  from being spread radially outwards, so that it locks the rod  33  axially in the interior of the piston  21  without any additional component. The rod  33  is therefore unable to escape axially from the piston  21 , while it remains able to rotate in three dimensions, by virtue of the seating  1   38  (which in this example is generally hemispherical in form) of the sleeve portion  38 , and the rear face  32 . It is of course possible to control the stress exerted by the piston skirt  42  on the sleeve portion  38  in such a way that the piston rod  33  can be removed manually. 
     All of this is made possible because the piston is made of plastics material and because of the flexibility of the sleeve portion  38 . 
     It is of course possible to secure the skirt  42  on the piston  21  by adhesive bonding, after the piston rod  33  has been introduced into the piston. In all cases, the piston  21  is, in accordance with one feature of the invention, configured internally so as to receive the head  40  of the piston rod  33 . The piston  21  has an internal seating for the head  40 . The seating is defined by the sleeve portion  38 , which is flexible because of its slots, and the rear face  32 . 
     The said head  40  may, in this connection, take another form, and the same is true for the interior of the sleeve portion  38  and seating. 
     The body  17  is adapted to be fixed on a fixed part of the vehicle, and is accordingly part of a fixed part of the emitter cylinder, while the piston  21  and the piston rod  33  are part of the movable part of the said cylinder. 
     For this purpose, the body  17  in this example has an integral intermediate plate  45  between the front part  26  and rear part  27  of the body  17 . 
     The plate  45  has two parallel edges which are joined together by two pointed lateral flanks. 
     The plate  45  is formed integrally by moulding with the front part  26  and rear part  27  of the body  17 , and projects transversely with respect to the tubular parts  26 ,  27 . In this example the plate  45  has two fastening ears  46  which project radially with respect to the parts  26 ,  27 . 
     These ears  46  have holes  47 , through which there pass fastening members, usually screws, for fastening the body  17 , and therefore the emitter cylinder, to a fixed part of the motor vehicle. 
     In FIG. 3, the distances D and L show the distance between the axes of the holes  47  and the distance between the summits of the lateral flanks of the plate  45 , respectively. Also, at  48 , there can be seen the wire clip which is carried by the connector  25 , and which serves for the rapid clip fastening of the end of the above mentioned duct in the connector  25 . 
     A tube  49 , fixed to the body  17 , can also be seen in FIG.  3 . 
     This tube  49  serves to connect the interior of the body  17  with a main feed reservoir which is mounted on the outside of the emitter cylinder, given that the hydraulic chambers of the emitter and receiver cylinders are filled with a hydraulic fluid which in this example is oil. The reservoir is partially filled with this hydraulic fluid, and is connected through a duct, not shown, to the tube  49  which is open into an aperture  50  formed in the front of the plate  45  which is integral, by moulding, with the body  17 . The aperture  50  is connected to a duct  51  extending through the plate  45  (FIG.  6 ). In this case several ducts  51  are provided. 
     These ducts  51  are open at the front into a feed groove  52  which is in communication with the aperture  50 , while the ducts  51  are open at the rear end, within the rear part  27  of the body  17 , and more precisely in the bore  23  of the rear part  27 . 
     The aperture  50  is annular in form, and the tube  49  has at its base a shouldered ring portion  53  which is complementary to the aperture  50  and penetrates into the latter, in such a way that the tube can be secured by sealed adhesive bonding to the plate  45 . 
     In this example, fastening of the tube  49  to the plate  45  is obtained by insertion of the ring portion  53  into the aperture  50 , and then by making an ultrasonic weld between the annular edge of the aperture  50  and the outer periphery of the ring portion  53 . 
     To this end, the tube  49  is of course made of a plastics material which is compatible with the body  17 . 
     In all cases, whether adhesive bonding or ultrasonic welding is used, a sealed fastening is obtained, without any additional seal, of the tube  49  on the plate  45 , the tube  49  being inclined with respect to the plate  45 . As a result of this, the ducts  51  and the groove  52  are easily formed by moulding. In another version, the tube  49  is replaced, as can be seen in FIG. 9, by a plug  149  which obturates the aperture  50 , and which is secured by sealed adhesive bonding or sealed ultrasonic welding to the plate  45 , like the tube  49  in FIG.  6 . The body  17  is accordingly standardised. 
     In this example the interior of the tube is of stepped diameter, to define three bores  22 ,  23  and  123 . 
     In the manner described above, the bore  22  having the smallest diameter is formed in the front part  26  of the body  17 , while the other two bores  23 ,  123  are made, respectively, in the rear part  27  and in the plate  45  of the body  17 . 
     The bore  123  is formed centrally in the plate  45 , and it has an intermediate diameter with respect to the bore  23  formed in the rear part  27 . 
     Because of the bores  23 ,  123 , a fastening and guiding skirt  54  can be fitted in the body  17 . This skirt  54  is thicker than the piston skirt  42 . The skirt  54  is made of plastics material, and has an internal bore the diameter of which is equal to that of the bore  22  in the front part of the body  17 . The skirt  54  is fixed to the body  17 . Thus the said skirt  54  is fixed, while the piston skirt  42  is movable axially. 
     The skirt  54  guides the piston skirt  42 . More precisely, the inner periphery of the skirt  54  of plastics material is in intimate contact with the outer periphery of the metal piston skirt  42 . The skirt  54  has a stepped diameter externally, so that the said skirt  54  is fitted by insertion into the rear part  27  and into the plate  45 , the outer periphery of the skirt being in cooperation with the inner periphery of the plate  45  and that of the rear part  27  of the body  17 . The said skirt  54  is therefore inserted in a complementary manner in the bores  123 ,  23 . 
     The tubular skirt  54  has, integral with each other, a front portion  55 , an intermediate portion  56  and a rear portion  57 . 
     The front portion  55  is received in the complementary bore  123 , while the portion  56  is received in the complementary bore  22 . 
     After the skirt  54  has been inserted in the body  17 , the skirt is fastened sealingly within the body by ultrasonic welding in the region of the outer periphery of the portion  56  and the inner periphery of the rear portion  27 , as is indicated at  58  in FIG.  6 . 
     In another version, the skirt  54  is sealingly attached by adhesive bonding on the body  17 . 
     The intermediate portion  56  partly surrounds the rear portion  57 , and there is a large annular space between the portions  56  and  57 . 
     The rear portion  57  has an outer diameter which is smaller than that of the front portion  55 , the outer diameter of the latter being itself smaller than that of the intermediate portion  56 . The portion  57  is longer in the axial direction than the portion  56  or  55 , so as to give good guidance to the movable skirt  42 . The lengths of the portions  55 ,  56  are a function of the length of the bores  23 ,  123 . 
     Sealing of the hydraulic chamber  20  with respect to the outside is provided by a dynamic seal  59  which is carried by the cylinder body  17 , and which cooperates with the outer periphery of the piston skirt  42 , that is to say with the cylindrical outer surface of the skirt  42 . 
     The seal  59  is mounted within the bore  123 , and is therefore in contact with the inner periphery of the plate  45  which defines the bore  123 . The seal  59  is in axial contact with the front face of the front portion of the guide skirt  54 , and with axial bosses  60  which are formed integrally by moulding with the transverse shoulder  61  that joins the bore  22  to the bore  123 . 
     Thus the seal  59 , which is a so-called secondary seal, makes a good abutment on the bosses  60 , and is preserved. In this connection, in the case where the seal  59  is in contact with the shoulder  61 , a deformation of the seal  59  occurs in the region of the radius joining the shoulder  61  to the inner periphery of the plate  45 . The seal  59  is a lip seal. The piston skirt  42  has at least one hole for providing communication between the inside and the outside of the skirt  42 . 
     This hole  62  is located as a function of the axial movement of the piston  21  and skirt  42  on either side of the lip seal  59 . Then, this hole  62 , when the clutch is engaged, provides communication between the hydraulic control chamber  20  and the external main reservoir. 
     During actuation of the clutch, this communication is interrupted because the hole  62  is displaced on the other side of the seal  59 . 
     In this example several holes  62  are provided in order to give a greater flow of fluid and to prevent orientation during fitting. 
     The holes  62  are spaced apart at regular intervals. 
     It will be noted that the skirt  54  immobilises the seal  59  axially, so that it constitutes an immobilizing skirt. 
     This skirt  54  has at its inner periphery at least one axial groove  63  which is accordingly open in the region of the lip of the seal  59 . In this example several axial grooves  63  are provided. 
     As can be seen in the upper part of FIGS. 5 and 6, in the advanced position, with the clutch engaged, there is communication between the chamber  20  and the grooves  63  via the holes  62 , the inclined lip of the seal  59  enabling such communication to exist. The holes  62  are adapted to be displaced on either side of the second seal  59 . 
     All of this enables communication to be obtained with the external reservoir, and enables leaks to be recovered. 
     More precisely, the external reservoir is connected to the tube  49  in communication with the aperture  50 , the groove  52  and the internal bore  23 , via the ducts  51  which are open into a space bounded by the transverse shoulder  64  joining the bore  123  to the bore  23 , through the front face of the intermediate portion  56 , and through the outer periphery of the portion  55  and the inner periphery of the rear portion  27 . 
     Passages  65  are formed axially in the intermediate portion  56  so as to put the fluid coming from the external reservoir into communication with the annular space that exists between the portions  56 ,  57  and the grooves  63  which have a closed base. 
     In order to avoid escape of the fluid outwards, a sealing bellows  66  is provided, which is fitted between the rear portion  27  of the body  17  and the rear of the piston rod  33 . In this example, the liquid is able to pass through the plate  45  and the fastening and guide sleeve  54 , so as to reach the axial grooves  63  and to communicate with the chamber  20  via the passage holes  62 . 
     In accordance with one feature, the skirt  54  limits axial displacement in one direction of the unit consisting of the piston  21  and piston skirt  42 . 
     For this purpose, the fixed skirt  54  has at its rear end at least one abutment  67  (FIG.  7 ), and in this example a plurality of abutments, orientated transversely and allowing fluid to pass. Thus, the piston skirt  52  which is movable axially is able to come into contact, in the advanced position, through its rear face with the abutments  67 . The rear end of the piston  21  projects axially with respect to the rear end of the skirt  42 , which is subjected to small forces when it is in contact with the abutment  67 , because the chamber  20  is depressurised. 
     It will be noted that there is fluid communication between the grooves  63  and the space outside the skirt  54 , because of the radial clearance that exists between the piston  21  and the abutments  67 . 
     The bellows  66 , which in this example is of elastomeric material, has at each of its ends transverse beads  68 ,  69  which are fixed, respectively, on the body  17  and on the piston rod  33 , with the aid of fastening members  70 ,  71  which in this example are of metal. 
     More precisely, the rear end of the rod  33  has two grooves  72 ,  73  which are separated axially. The groove  72  is arranged to receive the bead  69 , while the groove  73 , which is the one furthest away from the body  17 , is arranged to receive pads  74  of the tubular fastening member  71 , as can be best seen in FIG.  2 . This member  71  consists of a collar piece of stepped diameter (FIG.  2 ), with a continuous rear portion which is extended by a front portion  76  of larger diameter and divided by slots into radially elastically deformable lugs  77 . The pads  74  are part of the lugs  77 , and project inwards from the member  71 . 
     Thus, the rear end of the bellows  66  is surrounded by the front portion  76  of the component, being trapped between this portion  76  and the rod  33 , with the bead  69  being lodged within the groove  72 . 
     The pads  74  penetrate into the groove  73  so as to locate the collar piece  71  axially. 
     The member  70  is in the form of a collar piece with a base  77  which extends generally radially. 
     Like the collar piece  71 , the collar piece  70  has lugs  79  with deformations  80  which are engaged in a groove (not given a reference numeral) which is formed at the outer periphery of the rear part  27  of the body  17 . 
     The bead  68  is trapped and gripped in a cavity which is bounded by the rear face of the body  17 , the outer periphery of the intermediate portion  56  and the base  78 . 
     The bellows  66  is of stepped diameter, and comprises a rear part of smaller diameter which is adapted to come into engagement against the rear face of the portion  57 , and a front part of larger diameter which surrounds the portion  57 . The rear and front parts of the bellows are of accordion form. 
     The portion  57  has external blind slots  83  for receiving a tool for fitting and centring the skirt  54 . The slots  83  are open at the rear of the portion  57  and are closed at the front by the portion  56 . 
     Thus the fitting tool surrounds and centres the portion  57 , and it centres the piston skirt  42  by virtue of projections extending through the slots  83  so as to come into engagement with the skirt  42 . 
     It is thus possible to insert the assembly consisting of the skirt  54 , the skirt  42  and the piston  21  in the body  17  and to proceed to fastening the skirt  54  to the body  17  by ultrasonic welding. It is of course possible to replace the axially orientated blind slots  83  with oblong holes. In general terms, the rear portion has apertures for passage of the projecting elements of the fitting tool. 
     The low cost of the hydraulic control device in accordance with the invention, and its ease of assembly, will be appreciated. 
     In this connection, the skirt  42  is fitted over the piston  21 . Similarly, the tube  49  and the skirt  54  are inserted respectively in the aperture  50  and the bores  23 ,  1   23 . 
     The fastening members  70 ,  71  are fitted respectively over the portion  57  and over the rod  33 . The bellows  66 , with the body  17  and the rod  33 , defines an integral working reservoir. Ultrasonic welding between the body  17  and the fixed skirt  54  gives sealing. 
     Thus, the emitter cylinder in accordance with the invention makes use of a single dynamic seal, namely the seal  59 , together with static seals, namely the seal  41  and the beads  68 ,  69  of the bellows  66 . The seals  59  and  41  are protected because they are submerged in the control fluid. 
     One dynamic seal is eliminated by virtue of the holes  62 , grooves  63  and the bellows  66  with its beads  68 ,  69 . 
     In FIG. 1, the rear part of the bellows  66  is never fully compressed. The initial volume of fluid contained in this rear part is transferred into the interior of the body  17  by the axial displacement of the piston  21  It is then possible to omit the external main reservoir. This is what is shown in FIG. 9, by virtue of the plug  149 . It is of course necessary to have regard to wear in the friction liners of the clutch in order to provide the appropriate volume within the integral reservoir which is defined by the bellows and which is interposed between the body  17  and the rod  3 . This reservoir therefore constitutes the only reservoir. 
     In FIG. 8, the rear part of the bellows  66  is fully compressed, so that it is essential to provide an external reservoir, the working reservoir being no longer present. 
     Only the rear part of the bellows is of accordion form, the front part of the bellows being fitted over the portion  57 . 
     It is of course possible to omit the bellows, as can be seen in FIG.  10 . 
     In that case, a protective cap  1   66  is provided for giving resistance to shocks. The cap, which is of cylindrical form, surrounds the fixed skirt  54  and is provided at its rear end with a base portion  1   67  which has a central hole through which the piston rod passes. At its front end, the cap  166  has an annular radial flange  168  which extends outwards. 
     This flange  168  is fixed on the fixed skirt  54  by snap-fitting, which is obtained by means of a bead engaged in a groove in the body  17 . 
     In this case the fixed skirt  54  only has an intermediate portion  56  and a rear portion  57 . 
     The front portion of FIGS. 1 to  9  is replaced by a locating ring  155  in order to enable a second dynamic seal  159  to be fitted between the locating ring  155  and the portion  56 , which is formed with a rebate in its inner periphery to accommodate the seal  159 . The locating ring  155  has radial grooves (not shown) at its rear end. The locating ring  155  has internal axial grooves which replace the grooves  63  of FIGS. 1 to  9 . Thus, liquid is in communication with the tube  49  through the holes  62 . 
     In FIG. 10 the seal  159  is an additional seal which prevents leakage to the outside. 
     The cap  166  can of course surround the bellows  66  of FIGS. 1 to  9 . For this purpose, the fastening member  70  may be extended rearwards by a tubular portion, at least partly surrounding the bellows. In another version, the fastening member  71  is extended by a tubular portion at the front for partly surrounding the bellows. 
     In a further version, the tube  49  and the fixed skirt  54  are of course mounted in the body  17  by clip-fastening with the aid of an elastic wire clip fastener similar to the elastic clip  48 , with passages for the clip being again provided, respectively, in the edge of the aperture  50  and in the rear part  27 . In another version the members  49 ,  54  are fitted in the body  17  by means of a mounting of the bayonet type. 
     In these variants, an additional seal must be provided. 
     The present invention is of course applicable to the receiver cylinder. 
     In all cases the piston skirt encloses the piston and forms a sub-assembly with the latter. 
     The piston skirt  42  penetrates into the hydraulic chamber  20  and bounds the latter. More precisely, the skirt  42  penetrates into the bore  22  of the front part  26 ; the bore  22  is extended axially by the internal bore of the fixed skirt  54 , which can be referred to as the first skirt. 
     Because of the projecting element  28 , the inlet port  18 , which constitutes a connecting port, can in another version be open into the front portion  26  between the base  24  and the piston skirt  42 , having regard to the fact that this skirt is spaced away from the base when the piston  21  is in its retracted position. 
     The transverse wall  30  of the piston can of course be thicker, so that the seating  138  for receiving the head  40  of the rod  33  (FIG. 6) can be formed as a hollow in the transverse wall  30 . 
     In the Figures shown, the seating  138  extends in cross section over a little more than 180° so as to retain the rod  33  axially. 
     In another version, the seating  138  extends over 180° at least, and an attached ring, such as an open circlip, is attached in the piston  21  so as to locate the rod  33  axially. 
     This arrangement makes it possible to use a sleeve  38  which is continuous and rigid. The formation of a slotted sleeve  38  does of course facilitate the fitting of the piston skirt  42  over the piston  21 . 
     The piston  21  is located axially at its outer periphery on the piston skirt  42 , which can be referred to as the second skirt. For this axial location, the second skirt  42  may have a single lug, for example in the form of a bead which is engaged in a continuous groove of the piston, which in another version may be formed in the transverse wall  30  of the piston  21 . 
     A single lug and a single recess can therefore be provided on one of the elements consisting of the second skirt  42  and the piston  21 . 
     The metal second skirt  42  is fixed at least axially to the piston  21 , while the fixed first skirt  54  is fixed to the body  17 , and is of mouldable plastics material. 
     In FIGS. 1 to  9 , a flexible element of elastic material defines a reservoir which is incorporated in the emitter cylinder. This element consists of a bellows  66  made of an elastomer such as rubber. 
     The second skirt  42  slides axially in the first skirt  54 , while the seal  59  is in dynamic cooperation with the second skirt  42 . 
     The chimney element  34  gives access to the outer periphery of the front face  31  of the piston  21 , while the sleeve portion  38  enables access to be obtained to the central portion of the rear face  32 . 
     The fastening members  70 ,  71  grippingly cover the ends of the flexible element of elastic material, which in this example is the bellows. 
     In FIG. 8 the base  78  of the fastening member  70  extends further inwards in the radial direction. 
     In all the Figures, axial ribs, not given a reference numeral, join the portion  56 ,  57  radially together, while allowing the fluid coming from the external or internal reservoir to pass through. 
     In FIG. 10, the cap  166  for protecting the first skirt  54  is fixed on the intermediate portion of the latter. The same can be true in FIGS. 1 to  9 . To this end, it is sufficient, firstly, to form a rebate in the rear end of the intermediate portion  56  so that the bead  68  can be fitted, and secondly, to make the groove for receiving the pads of the member  70  at the outer periphery of the portion  56 . 
     It is of course possible to provide only a single pad on the members  70  and  71 , in the form of a bead. 
     The member  70  is fixed on one of the elements consisting of the cylinder body  17  and the first skirt  54 . 
     It then becomes possible to omit the ears  46  of the plate  45 , and to fasten the cylinder body  17 , using a bayonet fitting, on a fixed part in the manner described in the document GB-A-1 539 879, to which reference should be made for more detail. 
     This fitting may of course be obtained using one of the portions  26 ,  27  or the plate  45 . The plate  45  can then be without any means for fastening it on a fixed part. The plate  45  is then able to be annular. 
     The apertures  83 , which in this case are in the form of blind holes, formed in the rear portion  57  of the skirt  54 , are openings giving access to the second skirt  42  so that the fitting tool can be engaged with the second skirt  42 , this tool being in engagement with the first skirt  54 . All combinations are possible. 
     In FIG. 8 the bellows  66  may consist only of a rear part of accordion form, fixed at the front on a protective cap surrounding the portion  57  and attached sealingly on the rear portion  27 . 
     The second skirt  54  and the tube can be attached by screw fastening on the body  17 . 
     The fastening member  71  may be replaced by a gripping collar. This member  71  can be fixed by seaming on the rod. The same is true for the member  70 . 
     In another version, fastening of the member  70  is obtained with the aid of screws. 
     It is of course possible to replace the ultrasonic welds by means of sealed spot welds by indirect application of heat using at least one source of energy of the laser type. Radiation, in the form of a beam emitted by the laser, is for example in the infra-red radiation range. 
     One of the elements to be welded is of a material which is transparent to infra-red radiation, while the other one of these elements is so configured as to absorb this said radiation. For example, this other element is in material which is transparent to the said radiation and includes at least one additive for absorbing the said radiation. This other element can of course be one that is absorbent to that radiation. 
     Thus, fastening of the tube  49  to the plate  45  can be obtained by this welding by indirect application of heat, giving sealed fastening. The tube  49  is so configured as to absorb the said radiation, and is for example of material transparent to the said radiation and having an additive to make it transparent. The additive is for example in the form of pigments such as carbon in a proportion of 1 to 2%. It is the component that is closest to the laser that is made of transparent material, while the other is of absorbent material. Preferably, the laser is turned with respect to the element to be welded. The inverse is of course possible. Thus the plate  45  is of transparent material. 
     This type of welding is more advantageous than ultrasonic welding, because it protects the seal  59  together with the components to be welded, in such a way that the control cylinder is more reliable. Because of this, the ducts  51  and the groove  52  are easily made. The sealed fastening at  58  can be obtained using welding by indirect application of heat. 
     The plug  149  may be secured using this type of welding. 
     Fastening of the skirt  54  in the body  17  can be effected by indirect application of heat, with the plate  45  being of material transparent to the said radiation, while the other components to be welded are of material so configured as to absorb the said radiation. 
     In place of sealed ultrasonic welding, or welding by indirect application of heat, it is of course possible, for example at  58 , to carry out sealed friction welding and sealed high frequency welding. In another version, the sealed fastening is a fastening of the mirror welding type or the type in which ferromagnetic welds are made by induction welding. 
     As will be clear from the description and drawings, the skirt  54 , or so-called first skirt, and the piston are of plastics material, while the first skirt  54  has at least one abutment element  67  which is adapted to cooperate with a rear end of the piston so as to limit the rearward axial displacement of the piston. 
     This abutment is arranged to cooperate with the rear end of the skirt  42  which is called the second skirt. 
     In practice, the axial rear end of the first skirt has a plurality of transversely orientated abutments which extend radially inwards. These abutments  67  are spaced apart angularly at regular intervals so as to enable fluid to pass through. 
     With this design for axial retention of the piston towards the rear, reduced tolerance is obtained in the value of the course of dead travel of the piston, in so far as it results from the aggregate of the tolerances of three components, that is to say the piston skirt  42 , the external skirt  54  and the cylinder body. 
     In the embodiments described, the cylinder body  17  carries the intermediate plate, with which it is integral. 
     In another version the plate  45  is attached on the body  17 . 
     Thus, the control device comprises the cylinder body  17 , the skirt  54  and a central member which is interposed between the cylinder body  17  and the skirt  54 . This components constitutes the plate  45 . The dynamic seal is lodged within this intermediate member. The tube  49  is fixed on the plate  45  and is connected to a transverse duct formed within the plate. This duct then opens into a central hole of the plate  45  facing the external portion of the skirt  45 . Two dynamic seals can then be mounted within the plate. This plate then includes housings for mounting dynamic seals of configurations similar to the seal  59 . Fastening of the plate  45  on the body  17  is obtained in a manner similar to the fastening of the skirt on the plate  45 , that is to say using welding of the same type as the weld  58 , the plate  45  having a front portion similar to the portion  27  for the welding operation. The plate  45  does not serve for guiding the skirt  42 , there being a radial clearance between the skirt and the contour of the hole in the plate, so that the duct connected to the tube is able to open into a cavity closed by the two dynamic seals. It is the cylinder body and the skirt that guide the piston skirt  42 . For more detail, reference should be made to the document FR 99.024.26 filed on Feb. 26, 1999.