Abstract:
A clutch control device ( 10 ) comprising a cylinder body ( 17 ) and a piston ( 21, 42 ) axially sliding in an inner bore ( 22 ) of the cylinder body ( 17 ) and at least a dynamic seal ( 59 A,  59 C) borne by the cylinder body ( 17, 17 B). The cylinder body ( 17 ) comprises, axially arranged successively from the front rearwards, a blind front component ( 17 A) comprising a base ( 24 ), a central component ( 17 B) and a rear component ( 17 C) open rearwards made of plastic material and assembled together sealed by welding ( 58 A,  58 C) with indirect heat input, and the inner bore ( 22 ) comprises a front portion ( 22 A) and a rear portion ( 22 C), and the dynamic seal ( 59 A,  59 C) is housed in the central component ( 17   b ) of the cylinder body ( 17 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus for hydraulic brake or clutch control, in particular for a motor vehicle, comprising at least one emitter or receiver cylinder having a piston. 
     More particularly, the invention relates to apparatus for hydraulic brake or clutch control for a motor vehicle, comprising at least one cylinder which includes a cylinder body having at the front a base portion and being open at the rear, a piston sliding axially in an internal bore of the cylinder body, and at least one dynamic sealing member carried by the cylinder body for cooperation with the outer periphery of the piston, wherein the cylinder body is of synthetic material such as plastics material, and wherein the piston has a front face bounding a hydraulic chamber. 
     2. Description of Related Art 
     An apparatus of that kind is described, for example, in the documents DE-U-29516488, FR-B-274920 or in French patent application No. 98 07213 of Jun. 9, 1998. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to propose a control apparatus of the type mentioned above, having reduced manufacturing costs and simplifying the design of the cylinder body. 
     With this in view, the control apparatus in accordance with the invention is characterised in that the cylinder body includes, arranged consecutively in the axial direction from front to rear, a blind front piece which includes the said base portion, a central piece, and a rear piece which is open towards the rear, the said pieces being joined together in a sealed manner, in that the internal bore consists of a front portion and a rear portion, which are formed in the front piece and rear piece respectively of the cylinder body, and in that the said dynamic sealing member is mounted in the central piece of the cylinder body. 
     With such a design it is in particular possible to standardise the front and rear pieces of the cylinder body and to create a range of products by making a range of central pieces which are different from each other, in particular for the purpose of incorporating means for feeding the hydraulic chamber and means for fastening the control apparatus, which vary from one apparatus to another, especially as a function of the model and type of vehicle that it is to equip. 
     In accordance with further features of the invention: 
     the said sealed joint is an adhesively bonded joint; 
     the said sealed joint is a joint which is made by welding with indirect application of heat; 
     the said sealed joint is a joint which is made by welding with the aid of at least one energy source of the laser type; 
     the said sealed joint is a welded joint made by infrared radiation, for example of the laser type; 
     the said sealed joint is a joint made by ultrasonic welding; 
     the said sealed joint is a joint made by friction welding; the said sealed joint is a joint made by high frequency welding; 
     the said sealed joint is a welded joint made by induction welding of ferromagnetic members; 
     the sealed joint is obtained by mirror welding; 
     the piston extends through the central piece of the cylinder body with a radial clearance; 
     an axially oriented tubular rear portion and an axially oriented tubular front portion of the front piece and rear piece of the cylinder body, respectively, are welded to a complementary front portion and rear portion, respectively, of the central piece of the cylinder body; 
     the outer peripheral surface of the said tubular portion is fixed sealingly, by welding with indirect application of heat, to an internal peripheral surface of the said complementary portion of the central piece of the cylinder body; 
     the inner peripheral surface of the said tubular portion is fixed sealingly, by welding with indirect application of heat, to an external peripheral surface of the said complementary portion of the central piece of the cylinder body; 
     the said tubular portion is in axial abutment against a radial shoulder of the central piece of the cylinder body; 
     the central piece includes a transverse duct which is open in a substantially radial direction into an axially oriented central hole of the central piece in facing relationship with the outer periphery of the piston; 
     the central piece of the cylinder body is configured as a transverse plate for fastening the control apparatus; 
     the apparatus includes two dynamic sealing members, namely a front member and a rear member, which are mounted within the central piece of the cylinder body; 
     the front dynamic sealing member and the rear dynamic sealing member are each, respectively, disposed in a housing formed in the central piece of the cylinder body, which is open radially inwards; 
     the rear end of the rear piece of the cylinder body includes at least one abutment adapted to cooperate with a rear end of the piston so as to limit rearward axial displacement of the piston; 
     the piston includes a peripheral piston jacket which is fixed axially to the piston and which slides axially in the internal bore of the cylinder body; 
     the piston jacket has at least one hole for providing communication between the inside and the outside of the piston jacket, and the hole is adapted to be displaced to either side of a dynamic sealing member; 
     the piston jacket is of metal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will appear on a reading of the following detailed description, for an understanding of which, reference will be made to the attached drawings in which: 
     FIG. 1 is a view in longitudinal axial cross section of a first embodiment of an emitter cylinder made in accordance with the main feature of the invention, and in which the piston is shown in a retracted position; 
     FIG. 2 is a detail view, on a larger scale, of the central part of the emitter cylinder of FIG. 1, showing the sealed fastening of the three components of the cylinder body; and 
     FIG. 3 is a view similar to that in FIG. 2, but shows a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, those elements which are identical, similar or analagous to each other will be designated by the same reference signs. 
     The drawings show a control cylinder in the form of an emitter in 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 the emitter cylinder. 
     Each receiver or emitter cylinder comprises a piston which is movable axially inside a cylinder body so as to define a variable volume hydraulic chamber. A connecting port, into which the duct exhausts, is open into the hydraulic chamber. 
     The emitter cylinder includes a piston rod which is coupled for example to a clutch pedal operated by the driver, or to an actuator which is started in accordance with predetermined programmes. 
     The piston of the emitter cylinder is arranged to expel a fluid such as oil, which is contained in the hydraulic chamber, towards the channel or duct connecting 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. 
     In the operation of disengaging 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 works on a rod which acts for example on the declutching fork that actuates the clutch release bearing which is part of the friction clutch. 
     In another version, the piston works in traction on the said fork through a cable or other means. 
     Each of the emitter and receiver cylinders includes a spring which works between the piston and the base of the cylinder body. When the driver releases pressure on the clutch pedal, or when the actuator returns to its initial clutch-engaged position, the return spring of the emitter cylinder expands so as to return the piston to its initial position, while the return spring of the emitter cylinder is compressed by the clutch spring, such as a diaphragm, which thereby returns the piston of the receiver cylinder to its initial position. 
     In the drawings, the emitter cylinder  10  is shown in a delivery position and comprises a cylinder body  17  which has an inlet port  18 , closed off by a plug  19 . It is through this inlet port that the duct or channel (not shown) which connects the receiver cylinder (not shown) to the emitter cylinder  10 , is open into the hydraulic chamber  20  of the emitter cylinder. 
     The chamber  20  has a variable volume, and it is bounded by the body  17  and the piston  21  which is mounted for axial movement within the cylinder body  17 . 
     The cylinder body  17  is of generally tubular form, and in accordance with the features of the invention it consists of three distinct pieces, which are made separately by moulding in plastics materials, and which are disposed in axial succession from front to rear, that is to say from left to right in the drawings, and which consist of a front piece  17 A which includes the orifice  18  and the plug  19  at its front axial end, a central or intermediate piece  17 B, and a rear piece  17 C. 
     The cylinder body  17 , in its three pieces  17 A,  17 B and  17 C, has an internal bore  22  which is generally blind and formed on an axis X-X corresponding to the general axis of symmetry of the cylinder body  17 , with, at the front, a transversely oriented base portion  24  in which the inlet port  18  is open, and which is part of a connecting portion  25  for connection of the above mentioned duct, for example by seaming, the connecting portion  25  having for this purpose internal passages  26  for receiving a clip which fastens the duct to the emitter cylinder. In another version, the connecting portion may be of the screw type. 
     The cylinder body  17  is generally open at the rear, that is to say its rear piece  17 C is itself open towards the rear. 
     Each of the two axial end pieces, that is to say the front piece  17 A and the rear piece  17 B, is in the general form of a tubular member of generally simple design, easy to make by moulding in plastics material. 
     The internal bore  22 , for guiding the piston  21  in sliding movement, comprises, in accordance with the features of the invention, a front portion  22 A formed in the front piece  17 A, and a rear portion  22 C which is formed in the rear piece  17 C, the two portions, namely the front portion  22 A and rear portion  22 C, being aligned and coaxial with each other. 
     The piston  21  is arranged to slide in the cylinder body  17 , and more precisely in the internal bore  22  of the latter, forward from the rear, starting from the retracted or declutched position shown in FIG. 1, towards an advanced or clutch engaged position (not shown). 
     In the retracted or declutched position shown in FIG. 1, the hydraulic chamber  20  and the control fluid are pressurised, while in the advanced position, the chamber  20  and the control fluid are depressurised. 
     A generally frusto-conical projection  28  is formed integrally by moulding with the transverse base portion  24 , and it extends axially towards the rear within the bore  22 , so as to constitute a centre for centring a return spring  29 , which in this example is a helical spring acting between the cylinder body  17  and the piston  21 , and more precisely between the transverse base portion  24  and the front of the piston  21 . 
     The port  18  is of course formed through the base portion  24 , and is open at the side of the projection  28  which is formed integrally by moulding with the base portion  24 . 
     The body of the piston  21  is a component moulded in plastics material in a generally tubular form, and it has in its front part a transverse wall  30 , the front transverse face  31  of which bounds the hydraulic chamber  20  axially at the rear, while its rear face  32  is in cooperation with the head of a piston rod  33 . 
     The body of the piston  21  is extended axially forward from its front transverse face  30  by a chimney portion  34  which has a frusto-conical external profile, with a blind central hole  35  of cylindrical form, the axially oriented base of which is defined by the front face  31  of the wall  30 . 
     The helical return spring  29  is mounted around the chimney portion  34 , the base of which serves for centring the return spring  29 , and the rear axial end of the spring bears on the front face  31  of the transverse wall  30  of the piston  21 . 
     The spring  29  therefore works axially between the base portion  24  and the front face  31 . 
     The rear axial transverse end face  36  of the projection  28  may constitute an abutment with which the transverse front axial end face  37  of the chimney portion  34  is arranged to cooperate. 
     More precisely, in the deployed position the piston  21  comes into contact with the face  36  of the projection  28  through the front face  37  of the chimney portion  34 , the cylinder body  17  accordingly having an abutment  28  which limits the axial forward displacement of the piston  21 . 
     The tubular body of the piston  21  of plastics material is extended axially towards the rear from the transverse wall  30  by a sleeve portion  38  which has a bore  39 , the front base portion of which is defined by the rear face  32  of the transverse wall  30  of the piston  21 . 
     The base  32  of the bore  39  is of generally hemispherical form, for receiving the complementary head  40  formed at the axial front end of the piston rod  33 . 
     The head  40 , and therefore the piston rod  33 , is retained axially within the bore  39 , and is therefore held with respect to the piston  21 , by an elastically deformable element  26  which will not be described in any greater detail here. 
     The rod  33  is accordingly mounted for rotation within the piston  21 , and more precisely with respect to the base  32  of the sleeve portion  38 . 
     The piston  21  carries at its cylindrical outer periphery a static seal  41  for sealing the piston, which in this example is an O-ring seal mounted in an internal radial groove formed in the cylindrical outer periphery in line with the transverse wall  30 . Other forms of static seals may be envisaged. 
     The static seal  41  is arranged to cooperate with the cylindrical inner peripheral surface of a piston jacket  42  which in this example is of metal. 
     The piston jacket  42  is a tubular element which surrounds the piston  21 , the latter being received within the jacket  42 . 
     The piston jacket  42  is made for example of anodised aluminium or treated steel. The piston jacket  42  extends forward, in axial projection with respect to the chimney portion  34 , in such a way that, in the advanced position which is not shown, the chimney portion  34  is in contact with the projection  28 , while the transverse front terminal edge  42 A is spaced axially away from the transverse base portion  24  that faces it. The dimensions of the piston jacket  42  are such that the chimney portion  34  and the return spring  29  are received radially, with a clearance, within the piston jacket  42 . 
     The piston jacket  42  is in intimate contact on its cylindrical inner periphery with the cylindrical outer periphery of the piston  21  and with the outer periphery of the static seal  41 . 
     The piston jacket  42  is fixed axially with respect to the piston  21 , this being achieved firstly by a radial gripping effect between the piston  21  and the piston jacket  42 , with cooperation of the static seal  41 . In order to complete this relative axial immobilisation of the piston  21  with respect to the piston jacket  42 , and their immobilisation in rotation, it is in this example proposed that the piston jacket  42  has, in the region of its rear axial terminal edge  42 C, lugs  44  which are received in engagement within a fastening groove  43  which is formed radially, facing towards the axis X-X, in the rear end portion  38 C of the sleeve portion  38  of the piston  21 . 
     Other means for coupling the piston  21  to the piston jacket  42 , axially and in rotation, may be envisaged without departing from the scope of the invention. 
     In all cases, there is provided in this way a unitary sub-assembly consisting of the piston  21  and piston jacket  42 , constituting a unit which is movable axially in the internal bore  22  of the cylinder body  17 , the piston jacket  42  and the piston  21  being components which are concentric and coaxial with the axis X-X. 
     The cylinder body is arranged to be fixed on a fixed part of the vehicle, and is therefore part of the generally fixed portion of the emitter cylinder  10 , while the piston  21  with its piston rod  33  is part of the movable portion of the emitter cylinder. 
     In accordance with the features of the invention, the central portion  17 B of the cylinder body  17  is made in the form of an intermediate plate which is disposed axially between the front portion  17 A and the rear portion  17 C of the body  17 . 
     The plate  17 B extends transversely, in a radially oriented plane at right angles to the axis X-X. 
     The central piece  17 B constitutes a fastening plate and is moulded in plastics material, and it may for example have fastening ears  46  which include holes  47  for the passage through them of fastening members, usually screws, for securing the cylinder body  17  and therefore the emitter cylinder  10 , on a fixed part of the motor vehicle. 
     FIG. 1 also shows a tube  49  fixed to the central piece  17 B of the cylinder body  17 . 
     The tube  49  enables the interior of the cylinder body  17  to be connected to a main feedback reservoir (not shown), which is mounted outside the emitter cylinder  10 , the hydraulic chambers of the emitter and receiver cylinders being filled with a hydraulic fluid, such as oil for example. 
     The reservoir is partly filled by the hydraulic fluid and it is connected through a duct, not shown in the drawings, to the tube  49 , with the latter opening into an aperture  50  which is formed in the central piece  17 B in its transverse front end face  45 A. The aperture  50  is one end of a duct  51  which is oriented at an inclination to the axis X-X, and which extends through the central piece  17 B from its transverse front face  45 A, to open towards the rear and axially inwards into a cylindrical central hole  53 , oriented axially and formed in the solid central portion  54 , which is of generally cylindrical annular form, of the central piece  17 B, and which, as will be explained in detail later herein, is substantially coaxial and concentric with the front portion  22 A and rear portion  22 C of the internal bore  22 , the internal diameter of the cylindrical hole  53  being in this example greater than the internal diameter of the bore  22 , so that the cylindrical outer peripheral surface of the piston jacket  42  extends through the hole  53  with a radial clearance between these two elements. 
     The tube  49  is fastened to the central piece  17 B by force-fitting of its rear end in a complementary housing formed in the transverse front face  45 A of the central piece  17 B, and then for example by a welding operation with indirect heating. 
     With this in view, the tube  49  is of course made in a plastics material which is compatible with that in which the central piece  17 B of the cylinder body  17  is moulded. 
     The first embodiment, according to the invention, of the means for joining and sealingly fastening the three components which constitute the cylinder body  17  together, will now be described with reference to FIGS. 1 and 2. 
     In the first embodiment shown in FIGS. 1 and 2, the axial rear end portion  56 A of the front piece  17 A, and the axial front end portion  56 C of the rear piece  17 C, are each formed as a solid tubular portion, the radial thickness of which is slightly greater than that of the tubular main portion of the front piece  17 A or rear piece  17 C respectively, the two tubular portions  56 A and  56 C preferably having the same radial dimensions, that is to say, in particular, the same outer and inner diameters. 
     Each tubular portion  56 A,  56 C is received axially in a complementary housing formed in the central piece  17 B in facing relationship with it. 
     More precisely, the solid central portion  54  of the central piece  17 B in the form of a plate includes two sleeve portions or bushes, namely a front sleeve portion  27 A and a rear sleeve portion  27 B, which extend axially forwards and rearwards respectively from the opposed front transverse face  45 A and rear transverse face  45 C respectively of the central piece  17 B. 
     Each bush  27 A,  27 C is of generally cylindrical annular form and is formed integrally by moulding with the solid central portion  54  of the central piece  17 B, and it defines an internal cylindrical peripheral surface  23 A,  23 C, which are open axially towards the front and rear respectively, and which are bounded axially towards the rear and the front respectively by a transverse front shoulder  64 A and a transverse rear shoulder  64 C respectively, each of which constitutes an axial abutment which determines the relative axial position of the front and rear pieces  17 A and  17 C respectively with respect to the central piece  17 B, because of the abutting engagement against the said shoulder  64 A,  64 C of the respective transverse end face, namely the rear face  156 A of the rear tubular portion  56 A and the front face  156 C of the tubular front portion  56 C. 
     The cylindrical housings  23 A and  23 C thus precisely determine the relative axial position of the front piece  17 A and the rear piece  17 B respectively of the cylinder body  17 , and also the alignment and perfect coaxiality of the front portion  22 A and rear portion  22 C of the internal bore  22  of the cylinder body  17  in which the piston jacket  42  is mounted for sliding movement. 
     Each tubular portion  56 A,  56 C is force-fitted axially in the corresponding sleeve portion  27 A,  27 C until the faces  156 A,  156 C come into axial engagement with the shoulders  64 A and  64 C. 
     The sealed fastening of the front piece  17 A and rear piece  17 C on the central piece  17 B is, in accordance with the features of the invention, achieved by welding the tubular portions  56 A,  56 C in the housings  23 A,  23 C with indirect application of heat. 
     Thanks to the design according to the invention, the sealed welding zone  58 A,  58 C corresponds to the interface between the cylindrical outer peripheral surfaces of the tubular portions  56 A,  56 C and the cylindrical inner peripheral surfaces  23 A,  23 C of the sleeve portions  27 A,  27 C. 
     The welding zone  58 A,  58 C, in which the operation of welding by indirect heating is effected, is thus very precise and is limited without the welding operation having any detrimental impact on the other parts and other components of the emitter  10 . 
     The design in accordance with the embodiment shown in FIGS. 1 and 2 is more particularly well adapted to achieving an assembly with sealed joints, by the use of a welding operation with infrared radiation. 
     To this end, the central piece  17 B according to the invention is made by moulding in a plastics material which is transparent to infrared rays, while the front end piece  17 A and rear end piece  17 B are preferably made in a plastics material that absorbs infrared rays, for example the transparent plastics material identical to the piece  17 B, and to which is added at least one additive such as carbon in a proportion of 1 to 2% so as to make it absorbent to the said infrared rays. 
     By way of variant, and making use of twin-material moulding methods, only the parts which are arranged to be welded together are made in the materials transparent or absorbent to infrared rays. 
     The infrared welding operation is then for example carried out using a laser beam which causes the interface zone  58 A,  58 C to be welded. 
     The invention is not limited to the operation of making a sealed weld by infrared radiation. 
     It is also possible to achieve this sealed fastening by ultrasonic welding, high frequency welding, or induction welding to form joints between ferromagnetic members, with the joints being able in particular to be formed by introducing small ferromagnetic particles directly into the components to be welded together. 
     In another version, the joint is made by mirror welding, that is to say welding with the aid of an intermediate heating piece which is withdrawn afterwards. 
     The effectiveness with which the hydraulic chamber  20  is sealed with respect to the outside is in this example obtained by means of two dynamic seals, namely a front seal  59 A and a rear seal  59 C, which in accordance with one feature of the invention are carried by the central piece  17 B of the cylinder body  17 , and each of which cooperates with the outer periphery of the piston jacket  42 , that is to say with the cylindrical outer surface of the said piston jacket  42 . 
     Each dynamic seal, namely the front seal  59 A and rear seal  59 C, is in this example a lipped channel seal which is mounted in a complementary seating  123 A,  123 C formed in the solid central portion  54  of the central piece  17 B, and open radially inwards so that each dynamic seal  59 A,  59 C is able to cooperate through its radially inner lip with the outer cylindrical surface of the piston jacket  42 . In another version, each dynamic seal may be a composite sealing member. 
     The central piece  17 B thus serves, by virtue of its solid central portion  54 , as a seal box which, in this example, carries two dynamic sealing members, though a single dynamic sealing member may be provided without departing from the scope of the invention. 
     The seating  123 A is open axially at the front, in such a way that the front dynamic sealing member  59 A is in axial facing relationship with the rear transverse face  156 A of the tubular portion  56 A. 
     The piston jacket  42  has at least one hole  62  for putting the interior and exterior of the piston jacket  42  into communication with each other. 
     The hole  62  is located, as a function of the axial movement of the piston  21  with its piston jacket  42 , on either side of the lip of the front dynamic sealing member  59 A. When the clutch is engaged, the hole  62  provides communication between the hydraulic control chamber  20  and the external main reservoir. During actuation of the clutch, the communication is interrupted because the hole  62  is displaced with respect to the seal  59 A, forwards with reference to FIG.  2 . 
     Several holes  62  may of course be provided in a common transverse plane, so as to give a larger flow of fluid and to avoid the need to adopt an angular orientation during fitting, the holes  62  being spaced apart circumferentially at regular intervals. 
     Communication between the hydraulic chamber  20  and the reservoir takes place through the holes  62  and the annular axial space delimited between the external cylindrical surface of the piston jacket  42  and the internal cylindrical surface of the hole  53  of the solid central portion  54  of the central piece  17 B. 
     In accordance with an aspect which is not shown in the drawings, and so as to prevent fluid from escaping to the outside, a sealing bellows may be provided, interposed between the cylinder body  17  and the piston rod  33 , for example between the central piece  17 B and the piston rod. 
     The presence of two dynamic sealing members  59 A,  59 C can however enable the need for a complementary sealing bellows to be avoided. 
     In that case a complementary cap (not shown) can if necessary be provided for protecting the rear piece  17 C, which is for example fixed around the rear sleeve portion  27 C of the central piece  17 B, and which surrounds the rear piece  17 C, with the piston rod  33  passing through it. 
     Such a protective cap can of course surround a sealing bellows. 
     The second embodiment shown in detail in FIG. 3 differs from the foregoing as regards the arrangement of the tubular portions and the complementary sleeve portions, so that it proposes an embodiment which is more particularly adapted for a friction welding operation to achieve a sealed fastening. 
     To this end, the front sleeve portion  27 A and rear portion  27 C are received axially within complementary portions of the tubular portions  56 A,  56 C which are arranged radially on the outside of the sleeve portions  27 A,  27 C, that is to say each of the two sealed welding zones  58 A,  58 C corresponds to the interface zone between the external cylindrical surfaces of the sleeve portions  27 A,  27 C and the complementary internal cylindrical surfaces of the external radial parts of the tubular portions  56 A,  56 C, which are formed accordingly. 
     The sealed weld in this example is preferably made by friction welding in the region of the zones  58 A and  58 C. Welding is carried out externally with respect to the sleeve portions  27 A,  27 C by vibration and relative rotation of the front piece  17 A and rear piece  17 C with respect to the central piece  17 B. 
     In another version not shown, and without departing from the scope of the invention, one of the outer axial pieces  17 A or  17 C can be designed as in the first embodiment, while the other outer axial piece  17 C or  17 A is designed as in the second embodiment. 
     The two outer axial pieces  17 A,  17 C may be made by moulding in the same plastics material or in a different plastics material, depending, in particular, on what technique is used for making the sealed weld joining each outer axial piece to the central piece  17 B. 
     Without departing from the scope of the invention it is also possible to design the three pieces in mechanical inversion, that is to say the central piece  17 B includes tubular portions which extend axially outwards and which are received in complementary sleeve portions of the two outer axial pieces  17 A and  17 C. 
     In another version, the joint is made by adhesive bonding. It is of course possible to fix one of the pieces  17 A,  17 C by adhesive bonding on the central piece  17 B and to secure the other one of the pieces  17 A,  17 C by welding on the piece  17 B. Thanks to the invention it is possible to fix in position, for example, the pieces  17 A,  17 B in one place of manufacture, and then to secure the piece  17 C finally in another manufacturing location. In all cases, the central piece constitutes a seal carrier. The presence of the spring  29  is of course not essential, as the return function may be ensured by means of a spring associated with the brake or clutch pedal. The piston may be made entirely of plastics material, but metallised using surface treatment for makind contact with the seals. Friction welding is to be understood to mean, for example, vibration welding, rotation welding, or mirror welding. Preferably the pieces  17 A,  17 B are force-fitted into the piece  17 C so as to enable the weld or adhesive bond to be optimised. It is of course possible to carry out a conical force-fit. During the welding operation, heat may be applied directly to at least one of the pieces to be welded.