Abstract:
A hydrokinetic coupling appliance, in particular for a motor vehicle, comprising a housing ( 30 ) provided with a transverse wall ( 3 ), designed to be coupled in rotation to an input shaft, a turbine wheel ( 12 ) housed inside the housing ( 30 ) and integral with a hub ( 14 ), designed to be coupled in rotation to an output shaft, a first bearing ( 1 ) integral with the transverse wall ( 3 ) of the housing ( 30 ), a locking clutch interposed between said turbine wheel ( 3 ) and the transverse wall ( 13 ). A piston ( 4 ) carries a second bearing ( 2 ) extending opposite the first bearing ( 1 ) to be linked self-disengaging to the transverse wall, and wherein friction elements ( 60 ) operate between a transverse bearing ( 15 ) of the hub ( 14 ) and the piston ( 4 ), the piston ( 4 ) being shaped to bear the friction elements ( 60 ), and the hub ( 14 ) having an axially oriented annular portion ( 16 ) facing towards the transverse wall ( 3 ) and enclosed by the piston ( 4 ) mounted axially mobile relative to the annular portion.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates, most particularly, to a hydrokinetic coupling apparatus for a motor vehicle. 
   2. Description of Related Art 
   Such a hydrokinetic coupling apparatus is described in the document FR96-05722 published under the No. FR-A 2 748 539. In that document, the apparatus comprises an input element, in the form of a casing having a generally transversely oriented wall, and an output element comprising a moving assembly of a turbine wheel and hub mounted inside the casing. A piston is mounted between the said moving assembly and the transverse wall. This piston is mounted for axial movement with respect to the transverse wall and is coupled in rotation to the latter. 
   The input element is arranged to be coupled in rotation to a driving shaft through its transverse wall, while the output element is arranged to be coupled in rotation to a driven shaft through its hub. 
   The piston has at its outer periphery a surface referred to as a second surface, while the transverse wall has, facing the second surface, a surface which is called the first surface. These surfaces are in this case friction surfaces, with a friction disc being interposed between the two surfaces. By varying the pressure on either side of the piston, the latter is displaced axially in one direction or the other. The piston is accordingly movable axially with respect to the hub. In order to prevent any direct contact between the piston and the hub, the said document provides for a friction means to be interposed between piston and hub. 
   Because of this friction means, which constitutes an end stop that limits the movement of the piston, any direct contact between the piston and the hub is avoided. The friction means enables friction between the piston and hub to be controlled. 
   Accordingly, it is possible according to the application to choose an appropriate coefficient of friction for the friction means, which can therefore be small so that the friction means constitutes a bearing. 
   However, there is a problem because the friction means is carried by the hub, which makes it necessary to carry out additional operations on this treated component so that it can slide along the driven shaft. 
   In order to overcome this problem it could be thought that the friction means would be attached to the guide sleeve, which is also a machined component. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to mitigate these drawbacks in a simple and inexpensive way. 
   According to the invention, an apparatus of the type mentioned above is characterised in that the piston is so configured as to carry the friction means. 
   In addition, the hub has an axially oriented annular portion which is directed towards the transverse wall and surrounded by the piston which is mounted for axial movement with respect to the said portion. 
   In another embodiment of the invention, a hydrokinetic coupling apparatus of the type mentioned above, which further includes a turbine wheel having an annular ring which may be of divided form, and which is fixed to the hub by means of at least one rivet, and a friction means acting between the hub and the piston, is characterised in that the friction means is carried by at least one rivet. 
   In one embodiment, the friction means or the piston has at least one projecting element engaged in a complementary hole of the other one of the elements consisting of the piston and friction means. 
   Preferably, the projecting element is part of the piston and has a closed base. The projecting element extends in a first embodiment towards the transverse wall, and in a second embodiment towards the piston. This projecting element is easily made by press-forming in the piston, and, on the one hand, it preserves the seal of the piston, while on the other hand it enables the friction means to be fitted, especially by telescoping, snap-fitting, riveting, in situ moulding, swaging, screwing, seaming or, again, by a bayonet fitting. 
   Preferably, a passage is established for the fluid between the inner and outer peripheries of the friction means, and this passage may be obtained either by recessing the piston, or by giving the friction means grooves, or again by recessing the turbine hub in order to renew the oil film between the friction means and the transverse surface of the hub, thereby reducing wear in the friction means while preventing any danger of sticking. 
   In one embodiment, the friction means is mounted with respect to the piston with a clearance so as to give good contact with the transverse surface. 
   In another embodiment, the piston is mounted with respect to the hub with a radial clearance, and with a sealing ring interposed. In this embodiment, axially resilient tongues for allowing the piston to be displaced axially couple the piston in rotation to the outer periphery of the casing of the hydrokinetic coupling apparatus. 
   Thanks to this arrangement, the friction means has some capacity for movement so that its surface of contact with the transverse surface of the hub is maximised. In another version, the piston may be mounted for axial sliding movement along the hub, and radial and axial resilient means are interposed between the piston and the friction means in order to allow displacement of the friction means, so as to maximise the contact surface area between the friction means and the transverse surface. The resilient means may be part of either the friction means or the piston. 
   All these arrangements tend to lead to an increase in the useful life of the friction means and also the reliability of the whole. 
   The invention enables the guide sleeve to be omitted and the friction means to be attached to the piston, the latter being preferably of metal. 
   Thus it is no longer necessary to machine the hub in order to form housings for projecting elements carrying the friction ring, and as a result the elements of the hydrokinetic coupling apparatus are simplified, and costs are reduced without any adverse effect on performance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     It will be well understood from the following description that the features of the present invention may be applied in all types of hydrokinetic coupling apparatus, especially in apparatuses equipped with a bridging clutch of the monoface or biface type, that is to say a clutch having at least two friction surfaces. 
     The following description illustrates the invention with reference to the attached drawings, in which: 
       FIG. 1  is a half view in axial cross section of a fluid coupling apparatus according to the invention, 
       FIGS. 2 and 3  are exploded views of the hub, the lock-up clutch, the casing element and the torsion damper of the hydrokinetic coupling apparatus, 
       FIG. 4  is a view on an enlarged scale, from the lower part of  FIG. 1 , showing the friction means according to the invention, 
       FIG. 5  is a scrap view in cross section on the line  5 — 5  in  FIG. 4 , without the hub and without the friction means, 
       FIGS. 6 ,  8 ,  11 ,  14 ,  16 ,  18 ,  20 ,  22 ,  25 ,  27 ,  29 ,  31 ,  33 ,  35 ,  38 ,  41 ,  43 ,  45 ,  48 ,  50 ,  53  and  56  are views similar to  FIG. 1  for other embodiments of the invention, 
       FIG. 7  is a scrap view in the direction of the arrow  7  in  FIG. 6 , without the torque converter, 
       FIGS. 9 ,  12 ,  15 ,  17 ,  19 ,  21 ,  23 ,  26 ,  28 ,  30 ,  32 ,  34 ,  36 ,  39 ,  42 ,  44 ,  46 ,  49 ,  51 ,  54  and  57  are views similar to  FIG. 4 , for the various embodiments shown respectively in  FIGS. 8 ,  11 ,  14 ,  16 ,  18 ,  20 ,  22 ,  25 ,  27 ,  29 ,  31 ,  33 ,  35 ,  38 ,  41 ,  43 ,  45 ,  48 ,  50 ,  53  and  56 , 
       FIG. 10  is a view in cross section taken on the line  10 — 10  in  FIG. 9 , 
       FIG. 13  is a view in cross section taken on the line  13 — 13  in  FIG. 12 , 
       FIG. 24  is a view in cross section taken on the line  24 — 24  in  FIG. 23 , 
       FIG. 37  is a scrap view in the direction of the arrow  37  in  FIG. 36 , without the hub, 
       FIG. 40  is a view in cross section taken on the line  40 — 40  in  FIG. 39 , 
       FIG. 47  is a view showing the distribution of the rivets having friction means, 
       FIG. 52  is a scrap view in the direction of the arrow  52  in  FIG. 51 , without either the piston or the hub, 
       FIG. 55  is a scrap view in the direction of the arrow  55  in  FIG. 54 , without either the piston or the hub, 
       FIG. 58  is a scrap view in the direction of the arrow  58  in  FIG. 57 , without either the piston or the hub. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the drawings, those elements which are common will for simplicity be given the same reference signs. 
   Thus, in  FIGS. 1 to 4  a first surface and a second surface are shown at  1  and  2  respectively. These surfaces  1  and  2  are oriented transversely and are formed at the outer periphery of a generally transversely oriented wall  3  and a piston  4  respectively. 
   The surfaces  1 ,  2  in this example are integral with the wall  3  and piston  4 . In another version, at least one of the surfaces  1 ,  2  may be part of an additional member which is secured, for example by welding, on the wall  3  or piston  4  as the case may be. 
   The piston  4  is mounted while being axially displaceable with respect to the wall  3 , which has a central, generally tubular, axially projecting centring nose  5 . 
   The wall  3  is extended at its outer periphery by an axially oriented annular first flange  6 , to define a first casing element  3 ,  6 . An axially oriented annular second flange  7  is centred in the region of its free edge on the free end of the first flange  6 . This second flange  7  is therefore in intimate contact through its inner periphery with the outer periphery of the first flange  6 , and in this example it is welded on the first flange  6 . The second flange  7  is extended by a semi-toroidal envelope  8 , on the inside of which the vanes  9  of an impulse wheel  10 , which are in facing relationship with the vanes  11  of a turbine wheel  12 , are fixed. The flange  7  and envelope  8  are part of a second casing element  7 ,  8 . The piston  4  lies between the turbine wheel and the wall  3 . 
   The turbine wheel  12  has an internal annular ring portion  13 , which may be of divided form and by which it is fixed, in this example by rivets  59  but, in another version, by welding, on the outer periphery of a hub  14 , which is generally L-shaped and is directed axially towards the wall  3 . It is therefore on the transverse portion  15  of the hub  14 , this portion being in the form of a radial plate, that the ring  13  is fixed, while the axial portion  16  of the hub  14  is splined internally for coupling the hub  14 , and therefore the turbine wheel  12 , with a driven shaft not shown. This driven shaft is, in the known way, formed with a central duct for feeding a hydraulic control chamber  17  delimited axially by the piston  4  and wall  3 , and radially, internally, by the axial portion  16  of the hub  14 . For this purpose, at least one passage  18  is formed between the free end of the axial portion  16  and the wall  3  for passage of the control fluid, which is oil in this example and which comes from the duct in the driven shaft. The portion  16  is generally tubular, and has external splines  19  in the vicinity of its free end. The axial portion  16  has, between the plate  15  and splines  19 , an external smooth surface  20 , the diameter of which is slightly greater than that of the splines  19 , so that the axial portion  16  is of stepped diameter. A sealing ring  21  is fitted in a groove (which has no reference numeral) formed in the surface  20 . 
   This seal  21  co-operates with an axially oriented flange  22  formed on the piston  4  at its inner periphery. 
   A seal is thereby created at this level, and according to one feature the surface  20  is not a guide surface, so that this surface has no need to be machined, which reduces the cost of the hub  14 . 
   In this connection, there is a radial clearance between the surface  20  and the flange  22 . The piston  4  is coupled in rotation, with axial displacement being possible, to one of the flanges  6 ,  7  by means of a coupling with axially elastic tongues  23 . 
   The number of tongues  23  depends on the application, these latter being spaced apart circumferentially at regular intervals in several sets of tongues  23 , and in this example each set consists of at least one tongue. In this embodiment there are three sets of tongues  23 . 
   The tongues  23  may be oriented transversely, being for example of triangular or rectangular form, or even tangential. The tongues  23  are interposed between a first member  24  fixed to the piston  4  and a second member  25  fixed to one of the flanges  6 ,  7 . 
   In the embodiment shown in  FIG. 1 , the tongues  23  are fixed at each of their ends by means of rivets  26  on the first member  24  and the second piece  25  respectively, as can be seen in particular in  FIG. 7 . In this version, the first member  24  is integral with the piston  4  and consists of a plurality of lugs which project radially outwards from the outer periphery of the piston  4 . In  FIGS. 6 and 7  the member  24  is a separate piece from the piston  4 . To this end, the piston  4  has at its outer periphery an axially oriented annular first skirt  27  which extends axially away from the flange  22 , that is to say towards the turbine wheel  12 . 
   The skirt  27  stiffens the piston  4 . In all cases the piston  4 , radially inwardly of its surface  2 , matches the form of the turbine wheel  12  and hub  14 , so as to reduce the overall size of the hydrokinetic coupling apparatus, which comprises the turbine wheel  12 , impulse wheel  10 , piston  4 , hub  14 , and a torsion damper  28  which will be described below. The apparatus has an axial axis of symmetry and rotation X–X′. In this example, the apparatus also includes a reaction wheel  29  so that it constitutes a torque converter in the known way. 
   The casing elements  7 ,  8 , and  3 ,  6  constitute a sealed casing  30  which is filled with oil; inside which are the components  29 ,  10 ,  12 ,  4 ,  14  and the damper  28  which, in this case, is of the standard type. More precisely, the damper  28  comprises an input element  31  in the form of a disc arranged to be gripped between the surfaces  1 ,  2 . This element  31  is coupled elastically to a central damper plate  35 , which is splined internally for mounting on the external splines  19  of the hub  14 . The damper plate  35  in this case meshes without any circumferential clearance with the hub  14 ; the splines  19  of the hub  14  and the internal splines  32  of the damper plate  35  are of complementary forms. 
   As will have been understood, and in a known way, by varying the pressure on either side of the piston  4 —for example by varying the pressure in the hydraulic control chamber  17  using the supply duct in the driven shaft and the passage  18 —the piston  4  is displaced towards the wall  3  or away from the wall  3  so that, in one case, it grips the disc  31  between the surfaces  1 ,  2 , or releases the disc  31 . 
   When the disc  31  is gripped, the lock-up clutch, which comprises the surfaces  1 ,  2  and the torsion damper  28 , is said to be engaged or bridged, in such a way that the rotary driving motion is transmitted directly from the driving shaft—the crankshaft of a motor vehicle in the case of application to a motor vehicle—to the driven shaft through the lock-up clutch without any relative sliding between the turbine wheel  12  and impulse wheel  10 , and this reduces the fuel consumption of the vehicle. 
   When the disc  31  is released, the lock-up clutch is said to be disengaged or unbridged, so that the rotary driving motion is transmitted from the driving shaft to the driven shaft through the torque converter by virtue of the flow of oil between the vanes  9 ,  11  of the impulse wheel  10  and turbine wheel  12 . This is what happens when the motor vehicle is started. 
   The tongues  23  enable the piston  4  to move axially with respect to the wall  3  during the change of the lock-up clutch  1 ,  2 ,  28  from one position to the other. 
   In this case, the disc  31  carries friction liners  33  fixed on each of its faces. In another version, the liners  33  are fixed to the surfaces  1 ,  2 ,  7  which are accordingly fastening surfaces. In another version, the disc  31  is embedded at its outer periphery in a friction liner. In a further version, the disc  31  is in direct frictional contact against the surfaces. 
   Preferably, the liner or liners  33  are provided with grooves extending from their inner periphery to their outer periphery so as to provide a good cooling effect, the grooves being in contact with the surfaces  1 ,  2  or with the disc  31 . 
   In  FIGS. 1 to 5 , the tongues  23  are radially inside the liners  33  and surface  2 . In  FIGS. 6 and 7 , they lie facing the surface  2  and liners  33 . 
   In this case, the disc  31  is fixed at its inner periphery by means of rivets  34  to external peripheral annular ring portions  55 ,  155  which are part of guide rings  36 ,  37  disposed on either side of the damper plate  35 . 
   The ring portions  55 ,  155  are in contact with each other. The damper plate  35  and the guide rings  36 ,  37  are provided with windows  38 ,  39 , facing each other for holding resilient members  40  which in this example consist of coil springs or concentric helical springs. An axially acting spring  40  bears on the guide ring  37  which is proximal to the wall  3 , for action on the damper plate  35  and for gripping the latter in contact with the other guide ring  36  which is proximal to the piston  4 . 
   For this purpose, the ring  36  has a serpentine form with a surface radially outside the springs  40 . The damper plate  35  has at its outer periphery radial lugs  43 , each of which penetrates, with a circumferential clearance, into a notch  42  which, in  FIGS. 2 and 3 , is formed in the outer periphery of the guide ring  37 , so that the latter has a discontinuous ring portion  155 . The lugs  43 , by co-operating with the appropriate side edges of the notches  42 , limit the relative angular displacement between the damper plate  35  and the guide rings  36 ,  37 . 
   In  FIGS. 1 and 6  the structures are reversed, with the notches being formed in the guide ring  36 . 
   During the said displacement, the damper plate  35  rubs frictionally on the ring  41 , which in this example is a Belleville ring but which in another version is a corrugated ring. 
   The damper  28  is therefore preferably in the form of a conventional friction clutch, the guide rings  36 ,  37  of which have, in this case at their inner periphery, collar portions, which project axially towards the plate  15  in  FIG. 6  and towards each other in  FIG. 1 . In  FIGS. 2 and 3 , only the guide ring  36  has a collar portion, and all combinations are possible. Each collar portion is substantially in contact with the outer periphery of the splines  19 , and its function is stiffening of the whole. 
   The second connecting member  25  for the tongues  23  has, in  FIGS. 1 to 3 , the form of a stepped tongue with an ear for fastening one end of the tongue  23  by means of the rivets  26 . This ear is joined through a rounded portion to a strip having two holes for fastening the member  25  by riveting to the outer periphery of the wall  3 . Threaded plates (not given a reference numeral) are secured on the outer face of the wall  3 , in this example by welding. 
   In the known way, a radial plate  15  is screwed on these plates so as to couple the first casing element  3 ,  6  in rotation to the driving shaft. 
   The connecting members  25  extend tangentially with respect to the outer periphery of the piston, and are riveted to the outer periphery of the wall  3 , so that the friction liners  33  have a large-outer diameter, which is favourable to torque transmission. It is of course possible to increase the outer diameter of the liners  33  even more, so as to transmit even more torque, as can be seen in  FIG. 6 . 
   Thus, in  FIGS. 6 and 7 , the second connecting member  25  for the tongues  23  is L-shaped, and accordingly has a transversely oriented portion on which the tongues  23  are fixed, together with an axially oriented portion constituting a skirt and having transverse lugs  53  at its outer periphery. 
   Each of the lugs  53  is engaged in a complementary notch  45  which is formed, in this example, in the free end of the flange  6 . The notches  45  and the lugs  53  are overlaid by the flange  7 . Similarly, the first member  24 , which is flat in this case, has at its outer periphery lugs  44 , each of which is engaged in a complementary notch  46  formed in the free end of the skirt  27 . Two lugs  53 ,  44  are provided for each of the respective members  25 ,  24 . This number does of course depend on the application. Using cold or hot working of the lateral edges of the notches  45 ,  46 , the metal of the flange  6  and skirt  27  respectively is caused to flow so as to immobilise the lugs  43 ,  44  axially between the bases of the notches  45 ,  46  and the material reflowed during plastic flow of the side edges of the notches  45 ,  46 . Thus, the members  25 ,  24  are seamed, in this case on the flange  6  and on the piston  4 . 
   In another version, the flange  6  can partially surround the flange  7  so that the notches  45  can be formed in the flange  7 . In a further version, the lugs  53 ,  54  are secured by welding or adhesive bonding on one of the flanges  6 ,  7  and on the piston  4 . 
   In yet another version, the skirt of the member  25  is secured on the flange  6  by welding, for example by laser type transparency welding. 
   The first member  24  can be welded directly on the piston  4 , and this component then comprises a first portion for fastening the appropriate end of a set of tongues, and a second portion which is axially offset for fastening the first member  24 . 
   The piston  4  can also have at its outer periphery a transverse return projecting towards the axis X–X′ and enabling the first member  25  to be secured, for example using pop rivets. 
   In  FIGS. 1 to 6  a friction means  60  acts between a transverse surface formed in the radial plate  15  of the hub  14  and the piston  4 , which is coupled releasably to the transverse wall by means of the second surface  2 , lying in facing relationship with the first surface  1  as described above. 
   The friction means  60  prevents any direct contact occurring between the piston  4  and the transverse surface of the hub  14 , and limits axial displacement of the piston  4 , thereby preventing the latter from coming into contact with the turbine wheel  12 . This friction means  60  comprises at least one friction element, for example having a low coefficient of friction. This friction element is preferably of synthetic material such as plastics material, which is preferably reinforced with fibres and/or beads such as glass fibres and/or beads. 
   In accordance with the invention, the piston  4  is so configured as to carry the friction means  60 , and the hub  14  has an axially oriented annular portion directed towards the transverse wall  3  and surrounded by the piston  4 , which is movable axially with respect to the said portion. 
   With this arrangement, the radial plate  15  of the hub  14  does not need to be given any additional machining operation, since the friction means  60  is carried by the piston  4 . Another result is that the mechanical strength of the radial plate  15  is preserved, and in addition the solution is simple and inexpensive because the piston  4 , which is preferably of metal, is a component that is easy to shape. 
   All of this goes well with the damper  28 , because the piston  4  is fitted axially between the damper and the assembly consisting of the turbine wheel  12  and hub  14 , coming as close as possible to the said assembly and in particular to the plate  15 . In addition, the piston  4  is coupled in rotation to the outer periphery of one of the two casing elements  30 , which enables a radial clearance to be formed between the flange  22  and the surface  20 , so that risks of jamming of the piston  4  are minimised. 
   The piston  4  co-operates through its flange  22  solely with the seal  21  which is fixed axially to the hub  14 , so that the friction means  60  have a good surface contact with the plate  15 , since, because of the tongues  23  and seal  21 , the piston  4  is able to be displaced, in particular axially and circumferentially, so that the contact surface between the friction means  60  and radial plate  15  are always maximised. Moreover, a higher torque is transmitted because of the location of the tongues  23  at the outer periphery of the casing  30 , so that the liners  33  can have a large outside diameter. 
   In this example, either the friction means  60  or the piston  4  has at least one projecting element which is engaged in a complementary hole in the other one of these elements, i.e. the piston or friction means. 
   This method of coupling in mating cooperation is simple and inexpensive to achieve, and enables the friction means  60  to be properly centred while giving coupling in rotation. 
   The hole is preferably blind so that sealing of the chamber  17  is preserved. The blind hole is preferably formed by press-forming, or by drilling without drilling through, or by extrusion, and accordingly, in  FIGS. 1 to 6 , the friction means  60  consists of a ring which is adapted to come into contact with a transverse surface of the hub  14  formed in the radial plate  15 , radially inside the fastening rivets  59 . This surface faces towards the wall  3 . The ring  60  has a plurality of bosses  61 , each of which is engaged, with axial and radial clearance in this case, in a blind hole  62  formed by extruding the metal of the piston locally towards the wall  3 . 
   The bosses  61  and holes  62  are cylindrical, being of circular cross section here but of square cross section or in any other form in other versions. The contact surface between the radial plate  15  and ring  60  is a maximum because of the axial and radial clearance in the fitting of the bosses  61  in the holes  62 . 
   The structures can be reversed, see  FIGS. 8 to 10 , so that the blind hole  66  is formed in the friction ring  60  while the boss  166  is formed in the piston  4  by extrusion and plastic flow of material towards the plate  15 . In another version, the boss  166  can be open, and can for example be in the form of a chimney. Preferably in that case, the friction ring  60  is force-fitted on the bosses  166  so as to preserve the seal of the piston  4 . 
   The number of the holes depends on the application, and accordingly in  FIGS. 1 ,  4  and  5 , there are three holes  62  and three bosses  61 , spaced apart at regular intervals on the circumference of the friction ring  60 , while in  FIGS. 8 to 10  there are two bosses  166  and two holes  66 , diametrically opposed to each other. In that case, the holes  66  and bosses  166  are oblong. 
   The form and number of the holes and bosses respectively can of course be combined, and preferably there are at least two of the latter. 
   In another version, the bosses can penetrate into the holes with radial fitting clearance or without any clearance, that is to say force-fitted having regard to the fact that the piston may have rotational movement or angular displacement as described above. 
   As a result, the friction means  60  may consist of a ring divided into a plurality of annular sectors, engaged with the piston  4  preferably by at least two bosses and two holes. 
   In  FIGS. 1 to 10  the friction means  60  has a maximum surface area of contact with the transverse surface of the hub  14  and piston  4 , the said piston  4  being fully sealed. 
   In another version which is shown in  FIGS. 11 to 13 , sealing of the piston  4  can be preserved, with each of the bosses  166  penetrating into a through hole  164  formed in the friction ring  60 , and the latter may be of divided form so that the friction surface is slightly reduced. 
   The through hole  164  can of course be formed in the piston  4 , as can be seen in  FIGS. 14 and 15 , in which each of the bosses  61  penetrates into a through hole  161  in the piston  4 . 
   As explained above, the friction means  60  comprise at least one friction element such as the ring visible in  FIG. 4 , and this element is preferably made of plastics material. Thus in the embodiments in  FIGS. 16 and 17 , the bosses  61  are extended so as to traverse the holes  161 , the free ends of the bosses  61  being hot worked to form a rivet head  261 . In this way the friction means  60  becomes riveted on the piston  4 , whereby to ensure sealing. 
   In a further version, the piston  4  carries at least one rivet for fastening the friction means  60 , and this rivet is integral with the piston  4 . Thus, in  FIGS. 18 and 19 , the projecting element  166  is extended so as to extend through an aperture  266  in the friction ring  60  and to be deformed so as to form a rivet head  366  lodged within a hollow housing  466 , the outer diameter of which is greater than the diameter of the aperture  266 , so that a shoulder  566  is formed at the change of diameter, the head  366  being in contact with the said shoulder  566 . 
   In a further version which is shown in  FIGS. 20 and 21 , the rivet  666  is mounted on the piston  4 . This rivet  666  passes through the aperture  266  and its head is deformed into contact with the shoulder  566  of the housing  466 , so as to achieve the riveting operation. During the riveting operation the rivet shank expands radially so as to provide the seal between the edges of the aperture  266  and the piston  4 . 
   The rivet does of course not necessarily extend through an aperture in the friction means  60 . Thus, in  FIGS. 22 to 24  the boss  166  is deformed into contact with a shoulder  766  defined by a rebate  866  formed in the friction ring  60  at its outer periphery, the bosses  166  thereby centre the ring  60 , and the head  966  of the rivet is engaged in a rebate  866  of the friction means  60 , preferably after cold working, so that the said friction means  60  is immobilised axially and/or in rotation between the head  966  of the rivet on the one hand and the piston  4  on the other. 
   As will have been clearly understood, it is preferable that the projecting extends directly from the piston  4 , this projection being either directed towards the wall  3  so as to define the blind hole  62  or else directed in the opposite direction as can be seen in  FIGS. 22 to 24 . 
   Accordingly, connections can be made by riveting, but they can also be made by in situ moulding or by snap-fitting. In  FIGS. 25 and 26 , the projecting portion  1066  of the piston  4  has a terminal collar portion  1166  of enlarged diameter, so that the friction means  60  can be formed by moulding over the projecting portion  1066  of the piston  4 . In this case, the material of the friction means fills the space between the piston  4  and the collar portion  1166 . The collar portion  1166  is thus encapsulated in the friction means  60 . In another version, the friction means  60  can be snap-fitted on the projecting portion  1066 , the collar portion  1166  of which constitutes a bead. 
   Thus the projecting portion  1066  is snap-fitted in a hole  1266  in the friction means  60 , of castellated contour for example. As is best seen in  FIG. 26 , the friction means  60  includes at least one lip  1366  engaged in a groove of a projecting portion  1066  of the piston  4 , this groove being delimited by the piston  4  and the bead  1166 . In another version, the friction means  60  has at least one resilient lug engaged in the hole  161  in the piston  4  and provided with claws at its free end. 
   Thus in  FIG. 28 , the ring  60  has a plurality of lugs  1466  with claws, extending through the hole  161 , and this hole is preferably flared towards the radial plate  15  so that the lugs  1466  are able to be retracted during their forcible engagement in the hole  161 , and to be then deployed towards the outside once the claws of the lugs  1466  have come out on the other side. These claws come into engagement with the face of the piston that faces towards the wall  3 . 
   Snap-fitting means are thus interposed between the piston  4  and the friction means  60 . Seaming means can of course be interposed between the piston  4  and the friction means  60 . Thus in  FIGS. 29 and 30 , the friction ring  60  has a form identical with that of  FIGS. 22 to 24 , and the projecting portion  1066  has ridges which are designed to cut grooves in the outer periphery of the ring  60 . 
   Thus the ring is forcibly engaged on the projecting portion so that the ring is coupled in rotation to the said projecting portion, which is harder than it is itself, with grooves being formed in the outer periphery of the ring  60 . The material at the end of the projecting portion can then be upset into contact with the shoulder  766 . The piston  4  has at least one projecting portion which is deformed by plastic flow of the material into contact with a surface (the shoulder  766 ) of the friction means  60  facing away from the piston. The surface  766  is defined by a reduction in thickness. This reduction in thickness can of course be formed, as visible in  FIGS. 31 and 32 , at the inner periphery of the ring  60 , the rebate being then formed at the inner periphery of the ring  60 . The projecting portion  2066  then consists of an axially oriented annular flange on the inner periphery of the piston  4 . This projecting portion is splined and deformed into contact with the shoulder of the ring  60  facing towards the plate  15 . The flange may serve only for the purpose of centring the ring  60  as visible in  FIGS. 33 and 34 , the ring  60  then having no reduction in thickness. 
   In a further version, the flange can have an external thread, while the friction ring has an internal thread, thereby giving a screw and nut type fastening. 
   The piston  4  may have local recesses  400 , extending radially as can be seen in  FIGS. 35 to 37 , the recesses  400  being formed in the flange  2066  and overlapping radially with respect to the ring  60 , thereby enabling oil to flow between the inner and outer peripheries of the ring  60 . This oil flow creates an oil film between the piston  4  and the friction ring  60 , so that wear is diminished between these two elements, with the friction ring  60  preferably rubbing on the plate  15 . The movement of the piston  4  towards the plate  15  is easy and is not hindered by the presence of oil between the piston  4  and plate  15 . Such an arrangement is also applicable to the other embodiments. The face of the ring  60  facing towards the plate  15  may be given grooves to permit such an oil flow. It is of course possible to provide grooves on both faces of the friction ring  60 . Where the friction ring  60  is of divided form, oil flow automatically results between the outer and inner peripheries of the friction means, and therefore so also does the creation of an oil film between the friction means and the transverse surface of the hub. 
   Thus the friction means has at least one passage between its inner and outer peripheries to enable a fluid to flow. In one embodiment, the friction means consists of a ring having on at least one of its faces a passage such as a groove extending from its inner periphery to its outer periphery. 
   In  FIGS. 38 to 40 , the flange  2066  may be given projecting elements  2067 , each of which is engaged in a complementary notch  2068  formed in the inner periphery of the ring  60 , which is thereby coupled in rotation to the piston  4  in mating cooperation, being carried by the latter as can be seen in  FIGS. 33 to 37 . 
   All combinations or modifications are of course possible. 
   In  FIGS. 41 and 42 , the bead of the projecting portion  1066  is not necessarily in the form of a collar portion, but may be formed in a rounded profile as visible at  1067  in  FIG. 42 , and the internal bore  1068  of the passage hole formed in the ring  60  for receiving the projecting portion  1066  can have a central point such that fitting of the ring  60  is achieved by applying a pressure on the ring  60 , leading to an eversion, that is to say one particular form of snap-fitting. 
   The ring  60  may be chamfered at its outer periphery so that it can be secured by riveting to the piston  4  as can be seen in  FIGS. 43 and 44 . The embodiment in  FIGS. 43 and 44  also enables snap-fitting to be obtained because the outer periphery of the ring  60  comprises a rounded portion joined to the chamfer which is adapted for cooperation with the bead of the projecting portion  1066 . Thus the point, after being formed into a button, is engaged in a groove bounded by the piston  4  and the bead  1067  of the piston. 
   The friction means  60  may be carried by the hub  14 , and this friction means  60  may be coupled in rotation to the radial plate  15  by means of projecting elements engaged in blind holes in the plate  15 , though preferably, and as in the foregoing Figures, so as to avoid machining of the hub and therefore to simplify the elements of the apparatus while reducing cost without adversely affecting performance, the friction means  60  will be carried by at least one of the rivets  59  by which the turbine wheel  12  is fastened to the plate  15 . More precisely, and as described above, the rivet  59  serves to secure the ring  13 , which has at its inner periphery the turbine wheel  12 , to the plate  15 , which has a rebate for this purpose as can best be seen in  FIGS. 45 to 47 . To this end, the rivet  59  has a projecting head which has a thickened portion  159  for fastening the friction means  60 . In  FIGS. 45 to 47  the thickened portion  159  is also of constant width. 
   In the embodiment in  FIGS. 45 to 47 , the friction means  60  is moulded on the thickened portion  159 , given that in this embodiment, all of the rivets, which are spaced apart circumferentially at regular intervals, have such a thickened portion  159 . 
   In another version, some of the rivets  59  do not have such a head. In a further version, the thickened portion  159  is narrower and is joined to the free end through a portion  259  of penetrating form. In  FIGS. 48 and 49 , the portion  259  is of generally chamfered form. The friction means is accordingly snap-fitted on the thickened portion of the head  159 ,  259 . The friction means  60  includes for this purpose a blind cavity  359  which is open towards the plate  15  of the hub  14  so as to accommodate the thickened portion. The cavity  359  is bounded by L-shaped lugs  459 , these lugs  459  being elastically deformable transversely and arranged to come into contact with the face of the thickened portion  159  that faces away from the piston  4 . The portion  159  thus makes contact with the base of the cavity  359 . 
   In  FIGS. 45 to 49 , the friction means  60  comprises a plurality of elements fixed to the rivet heads, or else a single friction means in the form of a ring, or again, at least one annular sector fixed to at least one rivet head. 
   In  FIGS. 50 to 52 , the thickened portion  159  may be of constant width, and the elements  160  in the form of annular sectors have an oblong circumferential cavity  360  in which the thickened portions  159  are engaged. The cavity  360  is open axially towards the ring  13  and therefore towards the plate  15 , and its aperture is bounded by a lip  460  which is arranged to cooperate with the face of the thickened portion facing towards the ring  13 . The cavity  360  is preferably open towards the piston  4  so as to enable the rivets  59  to be passed through the said cavity  360  and secured. The corresponding aperture  362  defining a shoulder  361  for engagement of the thickened portion is thus arranged to be trapped between the shoulder  461  and the lip  460 . 
   The latter may of course comprise one or more elements in the form of annular sectors  160  as can be seen in  FIGS. 50 to 52 . The rivets  59  are introduced into the holes  363 , and then the heads having the thickened portions  159 , which are guided into the cavity  360 . The rivets are thus positioned in the friction means  60 , and the rivets can with advantage be held in position by clipping before riveting of the assembly thus constituted, namely the friction means  60  and rivets  59 . 
   In another version, the friction means  60  may be mounted on a bayonet type fitting, the rivet heads being positioned in holes in the friction means  60 , and the whole is then positioned by rotation or relative circular displacement of one with respect to the other, that is to say of the rivets  59  with respect to the friction means  60 . The assembly could of course be held in position by clips. A passage  363  is formed in the lip, this passage having a dimension, the diameter in this case, which is sufficient to enable the thickened portion to penetrate, the ring  13  being then turned after the thickened portion has penetrated so that a bayonet type fitting is obtained. This type of bayonet fitting is applicable to the case where the piston carries the friction means  60 , the rivets being then fixed to the piston, and the friction means may be divided into annular sectors. 
   In another version, in  FIGS. 53 to 55 , the head of the rivet  59  is engaged in an aperture  364  formed in the friction means, and in this case the aperture  364  is bounded by the adjacent ends of two successive sectors, these ends having a semicircular notch such that the aperture  364  is in the form of a cylindrical hole whereby each sector  160  is fitted over two rivet heads. The same is true in  FIGS. 56 to 58 , in which the apertures  464  are wider at the outer periphery than at their inner periphery, so that each sector could be force-fitted over at least one rivet head, or preferably two heads as in the preceding Figures, and these heads can be successive heads. The ends of the sectors  160  may be of any form whatever which enables them to be fitted over two rivet heads, for example each end of each element  60  may have a V-shaped recess, the dimension of which will be a function of the dimension of the rivet head. The rivet head must be large enough to prevent the friction means  60  from escaping when the piston  4  is displaced away from the friction means. The ring  13  may be fixed on the other side of the plate  15 , and it is then the foot of the rivet that extends between the piston  4  and the plate  15 , and this foot may be extended so that it will engage in an aperture in the friction means, for example of the same type as that in  FIGS. 53 to 58 . 
   It will be clear that the tongues in  FIGS. 8 to 58  may be replaced by those of  FIGS. 6 and 7 . The tongues  23  then lie radially outside the second surface  2 , or facing the latter as in  FIGS. 6 and 7 . 
   The friction means  60  does not necessarily cooperate with the radial plate  15 , so that the friction means  60  may cooperate with a transverse surface formed in the axial part of the hub  14 , for example at a change in diameter of the latter, so that the transverse surface is not necessarily part of the plate  15 . The flange  22  may slide along the surface  20  with a fitting clearance, and in that case the friction means  60  may included elastic elements so that it can always remain in contact with the associated transverse surface of the hub  14 , for example in the embodiment in  FIG. 4 , in which each boss can be surrounded by a sleeve of a material such as an elastomer. The sleeve is then interposed between the perimeter of the hole  62  and the projecting portion  61 , which enables the ring  60  to move with respect to the piston  4 . 
   In all of the drawings a groove is provided in the region of the root of radial plate  15  on the axial portion  16  of the hub  14 . This groove reduces mechanical stresses and avoids any interference with the piston  4 , especially where the latter includes a flange  2066  as shown in  FIGS. 34 ,  37  and  40 . In another version, a relief may be provided in the region of the root of the axial portion of the hub  14  in the radial plate  15 , the said relief being preferably combined with a chamfer on the piston and/or the friction means, with a view to preventing any interference. 
   The seal  21  may consist of an elastically deformable segment, and it is in order to facilitate fitting of the piston  4  and seal  21  that the piston  4  is chamfered in the region of its flange  22 , as shown in all of the drawings. 
   In  FIGS. 48 and 49 , and  50  to  52 , the friction means  60  may be out of engagement on the rivet head, but it is preferably in direct engagement on the turbine hub or turbine wheel, so that the friction means  60  is mounted with an axial clearance with respect to the thickened portion  159 , and manufacturing tolerances are thus reduced.