Abstract:
The invention concerns a clutch friction, comprising a reaction plate ( 202 ) designed to be locked in rotation on an input shaft a friction disk ( 200 ), bearing at its external periphery at least one friction lining ( 201 ), designed to be locked in rotation on an output shaft, a pressure plate ( 1 ), a cover ( 2 ) fixed on the reaction plate ( 202 ), clutch means with axial action ( 3 ) controlled by releasing means and which operate between the cover ( 2 ) and the pressure plate ( 1 ) via support means ( 14 ), the pressure plate ( 1 ) being interlocked in rotation with the cover ( 2 ) while being capable of moving axially relative thereto and subjected to the action of elastic return means returning the pressure plate ( 1 ) axially towards the cover ( 2 ). Said friction clutch further comprises a wear take-up device, one of the supports of the clutch means ( 3 ) being capable of being axially offset the wear take-up device further comprises means, called diverging means, adapted to modify rapidly, for a predetermined degree of wear of the friction lining ( 201 ), the position of the elements controlling the releasing means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a friction clutch, in particular for a motor vehicle, and more particularly it relates to a clutch equipped with a device for compensating for wear, particularly in at least one friction liner, referred to hereinafter as a wear take-up device. 
     2. Description of Related Art 
     A conventional friction clutch generally includes a reaction plate, which may be in two parts so as to constitute a damping flywheel or a flexible flywheel, which is mounted in rotation on a first shaft, usually a driving shaft such as a crankshaft of the internal combustion engine, and which supports through its outer periphery a cover plate to which at least one pressure plate is attached. 
     The pressure plate is fixed with respect to the cover plate and reaction plate for rotation therewith, while being displaceable axially under the biasing action of axially acting resilient clutch engaging means controlled by declutching means; the clutch engaging means may consist of helical springs, or one or two Belleville rings mounted in series or in parallel and subjected to the action of declutching levers which constitute the declutching means; generally, the clutch engaging means and declutching means are part of the same component, for example a metallic diaphragm bearing on the cover plate; the diaphragm may be mounted in series or in parallel with a Belleville ring so as to assist-the declutching force, in the manner described for example in the document FR-A-2 753 758. 
     A friction disc, carrying one or more friction liners at its outer periphery, and rotatable with a shaft, usually a driven shaft such as the input shaft of the gearbox, is interposed between the pressure plate and reaction plate so as to be gripped between them when the clutch is engaged. The clutch engaging means control the axial displacement of the pressure plate when actuated by a clutch release bearing through the declutching means. 
     In the course of the useful life of such a clutch, the friction liner or liners and the materials of the cooperating components, namely the pressure plate and reaction plate, become worn, which causes a variation in the position of the pressure plate and the positions of the axially acting clutch engagement means and clutch release bearing, as a result of which there is, firstly, a variation in the force with which the friction disc is gripped between the pressure plate and reaction plate due to the modifications in the working conditions of the clutch engaging means, and secondly, the force necessary to disengage the clutch is affected. By giving such a clutch a wear take-up device, these disadvantages are avoided since the clutch engaging means and the clutch release bearing, which is usually in constant engagement on the declutching means, occupy the same position when the clutch is engaged. 
     A friction clutch equipped with a wear take-up device is arranged in such a w ay that one of the abutments of the clutch engaging means, whether the abutment is coupled directly or indirectly to the cover plate or whether the abutment is coupled directly or indirectly to the pressure plate, is displaceable axially, and a so-called compensating means is provided for performing this displacement in association with a so-called actuating means which starts, and in fact actuates, the said compensating means, and a triggering means which itself controls the actuating means by authorising or not authorising the starting of the actuating means. A wear take-up device is of course governed by a so-called detection means which is responsive to wear in the liner or liners, directly or indirectly, that is to say it is for example responsive to the position of at least one of the components of the clutch influenced by the said wear, with the said detection means acting on one of the said means consisting of the compensating means, actuating means and triggering means. 
     Various types of wear take-up device are known. 
     For example, document FR-A-2 753 503 describes a clutch in which the wear take-up device comprises ramp means disposed circumferentially and constituting the compensating means, being placed axially between the displaceable abutment and the pressure plate and being adapted to be driven in rotation by a set of teeth which they carry at their periphery and with which a tangentially disposed worm cooperates, means being provided for driving the worm in rotation and being rendered operational by wear in the friction liner or liners, together with non-return means that prevent the worm from rotating in the opposite direction from that in which it is driven in rotation by the rotational driving means when these are operational, the worm, the non-return means and the rotational driving means for the worm being carried by a support which is fixed to the cover plate, and the axially acting resilient clutch engaging means consisting of a diaphragm. The means for driving the worm in rotation are controlled by the diaphragm; the ramp means consist of an annular ring which has the displaceable abutment for the diaphragm and ramps which are inclined and spaced apart circumferentially, while the pressure plate has pads or ramps which are adapted to cooperate with the ramps of the ramp means; the worm is mounted for sliding movement on its axis, being subjected to the action of a so-called wear take-up resilient means constituting the actuating means, and when it is displaced on its axis, it drives the said ramp means in circumferential rotation. The means for driving the worm in rotation consist of a ratchet wheel fixed to the worm, and the non-return means consist of a pawl that cooperates with the ratchet wheel. In this case the detecting means is a tongue which is actuated by the diaphragm and adapted to cooperate with the ratchet wheel, with its course of travel increasing with wear until, with the clutch engaged, it can push on one tooth of the ratchet wheel and so cause the worm to turn, thereby compressing the elastic wear take-up means; the triggering means is represented by a reduction in the load on the band of the elastic wear compensating means on which engagement takes place during the declutching operation, when its load is sufficiently high to overcome the friction forces, thereby enabling the inclined ramps to be displaced and therefore enabling the plate abutment to be displaced axially. 
     Thus, the detection means, in this case the tongue controlled by the diaphragm, acts via the ratchet wheel on the actuating means which here comprise the resilient wear take up means, thus causing an actuating force to appear; triggering takes place during a subsequent declutching operation. 
     In the document GB-A-2 294 983, the clutch engaging means consist of a diaphragm and the displaceable abutment is again the plate abutment; the compensating means again consists of the ramps; the actuating means is a spring which acts permanently between ramps and counter ramps in the direction of increase of the axial thickness; here again, the triggering action takes place during the declutching operation by reduction in the load on the band of engagement, while the pressure plate is displaced as a function of wear with respect to a pin in engagement on the reaction plate, with the said pin, which projects frictionally through the pressure plate, constituting the detection means. This pin carries an abutment in the form of a lever that cooperates with the ramp means; the detection means then acts on the compensating means. 
     In the document GB-A-2 294 301, the clutch engaging means once again consist of a diaphragm, the means for compensating by means of ramps and the spring actuating means acting permanently between the ramps and, in this case, the cover plate; in this case the displaceable abutment is the primary abutment carried by the cover plate; the clutch is of the push-to-release type, and triggering is obtained by lowering of the secondary abutment which is situated facing the primary abutment on the same side as the pressure plate and mounted elastically, while the maximum declutching load increases with wear and this constitutes- the detection means, the descent of which constitutes the triggering means. The detection means accordingly acts on the triggering means, and a clearance appears between the primary and secondary abutments as the secondary abutment descends, the said clearance being taken up by a movement of the primary abutment under the action of the actuating means. 
     In the document U.S. Pat. No. 5,564,541, in the version shown in FIG. 10, the detection means is a pin which is disposed with a clearance between the cover plate and the reaction plate and which extends frictionally through the pressure plate, the force necessary for displacement of the pin with respect to the pressure plate being greater than the return force of the pressure plate which in this case is due to the tangential tongues; the actuating means is a spring that acts permanently between the pressure plate and the ramps that constitute the compensating means; it is the plate abutment which is displaceable, the clutch as a whole being of the pull-to-release type. In this case the course of travel for disengaging the clutch varies: the fully declutched position is fixed in such a way that triggering takes place at the end of this declutching operation, after wear has been detected and a clearance has appeared, this clearance being subsequently filled under the action of the actuating means. 
     In the document FR-A-2 599 446, in the version in FIG. 5, the elements are analogous to those in the preceding document, except that the compensating means is a free wheel working in only one axial direction, and the actuating means is a spring which permanently biases the balls into engagement against their ramps. 
     In the version in FIG. 3 of the document U.S. Pat. No. 5,564,541, the detection means is a pin which is disposed with a clearance between the cover plate and the pressure plate and which extends frictionally through the cover plate, the force necessary for displacing the pin with respect to the cover plate being greater than the return force of the pressure plate, which in this case is due to the tangential tongues; the actuating means is a spring which acts between the cover plate and an annular rotary ring having ramps adapted to cooperate with counter ramps; it is therefore the cover plate abutment which is displaceable, the clutch being of the pull-to-release type. The triggering action takes place at the end of the declutching operation, after wear has been detected and a clearance has appeared, the clearance being taken up under the action of the actuating means; as in FIG. 10, the position of full declutching is fixed, the clutch engaging means comprising declutching levers associated with helical springs which bear on the cover plate. 
     When a clutch is equipped with a wear take-up device which operates automatically, it is relevant for the user to have some idea of the degree of wear in the friction liners; in this connection, without this information he runs the risk of using his friction disc until the friction liners are substantially worn out, and this situation can lead to serious damage to the friction surfaces of the flywheel and/or the pressure plate, which are expensive to replace; this is the case for example when, the friction liners of the friction disc being held in place by rivets, the latter start to rub against the pressure plate and to damage the friction surface of that plate. 
     It has in the past been proposed, for example in the document FR-A-2 496 204, to monitor the position of the friction disc which is displaced axially as a function of wear, for the information of the user; such an arrangement makes use of induction coils and requires space to be available in the centre and increases the axial size of the clutch. 
     Given that the control members for the clutch engaging means also shift as a function of wear, it has been proposed to measure this shift, especially in the documents U.S. Pat. No. 3,982,614 or FR-A-2 712 947; not only are solutions of this kind not simple, but above all they have the disadvantage that they cannot be applied to a clutch equipped with a wear take-up device. 
     In the French patent application filed on Jun. 17, 1998 under the No. 98 07639, the wear take-up device itself is made use of so as to inform the user of the state of wear in the friction liners; in this case, it includes circuit breaking means for putting it out of circuit before the liners are fully worn; as wear continues, the position of the diaphragm changes in the engaged state of the clutch, and therefore its load and the force needed for declutching increases, which warns the user that he has passed the maximum safe value of wear, for example 80%. 
     Such an arrangement operates well; it does however suppose that the user has some familiarity with the driving of his vehicle. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to overcome these drawbacks, while also taking advantage of the wear take-up device itself, and more precisely its operation, for the purpose of warning the user of a certain degree of wear, by creating a sharp break in the progress of the wear take-up operation which is adapted to modify, rapidly and significantly, the position of the control members for the declutching means; thus, the user is very clearly warned, regardless of his state of familiarity. 
     Thus, according to the invention, a friction clutch, in particular for a motor vehicle, of the kind comprising a reaction plate designed to be mounted in rotation on a driving shaft, a friction disc carrying at its outer periphery at least one friction liner, adapted to be mounted in rotation on a driven shaft, a pressure plate, a cover plate fixed on the reaction plate, axially acting clutch engagement means which are controlled by declutching means and which act between, firstly, the cover plate and, secondly, the pressure plate through interposed abutment means, the pressure plate being fixed to the cover plate for rotation with the latter while being displaceable axially with respect to it and being subjected to the action of elastic return means biasing the pressure plate axially towards the cover plate, the said clutch also including a wear take-up device, one of the abutments of the clutch engaging means being displaceable axially and the wear take-up device including a compensating means actuated by an actuating means and arranged to effect the said displacement, the said actuating means being controlled by a triggering means in association with a detection means responsive to wear in the friction liner and acting on at least one of the actuating means, compensating means and triggering means, is characterised by the fact that the wear take-up device further includes so-called divergence means carried by one of the elements in the group consisting of the compensating means, actuating means, triggering means and detection means, adapted to modify rapidly, for a predetermined amount of wear in the friction liner, the position of the control members for the declutching means. 
     The change is felt by the user and/or can be detected in the control of the declutching means. 
     The predetermined amount of wear may be the total or substantially the total wear of the friction liner or liners. In one embodiment, the rapid change is an abrupt change. 
     Further features of the invention are defined in the dependent Claims at the end of the description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention can be more clearly understood, some embodiments will now be described by way of purely illustrative and non-limiting example, these embodiments being shown in the attached drawings, in which: 
     FIG. 1 is a top plan view, shown partly cut away, of part  4  clutch equipped with a wear compensating device according too the invention, when the liners are new; 
     FIG. 2 is a scrap view of the clutch, in cross section on II—II in FIG. 1; 
     FIG. 3 is a view similar to FIG. 2 when the clutch is disengaged; 
     FIGS. 4 and 5 are views similar to FIGS. 2 and 3 respectively, after the liners have become worn; 
     FIG. 6 is a scrap view in the direction of the arrow VI in FIG. 1; 
     FIGS. 7 to  9  show the unitary sub-assembly consisting of the support, worm, ratchet wheel, spring, spindle, and resilient member; FIG. 7 is a view in the direction of the arrow VII in FIG. 9, which is a view in cross section on IX—IX in FIG. 7; FIG. 8 is a view in the direction of the arrow VIII in FIG. 7; 
     FIGS. 10 to  12  are scrap views seen from the side, showing the arrangement of the ramp means in the clutch of FIGS. 1 to  9 , and their cooperation with the pads of the pressure plate; in FIG. 10, the clutch is in its new state, FIG. 11 shows the end of first-level wear take-up, and FIG. 12 shows the end of the detection phase and the beginning of second-level wear take-up; 
     FIG. 13 shows the development of the height of the clutch during wear; 
     FIGS. 14 and 15 are similar to FIGS. 1 and 10 and correspond to another version, with the clutch in its new state; 
     FIGS. 16 and 17, and FIGS. 18 and 19, are similar to FIGS. 14 and 15, and show the clutch, respectively, at the end of the wear take-up operation in FIGS. 16 and 17, and in a phase with no wear take-up in FIGS. 18 and 19; 
     FIG. 20 shows the development of the height of the clutch of FIGS. 14 to  19 , during wear; 
     FIG. 21 is similar to FIG.  6  and shows a modified divergence means; 
     FIG. 22 is a view in the direction of the arrow F 2  in FIG. 21, showing the resilient member cooperating with the detection ramp in the detection phase; 
     FIG. 23 is similar to FIG.  22  and corresponds to the end of the wear take-up phase; 
     FIG. 24 shows the development of the height of the clutch in FIGS. 21 to  23 , during wear; 
     FIG. 25 is similar to FIG. 24, and shows the development of the height of a modified version of the clutch in FIGS. 21 to  23 ; 
     FIGS. 26 and 27 are partial views similar to FIG. 21, each showing a modified embodiment of the detection ramps and second-level wear take-up; 
     FIG. 28 is a diagrammatic view showing part of a clutch of the push-to-release type equipped with divergence means in accordance with the invention, the clutch being here in its new state; 
     FIGS. 29 and 30 are similar to FIG. 28, and show the same clutch just before detection of a predetermined amount of wear, FIG. 29, and after detection of the same wear in FIG. 30; 
     FIGS. 31 to  33  show on a large scale the relative positions occupied by the ramp means and counter-ramps of the clutch in FIGS. 28,  29  and  30  respectively; 
     FIG. 34 shows the development of the height of the clutch in FIGS. 28 to  33  as a function of wear; 
     FIGS. 35 and 36 show the load curves for the detection spring and the diaphragm, respectively, in the clutch of FIGS. 28 to  34 ; 
     FIG. 37 is similar to FIG.  30  and shows a modified clutch; 
     FIGS. 38 to  43  are similar to FIGS. 31 to  36  respectively and relate to the clutch of FIG. 37; 
     FIGS. 44 to  46  show another version of the invention applied to a clutch equipped with ramp means and counter-ramps, these means being shown, respectively, in the position which they occupy when new in FIG. 44, at the end of the wear take-up phase in FIG. 45, and in the detection phase in FIG. 46; 
     FIGS. 47 and 48 show the development of the height of a clutch equipped with the means of FIGS. 44 to  46 , the clutch being of the push-to-release type, FIG. 47, or the pull-to-release type, FIG. 48; 
     FIGS. 49 to  53  are similar to FIGS. 44 to  48  respectively, and correspond to a further version; 
     FIG. 54 is a view in cross section showing part of a clutch of the type described with reference to FIGS. 44 to  46 , or  49  to  51 , equipped with another version of divergence means according to the invention; 
     FIG. 55 is a view which partially repeats FIG. 54, and in which the divergence means are operational; 
     FIGS. 56 and 57 show the development of the height of the clutch equipped with the means according to FIGS. 54 and 55, the clutch being of the push-to-release type, FIG. 56, or the pull-to-release type, FIG.  57 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 to  27  the diaphragm clutch  3  has a hollow cover plate  2 . 
     The cover plate in this example is a metal pressing. 
     The cover plate has a base and means for fastening it on a reaction plate, which may be of divided form, to constitute a damping flywheel or a flexible flywheel as can be seen for example in the document GB-A-2 294 301, to which reference should be made for more detail. 
     In the embodiments in FIGS. 1 to  27 , the means for fastening the cover plate consist of an axially oriented annular skirt which is an extension of a radial flange having holes for passage of the fastening members, such as screws, which secure the cover plate  2  to the reaction plate. 
     In another version, the cover plate is generally in the form of a hollow dish and has at its outer periphery a radial flange constituting means for fastening the cover plate to the reaction plate, the said flange being provided with holes for passage of fastening members such as screws for securing the cover plate to the reaction plate. 
     In the embodiments of FIGS. 1 to  27 , the clutch is of the pull-to-release type, so that it is necessary to exert a tractive force on the inner ends of the fingers of the diaphragm  3  so as to disengage (declutch) the clutch. The peripheral outer portion of the Belleville ring of the diaphragm bears on the base of the cover plate, on a thrust ring, not given a reference numeral, which is carried by the outer periphery of the base of the cover plate, or, in another version, on a press-formed element formed in the said base. The inner peripheral portion of the said Belleville ring is in contact with a so-called tertiary abutment  14 , to be described later herein. 
     In the embodiment in FIGS. 28 to  57 , the clutch is of the push-to-release type, that is to say it is necessary to exert a thrust with the aid of a clutch release bearing, not shown, on the inner ends of the fingers of the diaphragm  103 , so as to disengage (declutch) the clutch. For this purpose, the base of the cover plate  108  carries, firstly, a primary abutment consisting for example of a press-formed element formed in the base of the cover plate at the inner periphery of the latter, and secondly, facing the primary abutment, a secondary abutment in the form of the heads of short posts, or a rolled crown carried by short posts, or any other means.. The diaphragm  103 , through the inner periphery of its Belleville ring, is mounted for tilting between the said primary and secondary abutments. It is in contact through the outer periphery of its Belleville ring with abutment zones to be described below and constituting the tertiary abutment. 
     With reference to FIGS. 1 to  10 , these show a friction clutch mechanism, especially for a motor vehicle, which comprises a pressure plate  1  adapted to cooperate with a friction disc  200  which carries at its outer periphery friction liners  201 , and which itself cooperates with a reaction plate  202 ; for reasons of simplification, the friction disc  200 , the friction liners  201  and the reaction plate  202  are shown in outline only in FIG.  3 . The reaction plate  202  is arranged to be mounted in rotation on a driving shaft such as the crankshaft of the internal combustion engine. The friction disc  200  is fixed in rotation to a driven shaft such as the input shaft of the gearbox. More precisely, the liners  201  are fixed to a support which is rigidly or flexibly coupled to a hub which is mounted in rotation on the input shaft and which constitutes the output element of the friction disc  200 ; in the region of the friction liners  201 , the support is preferably elastic in the axial direction as described for example in the document GB-A-2 294 301, so as to constitute a progressive friction disc. In another version, the friction disc is not progressive. For example, the support may be embedded within a friction liner. 
     In all cases the friction disc has two friction faces for disengageably cooperating with the pressure plate  1  and reaction plate  202  respectively. 
     The pressure plate  1  is fixed in rotation to a cover plate  202  through interposed tangential tongues, not shown in the drawings, which are elastic and which at the same time constitute means for returning the pressure plate  1  towards the cover plate  2 , the latter being in this example a metal pressing. 
     The pressure plate  1 , while being fixed in rotation to the cover plate  2 , is accordingly displaceable axially with respect to the cover plate  2  under the biasing action of controlled axially acting resilient means, which in this case consist of a diaphragm  3  bearing on the cover plate  2  through its outer periphery, the clutch being here of the pull-to-release type with operation of the clutch being effected by acting on the ends of the fingers of the diaphragm  3  in the direction of the arrow F in FIG.  2 . 
     In order, when the clutch is engaged, to hold the axially acting resilient means  3  in a position independent of wear of the liner or liners of the friction disc, and to a lesser extent wear in the pressure plate  1  and reaction plate, the so-called friction faces of which undergo wear in contact with the liner or liners of the disc, a wear take-up device  10  is provided. 
     The wear compensating device  10  comprises ramp means  11  which are disposed circumferentially; more precisely, these ramp means  11  consist of an annular ring on which ramps  15  are disposed circumferentially; the said ring also has abutment means  14  consisting of an upper edge which is rounded or, in another version, chamfered, and which is centred on the axis of the clutch, this edge being located axially towards the outside with respect to the ramps  15 . 
     The pressure plate  1  has pads  4  which in this example are formed by moulding and machining, on its face that faces towards the base of the cover plate  2 , these pads being spaced apart circumferentially at a distance from each other that corresponds to the distance which circumferentially separates two successive ramps  15 , with each pad  4  being arranged to cooperate with a ramp  15 . 
     The ramp means  11  are located axially between the diaphragm  3  and the pressure plate  1 , so that the pads  4  receive the ramps  15 , and the diaphragm  3  cooperates with the abutment zones  14  which thus constitute the abutment means through which the diaphragm  3  acts on the pressure plate  1 . 
     The ramp means  11  have at their outer periphery a cylindrical wall  17  parallel to the axis of the clutch and cooperating with a cylindrical, coaxial recess  5  formed on the pressure plate on its surface that faces towards the base of the cover plate  2 , so that the ramp means  11  are centred with respect to the pressure plate  1 . The ramp means  11  constitute a compensating means, as will be described below. 
     The wear take-up device  10  further includes a ratchet wheel  20  with inclined teeth  21 , which is carried by a spindle  22  that also carries a worm  13 . 
     The spindle  22  of the ratchet wheel  20  is carried by a support  12 , which can be seen best in FIGS. 7 to  9  and which in this example is of sheet metal press-formed and bent, or which, in another version, is of moulded material, being U-shaped with a spine portion  19  and two wing portions  23 ,  24  adapted to support the spindle  22 ; for this purpose, each of the wing portions has a circular hole which is matched to the diameter of the spindle  22 . 
     The wing portions  23 ,  24  of the support  12  are extended by arms  27 ,  28  in an L shape, which are spaced apart from each other at right angles to the spine portion  19 . 
     The support  12  is adapted to receive a resilient member  31  which in this example is of metal and which is generally C-shaped, having an end portion  32  which is used for fastening, in this case by means of rivets  34 , the resilient member  31  on a return  16  of the spine portion  19  of the support  12 , this return  16  extending in this example at right angles to the spine portion  19  and in alignment with the arm  28 . The other end portion  33 , which is T-shaped, of the resilient member  31  carries a terminal control tongue  35 . 
     When the resilient member  31  and the ratchet wheel  20  are mounted on the support  12 , the control tongue  35  cooperates with a tooth root of the ratchet wheel  20 ; a helical compression spring  40  is placed between the ratchet wheel  20  and the wing portion  23  of the support  12 , being wound around the spindle  22 ; the worm  13  and the ratchet wheel  20  are in one piece with each other; as can be seen, the support  12  equipped with this component and with the resilient member  31 , spring  40  and spindle  22 , together constitutes a subassembly ready to be fitted. 
     The helical spring  40  constitutes the resilient take-up means which constitutes an actuating means in the manner to be described below. 
     The thread and pitch of the worm  13  are matched to a set of teeth  18 ; the worm  13  is arranged to cooperate with the teeth  18  under conditions which will be described below. 
     The set of teeth  18 , which is best seen in FIGS. 1 and 6, is carried by a member bent from sheet metal into a generally flattened Z shape, and fixed through its ends, FIG. 1, on bosses of the pressure plate  1  by means of screws  25 ; these ends are disposed transversely with respect to the axis of the clutch, while the inclined portion that joins them together is parallel to the ramps  15 . 
     The support  12 , carrying the ratchet wheel  20 , the worm  13  and the helical spring  40 , being fixed to the ramp means  11 , for example by means of screws  26 , the threaded portion of which passes through passes  27 A,  28 A formed respectively in the arms  27 ,  28 , the diaphragm  3  is displaced with respect to it during the operations of declutching and reclutching; it will be understood that, with this arrangement, during tilting of the diaphragm  3  in operations of reengaging the clutch, the latter displaces the control tongue  35  which, by cooperation with the teeth  21  of the ratchet wheel  20 , is caused to rotate the ratchet wheel  20  clockwise; on the return movement, the elasticity of the resilient member  31  and the inclination of the teeth  21  causes the control tongue  35  to be displaced towards the right with respect to FIGS. 2 to  5 , riding up on the teeth  21 . As a function of wear in the liners  201 , the inclination of the diaphragm  3  varies so that the control tongue  35  associated with the diaphragm  3  constitutes a detection means which is sensitive to the state of wear in the friction liners  201 . 
     The wear compensating device just described operates as follows. 
     FIG. 2 shows the clutch engaged, with the liners of the friction disc being new and gripped between the reaction plate, not shown, and the pressure plate  1 . In this position the end of the control tongue  35  is at the root of one tooth of the ratchet wheel  20 . 
     When the clutch is disengaged, the diaphragm  3  tilts, and during this displacement it progressively releases the control tongue  35 , the end of which moves along the inclined ramp defined by the tooth at the root of which it was located in the previous step; the end of the control tongue  35  is positioned with relation to the teeth  21  of the ratchet wheel  20  in such a way that, during this declutching operation with new liners, it does not jump over a tooth, as can be seen in FIG. 3; this is achieved by the end portion  33  of the resilient member  31  coming into abutment against the support  12 , and more precisely against the faces  36 A that face towards the ratchet wheel  20  of the flanks  36  of a slot  37  formed in the spine portion  19  of the support  12 , these faces constituting a control abutment and limiting the course of return travel of the control tongue  35  regardless of that of the diaphragm  3 , as is shown in FIG. 9, the end portion  33  being in the form of a T, the upright part of which is displaced into the slot  37  while its cross piece constitutes the control tongue  35 . 
     As the liners become worn, and in the known way, the pressure plate  1 , which is always subjected to the axial force of the diaphragm  3 , is displaced towards the reaction plate which is fixed axially, as is the cover plate  2 ; the diaphragm  3  therefore approaches the reaction plate, that is to say the open end of the cover plate  2 , carrying with it the control tongue  35 ; the ratchet wheel  20  rotates clockwise about its axis; with the clutch engaged and the liners worn, this is shown in FIG.  4 . This driving of the ratchet wheel  20  in rotation leads the worm  13  also to rotate on its axis; because the ramp means  11  are immobilised with respect to the cover plate  2  under the effect of the load exerted by the diaphragm  3 , the worm  13 , meshing with the set of teeth  18 , exerts a kind of screw action on the set of teeth  18 , slightly compressing the helical spring  40 , the direction of the thread of the worm  13  being determined accordingly. 
     During the declutching operation that follows the wearing phase just described, the pressure plate  1  releases the liners, moving further away from the latter under the action of the tangential tongues, which return the pressure plate  1  and the ramp means  11  towards the base of the cover plate  2 , the ramp means being in contact with the diaphragm  3  through their abutment zones  14  and with the pads  4  of the pressure plate  1  through their ramps  15 , the end of the tongue jumping over the tooth, 
     The helical spring  40  biases the worm  13  towards the wing portion  24  of the support  12 , from which it became separated during the wearing phase, that is to say from left to right with respect to FIGS. 7 and 8; because the diaphragm  3  no longer exerts a force on the ramp means  11 , the only force to be overcome in order to enable the ramp means  11  to rotate with respect to the pressure plate  1  is the friction force generated by the return force of the tangential tongues; if the force of the helical spring  40  is enough to overcome this force, the spring  40  then bearing on the worm  13  through, in this example, the ratchet wheel  20 , then the worm  13 , bearing on the set of teeth  18 , rotates the support  12  and therefore the ramps  15 : by rotating on themselves, the ramps  15  of the ramp means  11 , by cooperating with the pads  4  of the pressure plate  1 , will displace the pressure plate  1  further from the base of the cover plate  2 , thus increasing the axial distance between the working surface of the pressure plate  1  and the abutment zones  14  of the ramp means  11 , and thereby taking up at least partly the displacement of the pressure plate  1  due to wear in the liners. Because the load of the helical spring  40  is a function of the amount by which it will have been compressed by the worm  30  during the wearing phases, it can be seen that several clutch engaging and disengaging operations may be necessary before the wear compensating device  10  enters a take-up phase proper, this being a function of the relative dimensioning of the components; by way of example, the latter may be made in such a way that the take-up phase proper only happens for the first time after a certain amount of wear has taken place; in FIG. 5, which applies to the case where the clutch is disengaged with worn liners, it is supposed that no wear has been taken up; it is also supposed that the amount of wear has been enough for the ratchet wheel  20  to have rotated sufficiently far for the control tongue  35  to have jumped over one tooth on the return. 
     Thus, it is during the clutch engaging operation that the control tongue  35 , pushed by the diaphragm  3 , rotates the ratchet wheel  20  which compresses the spring  40 , and during the subsequent declutching operation the ramp means  11 , which are not gripped by the diaphragm  3 , turn under the action of the actuating means which in this example consist of the spring  40 , if its load is sufficient, and so compensate for the displacement of the plate due to wear; at the end of the take-up operation, the diaphragm  3 , the ratchet wheel  20  and the control tongue  35  revert to the position that they occupied in FIG.  2 . 
     In accordance with the invention, the clutch is equipped with a wear take-up device which includes divergence means adapted to modify rapidly, after a predetermined amount of wear has occurred, the position of the control members of the declutching means. The predetermined amount of wear may be the total wear, or substantially total wear, in the friction liners. 
     In this example, the divergence means abruptly modify the position of the members that control the declutching means before the friction liners have become fully worn. These divergence means consist, see FIG. 10, of a so-called detection ramp  41  which simulates an amount of wear and which follows each ramp  15  of the ramp means  11 ; the detection ramp  41  is inclined in the opposite direction from the associated ramp  1   5 , and is itself followed by a so-called second-level ramp  42  which is inclined in the same direction as the ramp  15 , in this case parallel to the ramp  15 , the latter being referred to for convenience as the first-level ramp  15 . 
     In the new state, the pads  4  of the pressure plate  1  cooperate with the first-level ramps  15  on the left hand portion of the latter in FIG. 10; in the course of wear and as described above, the first-level ramps  15  climb on the pads, on their right-hand portion with respect to FIGS. 10 and 11, the length of the first-level ramps  15  corresponding to the value of the predetermined amount of wear to be detected, for example 80 per cent of the total amount of wear; during this first-level take-up phase, the height of the mechanism, which is shown for example by the axial distance separating the ends of the fingers of the diaphragm  3  from the friction face of the reaction plate  202 , remains constant as explained above; this height is the height H represented in FIG. 13, in which U D  is the value of the wear to be detected. 
     As the wearing process continues, the pads  4  of the pressure plate  1  then cooperate with the detection ramp  41 ; because of its reverse slope in relation to the first-level ramps  15 , a high degree of wear is thus simulated and the diaphragm  3  inclines in one direction so that the ends of its fingers approach the reaction plate  202 , with a rapidity and suddenness which is greater as the angle of inclination of the detection ramps  41  is greater with respect to a transverse plane at right angles to the axis of the clutch; the new height of the clutch is then the height H 2 , FIG. 13, which is smaller than H 1 ; at the end of the detection phase, the pads  4  occupy the position shown in FIG. 12; after this, in cooperation with the second-level ramps  42 , a new wear take-up process takes place, the starting point of which is shown in FIG.  12  and which maintains the diaphragm  3  in its new position closer to the reaction plate  202 ; since the diaphragm  3  is caused to displace the resilient member  31 , it is necessary that, for the second-level wear take-up phase, the relative positions of the diaphragm and of the said resilient members  31  should be identical to those which they occupied during the first take-up phase; to this end, a press-formed element  43 , FIG. 6, which is directed axially towards the base of the cover plate  2  and which is therefore open towards the reaction plate  202 , is formed on at least one of the fingers of the diaphragm  3 ; the depth of this element  43  is proportional to the difference between the heights H 1  and H 2  corrected by the ratio of the radii; when the pads  4  engage the second level ramp  42 , FIG. 12, the resilient member  31  is in alignment with the press-formed element  43  so that it then enters the element  43 , which corresponds to the new stabilising height H 2 . 
     As will have been understood, the substantial change in the position of the declutching means during the wear detecting phase U D  greatly modifies the position of control of the clutch, and the user will sense this modification very markedly. What occurs is a downward adjustment in which the height of the clutch changes from the height H 1  to the height H 2 , the height H 2  being smaller than the height H 1 . 
     The change is sensed by the user and/or is detectable in the control of the declutching means, in any appropriate way. 
     FIGS. 14 and 20 show another version in which the divergence means are similar to those described with reference to FIGS. 1 to  13 , except that here, no second level wear take-up is provided. 
     More precisely, in the new state, the relevant components of the clutch occupy the position shown in FIGS. 14 and 15, in which the pads  4  of the pressure plate  1  cooperate with the initial left hand part of the ramps  15  of the ramp means  11 , FIG. 15; when wear has been taken up, the pads  4  move along the ramps  15  and the clutch has a constant height H 1 , FIG.  20 . In this example, in the manner described in the French patent application previously cited, the length of the set of teeth  18  with which the worm  13  cooperates enables the latter to take up only an amount of wear which generally corresponds to about one tooth, for example the predetermined amount of wear U D  which it is desired to detect. 
     At the end of the wear take-up operation, the relevant components of the clutch occupy the position shown in FIGS. 16 and 17; the detection ramp  41  is then brought into use very rapidly, the diaphragm  3  is displaced into a wear position and the height of the clutch becomes H 2 , FIG. 20, which is less that H 1  as before, indicating to the user that-wear has reached the amount U D . 
     Finally, at the end of the set of teeth, a recess  30  which can be seen in FIGS. 14,  16  and  18  enables the worm  13  to disengage at this it  20  position from the set of teeth  18 , FIGS. 18 and 19, so that the worm  13  no longer acts on the teeth  18 , the clutch operates like a clutch which is not equipped with a wear compensating device, and its height develops in accordance with the curve H 3  in FIG. 20, with the ends of the fingers of the diaphragm  3  moving closer to the reaction plate  202  as a function of wear. 
     In FIGS. 1 to  20 , a downward adjustment is thus effected due to the detection ramps  41  simulating wear. 
     The rapidity of this downward adjustment depends on the angle of inclination of the ramps. The greater this inclination is, the more severe is the effect. With a gentler slope, the effect is still rapid but less abrupt. 
     Because of the recess  30 , more than 80% of the wear can take place in combination with the detection ramps. 
     In general terms, in FIGS. 1 to  20 , the predetermined wear may be substantially the total amount of wear in the friction liners. The downward adjustment leads to modification of the declutching force and also to modification of the position of the ends of the fingers of the diaphragm. This may be detected by means of a force and/or position sensor. 
     For example, the clutch release bearing which is here arranged to act in a pulling mode on the fingers of the diaphragm for disengaging the clutch, may be equipped with a force and/or position sensor. 
     The position of the abutment zones may also be detected. 
     Thanks to these sensors it is possible to activate warning means such as a lamp, a voltmeter, an ammeter or the like, located on the fascia. 
     These arrangements can be applied when the information as to the degree of wear is obtained through an upward adjustment, in the manner described below. 
     Thus, in accordance with the version in FIGS. 21 to  24 , the detection ramp  141  and the second level wear take-up ramp  142  are carried by the diaphragm  3 , the resilient member  31  being adapted to cooperate with them as wear takes place; in this example, these ramps consist of a metal stirrup piece  44  which is suitably bent and clipped on one of the fingers of the diaphragm  3 ; as previously stated, the detection ramp  141  has an inclination opposite to that of the ramps  15 , and develops towards the pressure plate  1 , the second level take-up ramp  42  extending transversely with respect to the axis of the clutch. 
     Thus, the detection ramp  141  carries out to some extent an upwardly adjusting wear take-up phase in which the height of the clutch changes from a height H 1  to a height H 4  greater than H 1  as is shown in FIG.  24 . 
     FIG. 22 shows the resilient member  31  cooperating with the detection ramp  141  in the detection phase; FIG. 23, similar to FIG. 22, shows the end of the second level wear take-up phase. 
     Here again, the second level take-up ramp  142  may of course be omitted, as in the version in FIGS. 14 to  20 , and a recess  30  may be provided at the end of the set of teeth  18 ; thereafter, after the amount of wear U D  has been detected, there is no further take-up and the height of the clutch varies in accordance with H 5 , FIG.  25 . 
     FIGS. 26 and 27 each show an embodiment of the detection and second level ramps; in FIG. 26, they are formed respectively at  241  and  242  on the outer face of the head of a rivet  45  which is riveted on one finger of the diaphragm  3 ; in FIG. 27 it is a finger  46  of the diaphragm  3  that is press-formed so as to have a detection ramp  341  and a second level ramp  342  respectively. 
     FIGS. 28 to  33  show part of a clutch of the same type as that described in FIGS. 15 to  17  of the document GB-A-2 294 301, to which reference should be made for more detail; let us simply recapitulate that, here again, the clutch means consist of a diaphragm  103  which is articulated with respect to the cover plate, the clutch being of the push-to-release type; here, this articulation includes a primary abutment  110  on the cover plate side and a secondary abutment  111  on the same side as the pressure plate  101 , situated in facing relationship with the primary abutment  110 , the primary abutment  110  and secondary abutment  111  being carried by short posts  109  which are spaced apart circumferentially and which are fixed to the cover plate  108 ; in this example, the disengageable abutment is the primary abutment  110 , and triggering is obtained by lowering the secondary abutment  111  which is in cooperating engagement with a resilient so-called triggering means  121  that is itself in engagement on the cover plate  108 . 
     In FIGS. 28 and 30, the resilient triggering means  120 , which in this case is in the form of a diaphragm, also acts as a detection means; it is in engagement on the cover plate  108  through one or more lugs  112  projecting from the cylindrical skirt of the cover plate  108  and extending radially into the interior of the latter; while wear take-up operations are taking place, the abutments  110  and  111  are displaced towards the reaction plate and the resilient detecting means  120  is progressively tilted. 
     In this case the ramp means consist of resilient rings  105  surrounding the short posts  109  and located axially between the base of the cover plate  108  and the primary abutment  110 ; the transverse face of the rings  105  located facing the base of the cover plate  108  carries ramps  115  which can be seen in FIGS. 31 to  33  on a larger scale, and which cooperate with counter-ramps  118  formed in the base of the cover plate  108 , in this example by deformation of the latter; a spiral spring  113  biases the rings  105  in rotation with respect to the cover plate  108  in the sense in which the primary abutment  110  is displaced axially away from the base of the cover plate  108 . 
     FIG. 35 shows the load characteristic of the detection means  120  as a function of its deformation and therefore as a function of wear; FIG. 36 shows the same characteristic for the diaphragm  103 . 
     FIGS. 28 and 31 show the position of the components of the clutch when new; in this position, it will be noticed that the ramps  115  are partly offset with respect to the counter-ramps  118 , so that the axial distance  100 , FIG. 31, which represents the distance from the primary abutment  110  to the cover plate  108 , is greater than the same distance when measured, see FIG. 33, when the ramps  115  and the counter-ramps  118  are in exact register with each other. 
     In the new state, the operating points of the detection means  120  and the diaphragm  103  are the points C 1  and D 1  respectively in FIGS. 35 and 36, the load at C 1  being greater than that at D 1 . 
     In the course of the wearing process, the detection means  120  is deformed and its operating point moves from C 1  to C 2  on its chosen load curve, FIG. 35, so that during this change there is little variation in its load; having regard to the presence of the wear take-up device, the operating point of the diaphragm remains as D 1 , FIG.  36 . 
     In the vicinity of the predetermined wear U D  to be detected, and therefore just before the detection phase proper, the components of the clutch occupy the position shown in FIGS. 29 and 32; up to this stage, the height H of the clutch has the value H 1 , FIG. 34, which is substantially constant, that is to say it is subject to the variation in the axial distance  100  mentioned above. 
     As wear proceeds, the ramps  115  jump sharply over the counter-ramps  118 , and the axial distance  100 , FIG. 33, becomes a minimum: the detection means  120  and the diaphragm  103  tilt significantly, FIG. 30, and the height of the clutch becomes H 4 , FIG. 32, which is greater than H 1 ; in this position, the operating points of the detection means  120  and diaphragm  103  are E 3  and D 2  respectively, FIGS. 35 and 36; the load curves are so chosen that the load C 3  is greater than the load D 2 , so that the secondary abutment  111  is not displaceable; the wear take-up device no longer operates, and as wear proceeds, the height of the clutch increases in accordance with H 6 , FIG.  34 . An upward adjustment therefore takes place. 
     FIGS. 37 to  43  show another version of the clutch which has been described with reference to FIGS. 28 to  36 . 
     Here, the course of the travel of the detection means  120  is limited by an end stop  122  carried by at least one short post  109  in line with the primary abutment  110  and secondary abutment  111 , on the same side as the pressure plate  101 . 
     The chosen load curves are shown in FIGS. 42 and 43, which relate respectively to the detection means  120  and to the action of the diaphragm  103  on the secondary abutment during its actuation by the end stop. 
     In the new state and in the worn state just before detection, the detection means  120  and the diaphragm  103  occupy positions identical to those shown in FIGS. 28 and 29 respectively of the previous version, the inner periphery of the detection means  120  being spaced apart axially from the end stop  122 , the load of C 1  to C 2  being greater than that at D 1 , the operating point of the diaphragm; in these two states, new and at the threshold of the wear take-up phase, the ramp means occupy the positions shown in FIGS. 38 and 39. 
     As FIG. 42 shows, beyond the point C 2 , the load of the detection means  120  decreases very rapidly; wear continues, the load C of the detection means  120  becoming smaller than D 1  and, rapidly and abruptly, the abutment  111  descends until the inner periphery of the detection means  120  comes into contact with the end stop  122 . 
     Following this descent, which corresponds to a downward adjustment, the operating point of the diaphragm  103  passes from D 1  to D 2 , FIG. 43; in parallel with this, the height of the clutch, which up to this point was the substantial constant amount H 1 , becomes H 2  smaller than H 1 , see FIG. 41; after detection, the ramps  115  occupy the position shown in FIG.  40 . 
     After detection, the wear take-up device no longer operates: the height of the clutch then varies in accordance with H 7 , FIG.  41 . 
     FIGS. 44 to  48  show the application of the invention to a wear takeup device of the kind described in the document GB-A-2 294 983: the clutch engaging means are a diaphragm and the displaceable abutment is the plate abutment; the compensating means consist of ramps  82  carried by ramp means  81  and adapted to cooperate with counter ramps  84  carried by counter ramp means  83 ; the ramp means  81  and counter ramp means  83  are conventionally biased in circumferential relative displacement of one with respect to the other by means of a helical spring which constitutes the actuating means for the wear take-up device, in the sense of increasing the axial thickness  80  of the assembly which consists of the ramp means and counter ramp means; the pressure plate is displaced as a function of wear with respect to a pin which is in engagement on the reaction plate and which carries an abutment  85  cooperating with the ramp means  81 , by virtue of a lateral slot  86  of the latter in which the said abutment  85  is caused to undergo relative displacement. 
     The relative position of the ramp means  81  and the counter ramp means  82  in the new state is shown in FIGS. 44, in which it can be seen that the abutment  85  is in line with one edge of the slot  86 , in this case the right hand edge in the Figure. 
     As wear takes place, the ramps  82  of the ramp means  81  climb on the ramps  84  of the counter ramp means  83 , with the axial thickness  80  increasing to a maximum as shown in FIG. 45, which corresponds to the wear U D  to be detected; because of the wear compensating device, during this take-up phase, the diaphragm keeps its position and the height H 1  of the clutch, FIG. 47, stays constant; in this position the slot  86  is displaced with respect to the abutment  85 , and the latter is spaced away from the edge of the slot  86  which is on the left in FIG.  45 . 
     Following a subsequent short period of wear, the ramps  82  of the ramp means  81  jump over the ramps  84  of the counter ramp means  83 ; the ramps  82 ,  84  of the said means  81 ,  83  are separated by ramp tails  87 ,  88  respectively, which are inclined in the opposite direction from the latter; because of this arrangement, after the above mentioned jump between the ramps  82  and  84 , the ramp tails  87 ,  88  cooperate, and this cooperation leads to a reduction in the axial thickness  80 ; this reduction is stopped by the left hand edge of the slot  86  coming into engagement against the abutment  85 ; the slopes of the ramp tails  87 ,  88  and the length of the slot  86  are so chosen that in this position, see FIG. 46, the axial thickness  80  is greater after detection than that which corresponds to the new state as in FIG. 44; the diaphragm has thus rapidly, and in this case abruptly, tilted in a significant way. In another version, the reduction is stopped by any other means. 
     When these arrangements are applied to a clutch of the push-to-release type, the height of the clutch then assumes a value H 4 , FIG. 47, which is greater than H 1 ; after detection, the wear compensating device no longer operates and this height increases according to H 6 . 
     In the case of a clutch of the pull-to-release type, the detection of U D  leads to a height H 2  smaller than H 1 ; it then decreases according to H 3 , FIG.  48 . 
     FIGS. 49 to  53  show a further version similar to that in FIGS. 44 to  48 ; in this case, the ramps  82 ,  84  are not separated by ramp tails, and arrangements are made to prevent ramps from jumping. The sharp variation in the axial thickness  80  after detection is obtained by means of a slot  89  formed in the recess  86  and extending from its left hand edge, FIGS. 49 to  51 ; FIG. 49 corresponds to the new state and FIG. 50 to the end of the wear take-up phase; and in FIG. 51, which corresponds to the detection of U D , the axial thickness  80  has sharply increased as compared with that in FIG.  50 . 
     FIGS. 52 and 53 show the development of the height of a clutch of the push-to-release type and a clutch of the pull-to-release type, respectively; they are similar to FIGS. 41 and 25 respectively. 
     FIG. 54 shows in cross section part of a clutch of the same type as the preceding version in which, as stated above, a pin  90 , in engagement on the reaction plate, extends frictionally through the pressure plate  201 , to constitute the detection means of the wear compensating device, the said pin  90  carrying the abutment  85  which cooperates with the ramp means  81 . 
     In this example, the frictional traverse of the pressure plate  201  is obtained by means of a radially elastic  0 -ring  91  fitted in a groove formed on the pin  90 ; in accordance with the extent of wear, the pressure plate  201  is displaced with respect to the pin  90  up to the moment when the ring  91  leaves the pressure plate  201 , FIG. 55; the pin  90  is then free, since it is no longer held by the pressure plate  201 ; the same is then true for the abutment  85  which no longer cooperates with the ramp means  81 , and the latter are abruptly displaced, so increasing the axial thickness of the assembly which consists of the ramp means  81  and the counter ramp means  83 ; the result of these arrangements, illustrated in FIGS. 56 and 57, is identical to that which is obtained in the preceding version, so that FIGS. 56 and 57 are identical to FIGS. 52 and 53 respectively.