GB2317207A

GB2317207A - Automatically adjusting clutch

Info

Publication number: GB2317207A
Application number: GB9619316A
Authority: GB
Inventors: Alastair John Young
Original assignee: Automotive Products PLC
Current assignee: Automotive Products PLC
Priority date: 1996-09-14
Filing date: 1996-09-14
Publication date: 1998-03-18
Anticipated expiration: 2016-09-14
Also published as: GB9619316D0; GB2317207B

Abstract

An automatically adjusting clutch including a pressure plate 213. The pressure plate has a first part 230 adjacent a driven plate 215 and a second part 240 situated in the force bias path between a clutch engaging spring 212 and the first part when the clutch is engaged. The first part is limited in its disengagement movement by a stop ring 271, and the second part moves axially relative to the first part to adjust the effective thickness of the pressure plate. During disengagement of the clutch following wear of the facings, the effective axial thickness of the pressure plate 213 is increased and subsequently during re-engagement of the clutch the stop ring is adjusted to ensure the disengagement movement of the first part remains substantially constant throughout the life of the clutch. The second part 240 and the stop ring may each rotate relative to the first part 230. The automatically adjusting clutch may have resilient retraction means 214 which assist in moving the first part away from the flywheel during clutch disengagement and when the first part reaches the limit of its movement the load of the resilient retraction means is reacted by the stop ring 271.

Description

AUTOMATICALLY ADJUSTING CLUTCH The present invention relates to automatically adjusting clutches in particular though not exclusively for automatically adjusting clutches for motor vehicles.

It is known that the engaged position of a pressure plate of a conventional non adjusting clutch moves towards a counter pressure plate (or flywheel) as the clutch wears. The majority of this wear is associated with the friction facings of a driven plate situated between the pressure plate and flywheel. This wear changes the attitude of a bias means (e.g. a belville spring ) which biases the pressure plate towards the driven plate thus causing a variation in the required clutch release load.

Some form of automatically adjustable clutches compensate for this wear by increasing the effective thickness of the pressure plate to restore the bias means to approximately its pre worn attitude thus ensuring a substantially constant clutch release load through the life of the clutch.

It is an object of the present invention to produce an improved form of a self adjusting clutch in which the effective thickness of the pressure plate is increased.

According to the present invention there is provided an automatically adjusting clutch including a pressure plate which is biased axially towards a flywheel by a main clutch engaging spring means to clamp a driven plate between the pressure plate and flywheel to engage the clutch, the pressure plate having a first part adjacent the driven plate and a second part situated in the force bias path between the clutch engaging spring means and the first part when the clutch is engaged in which the first part is limited in its disengagement movement by stop means, and the second part moves axially relative to the first part to adjust the effective thickness of the pressure plate, the arrangement being such that, during disengagement of the clutch following a reduction in the distance between the flywheel and first part of the pressure plate as a result of wear of the clutch, the effective axial thickness of the pressure plate is increased and subsequently during re-engagement of the clutch the stop means is adjusted to ensure the disengagement movement of the first part of the pressure plate remains substantially constant throughour the life of the clutch.

The second part of the pressure plate may rotate relative to the first part of the pressure plate.

The stop means may rotate relative to the first part of the pressure plate.

The automatically adjusting clutch may have resilient retraction means which assist in moving the first part of the pressure plate away from the flywheel during clutch disengagement and when the first part of the pressure plate reaches the limit of its movement the load of the resilient retraction means is reacted by the stop means.

There may also be provided an automatically adjusting clutch in which the disengagement movement of the second part of the pressure plate is limited by further stop means.

The present invention has the advantage of reducing the forces required to increase the effective thickness of the pressure plate in an automatically adjusting clutch. Furthermore the present invention ensures that the forces required to adjust the pressure plate thickness vary less throughout the life of the clutch when compared with heretofore known automatically adjusting clutches. This results in a more reliable operation of the automatically adjusting feature.

The invention will be described by way of example only with reference to the accompanying drawings in which: Figure 1 is an axial partially cutaway view of an automatically adjusting clutch according to the present invention looking towards an associated engine, Figure 1A is an enlarged cutaway view of part of figure 1 Figure 2 is a radial view of the clutch of figure 1 taken in the direction of the arrow J of figure 1A; Figure 3 is a cross section view of part of the clutch of figure 1 taken along the line KK of figure 1A with the spring 274 not shown for clarity; Figure 4 is a developed view of the pressure plate of figure 1 taken along the line MM of figure; Figure 5 is an enlarged view of part of figure 4; Figure 6 is a view ofpart of figure 3 showing a modified clutch cover 221'; Figure 7 is a view of part of figure 3 showing a modified clutch assembly; Figure 8 is a cutaway view of the modified clutch assembly of figure 7 taken in the direction of arrow N.

Figure 9 is a cross section view of part of a modified clutch.

With reference to figures 1 to 5 there is illustrated an automatically adjusting clutch 210 which includes a flywheel 211, a clutch cover assembly 220 and a driven plate 215. The flywheel 211 is fixed to the end of a crankshaft (not shown) of an associated internal combustion engine.

The clutch cover assembly 220 comprises a clutch cover 221, a diaphragm spring 212, a pressure plate 213, torque straps 214 and a pawl mechanism 250 which form part of an adjuster means. The clutch cover 221 is fixed rotationally and axially fast to the flywheel 211 by bolts (not shown) and supports the diaphragm spring 212 via two support rings 222 situated one on each axial side ofthe diaphragm spring 212 in a manner well known in the art. The diaphragm spring biases the pressure plate 213 towards the flywheel 211.

The clutch driven plate 215 is situated between the pressure plate 213 and flywheel 211 and is connected to the input shaft of a gear box (not shown). When the clutch is engaged i.e. when the diaphragm spring 212 biases the pressure plate 213 towards the flywheel, thus clamping the driven plate, power can be transmitted between the associated engine and gearbox.

By applying an axial force to the fingers 21 2A of the diaphragm spring 212 towards the flywheel 211 the clutch can be disengaged in a manner well known in the art.

The pressure plate 213 comprises a first part in the form of an annular mass 230 coaxial with a second part in the form of a pivot ring 240. A stop means in the form of a stop ring 271 is mounted on the annular mass 230.

Annular mass 230 (see figure 3) has significant thermal mass and is thus capable of absorbing heat generated by frictional contact with the adjacent friction facing 216 ofthe driven plate 215 during engagement and disengagement of the clutch 210. On the radially outer periphery of the annular mass 230 there are three circumferentially equi-spaced lugs 231. Each lug is fixed to one end 214A of a tangentially orientated torque strap 214 by a rivet 218. The other end 21 4B of the torque strap 214 is fixed to the clutch cover 221 by a rivet 219. The torque straps 214 ensure the annular mass 230 remains concentric with and rotationally fast with the clutch cover 221 but allow axial movement of the annular mass 230 relative to the clutch cover 221. When the clutch is engaged the torque straps 214 are stressed such that they bias the annular mass 230 away from the flywheel. This biasing assists in separating the annular mass 230 from the driven plate 215 when the clutch is disengaged. The torque straps thus act as resilient retraction means.

On the axial side ofthe annular mass 230 remote from the flywheel 211 there is a circumferentially arranged annular array of nine undulations 232 (see figures 1 and 4), facing towards the diaphragm spring 212. In cross section each undulation consists of a relatively short flat section 232A, a relatively long ramp section 232B of a relatively shallow gradient and a relatively short ramp section 23 2C of relatively steep gradient which joins adjacent ramp sections 232B and flat sections 232A of adjacent undulations 232.

A portion of the outer periphery of the annular mass remote from the flywheel is formed as an annular spigot 233.

Pivot ring 240 is annular in shape and may be made as a pressing. The radially outer section 241 (see fig 3) (is formed parallel to the axis ofthe clutch 210. The radially inner surface 242 of the outer section 241 is formed as an annular recess which engages with the spigot 233 to keep the pivot ring 240 concentric with the annular mass 230. The outer surface 243 of the outer section 241 has a circumferentially orientated array of pivot ring adjuster teeth 244 each tooth being of part helical form. In this case the teeth 244 are formed on the pivot ring only in the region local to the pawl means 250 and not over the whole circumference of the pivot ring. Each tooth has a flank portion 244A of relatively low gradient and an edge portion 244B of relatively steep gradient (see figure 1A).

The radially outer section 241 is connected to a pivot section 245 of the pivot ring 240.

This pivot section 245 is contacted by the radially outer portion 212B of the diaphragm spring 212 and as the clutch 210 is engaged and disengaged the diaphragm spring rotates about the pivot section 245.

The pivot section 245 is connected to a radially inner section 246 which has a circumferentially arranged annular array of 9 undulations 247 which face and contact the undulations 232.

When the clutch is assembled with new, unworn components flat section 247A, ramp section 247B and ramp section 247C of each undulation 247 face corresponding flat section 232A, ramp section 232B and ramp section 232C of undulations 232.

Stop ring 271 consists of an annular ring portion 272 with a plurality of radially outwardly projecting tabs 273 which are twisted out of the plane of the annular ring portion 272 to conform to the angle of the ramp section 232B. Some tabs 273' have rectangular plates 276 fixed to them by rivets 277(see figs 4 & 5).

The stop ring is mounted on the annular mass 230 radially inboard ofthe pivot ring 240.

Furthermore a portion 247D (see fig 5) of the pivot ring 240 is located with a minimal axial clearance C2 axially between corresponding rectangular plates 276 and ramp section 232B.

The tabs 273 engage with circumferential clearance, but minimal radial clearance, in corresponding slots 248 (see figs 1A, 4 & 5) formed in the array of undulations 247. The tabs 273 also engage corresponding ramp portion 232B.

A plurality of springs 274 (see fig 4) engage corresponding edges 248A of slot 248 and edges 273A of some tabs 273" to bias corresponding edges 248B of slots 248 towards edges 273B of tabs 273.

It will be apparent that relative rotation of the pivot ring 240 in the direction of arrow F2 of figure 4 relative to the annular mass 230 will cause ramp sections 247B to slide across ramp sections 232B and the effective axial thickness T ofthe pressure plate 213 will increase. The design is such that when the driven plate friction facings 216 are worn to their design limit there is still sufficient overlapping contact of ramp sections 232B and 247B.

When ramp sections 232B are in contact with ramp sections 247B and the clutch is engaged, the clamp load path of the diaphragm spring passes from ramp section 232B to ramp section 247B. This clamp load can tend to rotate the pivot ring 240 relative to the annular mass 230 to reduce the effective axial thickness of the pressure plate 213. The design must ensure that this potential reduction in pressure plate thickness is resisted by friction between ramp sections 232B and 247B and friction between the pivot 245 and diaphragm spring 212.

In a modified embodiment ramp sections 232B and 247B and/or pivot 245 and diaphragm spring 212 can have inter-engaging serrations, similar to those described in the applicants co-pending British application GB95189961, to prevent reverse rotation of the pivot ring.

In a further modification an independent ratchet mechanism could be used to prevent back rotation of the pivot ring or a ratchet mechanism could utilise the teeth 244 to prevent back rotation of the pivot ring.

In operation the relative rotation of the annular mass and pivot ring is effected by pawl mechanism 250 which causes the pressure plate to increase in effective thickness (T2) incrementally by an amount substantially similar to the decrease in thickness of the friction facings 216 as wear takes place.

The single mechanism 250 comprises pawl means 260 riveted via pawl rivet 270 to a bracket 251.

The pawl means 260 is made from a resilient material, typically sprung steel, and consists of three pawl teeth 265, 266, 267 mounted on respective arms 263, 264, 264A. The arms are all cantilevered from a common fixing portion 268 and can move independently to a limited extent relative to each other in the direction of the axis of pawl rivet 270.

Bracket 251 comprises rivet portion 251joined to and substantially parallel with flat portion 251B via a curved portion 251 C (which acts as a pawl bias means). Additionally flat portion 251 B has a pawl means support portion 251 D bent at 90 degrees to it and a further tab portion 251E is bent at 90 degrees to the support portion 251D.

Rivet 219 passes through a hole 252 in rivet portion 251A to secure the spring bracket 251 and end 214B of strap 214 to the clutch cover 221. Curved portion 251C allows flat portion 251B, support portion 251D and tab portion 251E to pivot together to a limited degree relative to rivet portion 251A. Such pivotal movement is limited in the direction of arrow G2 of figure 2 by tab portion 251 E contacting end 21 4B of strap 214. Indeed when the bracket 251 is manufactured and assembled onto the clutch cover 221 tab portion 251 E is arranged to be pre-loaded into contact with end 21 4B of strap 214 by virtue of stresses in curved portion 251 C.

Pawl means 260 is riveted via rivet 270 to sprung bracket 251 to form the pawl mechanism 250.

Pawl mechanism 250 carries out the function of rotating pivot ring 240 when adjustment of the clutch is required.

Each pawl tooth 265, 266 and 267 is permanently engaged with a corresponding adjuster tooth 244 of the pivot ring 240 of the pressure plate 213. The spacing S2 between teeth 265 and 266 and between teeth 266 and 267 is 1/3 of the pitch P2 of the adjuster teeth 244 (see fig 2).

Operation of the automatically adjusting clutch is as follows : Considering pawl mechanism 250 with the clutch in an unworn condition and engaged, pawl teeth 265, 266 and 267 lie on appropriate flanks 244A of adjuster teeth 244.

As the clutch is disengaged the pressure plate 213 moves axially away from the flywheel 211 under the influence ofthe torque straps 214 by lift distance L2 (see figure 3) until the annular ring portion 272 of stop ring 271 contacts portions 223 of tabs 223A which retain the support ring 222 and diaphragm spring 212 on the clutch cover 221. Under these circumstances the pivot ring adopts the position D2 shown in dotted outline in figure 3.

Subsequent further disengagement causes radially outer portion 212B of the diaphragm spring 212 to come out of contact with the pivot ring 240. Thus the torque strap loads on the fully disengaged clutch 210 are taken through the stop ring 271 and not through the pivot ring 240.

However the ramp portions 247B of the pivot ring remain substantially engaged with corresponding ramp portion 232B ofthe annular mass because portion 247D remains between corresponding rectangular plates 276 and ramp section 232B. The minimal axial clearance C2 is designed to just allow the pivot ring 240 to rotate relative to the stop ring 271 when required for adjustment of the clutch without there being any significant axial free play ofthe pivot ring 240. Thus the rectangular plates 276 act as further stop means to limit the axial movement of the second part of the pressure plate (see below) During the above mentioned disengagement, the pawi teeth 265, 266 and 267 slide across corresponding flanks 244A of adjuster teeth 244. During the whole of the disengagement movement of the pivot ring the teeth 265,266 and 267 remain on their appropriate flank 244A.

When the clutch is engaged the pressure plate 213 moves towards the flywheel and the pawl teeth slide relative to the corresponding flanks 244A on adjuster teeth 244 until the fully engaged position is achieved.

When wear of the friction facings 216 has taken place and the clutch is engaged, the pivot ring 240, annular mass 230 and stop ring 271 will all be slightly closer to the flywheel by an amount equal to the amount of wear of the friction facings and during the subsequent disengagement of the clutch, the disengagement movement or lift distance L2 of the pivot ring will be slightly greater by the amount of wear that has taken place.

Once a predetermined amount of wear of the friction facings 216 has taken place the engaged position of the pivot ring 240 is sufficiently close to the flywheel for the pawl tooth 267, to slide past the corresponding: flank 244A and engage with the adjacent edge 244B of the adjuster tooth, in other words the pawl tooth 267 engages behind the corresponding adjuster tooth. This effectively senses that a predetermined amount of wear has taken place.

On the next disengagement of the clutch the pivot ring is moved away from the flywheel under the influence of the torque straps but the pawl tooth 267 is unable to slide past the edge 244B of its engaging adjuster tooth and therefore the pawl means 250 is rotated against the preload stresses of curved portion 251 C of bracket 251. Also, at the same time the corresponding tab portion 251E disengages the corresponding end 214B of strap 214. Thus the pawl bias means i.e. curved portion 251C of bracket 251 provides a potential adjustment force.

During disengagement the stress in curved portion 251 C is being reacted at the adjuster tooth edge 244B against which the pawl tooth 267 is contacting. Since this tooth edge is helical there is a component of the reaction force which biases the pivot ring towards the flywheel. The design must ensure that the pressure plate disengages the driven plate correctly, in particular the force created by the torque straps biasing the pressure plate away from the flywheel must be greater than the component of force on the pressure plate towards the flywheel created by the appropriate pawl tooth (in this example tooth 267) of any pawl means which is about to cause adjustment of the pivot ring.

During the first part of the clutch disengagement the potential adjustment force is unable to rotate the pivot ring since the torque straps are acting to force the annular mass 230 towards the diaphragm spring 212 and thus creating friction between the annular mass 230 and the pivot ring 240 and between the diaphragm spring 212 and the pivot ring 240. Since the pivot ring can not rotate the stop ring is also unable to rotate at this stage.

When the clutch is fully disengaged and the torque strap loads are being taken through the stop ring 271 the pivot ring 240 is no longer clamped between the annular mass 230 and the diaphragm spring 212 and is now able to rotate about the clutch axis relative to the first portion under the influence of the stressed curved portion 251C (or pawl bias means) until the corresponding tab portion 251E re-engages with corresponding end 214B of strap 214.

However the stop ring 271 is unable to rotate at this stage since it is now clamped between the abutments 223 ofthe clutch cover 221 and the annular mass 230. Thus a gap is created between corresponding slot edge 248B and tab edge 273B. The initial gap between slot edge 248A and tab edge 273A prior to the pivot ring rotating is sufficient to not limit the rotation of the pivot ring 240 when adjustment is taking place.

It is important to note that with the clutch in the fully dis-engaged position the pivot ring 240 has rotated to a new position but the stop pin has not yet rotated.

Only when the clutch is subsequently re-engaged can the stop ring rotate. This occurs since the stop ring 271 disengages the abutment 223 and rotates under the influence of springs 274 until corresponding slot edges 248B again contact tab edges 273B. This rotation of the stop ring causes the tabs 273 to slide up the ramps 232B and thus the lift L2 of the pressure plate upon subsequent clutch disengagement is reset to its pre-worn amount. During this engagement the pawl teeth again slide relative to the adjuster teeth.

The clutch is designed such that the amount by which the pivot ring 240 is caused to rotate by the pawl means 250 relative to the annular mass 230 produces an increase in effrtive thickness (T2) ofthe pressure plate 213 equal to the amount of wear ofthe friction facings 216. Thus when the clutch is re-engaged following an adjustment, the pivot ring 240 and pivot section 245 is axially in its "un-worn engaged" position.

Following a further pre-determined amount of wear, a further adjustment of the effective thickness of the pressure plate 213 takes place under the influence of pawl tooth 266.

The next adjustment takes place under the influence of pawl tooth 265. The adjustment sequence then starts again with pawl tooth 267.

It is apparent that the pawl mechanism 250 provides automatic adjustment of clutch 210.

It should be noted that, depending upon the state of disengagement of the clutch, the torque strap axial loads are reacted through either the pivot ring or the stop ring.

Furthermore under normal circumstances only when the torque strap loads are being reacted by the stop ring can the pivot ring rotate and vice versa. Thus, under normal circumstances, at no stage can the stop ring and pivot ring rotate together.

It should also be noted that as the friction facings wear and the annular mass 230 moves towards the flywheel, the axial stresses in the torque straps increase as ends 214A ofthe torque straps move relative to ends 214B. However there is not a corresponding increase in force required to rotate the pivot ring when adjustment is required since when the pivot ring is rotating the increased torque strap loads are being reacted through the stop ring and do not affect the pivot ring. Similarly when the stop ring is rotating any increase in torque strap axial load is reacted through the pivot ring and does not affect the stop ring.

It should also be noted that under exceptional circumstances, such as severe axial vibration, the annular mass 230 can overcome the torque strap loads and move towards the flywheel with the clutch in the disengaged position. Under such circumstances the pivot ring and stop ring are both free to rotate together should adjustment be required.

Thus the clutch is still able to adjust correctly.

Figure 6 shows a modified embodiment of a clutch 210' in which when the clutch is in its fully disengaged position the pivot ring adopts the position D2' shown dotted in figure 6.

The stop ring (not shown) of clutch 210' is similar to stop ring 271 except that it has no rectangular plates 276 or corresponding rivets 277.

There is a small clearance C2' between the pivot ring and each of the three circumferentially spaced tabs 221A (only one shown) of modified clutch cover 221' which (similar to the clearance C2) ensures that the ramp portions 247B remain substantially engaged with corresponding ramp portions 232B when the clutch is fully disengaged but also is sufficiently large enough to allow the pivot ring 240 to rotate freely and move axially when the clutch is disengaged and adjustment of the clutch is necessary.

In all other respects the clutch 210' is identical to clutch 210.

It should be noted that each tab 221A (also known as further stop means) of the clutch cover 221' only acts to limit the distance the pivot ring can move away from the first portion of the pressure plate under the influence of vibrations (especially axial vibrations) generated by the associated engine. The action ofthe torque straps in the clutch 210' is reacted via the stop ring 271 through the tabs 223 A. At no stage do the tabs 221A axially react against the action of the torque straps.

In further embodiments there could be more or less than three tabs 221 A. Alternatively the stop ring could contact the diaphragm spring (or some other axially moveable component of the clutch) either radially inboard or radially outboard of the rings 222 instead of contacting the tabs 223. This can be advantageous in some circumstances.

Figures 7 and 8 show a further modified embodiment of a clutch 210" which has three springs 275 (also known as further stop means or resilient stop components) positioned at circumfernetially spaced locations (only one shown). Each spring 275 consist of a rivet portion 275A bent at 90 degrees to a support portion 275B which in turn is bent at 90 degrees to a leaf spring portion 275 C.

The rivet portion 275A is secured to the clutch cover by rivet 219 (which also secures the sprung bracket 251) and the support portion 275B is orientated parallel to the support portion 251 D of the sprung bracket 251. One end of the leaf spring portion 275C engages (at least when the clutch is fully disengaged) the pivot ring 240 between the pivot section 245 and the radially outer section 241 and biases the pivot ring towards the annular mass 230. The stop ring of clutch 210" is similar to stop ring 271 except that it has no rectangular plates 276 or corresponding rivets 277. In all other respects the clutch 210" is identical to clutch 210.

When the clutch is fully disengaged the-springs 275 ensure that the ramp section 247B and 232B (not shown) do not disengage and cause over adjustment ofthe effective thickness of the pressure plate.

Figure 9 shows a modified embodiment of clutch 210" in which the stop ring 271"' which is identical to stop ring (not shown) of clutch 210'(i.e. it has no equivalent rectangular plates 276 or corresoonding rivets 277) is supported on an axially fixed component of the clutch, in this case a set of circumferentially spaced rivets 278 (only one shown).Each rivet 278 is fixed to the cover 221"' of clutch 210"' by one of its ends 278A. The stop ring is sandwiched between the heads of the rivets 278B and adjacent portions 223"' of tabs 223A. It is apparent that the stop ring 271"' which moves circumferentially relative to cover 221"' fulfils the same function as stop ring 271 but stop ring 271"' is axially fixed relative to cover 221"' whereas stop ring 271 moves axially with the first part of the pressure plate 230 each time the clutch 210 is disengaged or engaged. Spring 274"' fulfils the same function as spring 274. Spring 275"' fulfils the same function as spring 275 and act as further stop means.

In an alternative embodiment of clutch 210"' the stop ring 271"' could be held axially fast but circumferentially movable relative to cover 221"' by means other than a rivet for example some (say three equi-spaced) tabs 223A"' could be modified to hold the stop ring 271"' axially fast instead of holding the rings 222"' in place.

In a further alternative embodiment of clutch 210"' the spring 275"' could be deleted and spring 274"' could be modified to additionally fulfil the function of spring 275"' and act as further stop means..

Alternatively the cover 221"' could be modified to provide tabs similar to tabs 221 A of clutch 210' which would also fulfil the function of spring 275"' and act as further stop means.

A further alternative embodiment of clutch 210"' the spring 275"' could be deleted and the stop ring 271"' could be replaced by stop ring 271 (which includes tabs 276 which would then act as further stop means and therefore fulfil the function of spring 275"' It should be noted that the total axial bias load created by the springs 275, or 275"' or spring 274"' when modified to fulfil the function of spring 275"' is relatively low since it only needs to ensure that the relatively light pivot ring 240 does not vibrate away from the annular mass 230. Thus this bias load creates a relatively small friction load between the pivot ring 240 and the annular mass 230 and in particular this friction load does not vary throughout the life of the clutch.

Similarly the springs 274 or 274"' only need to generate relatively light circumferential loads since they only have to rotate the relatively light stop ring 271 or 271"' once an adjustment has taken place and in particular these loads do not vary throughout the life of the clutch.

Thus since the forces resisting rotation of the pivot ring and stop ring are constant throughout the life of the clutch ,(and in particular the increase in torque strap load as the clutch wears does not effect the adjustment of the pivot ring or stop ring), the load the spring bracket 251 has to generate is constant and this leads to more reliable adjustment of the clutch.

It should also be noted that two mating ramp surfaces (such as ramp surfaces 247B and 232B) are not required to axially adjust the components associated with the ramps. Axial adjustment is possible with only one ramped component. For example the tabs 273 of the stop ring 271 could lie in the plane of the annular ring 272 and they could still be made to move up the ramp 232B as required during adjustment. Similarly it is possible to design the pivot ring without any undulations and still have it move up ramps 232B to effect adjustment as required. Alternatively it is possible to design the first portion of the pressure plate without any undulations or ramps but to ensure there are ramps on the pivot ring or any stop ring which cater for adjustment of the effective thickness of the pressure plate.

The invention is not restricted to being used in conjunction with one pawl mechanism 250. In particular the invention can be used with any of the pawl mechanisms described in the applicants co-pending application GB 9518991.6.

Also the invention can be used with other self-adjusting clutches which include a two piece pressure plate in which a first piece moves axially relative to a second part to increase the effective thickness of the pressure plate. However the axial adjustment takes place, the present invention can be used to reduce the friction loads at the second portion of the pressure plate to allow it to adjust at least axially when required.

Claims

1. An automatically adjusting clutch including a pressure plate which is biased axially towards a flywheel by a main clutch engaging spring means to clamp a driven plate between the pressure plate and flywheel to engage the clutch, the pressure plate having a first part adjacent the driven plate and a second part situated in the force bias path between the clutch engaging spring means and the first part when the clutch is engaged, the first part also being limited in its disengagement movement by stop means and the second part moving axially relative to the first part to adjust the effective thickness of the pressure plate, the arrangement being such that, during disengagement of the clutch following a reduction in the distance between the flywheel anrl first part of the oressure plate as a result of wear of the clutch. the effective axial thickness of the pressure plate is increased and subsequently during re-engagement of the clutch the stop means is adjusted to ensure the disengagement movement of the first part of the pressure plate remains substantially constant throughout the life of the driven plate.

2. An automatically adjusting clutch as defined in Claim 1 in which the stop means is supported on an axially moveable component.

3 An automatically adjusting clutch as defined in Claim 2 in which the stop means is supported on the first part of the pressure plate.

4. An automatically adjusting clutch as defined in Claim 1 in which the stop means is supported on an axially fixed component.

5. An automatically adjusting clutch as defined in Claim 4 in which the stop means is supported on the clutch cover.

6. An automatically adjusting clutch as defined in any previous claim in which resilient retraction means assist in moving the first part of the pressure plate away from the flywheel during clutch disengagement and when the first part of the pressure plate reaches the limit of its movement the load of the resilient retraction means is reacted by the stop means to allow the first part of the pressure plate to move relative to the second part.

7. An automatically adjusting clutch as defined in any previous claim in which the disengagement movement of the second part of the pressure plate is limited by further stop means.

8. An automatically adjusting clutch as defined in Claim 7 in which the further stop means is defined by a resilient stop component.

9. An automatically adjusting clutch as defined in Claim 8 in which the resilient stop component is mounted as an axially fixed component.

10. An automatically adjusting clutch as defined in Claim 8 in which the resilient stop component is supported on the stop means.

11. An automatically adjusting clutch as defined in Claim 7 in which with the clutch disengaged and the first part of the pressure plate at the limit of its disengagement movement, the second part of the pressure plate is limited to a relatively small amount of axial movement by the first part of the pressure plate and the further stop means.

12. An automatically adjusting clutch as defined in claims 7 or 11 in which the further stop means is defined by an axially fixed component.

13. An automatically adjusting clutch as defined in claims 7 or 11 or 12 in which the further stop means is supported on the stop means.

14. An automatically adjusting clutch as defined in any previous claim in which abutments on the first part of the pressure plate disengage abutments on the stop means during adjustment of the axial thickness of the pressure plate and the subsequent adjllstment of the stop means is limited by the said abutments on the stop means engaging the said abutments on the second part of the pressure plate.

15. An automatically adjusting clutch as defined in any preceding claim in which the stop means consists of a substantially annular component.

16. An automatically adjusting clutch as defined in Claim 15 when dependent upon claim 14 in which the abutments on the stop means consist of tabs on the periphery of the stop means.

17. An automatically adjusting clutch as defined in claim 16 in which the second part of the pressure plate is annular and the tabs on the periphery of the stop means engage in slots in the second part of the pressure plate.

18. An automatically adjusting clutch substantially as herein before described with reference to and as shown in figures 1 to 5, or 6, or 7 to 8, or 9 of the accompanying drawings.