AUTOMATICALLY ADJUSTING CLUTCHES

Abstract: An automatically adjusting clutch in which a pressure plate (13) is biased axially towards a flywheel (11) by a clutch engaging spring means (12) to clamp a driven plate (15) between the pressure plate and flywheel to engage the clutch. The pressure plate has a first part (30) and a second part (40) which is moveable by adjuster means (50) relative to the first part to increase the effective axial thickness (T) of the pressure plate to compensate for wear of the driven plate. The adjuster means has a circumferentially extending array of adjuster teeth (72) arranged on a component (40) which is rotatable about an axis parallel to the axis of rotation of the clutch and a pawl means (60) which moves relative to and in contact with the array as the pressure plate (13) moves relative to the flywheel. The teeth of the array (72) project in an axial direction of the clutch. The pawl means (60) and array of teeth (72) are arranged so that if the movement of the pressure plate towards the flywheel during clutch engagement exceeds a predetermined distance, indicating a predetermined amount of wear of the driven plate, the pawl means (60) moves sufficiently over the array of teeth (72) to engage behind a new tooth of the array so that, when the clutch is released during a subsequent clutch disengagement, the pawl means (60) moves the second part (40) of the pressure plate relative to the first part (30) to make the wear adjustment.

Description: AUTOMATICALLY ADJUSTING CLUTCHES The present invention relates to automatically adjusting friction clutches, and in particular to friction clutches for use on motor vehicles. Automatically adjusting clutches are known which compensate for wear of the friction surfaces of the clutch so that the force applied by the clutch engaging spring means on the pressure plate when the clutch is engaged is maintained substantially constant over the life of the clutch. It is an object of the present invention to provide an improved form of automatically adjusting clutch which is simple and cheap to produce and which is reliable in operation. Thus, in accordance with a first aspect of the invention there is provided an automatically adjusting clutch in which a pressure plate is biased axially towards a flywheel by a 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 and a second part which is moveable by adjuster means relative to the first part to increase the effective axial thickness of the pressure plate to compensate for wear of the driven plate, the adjuster means having a circumferentially extending array of adjuster teeth arranged on a component which is rotatable about an axis parallel to the axis of rotation of the clutch and a pawl means which moves relative to and in contact with the array as the pressure plate moves relative to the flywheel, the pawl means and array being arranged so that if the movement of the pressure plate towards the flywheel during clutch engagement exceeds a predetermined distance, indicating a predetermined amount of wear of the driven plate, the pawl means moves sufficiently over the array to engage behind a new tooth of the array so that when the clutch is released during a subsequent clutch disengagement the pawl means moves the second part of the pressure plate relative to the first part to make the wear adjustment, the arrangement being such that the teeth of the array project in an axial direction of the clutch. In accordance with a second aspect of the invention there is provided a friction clutch for a motor vehicle comprising a pressure plate which is biased axially towards a counter pressure plate by a clutch engaging spring means to clamp a driven plate between the pressure plate and the counter pressure plate to engage the clutch, the clutch further comprising an adjustment device which automatically compensates for wear of the friction faces of the clutch so that the force applied by the clutch engaging spring means on the pressure plate when the clutch is engaged is maintained substantially constant over the life of the clutch, the adjustment device comprising a pawl means which engages an array of adjuster teeth, wherein the pawl means comprises a first lever section pivotally connected at one end to a component of the clutch which is axially fixed relative to the counter pressure plate and a second lever section pivotally connected to the other end of the first lever section and which carries a pawl tooth for engagement with the array of adjuster teeth. Further aspects of the invention are defined in the dependent claims. Several embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is an axial view of a cover assembly of a friction clutch according to the invention looking away from an associated engine; Figure 2 is a cross sectional view of the clutch cover assembly of Figure 1 taken along the line AA of Figure 1, showing in addition the flywheel and a clutch driven plate; Figure 3 is an exploded developed view of part of the pressure plate of the clutch of Figure 1 taken in the direction of arrow B of Figure 2; Figure 4 is a perspective view showing in isolation the pivot ring and pawl mechanism of the clutch of Figure 1; Figures 5 A to 5D are a series of schematic radial views of the clutch of Figure 1 taken in the direction of arrow C on Figure 2 showing the following: Figure 5 A - the clutch having less than a predetermined amount of wear in the engaged and disengaged positions, Figure 5B - the clutch having a predetermined amount of wear in the engaged position showing engagement of the pawl behind a tooth of the array, Figure 5C - subsequent release movement of the clutch of Figure 5B, Figure 5D - the clutch of Figure 5C at the fully disengaged position showing the pawl means before and after making an adjustment; Figure 6 is a composite view similar that shown in Figures 5 A to 5D showing an alternative pawl mechanism; Figure 7 is a view similar to that of Figure 6 showing a further alternative pawl mechanism; and Figure 8 is a view similar to that of Figure 7 showing a modified version of the pawl mechanism of Figure 7. With reference to Figures 1 to 5 there is illustrated a first embodiment of a friction clutch 10 according to the invention. The clutch 10 includes a flywheel or counter pressure plate 11, a clutch cover assembly 20 and a driven plate 15. In use the flywheel is fixed to the end of a crankshaft (not shown) of an associated internal combustion engine (not shown). The clutch cover assembly 20 comprises a clutch cover 21, a diaphragm spring 12, a pressure plate 13, torque straps 14, three constant lift mechanisms 49 and an adjustment device 50. The clutch cover is fixed rotationally and axially fast to the flywheel 11 by bolts (not shown) and supports the diaphragm spring 12 via two support rings 22 situated one on each axial side of the diaphragm spring in a manner well known in the art. The diaphragm spring biases the pressure plate 13 towards the flywheel 11. The clutch driven plate 15 is situated between the pressure plate 13 and the flywheel 11 and in use is connected to the input shaft of a gear box (not shown). When the clutch is engaged, i.e. when the diaphragm spring 12 biases the pressure plate 13 towards the flywheel 11 to clamp the driven plate 15 between the pressure plate 12 and the flywheel 11, power gan be transmitted between the associated engine and gearbox. The clutch can be disengaged by applying an axial force in the direction of arrow W in Figure 2 to the fingers 12A of the diaphragm spring 12 to move the fingers towards the flywheel 11 in a manner well known in the art. The pressure plate 13 comprises a first part 30 coaxial with a second part in the form of a pivot ring 40. First part 30 is generally annular in shape and has a sigmficant thermal mass and is thus capable of absorbing heat generated by frictional contact with the adjacent friction facing 16 of the driven plate 15 during engagement and disengagement of the clutch 10. The first part 30 is connected to the clutch cover 21 by three tangentially orientated torque straps 14. One end 14A of each strap 14 is attached by a rivet 18 to a corresponding one of three equi-spaced lugs 31 formed on the radially outer periphery of the first part 30. The other end 14B of each strap is fixed to the clutch cover 21 by a further rivet 19. The torque straps 14 ensure that the first part 30 remains concentric with and rotationally fast with the clutch cover 21 but allow axial movement of the first part relative to the clutch cover 21 and the flywheel 11. When the clutch is engaged the straps 14 are stressed such that they produce a bias force tending to move the first part 30 way from the flywheel. This biasing assists in separating the pressure plate 13 from the driven plate 15 when the clutch is disengaged. Each strap 14 may comprise a plurality of superimposed strap members to form a multi-leaf strap as is known in the art. On the axial side of the first part 30 remote from the flywheel 11 there is formed an annular spigot 32. The spigot is castellated to provide an array of nine circumferentially equi-spaced ramp sections 33. Each ramp section 33 extends axially towards the diaphragm spring 12 and has circumferential end faces 33B, 33C and an axial end face which is inclined to provide a ramp surface 33D (see Figure 3). Pivot ring 40, which may be made as a pressing, is annular and in cross section is generally of a "U" shape (see Figures 2 and 4). The pivot ring 40 has a outer section or limb 41 which is parallel with and concentric to the axis of the clutch 10. The annular spigot 32 has three circumferentially equi-spaced regions of increased radial thickness (not shown) which contact the radially inner surface 41 A of the outer limb 41 to ensure th^t the pivot ring 40 remains concentric with the first part 30. Avmore detailed description of the above arrangement for ensuring the concentricity of the pivot ring 40 relative to the first part can be found in the Applicant's co-pending International patent application PCT/GB99/ 04220 to which the reader is referred. Pivot ring 40 also has an annular ridge forming a pivot section 45 which is contacted by a radially outer portion 12B of the diaphragm spring. As the clutch is engaged and disengaged the spring 12 pivots about the pivot section 45. The effective axial thickness T of the pressure plate 13 is determined by the axial distance between the pivot section 45 and the friction face 30A of the first part of the pressure plate 30. Pivot ring 40 also has a radially inner section or limb 46 having on an axial end face thereof a circumferentially arranged annular array of nine equi-spaced undulations 47 (see Figures 3 & 4) which face and contact the ramp surfaces 33D of first part 30 (see Fig. 3). Each undulation 47 comprises a flat section 47A, a relatively long ramp section 47B of a relatively shallow gradient, and a short ramp section 47C of relatively steep gradient which joins the ramp section 47B of one undulation 47 with the flat section 47 A of an adjacent undulation 47. When the clutch is assembled with new, unworn components, the short ramp sections 47C of undulations 47 abut or are in close proximity to the circumferential end faces 33C of corresponding ramp sections 33 such that the ramp surfaces 47B contact ramp surfaces 33D. It will be apparent that rotation of the pivot ring 40 relative to the first part 30 in the direction of arrow F of Figure 3 will cause ramp sections 47B to slide across ramp surfaces 33D thus increasing the effective axial thickness T of the pressure plate 13. The arrangement is such that there is sufficient overlap between the ramp sections 47B and the ramp surfaces 33D to ensure that adjustment can be effected throughout the design life of the clutch. When ramp surfaces 33D are in contact with ramp sections 47B and the clutch is engaged, the clamp load path of the diaphragm spring passes from ramp section 47B to ramp surface 33D. This clamp load tends to rotate the pivot ring 40 relative to the first part 30 in the opposite direction to arrow F and so to reduce the effective axial thickness T of the pressure plate 13. This potential reduction in pressure plate thickness is resisted by friction between ramp section 47B and ramp surface 33D, and between the-pivot 45 and diaphragm spring 12. It can be advantageous to manufacture each ramp surface 33D and ramp section 47B with serrations (not shown) which are preferably radial so as to prevent back rotation of the pivot ring 40 once the clutch is engaged. Similarly serrations could be formed on the pivot ring pivot 45 and the contacting surface of the belleville spring. It is preferable that the circumferential angle between adjacent serrations as measured at the centre of the clutch is equal to or less than the angle through which the pivot ring rotates as a result of one adjustment though this need not be the case. In a further modification which is also not shown an independent ratchet mechanism could be used to prevent back rotation of the pivot ring. The constant lift mechanisms 49 operate to limit movement of the first part 30 of the pressure plate away from the flywheel 11 during clutch disengagement to a substantially constant value over the life of the clutch in a manner known in the art. The constant lift mechanisms can be of any suitable type but it is preferred that the mechanisms be of the type described in the Applicant's co-pending patent application WO 00/11365. The constant lift mechanisms 49 will not be described in any further detail in this application but the reader is referred to the above referenced co-pending application for details of the constant lift mechanism described therein. The subject matter of the co-pending application in relation to the constant lift mechanisms is incorporated into the present application by reference and may be claimed. Rotation of the pivot ring 40 relative to the first part 30 in response to wear of the friction facings of the clutch is carried out by the adjustment device 50 which comprises a pawl mechanism 60 and an array of adjuster teeth 72. The array of adjuster teeth 72 are formed on a part annular component 71 which is attached by rivets 70 or any other suitable means to the inner surface 41 A of the radially outer section 41 of the pivot ring (see Figure 4). The adjuster teeth 72 are straight cut and are formed on an axial end face of the component 71 so as to project axially in the direction of arrow X of Figure 4 towards the flywheel 11. The array of adjuster teeth 72 extends circumferentially about the pivot ring 40 by an amount which ensures that adjustment can be provided over the design life of the clutch 10 and in particular the life of the friction facings 16 of the clutch driven plate 15. The pawl mechanism 60 (see Figures 2 & 4) comprises a journal 61 attached to the clutch cover 21 by rivets 62 or by any other suitable means such as screws. The journal 61 carries a shaft 63 which extends inwardly from the journal and has a head 63 A at its radially inward end. The shaft 63 is held in the journal 61 by a circlip 64 which engages in a groove formed about the surface of the shaft towards the end of the shaft opposite from the head 63A. One end of an elongate plate 65 is attached to the shaft 63 adjacent to the head 63 A so as to be rotationally fixed with the shaft. The elongate plate 65 forms a first lever section of the pawl which can pivot relative to the cover by means of the shaft 63 rotating in the journal 61. The plate 65 may be secured to the shaft by any suitable means such as welding. Alternatively, or in addition to welding, the plate 65 may have a through hole of non-circular shape which engages with a section of the shaft 63 having a correspondingly shaped outer surface. The elongate plate 65 carries at its other end a further shaft 66 which projects from the plate 65 radially inwardly of the clutch. The further shaft 66 has a head 66 A which abuts the radially outer side of the plate 65. As with shaft 63, the further shaft 66 is rotationally fixed relative to the plate 65. Pivotally mounted on the further shaft 66 is a slide 67 held in place by a circlip 66B. The slide 67 is an elongate member having a generally rectangular shape when viewed in the axial direction of the clutch as can be seen from Figure 4. A region of increased thickness is provided at a first end 67A to accommodate a through bore 67B to receive the further shaft 66. A pawl tooth 68 is provided on the slide 67 towards a second end 67E of the slide opposite from the first end 67 A. The slide forms a second lever section of the pawl. A coil spring 69 is arranged about the first shaft 63 between the journal 61 and the plate 65. A first end 69 A of the coil spring 69 is formed into a tang which engages part of the journal 61. The other end 69B of the spring 69 is formed into a generally U shaped tang which passes over a recess 65 A in an edge of the plate 65 and engages with a flat 67C formed on the surface of the slide 67 axially remote from the pawl tooth 68. TTie spring 69 acts on the slide 67 so as to pivot the plate 65 and the shaft 63 in a clockwise direction, as viewed in Figure 4, relalive to the journal 61. This biases axial surface 67D of the slide into contact with the end face 4 IB of the pivot ring, as shown in Figure 2, so that during normal non-adjusting operation of the clutch the slide 67 follows the axial movement of the end face 4 IB. An axial stop 80 (see Figure 2) is provided on the clutch cover 21 and is arranged so that it is in alignment with the axial end face 4 IB of the pivot ring 40 when the clutch is fully disengaged. The axial surface 67D of slide 67 contacts the axial stop 80 when the pressure plate 13 reaches the normal fully disengaged position with axial end face 4 IB in axial alignment with the stop 80. This ensures that the axial forces of the pawl can be taken through the stop 80 by the clutch cover 21 relieving the array 72 and the pivot ring 40 of these axial forces. This also serves to limit the movement of the pawl during clutch disengagement, ensuring that the spring 69 retains a minimum pre-load bias even in the event that the pivot ring 40 moves beyond its normal fully disengaged position due to an excessive movement of the clutch actuation system during normal disengagement. As can be seen best from Figure 5B, the second end 69B of the spring 69 acts on the first end 67 A of the slide 67 at a distance X from the centre 69E of the coil spring 69 which is less than the radial distance Y from the centre 69E of the spring 69 to the centre of the shaft 66 on which the slide 67 is mounted. This creates a turning moment Z tending to rotate the second end 67E of slide 67 around shaft 66 in a clockwise direction (as viewed in Figure 5) biasing the pawl tooth 68 into engagement with the array of adjuster teeth 72. Because the spring 69 has a pre-load bias force acting on the slide 67 throughout the entire range of movement of the pressure plate during clutch engagement and disengagement, there will always be a turning moment Z acting on the slide 67 to ensure that the pawl tooth 68 remains in contact with the array of adjusting teeth 72. It should be noted that turning moment Z acting on the slide 67 is small compared to the force of the spring 69 biasing the axial surface 67D of the slide 67 into contact with the axial end face 4 IB and/or the axial stop 80. Operation of the adjustment device will now be described with reference in particular to Figures 5 A to 5D. All references in the following description to the direction of rotation of the components of the pawl are taken with reference to the pawl mechanism as shown in Figures 5A to 5D. Figure 5 A shows the adjustment device in a clutch which has undergone no wear or only a limited amount of wear which is less than a pre-determined amount of wear required to initiate adjustment. The position of the pawl mechanism 60 and array of adjuster teeth 72 when the clutch is disengaged is shown in full lines whilst their position when the clutch is engaged is shown in dashed lines. With the clutch disengaged surface 67D of the slide 67 is in contact with the end face 4 IB and the axial stop 80, and the pawl tooth 68 is lying on face 72i of one of the teeth 72a of the array 72. As the clutch moves to the engaged position, movement of the end face 4 IB axially towards the flywheel 11 pivots the plate 65 and the shaft 63 clockwise relative to the journal 61 against the bias of the spring 69. This movement causes the slide 67 to slide to the left along the end face 4 IB. The leftward movement of the slide relative to the array 72 has the effect of drawing the pawl tooth 68 along the surface 72i towards the edge 72ii of the tooth 72a. As the pawl tooth 68 is drawn towards the edge 72ϋ, the second end 67E of the slide 67 is rotated anti-clockwise about the shaft 66 against the turning moment Z generated by the end 69B of spring 69. In this case there is insufficient wear to cause the pawl tooth to trip over the edge 72ii of pawl tooth 72a. When the clutch is subsequently released on the next disengagement, the slide 67 follows the axial movement of the end face 4 IB away from the flywheel 11 and slides back to the right along the end face 4 IB as the plate 65 and the shaft 63 rotate anti-clockwise under the bias of the spring 69. As the slide 67 moves back along the axial end face 4 IB to the right, the pawl tooth 68 moves back down the face 72i of the tooth 72a to the position shown in full lines in Figure 7 A when the clutch is fully released. In this position the axial surface 67D of the slide 67 will be in contact with both the end face 4 IB and the axial stop 80 and the axial loading of the pawl will be taken by the axial stop 80. The operation of the adjustment device in a clutch which has undergone a pre-determined amount of wear sufficient to cause an adjustment will now be described. Figure 5B shows the adjustment device when the clutch is in the engaged position. Movement of the pressure plate towards the flywheel 11 has caused the slide 67 to move closer to the flywheel 11 rotating the plate 65 and the shaft 63 clockwise against the bias of the spring 69. As described previously this results in the slide 67 sliding along the axial end face 4 IB to the left drawing the pawl tooth along the surface 72i towards the edge 72ii of the tooth 72a. The position of the slide 67 as the pawl tooth 68 reaches the tip 72ϋ is shown in dotted lines. Because wear has now taken place on the friction faces of the clutch, and in particular on the friction facings 16 of the driven plate 15, the pressure plate 13 has to move further towards the flywheel 11 by an amount which is equal to the amount of wear in order to engage the clutch. This increased axial movement of the pressure plate results in a corresponding increase in the movement of the slide 67 drawing the pawl tooth 68 further up the face 72i of the pawl tooth 72a. Once a predetermined amount of wear has taken place the movement of the pawl tooth 68 is sufficient for it to trip over the edge 72ii of the tooth 72a and to engage behind the tooth 72a in the position shown in full lines in Figures 5B and 5C. The turning moment which the spring 69 imparts to the slide 67 ensures that the pawl tooth 68 drops into full engagement behind the tooth 72a. When the pawl tooth 68 trips over the edge of the currently engaged tooth 72a, it abuts or at least lies close to the face 72iii of the tooth 72a. The increased bias force of the spring 69 which has been produced by the movement of the slide 67 towards the flywheel is reacted at this stage by the axial end face 4 IB of the pivot ring 40 which is in abutment with the axial surface 67D of the slide 67. During the subsequent disengagement of the clutch (see Figure 5C), the pressure plate 13 and the end face 4 IB of the pivot ring 40 will move axially away from the flywheel 11. In the initial stages of disengagement, the pawl spring 69 biases the plate 65 and shaft 63 to rotate anti-clockwise so that the slide 67 starts to move to the right. This brings the pawl tooth 68 firmly into engagement with the face 72iii of the tooth 72a of the array. Since the pivot ring 40 is subject to the clamp load of the diaphragm spring 12, the bias force of the spring 69 is not sufficient to rotate the pivot ring 40 relative to the first part of the pressure plate. As a result the bias force of the spring 69 is reacted through the pawl tooth 68 by the surface 72iϋ of tooth 72a which prevents the shde 67 from moving any further to the right. The plate 65 and shaft 63 are, therefore, only able to rotate anti-clockwise to the extent made possible by the geometry of the pawl during the remaining disengagement movement of the pressure plate 13 and the spring 69 is unable to unwind to the extent normally possihle during a non-adjusting disengagement. As the disengagement progresses, the first end 67 A of the slide 67 is not able to follow the axial movement of the end face 4 IB and an axial gap is produced between the first end 67 A and the end face 4 IB. The turning moment Z imparted to the slide 67 pivots the second end 67E of the slide in a clockwise direction about the shaft 66. This ensures that as the pressure plate 13 moves away from the flywheel 11 the pawl tooth 68 remains in engagement with surface 72ϋi of tooth 72a and the second end 67E of the slide remains in contact with the axial end face 4 IB of the pivot ring. The orientation of the pawl at this stage is shown in dashed lines in Figures 5C and 5D. As the pressure plate 13 approaches the fully released position the constant lift mechamsms 49 operate to stop further movement of the first part 30 of the pressure plate away from the flywheel 11. At this point the second end 67E of the shde will be in contact with the axial stop 80 so that the axial loading of the pawl is removed from the array of adjuster teeth 72 and the pivot ring 40. Further release movement of the diaphragm spring fingers 12a towards the flywheel 11 results in the axial loading of the diaphragm spring 12 being removed from the pivot ring 40 so that a tangential component of the bias force of the pawl spring 69 is able to move the pivot ring 40 relative to the first part 30 of the pressure plate. Adjustment then takes place as the pawl spring 69 rotates the plate 65 and shaft 63 anti-clockwise moving the slide 67 and the pivot ring 40 to the right, i.e. in the direction of arrow F. This movement continues until the first end 67 A of the slide 67 contacts the axial abutment 80 on the cover. This position is shown in full lines in Figure 5D. As discussed above, rotation of the pivot ring 40 relative to the first part 30 of the pressure plate in the direction of arrow F results in an increase in the effective thickness T of the pressure plate. The arrangement is such that on each adjustment the pivot ring 40 is rotated by an amount which causes an increase in the effective axial thickness T of the pressure plate which is substantially equal to the predetermined amount of wear required to initiate the adjustment. In this way it can be seen that the increase in effective thickness of the pressure plate T will compensate for the wear such that following an adjustment, the pivot section 45 of the pivot ring 40 will be substantially in the same axial position relative to the diaphragm spring 12 as it was before the wear had taken place. As will be understood by those skilled in the art, this will ensure that following an adjustment the cone angle of the diaphragm spring when the clutch is engaged will be substantially the same as it was before the wear had occurred. Following the wear adjustment as described above the adjustment device will take up the position shown in Figure 5A but with the pawl tooth 68 now lying on the surface 72i of the tooth 72b adjacent to the tooth 72a. When a further predetermined amount of wear has occurred the adjustment cycle will be repeated with the pawl tooth 68 engaging behind tooth 72b of the array and so on over the life of the clutch. If the increments between adjustment are suitably selected the effective thickness T of the pressure plate 13 and so the cone angle of the diaphragm spring 12 can be maintained substantially constant over the life of the clutch. If the cone angle is maintained substantially constant, then the pedal effort required by an operator to disengage the clutch is also maintained substantially constant. This is a significant advantage for motor vehicle manufacturers who wish to provide vehicles in which the pedal effort required of a driver to operate the clutch does not vary significantly as the clutch wears. In the above described construction, the pawl spring 69 functions not only to provide the bias force required to carry out an adjustment, but also to bias the pawl tooth 68 axially into engagement with the array of adjuster teeth 72. It will be understood that these functions could be performed by separate bias means if required. As has been described above, in the clutch 10 the first part 30 of the pressure plate 13 is connected to the clutch cover 21 by three torque straps 14 which permit the pressure plate to move axially relative to the clutch cover 21 and flywheel 11. It will be understood by those skilled in the art that as the pressure plate 13 moves towards the flywheel 11, the torque straps will arc causing the pressure plate 13 to rotate by a small amount relative to the * clutch cover 21. The amount of arcing and therefore amount of relative rotation increases as the friction surfaces 16 of the friction disc 15 wear and the first part 30 of the pressure plate moves axially closer to the flywheel 11 by an amount corresponding to the wear in order to fully engage the clutch. The direction of relative rotation between the pressure plate 13 and the clutch cover 21 will depend on the orientation of the torque straps 14. With the straps 14 orientated as shown in Figure 1, the pressure plate 13 will tend to rotate clockwise, as viewed in Figure 1, relative to the clutch cover 21 as the pressure plate 13 moves towards the flywheel 11 during clutch engagement. This relative rotation of the pressure plate 13 is indicated on Figures 5 A to 5D by the dash-dot line J. However, if the orientation of the straps 14 were to be reversed, the direction of relative rotation would also be reversed. Relative rotation between the pressure plate 13 and the clutch cover 21 will have an effect on the operation of the adjustment device 50 tending either to cause an over adjustment or an under adjustment depending on the direction of the relative rotation of the pressure plate 13 compared with the orientation of the array of adjuster teeth 72 and pawl mechanism 60. The effect of the torque strap arcing needs to be taken into consideration when working out the required geometry of the pawl mechanism 60, the array of adjuster teeth 72, and the ramp surfaces 33D, 47B of the first part 30 of the pressure plate and the pivot ring 40 to ensure proper adjustment over the life of the clutch. The arrangement described above in which the adjuster teeth are arranged to extend in an axial direction of the clutch has a number of advantages: i) the pawl loads act on the array and the pivot ring in an axial direction so that there are no radial loads generated by the pawl means which might tend to inhibit adjustment; ii) the pawl loads can be arranged to act axially on the pressure plate in a direction which assists the lift straps move the pressure plate away from the flywheel during clutch engagement; iii) centrifugal forces acting on the pawl means do not tend to make the pawl tooth move out of engagement with the array of adjuster teeth; iv) the axial loading of the pawl means on the pivot ring can be taken through a stop on the cover or some other axially fixed component when the clutch is substantially fully engaged, so that the array and the pivot ring are relieved of these axial loads to make adjustment easier. With reference to Figure 6 there is shown a modification to the clutch 10 in which the pawl mechanism 160 is in the form of a setf sphmg cranked lever. The clutch is otherwise as shown in Figures 1 to 3 except that the array of adjuster teeth 72 is attached the radially outer surface of limb 41 of the pivot ring 40 rather than the radially inner surface 41 A. The pawl 160 is made from a resilient material, such as spring steel, and comprises a first attachment portion 160 A which is aligned parallel to the flywheel 11 and is riveted to the clutch cover 21 by a rivet 19 which is also used to attach one end 14B of a torque strap 14 to the clutch cover. A second attachment portion 160B of the lever 160 extends axially from one end of the first attachment portion 160 A towards the flywheel 11 and is connected by a first resilient bend 160C to a first lever section 160D of the pawl 160. The first lever section 160D extends at an angle to the axis of the clutch away from the flywheel 11 and is connected by a second resilient bend 160E to a second lever section 160F. The second lever section 160F is shorter than the first lever section and extends generally parallel to the flywheel 11. A pawl tooth 168 is provided on the second lever section 160F for engagement with the array of adjuster teeth 72. Between the first attachment portion 160 A and the rivet 19 is a right angled backing piece 190 which supports the second attachment portion 160B to ensure that the first lever section 160D pivots relative to the clutch cover 21 at the bend 160C. The bend 160E between the first and second lever sections 160D, 160F sits on the axial end face 4 IB of the pivot ring during engagement movement of the clutch and contacts the axial abutment 80 on the clutch cover 21 when the clutch approaches full release. There is a built in pre-load in the first lever section 160D which biases the bend 160E into engagement with the axial end face 4 IB or abutment 80. The second lever section 160F also has a pre-load which tends to pivot the second lever section clockwise about the bend 160E so that the pawl tooth 168 is biased into engagement with the array of adjuster teeth 72. The pre-load of the second lever section 160F is designed to be less than that of the first lever section 160D. This can be achieved for example by making the width of the second lever section 160F less than that of the first lever section 160D and adjusting the pre-set position of the second lever section to keep the load down. The second lever section 160F has a further abutment 160G located at the end of the second lever section farthest from the bend 160E near to the pawl tooth 168. This further abutment 160G also contacts the axial end face 4 IB of the pivot ring 40 when the tooth is fully engaged in the array 72. When the clutch is fully disengaged, the further abutment 160G contacts the axial abutment 80 on the clutch cover to remove the axial engagement forces of the pawl from the pivot ring 40 in a manner similar to the contact of the axial surface 67D of the slide 67 with the abutment 80 in the embodiment shown in Figures 1 to 5. Operation of the pawl mechanism 160 is essentially the same as that of pawl mechanism 60 described earlier with the bend 160E sliding along the axial end face 4 IB of pivot ring to the left as viewed in Figure 6 as the clutch is engaged. This pivots the first lever section 160D clockwise (as viewed) relative to the cover 21 increasing the pre-load in the first lever section 160D and drawing the pawl tooth towards the edge of the tooth 72a on which it is currently engaged. When sufficient wear has taken place movement of the pressure plate towards the flywheel will cause the pawl tooth 168 to trip over the edge of the tooth 72a and to engage behind it. During the next disengagement, the bias load of the first lever section 160D is reacted through the pawl tooth 168 by the tooth 72a of the array. This prevents the first lever section 160D from pivoting anticlockwise about first resilient bend 160C so that the second resilient bend 160E is not able to follow the axial surface 4 IB of the pivot ring as the pressure plate 13 moves away from the flywheel 11. This results in an axial gap appearing between the bend 160E of the axial end face 41B of the pivot ring and the axial abutment 80 on the clutch cover as the clutch approaches full disengagement. Following actuation of the constant lift mechanisms, further disengagement movement of the diaphragm spring fingers 12a removes the clamp load of the diaphragm spring 12 from the pivot ring 40, allowing the tangential component of the increased pre-load in the first lever section 160D to rotate the pivot ring 40 relative to the first part 30 of the pressure plate. Rotation of the pivot ring 40 continues until the second resilient bend contacts the axial stop 80 on the clutch cover 21. As described above, the further abutment 160G contacts the axial abutment 80 on the clutch cover as the clutch * approaches full disengagement, this serves to remove all axial loading created by the pawl mechanism 160 from the pivot ring 40 making it easier for the tangential component of the pre-load in the first lever section 160D to rotate the pivot ring. Figure 7 shows a modified version 260 of the pawl mechanism 160. Features which perform the same function as those of the pawl means 160 are given the same reference numeral but increased by 100. Pawl mechanism 260 is similar to pawl mechanism 160 except that a roller 291 is provided at the bend 260E to contact the axial end face 4 IB of the pivot ring and the axial abutment 80 on the clutch cover. The roller 291 can be made of any suitable material such as a plastics material or metal and is mounted on a pin 292 which is attached by any suitable means to the lever 260. The pawl mechanism 260 also differs from pawl mechanism 160 in that the pawl tooth 268 is provided on a pawl tooth member 293 separately from the lever. In the embodiment shown in Figure 7 the pawl tooth member 293 has three pawl teeth 268, however, this is not essential and the pawl tooth member could have more or less teeth as required. The pawl tooth member 293 can be made of any suitable material and could, for example, be made of a plastics material. The pawl tooth member 293 is mounted on a roll pin or rivet 294 which is attached to the second lever section 260F by any suitable means. The roll pin or rivet 294 also carries a further pin 295 which contacts axial end face 4 IB of the pivot ring and which contacts the abutment 80 on the clutch cover when the clutch is fully released in place of the abutment 160G of pawl mechanism 160. Figure 8 shows modified version 360 of the pawl mechanism 260. Features which perform the same function as the features of pawl mechanism 260 are given the same reference numeral but increased by 100. In Figure 10 the circumferential orientation of pawl mechanism 360 and the teeth 72 of the array has been reversed such that the pawl mechanism 360 rotates pivot ring in the opposite direction to arrow F when making an adjustment. Consequently the direction of the co-operating ramp surfaces 33D, 47B on the first part of the pressure plate and the pivot ring respectively must also be reversed to ensure that rotation of the pivot ring relative to the first part, increases the effective thickness T of the pressure plate. With the circumferential orientation of the pawl mechanism 360 as shown in Figure 8, the pivotal movement of the first lever section 360D during clutch engagement is anti-clockwise as viewed and will be in the opposite direction to the rotation of the pressure plate 13 caused by the arcing J of the torque straps 14. This results in an effective increase in the movement of the first lever section 360D relative to the array 72 during clutch engagement and disengagement. As a result the angle of the first lever section 360D relative to the axis of the clutch can be reduced without reducing the resultant adjustment movement of pivot ring. This has the advantage that the pawl exerts less axial force on the pivot ring 40 than is the case with the steeper angled pawl 260 shown in Figure 7. The backing piece 390 in the Figure 8 embodiment has an additional section 390 A to control the point about which the first lever section 360D pivots at the bend 360C.

Claims: CLAIMS 1. An automatically adjusting clutch in which a pressure plate is biased axially towards a flywheel by a 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 and a second part which is moveable by adjuster means relative to the first part to increase the effective axial thickness of the pressure plate to compensate for wear of the driven plate, the adjuster means having a circumferentially extending array of adjuster teeth arranged on a component which is rotatable about an axis parallel to the axis of rotation of the clutch and a pawl means which moves relative to and in contact with the array as the pressure plate moves relative to the flywheel, the pawl means and array being arranged so that if the movement of the pressure plate towards the flywheel during clutch engagement exceeds a predetermined distance, indicating a predetermined amount of wear of the driven plate, the pawl means moves sufficiently over the array to engage behind a new tooth of the array so that when the clutch is released during a subsequent clutch disengagement the pawl means moves the second part of the pressure plate relative to the first part to make the wear adjustment, the arrangement being such that the teeth of the array project in an axial direction of the clutch. 2. A friction clutch as claimed in claim 1 in which the pawl means is biased axially into engagement with the array of adjuster teeth. 3. A friction clutch as claimed in claim 2 in which the axial bias force acts in the direction of movement of the pressure plate during clutch disengagement. 4. A friction clutch as claimed in any one of claims 1 to 3 in which during wear adjustment the second part of the pressure plate is rotated relative the first part of the pressure plate, at least one of the first or second parts having one of more circumferentially inclined ramp surfaces which engage corresponding abutments on the other of the first and second parts such that relative rotation between the first and second parts causes the second part to move axially relative the first part to increase the effective axial thickness of the pressure plate. 5. A friction clutch as claimed in claim 4 in which the corresponding abutments on the other of the first and second parts of the pressure plate comprise complementary circumferentially inclined ramp surfaces. 6. A fiiction clutch as claimed in any one of claims 4 to 6 in which the array of adjuster teeth are provided on the second part of the pressure plate. 7. A friction clutch as claimed in any previous claim in which the pawl means comprises lever means pivotally connected to a component which is axially fixed relative to the flywheel, the lever means carrying at least one pawl tooth for engagement with the array of adjuster teeth. 8. A friction clutch as claimed in claim 7 in which movement of the array towards the flywheel during clutch engagement pivots the lever means relative to the axially fixed component against the action of a pawl bias means so as to generate a potential adjustment force, the arrangement being such that following engagement of the pawl behind a new tooth of the array, the adjustment force acting through the pawl tooth is reacted by the engaged new tooth of the array such that when the clutch is substantially fully released during a subsequent clutch disengagement, the second part of the pressure plate is moved relative to the first part by the adjustment force acting on the engaged new tooth of the array to make the wear adjustment. 9. A friction clutch as claimed in claim 8 in which the adjustment force acts on the array to rotate the second part of the pressure plate relative to the first part of the pressure plate. 10. A friction clutch as claimed in claim 8 or claim 9 in which the pawl bias means in addition to providing the adjustment force also acts to bias the pawl means axially into engagement with the array of adjuster teeth. 11. A friction clutch as claimed in any one of claims 7 to 10 in which the lever means comprises a first lever section pivotally connected at one end thereof to the axially fixed component, the other end being pivotally connected to a first end of a second lever section, the at least one pawl tooth being provided on the second lever section. 12. A friction clutch as claimed in claim 11 in which the pawl bias means biases the first lever section to pivot about the pivotal connection with the axially fixed component in a first direction of rotation, the pivotal movement of the first lever means being controlled by engagement of the lever with a component that moves axially with the pressure plate during clutch engagement and disengagement and by contact of the lever with an axially fixed stop when the clutch is substantially fully disengaged. 13. A friction clutch as claimed in claim 12 in which the pawl bias means acts on the second lever section to pivot the second lever section about its pivotal connection with the first lever section in a second direction of rotation opposite to the first. 14. A friction clutch as claimed in any one of claims 11 to 13 in which the pawl bias means acts on the second lever section at point remote from the axis of rotation of the second lever section relative to the first lever section so as to generate a turning moment tending to rotate the second lever section about the axis so as to bias the pawl tooth into engagement with the array. 15. A friction clutch as claimed in any one of claims 7 to 14 in which the first lever section comprises an elongate plate one end of which is pivotally connected to the axially fixed component by a first shaft, the second lever section comprising an elongate slide member pivotally connected to the other end of the elongate plate by a second shaft. 16. A friction clutch as claimed in any one of claims 7 to 12 in which the pawl means comprises a self-sprung cranked lever made from a resilient material. 17. A friction clutch as claimed in claim 16 in which the self-sprung cranked lever comprises an attachment portion attached to the axially fixed component, a first lever section connected to the attachment portion by a first resilient bend and a second lever section connected to the first lever section by a second resilient bend. 18. A fiiction clutch for a motor vehicle comprising a pressure plate which is biased axially towards a counter pressure plate by a clutch engaging spring means to clamp a driven plate between the pressure plate and the counter pressure plate to engage the clutch, the clutch further comprising an adjustment device which automatically compensates for wear of the friction faces of the clutch so that the force applied by the clutch engaging spring means on the pressure plate when the clutch is engaged is maintained substantially constant over the life of the clutch, the adjustment device comprising a pawl means which engages an array of adjuster teeth, wherein the pawl means comprising a first lever section pivotally connected at one end to a component of the clutch which is axially fixed relative to the counter pressure plate and a second lever section pivotally connected to the other end of the first lever section and which carries a pawl tooth for engagement with the array of adjuster teeth. 19. A friction clutch as claimed in any previous claim in which the pawl means contacts a stop on an axially fixed component of the clutch when the clutch is substantially fully disengaged so that at least some of the axial forces generated by the pawl means are taken by the stop rather than the array of adjuster teeth. 20. A friction clutch as claimed in claim 18 in which substantially all of the axial forces generated by the pawl means are taken by the stop when the clutch is substantially fully disengaged. 21. An automatically adjusting clutch as hereinbefore described with reference to and as shown in Figures 1 to 5, or Figure 6, or Figure 7 or Figure 8 of the accompanying drawings.

Inventors: YOUNG ALASTAIR JOHN, YOUNG, ALASTAIR, JOHN
Applicants: AUTOMOTIVE PRODUCTS UK LTD, YOUNG ALASTAIR JOHN, AUTOMOTIVE PRODUCTS UK LIMITED, YOUNG, ALASTAIR, JOHN
Application year: 2001
Main classification: F16D13
IPC codes: F16D13