GB2501604A

GB2501604A - Clutch assembly having a replaceable insert of friction material

Info

Publication number: GB2501604A
Application number: GB1304530.7A
Authority: GB
Inventors: Dennis J Redmond
Original assignee: Smart Manufacturing Technology Ltd
Current assignee: Smart Manufacturing Technology Ltd
Priority date: 2012-03-15
Filing date: 2013-03-13
Publication date: 2013-10-30
Anticipated expiration: 2033-03-13
Also published as: CN103322079A; GB2501604B; GB201304530D0; GB201204604D0

Abstract

A clutch assembly having a pressure plate (14, fig 2), a clutch plate(16, fig 2) and a flywheel (12, fig 2). The pressure plate is arranged for clamping the clutch plate against the flywheel. At least one of said pressure plate, clutch plate and flywheel comprises friction material, and the friction material comprises a replaceable insert 26, having friction material 30 on a backing plate 32. The clutch assembly may comprise a plain rotor. The clutch assembly may also have a jet tube (70, fig 2) in communication with a compressed air source to direct air at the pressure plate, clutch plate and/or flywheel.

Description

Clutch assembly The present invention relates to a clutch assembly, more particularly, but not exclusively, a clutch assembly for use in automotive applications, such as between the engine and gearbox of a bus. The present invention further relates to a kit for a clutch assembly.

A conventional vehide clutch assemNy has a pressure plate acted upon by a powerful diaphragm spring, in order to clamp a clutch plate against an engine flywheel and thereby transmit engine torque to a gearbox input shaft. In conventional clutches for buses, the clutch plate includes a friction facing (e.g. provided by a friction pad attached to the clutch plate), whereas the engine flywheel and the pressure plate have a plain facing (i.e. of non-friction material).

A clutch cover is mechanically fixed for rotation with the flywheel and provides a housing for the clutch plate and pressure plate. The pressure plate is connected to the clutch cover, via straps. for rotation with the clutch cover. The straps allow the pressure plate to move axially relative to the clutch cover, e.g. for engaging and disengaging the clutch plate.

Upon selection of a gear to take up the drive to move the vehicle, a clutch contr& mechanism is used to gradually increase the force acting on the pressure plate via the diaphragm spnng, and thereby begin to clamp the clutch plate friction surfaces between the plain faces of the flywheel and the pressure plate. The resulting friction causes the clutch plate to begin to rotate. As the clamp load increases, the clutch plate becomes fully clamped and engaged.

The flywheel, clutch plate and pressure plate then rotate as one with no speed differential between them.

However, during the transition period between the start of dutch engagement (when the flywheel and pressure plate are rotating at high rpm) and the completion of engagement, the clutch will slip', that is to say that the clutch plate will be rotating at a different (usually slower) speed to the flywheel and the pressure plate. The slipping of the clutch causes significant amounts of heat to be generated at the interfaces between the friction facings of the clutch plate and the plain surfaces of the flywheel and pressure plate. If the driver has selected too high a pull-away gear, or is "holding" the vehicle on an incline by slipping the

I

clutch, the amount of heat generated can be very large. Excess heat can result in subsequent "burning" and accelerated wear of the friction facings. Eventually a clutch may start to slip constantly, to the point where it can no longer transmit torque and is said to be "burnt out".

The friction material is worn out preferentially to the pressure plate and flywheel. If the friction material is not replaced soon enough, or if the clutch is abused, the pressure plate and flywheel may also become unusable and need replacement.

When friction material is too worn it must be replaced. Where friction material is provided on a clutch plate, it is usual to replace the whole clutch plate. This is a difficult, time-i0 consuming and costly procedure due to the arrangement of a standard clutch, as follows. The engine crankshaft has a flange by which it is connected to the flywheel, and this flange supports the clutch assembly. A typical installation will have a bell-housing which is connected to the gearbox housing and is bolted to the engine block to torsionally ground the gearbox housing. The bell-housing normally completely encloses the clutch assembly iS between the end faces of the gearbox housing and the engine block. The bell-housing is only pierced by an actuation fork of a clutch release bearing. lii order to replace the clutch plate, therefore, removal of the gearbox and bell-housing is required, so that the clutch cover and the pressure plate can be removed and access gained to the clutch plate.

Wear of frictional facing on clutch plates is a particular problem for vehicles that are subjected to frequent gear-changes, such as buses used in an urban environment. Buses are required to frequently stop and start, and often travel slowly and in heavy traffic.

There is a need for an improved clutch assembly. particularly. but not exclusively, for use in automotive vehicles such as buses and coaches and the like.

According to one aspect of the present invenion there is provided a clutch assembly as set forth in claim I. In exemplary embodiments, the friction material may be replaceable without the removal of a clutch cover from the flywheel and a bell-housing from an engine block.

The fnction material may be a replaceable insert. The clutch cover may define at least one aperture configured to provide access for replacing said insert.

The replaceable insert may have a wide end and a narrow end for fitting into a complimentarily shaped recess on the pressure plate andlor flywheel.

The inseit may have opposing sides angled with respect to one another.

The opposing sides may be at an angle of less than 500 to one another. The sides may be at an angie of less than 400 to one another, or at an angle of tess than 330 to one another.

lO There is further provided a kit for a clutch assembly as set forth in claim 28.

There is yet further provided a clutch assembly as set forth in dairn 33.

Other aspects and preferred features of the invention will be readily apparent from the claims iS and following description of preferred embodiments made, by way of example only, with reference to the following drawings, in which: Figure i is a plan view of a clutch assembly according to an embodiment of the invention; Figure 2 is a cross-sectional view A-A of the clutch assembly of Figure 1 in a disengaged position; Figure 3 is a cross-sectional view A-A of the clutch assembly of Figures I and 2 in an engaged position; Figure 4 is a plan view of a friction pad for use in the clutch assembly of Figures ito 3; Figure 5 is a cross-sectional view C-C of the friction pad of Figure 4; Figure 6 is a cross-sectional view of the clutch assembly of Figures I to 3; and Figure 7 is a cross-sectional view B-B of the clutch assembly of Figures 1 to 4.

With reference to Figures 1 to 3, a clutch assembly is indicated generally at 10. The clutch assembly 10 of this embodiment is suitable for use in a vehicle such as a bus, but may be applicable for other types of automotive vehicle.

The clutch assemhly 10 includes an engine flywheel 12 and a pressure p'ate 14. A clutch plate 16 is supported by a gearbox input shaft 18 and is alTanged between the flywheel 12 and pressure plate 14. As will be described in more detail below, the pressure plate 14 is acted upon by a powerful diaphragm spring 15, in order to clamp the clutch plate 16 against the engine flywheel 12 and thereby transmit engine torque to the gearbox input shaft 18.

The flywheel 12 is mounted on a crankshaft flange (not shown) with bolts through bolt apertures 13 for rotation with an engine crankshaft (not shown). A crankshaft spigot bearing 19 supports an end of the shaft 18 in the flywheel 12. The shaft 18 supports a clutch release bearing 20. The bearing 19 enables rotation of the shaft 18 relative to the flywheel 12 and the iS pressure plate 14, for example when the clutch is disengaged. The clutch release bearing 20 allows rotation of the flywheel 12, the pressure plate 14 and the clutch cover 22 relative to a clutch rdease fork (not shown). In an alternative embodiment, the crankshaft spigot bearing 19 may be housed in the crankshaft flange rather than in the flywheel 12.

A clutch cover 22 is connected to the flywheel 12 and provides an enclosure for the pressure plate 14, diaphragm spring 15 and clutch plate 16. The clutch cover 22 is supported by the flywheel 12. The pressure plate 14 is connected to the cover 22 by drive straps (not shown) that allow axial movement of the pressure plate 14 relative to the cover 22, e.g. for engagement and disengagement of the clutch p'ate 16. In this embodiment the cover 22 is secured to the flywheel 12 with bolts (not shown) through bolt apertures 24. In alternative embodiments other means of securement may be used.

A bell-housing (not shown) encloses the clutch assembly. The bell-housing defines an access aperture that provides access to the clutch assembly, and a readily removable cover plate used to selectively close the access aperture.

The clutch assembly 10 is moveable between a disengaged position, shown in Figure 2, where torque is not transmitted between the flywheel 12 and the shaft 18, and an engaged position, shown in Figure 3. where torque is transmitted between the flywheel 12 and the shaft 18. In the engaged position, the pressure plate 14, actuated by a clutch control mechanism (not shown), clamps the clutch plate 16 against the flywheel 12. The clutch plate 16, and with it the shaft 18, are thus rotated with the flywheel 12 and the pressure plate 14.

In contrast to standard clutch arrangements, contact between the clutch plate 16 and the pressure plate 14 and flywheel 12 is through friction material attached to the pressure plate 14 and flywheel 12 rather than to the clutch plate 16.

Friction matenal in this application refers to increased-fnction material commonly used in clutches to create friction between components (e.g. organic compound resin, ceramic or the like), rather than plain metal.

The clutch plate 16 is a plain rotor with no friction material attached. The rotor of this embodiment is of steel, though other suitable materials may be used in alternative iS embodiments. The rotor 16 is cross-drilled with a number of apertures 17. The apertures 17 reduce the weight of the rotor 16 and increase cooling of the rotor 16.

The clutch assembly 10 includes a series of replaceable friction pads 26 arranged in the pressure plate 14 and flywheel 12 to provide frictional contact with either axial face of the clutch plate 16 when the clutch assembly is in the engaged position.

Each friction pad 26 (shown in Figures 4 and 5) consists of a friction lining material 30 supported on a backing plate 32. In this embodiment, rivets 34 are used to secure the friction lining material 30 to the backing plate 32. In alternative embodiments, other forms of securement may be used, such as other types of fastener or a suitable adhesive.

The backing plates 32 are in this embodiment dovetail-shaped, with an arced narrow end 36 and an arced wide end 38 parallel to the narrow end 36.

Each backing plate 32 has two sides 40 angled with respect to one another. As can be seen in Figure 4. the sides 40 are at an angle x to one another. The angle x may be in the range of substantially parallel (i.e. approximately 10) to radial (i.e. if extended the sides 40 would meet at a central longitudinal axis of the shaft 18). In exemplary embodiments, the sides 40 are at an angle of less than 500, less than 40° or less than 330 to one another. The backing plate 32 is chamfered at the sides 40.

in this embodiment, the friction lining material 30 corresponds to the shape of the backing plate 32, so is substantially parallel to the sides 40. However, in alternative embodiments, the sides of the friction lining material 30 are not parallel to the sides of the pad 26, and may be adjusted to optimise forces acting on the pad 26 during engagement. In this embodiment, the friction lining material 30 is substantially equidistant from both ends 36, 38. In aliernative embodiments, the friction lining material 30 may be closer to one or other of the ends 36, 38, lO in order to alter the direction of the moment acting on the pad 26 during engagement.

In alternative embodiments the backing plate 32 may include a radial protrusion at the wide end 38 for ease of purchase. The protrusion may define an aperture, also for ease of purchase by a tool or by hand. iS

The flywheel 12 has a clamping face l2a (shown in Figure 6) configured to oppose the clutch plate 16. The clamping face 12a defines a series of dovetail-shaped recesses 42 radially spaced about the clamping face 12a. Each recess 42 has an arced narrow end 44 and an arced wide end 46, and two sides 48 angled with respect to one another. The recesses 42, of which there are four in this embodiment, are positioned with the narrow ends 44 proximal the centre of the clamping face 12a. The wide ends 46 extend to a peripheral edge 50 of the clamping face 12a to create openings 47 in the perimeter of the flywheel i2. The openings 47 allow access to the recesses 42, and also advantageously provide a heat outlet for the clutch plate 16.

The pressure plate 14 has a clamping face (not shown) that also defines four recesses 42 extending to a peripheral edge (not shown). The recesses 42 of the pressure plate 14 oppose those of the flywheel 12.

Each recess 42 is configured to receive a friction pad 26, as shown in Figure 6. The recesses 42 are shaped to correspond to the shape of the backing plates 32. The sides 48 of each recess 42 are chamfered as shown in Figure 7 to receive the corresponding chamfered sides of a backing plate 32. The sides 48 are also at an angle of less than 50°, less than 40° or less than 300 to one another, depending on the angle x between the backing plate sides 40.

Each friction pad 26 can be slid radially inwards into its respective recess 42 from the peripheral edge 50 via the openings 47. The angle of the backing plate sides 40 and the colTesponding recess sides 48 being, for example. less than 500, allows the friction pad 26 to be easily slid into the recess 42, yet is slight enough to advantageously reduce radial forces acting to force the backing plate 32 from the recess 42, as the radial component of the force is kept to a minimum by the relatively slight angle.

The chamfered sides 40 of the backing plates 32 retain the backing plates 32 in an axial direction, ensuring that the pads 26 are held against the respective clamping faces 12a. The pads 26 being secured against the clamping faces ensures positive disengagement of the pads 26, allows good contact between the pads 26 and the clamping faces 12a, improving heat dissipation, and inhibits "rattle" of the pads 26 within the recesses 42. A heat outlet for the friction pads 26 is provided by the openings 47.

in alternative embodiments the recesses 42 and the friction pads 26 may be some shape other than dovetail-shaped. The shape of the recesses 42 may be adjusted to minimise stress concentration. The sides 48 of the recesses 42 may have a fillet radius, and the pads 26 a colTesponding chamfer or fillet radius to inhibit interference and minimise stress concentration.

The friction pads 26 of the flywheel 12 form opposing pairs with the friction pads 26 of the pressure plate 14, as the recesses 42 of those components oppose one another, and the flywheel 12 and pressure plate 14 rotate together at the same speed. The clutch assembly 10 includes a retainer 52 for each pair of friction pads 26. Each retainer 52 is configured to fit over the wide ends 38 of a pair of friction pads 26 at the peripheral edges 50. The retainers 52, shown in Figures 6 and 7, comprise a body 53 curved to correspond to the curve of the peripheral edges 50 and the wide ends 38 of the friction pads 26. A first end 56 of the body 53 is hooked, and a second end 58 defines a bolt aperture 60. The edge 50 of the flywheel 12 comprises a hook recess 62 corresponding to and configured to receive the hooked end 56, and a bolt aperture 64 configured to align with the bolt aperture 60. The retainers 52 are held in place by the hooked end 56 and recess 62, and secured by a bolt (not shown) extending through the bolt apertures 60, 64. In this embodiment cap head bolts with pre-applied micro-encapsulated sealant thread locking compound are used for the retainers 52. These bolts cannot be reused. In alternative embodiments other bolts or fasteners may be used.

Each retainer 52 comprises a resilient device in the form of a leaf spring 54 having a series of spring clips 57. In other embodiments, other types of resilient device or other types of spring may be used. The body 53 defines a slot 55 configured to receive a spring 54 and to locate that spnng 54 in an axial direction. The spring clips 57 secure the spring 54 in the slot 55, whilst allowing easy fitting of the spring 54.

lO The springs 54 pre-load their respective pairs of friction pads 26, and are in turn loaded by the retainers 52. The load from the springs 54 wedges the friction pads 26 into their respective recesses 42 to inhibit "rattle" and unwanted movement of the friction pads 26. In addition, the wedging effect ensures the pads 26 are pulled from the clutch plate 16 during disengagement, and good thermal contact is provided between the pads 26 and the clamping iS faces l2a. In this embodiment the springs 54 act on the backing plates 32 of the friction pads 26.

The clutch assembly lO is arranged so that the friction pads 26 face the clutch plate 16, as shown in Figures 2 and 3. When the clutch assembly 10 is in the disengaged position of Figure 2, the friction pads 26 are not in contact with the clutch plate 16. When the clutch assembly 10 is moved to the engaged position of Figure 3, the friction pads 26 are moved into clamping contact with the clutch plate 16, such that torque is transferred from the flywheel 12 to the friction pads 26. The recesses 42 transmit the full driving torque to the friction pads 26. Torque is transferred from the friction pads 26 to the clutch plate 16, and so to the shaft 18. In this way torque transfer is achieved without the use of friction material on the clutch plate 16.

Upon disengagement of the clutch the friction pads 26 held in the pressure plate 14 will retract from contact with the clutch plate 16 as force acting on the pressure plate is removed.

This retraction ensures clean disconnection of drive, and inhibits clutch drag.

The recesses 42 are arranged such that the whole of the friction lining material 30 extends axially beyond the respective clamping face l2a, and such that each backing plate 32 extends beyond the respective clamping face l2a. Some part of each friction pad 26 thus always extends axially outwardly of the pressure plate 14 and flywheel 12, regardless of the state of the friction lining material 32. The likelihood of damage to the flywheel 12 or pressure plate 14 is thus decreased, as they do not act as friction surfaces even if the friction lining material is entirely worn away (although the clutch assembly 10 will cease to function before this point).

Four recesses 42 holding four fnction pads 26 in each of the pressure plate 14 and flywheel 12 are used in this embodiment as there are four straps between the clutch cover 22 and the pressure plate 14 in a standard clutch assembly. In alternative embodiments more or fewer recesses 42 may be provided in the pressure plate 26 and the flywheel 12, so that more or fewer friction pads 26 may be fitted. For example. three pairs of friction pads 26 may be used, or five or more pairs of friction pads 26 may be used to increase the area of friction material available.

Additional cooling of the clutch assembly 10 is provided by compressed air directed into the clutch cover 22. The clutch assembly 10 includes ajet tube 70 sunounding the shaft 18. The jet tube 70 extends along the shaft 18 into the clutch cover 22. The clutch release bearing 20 surrounds the jet tube 70 and is axially movable relative to the jet tube 70. The jet tube 70 has a radially extending inlet 72 through which compressed air is directed into the jet tube 70, and a radial outlet 74 through which compressed air exits the jet tube 70. The outlet 74 is positioned within the clutch cover 22 such that compressed air enters the clutch cover 22 and so provides a powerful flow of cool air that circulates radially outwards through the clutch assembly 10.

The compressed air supply may be modulated externally to provide only the cooling required.

The hot air that has been used to cool the clutch cover 22 and its contents exits the bell-housing through a vent (not shown). The access aperture cover plate may be vented, or the bell-housing may be otherwise adapted to allow the exhaust of the hot air.

The jet tube 70 is rigidly attached to the interior of the bell-housing (not shown) around the shaft 18. As the jet tube 70 moves neither axially nor rotationally, it offers a stationary mounting point for the measurement of the axial position of the dutch plate 16 by a laser beam directed at a smooth, continuous area on a centre hub of the clutch plate 16. After self-calibration upon installation of a new clutch or replacement of the friction pads 26, the position can be continuously electronically monitored, and one or more warning lights can be provided as friction material is reduced and replacement becomes necessary.

As previously stated, the friction pads 26 are replaceable, and are intended to allow easy replacement of friction materiaL The remainder of the dutch assembly 10 is configured to provide easy access to the friction pads 26 so that replacement can take place. Access through the bell-housing is provided by the access aperture (not shown). The clutch cover 22 has a side wall 66 extending over the pressure plate 14 to the fiywhed 12. The side wall 66 defines a series of radially spaced access apertures 68, each aligned with a recess 42 in the lO pressure plate 14 (and thus also aligned with the opposing recess 42 in the flywheel 12).

Through the access aperture in the bell-housing, the apertures 68, and through the openings 47, access to the friction pads 26 may be gained, and the friction pads 26 may be rep'aced as follows.

iS For replacement of friction pads 26 in the clutch assembly 10 of this embodiment, a gearbox of a vehicle need not be removed. Access to the friction pads 26 is achieved through the aperture in the befi-housing and the access apertures 68. A suitable service tool is used to hold the clutch assembly 10 in the fully disengaged position whilst the friction pads 26 are changed. The vehicle engine is then turned until the retainer 52 and its bolt are accessible through the bell-housing aperture. The bolt is undone and the retainer 52 removed, with its spnng 54 and the bolt. The retainer 52 is discarded along with its spring 54 and bolt in order to prevent reuse of the bolt.

Access is now provided to the pair of friction pads 26 previously secured by the retainer 52.

The friction pads 26 are slid out of their respective recesses 42, and are replaced with new friction pads 26. Where in an alternative embodiment the backing plate 32 includes a radial protrusion at the wide end 38 for ease of purchase, and the protrusion defines an aperture, a slide hammer may be used to apply force to that aperture for removal of the friction pads 26 if required. A new retainer 52 is then hooked into place and secured by a retainer bolt.

A replacement friction pad kit is provided for convenient replacement of friction material.

The kit (not shown) comprises a pair of friction pads 26, a retainer 52 with leaf spring 54 attached and a bolt (not shown) held in place in the aperture 64 by a spire clip (not shown).

Providing a bolt with the kit ensures that new bolts are used. It is made clear on kit packaging that the bolts must not be reused.

The replacement procedure is repeated for the remaining three pairs of friction pads 26. The service tool is then removed.

In alternative embodiments the retainer may not be discarded, but may be reused with a new bolt. In embodiments where reusable boils are used, the retainer and bolt may both be reused.

The clutch assembly 10 of this embodiment can be retrofitted to a vehicle having a standard clutch arrangement. The gearbox of said vehicle is removed (as would be done when replacing a standard clutch plate not of this invention). An access aperture is cut into the bell-housing and a cover plate is fitted. Apertures 68 are cut into the standard clutch cover.

Alternatively, the standard clutch cover is replaced with a clutch cover 22 having apertures 68. The standard flywheel, pressure plate and clutch plate are removed and discarded, and are replaced with a flywhed 12 and pressure plate 14 having recesses 42, and a clutch plate 16 having no friction material. The friction pads 26 are then installed as described above, and secured by retainers 52.

The jet tube 70 can be retrofitted to a standard clutch arrangement. The standard clutch release bearing is replaced with a clutch release bearing 20 having a greater internal bore so that the jet tube 70 can be accommodated.

Worn friction material can be easily replaced without the need for replacement of the clutch plate 16, or of the pressure plate 14 or flywheel 12. The clutch assembly 10 can be retrofitted to existing vehicles.

Although described with reference to vehicles, and having particular advantage for use in buses and coaches and the like, the clutch assembly concepts described herein are applicable to other machines requiring a clutch mechanism.

Claims

Claims 1. A clutch assembly having a pressure plate, a clutch plate arid a flywheel, wherein the pressure plate is arranged for clamping the clutch plate against the flywheel, wherein at least one of said pressure plate. clutch plate and flywheel comprises friction material, and wherein the fnction material compnses a replaceable insert.
2. A clutch assembly according to claim I wherein the insert comprises at least one fnction pad having friction lining material and a backing plate. lo
3. A clutch assembly according to claim 2 wherein the backing plate has a wide end and a narrow end for fitting into a compfimentarfly shaped recess on the pressure plate and/or flywheel.iS
4. A clutch assembly according to claim 3 wherein one or both of the wide end and the narrow end is arced.
5. A clutch assembly according to claim 3 or claim 4 wherein the backing plate has opposing sides angled with respect to one another.
6. A clutch assembly according to claim 5 wherein the sides are at an angle of less than 500 to one another.
7. A clutch assembly according to claim 5 or dairn 6 wherein the sides are chamfered or filleted.
8. A clutch assembly according to any one of claims 2 to 7 further including a retainer configured to pre-load the backing plate into a recess on the pressure plate and/or flywheel.
9. A clutch assembly according to any one of claims 2 to 8 wherein the backing plate protrudes axially beyond the remainder of the pressure plate and/or flywheel.
10. A clutch assembly according to any preceding claim wherein one or both of the pressure plate and the flywheel defines at least one recess configured to receive an insert.
11. A clutch assembly according to claim 10 wherein the pressure plate and the flywheel define opposing pairs of recesses for inserts.
12. A clutch assembly according to claim 11 wherein the number of pairs of said recesses conesponds to a number of straps configured to connect the pressure plate to a clutch cover.
13. A clutch assembly according to any one of claims 10 to 12 wherein the or each said recess extends to the periphery of the pressure plate or flywheel.
14. A clutch assembly according to any one of claims 10 to 13 wherein the or each said recess has a wide end and a narrow end.
15. A clutch assembly according to claim 14 wherein one or both of the wide end and the narrow end is arced.
16. A clutch assembly according to claim 14 or claim 15 wherein the or each said recess is positioned with the wide end toward the periphery of the pressure plate or flywheel.
17. A clutch assembly according to any one of claims 11 to 16 wherein the or each said recess has opposing sides angled with respect to one another.
18. A clutch assembly according to claim 17 wherein the sides are at an angle of less than 500 to one another.
19. A clutch assembly according to claim 17 or claim 18 wherein the sides are chamfered or filleted.
20. A clutch assembly according to any preceding claim further comprising a retainer configured to retain the insert on said pressure plate and/or flywheel.
21. A clutch assemNy according to claim 20 wherein the retainer is configured to pre-load the insert into a recess on said pressure plate andlor flywheel.
22. A clutch assembly according to any preceding claim further comprising a clutch cover connected to the flywheel and including at least one access aperture configured to permit access to said friction material for refurbishment thereof
23. A clutch assembly according to any preceding claim further comprising a bell-housing configured to surround the remainder of the clutch assembly and including at least one access aperture configured to permit access to said friction material for refurbishment thereof.i0
24. A clutch assembly according to any preceding claim wherein the clutch plate comprises a plain rotor and at least one of said pressure plate and said flywheel comprises friction material configured for clamping contact with the p'ain rotor.
25. A clutch assembly according to claim 24 wherein the rotor is of steel construction. iS26. A clutch assembly according to any preceding claim further comprising a compressed air source configured to direct cool air toward the pressure plate, clutch plate and/or flywheel.27. A clutch assembly according to claim 26 further comprising a shaft supporting the pressure plate, clutch plate and flywheel, and a jet tube surrounding the shaft configured to transfer cool air.28. A kit for a clutch assembly, the kit comprising first and second friction pads, the first friction pad being configured for fitting in a recess on a flywheel and the second friction pad being configured for fitting in a recess on a pressure plate, a retainer configured for retaining the friction pads in said recesses, and at least one fastener configured to secure the retainer on a flywheel or pressure plate.29. A method of refurbishing a clutch assembly of the kind comprising a pressure plate, a clutch plate, and a flywheel, wherein the pressure plate is arranged for clamping the clutch plate against the flywheel, the method comprising the steps of: a) providing the clutch assembly with a pressure plate and/or flywhed having at least one recess in a face intended to engage the clutch plate, and a replaceable friction material insert fitted in said recess for clamping contact with the clutch plate, b) providing the clutch assembly with a clutch cover rotatably coupled with the flywheel and including at least one access aperture alTanged for access to said friction material insert; c) accessing the friction materiai insert via said access aperture; and d) replacing the friction material insert with another friction material insert via said access aperture.30. A method of cooling a clutch assemNy of the kind comprising a shaft supporting a pressure plate, clutch plate and a flywheel, the method compnsing the steps of: lO a) providing the clutch assembly with a jet tube in communication with a compressed air source; and b) directing compressed air from the jet tube to the pressure p'ate and/or the clutch plate andJor the flywheel.iS 31. A method of retro-fitting a clutch assembly of the kind having a clutch cover which encloses a pressure plate, clutch plate and a flywheel, wherein the pressure plate is arranged for clamping the dutch plate against the flywheel, the method comprising the steps of: a) creating at least one aperture in the clutch cover; b) replacing the pressure plate and flywheel with a respective pressure plate and flywheel defining at least one recess configured to receive a friction material insert; and c) fitting a friction material insert in each recess.32. The method of claim 31, further comprising the step of replacing the clutch plate with a plain rotor having no friction material.33. A clutch assembly having a pressure plate, a clutch plate and a flywheel, wherein the pressure plate is arranged for clamping the clutch plate against the flywheel, and wherein the clutch plate comprises a plain rotor and at least one of said pressure plate and said flywheel comprises friction material configured for clamping contact with the plain rotor.34. A clutch assembly. kit or method substantially as described herein and/or with reference to the accompanying drawings.