Field of the invention This present invention relates to a pressure plate forming part of a friction clutch assembly. More particularly, but not exclusively, the present invention relates to a pressure plate forming part of a friction clutch assembly in a manual transmission for a car or other automobile.

Background of the invention A friction clutch assembly or "clutch" of a car or other automobile having a manual transmission is generally located between the engine and the drive train. The assembly normally includes three adjacent annular plates, including a flywheel that is rotatably driven by the crank shaft, a clutch plate (otherwise known as a driven plate), and a pressure plate that is biased by energy storing devices, such as one or more springs, towards the clutch plate and flywheel to clamp the clutch plate between the flywheel and the pressure plate.

The frictional engagement of the coupling faces of the clutch plate with the adjacent rotating coupling faces of the flywheel and the pressure plate allow the clutch plate to transfer power generated by the engine to the remainder of the drive train. To facilitate this frictional engagement, both the coupling faces of the clutch plate are lined with a frictional material that exhibits substantially stable coefficients of static and dynamic friction over a wide range of operating temperatures, including cooler starting temperatures and significantly hotter running temperatures. The frictional material needs to resist wear, be strong enough to withstand frequent heavy impact loading, particularly during starting, stopping and changing gears, and be non-aggressive against the adjacent coupling faces of the flywheel and the pressure plate.

The clutch functions to regulate the power being transmitted from the engine to the transmission and the drive shaft. When the clutch is disengaged when starting, stopping and changing of gears by depressing an associated clutch pedal that moves the pressure plate away from the clutch plate and the flywheel against the bias of the spring(s), smooth slippage is allowed between the engine and the transmission. Conversely, when the clutch is actively engaged by releasing the clutch pedal so that the spring(s) again bias the pressure plate towards the clutch plate and flywheel, slippage is prevented to maximise the amount of torque that is able to be transmitted from the engine to the drive train.

Two factors limiting the performance of conventional clutches are the maximum power or torque that can be transmitted from the engine to the drive train, and the heat generated by the frequent frictional engagement and disengagement between the coupling faces of the clutch plate and the adjacent coupling faces of the flywheel and the pressure plate. These are particularly evident in high performance cars that generate considerable torque, during intentional aggressive slipping of the clutch, and during frequent riding of the clutch by lesser skilled drivers.

The maximum power or torque that can be transmitted from the engine to the drive train is constrained by the total area and the coefficients of friction of the coupling faces of the clutch plate and the coupling faces of the flywheel and the pressure plate, and the clamping force of the spring. The first of these may be addressed by increasing the areas of the coupling faces by increasing the size of the clutch. However, increasing the diameter of the clutch plate requires additional material, not only for the clutch plate, but also the associated flywheel and pressure plate, and the surrounding clutch housing (or "bell housing"). Further, smaller or compact design cars, particularly front wheel drives, may be unable to accommodate a larger clutch and/or associated larger bell housing.

Another option for increasing the power or torque is to stiffen the spring(s) to increase the clamping force acting on the clutch plate. Correspondingly however, this leads to an increase in the force required to depress the clutch pedal that a user may find undesirable. In extreme circumstances, this can lead to problems such as fire wall flex where the clutch pedal is mounted, or even broken linkages to the clutch where the linkages are unable to withstand the frequent increased forces being transmitted.

The second limitation of conventional clutches relates to the heat generated between the coupling faces of the clutch and the adjacent coupling faces of the flywheel and pressure plate that can result in clutch fade. Clutch fade is effectively a loss of friction force as a result of the heating of the frictional material lining the clutch plate, and is generally caused by the heat resulting from the frequent frictional engagement between the clutch plate and the adjacent rotated flywheel and pressure plate increasing the temperature of the frictional material such that it is within the temperature range at which binder and other constituent frictional materials of the clutch plate tend to melt down and vaporise. The vapour becomes trapped between the adjacent coupling faces of the clutch plate and the flywheel and the pressure plate causing the coupling faces of the clutch plate to glide on blankets of vapour resulting in increased slippage of the clutch.

It would be highly desirable to provide a smaller clutch assembly having increased power or torque capacity and reduced susceptibility to fade conditions.

Summary of the invention According to one aspect of the present invention, there is provided a pressure plate for use in a friction clutch assembly, the pressure plate comprising: a substantially flat annular coupling face for frictional engagement with a coupling face of a clutch plate; and one or more discontinuities formed in the coupling face of the pressure plate for decreasing a surface area of the coupling face of the pressure plate, the one or more discontinuities decreasing a greater relative proportion of the surface area closer to an inner radius of the coupling face of the pressure plate than an outer radius of the coupling face of the pressure plate to increase a mean effective radius of the surface area.

In one practical form, the pressure plate may comprise a plurality of the discontinuities, with each discontinuity defined by a respective groove or slot formed in the coupling face of the pressure plate.

Preferably, each groove generally extends from at or near the inner radius to at or near the outer radius. Preferably, the grooves are substantially equally radially spaced apart so as not to introduce an unbalance to the pressure plate.

The increased mean effective radius of the coupling face correspondingly increases a net torque capacity of a clutch assembly when a form of the pressure plate and a corresponding clutch plate are engaged. For example, the net torque capacity between the coupling face of the pressure plate and the coupling face of the clutch plate formed from a friction coupling material may be empirically estimated as: τ = FFr where : τ = net torque capacity Fp = maximumfrictional force acting on the clutch plate F = mean effective radius of the surface areas (or contacting portions) of the coupling faces of the pressure plate and the clutch plate when they are frictionafly engaged Hence, the net torque capacity, τ , may be increased by increasing either the frictional force, FF (as discussed above), or by increasing the mean effective radius, F , of the surfaces areas (or contacting portions) of the coupling faces.

The mean effective radius, 7 , of the surface area (or contacting portion) of the annular coupling face of the pressure plate is the area weighted average radius of the coupling face and may be approximated by: 7_rιaλ+r1a2+... + rnan ax+a2+... + an where : F = mean effective radius rn = radius at an an = area at rn

l(r) = 2πr-Nw(r) where : l(r) = circumferential perimeter length of the contact face of the pressure plate at a radius r w(r) = circumferential width of each groove at radius r N = number of radially spaced apart groove(s)

da - l(r) ■ dr = (Iπr - Nw(r)) ■ dr

\r • da

n where : r2 = outer radius of the annular coupling face of the pressure plate rx = inner radius of the annular coupling face of the pressure plate

_ 1

](2πr-Nw(r))-dr When each groove, the circumferential width of which determined by the function w(r) above, acts to remove proportionally more of the surface area of the coupling face of the pressure plate closer to the inner radius (smaller values of r) as compared to closer to the outer radius (larger values of r), the mean effective radius of the pressure plate is increased. This correspondingly increases the net torque capacity that is able to be applied to the clutch plate by the clamping action of the pressure plate thereupon. Advantageously, this facilitates the use of smaller clutches having a higher torque capacity than conventional clutches, which correspondingly facilitates a reduction in manufacturing costs, and more particularly material costs.

Preferably, each groove defines a respective arc extending from at or near the inner radius to at or near the outer radius such that a circumferential width of the groove relative to the coupling face of the pressure plate progressively changes from the inner radius to the outer radius. More preferably, an angle of incidence of the or each arc to a direction of rotation of the pressure plate progressively changes relative to a radius of the coupling face of the pressure plate so that the circumferential width of the groove progressively decreases from at or near the inner radius to at or near the outer radius. Still more preferably, the or each groove progressively changes from being generally tangential relative to the coupling face of the pressure plate at or near the inner radius to being generally radial relative to the coupling face of the pressure plate at or near the outer radius.

Alternatively, each groove may be substantially straight. Each of the straight grooves may extend from at or near the inner radius to at or near the outer radius at a radially offset angle.

Further alternatively, the or each groove may be a curved groove. For example, the pressure plate may include one continuous curved groove formed in the annular coupling face that spirals around the coupling face. The groove width of the grooves may be determined by considering an acceptable amount of unsupported facing material of the coupling face and its effect on wear. That is, if the grooves are too wide, they may act to allow the friction facing of the clutch plate to deflect and increase the load presented on the edges of the grooves and they may also act to increase the likelihood of shudder/NVH ("Noise Vibration Harshness"). Preferably, the grooves have a relatively high aspect ratio to minimise deflection of the friction facing of the coupling face of the clutch plate and their effect on the wear of the clutch plate.

The groove width of each groove may be substantially constant along a length of the groove. Advantageously, each constant width groove is able to be formed by a single pass of a machining tool. Preferably, the groove width of each groove is not greater than about 10 millimetres.

It will be understood though that the grooves do not have to be all the same length, and also that the groove width of each groove could vary along its length.

It will also be understood that each discontinuity formed in the coupling face need not be defined by a respective groove, and alternatively each discontinuity may be defined by a respective recess or depression formed in the coupling face of the pressure plate, for example. Advantageously, the pressure plate may have a greater concentration of recesses or depressions formed near to the inner radius than near to the outer radius to further increase the mean effective radius.

Preferably, each discontinuity presents a transitional interruption of the coupling face of the pressure plate to minimise point loading on an edge or edges of the discontinuity. The edge(s) may be radiused, and may have a radius of about 5 millimetres, for example.

Preferably, each groove is relatively shallow so as not to significantly compromise the structural integrity of the pressure plate. It has been found in practice that the depth of each groove is not required to be more than about 0.5 millimetres below the coupling face for it to be effective. Thus, the depth of each groove is preferably about 0.5 millimetres or greater. Preferably, the depth of each groove is less than about 2 to 5% of a total section thickness of the pressure plate.

In practice, it is expected that the selection of the depth of the grooves relative to the total section thickness of the pressure plate will be governed by the thermal fatigue characteristics of the particular pressure plate. It is contemplated that a material such as spheroidal graphite cast iron or nodular cast iron for example, may be used where structural integrity is restricting the application of grooves.

Preferably each groove extends from the inner radius to the outer radius to promote the passage and purging of air and wear debris trapped between the pressure plate and the clutch plate, and to allow volatile gases generated by frictional heating of the clamped together coupling faces of the pressure plate and the clutch plate in use to escape radially outwardly from between the adjacent faces and to effectively wipe and scrape clear and condition the coupling face of the clutch plate. The escaping/purging action is thought to be promoted by orienting the grooves such that a radially outer portion of each groove trails a radially inner portion of the groove with respect to the direction of rotation of the pressure plate in use. Advantageously, escaping/purging of the hot gases maintains the clutch at a cooler temperature during operation such that it is less susceptible to fade conditions.

The application and orientation of discontinuities formed in the coupling face of the pressure plate should be such that smooth modulation and engagement characteristics will be maintained. That is, the discontinuities should be formed in the coupling face of the pressure plate so as not to adversely affect either the balance, which may induce excessive shudder or NVH, or the structural integrity of the pressure plate. Brief description of the drawings The present invention is described, by way of non-limiting example only, with reference to the accompanying drawings in which: Figure 1 an elevation view of a pressure plate having a plurality of grooves formed in a coupling face thereof; Figure 2 is a partial section view of the pressure plate shown in Figure 1 taken along line A-A; Figure 3 is a partial section view of the pressure plate shown in Figure 1 taken along line B-B; and Figure 4 an elevation view of an alternative pressure plate having a plurality of recesses or depressions formed in a coupling face thereof.

Detailed description A pressure plate 10 is shown in Figures 1 to 3. The pressure plate 10 is formed by a generally annular plate 12 defined by an inner radius 16 and an outer radius 20, and includes several flange parts 22 having holes 24 formed therethrough by which the pressure plate 10 is able to be mounted to an associated clutch housing. The pressure plate 10 also includes a plurality discontinuities in the form of equally radially spaced grooves or slots 26 that each generally define an arc extending from the inner radius 16 to the outer radius 20, each of the grooves 26 formed in a flat planer friction surface 28 of a coupling face 30 of the pressure plate 10.

The arcuate grooves 26 decrease a greater relative proportion of a surface area or contacting portion of the coupling face 30 closer to the inner radius 16 than the outer radius 20 to increase a mean effective radius of the surface area. Each of the grooves 26 formed in the coupling face 30 follows a path defined by a portion of a circumference of a respective circle. For example, the circle followed by one groove 26 that is shown in the section view of Figure 2 is defined by the radius 32 in Figure 1. It will be understood that both an angle of incidence 34 of each groove 26 to a direction of rotation 36 of the pressure plate 10 and a circumferential width 38 of each groove 26 change along the length of each groove 26 relative to the radius of the coupling face 30. The angle of incidence 34 of each groove 26 progressively changes relative to the radius of the coupling face 30 so that the circumferential width 38 of the grooves 26 progressively decreases from at or near the inner radius 16 to at or near the outer radius 20. More specifically, the grooves 26 progressively change from being generally tangential relative to the coupling face 30 at or near the inner radius 16, where the circumferential width 38 of each groove 26 is largest to remove the most material closer to the inner radius 16, to being generally radial relative to the coupling face 30 at or near the outer radius 20, where the circumferential width 38 of each groove 26 is smallest to remove the least material closer to the outer radius 20. As such, the arcuate nature of the grooves 26 acts to further increase the mean effective radius of the surface area of the coupling face 30.

The groove width 40 of each groove 26 is substantially constant along the length of the groove from the inner radius 16 to the outer radius 20, and may be about 4 to 10 millimetres, for example. Advantageously, each groove 26 is formed by a single pass of a machining tool, and preferably has a depth 42 of at least 0.5 millimetres deep, but not greater than say 2 to 5% of the total section thickness 44 of the pressure plate 10.

Advantageously, corner portions 46 of the grooves 26 are radiused such that the grooves 26 present a transitional interruption of the surface 28 to minimise point loading on edges 48 of each groove 26 in use that can result in premature wear and possibly excessive shudder/NVH. In one preferred form of the pressure plate 10, the radiused corner portions 46 of the grooves 26 may have radii of about 5 millimetres, for example. - l i ¬

lt is thought that the escaping/purging of the hot gasses that are generated and become trapped between the coupling face 30 of the pressure plate 10 and an associated coupling face of a clutch plate (not shown) in use is promoted by a radially outer portion of each arcuate groove 26 trailing a radially inner portion of the groove 26 with respect to the direction of rotation 36 of the pressure plate 10 in use, as shown in Figure 1.

It is also thought that the escaping/purging action is further promoted by each groove 26 extending from the inner radius 16 to the outer radius 20. Alternatively, each groove 26 may not extend fully to the inner radius 16 or fully to the outer radius 20.

An alternative pressure plate 110 is shown in Figure 4. The pressure plate 110 is similar to the pressure plate 10, and the same reference numerals have been used to indicate common features. The pressure plate 110 differs from the pressure plate 10 however, in that instead of a plurality of arcuate grooves 26, the pressure plate 110 includes one or more discontinuities in the form of recesses or depressions 112 formed in the friction surface 28 of the coupling face 30.

The pattern of recesses 112 of the pressure plate 110 is formed on the coupling face 30 such that the recesses 112 act to decrease a greater relative proportion of the surface area or contacting portion of the coupling face 30 closer to the inner radius 16 of the coupling face 30 than the outer radius 20 of the coupling face 30 to increase the mean effective radius of the surface area. As is shown in Figure 4, preferably the pressure plate 110 has a greater concentration of recesses 112 formed near to the inner radius 16 than near to the outer radius 20 to further increase the mean effective radius.

It will be understood that while the recesses 112 are shown as circular depressions in Figure 4, they could take other shapes. Further, like the grooves 26 formed in the coupling face of the pressure plate 10, the number, spacing and dimensions of the formed recesses 112 may also be varied to suit specific requirements, subject to the requirements to maintain the balance and the structural integrity of the pressure plate 110.

In use in a clutch assembly of a manual transmission car (not shown) in the conventional manner, for example, the pressure plate 10 (or similarly pressure plate 110) is mounted to the clutch cover. The clutch cover is mounted to a flywheel that is in turn mounted on and rotationally driven by the engine crankshaft such that the flywheel is rotatably driven at the same speed as the engine.

A clutch plate having opposed friction faces in the form of coupling faces is mounted to a drive shaft that leads to the remainder of the drive train and is sandwiched between a coupling face of the flywheel and the coupling face 30 of the pressure plate 10. The clutch may be engaged and disengaged by using a clutch pedal connected to the clutch by an associated linkage, for example.

When the engine is running, the flywheel rotates at the same speed as the engine. When the clutch is engaged by releasing the clutch pedal, one or more springs disposed in the clutch housing act to bias the pressure plate 10 towards the flywheel and the clutch plate so that the rotating coupling face of the flywheel and coupling face 30 of the pressure plate 10 contact the coupling surfaces of the clutch plate to clamp the clutch plate therebetween, with the coupling faces of the clutch plate frictionally engaging the adjacent coupling face of the flywheel and the adjacent coupling face 30 of the pressure plate 10. The driven clutch plate is then able to transfer power from the engine to the drive shaft and the remainder of the drive train.

The grooves 26 formed in the coupling face 30 of the pressure plate 10 advantageously increase the mean effective radius of the surface area coupling face 30 of the pressure plate 10 by removing proportionally more material from the coupling face 30 of the pressure plate 10 closer to the inner radius 16 than the outer radius 20. In consequence, the net power or torque that is able to be transferred from the engine to the clutch plate is increased. This allows smaller clutches to be manufactured having an equivalent performance to larger conventional clutches. The pressure plate 10 having the grooves 26 may also be used in larger clutches to provide improved performance in higher performance vehicles.

It is also thought that the grooves 26 advantageously serve to maintain an operating temperature of the clutch in use at a lower temperature. This may result in the clutch being less susceptible to fade conditions.

When the clutch is disengaged by depressing the associated clutch pedal, a throwout bearing associated with the clutch acts against the spring(s) to bias the pressure plate 10 away from the flywheel and the clutch plate against the bias of the spring(s). This unlocks the clutch plate from rotation by the engine, to thereby allow smooth slippage between the engine and the clutch plate.

The forgoing describes the present invention by way of non-limiting example only, and it will be understood that modifications can be made without departing from the scope of the invention.