WO1998039574A1

WO1998039574A1 - Clutch spring assembly

Info

Publication number: WO1998039574A1
Application number: PCT/IB1998/000301
Authority: WO
Inventors: Yves J. Kemper
Original assignee: Kemper Yves J
Priority date: 1997-03-06
Filing date: 1998-03-05
Publication date: 1998-09-11
Also published as: AU6226398A

Abstract

An actuator for a power transmitting friction clutch having a rotatable power input originating in a flywheel (10), a pressure plate (14) rotatable with the power input, and an output connected friction disc assembly (16) between the pressure plate and the flywheel. The actuator comprises a diaphragm (22) having an outer annulus (21) defining a negative Belleville spring (21) and a plurality of radial control fingers (25) extending inwardly from the outer annulus, the outer annulus having an outer periphery engageable with the pressure plate and an inner fulcrum edge, and a reaction means opposing force exerted by the negative Belleville spring on the pressure plate. The reaction means consists essentially of positive spring means (30) connected to the flywheel and supporting the inner fulcrum edge of the outer annulus of the diaphragm so that absolute values of force between the negative Belleville spring and the pressure plate and force exerted between the negative Belleville spring and the positive spring means remain substantially equal during actuation of the clutch. Compensating springs (26) are provided for shifting the range of deflection of at least one of the negative Belleville spring and the positive spring as the thickness of the friction disc assembly is reduced by wear.

Description

CLUTCH SPRING ASSEMBLY

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application entitled CONTROLLED FORCE ACTUATOR SYSTEM, Serial No. 60/038,609, filed March 6, 1997, and U.S. Provisional Application entitled CONTROLLED FORCE ACTUATOR SYSTEM, Serial No. 60/041,307, filed March 19, 1997, the disclosures of which are incorporated by reference. This application is related to pending U.S. Applications Serial No. 08/746,887, filed November 15, 1996; Serial No. 08/791,524, filed January 30, 1997; Serial No. 08/888,832, filed January 30, 1997, and Serial No. 08/984,643, filed December 3, 1997, the disclosures of which are also incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to clutch actuating systems, and, more particularly, to actuating systems for automotive clutches in which a largely conventional diaphragm spring is combined with a positive spring device to reduce control force needed for clutch actuation to a predictable, relatively small, and essentially constant force value, and which provides an increased range of clutch actuating movement to account for wear of a friction disc assembly. Description of the Related Art

In pending U.S. Applications Serial No. 08/746,887, Serial No. 08/791,524, Serial No. 08/888,832, and Serial No. 08/984,643, the disclosures of which are incorporated by reference, control force actuating systems are disclosed in which a negative Belleville spring is arranged in series with a positive spring unit such that as force application energy is released from the negative Belleville spring, that energy is transferred between load and reaction members by the positive spring. The force/deflection characteristics of the negative Belleville spring and positive spring unit are. related so that the difference in energy stored in the respective springs throughout a range of force application, remains substantially the same. As a result, the control force required to operate the actuating system is related to the stored energy difference and can be kept very small . The relatively small control force required for such actuating systems represents a significant step toward automated actuation of an automotive clutch, for example, by using state-of-the-art electromechanical transducers exemplified by an electromagnetic solenoid.

In pending application Serial No. 08/984,643, an automotive starting clutch actuating system is disclosed in which the negative Belleville spring is embodied as a conventional clutch diaphragm spring that cooperates with a positive Belleville spring to develop clutch actuating normal forces between the pressure plate and the rigid back plate conventionally used in automotive starting clutches. The negative Belleville spring and the positive spring are in series between the pressure plate and the back plate so that the normal force exerted on the pressure plate is opposed by a reaction force in the back plate. Although the resulting clutch construction is both effective and relatively uncomplicated, there is room for improvement from the standpoint of further reducing the number of components without compromising effective operation of the clutch actuating system. The range of movement or deflection required to actuate a modern automotive starting clutch between a fully engaged condition for maximum torque transmission and a released condition, in which no torque is transmitted, is very small, e.g., less than one millimeter. The range of movement of the pressure plate required to accommodate wear in the friction disc assembly may exceed by two or three times the range of movement required to control its torque capacity. Although the force/deflection characteristics of the negative Belleville spring and of the positive spring unit stay related when the friction disc assembly wears, and the difference in energy stored in the respective springs throughout the range of force application remains substantially the same, such difference in energy will increase with the amount of wear of the friction disc if the starting clutch does not include a wear compensation mechanism, as described, for example, in US Pat 4,191,285. Although many diverse types of wear compensating mechanisms are available, such wear compensation devices complicate the overall clutch assembly and can be a source of operating failure .

Therefore, there is a need for a clutch actuating spring arrangement by which the desirable aspects of the related negative Belleville and positive spring arrangement disclosed in the cited co-pending applications can be retained but with increased range of deflection to account for wear in the clutch disc assembly.

SUMMARY OF THE INVENTION

The advantages and purpose of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.

To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is directed to an actuator for a power transmitting friction clutch having a rotatable power input originating in a flywheel, a pressure plate rotatable with the power input, and an output connected friction disc assembly between the pressure plate and the flywheel . The actuator comprises a diaphragm having an outer annulus defining a negative Belleville spring, and a plurality of radial control fingers extending inwardly from the outer annulus, the outer annulus having an outer periphery engageable with the pressure plate and an inner fulcrum edge, and a reaction means opposing force exerted by the negative Belleville spring on the pressure plate. The reaction means consists essentially of positive spring means connected to the flywheel and supporting the inner fulcrum edge of the outer annulus of the diaphragm so that absolute values of force between the negative Belleville spring and the pressure plate and force exerted between the negative Belleville spring and the positive spring means remain substantially equal during actuation of the clutch.

In another aspect the advantages and purpose of the invention are attained by an actuator for a power transmitting friction clutch having a rotatable power input originating in a flywheel, a backplate rotatable with the power input, a pressure plate between the power input and the backplate, and an output connected friction disc assembly between the pressure plate and the flywheel, in which the friction disc assembly has a thickness that is reduced by wear to change an axial position of engagement of the pressure plate to clamp the friction disc assembly against the flywheel. The actuator comprises a diaphragm with an outer annulus defining a negative Belleville spring having a force/deflection ratio in a range of deflection, and a plurality of radial control fingers extending inwardly from the outer annulus . The outer annulus having an outer periphery engageable with the pressure plate and an inner fulcrum edge. A positive spring unit is arranged in series with the negative Belleville spring between the inner fulcrum edge and the backplate, and has a force/deflection ratio related to the force/deflection ratio of the negative Belleville spring throughout the range of deflection. The diaphragm and the positive spring unit are supported in radial alignment on the backplate and so that movement of the inner fulcrum edge of the diaphragm loads and unloads the positive spring unit with absolute values of force between the negative Belleville spring and the pressure plate and force between the positive spring unit and the backplate remain substantially equal during actuation of the clutch. Compensating means is provided for shifting the range of deflection of at least one of the negative Belleville spring and the positive spring as the thickness of the friction disc assembly is reduced by wear.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings,

Figs. 1A, IB, IC, and ID are fragmentary cross-sectional views illustrating the components of a preferred embodiment of the invention in different conditions of operation;

Fig. 2 is a rear elevation of a backplate spring component of the preferred embodiment;

Fig. 3 is a front elevation of a negative Belleville spring diaphragm used in the preferred embodiment;

Figs. 4A, 4B, 4C, and 4D are fragmentary cross sections illustrating an alternative embodiment of the invention in different operating conditions;

Fig. 5 is an enlarged cross-section of parts located in the sight circle 4 of Fig. 4A;

Fig. 6 is a front elevation illustrating a wear compensation spring used in the alternative embodiment of Figs. 4A-4D; and

Fig. 7 is a graph showing force/deflection curves applicable to both embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

As in the disclosures of the above cited previously filed U.S. applications, the present invention incorporates a controlled force actuating system in which a "positive" spring and a "negative Belleville" spring are arranged in series between a load member and a reaction member. The terms "positive spring" "negative Belleville spring" are defined fully in the cited application Serial No. 08/746,887 and will not be described further in this text except as these terms apply to components of the actuator system of the present invention.

In each of Figs. 1A-1D and in Figs. 4A-4D, of the attached drawings, conventional functioning components of an automotive starting clutch are shown schematically to include a flywheel 10 having a friction surface 12, representing the power input to the clutch, a pressure plate 14 connected for rotation with the flywheel 10 and disposed for axial movement to and from the flywheel 10, a friction disc assembly 16 between the pressure plate 14 and the flywheel surface 12 and an output disc 18. All of these components are rotatable about a central axis 20.

In accordance with the present invention, a negative Belleville spring is provided in an outer annulus of a diaphragm having a plurality of radial control fingers extending inwardly from the outer annulus, the outer annulus having an outer periphery engageable with the pressure plate and an inner fulcrum edge. A positive spring back plate unit is connected to the flywheel and has radial spring fingers supporting the inner fulcrum edge of the outer annulus of the diaphragm so that absolute values of force between the negative Belleville spring and the pressure plate and force exerted between the negative Belleville spring and the positive spring unit remain substantially equal during actuation of the clutch. In the embodiment of Figs. 1A-3, the negative Belleville spring is defined as a continuous outer annulus 21 of a diaphragm 22, the inner edge of the annulus being delineated by a circular array of openings 23 at the outer ends of. slots 24 extending inwardly from the openings to define control fingers 25.

As shown in the radial cross sections of each of Figs. 1A-1D and in the rear elevation of Fig. 2, an annular positive spring unit 30 is shown in include a continuous outer peripheral portion 32 having a circular array of holes 34 to enable a rigid bolted connection of the outer continuous portion 32 to the rear end of an annular rim 36 on the flywheel 10. The continuous outer peripheral portion 32 connects the outer ends of a plurality of truncated sector- shaped springs 38, spaced by radial slots 39, and the outer ends of a like number of fingers 40 extending inwardly to axial tabs 42. The tabs 42 lie on a circle having a diameter intermediate the diameters of the inner and outer peripheral edges of the positive spring unit 30. The circle on which the tabs 42 lie also defines a boundary between inner and outer portions 38i and 38o, respectively, in each of the springs 38.

As shown in Figs. IC and ID, the radial cross-sectional shape of the springs 38 in their relaxed condition or state, is such that the outer portions 38o are arcuately concave at the front faces thereof and that the inner portions 38i extend linearly on tangents to the outer portions. The radial cross- sectional shape of the fingers 40 is the same as the outer spring portions 38o in their relaxed condition. The rigid connection of the peripheral edge by bolting, as described above, results in the springs 38 being cantilevered at the relatively wide outer edges thereof from the flywheel. As shown in Fig. 3, the diaphragm 22 has radial slots 24 extending inwardly from the holes 23 to delineate the control fingers 25, and also wear compensating spring fingers 26 to be described in more detail below. As shown in Fig. ID, for example, the control fingers lie generally in the plane of the negative Belleville spring 21. Also in Fig. 3, the control fingers 25 are shown to extend inwardly to a control bearing contact circle 44 and the shorter wear compensating springs 26 extend to a larger concentric contact circle 46. Another concentric contact circle 48 is positioned near the outer periphery of the diaphragm 22.

In the assembled actuating system shown in Figs. 1A-1D, the tabs 42 of the positive spring unit 30 extend forwardly through the holes 23 of the diaphragm 22 to retain the diaphragm 22 radially positioned relative to the positive spring unit 30. Stops 50 retain the diaphragm 22 against axial removal from the tabs 42 and a wire fulcrum bearing ring 52 lies between the inner edge of the negative Belleville spring 21 and each of the positive springs 38. The wire fulcrum bearing ring 52 also lies between the inner edge of the negative Belleville spring 21 and the inner ends of the radial fingers 40 outside of the tabs 42.

In Fig. 1A, the line X-X' lies on the circle 48 of Fig. 3 which represents the circle of contact between the pressure plate 14 and the negative Belleville spring 21; the line Y-Y' lies on the fulcrum ring 52; and the line Z-Z' lies on the circle 46 of contact between the inner ends of the wear compensating springs 26 and the inner ends of the positive springs 38. The lines X-X' and Y-Y' further define the ends of an actuating moment X-Y and the lines Y-Y' and Z-Z' define the ends of a compensating moment Y-Z.

Operation of the embodiment of Figs. 1A-3 with a new or unworn friction disc assembly 16, as shown in Figs. 1A,. IC, and ID, the clutch is operated between the fully closed condition of Fig. 1A and the fully open condition of Fig. ID without use of the wear compensating springs 26. Between the kissing point condition of Fig. IC and the fully closed condition of Fig. 1A, the full force of the negative Belleville spring is transmitted though the fulcrum ring 52 to the positive springs 38, and vice versa, by the moment X-Y.

In Fig. 1A, the pressure plate 14 is fully closed against the friction disc assembly 16 and the flywheel surface 10. In this condition, the negative Belleville spring 21 exerts the maximum force thereof against the pressure plate 14 and loads the positive spring unit 30. Conceptually, and assuming that the clutch in the condition of Fig. 1A is operated by a conventional foot pedal (not shown) , the foot pedal would be in its outer position and the automobile in which the clutch was used would be driven under power transmitted by the clutch.

To disengage the clutch from the condition shown in Fig. 1A, a control force Fc is applied to the inner end of the diaphragm fingers 25. The magnitude of the control force Fc, which accounts for the leverage of the control fingers 25, is a fraction of the difference between the force in the positive spring unit 30 and the force in the negative Belleville spring 21. Thus, the positive spring unit 30 assists the control force Fc in restoring energy into the negative Belleville spring 21 to release the clutch from the fully closed condition of Fig. 1A to the condition of Fig. IC.

It will be noted by a comparison of Fig. 1A and Fig. IC, that as energy is restored to the negative Belleville spring 21, the inner edge of the negative Belleville spring 21 is restrained from movement by the stop 50. This condition is illustrated in Fig. IC. In this condition, the positive spring unit 30 reaches minimal stored energy or a substantially relaxed condition. The condition of the clutch in Fig. IC is that the pressure plate 14 remains in engagement with the friction disc assembly 16 but under a very low or "kissing" normal force. To fully disengage the clutch to the condition illustrated in Fig. ID, the control force Fc continues to be applied to pivot the negative Belleville spring 21 about the retainer stop 50, adding further energy to the negative Belleville spring 21. At this point in the conceptualized clutch pedal analogy, the pedal is being retained in the clutch-open position by a foot applied force.

Upon release of the force applied to the pedal, the conditions of the positive spring unit 30 and the negative Belleville spring 21 will progress back to the fully engaged condition of Fig. 1A. In particular, the negative Belleville spring 21 will pivot on the fulcrum ring 52 from the condition of Fig. ID to the condition of Fig. IC with no or only minimal loading of the positive spring. As the release of the control fingers 24 continues to the condition shown in Fig. 1A, the positive spring unit 30 is loaded by the negative Belleville spring 21 until the two springs generate the same absolute force and Fc falls to zero. In accordance with the present invention, the negative Belleville spring has a negative force/deflection ratio related to the positive force/deflection ratio of the positive spring unit so that the positive spring unit and the negative Belleville spring oppose each other with a substantially constant force difference throughout a range of actuating force extending between minimum and maximum values. In Fig. 7, the force deflection characteristics of the negative Belleville spring 21 and the positive spring unit 30 for a new or unworn friction disc assembly are represented respectively by curves Nn and Pn. In particular, units of force on the ordinate of the curves are absolute values of force. The units on the abscissa of the curves, however, are units of spring travel resulting from relative values of defection of the positive and negative springs. The vertical distance between the two curves represents a difference in force exerted by the respective springs at common values of deflection. The points Kn, Kp represent a minimum value in a range of actuating force extending to a maximum value at the point Pe . Also, throughout the range of actuation between the points Kn,Kp and the point Pe, the positive spring unit is loaded so that the absolute values of force between the negative Belleville spring 21 and the pressure plate 14 and of force between the negative Belleville spring and the positive spring unit 30 remain substantially equal during actuation of the clutch.

Where the two curves intersect at the point Pe in the graph shown in Fig. 7, the condition of actuating system is as illustrated in Fig. 1A, that is, the clutch is fully engaged because the pressure plate 14 is fully closed against the friction disc assembly 16 and the flywheel surface 10. In this condition, the negative Belleville spring exerts the full energy capacity thereof between the pressure plate 14 and the positive spring unit 30 so that the disc assembly 16 is clamped between the pressure plate 14 and the flywheel friction surface 10. As mentioned above, the magnitude of the control force Fc amplified by leverage of the control fingers 24, in operation between the kissing point condition of Fig. IC and the fully closed condition of Fig. 1A, is a fraction of the difference between force exerted by the negative Belleville spring 21 of the diaphragm 22 and force exerted by the positive spring unit 30. It is to be noted that the curve Pn, representing the positive spring characteristics, is a composite essentially of a force deflection curve represented by the linear dashed line P, representing sector-shaped springs 38 of the positive spring unit 30 and any other flexibility that may exist in the system, and a curve Pen representing elastic deformation of the friction disc assembly 16. Also in Fig. 7, the curve Fc represents the magnitude of the control force applied to the inner ends of the diaphragm fingers 25 and the curve Fd represents the magnitude of difference between force exerted by the respective positive and negative springs.

The fully open condition of the clutch, as shown in ID, is represented at the lower right-hand corner of the graph in Fig. 7. Because the negative Belleville spring 21, in this condition, is loaded with energy and capable of releasing a force greater than the positive spring force, the open condition must be maintained by an applied control force Fc, such as depression of a conventional clutch pedal. Upon release of the clutch pedal, the negative Belleville spring 21 releases energy and deflects to the kissing point Kn represented in the graph of Fig. 7 by the horizontal position 7, at which point, the negative Belleville spring 21 begins to load the positive spring unit 30. Thereafter, operation proceeds to a point of intersection between the curves Nn, Pn in a fully closed position represented by Fig. 1A and at the upper left-hand corner of the graph in Fig. 7. At this point, the springs are in equilibrium. The control force applied in operating to or from the point of equilibrium is represented by the vertical distance in Fig. 8 between the negative curve Nn and the positive curve Pn.

In accordance with the present invention, the clutch actuating system, including the negative Belleville spring and the positive spring described above, further includes a provision to compensate for wear resulting in reduced thickness of the friction disc assembly between the pressure plate and the flywheel surface. In the above description of Figs. 1A-1D, reference was made only to Figs. 1A, IC, and ID which depict a "new" friction disc assembly 16. Fig. IB shows the operating system in the "kissing" condition like Fig. IC, but after the friction disc assembly 16 is worn to a reduced thickness. To compensate for such reduced thickness, the wear compensating spring spring fingers 26 are provided to ensure that relative force/deflection characteristics of the negative Belleville and positive springs are retained irrespective of such wear.

When the friction disc assembly 16 is reduced in thickness due to wear, as shown in Fig. IB, the increased deflection of the negative Belleville spring on line X-X' needed to reach the kissing point of the worn friction disc assembly, results in loading the wear compensating springs 26 against the inner ends of the positive springs 38. However, the reaction to such loading of the compensating springs 26 by the positive springs 38 is restricted to the inner portions 38i of the positive springs, or the force exerted by the moment Y-Z. Thus, loading of the cantilevered outer portions 38o of the positive springs 38 by the negative Belleville spring 21a is unaffected. However, the force of the negative Belleville spring is reduced by the loading of the wear compensating springs 26.

In the embodiment shown in Figs. 1A-1D, it is assumed that loading of the wear compensating springs 26 results in deflection only of these springs and a minimal deflection, if any, of the inner portions 38i of the positive springs 38. This assumption presumes that the inner portions 38i of the positive springs 38 are more rigid or stronger than the wear compensating springs 26. In the practice of the invention, however, either one or both of the springs 26 and the inner spring portions 38i may deflect under the wear compensation loading.

The purpose of the compensating springs 26 is to ensure that the relative force deflection curves of the negative Belleville spring 21 and the positive springs 32 remain constant irrespective of wear in the friction disc assembly 16. As may be appreciated from Fig. IB and Fig. 7, the reduction in thickness of the friction disc assembly 16, as a result of wear, results in larger initial deflection of the negative Belleville spring 21 on the diaphragm 22 before the kissing point is reached. The wear compensation spring fingers 26 are is not loaded when the friction disc assembly 16 is new or unworn. In the case of a worn friction disc assembly 16, as depicted in Fig. IB, however, upon deflection of the negative Belleville spring 21 to reach the kissing point, the wear compensation spring fingers 26 become loaded before any load is applied to the positive spring unit 30 because the angle α increases, causing the compensating springs 26 to bend. By doing so, the compensating springs 26 apply a moment to the diaphragm 22 that reduces the moment (and therefore the force) that the negative Belleville spring 21 delivers at this position. In this way, energy delivered by the negative Belleville spring 21, in deflecting from the condition of Fig. IC to that of Fig. IB, is essentially stored in the compensating springs 26 rather than being stored in the positive spring unit 30. Also, the compensating springs 26 remain loaded during operation between the kissing point and the fully loaded condition of Fig. IB, or a fully closed condition of the clutch with the worn friction disc assembly plate, and the angle between the negative Belleville spring 21 of the diaphragm and the positive spring unit remains substantially constant during that operation. The result of this loading of the compensation spring due to wear and reduced thickness of the friction disc assembly 16, is to reduce the net force exerted by the negative Belleville and to shift the force deflection characteristics of the negative Belleville from the curve Nn in Fig. 7, downwardly to the curve Nw. Since the relative positions of the positive spring unit 30 and the negative Belleville spring 21 remain substantially the same, the control force Fc throughout deflection from kissing point to the fully loaded position is the same for the worn friction disc assembly as it was for the new friction disc assembly.

In Fig. 4A-4D, an alternative embodiment is illustrated in which the positive spring unit 30a is in the nature of a modified back plate having inclined radial positive spring fingers 38a. As such the positive spring unit may be connected to a conventional flywheel. Also in this case, the compensating spring 26a is a component of the system separate from the diaphragm 22a.

As shown in Fig. 6 and in Figs. 4A-4D, the wear compensation spring 26a is an annular array of radiating spring fingers 26b preferably, but not necessarily, held together at their inner ends by a continuous integral ring portion 26c. The fingers 26b are bent approximately midway along their radial length to impart a frusto-conical shape apparent from Figs. 4A-4D and to provide a circular fulcrum 26d shown in Fig. 6. In the assembled actuating system, the outer ends of the compensating spring fingers 26b are retained by claws 60 relative to an outer rim 62 of the positive spring unit 30a. As shown in Fig. 5, the spring 26a is maintained axially and circumferentially against the diaphragm 22a at the outer ends of the slots defining the control fingers 25a (or at the inner edge of the negative Belleville spring 21a) by a ring 64 having a series of U- shaped brackets 66 as shown in Fig. 5. Further, the inner edge of the spring fingers 26b extend toward the axis 20 of rotation such that they may apply a load to the control fingers 25a of the diaphragm 22a.

The modified actuating system shown in Figs. 4A-4D operates in the same manner as that described above with respect to the embodiment of Figs. 1A-1D.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An actuator for a power transmitting friction clutch having a rotatable power input originating in a flywheel, a pressure plate rotatable with the power input, and an output connected friction disc assembly between the pressure plate and the flywheel, the actuator comprising: a diaphragm having an outer annulus defining a negative Belleville spring, and a plurality of radial control fingers extending inwardly from the outer annulus, the outer annulus having an outer periphery engageable with the pressure plate and an inner fulcrum edge; and reaction means opposing force exerted by the negative Belleville spring on the pressure plate, the reaction means consisting essentially of positive spring means connected to the flywheel and supporting the inner fulcrum edge of the outer annulus of the diaphragm so that absolute values of force between the negative Belleville spring and the pressure plate and force exerted between the negative Belleville spring and the positive spring means remain substantially equal during actuation of the clutch.

2. The actuator of claim 1, wherein the positive spring means comprises a plurality of radial springs cantilevered from the flywheel.

3. The actuator of claim 1, wherein the positive spring means comprises a continuous outer peripheral portion bolted to the flywheel, and a plurality of radial springs extending inwardly from the outer peripheral portion.

4. The actuator of any one of claims 2 or 3 , wherein the diaphragm is supported by axial tabs on the inner ends of the radial springs .

5. The actuator of claim 4, including a fulcrum bearing ring between the inner fulcrum edge .

6. The actuator of claim 1, wherein the positive spring unit has a positive force/deflection ratio and is initially in an energy receiving condition and the negative Belleville spring has a negative force/deflection ratio and is initially in a loaded energy storing condition, so that movement of the control fingers effects a transfer of stored energy from the negative Belleville spring to the positive spring, thereby to apply a range of applied force between the backplate and the pressure plate, the range of applied force varying between minimum and maximum values of applied force, and the negative and positive force deflection ratios being related so that the absolute values of the force deflection ratios differ by a control force in the control fingers that remains substantially the same for changes in deflection of the positive and negative springs substantially throughout the applied range of forces.

7. The actuator of claim 6, wherein the negative and positive force deflection ratios are further related so that the positive and negative springs are in equilibrium at maximum value of applied force.

8. An actuator for a power transmitting friction clutch having a rotatable power input originating in a flywheel, a backplate rotatable with the power input, a pressure plate between the power input and the backplate, and an output connected friction disc assembly between the pressure plate and the flywheel, the friction disc assembly having a thickness that is reduced by wear to change an axial position of engagement of the pressure plate to clamp the friction disc assembly against the flywheel, the actuator comprising : a diaphragm having an outer annulus defining a negative Belleville spring having a force/deflection ratio in a range of deflection, and a plurality of radial control fingers extending inwardly from the outer annulus, the outer annulus having an outer periphery engageable with the pressure plate and an inner fulcrum edge; a positive spring unit in series with the negative Belleville spring between the inner fulcrum edge and the backplate, the positive spring unit having a force/deflection ratio related to the force/deflection ratio of the negative Belleville spring throughout said range of deflection; means for supporting the diaphragm and the positive spring unit in radial alignment on the backplate and so that movement of the inner fulcrum edge of the diaphragm loads and unloads the positive spring unit with absolute values of force between the negative Belleville spring and the pressure plate and force between the positive spring unit and the backplate remain substantially equal during actuation of the clutch; and compensating means for shifting the range of deflection of at least one of the negative Belleville spring and the positive spring as the thickness of the friction disc assembly is reduced by wear.

9. The friction clutch actuator of claim 8, wherein the compensating means shifts the range of deflection of the negative Belleville spring.

10. The friction clutch actuator of claim 8, wherein the compensating means comprises a spring effective to reduce loading of the positive spring by the negative Belleville spring upon reduced thickness of the friction disc assembly,