Abstract:
A wet clutch assembly comprises an input shaft and a stack comprising interleaved first and second sets of clutch members. A hydraulic actuator piston is located concentrically around the input shaft, but is static. A member rotationally fast with one of the sets of clutch members is journalled to the actuator piston by means of a bearing whose inner race is integral with the piston.

Description:
BACKGROUND TO THE INVENTION 
     The present invention relates to a wet clutch assembly. 
     Wet clutch assemblies, that is incorporating clutches in which oil is used to cool and lubricate the clutch plates are well known. The clutches can be pressure engaged and spring disengaged as is usually the case for high horse power applications, or they can be spring engaged and pressure disengaged, for use in lower horse power applications. 
     Known wet clutch assembles suffer from the following disadvantage. The hydraulic piston used for disengagement of the clutch is located within the clutch drum and therefore rotates. This necessitates the use of expensive rotating seals for the hydraulic fluid connections which will permit some degree of leakage at least once partially worn. Such seals produce considerable friction compared with a conventional bearing. The leakage allowed by this type of seal results in a gradual loss of pressure, for example in a spring engaged clutch, if the clutch pedal is held down for any length of time. Therefore an additional control system, including a separate control valve, is needed to compensate for this leakage and thus maintain the clutch in a disengaged state. 
     It is an aim of the present invention to provide an improved wet clutch assembly. 
     SUMMARY OF THE INVENTION 
     The present invention provides a wet clutch assembly comprising two sets of substantially parallel clutch members interleaved to form a stack, the disengagement and/or engagement of the members being hydraulically actuable, the assembly further comprising an hydraulic actuator piston concentrically disposed with respect to the input shaft, but arranged not to rotate with it, and being connected to one set of clutch members so as to allow relative rotation between the piston and clutch members, e.g. via a bearing. The piston being rotationally stationary means that the need for expensive rotating seals, which tend to leak anyway, is removed. Normally, a member, such as a clutch pressure plate, which is rotationally fast with one set of clutch members would be journalled to the piston. In this case, the construction is simplified if the piston provides one of the bearings surfaces, the piston and bearing thus being an integral unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a vertical cross-section through a clutch assembly according to the present invention, with the components shown with the clutch released in the upper half and engaged in the lower half; and 
     FIG. 2 shows a front view of the clutch assembly of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 and 2, a clutch assembly 10 includes a clutch cover 12 and a mounting plate 14 bolted together by bolts 13. Input is by means of a clutch drive shaft 16 and clutch drive plate 18 driven by an input shaft 20, which is the power take off drive shaft. The clutch drive shaft 16 incorporates splines for connection to the input shaft 20. Output is by means of a clutch output hub 22, located on the mounting plate 14 via a clutch hub bearing 24, and splined to an output shaft 26. A thrust bearing 28 is located between the clutch drive shaft 16 and the clutch output hub 22. The clutch hub bearing and thrust bearing are both combined seals and bearings of a form described in co-pending U.S. Pat. application Ser. No. 08/446,956. 
     A clutch drum 30 and pressure plate 32, located between the clutch cover 12 and mounting plate 14, are bolted together by bolts 34 and are connected for rotation with the clutch drive shaft 16 but not with the clutch output hub 22. Between the clutch drum 30 and pressure plate 32 are a number of friction discs 36, connected for rotation with the clutch output hub 22, and interleaved with an equal number of intermediate counterplates 38, connected for rotation with the clutch drum 30. Engagement Belleville springs 40 are located between the clutch drive plate 18 and the pressure plate 32. Cushion Belleville springs 42 which provide for progressive engagement of the clutch are located between the clutch drive plate 18 and the first of the counterplates 38. The engagement and cushion Belleville springs 40 and 42 are selected as appropriate for the application for which the clutch assembly 10 is designed. FIG. 1 shows two Belleville springs in each location, however, the number used is also selected as appropriate for the application. Multiple cushion springs allow for very sensitive control as the clutch is initially engaged, followed by less sensitive control as the clutch pedal is further depressed. In an alternative design, progressive engagement could be provided in some other way, for example with the discs and counterplates biassed against the pressure plate by the resilience of the clutch drum 30. 
     A clutch release piston 44 is located inside the clutch cover 12 but outside the clutch drum 30. The piston 44 does not rotate permitting use of standard non-rotational seals in the form of o-rings. Pressurised hydraulic fluid for operation of the piston 44 is supplied from a clutch master cylinder (not shown) via a control port 50 and bore 52 in the clutch cover 12. The clutch release piston 44 acts on the pressure plate 32 via an axial clutch release bearing 54. The bearing 54 and piston 44 may be made as an integral unit (as shown in of FIG. 1) or as two separate items (as is well known in the art). 
     Oil for cooling and lubrication of the clutch discs 36 and counterplates 38 is supplied from a sump (not shown) via an oil supply tube 56 (shown in dashed lines in FIG. 1 and solid lines in FIG. 2) and a pump 58 located around the clutch drive shaft 16. The rotation of the clutch forces the oil outwards and it passes from the pump 58 through a passage 60 in the clutch cover 12, through passages 62 in the clutch drive plate 18 and then through passages 64 in the clutch output hub 22 to the clutch discs 36 and counterplates 38. After the oil has passed between the discs 36 and counterplates 38 it is forced outwards to the inner surface of the clutch cover 12 and then flows around the clutch cover under the influence of the rotating clutch drum 30 and passes out of the clutch cover 12 along oil outlet tube 66. As can be seen in FIG. 2, the oil outlet tube 66 is located adjacent a shoulder 67. Oil flowing around the inner surface of the clutch 12 in the clockwise direction in FIG. 2 impinges on the shoulder 67 causing a build up of static pressure and thereby forcing the oil out of the outlet tube 66. From the point identified by reference number 68 in FIG. 2, the clutch cover 12 is non-circular, providing a lead-in to the shoulder 67. 
     The operation of the wet clutch assembly 10 will now be described. Referring to the lower half of FIG. 1, the clutch is shown fully engaged. In this position the friction discs 36 and counterplates 38 are forced into close contact and drive is transmitted from the input shaft 20 to the output hub 22. In addition the oil flows as previously described. 
     When the operator of the vehicle or machine in which the assembly 10 is incorporated wishes to disengage the clutch the clutch control pedal (not shown) is depressed and operates a clutch master cylinder (not shown). The hydraulic fluid expelled from the clutch master cylinder passes to the clutch release piston 44 via the control port 50 and bore 52. The fluid forces the clutch release piston 44 to move to the right, to the position shown in the upper half of FIG. 1. The movement of the piston 44 also causes the axial bearing 54, clutch drum 30 and pressure plate 32 to move to the right and the engagement Belleville springs 40 to be compressed. The counterplates 38, connected for movement with the clutch drum 32 thus move away from the friction discs 36 and drive is no longer transmitted from the input shaft 16 to the output hub 22. 
     In intermediate positions of the clutch pedal, the degree of pressure between the clutch discs and counterplates, and therefore the amount of torque transmitted by the clutch, depends on the spring constant(s) of the cushion spring (or springs) 42 and the degree to which the cushion spring 42 is compressed. This is because pressure is only transferred from the pressure plate 32 to the discs and counterplates via the cushion spring 42. A weak cushion spring will therefore allow small changes in torque for relatively large movements of the clutch pedal, until the spring is completely flattened. A stronger cushion will, conversely, allow for a greater rate of change of torque as the clutch pedal is moved. It is envisaged that two or more cushion springs could be provided. The weakest spring would be compressed first, providing a very gradual initial engagement of the clutch, with a stronger spring then taking over to provide a greater rate of increase of torque. Further springs could be added to provide any number of stages.