Wet clutch assembly

Abstract: A wet clutch assembly comprises an input shaft (20) and a stack comprising interleaved first and second sets of clutch members (36,38). A hydraulic actuator piston (44) is located concentrically around the input shaft, but is static. A member (32) rotationally fast with one of the sets of clutch members (38) is journalled to the actuator piston by means of a bearing (54) whose inner race is integral with the piston (44).

Description: 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 assemblies 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. US 4646889 discloses a wet clutch assembly having a rotational axis and comprising an input shaft rotatable about said axis, two sets of substantially parallel clutch members interleaved to form a stack, and an actuator comprising a pneumatically actuable piston, the assembly comprising a thrust bearing mounted between one end of the actuator and one of the sets of clutch members permitting rotation therebetween. It is an aim of the present invention to provide an improved wet clutch assembly. The present invention provides a wet clutch assembly having a rotational axis and comprising an input shaft rotatable about said axis, a cover having a fluid inlet and a fluid outlet, two sets of substantially parallel clutch members interleaved to form a stack within said cover, said stack being rotatable about said axis and being hydraulically actuable, and a hydraulic actuator piston slidably mounted on said cover and concentric with said input shaft, wherein said piston is arranged not to rotate with said shaft, said piston being operably connected to one of said sets by means permitting relative rotation therebetween wherein the said piston is annular and has a stepped profile, the piston comprising end portions extending in opposite directions and substantially axially with respect to the said rotational axis, the end portions being joined by a mid portion, wherein the respective radially innerfaces of the end portions comprise sealing surfaces in engagement with circumferential fluid seals of said cover and wherein, a fluid chamber is defined between the radially inner surface of the radially outermost end portion, the mid portion, and the fluid outlet of said cover. 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. One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 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 Figure 2 shows a front view of the clutch assembly of Figure 1. Referring to Figures 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 15 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 European patent application no. 95107224. 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. Figure 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, eg 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 the upper half of Figure 1) or as two separate items (as shown in the lower half of Figure 1). 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 Figure 1 and solid lines in Figure 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 Figure 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 Figure 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 Figure 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 Figure 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 Figure 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.

Claims: A wet clutch assembly having a rotational axis and comprising an input shaft rotatable about said axis, a cover (12) having a fluid inlet (50) and a fluid outlet (53), two sets of substantially parallel clutch members (36,38) interleaved to form a stack within said cover (12), said stack being rotatable about said axis and being hydraulically actuable, and a hydraulic actuator piston (44) slidably mounted on said cover (12) and concentric with said input shaft (16), wherein said piston is arranged not to rotate with said shaft, said piston (44) being operably connected to one of said sets by means (54) permitting relative rotation therebetween wherein the said piston (44) is annular and has a stepped profile, the piston (44) comprising end portions extending in opposite directions and substantially axially with respect to the said rotational axis, the end portions being joined by a mid portion, wherein the respective radially innerfaces of the end portions comprise sealing surfaces in engagement with circumferential fluid seals (48) of said cover (12) and wherein, a fluid chamber is defined between the radially inner surface of the radially outermost end portion, the mid portion, and the fluid outlet (53) of said cover (12). A wet clutch assembly according to Claim 1 wherein said circumferential fluid seals (48) are located within respective grooves of said cover. A wet clutch assembly as claimed in Claim 1 or Claim 2 wherein engagement of the clutch is by spring (40) pressure and disengagement by means of the said hydraulic actuator piston (44). A wet clutch assembly as claimed in any preceding Claim, wherein said piston (44) is journalled to a member which is rotationally fast to a first set of clutch members, by means of a bearing having inner and outer races and rolling members (54) between the said races. A wet clutch assembly as claimed in Claim 4, wherein the said inner race and the said piston constitute a unitary member.

Inventors: LORRIETTE PATRICK, LORRIETTE, PATRICK
Applicants: AGCO SA
Application year: 1995
Main classification: F16D25
IPC codes: F16D25