WET CLUTCH VALVE ARRANGEMENT

Abstract

A wet clutch (10) for mounting on a shaft (12) having an internal shaft conduit (88) for a hydraulic fluid is provided. The wet clutch (10) comprises: a clutch hub (16) configured to be mounted on the shaft (12), a clutch basket (18) configured to be rotationally supported relative to shaft (12), and a clutch pack (20) operationally connecting the clutch hub (16) and the clutch basket (18). The wet clutch (10) further comprises: a front part (34) configured to be fixed relative to the shaft, an actuator (22) supported by the front part (34) and radially spaced apart from the shaft (12), wherein the actuator (22) is configured to operationally connected to the shaft conduit (88). The wet clutch (10) further comprises: one or more relief valves (102) connected to the actuator and configured to release the hydraulic fluid.

Claims

1-24: (canceled)

25. A wet clutch for mounting on a shaft having an internal shaft conduit for a hydraulic fluid, wherein the wet clutch comprises: a clutch hub configured to be mounted on the shaft; a clutch basket configured to be rotationally supported relative to the shaft; a clutch pack operatively connecting the clutch hub and the clutch basket; a front part configured to be fixed relative to the shaft; an actuator supported by the front part and radially spaced apart from the shaft, wherein the actuator is configured to receive hydraulic fluid from the shaft conduit; and one or more relief valves connected to the actuator and configured to release the hydraulic fluid, wherein each relief valve has a closed state and an open state, each relief valve being spring-biased to be in the closed state, each relief valve being configured to reduce the spring biasing of the relief valve at a defined rotational speed of the shaft.

26. The wet clutch according to claim 25, wherein each relief valve is biased to transition from the closed state to the open state at a defined rotational speed of the shaft.

27. The wet clutch according to claim 25, wherein each relief valve is configured to be biased toward the closed state at a defined dynamic pressure in the hydraulic fluid at the relief valve.

28. The wet clutch according to claim 25, wherein each relief valve comprises a valve spring biasing the relief valve to be in the closed state.

29. The wet clutch according to claim 25, wherein each relief valve comprises a valve seat and a valve member configured to cooperate with the valve seat, the valve member having a closed position relative to the valve seat and an open position relative to the valve seat, and wherein the valve member is spring biased to be in the closed position.

30. The wet clutch according to claim 29, wherein the valve member is biased to change from the closed position to the open position at a defined rotational speed of the shaft.

31. The wet clutch according to claim 29, wherein the valve member is biased to change from the open position to the closed position at a defined dynamic pressure in the hydraulic fluid at the relief valve.

32. The wet clutch according to claim 29, wherein the valve member is configured to move relative to the valve seat in a direction that is transverse to shaft.

33. The wet clutch according to claim 29, wherein the valve seat is positioned between the valve member and the shaft.

34. The wet clutch according to claim 29, wherein each relief valve comprises a valve spring biasing the valve member to be in the closed position.

35. The wet clutch according to claim 25, wherein the actuator is biased away from the clutch pack, and the one or more relief valves are positioned at the actuator.

36. The wet clutch according to claim 25, wherein each of the one or more relief valves is configured to release the hydraulic fluid above a first rotationally induced pressure in the hydraulic fluid, wherein the first rotationally induced pressure is below a pressure at which the hydraulic fluid causes the actuator to engage the clutch pack.

37. The wet clutch according to claim 25, wherein each of the one or more relief valves is configured to release the hydraulic fluid to a space surrounding the wet clutch.

38. The wet clutch according to claim 25, wherein the actuator comprises: a recess formed by the front part and configured to operationally connect to the internal shaft conduit; and a piston positioned in the recess and configured to move axially relative to the shaft and to engage the clutch pack; wherein the recess is annular and concentric with the shaft, and the piston is ring-shaped and concentric with the shaft.

39. The wet clutch according to claim 38, wherein the piston is mechanically biased to disengage the clutch pack.

40. The wet clutch according to claim 38, wherein each relief valve is positioned in the piston.

41. The wet clutch according to claim 38, wherein each relief valve comprises a valve body that is formed by the piston.

42. The wet clutch according to claim 41, wherein the valve member engages the valve seat in a closing movement toward the shaft and disengages the valve seat in an opening movement away from the shaft.

43. The wet clutch according to claim 41, wherein the valve member is spring biased toward the valve seat, the valve member is biased away from the valve seat at a defined rotational speed of the wet clutch, and the valve member is positioned upstream of the valve seat with respect to a flow of hydraulic fluid from the recess to a space surrounding the wet clutch.

44. The wet clutch according to claim 41, wherein, in each relief valve, the valve seat separates the relief valve into an input side and an output side, the input side being in fluid communication with the recess, the output side being in fluid communication with a space surrounding the wet clutch, and a flow of hydraulic fluid from the input side to the output side biases the valve member toward the valve seat.

45. The wet clutch according to claim 25, wherein each of the one or more relief valves is configured to restrict a flow of hydraulic fluid passing through the internal shaft conduit and the actuator.

46. A gear assembly for mounting on a shaft having an internal shaft conduit for a hydraulic fluid, wherein the shaft passes completely through the gear assembly, wherein the gear assembly comprises: a gear wheel configured to be rotationally supported with respect to the shaft; and a wet clutch configured to be mounted on the shaft and to operationally connect to the internal shaft conduit, wherein the wet clutch comprises: a clutch hub configured to be mounted on the shaft; a clutch basket attached to the gear wheel and configured to be rotationally supported relative to the shaft; a clutch pack operatively connecting the clutch hub and the clutch basket; a front part configured to be fixed relative to the shaft; an actuator supported by the front part and radially spaced apart from the shaft, wherein the actuator is configured to receive hydraulic fluid from the shaft conduit; and one or more relief valves connected to the actuator and configured to release the hydraulic fluid, wherein each relief valve has a closed state and an open state, each relief valve being spring-biased to be in the closed state, each relief valve being configured to reduce the spring biasing of the relief valve at a defined rotational speed of the shaft.

47. A shaft assembly, comprising: a shaft having an internal shaft conduit for a hydraulic fluid; and a gear assembly mounted on the shaft and operationally connected to the internal shaft conduit, wherein the gear assembly comprises: a gear wheel configured to be rotationally supported with respect to the shaft; and a wet clutch configured to be mounted on the shaft and to operationally connect to the internal shaft conduit, wherein the wet clutch comprises: a clutch hub configured to be mounted on the shaft; a clutch basket attached to the gear wheel and configured to be rotationally supported relative to the shaft; a clutch pack operatively connecting the clutch hub and the clutch basket; a front part configured to be fixed relative to the shaft; an actuator supported by the front part and radially spaced apart from the shaft, wherein the actuator is configured to receive hydraulic fluid from the shaft conduit; and one or more relief valves connected to the actuator and configured to release the hydraulic fluid, wherein each relief valve has a closed state and an open state, each relief valve being spring-biased to be in the closed state, each relief valve being configured to reduce the spring biasing of the relief valve at a defined rotational speed of the shaft.

48. A gearbox assembly for a road vehicle, comprising: a shaft assembly comprising a shaft having an internal shaft conduit for a hydraulic fluid; and a gear assembly mounted on the shaft and operationally connected to the internal shaft conduit, wherein the gear assembly comprises: a gear wheel configured to be rotationally supported with respect to the shaft; and a wet clutch configured to be mounted on the shaft and to operationally connect to the internal shaft conduit, wherein the wet clutch comprises: a clutch hub configured to be mounted on the shaft; a clutch basket attached to the gear wheel and configured to be rotationally supported relative to the shaft; a clutch pack operatively connecting the clutch hub and the clutch basket; a front part configured to be fixed relative to the shaft; an actuator supported by the front part and radially spaced apart from the shaft, wherein the actuator is configured to receive hydraulic fluid from the shaft conduit; and one or more relief valves connected to the actuator and configured to release the hydraulic fluid, wherein each relief valve has a closed state and an open state, each relief valve being spring-biased to be in the closed state, each relief valve being configured to reduce the spring biasing of the relief valve at a defined rotational speed of the shaft; wherein the gearbox assembly further comprises: an oil sump configured to collect hydraulic fluid released from the wet clutch of the gear assembly; and a hydraulic control system comprising a hydraulic pump operationally connected to the oil sump and to the internal shaft conduit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0095] A more complete understanding of the abovementioned and other features and advantages of the proposed technology will be apparent from the following detailed description of preferred embodiments of the proposed technology in conjunction with the appended drawings, wherein:

[0096] FIG. 1a is a schematic cross section pf an embodiment of shaft assembly,

[0097] FIG. 1b is a schematic cross section of a front part with a piston fitted with a relief valve,

[0098] FIG. 1c is a schematic cross section of a piston with a relief valve with a valve member in the closed position,

[0099] FIG. 1d is a schematic cross section of a piston with a relief valve with a valve member in the open position, FIG. 2 illustrates a perspective cross-section of an embodiment of a gear assembly shown in FIG. 1,

[0100] FIG. 3 illustrates a perspective cross-section of a part of the gear assembly shown in FIG. 2,

[0101] FIG. 4 illustrates a perspective cross-section of another part of the gear assembly shown in FIG. 2,

[0102] FIG. 5 schematically illustrates an embodiment of a gearbox including the shaft assembly shown in FIG. 1, and

[0103] FIG. 6 schematically illustrates an alternative embodiment of a piston and a relief valve.

DETAILED DESCRIPTION OF THE DRAWINGS

[0104] FIG. 1a schematically illustrates a shaft assembly 6 that has a shaft 12 with an internal shaft conduit 88 for a hydraulic fluid and an additional internal shaft conduit 14 intended for carrying a combined coolant and lubricant. It also has a gear assembly 8 that is mounted on the shaft 12 and connected to the additional internal shaft conduit 14. The gear assembly 8 as a whole forms a through hole 68 receiving the shaft 12, whereby the shaft 12 passes through the complete gear assembly 8. The gear assembly 8 has a gear wheel 80 that is rotationally supported with respect to shaft 12. It also has a wet clutch 10 that is mounted on the shaft 12 and operationally connected to the additional internal shaft conduit 14. Details of the wet clutch 10 are shown in FIGS. 2-4.

[0105] The gear wheel 80 and the wet clutch 10 are concentric with respect to the shaft 12. The gear wheel 80 has an axially extending flange 82 that is also concentric with the shaft 12. The clutch basket 18 and the flange 82 overlap at the flange 82. The outer side of the flange 82 conforms to the inner side of the clutch basket 18 at the overlap, whereby the clutch basket 18 is attached to the flange 82, and in extension to the gear wheel 80.

[0106] The wet clutch 10 is a multi-plate clutch and the shaft 12 passes through the complete wet clutch 10. The wet clutch 10 has a clutch hub 16 that is mounted on the shaft 12 and radially fixed relative to the shaft 12 by way of splines. It further has a clutch basket 18 that is rotationally supported relative to shaft 12 and a clutch pack 20 that connects the clutch hub 16 and the clutch basket 18. The wet clutch 10 also has a front part 34 that is juxtaposed and attached to the clutch hub 16 by way of bolts. This way, the front part 34 is mounted on and rotationally fixed relative to the shaft 12.

[0107] The clutch hub 16 and the clutch basket 18 are concentric with respect to the shaft 12. The clutch hub 16 forms a through hole 62 and the front part 34 forms another through hole 64. This means that the clutch hub 10 as a whole forms a through hole 66 that receives the shaft 12.

[0108] The gear assembly 8 has a radial spacer 84 that is rotationally fixed relative to the clutch hub 16 by way of bolts. Thus, it is also rotationally fixed relative to the shaft 12. The gear assembly 8 further has a radial rolling bearing 86 with an inner race attached to the radial spacer 84 and an outer race attached to the gear wheel 80. The gear wheel 80 has a central through bore with a cylindrical inner wall, and the outer race conforms to and engages the inner wall of the through bore. The radial spacer 84 has a ring-shaped partly hollow body.

[0109] Clamps (not shown) are positioned on the shaft 12 on either side of the gear assembly 8 that axially fix the wet clutch 10, and in extension the clutch hub 16, the clutch basket 18, and the front part 34, relative to the shaft 12.

[0110] The clutch hub 16 has been manufactured from a single piece of material. Similarly, the front part 34 has been manufactured from a single piece of material. This means that both components individually constitute a unitary body.

[0111] The wet clutch 10 has a number of individual clutch conduits 48, more precisely 15 clutch conduits 48. Each has a front part portion 52 formed by the front part 34 and a hub portion 50 formed by the clutch hub 16. The front part portion 52 has an inlet 54 that can receive the coolant. The hub portion 50 is coupled to the front part portion 52 and has three outlets at the clutch pack 20 through which the coolant can be released. The outlets 24 are distributed axially with respect to the clutch hub 16, which means that they are distributed lengthwise with respect to the shaft 12.

[0112] The hub portion 50 of each clutch conduit 48 is elongated and aligned with the shaft 12. Each hub portion 50 has a cylindrical portion 56 with a circular cross-section and an axis that is parallel to the axis 90 of the shaft 12, as can be seen in FIG. 3. This means that all cylindrical portions 56 have parallel cylinder axes.

[0113] The through hole 64 of the front part 34 that receives the shaft 12 has a circumferential inner wall portion 72 facing the shaft 12. The front part 34 forms a circumferential groove 70 in the inner wall portion 72 that can receive the coolant from the additional internal shaft conduit 14, and the inlet 54 of the front part portion 52 of each clutch conduit 48 connects to the groove 70. The inner wall portion 72 facing the shaft 12 is flush with the outer surface of the shaft 12. This way, the individual clutch conduits 48 form part of a conduit arrangement 26 that connects the additional internal shaft conduit 14 to the outlets 24. The conduit arrangement 26 allows a flow of coolant from the additional internal shaft conduit 14 to be distributed at the clutch pack 20, thus having the function of a manifold.

[0114] The front part portions 52 and the hub portions 50 of the clutch conduits 48 are evenly distributed around the shaft 12. They have a 24-degree separation with respect to the rotational axis 90 of the shaft 12 between neighboring clutch conduits 48.

[0115] The clutch hub 16 has a number of axially extending ridges 44 that form part of a spline joint with the clutch pack 20. The outlets 24 of each clutch conduit 48 are located between a pair of neighboring ridges 44, or more precisely at the bottom of the single groove between neighboring ridges 44. There are forty-five ridges 44 and fifteen clutch conduits 48, which means that there are three times more of the former than the latter. The axially extending ridges 44 form male splines 44 cooperating with female splines 46 formed by the clutch pack 20.

[0116] The clutch pack 20 has three states. In the first state, or the unengaged state, the clutch hub 16 and the clutch basket 18 are unlocked and can spin at different speeds. In extension, this means that the gear wheel 80 can spin freely relative to the shaft 12. In the second state, or the slipping state, the clutch hub 16 and the clutch basket 18 are partly locked together but can spin at different speeds. This means that some torque is transferred from the shaft 12 to the gearwheel 80. In the third state, or the engaged state, the clutch hub 16 and the clutch basket 18 are locked together and spin at the same speed. This means that all torque supplied to the shaft 12 is transferred to the gear wheel 80.

[0117] The wet clutch 10 has 15 valves 28. Each valve 28 controls the flow of coolant through a single clutch conduit 48. The wet clutch 10 further has a single actuator 22 supported by the front part 34 and an annular pressure plate 40 that is concentric with respect to the shaft 12, as is shown in FIGS. 1a and 2. The pressure plate 40 is positioned between the actuator 22 and the clutch pack 20 such that it can engage the clutch pack 20 when the actuator 22 is activated. Additionally, the pressure plate 40 forms part of each valve 28, which means that it simultaneously engages the clutch pack 20 and operates the valves 28.

[0118] When activated, the actuator compresses the clutch pack 20 axially and the clutch pack 20 changes from the unengaged state to the engaged state via the slipping state.

[0119] The clutch pack 20 is concentric with respect to the clutch hub 16 and the shaft 12. The clutch basket 18 is concentric with respect to the clutch pack 20, and in extension with respect to the clutch hub 16. The clutch pack 20 has an annular shape and extends both radially and axially with respect to the axis of the shaft 90.

[0120] The clutch pack 20 has eight inner plates 30 attached to the clutch hub 16, which constitutes an inner plate carrier, and seven interleaved outer plates 32 attached to the clutch basket 18, which constitutes an outer plate carrier. The inner plates 30 can move axially relative to the clutch hub 16 and are rotationally fixed relative to the clutch hub 16. Similarly, the outer plates 32 can move axially relative to the clutch basket 18 and are rotationally fixed relative to the clutch basket 18.

[0121] The inner and outer plates 30 and 32 are positioned alternately in the clutch pack 20. In the unengaged state there is no mechanical friction between the inner plates 30 and the outer plates 32, in the slipping state there is a kinetic friction between the inner plates 30 and the outer plates 32, and in the engaged state there is a static friction between the inner plates 30 and the outer plates 32.

[0122] The clutch pack 20 forms channels 92 in a square grid pattern on both sides of each inner plate 30. Even though not radially oriented, the square grid on the circular plate 30 mean that all the channels 92 to some extent extend radially with respect to the shaft 12, which enables the coolant to flow radially outwards through the clutch pack 20.

[0123] As mentioned above, the clutch hub 16 has a number of outer male splines 44 and each inner plate 30 has the same number female splines 46 that cooperated with the male splines 44. Similarly, the clutch basket 18 has female splines 76 and each of the outer plates 32 has male splines 78 cooperating with the female splines 76.

[0124] The valves 28 have been constructed to prevent the flow of coolant through the clutch conduits 48 when the clutch pack 20 is in its unengaged state. It further allows the flow of coolant when the clutch pack 20 is in its slipping state and its engaged state. In some embodiments, the flow of coolant is up to ten times greater when the clutch pack 20 is in its engaged state than in its unengaged state, this means that there is a flow of coolant even if the valves 28 are in the closed state.

[0125] The front part 34 forms a valve seat 96 at each coupling between the hub portions 50 and the front part portions 52 of the clutch conduits 48. The valve seat 96 is a hard seat integral to the front part 34.

[0126] The pressure plate 40 is disc-shaped, planar, and has a rotational symmetry with respect to the axis 90 of the shaft 12. It has a central through hole 94 and forms a number of protrusions 60, more precisely 15 protrusions 60, each extending radially inwards in the central hole 94 as illustrated in FIG. 4. Each protrusion 60 constitutes a valve member, or valve disc, of a single valve 28 and seals against one of the valve seats 96 when the wet clutch 10 is in its unengaged state. In its engaged state, the pressure plate 40 is pushed by the actuator 22 such that a gap is formed between the protrusion 60 and the valve seats 96, thus allowing a flow of the coolant past the protrusions 60 and into the hub portions 50, from where it is expelled via the outlets 24.

[0127] A compression coil spring 58 is positioned in the cylindrical portion 56 of each hub portion 50. Each spring 58 engages a single protrusion 60 of the pressure plate 40, and the springs 58 jointly bias the pressure plate 40 with respect to the clutch hub 16 and pushes the pressure plate toward the actuator 22, thus acting to close the valves 28.

[0128] A valve is closed when the clutch pack 20, or wet clutch 10, is in its unengaged state, at which the protrusions 60 block the front part portions 52 of the clutch conduits 48. This way, the coolant is prevented from flowing through the clutch conduits 48 and reaching the clutch pack 20.

[0129] The actuator 22 has an annular recess 36 formed by the front part 34 and concentric with the axis 90 of the shaft 12. The recess 36 is connected to and in fluid communication with the internal shaft conduit 88. It further has a ring-shaped piston 38 concentric with the shaft 12 positioned in the recess 36 and that can move axially relative to the shaft 12 at an increase of the pressure of a hydraulic fluid in the recess 36. The annular recess 36 is connected to the additional internal shaft conduit 88. The actuator 22 is activated by increasing the pressure of the hydraulic fluid, which causes the ring-shaped piston 38 to move toward the clutch pack 20 and engage the wet clutch 10.

[0130] The ring-shaped piston 38 engages the annular pressure plate 40. In the slipping state and in the engaged state, the piston 38 presses against and axially loads the pressure plate 40. The plurality of springs 58 provides a counter force pushing the pressure plate 40 against the ring-shaped piston 38. By way of the pressure plate 40, the actuator 22 is configured to simultaneously engage the clutch pack 20 and operate the plurality of valves 28.

[0131] The wet clutch 10 has three relief valves 102 that are positioned 120 apart relative to the shaft 12. The valves 102 are positioned in the piston 38. Only one of the relief valves is shown in the schematic cross section of the shaft assembly 6 of FIG. 1a. The other two relief valves are identical in form and function. This means that the valve 10 has a plurality of relief valves that are positioned in a rotationally symmetric manner with respect to the rotational axis 90 of the shaft 12.

[0132] Each relief valve 102 has a valve body 108 and a valve seat 110, the latter separating the relief valve 102 in an input side 112 and an output side 114. The input side 112 is in fluid communication with the recess 36 and the output side 114 is in fluid communication with the surroundings of the wet clutch 10. The valve body 108 and the valve seat 110 are formed by the piston 38. The relief valve 102 further has a valve member 116 that is a spherical steel ball located inside the valve body 108. This means that a flow of a hydraulic fluid from the input side 112 to the output side 114, or a dynamic pressure of a hydraulic fluid, biases the valve member 116 toward the valve seat 110 if the relief valve 102 is open. Additionally, a static pressure in the input side 112 biases the valve member 116 toward the valve seat 110 if the relief valve 102 is closed.

[0133] The valve seat 110 is positioned between the valve member 116 and the shaft 12. The valve member 116 is mechanically biased toward the valve seat 110 by a compression coil spring 118. The valve spring 118 biases the valve member 116 toward the shaft 12 at a right angle to the rotation axis 90. This means that the valve member 116 engages the valve seat 110 in a closing movement toward the shaft 12 and disengages the valve seat 110 in an opening movement away from the shaft 12.

[0134] When the wet clutch 10 is rotated, the rotationally induced centripetal force will force the valve member 116 outwards. This force will be counteracted by the force from the valve spring 118 and by the rotationally induced pressure of the hydraulic fluid on the input side 112 of the relief valve 102. At some rotational speed, the centripetal force will overcome the counteracting forces, and the valve member 116 will move away from the valve seat 110 and the relief valve 102 opens to release the hydraulic fluid. This way, the relief valve is configured to release hydraulic fluid depending on the rotational speed of the shaft 12. A closed relief valve 102 is shown in FIG. 1c and an open relief valve 102 is shown in FIG. 1d.

[0135] The wet clutch 10 has an inlet release conduit 104 for the hydraulic fluid formed by a bore in the piston 38. The inlet release conduit 104 connects the actuator 22 and the relief valve 102. The wet clutch 10 further has an outlet release conduit 106 for the hydraulic fluid formed in part by a bore in the piston 38 and an open channel facing the piston 38 in the pressure plate 40. The outlet release conduit 106 connects the relief valve 102 to the surroundings of the wet clutch 10. The portion of the outlet release conduit 106 in the pressure plate 40 releases the hydraulic fluid at the outer edge of the pressure plate 40. This way, the relief valve 102, and in extension the wet clutch 10, is configured to release the hydraulic fluid in the actuator 22 to the surroundings of the wet clutch 10, thereby deactivating the actuator 22 if the actuator 22 is activated.

[0136] The actuator 22 is radially spaced apart from the shaft 12 and is connected to the shaft conduit 88, which means that a rotation of the shaft 12 builds up a pressure in the actuator 22. In a basic model of the function of the wet clutch 10, it is assumed that no pressure is actively generated and supplied through the internal shaft conduit 88 for activating the wet clutch 10. It is further assumed that P.sub.rot is the static pressure acting on the piston 38 that is generated from the radial column of hydraulic fluid when the wet clutch 10 is rotated. If F.sub.piston,bias is the force generated by the springs 58 biasing the piston 38 in the unengaged state, and A.sub.piston is the effective area of the piston, then the piston 38 engages clutch pack 20 if:

F.sub.piston,bias<A.sub.piston*P.sub.rot, or P.sub.rot>F.sub.piston,bias/A.sub.piston.

[0137] The pressure of the hydraulic fluid in the actuator 22 must overcome the biasing of the springs 58 for the actuator to engage the clutch pack, by changing the actuator 22 from an unengaged to an engaged state.

[0138] If F.sub.valve,rotation is the centripetal force acting on the valve member 116 at the rotation of the wet clutch 10, F.sub.valve,bias is the force generated by the valve spring 118 biasing the valve member 116, and A.sub.valve is the effective area of the valve member 116, then the relief valve 102 is opened if:

F.sub.valve,rotation>F.sub.valve,bias+A.sub.valve*P.sub.rot.

[0139] In a simple model, the centripetal force F.sub.valve,rotation is given by:

F.sub.valve,rotation=m.sub.valve*r.sub.valve*.sup.2,

where m.sub.valve is the mass of the valve member 116 and the valve spring 118, r.sub.valve the radial position of the relief valve 102 relative to the rotational axis 90 of the shaft 12, and w is the angular velocity at which the shaft 12 rotates. If the valve should always be open at a rotational activation of the wet clutch, then:

F.sub.valve,rotation>F.sub.valve,bias+(A.sub.valve/A.sub.piston)*F.sub.piston,bias.

[0140] The relief valve 102 can be configured to open before the hydraulic fluid reaches a pressure that activates the wet clutch 10 by increasing the centripetal force F.sub.valve,rotation. This can be done my increasing the weight of the valve member 116, for example by using a heavier material, or by placing relief valve 102 at a greater distance from the rotational axis 90 of the shaft 12. The earlier opening can also be achieved by reducing the total biasing force F.sub.piston,bias of the springs 58, or by reducing the biasing force F.sub.valve,bias of the valve spring 118. Alternatively, it could also be achieved by reducing the effective area A.sub.valve of the valve member 116.

[0141] In the present embodiment, the wet clutch 10 is activated at a rotational speed of about 6400 rpm, and the relief valve 102 is configured to open at a rotational speed of about 4200 rpm. This way, the relief valve 102 is configured to release the hydraulic fluid above a first rotationally induced pressure that is below the pressure at which the hydraulic fluid causes the actuator 22 to engage the clutch pack 20. Additionally, the relief valve 102 is configured to release hydraulic fluid above a first rotational speed of the of the shaft 12, e.g. resulting in the first rotationally induced pressure.

[0142] When the relief valve 102 is open and the pressure is increased in the internal shaft conduit 88 to activate the actuator 22, the hydraulic fluid will start to flow through the release valve 102, resulting in a dynamic pressure acting on the valve member 116, which causes the relief valve 102 to close.

[0143] The piston 38 forms an outer piston surface 122 facing outwards and an inner piston surface 124 facing inwards. The recess 36 forms an outer recess surface 126 facing inwards and an inner recess surface 128 facing outwards. The four surfaces are concentric and rotationally symmetric with respect to the shaft 12. The outer piston surface 122 faces the outer recess surface 126 and that inner piston surface 124 faces the inner recess surface 128 when the actuator 22 is unengaged.

[0144] The wet clutch 12 has a mechanical seal between the piston 38 and the front part 34 configured to prevent the hydraulic fluid from passing, or escaping, between the piston 38 and the front part 34. This way, the hydraulic fluid cannot escape the actuator 22 other than through the one or more relief valves 102.

[0145] The piston 38 has an outer gasket recess 130 in the outer piston surface 122 and the recess 36 has an inner gasket recess 132 in the inner recess surface 128. An outer gasket 134 is positioned in and supported by the outer gasket recess 130, and an inner gasket 136 is positioned in and supported by the inner gasket recess 132. The outer gasket 134 prevents the hydraulic fluid from passing on the outside of the piston 38 between the outer piston surface 122 and the outer recess surface 126, and the inner gasket 136 prevents the hydraulic fluid from passing on the inside of the piston 38 between the inner piston surface 124 and the inner recess surface 128, thus jointly forming a mechanical seal between the piston 38 and the front part 34 that prevents hydraulic fluid from passing there between.

[0146] The outer piston surface 122 is bisected in a front portion 138 and a rear portion 140 of equal lengths parallel to the rotational axis 90 of the shaft 12. The front portion 138 is closer to the clutch pack 22 than the rear portion 140. The outer gasket recess 130 is position on the rear portion 140 of the outer piston surface 122.

[0147] The inner recess surface 128 is bisected in a front portion 142 and a rear portion 144 of equal length parallel to the rotational axis 90 of the shaft 12. The front portion 142 is closer to the clutch pack 22 than the rear portion 144 The inner gasket recess 132 is position on the front portion 142 of the inner recess surface 128.

[0148] The annular pressure plate 40 is mechanically biased by the springs 58 in a direction away from the clutch pack 20 and toward the recess 36. The pressure plate 40 in turn biases the piston 38. In extension, this means that the piston 38 is biased by the springs to rest in the recess 36.

[0149] The valve body 108, or piston 38, form a cylindrical spring bore 146 that starts at the front portion 138 of the outer piston surface 122 and extends inwards toward to shaft 12.

[0150] The valve spring 118 is positioned in and aligned with the cylindrical spring bore 146.

[0151] The valve seat 110 is positioned at the inner end of the spring bore 146 and a spring base support 148 is positioned at the outer end of the spring bore 146. The valve spring 118 is biased between the spring base support 148 and the valve member 116. The cylindrical spring bore 146 has a female threading at its outer end and the spring base support 148 has a cooperating male threading. The spring base support 148 can be engaged and turned by an Allen key, whereby the positioned of the spring base support 148 in the spring bore 146, and in extension the length and tension of the valve spring 118, can be manually adjusted if the piston 38 is free from the recess 36.

[0152] The internal shaft conduit 88 of the shaft defines the smallest, or minimum, transverse area of about 13 mm.sup.2 upstream of the relief valve 102, which is understood to include the actuator 22. This corresponds to an internal shaft conduit 88 having a diameter of about 4 mm. The three relief valves 102 may have a combined smallest, or minimum transverse area of about 9 mm.sup.2 that is defined by the combined areas of inlet release conduits 104, which corresponds to diameter of 1 mm of each release conduit 104. This means that the three relief valves 102 are configured to restrict the flow of hydraulic fluid that is released, and that the pressure in the actuator 22 can be increased to the point where it is activated, even if the relief valves 102 are open. The valve member 116 has a diameter of 2 mm, while the cylindrical spring bore has a diameter of 2.3 mm.

[0153] The wet clutch 10 further has a radially and outwardly extending back part 42 mounted on and concentric with the shaft 12. The back part 42 is juxtaposed to the clutch hub 16 and the clutch pack 20 is positioned between the back part 42 and the front part 34. The back part 42 is attached to the clutch hub 16 by way of bolts. The clutch pack 20 is pressed against the back part 42 when the clutch pack 20 is engaged by the actuator 22 in the slipping state and in the engaged state of the wet clutch 10.

[0154] The clutch basket 18 has a cylindrical shape without endplates, as can be seen in FIG. 3. It has several apertures 74 through which the coolant can escape the wet clutch 10 radially with respect to the axis 90 of the shaft 12. There is also a gap between the clutch basket 18 and the front part 34 through which the coolant can escape the wet clutch 10.

[0155] FIG. 5 schematically illustrates a gearbox assembly 2 including a gearbox 4 and a hydraulic control system 150. The gearbox 4 is intended to be used in a road vehicle and has a shaft assembly 6 as described above. The shaft 12 of the shaft assembly 6 is the input shaft of the gearbox 4 intended to receive torque from an internal combustion engine (not shown). The gearbox further has a gearbox case 152 in which the wet clutch 10 of the shaft assembly 6 is positioned. The gearbox 4 has an oil sump 154 forming part of the gearbox case 152 that collects hydraulic fluid and coolant released from the wet clutch 10 of the gear assembly 8. The hydraulic fluid and the coolant are mixed in the oil sump 154. This means that there is only one type of liquid inside the gearbox case 152, and that the difference in terminology reflects its different uses.

[0156] The hydraulic control system 150 has a high-pressure hydraulic pump 156 and a control valve 158 connected in series between the oil sump 154 and to the internal shaft conduit 88. The hydraulic pump 156 receives the liquid collected by the oil sump 154 and supplies it as hydraulic fluid to the internal shaft conduit 88, which in turn supplies the hydraulic fluid to the actuator 22 of the wet clutch 10. The control valve 158 controls and regulates the flow of the hydraulic fluid from the hydraulic pump 156 to the internal shaft conduit 88, and in extension to the wet clutch 10. The engagement and disengagement of the wet clutch 10 is controlled by the control valve 158.

[0157] The gearbox assembly 2 further has a low-pressure coolant regeneration pump 160 connected to the oil sump 154 and to the additional internal shaft conduit 14 of the shaft 12. The regeneration pump 160 receives the liquid collected by the oil sump 154 and supplies it as coolant to the additional internal shaft conduit 14.

[0158] FIG. 6 schematically illustrates an alternative embodiment of a piston 38 and a relief valve 102 in which the inlet release conduit 104 has an inlet at the outer end of the piston 38 and the outlet release conduit 106 has an outlet at the outer end of the piston 38. The outer end of the piston is on a radially distant portion of the piston 38 with respect to the shaft 12, as is shown in FIG. 6.

ITEM LIST

[0159] 2 gearbox assembly [0160] 4 gearbox [0161] 6 shaft assembly [0162] 8 gear assembly [0163] 10 wet clutch [0164] 12 shaft [0165] 14 additional internal shaft conduit for coolant [0166] 16 clutch hub [0167] 18 clutch basket [0168] 20 clutch pack [0169] 22 actuator [0170] 24 outlets [0171] 26 conduit arrangement [0172] 28 valve [0173] 30 inner plates [0174] 32 outer plates [0175] 34 front part or collar [0176] 36 ring-shaped recess [0177] 38 ring-shaped piston [0178] 40 pressure plate [0179] 42 back part or flange [0180] 44 male spline of clutch hub [0181] 46 female spline of inner plates [0182] 48 clutch conduit [0183] 50 hub portion of individual clutch conduits [0184] 52 collar portion of individual clutch conduits [0185] 54 inlet of clutch conduit [0186] 56 cylindrical portion [0187] 58 spring [0188] 60 protrusions of pressure plate [0189] 62 through hole of clutch hub [0190] 64 through hole of collar [0191] 66 through hole of wet clutch [0192] 68 through hole of gear assembly [0193] 70 circumferential groove of collar [0194] 72 inner wall portion of through hole of collar [0195] 74 apertures of clutch basket [0196] 76 female splines of clutch basket [0197] 78 male splines of outer plates [0198] 80 gear wheel [0199] 82 axially extending flange of gear wheel [0200] 84 radial spacer [0201] 86 rolling bearing [0202] 88 internal shaft conduit for hydraulic fluid [0203] 90 axis of shaft [0204] 92 channels of inner plates [0205] 94 through hole of pressure plate [0206] 96 valve seat [0207] 98 gaskets [0208] 100 gap [0209] 102 relief valve [0210] 104 inlet release conduit [0211] 106 outlet release conduit [0212] 108 valve body of relief valve [0213] 110 valve seat of relief valve [0214] 112 input side of relief valve [0215] 114 output side of relief valve [0216] 116 valve member of relief valve [0217] 118 biasing valve spring of relief valve [0218] 120 valve conduit [0219] 122 outer piston surface [0220] 124 inner piston surface [0221] 126 outer recess surface [0222] 128 inner recess surface [0223] 130 outer gasket recess [0224] 132 inner gasket recess [0225] 134 outer gasket [0226] 136 inner gasket [0227] 138 front portion of outer piston surface [0228] 140 rear portion of outer piston surface [0229] 142 front portion of inner recess surface [0230] 144 rear portion of inner recess surface [0231] 146 spring bore [0232] 148 spring base support [0233] 150 hydraulic control system [0234] 152 gearbox case [0235] 154 oil sump [0236] 156 hydraulic pump [0237] 158 control valve [0238] 160 low-pressure coolant regeneration pump