The present invention relates to clutches, and more particularly, but not exclusively, to clutches for automotive applications, such as dual clutch transmissions. Aspects of the construction and operation of the clutches described herein may equally be applied to braking systems where a rotary member is acted upon by friction members.

The durability of a friction clutch deteriorates with an increase in clutch plate temperature. It is therefore important to ensure that the clutch plate does not exceed a desired working temperature. In a wet clutch the clutch components are contacted by lubricating fluid to reduce friction and transfer heat. The friction reducing effect of the lubricating fluid can result in an increase in the size of the clutch to compensate. Where space is limited a more compact dry clutch arrangement may thus be preferable. However, without the cooling effect of a lubricant, management of the clutch plate temperature is of increased importance.

Dual clutch transmissions typically include a pair of clutches, each having a driving plate and a movable pressure plate, for clamping a friction plate therebetween. There is a need to optimise the packaging and weight of such transmissions. The means by which the force applied to the pressure plates of such a transmission may be complex and thus add to the overall size and cost of the transmission.

An object of the invention is to provide an improved dry dual clutch assembly. Another object of the invention is to provide alternative means for applying force to the pressure plates of a dual clutch assembly.

According to a first aspect of the present invention there is provided a clutch assembly including clutch comprising a pressure plate, a friction plate and a driving plate, the driving and pressure plates being connected to a clutch input shaft and the friction plate being connected to clutch output shaft, the clutch assembly further including an electromagnetic actuation mechanism operable to move the pressure plate in the direction of the driving plate so as to clamp the friction plate therebetween, wherein the actuation mechanism comprises a solenoid including an electromagnet and a movable armature connected to the pressure plate through an aperture of the driving plate. It will be appreciated that the amount by which the armature is required to move will vary over time as the clutch wears and, for example, the thickness of the friction plate decreases. Accordingly, the actuation mechanism may be provided with an adjustment mechanism that maintains the amount by which the armature is required to move as the clutch wears. The adjustment mechanism may, for example, move the electromagnet so as to accommodate wear related displacement of the armature starting position over time. The adjustment mechanism may further include an arrangement which permits the movement of the pressure plate in the direction of the driving plate to accommodate thinning of the friction plate. The adjustment mechanism may further include means to centre the friction plate between the pressure and driving plates .

The actuation mechanism of such an embodiment may include a lever which interacts with the pressure plate to facilitate movement of the pressure plate, wherein the armature is connected to the lever such that attraction of the armature by the electromagnet causes pivoting of the lever and movement of the pressure plate. The armature of the actuation mechanisms may be provided with a projection which is received with a clearance in a correspondingly shaped recess of the or each respective electromagnet. The pressure and driving plates of such an embodiment may define respective moving and fixed armatures of the solenoid.

According to a second aspect of the present invention there is provided a dual clutch assembly having an input shaft, concentric inner and outer output shafts, a first clutch and a second clutch, each clutch comprising a pressure plate, a friction plate and a driving plate, the driving and pressure plates being connected to the input shaft and each friction plate being connected to a respective output shaft, wherein the clutch assembly is further provided with a first actuation mechanism operable to move the pressure plate of the first clutch and a second actuation mechanism operable to move the pressure plate of the second clutch, where the first and second actuation mechanisms are electromechanically operated. In such an embodiment each actuation mechanism may include a solenoid having a fixed electromagnet and a movable armature, and a lever which interacts with the clutch pressure plate to facilitate movement of the pressure plate, wherein the armature is connected to the lever such that attraction of the armature by the electromagnet causes pivoting of the lever and movement of the pressure plate.

It will be appreciated that the amount by which each armature is required to move will vary over time as the clutch with which it is associated wears and, for example, the thickness of the friction plate decreases. Accordingly, each actuation mechanism may be provided with an adjustment mechanism that maintains the amount by which the armature is required to move as the clutch wears. The adjustment mechanism may, for example, move the electromagnet so as to accommodate wear related displacement of the armature starting position over time. The adjustment mechanism may further include an arrangement which permits the movement of the pressure plate in the direction of the driving plate to accommodate thinning of the friction plate. The adjustment mechanism may further include means to centre the friction plate between the pressure and driving plates.

The armature may be provided with a projection which is received with a clearance in a correspondingly shaped recess of the electromagnet.

The friction plate of the first clutch may be connected to the outer output shaft and the friction plate of the second clutch may be connected to the inner output shaft. In such an embodiment the armature and electromagnet of the second clutch actuation mechanism may be provided on opposing sides of the friction plate of the first clutch. The friction plate of the first clutch may thus include a web which is at least partially composed of a non-ferromagnetic material and which does not block or reduce the magnetic flux generated by the electromagnet. The web of the first clutch pressure plate may be composed of non magnetic stainless steel. In an alternative embodiment the web of the first clutch pressure plate may be composed of a plastics material or a composite material. In yet a further embodiment the web of the first clutch pressure plate may include an annular portion composed of a non-ferromagnetic material. The clutches are preferably dry clutches.

According to a third aspect of the present invention there is provided the combination of an engine, a gearbox and a dual clutch assembly according to either of the first or second aspects, where the gearbox is provided between the engine and the dual clutch assembly.

In such a combination, the input shaft may extend through the gearbox from the engine to the clutch assembly. By providing the gearbox between the engine and dual clutch assembly the gearbox acts to insulate the clutch assembly from heat generated by the engine. The input shaft may be supported by a bearing provided in the clutch assembly which supports the end of the input shaft which is remote from the engine. The input shaft may further be supported by a further bearing at or near the mid point of the input shaft within the gearbox. According to a fourth aspect of the present invention there is provided a clutch pressure plate actuation mechanism comprising a solenoid and a lever pivotable about a fulcrum positioned between the ends of the lever, where the solenoid includes a fixed electromagnet and an armature connected to a first end of the lever, the second end of the lever being movable to apply force to a clutch pressure plate, wherein attraction of the armature by the electromagnet causes pivoting of the lever about the fulcrum and the application of force to the pressure plate.

The armature may be provided with a projection which is received with a clearance in a correspondingly shaped recess of the electromagnet.

According to another aspect of the invention, there is provided a dual clutch assembly having an input shaft, concentric inner and outer output shafts, a first clutch and a second clutch, each clutch comprising a pressure plate, a friction plate and a driving plate, the clutch driving and pressure plates being connected to the input shaft and each friction plate being connected to a respective output shaft, the clutch assembly is further provided with a first actuation mechanism operable to move the pressure plate of the first clutch and a second actuation mechanism operable to move the pressure plate of the second clutch, where the first and second actuation mechanisms are electromechanically operated,

wherein the second actuation mechanism comprises a solenoid having a fixed electromagnet and a movable armature, and wherein at least part of the armature is movable within a recess of the electromagnet.

In exemplary embodiments, the armature is provided with a projection which is received with a clearance in a correspondingly shaped recess of the electromagnet.

In exemplary embodiments, the movable armature includes a tapered or conical part arranged to be received within a recess of the electromagnet.

In exemplary embodiments, the second actuation mechanism is operable to apply a magnetic field through the friction plate of the first clutch. In exemplary embodiments, the fixed electromagnet and movable armature are arranged on opposite sides of the friction plate of the first clutch.

In exemplary embodiments, the clutches are dry clutches. According to another aspect of the invention, there is provided a combination of an engine, a gearbox and a dual clutch assembly according to the above aspect of the invention, where the gearbox is provided between the engine and the dual clutch assembly. In exemplary embodiments, the input shaft extends through the gearbox from the engine to the clutch assembly. In exemplary embodiments, the input shaft is supported by a bearing provided in the clutch assembly which supports the end of the input shaft which is remote from the engine, and by a further bearing at or near the mid point of the input shaft within the gearbox.

According to another aspect of the invention, there is provided a clutch pressure plate actuation mechanism comprising a solenoid and a lever pivotable about a fulcrum positioned between the ends of the lever, where the solenoid includes a fixed electromagnet and an armature connected to a first end of the lever, the second end of the lever being movable into contact with a clutch pressure plate, wherein attraction of the armature by the electromagnet causes pivoting of the lever about the fulcrum and movement of the pressure plate towards an engaged position.

In exemplary embodiments, part of the armature is movable within a recess of the electromagnet.

In exemplary embodiments, the armature is provided with a projection which is received with a clearance in a correspondingly shaped recess of the electromagnet. In exemplary embodiments, the armature includes a conical or tapered portion arranged to be received within a recess of the electromagnet.

According to another aspect of the invention, there is provided a method of cooling a dry plate clutch assembly comprising the steps of:

providing a dry plate clutch assembly within an enclosed clutch basket; and passing a cooling fluid over the exterior of the clutch basket to cool the clutch assembly.

In exemplary embodiments, the cooling fluid is lubricating oil.

In exemplary embodiments, the lubricating oil is supplied from a gearbox to which the clutch assembly is connected. In exemplary embodiments, the method includes the steps of:

directing lubricating oil from a gearbox to the clutch basket; and

returning the lubricating oil to the gearbox after it has passed over the clutch basket. In exemplary embodiments, the step of directing the oil from the gear box to the clutch basket comprises the step of directing the oil through a clutch input shaft extending from the gear box to the clutch assembly.

In exemplary embodiments, the interior of the shaft is configured so as to promote the flow of oil therethrough.

In exemplary embodiments, the interior of the shaft is provided with one or more formations which promote or induce the flow of oil in a predetermined direction when the shaft is rotating.

According to another aspect of the invention, there is provided a method of heating the lubricant in a gearbox comprising the steps of;

providing a gearbox having a clutch assembly connected thereto;

directing lubricating oil from the gear box to the clutch assembly;

passing the lubricating oil over the exterior of the clutch assembly so that it becomes heated by heat emanating from the clutch assembly; and

returning the heated oil to the gearbox.

According to another aspect of the invention, there is provided a clutch assembly including clutch comprising a pressure plate, a friction plate and a driving plate, the driving and pressure plates being connected to a clutch input shaft and the friction plate being connected to a clutch output shaft, the clutch assembly further including an electromagnetic actuation mechanism operable to move the pressure plate in the direction of the driving plate so as to clamp the friction plate therebetween, wherein the actuation mechanism comprises a solenoid including an electromagnet wherein the pressure and driving plates define respective moving and fixed armatures of the solenoid. In exemplary embodiments, the clutch assembly includes a clutch basket, and wherein the driving plate is fixed to the clutch basket via a thermally conductive but magnetically insulative material. In exemplary embodiments, the clutch assembly includes a clutch basket, and wherein the pressure plate is fixed to the clutch basket via a thermally conductive but magnetically insulative material.

In exemplary embodiments, the electromagnet and driving plate are provided with projections which are received in complementarily shaped recesses of the driving plate and pressure plate respectively.

In exemplary embodiments, the friction plate defines a web shaped to accommodate the respective projection and recess of the driving and pressure plates.

In exemplary embodiments, the pressure plate and/or driving plate include air flow apertures to facilitate airflow to the friction plate.

According to another aspect of the invention, there is provided a dual clutch transmission comprising a clutch assembly according to the above aspect of the invention.

According to another aspect of the invention, there is provided a clutch assembly including clutch comprising a pressure plate, a friction plate and a driving plate, the clutch driving and pressure plates being connected to a clutch input shaft and the friction plate being connected to a clutch output shaft, the clutch assembly further including an electromagnetic actuation mechanism operable to move the pressure plate in the direction of the driving plate so as to clamp the friction plate therebetween, wherein the actuation mechanism comprises a solenoid including an electromagnet and a movable armature connected to the pressure plate through an aperture of the driving plate. In exemplary embodiments, the assembly further comprises a mechanism for selectively latching the pressure plate in an apply direction after energisation of the electromagnet. In exemplary embodiments, the mechanism comprises a bi-stable latching mechanism.

In exemplary embodiments, the bi-stable latching mechanism comprises a pair of cam rings, wherein the first ring is arranged for axial displacement, and the second ring is arranged for rotation in response to axial displacement of the first ring, to bring about bi-stable latching engagement between the second ring and a fixed latch tooth.

In exemplary embodiments, the latch mechanism is provided on a side of the friction plate distal from the electromagnet. According to another aspect of the invention, there is provided a clutch assembly including clutch comprising a pressure plate, a friction plate and a driving plate, the clutch driving and pressure plates being connected to a clutch input shaft and the friction plate being connected to a clutch output shaft, the clutch assembly further including an electromagnetic actuation mechanism operable to move the pressure plate in the direction of the driving plate so as to clamp the friction plate therebetween, wherein the electromagnetic actuation mechanism comprises a solenoid including an electromagnet and an armature arranged for movement if the electromagnet is energised, and wherein movement of the armature brings about movement of the pressure plate, and further comprising a mechanism for selectively latching the pressure plate in an apply direction after energisation of the electromagnet.

In exemplary embodiments, the mechanism comprises a bi-stable latching mechanism. In exemplary embodiments, the bi-stable latching mechanism comprises a pair of cam rings, wherein the first ring is arranged for axial displacement, and the second ring is arranged for rotation in response to axial displacement of the first ring, to bring about bi-stable latching engagement between the second ring and a fixed latch tooth. In exemplary embodiments, the armature is connected to the pressure plate through an aperture of the driving plate. In exemplary embodiments, the latch mechanism is provided on a side of the friction plate distal from the electromagnet.

Embodiments of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a schematic cross-sectional view of an engine and gearbox having a dual clutch assembly according to an aspect of the invention;

Figure 2 shows a schematic cross-sectional view of a dual clutch assembly according to an aspect of the invention;

Figure 3 shows a schematic cross-sectional view of a dual clutch assembly according to an aspect of the invention;

Figure 4 shows a schematic cross-sectional view of a dual clutch assembly according to an aspect of the invention;

Figures 5a and 5b show schematic cross-sectional views of solenoids forming part of a clutch actuation mechanism according to a further aspect of the invention;

Figures 6a and 6b show schematic cross-sectional views of solenoids forming part of a clutch actuation mechanism according to a further aspect of the invention;

Figure 7 shows a schematic cross-sectional view of a gearbox having a dual clutch assembly according to an aspect of the invention;

Figure 8 shows a schematic cross-sectional view of a clutch actuation system according to an aspect of the invention;

Figure 9 shows a schematic cross-sectional view of a clutch actuation system according to an aspect of the invention;

Figure 10 shows the clutch actuation system of Figure 9 including an adjustment mechanism;

Figure 11 is a graph showing force against travel for a solenoid; Figures 12a and 12b show schematic cross-sectional views of solenoids forming part of a clutch actuation mechanism according to another aspect of the invention;

Figure 13 shows a schematic cross-sectional view of a modified clutch actuation system according to an aspect of the invention; and

Figures 14 to 17 show a schematic view of a latching mechanism for holding the clutch in a closed condition, between a clutch open condition and a clutch closed condition. Referring firstly to Figure 1 there is shown an engine 10, a gearbox 12 and a dual clutch assembly 14 which are mounted transversely between opposing chassis rails 16 of a vehicle. The gearbox 12 is positioned intermediate the engine 10 and clutch assembly 14. A clutch input shaft 18 extends though the gearbox 12 to the clutch assembly 14. Concentric inner and outer gearbox input shafts 20,22 extend from the clutch assembly 14 to the gearbox 12. A torsional damper 24 is provided between the clutch input shaft 18 and the engine 10.

By positioning the clutch assembly 14 on the opposite side of the gearbox 12 to the engine 10 the clutch assembly 14 is insulated by the gearbox 12 from heat generated by the engine 10. The clutch assembly 14 is also rendered accessible for servicing and repair after removal of the clutch cover 26.

Referring now to Figure 2 the dual clutch assembly 14 is shown in greater detail. The assembly 14 includes a first clutch 30, a second clutch 32 and a basket 34. The basket 34 is connected to the clutch input shaft 18. The first clutch 30 includes a driving plate 36, a pressure plate 38 and a friction plate 40. The driving plate 36 is fixed to the basket 34 while the pressure plate 38 is movably connected to the basket 34 by straps 42, one of which is shown. It will be understood that a plurality of such straps 42 are provided around the periphery of the pressure plate 38. The friction plate 40 is rotationally connected to the outer gearbox input shaft 22, but is able to move axially on the input shaft 22, for example by virtue of a splined connection therebetween. The second clutch 32 includes a driving plate 44, a pressure plate 46 and a friction plate 48. The driving plate 44 is fixed to the basket 34 while the pressure plate 46 is movably connected to the first clutch driving plate 36 by straps 50 (only one of which is shown), which are provided around the periphery of the pressure plate 46. The second clutch pressure plate 46 may alternatively be movably connected to the basket 34. The second clutch friction plate 48 is rotationally connected to the inner gearbox input shaft 20 and axially movable thereupon.

The clutch input shaft 18 is supported by a first bearing 52 positioned at the end of the shaft distal to the engine 10. This bearing 52 is provided within the shaft 18 and is supported by the clutch cover 26. As can be seen from Figure 1, the clutch cover 26 is connected to the gearbox 12. As such, the bearing 52 is indirectly supported by the gearbox 12. The clutch input shaft 18 is further supported by an additional bearing 54 which is positioned between the first bearing 52 and the engine 10. The additional bearing is provided around the clutch input shaft 18. This additional bearing 54 may be provided at or in the region of the midway point of the clutch input shaft 18. The clutch input shaft 18 may be provided with a plurality of additional bearings 54 along its length.

The friction material provided on the opposing faces of each friction plate 40,48 is preferably bonded to the web 56,58 of each plate 40,48 as opposed to being riveted. Bonding of the friction material reduces the thickness of the plates 40, 48 and provides a degree of cushioning or compliance between the friction material and its respective plate 40,48. The pressure and driving plates 38,46,36,44 of the clutches 30,32 may be made from iron and specifically grey cast iron. The webs 56,58 of the or each friction plate 40,48 may include a rotational damper arrangement.

Each clutch 30, 32 is provided with an actuator generally designated 60 for the first clutch 30 and 62 of the second clutch 32. The first clutch actuator 60 includes a lever 64 which is pivotable about a fulcrum point 66 provided on the rear face 68 of the clutch basket 34, and a solenoid 70. The solenoid 70 comprises an armature 72 connected to the lever 64 and an electromagnet 74 mounted at a fixed position, for example on the casing of the gear box 12. The armature 72 is mounted to the radially inner end of the lever 64 while the radially outer end of the lever 64 contacts the pressure plate 38. The armature 72 may be annular. The electromagnet 74 may be annular. Alternatively, a plurality of separate electromagnets 74 may be provided and arranged in a circle having a diameter corresponding to that of the armature. The lever 64 is configured so as to be very stiff with minimal deflection in use. Energisation of the electromagnet 74 attracts the armature 72 and causes the lever 64 to pivot about the fulcrum point 66. The armature 72 is moved towards, but not into contact with, the electromagnet 74. The pressure plate 38 is thus urged against the friction plate 40, which in turn is urged against the driving plate 36. Sufficient force is applied to couple the basket 34 to the friction plate 40 with the result that torque is transferred from the input shaft 18 to the outer clutch assembly output shaft 22 (i.e. the outer gearbox input shaft 22). Variation of the current applied to the electromagnet 74 varies the force applied to the pressure plate 38 by the lever 64 and thus allows the clutch 30 to be slipped. The inherent resilience of the strap 42 biases the pressure plate 38 away from the friction plate 40 when the current applied to the electromagnet is reduced or stopped. The first clutch 30 may thus be considered to be normally open when no power is applied to the electromagnet 74.

The second clutch actuator 62 is similar to the first clutch actuator 60 in that it includes a solenoid 76 having an annular armature 78, an electromagnet 80 and a rigid lever 82. The armature 78 is connected to the lever 82, and the lever is pivoted about a fixed location 84 on the first clutch driving plate 36. A conical spring 86 is provided between the lever 82 and the first clutch driving plate 36 which urges the lever 82 against the second clutch pressure plate 46, and in turn clamps the friction plate 48 between the pressure and driving plates 44,46. In this configuration torque is transferred from the input shaft 18 to the inner clutch assembly output shaft 20 (i.e. the inner gearbox input shaft 20).

Energisation of the electromagnet 80 attracts the armature 78 and causes the lever 82 to pivot about the fixed location 84. The armature 78 is moved towards, but not into contact with, the electromagnet 80. The spring 86 is thus compressed and the inherent resilience of the strap 50 moves the pressure plate 46 away from the friction plate 48. Variation of the current applied to the electromagnet 80 varies the force applied to the pressure plate 46 by the lever 82 and thus allows the clutch 32 to be slipped. The second clutch 32 may thus be considered to be normally closed when no power is applied to the electromagnet 80.

It will be noted that the solenoid 76 of the second clutch actuator 62 acts through the web 56 of the first clutch friction plate 40, with the armature 78 being provided on one side of the web 56 and the electromagnet 80 on the opposing side. The web 56 of the first clutch friction plate 40 is therefore provided with a section in the region of the second solenoid that is non-ferromagnetic and which does not block or reduce the magnetic flux generated by the electromagnet 80. For example, the web 56 may be manufactured from non-magnetic stainless steel or have an annular section which, in use, is aligned with the solenoid of the second clutch actuator 62 which is of nonmagnetic stainless steel. The annular section of non-magnetic stainless steel may be provided between respective inner and outer annular sections of ferromagnetic material. The annular sections may be connected to one another by any appropriate means such as, for example, welding. In an alternative embodiment the web 56 may be manufactured at least partially from a non-metallic material having the required characteristics such as, for example, a plastics material or a composite material.

As the first clutch 30 is normally open and the second clutch normally closed, it will be appreciated that in the event of both solenoids 70,76 failing, the clutch assembly 14 will not be able to simultaneously engage drive to both output shafts 20,22. Instead, the second clutch 32 will be engaged arid the vehicle kept in a gear associated with the inner out put shaft 20. It will further be appreciated that no actuation power is needed to keep the second clutch 32 engaged as the necessary clamping force is applied by the spring 86.

Figure 3 shows an alternative arrangement of a clutch assembly generally designated 88. Features common to the earlier described embodiment are identified with like reference numerals. The clutch assembly 88 differs in that the solenoids 70,76 have been moved radially outward with the result that the levers 64,82 are shortened. This arrangement has the advantage that the levers 64,82 are less susceptible to bending. However, the rotational mass of the assembly 88 is increased due to the greater diameter of the armatures 72,78. The levers 64, 82 have a more convoluted shape, while the web 56 of the first clutch friction plate 40 is dished to accommodate the position of the second solenoid 76. The solenoid 70 for the first clutch 30 is radially inward of the solenoid 76 for the second clutch 32 which is the opposite of the arrangement shown in Figure 2. The arrangement shown in Figure 3 is more compact than that shown in Figure 2, with the result that a greater annular space is provided around the output shafts 20,22. This space may be utilised to, for example, receive one or more components of a hybrid drive arrangement.

Figure 4 shows an alternative arrangement of a clutch assembly generally designated 90. Features common to the earlier described embodiment are identified with like reference numerals. The clutch assembly 90 differs in that the solenoid armatures 72,78 are provided with tapered projections 92, 94. The fixed electromagnets 74, 80 each have a coil or windings 83 mounted within a stator body 75, 81. The stator bodies 75, 81 each define a recess 96, 98 which corresponds generally to the shape of the tapered projections 92, 94. The tapered projections 92, 94 are partially received in the recesses 96,98.

Arranging the moving armatures 72, 78 at least partially within a recess of the stator body 75, 81 improves the performance of the solenoid, allowing for improved control of the armatures 72, 78, compared with flat-faced closing air gap solenoid arrangements, e.g. of the kind illustrated in Figures 2 and 3. For example, flux coupling or flux linkage can occur between the overlapping portions.

Moreover, the tapered form of the armatures 72,78 (e.g. defining generally conical tips) increases the linearity of the force to travel characteristic of each solenoid 70,76.

In order to accommodate this kind of recessed electromagnet arrangement, the web 56 of the first clutch friction plate 40 has to be configured to enable at least the tip 94 of the moving armature 78 to be movably received within the recess 98. In the illustrated embodiment, the web 56 of the first clutch friction plate 40 includes a convoluted portion 100 which mirrors the shape of the projection 94 and recess 98. Figures 5a to 6b show different electromagnet and armature configurations. Figure 5a shows the arrangement of Figures 2 and 3 where there is no extension of the armature 78 into the body 81, and the web 56 of the first clutch friction plate 40 is planar. Figure 5b shows the arrangement of Figure 4 where a tapered projection 94 (e.g. of generally conical configuration) of the armature 78 is partially received in correspondingly shaped recesses 98 of the body 81, and the web 56 of the first clutch friction plate 40 is provided with a convoluted portion 100 which mirrors the shape of the projection 94 and recess 98. Figures 6a and 6b show alternative arrangements where the armature is provided with two projections 94a,94b and the stator body 81 has two correspondingly shaped recesses 98a,98b. In Figure 6a the projections 94a,94b and recesses 98a,98b are of substantially uniform cross-section along their length, while in Figure 6b, the projections 94a,94b are provided with a root portion of substantially uniform cross- section and a tapered tip portion. The recesses 98a,98b mirror this shape with an appropriate clearance. In Figures 5b to 6b the convoluted portion 100 of the web 56 can be seen. While Figures 5a to 6b show the arrangement of the second solenoid 76 where the components are provided on either side of the web 56, it will be understood that the armature and electromagnet configurations are equally applicable to the first solenoid 70.

Figures 12a and 12b show modified embodiments in which the web 56 of the driven plate of the first clutch includes ferromagnetic material (indicated at 200) specifically arranged within the intended magnetic flux path of the solenoid, e.g. between opposing surfaces of the moving armature 78 and the stator body 81. This helps to direct magnetic flux between the fixed electromagnet 80 and the moving armature 78, thereby increasing the magnetic force and reducing the current requirements of the solenoid. This may enable the size of the solenoid to be reduced. For the embodiment of Figure 12a, the stator body 81 defines an exposed end face 202, and the ferromagnetic material 200 is generally aligned with said exposed end face 202, but not with the coil 83 of the electromagnet 80. In exemplary embodiments, the ferromagnetic material comprises a ring of material arranged between the movable armature 78 and the exposed end face 202 of the stator body 81.

In the embodiment of Figure 12b, the ferromagnetic material 200 is provided on the convoluted region 100 of the driven plate (i.e. the part which extends into the recess 98), so as to be arranged between the tip 94 of the armature 78 and the internal wall of the recess 98.

In both embodiments, the ferromagnetic material 200 defines an annulus about a non- ferromagnetic section 204 arranged between the movable armature 78 and the coil 83 of the electromagnet 80.

The ferromagnetic material 200 helps to concentrate or entrain the flux path (e.g. as indicated generally at 206 in Figures 12a and 12b).

It will be understood that the electromagnets may be of annular configuration, e.g. arranged concentric with the centre line of the clutch. In alternative embodiments, each actuation mechanism may comprises a plurality of discrete electromagnets in a spaced array around the centre line of the clutch, e.g. three electromagnets at a 120 degree spacing or four electromagnets at a 90 degree spacing, etc.

With reference to the above described embodiments, it will be appreciated that as the amount by which each armature is required to move will vary over time as the clutch with which it is associated wears and, for example, the thickness of the friction plate decreases. Accordingly, each actuation mechanism or actuator may be provided with an adjustment mechanism 61 that maintains the amount by which the armature is required to move as the clutch wears. The adjustment mechanism 61 may, for example, move the electromagnet so as to accommodate wear related displacement of the armature starting position over time.

Turning now to Figure 7 there is shown a gearbox 12 and a clutch assembly 14. Features common to the above described embodiments are identified with like reference numerals. The clutch assembly 14 is cooled by the circulation of lubricating oil from the gearbox 12 around the clutch basket 34. Oil from the gearbox 12 is directed into the clutch input shaft 18 as indicated by arrow 102. The oil is then conveyed through the shaft 18 and the bearings 52,54 upon which the shaft 18 is supported. Formations 104 may be provided on the inner surface of the shaft 18 to move the oil in the direction of the clutch assembly 14 as a result of rotation of the shaft. The oil then passes to the end space 106 defined between the clutch basket 34 and the clutch cover 26. Centripetal force applied to the oil as a result of the rotation of the clutch basket 34 forces the oil over the end face 108 and side wall 110 of the clutch basket 34 before the oil returns to the gearbox 12. Appropriate seals are provided to prevent oil from entering the clutch basket and contaminating the clutches 30,32 The end face 108 and side wall 110 may be provided with formations 112,114 which promote the flow of oil in the directions indicated as a result of rotation of the clutch basket 34. When rotation of the clutch basket 34 ceases, for example upon shutdown of the engine, it will be understood that the flow of oil around the clutch basket 34 will also cease. It is therefore desirable for oil present upon the surface of the clutch basket 34 to drain from the basket 34 so as not to present a combustion risk. Oil present on the surface of the clutch basket 34 may be caused to ignite due to heat emanating from the clutch assembly after engine shutdown. The formations 112,114 provided on the end face 108 and side wall 110 may promote the draining of oil from the surface of the basket 34 when rotation has ceased. Alternatively or in addition, the surface of the basket may be provided with an oleophobic coating or surface. In addition to maintaining the clutch assembly 14 at a desired working temperature, the circulation of gearbox lubricating oil around the clutch assembly 14 heats the oil upon start up as more heat in generated in the clutch assembly 14 than the gearbox 12. The gearbox is thus brought up to operating temperature more quickly with an consequential reduction in C02 emissions from the vehicle. The reduced C02 emission results from reduced gearbox drag as a result of the heated oil having a lower viscosity than cold oil. Figure 8 shows a schematic representation of an exclusively electromagnetic arrangement, generally designated 116, for actuating a clutch 118. Features common to the previously described embodiments are identified with like reference numerals. The clutch 118 includes a friction plate 40 which is connected to the outer output shaft 22. A driving plate 120 is fixed to the clutch basket 34 and a pressure plate 122 is movably mounted to the clutch basket 34 by a strap 124. Both the driving plate 120 and pressure plate 122 define armatures of a solenoid which is completed by an electromagnet 126. Both the driving plate 120 and the pressure plate 122 are manufactured from soft iron or magnetically similar material with cast iron inserts to contact the friction plate 40. The driving plate 120 is fixed to the clutch basket 34 via a thermally conductive but magnetically insulative material such as aluminium. The pressure plate 122 is mounted with similar considerations. The electromagnet 126 and driving plate 120 are provided with projections 128,130 which are received in complementarily shaped recesses 132,134 of the driving plate 120 and pressure plate respectively. The projections 128,130 and recesses 132,134 permit flux coupling as mentioned above. The electromagnet 126, driving plate 120 and pressure plate 122 may be provided without the projections and recesses and have substantially flat faces. The web 56 of the friction plate 40 requires shaping to accommodate the respective projection 130 and recess 134 of the driving and pressure plates 120,122.

Upon energisation of the electromagnet 126 the pressure plate 122 is drawn towards the driving plate 120 and the friction plate 40 is clamped therebetween. Torque can thus be transmitted from the clutch input shaft 18 to the outer output shaft 22. Variation of the current applied to the electromagnet 126 varies the force applied by the pressure plate 122 to the friction plate 40 and thus allows the clutch 118 to be slipped. The resilience of the strap 124 moves the pressure plate 122 away from the friction plate 40 when current to the electromagnet 126 is reduced or stopped.

The pressure and driving plates 120,122 surround the friction plate 40 to a greater amount than in a conventional clutch. Apertures therefore need to be provided in the pressure and/or driving plates 120,122 to facilitate air flow to the friction plate 40 and the egress of wear dust. The electromagnetic arrangement 116 of Figure 8 may be used to actuate one or both of the clutches in the previously described dual clutch arrangements. The electromagnetic arrangement 116 of Figure 8 may further incorporate an adjustment 117 mechanism which takes into account the decrease in the thickness of the pressure plate 40 as the clutch 118 wears. The adjustment mechanism would seek to maintain the distance by which the pressure plate 122 is required to travel in order engage the clutch 118.

Figure 9 shows an alternative electromagnetic actuation arrangement, generally designated 136, for actuating a clutch 138. Features common to the previously described embodiments are identified with like reference numerals. The clutch 138 includes a friction plate 40 which is connected to the outer output shaft 22. A driving plate 140 is fixed to the clutch basket 34 and a pressure plate 142 is movably mounted to an armature 144 of a solenoid. The solenoid is completed by an electromagnet 126. The armature 144 has an extension 148 which extends through an aperture 150 of the driving plate 140. The electromagnet 126 is provided with projections 152 which are received in complementarily shaped recesses 154 of the armature 144. The projections 152 and recesses 154 permit flux coupling as mentioned above. The electromagnet 126 and armature 144 need not be provided with said projections and recesses and may thus have substantially flat faces. Upon energisation of the electromagnet 126 the armature 144 and pressure plate 142 are drawn towards the driving plate 140 and the friction plate 40 is clamped between the pressure and driving plates 142,140. Torque can thus be transmitted from the clutch input shaft 18 to the outer output shaft 22. Variation of the current applied to the electromagnet 126 varies the force applied by the pressure plate 142 to the friction plate 40 and thus allows the clutch 138 to be slipped. The electromagnetic arrangement 136 of Figure 9 may be used to actuate one or both of the clutches in the previously described dual clutch arrangements.

A modified variant is shown in Figure 13, which includes a mechanism 210 for latching the pressure plate 142 in an apply direction after the electromagnet has been energised. The mechanism 210 permits the load path to be local to the clutch system and avoids the requirement for sustained reaction of actuation loads against bearings within the gearbox or engine. The mechanism 210 also enables a reduction in the consumption of electrical power in the solenoid windings.

In general terms, the latch mechanism 210 is used to engage a fixed mechanical stop that acts to maintain the clamp load if the actuation force is reduced or removed. In exemplary embodiments, the latch mechanism 210 is of the kind described in WO2007/045841, and is configured to provide bi-stable states, for latching or releasing the clutch from an open to a closed state, and vice versa, in response to actuator forces from the electromagnet.

In Figure 13 (and as will be typical for most embodiments described herein), the latch mechanism 210 has first and second latch elements 212, 214 in the form of cam rings. These are spaced axially from one another and concentric with the clutch axis. The first latch element 212 is arranged for axial movement only (e.g. left to right as shown in Figure 13). The second latch element 214 is arranged for axial movement and rotation in a first direction. A resilient element 216 acts between the second latch element 216 and the pressure plate 142, e.g. to bias the second latch element in a return direction, to the left as viewed in Figure 13. The resilient element may be of annular configuration, and may take the form of a Belleville spring. The latch mechanism 210 also includes a ground element 218 which is fixed to the basket 34. The ground element 218 provides fixed stop features or teeth 220, for use in holding the clutch in a closed condition, as will now be described with reference to Figures 13 to 17. In Figures 14 to 17, the rings 212, 214 are shown 'unwound', for ease of illustration.

In Figure 14, the clutch is open, i.e. there is no electromagnetic force. The latch mechanism is in a first bi-stable state. The two rings 212, 214 are able to move axially (up and down, as viewed in Figure 14) and the second ring is constrained against rotation in said first by the teeth 220 on the ground element 218. More particularly, the second ring 214 defines a plurality of recesses 217 into which the teeth 220 may engage when the mechanism 210 is in its first bi-stable condition. If the electromagnet is energised, the first ring 212 moves axially, to the right as viewed in Figure 13 (or 'down', as viewed in Figures 14 to 17), via movement of the armature 144 and extension 148. As can be seen in Figure 15, a cam surface 213 on the first ring 212 pushes against a cam surface 215 on the second ring 214, against the bias of the resilient element 216. However, the second ring 214 is still constrained against rotation by the teeth 220 on the ground element 218.

If the armature 144 is pulled further in an apply direction (to the right as viewed in Figure 13), the first ring 212 pushes the second ring out of engagement with the teeth 220. The second ring 214 is then able to rotate, but is snapped back into engagement with the teeth 220, under bias from the resilient element 216. As can be seen in Figure 16, the teeth 220 on the ground element initially are in engagement with an angled surface 222 of a notch 224 forming part of the cam surface 215 on the second ring 214, until the ring 214 has rotated sufficiently to latch a vertical wall 223 of the notch 224 against the teeth 220 (see Figure 17), and the mechanism 210 is in a second bi-stable state. Ring 214 is prevented from further axial movement in the non-apply direction by the teeth 220, and so the clutch is latched in a closed condition, with the pressure plate 142 held in clamping engagement with the driven plate, for transmitting torque.

The solenoid can then be de-energised. Accordingly, the armature 144 moves in a return direction, allowing axial movement of the first ring 212 in a non-apply direction (to the left as viewed in Figure 13). In order to release the pressure plate 142 and permit the clutch to open, current is reapplied to the solenoid windings, causing the armature 144 to be pulled towards the electromagnet, causing axial movement of the first ring 212 in an apply direction. This again causes further rotation of the second ring 214, whereby the notch 224 on the second ring 214 is unlatched from the fixed teeth 220, so that the ring 214 rotates further (illustrated by movement to the right, as viewed in Figures 14-17), and the latch mechanism 210 is returned to a first bi-stable state. Accordingly, the pressure plate 242 can move in the opposite (non-apply) direction. In exemplary embodiments, one or more parts of the latch mechanism 210 may be made from non-ferromagnetic material, e.g. plastics material, for avoidance or reduction of magnetic flux transfer to the latch mechanism. Such a latch mechanism may be incorporated into any of the electromagnetic clutch actuation mechanisms described herein. Typically the latch mechanism 210 will be provided on the side of the driven plate distal from the electromagnet, e.g. as shown in Figure 13. In exemplary embodiments, the latch mechanism may be of mono-stable configuration, for allowing only partial removal of the electromagnetic force and ensuring a 'fail-open' operation (particularly important in at least one clutch of a dual clutch transmission). Figure 10 shows the clutch 138 and electromagnetic actuation arrangement 136 of Figure 9 and includes an adjustment mechanism which acts to ensure that the distance by which the armature 144 is moved by the electromagnet 126 to clamp the friction plate 40 between the driving and pressure plates 140,142 remains constant as the thickness of the friction material on the friction plate 40 reduces, in use. Features common to Figure 9 are identified with like reference numerals. The maintenance of a substantially constant travel distance for the armature 144 is desirable due to the force/travel characteristics of a solenoid. So as to be able to adequately control the force applied to the pressure plate 142 by a solenoid, advantage can be taken of the relatively linear portion of the force to travel relationship of the solenoid. This is indicated by arrow 156 on Figure 11, where force is approximately proportional to current supplied to the solenoid. The desirability to keep the solenoid travel within the aforementioned linear portion of the graph is common to all the above described embodiments. The desired travel of the armature in the embodiments described may be limited to the region of 1.5mm.

Referring again to Figure 10, the friction material 158 of the friction plate 40 is mounted to the web 56 of the plate 40 by cushion springs 160. The driving and pressure plates 140,142 are each provided with a centralisation spring 162. Each spring 162 extends from the respective plate 140,142 to the web 56 of the friction plate 40. The friction plate 40 is mounted to the output shaft 22 by a splined connection and thus is capable of axial movement, indicated by arrows 164,166, relative to the shaft 22 under the influence of the springs 162. The centralisation springs 162 are temperature sensitive, for example they may be of a bi-metallic construction, and hence are deflectable out of engagement with the web 56 once centralisation of the friction plate 40 has taken place. Heat generated by frictional contact of the springs 162 against the web 56 causes deflection of the springs 162. As an alternative to temperature sensitive springs 162, opposing sprung plungers may be used.

The pressure plate 142 is connected by a resilient strap 168 to an annular collar 170. The collar 170 is mounted on splines 172 of the clutch basket 34 and thus is coupled for rotation with the basket 34 but movable axially relative to the basket 34. The collar 170 is provided with a projection 174 which is contactable by the pressure plate 142. The collar 170 is further provided with a spring 176 which is connected to the clutch basket 34 and a sprag arm 178 in engagement with the clutch basket 34 which, in use, allows the collar 170 to move in the direction of the driving plate 140, but resists movement of the collar 170 in the direction of the pressure plate 142.

The electromagnet 126 is provided with an arrangement which permits it to be moved axially in the direction indicated by arrow 180. The electromagnet is mounted to a first collar 182 having a first ramp surface 184. The first collar is fixed rotationally but movable axially The first collar 182 opposes a second collar 186 having a second ramp surface 188. The second collar 186 is fixed axially but movable rotationally. The second collar 186 is moveable rotationally by a motor 190 and a worm/wheel arrangement 192. The ramp surfaces 184,188 mat comprise multiple discontinuous ramps or, alternatively, single or multiple start screw threads. The electromagnet 126 is further provided with distance keeper 194 in the form of a hook which reaches around the armature 144 and prevents the electromagnet 126 from exceeding the attraction distance between the electromagnet and armature 126,144. The adjustment sequence is as follows and is undertaken when the clutch 138 is not engaged to transmit drive torque. The electromagnet 126 is progressively engaged thereby causing movement of the armature 144 towards the electromagnet 126. This causes deflection of the resilient strap 168 and then extension of the spring 176 when the pressure plate 142 abuts the projection 174 of the collar 170. It will be understood that the spring 176 has a greater spring strength than the strap 168. In taking up any wear related clearance between the pressure plate 142 and the friction plate 40 the collar 172 is moved axially on the splines 172 in the direction of the driving plate 140. Finally, the cushion springs are compressed as the friction plate 40 is clamped between the pressure and driving plates 142,140. The axial position of the electromagnet 126 can then be adjusted by rotating the second ramp surface 188 relative to the first ramp surface 184. With the necessary adjustment made the electromagnet 126 can be de-energised and the resilient strap 168 provided sufficient restorative force to move the pressure plate 142 away from the friction plate 40 to disengage the clutch. The collar 170 is prevented from moving axially away from the driving plate 140 under the influence of the spring 176 by the sprag arm 178

It will be appreciated that amendment and/or modification of the embodiments described above may be made without deviating from the inventive concepts exhibited therein. For example, the various armatures may be composed of a ferromagnetic material such as iron, alternatively may themselves be permanently magnetic. The solenoid arrangements may, for example, be substituted for electromagnetic linear actuators. Examples of such actuators are described in, for example, WO 2007/034195. Arrangements to assist the movement of the armatures in the direction of the electromagnets may also be included. Examples of such arrangements are described in PCT/GB2009/050894 (WO2010/010385) and PCT/GB2009/051240 (WO2010/035030).

As has been described above, the current applied to the electromagnets may be varied so as to vary the force applied by the pressure plate and thereby permit the clutch to slip. In use, the engaged clutch may be slipped for the purpose of damping torsional vibrations. Figure 1 illustrates the arrangement where the gearbox is provided between the clutch assembly and the engine. It will be appreciated that in an alternative arrangement a clutch assembly according to an aspect of the present invention, or a clutch assembly including an actuator or adjustment mechanism according to an aspect of the present invention, may be provided in a more conventional position between the engine and the gearbox.