This invention relates generally to transmissions, more particularly dual clutch transmissions and more specifically, although not exclusively, to sequential dual clutch transmissions for use in land vehicles.

Traditional vehicle transmissions fall into one of two categories, namely manual transmission devices and automatic transmission devices. Manual transmission devices generally include a foot operated clutch for engaging and disengaging the driveline with the engine of the vehicle and a gearshift lever for selectively changing the gear ratio within the transmission. In such an arrangement, the driver must coordinate the actuation of the clutch, the movement of the gearshift lever and the actuation of the accelerator in order to ensure a smooth and efficient shift from one gear to the next. This requires a certain degree of skill to master, but nonetheless there will always be a period of interruption in the drive connection during the shifting procedure with such transmission devices.

Automatic transmission devices were developed in an attempt to mitigate the issues associated with manual transmission devices. Automatic transmission devices generally use hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween. Whilst such a device is much simpler for the driver to operate, the complexity of the transmission device and, more particularly the use of hydrokinetic devices, results in greater losses, reduced efficiency and a more expensive solution.

Developments have been made to both of these traditional types of transmission in an effort to combine the advantages of both approaches. Recent developments include manual-type transmission arrangements which shift automatically without any specific demand for a shift from the driver of the vehicle. One approach is to use two clutches configured cooperatively to draw power from a single engine flywheel. These arrangements combine two transmission assemblies into a single housing, wherein each assembly drives a single output shaft but is shifted and clutched independently. Thus, upshifting and downshifting can be achieved with minimal power transmission interruption.

Dual clutch transmission systems generally include a pair of clutches with respective electromechanical or hydraulic actuators for independent control. Power interruption is minimised by engaging the desired gear prior to the shift and subsequently engaging the relevant clutch. More specifically, when a shift is required, first the relevant gears in the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged. Thus, the transmission is able to offer two different gear ratios simultaneously while the clutches are used to control which of these gear ratios is engaged and transmitting power.

In order to accommodate upshifting and downshifting in the normal way, typical dual clutch transmission assemblies include two input shafts each of which is connected to the flywheel by a respective one of the clutches, e.g. dry disc clutches. The first input shaft carries the odd gears (e.g. first, third and fifth gears) and the second input shaft carries the even gears (e.g. second, fourth and sixth gears). The assembly also includes a single counter shaft with input mating gears and an output mating gear. Each of the input shafts is in constant engagement with a respective input mating gear and the output shaft is in constant engagement with the output mating gear. One of the input shafts will normally include a reverse gear which engages a further mating gear on the counter shaft via an intermediate gear mounted on a separate shaft to achieve reverse rotation. Power from the engine is transferred from one of the clutches to one of the input shafts, through the gear set to the counter shaft and finally to the output shaft.

As with conventional manual transmissions, one of the gears in each gear set is rotatably mounted to the shaft with a gear clutch, e.g. a dog clutch, and a synchroniser mounted on the shaft next to the rotatably mounted gear to selectively engage the gear with the shaft. Generally, the dog clutches and synchronisers are moved automatically using a dedicated actuator and their movement is coordinated with the actuation of the clutch.

It will be appreciated that such dual clutch transmissions generally require constant and accurate monitoring and control by an electronic control system. Such a control system must be fully integrated with several other aspects of the vehicle's control system in order to ensure optimal performance. Integration of these control systems is a daunting task that requires a great deal of coordination and resources owing to the nature of the automotive industry and the numerous organisations involved in the development of a new automobile.

It is therefore a general non-exclusive object of the invention to provide a dual clutch transmission which at least mitigates the issues associated with known designs. It is a further non-exclusive object of the invention to provide a dual clutch transmission which is more easily integrated within a vehicle's power transmission. It is a more specific nonexclusive object of the invention to provide a dual clutch transmission which is controlled mechanically rather than electronically.

Accordingly, one aspect of the invention provides a dual clutch transmission comprising an engine coupling, a first input shaft with at least one first ratio defining gear, a second input shaft with at least one second ratio defining gear, first and second clutches and an override, the first input shaft being connected, e.g. operatively connected, to the engine coupling via the first clutch, the second input shaft being connected, e.g. operatively connected, to the engine coupling via the second clutch, the first clutch being operatively connected to the second clutch such that an actuation force exerted, in use, by a clutch actuator to one of the clutches disengages the first clutch and engages the second clutch, wherein the override is operable to cause both of the first and second clutches to be disengaged, for example in a disengaged state, e.g. simultaneously.

Thus, shifting from one gear ratio to the next may be achieved by a single actuation force. This arrangement precludes the need for independent actuation of each of the clutches, thereby avoiding the need for complex electronic closed loop control systems. Incorporating the transmission of the invention into a vehicle therefore requires significantly less resources than the aforementioned transmissions, thus providing a significant advantage over known systems.

It will be appreciated that the term "engaged" as used herein with reference to the state of a friction clutch refers to that clutch being frictionally engaged. Similarly, the terms "fully engaged" or "partially engaged" in the same context refer to the clutch being engaged to a full or partial operational level such that it is able to transmit all or a portion of the intended or required torque capacity.

It will also be appreciated that the term "coupling" as used herein refers to any suitable form of connection means for connecting to an engine, e.g. the output shaft of an engine.

The override may be operable to disengage the first clutch without engaging the second clutch and/or to selectively prevent operation of the clutch actuator. In some embodiments, the override is operable to disengage the first clutch and to prevent operation of the clutch actuator simultaneously. One or both of the first and second clutches may comprise a friction clutch. Preferably, the transmission is configured such that both the first and second clutch are at least partially engaged at the same time, e.g. as the actuation force is exerted by the clutch actuator to the one clutch. The transmission may be configured such that the second clutch begins to engage, in use, while the first clutch is or remains at least partially engaged, e.g. when the actuation force is exerted by the clutch actuator to the one clutch. More preferably, the first comprises a friction clutch and/or the second clutches comprise a friction clutch.

The first clutch is preferably mechanically connected or coupled to the second clutch, for example by a linkage element. Each clutch may include two or more clutch parts, for example first and second clutch parts which may have cooperating and/or engaging and/or friction surfaces. One of the clutch parts of the first clutch may be connected, for example mechanically connected or fixed, to one of the clutch parts of the second clutch and/or for movement therewith, e.g. such that an actuation force exerted, in use, by a clutch actuator to one of the clutches disengages the first clutch and engages the second clutch. Each clutch may comprise one or more, e.g. a plurality of first clutch parts and/or one or more, e.g. a plurality of second clutch parts, for example each clutch may comprise a stack of plates, e.g. inner and outer discs which may radially engage, respectively, a shaft and a sleeve or drum. Each clutch may further include one or more platens, for example a pair of opposed platens, e.g. between which the clutch parts are or may be compressed, in use, when the clutch is engaged. The radial engagement may be provided by cooperating portions, for example one or more projections and/or depressions, e.g. one or more teeth or splines. The clutches may be co-axial and/or the first clutch may surround the second clutch, e.g. the second clutch may be radially nested in or within the first clutch, or vice versa.

A part or one of the parts of the first clutch may be mounted or connected or located on the second input shaft, e.g. via a bearing. This configuration may be such that the bearing will only be subjected to either high rotational speed or high axial load, but not both at the same time, which enables a much smaller bearing (or fewer bearings) to be used, thus reducing weight, inertia and improving efficiency.

The transmission may further comprise a clutch actuator configured to provide the actuation force. The actuator may comprise an actuating piston, which may be actuated hydraulically or electromechanically or other and/or which piston may be connected, for example operatively or mechanically connected, to one of the clutches, for example wherein the one clutch may incorporate the piston. The transmission may further comprise a pump, e.g. which may be suitable for providing or configured to provide pressurised hydraulic fluid to the actuator for actuating the piston, and/or an accumulator. The pump may also be configured to provide lubrication to the transmission, e.g. it may comprise a lubrication pump and/or it may be integral to the transmission. For example, the control valve and/or the main valve and/or the clutch actuator may be configured to exhaust excess hydraulic fluid into a lubrication system.

The transmission may further comprise a biasing means, e.g. a resilient biasing means such as a spring, for biasing or forcing the other one of the first and second clutches to or toward an engaged condition or position. The transmission or clutch actuator may be configured such that the actuation force is arranged to counter, in use, the force exerted by the resilient biasing means, for example by compressing or tensioning the biasing means, e.g. to disengage the first clutch, for example when the actuator exerts the actuation force on the one clutch, e.g. to engage the second clutch. The one clutch may comprise the first clutch and the other clutch may comprise the second clutch. Preferably however, the one clutch comprises the second clutch and the other clutch comprises the first clutch.

A second aspect of the invention provides a dual clutch transmission comprising:

a) an engine coupling;

b) a first input shaft with at least one first ratio defining gear, the first shaft being connected to the engine coupling via a first clutch;

c) a second input shaft with at least one second ratio defining gear, the second shaft being connected to the engine coupling via a second clutch;

d) a clutch actuator operatively connected to the second clutch and configured to exert, in use, an actuation force thereon; and

e) a biasing means configured to bias or force the first clutch toward an engaged condition,

wherein the first clutch is operatively connected to the second clutch such that the actuation force counters the force exerted by the biasing means, thereby to disengage the first clutch and engage the second clutch.

The transmission, for example the biasing means, may comprise a stop, e.g. for limiting the distance or extent to which the biasing means is compressed or tensioned and/or against which the actuator compresses or engages the second clutch. For example, where the clutches are radially nested and the transmission comprises a linkage element that incorporates or interconnects one of the platens of each clutch part, the biasing means preferably acts on the linkage element. The clutch actuator preferably acts on the second clutch to force the second clutch against the linkage element and against the biasing force exerted by the biasing means. In such a case, the stop may advantageously be configured to limit the movement of the linkage element and/or permit the actuator to compress or engage the second clutch thereagainst.

The first and second input shafts may be rotatably mounted on or about parallel axes or, more preferably, along a common axis, e.g. a first axis and/or may be mounted in series. The transmission may further comprise a further shaft, which may be an output shaft or a counter shaft and/or which may be rotatably mounted, e.g. on or about a further axis such as a second axis, which further or second axis may be parallel to and/or spaced from the first axis. The further shaft may comprise two or more mating gears, for example two or more input mating gears and/or one or more output mating gear, which may be rotatably mounted or connected thereto. Each of the mating gears, e.g. each of the input mating gears, may be operatively, and/or constantly, engaged or engageable or connected or connectable or meshed or meshable to or with a respective one of the ratio defining gears of the input shafts. The transmission may further comprise one or more gear clutches, for example one or more dog clutches or dog drive clutches, which may be configured to selectively engaged or couple or fix one or more of the mating gears, e.g. one or more of the input mating gears, to the further shaft. Alternatively, the mating gears may be coupled or secured or fixed to the further shaft with the ratio defining gears being rotatably mounted to their respective input shaft, wherein the one or more gear clutches may be configured to selectively engaged or couple or fix one or more of the ratio defining gears to their respective input shaft. Preferably, the transmission also comprises a synchroniser or synchroniser mechanism, for example a synchromesh, e.g. between the or each gear clutch and the or each mating gear or ratio defining gear, e.g. for facilitating a smooth transition during a shift.

The at least one first ratio defining gear may comprise a first set of ratio defining gears and/or the at least one second ratio defining gear may comprise a second set of ratio defining gears. The first set of ratio defining gears may comprise odd gears and/or the second set of ratio defining gears may comprise even gears. Alternatively, the first set of ratio defining gears may comprise even gears and/or the second set of ratio defining gears may comprise odd gears. The further shaft may comprise a mating gear, for example an input mating gear, operatively engaged or engageable or connected or connectable or meshed or meshable to or with each of the ratio defining gears of the input shafts. Each gear clutch may be configured to selectively engage or couple or fix one or more of the mating gears, e.g. one or more of the input mating gears, to the further shaft.

At least one of the gear clutches may comprise a member or plate or disc which may include one or more projections, e.g. dogs or dog projections, on one or both sides or major surfaces thereof. The or each gear clutch may be movably or slideably mounted or engaged or coupled to the further shaft, e.g. adjacent a first mating gear or between a first and second mating gear. The or each gear clutch may be movable or slideable between a first position in which the gear clutch engages or couples or fixes the or a first mating gear to the further shaft and a second position in which the gear clutch disengages or uncouples the first mating gear or frees the first mating gear to rotate with respect to the further shaft. Where the gear clutch is movably or slideably mounted or engaged or coupled to the further shaft between a first and second mating gear, the second mating gear may be rotatable or free to rotate when the gear clutch is in the first position and/or the gear clutch may be movable or slideable to a third position in which the gear clutch engages or couples or fixes the second mating gear to the further shaft and/or the first mating gear is disengaged or uncoupled or free to rotate with respect to the further shaft. Preferably, the transmission comprises a synchroniser between the or each gear clutch and the or each mating gear or ratio defining gear, e.g. for facilitating a smooth transition during a shift.

The transmission may further comprise a gear selector, for example which may be configured to move or slide or operate or actuate, in use, the one or more gear clutches. The gear selector may comprise a selector barrel, which selector barrel may comprise one or more guides, for example gear clutch guide or gear clutch guides, e.g. about its periphery. The barrel may be rotatably mounted adjacent the further shaft, e.g. rotatably mounted, e.g. on or about a further axis such as a third axis, which further or third axis may be parallel to and/or spaced from the first and/or second axes. Each guide may be configured or positioned to receive a portion, for example a peripheral portion, of a selector fork, e.g. slideable along a selector shaft, which selector fork may in turn receive a portion respective gear clutch. Each guide may be configured to move or slide the respective gear clutch, e.g. along the further shaft. In one embodiment, one or more of the guides includes a portion which is helical and/or at an angle to the circumferential direction and/or the axial direction, for example to cause the movement or sliding of the respective gear clutch. The one or more guides may also include a portion which extends along the circumferential direction, e.g. to provide allow rotation of the selector barrel without moving the gear clutch, e.g. while retaining or maintaining or keeping or leaving the gear clutch in a p re-determined one or the first, second or third positions. The gear selector may comprise or incorporate a delay means or lost motion mechanism or coupling, for example for distinguishing if an upward shift or upshift follows either a previous upward shift or upshift or a downward shift or downshift and/or whether a downshift follows either an upward shift or upshift or a previous downward shift or downshift, e.g. for providing a different angle of rotation of the selector barrel and/or control valve when a shift in a first direction following a shift in the same previous first direction is selected as compared to the angle of rotation thereof when a shift in the first direction follows a previous shift in a second direction opposite the first direction is selected.

The transmission or gear selector may also comprise control valve, e.g. for controlling the clutch actuator, which control valve may be operatively, e.g. mechanically, connected to the gear selector or selector barrel. In one embodiment, the control valve comprises a rotary valve, e.g. a rotary plate valve, which is configured to rotate with the selector barrel. Preferably, the control valve or rotary valve comprises a pilot or servo to a main valve, for example a spool valve, which main valve may operate and/or supply and/or be operatively connected to the clutch actuator. The override may be provided in part by a function of the main valve or control valve, which may comprise a bypass feature, for example a manual override actuator which may be operated by or operatively connected to a global manual override actuator, e.g. a clutch pedal. The override may be configured to close the valve and/or exhaust hydraulic pressure from the clutch actuator. The transmission may also comprise a further or second override or the override may comprise a second override feature, e.g. a piston which may be operated by or operatively connected to a manual clutch actuator or the global manual override actuator or clutch pedal, which further or second manual override or override feature may be configured to disengage the first clutch. The main valve may also include a manual bleed button, e.g. for directing fluid pressure to the clutch actuator. This enables the operator to bleed air from the clutch actuator, which may be necessary as part of the periodic maintenance of the transmission.

The transmission or gear selector may further comprise a conversion device such as a ratchet or ratcheting selector or rack and/or pinion mechanism, e.g. for transmitting a linear input movement, e.g. from the driver, to a rotary movement, for example of the selector barrel and/or control valve. Preferably, the conversion device or ratchet or ratcheting selector or rack and/or pinion mechanism is operable in two or both directions, for example to allow for or accommodate upward shifting or upshifting and/or downward shifting or downshifting. Alternatively, the transmission or gear selector may comprise two conversion devices or ratchets or ratcheting selectors or rack and/or pinion mechanisms or the conversion device may comprise two ratchets or ratcheting selectors or rack and/or pinion mechanisms, one of which may be operable in a first direction, e.g. to allow for or accommodate upward shifting or upshifting and/or the other of which may be operable in a second direction opposite the first direction, e.g. to allow for or accommodate downward shifting or downshifting. The transmission or gear selector may further comprise a shifter, for example a manual shifter such as a gear lever or electronic shifting means, e.g. connected to the or each conversion device or ratchet or ratcheting selector or rack and/or pinion mechanism.

The conversion device or devices may comprise or incorporate delay means or lost motion mechanism or coupling and/or a stop, e.g. for preventing over-travel or over- rotation of the selector barrel and/or control valve. For example, the shifter may be operatively connected to the selector barrel via the delay means or lost motion mechanism or coupling. The delay means or lost motion mechanism or coupling may be provided by a projection which is received within a slot, e.g. wherein the slot includes two abutment ends and the projection is movable or slideable within the slot between the two abutment ends. The transmission or gear selector may comprise a mating part which may be receivable or received within the selector barrel and/or control valve, and/or a portion of the selector barrel and/or control valve may be receivable or received within the mating part, wherein one of the mating part and the selector barrel and/or control valve incorporates or includes the projection and the other includes the slot.

The or one or both of the conversion device or devices may comprise an elongate member or rod or shaft with a pivotable ratchet arm. The or one or both of the conversion device or devices may further comprise a rotary element, for example which rotary element includes a plurality of recesses or gaps and/or includes two opposed members or plates interconnected by a plurality of pins which may be equally spaced thereabout. The pivotable ratchet arm may comprise opposed hook portions, e.g. for selectively operating the rotary element in the two directions, e.g. the first and/or second direction or directions. The delay means or lost motion mechanism or coupling may be incorporated in the spacing of the rotary element of the opposed hook portions of the ratchet arm. Preferably, the elongate member or ratchet arm includes at least one stop, e.g. for preventing over-travel or over-rotation of the rotary element and/or selector barrel and/or control valve, for example in each direction. More preferably, the elongate member or ratchet arm includes at least one further stop, e.g. for preventing over-travel or over- rotation of the delay means or lost motion mechanism or coupling.

One or both of the input shafts, for example the first or second input shaft, may comprise an air channel, e.g. a central air channel that may extend along its length, and may comprise one or more, e.g. a plurality, of holes which are preferably radial holes adjacent the first and/or second clutches, for example to cool the clutches and/or for extracting dust and/or debris. The clutches, for example the clutch parts, e.g. the plurality of clutch parts, may further comprise one or more holes or slots configured to permit passage of air therethrough, for example to cool the clutches and/or for extracting dust and/or debris.

One of the input shafts may comprise a reverse gear, e.g. for causing the further shaft to rotate in the opposite direction to the other ratio defining gears. The further shaft may comprise a reverse mating gear, which may be engaged or engageable or connected or connectable or meshed or meshable to or with the reverse gear via an intermediate gear. The transmission may further comprise an intermediate shaft which includes the intermediate gear and which may be rotatably mounted adjacent the further shaft and/or the one of the input shafts, e.g. rotatably mounted, e.g. on or about a further axis such as a fourth axis, which further or fourth axis may be parallel to and/or spaced from the first and/or second and/or third axes.

Where the further shaft comprises a counter shaft, the transmission may further comprise an output shaft which may include an output gear, e.g. coupled or fixed thereto or therewith. The output mating gear of the counter shaft may be engaged or engageable or connected or connectable or meshed or meshable to or with the output gear.

The transmission may further comprise a flywheel which may incorporate the engine coupling.

A further aspect of the invention provides a vehicle comprising a transmission according to the first aspect of the invention.

A yet further aspect of the invention provides a method of shifting gears using a transmission according to the first aspect of the invention.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a section view along a section line running through the centreline of the clutch input shaft, the counter shaft and the differential of a transmission according to one embodiment of the invention;

Figure 2 is an enlarged view of the region of the clutches of the transmission of Figure 1 ;

Figure 3 is a section view through part of the gear selector of the transmission of Figure 1 showing the barrel and spool valve;

Figure 4 is a schematic view of the ratchet conversion device of the gear selector of the transmission of Figure 1 ;

Figure 5 is a perspective view of the ratchet conversion device of Figure 4 from below;

Figure 6 is a schematic of the configuration of the guides on the selector barrel and channels on the control valve; and

Figurea 7a to 7c show graphs which illustrate the overlap of the engagement of the clutches.

Referring to Figures 1 and 2, there is shown a dual clutch transmission 1 which includes a flywheel 2 incorporating an engine coupling, a first input shaft 3 connected to the flywheel 2 via a first or primary clutch 4, a second input shaft 5 connected to the flywheel 2 via a second or secondary clutch 6, a counter shaft 7, an output shaft 8 and a housing 9. The secondary clutch 6 is radially nested within the primary clutch 4 and is operatively connected or secured thereto via a linkage element 46. The flywheel 2 is of a relatively standard configuration and includes a plurality of bolts 20 for connection with the output shaft of an engine (not shown) of a vehicle (not shown). The flywheel 2 is mounted at its centre to the end of a first hollow transfer shaft 10 via a bearing 21 and is bolt connected to a primary outer clutch carrier 22, wherein the flywheel 2 is rotatable about a first axis. The first transfer shaft 10 is rotatably mounted to the housing 9 about the first axis and includes a central air channel 10a extending along its length and a plurality of radial holes 10b adjacent the first and second clutches 4, 6 to cool them and for extracting dust and/or debris. The clutches 4, 6 also include holes and slots (not shown) configured to permit passage of air therethrough to cool them and for extracting dust and/or debris.

The first input shaft 3 is hollow and partially surrounds the first transfer shaft 10. The first input shaft 3 includes a first set of ratio defining gears 30a, 30b, 30c, 30d which correspond respectively to first, third and fifth gears 30a, 30b, 30c and a reverse gear 30d and are coupled thereto and arranged for rotation therewith. Each end of the first input shaft 3 is mounted to the housing 9 via a respective bearing 31 a, 31 b such that it is rotatable about the first axis. The first input shaft 3 also includes a plurality of internal splines 32 which engage corresponding external splines on the first transfer shaft 10.

The primary clutch 4 is a friction clutch in this embodiment and includes a plurality, four in this embodiment, of first or outer clutch plates 40a and a plurality, three in this embodiment, of second or inner clutch plates 40b. The primary outer clutch plates 40a include a plurality of outer teeth which slideably engage internal splines of the primary outer clutch carrier 22 with a respective one of the primary inner clutch plates 40b disposed between each pair of primary outer clutch plates 40a. The linkage element 46 also includes a plurality of outer teeth which slideably engage the internal splines of the primary outer clutch carrier 22. The primary inner clutch plates 40b include a plurality of teeth which slideably engage external splines 42a of a primary inner clutch carrier 41 . The primary inner clutch carrier 41 is in the form of an open ended ring shaped drum having an outer wall 42 with the external splines 42a on its outer surface and an inner wall 43 with internal splines 43a on its inner surface. The primary outer clutch carrier 22 also includes a platen 22a in the form of an abutment ring portion 22a which cooperates with a corresponding platen 46a of the linkage element 46 such that the stack of primary clutch plates 40a, 40b is compressed therebetween when the primary clutch 4 is engaged to provide a frictional engagement. The internal splines 43a engage corresponding external splines on the first transfer shaft 10.

Thus, the first set of ratio defining gears 30a, 30b, 30c, 30d are coupled to the primary inner clutch plates 40b and arranged for rotation therewith, while the primary outer clutch plates 40a are coupled to the flywheel 2 and arranged for rotation therewith.

The second input shaft 5 is also hollow and partially surrounds the first transfer shaft 10 such that the first and second input shafts 3, 5 are mounted co-axially. The second input shaft 5 includes a second set of ratio defining gears 50a, 50b, 50c which correspond respectively to second, fourth and sixth gears 50a, 50b, 50c and are coupled thereto and arranged for rotation therewith. Each end of the second input shaft 5 is mounted to the housing 9 via a respective bearing 51 a, 51 b such that it is rotatable about the first axis. The second input shaft 5 also includes a plurality of internal splines 5a which engage corresponding external splines on a second hollow transfer shaft 1 1 , which second transfer shaft 1 1 is also axially fixed to the second input shaft 5 by a snap ring 1 1 a.

The secondary clutch 6 is a friction clutch in this embodiment and includes a plurality, five in this embodiment, of first or outer clutch plates 60a and a plurality, five in this embodiment, of second or inner clutch plates 60b. The secondary outer clutch plates 60a include a plurality of teeth which slideably engage splines of the linkage element 46 with a respective one of the secondary inner clutch plates 60b disposed between each pair of secondary outer clutch plates 60a. The secondary inner clutch plates 60b include a plurality of teeth which slideably engage external splines 52b of a secondary inner clutch carrier 52. The secondary inner clutch carrier 52 is in the form of a hollow shaft and includes a piston 53a secured thereto adjacent one of its ends. The piston 53a is surrounded by a cylinder 52a and the cylinder 52a is movable along the secondary inner clutch carrier 52. The piston 53a is slideably received within a recess of the cylinder 52a such that a chamber 54 is defined therebetween. The chamber 54 includes an inlet for receiving hydraulic fluid, wherein the cylinder 52a is axially displaceable relative to the piston 53a and secondary inner clutch carrier 52 by applying a hydraulic pressure within the chamber 54, thus providing a clutch actuator 52a, 53a, 54. The cylinder 52a includes a platen 52c in the form of an abutment ring portion 52c which cooperates with a corresponding further platen 46b of the linkage element 46 such that the stack of secondary clutch plates 60a, 60b is compressed therebetween when the secondary clutch 6 is engaged to provide a frictional engagement. The secondary inner clutch carrier 52 includes a plurality of internal splines 53b which engage corresponding external splines on the second transfer shaft 1 1 .

Thus, the second set of ratio defining gears 50a, 50b, 50c are coupled to the secondary inner clutch plates 60b and arranged for rotation therewith, while the secondary outer clutch plates 60a are coupled to the flywheel 2 via the linkage element 46 and primary outer clutch carrier 22 and arranged for rotation therewith.

The transmission 1 also includes a biasing means in the form of a spring assembly 12 for biasing or forcing the primary clutch 4 toward an engaged condition. The spring assembly 12 includes a spring carrier 12a which is bolt connected about its periphery to the primary clutch carrier 22 and is rotatably mounted to the secondary inner clutch carrier 52 via a bearing 12b. The spring carrier 12a includes a conical travel stop 12a' configured to cooperate with a correspondingly shaped portion of the linkage element 46 in order to limit the movement of the linkage element 46 realtive to the outer clutch carrier 22. The spring assembly 12 also includes a disc spring 12c mounted to the spring carrier 12a such that it exerts a biasing force on the linkage element 46, thereby forcing the linkage element 46 to compress the stack of primary clutch plates 40a, 40b against the platen 22a of the primary outer clutch carrier 22 to engage the primary clutch 4.

When the clutch actuator 52a, 53a, 54 is activated by pressurising the chamber 54, the platen 52c of the cylinder 52a of the forces the stack of secondary clutch plates 60a, 60b against the corresponding further platen 46b of the linkage element 46. This compresses the secondary clutch plates 60a, 60b and starts to engage the secondary clutch 6. Simultaneously, the force exerted on the linkage element 46 urges it against the spring 12c which counteracts the biasing force and moves the platen 46a of the linkage element 46 away from the platen 22a of the primary outer clutch carrier 22, thereby disengaging the primary clutch 4. When the linkage element 46 contacts the travel stop 12a' of the spring carrier 12a, the primary clutch 4 is completely disengaged and the compression force exerted on the stack of secondary clutch plates 60a, 60b by the clutch actuator 52a, 53a, 54 increases until the secondary clutch 6 is fully engaged.

Thus, a smooth shift from one set of ratio defining gears 30a, 30b, 30c, 30d to the next set of ratio defining gears 50a, 50b, 50c is achieved by a single actuation force. This arrangement precludes the need for independent actuation of each of the clutches 4, 6, thereby simplifying the transmission 1 . It will also be appreciated that this configuration is such that the bearing 12b will only be subjected to either high rotational speed or high axial load, but not both at the same time, which enables a much smaller bearing 12b (or fewer bearings) to be used, thus reducing weight, inertia and improving efficiency.

The counter shaft 7 includes six input mating gears 70a, 70b, 70c, 70d, 70e, 70f and one reverse mating gear 70g, all of which are rotatably mounted to the counter shaft by a respective bearing, and one output mating gear 70h which is formed integrally with the counter shaft 7. Each of the input and reverse mating gears 70a, 70b, 70c, 70d, 70e, 70f, 70g is in constant mesh or with a respective one of the ratio defining gears 30a, 30b, 30c, 50a, 50b, 50c, 30d of the input shafts 3, 5. The counter shaft 7 is mounted to the housing 9 via three bearings 71 a, 71 b, 71 c such that it is rotatable about a second axis, which is parallel to and spaced from the first axis. The transmission 1 also includes four gear clutches 72a, 72b, 72c, 72d in the form of dog clutches 72a, 72b, 72c, 72d for selectively engaging or coupling the input and reverse mating gears 70a, 70b, 70c, 70d, 70e, 70f, 70g to the counter shaft 7 for rotation therewith. Each dog clutch 72a, 72b, 72c, 72d is slidably coupled to the counter shaft 7 by internal teeth which cooperate with external splines of the counter shaft 7. Three of the dog clutches 72a, 72b, 72c include dogs on both sides for selectively engaging a pair of the mating gears 70g and 70a, 70b and 70c, 70d and 70e, wherein each dog clutch 72a, 72b, 72c is movable between first, second and third positions. In the first and third positions, a respective one of the mating gears of the pair 70g and 70a, 70b and 70c, 70d and 70e is engaged while the other is disengaged and both mating gears of the pair 70g and 70a, 70b and 70c, 70d and 70e are disengaged in the second position. The remaining dog clutch 72d carries dogs on only one of its sides for selectively engaging the remaining mating gear 70f.

The output shaft 8 is mounted to the housing 9 adjacent the counter shaft 7 such that it is rotatable about a fifth axis parallel and spaced from the fourth axis. The output shaft 8 includes an output coupling 80 configured to be coupled with the drive train (not shown) of the vehicle (not shown) and an output gear 81 coupled or fixed to the output shaft 8 which output gear 81 is in constant mesh with the output mating gear 70h of the counter shaft 7.

Referring now to Figures 3 to 6, the transmission also includes a gear selector 74 for moving the dog clutches 72a, 72b, 72c, 72d between the first, second and third positions. The gear selector 74 includes a control valve 75 for controlling the clutch actuator 52a, 53a, 54, a ratcheting selector 76 and a selector barrel 77.

The control valve 75 includes a rotary plate valve 75a coupled to the selector barrel 77 which plate valve 75a controls a main spool valve 75b hydraulically connected to the clutch actuator 52a, 53a, 54. The rotary valve 75a is configured to rotate with the selector barrel 77 and includes a channel 75c which cooperates with a manifold 75d to control the passage of hydraulic fluid for controlling the spool valve 75b. The spool valve 75b includes an inlet port 75e, an outlet port 75f, an exhaust port 75g, an override input 75h and a manual bleed button 75i.

The inlet port 75e is fluidly connected to an accumulator (not shown), which is fed by a pump 14 which is also used for lubrication of the gearbox, while the outlet port 75f is fluidly connected to the chamber 54 of the clutch actuator 52a, 53a, 54. The override input 75h is operatively connected to a clutch pedal (not shown) and is part of the piston in a small hydraulic actuator (not shown). The override input 75h is configured, on actuation of the clutch pedal (not shown), to close the spool valve 75b and exhaust any hydraulic pressure from the clutch actuator chamber 54 and to operate a piston 15 that disengages the primary clutch 4 so that both clutches are disengaged, thereby providing the override of the invention. The manual operation of the manual bleed button 75i enables the operator to bleed air from the clutch actuator chamber 54, which may be necessary as part of the periodic maintenance of the transmission 1 .

The ratcheting selector 76 includes a translating rod 76a operated by a gear lever (not shown) to cause stepwise rotation of a rotary element 76b. The rod 76a includes a double sided ratchet arm 76c with opposed hook portions 76c' for selectively operating the rotary element 76b in opposite directions. The rotary element 76b is substantially cylindrical in shape with an oversized head portion and includes a plurality of pins 76d equally spaced about its periphery to provide regular gaps therebetween. Each of the hook portions 76c' of the ratchet arm 76c are shaped to cooperate with one of the pins 76d to cause the rotary element 76b to rotate when the rod 76a translates in either direction. The pins 76d also act as an over-rotation stop for the rotary element 76b by cooperating with a pair of stops 76f of the rod 76a for preventing over-rotation of the rotary element 76b in each direction during a shift.

The rotary element 76b is coupled to the selector barrel 77 by a lost motion coupling 13, which is shown more clearly in Figure 5. The rotary element 76b includes a projection 13a received in an aperture 13b of the lost motion coupling 13, wherein the aperture 13b is larger circumferentially than the projection 13a to permit limited rotation of the rotary element 76b relative to the lost motion coupling 13 and hence the barrel 77. This lost motion coupling 13 is configured to distinguish if an upward shift follows either a previous upward shift or a downward shift and/or whether a downshift follows either an upward shift or a previous downward shift. More specifically, this configuration provides a different angle of rotation of the barrel 77 and control valve 75 when a shift in a first direction following a shift in the same previous first direction is selected as compared to the angle of rotation thereof when a shift in the first direction follows a previous shift in a second direction opposite the first direction is selected. The function of the lost motion coupling 13 will be appreciated by those skilled in the art in light of the disclosure herein, particularly in light of the discussion below in relation to Figure 6. The lost motion coupling 13 also includes a plurality of pins 76e having substantially the same number, cross-section and spacing as the rotary element pins 76d. These pins 76e cooperate with a further stop 76f of the rod 76a for preventing over-rotation of the selector barrel 77 as a result of the lost motion configuration.

The selector barrel 77 has four guides 77a about its periphery and is mounted to the housing 9 adjacent the counter shaft 7 such that it is rotatable about a third axis parallel to and spaced from the first and second axes. Each guide 77a is configured and positioned to receive a peripheral portion of a respective one of four selector forks (not shown), each of which selector forks (not shown) receives a peripheral portion of a respective dog clutch 72a, 72b, 72c, 72d for moving or sliding the respective dog clutch along the counter shaft 7. The guides 77a include helical portions for causing the movement or sliding of the respective dog clutch 72a, 72b, 72c, 72d along the counter shaft 7 and portions which extend along the circumferential direction of the selector barrel 77 to allow rotation of the selector barrel 77 while retaining the dog clutch 72a, 72b, 72c, 72d in a pre-determined one of the first, second or third positions.

In order to provide for reverse rotation of the output shaft 8, the transmission 1 also includes an intermediate shaft (not shown) that includes an intermediate gear (not shown) and is mounted to the housing 9 adjacent the counter shaft 7 and the first input shaft 3 such that it is rotatable about a fourth axis parallel to and spaced from the first, second and third axes.

Turning now to Figure 6, there is shown a schematic of the sequence of operation of the transmission 1 , which is represented by the lost motion coupling 13, the profile of the guides 77a of the selector barrel 77 and the operation of the control valve 75. It will be appreciated that the selector barrel 77 rotates throughout the full travel of the rod 76a of the gear selector 76 when a shift in a first direction is followed by a second shift in the same first direction. It will also be appreciated that the selector barrel 77 only rotates through a portion of the travel of the rod 76a when a shift in the first direction is followed by a second shift in a second direction opposite the first direction by virtue of the lost motion coupling 13. A brief analysis of the schematic of Figure 6 shows that the interaction between the profiles provides substantially seamless sequential upshifting and downshifting by ensuring that the relevant gear ratio on the non-driven input shaft 3, 5 is engaged prior to the shifting between the primary and secondary clutches 4, 6. Referring now to Figures 7 A to 7C, there is shown three graphical representations of the transition from the full engagement to disengagement of the primary clutch 4 and disengagement to full engagement of the secondary clutch 6 as a function of the actuation force exerted by the clutch actuator 52a, 53a, 54. Engagement is represented as a proportion of maximum torque capacity, wherein a first line 104a, 104b, 104c illustrates the relationship of torque capacity of the primary clutch 4 as a function of the actuation force and a second line 106a, 106b, 106c illustrates the same relationship for the secondary clutch 6.

Figure 7 A illustrates a first configuration, wherein the torque capacity of both clutches 4, 6 is equal and approximately half of their maximum or fully engaged torque capacity when half of the maximum actuation force is exerted on the secondary clutch 6. As shown in this graph, full engagement of the secondary clutch 6, i.e. when the torque capacity 106a reaches its maximum, occurs at substantially the same actuation force as full disengagement of the primary clutch 4, i.e. when the torque capacity 104a reaches zero. This arrangement is advantageous in that there is a quick and effective shift with a substantially constant total torque capacity delivered throughout the shift. However, one disadvantage of this arrangement is that it could result in excessive wear of the clutches and/or other components of the transmission 1 since the output shaft 8 is driven simultaneously by two different gear ratios.

Figure 7B represents a second configuration, which is less aggressive than that of Figure 7A. In this configuration, the spring assembly 12 is configured such that the reduction in the primary clutch torque capacity 104b is steeper with respect to the actuation force. Thus, the torque capacity of both clutches 4, 6 is equal when roughly 40% of the maximum actuation force is exerted on the secondary clutch 6. The cross-over torque capacity is therefore lower than that of Figure 7A, leading to a less aggressive shift. It can also be seen that the primary clutch 4 is fully disengaged much earlier than in Figure 7A as well. More specifically, the primary clutch 4 reaches full disengagement when the secondary clutch 6 has reached only roughly two thirds torque capacity. This will result in a small dip in the total torque capacity delivered during the shift.

It will be appreciated by those skilled in the art that it may be advantageous to include a further biasing means, for example a further spring assembly 12 e.g. linking the clutch actuator directly to the linkage element 46, configured to delay the application of the actuation force on the secondary clutch 6 while starting to disengage the primary clutch 4. Figure 7C illustrates such an arrangement, wherein the primary clutch 4 begins to disengage, i.e. the torque capacity 104c starts to decrease, immediately on application of the actuation force while the engagement of the secondary clutch 6, i.e. an increase in the torque capacity 106c, is delayed by the further biasing means.

The mechanical or operative connection provided by the linkage element 46 in conjunction with the biasing spring assembly 12 therefore provides a degree of overlap during which both clutches 4, 6 are partially engaged, thereby minimising and/or effectively eliminating power interruption during the shift. It is possible to finely tune this overlap by altering the configuration of the spring assembly 12 and/or further spring assembly. It is also envisaged to provide an adjustable spring assembly to facilitate such fine tuning, for example using a pre-compressed spring assembly 12 whose degree of compression may be altered by, for example, an adjustable abutment member or an adjustable spacer or the like.

Thus, the invention provides a transmission in which shifting from one gear ratio to the next may be achieved by a single actuation force. The invention precludes the need for constant and accurate monitoring and control by an electronic control system. This arrangement may therefore be fully integrated with the engine and the rest of the vehicle without the need for a full scale integration of complex electronic control systems. It will be appreciated that the transmission of the invention provides several advantages over prior art designs, for example its integration into a vehicle requires significantly less resources than known transmissions, yet the transmission provides substantially seamless shifting between gears.

It will be appreciated by those skilled in the art that several variations to the embodiment described above are envisaged without departing from the scope of the invention. For example, the override may be provided by any suitable arrangement such as a mechanical linkage or a stationary pneumatic, hydraulic or electrical actuator, e.g. to provide a force that operates through a thrust bearing to oppose (and so reduce) the primary clutch spring force. Alternatively, a rotating actuator could act directly onto the clutch spring without a thrust bearing. Any such device could be linked to the valve assembly 75 to override the function of the clutch actuator 52a, 53a, 54.

Moreover, the clutch actuator 52a, 53a, 54 need not be actuated or controlled hydraulically and may comprise an electromechanical actuation means or any other suitable means for achieving the required function. The spring assembly 12 may be replaced with any suitable arrangement, for example a different biasing means or resilient biasing means. The dual direction ratcheting selector 76 may be replaced with a pair of ratcheting selectors, for example wherein two gear levers (not shown) are provided, each one being configured to operate a respective one of the ratcheting selectors 76. The lost motion function may be incorporated in the connection between the rotary element 76b and the selector barrel 77 as opposed to be incorporated in the ratcheting selector 76. The lubrication pump need not be integral to the transmission.

The transmission 1 may also include a synchromesh (not shown), for example between each dog clutch 72a, 72b, 72c, 72d and each of the input and reverse mating gears 70a, 70b, 70c, 70d, 70e, 70f, 70g for facilitating a smooth transition during a shift.

It will be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.