Actuator system for a gear selector assembly and transmission including the actuator system

The present invention relates to an actuator system for a gear selector assembly in a transmission system, and in particular, but not necessarily limited to, gear selector assemblies used in instantaneous type transmission systems, and a transmission system including the actuator system.

Instantaneous transmission systems are arranged such that a new gear can be selected whilst the current gear is still engaged. When the new gear has been selected the initial gear is released. Thus during a shift, for at least one shift type, torque is substantially continuously supplied to the output of the transmission. Hence such shifts are said to be instantaneous because there is no delay in selecting the new gear whereas for conventional transmissions it is necessary to deselect the current gear, move through a neutral phase, and then select the new gear.

Typically instantaneous transmissions have a plurality of gear trains for transmitting drive between transmission input and output shafts. For a first gear train, a first gear wheel is rotatably mounted on either a transmission input shaft or an output shaft and a second gear wheel is fixed to the other shaft, in mesh with the first gear wheel. A second gear train comprising third and fourth gear wheels is similarly arranged. The transmission also includes at least one gear selector mechanism that is located between the rotatably mounted gear wheels that is arranged to selectively lock them for rotation with the shaft on which they are mounted. When a gear wheel from a gear train is locked for rotation with the shaft, drive is transmitted between the input and output shafts via that gear train.

The arrangement of the transmission is such that when drive is transmitted between the input and output shafts via one of the gear trains the gear selector mechanism can select a new gear train under power without first disengaging the first gear train, by locking the rotatably mounted gear wheel of the second gear train to its shaft. Thus momentarily, two gear trains

are engaged by the selector mechanism simultaneously. The new gear train then overdrives the first gear train and the selector mechanism disengages the first gear wheel. Drive is then transmitted between the input and output shafts via the new gear train only. Since it is not necessary to open the clutch when changing gear the transmission provides uninterrupted power through a gearshift.

An instantaneous gear selector mechanism typically has four modes of operation with respect to each of the rotatably mounted gear wheels associated with it:

Fully engaged in both torque directions (fully in gear);

Disengaged in both torque directions (neutral);

Engaged in the forward torque direction while disengaged in the reverse torque direction;

Disengaged in the forward toque direction while engaged in the reverse torque direction.

The last two modes enable a discrete ratio gearbox to have the ability to shift up or down ratios instantly under load without torque interruption. In some embodiments it is not necessary to have a neutral position.

However, there is an inherent failure mode in some known instantaneous transmissions including a plurality of selector assemblies and having the last two modes of operation where it is necessary to operate both selector assemblies in order to select a new gear. That is, it is possible for two gears to be engaged simultaneously with opposing torque directions under some conditions, which can cause the transmission to lock up. One of the most dangerous scenarios is if the direction of torque changes during a shift. If torque has a constant known direction during a shift, the natural sequence of events prevents the above failure mode. During a sudden reversal of the direction of torque immediately prior to, or during a shift, there is potential for the above failure mode to occur.

For example, a transmission having four gear trains and a conventional layout with all the selector mechanisms mounted on the same shaft, typically requires two instantaneous gear selector mechanisms. The first gear selector mechanism is arranged to selectively engage the first and second gear trains and the second gear selector mechanism is arranged to selectively engage the third and fourth gear trains. Each gear selector mechanism includes first and second sets of engagement members having opposed ends with fixed opposed directions of torque transfer. This provides an inherent fail-safe arrangement against the above mentioned failure mode where the shift is from a gear on one side of the selector mechanism to a gear on the other side of the same selector mechanism, for example when the first selector mechanism selects between the first and second gears or when the second selector mechanism selects between the third and fourth gears.

The failure mode described above can only occur if a gearshift is from a gear that is engageable by one of the gear selector mechanisms to a gear that is engageable by the other gear selector mechanism, for example when changing between second and third gears in the four speed transmission mentioned above, since this requires movement of both the first and second gear selector mechanisms.

It has been found that a particular variant of this conflict problem can occur when the gear wheel that was initially engaged collides, with the set of engagement members as it is moving out of engagement with that gear wheel. The collision takes place because of the different rotational speeds of the engagement members and that gear wheel after the new gear wheel has been selected by the second gear selector mechanism. The force of this collision under some circumstances can move the formerly loaded set of engagement members to move through the neutral position, where it would ordinarily stop, into engagement with the gear wheel opposite. In this scenario, two gear wheels are locked for rotation with the shaft by separate gear selector mechanisms in opposite directions, which will cause the transmission to lock up.

Figures Ia to Ie illustrate this problem for a transmission system similar to that described above. In the example shown, the transmission layout has the first and second selector

mechanisms and the rotatably mounted gear wheels mounted on the output shaft, and the shift type is an accelerating upshift from second to third gear. It will of course be appreciated that the same problem can occur in layouts having the selector mechanisms mounted on the input shaft and for other shift types.

In Figure Ia, dogs on the second gear wheel drive the first set of engagement members from the first selector mechanism. The second set of engagement members is in an unloaded condition. The first and second sets of engagement members of the second selector mechanism are in the neutral position. The driver is accelerating in second gear.

When the driver, or an engine management system, calls for third gear, the unloaded set of engagement members of the first selector mechanism are moved out of engagement with second gear to a neutral position (see figure Ib). The second selector mechanism then engages third gear with the set of engagement members for accelerating in third gear. The other set of engagement members remains in neutral. The loaded set of engagement members of the first selector mechanism are still engaged with second gear and hence the gearshift is instantaneous since there is substantially no loss of drive.

Since the sets of engagement members of both the first and second selector members rotate with the output shaft, when third gear is selected, the sets of engagement members of both the first and second gear selector mechanisms are driven by third gear and have the same rotational speed as third gear. Second gear rotates more slowly than third gear due to the different gear wheel geometries, hence the set of engagement members that was driven by second gear become unloaded and move out of engagement with the dogs on the second gear wheel.

However, at very high rotational speeds it has been found that in some embodiments it may not be possible to move the formerly loaded set of engagement members axially sufficiently quickly to prevent the dogs of second gear from colliding with the ramped faces of the engagement members (see Figure Id). If a collision occurs the force can be sufficiently large to drive the engagement members into first gear such that the deceleration drive faces engage

the dogs thereon. Thus the first and third gears can be locked for rotation with the output shaft in opposite directions, and lock up will occur.

Accordingly the present invention seeks to provide an actuator system and a transmission system including the actuator system that mitigates the aforementioned problem.

According to one aspect of the invention there is provided an actuator system for actuating a gear selector assembly in a transmission system, said actuator system including an actuator member for controlling the movement of at least one set of engagement members in the gear selector assembly and a limiting system for controlling the movement of the actuator member so as to prevent the or each set of engagement members from engaging a new gear element as a result of a collision between one of the sets of engagement members and a currently selected gear element that drives that set of engagement elements toward the new gear.

According to another aspect of the invention there is provided an actuator system for actuating a gear selector assembly in a transmission system, said actuator system including a drive system having a driveshaft, an actuator member for controlling the movement of at least one set of engagement members in the gear selector assembly, said actuator member being mounted on the driveshaft such that it can move rotationally and axially relative thereto, and a limiting system for limiting the extent of allowable axial movement of the actuator member on the driveshaft, wherein the limiting system is arranged to controllably adjust the extent of allowable axial movement of the actuator member along the driveshaft.

Thus the actuator system includes means for preventing the selector mechanism from accidentally engaging a gear element, for example due to a collision with another gear element since it is able to control the extent to which the actuator member, and hence the engagement members driven thereby, can move, which provides a capability of arresting movement before the new gear is unintentionally engaged.

Advantageously the limiting system includes first and second blocking members and is arranged to controllably adjust the extent of allowable axial movement of the actuator member along the driveshaft according to the rotational orientation of the first and second

blocking members relative to the actuator member. Additionally, or alternatively, the limiting system can be arranged to controllably adjust the extent of allowable axial movement of the actuator member along the driveshaft according to the rotational orientation of the driveshaft.

Advantageously the first blocking member includes a profiled surface and the extent of allowable axial movement of the actuator member in a first axial direction is determined in accordance with the rotational orientation of the profiled surface relative to the actuator member. Preferably the first blocking member includes a substantially cylindrical body and the profiled surface is formed in, or attached to, an end face of the cylindrical body. The cylindrical body can be tubular and mounted onto the driveshaft or can be formed integrally therewith.

Advantageously the actuator member can include a first profiled surface that is arranged to interact with the profiled surface of the first blocking member to provide a variable separation between the members according to the relative rotational orientations. Advantageously the first profiled surface of the actuator member is arranged substantially complementary to the profiled surface of the first blocking member.

Advantageously the second blocking member includes a profiled surface and the extent of allowable axial movement of the actuator member in a second axial direction is determined in accordance with the rotational orientation of the profiled surface relative to the actuator member. Preferably the second blocking member includes a substantially cylindrical body and the profiled surface is formed in, or attached to, an end face of the cylindrical body. The cylindrical body can be tubular and mounted onto the driveshaft or can be formed integrally therewith.

Advantageously the actuator member can include a second profiled surface that is arranged to interact with the profiled surface of the second blocking member to provide a variable separation between the members according to the relative rotational orientations. Preferably the actuator member is mounted on the drive member via a sleeve and is located between the first and second blocking members. Preferably the end faces of the sleeve both include profiled surfaces that are similar and are symmetrically arranged. Advantageously the second

profiled surface of the actuator member can be arranged substantially complementary to the profiled surface of the second blocking member.

Advantageously the first and/or second blocking members are mounted on the driveshaft. Advantageously the first and second blocking members are fixed to the driveshaft such that they rotate therewith and their profiled surfaces face each other. Advantageously the profiled surfaces of the first and second blocking members are similar and they are similarly oriented on the driveshaft such that their protruding and recessed parts are aligned.

Advantageously the profiled surfaces can each include at least one part that is arranged helically with respect to the driveshaft. Advantageously each profiled surface includes first and second helical parts that slope in opposite directions. The first and second helical parts can be connected by first and second arcuate parts that extend substantially circumferentially with respect to the driveshaft.

Advantageously the limiting system includes resilient means arranged to oppose axial movement of the actuator member along the driveshaft in the first and / or second axial directions. For example, the resilient means can comprise spring elements mounted between the actuator member and the first and second blocking members.

Advantageously the limiting system can be arranged such that the maximum extent of allowable movement of the actuator member is when the engagement members engage a gear element, and the minimum extent of allowable movement of the actuator member is when the set of engagement members is in the neutral position.

Advantageously the drive system includes means for driving the driveshaft rotationally and axially. The means for driving the driveshaft rotationally and axially can include a cam drive system that is arranged to convert rotational movement into axial movement. Advantageously the driveshaft is slidably connected to a rotational drive source. Preferably the drive source is a motor that is arranged to be controlled by a transmission control unit.

Advantageously the cam drive system includes a cam track having at least one helical part for converting rotational movement of the driveshaft into axial movement thereof. The cam track

can include first and second helical parts for converting rotational movement of the driveshaft into axial movement thereof, wherein the first and second helical parts are connected by a r third part of the track that allows rotational movement of the driveshaft without causing the driveshaft to move axially. The first part of the cam track can be connected to a fourth part and the second part can be connected to a fifth part, wherein the fourth and fifth parts of the cam track are arranged to allow rotational movement of the driveshaft without causing the driveshaft to move axially. Advantageously the cam drive system can be mounted on the drive shaft and a cam follower is arranged to move along the cam track wherein the first and second parts of the cam track are arranged substantially helically with respect to the driveshaft. The third, fourth and fifth parts of the cam track are arcuate and extend substantially circumferentially with respect to the driveshaft.

Advantageously the drive system is arranged such that, when activated, the drive member is substantially continuously rotated and the translational movement is intermittent. Advantageously the drive means and the blocking means can be arranged such that at least one of the periods when the drive member is rotating but not moving translationally coincides with the minimum allowable extent of movement of the or each engagement member. Advantageously the drive means and the blocking means can be arranged such that at least one of the periods when the drive member is rotating but not moving translationally coincides with the maximum allowable extent of movement of the or each engagement member. Preferably the actuator system is arranged such that when the gear selector device is in the centrally located neutral position, rotation of the drive member in at least one of the clockwise and anti-clockwise directions does not initially cause translational movement of the drive member. Preferably the actuator system is arranged such that when the gear selector device engages a gear element, rotation of the drive member in at least one of the clockwise and anti- clockwise directions does not initially cause translational movement of the drive member. This is useful since it allows a fast response time since the motor does not have to move the driveshaft translationally while it speeds up.

Advantageously the drive system is arranged to rotate the drive shaft bi-directionally.

Advantageously the actuator system includes means for determining the axial position of the actuator member on the driveshaft.

According to another aspect of the invention there is provided a transmission system including an actuator system according to any one of the configurations described herein.

According to another aspect of the invention there is provided a transmission system including a selector assembly arranged to select between gear ratios instantaneously without substantial torque interruption, said selector assembly including an actuator system according to any one of the configurations described herein.

According to another aspect of the invention a transmission system including a first shaft, first and second gears rotatably mounted on the first shaft, a gear selector assembly having first and second sets of engagement members that are arranged to selectively lock the first and second gear elements for rotation with the shaft and at least one actuator system including an actuator member for controlling the movement of at least one of the sets of engagement members and a limiting system for controlling the movement of the engagement members to prevent the or each set of engagement members from engaging a new gear element as a result of a collision between one of the sets of engagement members and a currently selected gear element that drives that set of engagement elements toward the new gear.

According to another aspect of the invention there is provided a transmission system including a first shaft, first and second gear elements rotatably mounted on the first shaft, a gear selector assembly including first and second sets of engagement members arranged to selectively lock the first and second gear elements for rotation with the first shaft independently of each other, said selection being from operational modes that include: lock the gear element for rotation with the first shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the first shaft in the clockwise direction and not lock in the anti-clockwise direction; lock the gear element for rotation with the first shaft in the anti-clockwise direction and not lock in the clockwise direction, and at least one actuator system for controlling the movement of the first and second sets of engagement

members for selectively engaging the first and second gear elements to implement the operational modes, wherein the or each actuator system includes an actuator member for controlling the movement of at least one of the sets of engagement members and a limiting system arranged to controllably adjust the extent of allowable movement of the engagement ^■ members to prevent unintentional gear selections.

The invention enables gears to be selected instantaneously in the normal course of operation of the transmission system, while at the same time preventing unintentional gear selections from occurring due to accidental collisions between the gear elements and the engagement members since the allowable extent of movement of the engagement members is controllably adjusted according to the operating conditions.

Advantageously the limiting system includes first and second blocking members and is arranged to adjust the extent of allowable movement of the first and second sets of engagement members to control the separation between them and the first and second gear elements according to the rotational orientation of the first and second blocking members relative to the actuator member. Advantageously the limiting system is arranged to control the allowable axial movement such that the gap between the actuator member and the blocking member associated with the unengaged gear is less than the gap between the unengaged gear and the or each set of engagement members controlled by the actuator member.

Advantageously the transmission system can include a first actuator system according to any one of the configurations described herein to actuate the first set of engagement members and a second actuator system according to any one of the configurations described herein to actuate the second set of engagement members. Alternatively a single actuator system can be used to control the movement of the first and second sets of engagement members.

The first gear selector assembly can be arranged to select the following operational mode with respect to the first and second gear elements: not lock the gear element for rotation with the first shaft in the clockwise or anticlockwise directions. Thus the gear elements can be fully disengaged.

Advantageously the or each gear selector assembly is arranged such that when a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the engaged gear element, and the other set of engagement members is then in an unloaded condition.

Preferably the or each selector assembly is arranged such that when a decelerating force is transmitted the first set of engagement members engages the engaged gear element, and the second set of engagement members is in an unloaded condition, and when a driving force is transmitted the second set of engagement members drivingly engages the engaged gear element, and the first set of engagement members is then in an unloaded condition.

Advantageously the actuator system can be arranged to bias the loaded set of engagement members towards an unengaged gear wheel without disengaging the loaded set of engagement members from the engaged gear element.

The first gear selector assembly is preferably arranged to engage the second gear element by moving the unloaded set of engagement members out of engagement with the first gear element and into driving engagement with the second gear element whilst the first gear element is still engaged by the loaded set of engagement members to effect a gear change between the first and second gear elements. Thus the first gear selector assembly is arranged to selectively lock the first and second gear elements for rotation with the first shaft simultaneously, at least momentarily. Typically, this only happens for a very short period of time during the shift, since when the new gear has been selected the loaded element set becomes unloaded and the control system is arranged to disengage it from it gear element and move it into engagement with the new gear element.

The first gear selector assembly is preferably arranged to move the unloaded set of engagement members out of engagement with the second gear element and into driving engagement with the first gear element whilst the second gear element is still engaged by the loaded set of engagement members to effect a gear change between the first and second gear elements.

Advantageously the transmission includes an electronically programmable control system for controlling operation of the or each gear selector assembly. For example, the control system may include a processing device that is programmed to control operation of the selector assemblies.

Advantageously the transmission includes means for determining which gear(s) is/are engaged. Preferably this is achieved by means for detecting the positions of the first and second sets of engagement members for each gear selector assembly. The control system is arranged to determine which gear element(s) is/are engaged from the output of the means for detecting the positions of the first and second sets of engagement members.

The transmission may include a third gear element rotatably mounted on the first shaft and a second gear selector assembly and wherein the second gear selector assembly is arranged to selectively lock the third gear element for rotation with the first shaft, said selection being from operational modes that include: lock the gear element for rotation with the first shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the first shaft in the clockwise direction and not lock in the anti-clockwise direction; and lock the gear element for rotation with the first shaft in the anti-clockwise direction and not lock in the clockwise direction.

The transmission system can select the following operational mode with respect to the third gear element: not lock the gear element for rotation with the first shaft in the clockwise or anticlockwise directions.

Any practicable number of gear selector assemblies can be included in the system. Preferably the transmission includes at least three gear selector assemblies, which are similar to the first gear selector assembly. Typically, each gear selector assembly will selectively lock two gear elements for rotation with a shaft. Typically, each rotatably mounted gear element will form part of a gear train that transfers drive between the first shaft and a second shaft. Preferably transmissions include between three and ten gear trains, and more preferably between four and six gear trains. For example, the first gear element can be part of a first gear train that

includes a fourth gear wheel fixed to the second shaft. The second gear element can be part of a second gear train that includes a fifth gear wheel fixed to the second shaft and the third gear element can be part of a third gear train that includes a sixth gear wheel fixed to the second shaft.

According to another aspect of the invention there is provided a transmission system including first and second rotatable shafts, and means for transferring drive from one of the shafts to the other shaft including first and second gear elements each rotatably mounted on the first shaft and having drive formations formed thereon, a gear selector assembly for selectively transmitting torque between the first shaft and the first gear element and between the first shaft and the second gear element, said selector assembly including first and second sets of engagement members that are moveable into and out of engagement from the first and second gear elements and at least one actuator system for controlling the movement of the first and second sets of engagement members, wherein the gear selector assembly is arranged such that when a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the engaged gear element, and the other set of engagement members is then in an unloaded condition, and the or each actuator system is arranged to move the unloaded set of engagement members to engage the new gear, wherein the or each actuator system includes an actuator member for controlling the movement of at least one of the sets of engagement members and a limiting system arranged to controllably adjust the extent of allowable movement of the engagement members to prevent unintentional gear selections.

Advantageously the limiting system includes first and second blocking members for preventing unintentional gear engagements by limiting the extent of allowable movement of the engagement members, wherein the limiting system is arranged to adjust the extent of allowable movement of the first and second sets of engagement members to control the separation between them and the first and second gear elements according to the rotational orientation of the first and second blocking members relative to the actuator member.

Advantageously the transmission system can include a first actuator system according to any one of the configurations described herein to actuate the first set of engagement members and a second actuator system according to any one of the configurations described herein to actuate the second set of engagement members. Alternatively, a single actuator system can be arranged to operate both the first and second sets of engagement members.

The first gear selector assembly is preferably arranged to engage the second gear element by moving the unloaded set of engagement members out of engagement with the first gear element and into driving engagement with the second gear element whilst the first gear element is still engaged by the loaded set of engagement members to effect a gear change between the first and second gear elements. Thus the first gear selector assembly is arranged to selectively lock the first and second gear elements for rotation with the first shaft simultaneously, at least momentarily. Typically, this only happens for a very short period of time during the shift, since when the new gear has been selected the loaded element set becomes unloaded and the control system is arranged to disengage it from it gear element and move it into engagement with the new gear element.

The first gear selector assembly is preferably arranged to move the unloaded set of engagement members out of engagement with the second gear element and into driving engagement with the first gear element whilst the second gear element is still engaged by the loaded set of engagement members to effect a gear change between the first and second gear elements.

Advantageously the first and second sets of engagement members are arranged to rotate, in use, with the first shaft. Preferably the first shaft is an input shaft and the second shaft is an output shaft and drive is transferred from the input shaft to the output shaft. Preferably the selector assembly is arranged such that when the first and second sets of engagement members engage one of the first and second gear elements the backlash when moving between acceleration and deceleration is less than or equal to five degrees.

Preferably the drive formations on the first and second gear elements comprise first and second groups of dogs respectively. For example, the first and second groups of dogs each comprise between two and eight dogs, evenly distributed on the first and second gears respectively. Preferably the first and second groups of dogs each comprise between two and four dogs, and more preferably three dogs.

The first and second sets of engagement members preferably comprise between two and eight members, more preferably between two and four members, and more preferably still three members.

Advantageously the first shaft may include keyways arranged such that the first and second sets of engagement members can slide axially along the keyways and to radially restrain the positions of the sets of engagement members. Preferably the cross-section of the keyways is one of T-shaped, slotted, and dovetailed.

Preferably the actuator system includes at least one resiliently deformable means arranged to move at least one of the first and second sets of engagement members into engagement with the first and second gear elements when the engagement members are in unloaded conditions. Preferably the or each resiliently deformable means is arranged to bias at least one of the first and second sets of engagement members towards the first or second gear element when the engagement members are drivingly engaged with a gear element.

The transmission system may further include third and fourth gears mounted on the first shaft and a second selector assembly to provide additional gear ratios between the first and second shafts.

According to another aspect of the invention there is provided a method for preventing unintentional gear engagements in a transmission system having a first shaft, first and second gear elements rotatably mounted on the first shaft, a gear selector assembly including first and second sets of engagement members arranged to selectively lock the first and second gear elements for rotation with the first shaft independently of each other, said selection being

from operational modes that include: lock the gear element for rotation with the first shaft in the clockwise and anti-clockwise directions; lock the gear element for rotation with the first shaft in the clockwise direction and not lock in the anti-clockwise direction; lock the gear element for rotation with the first shaft in the anti-clockwise direction and not lock in the clockwise direction, and at least one actuator system having an actuator member for controlling the movement of at least one of the sets of engagement members and a limiting system for controllably adjusting the extent of allowable movement of the or each set of engagement members to control the separation between the engagement members and the first and second gear elements, wherein when one of the sets of engagement members is driven towards a new gear element as a result of a high speed collision between a currently selected gear element and that set of engagement members, the method includes using the or each actuator system to arrest the movement of the driven set of engagement members before it engages the new gear.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which like references indicate equivalent features, wherein:

Figures la-e shows a shift sequence to illustrate the problem that the current invention addresses;

Figure If is a sectional view of a general arrangement of a transmission system in accordance with the present invention;

Figure 2 is a schematic that illustrates the arrangement of a group of dogs on a side of a gear (teeth omitted for clarity);

Figure 3 is a schematic that illustrates the interaction of a selector mechanism and the dogs on the side of a gear wheel;

Figure 4 is a perspective view of an engagement element from the selector mechanism;

Figure 5a-f illustrate diagrammatically operation of the selector mechanism;

Figures 6 to 8a show various views of an actuator mechanism;

Figure 8b shows an exploded view of the actuator mechanism of Figures 6 to 8a;

Figure 9 illustrates diagrammatically layout of a cam track;

Figures 10 and 11 show the relative orientations of blocking collars and a sleeve on which is mounted an actuating fork (Figure 10 shows the relative orientations when the selector mechanism is in neutral and Figure 11 shows the relative orientations when the selector mechanism engages a gear); and

Figures 12a-l show the relative orientations of one of the blocking collars and the sleeve under certain operational conditions: Figure 12a - neutral, Figures 12b-e engagement element on dog mis-engagement, Figures 12f-g engagement, Figures 12h-k actuator attempting disengagement but not achieved and Figure 121 disengagement and return to neutral.

Figure 1 shows the layout of the transmission system 88, which includes an output shaft 1, an input shaft 3 and first, second, third, fourth, fifth and sixth gear trains (or gear ratios)

5,7,9,11,12,14 (1 ^st , 2 ^nd , 3 ^rd , 4 ^th , 5 ^th and 6 ^th ) arranged to transmit drive between the input and output shafts 3,1. The first gear train 5 comprises a first gear wheel 13 rotatably mounted on the output shaft 1 via a bearing and a second gear wheel 15 fixed to the input shaft 3 in mesh with the first gear wheel 13. The second gear train 7 comprises a third gear wheel 17 rotatably mounted on the output shaft 1 and a fourth gear wheel 19 fixed to the input shaft 3 in mesh with the third gear wheel 17. The third gear train 9 comprises a fifth gear wheel 21 rotatably mounted on the output shaft 1 and a sixth gear wheel 23 fixed to the input shaft 3 in mesh with the fifth gear wheel 21. The fourth gear train 11 comprises a seventh gear wheel 25 rotatably mounted on the output shaft 1 and an eighth gear wheel 27 fixed to the input shaft 3 in mesh with the seventh gear wheel 25. The fifth gear train 12 comprises a ninth gear wheel

16 rotatably mounted on the output shaft 1 and a tenth gear wheel 18 fixed to the input shaft 3

in mesh with the ninth gear wheel 16. The sixth gear train 14 comprises an eleventh gear wheel 22 rotatably mounted on the output shaft 1 and a twelfth gear wheel 24 fixed to the input shaft 3 in mesh with the seventh gear wheel 25.

First, second and third selector mechanisms 29,31,33 are also mounted on the output shaft 1. Each selector mechanism 29,31,33 is arranged to selectively transmit drive between the output shaft 1 and input shaft 3 via the gear trains by selectively locking the gear wheels rotatably mounted on the output shaft 1 for rotation with the output shaft 1. The first selector mechanism 29 is arranged to selectively lock the first gear wheel 13 from the 1 ^st gear ratio and third gear wheel 17 from the 2 ^nd gear ratio for rotation with the output shaft 1. The second selector mechanism 31 is arranged to selectively lock the fifth gear wheel 21 from the 3rd gear ratio and the seventh gear wheel 25 from the 4th gear ratio for rotation with the output shaft 1. The third selector mechanism 31 is arranged to selectively lock the ninth gear wheel 16 from the 5th gear ratio and the eleventh gear wheel 22 from the 6 ^th gear ratio for rotation with the output shaft 1.

When a gear wheel is engaged by a gear selector mechanism it is locked for rotation with the output shaft 1. So, for the third gear train 9, when the second gear selector mechanism 31 engages the fifth gear wheel 21 and the first and third gear selector mechanisms 29,33 are in neutral (no gear wheels engaged) drive is transmitted between the input and output shafts 3,1 via the third gear train 9.

Each selector mechanism 29,31,33 is similar and is mounted on the output shaft 1 in a similar manner. The structure of the first gear selector mechanism 29 and the way that it selectively engages the first and third gear wheels 13,17 will now be described. However the general structure and principles of operation are applicable to the second and third gear selector mechanisms 31,33 and their respective gear wheels.

The gear selector mechanism 29 is arranged to engage drive formations 20 located on the first and third gear wheels 13,17. The drive formations 20 on each gear wheel 13,17 comprise

groups of dogs (similar drive formations are located on the fifth, seventh, ninth and eleventh gear wheels 21,25,16,22).

The first dog group 20 is located on one side of the first gear wheel 13. The dogs are preferably formed integrally with the first gear wheel, but this is not essential. The first dog group 20 comprises three dogs evenly circumferentially distributed about the gear face, i.e. the angle subtended between the centres of a pair of dogs is approximately 120° (see Figures 2 and 3). The second dog group 20, comprises three dogs and is similarly arranged on one side of the third gear wheel 17. Three dogs are used because this arrangement provides large engagement windows, that is the spaces between the dogs, to receive the engagement elements. Large engagement windows provide greater opportunities for the first gear selector mechanism 29 to fully engage the gear wheels 13,17 before transmitting drive thereto. If the first gear selector mechanism 29 drives a gear wheel when only partially engaged it can lead to damage of the dogs and / or the first gear selector mechanism 29.

The first and third gear wheels 13,17 are mounted spaced apart on the output shaft 1 and are arranged such that the sides including the first and second dog groups face each other.

The first gear selector mechanism 29 includes first and second sets of engagement elements 35,36 and an actuator assembly 38.

The first and second sets of engagement elements 35,36 are mounted on the output shaft 1 between the first and third gear wheels 13,17. The first set of engagement elements 35 comprises three elements 28 that are evenly distributed about the output shaft 1 such that their bases face inwards, and the axes of the elements 28 are substantially parallel. The second set of engagement elements 36 comprises three elements 30 which are similarly arranged about the output shaft 1.

The sets of engagement elements 35,36 are mounted on a sleeve 34 which is mounted on the output shaft 1 between the first and second gear wheels 3,5 (see Figures 1 and 3). The sets of engagement elements 35,36 are arranged to rotate with the output shaft 1 but are able to slide axially along the sleeve 34 and the output shaft 1 in response to a switching action of the

actuator assembly 38. To facilitate this, the sleeve 34 includes six keyways 41 formed in its curved surface with each engagement element 28,30 having a complementary formation in its base. The keyways 41 may have substantially T-shaped profiles such that the elements are radially and tangentially (but not axially) restrained within the keyways 41 (see Figure 2). Alternatively, the keyways 41 can have slotted or dovetailed profiles to radially restrain the elements.

Preferably the elements are configured to be close to the output shaft 1 to prevent significant cantilever effects due to large radial distances of loaded areas thus reducing the potential for structural failure.

The arrangement of the element sets 35,36 is such that elements of a particular set are located in alternate keyways 41 and the element sets 35,36 can slide along the sleeve 34. The elements in each element set are rigidly connected to each other by an annular connector member 100 and move as a unit. Each element set can move independently of the other. The connector member 100 has a groove 102 formed in its outer curved surface that extends fully around the connector member. The elements 28 in the first set of engagement elements 35 are preferably integrally formed with its connector member 100, though this is not critical. The elements 28 are evenly distributed about the connector member 100. The second set of engagement elements 36 comprises three elements 30, which are held in a similar fixed arrangement by a second connector member 100. When there is relative movement between the first and second sets of elements 35,36, the connector member 100 of the first element set 35 moves over the second set of elements 36 and the connector member 100 of the second element set 36 slides over the first set of elements 35.

Each element 28 in the first element set 35 has a first end 28a arranged to engage the first group of dogs 20 attached to the first gear wheel 13 and a second end 28b arranged to engage the second group of dogs 20 on the third gear wheel 17. The first and second ends 28a,28b typically have the same configuration but are opposite handed, for example the first end 28a is arranged to engage the first group of dogs 20 during deceleration (reverse torque direction) of the first gear wheel 13 and the second end 28b is arranged to engage the second group of dogs 20 during acceleration (forward torque direction) of the third gear wheel 17. Each element 30

in the second element set 36 is similarly arranged, except that the first end 30a is arranged to engage the first group of dogs 20 during acceleration of the second gear wheel 15 and the second end 30b is arranged to engage the second group of dogs 20 during deceleration of the third gear wheel 17.

When both the first and second sets of engagement elements 35,36 engage a gear wheel drive is transmitted between the input and output shafts 3,1 whether the gear is accelerating or decelerating.

The first and second ends 28a,30a,28b,30b of each element include an engagement face 43 for engaging the dogs 20, a ramp 45, an end face 42 and may include a shoulder 44 (see Figure 4). The end faces 42 limit the axial movement of the engagement elements 28,30 by abutting the sides of the gear wheels. The engagement faces 43 may be angled to complement the sides of the dogs 20a so that as the engagement elements 28,30 rotate into engagement, there is face-to-face contact to reduce wear. Each ramp 45 is preferably helically formed and slopes away from the end face 42. The angle of inclination of the ramp 45 is such that the longitudinal distance between the edge of the ramp furthest from the end face 42 and the plane of the end face 42 is larger than the height of the dogs 20. This ensures that the transmission does not lock up when there is relative rotational movement between the engagement elements 28,30 and the dogs 20 that causes the ramp 45 to move towards engagement with the dogs 20. The dogs 20 do not crash into the sides of the engagement elements 28,30 but rather engage the ramps 45. As further relative rotational movement between the dogs 20 and the engagement elements 28,30 occurs, the dogs 20 slide across the ramps 45 and the helical surfaces of the ramps cause the engagement elements 28,30 to move axially along the output shaft 1 away from the dogs 20 so that the transmission does not lock up.

The arrangement of the gear selector mechanism is such that it inherently prevents lockup of the transmission occurring when selecting a new gear.

When the elements of the first and second sets 35,36 are interleaved, as in Figure 3, the engagement faces 43 of the first ends 28a of the first set of elements 35 are adjacent the engagement faces 43 of the first end 30a of the second set of elements 36. When the first and second sets of elements 35,36 are fully engaged with a gear, a dog 20 is located between each pair of adjacent engagement faces 43. The dimensions of the dogs 20 and the ends of the elements are preferably such that there is little movement of each dog between the engagement face 43 of the acceleration element and the engagement face 43 of the deceleration element when the gear moves from acceleration to deceleration, or vice versa, to ensure that there is little or no backlash in the gear.

The actuator assembly 38 includes first and second actuator mechanisms 46,64 that are arranged to actuate the first and second sets of engagement elements 35,36 respectively, and independently of each other. The first and second actuator mechanisms 46,64 are shown diagrammatically as 'black boxes' in Figure If for clarity. The first and second actuator mechanisms 46,64 are similar and the detailed arrangement of each of them is shown in Figures 6 to 11. The first actuator mechanism 46 will now be described, however the arrangement and principles of operation are applicable to the second actuator mechanism 64.

The first actuator mechanism 46 includes an electric motor 200 that is controlled by a transmission control unit (not shown), a drive shaft 202, a first cam system 204 for adjusting the axial position of the drive shaft 202, a fork 206 for driving the first set of engagement elements 35, a second cam system 208 for limiting axial movement of the fork 206 relative to the drive shaft 202 and helical springs 210 for biasing the position of the fork 206.

The motor 200 is arranged to rotate the drive shaft 202 in clockwise and anti-clockwise directions. The first cam system 204 is arranged to limit the extent of rotation in the clockwise and anti-clockwise directions of the drive shaft 202 and at the same time adjust the axial position of the drive shaft 202 according to the position of a cam follower 216 along a cam track 218 defined by first and second cam members 220,222 having first and second cam surfaces 224,226 respectively.

The first and second cam members 220,222 are fixed for rotation with the drive shaft 202. The first and second cam surfaces 224, 226 define the cam track 218 (shown diagrammatically in Figure 9), which comprises a groove having first, second and third circumferentially extending portions 228,230,232, and first and second helically extending portions 234,236. The first and second 228,230 and second and third 230,232 circumferentially extending portions are connected by the first and second helically extending portions 234,236 respectively. The full rotational extent of the cam track 218 is approximately 360 degrees, with approximately 180 degrees rotation in each direction from the central position of the second circumferential portion 230 to the ends of the track.

The cam follower 216 has a fixed translational position but is able to rotate freely within the cam track 218. When the cam follower 216 is positioned along the second circumferentially extending portion 230, the fork 206 and hence the first set of engagement elements 35 are in a neutral position. To engage first gear, the transmission control unit activates the motor 200 to rotate the drive shaft 202 anti-clockwise. The cam follower 216 rotates along the cam surfaces 224,246 moving along the second circumferential portion 230 and onto the first helical portion 234. As the cam follower 216 moves along the first helical portion 234, the drive shaft 202 moves translationally along its longitudinal axis since the translational position of the cam follower 216 is fixed. As the axial position of the drive shaft 202 is adjusted so to is the axial position of the fork 206 and hence the first set of engagement elements 35. However, the fork 206 is rotatably mounted on the drive shaft 202, and therefore the orientation of the fork remains substantially constant whilst the drive shaft 202 rotates.

The motor 200 continues to rotate the drive shaft 202 until the cam follower 216 reaches the end of the cam track 218. When the cam follower 216 reaches the first circumferential portion 228, the drive shaft 202 ceases to move axially and the first set of engagement elements 35 has moved into engagement with the first gear wheel 13. However, the motor 200 continues to rotate the drive shaft 202 until the cam follower reaches the end of the cam track.

To disengage the first gear wheel 13, the motor 200 is activated to rotate the drive shaft 202 in the clockwise direction. The cam follower 216 moves along the first circumferential portion

228, the first helical portion 234 wherein the axial position of the drive shaft 202, and hence the first set of engagement members 35, moves axially away from the first gear wheel 13 until it reaches the second circumferential portion 230, wherein axial movement ceases. However, the motor 200 continues to rotate the drive shaft 202 at least until the cam follower 216 reaches the central position of the second circumferential portion 230.

To engage the third gear wheel 17 (second gear), the motor 202 further rotates, or continues to rotate, the drive shaft 202 in the clockwise direction. The cam follower 216 moves along the remainder of the second circumferential portion 230, the second helical portion 236 and the third circumferential portion 232. As the cam follower 216 moves along the second helical portion 236 the drive shaft 202 moves axially thereby moving the fork 206 and first set of engagement elements 35 out of neutral and into engagement with the third gear wheel 17. The motor 200 continues to rotate the drive shaft 202 until the cam follower 216 reaches the end of the cam track 218.

A particular advantage of the cam track 218 layout is that the circumferential portions 228,230,232 enable the motor 200 to get up to speed quickly since during those periods it does not have to overcome the inertia of the translational movement of the drive shaft 202 when a gear shift is requested. This is because when a gear shift is initiated the cam follower

216 is likely to start from one of the circumferential portions. This provides a quick response time. In situations where a new shift is requested when the cam follower 216 is positioned on a helical portion 234,236 the motor 202 is already up to speed.

The connection between the motor 200 and the drive shaft 202 is such that the motor 200 is able to rotate the drive shaft 202 and move it along its entire extent of axial movement in both directions that is allowed by the cam track 218, without disengaging the shaft 202. This is achieved by the output shaft of the motor having a square section that is arranged to slide within a complementary axial bore formed in the near end of the drive shaft 202.

The second cam system 208 includes two collars 238 and a fork mounting sleeve 240. Each collar 238 includes a flange 242 at one end and a third cam surface 244 on the side opposite

the flange 242. The collars 238 are fixed to the drive shaft 202 spaced apart and are arranged such that the cam surfaces 244 face each other and have the same orientation. The third cam surfaces 244 have fourth and fifth circumferential portions 246,248 and third and fourth helical portions 250,252.

The mounting sleeve 240 is mounted on the drive shaft 202 and is located between the collars 238. The sleeve 240 can move freely rotationally and translationally relative to the drive shaft 202. The sleeve 240 includes two fourth cam surfaces 254. The fourth cam surfaces 254 are formed in the end faces of the sleeve 240 and have similar profiles to the third cam surfaces 244. The fourth cam surfaces 246 are arranged substantially symmetrically to one another about an axis running through the fork 206. The fork 206 is fixed to the sleeve 240 and therefore the orientation of the fork 206 is fixed in relation to the fourth cam surfaces 254.

The fork 206 is arranged to extend approximately 180 degrees around the groove 102 of the first set of engagement elements 35 and includes a semi-annular part that is located within the groove 102 . The first set of engagement elements 35 can rotate relative to the fork 206 and is caused to move axially along the output shaft 1 by the fork 206 applying a force to the connector member 100.

The orientation of the cam surfaces 244 on the collars 238 is set in relation to the cam track 218 to give the appropriate blocking action to the movement of the sleeve 240, and hence the fork 206, to prevent the first set of engagement elements 35 from being driven into engagement with a gear wheel unintentionally due to the collision of dogs 20 with the ramped faces 45 of the first element set 35. Figure 10 shows the orientation of the collars 238 when the first set of engagement elements 35 is in the centrally located neutral position and wherein the cam follower 216 is centrally located on the second circumferential portion 230. Figure 11 shows the orientation of the collars 238 when the first set of engagement elements 35 engage either the first or third gear wheels 13,17 and wherein the cam follower 216 has reached either end of the cam track 218. It is to be noted that in both Figures the orientation of the sleeve 240 and hence the fork 206 has not changed since it rotates freely with respect to the drive shaft 202. Similarly, the sleeve is located substantially equidistantly between the collars 238

and typically remains in that position due to the action of the helical springs 210. Accordingly, the separation between the collars 238 and the sleeve 240 changes with the rotational orientation of the drive shaft 202. The separation is at a maximum when the first or second gear is selected (see Figure 11) and at a minimum when in neutral (see Figure 10). In each case, the amount of separation between the collars 238 and the sleeve 240 is such that it is smaller than the distance between the first set of engagement elements 35 and the dogs 20 on the gear wheels so that the collars 238 will always block the movement of the engagement element set 35 to prevent unintentional engagements.

For example, when the first element set is engaged with the third gear wheel 17 (in second gear), the distance between the dogs 20 on the first gear wheel 13 and the unengaged ends of the first set of engagement members 35 is 5.5mm and the separation between the collar 238 and the sleeve 240 is 5mm. Thus if the dogs on the third gear wheel 17 collide with the first element set 35 and cause it to move axially towards the first gear wheel 13, the movement of the sleeve 240 is arrested when it collides with the collar 238 after 5mm. Thus the first element set 35 is held 0.5mm out of engagement with the dogs 20 on the first gear wheel 13. This prevents the conflict situation arising. When the first element set 35 is in neutral, the separation between the element set 35 and the dogs 20 of each gear wheel is approximately 1.5mm. Accordingly, the separation between the sleeve 240 and the collars 238 in this condition is typically lmm.

The helical springs 210 bias the fork 206 and sleeve 240 back towards a centrally located equilibrium position. The helical springs 210 are also used to bias the fork 206, and hence the first element set 35, to move out of engagement with the engaged gear ratio during shift when the first element set 35 is loaded and is not free to move. Thus when the new gear is engaged by the unloaded set of engagement elements and the currently engaged set becomes unloaded, the resiliency of one of the springs 210 urges it out of engagement.

Figures 12a-l show the relative positions of the sleeve 240 and one of the collars 238 when a mis-engagement initially occurs (Figures 12a-f) and when the engagement elements do not initially disengage a gear wheel when a gearshift is initiated (Figures 12g-l).

When performing a gearshift, one of the possible mis-engagements that can occur is the engagement elements initially colliding with the tops of the dogs 20 instead of entering the spaces between the dogs 20. Figure 12a shows sleeve 240, and hence engagement elements, in the neutral position at the time when a gearshift is initiated, for example when selecting the third gear wheel 17 (second gear). The motor 200 rotates the drive shaft 202 and the first cam system 204 causes the drive shaft 202 to move axially. However, when the mis-engagement described above occurs, the axial movement of the engagement elements and hence fork- sleeve assembly 206,240 is restricted but the drive shaft 202 continues to rotate and move axially and hence the translational and rotational position of the collar 238 relative to the sleeve 240 continues to adjust (see Figures 12b-e). Advantageously the arrangement of the surfaces 246,248,250,254 of the sleeve and collar are such that in the event of a collision between the engagement members and the dogs 20 on the third gear wheel 17 forcing the engagement elements towards the first gear wheel 13, engagement of the first gear wheel 13 would be prevented since axial movement of the sleeve 240 would be blocked by the collar 238. In other words, at all times, the distance A between the sleeve 240 and collar 238 is smaller than the axial distance between the engagement elements and the dogs 20 on the first gear wheel 13. Therefore the actuator system protects the transmission system from dangerous engagements even when mis-engagements occur.

Figures 12f and 12g show the relative positions of the sleeve 240 and collar 238 when the engagement elements engage the third gear wheel 17 since the elements have now moved into the spaces between the dogs. The distance A between the sleeve 240 and collar 238 is smaller than the axial distance between the engagement elements and the dogs 20 on the first gear wheel 13. Thus the collar 238 would block engagement with the first gear wheel 13 in the event of the engagement members being forced towards that gear wheel 13.

Figures 12h-k illustrate the action of the blocking mechanism in a scenario wherein the actuator is attempting to disengage the third gear wheel 17 but is unable to do so due to the operational conditions. Accordingly, the collar 238 rotates and move translationally relative to the fork-sleeve assembly 206,240 however the distance A between the sleeve 240 and collar 238 is smaller than the axial distance between the engagement elements and the dogs 20 on

the first gear wheel 13 at all times and therefore the collar 238 blocks engagement with the first gear wheel 13. The engagement elements remain engaged with the third gear wheel 17 until the force of the spring 210 overcomes the retention force. When this occurs, the fork- sleeve assembly 206,240 moves axially towards the first gear wheel 13 into a neutral position (see Figure 121). If the forces are sufficiently large, the sleeve 240 may collide with the collar 238 before settling in the neutral position.

Optionally, the actuator assembly 38 may include first and/or second sensors 212,214. The first and/or second sensors 212,214 can be used to determine the position of the first set of engagement elements 35. The first sensor 212 does this by detecting the rotational orientation of the drive shaft 202. The position of the first set of engagement elements 35 can be calculated by from an understanding of the geometry of the actuator system 38. The second sensor 214 is contactless. It is arranged to detect the position of a magnet 256, which is mounted on top of the fork 206. The sensors 212,214 can be integrated into a control system that controls operation of the transmission system. For example, the sensors 212,214 can send signals to the transmission control unit and the transmission control unit can use the information provided to determine the operational status of the first selector mechanism 29, such as whether or not a gear wheel is engaged, and to initiate appropriate control signals.

Preferably the operation of the first and second actuator mechanisms 46,64, and hence movement of the first and second sets of engagement elements 35,36 are controlled by the transmission control unit. The transmission control unit is an electronic logic control system driven by software. It is the sequence programing that enables the transmission control unit to automatically control the operation of the transmission to undertake gearshifts in the most appropriate manner. The transmission can be fully automatic, that is gear selections are made by the transmission control unit when an engine control unit detects predetermined operational conditions, for example when the engine reaches a particular speed in a particular gear. Alternatively, gear selection can be made by the user of the drive system by initiating a gear selection input device, for example a gear lever (manual) or switches located adjacent the steering wheel (semi-automatic). The transmission can be arranged such that it is possible to select between the automatic and manual modes.

The first and second sensors 212,214 are not required when the stiffness of the helical springs 210 is high such that the movement of the sleeve-fork 240,206 arrangement is negligible. Also, having high spring stiffness can increase the accuracy of the transmission control units determination of the position of the first element set 35 based on readings obtained from the first sensor 212.

The operation of the first gear selector mechanism 29 will now be described with reference to Figures 5a-5f which for clarity illustrate diagrammatically the movement of the first and second element sets 35,36 by the relative positions of only one element from each set.

Figure 5a shows the first and second element sets 35,36 in a neutral position, that is, neither element set is engaged with a gear wheel. Figure 5b shows the first and second element sets moving into engagement with the first gear wheel 13 under the action of the first and second actuator mechanisms 46,64 in response to a gearshift request. Preferably, the clutch is opened for selecting first gear from neutral.

Figure 5c shows a condition when the first gear wheel 13 is fully engaged, that is, the element sets 35,36 are interleaved with the first group of dogs 20. The first and second actuator mechanisms 46,64 are arranged such that the actuator members 48,58 maintain the first and second element sets 35,36 in engagement with the first gear wheel 13. Accordingly, drive is transferred from the input shaft 3 through the second gear wheel 15, the first gear wheel 13 to the output shaft 1 via the first element set 35 when decelerating and via the second element set 36 when accelerating.

Whilst accelerating (first gear wheel 13 rotating in the direction of arrow B in Figure 5c) using the first gear train 5, the engagement faces 43 of the elements of the first element set 35 are not loaded, whilst the engagement faces 43 of the elements of the second element set 36 are loaded. When a user, or an engine control unit (not shown) wishes to engage the second gear train 7 the transmission control unit actuates the first actuator mechanism 46 to drive the first fork 206, which causes the elements 28 of the first element set 35 to slide axially along

the keyways 41 in the sleeve 34 thereby disengaging the first element set 35 from the first gear wheel 13 (see Figure 5d).

The second actuator mechanism 64 is activated to move the second fork 206 and hence the second element set 36 towards the third gear wheel 17. However, because the second element set 36 is loaded, i.e. is driving the first gear wheel 13, it cannot be disengaged from the first gear wheel 13, and the second element set 36 remains stationary, with the second actuator mechanism 64 biasing it towards the third gear wheel 17.

When the first element set 35 slides axially along the output shaft 1, the engagement faces 43 engage the second group of dogs 20 (see Figure 5e). The third gear wheel 17 then begins to drive the elements 28 in the direction of Arrow C in Figure 5e and drive is transmitted between the input and output shafts 3,1 via the second gear train 7. As this occurs, the second element set 36 ceases to be loaded, and is free to disengage from the first group of dogs 20. Since the second element set 36 is biased by the second actuator mechanism 64 it slides axially along the keyways 41 in the sleeve 34 thereby completing the disengagement of the first gear wheel 13 from the output shaft 1. The second element set 36 slides along the keyways 41 until it engages the third gear wheel 17, thereby completing engagement of the third gear wheel 17 with the output shaft 1 (see Figure 5f).

This method for selecting gear trains substantially eliminates torque interruption since the second gear train 7 is engaged before the first gear train 5 is disengaged, thus momentarily, the first and second gear trains 5,7 are simultaneously engaged and locked for rotation with the output shaft 1, until the newly engaged gear wheel overdrives the original gear wheel. Thus gear shifting is instantaneous.

When a gear wheel is engaged by both the first and second element sets 35,36 it is possible to accelerate or decelerate using a gear wheel pair with very little backlash occurring when switching between the two conditions. Backlash is the lost motion experienced when the dog moves from the engagement face 43 of the acceleration element to the engagement face 43 of the deceleration element when moving from acceleration to deceleration, or vice versa. A

conventional dog-type transmission system has approximately 30 degrees of backlash. A typical transmission system for a car in accordance with the current invention has backlash of less than five degrees.

Backlash is reduced by minimising the clearance required between an engagement member and a dog during a gearshift: that is, the clearance between the dog and the following engagement member (see measurement ¹ A' in Figure 5b). The clearance between the dog and the following engagement member is typically in the range 0.5mm - 0.03mm for a car and is typically less than 0.2mm. Backlash is also a function of the retention angle, that is, the angle of the engagement face 43, which is the same as the angle of the undercut on the engagement face of the dog 20a. The retention angle influences whether there is relative movement between the dog and the engagement face 43. The smaller the retention angle, the less backlash that is experienced. The retention angle is typically between 2.5 and 15 degrees.

Transition from the second gear train 7 to the first gear train 5 whilst decelerating is achieved by a similar process.

Whilst decelerating in the second gear train 7 the engagement surfaces 43 of the elements of the first element set 35 are not loaded, whilst the engagement surfaces 43 of the elements of the second element set 36 are loaded. When a user, or an engine control unit (not shown) wants to engage the first gear train 5 the transmission control unit actuates the first actuator mechanism 46 to move the first actuator member 48 axially, causing the first element set 35 to slide axially in the keyways 41 along the output shaft 1 in the direction of the first gear wheel 13, thereby disengaging the first element set 35 from the third gear wheel 17.

The transmission control unit activates the second actuator mechanism 64 however since the second element set 36 is loaded, i.e. it is drivingly engaged with the dogs 20 on the third gear wheel 17, it remains stationary but is urged towards the first gear wheel 13. As the first element set 35 slides axially in the keyways 41 and engages the dogs 20 on the first gear wheel 13 and the first gear wheel 13 drives the first element set 35 such that energy is transmitted between the input and output shafts 3,1 by way of the first gear train 5. As this

occurs, the second element set 36 ceases to be loaded and biasing of the second actuator mechanism 64 causes it to slide axially within the keyways 41 along the output shaft 1 towards the first gear wheel 13, thereby completing disengagement of the third gear wheel 17. The second element set 36 continues to slide within the keyways 41 along the output shaft 1 until it engages the first gear wheel 13, thereby completing engagement of the first gear wheel 13 with the output shaft 1.

Kick-down shifts, that is a gearshift from a higher gear train to a lower gear train but where acceleration takes place, for example when a vehicle is travelling up a hill and the driver selects a lower gear to accelerate up the hill, require a brief torque interruption to allow disengagement of the driving element set.

In the transmission configuration shown in Figure If all the selector mechanisms 29,31,33 are mounted on the output shaft 1. The transmission is arranged such that the deceleration drive faces of the first set of engagement elements 35 of the first gear selector mechanism 29 are engageable with the first gear wheel 13 and the acceleration drive faces are engageable with the third gear wheel 17 (see Figure 1). The second set of engagement elements 36 of the first gear selector device 29 is arranged such that the acceleration drive faces are engageable with the first gear wheel 13 and the deceleration faces are engageable with the third gear wheel 17.

The first set of engagement elements 35 of the second selector mechanism 31 is arranged such that the deceleration drive faces are engageable with the fifth gear wheel 21 and the acceleration drive faces are engageable with the seventh gear wheel 25. The second set of engagement elements 36 is arranged such that the acceleration drive faces are engageable with the fifth gear wheel 21 and the deceleration faces are engageable with the seventh gear wheel 25.

The first set of engagement elements 35 of the third selector mechanism 33 is arranged such that the deceleration drive faces are engageable with the ninth gear wheel 16 and the acceleration drive faces are engageable with the eleventh gear wheel 22. The second set of engagement members 36 is arranged such that the acceleration drive faces are engageable

with the ninth gear wheel 16 and the deceleration faces are engageable with the eleventh gear wheel 22.

When a gear shift takes place that requires the movement of two of the gear selector mechanisms 29,31,33, such as from 2 ^nd to 3 ^rd gear which requires movement of the first and second gear selector mechanisms 29,31, the conflict problem described above cannot occur because each actuator assembly 38 for each set of engagement elements 35,36 includes blocking elements (collars 238) for limiting the axial movement of the sets of engagement elements 35,36. For example, at the time of the collision of dogs 20 of the third gear wheel 17 on one of the sets of engagement elements 35,36 (see Figure Id) the collars 238 are oriented substantially as shown in Figure 11. This enables the set of engagement elements 35,36 to move under the force of the collision towards the first gear wheel 13, and hence the fork- sleeve arrangement 206,240 moves translationally relative the shaft from its equilibrium position and compresses one of the helical springs. However, engagement of the first gear wheel 13 does not occur since the sleeve 240 collides with one of the collars 238 prior to engagement, which arrests the movement of the set of engagement elements 35,36.

Subsequently the reaction of the compressed helical spring 210 moves the fork-sleeve arrangement 206,240 back to its equilibrium position substantially equi-distant between the collars 238 and the set of engagement elements 35,36 is held in neutral.

It will be appreciated by the skilled person that adaptations can be made to the above embodiment that fall within the scope of the invention. For example, different cam surface profiles may be used. Also, it will be appreciated that whilst the above embodiment relates to instantaneous transmission systems having individual fork control the actuator system is equally applicable to dual fork type instantaneous transmission systems, for example of the type described in WO 2004/099654, WO 2005/005868, WO 2005/005869 and WO 2005/024261.

Although the actuator mechanism described above has been shown in conjunction with an instantaneous transmission system, it is envisaged that at least one aspect of the invention can be used on other types of transmission systems.

The transmission system can be used in any vehicle for example, road cars, racing cars, lorries, motorcycles, bicycles, trains, trams, coaches, earth removal vehicles such as bulldozers and diggers, cranes, water craft such as hovercraft and ships, aircraft including aeroplanes and helicopters, and military vehicles. The system can also be used in any machine that has first and second rotatable bodies wherein drive is to be transmitted from one of the rotatable bodies to the other with variable speed and torque characteristics, such as transportation systems and manufacturing equipment including lathes, milling machines and dedicated production systems.

Use of instantaneous type gear selector mechanism leads to improved performance, lower fuel consumption and lower emissions since drive interruption during gear changes is substantially eliminated. Also the system is a more compact design than conventional gearboxes leading to a reduction in gearbox weight.