Mechanism for limiting the transmission of torque The present invention relates mechanisms for limiting the transmission of torque, for example for use in transmission systems. The present invention can be used in conjunction with transmission systems of the type described in PCT/GB2004/001976, and in PCT/GB2004/and PCT/GB2004/, filed in the name of Zeroshift Limited onttJuly 2004, though the invention is not to be considered as being limited to transmissions. The features of each of those documents are hereby incorporated by reference.

In many machines drive is transferred from a rotating body such as a shaft to a component or assembly mounted on the rotating body. Often it is desirable to fix the component or assembly onto the shaft such that it is locked for rotation therewith, for example a gear wheel selector assembly in a transmission system. A gear wheel selector assembly may be mounted on a shaft between gear wheels to selectively engage different gear ratios. With such an arrangement, if torque spikes or torque reversals occur in the transmission the full effect of the torque spike or torque reversal will be transmitted between the selector assembly and shaft because the components are fixed to each other.

Other components that may be fixed to rotatable bodies include gears, shafts, pulleys, arms, cranks, wheels, levers, clutches, etc.

In transmission systems for vehicles where the selection of a new gear ratio takes place almost instantaneously without substantial power interruption, such as the transmission described in PCT/GB2004/001976, large torque spikes can be generated when the new gear is engaged under certain shift conditions. These torque spikes cause shock waves to propagate through the transmission that can be heard and felt by the occupants of the vehicle. The shockwaves can produce a jerky ride for the car occupants and can lead to wear of transmission components and the possibility of components failing. Nevertheless it is desirable to use such a transmission in vehicles since it is more efficient than conventional transmissions thereby requiring less fuel to

operate, produces lower emissions and increases the performance of the car since the application of power is substantially uninterrupted.

The problem of torque spikes in such transmission systems has been addressed by the inventions of PCT/GB2004/and PCT/GB2004/by means of control systems that limit the level of torque in the transmission when a new gear is selected. The control systems provide effective means for limiting the effect of torque spikes: however, the control systems are complex and require computer processors to regulate the operation of the drive source and clutch.

Accordingly the present invention seeks to provide alternative or additional means of limiting the effects of torque spikes and rapid torque reversals between components.

According to one aspect of the present invention there is provided a mechanism for limiting the transmission of torque, said mechanism including first and second components and at least one pair of complementary cam surfaces for transmitting torque between the first and second components, wherein at least one of the components is arranged to resiliently deform with the level of torque transmitted between the components.

The mechanism can be used, for example as a damping mechanism to absorb shock levels of torque in rotary machines thus attempting to prevent, or at least reduce, the torque being transmitted to other components or assemblies in the rotary machine. As the level of torque transmitted increases so does the amount of resilient deformation of at least one of the components to absorb the shock. This can limit the damaging effects of torque spikes or rapid torque reversals in rotary machines such as transmission systems. Preferably the mechanism is arranged such that the amount of resilient deformation is substantially proportional to the level of torque transmitted.

Preferably the mechanism includes at least two pairs of cam surfaces for transmitting torque between the first and second components. Preferably both the first and second components include both male and female cam surfaces.

Advantageously the second component is mounted rotatably about the first component and the components are arranged for limited relative rotational movement caused by the torque.

Preferably the first component is a shaft and the second component is a sleeve mounted on the shaft. As the first and second components rotate relative to each other the angular displacement

between them increases. The components may be arranged such that the angular displacement between the components is substantially proportional to the magnitude of torque causing the displacement. Preferably the first and second components are arranged such that relative rotational movement only takes place between the components after the torque has exceeded a predetermined value.

Preferably one of the pair of complementary cam surfaces is arranged on an outer surface of the first component and the other cam surface on an inner surface of the second component.

Advantageously the first and second components are arranged such that, in use, as the angular displacement between the first and second components increases the amount of deformation of at least one of the components increases.

Advantageously at least one of the first and second components is arranged to defonn. For example, the first component may be a hollow shaft. Advantageously at least one of the first and second components may include defonnable formations to allow the first or second components to deform more easily. For example, slots may be formed in the components that allow them to expand or compress more easily.

Preferably the components are arranged such that when the level of torque in the mechanism reduces below apredetennined value the resiliency of at least one ofthe components substantially returns the components to a position determined by the level of torque. The geometry of the components is such that they do not jam when the components move rotationally relative to each other thereby allowing the components to return to a neutral position.

Advantageously the mechanism can include resilient means arranged to bias the first and second components towards a neutral position. Preferably the resilient means includes at least one resilient member, such as a spring. The mechanism may include a plurality of resilient members, or resilient materials to bias the first and second components towards the neutral position.

Advantageously the mechanism can include means for delivering lubricant. Preferably the means for delivering lubricant includes channels formed in the sleeve.

According to another aspect of the present invention there is provided a transmission system having a plurality of gear ratios, selector means for selectively engaging the gear ratios mounted on a shaft by a sleeve, and at least one pair of complementary cam surfaces for transmitting

torque between the shaft and sleeve, wherein at least one of the components is arranged to resiliently defonn with the level of torque transmitted between the components.

According to another aspect of the present invention there is provided a rotary machine including a mechanism or transmission substantially as described above.

The mechanism for limiting the transmission of torque may be included in transmission systems for vehicles. For example, the transmission system may be of the type that includes first and second rotatable shafts, and means for transferring drive from one of the shafts to the other shaft including first and second gear wheels each rotatably mounted on the first shaft and having drive formations formed thereon, a selector assembly for selectively transmitting torque between the first shaft and the first gear wheel and between the first shaft and the second gear wheel. The selector assembly includes an actuator assembly and first and second sets of engagement members that are moveable into and out of engagement with the first and second gear wheels independently of each other, said selector assemblybeing arranged suchthatwhen a driving force is transmitted, one of the first and second sets of engagement members driving engages the engaged gear wheel, and the other set of engagement members is then in an unloaded condition.

The actuator assembly is arranged to move the unloaded set of engagement members into driving engagement with the unengaged gear wheel to effect a gear change.

The transmission system of that type performs accelerating up-shifts and braking down-shifts substantially without torque interruption.

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. Alternatively, the first shaft can be the output shaft and the second shaft is the input shaft.

The selector assembly is arranged such that when a braking force is transmitted the first set of engagement members driving engages the engaged gear wheel, 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 driving engages the engaged gear wheel, and the second set of engagement members is then in an unloaded condition.

The actuator assembly is arranged to bias the loaded set of engagement members towards the unengaged gear wheel without disengaging the loaded set of engagement members from the

engaged gear wheel.

The first and second sets of engagement members are arranged to rotate, in use, with the first shaft. The selector assembly is arranged such that the first and second sets of engagement members can move axially relative to each other along the first shaft. The first and second sets of engagement members are axially aligned when both sets engage the first or second gear wheels, and preferably when in a neutral condition. The first and second sets of engagement members are axially offset during gear change operations.

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: Figure 1 is a side section general arrangement of a transmission system in accordance with the present invention; Figure 2a is a perspective view of part of a selector assembly ; Figure 2b is a sectional view of a shaft with a selector assembly mounted thereon via a sleeve; Figure 2c is a sectional view of a shaft with a sleeve mounted thereon in a neutral position; Figure 2d is a sectional view of a shaft with a sleeve mounted thereon angularly displaced from the neutral position in a first direction; Figure 2e is a side view of a spring member ; Figure 2f is a sectional view of part of a shaft with a sleeve mounted thereon in a neutral position with a spring member for biasing the sleeve and shaft towards the neutral position; Figure 2g is a sectional view of part of a shaft with a sleeve mounted thereon angularly displaced from the neutral position in the first direction showing compression of the spring member;

Figure 2h is a sectional view of a shaft with a sleeve mounted thereon angularly displaced from the neutral position in a second direction; Figure 2i is a sectional view of part of a shaft with a sleeve mounted thereon angularly displaced from the neutral position in the second direction showing compression of the spring member ; Figure 3 illustrates the arrangement of a group of dogs on a side of a gear; Figure 4 is a plan of a disc spring ; Figures 5a-f illustrate diagrammatically operation of the selector assembly; and Figure 6 is a plan view of a disc spring for a second embodiment of the invention.

Figure 1 shows a transmission system in accordance with the invention. The transmission system comprises an input shaft 1 having first and second gear wheels 3,5 mounted thereon, an output shaft 7 having third and fourth gear wheels 9, 11 mounted thereon and a selector assembly 13. The first and second gear wheels 3,5 are rotatably mounted on the input shaft 1 and the third and fourth gear wheels 9,11 are fixedly mounted on the output shaft 7. The first and second gear wheels 3,5 mesh with third and fourth gear wheels 9,11 respectively to form first and second gear wheel pairs 15,17.

Rotational drive may be transferred from the input shaft 1 to the output shaft 7 via either the first or second gear wheel pairs 15,17, with selection of the gear wheel pairs being detennined by the position of the selector assembly 13. The selector assembly 13 engages first and second groups of drive formations 19, 21 located on the first and second gear wheels 3, 5 respectively. The drive formations each comprise groups of dogs.

The first dog group 19 is located on one side of the first gear wheel 3. The dogs are preferably formed integrally with the first gear wheel, but this is not essential. The first dog group 19 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 Figure 3). The second dog group 21 comprises three dogs and is similarly arranged on one side of the second gear wheel. Three dogs are used because this arrangement provides large engagement windows, that is the spaces between the dogs, to receive the selector assembly 13. Large engagement windows

provide greater opportunities for the selector assembly to fully engage the gear wheels 3,5 before transmitting drive thereto. If the selector assembly 13 drives a gear wheel when only partially engaged it can lead to damage of the dogs and/or the selector assembly 13.

The first and second gear wheels 3,5 are mounted spaced apart on the input shaft 1 on roller bearings 23,25 and are arranged such that the sides including the first and second dog groups face each other.

The selector assembly 13 includes first and second sets of engagement bars 27,29 and an actuator assembly 31 in the form of a fork assembly 33 and a selector rod 35.

The first and second sets of engagement bars 27,29 are mounted on the input shaft 1 between the first and second gear wheels 3,5. Referring specifically to Figure 2, the first set of engagement bars 27 comprises three bars 28 attached to a first connector ring, for example using grub screws.

The first connector ring 37 holds the bars in a fixed arrangement. The bars 28 are evenly distributed about the inner circumference of the first connector ring such that their bases face inwards, and the bars 28 are arranged substantially parallel. The second set of engagement bars 29 comprises three bars 30 which are held in a similar fixed arrangement by a second connector ring 39.

The first and second engagement bar sets 27,29 are mounted on a sleeve 2 which is mounted on the input shaft 1 between the first and second gear wheels 3,5 (see Figure 2b). The sets of engagement bars 27,29 are arranged to rotate with the input shaft 1 but are able to slide axially along the sleeve 2, and hence the input shaft 1, in response to a switching action of the actuator assembly 31. To facilitate this, the sleeve 2 includes six keyways 41 formed in its curved surface with each engagement bar 28, 30 having a complementary formation in its base. The arrangement of the bar sets 27,29 is such that bars of a particular set are located in alternate keyways 41 and the bar sets 27, 29 can slide along the sleeve 2. Each bar set 27,29 moves as a unit and each bar set can move independently of the other. When there is relative movement between the first and second sets of bars 27,29, the second connector ring 39 slides over the first set of bars 27 and the first connector ring 37 slides over the second set of bars 29.

The keyways 41 have substantially T-shaped profiles such that the bars are radially and tangentially (but not axially) restrained within the keyways (see Figure 2b). Alternatively, the keyways 41 can have slotted or dovetailed profiles to radially restrain the bars. Since the

engagement bars are radially restrained by the profiles of the keyways 41, the connector rings 37,39 are not strictly necessary and therefore it is possible to omit the connector rings 37,39 and reduce the length of the engagement bars 28, 30 thereby producing a more compact transmission system. (The engagement bars shown in Figure 2 are shown with inner profiles that are arranged to fit into splines rather than the profiled keyways 41 shown in Figure 2b. In that instance, the connector rings 37,39 are necessary since splines do not radially restrain the engagement bar sets 27,29).

The input shaft 1 and sleeve 2 are connected to each other via a damping mechanism 4 (see Figure 2b). The damping mechanism 4 is arranged integrally with the sleeve 2 and input shaft 1.

The internal wall of the sleeve is profiled to incorporate sixteen cam surfaces 6,8. Each cam surface extends along the full length of the sleeve 2 parallel to the longitudinal axis of the sleeve.

Eight of the cam surfaces are convex 6 and eight are concave 8. The convex and concave surfaces 6, 8 are arranged alternately such that, in section, they produce a symmetrical wavy pattern of ridges and channels extending around the inner circumference of the sleeve (see Figure 2c. In Figures 2c, 2d and 2g to 2i the outer features of the sleeve are omitted for clarity). Each of the convex cam surfaces 6 has an identical geometry. The radius of curvature of each convex surface 6 is approximately 8. 4mm. Each convex surface 6 blends into the adjacent concave cam surfaces 8. The concave surfaces 8 each have a radius of curvature of approximately 4. 5mm. Each of the concave surfaces 8 has an identical geometry.

Each of the concave cam surfaces 8 has a groove 10 that runs along the length of each channel.

The groove allows a lubricant to be delivered between the input shaft 1 and the sleeve 2.

The part of the input shaft 1 on which the sleeve 2 is mounted includes sixteen cam surfaces 12,14 arranged longitudinally on the shaft. Eight of the cam surfaces are convex 12 and eight are concave 14. The convex and concave surfaces 12, 14 are arranged alternately such that, in section, they produce a symmetrical wavy pattern of ridges and channels extending around the outer circumference of the input shaft (see Figure 2c). Each of the convex cam surfaces 12 has an identical geometry. The radius of curvature of each convex surface is approximately 4mm. Each convex surface 12 blends into the adjacent concave cam surfaces 14. Each concave cam surface 14 has a radius of curvature of approximately 141nm. Each of the concave surfaces 14 has an identical geometry.

The sleeve 2 is mounted on the input shaft 1 such that the convex cam surfaces of the input shaft 12 mesh with the concave surfaces 8 of the sleeve, and vice versa, and engage therewith to transmit drive between the components. The geometry of the cam surfaces 6,8, 12,14 is such that there are gaps 16 between the cam surfaces of the shaft 12,14 and the sleeve 2 to allow limited relative rotation between the shaft 1 and sleeve 2, up to around ten degrees, when there is sufficient torque in the transmission to cause resilient deformation of the sleeve 2 and/or shaft, including the cam surfaces. The resistance to relative rotationalmovementbetween the input shaft 1 and the sleeve 2 increases as the angular displacement between the components 1, 2 increases.

The increase is gradual to prevent jamming and sudden shocks occurring.

Starting from a neutral position (see Figure 2c), as the torque acting on the input shaft 1 increases past a threshold level, for example due to a torque spike caused by a new gear ratio selection or rapid torque reversals when a drive source changes from acceleration to deceleration or vice versa, the input shaft 1 rotates relative to the sleeve 2 (this is shown as clockwise rotation of the input shaft 1 in Figure 2d) and the convex cam surfaces 12 of the input shaft cause the sleeve 2 to defonn resiliently. The resilient deformation absorbs the torque spike energy to prevent it from propagating through the transmission, thus providing torque damping. When the torque spike has been dissipated and the level of torque in the transmission reduces, the resiliency of the sleeve 2 causes relative rotation of the input shaft 1 and sleeve 2 in the opposite direction causing them to return to a position dictated by the amount of torque being transmitted. If no torque is being transmitted the sleeve will rotate to a neutral position. This can be assisted by including a spring member 22 in the damping mechanism that is arranged to bias the shaft 1 and sleeve 2 to return to the neutral position (see Figures 2e to 2g. In Figures 2g, 2f and 2i only part of the input shaft <BR> <BR> 1 and the sleeve 2 are shown for clarity. ) The spring member 22 is a leaf spring having a wavy profile (see Figure 2e) that is located in recesses 18, 20 in the sleeve 2 and input shaft 1 respectively (see Figure 2f). The leaf spring can comprise a single layer or a plurality of layers.

When there is relative rotational movement between input shaft I and the sleeve 2, the spring member 22 is compressed (see Figure 2g). As the torque reduces in the transmission the resiliency of the spring member 22 and the structure of the sleeve 2 returns the input shaft 1 and the sleeve 2 towards the neutral position (see Figure 2f). The spring member 22 also reduces the amount of backlash in the transmission.

The damping mechanism is bi-directional. If a torque spike in the transmission causes the input shaft 1 and sleeve 2 to rotate relative to each other in the opposite direction to that previously

described (this is shown as anti-clockwise rotation of the input shaft 1 in Figures 2h and 2i) The operation of the damping mechanism in the opposite direction is similar to that described above.

Preferably the sleeve 2 is made from steel having good elastic properties and a high toughness so that it is able to absorb the energy from the torque spikes.

The value of torque at which the input shaft 1 and the sleeve 2 rotate relative to one another is determined by the arrangement of the input shaft 1 and sleeve 2. For example, the geometry of the cam surfaces, the materials used and the relative sizes of the outside diameter of the shaft and internal diameter of the sleeve. The arrangement can be controlled to determine the value of torque at which the damping mechanism operates.

Each bar 28 in the first bar set 27 has a first end 28a arranged to engage the first group of dogs 19 attached to the first gear wheel 3 and a second end 28b arranged to engage the second group of dogs 21 on the second gear wheel 5. The first and second ends 28a, 28b typically have the same configuration but are opposite handed, such that the first end 28a is arranged to engage the first group of dogs 19 during deceleration of the first gear wheel 3 and the second end 28b is arranged to engage the second group of dogs 21 during acceleration of the second gear wheel 5, for example during engine braking in automotive applications. Each bar 30 in the second bar set 29 is similarly arranged, except that the first end 30a is arranged to engage the first group of dogs 19 during acceleration of the first gear wheel 3 and the second end 30b is arranged to engage the second group of dogs 21 during deceleration of the second gear wheel 5.

When both the first and second sets of engagement bars 27,29 engage a gear wheel drive is transmitted from the input shaft 1 to the output shaft 7 whether the gear is accelerating or decelerating.

The first and second ends 28a, 30a, 28b, 30b of eachbar include a substantially vertical face 43 for engaging dogs 19,21 and a ramp 45 that slopes in the direction of the engagement face 43 to ensure that the bars 28, 3 0 disengage from the dogs 19,21 to prevent the transmission from locking up. When the bars of the first and second sets 27,29 are interleaved, as in Figure 2, the dog engagement faces 43 of the first end 28a of the first set of bars 27 are adjacent the dog engagement faces 43 of the first end 30a of the second set of bars 29. When the first and second sets of bars 27,29 are fully engaged with a gear a dog is located between each pair of adjacent engagement faces 43. The dimensions of the dogs 19, 21 and the ends of the bars are preferably

such that there is little movement of a dog between the engagement face 43 of the acceleration bar and the engagement face 43 of the deceleration bar when the gear moves from acceleration to deceleration, or vice versa, to ensure that there is little or no backlash in the gear.

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

The actuator assembly 31 is arranged such that the fork assembly 33 is mounted on the selector rod 35, and the selector rod is provided parallel to the input shaft 1 and adjacent thereto. The fork assembly 33 includes a fork 46 and first and second annular disc springs 47,49 mounted about the input shaft 1 (see Figure 1). The first and second disc springs 47,49 have three arms, with each arm having a first part that extends circumferentially around a part of the spring and a second part that extends radially inwards (see Figure 4).

The fork 46 has a first pair of arcuate members 51 arranged to engage the first disc spring 47. The arcuate members 51 are arranged such that the first disc spring 47 can rotate with the input shaft 1 between the arcuate members 51 and such that axial movement of the fork 46 parallel to the input shaft 1 moves the arcuate members 51 and hence the first disc spring 47 axially along the shaft if the first disc spring 47 is free to move, or biases the first disc spring 47 to move in the same direction as the fork 46 if the first disc spring 47 is unable to move. The fork 46 has a second pair of arcuate members 53 arranged to engage and act upon the second disc spring 49 in a similar manner.

The position of the fork 46 relative to the first and second gear wheels 3,5 can be adjusted by movement of the selector rod 35 in the axial direction.

The inner edges of the first disc spring 47 are fixed to the bars 28 in the first bar set 27 and the inner edges of the second disc spring 49 are fixed to the bars 30 in the second bar set 29. When the fork 46 moves, thereby moving or loading the disc springs 47,49, the engagement bar sets 27,29 are likewise moved or biased to move.

The operation of the selector assembly 13 will now be described with reference to Figures 5a-5f which for clarity illustrate diagrammatically the movement of the first and second bar sets 27,29 by the relative positions of only one bar from each set.

Figure 5a shows the first and second bar sets 27,29 in a neutral position, that is, neither bar set is engaged with a gear wheel. Figure 5b shows the first and second bar sets moving into engagement with the first gear wheel 3 under the action of the fork 46.

Figure 5c shows a condition when the first gear wheel 3 is fully engaged, that is, the bars 28,30 are interleaved with the first group of dogs 19. The selector rod 35 is located such that the fork 46 maintains the first and second bar sets 27,29 in engagement with the first gear wheel 3.

Accordingly, power is transferred from the input shaft 1, to the first gear wheel 3 by the first bar set 27 when decelerating and the second bar set 29 when accelerating via the first group of dogs 19. Power is transmitted to the output shaft 7 put via the third gear wheel 9.

Whilst accelerating (first gear wheel 3 rotating in the direction of arrow B in Figure 5c) using the first gear wheel pair 15, the engagement faces 43 of the bars of the first bar set 27 are not loaded, whilst the engagement faces 43 of the bars of the second bar set 29 are loaded. When a user, or an engine management system (not shown) wishes to engage the second gear wheel pair 17, the selector rod 35 is moved such that the fork 46 acts on the first disc spring 47, causing the bars of the first bar set 27 to slide axially along the keyways 41 in the input shaft 1 thereby disengaging the bars from the first gear wheel 3 (see Figure 5d).

The fork 46 also acts on the second disc spring 49 to bias the bars of the second bar set 29 to move towards the second gear wheel 5. However, because the bars of the second bar set 29 are loaded, i. e. are driving the first gear wheel 3, they cannot be disengaged from the first gear wheel 3, and therefore the bars of the second bar set 29 remain stationary.

When the bars of the first bar set 27 slide axially along the input shaft 1, the engagement faces 43 engage the second group of dogs 21 (see Figure 5e). As this occurs the rotation of the second gear wheel 5 is substantially instantaneously locked to the rotation of the input shaft 1, which generates a torque spike in the transmission. The torque spike may be of sufficient magnitude that it causes the sleeve 2 and input shaft 1 to move rotationally relative to each other, thereby causing the sleeve 2 to deform resiliently to absorb the energy of the torque spike and prevent or at least reduce the magnitude of shockwaves propagating through the transmission.

The bars then begin to drive the second gear wheel 5 in the direction of Arrow C in Figure 5e and energy is transmitted from the input shaft 1 to the output shaft 7 by way of the second gear wheel pair 17. As this occurs, the bars of the second bar set 29 cease to be loaded, and are free to

disengage from the first group of dogs 19. Since the second disc spring 49 is biased by the fork 46, the bars of the second bar set 29 slide axially along the keyways 41 in the input shaft 1 thereby completing the disengagement of the first gear wheel 3 from the input shaft 1. The bars of the second bar set 29 slide along the keyways 41 in the input shaft until they engage the second gear wheel 5, thereby completing engagement of the second gear wheel 5 with the input shaft l (see Figure 5f). This method of selecting gear wheel pairs substantially eliminates torque interruption since the second gear wheel pair 17 is engaged before the first wheel pair 15 is disengaged, thus momentarily, the first and second gear wheel pairs 15,17 are simultaneously engaged.

When a gear wheel is engaged by both the first and second bar sets 27,29 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 bar to the engagement face 43 of the deceleration bar 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 four degrees.

Backlash is reduced by minimising the clearance required between an engagement member and a dog during a gear shift: that is, the clearance between the dog and the following engagement member (see measurement'A'in Figure 5b). The clearance between the dog and the following engagement member is in the range 0. 5mm-0. 03mm 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. 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, and preferably is 15 degrees.

Transition from the second gear wheel pair 17 to the first gear wheel pair 15 whilst decelerating is achieved by a similar process.

Whilst decelerating in the second gear wheel pair 17 the engagement surfaces 43 of the bars of the first bar set 27 are not loaded, whilst the engagement surfaces 43 of the bars of the second bar set 29 are loaded. When a user, or an engine management system (not shown) wishes to engage

the first gear wheel pair 15, the selector rod 35 is moved such that the fork 46 slides axially relative to the input shaft 1. The fork 46 acts on the first disc spring 47 attached to the first bar set 27, causing the bars of the first bar set 27 to slide axially in the keyways 41 along the input shaft 1 in the direction of the first gear wheel 3, thereby disengaging the first bar set 27 from the second gear wheel 5.

The fork 5 also acts on the second disc spring 49 but since the bars of the second bar set 29 are loaded, i. e. they are drivingly engaged with the dogs 21 on the second gear wheel, the second bar set 29 remains stationary, however the second disc spring 49 is biased by the fork 46 to move the second bar set 29 towards the first gear wheel 3.

As the bars of the first bar set 27 slide axially in the keyways 41, the bars 28 engage the dogs 19 on the first gear wheel and begin to drive the first gear wheel 3 such that energy is transmitted from the input shaft 1 to the output shaft 7 by way of the first gear wheel pair 15. As this occurs the rotation of the first gear wheel 3 is substantially instantaneously locked to the rotation of the input shaft 1, which generates a torque spike in the transmission. The torque spike may be of sufficient magnitude that it causes the sleeve 2 and input shaft 1 to move rotationally relative to each other, thereby causing the sleeve 2 to defonn resiliently to absorb the energy of the torque spike and prevent or at least reduce the magnitude of shockwaves propagating through the transmission.

The bars of the second bar set 29 cease to be loaded. The second disc spring 49 acts on the bars of the second bar set 29, causing it to slide axially within the keyways 41 along the input shaft 1 towards the first gear wheel 3, thereby completing disengagement of the second gear wheel 5.

The second bar set 29 continues to slide within the keyways 41 along the input shaft 1 until it engages the first gear wheel 3, thereby completing engagement of the first gear wheel 3 with the input shaft 1.

Kick-down shifts, that is a gear shift from a higher gear ratio to a larger gear ratio 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, have a brief torque interruption to allow disengagement prior to the shift.

A plurality of selector assemblies can be mounted on the input shaft with corresponding pairs of gear wheels to provide a larger number of gear ratios between the output shaft and the input shaft.

It is also possible to have transmission systems with more than two shafts to provide additional gear ratios.

Use of the transmission system leads to improved performance, lower fuel consumption and lower emissions since drive interruption has substantially been eliminated. Also the system is a more compact design than conventional gearboxes leading to a reduction in gearbox weight.

For embodiments of the invention wherein the keyways 41 have dovetailed, slotted or T-shaped profiles without the connector rings 37,39 arrangements of the invention are possible that include only one disc spring 147 (see Figure 6) connecting all six bars together, i. e. bars from the first and second sets, with the actuator arrangement being adapted accordingly. In use, three of the bars would be loaded when the first gear is accelerating and three not loaded, and moving the fork to bias the disc spring towards the second gear will move the three unloaded bars out of engagement with the first gear wheel, leaving three bars still in engagement. Once the bars have engaged with the second gear wheel, the remaining three bars will disengage from the first gear wheel, and under the loading of the disc spring move into engagement with the second gear wheel. This configuration provides a highly compact arrangement leading to smaller, lighter gearboxes. The axial space between the first and second gears to accommodate the selector mechanism may be reduced to around 20mm for typical road car applications.

Figure 5a shows a recess 28c in the top of each bar of the first bar set and a recess 30c in the top of a bar from the second bar set. The recesses 28c, 30c allow connections to be made between the bars of the first and second bar sets 27,29 with the anus of the first and second disc springs 47,49 respectively. The shape of the recesses 28c, 30c is such that the recesses allow each spring arm to move to a non-perpendicular angle relative to the bars 28,30 during a gear shift.

The recesses 28c, 30c shown in Figure 5a are for a two disc spring configuration. For embodiments having only one disc spring 147 the recesses 28c, 30c are located more centrally along the length of the bars 28, 30.

When a ring is not used to fix the positions of the bars in a set, the bars in a set can move a small amount relative to each other in the axial direction. This is because the only connection between the bars in a set is provided by a defonnable disc spring. A single bar is attached to each disc spring arm and each arm can deform independently of the others, thereby allowing the relative movement between the bars. The bars in a set will nevertheless essentially move in unison.

It will be appreciated by the skilled man that various modifications can be made to the above embodiment that are within the scope of the current invention, for example the sleeve 2 may be arranged to include a series of expansion slots or have a concertinaed arrangement to assist the damping mechanism to absorb torque spikes in the transmission.

Alternatively, or additionally, the input shaft 1 can be arranged to defonn resiliently, for example by making at least part of the shaft from resilient material such as steel having good elastic properties or by making the shaft hollow. At least some of the loading caused by torque spikes in the transmission is then dissipated by resilient deformation of the shaft. The shaft may also include slots formed in its structure to allow it to resiliently defonn.

When the damping mechanism 4 is used in transmission systems or other rotating machines the sleeve 2 may be arranged to transmit drive to the shaft on which it is mounted. For example, a selector mechanism could be mounted on the output shaft of a transmission wherein drive is transmitted from an input shaft via a gear ratio to the selector mechanism and then to the output shaft.

Any practicable number of cam surfaces can be used. For example, the shaft could include a single convex cam surface and the sleeve 2 a single concave cam surface. Preferably a plurality of cam surfaces is used that are uniformly distributed about the shaft and sleeve 2.

More than one spring member 22 can be used. For example, a plurality of spring members 22 can be distributed about the circumferences of the sleeve and input shaft. The leaf springs may be replaced with resiliently deformable materials such as rubber or neoprene.

The geometry of the cam surfaces can also be adjusted to suit different applications. The cam surfaces which are used to transmit torque between the shaft and sleeve 2 could be replaced by attaching the sleeve 2 to the shaft with a key and keyway arrangement. The keys and keyways are shaped to mimic the cam shapes above.

The invention may also be used in machines wherein the components transmit torque without rotating, until relative rotational movement takes place between the first and second components

after the predetermined torque threshold has been exceeded.

The transmission system can be used in any vehicle for example, road cars, racing cars, lorries, motorcycles, bicycles, earth removal vehicles such as bulldozers, cranes, military vehicles, aircraft such as aeroplanes and helicopters, watercraft such as boats, ships and hovercrafts. 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, for example in lathes, milling machines and other rotary machines.

It will also be appreciated by the skilled person that the transmission system can be adapted such that the selector assembly and the first and second gear wheels are mounted on the output shaft and the fixed gear wheels are mounted on the input shaft.