GB2358444A

GB2358444A - A gear engaging mechanism having a single actuator that selects and engages a gear

Info

Publication number: GB2358444A
Application number: GB0100675A
Authority: GB
Inventors: David Anthony Harries; Belinda Jane Hughes
Original assignee: LuK Lamellen und Kupplungsbau GmbH
Current assignee: LuK Lamellen und Kupplungsbau GmbH
Priority date: 2000-01-24
Filing date: 2001-01-11
Publication date: 2001-07-25
Anticipated expiration: 2021-01-11
Also published as: GB0100675D0; DE10063502A1; GB2358444B; GB0001364D0; FR2804194A1; ITMI20010118A1; FR2804194B1

Abstract

A gear engaging mechanism comprises a single actuator (214, fig 8) which rotates and moves, linearly and transversely with respect to shift rails 111, 112, 113, a selector member 110 so as to engage projections 130 on the shift rails 111, 112, 113. Rotary motion of the selector member 110, which causes axial movement of the shift rails 111, 112, 113 so as to engage a gear, is produced by a formation 160 on the selector member 110 that engages with one of two paths 150, 152 cut into a stationary arcuate plate 132. Formation 160 is pivoted about its axis by a solenoid (170, fig 4) so that a leading edge may follow the different paths 150, 152 and engage in different branches 136, 138, 140, 142 of the path, thereby engaging different gears. A projection 148 in the middle of the paths 150, 152 acts as a guide for the formation 160. Linear movement of the selector member 110 may be effected by a hydraulic cylinder (see fig 8), an electric motors or a solenoid.

Description

1 2358444 GEAR ENGAGEMENT MECHANISM The present invention relates to a

gear engagement mechanism and in particular a gear engagement mechanism for an automatic or semi automatic transmission system, including a multi-ratio gearbox in which gear ratios are engaged using actuators under the control of an electronic control unit.

Hitherto, automatic or semi-automatic transmission systems of this type, for example as disclosed in W097/0541 0 or W097/40300, whose content is expressly incorporated in the disclosure content of the present application, have utilised a conventional gear selection mechanism, as used with a manual gearbox, movement of the selector means being controlled by a pair of double-acting actuators to control movement of the selector means in a first direction to select a pair of gear ratios and in a second direction to engage one of the selected pair of gear ratios.

An object of the invention is to provide a gear engagement mechanism, for use in an automatic or semi-automatic transmission system, of simplified construction and operation.

According to one aspect of the present invention, a gear engagement mechanism for an automated transmission system comprises a gear box with a plurality of distinct gear ratios, the gear engaging mechanism comprising; a selector member and a plurality of shift rails, each shift rail being adapted to be moved axially, a different pair of gear ratios being associated with each of the shift rails, axial moveffient of the shift rail in one direction causing engagement of one of said associated gear ratios and axial movement of the shift rail in the opposite direction causing engagement of the other of said associated gear ratios; means being provided to move the selector member along an axis transverse to the 2 longitudinal axes of the shift rails, whereby the selector member may 0( indexed with and engage a selected one of the shift rails; the selector member having a formation for engagement of a track, the track definn two paths, engagement of the formation on the selector member in tho track, upon movement of the selector member along the transverse a)isl,.

causing the selector member to be pivoted in one direction or the othE,r about the transverse axis, depending on the path followed, as the selEl4pl or member moves to engage each of the shift rails, thereby causing axiali movement of an engaged shift rail to engage a gear associated therevit 1; and means being provided to selectively switch between one path andl' t e other in accordance with the gear ratio to be selected.

According to a preferred embodiment of the invention, the formation qn the selector member is shaped to engage formations on the track, by which it is deflected to follow one path or the other, the formation beio:, 15 switchable between a first orientation in which it will be deflected aion !g 0 first path and a second orientation in which it will be deflected along iz,,:i second path.

Preferably, the formation on the selector member is biased to the first orientation, means being provided to switch the formation to its secom 1 orientation, as required.

An embodiment of the invention is now described, by way of example' only, with reference to the accompanying drawings, in which:- Figure 1 shows diagrammatically, a semi-automated transmission systi,ry in accordance with the present invention; Figure 2 illustrates a gear ratio selector gate and associated shift rails used in the gear engagement mechanism of the present invention.

3 Figure 3 shows a sectional isometric view of the gear engagement mechanism of the present invention; Figure 4 shows a sectional plan view of the mechanism illustrated in Figure 2; Figure 5 shows a sectional side elevation of the mechanism illustrated in Figure 2; Figure 6 shows a partial end elevation of a mechanism illustrated in Figure 2; Figures 7A to 717 illustrate in plan and side elevation the different positions of the mechanism illustrated in Figures.3 to 6, for selection of each gear 15 ratio; and Figure 8 illustrates diagrammatically an electro/hydraulic control circuit for the gear engagement mechanism illustrated in Figure 3.

Figure 1 of the accompanying drawings shows an engine 10 with a starter and associated starter circuit 1 Oa which is coupled through the main drive friction clutch 14 to a multi-speed synchromeshed lay shaft-type gearbox 12, via a gearbox input shaft 15. Fuel is supplied to the engine by a throttle 16 which includes a throttle valve 18, operated by accelerator 25 pedal 19. The invention is equally applicable to electronic or mechanical fuel injection petrol or diesel engine. The clutch 14 is actuated by a release fork 20 which is operated by a hydraulic slave cylinder 22, under the control of a clutch actuator control 30 means 38.

4 A gear selector lever 24 operates in a gate 50 having two limbs 51 and 52 joined by a cross track 53 extending between the end of limb 52 an( intermediate of the ends of limb 51. The gate 50 defines five position,s; "R" at the end of limb 52; "N" intermediate of the ends of the cross til, o( k a l it 53; "S" at the junction of limb 51 with the cross track 53; and " + ahd; i 1 1 1 j xt /f - at the extremities of limb 51. In limb 51 the lever 24 is biased to th 3 5 g 1 s central "S" position. The "N" position of the selector lever 24 t 1 corresponds to neutral; "R" corresponds to selection of reverse gear; 'IS ' corresponds to selection of a forward drive mode; momentary movem(bn t,, of the lever to the + position provides a command to cause the gea,,b x to shift up one gear ratio; and momentary movement of the gear lever 2 to the position provides a command to cause the gearbox to shift down one gear ratio.

The positions of the lever 24 are sensed by a series of sensors, for example micro switches or optical sensors, positioned around the gatet Signals from the sensors are fed to an electronic control unit 36. An output from the control unit 36 controls a gear engaging mechanism 25 which engages the gear ratios of the gearbox 12, in accordance with movement of the selector lever 24 by the vehicle operator.

In addition to signals from the gear selector lever 24, the control unit 6 receives signals from:

sensor 1 9a indicative of the degree of depression of the accelerti r pedal 19; sensor 30 indicative of the degree of opening of the throttle contffil valve 18; sensor 26 indicative of the engine speed; sensor 42 indicative of the speed of the clutch driven plate; sensor 34 indicative of the clutch slave cylinder position; and sensor 32 indicative of the gear ratio selected.

The control unit 36 utilises the signals from these sensors to control actuation of the clutch 14 during take-up from rest and gear changes, for example as described in patent specifications EP0038113, EP0043660,

EP0059035. EP01 01220 and W092/13208, whose content is expressly incorporated in the disclosure content of the present application.

In addition to the above mentioned sensors, control unit 36 also receives signals from a vehicle speed sensor 52, ignition switch 54 and brake switch 56 associated with the main braking system, for example the footbrake 58 of the vehicle.

A buzzer 50 is connected to the control unit 36 to warnlindicate to the vehicle operator as certain operating conditions occur. In addition or in place of the buzzer 50 a flashing warning light or other indicating means may be used. A gear indicator 60 is also provided to indicate the gear ratio selected.

As illustrated in Figures 2 to 7, the gear engagement mechanism 25 comprises three shift rails 111, 112,113 mounted parallel to one another for movement in an axial direction. Each shift rail 111, 112,113 is associated with two of the gear ratios of the gearbox 12, via a selector fork and synchromesh unit in conventional manner, so that movement of the shift rails 111, 112,113 in one axial direction will cause engagement of one of the associated gear ratios and axial movement of the shift rail 111, 112,113 in the opposite axial direction will cause engagement of the other associated gear ratio.

As illustrated in Figure 2, typically; first and second gear ratios are associated with shift rail 111, so that movement of the shift rail 111 in a first direction will engage first gear or movement of shift rail 111 in a second direction will engage second gear; third and fourth gear ratios are associated with shift rail 112, so that movement of shift rail 112 in the 6 first direction will engage third gear or movement of shift 112 in a se(p d direction will engage fourth gear; and fifth and reverse gear ratios are associated with shift rail 113, so that movement of shift rail 113 in th first direction will engage fifth gear while movement of shift rail 113 in the second direction will engage reverse gear.

As illustrated in Figures 3 to 8, a selector member 110 is mounted on a shaft 120 for movement with the shaft 120 along an axis transverse -to the longitudinal axis of the shift rails 111, 112,113, the shaft 120 being slidably and rotatably located in bearing formations in end plates 122 6f 0 housing 124. The selector member 110 is of generally cylindrical configuration, having an axially extending lug formation 126. Collar formations 128 mounted on the shift rails 111, 112,113 each define a'.p $ir of axially separated projections 130, between which the lug f ormation 126 on selector member 110 is adapted to engage, as the selector member 110 is moved transversely of the shift rails 111, 112,113.

l i 1 I i The housing 124 includes an arcuate plate 132 which extends concentrically of the shaft 120 in juxtaposed relation to the path of tho selector member 110. The arcuate plate 132 has an aperture 133 whic ' defines a track 134. The track 134 is branched at both ends, branch(' 136 and 138 at one end corresponding to shift rail 111 and branches 1, O and 142 at the other end corresponding to shift rail 113. The track 1 is generally of uniform width having an enlarged width portion 144 corresponding to the shift rail 112. A guide 146 is mounted on the arcuate plate 132, the guide 146 extending co-axially of the shaft 1 2( i centrally of the track 134. The guide 146 has a diamond-shaped projection 148 which extends down into the enlarged width portion 14z.:

of the track 134, to divide the track 134 into two paths 150 and 1 52'l 61f equal width to the other portions of the track 134. The guide 146 alcp 1 i 1 ' i 1 defines a rail formation 154 which extends longitudinally of the track I, 1,4, j 7 between the projection 148 and the branched end of track 134 corresponding to shift rail 113.

A rhombus-shaped follower 160 is pivotally mounted on the selector member 110 f or engagement of the track 134. The f ollower 160 is mounted in a recess 162 in the selector member 110, whereby pivotal movement of the follower 160 is limited between; a first position in which opposed parallel sides of the follower 160 are parallel to the sides of the track 134, the other sides extending forwardly and to opposite sides of the track 134 relative to opposite directions of movement of the selector member 110; and a second position in which said other sides are parallel to the sides of the track 134 and said opposed parallel sides extend forwardly and to opposite sides of the track 134 relative to opposite directions of movement of the selector member 110; the forwardly extending sides of the follower, relative to movement in opposite directions, extending towards opposite sides of the track in the first and second positions. The follower 160 is biased to its first position by means of a torsion spring 163. A projection 164 extends from the upper surface of the follower 160, from a leading end thereof, relative to movement of the selector member 110 in one direction. The projection 164 extends beyond the outer surface of the arcuate plate 132, so that it will engage the rail formation 154 of guide 146.

A solenoid 170 is located intermediate of the branches 136,138 corresponding to shift rail 111 and paths 150,152 corresponding to shift rail 112. A plunger 17 2 is controlled by solenoid 170. The plunger 172 is arranged such that, when the solenoid 170 is de-energised, the plunger 172 will engage the projection 164 when the follower 160 is in its first position and upon energisation of the solenoid 170, the plunger 172 will act upon the projection 164, to switch the follower 160 to its second position, against the biasing force of spring 163.

8 A double-acting hydraulic actuator 214, as illustrated in Figure 8, actst, 0 1 the shaft 120 to move the shaft 120 and selector member 110 moun.te i thereon, axially. A rotary connection 180 is provided in the shaft 1 20i W as to permit f ree-rotational movement of the selector member 110 as it Is moved axially by the actuator 214. Alternatively, the selector membet may be fixed axially with respect to shaft 120 but rotatable with.

respect thereto, or shaft 120 may be fixed, the selector member 110 being mounted thereon for axial and rotational movement with respec thereto, the actuator 214 acting directly on the selector member 110 using a suitable linkage mechanism.

Starting from the position illustrated in Figure 4, with the solenoid 17(l energised, movement of the selector member 110 to the left (as illustrated in Figure 8) will cause the leading edge 182 of the follower 1 30 to engage branch 136, as the lug 126 engages between projections 1'.3()' on the collar formation 128 mounted on shift rail 111. Engagement o,,. i he follower 160 in branch 136 will cause the selector member 110 to ro anti-clockwise, moving shift rail 111 axially to the right (as illustrated jn Figure 3) and engaging second gear as illustrated in Figure 7B.

Movement of the selector member 110 to the right, with the solenoid i energised, will cause the leading edge 184 of the follower 160 to engo, 0 1, path 152, as the lug 126 engages between projections 130 of the coOlla formation 128 mounted on shift rail 112. Engagement of the f ollowei 1, 160 with path 15 2, will cause the selector member 110 to pivot clockwise, moving shift rail 112 to the left and engaging third gear, a:

illustrated in Figure 7C.

Further movement of the selector member 110 to the right will then c:,,, 'ot $e follower 160 to follow path 152 so that the selector member 110 is -clockwise to return the lug formation 126 back to a neut 0 pivoted anti plane. Continued movement to the right will then cause projection 104 to 9 engage rail 154, switching the follower 160 so that as the lug 126 engages between projections 130 of the collar formation 128 mounted on the shift rail 113, the leading edge 184 of the follower 160 will engage branch 140, causing the selector member 110 to be pivoted anti clockwise and the shift rail 113 to be moved to the right to engage reverse gear, as illustrated in Figure 7F.

Starting from the position illustrated in Figure 4, energisation of solenoid will switch the follower 160 to its second position. Movement of the selector member 110 to the left (as illustrated in Figure 8) will now cause the leading edge 182 of the follower 160, to engage branch 138 as the lug 126 engages between the projections 130 of the collar formation 128 mounted on shift rail 111, causing the selector member 110 to pivot clockwise, moving shift rail 111 to the left and engaging first gear, as illustrated in Figure 7A.

Movement of selector member to the right with solenoid 170 energised, will cause the leading edge 184 of follower 160 to engage path 150, as the lug 126 engages between the projections 13 of the collar formation 128 mounted on shift rail 112. Engagement of the follower 160 in path will cause selector member 110 to rotate anti-clockwise, moving shift rail 112 to the right to engage fourth gear, as illustrated in Figure 7D.

As soon as the formation 164 clears the plunger 172, the spring 163 will cause the follower 160 to return to its first position. Further movement of selector member 110 to the right, will first cause the selector member to pivot clockwise so that the lug formation 126 is returned to the neutral plane. Then, as the lug formation 126 engages between the projections 130 on the collar formation 128 on shift rail 113, the leading edge 184 of follower 160 engages branch 142. Engagement of the follower 160 in branch 142, will pivot the selector member clockwise, moving shift rail 113 to the left and engaging fifth gear, as illustrated in Figure 7E.

Consequently to change up through the gearbox, starting from the netti $I position illustrated in Figure 4:- a) to engage first gear, solenoid 170 is energised and the actuator 214 is controlled to move the selector member to the let, to a position corresponding to the shift rail 111; b) to move from first gear to second gear, solenoid 170 is denergised and actuator 214 is controlled to move the selector member 110 to the right back to the neutral position llustrated in Figure 4, the actuator 214 is then controlled to move the selector member 110 to the left, to a position corresponding to shift rail:! 111; i i I i c) to move from second gear to third gear, the actuator 21 zi' controlled to move selector member 110 to the right, through t neutral position illustrated in Figure 4, to a position correspondi, 9 to shift rail 112; d) to move from third gear to fourth gear, the actuator 214 js controlled to move the selector member 110 to the left back to Itt neutral position illustrated in Figure 4 and solenoid 170 is energ i S d to switch the follower 160 to its second position, actuator 214:

then controlled to move the selector member 110 to the right, l.a to a position corresponding to shift rail 112; e) finally, to move from fourth gear to fifth gear, actuator 2, is controlled to move the selector member 110 to the right to a,L position corresponding to shift rail 113.

To engage reverse gear from the neutral position illustrated in Figure 3, the solenoid 170 is de-energised and actuator 214 is controlled to move the selector member 110 to the right, to a position corresponding to shift rail 113.

It will be appreciated that other shifts, for example from first gear to third gear or second gear to fourth gear may be achieved by suitable control of the solenoid 170 and actuator 214, the selector member 110 being brought back to the neutral position illustrated in Figure 4, when it is required to change the path followed by the follower 160.

Sensing means is provided to indicate when the selector member 110 is in the neutral position illustrated in Figure 4, so that the solenoid 170 may be energised when required, to switch the follower 160 from its first to its second position. Such sensing means may comprise a rheostat 227 which senses the position of the actuator 214, as illustrated in Figure 8.

Alternatively a proximity switch or similar means may be provided, which is actuated by the selector member 110 when it is in the neutral position illustrated in Figure 4.

As illustrated in Figure 8, a hydraulic circuit for control of the clutch 14 and gear engagement mechanism 25 of the transmission system disclosed above, comprises a main solenoid-operated flow control valve 220 for selectively connecting the clutch slave cylinder 22 and/or hydraulic gear engagement actuator 214 to a hydraulic pressure accumulator 224 or to a reservoir 226. Accumulator 224 is charged to a predetermined pressure and is maintained above a minimum operating pressure, when the system is keyed-on, by means of an electrically driven pump 223, via a non- return valve 224a. A pressure transducer 260 monitors the pressure in the accumulator 224 and the pump 223 is controlled by the electronic control 12 unit 36, to maintain pressure in the accumulator 224 above the predetermined minimum operating pressure.

The main control valve 220 has a spool 250 which is displaced againE,-t spring 251, by means of a solenoid 220a. The spool 250 is displacedi:b solenoid 220a under control of control unit 36, to one of three positiocls.

In a first position of the main control valve 220, as illustrated in Figur(' 3,; the working chamber of the clutch slave cylinder 22 is vented to reser, (Ir 226 around land 225 of spool 250, via flow path Z and supply line 2615 connected to the reservoir 226 between lands 257 and 258.

In a second position of the control valve 220, the spool 250 is displacoc to the right so that: the working chamber of clutch slave cylinder 22 remains vented to reservoir 226 via flow path Z, while land 265 is isolated from reservoir 226 by land 258 and is connected to the presspr accumulator 224 between lands 257 and 258.

In a third position of the control valve 220, the spool 250 is further displaced to the right so that: flow path Z is closed and the working chamber of clutch slave cylinder 22 is connected to the pressure accumulator 224, between lands 256 and 257, while line 265 remain:$ connected to the accumulator 224.

Line 265 connects the gear engaging mechanism 25 to the hydraulic pressure accumulator 224 or to reservoir 226, depending upon the position of the main control valve 220.

The selector member 110 is moveable transversely of the shift rails 111, 112,113 by means of the fluid pressure actuator 214, to align tho i 1 i P i 1 1 j selector member 110 with one of the shift rails 111, 112,113 and the,o ' i select the pair of gears associated with that selector rail.

13 The actuator 214 comprises a double-acting ram with an operating rod 214a which is operatively connected to the shaft 120 on which the selector member 110 is mounted, with a pivotal connection 180 therebetween. The operating rod 214a extends from one side of a piston 216 of actuator 214, so that the working surface on the rod end of piston 216 is smaller than that on the opposite head end. In view of the differential areas on the sides of piston 216, when both sides of the piston 216 are connected to the accumulator 224, the piston 216 will be displaced along the actuator 214, to extend the associated operating rod 214a. If however the head end side of piston 216 is connected to the reservoir 226, while the rod end side is connected to the accumulator 224, the pressure differential across the piston 216 will cause it to be displaced along the actuator 214 to contract the associated operating rod 214a.

The supply of hydraulic fluid to the rod end of piston 216 is controlled directly by the main control valve 220. The supply of hydraulic fluid to the head end of piston 216 is controlled from the main control valve 220 by a further solenoid operated proportional flow control valve 221. The valve 221 connects the head end side of piston 216 to the main control valve 220 or to the reservoir 226.

The proportional flow control valves 220 and 221 are controlled by the electronic control unit 36, to apply appropriate pressures to opposite sides of piston 216 to control movement of the piston 216 and the associated operating rod 21 4a, to select the required shift rail 111, 112 or 113, as disclosed in W097/0541 0 or W097/40300, whose content is expressly incorporated in the disclosure content of the present application.

A potentiometer 127 is connected to the operating rod 214a to provide a signal indicative of the position of the operating rod and the selector 14 member 110 connected thereto. Signals from the potentiometer 227 or fed to the control unit 36 to provide an indication of position of the operating rod 21 4a and selector member 110. The transmission syste 1 may thus be calibrated so that predetermined position signals from potentiometer 127 correspond to engagement of the selector member with each of the shift rails 111, 112 and 113.

Measurements from the potentiometer 127 may then be used in a clos.,,0'( loop control system, to control valves 220 and 221 to move the selec-'pl member 110 to the predetermined positions to engage the desired shift, rails 111,112 and 113.

With the main control valve 220 in the position illustrated in Figure 8, which represents the de-energised condition of solenoid 220a, both ciijt(h slave cylinder 22 and gear engagement mechanism 25 via line 265, wi[ be connected to the reservoir 226. The clutch slave cylinder 22 and actuator 214 will thereby be depressurised. t Upon movement of the control valve 220 to its second position, the clutch actuator 22 remains depressurised but the gear engagement i mechanism 25 is pressurised. Appropriate control of the solenoid 221 by control unit 36 as disclosed in W097/0541 0 or W097/40300 may then cause actuator 214 to select an appropriate shift rail 111, 112 or 113. The control unit 36 will also selectively actuate the solenoid 1 7C, t switch the follower 160 to the appropriate path, so that the appropriato gear is selected.

Movement of the main control valve 220 to its third position connects tio working chamber of the clutch slave cylinder 22 to the accumulator 224 thereby disengaging the clutch 14.

The clutch slave cylinder 22 and gear engagement mechanism 25 may thus be controlled by appropriate manipulation of solenoids 220a,221 a and 170, by the control unit 36, to effect gear changes at take-up from rest, while the vehicle is on the move or when the vehicle is coming to a stop.

Various modifications may be made without departing from the invention.

For example, while the invention has been described with reference to a semi-automatic transmission system in which both the clutch and gear engagement mechanism are hydraulically actuated, the invention may also be used with fully automated transmission systems.

In accordance with a further embodiment, the solenoid 170 may be mounted for movement with the selector member 110, so that the follower 160 may be switched at one of two positions; a first position intermediate of the first and second shift rails 111, 112, as illustrated in Figure 4; and a second position intermediate of the second and third shift rails 112,113.

Furthermore, while in the above embodiment, hydraulic means are used for actuation of the clutch and movement of the gear selector member 110, hydraulic, pneumatic and/or electrical actuators, for example electric motors or solenoids, may be used, for example as disclosed in DE1 9504847; W097/01456 or DE1 9734023,, whose content is expressly incorporated in the disclosure content of the present application.

The patent claims submitted with the application are proposed formulations without prejudice to the achievement of further patent protection. The applicant reserves the right to submit claims for further combinations of characteristics, previously only disclosed in the description and/or drawings.

16 References back used in sub-claims refer to the further developmei t of the subject of the main claim by the characteristics of the respective claim; they are not to be understood as a waiver with regard to achiqvi g independent item protection for the combination of characteristics irl 1 he related sub-claims.

Since the subject of the sub-claims can form separate and indepenldf Ot inventions with reference to the prior art on the priority date, the appliCi tit reserves the right to make them the subject of independent claims 6r f 1 () division declarations. Furthermore, they may also contain indepen,040t inventions which demonstrate a design which is independent of one of 11he objects of the preceding sub-claims.

The embodiments are not to be considered a restriction of the inventic1h Rather, a wide range of amendments and modifications is possible withi J the scope of the current disclosure, especially those variations, elemenlib; and combinations and/or materials which, for example, the expert can learn by combining individual ones together with those in the general description and embodiments in addition to characteristics and/or elements or process stages described in the claims and contained in the drawings with the aim of solving a task thus leading to a new object or new process stages or sequences of process stages via combinable characteristics, even where they concern manufacturing, testing and vV rk processes.

17

Claims

1. A gear engagement mechanism for an automated transmission system comprising a gear box with a plurality of distinct gear ratios, the gear engaging mechanism comprising; a selector member and a plurality of shift rails, each shift rail being adapted to be moved axially, a different pair of gear ratios being associated with each of the shift rails, axial movement of the shift rail in one direction causing engagement of one of said associated gear ratios and axial movement of the shift rail in the opposite direction causing engagement of the other of said associated gear ratios; means being provided to move the selector member along an axis transverse to the longitudinal axes of the shift rails, whereby the selector member may be indexed with and engage a selected one of the shift rails; the selector member having a formation for engagement of a track, the track defining two paths, engagement of the formation on the selector member in the track, upon movement of the selector member along the transverse axis, causing the selector member to be pivoted in one direction or the other about the transverse axis, depending on the path followed, as the selector member moves to engage each of the shift rails, thereby causing axial movement of an engaged shift rail to engage a gear associated therewith; and means being provided to selectively switch between one path and the other in accordance with the gear ratio to be selected.

2. A gear engagement mechanism according to Claim 1 in which the formation on the selector member is shaped to engage formations on the track, whereby the formation is directed to follow one path of the track or the other path of the track, depending on the orientation of the formation.

3. A gear engagement mechanism according to Claim 1 or 2 in which the formation on the selector member is biased to a first orientation, 18 means being provided to switch the formation from the first orientatioill a second orientation.

4. A gear engagement mechanism according to Claim 3 in which a',, solenoid is provided to switch the formation on the selector member f,,:o Y1 its first orientation to its second orientation.

5. A gear engagement mechanism according to any one of Claims:2 to 4, in which the formation on the selector member comprises a rhomblir> shaped follower having two pairs of parallel sides and diagonally oppos'. e acute corners, the follower being pivotally attached to the selector member and moveable between a first orientation in which one pair of, parallel sides are disposed parallel to the walls of the track and a secold orientation in which the other pair of parallel sides are disposed parallel; 10 the walls of the track, the acute corners of the follower forming the leading edge of the follower, depending on the direction of movement 0 the selector member, in the first orientation of the follower, the leadin edges of the follower being disposed on opposite sides of the track totl 0 leading edges of the follower when the follower is in its second orientation; the track being divided into different paths, the track being, divided centrally thereof, so that the path taken by the follower will depend on the side of track upon which the leading edges of the f oiio\E r are disposed.

6. A gear engagement mechanism according to Claim 5 in which tt follower is pivotally mounted to the selector member in a recess, the recess limiting movement of the follower between its first orientation:n 1 its second orientation.

ic

7. A gear engagement mechanism according to Claim 4 or 5 in whi, 1 spring means biases the follower to its first orientation.

19

8. A gear engagement mechanism according to any one of Claims 5 to 7 in which a- projection extends from the upper end of the follower adjacent one of the leading edges thereof, the projection being adapted to be engaged by a solenoid operated plunger, the solenoid operated plunger acting when energised to switch the follower from its first orientation to its second orientation.

9. A gear engagement mechanism according to Claim 8 in which the projection on the follower is adapted to engage a guide rail by which the follower may be switched from its first orientation to its second orientation.

10. A gear engagement mechanism according to Claim 8 in which the solenoid operated plunger is mounted for movement with the selector member and is adapted to switch the follower from its first orientation to its second orientation at different points along the track.

11. A gear engagement mechanism according to any one of the preceding claims for use with a five-speed gearbox, the gear engagement mechanism including three shift rails, the track extending co-axially of the transverse axis of movement of the selector member and having a pair of branches at each end corresponding to a first and a third shift rail, the track being divided intermediate of its ends at a position corresponding to a second shift rail.

12. A gear engagement mechanism according to Claim 11 in which the track is formed by an aperture in an arcuate path which is arranged in juxtaposed relationship to the path of movement of the selector member, the aperture defining the branched ends of the track and a central portion of enlarged width, a guide being mounted on the arcuate plate, the guide having a diamond-shaped projection which projects down into the enlarged width portion of the track to divide the track into two paths.

13. A gear engagement mechanism according to Claim 12 in whichIP 0 guide is mounted co-axially of the aperture in the arcuate plate and defines a guide rail which extends between the diamond-shaped proje(.It' n and the branched end corresponding to the third shift rail, the guide ra:, 1 being clear of the track but being adapted to be engaged by the formai(, tl on the selector member to effect switching of the path followed thereb.

1 l: j

14. A gear engagement mechanism according to any one of Claims to 13, in which a solenoid operated plunger is disposed between the branched end of the track corresponding to the first shift rail and the intermediate division of the track corresponding to the second shift raJ, the solenoid operated plunger being arranged, upon actuation of the solenoid to switch the formation on the selector member between a fic's is orientation and a second orientation, when the selector member is positioned between the first and second shift rails.

15. A gear engagement mechanism according to any one of the preceding claims in which the selector member is mounted on a shaftfcr, movement along an axis transverse of the longitudinal axis of the shif.t rails, the shaft permitting free rotational movement of the selector member as it is moved axially.

16. A gear engagement mechanism according to Claim 15, in whiclil i n 1 actuator means is mounted for rotational movement relative to the shOl the selector member moving axially with the shaft.

17. A gear engagement mechanism according to Claim 15, in whichi e selector member is moveable axially and rotationally of the shaft, an actuator acting on the selector member to move the selector member axially of the shaft.

21

18. A gear engagement mechanism according to any one the preceding claims in which the selector member is moved along an axis transverse to a longitudinal axis of the shift rails by means of a double-acting hydraulic or pneumatic actuator or an electrical actuator.

19. A gear engagement mechanism according to any one of the preceding claims in which an electronic control unit is provided to control movement of the selector member along an axis transverse to longitudinal axis of the shift rails and for switching the path followed by the formation on the selector member, to effect the required changes in gear ratio.

20. A gear engagement mechanism substantially as described herein and with reference to and as shown in Figures 1 to 8 of the accompanying drawings.