WO1996010705A1

WO1996010705A1 - A ratio selector mechanism of a synchromesh vehicle transmission

Info

Publication number: WO1996010705A1
Application number: PCT/GB1995/002175
Authority: WO
Inventors: Andrew Julian Burrows; Andrew Robert Gray
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1994-09-30
Filing date: 1995-09-15
Publication date: 1996-04-11
Anticipated expiration: 1997-03-30
Also published as: GB9701633D0; GB2304836B; AU3480795A; GB2304836A; GB9419741D0

Abstract

The mechanism comprises a locking member (42) mounted for translational movement in a direction transverse to the gear select direction and an abutment surface (66) which limits the translational movement of the locking member (42). During selection of reverse ratio by a selector (26, 126) the locking member (42) is brought into contact with part (58) of a forward ratio selector (20) so as to cause the locking member (42) to undergo said translational movement until it contacts the abutment surface (66). Continued movement of the reverse ratio selector (26, 126) causes the locking member (42) to shift the forward ratio selector (20) so as to operate a synchroniser (28) of the forward ratio synchromesh prior to the engagement of reverse ratio and thereby prevent gear clashing when selecting reverse.

Description

A RATIO SELECTOR MECHANISM OF A SYNCHROMESH VEHTCI,F.

TRANSMISSION

The invention relates to a ratio selector mechanism of a synchromesh vehicle transmission having a synchromesh operating device for use when selecting reverse ratio to prevent clashing.

The use on a vehicle of a transmission having a synchromesh arrangement to assist engagement of its forward ratios is commonplace. However, in the vast majority of cases, reverse ratio is not selected with the aid of synchromesh.

When selecting reverse gear with the engine running and with the vehicle stationary, depression of the clutch to permit engagement of reverse leaves the transmission input shaft, lay shaft (s) and constant mesh gearing in motion. Therefore, rapid engagement of reverse ratio can cause clashing of the usual sliding pinion reverse gear against one of the gears which is still moving under its own momentum.

It has been proposed hitherto to brake the rotating parts in a synchromesh gearbox when selecting reverse ratio. For example in EP-A-459859, braking is effected by bringing into operation the synchroniser of one of the forward ratios without actually causing the forward ratio to be engaged. In that particular case, a load is applied to the synchroniser through a spring. By using a spring in that way, there is a risk of "beating" the action of the forward ratio synchroniser if reverse ratio is selected very quickly and gear clashing will then occur. A similar problem can occur with a system described in US-A-4 467 665 where an inclined surface on a selector makes contact with a spring loaded pin to operate a forward ratio synchroniser when selecting reverse. In US-A-5 119 917 a forward ratio synchroniser is also operated through a spring, a torsion spring in this case. Again, there is the real possibility of beating the operation of the forward ratio synchroniser during rapid selection of reverse ratio leading to gear clashing.

Attempts to overcome the problem of gear clash when selecting reverse have also been described in GB-A-2 110

778, US-A-4 181 036 and GB-A-1 461 328. In each case, the system described uses the operation of a forward ratio synchroniser to brake rotating parts of the gearbox to a standstill before engaging reverse ratio. However, the forward ratio synchroniser in each case is operated by moving a gear lever across the gate of the gear box into a position which will then enable the gear lever to be moved in a gear shift direction for selection of reverse ratio.

Such a arrangement is disadvantageous in that movement across the gate is normally a smooth movement whereas considerable resistance will be encountered when hitting a ramp, cam or other surface used to transmit force to operate the forward ratio synchroniser. There is then a risk that the resistance will provide a "feel" to the driver normally indicating that a point in the cross-gate movement has been reached where the gear lever can be moved in a gear shift direction to select reverse. However, it will not be possible to shift the gear lever to select reverse until the gear lever has been moved further through the gate with sufficient force to operate the forward ratio synchroniser. The force involved is likely to be considerable as it is necessary with such arrangements to convert the movement of gear lever in a cross-gate direction into a movement in the gear select direction in order to operate the forward ratio synchroniser. In the cases described, the conversion of movement takes place through a ramp or cam arrangement which generates considerable frictional resistance.

The problem of clashing when selecting reverse ratio has also been addressed in US-A-4 510 818. The arrangement described therein provides a direct operation of a forward ratio synchroniser momentarily to brake the rotating part of the gear box immediately prior to the engagement of reverse ratio. Although the arrangement avoids the problem of operating the synchroniser during cross-gate movement, a selector mechanism of complex form is required in an attempt to avoid the risk of engaging the forward ratio following operation of its synchroniser. Also, it is essential to disengage a selector lever from a surface of a shift head used to select the said forward ratio to ensure that only the reverse ratio will actually engage. A spring is provided to effect that disengagement. However, when selecting reverse ratio, the selector lever forcibly abuts simultaneously both the forward ratio shift head and a shift head for reverse ratio and the force will increase in proportion to the force applied by the driver when selecting reverse ratio. Bearing in mind that the spring load is constant, there is a risk that the force applied by the spring will not be sufficient to move the selector lever off the forward ratio shift head when an attempt is made to select reverse gear rapidly, thereby resulting in jamming.

An object of the present invention is to provide an improved ratio selector mechanism of a synchromesh vehicle transmission for preventing gear clash when selecting reverse ratio.

According to the invention there is provided a selector mechanism of a synchromesh vehicle transmission comprising a selector for engaging a synchromesh forward ratio, a selector for engaging a non-synchromesh reverse ratio and a shift member movable in gear select directions from a neutral position for selecting the ratios, the mechanism being arranged such that by moving the shift member from the neutral position to select reverse ratio the forward ratio synchromesh will be operated prior to the engagement of reverse ratio to prevent clashing, characterised in that a locking member is provided mounted for translational movement and an abutment surface is provided which limits the translational movement of the locking member, the locking member being arranged such that during selection of reverse ratio, the locking member is brought into contact with part of the forward ratio selector so as to cause the locking member to undergo said translational movement until it contacts the abutment surface whereupon continued movement of the shift member to select reverse ratio causes the locking member to shift the forward ratio selector so as to operate a synchroniser of the forward ratio synchromesh prior to the engagement of reverse ratio.

The mechanism of the invention does not operate the forward ratio synchronizer through the action of a spring as described in the prior systems mentioned above and does not suffer from the disadvantages of such systems where it is possible to "beat" the operation of the forward ratio synchroniser. Moreover, the operation of the forward ratio synchroniser takes place during the selection of reverse ratio and not during cross gate movement and the mechanism of the invention is particularly advantageous in that respect .

Preferably, the abutment surface is arranged to inhibit further translational movement of the locking member after the locking member has been moved into contact with the abutment surface. Preferably, the locking member is arranged to shift the forward ratio selector so as to operate the forward ratio synchromesh until the locking member clears the abutment surface whereupon the operation of the forward ratio synchromesh ceases prior to the engagement of reverse ratio. In that way, the mechanism is always operable to ensure that the forward ratio cannot actually be selected after operation of its synchroniser prior to the engagement of reverse ratio. Moreover, once the locking member moves into the clearance space, operation of the forward ratio synchroniser will cease and the forward ratio selector can then return to, eg, a neutral position.

The translational movement of the locking member may take place against a resilient bias means such as a torsion spring.

The locking member may be arranged to contact a ramp- like surface of the forward ratio selector during the selection of the reverse ratio, contact with the ramp-like surface being arranged to effect the translational movement of the locking member.

The locking member may be mounted for tipping movement whereby during disengagement of the reverse ratio the locking member can tip to enable it to move over the abutment surface. Bias means may be provided against which the locking member tips. The bias means may be the same bias means as that against^• which the locking member moves during its translational movement.

A return device may be provided for returning the forward ratio selector towards a neutral position after the operation of the forward ratio synchroniser. The return device may comprise a ramp and follower arrangement which provides a resilient return bias. In such a case the follower may comprise a spring-loaded ball which co¬ operates with the ramp. Additionally or alternatively in one embodiment, the return device may comprise an arm which is movable into a position in which it can co-operate if necessary with a portion associated with the forward ratio selector to ensure that the forward ratio selector returns towards the neutral position. The movable arm may be co- operable with said portion associated with the forward ratio selector as the reverse ratio selector disengages the reverse ratio.

Preferably, the movable arm is arranged to be moved by part of the selector mechanism which is shifted to select reverse ratio. The movement of the arm in that way enables the arm to be correctly positioned in a convenient manner relative to the said portion of the forward ratio selector. Preferably, the said part of the selector mechanism acts on a surface of the arm to cause the arm to move. The surface may comprise an edge of a slot formed in the arm. The arm may have a hook-like end arranged to co¬ operate, if necessary, with the said portion of the forward ratio selector.

The aforesaid movable arm may be mounted for movement on fixed part of the transmission, for example a member fixed to a casing of the transmission.

The movable arm may be operatively connected to a selector member whereby selection of the reverse ratio causes the selector member to move with the arm and engage reverse ratio. Preferably, the selector member and the arm extend one each side of a pivot therefor about which the arm and selector member move when selecting reverse ratio.

In an alternative embodiment, the return device may comprise a surface associated with the forward ratio selector which is engaged by a ratio selector lever used to select the forward and reverse ratios. In such a case, the engagement of the ratio selector lever and the said surface associated with the forward ratio selector provides a positive return movement instead of or in addition to the above mentioned ramp and follower arrangement.

The surface associated with the forward ratio selector is preferably oblique to a co-operable part of the lever to enable the lever to effect a camming action on the surface to move the forward ratio selector towards its neutral position. Conveniently, the selector lever will normally be biased by a spring into a gate position corresponding to the said forward ratio.

Preferably, the first said forward ratio selector is arranged to select third and fourth ratios of a transmission having four or more forward ratios. In such a case, the locking member is preferably operable to shift the first said forward ratio selector so as to effect operation of the fourth ratio synchroniser when reverse ratio is being selected.

The abutment surface may be provided on a shaft which supports a selector for selecting a further forward ratio.

A selector mechanism of a synchromesh vehicle transmission in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which: -

Fig 1 is a cross-section through part of a selector mechanism of a synchromesh vehicle transmission in accordance with the invention,

Fig 1A is a perspective view of showing a locking member and abutment surface forming part of the selector mechanism shown in Fig 1

Fig IB is a perspective view of the locking member of Figs 1 and 1A, Figs 2 to 6 show diagrammatically step-by-step operation of a synchroniser operating device used in the mechanism shown in Fig 1 and

Figs 7, 8 and 9 show diagrammatically various position of the locking member of the selector mechanism shown in Fig 1.

With reference to Figs 1 and 1A, a transmission in the form of a gearbox indicated generally at 10 has a casing 12 which supports a fixed shaft 14 and an axially slidable shaft 16. The shaft 14 slidably supports a selector 18 for first and second forward ratios of the gearbox and the shaft 16 slidably supports a selector 20 for selecting third and fourth ratios of the gearbox. The shaft 16 is connected via a resilient detent in the form of a spring loaded ball 22 to a selector 24 for selecting a fifth ratio of the gear box. The ball 22 locates in a recess 16a in the shaft 16. The shaft 16 is arranged to operate a selector generally indicated at 26 for selecting a reverse ratio. The ratios 1 to 5 are forward ratios and the selectors 18, 20 and 24 select the forward ratios through respective synchronisers of known kind. A synchroniser for use in selecting fourth ratio is indicated at 28.

A collar 30 on the shaft 16 carries a synchroniser operating device 31. The shaft 16 is drivably connected to the collar 30 by means of a diametral pin 32 passing through the shaft and the collar. A support arm 34 extends from the collar 30 and terminates at a fork 36 through which the shaft 14 extends. The fork 36 is slidable on the shaft 14 and prevents movement of the support arm 34 transverse to the shaft 14. The support arm 34 has a pair of spaced apart flanges 38 which are formed with slots 40 which support a locking member 42 for translational and pivotal movement. The locking member 42 has a pin 44 projecting from each side thereof and the pins 44 slidably locate in the respective slots 40. The locking member 42 is formed with a step 46 for locating part of a torsion spring 48. The spring 48 has a portion 48a which extends along the step 46 and two side sections 48b extending therefrom each of which is bent through a loop 48c. Free ends 48d of the spring 48 are attached to the respective sides of the support arm 34 by studs 49. The spring 48 biases the locking member 42 against a stop surface 50 on the arm 34. The locking member 42 is formed with a flat surface 52 at the opposite end to the pins 44. The locking member 42 has an oblique surface 54 adjacent the pins 44.

The selector 20 has a sleeve 56 formed with a ramp like surface 58 which is aligned with the surface 54 in the axial direction of the sleeve 56. The selector 20 defines a socket 60 housing a spring loaded ball 62 which contacts opposing ramps 64a, 64b formed on the shaft 14 when the selector 20 occupies a neutral position as shown in Fig 1. The shaft 14 is provided with an abutment surface 66 for co-operation with the surface 52 of the locking member 42 as described below with reference to Figs 2 to 6.

A bracket 61 is mounted rigidly on the casing 12 and supports by means of a pivot 61a an arm 63 which is fast with a selector member 65. The arm 63 is formed with an L- shaped slot 63a having a first limb 63b and a second limb 63c. The pivot 61a passes through an aperture 61b in the arm 63. The arm 63 is formed with a hook-like terminal end 67 (having an internal edge 67a) which, as described below, can make contact with a projection 69 on the sleeve 56 of the selector 20. The projection 69 extends outwardly from the sleeve 56. The selector member 65 is formed with a fork end 65a which slidably locates a sliding idler gear 65b. The idler gear 65b can be slid into and out of engagement with a layshaft gear 65c and a further gear 65d of the transmission to select reverse ratio. The gears 65b, 65c and 65d are illustrated diagrammatically in Fig 1. The collar 30 carries a pin 77 which projects slidably into the slot 63a.

The selector 18, the shaft 16 and the selector 20 are axially fast with selector rods 70, 72 and 74 respectively. The selector rods 70, 72 and 74 are shown diagrammatically in Fig 1 and are movable in the directions indicated by arrows A in Figs 1 and 7 by means of a shift mechanism 76 shown in detail in Figs 7, 8 and 9. The operation of the transmission will now be described with particular reference to Figs 2 to 6. In Figs 2 to 6 the position of the arm 63 is shown in broken lines.

By shifting the selector rod 70 from a neutral position, the selector 18 will select either the first or second ratio. By shifting selector rod 74 from a neutral position, the selector 20 will select either third or fourth ratio and by shifting selector rod 72 from its neutral position, fifth ratio or reverse ratio will be selected. A gate 71 showing a neutral path N and select directions for ratios 1 -5 and R from neutral path is illustrated in Fig 7.

The present invention is particularly concerned with the selection of reverse ratio without clashing of gears.

When selecting reverse ratio, the shaft 16 is moved upwards by selector rod 72 as viewed in Fig 1 and carries with it the locking member 42. Such movement causes the ball 22 to move away from the shaft 16 and disengage the recess 16a in the shaft 16 to permit shaft 16 to move upwards relative to the fifth ratio selector 24 which is prevented from moving upwards from the Fig 1 position by part of the casing 12. The selector 24 is retained in the Fig 1 position by a further ball 22a which locates in a recess 114 in the shaft 14. Fig 2 shows the locking member 42 in the Figs 1 and 1A neutral position immediately prior to shifting the shaft 16 to select reverse ratio. In Fig 3, upward movement of shaft 16 has moved the locking member 42 upwardly to bring the oblique surface 54 into contact with the ramp surface 58 on the fourth ratio selector 20. The surfaces 54, 58 may be inclined at approximately 45°. The flanges 38 pass one each side of the ramp surface 58. Continued upward shift of the shaft 16 causes the locking member 42 to undergo a translational movement as it is displaced transversely to the shift direction by the oblique surface 54 sliding up the ramp surface 58, the pins 44 (which are coaxial) sliding along the slots 40. Such translational sliding movement continues until the surface 52 of the locking member 42 makes contact with the abutment surface 66 as shown in Fig 3. Continued upward movement of the shaft 16 as viewed in Fig 3 then causes the locking member 42 to transmit the upward movement to the selector 20 by virtue of contact between the oblique surface 54 and the ramp surface 58. Such movement of the selector 20 causes the selector to operate the synchroniser 28 associated with the fourth ratio. If the layshaft(s) and constant mesh gearing (not shown) of the gearbox 10 are rotating with the vehicle stationary, then the operation of the synchroniser 28 will bring the layshaft(s) and constant mesh gearing of the gearbox to a standstill. As the selector 20 moves upwardly as described above, the ball 62 moves against the ramp 64a and creates a load tending to bias the selector 20 back to the Fig 1 position. Upward movement of the collar 30 with the shaft 16 causes the pin 77 to contact the upper edge of the limb 63b of slot 63a. Continued upward movement causes the pin 77 to turn the arm 63 about the pivot pin 61a and the pin 77 moves along the limb 63b. It will be noted that the limb 63b of the slot 63a is arranged at an oblique angle to the direction of upward movement of the collar 30. In that way, the inner surface 67a will be moved into a position such that it lies above the projection 69 as shown in Fig 4. The fork 65a also moves anticlockwise and slides the idler gear 65c towards the gear 65e.

Further upward movement of the shaft 16 from the Fig 3 position to the Fig 4 position causes the surface 52 to clear the abutment surface 66. Once that occurs, the loading of the oblique surface 54 against the ramp surface 58 of the selector 20 causes the locking member 42 to move into a clearance space 78 against the bias of the spring 48 as shown in Fig 5.

Continued upward movement of the shaft 16 so as to cause fork 65 to engage the reverse ratio causes the oblique surface 54 to move completely off the ramp surface 58 as shown in Fig 5 so that there is no longer any upward force applied to the selector 20 by the locking member 42. The ramp surface 58 then lies in a channel 38a (Fig 1A) between the flanges 38. In that way, the selector 20 operates the synchroniser 28 but cannot actually move sufficiently far to engage the fourth ratio. However, continued upward movement of the shaft 16 causes the reverse ratio idler gear 65c to move further into mesh with the gear 65e.

The return bias on the selector 20 created by the ball

62 riding up the ramp 64a then normally causes the selector 20 to move downwards and return towards the Fig 1 position.

When disengaging reverse ratio, the shaft 16 is moved downwardly by selector rod 72 as viewed in Figs 1 to 6 thereby causing the arm 63 and fork 65a to move clockwise about the pivot pin 61a. In order to enable the locking member 42 to clear the abutment surface 66 on its return movement, the abutment surface 66 causes the locking member 42 to pivot about the axis of pins 44 against the bias of the spring 48 as shown in Fig 6. Eventually, the locking member 42 rides over the abutment surface 66 and the spring 48 draws the locking member back into the Fig 1 neutral position.

As mentioned above, the bias created by the ball 62 riding up the ramp 64a will normally cause the selector 20 to move downwards back to the Fig 1 position. In the unlikely event of the selector 20 becoming stuck so that the ball 62 is unable to move the selector downwardly, it will be noted from Fig 5 that the internal surface 67a of the hook-like end 67 of the arm 63 lies over the projection 69. Therefore, as the shaft 16 continues to move downwards to disengage reverse ratio, the internal surface 67a will make contact with the projection 69 as in Fig 6, the upper position of which is shown in broken lines in Fig 6, and will draw the selector 20 downwards towards the Fig 1 position. Therefore, there is no possibility of the 4th gear synchroniser 28 remaining under load from selector 20 after disengaging reverse ratio.

The shift mechanism 76 is shown in detail in Figs 7, 8 and 9.

In Fig 7, it can be seen that the selector rod 72 is formed with a recess 80 for receiving a selector lever 82 (constituting the aforesaid shift member) . The selector rods 70 and 74 are formed with similar recesses 84, 86. A blocking device 88 comprises three fingers 90, 92 and 94. The finger 90 is connected to the fingers 92, 94 and the fingers are mounted for movement in unison from side to side as indicated by the arrows B in Fig 7. The selector lever 82 is defined by or connected to a gear shift lever

(not shown) for the vehicle and movement of the gear shift lever across the gate 71 along the neutral path N shown in

Fig 7 will cause the blocking device 88 to be moved in either of the directions indicated by arrows B.

When reverse ratio is to be selected, the shift mechanism 76 will be in the condition shown in Fig 7 with the selector lever 82 positioned within the recess 80 enabling the selector rod 72 to move the shaft 16. In that condition, finger 92 occupies only part of recesses 84 and 86. The finger 94 abuts a face 87 on the selector rod 70. In that way, the blocking member 88 permits free travel of the selector rod 72, limited travel of the selector rod 74 and blocks travel of the selector rod 70. Therefore, as the selector rod 72 is shifted upwardly as viewed in Fig 7 to select reverse ratio R by moving the selector lever 82 in a gear select direction A from the neutral path N, the finger 92 permits the rod 74 and hence the selector 20 to move upwardly by sufficient distance to enable the locking member 42 to move into the Fig 4 position so that the selector 20 can operate the synchroniser 28. ^• Movement of the rod 72 in the opposite direction will cause the selector 24 to engage fifth ratio. When selecting fifth ratio, the shaft 16 is moved downwardly as viewed in Fig 1 and a shoulder 113 on the shaft 16 causes the selector 24 to be moved downwardly with it, the ball 22a leaving the recess 114. Downward movement of the shaft 16 also moves the collar 30 downwardly as it is connected to the shaft 16. The provision of the limb 63c of the slot 63a, permits the pin 77 of the collar 30 to move downwards so that fifth ratio can be selected leaving the arm 63 and selector member 65 in the Fig 1 position.

When the selector mechanism 76 is positioned to select third or fourth ratio as shown in Fig 8, the selector lever 82 occupies the recess 84, the finger 90 of the blocking member 88 occupies the recess 80 and the finger 94 occupies the recess 87. In that way, the blocking member 88 permits free movement of the selector rods 74 but blocks movement of the selector rods 70, 72. When selecting third ratio, the selector 20 is moved downwardly from the Fig 1 position and the ramp surface 58 will move into contact with the oblique surface 54 of the locking member 42. The continued downward movement of the selector 20 causes the locking member 42 to undergo translational movement towards the shaft 14. However, it will be noted that the shaft 14 is formed with a further clearance space 110 below the abutment surface 66 and the locking member 42 enters the space 110 so that the third ratio can be freely selected.

To enable the selector mechanism 76 to select the first and second ratios, the selector lever 82 occupies the recess 86 as shown in Fig 9, the finger 90 occupies the recesses 80 and 84 and the fingers 94, 92 clear the recesses 86, 87 completely. Therefore, the blocking member 88 allows free movement of the selector rod 70 but blocks movement of the selector rods 72 and 74.

As described with reference to Figs 1 to 6, the ball 62 and ramp 64a provide a resilient return bias for the selector 20 and, additionally, the arm 63 having the hook 67 can be provided. Instead of using an arm 63 and fork 65, the shaft 16 can be arranged to operate an alternative reverse gear arrangement 120 through an alternative reverse ratio selector 126 (shown diagrammatically in broken lines in Fig 1) drivably connected to the shaft 16 and hence the collar 30. In order to provide a positive mechanical return force on the selector 20 in addition to the ball 62 and ramp 64a, part of the selector rod 74 defining a lower left corner of the recess 84 as viewed in Figs 7, 8 and 9 is formed with a chamfer 100. The chamfer 100 is co- operable with an inclined surface 101 on the selector lever 82. When the selector 20 has been moved into the Fig 5 position having operated the synchroniser 28, the chamfer 100 occupies a position shown in broken lines in Fig 7. The position of the selector lever 82 when reverse ratio is selected is also shown in broken lines. When the selector lever 82 is operated to disengage reverse ratio, the selector lever 82 will eventually occupy the full line position as shown in Fig 7 ie its neutral position. Should the selector 20 "stick" in the Fig 5 position instead of returning under the influence of the ba and ramp 62, 64a, the inclined surface 101 will make contact with the chamfer 100 when the selector lever moves to the Fig 8 position. In that way, the selector lever 82 effects a camming action which causes the selector rod 74 to move into its full line position in Fig 7. Normally, the selector lever 82 will be resiliently biased in known manner into the third/fourth gate position shown in Fig 7, ie into the recess 84, so that there is no risk of the selector 20 not returning to the Fig 1 neutral position after reverse ratio has been disengaged.

Although the terms "upward" and "downward" have been used in the specification, it will be understood that such terms are intended for explanatory purposes only and do not limit in any way the orientation of the gearbox 10 in use.

Claims

C AI .

1. A ratio selector mechanism of a synchromesh vehicle transmission comprising a selector (20) for engaging a synchromesh forward ratio, a selector (26,-126) for engaging a non-synchromesh reverse ratio and a shift member (82) movable in ratio select directions from a neutral position for selecting the ratios, the mechanism being arranged such that by moving the shift member (82) from the neutral position to select reverse ratio the forward ratio synchromesh (28) will be operated prior to the engagement of the reverse ratio to prevent clashing, characterised in that a locking member (42) is provided mounted for translational movement and an abutment surface (66) is provided which limits the translational movement of the locking member (42) , the locking member (42) being arranged such that during selection of reverse ratio, the locking member (42) is brought into contact with part (58) of the forward ratio selector (20) so as to cause the locking member (42) to undergo said translational movement until it contacts the abutment surface (66) whereupon continued movement of the shift member (82) to select reverse ratio causes the locking member (42) to shift the forward ratio selector (20) so as to operate a synchroniser (28) of the forward ratio synchromesh prior to the engagement of reverse ratio.

2. A ratio selector mechanism according to Claim 1 characterised in that the abutment surface (66) is arranged to inhibit further translational movement of the locking member (42) after the locking member (42) has been moved into contact with the abutment surface.

3. A ratio selector mechanism according to Claim 1 or 2 characterised in that locking member (42) is arranged to shift the forward ratio selector (20) so as to operate the forward ratio synchromesh (28) until the locking member (42) clears the abutment surface (66) whereupon the operation of the forward ratio synchromesh (28) ceases prior to the engagement of reverse ratio.

4. A ratio selector mechanism according to Claim 1, 2 or 3 characterised in that the translational movement of the locking member (42) takes place against a resilient bias (48) .

5. A ratio selector mechanism according to any preceding claim characterised in that the locking member (42) is arranged to contact a ramp-like surface (58) of the forward ratio selector (20) during the selection of the reverse ratio, contact with the ramp-like surface (58) being arranged to effect the translational movement of the locking member (42) .

6. A ratio selector mechanism according to Claim 5 characterised in that movement of the reverse ratio selector (26,-126) in the select direction causes the locking member (42) in contact with the ramp surface (58) to shift the forward ratio selector (20) .

7. A ratio selector mechanism according to any preceding claim characterised in that abutment surface (66) terminates to provide a clearance space (78) into which the locking member (42) moves following operation of the forward ratio synchroniser (28) to prevent selection of the forward ratio by the locking member (42) .

8. A ratio selector mechanism according to any preceding claim characterised in that locking member (42) is mounted for tipping movement whereby during disengagement of reverse ratio, the locking member (42) can tip to enable it to move over the abutment surface.

9. A ratio selector mechanism according to Claim 8 characterised in that locking member tips against a resilient bias (48) .

10. A ratio selector mechanism according to Claim 9 when appendant to Claim 4 characterised in that resilient bias (48) is the same as that against which the locking member (42) moves during its translational movement.

11. A ratio selector mechanism according to any preceding claim characterised in that a return device (62,64a) is provided for returning the forward ratio selector (20) towards the neutral position after the operation of the forward ratio synchroniser (28) .

12. A ratio selector mechanism according to Claim 11 characterised in that return device comprises a ramp (64a) and follower (62) arrangement which provides a resilient return bias.

13. A ratio selector mechanism according to Claim 11 or 12 characterised in that the return device comprises an arm (63) which is movable into a position in which it can co-operate if necessary with a portion (69) associated with the forward ratio selector (20) to ensure that the forward ratio selector returns (20) towards the neutral position.

14. A ratio selector mechanism according to Claim 13 characterised in that the movable arm (63) is co- operable with said portion associated with the forward ratio selector as the reverse ratio selector (26) disengages the reverse ratio.

15. A ratio selector mechanism according to Claim 13 or 14 characterised in that the movable arm (63) is moved by part (77) of the selector mechanism which is shifted to select reverse ratio.

16. A ratio selector mechanism according to Claim 15 characterised in that the part (77) of the selector mechanism acts on a surface of the arm (63) during selection of reverse gear to cause the arm (63) to move.

17. A ratio selector mechanism according to any of Claims 13 to 16 characterised in that the arm (63) is mounted for movement on a fixed part (61) of the transmission.

18. A ratio selector mechanism any of Claims 13 to 17 characterised in that the movable arm (63) is operatively connected to a selector member (65) whereby selection of reverse ratio causes the selector member (65) to move with the arm (63) and engage reverse ratio.

19. A ratio selector mechanism according to Claim 18 characterised in that the selector member (65) and the arm (63) extend one each side of a pivot (61a) therefor about which the arm and selector member when selecting reverse ratio.

20. A ratio selector mechanism according to Claim 11 or 12 characterised in that return device comprises a surface (100) associated with the forward ratio selector (20) which is engaged by the shift member (82) used to select the forward and reverse ratios.

21. A ratio selector mechanism according to Claim 20 characterised in that the shift member (82) is arranged to co-operate with the surface (100) and move the forward ratio selector (20) into its neutral position as the shift member (82) moves from a position in which it can select the reverse ratio to a position in which it can select the forward ratio.

22. A ratio selector mechanism according to Claim 20 or 21 characterised in that surface (100) is oblique to a co-operable part (101) of the shift member (82) to enable the lever to effect a camming action on the surface (100) to move the forward ratio selector (20) towards its neutral position.

23. A ratio selector mechanism according to any preceding claim characterised in that a further selector (24) is provided for enabling another forward ratio to be selected and a blocking device (88) is provided for blocking movement of the further selector (24) when the reverse ratio or the first said forward ratio is being selected.

24. A ratio selector mechanism according to Claim 23 characterised in that blocking device (88) is operable to block movement of the first said ratio selector (20) and the reverse ratio selector (26,126) when the other forward ratio is being selected.

25. A ratio selector mechanism according to any preceding claim characterised in that first said forward ratio selector (20) is arranged to select 3rd and 4th ratios of a transmission having four or more forward ratios.

26. A ratio selector mechanism according to Claim 25 characterised in that the locking member (42) is operable to shift the first said forward ratio selector (20) so as to effect operation of the fourth ratio synchroniser when reverse ratio is being selected.

27. A ratio selector mechanism according to any preceding claim characterised in that the locking member (42) is carried by a member (30) which moves simultaneously with the reverse ratio selector (26,126) .

28. A ratio selector mechanism according to any preceding claim characterised in that abutment surface (66) is provided on a shaft (14) which supports a selector (18) for selecting a further forward ratio.

29. A ratio selector mechanism constructed and arranged substantially as described herein with reference to the accompanying drawings.