GB2267940A

GB2267940A - Gear engagement assembly

Info

Publication number: GB2267940A
Application number: GB9213213A
Authority: GB
Inventors: David Benjamin Hirons
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-06-20
Filing date: 1992-06-20
Publication date: 1993-12-22
Anticipated expiration: 2012-06-20
Also published as: GB2267940B; GB9213213D0

Abstract

A gear engagement assembly comprises a selector hub, Fig 4, a driving gear, Fig 2, first and second primary gear engagement means (4) and (13) respectively, and secondary gear engagement means (5). In operation, the selector hub and the driving gear are engaged with each other by moving the selector hub and the driving gear towards each other so that in a first gear engagement stage the first and second primary gear engagement means (4, 13) engage with each other and so that on further movement of the members towards each other in a second gear engagement stage the primary gear engagement means of one of the selector hub and the driving gear engages between the primary gear engagement means and the secondary gear engagement means of the other of the selector hub and the driving gear. The selector hub may include short dogs 14 forming secondary gear engagement means. In a modification, (Figures 58) lands between recesses comprise first primary engagement means and the bottoms of arcuate grooves comprise secondary gear engagement means. Dogs (29) on the selector hub comprise second primary gear engagement means. <IMAGE>

Description

GEAR ENGAGEMENT ASSEMBLY The present invention relates to a gear engagement assembly for example for use with an automotive gear assembly and more particularly but not exclusively relates to a gear engagement assembly such as may be used in a competition gear assembly.

Competition gear assemblies are conventionally engaged and disengaged by means of a selector hub which carries a number of axially protruding dogs arranged to engage between corresponding axially protruding dogs provided on a selected driving gear to be engaged. In order to facilitate engagement between the dogs on the selector hub and the dogs on the driving gear to be engaged the circumferential spacing between the dogs is generally in the range from 1.5 to 2 or more times the circumferential dimension of each of the dogs. The spacing is required to permit the selector hub to be moved from a position in which the dogs of the selector hub abut but do not engage the dogs of the driving gear to a position in which the dogs of the selector hub engage sufficiently between the dogs of the driving gear such that the driving gear can successfully be engaged. Gear engagement inevitably takes place with the gears rotating at differential speeds. The speeds involved inevitably make it relatively difficult to effect sufficient axial movement in the short time available in order to secure engagement between the dogs and this makes it more difficult to engage gear and results in considerable stress being generated within the gear selector mechanism. Moreover, once the dogs have been engaged there is considerable freedom of movement between the selector hub and the driving gear which gives rise to high levels of backlash as a result of torque reversals, i.e. when the engine torque changes between power and overrun conditions. The high levels of backlash and the inertial forces generated lead to high rates of wear in the gears and in other components of the vehicle and there is clearly a demand for an improved gear engagement assembly for use in competition and other vehicles.

It is therefore an object of this invention to provide a gear engagement assembly which has reduced backlash and which may also involve lower inertial forces.

According to the present invention there is provided a gear engagement assembly which comprises: a selector hub adapted for rotation with a shaft of a gear assembly; a driving gear adapted to be rotatable on the shaft adjacent to and coaxial with the selector hub and adapted for engagement with an input gear provided on an input shaft of the gear assembly; first primary gear engagement means incorporated in or provided on the driving gear and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the driving gear; second primary gear engagement means incorporated in or provided on the selector hub and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the selector hub; and secondary gear engagement means incorporated in or provided on one of the selector hub and the driving gear and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the selector hub and the driving gear, the secondary gear engagement means being positioned in a disengaged condition of the selector hub and the driving gear on that side of the primary gear engagement means of said one of the selector hub and the driving gear which is remote from the primary gear engagement means of the other of the selector hub and the driving gear, the arrangement being such that the selector hub and the driving gear are engaged with each other by moving the selector hub and the driving gear towards each other so that in a first gear engagement stage the first and second primary gear engagement means engage with each other and so that on further movement of the selector hub and the driving gear towards each other in a second gear engagement stage the primary gear engagement means of said other of the selector hub and the driving gear engages between the primary gear engagement means and the secondary gear engagement means of said one of the selector hub and the driving gear.

The second primary gear engagement means may comprise a plurality of dogs extending axially a predetermined distance from the selector hub towards the driving gear.

The first primary gear engagement means may comprise a plurality of land areas provided between at least some of a plurality of recesses formed in the driving gear and positioned in an annular region coaxial with the driving gear. In this case the secondary gear engagement means may comprise the base areas of a plurality of arcuate slots provided between some of the recesses formed in the driving gear and positioned in the annular region.

Alternatively, the first primary gear arrangement means may comprise a plurality of dogs extending axially a predetermined distance from the driving gear towards the selector hub. In this case the secondary gear arrangement means may comprise a plurality of further dogs positioned between the dogs of the first primary gear arrangement and extending axially a predetermined distance less than the distance of the dogs of the first primary gear arrangement towards the selector hub.

Secondary gear engagement means may be incorporated in or provided on each of the selector hub and the driving gear and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the selector hub and the driving gear such that, in a disengaged condition of the selector hub and the driving gear, the secondary gear engagement means of the driving gear is positioned on that side of the first primary gear engagement means which is remote from the second primary gear engagement means and the secondary gear engagement means of the selector hub is positioned on that side of the second primary gear engagement means which is remote from the first primary gear engagement means, the arrangement being such that in the second gear engagement stage the first primary gear engagement means engages between the second primary gear engagement means and the secondary gear engagement means of the selector hub and the second primary gear engagement means engages between the first primary gear engagement means and the secondary gear engagement means of the driving gear.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is an end elevational view of one embodiment of a driving gear for use in a gear engagement assembly according to the present invention; Figure 2 is a side elevational view of the driving gear shown in Figure 1; Figure 3 is an end elevational view of a selector hub for use in conjunction with the driving gear of Figures 1 and 2 in a gear engagement assembly according to the present invention; Figure 4 is a side elevational view of the selector hub shown in Figure 3; Figure 5 is an end elevational view of another embodiment of a driving gear for use in a gear engagement assembly according to the present invention; Figure 6 is a cross-sectional view taken along the line VI VI shown in Figure 5; Figure 7 is an end elevational view of a selector hub for use in conjunction with the driving gear of Figures 5 and 6 in a gear engagement assembly according to the present invention; and Figure 8 is a side elevational view of the selector hub shown in Figure 7.

The driving gear shown in Figures 1 and 2 is provided with a plurality of straight cut teeth 1 around its outer periphery, for constant engagement with a corresponding gear on an input shaft (not shown) of the gear assembly, and with a smooth bore 2 so as to be rotatable on a main shaft (not shown) of the gear assembly. It should be noted that helical cut teeth, for example, can be provided in place of the straight cut teeth 1 if desired. An axially extending ring 3 is provided on one face of the driving gear and is provided with radially protruding dogs 4 and 5 as will be explained in more detail hereinafter.

In the driving gear shown there are seven dogs 4 and seven dogs 5, with the dogs 4 and 5 being positioned alternately around the periphery of the ring 3. However, the number of dogs may be varied depending on the dimensions of the gear and of the dogs.

The dogs 4 comprise first primary gear engagement means and extend axially substantially from the exposed face 6 of the ring 3 and terminate a short distance from the face of the gear, whereas the dogs 5 comprise secondary gear engagement means and are set back from the exposed face 6 of the ring 3, for example by about half the axial extent of the dogs 4, and terminate a short distance from the face of the gear as with the dogs 4.

The dogs 4 and 5 are tapered in the radial direction in such a way that the circumferential dimension of the dogs decreases with increasing radial dimension so as to cooperate with corresponding dogs provided on the selector hub as will be explained in more detail hereinafter. The dogs 4 and 5 are also tapered in the axial direction such that the circumferential extent of each dog 4, 5 increases with increasing axial distance from the face of the gear to the exposed face of the ring 3. The taper serves to reduce the likelihood of the gear assembly jumping out of gear and additionally assists the speed of gear engagement. The exposed face 7 of each of the longer dogs 4 is substantially planar to assist gear engagement, although it may be inclined in the radial direction such that the radially outer portion of the face 7 is set back from the exposed face 6 of the ring 3 relative to the radially inner portion of the face 7. The dogs 4 may be chamfered at 10 where the exposed face 7 meets the axial faces 8, 9 which form the sides of each dog in order to improve the metallurgical properties of the metal and to reduce the likelihood of damage at this point, but not to such an extent as to detract substantially from the planar nature of the face 7.

The selector hub is shown in Figures 3 and 4 and comprises a short cylindrical hub provided with a peripheral recess 11 for receiving the selector fork of a gear change mechanism (not shown) and with a splined bore 12 for transferring drive from the driving gear to a main shaft (not shown) of the gear assembly. The selector hub is provided on each face thereof with a circular array of dogs 13, 14 which will be described in more detail hereinafter, the dogs on one face being provided to engage one driving gear and the dogs on the other face being provided to engage another driving gear, the particular gear to be engaged, if any, being determined by the driver of the vehicle by way of the gear selector mechanism in known manner. Clearly, however, if the arrangement of the gear assembly is such that only a single driving gear requires to be engaged or disengaged then the dogs 13, 14 need only be provided on one face of the selector hub.

The selector hub is provided with the same number of dogs 13 and with the same number of dogs 14 as the numbers of dogs 4, 5 respectively on the driving gear. The dogs 13 comprise second primary gear engagement means and extend axially towards the driving gear by an amount corresponding substantially to the axial dimension of the dogs 4, while the dogs 14 comprise secondary gear engagement means and extend axially towards the driving gear by an amount corresponding substantially to the axial dimension of the dogs 5 and are thus set back with respect to the dogs 13.

The dogs 13, 14 are tapered in the radial direction in such a way that the circumferential dimension of the dogs decreases with decreasing radial dimension so as to cooperate with the dogs 4, 5 provided on the driving gear, the arrangement being such that when the dogs 4, 5 and the dogs 13, 14 are fully engaged, the long dogs 4 of the driving gear extend closely between adjacent dogs 13, 14 of the selector hub, while the long dogs 13 of the selector hub extend closely between adjacent dogs 4, 5 of the driving gear. In this way, the levels of backlash generated as a result of torque reversals, i.e. when the engine torque changes between power and over-run conditions, are very substantially reduced.

Although it is not shown in the drawings, the dogs 13, 14 are also tapered in the axial direction such that the circumferential extent of each dog 13, 14 increases with increasing axial distance from the face of the selector hub so as to co-operate with the dogs 4, 5 in order to reduce the likelihood of the gear assembly jumping out of gear and to assist the speed of gear engagement. As with the dogs 4, the exposed face of each dog 13 is substantially planar, although it may be inclined in the radial direction such that the radially inner portion of the face 15 is set back relative to the radially outer portion thereof. The dogs 13 may be chamfered at 16 where the exposed face 15 meets the axial faces 17, 18 which form the sides of each dog in order to improve the metallurgical properties of the metal and to reduce the likelihood of damage at this point, but not to such an extent as to detract substantially from the planar nature of the face 15.

In use of the gear engagement assembly shown in Figures 1 to 4, when the selector hub is to be engaged with the driving gear the driver of the vehicle operates the gear selector mechanism so as to urge the selector hub towards the driving gear coaxial therewith. Because every other dog in each of the sets of dogs 4, 5 and 13, 14 is set back relative to the adjacent dog, the initial spacing between the dogs is in excess of twice the circumferential extent of each dog thus providing adequate opportunity for long dogs 13 of the selector hub to be moved axially in a first gear engagement stage a sufficient distance to engage with and bear against the long dogs 4 of the driving gear so as to engage the first and second primary gear engagement means. In this way the dogs 13, 14 of the selector hub and the dogs 4, 5 of the driving gear are self-aligning and, once the selector hub and the driving gear have engaged, the inclinations on the dogs will encourage gear engagement and will generally prevent any subsequent jumping out of gear. Thereafter, the selector hub can be moved axially in a second gear engagement stage so as to complete the engagement of the chosen gear. As the selector hub advances, the long dogs 13 of the selector hub engage between adjacent dogs 4, 5 of the driving gear and the long dogs 4 of the driving gear engage between adjacent dogs 13, 14 of the selector hub thereby very substantially reducing any backlash.

The greater initial circumferential spacing between the long dogs 4 and 13 enables a gear to be selected more quickly and reliably than the prior art arrangement, while at the same time reducing the stress on the selector mechanism, and moreover, as noted above, the close coupling between the dogs 4, 5 and 13, 14 when the gear is fully engaged substantially eliminates backlash and correspondingly largely eliminates shock loadings due to torque reversal.

Although the gear engagement assembly of the present invention has been described in relation to Figures 1 to 4 as incorporating long dogs and short dogs on both the selector hub and on the driving gear, it is not essential that the short dogs are provided on both the selector hub and the driving gear. Thus, if desired, the short dogs can be omitted from either the selector hub or the driving gear with no difference in operation except that the long dogs of the other of the driving wheel and the selector hub will not engage between adjacent dogs. The advantage of such an arrangement would be reduced weight, albeit at a cost of somewhat reduced strength. Moreover, although the gear engagement assembly of the present invention has been described in relation to Figures 1 to 4 as having alternate long and short dogs, it is possible to provide more than one short dog between each pair of long dogs with the proviso that the spacing between the long dogs should be the same in the selector hub as in the driving gear. Thus, for example, two short dogs may be provided between each pair of long dogs. Such an arrangement would render gear engagement even easier, but would reduce the torque transmission capacity of the available long dogs. A further modification of the gear engagement assembly of the present invention described in relation to Figures 1 to 4 involves mounting the selector hub on the main shaft by way of a gear secured thereon. In such a modification the selector hub is generally annular and the dogs 13, 14 extend radially inwardly from the inner peripheral surface of the selector hub so as to engage and be rotatable with the gear mounted on the main shaft. The selector hub is slidable axially on the gear by the gear selector mechanism such that movement of the selector hub in one direction first engages with one driving gear the dogs 13 and then the dogs 14 of one set of dogs 13, 14 with the other set of dogs 13, 14 transferring drive to the gear mounted on the main shaft. Movement of the selector hub in the opposite direction first engages with the other driving gear the dogs 13 and then the dogs 14 of the other set of dogs 13, 14 with the one set of dogs 13, 14 transferring drive to the gear mounted on the main shaft.

The gear engagement assembly described in relation to Figures 1 to 4 represents a substantial improvement over prior art gear engagement assemblies for use in competition gear assemblies. However, there is a constant demand in this field for more effective equipment and I have therefore modified the gear engagement assembly of Figures 1 to 4 so as to provide in the embodiment of Figures 5 to 8 a gear engagement assembly which has several additional advantages. Because the gear engagement mechanism takes up less space and involves the removal of material from the driving gear rather than the provision of an axially extending ring, the overall gear assembly can be made more compact and can involve lower inertial forces.

Alternatively, for a given available space, the driving gears can be more robust making the gear assembly more reliable and longer lasting.

Figures 5 and 6 show a driving gear which is provided with a plurality of straight cut teeth 21 around its outer periphery, for constant engagement with a corresponding gear on an input shaft (not shown) of the gear assembly, and with a smooth bore 22 so as to be rotatable on a main shaft (not shown) of the gear assembly. As with the embodiment of Figures 1 to 4, helical cut teeth, for example, can be provided in place of the straight cut teeth 21 if desired.

The driving gear shown in Figures 5 and 6 is not provided with dogs in the generally accepted sense, but an even plurality of axially extending recesses 23, 23a are formed around an annular region of the face 25 of the driving gear which in use lies adjacent to the selector hub. The recesses 23, 23a are grouped into two sets, the recesses of each set being equally spaced from each other in the circumferential direction, with a recess 23a being positioned between each pair of recesses 23. It is not necessary that the recesses 23 and 23a are all equally spaced from each other. The recesses are preferably circular in cross-section and can conveniently be formed by an aperture extending entirely through the driving gear, but this is not essential. Preferably, the recesses 23, 23a are tapered in the axial direction such that the crosssectional area of each recess increases with increasing distance from the face 25 of the driving gear so as to encourage gear engagement and to reduce the likelihood of the gear assembly jumping out of gear. Each pair of recesses comprising one recess 23 and one recess 23a is interconnected by an axially extending arcuate groove 24 which comprises secondary gear engagement means and which extends from the face 25 of the driving gear a smaller distance than the depth of the recesses 23, 23a.

Looking at this arrangement in reverse, the lands 26 between adjacent recesses 23, 23a of different pairs of recesses can be regarded as long dogs and comprise the first primary gear engagement means, the bottoms of the arcuate grooves 24 can be regarded as short dogs and comprise the secondary gear engagement means and the recesses 23, 23a can be regarded as the gaps between adjacent dogs for receiving the corresponding dogs of the selector hub. Thus the dogs are incorporated into the driving gear rather than extend axially outwards from the driving gear. The edges formed at the mouth of each recess 23, 23a and at the mouth of each arcuate groove 24 may be chamfered, but only to the extent necessary to improve the metallurgical properties of the metal and to reduce the risk of damage at this point.

Figures 7 and 8 show a selector hub for use in conjunction with the driving gear of Figures 5 and 6. As with the selector hub of Figures 3 and 4, the selector hub shown in Figures 7 and 8 comprises a short cylindrical hub provided with a peripheral recess 27 for receiving the selector fork of a gear change mechanism (not shown) and with a splined bore 28 for transferring drive from the driving gear to a main shaft (not shown) of the gear assembly. The selector hub is provided on each face thereof with a circular array of dogs 29 which will be described in more detail hereinafter, the dogs on one face being provided to engage one driving gear and the dogs on the other face being provided to engage another driving gear, the particular gear, if any, to be engaged being determined by the driver of the vehicle by way of the gear selector mechanism in known manner. Clearly, however, if the arrangement of the gear assembly is such that only a single driving gear requires to be engaged or disengaged then the dogs 29 need only be provided on one face of the selector hub.

The selector hub is provided with the same number of dogs 29 as the number of recesses 23 or 23a, the dogs 29 comprising the second primary gear engagement means and extending axially towards the driving gear by an amount not greater than the depth of the recesses 23, 23a. The dogs 29 are shaped so as to correspond to the shape of the recesses 23, 23a. For example, where the recesses are circular in cross-section as illustrated, the dogs 29 are preferably circular in cross-section. Thus, when the dogs 29 are fully engaged in the recesses 23 or 23a, the dogs 29 are closely adjacent to the surfaces of the recesses in order to minimise the levels of backlash generated as a result of torque reversals. The dogs 29 are tapered in the axial direction such that the cross-sectional area of each dog increases with increasing distance from the face of the selector hub so as to co-operate with the taper formed in the recesses 23, 23a in order to encourage gear engagement and to reduce the likelihood of the gear assembly jumping out of gear. The exposed face of each dog 29 is substantially planar, although the edges may be chamfered, but only to the extent necessary to improve the metallurgical properties of the metal and to reduce the risk of damage at this point.

In use, the gear engagement assembly of Figures 5 to 8 operates in a very similar manner to the gear engagement assembly of Figures 1 to 4 described above. When the selector hub is to be engaged with the driving gear the driver of the vehicle operates the gear selector mechanism so as to urge the selector hub towards the driving gear coaxial therewith. As can be seen from Figure 5, because there is considerable angular spacing between the recesses 23, 23a which are interconnected by an arcuate groove 24, there is adequate opportunity for the dogs 29 of the selector hub to be moved a sufficient axial distance in a first gear engagement stage to engage with and bear against the surface of one of the recesses 23, 23a. In this way the dogs 29 of the selector hub are self-aligning and, once the selector hub and the driving gear have engaged, the tapering surfaces will generally encourage gear engagement and will prevent any subsequent jumping out of gear.

Thereafter, the selector hub can be moved axially in a second gear engagement stage so as to complete the engagement of the chosen gear. As the selector hub advances, the dogs 29 of the selector hub engage in the recesses 23 or 23a beyond the depth of the arcuate groove 24 thereby very substantially reducing any backlash.

As with the embodiment of Figures 1 to 4, the circumferential extent of the arcuate groove 24 enables a gear to be selected more quickly and reliably than the prior art arrangement, while at the same time reducing the stress on the selector mechanism, and moreover, as noted above, the close coupling between the dogs 29 and the recesses 23 or 23a when the gear is fully engaged substantially eliminates any backlash and correspondingly largely eliminates shock loadings due to torque reversal.

It will be noted that the gear engagement assembly described with reference to Figure 5 to 8 does not incorporate both long dogs and short dogs on the selector hub. This is because the recesses 23 and 23a are not all equally spaced in the circumferential direction and no advantage is to be had from the provision of short dogs on the selector hub. If the recesses 23 and 23a were to be equally spaced in the circumferential direction, short dogs could be provided on the selector hub intermediate the dogs 29 such that as the chosen gear is fully engaged the dogs 29 engage in one of the sets of recesses 23 or 23a beyond the depth of the arcuate groove 24 and the additional short dogs engage in the other set of recesses 23a or 23, preferably beyond the depth of the arcuate groove 24, so as to provide additional power transfer strength and additional support to resist torque reversal forces. If desired, additional short dogs can be provided on the selector hub, for example such that there are two short dogs between each of the dogs 29, the additional short dogs being of a length sufficiently short so as not to detract from the advantages of quick and reliable gear selection, but of a sufficient length to engage with additional recesses provided in the arcuate grooves between the recesses 23, 23a. Such an arrangement would provide further power transfer strength, albeit at the cost of additional weight.

Although the gear engagement assembly of the present invention has been described herein in relation to manually operated gear selector mechanisms, it should be borne in mind that the gear engagement assembly of the present invention also has application to semi-automatic and automatic gear selector mechanisms which may be operated, for example, pneumatically or hydraulically.

Claims

1. A gear engagement assembly which comprises: a selector hub adapted for rotation with a shaft of a gear assembly; a driving gear adapted to be rotatable on the shaft adjacent to and coaxial with the selector hub and adapted for engagement with an input gear provided on an input shaft of the gear assembly; first primary gear engagement means incorporated in or provided on the driving gear and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the driving gear; second primary gear engagement means incorporated in or provided on the selector hub and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the selector hub; and secondary gear engagement means incorporated in or provided on one of the selector hub and the driving gear and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the selector hub and the driving gear, the secondary gear engagement means being positioned in a disengaged condition of the selector hub and the driving gear on that side of the primary gear engagement means of said one of the selector hub and the driving gear which is remote from the primary gear engagement means of the other of the selector hub and the driving gear, the arrangement being such that the selector hub and the driving gear are engaged with each other by moving the selector hub and the driving gear towards each other so that in a first gear engagement stage the first and second primary gear engagement means engage with each other and so that on further movement of the selector hub and the driving gear towards each other in a second gear engagement stage the primary gear engagement means of said other of the selector hub and the driving gear engages between the primary gear engagement means and the secondary gear engagement means of said one of the selector hub and the driving gear.

2. A gear engagement assembly as claimed in claim 1, wherein the second primary gear engagement means comprises a plurality of dogs extending axially a predetermined distance from the selector hub towards the driving gear.

3. A gear engagement assembly as claimed in claim 1 or 2, wherein the first primary gear engagement means comprises a plurality of land areas provided between at least some of a plurality of recesses formed in the driving gear and positioned in an annular region coaxial with the driving gear.

4. A gear engagement assembly as claimed in claim 3, wherein the secondary gear engagement means comprises the base areas of a plurality of arcuate slots provided between some of the recesses formed in the driving gear and positioned in the annular region.

5. A gear engagement assembly as claimed in claim 1 or 2, wherein the first primary gear arrangement means comprises a plurality of dogs extending axially a predetermined distance from the driving gear towards the selector hub.

6. A gear engagement assembly as claimed in claim 5, wherein the secondary gear arrangement means comprises a plurality of further dogs positioned between the dogs of the first primary gear arrangement and extending axially a predetermined distance less than the distance of the dogs of the first primary gear arrangement towards the selector hub.

7. A gear engagement assembly as claimed in any preceding claim, wherein secondary gear engagement means is incorporated in or provided on each of the selector hub and the driving gear and terminating in a predetermined plane substantially perpendicular to the axis of rotation of the selector hub and the driving gear such that, in a disengaged condition of the selector hub and the driving gear, the secondary gear engagement means of the driving gear is positioned on that side of the first primary gear engagement means which is remote from the second primary gear engagement means and the secondary gear engagement means of the selector hub is positioned on that side of the second primary gear engagement means which is remote from the first primary gear engagement means, the arrangement being such that in the second gear engagement stage the first primary gear engagement means engages between the second primary gear engagement means and the secondary gear engagement means of the selector hub and the second primary gear engagement means engages between the first primary gear engagement means and the secondary gear engagement means of the driving gear.

8. A gear engagement assembly substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 4 or Figures 5 to 8 of the accompanying drawings.