Abstract:
A mainshaft assembly for a gearbox and a gearbox embodying such a mainshaft assembly are disclosed. The mainshaft assembly comprises a mainshaft, and several drive gears, each carried for rotation about the mainshaft, each drive gear having a different number of teeth. The assembly includes first and a second hub, each being associated with a respective drive gear, each hub having a dog ring operable to selectively couple with the drive gear causing it to rotate with the hub or uncouple from the drive gear to allow the drive gear to rotate with respect to the hub. A respective drive connection means comprising a cage and rollers is associated with each hub and is operative to connect or disconnect the hub to the mainshaft for rotation with it or to allow rotation with respect to it. Upon connection of both first and second hub by their respective dog rings to each corresponding drive gear, the drive connection means operates to connect one of the hubs to the mainshaft when torque is applied to the mainshaft in a first direction and to connect the other one of the hubs to the mainshaft when torque is applied to the mainshaft in the opposite direction. This enables selection of a gear ratio provided by one or other drive gear to be achieved by reversal of the torque being handled by the gearbox (as happens, for example, when a vehicle goes through a transition from accelerative drive to coasting). There is no delay while drive engagement means take up or disengage drive.

Description:
BACKGROUND TO THE INVENTION 
   1. Field of the Invention 
   This invention relates to a gearbox. It has particular, but not exclusive, application for use in a high-performance motor vehicle such as a sports car or a racing car. 
   2. Summary of the Prior Art 
   A conventional manual automotive gearbox has one particular disadvantage when applied to a vehicle from which maximum performance is to be extracted. It is necessary to remove engine torque from the input to the gearbox when the gear ratio is to be changed, typically by interrupting drive through a friction clutch. This results in the acceleration of the vehicle being interrupted during the period for which the clutch is open. In a conventional gearbox, it is necessary to remove torque from immediately before a currently-selected gear is disengaged until a new gear is selected. 
   The most common arrangement in general automotive use mounts a gear onto a hub using a bearing or bush arrangement. The hub is joined to the gear shaft through a splined or similar coupling. Mounted on the hub is a sliding ring system which can slide on the hub to engage a gear in order to couple that gear to the hub for rotation, thus permitting drive to pass from the gear to the shaft. In some instances the hub may be integral with the gear shaft. The sliding ring system can be either a dog clutch ring or a synchronizer ring assembly; many different sizes and types are available. In a sequential gearbox, the sliding ring system is actuated by a selector fork, which in turn is actuated by the rotation of a gearchange barrel upon which is a cam profile. As the barrel is rotated the cam profile causes the correct selector fork to move at the correct time. 
   In operation of such a system, to effect a gearchange, one gear is de-selected, and then the subsequent gear selected. In order for the sliding ring system to engage and disengage with the gear the drive torque needs to be cut, this is typically done through the engine to gearbox clutch and/or an electronic engine cut. A cut in the engine torque for the required time to allow the gear to disengage results in the rate of vehicle acceleration being reduced. In certain applications, for example in motor sport, it is not desirable for the vehicle acceleration rate to reduce during a gear change. 
   SUMMARY OF THE INVENTION 
   An aim of this invention is to provide a gearbox that enables gear changes to be made with the minimum of reduction in the time period for which engine torque need be reduced. 
   From a first aspect, this invention provides a mainshaft assembly for a gearbox, the mainshaft assembly comprising:
         a mainshaft;   a first and a second drive gear, each carried for rotation about the mainshaft, each drive gear having a different number of teeth;   a first and a second hub, each hub being associated with a respective drive gear, each hub having engagement means operable to selectively couple with the drive gear causing it to rotate with the hub or uncouple from the drive gear to allow the drive gear to rotate with respect to the hub;   respective drive connection means associated with each hub being operative to connect or disconnect the hub to the mainshaft for rotation with it or to allow rotation with respect to it;   wherein, upon connection of both first and second hub by their respective engagement means to each corresponding drive gear, the drive connection means operates to connect one of the hubs to the mainshaft when torque is applied to the mainshaft through the hub and to connect the other one of the hubs (typically exclusively) to the mainshaft when torque is applied to the hub through the mainshaft.       

   This enables selection of a gear ratio provided by one or other drive gear to be achieved by reversal of the torque being handled by the gearbox (as happens, for example, when a vehicle goes through a transition from accelerative drive to coasting). There is no delay while drive engagement means take up or disengage drive. 
   For example, when the gearbox is transmitting torque in a direction that corresponds to acceleration of a vehicle to which it is fitted, the connection means operates to connect the hub that is associated with the drive gear that has the higher drive ratio. Thus, when the vehicle is accelerating, engagement of a second hub with its drive gear effects an up-change, and an automatic down-change when the vehicle is coasting. 
   In typical embodiments, at least one hub is associated with two drive gears, the engagement means being operable to selectively couple with one or other drive gear or uncouple from both drive gears. This can be used to increase the number of ratios provided in a gearbox while keeping the number of components to a minimum. In such embodiments, drive gears that provide neighbouring ratios do not share a hub. The drive gears for successive speeds may be carried on successive hubs disposed along the mainshaft. For instance, in the case of a four-speed gearbox, ratios 1 and 3 are provided by drive gears associated with a first hub and ratios 2 and 4 are provided by drive gears associated with a second hub. 
   Suitably, each engagement means includes a dog clutch that can engage with or disengage from dogs on a drive gear. The dog clutch may be embodied by a dog ring that is carried on the hub. Typically, the dog ring is carried such that it can slide axially on the hub and is constrained to rotate with the hub, the dog ring being coupled or uncoupled by sliding upon the hub. For example, the dog ring may be splined to the hub. 
   In preferred embodiments, the connection means includes connection elements movable between a deployed position in which they prevent relative movement between the hub and the mainshaft and a withdrawn position in which such relative movement is allowed. For example, the connection elements may be cylindrical metal rollers. In the withdrawn position, each connection element may be received within a respective recess in one of the mainshaft and the hub. In the deployed position, the connection elements may project from the recesses to engage with formations in the other of the mainshaft and the hub. The connection means typically further includes a control component to move the connection elements between their withdrawn and their deployed position. The control component may comprise a hollow cylindrical cage that surrounds the mainshaft and which extends in an annular space between the hubs and the mainshaft. The cage may include slots within which the connection elements are located. The connection elements may be moved between their withdrawn and their deployed position by rotation of the cage with respect to the mainshaft. Rotation of the cage in a first direction may be effected by relative movement between the cage and the mainshaft caused by the connection elements being rotated by the hub and rotation of the cage in the opposite direction is effective by relative movement between the cage and a hub. In alternative embodiments, the control component may be a ring-shaped component having lobes between which the connection elements are retained. Typically, the lobes have a greater circumferential extent around the ring than the connection elements. Typically, in such embodiments, a limited amount of relative rotational movement is provided between the ring-shaped components. 
   From a second aspect, this invention provides a gearbox that includes a mainshaft assembly that embodies the first aspect of the invention. 
   In such a gearbox, each of the drive gears of the mainshaft assembly is in mesh with a respective laygear. Typically, the laygears are constrained to rotate together. 
   A gearbox embodying this aspect of the invention typically includes a selector assembly operable to engage or disengage the engagement means. The selector assembly typically operates sequentially. This is the preferred form of selector for use in competition. 
   A typical configuration of the gearbox uses the layshaft as its input and the mainshaft as its output. However, in alternative embodiments, the opposite arrangement may be adopted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the main internal components of a gearbox being a first embodiment of the invention; 
       FIG. 2  is an end view of the main internal components of the gearbox of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the main internal components of the gearbox of  FIG. 1 ; 
       FIG. 4  is an exploded view of the mainshaft assembly of the embodiment of  FIG. 1 ; 
       FIG. 5  is a cross-sectional view of a mainshaft assembly of the embodiment of  FIG. 1  with no gear selected (i.e., in neutral); 
       FIG. 6  is an end view of the mainshaft assembly of the embodiment of  FIG. 1 ; 
       FIGS. 7 to 13  represent a sequence of operation of a mainshaft assembly embodying the invention; 
       FIG. 14  is an exploded view of a mainshaft assembly of a second embodiment of the invention; 
       FIG. 15  is a cross-sectional view of a mainshaft assembly of the second embodiment with no gear selected (i.e., in neutral); 
       FIG. 16  is a detail of Section B-B from  FIG. 14  that shows modified components of this embodiment; 
       FIGS. 17   a  and  17   b  show a roller cage, being a component of the second embodiment of the invention; 
       FIG. 18  shows a mainshaft inner track, being a component of the second embodiment of the invention; and 
       FIGS. 19 and 20  are views of Section B-B from  FIG. 14  that show a sequence of operation of the embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings. 
   Each embodiment described is a four-speed gearbox intended for competition use. However, it will be seen that the principles of its construction could be extended in a straightforward manner to a gearbox having a larger number of speeds or applications. The embodiment described may also provide four forward speeds in a gearbox in which further forward speeds are provided using conventional means. 
   With reference first to  FIGS. 1 to 3 , the gearbox comprises two principal shaft assemblies—a mainshaft assembly  10  and a layshaft assembly  12 —and a selector assembly  14 . Drive from the engine passes through the clutch and enters the gearbox to drive the layshaft assembly  10 . The output of the gearbox is taken from the mainshaft assembly  10 . Ratio selection is performed on the mainshaft and is controlled by the selector assembly  14 . 
   The layshaft assembly  12  comprises four differently-sized spur gears  20 ,  22 ,  24 ,  26  carried upon a rotatable shaft  80  for rotation about an axis within the gearbox. The spur gears  20 ,  22 ,  24 ,  26  and the layshaft are coupled by splines so as they rotate together; that is to say, relative rotation between the spur gears is prevented. 
   The mainshaft assembly  10  has four spur gears  30 ,  32 ,  34 ,  36  each of which is in mesh with a respective spur laygear of the layshaft assembly  12 . The sizes of the spur gears  30 ,  32 ,  34 ,  36  are such that they are arranged along a straight axis. The mainshaft assembly is central to the operation of this embodiment, so it will now be described in detail with reference to  FIGS. 4 and 5 . 
   The four spur gears  30 ,  32 ,  34 ,  36  of the mainshaft gears provide 1 st  to 4 th  speeds, the four speeds being each incrementally higher in ratio than the previous gear, i.e., 4 th  is higher than 3 rd , and so forth. The gears are not arranged in ratio-order as is common in most gearboxes. Rather, they are arranged such that a first adjacent pair of gears (referred to as the even-speed pair)  30 ,  32  provide 4 th  and 2 nd  gears respectively, while the second pair (referred to as the odd-speed pair)  34 ,  36  provide 3 rd  and 1 st  gears respectively. The requirement in general is that adjacent speeds should not share a hub so that a speed change can be effected by changing which one of two hubs is transmitting drive to the mainshaft. 
   Each gear  30 ,  32 ,  34 ,  36  is supported on a respective bearing  40 , which in turn is mounted on a cage  42  that extends under all four gears. The cage  42  is carried on a rotatable mainshaft  44 . (The bearings  40  could be bushes rather than rotating element bearings, as in this embodiment.) The cage  42  is carried directly on the mainshaft  44  such that it can rotate upon the mainshaft  44 . In alternative embodiments, bushes or bearings may be disposed between the cage  42  and the mainshaft  44 . 
   Mounted on the cage between the gears of the even-speed pair and the odd-speed pair, concentric to the mainshaft  44 , is a respective hub  50 ,  52 . Mounted on each hub  50 ,  52  is a respective dog ring  54 ,  56 , in this embodiment, connected through a spline drive. The number of splines can vary between embodiments; in this case there are six. Thus, the dog rings  54 ,  56  can slide axially with respective to the corresponding hub  52 ,  54 , but cannot rotate with respect to it. Each dog ring can slide between three operative positions: a central neutral position (shown in  FIG. 5 ) in which it is spaced axially from both of the corresponding spur gears, or displaced from the central position in one or other direction to a respective drive position to engage one or other spur gear. When in a drive position, the dog ring engages with dog formations on the corresponding spur gear to lock that gear to the hub upon which it is carried such that the hub and the gear rotate together. This sliding movement is effected by selector forks  70  of the selector assembly  14 . 
   Each hub  50 ,  52  has a series of internal axially-aligned grooves  58  and is mounted on the cage  42 . In this example, there are six such grooves, but other embodiments may have more or fewer. The cage  42  has a series of rectangular slots  60 , each slot  60  being disposed approximately radially inwardly from a respective one of the grooves  58 . Likewise, the mainshaft  44  has a series of grooves  62 , each being disposed approximately radially inwardly from a respective one of the slots  60 . Thus, a space is enclosed between the internal grooves  58  of the hubs, the slots  60  and the grooves  62  of the mainshaft  44  and located within each space is a respective cylindrical roller  64 . 
   The grooves  58  of the hubs  50 ,  52  are curved such that each roller  64  fits closely to the base of its groove  58 . The bases of the grooves  62  of the mainshaft  44  are also curved with a radius similar to that of the rollers  64 . However, each of the grooves  62  of the mainshaft  44  has sloping sidewalls upon which the rollers  64  can slide, thus allowing the rollers  64  a small amount of angular rotation around the mainshaft. The width of each slot  60  is slightly greater than the diameter of a roller  64 . When a roller is located in the base of its mainshaft groove  62 , its radially outermost extent does not project beyond the radially outer surface of the cage  42 . 
   The slots  60  in the cage  42  are sized to allow the rollers  64  to disengage from the hubs  50 ,  52  as required during operation. The cage  42  ensures that all of the rollers  64  under any one hub are aligned on the same side of the grooves  62  in the mainshaft  44  as required during operation of the system. The purpose of the rollers is to allow the hubs  50 ,  52  to be coupled to or uncoupled from the mainshaft  44 , whereby when coupled, a hub is caused to rotate with the mainshaft and when uncoupled can rotate with respect to it. The mechanism by which this occurs will now be described. 
   Consider first the state of the mainshaft assembly  10  as shown in  FIG. 7 . Both dog rings  54 ,  56  are in their central positions, so all of the spur gears  30 ,  32 ,  34 ,  36  are free to rotate with respect to the mainshaft  44 , and the cage  42  and rollers  60  are free. Thus, the gearbox is in neutral, no drive being transmitted from the layshaft assembly  12  to the mainshaft assembly  10 . 
   To engage 1 st  gear on drive the dog ring  56  of the odd-speed pair is moved across to its drive position with respect to the 1 st  spur gear  36 , as shown in  FIG. 8 . Section B-B through 1 st  gear in  FIG. 8  shows the drive torque direction where the torque is taken through the hub  52  and the rollers  60  into the mainshaft  44 . The cage  42  is rotated fully in the direction of the drive torque by the rollers  60 . 
   If the vehicle is accelerating, it is likely that the next gear to be required will be 2 nd , and the way in which this is achieved is shown in  FIG. 9 . As will be seen, 2 nd  spur gear  32  has now been engaged on drive by dog ring  54  of the even-speed pair. Note that 1 st  gear  36  is also still engaged by the dog ring  56  of the odd-speed pair. Section A-A through 2 nd  gear  32  shows that the drive torque is taken through the even-speed hub  52  and rollers  60  to the mainshaft  44 . The rollers  60  have rotated the cage  42  fully in the direction of the drive torque due to the drive torque. Section B-B through 1 st  gear shows that, due to the fact that the mainshaft  44  is now rotating faster than 1 st  gear (which is still engaged) the odd-speed hub  50  is now rotating slower than the even-speed hub  52 . As the cage  42  is forced fully in the direction of the drive torque by the even-speed hub  52  and rollers  60  there is now sufficient space for the 1 st  gear rollers  60  to be forced into the grooves in the mainshaft  44 . This allows the odd-speed hub  50  to rotate slower than even-speed hub  52  and therefore not transmit drive. Thus, 2 nd  gear has been engaged without the need to disengage 1 st  gear. 
   Once 2 nd  gear has been engaged, as described above, the dog ring  56  of the odd-speed pair is withdrawn from engagement with the 1 st  speed spur gear  36  to its central neutral position, as shown in  FIG. 10 . The even-speed hub dog ring  54  remains in engagement with the 2 nd  spur gear  32 . The 1 st  spur gear  36  is now free to rotate on its bearing  40  as it is no longer joined to the odd-speed hub  52  by dog ring  56 —this is essential because there is no relative rotational motion between the odd-speed hub  52  and mainshaft  44 . 
   This procedure can be repeated, using further components of the mainshaft assembly, to perform further changes to 3 rd  and 4 th  gears. 
   The sequence of operation to accomplish downchanges will now be described. Before a downchange, engine torque is reduced such that it is now imposing a drag on the vehicle —that is, the direction of torque being transmitted by the gearbox is reversed. 
     FIG. 11  shows the gearbox with 2 nd  gear engaged but no drive torque being applied, instead a coast torque is being applied to the gearbox, for example by the vehicle slowing. When compared to  FIG. 10 , section A-A through 2 nd  gear shows that the even-speed hub  52  has now moved rotationally in the opposite direction to the direction of rotation relative to the mainshaft  44 , and this has also rotated the rollers  60  in the same relative direction. The coast torque is transmitted from the mainshaft  44  through the rollers  60  into the even-speed hub  52  and thence to the even-speed dog ring  56  and the 2 nd  speed spur gear  32 . The change in torque direction has also forced the cage  42  to move to a coast position. 
   When the change to 1 st  gear is initiated, the components adopt the positions shown in  FIG. 12 . The odd-speed dog ring  54  has now also been engaged with the first speed spur gear  36  on coast. When compared to  FIG. 11 , the coast torque is now being taken by the slowest gear, the torque path being from 1 st  gear  36  through the odd-speed dog ring  54 , the odd-speed hub  50  through rollers  60  into the mainshaft  44 , as shown in section B-B. The rollers  60  are forced in the opposite direction to the direction of rotation by the odd-speed hub  50 . The cage  42  is already in the coast position because of the drive direction. The even-speed hub  52  is rotating faster than the odd-speed hub  50  and therefore forces the rollers  60  into the grooves  62  in mainshaft  44  which allows the hub to rotate with respect to the mainshaft  44 , as shown in section A-A. 
   The even-speed dog ring  56  is then disengaged from the 2 nd  speed spur gear  32  to its neutral position, meaning that only the 1 st  speed spur gear  36  is engaged to transmit torque in either coast or drive. This is shown in section B-B of  FIG. 13 . 
   The sequence can now restart (i.e., changing up from 1 st  gear to 2 nd  gear and so on), and then back down to 1 st  gear as required by the vehicle operation. 
   The selector assembly  14  is substantially conventional, using selector forks  70  slidably mounted on a selector shaft  76  to slide the dog rings  54 ,  56  on the hubs  50 ,  52 . The selector forks  70  are moved by a rotatable shaft  72  having cam grooves  74  that are tracked by cam followers on the selector forks  70 . Rotation of the selector shaft  76  in a first direction effects upchanges and rotation of the selector shaft in the opposite direction effects downchanges. Clearly, the cam grooves must be shaped to ensure that the dog rings are engaged and disengaged in the sequence required to effect the changes as described above. 
   In alternative embodiments, the alternate gears could also be mounted on different shafts and the shift completed between shafts using this cage concept. In place of rollers  60 , spherical balls could be used, the slots in the cage being round to accommodate them. The rollers  60  could be shapes in section other than round, for example elliptical, rectangular or square. The grooves in mainshaft  44  and hubs  50 ,  52  are formed to have an appropriate shape. 
   A second embodiment of the invention will now be described. This embodiment is a modification of the first embodiment, therefore only those of its features that have been modified will be described in full. 
   In this embodiment, cylindrical rollers  164  are axially shorter and of larger diameter than the functionally-equivalent cylindrical rollers  64  of the first embodiment. This provides additional space within the hubs  150 ,  152  for other components. One effect of this is that the dog rings  154 ,  156  are each carried on a respective hub  150 ,  152  by a respective pair of ball bearings  140  to provide good support for the dog rings  154 ,  156  and ensure that no radial loads from the roller drive system will result in misalignment between the dog engagement system of the dog ring and mainshaft gear. 
   As in the first embodiment, a respective set of rollers  164  is provided for each hub  150 ,  152 . However, in this embodiment, each set of rollers is disposed within a respective roller cage  142 ,  143 , which replaces the single cage  42  of the first embodiment. 
   Each roller cage  142 ,  143 , as shown in  FIG. 17 , is a generally ring-shaped one-piece component. It comprises a plurality of lobes  160  (same in number as one set of rollers  164 ) interconnected by comparatively thin elements. Mutually facing surfaces of adjacent lobes  160  are generally parallel to one another and to a radius of the cage, and are spaced apart by a distance that is a small amount greater than the diameter of the rollers  164 . Each roller cage  142 ,  143  surrounds the mainshaft  144  and can rotate about it concentrically with the axis of the mainshaft  144 . 
   Each of the rollers  164  is retained between a pair of lobes  160 , and is located within a groove  158  in the hub and a groove  162  in the mainshaft. Thus, the rollers  164  are located in a manner similar to the rollers  64  in the first embodiment. However, the roller cages constrain the rollers more closely in their correct positions than can the roller cage  60  of the first embodiment. 
   The grooves  162  within the main shaft  144  of this embodiment have a cross-sectional shape that can be seen most clearly in  FIG. 16 . A central part of each groove  162  is arcuate and has a radius that is substantially the same as that of the roller. Each groove has opposite radiused regions that are also of radius similar to that of the roller  164 . 
   This resists any tendency for the rollers  164  to become disengaged during transmission of high-torque loads. 
   Since the cage  60  is omitted from this embodiment, the adjacent hub assemblies are interconnected by a separate tubular component referred to as the ratio inner track  194 , shown in  FIG. 18 . This ratio inner track  194  is located on the outside of the mainshaft  144  and is free to turn relative to the mainshaft assembly. The two adjacent mainshaft gears  132 ,  134  are each located on the outside of the ratio inner track  194  using respective needle roller bearings  140 . The ratio inner track  194  is indexed to each of the two adjacent roller cages  142 ,  143  by face dogs  190 ,  192  formed on axial end surfaces of each the ratio inner track  194  and the roller cages  142 ,  143 . The dogs on the roller cages are formed by faces of adjacent lobes  160 . This dog drive system allows a controlled amount of backlash between the two roller cages  142 ,  143  such that this embodiment can operate without loose circumferential control of the rollers present in the first embodiment. 
   Operation of the embodiment will now be described with reference to  FIGS. 19 and 20 . 
   In  FIG. 19 , 2 nd  gear has just been engaged, on an upchange from 1 st , using the dog ring  154 , causing torque to be applied to the even-speed hub  152 . This torque input will make the even-speed hub  152  rotate faster than the mainshaft  144 , so the roller cage  143  will be dragged forwards causing the drive rollers  164  to be locked into position between the external grooves  162  of the mainshaft  144  and the internal grooves  158  of the even speed hub. Once this occurs, the drive torque will be passed directly to the mainshaft  144  and the newly-selected gear will become operational. 
   Once 2 nd  gear has become engaged as described above, the mainshaft  144  speed will increase to match the 2 nd  gear input speed, so the mainshaft  144  will turn faster than the hub of the next-lower gear 1 st . The dog drive arrangement between the even-speed roller cage  143 , ratio inner track  194  and odd-speed roller cage  142  means that the odd-speed roller cage  142  will quickly increase in speed to match that of the mainshaft  144 . As the odd-speed hub  150  will be rotating slower than the mainshaft  144 , this will cause drag on the odd-speed roller cage  142 , but the limited backlash between the cages  142 ,  143  and the ratio inner track  194  will force the odd-speed hub  150  to fall only a small angle behind the rotation of the mainshaft  144 . This disposition is shown in  FIG. 20 . At this position relative to the mainshaft, the odd-speed roller cage  142  will prevent the drive rollers  164  from engaging on the overrun side of the mainshaft grooves  162 . The mainshaft  144  will therefore ‘freewheel’ inside the slower-rotating odd-speed hub  150 , and no torque will be transferred between the odd-speed hub  150  and the mainshaft  144 . 
   The gearshift from 1 st  to 2 nd  is subsequently completed by movement of the selector fork to disengage the dogs of the previously engaged gear (1 st ) from the adjacent dog ring. At this point, all components of the odd-speed hub assembly will be free to rotate at the same speed as the gearbox mainshaft  144 . 
   As with the first embodiment, the gear selection mechanism will also function in the overrun (deceleration) condition, but with all torque directions reversed. 
   The embodiments described offer four forward drive ratios. However, in practice, a gearbox that embodies the invention may have additional ratios. For example, a gearbox with six forward speeds and reverse may use the four-speed arrangements described above to provide forward gears  2  to  5 , with forward gears  1  and  6  and reverse being provided by conventional means. It is in these intermediate gears that greatest advantage can be gained through the increase in gear-change speed that the invention provides.