Abstract:
A mainshaft assembly for a gearbox includes a mainshaft ( 44 ) and drive gears ( 31, 36 ) carried for rotation about the mainshaft. First and second hubs ( 50, 50 ′) are associated with respective drive gears, each hub being operable to selectively couple or uncouple with the drive gear causing it to rotate with the hub or with respect to the hub. A drive connection mechanism ( 42, 64, 66 ) associated with each hub selectively connects the hub to the mainshaft. Upon connection of the hubs to the drive gears, the drive connection operates to connect one or other of the hubs to the mainshaft when torque is applied to the mainshaft in a first direction or an opposite direction. This enables a gear ratio to be selected by reversing the torque being handled by the gearbox.

Description:
FIELD OF 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. 
     BACKGROUND 
     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. 
     A gearbox that allows a driver to make a gear change without the requirement to remove drive torque was disclosed by the present applicant in EP-A-1 736 678. Such a gearbox allows a driver to perform gear changes without interrupting drive power by arranging for drive hubs to be selectively connected to and disconnected from a drive gear in accordance with the rotational direction of torque between the hub and the gear: in other words, in accordance with whether the gear is tending to drive the hub or the gear is overrunning the hub. 
     SUMMARY 
     An aim of this invention is to improve the operation of the gearbox disclosed in EP-A-1 736 678. 
     To this end, from a first aspect, this invention provides a mainshaft for a gearbox assembly, 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 the hub to the mainshaft for rotation with it or to allow rotation with respect to it, the connection means including connection elements having 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;   in which, 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 to the mainshaft when torque is applied to the hub through the mainshaft; wherein   the drive connection means includes control means which, in an engaged condition, causes the connection elements to adopt their deployed position, and in a disengaged condition, allows the connection elements to be moved against a biasing force to their withdrawn position.       

     The presence of the biasing force ensures rapid engagement of the drive connection means when they are required to transmit drive. 
     Each connection element is typically a generally cylindrical pawl. 
     In a typical embodiment, in the withdrawn position, each connection element can be received within a respective recess in one of the mainshaft and the hub. 
     At least part of the biasing force may be provided by one or more springs within the hub. For example, each spring may be generally C-shaped and extend circumferentially partly around the mainshaft. Preferably, each spring is prevented from rotating about the mainshaft by one of the connection elements. Alternatively or additionally, at least part of biasing force may be provided by a helper assembly located within the mainshaft. The helper assembly may comprise a resiliently biased plunger that acts upon a pawl. 
     The control means may comprise a hollow cylindrical cage that surrounds the mainshaft and which extends between the hubs and the mainshaft. The cage typically includes slots within each of which a connection element is located. The cage may be formed of two coaxial components interconnected such that limited backlash movement can take place between them. In such embodiments, the slots may have a circumferential extent that is marginally greater than the diameter of the connection elements. Alternatively, the cage may formed of two coaxial components interconnected such that minimal backlash movement can take place between them or of a single coaxial component. In such embodiments, the slots may have a circumferential extent that is greater than the diameter of the connection elements to allow backlash movement between the cage and the connection elements. 
     Each engagement means typically includes a dog clutch that can engage with or disengage from dogs on a drive gear. 
     From a second embodiment, the invention provides a gearbox that includes a mainshaft assembly embodying the first aspect of the invention. 
     In such a gearbox, the drive gears of the mainshaft assembly may be in mesh with a respective laygear. The laygears are typically constrained to rotate together on a layshaft. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: 
         FIG. 1  is a longitudinal cross-section of layshaft and mainshaft assemblies of a six-speed gearbox being a first embodiment of the invention; 
         FIG. 2  is an exploded view of the mainshaft assembly shown in  FIG. 1 ; 
         FIG. 3  is an exploded view of a hub assembly shown in  FIG. 2 ; 
         FIG. 4  is a section on B-B of  FIG. 1 , showing the odd-speed hub of the first embodiment in a drive condition; 
         FIG. 5  is a section on C-C of  FIG. 1 , showing the even-speed hub of the first embodiment in a ratchet condition; 
         FIGS. 6 and 7  show cage assemblies being two alternative components of the embodiment of  FIG. 1 ; 
         FIG. 8  shows a helper assembly for use in embodiments of the invention; and 
         FIG. 9  shows the helper assembly of  FIG. 8  installed in a mainshaft. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments described are five-speed or six-speed gearboxes 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 smaller or larger number of speeds and different applications. The embodiments described may also provide some 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 . Drive from the engine passes through the clutch and enters the gearbox to drive the layshaft assembly  12 . The output of the gearbox is taken from the mainshaft assembly  10 . Ratio selection is performed on the mainshaft and is controlled by a selector assembly. 
     The layshaft assembly  12  comprises six differently-sized forward-speed spur laygears  21  . . .  26  and a reverse spur laygear  27  carried upon a rotatable shaft  80  for rotation about an axis within the gearbox. The spur gears  21  . . .  27  and the layshaft  80  are coupled by splines so that they rotate together; that is to say, relative rotation between the spur gears is prevented. 
     The mainshaft assembly  10  has six forward-speed spur gears  31  . . .  36  each of which is in mesh with a respective spur laygear  21  . . .  26  of the layshaft assembly  12 , and a reverse-speed spur gear  37  that can be brought into mesh with the reverse spur laygear  27  through an idler gear (not shown) when reverse gear is selected. The sizes of the spur gears  31  . . .  37  are such that they are arranged along the straight axis of a mainshaft  44  that is parallel to the axis of the layshaft  80 . The mainshaft assembly is central to the operation of this embodiment, so it will now be described in detail with reference to  FIGS. 2 and 3 . 
     The six forward-speed spur gears  31  . . .  36  of the mainshaft assembly provide 1 st  to 6 th  speeds, the six speeds being each incrementally higher in ratio than the previous gear, i.e., 6 th  is higher than 5 th , 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  31 ,  36  provide 1 st  and 6 th  gears, a second adjacent pair of gears (referred to as the even-speed pair)  32 ,  34  provide 4 th  and 2 nd  gears respectively, while a third pair (referred to as the odd-speed pair)  33 ,  35  provide 3 rd  and 5 th  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. 
     Note that, in this embodiment, the pair of gears  31 ,  36  that provide the 1 st  and 6 th  speeds are connected to the mainshaft  44  using a conventional selector hub  38 , and do not make use of the change system provided by this invention; nor does the reverse gear  37 . Therefore, the following description will describe the operation of the odd-speed pair and the even-speed pair, which do benefit from the arrangement of the present invention. 
     Each spur gear  32  . . .  35  is supported on a respective bearing  40 ,  40 ′ which in turn is mounted on a two-component cage  42 ,  42 ′ that extends under all four gears. The cage  42  is carried on the rotatable mainshaft  44 . (The bearings  40 ,  40 ′ could be bushes rather than rotating element bearings, as in this embodiment.) The components  42 ,  42 ′ of the cage are carried on bearings  46  mounted on the mainshaft  44  such that they can rotate upon the mainshaft  44 . In alternative embodiments, bushes might replace the bearings, or the cage may be carried directly upon the mainshaft  44 . 
     Mounted on each component  42 ,  42 ′ of the cage between the gears of the odd-speed pair and the even-speed pair, concentric to the mainshaft  44 , is a respective hub  50 ,  50 ′. Mounted on each hub  50 ,  50 ′ is a respective dog ring  54 ,  54 ′ which, in this embodiment, is connected to the hub through a spline drive. Thus, the dog rings  54 ,  54 ′ can slide axially with respective to the corresponding hub  50 ,  50 ′, but cannot rotate with respect to it. Each dog ring  54 ,  54 ′ can slide between three operative positions: a central neutral position 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  54 ,  54 ′ engages with dog formations on the corresponding spur gear to lock that gear to the corresponding hub  50 ,  50 ′ upon which it is carried such that the hub and the gear rotate together. This sliding movement is effected by selector forks of the selector assembly. 
     Each hub  50 ,  50 ′ has a series of internal axially-aligned grooves  58  and is mounted on a respective component of the cage  42 ,  42 ′. In this example, there are five such grooves, but other embodiments may have more or fewer. Each cage component  42 ,  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  50 ,  50 ′, the slots  60  and the grooves  62  of the mainshaft  44  and located within each space is a respective pawl  64 ,  66  of generally cylindrical outer shape. 
     The cage may be in several configurations, two of which are shown in  FIGS. 6 and 7  respectively. Proximal end faces of the components  42 ,  42 ′ of the cage have castellated formations at  74  that couple them together. In the first arrangement, shown in  FIG. 6 , the slots  60  have a width just greater than the diameter of the pawls  64 ,  66 . Thus, the pawls  64 ,  66  are constrained to rotate about the mainshaft  44  along with the cage component  42 ,  42 ′ in which they are located. The castellated formations  74  are configured to allow some limited rotational backlash movement between the components  42 ,  42 ′ of the cage. In the second arrangement, shown in  FIG. 7 , the slots  60  have a width somewhat greater than the diameter of the pawls  64 ,  66 . Thus, the pawls  64 ,  66  are caused to rotate about the mainshaft  44  along with the cage component  42 ,  42 ′ in which they are located, with some backlash as the pawls  64 ,  66  move from one side of their slots  60  to the other. The castellated formations  74  are configured to minimise rotational backlash movement between the components  42 ,  42 ′ of the cage. 
     The grooves  58  of the hubs  50 ,  50 ′ are curved such that each pawl  64 ,  66  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 pawls  64 ,  66 . However, each of the grooves  62  of the mainshaft  44  has sloping sidewalls upon which the pawls  64 ,  66  can slide, thus allowing the pawls  64 , 66  a small amount of angular rotation around the mainshaft. The width of each slot  60  is slightly greater than the diameter of a pawl  64 ,  66 . When a pawl is located in the base of its mainshaft groove  62 , its radially outermost extent does not project beyond the radially outer surface of the components  42 ,  42 ′ of the cage. 
     Each pawl  64 ,  66  is generally cylindrical, and has a transverse slot  90 , 92  formed in it. Two of the pawls  66  have a slot  90  that is of width approximately a quarter of the length of the pawl  66 , and offset such that one transverse edge of the slot  90  is approximately at the mid-point of the length of the pawl  66 . These are referred to as “anti-rotation pawls”. The remaining pawls  64  have a slot  92  that is of width approximately a half of the length of the pawl  64 , and is substantially mid-way along of the length of the pawl  64 . These are referred to as “locking pawls”. When assembled, the anti-rotation pawls  66  are separated from one another by at least one locking pawl  64 , and the slots  90  of the anti-rotation pawls are offset in axially opposite directions along the mainshaft  44 . 
     A pair of C-shaped springs  68  surrounds the mainshaft  44 . The natural diameter of the springs  68  is somewhat larger than that of the mainshaft  44 . When compressed to closely surround the mainshaft  44 , the free ends of the springs  68  are spaced from one another by a distance greater than the diameter of the pawls  64 ,  66 . Both springs  68  pass within the slots  92  of all of the locking pawls  64 . However, the slots  90  of the anti-rotation pawls  66  are wide enough to receive just one spring  68 . Thus, each spring  68  passes through the slot  90  of one anti-rotation pawl  66 , and its free ends are disposed on opposite sides of the other anti-rotation pawl  66 . This prevents rotation of the springs  68  about the mainshaft  44 . 
     The slots  60  in the components  42 ,  42 ′ of the cage are sized to allow the pawls  64 , 66  to disengage from the hubs  50 ,  50 ′ as required during operation. The cage ensures that all of the pawls  64  under any one hub  50 ,  50 ′ 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 pawls  64 ,  66  is to allow the hubs  50 ,  50 ′ to be coupled to or uncoupled from the mainshaft  44 , whereby when coupled, a hub  50 ,  50 ′ is caused to rotate with the mainshaft  44  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. 1 . Both dog rings  54 ,  54 ′ are in their central positions, so all of the spur gears  31  . . .  36  are free to rotate with respect to the mainshaft  44 , and the cage components  42 ,  42 ′ and pawls  64 ,  66  are free. Thus, the gearbox is in neutral, no drive being transmitted from the layshaft assembly  12  to the mainshaft assembly  10 . 
     When driving in 4 th  gear, the dog ring  54 ′ of the even-speed pair is located in its drive position with respect to the 4th spur gear  34 . Section B-B through 4th gear in  FIG. 4  shows the drive where the torque is taken through the even-speed hub  50 ′ and the pawls  64 ,  66  into the mainshaft  44 . The cage  42  is rotated fully in the direction of the drive torque by the pawls  64 ,  66 . 
     If the vehicle is accelerating, it may be necessary to change up a gear, for example from 3 rd  gear to 4 th  gear. Initially, the 3 rd  gear  33  will be engaged through the odd-speed hub  50 . To perform the gearchange, the 4 th  spur gear  34  is engaged on drive by dog ring  54 ′ of the even-speed pair. Note that 3 rd  gear  36  is also still engaged by the dog ring  54  of the odd-speed pair. Section B-B through the even speed hub  50 ′ shows that as the 4 th  gear is engaged the drive torque is taken through the even-speed hub  50 ′ and pawls  64 ,  66  to the mainshaft  44 . The pawls  64 ,  66  have rotated the cage  42 ,  42 ′ fully in the direction of the drive torque due to the drive torque. Section C-C in  FIG. 5  through the odd-speed hub  50  shows that, due to the fact that the mainshaft  44  is now rotating faster than 3 rd  gear (which is still engaged) the odd-speed hub  50  is now rotating slower than the even-speed hub  50 ′. As the cage  42  is forced fully in the direction of the drive torque by the even-speed hub  50 ′ and pawls  64 ,  66  there is now sufficient space for the 3 rd  gear pawls  64 ,  66  to be forced into the grooves in the mainshaft  44  against the action of the springs  68 . This allows the odd-speed hub  50  to rotate slower than even-speed hub  50 ′ and therefore not transmit drive. Thus, 4 th  gear has been engaged without the need to disengage 3 rd  gear. 
     Once 4 th  gear has been engaged, as described above, the dog ring  54  of the odd-speed pair is withdrawn from engagement with the 3 rd  forward-speed spur gear  33  to its central neutral position. The even-speed hub dog ring  54 ′ remains in engagement with the 4 th  spur gear  34 . The 3 rd  spur gear  33  is now free to rotate on its bearing  40  as it is no longer joined to the odd-speed hub  50  by dog ring  54 —this is essential because there is no relative rotational motion between the odd-speed hub  52  and mainshaft  44 . 
     The sequence of operation to accomplish downchanges, with the example being from 4 th  to 3 rd  speeds, 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. Once drive torque is removed, the even-speed hub will move rotationally in the opposite direction to the direction of rotation relative to the mainshaft  44 , and this will also rotate the pawls  64 ,  66  in the same relative direction. The pawls  64 ,  66  are urged radially outwardly by the springs  68 , ready to transmit drive. The coast torque is transmitted from the mainshaft  44  through the pawls  64 ,  66  into the even-speed hub  50 ′ and thence to the even-speed dog ring  54 ′ and the 4 th  speed spur gear  34 . The change in torque direction also forces the cage  42  to move to a coast position. The coast torque is now being taken by the slower gear, the torque path being from 3 rd  gear  33 , through the odd-speed dog ring  54 , the odd-speed hub  50  and the pawls  64 ,  66  into the mainshaft  44 . The pawls  64 ,  66  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  50 ′ is rotating faster than the odd-speed hub  50  and therefore forces the pawls  64 ,  66  into the grooves  62  in mainshaft  44  against the action of the springs  68  which allows the hub to rotate with respect to the mainshaft  44 , as shown in section C-C. 
     A helper assembly  70  that can be incorporated into the mainshaft  44  is shown in  FIGS. 8 and 9 . This helper assembly  70  serves to apply a radial force to the pawls  64 ,  66  additional to that of the springs  68 . 
     The helper assembly  70  comprises a tubular casing  72  that has an axial through bore and that is symmetrical about a centre plane normal to the axis. The bore has a central region of reduced diameter, and its end portions are internally threaded. The casing has an external diameter that is a close fit within the mainshaft  44 . A ball-spring plunger  92  is located centrally in the casing  72 , projecting radially outwardly from it. The ball-spring plunger cooperates with an internal groove of the mainshaft  44  to resist axial movement of the helper assembly  70  within the bore of the mainshaft  44 . 
     At positions that are radially inward of the pawls  64 , 66  when the helper assembly is installed for use, radial apertures  76  are formed through the casing  72  to communicate with the bore. A respective plunger  80  is located within each radial aperture  76 . Each plunger  80  has a tapered portion that is directed into the bore of the mainshaft  44 . 
     Each plunger  80  is acted upon by two expander elements  84  that are located to slide within the bore of the casing  72 . Each of the two expander elements  84  has a chamfered axial face that face one another and which bear upon the tapered portions of the plungers  80 . Each expander element  84  is acted upon by a pair of coaxial helical springs or a series of Belleville compression springs  86 ,  88 , whereby the two expander elements  84  that are associated with each plunger  80  are urged towards one another. The compression springs  86 ,  88  that are associated with the expander elements  84  closest to the centre plane of the casing  72  bear upon the central region of reduced diameter. The compression springs  86 ,  88  that are associated with the expander elements  84  furthest from the centre plane of the casing  72  are retained by plugs  90  secured within the threaded end portions of the bore of the casing  72 . Alternatively, the bore might have a groove within which a circlip can be located to retain the springs. The action of the expander elements  84  acting on the plungers  80  causes the plungers  80  to be urged radially outwardly from the housing to bear upon the pawls  64 ,  66 , thereby urging the pawls radially outwardly to supplement the action of the springs  68 .