GB2426302A

GB2426302A - Gearbox

Info

Publication number: GB2426302A
Application number: GB0510421A
Authority: GB
Inventors: Adrian Moore; Paul Pomfret; Andrew Heard; Andrew Mcdougall
Original assignee: Xtrac Ltd
Current assignee: Xtrac Ltd
Priority date: 2005-05-21
Filing date: 2005-05-21
Publication date: 2006-11-22
Also published as: GB0510421D0; US20070034032A1; EP1736678B1; EP1736678A2; DE602006010480D1; US7428854B2; EP1736678A3

Abstract

A mainshaft assembly for a gearbox is disclosed. The mainshaft (44) comprises several gears (30, 32, 34, 36) all arranged coaxially on the mainshaft. The assembly comprises first and second hubs (50, 52), each being associated with a respective drive gear. The hubs having dog rings (54, 56) operable to selectively couple with the drive gear. A drive connection comprising a cage (42) and rollers (60) is associated with each hub and operative to connect or disconnect the hub to the mainshaft. Upon connection of first and second hub by their dog rings, the drive connection connects one of the hubs the mainshaft when torque is applied to the mainshaft in a first direction. When torque is applied in opposite direction the second hub engages the mainshaft. This enables selection of gear without delay during engagement and disengagement of drive gears.

Description

Gearbox 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.

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 deselected, 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.

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 mainshafi 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.

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 mainshafi. For instance, in the case of a four-speed gearbox, ratios I 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 andahub.

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.

An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the main internal components of a gearbox being a first embodiment of the invention; Figure 2 is an end view of the main internal components of the gearbox of Figure 1; Figure 3 is a cross-sectional view of the main internal components of the gearbox of Figure 1; Figure 4 is a cross-sectional view of a mainshaft assembly of the embodiment of Figure 1 with no gear selected (i.e., in neutral); Figure 5 is an exploded view of the mainshaft assembly of the embodiment of Figure 1; Figure 6 is an end view of the mainshaft assembly of the embodiment of Figure 1; and Figures 7 to 13 represent a sequence of operation of a mainshaft assembly embodying the invention.

The 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.

With reference first to Figures 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, 60 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 Figures 4 and 5.

The four spur gears 30, 32, 34, 36 of the mainshaft gears provide 1st to 4th speeds, the four speeds being each incrementally higher in ratio than the previous gear, i.e., 4th is higher than 3', 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 4th and 2' gears respectively, while the second pair (referred to as the odd-speed pair) 34, 36 provide 3rd and 1St 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 oddspeed 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 Figure 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 locks 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 Figure 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 15t gear on drive the dog ring 56 of the odd-speed pair is moved across to its drive position with respect to the 1St spur gear 36, as shown in Figure 8. Section B-B through 1st gear in Figure 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'', and the way in which this is achieved is shown in Figure 9. As will be seen, 2'' spur gear 32 has now been engaged on drive by dog ring 54 of the even-speed pair. Note that 1st gear 36 is also still engaged by the dog ring 56 of the odd- speed pair. Section A-A through 2nd 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 15t gear shows that, due to the fact that the mainshaft 44 is now rotating faster than 15t gear (which is still engaged) the odd-speed hub 50 can 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 1St gear rollers 60 to be forced into the grooves in the mainshafi 44. This allows the odd-speed hub 50 to rotate slower than even-speed hub 52 and therefore not transmit drive. Thus, 2" gear has been engaged without the need to disengage 15t gear.

Once 2'' gear has been engaged, as described above, the dog ring 56 of the odd-speed pair is withdrawn from engagement with the 1st speed spur gear 36 to its central neutral position, as shown in Figure 10. The even- speed hub dog ring 54 remains in engagement with the 2 spur gear 32. The 1st spur gear 36 is now free to rotate on its bearing 40 as is no longer joined to hub 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' and 4th 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.

Figure 11 shows the gearbox with 2' 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 Figure 10, section A- A through 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 to the and 2nd 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 lS gear is initiated, the components adopt the positions shown in Figure 12. The odd-speed dog ring 54 has now also been engaged with the first speed spur gear 36 on coast. When compared to Figure 11, the coast torque is now being taken by the slowest gear, the torque path being from 1st gear 36 through the odd-speed dog ring 54, the odd-speed hub 50 through rollers 60 into the mainshafI 44, as shown in section B-B. The rollers 60 are forced in the opposite direction of 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 odd-speed hub 52 is rotating faster than the odd-speed hub 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 speed spur gear 32 to its neutral position, meaning that only the lS speed spur gear 36 is engaged to transmit torque in either coast or drive. This is shown in section B-B of Figure 13.

The sequence can now restart (i.e., changing up from 1st gear to 2nd 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 - 10 example elliptical, rectangular or square. The grooves in mainshafi 44 and hubs 50, 52 are formed to have an appropriate shape.

Claims

Claims 1. 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 to the mainshaft when torque is applied to the hub through the mainshaft.
2. A mainshaft assembly according to claim 1 in which, 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 on drive and lower drive ratio on coast.
3. A mainshaft assembly according to claim I or claim 2 in which 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.
4. A mainshaft assembly according to claim 3 in which drive gears that provide neighbouring ratios do not share a hub.
5. A mainshaft assembly according to claim 4 in which the drive gears for successive speeds may be carried on successive hubs disposed along the mainshaft.
6. A mainshaft assembly according to any preceding claim in which each engagement means includes a dog clutch that can engage with or disengage from dogsonadrivegear.
7. A mainshaft assembly according to claim 6 in which the dog clutch is embodied by a dog ring that is carried on the hub.
8. A mainshaft assembly according to claim 7 in which the dog ring is carried such that it can slide axially on the hub and is consfrained to rotate with the hub, the dog ring being coupled or uncoupled by sliding upon the hub.
9. A mainshaft assembly according to claim 8 in which the dog ring is splined to the hub.
10. A mainshaft assembly according to any preceding claim in which 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.
11. A mainshaft assembly according to claim 10 in which the connection elements are cylindrical metal rollers.
12. A mainshaft assembly according to claim 10 or claim 11 in which in the withdrawn position, each connection element may be received within a respective recess in one of the mainshaft and the hub.
13. A mainshaft assembly according to claim 12 in which, in the deployed position, the connection elements project from the recesses to engage with fonnations in the other of the mainshaft and the hub.
14. A mainshaft assembly according to any one of claims 10 to 13 in which the connection means further includes a control component to move the connection elements between their withdrawn and their deployed position.
15. A mainshaft assembly according to claim 14 in which the control component comprises a hollow cylindrical cage that surrounds the mainshaft and which extends between the hubs and the mainshaft.
16. A mainshaft assembly according to claim 15 in which the cage includes slots within which the connection elements are located.
17. A mainshaft assembly according to claim 15 or claim 16 in which the connection elements are moved between their withdrawn and their deployed position by rotation of the cage with respect to the mainshaft.
18. A mainshaft assembly according to claim 17 in which rotation of the cage in a first direction is effected by relative movement between the cage and the mainshaft and rotation of the cage in the opposite direction is effective by relative movement between the cage and a hub.
19. A mainshaft assembly for a gearbox substantially as described herein with reference to the accompanying drawings.
20. A gearbox that includes a mainshaft assembly according to any preceding claim.
21. A gearbox according to claim 20 in which each of the drive gears of the mainshaft assembly is in mesh with a respective laygear.
22. A gearbox according to claim 21 in which the laygears are constrained to rotate together.
23. A gearbox according to any one of claims 20 to 22 including a selector assembly operable to engage or disengage the engagement means.
24. A gearbox according to claim 23 in which the selector assembly is a sequential selector.
25. A gearbox according to any one of claims 20 to 24 in which the layshaft is its input and the mainshaft as its output.
26. A gearbox substantially as described herein with reference to the accompanying drawings.