GB2461283A

GB2461283A - Dual clutch transmission

Info

Publication number: GB2461283A
Application number: GB0811666A
Authority: GB
Inventors: James William Griffith Turner
Original assignee: Lotus Cars Ltd
Current assignee: Lotus Cars Ltd
Priority date: 2008-06-25
Filing date: 2008-06-25
Publication date: 2009-12-30
Anticipated expiration: 2028-06-25
Also published as: GB0811666D0; GB2461283B; WO2009156744A1

Abstract

A dual clutch transmission comprises a first set of input gears 206, 207, 208, 209 each of which meshes with one of a first set output gears 220, 221, 222, 223 a second set of input gears 210, 211, 212, 213 each of which meshes with a respective one of a second set of output gears 224, 225, 226, 227. A first clutch 215 is associated with the input gears 206, 207, 208, 209 and a second clutch 216 is associated with the input gears 210, 211, 212, 213 and the output gears 210, 211, 212, 213. At least one of the input and output gears can freely rotate on a shaft and a gear ratio selection mechanism 231 selects a gear ratio by locking the gear to the shaft. In use either the first clutch 215 is engaged and the second clutch 216 disengaged and drive is transmitted via a selected pair of the first set of input and output gears or the second clutch 216 is engaged and the first clutch 215 disengaged and drive is transmitted via a selected pair of the second set of input and output gears.

Description

A TRANSMISSION UNIT

The present invention relates to a multi-speed transmission unit, e.g. for relaying drive from an internal combustion engine to driven wheels of an automobile.

Conventional transmission units, sometimes called gearboxes, have many parts and require face dogs, the use of which dictates that the units must have a certain size.

There is a need for a more compact design of transmission unit, with fewer parts.

The present invention in a first aspect provides a transmission unit as claimed in claim 1.

The present invention in a second aspect provides a transmission unit as claimed in claim 11.

The present invention in a third aspect provides a transmission unit as claimed in claim 14.

The present invention in a fourth aspect provides a transmission unit as claimed in claim 17.

The present invention provides in a fifth aspect a transmission unit as claimed in claim 20.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic drawing which illustrates a principle of operation of the present invention; Figure 2 shows a layout of a transmission unit of the present invention suited for a 2-wheel drive vehicle with driven front wheels; Figure 3 shows a layout of a transmission unit of the present invention suited for a 2-wheel drive vehicle with driven rear wheels; Figure 4 shows a layout of a transmission unit of the present invention with two output shafts; Figure 5 is shows a layout of a transmission unit of the present invention with two input shafts; and Figure 6 shows a layout of a further embodiment of a transmission unit of the present invention with two input shafts.

Turning first to figure 1, there can be seen eight gears 10-17 on an output shaft of a transmission unit. For simplicity the gears are shown identically sized, but in practice will be differently sized, as shown in figures 2 to 4. The gears 10-17 are all rotatably mounted on a slidable selector tube 18.

Each gear has a pair of annular bosses extending one each from the opposed faces of the gear, e.g. the gear 10 has two annular bosses lOa, lOb. Each annular boss provides an annular bearing surface, for instance the annular boss lOa provides an annular bearing surface lOc which abuts a facing annular bearing surface lic of an annular boss lib of gear 11. The abutting annular bearing surfaces provide for small rubbing surface areas between the gears.

The slidable selector tube 18 has two axially spaced apart integral teeth 19, 20 extending radially out therefrom. Each gear has a slot extending radially outward from a radially innermost portion adjacent the sleeve 18.

Slots lOd, ild, 12d, 13d, 14d, l5d, 16d and l7d are seen in figure 1. The tooth 19 can be slid into each of the slots lOd, ild, 12d and l3d to engage the sleeve 18 respectively with the gears 10, ii, 12 and 13. The tooth 20 can be slid into each of the slots l4d, 15d, 16d, l7d to engage the sleeve 18 respectively with the gears 14, 15, 16 and 17.

The position illustrated in figure 1 is a neutral position in which no gear is selected. Sliding the selector tube to the right initially engages the tooth 20 with the slot 14d to fix the gear 14 to the sleeve 18 and thereby select a first gear ratio; the tooth 19 remains in a gap between gears 10 and 11.

Sliding the selector tube 18 further to the right then slides the tooth 19 into engagement with the slot ild, while the tooth 20 remains in slot 14d; changing up from the first gear ratio to a second gear ratio can now be achieved by swapping drive from the gear 14 to the gear 11 using a twin clutch engagement (as will be described with reference to the later figures) . Whilst both gears 14 and 11 are engaged by the teeth 20 and 19 respectively, initially first gear is selected by relaying drive via a first clutch to gear 1 whilst a second clutch for gear 2 remains disengaged, then the clutch for gear 14 is disengaged and the clutch for gear 11 is engaged to relay via gear 11.

Sliding the selector tube 18 still further to the right slides the tooth 20 out of slot 14d, into a gap between gears 13 and 15, while the tooth 19 remains in slot lid.

This will be done shortly after changing the first gear ratio to a second gear ratio.

Further sliding the sleeve 18 to the right slides the tooth 20 into slot 15d while the tooth 19 remains in slot lid. Now it is possible to change up from the second gear ratio to a third gear ratio, by swapping between the two clutches; initially the clutch relaying drive to gear 11 will be disengaged and then the clutch relaying drive to gear 15 will be engaged.

Continuing to slide the selector tube 18 to the right, the tooth 19 is slid out of the slot lid, to occupy a gap between the egars 11 and i2, while the tooth 20 remains in the slot i5d, this occurring shortly after selection of the third gear ratio.

Sliding the selector tube further to the right slides the tooth 19 into the slot 12d while the tooth 20 remains in the slot 15d. Then selection of a fourth gear ratio is enabled, with gear ratio selection occurring by disengagement of the clutch which relays drive to the gear and engagement of the clutch which relays drive to the gear 12.

Sliding the selector tube 18 further to the right keep the tooth 19 in the slot i2d while the tooth 20 is slid out of slot 15d to the gap between gears 15 and 16; this will happen shortly after selection of the fourth gear ratio.

Further sliding the selector tube 18 to the right will slide the tooth 20 into the slot 16d while the tooth 19 remains engaged in the slot 12d. This then will permit the selection of a fifth gear ratio by the disengagement of the clutch associated with gear 12 and the engagement of the clutch associated with gear 16.

Continued sliding of the selection tube to the right will slide the tooth 19 out of slot 12d into the gap between the gear 12 and 13; this will usually happen shortly after the selection of the fifth gear ration.

Additional sliding of the selection tube to the right will engage the tooth 19 in slot 13d while the tooth 20 remains engaged to slot 16d. Then selection of a sixth gear ration is then possible by disengagement of the clutch associated with gear 16 and engagement of the clutch associated with the gear 13.

Further sliding of the selection tube to the right slides the tooth 20 out of the slot 16d into the gap between gears 16 and 17, while the tooth 19 remains engaged in slot 13.

Sliding the selection tube further right engages the tooth 20 in a slot 17d while the tooth 19 remains engaged in slot 13d. This enables selection of a seventh gear ration by disengagement of the clutch associated with the gear 13 and engagement of the clutch associated with the gear 17.

Sliding the selector tube 18 to its extreme right position will slide the tooth 19 out of engagement with the slot 13d into the gap between gears 13 and 14, while retaining the tooth 20 in slot 17d; this will be done shortly after the seventh gear ratio has been selected.

To change down gear ratios, the above sequence is reversed, with the selector rod 18 slid to the left.

Sliding the selector rod 18 to the left of its figure 1 position engages the tooth 19 into the slot lOd to select a reverse transmission ratio (with the selector tube 18 subsequently slid back to the right to disengage reverse gear) The gears 10, 11, 12 and 13 are axially separated from the gears 14, 15, 16, 17 by a central bearing panel 21 which has a roller bearing 22 engaging a central part of the sleeve 13 and rotatably supporting the sleeve 13.

The transmission ratio selection is sequential in nature, with the sleeve 13 moving progressively to the right from the figure 1 neutral position to sequentially select the first to seventh transmission ratios.

The gears 10-17 remain in abutment with each other along the bearing surfaces provided by their bosses and do not slide relative to the sleeve 13, therefore the length of the casing of the transmission unit does not have to accommodate both the gear widths and the widths of the gaps between them and is more compact than a unit with face dogs.

Also there is no need for an arrangement to slide the gears relative to each other and this allows the transmission unit to have a smaller part count than conventional transmission units. The switch between adjacent gear rates is achieved with two gears simultaneously engaged by the teeth 19, 20 and the switching effected by disengaging one clutch and engaging another; this will now be further explained below with reference to Figures 2 to 6.

A practical implementation of the principal illustrated in figure 1 is shown in figure 2, which shows a single-shaft rear-wheel drive transmission unit 200 having a clutch mechanism 214 receiving drive from a prime mover. This has a pair of coaxial input shafts 201, 202, the outer input shaft 201 comprising a sleeve surrounding the inner input shaft 202 and extending along a part of its length. The outer input shaft 201 is directly mounted in a pair of bearings 203, 204. The inner input shaft 202 is mounted directly in a bearing 205 and indirectly (via the outer input shaft 201) in the bearings 203, 204. A first set of four input gears 206, 207, 208 and 209 are fixedly attached to the outer input shaft 201 for rotation therewith. A second set of four input gears 210, 211, 212 and 213 are fixedly attached to the inner input shaft 202 for rotation therewith.

Although not shown, each gear will abut its neighbouring gear(s) and will be provided with a boss or bosses which define the abutment surface (as described above) . The twin clutch arrangement 214 is provided for the input shafts 201, 202 with the outer input shaft 201 connected to a first clutch plate 215 for rotation therewith and the inner input shaft 202 connected to a second separate clutch plate 216.

The first set of input gears 206, 207, 208, 209 are in constant mesh respectively with output gears 220, 221, 222, 223 of a first set of output gears and the second set of input gears 210, 211, 212, 213 are in constant mesh respectively with output gears 224, 225, 226, 227 of a second set of output gears. The output gears 220-227 are all mounted around an output shaft 230 and a selector tube 231 is interposed between them and the output shaft 230. Each output gear 220-227 will abut at least one of the output gears 220-227 and each will be provided with at least one boss to provide a bearing surface along which the abutment occurs. The selector tube 231 has two external teeth and functions as the sleeve 18 of figure 1. The selector tube 231 is slidable along its axis as indicated by the arrow in figure 3 to sequentially engage the gears 220-227 and interlock them with to the output shaft 230 so that the shaft rotates with the selected gear. The output shaft 230 extends rearwardly of the transmission unit to relay drive to rear wheels of the vehicle.

The figure 2 transmission unit works by operation of the sliding sleeve 231 and the double clutch 214. To select reverse, the sleeve 231 locks the gear 220 to the output shaft 230 and the clutch 214 engages the plate 215 so that drive is relayed from the prime mover via the clutch 214 to the input shaft 201 and then via the gear 206 and gear 220 to the output shaft 230. A reversing idler gear 240 is interposed between input gears 206 and output gear 220.

To select the first gear ratio the selector tube 231 is slid to disengage the gear 220 and to lock the gear 224 to the output shaft 230 and the clutch 214 disengages clutch plate 215 and engages the clutch plate 216 to relay drive to the input shaft 202.

To select the second gear ratio the selector tube 231 is slid to lock the gear 221 to the output shaft 230 whilst the gear 224 is still also locked to the output shaft 230.

Then the clutch 214 disengages clutch plate 216 and engages the clutch plate 215 to relay drive to the input shaft 201.

After selection of the second gear ratio the selector tube 231 can be slid to disengage the gear 224, to leave only gear 221 engaged.

To select the third gear ratio the selector tube 231 is slid to interlock the gear 225 with the output shaft 230 whilst the gear 221 remains also locked to the shaft 230.

Then the clutch 214 disengages the clutch plate 215 and engages the clutch plate 216 to relay drive to the input shaft 202. After selection of the third gear ratio the selector tube 231 can be slid to unlock gear 221 to leave only gear 225 interlocked.

To select the fourth gear ratio the selector tube 231 is slid to interlock the gear 222 with the output shaft 230 whilst the gear 225 remains also locked to the shaft 230.

Then the clutch 214 disengages the clutch plate 216 and engages the clutch plate 215 to relay drive to the input shaft 201. After selection of the fourth gear ratio the selector tube 231 can be slid to disengage the gear 225, to leave only gear 222 interlocked.

To select the fifth gear ratio the selector tube 231 is slid to interlock the gear 226 with the output shaft 230 whilst the gear 222 remains interlocked with shaft 230.

Then the clutch 214 disengages the clutch plate 215 and engages the clutch plate 216 to relay drive to the input shaft 202. After selection of the fifth gear ratio the selection tube 231 can be slid to disengage the gear 222, to leave only gear 226 interlocked. -10-

To select the sixth gear ratio the selector tube 231 is slid to interlock the gear 223 with the output shaft 230 whilst the gear 226 remains interlocked with the shaft 230.

Then the clutch 214 disengages the clutch plate 216 and engages the clutch plate 215 to relay drive to the input shaft 201. Once the sixth gear ratio has been selected then the selection tube 231 can be slid to disengage the gear 226, to leave only gear 223 interlocked.

To select the seventh gear ratio the selector tube 231 is slid to lock the gear 227 to the output shaft 230 whilst the gear 223 remains interlocked to the shaft 230. Then the clutch 214 disengages the clutch plate 215 and engages the clutch plate 216 to relay drive to the input shaft 202.

After selection of the seventh gear ratio the selection tube 231 can be slid to disengage the gear 223, to leave only gear 227 interlocked.

For each gear change there is a swap from engagement of one clutch plate to the other and thereby a swap from one of the first set of input gears 206-209 to one of the second set of input gears 210-213. The selector tube 231 is slid sequentially in one axial direction to effect the gear ratio change. Between gear changes the selection tube can be slid to a position in which only one gear is interlocked with the selection tube 230. During gear changes two gears are interlocked with the selection tube 230. Described above are the upward gear ratio changes -the downward gear ratio changes are the same in reverse.

A second practical implementation of the principal illustrated in figure 1 is shown in figure 3, which shows a single-shaft front-wheel drive transmission unit 300. This has a pair of coaxial input shafts 301, 302, the outer input shaft 301 comprising a sleeve surrounding the inner shaft 302. The outer input shaft 301 is directly mounted in a pair of bearings 303, 304. The inner input shaft 302 is mounted at the end thereof directly in a bearing 305 and indirectly (via the outer input shaft 301) in the bearings 303, 304. A first set of four input gears 306, 307, 308 and 309 are fixedly attached to the sleeve 301 for rotation therewith.

A second set of four input gears 310, 311, 312 and 313 are fixedly attached to the shaft 302 for rotation therewith.

Each input gear will abut at least one other input gear and each input gear is provided with at least one hub which provides an abutment surface, as described above in relation to figure 1. A twin clutch arrangement 314 is provided for the input shafts 301, 302. The outer sleeve shaft 301 is connected to a first clutch plate 315 for rotation therewith and the inner shaft 302 is connected to a second separate clutch plate 316.

The first set of input gears 306, 307, 308, 309 are in constant mesh respectively with output gears 320, 321, 322, 323 of a first set of output gears and the second set of input gears 310, 311, 312, 313 are in constant mesh respectively with output gears 324, 325, 326, 327 of a second set of output gears. The output gears 320-327 are all mounted around an output shaft 330 and a selector tube 331 is interposed between them and the output shaft 330.

Each output gear will abut at least one other output gear and will have at least one boss providing an abutment -12 -surface, as described above in relation to Figure 1. The selector tube 331 has two external teeth and functions as the selector tube 18 of figure 1. The selector tube 331 is slidable along its axis as indicated by the arrow in figure 3 to sequentially engage the gears 320-327 and interlock them with the output shaft 330 so that the output shaft 330 rotates with the selected gear. The transmission unit 300 of figure 3 is identical in layout to the transmission unit 200 of figure 2, except that the output shaft 330 extends forwardly of the transmission unit 300 to a final drive arrangement of intermeshing gears 340,341.

The figure 3 gearbox works identically to the figure 2 gearbox by sliding the selector tube 331 and operating the double clutch 314. For each forward gear ratio change two output gears are simultaneously interlocked with the output shaft and there is a swap from engagement of one clutch plate to the other and thereby a swap from one of the input gears 306-309 of the first set to one of the input gears 310-313 of the second set. The selector tube 331 is slid sequentially in one axial direction to effect the gear ratio change. Between gear changes the selector tube can be slid to a position in which only one output gear is interlocked with the output shaft. Described above are the upward gear ratio changes -the downward gear ratio changes are the same in reverse, with the selector tube 331 slid in an axial direction opposite to the direction of movement during the upward gear ratio changes. A reversing gear 342 is interposed between input gear 306 and output gear 320.

A third practical implementation of the principal illustrated in figure 1 is shown in figure 4, which shows a -13 -twin-shaft transmission unit 400 with a clutch arrangement 416 receiving drive from a prime mover. The transmission unit has a pair of coaxial input shafts 401, 402, the outer input shaft 401 comprising a sleeve surrounding the inner input shaft 402. The outer input shaft 401 is directly mounted in a pair of bearings 403, 404. The inner input shaft 402 is mounted at one end thereof directly in a bearing 405 and indirectly (via the outer input shaft 401) in the bearings 403, 404. A first set of four input gears 406, 407, 408 and 409 are fixedly attached to the sleeve 401 for rotation therewith. A second set of four input gears 410, 411, 412 and 413 are fixedly attached to the shaft 402 for rotation therewith. Each input gear will abut at least one other input gear and will be provided with at least one hub which provides an abutment surface, as described above in relation to figure 1. A twin clutch arrangement 414 is provided for the input shafts 401, 402. The outer input shaft 401 is connected to a first clutch plate 415 for rotation therewith and the inner shaft 402 is connected to a second separate clutch plate 416.

A first set of input gears 406, 407, 408, 409 are in constant mesh respectively with output gears 420, 421, 422, 423 of a first set of output gears and the second set of input gears 410, 411, 412, 413 are in constant mesh respectively with output gears 424, 425, 426, 427 of a second set of output gears. The first set of output gears 420-423 are all mounted around a first output shaft 430 and a first selector tube 431 is interposed between them and the output shaft 430. The selector tube 431 has at least one external tooth and functions as described above in relation to figure 1. The selector tube 431 is slidable along its axis as indicated by the arrow in figure 4 to sequentially engage the gears 420-423 and interlock them with the output shaft 430 so that the output shaft 430 rotates with the selected gear. The second set of output gears 424-427 are all mounted around a second output shaft 432 and a second selector tube 433 is interposed between them and the output shaft 430. The selector tube 433 has at least one external tooth and functions as described above in relation to figure 1. The selector tube 433 is slidable along its axis as indicated by the arrow in figure 4 to sequentially engage the gears 424-427 and interlock them with the output shaft 432 so that the output shaft 432 rotates with the selected gear. Each of the output gears 420-427 will abut at least one other output gear and each is provided with at least one hub which provides an abutment surface, as described above in relation to figure 1.

The figure 4 transmission unit works in a similar fashion to the figure 2 and figure 3 embodiments and for each forward gear ratio change an output gear of the set 424-427 is engaged the selection tube 433 and an output gear of the set 420-423 is simultaneously engaged by the selection tube 431 and there is a swap from engagement of one clutch plate to the other and thereby a swap from one of input gears 406-409 of the first set to one of input gears 410-413 of the second set. Between gear ratio changes only one output gear of one set will be engaged with its respective selection tube. Whilst in the figure 2 and 3 embodiments only one selector tube is used in each embodiment, in the figure 4 embodiment two selector tubes 431 and 433 are used.

-15 -To select a reverse gear ratio, the sleeve 431 interlocks the gear 420 with the output shaft 430 and the clutch 414 engages the plate 415 so that drive is relayed from the prime mover via the clutch 414 to the input shaft 401 and then via the input gear 406 and a reversing gear 438 to output gear 420 to the output shaft 430. The selector tube 433 is either kept in a neutral position with none of the second set of output gears 424-427 engaged or pre-selects the first gear ratio by interlocking the output gear 424 with the selector tube 433.

To select a first gear ratio the clutch 414 disengages the clutch plate 415 and engages the clutch plate 416 to relay drive to the input shaft 402. The gear 424 has already been locked to the selector tube 433. While the clutch plate 415 is disengaged the sleeve 431 is slid to disengage the output gear 420 and to initially assume a neutral condition in which none of the first set of output gears 420-423 are engaged and then to pre-select a second gear ratio by interlocking the output gear 421 with the selector tube 431.

To select a second gear ratio the clutch 414 disengages the clutch plate 416 and engages the clutch plate 415 to relay drive to the input shaft 401. The output gear 421 has already been interlocked with the output shaft 430. After selection of the second gear ratio, while the clutch plate 416 is disengaged the selector tube 433 can be slid to disengage the gear 424 and to assume first a neutral condition in which none of the second set of gears 424-427 are engaged and then to pre-select a third gear ratio by interlocking the gear 425 with the selector tube 433. -16-

To select a third gear ratio the clutch 414 disengages the clutch plate 415 and engages the clutch plate 416 to relay drive to the input shaft 402. The output gear 425 has already been interlocked with the selector tube 433. After the selection of the third gear ratio, while the clutch plate 415 is disengaged, the sleeve 431 can be slid to disengage the gear 421 and to assume first a neutral condition in which none of the first set of output gears 420-423 are engaged and then pre-select a fourth gear ratio by interlocking the output gear 422 with the selector tube 431.

To select a fourth gear ratio the clutch 414 disengages the clutch plate 416 and engages the clutch plate 415 to relay drive to the input shaft 401. The output gear 422 has already been interlocked with selector tube 431. After selection of the fourth gear ratio, while the clutch plate 416 is disengaged, the selector tube 433 can be slid to disengage the gear 425 and to assume first a neutral condition in which none of the second set of output gears 424-427 are engaged and then pre-select a fifth gear ratio by interlocking the output gear 426 with the selector tube 433.

To select a fifth gear ratio the clutch 414 disengages the clutch plate 415 and engages the clutch plate 416 to relay drive to the input shaft 402. The output gear 426 has already been interlocked with the selector tube 433. After selection of the fifth gear, while the clutch plate 415 is disengaged, the selector tube 431 can be slid to disengage the gear 408 and to assume first a neutral condition in which none of the first set of output gears 420-423 are -17-engaged and then pre-select a sixth gear ratio by interlocking the output gear 423 with the selector tube 431.

To select a sixth gear ratio the clutch 414 disengages the clutch plate 416 and engages the clutch plate 415 to relay drive to the input shaft 401. The output gear 423 has already been interlocked with the selector tube 433. After selection of the sixth gear ratio, while the clutch plate 416 is disengaged, the selector tube 433 can be slid to disengage the gear 426 and to assume first a neutral condition in which none of the second set of output gears 424-427 are engaged and then pre-select a seventh gear ratio by interlocking the output gear 427 with the selector tube 433.

To select a seventh gear ratio the clutch 414 disengages the clutch plate 415 and engages the clutch plate 216 to relay drive to the output shaft 402. The output gear 427 has already been interlocked with the selection tube 433.

The downshifting of gear ratios also happens sequentially in a manner which is the reverse of that described above.

It will be understood that in operation of the figure 4 transmission unit changing gear ratio involves not only swapping engagement of the clutch plates and therefore swapping drive from one input shaft to the other, the changing of gear ratio also involves swapping from one output shaft to the other. The output shafts 430 and 432 -18 -have output gears 439 and 440 respectively which both engage a common final drive output gear 441.

All of the embodiments described above have the advantage that a gear ratio can be pre-selected on an output shaft whilst the associated input shaft is disconnected from the drive by the clutch 414. This both allows for greater time to effect the unlocking and locking of output gears and achieves instant gear change on swapping from one clutch to the other.

All of the transmission units described allow for the use of a much simpler actuation mechanism than in traditional dog-clutch gearboxes. Only a simple hydraulic or pneumatic actuator is needed for the selector tubes in figures 2 and 3. The figure 4 transmission unit will preferably uses two hydraulic or pneumatic actuators, one for each selector tube, although it is envisaged that a suitable mechanical linkage could allow one actuator to slide both selector tubes. Pneumatic actuation may be preferred since the compliance of the gas (e.g. air) can cushion impact of the selector teeth with the gears. In each embodiment two further actuators will be needed, one for each of the clutch plates. The limited number of actuators reduces in complexity the control of the operation of the transmission units.

Compared to traditional designs, the elimination of the use of face dogs on the gears greatly reduces the length of the transmission unit. Although in the transmission units described there are frictional losses due to adjacent gears abutting each other, these are minimised by the use of the hubs to reduce the rubbing contact area between the gears and by positioning the gears so that that the rotational speed difference between adjacent abutting gears is minimised.

Figure 5 shows a further embodiment 500 of a transmission unit according to the present invention, which employs the principles applied above with and co-axial with reference to Figure 1. The embodiment 500 has two separate clutches 501 and 502. Clutch 501 is a associated with and co-axial with a first input shaft 500 and the second clutch 502 is associated with and co-axial with a second input shaft 504. The input shaft 503 is mounted in three bearings 505, 506 and 507 and has mounted thereon a reverse gear 508 and three forward gears 509, 510, 511, all of which rotate with rotation of the shaft 503. The input shaft 504 has four gears 512, 513, 514, 515 mounted thereon for rotation therewith. The shaft 503 is parallel to and spaced apart from the shaft 504. The gears 508-511 are mounted at the end of the shaft 503 nearest the clutch 501, whilst the gears 512-515 are mounted on the shaft 504 at the end of the shaft 504 furthest distant from the clutch 502, so that the set of input gears 508-511 are axially spaced from the set of input gears 512-515. The input shaft 504 is mounted in three bearings 516, 517, 518.

Located between the two input shafts 503,504 is an output shaft 519. This output shaft 519 is surrounded by coaxial selector tube 520, of the form described above in relation to Figure 1. The selector tube 520 has a sleeve portion through which the output shaft 519 extends, with the output selector tube 520 being able to slide axially -20 -relative to the output tube 519, whilst the selector tube 520 is engaged, e.g. by splines, with the output shaft 519 so that they both rotate together.

On the selector tube 520 there are provided a first set of output gears 521, 522, 523, 524 each in permanent meshed engagement respectively with input gears 508, 509, 510, 511.

The output gear 521 meshes with the input gear 508 via a reversing gear 530, which is a idling gear.

A second set of output gears 525, 526, 527, 528 are mounted on the exterior of the selector tube 520 in a manner described in Figure 1 above. The output gears 525-528 are in permanent meshed engagement respectively with the input gears 512-515.

To select reverse the clutch 501 is engaged and drive is relayed via the reversing gear 508 and idling gear 530 to the output gear 521,which is engaged with the selector tube 520 by a tooth (not shown) so that the tube 520 and therefore the output shaft 519 rotate with rotation of the gear 521.

To select first gear the clutch 502 is engaged and the clutch 501 disengaged and drive is relayed via the input shaft 504 and input gear 512 to the output gear 525, which is selectively engaged by the selector tube 520 and therefore rotates with the output shaft 519.

Prior to switching from the first gear ratio to the second gear ratio the selector tube 520 will be slid to a position in which both of the gears 522 and 525 are -21 -simultaneously interlocked with the selector tube 520 and thereby the output shaft 519. Moving from selection of the first gear ratio to the second gear ratio then involves disengaging the clutch 502 and engaging the clutch 501 so that power is relayed from the clutch 501 via the input shaft 503, input gear 509 and output gear 522 to the selector tube 520 and therefore the output shaft 519.

To switch from the second gear ratio to a third gear ratio the selector tube 520 will be slid to a position in which the output gears 522 and 526 are simultaneously engaged with the selector tube 520. The clutch 501 is disengaged and the clutch 500 to engage so that drive is transmitted via the clutch 502 to the input shaft 504, the input gear 513, the output gear 526 and therefore to the selector tube 520 and output shaft 519.

To move from the third gear ratio to a fourth gear ratio the selector tube 520 is slid to a position in which both of the gears 523 and 526 are simultaneously engaged with the output tube 520. The clutch 502 is disengaged and the clutch 501 engaged so that drive is transmitted by the clutch 501 to the input shaft 503, input gear 510, output gear 523 and therefore to the selector tube 520 and thereby the output shaft 519.

To move from the fourth gear ratio to a fifth gear ratio the selector tube 520 is slid to a position in which the output gears 523 and 527 are simultaneously engaged with the selector rod 520 and therefore the output shaft 519.

The clutch 501 is disengaged and the clutch 502 engaged to relay drive via input shaft 504, the input gear 514, output -22 -gear 527 and the selector rod 520 to the output shaft 519.

To move from the fifth gear ratio to a sixth gear ratio the selector rod 520 is moved to simultaneously engage the output gear 524 and the output gear 514 with the selector rod 520. Then the clutch 502 is disengaged and the clutch 501 engaged to relay drive via the input shaft 503, input gear 511 and output gear 524 to the selector rod 520 and output shaft 519.

To select a seventh gear ratio the selector rod 520 is moved to simultaneously engage output gears 524 and 528 with a selector rod 520. Then the clutch 501 is disengaged and the clutch 502 engaged to relay drive from the clutch 502 via the input shaft 504, the input gear 515 and the output gear 528 to the selector rod 520 and the output shaft 519.

The gear selection is sequential. The down shifting of gear ratios happens also sequentially in manner which is the reverse of that described above.

Once the gear ratio has been selected the selector rod 520 can move to a "neutral" position in which only one of the output gears 521-528 is engaged to the selector rod 520, but immediately prior to, during and immediately after the change of a gear ratio the selector rod 520 will be in a position in which two different output gears are simultaneously engaged with the selector rod 520. These two output gears will comprises one output gear of a first set of output gears meshed with the first set of input gears and one output gear of a second set of output gears meshed with the second set of input gears.

-23 -Figure 6 shows a further embodiment of the invention, similar in many ways to the embodiment of Figure 5, but with the mode of operation reversed by having selector tubes associated directly with each of the two sets of input gears rather than being associated directly with output gears.

The Figure 6 shows a transmission 600 which has two separate clutches 601, 602. The clutch 601 can relay drive to an input shaft 603 and the clutch 602 can relay drive to an input shaft 604. The input shaft 603 is an inner shaft over which is slidable a selector rod 605 in the form of a sleeve. The rod 605 is slidable axially along the input shaft 603, but rotates with the rod 603 (e.g. is connected by way of a splined connection) . The input shaft 603 is mounted at one end directly in a bearing 606. The selector rod 605 is mounted directly in two bearings 607, 608 and the input shaft 606 is mounted indirectly in the bearings 607, 608, via the selector rod 605.

In a similar fashion the input shaft 604 is a inner shaft and a selector rod 609 takes the form of a sleeve slidable over the exterior of the input shaft 604 with the selector rod 609 slidable axially along the input shaft 604, but being mounted on the input shaft 604 for rotation therewith, e.g. by the use of spl.ines. The input shaft 604 is mounted at one end directly in a bearing 610 and then is mounted indirectly in bearings 611, 612, which also support the selector rod 609.

A first set of input gears 617, 618, 619, 620 are mounted externally of the selector rod 609 and therefore -24 -externally of the input shaft 604. The selector rod 609 will have one or more teeth capable of separately interlocking each of the gears 617-620 to the selector rod 609 for rotation therewith.

A second set of input gears 621, 622, 623, 624 are mounted externally of the selector rod 605 and can be sequentially engaged by the selector rod 605 in the manner described in Figure 1, with the selector rod 605 having one or more teeth thereon.

The transmission unit 600 has an output shaft 613 mounted in three bearings 614, 615, 616. The first set of input gears 617-620 respectively engage a first set of output gears 625, 626, 627, 628 mounted on the output shaft 613 for rotation therewith. The input gear 617, which is a reversing gear, engages the output gear 625 via an idler gear 629. A second set of output gears 630, 631, 632, 633 are also mounted on the output shaft 613 for rotation therewith and are respectively in continuous meshing engagement with the second set of input gears 621-624. The first set of output gears 625-628 are axially separated from the second set of output gears 630-633 with the gears 628- 630 separated by the bearing 615. The output shaft 613 is provided with a final drive gear 634 via which drive is relayed outside the transmission unit.

To select reverse gear the selector rod 609 engages the input gear 617 and therefore allows drive to be relayed through the clutch 602 and the input shaft 604 to the gear 617 and then by the idler gear 629 to the output gear 625 and hence the output shaft 613. The clutch 602 is engaged to relay drive while the clutch 601 is disengaged.

To select first gear the selector rod 605 is slid to a location in which the gear 621 is locked to the selector rod 605 and therefore to the input shaft 603. The clutch 602 is disengaged and the clutch 601 engaged to relay drive via the input shaft 603 and the input gear 621 to the output gear 630 and therefore the output shaft 613.

To move from the first gear ratio to a second gear ratio the selector tube 609 interlocks the input 618 with the selector tube 609 whilst the input gear 621 is still interlocked with the selector tube 605. Then the clutch 601 is disengaged and the clutch 602 engaged so that drive is relayed from the clutch 602 via the input gear 618 to the output gear 626 and therefore to the output shaft 613.

To move from the second gear ratio to a third gear ration the selector tube 605 is moved (with clutch 601 disengaged) to interlock the input gear 622 with the tube 605, simultaneously with interlocking of the input gear 618 with the selector rod 609. Then the clutch 602 is disengaged and the clutch 601 engaged so that drive is transmitted from the clutch 601 via the input shaft 603, the input gear 622 and the output gear 631 to the output shaft 613.

To shift from the third gear ratio to a fourth gear ratio the selector rod 609 is moved to interlock the gear 619 with the selector rod 609, with the input gear 622 still engaged by the selector rod 605. Then the clutch 601 is -26 -disengaged and the clutch 602 engaged so that drive is transmitted via the input shaft 604 and input gear 619 to the output gear 627 and therefore the output shaft 613.

To move from the fourth gear ratio to a fifth gear ratio the selector rod 605 is slid to interlock the input gear 623 with the selector rod 605 whilst the input gear 619 still engaged by the selector rod 609. Then the clutch 602 is disengaged while the clutch 601 is engaged so that drive is relayed by input shaft 603 and input gear 623 to the output gear 632 and therefore the output shaft 613.

To shift from the fifth gear ratio to a sixth gear ratio the selector rod 609, whilst clutch 602 is disengaged and whilst the input gear 623 is still interlocked with the selector rod 605, moves to interlock the input gear 620 with the selector rod 609. then the clutch 601 is disengaged and the clutch 602 engaged so that drive is transmitted via the input shaft 604 and input gear 620 to output gear 628 and therefore to the output shaft 613.

Finally, to move from the sixth gear ratio to a seventh gear ratio, the selector rod 605 is moved so that the input gear 624 is interlocked with the selector rod 605, this occurring whilst the clutch 601 is disengaged and whilst the input gear 620 continues to be interlocked with the selector rod 609. Then the clutch 602 is disengaged and the clutch 601 is engaged so that drive is transmitted via the input shaft 603, the input gear 624 and the output gear 633 to the output shaft 613.

The gear selection is sequential through the gears.

-27 -Downshifting through the gears will be the reverse process to that described above. Whilst immediately prior to, during and immediately after the shifting between gear ratios the two selector rods 605 and 609 will both interlock a gear therewith, at other times one of the selector rods 605, 609 can be moved to a "neutral" position in which only one input gear of one set is engaged with its associated selector rod.

Claims

Claims 1. A transmission unit comprising: *a first set of shaft-mounted input gears each of which meshes with a respective one of a first set of shaft-mounted output gears; a second set of shaft-mounted input gears each of which meshes with a respective one of a second set of shaft-mounted output gears; a first clutch associated with the first set of input gears and the first set of output gears; a second clutch associated with the second set of input gears and the second set of output gears; wherein: at least one of each meshed pair of input and output gears can freely rotate on the shaft on which the gear is mounted and a gear ratio selection mechanism is provided to select a gear ratio by locking a freely rotatable gear to the shaft on which the gear is mounted; in use of the transmission unit either the first clutch is engaged and the second clutch disengaged and drive is transmitted via a selected pair of the first set of input and output gears or the second clutch is engaged and the first clutch disengaged and drive is transmitted via a selected pair of the second set of input and output gears; and in changing between at least the majority of gear ratios, the gear selection mechanism unlocks a first shaft-locked gear of a set of gears associated with a disengaged clutch and then locks a second gear of the same set to the respective shaft while the associated clutch remains disengaged and then the said disengaged clutch is engaged concurrently with disengagement of the other clutch.

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2. A transmission unit as claimed in claim 1 wherein: the first set of gears are mounted on a first shaft for rotation therewith; the second set of input gears are mounted on a second input shaft for rotation therewith, the second input shaft comprising a sleeve surrounding and co-axial with the first input shaft; the first input shaft is connected to the first clutch: the second input shaft is connected to the second clutch; and the first and second sets of output gears are mounted around a common output shaft and each can freely rotate relative to the output shaft and interposed between the output shaft and the output gears is a selector rod having a sleeve portion which surrounds the output shaft whilst being coupled to the output shaft to rotate therewith and the selector rod has gear engagers for selectively interlocking either one or a pair of output gears with the selector rod to rotate therewith, gear selection being effected by sliding of the selector rod along the output shaft relative to the output gears.
3. A transmission unit as claimed in claim 1 wherein: the first set of input gears are mounted on a first input shaft for rotation therewith; the second set of input gears are mounted on a second input shaft for rotation therewith, the second input shaft comprising a sleeve surrounding and co-axial with first input shaft; the first input shaft is connected to the first clutch; the second input shaft is connected to the second clutch; the first set of output gears are mounted around a first output shaft and each can freely rotate relative to the first output shaft and interposed between the first output shaft and the first set of gears is a first selector rod having a sleeve portion which surrounds the first output shaft, the first selector rod having a sleeve portion which surrounds the first output shaft, the first selector rod being slidable axially along the first output shaft while being coupled to the output shaft to rotate therewith and the first selector rod having a gear engager for selectively interlocking a selected one of the first set of output gears to the selector rod to rotate therewith, gear selection being effected by sliding of the first selector rod along the first output shaft relative to the gears of the first set of output gears; and the second set of output gears are mounted around a second output shaft and each can freely rotate relative to the second output shaft and interposed between the second output shaft and the second set of gears is a second selector rod having a sleeve portion which surrounds the second output shaft, the second selector rod being slidable axially along the second output shaft while being coupled to the second output shaft to rotate therewith and the second selector rod having a gear engager for selectively interlocking a selected one of the second set of output gears to the second selector rod to rotate therewith, gear selection being effected by sliding of the second selector rod along the second output shaft relative to the gears of the second set of output gears. -31 -
4. A transmission unit as claimed in any one of the preceding claims wherein the first and second clutches are located side by side and are co-axial.
5. A transmission unit as claimed in claim 1 wherein: the first set of input gears are mounted on a first input shaft for rotation therewith; the second set of input gears are mounted on a second input shaft for rotation therewith, the second shaft being mounted parallel to and spaced apart from the first input shaft; the first clutch is connected to the first input shaft; the second clutch is connected to the second input shaft; the first and second sets of output gears are mounted around a common output shaft and each can freely rotate relative to the output shaft and interposed between the output shaft and the output gears is a selector rod having a sleeve portion which surrounds the output shaft whilst being coupled to the output shaft to rotate therewith and the selector rod has gear engager for selectively interlocking either one or a pair of output gears with the selector rod to rotate therewith, gear selection being effected by sliding of the selector rod along the output shaft relative to the output gears.
6. A transmission unit as claimed in claim 1 wherein: the first and second set of output gears are mounted on a common output shaft for rotation therewith; the first set of input gears are mounted on a first input shaft and each can freely rotate relative to the first input shaft and interposed between the first input shaft and -32 -the first set of input gears is a first selector rod having a sleeve portion which surrounds the first output shaft, the first selector rod being slidable axially along the first input shaft whilst being coupled to the first input shaft to rotate therewith and the first selector rod having a gear engager for selectively interlocking an input gear of the first set with the first selector rod to rotate therewith, gear selection being effected by sliding of the first selector rod along the first input shaft relative to the first set of input gears; the second set of input gears are mounted on a second input shaft, the second input shaft being mounted parallel to and spaced from the first input shaft, and each input gear of the second set can freely rotate relative to the second input shaft and interposed between the second input shaft and the second set of input gears is a second selector rod having a sleeve portion which surrounds the second input shaft whilst being coupled to the second input shaft to rotate therewith and the second selector rod having a gear engager for selectively interlocking an input gear of the second set with the second selector rod, gear selection being effected by sliding of the second selection rod along the second output shaft relative to the second set of input gears; the first input shaft is connected to the first clutch; and the second input shaft is connected to the second clutch.
7. A transmission unit as claimed in claim 5 or claim 6 wherein the first and second clutch are separate and independent from each other.

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8. A transmission unit as claimed in any one of the precding claims in which an input gear meshes with an output gear via an idler gear.
9. A transmission unit as claimed in any one of the preceding claims wherein each input gear of each set abuts at least one other axially adjacent input gear of the same set and the abutting gears are provided with hubs which provide the meshing surfaces across which the abutting gears make contact.
10. A transmission unit as claimed in any one of the preceding claims wherein each output gear of each set abuts at least one other axially adjacent output gear of the same set and the abutting gears are provided with hubs which provide the rubbing surfaces across which the abutting gears can make contact.
11. A transmission unit comprising: a clutch arrangement which receives drive input to the transmission unit; a first input shaft having mounted thereon for rotation therewith a first set of input gears; a second input shaft in the form a sleeve external of and coaxial with the first input shaft and extending along only a part of the first input shaft, the second input shaft having mounted thereon for rotation therewith a second set of input gears; an output shaft; a selector tube in the form of a sleeve external of and coaxial with the output shaft, the selector tube being -34.-mounted on the output shaft to rotate therewith whilst being slidable axially along the output shaft; a first set of output gears mounted around the selector tube with each output gear of the first set of output gears meshing with an associated gear of the first set of input gears; a second set of output gears mounted around the selector tube with each output gear of the second set of output gears meshing with an associated gear of the second set of input gears; interlocking means provided on each of the output gears and on the selector tube which enables selective interlocking of an output gear with the selector tube so that the interlocked gear and selector tube rotate together; and an actuator for sliding the selector tube axially relative to the output shaft; wherein: the clutch arrangement can switch between relaying drive to the first input shaft and relaying drive to the second input shaft; the actuator by sliding the selector tube axially along the output shaft can select which of the output gears is interlocked with the selector tube; and to shift from a selected transmission ratio to a next closest higher or lower transmission ratio the actuator slides the selector tube to simultaneously interlock a selected output gear of each set of output gears with the selector tube and the clutch switches drive from one input shaft to the other to switch drive from an input gear of one set of input gears to an input gear of the other set of input gears.
12. A transmission unit as claimed in claim 11 wherein the interlocking means comprises: a slot in each output gear extending radially outwardly from a radially innermost surface of the output gear; and first and second axially spaced apart external teeth on the selector tube, the first tooth engagable in slots of the output gears of the first set of output gears and the second tooth engagable in slots of the output gears of the second set of output gears.
13. A transmission unit as claimed in claim 12 wherein the axial spacing of the first and second teeth and the axial spacing of the output gears is chosen so that when the first tooth engages an output gear of the first set of output gears then the second tooth can assume a neutral position out of engagement with all gears of the second set of output gears and vice versa.
14. A transmission unit comprising: a clutch arrangement which receives drive input to the transmission unit; a first input shaft have mounted thereon for rotation therewith a first set of input gears; a second input shaft in the form a sleeve external of and coaxial with the first input shaft and extending along only a part of the first input shaft, the second input shaft having mounted thereon for rotation therewith a second set of input gears; a first output shaft; a first selector tube in the form of a sleeve external of and coaxial with the first output shaft, the first selector tube being mounted on the first output shaft to -36 -rotate therewith whilst being slidable axially along the first output shaft; a first set of output gears mounted around the first selector tube with each output gear of the first set of output gears meshing with an associated gear of the first set of input gears; a second output shaft; a second selector tube in the form of a sleeve external of and coaxial with the second output shaft, the second selector tube being mounted on the second output shaft to rotate therewith whilst being slidable axially along the second output shaft; a second set of output gears mounted around the second selector tube with each output gear of the second set of output gears meshing with an associated gear of the second set of input gears; first interlocking means provided on each of the first set of output gears and on the first selector tube which enables selective interlocking of an output gear of the first set with the first selector tube so that the interlocked gear and first selector tube rotate together; second interlocking means provided on each of the second set of output gears and on the second selector tube which enables selective interlocking of an output gear of the second set to the second selector tube so that the interlocked gear and second selector tube rotate together; and actuator means for sliding each selector tube axially relative to a respective output shaft; wherein: the clutch arrangement can switch between relaying drive to the first input shaft and relaying drive to the second input shaft; -37 -the actuator means by sliding the first and second selector tube axially along the first and second output shafts can select which of the output gears is interlocked with each selector tube; and in shifting from a selected transmission ratio to a next closest higher or lower transmission ratio the clutch switches drive from one input shaft to the other and hence from one set of input gears to the other and hence also from an interlocked output gear of one set of output gears on one output shaft to an interlocked output gear of the other set of output gears on the other output shaft.
15. A transmission unit as claimed in claim 14 wherein during shifting between gear ratios and prior to switching of drive from one input shaft to the other input shaft, the actuator means slides the selector tube associated with the non-driven output shaft to unlock an interlocked output gear from the non-driven output shaft and to interlock a further output gear of the same set of output gears with the non-driven output shaft.
16. A transmission unit as claimed in claim 14 or claim 15 wherein the interlocking means comprises: a slot in each output gear extending radially outwardly from a radially innermost surface of the output gear; and an external tooth on each selector rod, the external tooth of the first selector rod being engagable in slots of the output gears of the first set of output gears and the external tooth of the second selector rod being engagable in slots of the output gears of the second set of output gears.
17. A transmission unit comprising: a clutch arrangement which receives drive input to the transmission unit; a first input shaft having mounted thereon for rotation therewith a first set of input gears; a second input shaft, parallel to and spaced from the first input shaft, having mounted thereon for rotation therewith a second set of input gears; an output shaft; a selector tube in the form of a sleeve external of and coaxial with the output shaft, the selector tube being mounted on the output shaft to rotate therewith whilst being slidable axially along the output shaft; a first set of output gears mounted around the selector tube with each output gear of the first set of output gears meshing with an associated gear of the first set of input gears; a second set of output gears mounted around the selector tube with each output gear of the second set of output gears meshing with an associated gear of the second set of input gears; interlocking means provided on each of the output gears and on the selector tube which enables selective interlocking of an output gear with the selector tube so that the interlocked gear and selector tube rotate together; and an actuator for sliding the selector tube axially relative to the output shaft; wherein: the clutch arrangement can switch between relaying drive to the first input shaft and relaying drive to the second input shaft; the actuator by sliding the selector tube axially along the output shaft can select which of the output gears is interlocked with the selector tube; and to shift from a selected transmission ratio to a next closest higher or lower transmission ratio the actuator slides the selector tube to simultaneously interlock a selected output gear of each set of output gears with the selector tube and the clutch switches drive from one input shaft to the other to switch drive from an input gear of one set of input gears to an input gear of the other set of input gears.
18. A transmission unit as claimed in claim 17 wherein the interlocking means comprises: a slot in each output gear extending radially outwardly from a radially innermost surface of the output gear; and first and second axially spaced apart external teeth on the selector tube, the first tooth engagable in slots of the output gears of the first set of output gears and the second tooth engagable in slots of the output gears of the second set of output gears.
19. A transmission unit as claimed in claim 18 wherein the axial spacing of the first and second teeth and the axial spacing of the output gears is chosen so that when the first tooth engages an output gear of the first set of output gears then the second tooth can assume a neutral position out of engagement with all gears of the second set of output gears and vice versa.
20. A transmission unit comprising: a clutch arrangement which receives drive input to the transmission unit; -40 -an output shaft having mounted thereon for rotation therewith first and second sets of output gears; a first input shaft; a first selector tube in the form of a sleeve external of and coaxial with the first input shaft, the first selector tube being mounted on the first input shaft to rotate therewith whilst being slidable axially along the first input shaft; a second input shaft parallel to and spaced apart from the first input shaft; a second selector tube in the form of a sleeve external of and coaxial with the second input shaft, the second selector tube being mounted on the second input shaft to rotate therewith whilst being slidable axially along the second input shaft; a first set of input gears mounted around the first selector tube with each input gear of the first set of input gears meshing with an associated gear of the first set of output gears; a second set of input gears mounted around the second selector tube with each output gear of the second set of input gears meshing with an associated gear of the second set of output gears; first interlocking means provided on each of the first set of input gears and on the first selector tube which enables selective interlocking of an input gear of the first set with the first selector tube so that the interlocked gear and first selector tube rotate together; second interlocking means provided on each of the second set of input gears and on the second selector tube which enables selective interlocking of an input gear of the second set to the second selector tube so that the interlocked gear and second selector tube rotate together; and actuator means for sliding each selector tube axially S relative to a respective input shaft; wherein: the clutch arrangement can switch between relaying drive to the first input shaft and relaying drive to the second input shaft; the actuator means by sliding the first and second selector tube axially along the first and second input shafts can select which of the output gears is interlocked with each selector tube; and in shifting from a selected transmission ratio to a next closest higher or lower transmission ratio the clutch switches drive from one input shaft to the other and hence from an interlocked input gear of one set of input gears on one input shaft to an interlocked input gear of the other set of input gears on the other input shaft.
21. A transmission unit as claimed in claim 20 wherein during shifting between gear ratios and prior to switching of drive from one input shaft to the other input shaft, the actuator means slides the selector tube associated with the non-driven input shaft to unlock an interlocked input gear from the non-driven input shaft and to interlock a further input gear of the same set of input gears with the non-driven output shaft.
22. A transmission unit as claimed in claim 20 or claim 21 wherein the interlocking means comprises: a slot in each input gear extending radially outwardly from a radially innermost surface of the input gear; and an external tooth -42 -on each selector rod, the external tooth of the first selector rod being engagable in slots of the input gears of the first set of output gears and the external tooth of the second selector rod being engagable in slots of the input gears of the second set of input gears.
23. A transmission unit as claimed in any one of claims 11 to 22 wherein each input gear of each set abuts at least one other axially adjacent input gear of the same set and the abutting gears are provided with hubs which provide the rubbing surfaces across which the abutting gears make contact.
24. A transmission unit as claimed in any one of claims 11 to 23 wherein each output gear of each set abuts at least one other axially adjacent output gear of the same set and the abutting gears are provided with hubs which provide the rubbing surfaces across which the abutting gears make contact.
25. A transmission unit as claimed in any one of claims 11 to 24 wherein the clutch arrangement comprises a first clutch plate attached to the first input shaft and a second clutch plate attached to the second input shaft and one or more clutch actuator(s) for selectively engaging the first and second clutch plates with input plates of the clutch arrangement.
26. A transmission unit substantially as hereinbefore described with reference to and as shown in the accompanying drawings.