WO2014096450A1

WO2014096450A1 - Agricultural vehicle transmission

Info

Publication number: WO2014096450A1
Application number: PCT/EP2013/077922
Authority: WO
Inventors: Richard Heindl
Original assignee: AGCO International GmbH
Current assignee: AGCO International GmbH
Priority date: 2012-12-21
Filing date: 2013-12-23
Publication date: 2014-06-26
Anticipated expiration: 2015-06-21
Also published as: GB201223538D0

Abstract

A continuously variable transmission for an agricultural vehicle, the transmission comprising a transmission casing and transmission components, the casing having a front wall and a rear wall, the front wall having a first aperture for receiving an input shaft and a second aperture for receiving a front axle drive shaft, the rear wall having a third aperture for receiving a first rear power take off shaft and/or a first rear axle drive shaft.

Description

AGRICULTURAL VEHICLE TRANSMISSION

The present invention relates to vehicle transmissions and in particular, but not exclusively, to a transmission adaptable for use with a variety of configurations of agricultural vehicles, such as tractors.

Transmissions form part of the driveline tractors. The purpose of the driveline is to transmit torque from the engine (most usually an internal combustion engine and/or or electric motor) to the wheels (or tracks as may be the case in larger tractors). In addition to the engine and transmission, the driveline may also include a flywheel; one or more clutches; a transfer box; and front, centre and rear differentials.

The configuration of the driveline depends on the specific type of tractor, for example whether the vehicle is rear wheel drive or four wheel drive, front wheel steered or centrally articulated, tracked or provided with conventional wheels and tyres. Each variation of driveline requires a different transmission layout.

Additionally, the transmission may be required to drive front and/or rear power take off (PTO) shafts to allow the tractor to operate ancillary equipment such as seed drills and bailers.

By way of an example, a rear wheel drive tractor with steered front wheels would not require a front differential or a front output shaft from the transmission, but would require a rear output shaft and a rear differential. In addition it may be provided with front and/or rear PTO shafts.

In contrast, a tracked tractor would not require an output shaft from the transmission to the front wheels, but would require a rear drive shaft and rear differential capable of splitting the torque between the left and right tracks in order to steer the tractor.

Historically, the provision of different transmissions for each configuration of vehicle has necessitated the design and development of a distinct transmission for each particular application. This has led to the parallel development of transmissions for each of the following commonly provided configuration of tractor: a) standard tractors with smaller diameter, steered front wheels and larger diameter, driven rear wheels;

four wheel drive standard tractors with smaller diameter, steered and driven front wheels and larger diameter, driven rear wheels; and

tracked tractors where only the rear track wheels, not the front idler wheels, are driven.

It will therefore be appreciated that there is at present a considerable duplication of effort in designing and developing multiple variations of transmissions to fit the various configurations of vehicle.

To exacerbate the problem, different configurations of vehicle require the output shafts to be positioned at differing heights relative to the input shaft. Furthermore, transmissions for agricultural vehicles such as self-propelled harvesting or application machinery may require an even greater variation in shaft vertical position than that required across the range of tractor configurations listed above.

This variation in vertical shaft position presents an additional problem. Unlike passenger cars and trucks where the transmission is mounted on a chassis, in agricultural vehicles the transmission is often an integral (and structural) part of the chassis. As a result the space envelope for the transmission is severely restricted by the transmission casing which forms an integral part of the chassis. Furthermore, since the transmission is part of the structure of the chassis, each variation in transmission design requires structural testing thereby increasing the cost and complexity of transmission development.

To overcome this problem, it is well known to provide an additional gearbox which serves the sole purpose of adapting the vertical displacement of the output shaft (to which the axle is connected via a differential) relative to the input shaft. This arrangement is shown in Figure 1 in which a tractor 1 has an engine 2 providing drive to a transmission 3 which in turn has a rear PTO shaft 4 and a rear output shaft 5. The rear out put shaft 5 drives a transfer box 6 and a rear axle 7 via a rear differential 11. The transfer box 6 has a front drive shaft 8 which drives a front axle 9. It can be seen that the transfer box 6 does not form part of the transmission 3 and serves to drop the front drive shaft 8 by a vertical distance A relative to the rear output shaft 5.

Figures 2A to 2D show alternative uses of the "bolt-on" transfer box 6 to alter the relative vertical displacements of the output shafts. Figure 2 A replicates part of the view of Figure 2 and once again shows the relative vertical distance A between front drive shaft 8 and the rear output shaft 5 for ease comparison with Figures 2B to 2D. Figure 2D shows the use of a second configuration of transfer box 6B which raises the height of the rear differential 11 by a distance B relative to the rear output shaft 5. Figure 2C shows use of a second transfer box 6' in front of the transmission 3 in order to raise the height of the front differential (not shown for clarity) by a distance C relative to the front drive shaft 8. Lastly, Figure 2D shows an arrangement similar to Figure 2B in which transfer box 6D raises the height of the rear differential 11 by a distance D relative to the rear output shaft 5.

It will be evident that this solution requires additional hardware to be selectively designed and installed on the vehicle, hardware which is additional to a potentially wide range of different transmissions. This increases the cost of manufacture and the maintenance requirements of the vehicle.

It is an objective of the present invention to at least mitigate one or more of the above problems.

According to the invention there is provided continuously variable transmission for an agricultural vehicle, the transmission comprising a transmission casing and transmission components,

the casing having a front wall and a rear wall, the front wall having a first aperture for receiving an input shaft and a second aperture for receiving a front axle drive shaft,

the rear wall having a third aperture for receiving a first rear power take off shaft and/or a first rear axle drive shaft.

Advantageously, the invention provides a transmission unit which is readily adaptable for each of the required tractor configurations. This provides a considerable technical advantage over known transmission units in that the extent of adaptation required between the different configurations is minimized.

The provision of a transmission casing having multiple apertures for receiving various combinations of power take off and drive shafts eliminates the necessity to design a specific transmission unit for each configuration of vehicle. This greatly reduces the transmission design and manufacturing expense and complexity over prior art transmissions. Preferably, the transmission includes fourth and fifth apertures for receiving second and third rear axles hafts respectively.

Preferably, the transmission casing includes one or more blanking plates which cover one or more of the first to fifth apertures when not required to receive a drive shaft.

Preferably, the rear power take off shaft and first rear drive shaft are co-axial to oneanother and pass through the second aperture in the transmission casing.

Preferably, the transmission further includes

a mechanical transmission branch which transfers torque mechanically from the input shaft to the power take off shaft and axle drive shafts, and

a hydrostatic transmission branch having a hydraulic pump driven by the input shaft and at least one hydraulic motor hydraulically driven by the pump,

wherein the axle drive shafts are additionally driven by the at least one hydraulic motor so as to provide the axle drive shafts with continuously variable drive.

Preferably, the front axle drive shaft is mechanically connected to one or more of the rear axle drive shafts.

Preferably, the transmission includes a planetary gear assembly to split the torque from the input shaft between the mechanical and hydraulic transmission branches. Further to Figures 1 to 2D (described above) which depict prior art relevant to the invention, the invention will now be described, by way of example only, and with reference to the following drawings, in which: Figure 3 is a representation of a standard tractor including a transmission according to the present invention;

Figure 4 is a schematic representation of the driveline of the tractor of Figure 3, including a first configuration of the transmission of the present invention;

Figure 5 is a plan view of the layout of the first transmission configuration of Figure 4;

Figure 6 is an elevated rear view of the layout of the first transmission configuration of Figure 4;

Figure 7 is a rear view of the layout of the first transmission configuration of Figure 4;

Figure 8 is a schematic representation of an alternative tractor driveline including a second configuration of the transmission of the present invention;

Figure 9 is a plan view of the layout of the second transmission configuration of Figure 8; Figure 10 is an elevated rear view of the layout of the second transmission configuration of Figure 8;

Figure 11 is a rear view of the layout of the second transmission configuration of Figure 8;

Figure 12 is a schematic representation of a third transmission configuration of the transmission of the present invention; Figure 13 is a schematic representation of a fourth transmission configuration of the transmission of the present invention;

Figure 14 is a schematic representation of a fifth transmission configuration of the transmission of the present invention;

Figure 15 is a schematic representation of a sixth transmission configuration of the transmission of the present invention; Figure 16 is a schematic representation of a seventh transmission configuration of the transmission of the present invention;

Figure 17 is a schematic rear view of the transmission of the present invention; Figure 18 is a schematic front view of the transmission of the present invention; and

Figure 19 is a schematic side view of the transmission of the present invention.

Figure 3 shows an agricultural vehicle, in the form of a tractor 110, having front wheels 112, rear wheels 114, an engine cover 116 and operator cab 118. The tractor 110 has a chassis 120 (not shown in Figure 3 for clarity) and provides support for the tractor and a driveline 121 which provides propulsive drive. In some tractors the driveline may form part of the chassis, for example where the outer casing of a transmission and a rear transaxle is structurally integral to the chassis.

Figure 4 shows the driveline 121 having a prime mover in the form of an internal combustion engine 124 and a first configuration of transmission 100 (indicated schematically in Figure 4 by dotted line 100). The transmission 100 has an inner transmission casing 199 (not shown in Figure 4) which is mounted inside an outer structural housing (not shown) which forms part of the chassis. Accordingly, the transmission 100, specifically the casing 199, supports or houses all of the components required to provide the full transmission output power with variable torque, variable vehicle speed and variable driving direction over a full range of vehicle speeds. It will be appreciated that whilst the example of prime mover given is that of an internal combustion engine, it is conceivable within the scope of the invention that the prime mover could be an electric motor or other form of propulsive engine.

The configuration of the tractor 110 shown in Figures 3 to 7 is a four wheel drive tractor with steered front wheels. Alternatively, the tractor may be rear wheel drive only in which case the driveline 121 would not include a front differential assembly while the transmission 100 would be otherwise unchanged.

The engine 124 provides drive to the transmission 100 via a flywheel 146. The transmission 100 drives the front and rear axle drive shafts 134,136 to provide propulsive drive to the wheels 112, 114. In addition to providing propulsive drive, the transmission 100 also provides drive to a rear power take-off drive shaft 132.

The engine 124 is connected to an input shaft 102 of the transmission 100 via the flywheel 146. The input shaft 102 is connected at its inboard end to a planetary gear assembly indicated generally 148. The purpose of the planetary gear assembly 148 is to split the torque provided by the input shaft 102 between a mechanical branch indicated generally at 150 and a hydrostatic branch indicated generally at 152. On the opposite side of the planetary gear assembly to the input shaft 102 is a rear power take off shaft 132.

The hydrostatic branch 152 drives a hydraulic pump 140. The mechanical branch 150 is connected to a front axle drive shaft 134 and a rear axle drive shaft 136 as follows. Torque is transmitted from the mechanical branch 150 of the planetary gear assembly 148 to the rear axle drive shaft 136 via a rear axle gear 154. Mounted on the same shaft as the rear axle drive gear 154 is an intermediary gear 156 which in turn drives a front axle drive gear 158 which selectively drives the front axle drive shaft 134. A clutch 160 is provided to selectively engage and disengage the front axle drive shaft 134 from the rear axle drive shaft 136 or to control the ratio of torque distribution between the two axles. This allows grip to be optimised dependant on the ground conditions. In addition to the mechanical drive path described above, the hydraulic pump 140 is hydraulically connected (not shown in Figure 4 for clarity) to a first hydraulic motor 142 which is driveably connected to the rear axle drive shaft 136 in order to provide hydraulic drive to the rear wheels. The hydraulic pump 140 is also connected to a second hydraulic motor 144 in order to provide hydraulic drive to the front axle drive shaft 134 as follows. The motor 144 is driveably connected to the front axle drive shaft 134 via first and second hydraulic motor gears 162, 164. A clutch 166 allows the second hydraulic motor 144 to be selectively engaged and disengaged from the front axle drive shaft 134. This allows hydraulic drive to be provided to the front axle drive shaft 134 by the second hydraulic motor 144 in addition to, or alternatively to, the drive delivered to the front axle drive shaft 134 from the rear axle drive shaft 136 via the intermediary and front axle drive gears 156, 158, depending on the extent of engagement of the clutch 160. Second hydraulic motor 144 is connected to front axle drive shaft 134 by gears 162, 164 having a high transmission ratio. This allows motor 142 to provide high torque at a limited, lower range of vehicle speeds. Consequently, at higher vehicle speeds, the motor 144 may be disconnected from driveline 121 via clutch 166. Due to the layout, the first hydraulic motor 144 is provided for delivering lower torque but over the full range of vehicle speeds. However, in combination, both motors 142, 144 enable the transmission to provide a full transmission output power with variable torque, variable vehicle speed and variable driving direction over a full range of vehicle speeds.

The rear axle drive shaft 136 provides drive to the rear wheels 114. Similarly, the front axle drive shaft 134 provides drive to the front wheels 112.

By way of further explanation, various drive configurations are possible depending on the operating positions of clutches 160 and 166: With the clutch 160 disengaged, and the clutch 166 engaged, motor 142 drives the rear axle drive shaft 136 and thereby rear axle 114, and motor 144 drives the front axle drive shaft 134 and thereby front axle 112. With the clutch 160 disengaged, and the clutch 166 disengaged, motor 142 drives the rear axle drive shaft 136 and thereby rear axle 114. The front axle drive shaft 134 and thereby front axle 112 is not driven. With the clutch 160 engaged, and the clutch 166 disengaged, motor 142 drives the rear axle drive shaft 136 and thereby rear axle 114. The motor 142 also drives the front axle drive shaft 134 and thereby front axle 112.

With the clutch 160 engaged, and the clutch 166 engaged, motor 142 and motor 144 both drive the rear axle drive shaft 136 and thereby rear axle 114. The motors 142, 144 also drive the front axle drive shaft 134 and thereby front axle 112.

With the clutch 160 variably controlled, and the clutch 166 engaged, torque supplied by motor 142 to the front axle drive shaft 134 and thereby front axle 112 can be adapted.

Referring now to Figures 5, 6 and 7, the input shaft 102, rear power take-off drive shaft 132, front axle drive shaft 134 and a rear axle drive shaft 136 are visible at the exterior of the an inner transmission casing 199. The transmission 100 has a blanking cap 138 which covers an aperture in the inner transmission casing 199 for an additional drive shaft not used in this first configuration of transmission. In addition, the hydraulic pump 140, first hydraulic motor 142 and second hydraulic motor 144 are shown mounted to the inner transmission casing 199. In use, the operator is able to vary the division of torque provided by the transmission 100 either mechanically or hydraulically by varying the amount of hydraulic assistance provided by the first and second hydraulic motors.

The transmission of the present invention is also suitable for a tracked tractor as will now be described in further detail.

Figure 8 shows the driveline 221 for a tracked tractor having an engine 224 which drives a second configuration of transmission 200 which in turn provides drive to a rear axle drive shaft 236. The rear axle drive shaft 236 provides drive to the rear drive wheels (not shown) which in turn drive the track (not shown). As a result there is no requirement for a front axle drive shaft since no drive is transmitted to the front wheels. The engine 224 provides drive to the transmission 200 via a flywheel 246. In addition to providing propulsive drive to the rear axle drive shaft 236, the transmission 200 also provides drive to a rear power take-off drive shaft 232. The engine 224 is connected to the input shaft 202 of transmission 200 via the flywheel 246. The input shaft 202 is connected at its inboard end to a planetary gear assembly indicated generally 248. The purpose of the planetary gear assembly 248 is to split the torque provided by the input shaft 202 between a mechanical branch indicated generally at 250 and a hydrostatic branch indicated generally at 252. On the opposite side of the planetary gear assembly 248 to the input shaft 202 is the rear PTO drive shaft 232. The hydrostatic branch 252 drives first and second hydraulic motors 242, 244 via hydraulic pump 240 while the mechanical branch 250 provides drive to the rear axle drive shafts 236 as follows. Torque is transmitted from the mechanical branch 250 of the planetary gear assembly 248 to the rear axle drive shaft 236 via initial gear 254. Mounted on the same shaft as the initial gear 254 is an intermediary gear 256 which in turn drives a rear axle drive gear 258. The rear axle drive gear 258 is mounted on and thereby drives the rear axle drive shaft 236. A steering differential (not shown) is provided to split the torque delivered by the rear axle drive shaft 236 between the left and right rear drive wheels in order to steer the tractor 210. In addition to the mechanical drive path described above, the hydraulic pump 240 is hydraulically connected (not shown in Figure 5 for clarity) to the first hydraulic motor 242 which is driveably connected to the rear axle drive shaft 236 via the intermediary gear 256 and rear axle drive gear 258 in order to provide supplementary hydraulic drive to the rear drive wheels 212. The hydraulic pump 240 is also connected to the second hydraulic motor 244 which is driveably connected to the rear axle drive shaft 236 via second hydraulic motor gear 264. A clutch 266 allows the second hydraulic motor 244 to be selectively engaged and disengaged from the rear axle drive shaft 236. This allows hydraulic drive to be provided to the rear axle drive shaft 236 by the second hydraulic motor 244 in addition to, or alternatively to, the drive delivered by the first hydraulic motor 242 and the mechanical branch 250 of the planetary gear assembly 248.

Referring now to Figures 9, 10 and 11 the input shaft 202, rear power take-off drive shaft 232, and rear axle drive shaft 236 are visible at the exterior of the inner transmission casing 299. It will be noted that the transmission 200 does not have the blanking cap 138 of the transmission 100. Instead the transmission 200 uses the aperture covered by the blanking cap 138 in transmission 100 to provide access for the rear axle drive shaft 236. Also the transmission 200 has no front axle drive shaft as the tracked tractor 210 requires no drive to the front wheels. Furthermore, the transmission 200 is not provided with clutch 166 of transmission 100 as there is no need for torque distribution.

In addition, the hydraulic pump 240, and the first and second hydraulic motors 242, 244 are shown mounted to the inner transmission casing 299.

It will be appreciated that the transmission of the present invention can be configured to provide a configuration of transmission 100 for tractor 110 or transmission 200 for tractor 210 wherein the drive shafts 136, 134, 236 can be positioned at a different vertical distance to e.g. the input shaft 202. Furthermore the drive shafts 136, 134, 236 are positioned at the appropriate vertical shaft level and provided with the respective transmission ratio/output speed to meet the requirements of different wheel sizes over different vehicles. Drive shaft 136 is provided with the respective gear ratio suitable to drive a big size rear wheel of a standard tractor. Drive shaft 134 is provided with the respective gear ratio suitable to drive a smaller rear wheel of a standard tractor. Drive shafts 136 is provided with the respective gear ratio suitable to drive a mid size drive wheel to drive the track of a track-type tractor. This design eliminates the requirement for transfer boxes when used in different vehicle types. In each configuration, the inner transmission casing 199, 299 and many of the internal components remain the same. This provides significant advantages, for example a reduction in the complexity of production of the transmissions across a range of tractors typically offered by agricultural manufacturers. The transmission unit 26 can provide further configurations of transmission to suit a large range of applications. Further alternative transmission configurations of transmission will now be described with reference to Figures 12 to 19. Each transmission is similar in construction and operation to transmissions 100, 200 described above and all provide at least two continuously variable output drive shafts. The modes of operation of each of the transmissions of Figures 12 to 19 will only be described in detail where they differ significantly from transmissions 100, 200 described above. Referring initially to Figure 12, a configuration of transmission 400 has an input shaft 402 which drives a planetary gear system 448 with a mechanical branch 450 and a hydraulic branch 452. The hydraulic branch 450 drives a hydraulic pump 440 which provides hydraulic power to a hydraulic motor 442. The input shaft 402 passes through the planetary gear system 448 and exits the transmission at the rear as a rear PTO drive shaft 432. The rear PTO drive shaft 432 is co-axial with, and positioned diametrically within, a rear axle drive shaft 436. A front axle drive shaft 434 is driven by the hydraulic motor 442 and the mechanical branch 450 via gears 474, 476. The hydraulic motor 442 is also able to power the rear axle drive shaft 436 via gears 474, 476.

Figure 14 shows a further configuration of transmission 500. The transmission 500 has an input shaft 502, front axle drive shaft 534, rear axle drive shaft 536, gears 574, 576, hydraulic pump 540 and hydraulic motor 542 in common with transmission 400. Compared to the transmission 400 shown in Figure 12, no PTO shaft is provided as the hollow shaft extending to the rear of the planetary gear system 548 is connected to the rear axle drive shaft 536. This arrangement may be suitable for a truck not requiring a rear PTO. As trucks require a transmission with a lower output speed to better fit to the rest of the driveline, the planetary gear system 548' may be adapted so that the rear axle drive shaft 536 is drivingly connected to a component of the planetary gear system 548' which rotates with lower speed.

The transmission 600 of Figure 14 differs from transmission 400 (with identical components being numbered 200 higher in Figure 14 than in Figure 12) in that it has a second hydraulic motor 644 which acts in cooperation with the first hydraulic motor 642 to drive the front axle drive shaft 634 and rear axle drive shaft 636. The second motor 644 could also be provided in the embodiments shown in Figure 12 and 13.

Referring now to Figure 15, the transmission 700 has an input shaft 702 driving a planetary gear system 748 with a mechanical branch 750 and a hydraulic branch 752. The hydraulic branch 752 drives a hydraulic pump 740 which provides hydraulic power to first and second hydraulic motors 742, 744. The input shaft 702 passes through the planetary gear system 748 and exits the transmission at the rear as a rear PTO drive shaft 732. The rear PTO drive shaft 432 is co-axial with, and positioned diametrically within, a first rear drive shaft 736. A second rear drive shaft 790 is driven by the first hydraulic motor 742 and the mechanical branch 750 via gears 774, 776. The first hydraulic motor 742 is also able to power the first rear drive shaft 736 via gears 774, 776. A front axle drive shaft 734 is driven by the first hydraulic motor 742 and mechanical branch 750 of the planetary gear system 748 via gears 792, 794 and clutch 796. Attached to the other end of the front axle drive shaft 734 is a third rear drive shaft 798. The front axle drive shaft 734 and third rear drive shaft 798 are selectively driven by the second hydraulic motor 744 via clutch and gears 795, 797. In such a manner the clutch 766 can be engaged to enable the second hydraulic motor 744 to drive the first and second rear drive shafts 736, 790 via the gears 795, 797, 794, 792, 776 and 774. The clutch 796 can be engaged to mechanical drive and hydraulic drive to the front axle drive shaft 734 and third rear drive shaft 798. As the clutch 796 is summing up the torque delivered by motors 742 and 744 via circumferential toothed portions, the front axle drive shaft 734 can extend through clutch 796 to provide third rear drive shaft 798.

The configuration of transmission 700 may be used to drive a vehicle with more than two driven axles as follows. In a vehicle with three axles, the front axle of the vehicle may be connected to front axle drive shaft 734, a mid vehicle axle may be connected to third rear drive shaft 798 while the rear vehicle axle may be connected to second rear drive shaft 790. In addition, rear PTO drive shaft 432 may drive a PTO. In a vehicle with four axles, the front (first) axle of the vehicle may be connected to front axle drive shaft 734, a mid (second) vehicle axle may be connected to third rear drive shaft 798 while the first rear (third) vehicle axle may be connected to first rear drive shaft 736. In addition, the second rear (fourth) vehicle axle may be connected to first rear drive shaft 736. The rear PTO drive shaft 432 would not be used in this case.

The transmission 800 of Figure 16 differs from transmission 700 (with identical components being numbered 100 higher in Figure 16 than in Figure 15) in that second hydraulic motor 844 is positioned in line with the front axle drive shaft 834 in order to selectively (via clutches 866 and 896) drive the front axle drive shaft 834 and rear drive shafts 836, 890. The second hydraulic motor 844 is shown in an alternative location 844" by dashed lines in Figure 16 in which embodiment the front axle drive shaft 834 is removed in order to provide access for the motor 844" and a third rear drive shaft 898 is provided instead.

In Figure 17, the rear wall of the transmission unit 26 is depicted showing the shaft positions of configurations of transmission 100, 200, 400, 500, 600, 700 and 800. Figures 18 and 19 show the shaft positions in front wall and side walls, respectively. The input shaft 102, 202, 402, 502, 602, 702, 802 enters the transmission unit 26 through aperture Al . The rear PTO drive shaft 132, 232, 432, 632, 732, 832 is positioned coaxially within the rear axle drive shaft 436, 536, 636, 736, 836 at position A exiting the transmission unit through aperture A3. Alternatively (or additionally in the case of transmissions 700, 800) the rear axle drive shaft 136, 790, 890 is provided at an intermediate position B exiting the transmission unit through aperture A4. In transmissions 400, 500 and 600 the front axle drive shaft 434, 534, 634 exits the front of the transmission casing at position B through aperture A6. Alternatively (or additionally in the case of transmissions 700, 800) the rear axle drive shaft 236, 798, 898 is provided at a low position C exiting the transmission unit through aperture A5. Alternatively (or additionally in the case of transmissions 700) the front axle drive shaft 134, 734, 834 exits the front of the transmission casing at position C through aperture A2. Accordingly, the configurations of transmission described above provide the full transmission output power with variable torque, variable vehicle speed and variable driving direction over a full range of vehicle speeds. This is in contrast to prior art transmissions (see Figures 1 and 2A to 2D) in which only a single drive shaft is provided which can achieve a continuously variable torque delivery across the range of vehicle speeds. Rather, in the prior art devices (depicted in Figures 1 and 2 A to 2D) a supplementary transfer box 6 must be provided in order to split the drive from the single output shaft 5 to the front and rear axles. The transmission of the present invention therefore achieves considerable space saving and a reduction in mechanical components over the prior art transmissions as the requirement for a separate transfer box is eliminated by the provision of two continuously variable output shafts, one for providing drive to the rear wheels the other for providing drive to the front wheels.

The embodiments of transmission described above all have a hydrostatic branch defined by a pump (driven by the input shaft) and two motors which are driven by the pump. In an alternative embodiment the hydrostatic branch is replaced by an electrical branch in which the hydraulic pump is replaced by an electrical generator (driven by the input shaft) and the hydraulic motors are replaced by electric motors.

In the specific embodiments the transmission unit 26 is surrounded by an additional structural housing integral with the chassis of the tractor. This is especially advantageous for assembly and maintenance reasons.

Alternatively, and without departing from the scope of the invention, the transmission unit 26 could incorporate the structural housing in place of the inner transmission casing 199. In this case, all the components included in each configuration of the transmission unit 26 described above would be directly assembled to the structural housing. Furthermore, the embodiments described above are based on an orientation of the transmission which is common in agricultural vehicles such as tractors, that is to say that the engine is at the front and the transmission is positioned rearwards of the engine relative to the driving direction. This results in the front axle being situated in front of the transmission while the rear axle is behind the transmission. However it is conceivable that the orientation of the transmission or the engine could be altered, e.g. for bucket loaders having a rear engine, would result in a different orientation of the vehicle front and rear axle(s) in relation to the transmission. So the reference of to front or rear axle must be more seen with reference to the transmission than strictly to the vehicle.

Claims

1. A continuously variable transmission for an agricultural vehicle, the transmission comprising a transmission casing and transmission components, the casing having a front wall and a rear wall, the front wall having a first aperture for receiving an input shaft and a second aperture for receiving a front axle drive shaft,

2. The transmission of claim 1 wherein the transmission includes fourth and fifth apertures for receiving second and third rear axles hafts respectively.

3. The transmission of claim 1, 2 or 3 wherein the transmission casing includes one or more blanking plates which cover one or more of the first to fifth apertures when not required to receive a drive shaft.

4. The transmission of claim 1, 2 or 3 wherein the rear power take off shaft and first rear drive shaft are co-axial to one another and pass through the second aperture in the transmission casing.

5. The transmission of any one of claims 1 to 4 wherein the transmission further includes:

a hydrostatic transmission branch having a hydraulic pump driven by the input shaft and at least one hydraulic motor hydraulically driven by the pump, wherein the axle drive shafts are additionally driven by the at least one hydraulic motor so as to provide the axle drive shafts with continuously variable drive.

6. The transmission of claim 5 wherein the front axle drive shaft is mechanically connected to one or more of the rear axle drive shafts.

7. The transmission of claim 5 or 6 wherein the transmission includes a planetary gear assembly to split the torque from the input shaft between the mechanical and hydraulic transmission branches.