GB2169388A - Hydraulic drive system for a towed vehicle - Google Patents

Abstract

By means of a distributor (7) a pump (1) feeds hydraulic drive means such as a motor (8) mechanically coupled to wheel shafts (14) of a trailer. The pump is of the constant pressure type with automatic delivery regulation. There is a motor (8) for each wheel. Valves (43, 44) simultaneously actuated by a pilot conduit (42) selectively connect the motors in series or parallel. Dog clutches (23) with a return spring (28) decouple the motors (8) from the shafts (14) in the absence of any pressure at the pump output. The apparatus allows automatic matching of the trailer drive speed to a tractor drive speed irrespective of the transmission ratio used on the tractor. <IMAGE>

Description

SPECIFICATION Hydraulic drive system for a towed vehicle, and a vehicle thus equipped This invention relates to a hydraulic drive system for a towed vehicle, more particularly a vehicle used in agriculture or forestry, and adapted for hitching to a tractor, such as an agricultural tractor, the system comprising a hydraulic pump feeding hydraulic drive means, and means for mechanically coupling the drive means to the vehicle wheels.

The invention also relates to a vehicle thus equipped.

In agricultural or forestry operations, the wheels of some vehicles designed primarily as trailers are often required to be convertible to drive wheels in order to increase adhesion in very difficult ground, or to reduce slipping, which damages agricultural land.

Systems of this kind are described in Belgian patents 694 752 and 835 958, and also in European patent 54 500. These patents relate to trailers for use on roads or building sites. They give rise to a simpler problem, because all that is required is for the hydraulic motor of the trailer to be adapted to one or two transmission ratios of the tractor. In the event of the vehicle actually being bogged down or there being any risk of this, the driver operates his tractor on the transmission ratio to which the hydraulic motor is adjusted.

In other cases, e.g. on roads, the hydraulic motor is rendered inoperative.

Agricultural or forestry equipment gives rise to much more complex problems. More particularly, unlike trailers for use on roads or building sites, where the same tractor practically always tows the same trailer, there is a frequent change of tractors in agriculture and the number of makes available on the market is very much larger. It is therefore impossible to take as a basis any match between the hydraulic motor of the trailer and the transmsision ratios of the tractor.

Tractors for agricultural or forestry use have a very wide range of transmission ratios which can be used for operation and the hydraulic drive of the trailer must be guaranteed irrespective of the operating ratio engaged on the tractor.

Some vehicles have a power take-off, the speed of rotation of which is in a constant ratio to the speed of movement of the vehicle, and one might therefore consider using this power take-off to drive a pump to feed the hydraulic motor for the wheels, thus providing synchronization between the trailer wheel drive speed and the speed of movement of the vehicle. However, power take-off systems of this kind are not available on tractors for agricultural or forestry use. On these the speed of the power take-off is in a constant ratio to the speed of rotation of the motor. It is therefore also necessary to solve the probiem involved when the tractor stops with the engine running; in these conditions the trailer must not be able to push the tractor.

The object of the invention is thus to adapt the said system to operation for agricultural or forestry use.

According to the invention, the system is characterised in that the pump is of the type outputting a substantially uniform pressure by automatic adjustment of its delivery.

In operation, the pressure is constant and the delivery is adapted to the speed of movement of the tractor. If this pressure is such as to give an inadequate torque for full propulsion of the trailer, the additional power is supplied by the mechanical hitch to the tractor. If, on the other hand, the constant pressure delivered by the pump is equivalent to an excessive torque for moving the trailer, the latter tends to push the tractor, which thus recovers a certain mechanical power.

When the tractor is at a standstill with the engine running, the trailer tends to push the tractor, but since the latter is braked the constant pressure is supplied by a pump at a very low delivery corresponding to the leakages, so that the absorbed power is minimal.

Thus the invention is based on the inventive idea that there is no point, and that it is a complex matter, to try to drive the vehicle wheels at a speed matching that of the tractor and that it is preferable to deliver a given torque at a speed which itself adjusts to the tractor speed, to an extent such that the mechanical energy may be exchanged between the tractor and the trailer through the hitch which connects them.

Preferably, the substantially uniform pressure delivered by the pump is controllable for adjustment to the different types of work to be carried out.

Preferably, the drive means comprise two hydraulic motors adapted each to drive at least one respective wheel of the vehicle. In this way it is not necessary to provide a differential between the two drive wheels of the vehicle.

According to a second aspect, the invention also relates to a vehicle for agricultural or forestry use and having at least four wheels, adapted to be hitched to a tractor and using a hydraulic drive system of the aforesaid kind, in which two wheels are disposed one behind the other on each side of the vehicle.

According to this second aspect, the vehicle is characterised in that the two wheels on each side are connected by a rotary coupling means and are driven together by one of the motors of the hydraulic drive system. This is a very simple way of providing a vehicle with four drive wheels.

Other features and advantages of the invention will be apparent from the following de scription.

In the accompanying drawings, which are given by way of example without limiting force: Fig. 1 is a synoptic diagram of the system according to the invention.

Fig. 2 is an axial section of a jack for controlling the dog clutch engagement of the system shown in Fig. 1, and Fig. 3 is a diagrammatic plan view of a vehicle with four drive wheels using the system shown in Fig. 1.

In the example shown in Fig. 1, the system according to the invention comprises a hydraulic pump 1 driven by the heat engine 2 of the tractor through a speed step-up transmission 3. The pump 1 draws oil from a sump 4 to deliver it via output 6 to a distributor 7 shown diagrammatically. The distributor may occupy four positions, namely three positions in which the oil delivered by the pump 1 is fed to hydraulic drive means 8, which drive two wheel half-shafts 14 and from which the oil returns to the sump 4 through the distributor 7, and a position (the one illustrated) in which the drive means 8 are inoperative because they are short-circuited by an internal path of the distributor which returns the oil directly to the sump 4. The hydraulic circuit is therefore of the open type, i.e. the pump feeds from the sump and the motors deliver into the sump.

According to the invention, the pump 1 is a pump whose output 6 delivers a uniform pressure by automatic regulation of its delivery.

This uniform pressure is controllable by theuserto adapt it to the nature of the work to be carried out or to the nature of the ground. A pressure gauge 9 provides a check on proper operation of the pump 1.

There is no need to describe the pump 1 in detail, because controllable pumps of the type having a controllable uniform pressure and automatic delivery regulation are available on the market.

Although the pressure delivered by the pump 1 is theoretically uniform, over-pressure is possible in transient states. To avoid their harmful effects, an excess pressure valve 11 is provided between the output 6 of the pump 1 and the conduit 12 providing the return of oil to the sump 4 from the distributor 7. Conduit 12 extends through a filter 13 between the connection to the output of the valve 11 and to the sump 4.

According to one important feature of the invention, th drive means comprise two hydraulic motors 8 which are identical and mechanically independent from one another. Each of the motors 8 drives a respective half-shaft 14 through its respective reduction gear 16.

Each half-shaft 14 mechanically rotates at least one side wheel of the vehicle on one respective side thereof.

The reduction gears 16 are required because the hydraulic motors 8 conventionally available on the market have nominal speeds of rotation much higher than that of the half-shafts 14. However, the need for the reduction gears has been put to good use in order to provide the system with a dog clutch disengagement system.

To this end, each gear 16 comprises two reducing gears. A first of these comprises a pinion 17 driven by the motor 8 and meshing with a larger-diameter intermediate gearwheel 18 connected for rotation to a fluted shaft 19.

The second gear comprises a pinion 21 mounted for loose rotation on shaft 19 and meshing with an output gearwheel 22 fixed to the half shaft 14. A dog clutch 23 provided with internal grooving is mounted on the shaft 19 so as to be slidable along the same and rotatable thereby. A fork 24 controls the movements of the dog clutch along the shaft 19 between a position remote from the loose gearwheel 21, in which the latter is not driven by the shaft 19, and a dog clutch engagement position in which the dog clutch 23 meshes with a dog clutch engagement ring 26 connected to the loose gearwheel 21 so that the same is engaged on the shaft 19 by means of the ring 26 and the dog clutch 23.

The movements of each fork 24 are controlled, in the direction of dog clutch engagement, by a single-acting jack 27, and in the direction of dog clutch disengagement, by a return spring 28, the whole being shown diagrammatically in Fig. 1.

The two jacks 27 are fed by a common conduit 29 connected to the output 6 of pump 1. For the dog clutch 23 to be able to remain in the engaged position even when the distributor 7 is in the position illustrated, in which the pump 1 delivers to the sump 4, a pressure limiter valve 31 is fitted in the conduit 12 between the output of the pressure limiter 11 and the filter 13. Valve 31 opens when the pressure in the conduit 12 upstream is, for example, 3 bars. Thus when pump 1 is in operation conduit 29 is at a pressure of at least 3 bars, which is sufficient to hold the dog clutches 23 in the engaged position.

Means are also provided for the springs 28 to be able to return the dog clutches 23 to the disengaged position when the pump 1 is not in operation. To this end, conduit 29 is connected to sump 4a (which is shown separately from sump 4 in order to clarify the drawing) through the agency of a flow limiter 32. This allows a very small leakage (e.g. 3 litres per minute) to the sump 4a during operation. Such leakage is acceptable. Consequently, when pump 1 is stopped the springs 28 can urge the forks 24 back into the disengagement position, while the oil contained in the jacks 27 is delivered to the sump 4a through the flow limiter 32.

As shown in Fig. 2, each jack 27 comprises a piston 33 subject to the pressure in the conduit 29, and which transmits the force resulting from this pressure to a push rod 34 through a spring 36. Rod 34 is mechanically connected to fork 24. The hardness of the spring 36 is such that it is not significantly compressed unless the spring 28 is prevented from being compressed. In the example illustrated, the springs 28 and 36 are disposed one after the other around the rod 34 on either side of a collar 37 on the latter. The travel of piston 33 in the direction of compression of the springs 28 and/or 36 is limited by a shoulder 38. These features enable the declutching systems to respond to a fairly low pressure of the order of 3 bars without any risk of being damaged when the pressure in the conduit 29 is equal to the full pressure that the pump 1 can deliver, e.g. 200 bars.

The pressure in the conduit 29 is taken by the shoulder 38 and does not therefore result in an axial thrust against the engaged dog clutch 23. In clutching and declutching systems using dog clutches, the internal teeth of the dog clutch 33 may unpredicatably abut the teeth of the engagement ring 26 during an engagement operation, thus preventing engagement of the dog clutch and preventing the dog clutch 23 from reaching the engaged position. In such cases, the rod 34 is locked in the intermediate position, but if the pressure in the conduit 29 is considerable the piston 33 reaches the shoulder 38 by compression of the hard spring 36. This thus prevents an inadmissible force being transmitted to the fork 24.

The four regions of the distributor 7 are designated 7a, 7b, 7c and 7d respectively.

Each of them, when brought to the operative position, establishes certain connections between the pump output 6, the return 12 to the sump 4, two feed conduits 39 and 41 for the motors 8, and a pilot conduit 42.

The pilot conduit 42 is connected to the pilot input of each of two valves 43, 44 which operate simultaneously with one another to connect the two motors 8 either in series or in parallel to one another. In the example illustrated, the two motors 8 are in series with one another when the conduit 42 is pressurized, i.e. connected to the output 6 of the pump, and in parallel with one another when the conduit 42 is connected to the return 12 to the sump 4.

The switching valve 43 provides selective connections between the four ports of the two motors 8 while one of the ports of one of the motors 8 is connected to the conduit 41 and the port of the other motor 8 having the opposite function (intake or exhaust) is connected to the conduit 39 through a complex connection 46 which inciudes the valve 44 and will be described hereinafter. In order to understand the function of the valve 43 it is sufficient to consider the connection 46 as a simple junction point.

In the absence of pressure in the conduit 42 (as shown in the drawing) valve 43 connects the port of each motor 8 to the port of the other motor having the same function (intake or exhaust). Thus the connection point 46 communicates with the two intake ports or the two exhaust ports depending on the direction of operation of the motors 8 and the conduit 41 communicates with the other two ports which are exhaust or intake ports respectively depending upon the direction of operation of the motors 8. The reason for this is that the motors 8 are of the type which can operate in both directions depending upon the direction of the oil flow through them. It will be apparent that in the position shown for the valve 43 the motors 8 are in parallel with one another.

In its other position valve 43 provides a direct connection between the two motor ports which are not directly connected to the conduit 41 and respectively to the connection 46, and closes the output to the conduit 41 and the output to the connection 46. In these conditions, the two motors 8 are connected in series with one another between the connection 46 and the conduit 41.

When the two motors 8 are in series with one another, the flow through them is strictly the same so that the half-shafts 14 are driven strictly at the same speed, subject to features which will be decribed hereinafter to enable curves to be negotiated. The structure of the connection 46 is intended to provide the same condition when the motors operate in parallel. In the situation shown in Fig. 1, which corresponds exactly to connection in parallel, valve 44 connects conduit 39 to the input of a flow splitter 47. The latter delivers equally to the conduit 48 extending to valve 43 and to the conduit 49 extending to motor 8, in accordance with the arrangement described hereinbefore, or alternatively, depending upon the direction of operation of the motors 8, accepts only equal flows of oil delivered by the conduits 48 and 49 to the conduit 39.In these conditions, the flows through the motors 8 are equal in both directions of operation.

In its other position, valve 44 provides direct communication between conduit 39 and conduit 49 and closes the input to the splitter 47, the output 48 of which is then also closed by the valve 43 as described hereinbefore.

The regions 7a and 7b of the distributor 7 correspond to two forward positions in which the conduit 41 is connected to the output 6 of the pump and the conduit 39 to the return 12 to the sump 4. When region 7a is in operation, conduit 42 communicates with output 6 of pump 1 so that the valves 43 and 44 are urged into their position in which the two motors B are connected in series. On the other hand, when region 7b is in operation conduit 42 communicates with the return 12 and springs (not shown) return the switching valves 43 and 44 to their position shown in Fig. 1, to provide the parallel connection of the motors 8.

The region 7c is the one which renders the motors 8 inoperative. The conduits 39 and 41 are connected to one another and are isolated from both the output 6 and the return 12.

Conduit 42 is connected to output 6 and to return 12, thus providing the parallel connection (alternatively the series connection could be provided in this situation).

When region 7d is in operation, the output 6 of pump 1 is connected to the conduit 39 and the return 12 is connected to the conduits 41 and 42, thus providing the parallel connection of the motors 8 and reversing the direction of flow of oil in the motors 8 so as to drive the half-shafts 14 in reverse.

Steps must be taken to ensure that there is no lack of oil at the input to the motors 8.

This might occur if the vehicle were driven at a speed higher than that corresponding to maximum delivery of the pump 1. To this end, each portof the same motor 8 is connected to the sump 4a or 4b through an intake valve 51.

Steps must also be taken to prevent the motors 8 experiencing any over-pressure on intake. To this end, each port of each motor is also connected by a respective delivery valve 52 to a pressure limiter 53 outputting to the sump 4a or 4b. The pressure limiters 53 can render the limiter 11 inoperative.

The system shown in Fig. 1 operates as follows: When pump 1 is operating, with distributor 7 in the position 7c, the pump delivery is maximum but the pressure is limited to the pressure determined by the limiter 31 so that the absorbed power is very small.

If the pump is in operation with the distributor in one of its positions 7a, 7b or 7d, the tractor being stopped and braked, the pressure rises in the circuit and in the motors 8 and the torque is transmitted to the vehicle wheels. This torque is not sufficient to push the tractor and the braked vehicle. The pressure rises to the cut-off pressure while the flow is limited to that required to compensate for the circuit leaks. The absorbed power is very small because of the small delivery.

When the pump is in operation with the distributor in one of the positions 7a, 7b or 7d, with the tractor running in the corresponding direction, the pump feeds the motors. The latter rotate and the delivery to them from the pump increases until the vehicle tends to go faster than the tractor. From then on the torque delivered by the motors 8 is inadequate to push the combination so that the pressure rises in the circuit to the uniform pressure as set at the pump 1. The pressure delivered to the motors 8 is constant irrespective of the speed of the combination provided that it does not exceed the speed corresponding to maximum pump delivery.

When the motors 8 are connected in series, each motor has all the delivery flowing through it and for a given maximum pump delivery the maximum possible speed for the vehicle wheels is twice that permitted when the motors 8 are connected in parallel and hence have half the delivery flowing through each. On the other hand, when the motors 8 are in series, they must share the pressure delivered by the pump 1, so that the torque delivered to the wheels is substantially half the torque delivered when the motors 8 connected in parallel each experience the total pressure delivered by the pump 1. It should be noted that in reverse operation only the parallel connection has been provided because in that direction of operation the speed is always reduced.

On turns, in which the vehicle wheels rotate at different speeds, the motors 8 have to have different flows. This is possible by return flow means 51 and/or excess pressure valves 53. At high speeds, e.g. on roads, the motors 8, even connected in series, are driven at a speed corresponding to a delivery greater than the maximum possible pump delivery. Motors 8 are converted to pumps fed partially by pump 1 and partially through the return flow valves 51 and deliver to the sump 4. In this situation it is advisable for the driver to stop the pump 1, in which case the pressure drops in the conduit 29, thus disengaging the half shafts 14 relatively to the motors 8 and stopping unnecessary energy consumption in this way.

In the example shown in Fig. 3, each halfshaft 14 does not drive one wheel directly, but drives two wheels 56 through the medium of a chain 54, said wheels being mounted one behind the other on a rocker 57 pivoted to the chassis 58 of the vehicle along an axis 59 extending parallel to the chassis plane transversely of the chassis. Chain 54 extends round a gearwheel 61 connected to the end of the half shaft 14 and two gearwheels 62 each connected to one of the wheels 56 and disposed between the latter and the rocker 57. The half shafts 14 extend along the axis 59 and are therefore fixed with respect to the chassis 58 despite the movements of the wheels 56 with respect thereto.

Of course the invention is not limited to the examples described and illustrated, and numerous modifications may be made to these examples without departing from the scope of the invention.

For example, zone 7c of the distributor 7 could be designed to isolate the output 6 from the return 12 and the conduits 39, 41, 42 instead of connecting it to the return 12.

In these conditions, when zone 7c is in operation, pump 1 delivers its uniform pressure at a very small rate of flow instead of providing its maximum delivery at low pressure, but the power consumption is subtantially the same.

For mechanical simplification purposes, the valves 43 and 44 which always operate together can be regrouped in a single valve.

A vehicle having four driving wheels could alternatively be embodied by connecting each half-shaft 14 directly to a respective wheel, which is in turn connected by a transmission means, such as a chain, to the other wheel situated on the same side of the vehicle. If the wheels are mounted on a rocker, each hydraulic motor can then be fixed to one of the rockers.

In a variant of the system, just a single hydraulic motor may be provided which drives the input of a differential, the outputs of which are in the form of the half shafts, e.g.

14, each driving one wheel, for example, or two wheels connected by a transmission means.

Claims (18)

1. A hydraulic drive system for a towed vehicle, more particularly a vehicle used in agriculture or forestry, and adapted for hitching to a tractor, such as an agricultural tractor, the system comprising a hydraulic pump feeding hydraulic drive means and means for mechanically coupling the drive means to the vehicle wheels, characterised in that the pump is of the type outputting a substantially uniform pressure by automatic adjustment of its delivery.

2. A system according to claim 1, characterised in that the substantially uniform pressure delivered by the pump is controllable.

3. A system according to claim 1 or 2, characterised in that the mechanical coupling means comprise dog clutch disengagement means.

4. A system according to claim 3, characterised in that the dog clutch disengagement means are sensitive to the hydraulic pressure upstream of the drive means.

5. A system according to claim 4, characterised in that the dog clutch disengagement means are actuated, in the direction of dog clutch engagement, by at least one jack which is actuated by the hydraulic pressure upstream of the hydraulic drive means and through the agency of a force limiting spring.

6. A system according to claim 4, characterised in that dog clutch disengagement means are controlled in the direction of dog clutch engagement by at least one jack which is returned to the dog clutch disengagement position by a spring and connected to the hydraulic circuit upstream of the drive means by a conduit connected to the sump through calibrated leakage means.

7. A system according to any one of claims 1 to 6, characterised by means for reversing the direction of the flow of oil in the drive means so as to allow operation in both directions.

8. A system according to any one of claims 1 to 7, characterised in that the drive means comprise two hydraulic motors adapted each to drive at least one respective wheel of the vehicle.

9. A system according to claim 8, characterised by means for feeding the two motors in parallel.

10. A system according to claim 9, characterised in that the means for feeding the two motors in parallel comprise a flow splitter connected to a junction between the paths over which the two motors are fed in parallel.

11. A system according to claim 8 or 10, characterised in that it comprises hydraulic switching means enabling the motors to be fed in series or parallel at choice.

12. A system according to any one of claims 8 to 11, characterised in that it comprises, at the intake of each motor, an intake pressure limiter and/or means for drawing oil from the sump in parallel with the pump.

13. A vehicle for use in agriculture or forestry and having at least four wheels, adapted for hitching to a tractor and using a system according to any one of claims 8 to 12, in which two wheels are disposed one behind the other on each side of the vehicle, characterised in that the two wheels on each side are connected by a rotary coupling means and are driven together by one of the motors of the hydraulic drive system.

14. A vehicle according to claim 13, characterised in that the two wheels on each side are borne by a common rocker mounted to oscillate along a transverse axis of the vehicle.

1 5. A vehicle whenever comprising a hydraulic drive system in accordance with any one of Claims 1 to 12.

16. A hydraulic drive system, substantially as described in Figures 1 and 2 of the accompanying drawings.

17. A vehicle, substantially as described in Figures 1 to 3 of the accompanying drawings.

18. Any novel feature or combination of features described herein.