A HYDRAULIC DRIVE SYSTEM This invention relates to hydraulic drive systems and more particularly but not solely to the use of variable volume hydraulic motors to drive the wheels of a vehicle. The present invention is concerned with obviation of wheel slip which can occur when a vehicle starts from rest but which is particularly evident as a vehicle changes from running on a horizontal surface to climbing a hill. In the latter circumstances the weight on the front axle reduces and that on the rear axle increases. The tractive force that can be transmitted by a wheel is a function of the weight on the wheel, and therefore the torque that can be applied to the wheel without the wheel exhibiting slip will be a function of the weight on the wheel, the diameter of the wheel and of course the coefficient of friction between the wheel and ground.

The torque that a hydraulic motor can deliver is directly proportional to the swept volume of the motor and the pressure difference across the motor. Thus when a vehicle is travelling on the level there is an optimum hydraulic motor swept volume per axle per revolution of the wheel. If the pressure demand of each of the wheels is arranged to be an optimum value for level ground, it will be appreciated that the front wheels of the vehicle will be inclined to slip when the vehicle climbs a hill due to the reduction of weight on the wheel. The present invention seeks to overcome or at least substantially reduce this problem and to provide a hydraulic drive system suitable for use in vehicles but which is also applicable to hydraulic

drive systems where similar risk of slip can result due to different varying loadings.

According to the invention there is provided a hydraulic drive system comprising a plurality of wheels drivable by hydraulic motors from a source of hydraulic power, characterised in that the torque supplied by at least one of the motors is arranged to be varied relative to the torque of at least one of the other motors in dependence upon the operation pressure generated by the hydraulic source.

The invention resulted from the observation that in a hydraulic vehicle drive system the pressure increases as the vehicle starts to ascend a slope and is a function of the inclination of the slope. By sensing this pressure and using it to control the torque supplied to wheels susceptible to slip it is possible to provide a much improved drive system enabling the tractive effort to be increased and its distribution optimised.

A particularly convenient way of implementing the invention is to arrange that said at least one of the hydraulic motors is a variable volume motor the swept volume of which is variable in dependence upon the operation pressure. At least one of the other hydraulic motors may be a fixed displacement motor. In a refinement of the invention the, or each, variable swept volume motor is connected between feed and return lines of a hydraulic pressure source, the displacement control mechanism of the motor is coupled with a piston and cylinder arrangement in which a piston is biased to an intermediate position in the cylinder and the cylinder has opposite ends coupled one to the feed and one to the return lines such that relative displacement between the piston and cylinder effects adjustment of the displacement control. Alternatively, an electric servomotor may be arranged to vary the swept volume in dependence upon an electrical control signal derived from a pressure sensitive transducer coupled with the feed line of the hydraulic pressure source.

In a practical application of the invention there

would be provided a variable delivery pump for providing the source of hydraulic pressure generation for controlling torque supplied by the hydraulic motors.

The present invention also extends to the provision of a vehicle incorporating a hydraulic drive system as previously described having at least two wheels spaced along the vehicle and each driven by one of the hydraulic motors such that the torque supplied to one of the wheels is varied relative to the torque supplied to the other of the wheels in dependence upon the operating pressure supplied by the hydraulic source. Two pairs of wheels may be provided each of which pairs is oppositely disposed on the vehicle, which pairs are spaced mutually apart along the vehicle. At least one of the pairs of wheels may be driven by a single motor either directly or via a differential gear. The wheels of at least one of the pairs may be driven by individual hydraulic motors.

In order that the invention and its various other preferred features may be understood more easily, embodiments thereof will now be described, by way of example only, with reference to the drawings in which:-

Figure 1 is a schematic illustration of a four wheel vehicle incorporating a hydraulic drive system without traction control, Figure 2 is a schematic hydraulic circuit diagram illustrating a hydraulic drive system with traction control constructed in accordance with the invention,

Figure 3 is a graphical illustration to illustrate how speed, swept volume and operating pressure of the hydraulic system varies in dependence upon the slope when negotiated by a typical hydraulically powered vehicle, and

Figure 4 is a cross sectional illustration of an alternative hydraulic servo device suitable for use in the configuration of Figure 2. Referring now to Figure 1 a vehicle 10 has a pair of oppositely disposed front wheels 11 and 12 and a pair of oppositely disposed rear wheels 13 and 14. An engine driven variable delivery pump 15 provides a pressure source for driving all four wheels and also drives a booster pump 18

for replacing hydraulic fluid lost from the system. The rear wheels 13 and 14 are driven by a hydraulic motor 16 via a differential gear 17. The pump 15 also supplies hydraulic fluid to individual hydraulic motors 19 and 20 for driving wheels 11 and 12. In normal circumstances the motors will be designed to provide similar tractive forces to each of the wheels. It will be appreciated that during rapid acceleration and deceleration, or upon negotiating an uphill or downhill gradient, slipping of some of the wheels is likely to occur.

Referring now to Figure 2 this shows symbolically a modification of the system of Figure 1 incorporating the features of this invention. For simplicity similar elements of the system have been given the same numbers as those elements in Figure 1. Instead of the conventional fixed volume motor 16 for driving the rear wheels the system of Figure 2 employs a variable volume motor 21 in which the swept volume may be varied by varying the angle of the swash plate. The larger the angle the greater will be the swept volume. A typical type of motor for this purpose is an axial piston variable displacement, swash plate unit but other types of displacement changing control mechanism may be employed. In the former type, the angle of the swash plate is controlled by a hydraulic servo device 22, which comprises a cylinder 23 and a shuttle piston 24. The piston 23 is jointed to the swashplate by the coupling 25. The piston is held in a median position in the cylinder by two springs 26 and 27 which are preferably not preloaded. The two ends of the shuttle cylinder are connected hydraulically to the respective supply and return sections of the hydraulic 'system' .

The springs 26 and 27 may be of differing load/deflection characteristic and the effective piston areas 28 and 29 although shown symbolically to be equal may be different. These springs and piston areas are proportioned to provide the optimum swash-plate deflection in response to supply and return circuit pressure magnitudes and rate of change charcteristics.

The median 'or node' position of the servo shuttle

piston 24, and hence variable motor swash plate angle, is predetermined by the required displacement of motor 21, for optimum performance when operating at constant velocity on level ground. The circuit also includes normal hydraulic circuit components including dual relief valves 30 and 31, which open if the pressure rises above a certain level in the hydraulic circuit, each one of the valves operating in a different direction as the pump can be arranged to deliver fluid in either direction round the hydraulic circuit to drive the vehicle forwards or in reverse. Another normal arrangement is a boost purge system 32 for permitting purging of excess fluid from the system to a reservoir 33 whilst the booster pump 18 provides injection of fluid to the system to replace losses in response to boost inlet check valves 34 and 35.

In operation, when the vehicle is being driven onto an ascending gradient, the pressure on the forward side of the hydraulic system e.g. say, the righthand side as illustrated in the drawing, is caused to increase. The increase of pressure to the area 29, causes the piston shuttle 24 to be deflected against the bias of spring 26 to the left thereby increasing the swept volume of the motor 21 and increasing the torque delivered by the rear axle as the gradient increases. In this way as additional weight is transferred to the rear axle, additional torque is delivered to the rear wheels proportioned in such a way as to prevent wheel slip. Similarly when the vehicle commences a down gradient and drive is reduced to induce a braking effect, pressure in the system is reversed such that a higher pressure exists on the lefthand side of the system compared with the righthand side of the system. In this circumstance the high pressure fed to the lefthand side of the cylinder 23 to the area 28 of the piston causes shuttle 24 to be urged towards the right against the bias of the spring 27 which reduces the swept volume of the motor 21 and thus the proportion of the torque delivered to the rear axle is reduced. This system also compensates automatically for operation of the vehicle in reverse and also for accelerating and decelerating forces

on the level in both directions of travel.

The graph of Figure 3 is an illustrative example of calculated values of speed, swept volume and operating pressure for a grass cutting machine plotted against 5 downhill and uphill gradients.

Referring now to Figure 4 there is illustrated in detail an alternative hydraulic servo device providing a similar function to the hydraulic servo device 22 and which constitutes a further refinement of the invention. A 10 cylindrical housing 40 contains a cylindrical piston 41 slidably mounted therein. The housing at one end of the cylinder is provided with an inlet nozzle 42 for connection to one side of the hydraulic circuit shown in Figure 2. The nozzle is cylindrical and contains a piston 43 which bears 15 on the closed end of the cylindrical piston 41 and serves to displace the piston, upwardly in the illustrated configuration.

The housing at the other end has a closure plate 45 which is provided with an elongate inlet nozzle 46 for 20 connection with the other side of the hydraulic circuit shown in Figure 2. The nozzle is cylindrical and contains a piston 47 which bears on the inside blind end of the cylindrical piston 41 and serves to displace the piston, downwardly in the illustrated configuration. The nozzle 46 * 25 is screw threaded into the closure plate 45 and has an enlarged end 48 inside the housing which serves as an end stop for a ferrule 49 which is slidably mounted on the nozzle 46 inside the piston 41. A second ferrule 50 is similarly mounted on the nozzle 46. Each ferrule has 30 enlarged end at remote ends of the cylinder 41 which serve to retain a coil spring 51 therebetween. The spring bears against the enlarged ends of the ferrule 49 and the ferrule 50. The assembly of ferrules 49,50 and spring 51 mounted on the nozzle 46 is retained in the cylindrical piston 41 by 35 means of a circlip 52. Spring ferrule 49 is constrained from downward movement by the enlargement 48 of nozzle 46. The spring ferrule 50 is constrained from upward movement by the spacer 57. The length of spacer 57 is predetermined by the required 'node' position of the motor swash plate and

hence position of piston 41. A lever 44 has a peg 53 which engages with the piston 41 through a slot in the housing and is actuated by movement of the piston to adjust the displacement of the variable volume motor 21 shown in Figure 2. The inlet nozzle 46 is adjustable in and out of the housing to set a predetermined bias of the spring (or no bias) and to determine the steady state position of the piston 41 when there is no differential pressure between the two inlet nozzles. In this way is controlled the unloaded displacement of the variable volume motor and also the response of the motor to differential pressure. Once set the nozzle is secured by a lock nut 54. Adjustable end stops 55 and 56 are provided at opposite ends of the housing which serve to limit the travel of the piston 41. In operation when the pressure increases in the nozzle 46 relative to the nozzle 42 the piston 47 moves the piston 41 downwardly, in the illustrated configuration against the retroactive force of the spring, until the piston hits the end stop 56. This movement via the linkage with the variable displacement motor effects adjustment of the displacement of the motor. Similarly if the differential pressure between the two nozzles is in the opposite sense the piston 43 moves the piston 41, upwardly, in the illustrated configuration, until the skirt of the piston 41 hits the end stop 56. This movement via the linkage with the variable displacement motor again effects adjustment of the displacement of the motor but in the opposite sense.

Although the specific description has been directed to a four wheel vehicle it will be appreciated that the principals of this invention can be applied to any hydraulic drive system which employs a plurality of drive wheels which are likely to experience different resistive forces e.g. in hydraulically powered processing machinery. Insofar as its application to vehicles is concerned it will be appreciated that the invention is applicable to vehicles having two, three, four or more wheels and the term vehicle is intended to extend to non passenger or goods carrying machines e.g. pedestrian controlled vehicles such as lawn mowers.

An alternative method of varying the swept volume of

the motor 20 in dependence upon pressure in the system is to employ an electrical servo mechanism linked to the swash plate which mechanism is responsive to an electrical control signal derived from one or more pressure sensitive electrical transducers coupled with the hydraulic line. The transducers could be arranged to provide for example a voltage related to the pressure difference between opposite sides of the hydraulic circuit and this voltage could be used as a control voltage for actuating the servo mechanism to move the swash plate to a predetermined angle to control the proportion of the torque delivered by the rear axle.

Although the embodiment described with reference to

Figure 2 employs a differential drive to the rear wheels it will be appreciated that a single motor could be employed to provide a direct drive to those wheels or alternatively individual motors could be employed for each wheel. Any combination of individual motors and wheels driven by a single motor via a differential drive can be employed without departing from the scope of this invention.