Patent Description:
Modern agricultural tractors are equipped with various hydraulic consumers requiring a supply of pressurised hydraulic fluid. One important hydraulic consumer is the steering system which is supplied by various pumps to ensure steering capability in all operating conditions whilst the vehicle is moving, even if the engine or other prime move is shut down by damage. A typical supply system is briefly described below:
A main pump supplies the steering system as well as auxiliary hydraulics (different consumers on the tractor, for example front and/or rear linkages, main valve manifold and a Power-Beyond Connection). If the main supply pump is overloaded (which may occur if the steering moves very fast or the main supply circuit and connected consumers of the auxiliary hydraulics demand too much oil flow) and is not capable of providing the fluid pressure required to maintain the steering function, a secondary pump, the steering pump, is utilised to additionally supply the steering system to ensure that the vehicle can be steered. The main and secondary pumps are driven by the vehicle prime mover, which is often an internal combustion engine but may be an electric motor or a hybrid combination of the two. Should the prime mover fail, this would cut the supply of pressurised fluid and disable the hydraulic steering system. Unlike on a conventional car where the vehicle can often be steered mechanically in the event that a fluid power assisted steering system fails whilst the vehicle is moving, agricultural vehicles cannot usually be steered adequately without fluid power assistance. Therefore, many agricultural vehicles have a third pump, referred to as emergency steering pump, which is driven by the ground-engaging wheels and so is able to provide a supply of pressurised fluid to the steering system in the event that the prime mover fails whilst the vehicle is moving. This provides emergency redundancy cover for the main and secondary pumps to provide continuation of the steering function whilst the vehicle is moving.

The number of hydraulic consumers used on modern tractors is increasing. For example, the increased demand for cooling capacity has resulted in the introduction of hydraulic drive systems for cooling fans. In such drive system, a hydraulic motor which drives the cooling fan is supplied by a pump assigned only for this purpose. Such a system is described in applicant's published patent application <CIT>. As a consequence, fluid supply systems of tractors are becoming more and more complex requiring an increasing number of pumps. Increasing the number of pumps installed on a tractor increases costs and requires valuable installation space. Furthermore, increasing the number of pumps in a fluid supply system generally means reducing efficiency as each pump is circulating oil even when the demand made on it is zero.

<CIT>, on which the preamble of claim <NUM> is based, describes a hydraulic system for an earth moving machine having a primary pump for supplying the hydraulic steering system and a ground speed pump which can be used to provide dynamic braking. A prioritisation valve is operable to directed fluid from the ground speed pump to the steering system when the fluid pressure in the steering system falls below a predetermined level.

<CIT> discloses a hydraulic system for a vehicle having a first pump for supplying a primary consumer such as a hydrostatic steering system and a second pump for supplying a secondary consumer. A prioritisation valve is configured to direct fluid from the second pump to a supply line for the primary consumer if the fluid pressure in the supply line falls below a certain value.

It is an object of the present invention to provide a fluid supply system that addresses the issue of costs, installation space and efficiency.

Furthermore, the object of the invention is to provide a control system for a hydraulic fan drive which enables the use of a fluid supply system according the invention.

In accordance with an aspect of the invention there is provided a pressurised fluid supply system for an agricultural vehicle, comprising:.

wherein the cooling fan pump is fluidly connectable with the steering circuit so as to be able to provide a second a source of pressurised fluid to the steering system, the prioritisation valve being configured to adopt a first operative configuration in which fluid flow from the cooling fan pump to the cooling fan motor through the prioritisation valve is substantially unrestricted when the pressure in the steering circuit is at or above a first threshold value and to adopt a second operative configuration in which fluid flow from the cooling fan pump to the cooling fan motor through the prioritisation valve is partially restricted when the pressure in the steering circuit is below the first threshold value; characterized in that one of the cooling fan pump and the cooling fan motor is a variable displacement machine, the fluid supply system having a control system operative to adjust displacement of the variable displacement machine to maintain the input drive power provided by the cooling fan motor to the cooling fan substantially constant or at a demanded value and/or to vary the speed of the fan in accordance with cooling demand.

In a fluid supply system according to the invention, the cooling fan pump acts as a secondary pump for the steering system and drives a cooling fan. This helps to keep the number of pumps in the system to a minimum and improves overall system efficiency, whilst ensuring continuity of supply for the steering system to meet safety requirements. Since the flow to cooling fan is only partially restricted when the prioritisation valve is in its second configuration, a supply of pressurised fluid is maintained to the cooling fan in most circumstances. The control system is operative to adjust the displacement of the variable displacement pump or motor to compensate for the loss in pressure in the fluid supplied to the cooling fan motor when the cooling fan pump is called upon to supply the steering system so as to maintain effective cooling.

In an embodiment, the prioritisation valve may has a third operative configuration in which fluid flow from the cooling fan pump to the cooling fan motor through the prioritisation valve is substantially blocked, the prioritisation valve being configured to adopt its second operative configuration when the fluid pressure in the steering circuit is below the first threshold value but at or above a second threshold value lower than the first threshold value and to adopt its third operative configuration when the fluid pressure in the steering circuit is below the second threshold value.

The cooling fan circuit may be fluidly connected to the steering circuit at a position upstream of the prioritisation valve through a check valve which prevents fluid flowing in a direction from the steering circuit towards the cooling fan pump. The arrangement may be configured such that, in use when the prioritisation valve is in its second or third operative configuration, the fluid pressure in the cooling fan circuit upstream of the prioritisation valve increases causing the check valve to open to admit pressurised fluid from the cooling fan circuit into the steering circuit when the pressure in the cooling fan circuit upstream of the prioritisation valve is higher than the pressure in the steering circuit by a threshold amount.

The fluid supply system may also comprise an emergency steering pump fluidly connectable with a fluid tank or reservoir through an emergency steering pump circuit, the prioritisation valve being operative to regulate the flow of fluid in emergency steering pump circuit from the emergency steering circuit to the fluid tank or reservoir, the emergency steering pump circuit being fluidly connected with the steering circuit at a position upstream of the prioritisation valve, wherein the flow of fluid from the emergency steering pump through the prioritisation valve to the tank or reservoir is substantially unrestricted when the prioritisation valve is in either of its first and second operative configurations. Where the prioritisation valve has a third operative configuration, the flow of fluid from the emergency steering pump through the prioritisation valve may be substantially blocked when the prioritisation valve is in its third operative configuration.

The emergency steering circuit may be fluidly connected to the steering circuit through a check valve which prevents fluid flowing in a direction from the steering circuit to the emergency steering pump. The arrangement may be configured such that, in use when the prioritisation valve is in its third operative configuration and the vehicle is moving, the fluid pressure in the emergency steering circuit upstream of the prioritisation valve increases causing the check valve to open and admit pressurised fluid to flow from the emergency steering circuit into the steering circuit when the pressure emergency steering circuit upstream of the prioritisation valve is higher than the pressure in the steering circuit by a further threshold amount.

In an embodiment, the prioritisation valve is a spool valve, in which the valve spool has at least two positions corresponding to the at least two operative configurations, wherein fluid pressure in the steering circuit is applied to the spool to drive spool movement in a first direction and wherein fluid pressure in a steering system load sensing circuit and a biasing force from at least one resilient member is applied to the spool to drive the spool in a second direction opposite to the first. Where the prioritisation valve has a third operative configuration, the valve spool may have three positions, each corresponding to a respective one of the three operative configurations, and the at least one resilient member may be a two-stepped spring set.

The prioritisation valve may be configured such that the degree to which the flow of fluid through the valve from the cooling fan pump to the cooling fan motor is restricted when in its second operative configuration is variable, dependent on the fluid pressure in the steering circuit.

The control system may additionally be operative to adjust the displacement of the variable displacement machine to vary the speed of the fan in accordance with cooling demand. The control system may include means to determine the input drive power provided by the cooling fan motor to the cooling fan, which may comprise a pressure sensor for monitoring the pressure of the fluid supplied to the cooling fan motor and/or a speed sensor for determining the speed of the fan.

The control system may have a pressure sensor for monitoring the pressure of fluid flowing from the prioritisation valve to the cooling fan motor and/or a fan speed sensor for determining the speed of the cooling fan and may be configured to adjust the displacement of the variable displacement machine in dependence on the pressure of the fluid flowing from the prioritisation valve to the cooling fan motor and/or the speed of the fan.

The control system may comprise one or more controllers which may collectively comprise an input (e.g. an electronic input) for receiving one or more input signals (e.g. the pressure signal and/or the fan speed signal) indicative of the input drive power provided by the cooling fan motor to the cooling fan. The one or more controllers may collectively comprise one or more processors (e.g. electronic processors) operable to execute computer readable instructions for controlling operation of the control system, for example to determine the input drive power provided by the cooling fan motor to the cooling fan. The one or more processors may be operable to generate one or more control signals for controlling the displacement of the variable displacement machine. The one or more controllers may collectively comprise an output (e.g. an electronic output) for outputting the one or more control signals.

In an embodiment, cooling fan pump is a variable displacement pump and the cooling fan motor is a constant displacement motor. In an alternative embodiment, the cooling fan pump is a constant displacement pump and the cooling fan motor is a variable displacement motor.

The control system may include a sensor arrangement for monitoring movement of a steering system in a vehicle to which the fluid supply system is mounted and be configured to predict, based on an input or inputs from said sensor arrangement, when the steering system is expected to place an increased demand on the main pump likely to result in the prioritisation valve moving from its first operative configuration to its second operative configuration and to adjust the displacement of the variable displacement machine in advance of the prioritisation valve moving from its first operative configuration to its second operative configuration. The sensor arrangement may be configured to sense the rotary position and/or rotary movement of a steering wheel or other steering control of the vehicle.

The main pump may be connected to the steering circuit though a check valve which prevents fluid flowing in a direction from the steering circuit to the main pump.

The fluid supply system may be a hydraulic fluid supply system.

In accordance with a further aspect of the invention, there is provided an agricultural vehicle including a pressurised fluid supply system according to the various aspects of the invention set out above.

Within the scope of this application, as defined by the appended claims, it should be understood that the various aspects, embodiments, examples and alternatives set out herein, and individual features thereof may be taken independently or in any possible and compatible combination. Where features are described with reference to a single aspect or embodiment, it should be understood that such features are applicable to all aspects and embodiments unless otherwise stated or where such features are incompatible.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:.

The consumers in a pressurised fluid supply system are connected with fluid lines or conduits which are also named as circuits in the following description. It will be understood that the term "circuit" as used herein is not limited to a closed loop arrangement of lines or conduits and may refer to arrangements as simple as a single fluid line linking two components or consumers.

Furthermore it will be understood that the term "motor" and "pump" as used herein in reference to a pressurised fluid supply system describe fluid machines which may be hydrodynamic or hydrostatic, unless the context requires otherwise.

<FIG> illustrates schematically a first embodiment of a fluid supply system <NUM> in accordance with an aspect of the invention and which is suitable for use in an agricultural vehicle, such as an agricultural tractor represented at <NUM>.

The fluid supply system is a hydraulic system and includes three pumps, <NUM>, <NUM>, <NUM> for supplying pressurised hydraulic fluid (oil) to different circuits on the agricultural vehicle such as a tractor <NUM>. These are a main supply pump <NUM>, a cooling fan pump <NUM>, and an emergency steering pump <NUM>.

The main supply pump <NUM> is a variable displacement type pump and is operable to generate a fluid pressure which may be up to <NUM> bar or more, for example. The main pump <NUM> is driven by the prime mover of the vehicle and supplies auxiliary hydraulic systems (indicated schematically at <NUM>) which include various hydraulic consumers on the tractor, for example front and/or rear linkages, main valve manifold and/or a power beyond supply system, via main supply circuit <NUM>. In addition, main supply pump <NUM> delivers oil to a steering system, indicated schematically at <NUM>, via a steering circuit <NUM>. The main supply pump <NUM> is connected to the steering circuit though a first check valve <NUM> which prevents pressurised fluid flowing to the main pump <NUM> from the steering circuit and prevents the other pumps <NUM>, <NUM> counteracting the main supply pump <NUM>.

The displacement (and thereby mass-flow) of main supply pump <NUM> is controlled by a load-sensing circuit <NUM>. The load-sensing circuit <NUM> is connected to the major consumers and is configured to regulate the delivery of the main pump <NUM> according to current requirements. The load sensing circuit <NUM> is connected to the auxiliary hydraulic systems <NUM> by an auxiliary hydraulic system load sensing circuit 28a and to the steering system <NUM> by a steering system load sensing circuit 28b. The load sensing circuit <NUM> is connected to a pressure limiting valve <NUM> which opens a vent connection to a hydraulic fluid storage tank or reservoir <NUM> if pressure in the load sensing circuit exceeds a predetermined limit. For example, the pressure limiting valve <NUM> may be set to open at a pressure in the region of <NUM> bar. The load sensing circuit <NUM> is connected with the auxiliary hydraulic systems <NUM> load sensing circuit 28a and the steering system <NUM> load sensing circuit 28b by means of a shuttle valve <NUM>. The shuttle valve <NUM> provides a hydraulic OR functionality to forward the highest load sensing signal/pressure coming from either the steering system <NUM> or from the auxiliary hydraulic systems <NUM> to the main supply pump <NUM>. The shuttle valve also ensures that the load sensing signals from the steering system <NUM> and the auxiliary hydraulic systems <NUM> do not counteract one another. This ensures that the highest load sensing pressure/signal coming from the steering system <NUM> or the auxiliary hydraulic systems <NUM> is used to determine the required output from the pump <NUM>.

The cooling fan pump <NUM> is operative to supply a cooling fan circuit <NUM>. The cooling fan pump may also supply fluid up to a pressure in the region of <NUM> bar or more. The cooling fan circuit <NUM> includes a hydraulic cooling fan motor <NUM> drivingly connected to a cooling fan <NUM> and a cooling fan circuit heat exchanger <NUM>. The cooling fan circuit heat exchanger <NUM> is provided for cooling the fluid in the cooling fan circuit <NUM>. It is shown in <FIG> spaced from the cooling fan <NUM> for clarity reasons. However, in a vehicle <NUM> the cooling fan circuit heat exchanger <NUM> is installed in the airflow from the cooling fan <NUM>. It may be installed as part of a main cooling package having several heat exchangers for cooling various systems of the vehicle which might include, as examples only, engine cooling fluid, combustion air, or exhaust gas. After passing through the cooling fan circuit heat exchanger <NUM> the fluid is returned to fluid tank <NUM>. The flow of pressurised fluid in the cooling fan circuit from the cooling fan pump <NUM> to the cooling fan motor <NUM> is directed through a prioritisation valve <NUM>.

In this embodiment, the cooling fan pump <NUM> is a variable displacement pump and the cooling fan motor <NUM> is a constant displacement motor. The cooling fan pump <NUM> is driven by the vehicle prime mover and its displacement (and thereby mass-flow) is controlled by a solenoid valve (generally indicated at <NUM>) integrated in cooling fan pump <NUM>. Control of the displacement of cooling fan pump <NUM> will be described in more detail below.

In addition to supplying the cooling fan circuit <NUM>, the cooling fan pump <NUM> also delivers fluid into a secondary steering circuit <NUM> which is connected to the steering circuit <NUM>. This enables the cooling fan pump <NUM> to support the main supply pump <NUM> in delivering oil to the steering system <NUM> under certain circumstances. This would typically be required if the main supply pump <NUM> becomes overloaded and is not capable of providing the required fluid pressure to maintain the steering function adequately. This might occur, for example, if the steering system is moved very fast or if the demand for pressurised fluid from the main supply circuit <NUM> and connected auxiliary consumers increases too much for the main pump to cope with. A second check valve <NUM> is provided between the secondary steering circuit <NUM> and the steering circuit to prevent fluid flowing from the steering circuit towards the cooling fan pump <NUM> and into the cooling fan circuit <NUM>. This avoids the cooling fan pump <NUM> and main supply pump <NUM> counteracting each other. The flow of pressurised fluid from the cooling fan pump <NUM> to the cooling fan motor <NUM> and the steering system <NUM> is controlled by the prioritisation valve <NUM>. The secondary steering circuit <NUM> is connected to the cooling fan circuit <NUM> at a position upstream of the prioritisation valve <NUM>.

The emergency steering pump <NUM> is connected to an emergency steering pump circuit <NUM> which usually directs fluid from the pump back to the fluid tank <NUM> through the prioritisation valve <NUM>. The emergency steering pump <NUM> may also supply fluid up to a pressure in the region of <NUM> bar or more. As illustrated in <FIG>, the flow from the emergency steering pump <NUM> can be directed from the prioritisation valve <NUM> to join the cooling fan circuit <NUM> downstream of the cooling fan motor <NUM> but upstream of the cooling fan circuit heat exchanger <NUM> so as to pass through the heat exchanger before being returned to the tank. However, this is not essential and it can be routed from the valve <NUM> to the tank <NUM> in any suitable manner.

The emergency steering pump <NUM> is also connected with the steering circuit <NUM> through an emergency steering supply line <NUM>. A third check valve <NUM> is provided between the emergency steering supply line <NUM> and the steering circuit <NUM> to prevent fluid flowing in a reverse direction from the steering circuit <NUM> into the emergency steering pump circuit <NUM> and avoids the main supply pump <NUM> and the emergency steering pump <NUM> counteracting each other. Flow of pressurised fluid from the emergency steering pump <NUM> to the tank <NUM> also passes through and is regulated by the prioritisation valve <NUM>. The emergency steering supply line <NUM> is connected to the emergency steering pump circuit <NUM> at a position upstream of the prioritisation valve <NUM>.

Unlike the main supply pump <NUM> and the cooling fan pump <NUM> which are driven from the prime mover, the emergency steering pump <NUM> driven by the ground-engaging members (wheels or tracks). Accordingly, in the event the prime mover should fail, for example if the prime mover is an engine and it stalls, or should both the main pump <NUM> and the cooling fan pump <NUM> fail, the emergency steering pump will continue to be driven by the vehicle ground engaging members so long as the vehicle is moving and so is able to provide pressurised fluid to the steering system as an emergency backup. The emergency steering pump <NUM> can be described as a ground-speed driven pump. The emergency steering pump <NUM> is generally a smaller pump than the main supply pump or the cooling fan pump as it is only required to provide a basic emergency steering function which requires the driver to apply higher steering forces than during normal operation of the vehicle.

Operation of the prioritisation valve <NUM> to regulate the supply of pressurised fluid from the cooling fan pump <NUM> and the emergency steering pump <NUM> will now be described.

The prioritisation valve <NUM> according to the present embodiment is a spindle valve in which the valve spindle (not shown) has three has three positions 48a, 48b, 48c, which correspond with three operative configurations of the valve. The valve spindle is biased in a first direction, to the right as the valve is illustrated in <FIG>, by the fluid pressure in the steering circuit <NUM>, which connected to a first control port on one side of the valve by a fluid line <NUM>. The valve spindle is biased in the opposite direction (to the left as shown in <FIG>) by the fluid pressure (the load-sensing signal) in the load sensing circuit 28b, which is connected to a control port on the opposite side of the valve, and by a two stepped spring set <NUM>, which in this embodiment are set to be equivalent to <NUM> bar (48a) and <NUM> bar (48b). The load sensing signal to the prioritisation valve <NUM> is taken from load sensing circuit 28b on the steering system side of the shuttle valve <NUM> and so is representative of the load sensing signal (fluid pressure) coming from the steering system.

The operating conditions under which the three positions or operative configurations of the prioritisation valve <NUM> are selected are as follows:.

It will be appreciated that the threshold pressures in the steering circuit <NUM> which result in the prioritisation valve spindle moving between the various operative positions 48a, 48b, 48c can be selected as appropriate for any given fluid supply system. It will also be appreciated that other prioritisation valve arrangements could be adopted provided that the valve can be placed in different operative configurations so as to either allow unrestricted flow of fluid from the cooling fan pump to the cooling motor or to restrict this flow when fluid flow from the cooling fan pump <NUM> is required to supplement the flow from the main pump <NUM> to the steering system <NUM>. Furthermore, the flow of fluid from the emergency steering pump <NUM> to the tank <NUM> need not be regulated by the same valve as the flow from the cooling fan pump but could be regulated by a separate valve. In this case, the prioritisation valve <NUM> might only have two operative configurations, a first 48a in which the fluid flow from the cooling fan pump <NUM> to the cooling fan motor is unrestricted by the valve and a second 48b in which the flow is restricted. The prioritisation valve in this case may be a proportional valve in which the degree by which the fluid flow is restricted varies in dependence on the pressure of the fluid in the steering system circuit when the valve is in its second operative configuration.

The pressurised fluid supply system <NUM> according the present invention has the major advantage that the cooling fan pump <NUM> can be used to both drive the cooling fan motor <NUM> and act as a secondary steering pump thus reducing the number of pumps required where a hydraulically driven cooling fan is utilised. Furthermore, whilst in a conventional fluid supply system the secondary steering pump just returns fluid to the tank when main supply pump <NUM> is operationally capable of maintaining the required pressure for the steering function, in the supply system according to the invention, the cooling fan pump <NUM> will be used to supply the cooling fan when it is not required to support the steering system and so is functionally operative most or all of the time. This improves the overall efficiency of the system.

Furthermore, the use of a common pump <NUM> to supply a cooling fan motor <NUM> and the steering system <NUM> does not impair the function of the supply system <NUM> as during normal operation, a permanently high steering demand with simultaneous undersupply by the main supply pump <NUM> is unlikely so that the cooling function provided by the cooling fan pump <NUM> is only reduced for short periods at most.

The fluid supply system <NUM> according to the invention therefore provides increased efficiency at lower costs while providing full safety with regard to steerability.

At least when the prioritisation valve <NUM> is in its intermediate position 48b, it may function as a proportional valve to direct some of the pressurised fluid from the cooling fan pump <NUM> to the steering system <NUM> whilst still maintaining a supply of fluid from the cooling fan pump to the cooling fan motor <NUM>. Accordingly, the prioritisation valve restricts the flow of pressurised fluid from the cooling fan pump to the cooling fan motor <NUM> but not fully block it. The degree of restriction may be variable so that the amount of pressurised fluid from the cooling fan pump <NUM> which is diverted to the steering system <NUM> increases proportionally to the amount by which the pressure in the steering circuit <NUM> falls below the threshold to move the valve to its second position 48b. Thus, the greater the drop in pressure in the steering circuit <NUM>, the more pressurised fluid is diverted from the cooling fan pump circuit to the steering system to maintain steering function. In this sense, the valve spindle may have a range of movement when the prioritisation valve is in its second, intermediate position 48b.

As the flow of pressurised fluid from the cooling fan pump <NUM> to the steering system is prioritised over the flow to the cooling fan motor <NUM>, it is preferable to have a control system to ensure sufficient supply to the cooling fan motor <NUM> in all, or at least most, non-emergency operating conditions.

When using the cooling fan pump <NUM> to provide a secondary supply for the steering system <NUM>, there may be circumstances in which the steering system <NUM> is taking a high level of fluid supply from the cooling fan pump <NUM> which would otherwise leave insufficient fluid pressure to drive the cooling fan motor <NUM> and cooling fan <NUM> in order to maintain a desired level of, or at least adequate, cooling. This would result in low cooling capacity and could lead to damage to various systems of the vehicle. To overcome this issue, according to the invention, the fluid supply system is provided with an electronic control system (indicated generally at <NUM>) configured to maintain the input drive power delivered by the cooling fan motor <NUM> to the cooling fan <NUM> substantially constant or at a demanded value where possible.

In the present embodiment in which the cooling fan pump <NUM> is a variable displacement pump, the displacement of the pump can be changed to vary the mass-flow output of the pump. Accordingly, when the steering system <NUM> makes a demand on cooling fan pump <NUM> and the prioritisation valve <NUM> moves to its second position 48b, the control unit <NUM> is operative to increase the displacement of the pump, and so increase the mass-flow from the pump, to compensate for the additional demand being made on it without compromising operation of the cooling fan motor <NUM> and cooling fan <NUM>. When the demand from the steering system <NUM> stops and the prioritisation valve <NUM> returns to its first position 48a, the control system is operative to reduce the displacement of the cooling fan pump <NUM> so that the fluid flow through the cooling fan motor <NUM> is maintained substantially constant.

In a first embodiment, the control system <NUM> includes a pressure sensor <NUM> for monitoring the pressure in the cooling fan circuit <NUM> between the prioritisation valve <NUM> and the cooling fan motor <NUM>. The fluid pressure in the cooling fan circuit is related to the flow rate such that if the flow rate of fluid through the cooling fan circuit falls, the pressure in the circuit will also fall. The pressure sensor provides an input to an electronic control unit (ECU) which forms part of the control system and which is operatively connected with the solenoid valve <NUM> (as indicated at <NUM>) for varying the displacement of the cooling fan pump <NUM>. The ECU is configured to regulate the displacement of the cooling fan pump <NUM> in dependence on the fluid pressure in the cooling fan circuit between the prioritisation valve <NUM> and the motor <NUM> as determined by the pressure sensor <NUM>. If the fluid flow from the cooling fan pump <NUM> to the cooling fan motor <NUM> drops, say due the prioritisation valve <NUM> moving to its second position 48b to direct some of the fluid from the cooling fan pump <NUM> to the steering system <NUM>, the drop in pressure is detected by the pressure sensor <NUM> and the ECU in dependence on an input for the pressure sensor <NUM> actuates the solenoid valve <NUM> to increase the displacement of the cooling fan pump <NUM> to provide a higher mass-flow from the pump to restore, so far as possible, the flow rate of fluid through the motor <NUM> and hence maintain the input drive power delivered by the cooling fan motor <NUM> to the cooling fan <NUM> constant or at a demanded value. Should the pressure in the cooling fan circuit <NUM> reach or exceed a target value sufficient to maintain the input drive power delivered by the cooling fan motor <NUM> to the cooling fan <NUM> constant or at a demanded value (e.g. should the prioritisation valve <NUM> move back to its first position when the steering system <NUM> is no longer showing demand on the cooling fan pump), the control system adjusts the cooling fan pump <NUM> to reduce displacement until the pressure in the cooling fan circuit is at a suitable level.

In an alternative embodiment, the control system <NUM> monitors the speed of the cooling fan <NUM> using a fan speed sensor <NUM> as part of a closed loop control such that the displacement of the cooling fan pump <NUM> is varied to maintain the speed of the fan constant or to vary the fan speed to meet a cooling demand. In a further alternative, the control system may monitor both the fluid pressure in the cooling fan circuit <NUM> using a pressure sensor <NUM> and the speed of the fan using a fan speed senor <NUM> and vary the displacement of the pump <NUM> to regulate the displacement of the pump as required to maintain adequate cooling.

In a still further alternative embodiment, the control system may be configured to vary the displacement of the cooling fan pump <NUM> hydraulically by means of a load sensing line (not shown) to feedback the demand of the cooling fan motor <NUM> in a similar manner to that described above for the main supply pump <NUM>. Such a system may only deliver a constant input power to the cooling fan <NUM> or require means to adapt the feedback load sensing signal from cooling fan motor <NUM> before adjusting the cooling fan pump <NUM>.

In the control system <NUM> as described above, the cooling fan pump <NUM> is a variable displacement pump and the cooling fan motor <NUM> is a fixed displacement motor. <FIG> illustrates a further embodiment of the fluid supply system <NUM>' which is substantially identical to that of <FIG> except that the cooling fan pump <NUM>' is a fixed displacement pump and the cooling fan motor <NUM>' is a variable displacement motor. In all other respects, the fluid supply system <NUM>' is constructed and operates substantially the same as the previous embodiment illustrated in <FIG>. Accordingly, only those aspects of the supply system <NUM>' which differ from those of the previous embodiment <NUM> will be described in detail. The reader should refer to the above description of the previous embodiment <NUM>.

The supply system <NUM>' operates in exactly the same way as the previous embodiment described above except that the control system <NUM>' is operative to vary the displacement of the cooling fan motor <NUM>' using a solenoid valve <NUM>'incorporated into the motor in order to maintain the drive to the cooling fan <NUM> substantially constant or to vary the drive to meet a cooling demand. In this case, in response to a fall in the fluid flow through the motor <NUM>', leading to a reduction in pressure in the cooling fan circuit and/or a reduction in fan speed, the control system compensates by reducing the displacement of the motor <NUM>' so that the internal pressure in the fan motor is increased, so as to keep the input drive power delivered to the cooling fan <NUM> constant or at a demanded value. Should the pressure in cooling fan circuit <NUM> reach or exceed a target value again (e.g. when steering system <NUM> is no longer placing a demand on the cooling fan pump), the displacement of the cooling fan motor is increased to maintain the input drive power to the cooling fan <NUM> at the desired value.

As in the previous embodiment, the control system may use a pressure sensor <NUM> for monitoring the pressure in the cooling fan circuit and/or a fan speed sensor <NUM> for monitoring the speed of the fan <NUM> to regulate the fluid flow through the cooling fan motor <NUM>'.

Furthermore, in both the fluid supply system <NUM> of <FIG> and the fluid supply system <NUM>' of <FIG>, the control system <NUM>, <NUM>' may proactively monitor the demand of the steering system <NUM> and vary the displacement of the pump <NUM> or the motor <NUM>' prior to any change being felt in the cooling fan circuit <NUM>.

The control system <NUM>, <NUM>' may for example monitoring the movement of a steering wheel, or other steering control, say by means with a respective angle sensor on the steering wheel. If the steering wheel is moved by a certain amount or at a certain velocity which is likely to give rise to the steering system <NUM> placing a demand on the cooling fan pump <NUM>, <NUM>', the control system <NUM>, <NUM>' is operative to vary the displacement of the variable displacement cooling fan pump <NUM> or motor <NUM>' before the prioritisation valve <NUM> moves to the second position 48b and fluid supply from the cooling fan pump to the cooling fan motor drops. The change in displacement may thereby be made dependent on the pressure change determined by pressure sensor <NUM> and/or the change in fan speed detected by the fan speed sensor <NUM> and/or the steering wheel movement. This ensures steering capability and sufficient cooling capacity in all situations. It will be appreciated that movement of other parts of the steering system could be used to predict an increased fluid demand by the steering system <NUM>.

Whilst the control system <NUM>, <NUM>' is primarily provided to maintain the drive to the cooling fan <NUM> within limits despite a variable demand being placed on the cooling fan pump <NUM>, <NUM>' by the steering system <NUM>, it can also be used to vary the cooling fan speed to meet differing cooling demands. Thus the control system <NUM>, <NUM>' could be configured to adjust displacement of the cooling fan pump <NUM> or the cooling fan motor <NUM>', as appropriate, in order to increase the speed of the cooling fan <NUM> when there is a high demand for cooling and/or to reduce the speed of the fan when the cooling demand is lower.

It is envisaged that the invention described above may be additionally provided with alternative embodiments such providing a pilot control for connection with the main supply circuit <NUM> and the steering circuit <NUM> when a constant displacement cooling fan pump <NUM>' as in the embodiment of <FIG> is installed. Such a system is described in applicant's published patent applications <CIT> without leaving the scope of the invention.

As is well known in the art, the various circuits <NUM>, <NUM>, <NUM> in a fluid supply system <NUM>, <NUM>' according to the invention may be provided with pressure limiting valves (not shown) which ensure that the pumps and other components, including the supplied consumers of the auxiliary hydraulic systems <NUM> in the case of the main circuit <NUM>, are protected from unintended high pressure in the system, for example if hoses are squeezed and oil flow is blocked which would otherwise result in damage. Typically, hydraulic systems on an agricultural vehicle operate in the region of <NUM> bar, in which case the pressure limiting valves may be set to open at around <NUM> bar. However, it will be appreciated that in fluid supply system <NUM>, <NUM>' according to the invention the operating pressure may be higher or lower than <NUM> bar and that the pressure at which the pressure limiting valves are set to open selected accordingly. It will also be appreciated that the fluid flow rates provided by the various pumps <NUM>, <NUM> and <NUM> can be selected as appropriate to the vehicle requirements.

Claim 1:
A pressurised fluid supply system (<NUM>; <NUM>') for an agricultural vehicle (<NUM>), comprising:
a. a main supply pump (<NUM>) providing a first source of pressurised fluid to a steering system (<NUM>) of the vehicle through a steering circuit (<NUM>) and to other consumers (<NUM>) through a main circuit (<NUM>);
b. a cooling fan pump (<NUM>; <NUM>') providing a source of pressurised fluid to a cooling fan motor (<NUM>; <NUM>') driving a cooling fan (<NUM>) through a cooling fan circuit (<NUM>);
c. a prioritisation valve (<NUM> for regulating the flow of fluid from the cooling fan pump (<NUM>; <NUM>') to the cooling fan motor (<NUM>; <NUM>'), the prioritisation valve having at least two operative configurations (48a, 48b) and movable between the two operative configurations in dependence on the fluid pressure in the steering circuit;
wherein the cooling fan pump (<NUM>, <NUM>') is fluidly connectable with the steering circuit (<NUM>) so as to be able to provide a second a source of pressurised fluid to the steering system (<NUM>), the prioritisation valve (<NUM>) being configured to adopt a first operative configuration (48a) in which fluid flow from the cooling fan pump (<NUM>; <NUM>') to the cooling fan motor (<NUM>; <NUM>') through the prioritisation valve (<NUM>) is substantially unrestricted when the pressure in the steering circuit (<NUM>) is at or above a first threshold value and to adopt a second operative configuration (48b) in which fluid flow from the cooling fan pump (<NUM>; <NUM>') to the cooling fan motor (<NUM>; <NUM>') through the prioritisation valve (<NUM>) is partially restricted when the pressure in the steering circuit (<NUM>) is below the first threshold value;
characterized in that one of the cooling fan pump (<NUM>; <NUM>') and the cooling fan motor (<NUM>; <NUM>') is a variable displacement machine, the fluid supply system having a control system (<NUM>) operative to adjust displacement of the variable displacement machine to maintain the input drive power provided by the cooling fan motor (<NUM>; <NUM>') to the cooling fan (<NUM>) substantially constant or at a demanded value and/or to vary the speed of the fan in accordance with cooling demand.