Abstract:
A fan drive system includes a pump, at least one implement arrangement in fluid communication with the pump and a fan drive unit driveably connected to the pump. The fan drive unit is in fluid communication with the pump through a modulation valve. A signal operator is structured and arranged to receive at least one implement signal and a fan drive signal and generate a load signal output, wherein the pump is adapted to modify an output flow of the pump in response to the load signal. A control valve is urged to respond under the influence of a sensed condition and is in fluid communication with the modulation valve and the signal operator through a signal conduit.

Description:
TECHNICAL FIELD  
         [0001]    This invention generally relates to hydraulically driven implement systems and more particularly to combining a fan drive system with multiple implement control systems such that a common source of pressurized hydraulic fluid is utilized.  
         BACKGROUND  
         [0002]    In machines, such as earth moving equipment having hydraulic implement systems, it is desirable that a dedicated hydraulic system be employed to charge a braking system. Typically, the dedicated hydraulic system directs pressurized fluid to mechanical brake assemblies which, in turn, are attached to ground engaging wheels to reduce the ground speed of the machine. Additionally, these machines also employ an auxiliary or secondary hydraulic system to drive a hydraulically operated fan motor, for example, to control the temperature of heat generating equipment such as an internal combustion engine.  
           [0003]    It is known to combine various hydraulic implement control systems into a common hydraulic circuit such that a single source of pressurized fluid is provided to animate the various implement systems. For example, U.S. Pat. No. 6,314,729 B1, issued Nov. 13, 2001 to Crull et al. discloses a fan drive system including a pump hydraulically connected to a load sense circuit which provides fluid to first and second work circuits in addition to supplying fluid to a hydraulically driven fan unit. The fan drive system employs an electronic controller which, depending on the sensed temperature to be controlled, directs an electrical current to a proportional valve to modify the pressure drop across the fan motor.  
           [0004]    However, the system disclosed by Crull et al. may lack suitable response performance since the signal circuitry of the proportional valve provides flow-modifying feedback to the load sensing pump and the proportional relief valve through the supply line. As a result, the fan motor may continue to run at an unwarranted level due to response performance. In fact, it has become imperative that the fan motor operates as sparingly as acceptable since the fan drive unit typically emits significant levels of noise which are undesirable to the operator. Moreover, the load sensing signals directed to the pump are similarly configured such that load communication between the work circuits, the fan drive system and the pump cause lethargic circuit response and as a result the system may be prone to inefficient operation. Additionally, the fan drive system is in continuous communication with the pump through a pressure reduction valve which leads to inefficient operation of the fan circuit. Such inefficient operation typically results in increased costs associated with ineffective operation and increased maintenance of the fan drive system in addition to the unwarranted noise generated by such a system.  
           [0005]    Accordingly, it would be desirable to provide an efficient hydraulic fan drive system which may overcome one or more of the problems or disadvantages as set forth above.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention relates to a fan drive system including a pump, at least one implement arrangement in fluid communication with the pump, a fan drive unit driveably connected to the pump, a modulation valve, a signal operator and a control valve. The fan drive unit is in fluid communication with the pump through the modulation valve. The signal operator is structured and arranged to receive at least one implement signal and a fan drive signal and generate a load signal output, wherein the pump is adapted to modify an output flow of the pump in response to the load signal. The control valve is urged to respond under the influence of a sensed condition and is in fluid communication with the modulation valve and the signal operator through a signal conduit.  
           [0007]    The present invention further relates to a fan drive system including a pump, at least one implement arrangement in fluid communication with the pump, a fan drive unit driveably connected to the pump, a modulation valve, a signal operator, a control valve and a priority operator. The fan drive unit is in fluid communication with the pump through the modulation valve. The signal operator is structured and arranged to receive at least one implement signal and a fan drive signal and generate a load signal output, wherein the pump is adapted to modify an output flow of the pump in response to the load signal. The control valve is urged to respond under the influence of a sensed condition and is in fluid communication with the modulation valve and the signal operator through a signal conduit. The pump is in communication with the at least one implement arrangement through the priority operator and the priority operator is adapted to divide flow between the at least one implement arrangement and the fan drive unit.  
           [0008]    The present invention further relates to a method of operating a fan drive system, comprising: causing fluid urged from a pump to be directed to at least one implement arrangement and a fan drive unit; causing the fan drive unit to modify its demand based on a sensed condition signal from a control valve; causing the implement arrangement to modify its demand based on a pressure condition signal of the implement arrangement; causing the pump to modify its demand based on a load condition signal of the pump; directing the sensed condition signal, the pressure condition signal and a load condition signal into a signal operator; and communicating demand of one of the implement arrangement and the fan drive unit to the pump through the signal operator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several exemplary embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
         [0010]    [0010]FIG. 1 is a schematic representation of a first embodiment of a fluid system according to the present invention;  
         [0011]    [0011]FIG. 2 is a schematic representation of a second embodiment of a fluid system according to the present invention; and  
         [0012]    [0012]FIG. 3 is a schematic representation of a third embodiment of a fluid system according to the present invention. 
     
    
       [0013]    The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION  
       [0014]    Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or corresponding reference numbers will be used throughout the drawings to refer to the same or corresponding parts.  
         [0015]    Referring to FIG. 1, a hydraulic fan drive system  10  is shown and includes a source of pressurized fluid  12 , such as a pump which draws fluid from a reservoir  14 , for example. The fan drive system  10  further includes a fluid circuit  16  hydraulically connected to a primary implement arrangement  20 , a secondary implement arrangement  22  and a fan drive unit  18 , all of which are hydraulically energized via the pump  12 .  
         [0016]    The fluid circuit  16  includes a priority valve  24  fluidly connected to the pump  12  through a conduit  28 . The priority valve  24  may be a normally closed, infinite position pressure relief valve having a pilot assist signal conduit  26  which directs signal fluid to a spring end of the valve  24 . A conduit  30  is positioned immediately downstream of the priority valve  24  to fluidly connect the priority valve with a proportional modulation valve  32 .  
         [0017]    The fluid circuit  16  includes the modulation valve  32  positioned downstream of the priority valve  24 . The modulation valve  32  includes a first signal conduit  34  and a second signal conduit  36 . First and second orifices  38 ,  40 , which may have 0.8 mm diameters, for example, may be respectively provided within the first and second signal conduits  34 ,  36 . A check valve  41 , provided in a signal conduit  50  connected to signal conduit  36 , is positioned upstream of a control valve  48 . The modulation valve  32  includes a moveable internal member or spool  33  having an internal flow passage  42  at an end thereof and a flow-blocking portion  44  is provided on the other end of the spool  33 . A conduit  46  extends between the modulation valve  32  and the fan drive unit  18 .  
         [0018]    The fluid circuit  16  of the fan drive system  10  includes a control valve  48  which may be, for example, a solenoid controlled, variable position, normally open relief valve  32 . A signal conduit  52  fluidly connects the downstream portion of the valve  48  with other signal conduits leading to reservoir  14 . In an exemplary embodiment the electronic control valve  48  is operative in response to receipt of an electrical signal indicative of engine water jacket temperature, induction manifold temperature, retarder oil temperature or any other temperature based parameter which is known to those having ordinary skill in the art. It is further envisioned that the electrical signal, indicative of any one of said temperature sources, may be obtained by a temperature sensor in communication with an electronic control module, as is customary.  
         [0019]    A signal operator  54  is in fluid communication with the signal conduit  50  of the electronic control valve  48  and includes a first port  56 , a second port  58 , an outlet port  57  and a moveable member  60  therein which alternatively blocks ports  56 ,  58  depending on signal strength. In an exemplary embodiment the signal operator  54  is a shuttle valve, for example. It may be seen that the pump  12  may be a pressure compensated pump having a variable displacement, controllable element  59  reactive to fluid signal pressure communicated through load signal conduit  61 . In turn, load signal conduit  61  is fluidly connected to the output  57  of the signal operator  54 .  
         [0020]    A charge valve  62 , which may be a two-position, pilot operated valve for example, provides pump flow from a conduit  66  to a signal conduit  64  when the valve is in a first position (as shown in FIG. 1). A first portion  76  of the valve  62 , which is operative in the first shown valve position, includes a flow passage  78  to fluidly connect the signal conduit  74  with the conduit  66 . In a second position, a second portion  80  of the valve  62  includes a signal passage  82  which fluidly connects a signal conduit  85  with the signal conduit  64 . Upstream of the charge valve  62 , there is provided a filter  70 , an orifice  72  and a check valve  68  to ensure sufficiently pressurized, stable and non-contaminated fluid passes to the charge valve in addition to the remaining circuitry downstream of the charge valve  62 . Fluid ultimately passing through the check valve  68  will be communicated to a pilot  77  on the charge valve  62  through a signal conduit  75 . A predetermined pressure level within the signal conduit and the pilot  77  causes the valve  62  to shift to its second position. Notably, in the second valve position fluid pressure delivered by the pump  12  to the signal conduit  64  is blocked by a blocked port  84  within the second portion  80  of the valve  62 .  
         [0021]    A standby pressure-reducing valve  86  is included in the fluid circuit  16  and may include a normally open biasing relief valve. Valve  86  includes a signal conduit  88  and spring bias to urge the valve in an open position. A downstream signal conduit  89  provides fluid pressure to act on the respective end of the valve  86  to urge the valve closed. A conduit  90  supplies pump pressure to the valve  86  and a flow passage  92  within the valve  86  fluidly connects the pump pressure from the conduit  90  to the secondary implement arrangement  22 . The secondary implement arrangement may be, for example, a source of pilot pressure for a hoist valve actuator for an off-highway dump truck or any other suitable secondary implement arrangement.  
         [0022]    A pressure relief valve  94  is provided within the fluid circuit  16  to prevent an over pressure condition of the fan drive system  10 . A conduit  96  connects the pump pressure within the conduit  28  to the relief valve  94  and a conduit  98  connects the relief valve  94  with the reservoir  14 .  
         [0023]    The fan drive unit  18  includes a fluid actuator  100 , which may be a bi-directional fluid motor, for example, fluidly connected to conduit  46  at a position upstream of the motor  100 . An outlet  101  of the motor  100  is fluidly connected to the reservoir  14 . An anti-cavitation conduit  102  is provided in parallel with the motor  100 , between the reservoir  14  and an inlet  103  of the motor, to provide make-up fluid to the motor inlet should the outlet pressure exceed the motor inlet pressure. A check valve  104  is provided within the conduit  102 , as is customary, to provide one-way flow of fluid from the motor outlet  101  to the motor inlet  103 .  
         [0024]    The fluid circuit  16  is hydraulically connected to the priority implement arrangement  20  which, in an exemplary embodiment, is an actuation and charging system such as, for example, a hydraulic brake system. Alternatively, it is envisioned that the priority implement arrangement  20  may be a hydraulic steering system, lubrication system, hydrostatic transmission system or any other hydraulic system requiring priority fluid pressure known to those having ordinary skill in the hydraulically activated implement or hydraulic work system arts.  
         [0025]    The priority implement arrangement  20  includes an inverse shuttle circuit  108  fluidly connected to a hydraulic brake circuit  110 . A hydraulically activated parking brake system  112  and front and rear brake systems  116 ,  118  may be hydraulically connected, as illustrated, with the priority implement arrangement  20  to provide a complete braking system for a mobile machine such as an agricultural or construction vehicle, such as a truck or skid steer loader, for example.  
         [0026]    The inverse shuttle circuit  108  of the primary implement arrangement  20  includes a first two-position valve  120  and a second two-position valve  122 . The first and second valves  120 ,  122  respectively include upstream signal ports  124 ,  126  and downstream signal ports  128 ,  130 . Finally, bias members  132 ,  134  are respectively included with each of the first and second valves  120 ,  122  to close each valve when the downstream pressure is substantially the same as the upstream pressure. Alternatively, the bias members  132 ,  134  respectively assist closing valves  120 ,  122  when the upstream pressure is substantially less than the downstream pressure. In operation, if the pressure in one of the downstream signal ports significantly decreases, such as the pressure in port  128 , the valve  120  will shift open and allow pump pressure to be communicated downstream to a front brake accumulator  136 . Similarly, if the pressure in port  130  were suddenly decreased, the valve  122  would shift open and allow pump pressure to be communicated downstream to a rear brake accumulator  138 . Moreover, the inverse shuttle circuit  108  provides the lowest accumulator pressure to be sensed in conduit  140  which is connected to the sensor switch  114  and the charge valve  62 .  
         [0027]    The inverse shuttle circuit  108  receives fluid pump pressure through the conduit  140  whereas fluid pressure discharged from this circuit is branched between the front and rear brake systems  116 ,  118  through respective conduits  142 ,  144 .  
         [0028]    The primary implement arrangement  20  further includes the hydraulic brake circuit  110 . The hydraulic brake circuit  110  includes a front brake valve  146  and a rear brake valve  148 . Front and rear brake valves may be three position valves or any other valve combination known to those having ordinary skill in the hydraulic brake system arts. The front brake valve  146  may be engaged by a lever  150  such as a foot pedal, for example. The front brake valve  146  further includes a downstream signal port  152 . The rear brake valve  148  includes a first signal port  154  in fluid communication with the downstream signal port  152  of the front brake valve  146 . The rear brake valve  148  also includes a second signal port  156 . The rear brake valve  148  includes a bias member  158  which provides a force that complements the force imposed by pressure impinging on a spool end (not shown) in the second port  156  and a opposes the force imposed by pressure impinging on the other end of the spool (not shown) in the first port  154  of the rear brake valve  148 . Front and rear brake conduits  160 ,  162  respectively connect the front brake and the rear brake valves to the front and the rear brake systems  116 ,  118 .  
         [0029]    Operation of the hydraulic brake circuit  110  of the priority implement arrangement  20  will now be described. In operation, manipulation of the lever  150  causes the front brake conduit  160  to be successively blocked from the reservoir  14  and thereafter, open to the associated front brake accumulator  136 . This accumulator pressure is transmitted to both the downstream signal port  152  and the first signal port  154  in the rear brake valve  148 . As a result, the rear brake valve  148  successively blocks the tank and opens to the associated rear brake accumulator  138 . Once the lever  150  is released, the front brake valve returns  146  to its original position with the pressure in the conduit  160  being relieved to tank  14  which, in turn, causes repositioning of the rear brake valve  148  to its original position. Accordingly, pressure in the rear brake conduit  162  is then relieved to the tank  14  through the rear brake valve  148 .  
         [0030]    Referring to FIG. 2, shown is a second embodiment of a hydraulic fan drive circuit wherein certain corresponding elements are denoted by primed reference numerals. A hydraulic fan drive circuit  10 ′ includes a pump  12  which draws fluid from a reservoir  14 , for example. The fan drive system  10 ′ further includes a fluid circuit  16 ′ hydraulically connected to a primary implement arrangement  20 ′ and a fan drive unit  18 , all of which are hydraulically energized via the pump  12 .  
         [0031]    The fluid circuit  16 ′ includes a priority valve  24  fluidly connected to the pump  12  through a conduit  28 . The priority valve  24  may be a normally closed, infinite position pressure relief valve having a pilot assist signal conduit  26  which directs signal fluid to a spring end of the valve  24 . A conduit  30  is positioned immediately downstream of the priority valve  24  to fluidly connect the priority valve with a proportional modulation valve  32 .  
         [0032]    The fluid circuit  16 ′ includes the modulation valve  32  positioned downstream of the priority valve  24 . The modulation valve  32  includes a first signal conduit  34  and a second signal conduit  36 . First and second orifices  38 ,  40 , which may have 0.8 mm diameters, for example, may be respectively provided within the first and second signal conduits  34 ,  36 . A check valve  41  is provided in a signal conduit  50  upstream of a control valve  48 . The modulation valve  32  includes a moveable internal member or spool  33  having an internal flow passage  42  at an end thereof and a flow-blocking portion  44  is provided on the other end of the spool  33 . A conduit  46  extends between the modulation valve  32  and the fan drive unit  18 .  
         [0033]    The fluid circuit  16 ′ of the fan drive system  10  includes the control valve  48  which may be, for example, a solenoid controlled, variable position, normally open relief valve  32 . A signal conduit  52  fluidly connects the downstream portion of the valve  48  with the reservoir  14 . In an exemplary embodiment, the electronic control valve  48  is operative in response to receipt of an electrical signal indicative of engine water jacket temperature, induction manifold temperature, retarder oil temperature or any other temperature based parameter which is known to those having ordinary skill in the art. It is further envisioned that the electrical signal, indicative of any one of said temperature sources, may be obtained by a temperature sensor in communication with an electronic control module, as is customary.  
         [0034]    A signal operator  54  is in fluid communication with the control valve  48  through the signal conduit  50  and includes a first port  56 , a second port  58 , an outlet port  57  and a moveable member  60  therein which alternatively blocks ports  56 ,  58  depending on signal strength. In an exemplary embodiment the signal operator  54  is a shuttle valve, for example. It may be seen that the pump  12  may be a pressure compensated pump having a variable displacement, controllable element  59  reactive to fluid signal pressure communicated through load signal conduit  61 . In turn, load signal conduit  61  is fluidly connected to the output  57  of the signal operator  54 .  
         [0035]    The fluid circuit  16 ′ provides continuous pressure supply to the priority implement  20 ′ and utilizes feedback from the priority implement  20 ′ via the signal conduit  64 ′ to limit pressure to the fan during a charge or demand mode. The feedback of the priority implement  20 ′ may include a signal port  164  in fluid communication with the signal operator  54  through the signal conduit  64 ′. Moreover, an orifice  72  is provided in the line  28  to dampen pressure pulses and to provide overall system stability. It may be seen that the priority valve  24  is adapted to close coincident with the priority implement  20 ′ being under a pressure demand or charging mode. Hence, the fan  18  is allowed to be substantially blocked from the supply pressure when the priority implement  20 ′ is being charged.  
         [0036]    Referring to FIG. 3, shown is a third embodiment of a hydraulic fan drive circuit  10 ″ which includes a fluid circuit  16 ″ hydraulically connected to a primary implement arrangement  20 ″ and a fan drive unit  18 , all of which are hydraulically energized via the pump  12 .  
         [0037]    The fluid circuit  16 ″ includes a priority operator  166  fluidly connected to the pump  12  through a conduit  28 . The priority operator  166  may be a proportional, pilot operated valve for example, which directs pump flow from the conduit  28  to the conduit  168  when the operator is in a first position (as shown in FIG. 3). A first portion  170  of the operator  166  includes a flow passage  172  to fluidly connect the conduit  28  with the conduit  168 . In a second position of the priority operator  166 , a second portion  174  of the valve  166  includes a flow passage  176  which fluidly connects the conduit  28  with the conduit  30 .  
         [0038]    The priority operator  166  includes a first signal conduit  178  and a second signal conduit  180 . First, second and third orifices  182 , which may have 0.8 mm diameters, for example, may be respectively provided within the first and second signal conduits  178 ,  180 . The signal conduit  180  is connected to the port  58  of the signal operator and an end  184  of the priority operator  166  includes a spring  186  to urge the valve into its first position coinciding with supply pressure being directed to the priority implement  20 ″. The priority operator  166  also includes opposing pilots  188 ,  190  in fluid communication through the signal conduit  178 .  
         [0039]    The fluid circuit  16 ″ includes the modulation valve  32  positioned downstream of the priority operator  166 . The modulation valve  32  includes a first signal conduit  34  and a second signal conduit  36 . First and second orifices  38 ,  40 , which may have 0.8 mm diameters, for example, may be respectively provided within the first and second signal conduits  34 ,  36 . A check valve  41  is provided in a signal conduit  50  upstream of the control valve  48 . The modulation valve  32  includes a moveable internal member or spool  33  having an internal flow passage  42  at an end thereof and a flow-blocking portion  44  is provided on the other end of the spool  33 . A conduit  46  extends between the modulation valve  32  and the fan drive unit  18 .  
         [0040]    The fluid circuit  16 ″ of the fan drive system  10 ″ includes the control valve  48 . A signal conduit  52  fluidly connects the downstream portion of the valve  48  with the reservoir  14 . In an exemplary embodiment, the control valve  48  is operative in response to receipt of an electrical signal indicative of engine water jacket temperature, induction manifold temperature, retarder oil temperature or any other temperature based parameter which is known to those having ordinary skill in the art.  
         [0041]    A signal operator  54  is in fluid communication with the signal conduit  50  of the electronic control valve  48  and includes a first port  56 , a second port  58 , an outlet port  57  and a moveable member  60  therein which alternatively blocks ports  56 ,  58  depending on signal strength. In an exemplary embodiment the signal operator  54  is a shuttle valve, for example. It may be seen that the pump  12  may be a pressure compensated pump having a variable displacement, controllable element  59  reactive to fluid signal pressure communicated through load signal conduit  61 . In turn, load signal conduit  61  is fluidly connected to the output  57  of the signal operator  54 .  
         [0042]    The hydraulic fan drive circuit  10 ″ differs from the fan drive circuit  10 ′ of FIG. 2 in several respects; one respect may include that the fluid circuit  16 ″ directs supply fluid between the priority implement  20 ″ and the fan  18  through the priority operator  166 . Since flow through the priority operator  166  is proportional, in the exemplary embodiment, then the flow is continually being divided between the priority implement  20 ″ and the fan  18  based on the requirements of the priority implement.  
       Industrial Applicability  
       [0043]    In the operation of the embodiment set forth in FIG. 1, pressurized fluid from the pump  12  is transmitted to the priority implement arrangement  20  through the fluid circuit  16 . The check valve  68  acts to ensure that a predetermined pressure level is maintained in the conduit  140  upstream of the inverse shuttle circuit  108 . In so doing, this will ensure that the priority implement arrangement  20  is always supplied with a volume of fluid at a predetermined pressure level. In the exemplary embodiment, since it is generally desirable to ensure that a minimum pressure level is always present for proper operation of the brakes, accumulators  136 ,  138  are employed. The accumulators  136 ,  138  act to store a volume of pressurized fluid in a known manner to further ensure that ample pressurized fluid is always available for the brake systems  116 ,  118 . The pressure sensor or switch  114  is positioned within the hydraulic fan drive system  10  to continuously monitor the pressure of the fluid in the pressure conduit  140 . It is envisioned that the signal provided by sensor/switch  114  may be communicated to an electronic controller (not shown), such as an electronic control module (“ECM”). The controller may be programmed to divert the required pressure from alternate sources of pressure to the brake system if the operating brake pressure were to decrease below a threshold amount. Additionally, an operator alarm or alert warranting immediately attention to the brake system is contemplated by the present hydraulic fan drive system.  
         [0044]    In one mode of operation, the priority implement arrangement  20  will be under certain demand and require a significant portion of the pump&#39;s generated pressure, such as when the brake system requires to be charged. Specifically, during charge mode, the charge valve  62  senses that pressure in conduit  75  has dropped and shifts to allow accumulator pressure to travel to line  64  which provides the pressure signal to the pump. At the same time the fan drive circuit  18  may include a requirement for fluid flow from the pump to accordingly cool select heat generating componentry, as is customary. During a charging event the pump&#39;s flow is directed to the charge valve  62  through the conduit  28 . The spring in the charge valve  62  is designed to retain the valve in its charging position (shown) via the spring force until a predetermined pressure is obtained. Once obtained, the pressure in the signal port  75  creates a force on the valve member which overcomes the spring bias. Consequently, the charge valve shifts to a bypass position and accordingly the signal conduit  74  is fluidly connected to the signal operator  54  through the charge valve  62 . Notably, in the charge mode, the brake load is communicated to the signal operator  54  through the charge valve  62  and the fan load is communicated directly to the signal operator  54  at all times. In contrast, when the charge valve  62  is in the bypass mode the load of the secondary implement arrangement  22  substitutes the brake load and is communicated to the signal operator via the charge valve  62 .  
         [0045]    The fan drive circuit  18  is controlled via the modulation valve  32  being controlled based on the electronic control valve  48  sensing temperature. The flow the fan drive circuit  18  is dependent on the pressure commanded by the control valve  48  which communicates with the pump in the uncharging mode. In the charging mode the fan flow is dependent on the pressure commanded by  48  and the modulation of valve  32 . The priority valve  24  can also limit flow to the fan if the pressure differential across  72  does not correspond to the required charge flow desired.  
         [0046]    The priority valve  24  is normally closed and may be opened if the upstream pressure (in conduit  24 ) exceeds the sum of the force due to the signal pressure in signal conduit  26  and the biasing force of the spring within the priority valve  24 . Thus, it may be seen that during charging, the pressure in conduit  26  will likely be at its greatest value and the priority valve  24  will be nearly closed allowing the pump&#39;s delivery to be almost exclusively to the brake system  20 . In so doing, the priority valve  24  ensures that the flow across the orifice  72  is at the desired level. If the flow is too low then the low-pressure differential will cause the priority valve  24  to modulate closed to allow increased flow across the orifice  72 .  
         [0047]    During charging, the signal operator  54  receives signal flow, indicative of brake load, from the brake system  20  via shuttle port  58 . Additionally, the signal operator  54  receives signal flow, indicative of fan load, from the fan drive circuit  18  via signal operator port  56 . The stronger of the two signals will cause the signal operator  54  to provide the stronger load signal to the load conduit  61  of the load-sensing pump  12 .  
         [0048]    Conversely, when the brake system  20  is not charging, the pressure within the signal conduit  74 , indicative of the load of the secondary implement arrangement, is communicated to the port  58  of the signal operator  54  via the charge valve  62 . In so doing, the greater of the fan and secondary implement system will be conveyed to the load sensing conduit  61  of the load sensing pump  12 . Accordingly, the pump  12  will effectively satisfy the demand based on the larger of the loads between the secondary implement and the fan systems.  
         [0049]    The relief valve  94  is a normally closed valve and allows fluid to pass therethrough when a predetermined high pressure is attained to protect system components from overpressure. Other protective features of system  10  include the electronically controlled valve  48  being set to wide-open during an electricity failure to ensure enough flow is directed to the fan. Furthermore, the check valve  41  is sized to ensure that the fan rotates at all times by causing a predetermined pressure to be imposed on the biasing end of the valve spool (not shown) of the modulation valve  42  which results in the modulation valve  42  being “cracked open” with no demand on the fan drive circuit  18 . Therefore, when the controller is not calling for cooling, the modulation valve  32  is being controlled by the check valve  41  and not the electronic control valve  48 .  
         [0050]    The fan drive is protected from overspeeding since the modulation valve  32  will reduce the pressure to coincide with the pressure commanded by the control valve  48 . Valve  48  usually commands the pump in the uncharging mode but in the charging mode, valve  48  controls the modulation valve  32  to prevent the fan drive circuit  18  from overspeeding. Furthermore, the orifice  38  provided in conduit  34  of the modulation valve  32  and the orifice  40  provided in the conduit  36  downstream of the modulation valve  32  ensure that the fan operation remains stable and instabilities due to pressure fluctuations are minimized.  
         [0051]    By combining the priority implement arrangement  20  and the fan drive circuit  18  within a common hydraulic system and utilizing a variable displacement pump  12 , the operation of the pump becomes significantly more efficient over known combined systems. Since the system delivers the flow to the brake-charging portion of the circuit only when it is needed, the pump operates with high efficiency. Since the pump is required to operate only as much as needed it infrequently sustains continual operation at high pressures which significantly increases pump life and decreases the frequency of system maintenance at a significant cost savings.  
         [0052]    Referring to FIG. 2, the operation of the hydraulic fan drive system  10 ′ will be described. When the priority implement  20 ′ is under demand or charge conditions, the priority implement  20 ′ may require a significant portion of the pump&#39;s generated pressure, such as when a brake system requires to be charged, for example.  
         [0053]    The fan drive circuit  18  is controlled via the modulation valve  32  being controlled based on the electronic control valve  48  sensing temperature. The fan drive circuit  18  is dependent on the pressure commanded by the control valve  48  which communicates with the pump in the uncharging mode. In the demand or charging mode the fan flow is dependent on the pressure commanded by  48  and the modulation of valve  32 . The priority valve  24  can also limit flow to the fan if the pressure differential across  72  does not correspond to the required charge flow desired.  
         [0054]    The priority valve  24  is normally closed and may be opened if the upstream pressure (in conduit  24 ) exceeds the sum of the force due to the signal pressure in signal conduit  26  and the biasing force of the spring within the priority valve  24 . Thus, it may be seen that during charging, the pressure in conduit  26  will likely be at its greatest value and the priority valve  24  will be nearly closed allowing the pump&#39;s delivery to be almost exclusively to the priority implement  20 ′. In so doing, the priority valve  24  ensures that the flow across the orifice  72  is at the desired level. If the flow is too low then the low-pressure differential will cause the priority valve  24  to modulate closed to allow increased flow across the orifice  72 .  
         [0055]    During charging, the signal operator  54  receives signal flow, indicative of implement load, from the priority implement  20 ′ via shuttle port  58 . Additionally, the signal operator  54  receives signal flow, indicative of fan load, from the fan drive circuit  18  via signal operator port  56 . The stronger of the two signals will cause the signal operator  54  to provide the stronger load signal to the load conduit  61  of the load-sensing pump  12 .  
         [0056]    Referring to FIG. 3, the operation of the hydraulic fan drive system  10 ″ will be described. Fluid ultimately passing through the priority operator  166  will be communicated to opposing pilots  188 ,  190  on the priority operator  166  through the signal conduit  178 . During priority mode operation, a substantially uniform pressure may preside within the signal conduits  178  and  180  feeding the respective pilots  188 ,  190  which results in a canceling of the pressure induced forces on the priority operator  166 . As a result, the spring  186  invokes a spring bias on the priority operator  166  to urge the same to its first or priority position. Notably, if pressure in the conduit  180  deteriorates, which may be indicative of decreased pressure demand of the primary implement  20 ″ then the primary operator  166  shifts to its second position and fluid pressure is delivered by the pump  12  to the fan  18  through the modulation valve  32 .  
         [0057]    It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed hydraulic fan drive system without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only.