FIELD OF THE DISCLOSURE 
     The present disclosure relates to hydraulic circuits and more particularly to a means to exhaust flow from a hydraulic circuit that is operating in a non-powered (static) condition. 
     BACKGROUND 
     In a closed loop hydraulic or hydrostatic circuit, flow is exhausted from the working circuit for the purposes of cooling and filtering. If this is the only system contributing to a particular work function (a hydrostatic drive, for example), exhausting this oil only during powered operation is generally acceptable. If the system is assisted or provides assistance to another system (a part time hydrostatic drive on a machine that is primarily driven by a traditional transmission, for example) a means of exhausting oil in a non-powered (standby) condition may be necessary. 
     Previous efforts include splitting the traditional hydrostatic flushing system between two separate devices. For example, a motor grader vehicle can include a hydrostatic loop flushing system that is independent from the drive motor case flushing system. Typical hydrostatic loop flushing exhausts flow from the low pressure side of the work circuit prior to sending it to the hydraulic oil cooler and reservoir. A separate system utilizes a small pump to send constant flow to the cases of the drive motors. This provides benefits similar to an open center loop flushing system, but does so by utilizing extra components and hydraulic circuits which add costs and complexity. 
     It would be desirable to have a hydrostatic loop flushing system that always flushes fluid through the system whether the hydrostatic circuit is in a powered or unpowered state. 
     SUMMARY 
     An open center flushing valve is disclosed for use in a hydraulic system that includes a hydraulic circuit and a fluid flushing system. The open center flushing valve includes a spool, first and second work ports and an outlet port. The spool is located in the interior of the flushing valve, and is movable between a plurality of spool positions. The first work port hydraulically couples the flushing valve to a first side of the hydraulic circuit, the second work port hydraulically couples the flushing valve to a second side of the hydraulic circuit, and the outlet port hydraulically couples the flushing valve to the fluid flushing system. When the spool is in a first spool position, it allows flow between the first work port and the outlet port and blocks flow between the second work port and the outlet port. When the spool is in a second spool position, it allows flow between the first work port and the outlet port and blocks flow between the second work port and the outlet port. When the spool is in a third spool position, it allows flow between the first work port and the outlet port and also allows flow between the second work port and the outlet port. 
     The open center flushing valve can also include a power mechanism for moving the spool between the plurality of spool positions, where the power mechanism has an unpowered state and a plurality of powered states. In the unpowered state, the power mechanism does not provide an external force on the spool and the spool rests in the third position. In a first powered state, the power mechanism forces the spool into the first spool position, and in a second powered state, the power mechanism forces the spool into the second spool position. 
     The spool can include a first end portion, a middle portion and a second end portion, where the middle portion is located between the first and second end portions. In the first powered state, the power mechanism can force the spool into the first spool position where the first end portion of the spool blocks flow between the first work port and the outlet port and the middle portion of the spool allows flow between the second work port and the outlet port. In the second powered state, the power mechanism can force the spool into the second spool position where the second end portion of the spool blocks flow between the second work port and the outlet port and the middle portion of the spool allows flow between the first work port and the outlet port. In the unpowered state, the spool can rest in the third position where the middle portion of the spool allows flow between the first work port, the second work port and the outlet port. The first and second end portions of the spool can have a greater diameter than the middle portion of the spool. 
     The open center flushing valve can also include an interior cavity, where the spool is located in the interior cavity, and the first and second work ports and the outlet port are connected to the interior cavity. The length of the middle portion of the spool can be greater than the distance between the connections of the first and second work ports to the interior cavity. The interior cavity can have a first cavity diameter where the first work port connects to the interior cavity and a second cavity diameter where second work port connects to the interior cavity. The first end portion of the spool can have a first spool diameter that is substantially equal to the first cavity diameter, and the second end portion of the spool can have a second spool diameter that is substantially equal to the second cavity diameter. The first and second end portions of the spool can have a greater diameter than the middle portion of the spool. The interior cavity can have a substantially uniform diameter where the first cavity diameter is substantially equal to the second cavity diameter. 
     An open center flushing valve is disclosed for use in a hydraulic system that includes a hydraulic circuit and a fluid flushing system. The open center flushing valve includes a plurality of positions where each of the plurality of positions has a fluid exhaust flow path through the flushing valve. When the flushing valve is in the hydraulic system, any particular position of the plurality of positions of the flushing valve couples the hydraulic circuit to the fluid flushing system through the fluid exhaust flow path for that particular position. The plurality of positions can include an unpowered position and at least one powered position, where the flushing valve remains in the unpowered position except when the hydraulic circuit pressurizes the flushing valve which shifts the flushing valve into a powered position. 
     First and second sides of the hydraulic circuit can be separately coupled to the flushing valve, such that in any powered position only one of the first and second sides of the hydraulic circuit is coupled to the fluid flushing system through the flushing valve, and in the unpowered position both of the first and second sides of the hydraulic circuit are coupled to the fluid flushing system through the flushing valve. In any powered position, the lower pressure side of the hydraulic circuit can be coupled to the fluid flushing system through the flushing valve. 
     First and second sides of the hydraulic circuit can be separately coupled to the flushing valve such that when the first side of the hydraulic circuit is pressurized the flushing valve shifts to a first powered position with a first fluid exhaust flow path through the flushing valve coupling the hydraulic circuit to the fluid flushing system; when the second side of the hydraulic circuit is pressurized the flushing valve shifts to a second powered position with a second fluid exhaust flow path through the flushing valve coupling the hydraulic circuit to the fluid flushing system; and when neither the first or second sides of the hydraulic circuit are pressurized the flushing valve remains in the unpowered position with a third fluid exhaust flow path through the flushing valve coupling the hydraulic circuit to the fluid flushing system. In the first powered position, the first fluid exhaust flow path can couple the second side of the hydraulic circuit to the fluid flushing system. In the second powered position, the second fluid exhaust flow path can couple the first side of the hydraulic circuit to the fluid flushing system. In the unpowered position, the third fluid exhaust flow path can couple both the first and second sides of the hydraulic circuit to the fluid flushing system. 
     A hydraulic system flushing method is disclosed that includes coupling a hydraulic circuit to a fluid flushing system through an open center flushing valve having a plurality of positions; selectively pressurizing the flushing valve using pressure from the hydraulic circuit to shift the flushing valve between the plurality of positions; and regardless of the position of the flushing valve, coupling the hydraulic circuit to the fluid flushing system via a fluid exhaust flow path through the flushing valve. The plurality of positions of the flushing valve can include an unpowered position and at least one powered position, and the method can also include shifting the flushing valve to one of the at least one powered position when the hydraulic circuit pressurizes the flushing valve; and otherwise maintaining the flushing valve in the unpowered position. First and second sides of the hydraulic circuit can be separately coupled to the flushing valve, and the method can also include in any powered position, coupling only one of the first and second sides of the hydraulic circuit to the fluid flushing system through the flushing valve; and in the unpowered position, coupling both of the first and second sides of the hydraulic circuit to the fluid flushing system through the flushing valve. The method can also include, when the first side of the hydraulic circuit is pressurized relative to the second side, shifting the flushing valve to a first powered position with a first fluid exhaust flow path through the flushing valve coupling the hydraulic circuit to the fluid flushing system; when the second side of the hydraulic circuit is pressurized relative to the first side of the hydraulic circuit, shifting the flushing valve to a second powered position with a second fluid exhaust flow path through the flushing valve coupling the hydraulic circuit to the fluid flushing system; and when neither the first or second sides of the hydraulic circuit are pressurized relative to one another, maintaining the flushing valve in the unpowered position with a third fluid exhaust flow path through the flushing valve coupling the hydraulic circuit to the fluid flushing system. The method can also include, in the first powered position, coupling the second side of the hydraulic circuit to the fluid flushing system via the first flow path; in the second powered position, coupling the first side of the hydraulic circuit to the fluid flushing system via the second flow path; and in the unpowered position, coupling both the first and second sides of the hydraulic circuit to the fluid flushing system via the third flow path. 
     The above and other features will become apparent from the following description and the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description of the drawing refers to the accompanying figures in which: 
         FIG. 1  illustrates an exemplary motor grader including six wheels that are powered by two independent drive systems; 
         FIG. 2A  illustrates an exemplary closed-center valve in a hydrostatic system with an A-side and a B-side where the A-side is the high-pressure side; 
         FIG. 2B  illustrates the exemplary closed-center valve of  FIG. 2A  where the B-side is the high-pressure side; 
         FIG. 2C  illustrates the exemplary closed-center valve of  FIG. 2A  in the neutral position; 
         FIG. 3A  illustrates an exemplary open-center valve in a hydrostatic system with an A-side and a B-side where the A-side is the high-pressure side; 
         FIG. 3B  illustrates the exemplary open-center valve of  FIG. 3A  where the B-side is the high-pressure side; 
         FIG. 3C  illustrates the exemplary open-center valve of  FIG. 3A  in the neutral position; 
         FIG. 4A  illustrates an exemplary embodiment of a closed-center valve including two work ports and an outlet port where the A-side is pressurized (powered); 
         FIG. 4B  illustrates the exemplary embodiment of the closed-center valve of  FIG. 4A  where the B-side is pressurized (powered); 
         FIG. 4C  illustrates the exemplary embodiment of the closed-center valve of  FIG. 4A  in the neutral position; 
         FIG. 5A  illustrates an exemplary embodiment of an open-center valve including two work ports and an outlet port where the A-side is pressurized (powered); 
         FIG. 5B  illustrates the exemplary embodiment of the open-center valve of  FIG. 5A  where the B-side is pressurized (powered); and 
         FIG. 5C  illustrates the exemplary embodiment of the open-center valve of  FIG. 5A  in the neutral position. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary motor grader  100  which includes six wheels that are powered by two independent drive systems. The four rear wheels  110  are operated in tandem by a transmission that is driven directly by the machines engine. The front two wheels  120  are driven independently by two separate closed loop hydrostatic systems. The drive motors contained within this system are connected to a final drive hub that contains a clutch. When the clutch is engaged, the motors are tied to the wheels. When the clutch is disengaged, the wheels are allowed to spin freely. When engaged, the front wheel speeds are typically controlled to match the speeds of the rear wheels. When set to an “overly-aggressive” condition, the front wheel speeds are set higher than the rear wheel speeds (thus leading to more front wheel pull effort). When set to an “under-aggressive” condition, the front wheel speeds are set lower than the rear wheel speeds (thus causing the front wheels to pull only when the rear wheels are slipping). With a closed-center hydrostatic loop flushing circuit, when the hydrostatic systems driving the front wheels  120  are in an unpowered state (for example, when the rear wheels  110  are moving the motor grader  100 ) then there will not be any flushing of the front wheel hydrostatic systems. 
     A typical closed-loop hydraulic or hydrostatic system requires a means by which to flush fluid out of the closed circuit. This is typically accomplished through the use of a shuttle valve or spool. The pressure differential in the working circuit shifts the shuttle valve in such a way that the low pressure side of the hydrostatic loop is connected to a circuit intended to relieve flow (through the use of an orifice and/or pressure relief valve). Flow can be replenished in the circuit through an element aimed at maintaining a specific pressure in the low pressure side of the closed loop. The exhausted flow can then be directed to the case of one of the major hydraulic components (for example, the drive motor) and ultimately routed to the machine oil cooler. This process allows the oil to carry heat away from the hydraulic component that it is routed through. The replenishing flow can be filtered prior to being reintroduced into the closed loop circuit. 
     This type of circuit requires a pressure differential to shift the shuttle spool. In certain hydrostatic systems, there can be operating conditions when a pressure differential is not present while circuit cooling is still necessary. One example of this is the hydrostatic drive assist system for the front wheels  120  of the motor grader  100  with the traditional primary drive transmission for the rear wheels  110 . Under certain conditions, the hydrostatic drive might be engaged but set in such a manner that it only provides power to the ground under low tractive conditions (the “under-aggressive” condition). Under this condition, the hydrostatic motors may still be connected to the ground and spinning in a non-powered manner. There would not be adequate pressure to shift the loop flushing shuttle spool, thus leading to a lack of cooling flow. 
     The typical loop flushing circuit can also have performance issues in cold weather. In cold weather conditions, when the system is sitting idle the flow through this circuit will remain stagnant. This allows for the oil to cool down (or remain cool) prior to the system being operated. When the system is then used, the cooled oil in the circuit can lead to an excessive amount of restriction, which can lead to the over-pressurization of any area through which this flushing oil flows (a drive motor case, for example). This over-pressurization can cause damage to system components. This can affect any type of hydrostatic system, but it especially affects part-time systems that can spend considerable amounts of time de-activated. 
       FIG. 2  illustrates an exemplary closed-center flushing valve  200  with an A-side and a B-side. The closed-center flushing valve  200  is part of a closed loop hydraulic or hydrostatic circuit where the A-side and B-side are part of the closed loop. The hydrostatic system also includes a pressure relief valve  210 . The pressure relief valve in this and other embodiments could be replaced by an orifice or other method for flushing flow that are known to those of skill in the art. The pressure differential between the A-side and B-side of the closed loop hydrostatic circuit shifts the flushing valve  200  in such a way that the low pressure side of the hydrostatic loop is connected to the pressure relief valve  210 . 
       FIG. 2A  shows the case where the A-side is pressurized, which shifts the flushing valve  200  to the right, which allows the flushing of fluid from the low-pressure B-side through the flushing valve  200  and into the pressure relief valve  210 .  FIG. 2B  shows the case where the B-side is pressurized, which shifts the flushing valve  200  to the left, which allows the flushing of fluid from the low-pressure A-side through the flushing valve  200  and into the pressure relief valve  210 .  FIG. 2C  shows the neutral case where the A-side and B-side are at substantially the same pressure, leaving the flushing valve  200  in the center position, which does not allow the flushing of fluid through the flushing valve  200 . 
       FIG. 3  illustrates an exemplary open-center flushing valve  300  with an A-side and a B-side. The open-center flushing valve  300  is part of a closed loop hydraulic or hydrostatic circuit where the A-side and B-side are part of the closed loop. The hydrostatic system also includes a pressure relief valve  310 . The pressure differential in the A-side and B-side of the closed loop hydrostatic circuit shifts the flushing valve  300  in such a way that the low pressure side of the hydrostatic loop is connected to the pressure relief valve  310 . In addition; when the pressures are substantially equal in the A-side and B-side of the closed loop hydrostatic circuit, the flushing valve  300  allows flow from both sides of the hydrostatic loop to the pressure relief valve  310 . 
       FIG. 3A  shows the case where the A-side is pressurized, which shifts the flushing valve  300  to the right, which allows the flushing of fluid from the low-pressure B-side through the flushing valve  300  and into the pressure relief valve  310 .  FIG. 3B  shows the case where the B-side is pressurized, which shifts the flushing valve  300  to the left, which allows the flushing of fluid from the low-pressure A-side through the flushing valve  300  and into the pressure relief valve  310 .  FIG. 3C  shows the neutral case where the A-side and B-side are at substantially the same pressure, leaving the flushing valve  300  in the center position, which allows the flushing of fluid from both the A-side and B-side through the flushing valve  300  and into the pressure relief valve  310 . 
     Thus, in a powered condition, when higher pressure on one side shifts the valve, both the open-center and closed-center flushing valves allow flow from the low-pressure side of the hydrostatic loop. However, in an unpowered condition, when pressure is substantially equal on both sides of the valve, the open-center valve enables flushing from both sides of the hydrostatic loop, while the closed-center valve stops flow in the hydrostatic loop. Thus, there is always flow bleeding through the open-center valve whether the hydrostatic circuit is powered or unpowered. 
     The downstream valve  210  in  FIGS. 2A-2C and 310  in  FIGS. 3A-3C  can be used to regulate the amount of flow that can be exhausted from the circuit in a powered condition. This type of downstream valve can be used in both a closed-center and open-center configuration to prevent the flushing of an excessive amount of flow from the circuit.  FIGS. 2 and 3  illustrate the downstream valve as relief valve  210 ,  310 , respectively, but it can be implemented in a number of different ways. The relief valve  210 ,  310  requires enough pressure to overcome a spring force before it will open and relieve flow. Alternative implementations can include, for example, an orifice valve or a spring loaded check valve. Depending upon the system design, it may also be possible to implement this function without an added valve. 
       FIG. 4  illustrates an exemplary embodiment of a closed-center valve  400 . The closed-center valve  400  includes work ports A and B, an outlet port  410 , a first spring  420 , a second spring  422  and a spool  430 . The first and second springs  420 ,  422  bias the spool  430  to a neutral position in the center of the valve cavity (see  FIG. 4C ) when there is no other external forces acting on the spool  430 . The spool  430  of the closed-center valve  400  is designed to prevent flow from both of the work ports A and B to the outlet port  410  when the spool  430  is in the center (neutral) position. When one of the work ports A or B are pressurized (a powered condition), the pressure forces the spool  430  to shift towards the non-pressurized (non-powered) work port. Notches can be cut into the spool  430  or other mechanisms used in the closed-center valve  400  to provide a flow path connecting the non-powered work port A or B to the outlet port  410 , thus allowing flow to be flushed from the circuit through the non-pressurized (non-powered) work port. 
       FIG. 4A  illustrates a state when the A-side of the closed-center valve  400  is powered. In this state, the spool  430  is pushed away from the A-side which compresses the second spring  422 , blocks the work port A, and opens up a flow path indicated by arrow  440  from the work port B to the outlet port  410 . 
       FIG. 4B  illustrates a state when the B-side of the closed-center valve  400  is powered. In this state, the spool  430  is pushed away from the B-side which compresses the first spring  420 , blocks the work port B, and opens up a flow path indicated by arrow  442  from the work port A to the outlet port  410 . 
       FIG. 4C  illustrates a neutral state when neither the A nor B-sides of the closed-center valve  400  are powered. In this state, the first and second springs  420 ,  422  bias the spool  430  to the center of the valve cavity where the spool  430  blocks flow from both the work ports A and B to the outlet port  410 . 
       FIG. 5  illustrates an exemplary embodiment of an open-center valve  500 . The open-center valve  500  includes a valve body  502 , an interior cavity  504 , work ports A and B, an outlet port  510 , a first spring  520 , a second spring  522  and a spool  530 . The work ports A and B, and the outlet port  510  connect the exterior of the valve body  502  to the interior cavity  504 , and the spool  530  is located in the interior cavity  504 . The first and second springs  520 ,  522  are part of a power mechanism that biases the spool  530  to a neutral position in the center of the valve cavity (see  FIG. 5C ) when there are no other external forces acting on the spool  530 , and moves the spool  530  when power or pressure acts on the spool  530 . 
     In this exemplary open-center valve  500 , the spool  530  includes a first end portion  532 , a second end portion  536  and a middle portion  534  between the first and second end portions  532 ,  536 ; and the interior cavity  504  has a substantially uniform cavity diameter CD. The outside diameter of the first and second end portions  532 ,  536  of the spool  530  is substantially equal to inside diameter CD of the interior cavity  504 , and the outside diameter of the middle portion  534  of the spool  530  has a smaller outside diameter than the end portions  532 ,  536 . The spool  530  of the open-center valve  500  is designed to allow flow from both of the work ports A and B to the outlet port  510  when the spool  530  is in the center (neutral) position. An exemplary way to achieve this, as shown in  FIG. 5 , is to make the length of the middle portion  534  of the spool  530  longer than the distance WPD between the work ports A and B. When one of the work ports A or B are pressurized (a powered condition), the pressure forces the spool  530  to shift towards the non-pressurized (non-powered) work port which blocks the pressurized (powered) work port with one of the first and second end portions  532 ,  536  of the spool  530  and opens a flow path around the middle portion  534  of the spool  530  between the non-pressurized (non-powered) work port and the outlet port  510 . Notches can be cut into the middle portion  534  of the spool  530  or other mechanisms used in the open-center valve  500  to provide a flow path connecting the non-powered work port A or B to the outlet port  510 , thus allowing flow to be flushed from the circuit through the non-pressurized (non-powered) work port. 
       FIG. 5A  illustrates a powered state when the A-side of the open-center valve  500  is powered. In this state, the spool  530  is pushed away from the A-side which compresses the second spring  522 . This blocks the work port A with the first end portions  532  of the spool  530 , and opens up a flow path indicated by arrow  540  around the middle portion  534  of the spool  530  connecting the work port B to the outlet port  510 . 
       FIG. 5B  illustrates a powered state when the B-side of the open-center valve  500  is powered. In this state, the spool  530  is pushed away from the B-side which compresses the first spring  520 . This blocks the work port B with the second end portion  536  of the spool  530 , and opens up a flow path indicated by arrow  542  around the middle portion  534  of the spool  530  connecting the work port A to the outlet port  510 . 
       FIG. 5C  illustrates a neutral (unpowered) state when neither the A nor B-sides of the open-center valve  500  are powered. In this state, the first and second springs  520 ,  522  bias the spool  530  to the center of the interior cavity  504  which opens up flow paths indicated by arrows  546  and  548  around the middle portion  534  of the spool  530  connecting both of the work ports A and B, respectively, to the outlet port  510 . 
     The invention above has been described largely around a hydrostatic drive system. However, this type of flushing system could be applied to any closed loop hydraulic application. This could include, for example, hydrostatic fan drive systems or closed loop hydraulic conveyer systems. 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that illustrative embodiment(s) have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. It will be noted that alternative embodiments of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the present invention as defined by the appended claims.
An open center flushing valve for a hydraulic system including a hydraulic circuit and a fluid flushing system, where the flushing valve has multiple positions that each include a fluid exhaust path through the flushing valve. Any of the flushing valve positions couples the hydraulic circuit to the fluid flushing system through a fluid exhaust path for that position. The flushing valve can include unpowered and powered positions. The hydraulic circuit can have two sides separately coupled to the flushing valve where in a powered position, only one side of the hydraulic circuit is coupled to the fluid flushing system through the flushing valve, and in an unpowered position both sides of the hydraulic circuit are coupled to the fluid flushing system through the flushing valve. In a powered position, the lower pressure side of the hydraulic circuit can be coupled to the fluid flushing system through the flushing valve.