Patent Publication Number: US-2023146852-A1

Title: Pressure control valve with reduced pilot flow and hydraulic control system with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of priority to U.S. Provisional Pat. Application No. 63/273,629, filed Oct. 29, 2021, and entitled, “Pressure Control Valve with Reduced Pilot Flow and Hydraulic Control System with the Same,” which is incorporated in its entirety herein by this reference. 
    
    
     TECHNICAL FIELD 
     This patent disclosure relates generally to a hydraulic valve and, more particularly, to a hydraulic cartridge valve comprising a pilot-operated proportional pressure control valve that uses a leakage control element in the pilot stage to reduce the pilot flow between a supply pressure port and a tank port when the valve is in a neutral position. 
     BACKGROUND 
     Mobile machines, such as, e.g., excavators and backhoe loaders, typically include pilot-operated proportional pressure control valves in a hydraulic circuit configured for selective actuation of transmission clutches. One problem associated with such valves is the high pilot oil flow between the supply pressure port to the tank port when the valve is in the neutral position (i.e., with no current applied to the coil). The pilot flow, also known as leakage, is a function of supply pressure in which the higher the supply pressure the higher the leakage. In many applications with a supply pressure of 30 bar applied to the supply pressure port, the pilot flow can exceed one liter per minute in the neutral position. This unused pilot oil results in energy loss. In many transmission applications, there are typically more than one such valve used in the hydraulic circuit which significantly increases the amount of unused oil. 
     One solution to reduce the pilot flow leakage is to decrease the size of the pilot stage orifice. The downside to this approach is the response time of the valve is negatively affected, thereby causing the main spool to shift much slower when moving between the supply pressure port and the work port. The smaller orifice sizes can be difficult to machine, thereby increasing their cost and/or rendering them impractical to manufacture, and are also more prone to blockage from contamination in the system. 
     Typical two stage pressure control valves can be sensitive to contamination. This type of valve uses pilot flow to control the main stage spool. Over time, the filters used to protect the valve from contamination break down and are ineffective in preventing large particles into these orifices. If theses orifices or passages are exposed to contamination, then the particles may not pass through and eventually block the orifice. A blocked orifice is a common failure mode in a hydraulic system. 
     U.S. Pat. No. 9,915,276 discloses an example of a valve available on the market which uses a leakage reducing valve integrated into the pilot stage to control the leakage. This secondary valve is essentially a miniature spool valve that is moved between an open and closed position by actuating the solenoid coil. When the coil is de-energized, the leakage reducing valve returns to the closed position by use of a secondary spring to restrict the pilot flow. When the coil is actuated, the reducing valve spool moves to the open position. This is a costly and complex solution to reduce the pilot flow leakage. 
     There is a continued need in the art to provide additional solutions to enhance the use and efficiency of hydraulic circuits over a range of conditions. For example, there is a continued need for a hydraulic cartridge valve, specifically a pilot-operated proportional pressure control valve that operates with reduced pilot flow between the supply pressure port and the tank port when the valve is in a neutral position. 
     It will be appreciated that this background description has been provided to aid the reader, and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some aspects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims, and not by the ability of any disclosed feature to solve any specific problem noted herein. 
     SUMMARY 
     The present disclosure, in one aspect, is directed to embodiments of a pressure control valve. In embodiments, a pressure control valve includes means for reducing a pilot flow between a supply pressure port and a tank port thereof when the pressure control valve is in a neutral position. 
     In one embodiment, a pressure control valve includes a body, a spool, a biasing element, a pilot flow valve assembly, and a pilot flow control assembly. The body defines an axial bore, a supply port, a work port, and a tank port. Each of the supply port, the work port, and the tank port are in fluid communication with the axial bore. 
     The spool is disposed within the axial bore of the body and axially movable over a range of travel between: (i) a neutral position, in which the supply port and the work port are in fluid isolation from each other and the work port and the tank port are in fluid communication with each other, and (ii) a shifted position, in which the supply port and the work port are in fluid communication with each other and the work port and the tank port are in fluid isolation from each other. At least one of the spool and the body defines a pilot flow passage. The pilot flow passage is in fluid communication with the supply port and the tank port when the spool is in the neutral position. The biasing element is operatively arranged with the spool to bias the spool to the neutral position. 
     The pilot flow valve assembly is configured to selectively occlude the pilot flow passage. The pilot flow valve assembly includes a closure element movable between an open position in which the pilot flow passage is open and a closed position in which the pilot flow passage is occluded. 
     The pilot flow control assembly is disposed in the pilot flow passage. The pilot flow control assembly includes a first control element and a second control element. The first control element is secured relative to the spool, and the second control element is secured relative to the body such that the first control element is axially movable with respect to the second control element upon axial movement of the spool. The first control element and the second control element define, when the spool is in the neutral position, a restricted pilot flow path along the pilot flow passage including a restriction. The restriction varies as a function of the spool position over the range of travel such that the flow of hydraulic fluid through the pilot flow passage is variably restricted along the range of travel of the spool. 
     In another embodiment, a pressure control valve includes a body, a spool, a biasing element, a pilot flow valve assembly, and means for restricting pilot flow through the pilot flow passage. The body defines an axial bore, a supply port, a work port, and a tank port. Each of the supply port, the work port, and the tank port are in fluid communication with the axial bore. 
     The spool is disposed within the axial bore of the body and axially movable over a range of travel between: (i) a neutral position, in which the supply port and the work port are in fluid isolation from each other and the work port and the tank port are in fluid communication with each other, and (ii) a shifted position, in which the supply port and the work port are in fluid communication with each other and the work port and the tank port are in fluid isolation from each other. At least one of the spool and the body defines a pilot flow passage in fluid communication with the supply port and the tank port when the spool is in the neutral position. The biasing element is operatively arranged with the spool to bias the spool to the neutral position. 
     The pilot flow valve assembly is configured to selectively occlude the pilot flow passage. The pilot flow valve assembly includes a closure element movable between an open position in which the pilot flow passage is open and a closed position in which the pilot flow passage is occluded. The means for restricting pilot flow define, when the spool is in the neutral position, a restricted pilot flow path along the pilot flow passage including a restriction, the restriction being configured to variably restrict flow of hydraulic fluid through the pilot flow passage, the restriction varying as a function of the spool position over the range of travel. 
     In still another aspect, embodiments of a hydraulic control system are disclosed. In one embodiment, a hydraulic control system includes a pump, a tank, an actuator, and a pressure control valve. The pump is adapted to provide a source of pressurized fluid, and the tank is adapted to hold a reservoir of fluid. The tank is in fluid communication with the pump. The actuator is in selective fluid communication with the pump. The actuator defines a chamber therein adapted to receive pressurized fluid. The pressure control valve is in fluid communication with the pump, the tank, and the actuator with the pressure control valve interposed between the pump and the actuator and between the actuator and the tank. 
     The pressure control valve includes a body, a spool, a biasing element, a pilot flow valve assembly, and a pilot flow control assembly. The body defines an axial bore, a supply port, a work port, and a tank port. Each of the supply port, the work port, and the tank port are in fluid communication with the axial bore. The supply port is in fluid communication with the pump. The work port is in fluid communication with the chamber of the actuator. The tank port is in fluid communication with the tank. 
     The spool is disposed within the axial bore of the body and axially movable over a range of travel between (i) a neutral position, in which the supply port and the work port are in fluid isolation from each other and the work port and the tank port are in fluid communication with each other to thereby fluidly connect the chamber of the actuator to the tank, and (ii) a shifted position, in which the supply port and the work port are in fluid communication with each other to thereby fluidly connect the pump to the chamber of the actuator and the work port and the tank port are in fluid isolation from each other. At least one of the spool and the body defines a pilot flow passage in fluid communication with the supply port and the tank port when the spool is in the neutral position. The biasing element is operatively arranged with the spool to bias the spool to the neutral position. 
     The pilot flow valve assembly is configured to selectively occlude the pilot flow passage. The pilot flow valve assembly includes a closure element movable between an open position in which the pilot flow passage is open and a closed position in which the pilot flow passage is occluded. The pilot flow control assembly is disposed in the pilot flow passage. The pilot flow control assembly includes a first control element and a second control element. The first control element is secured relative to the spool, and the second control element secured relative to the body such that the first control element is axially movable with respect to the second control element upon axial movement of the spool. The first control element and the second control element define, when the spool is in the neutral position, a restricted pilot flow path along the pilot flow passage including a restriction, the restriction varying as a function of the spool position over the range of travel such that the flow of hydraulic fluid through the pilot flow passage is variably restricted along the range of travel of the spool. 
     Further and alternative aspects and features of the disclosed principles will be appreciated from the following detailed description and the accompanying drawings. As will be appreciated, the pressure control valves and hydraulic control systems disclosed herein are capable of being carried out in other and different embodiments, and capable of being modified in various respects. Accordingly, it is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and do not restrict the scope of the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an elevational view, partially in section, of an embodiment of a hydraulic cartridge valve constructed in accordance with principles of the present disclosure, illustrating the hydraulic cartridge valve illustrated in a neutral position. 
         FIG.  2    is an enlarged detail view of the hydraulic cartridge valve taken from  FIG.  1   , illustrating an embodiment of a pilot flow control assembly constructed in accordance with principles of the present disclosure. 
         FIG.  3    is an elevational view, in section, of a leakage control element of the hydraulic cartridge valve of  FIG.  1   . 
         FIG.  4    is a top plan view of the leakage control element of  FIG.  3   . 
         FIG.  5    is a view of the hydraulic cartridge valve as in  FIG.  1   , but illustrating the hydraulic cartridge valve in an intermediate position. 
         FIG.  6    is a view of the hydraulic cartridge valve as in  FIG.  1   , but illustrating the hydraulic cartridge valve in a shifted position. 
         FIGS.  7 - 12    are each an elevational view, in section, of a respective embodiment of a pilot flow control assembly suitable for use in an embodiment of a hydraulic cartridge valve constructed in accordance with principles of the present disclosure. 
         FIG.  13    is a fragmentary, top plan view of the leakage control assembly of  FIG.  12   , illustrating a notch feature defined therein. 
         FIGS.  14  and  15    are each an elevational view, in section, of a respective embodiment of a pilot flow control assembly suitable for use in an embodiment of a hydraulic cartridge valve constructed in accordance with principles of the present disclosure. 
         FIG.  16    is an elevational view, partially in section, of an embodiment of a hydraulic cartridge valve constructed in accordance with principles of the present disclosure, illustrating the hydraulic cartridge valve in a neutral position. 
         FIG.  17    is an enlarged, detail view taken from  FIG.  16    as indicated by circle XVII— XVII. 
         FIG.  18    is an elevational view, partially in section, of an embodiment of a hydraulic cartridge valve constructed in accordance with principles of the present disclosure, illustrating the hydraulic cartridge valve in a neutral position. 
         FIG.  19    is an enlarged, detail view taken from  FIG.  18    as indicated by circle XIX— XIX. 
         FIG.  20    is a generally schematic view of an embodiment of a hydraulic control system constructed in accordance with principles of the present disclosure, the hydraulic control system including a pair of hydraulic cartridge valves constructed in accordance with principles of the present disclosure respectively associated with a pair of actuators in the form of clutches. 
     
    
    
     It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of this disclosure or which render other details difficult to perceive may have been omitted. It should be understood that this disclosure is not limited to the particular embodiments illustrated herein. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of a pressure control valve constructed in accordance with principles of the present disclosure include means for reducing a pilot flow between a supply port and a tank port thereof when the pressure control valve is in a neutral position. Embodiments of a pressure control valve constructed in accordance with principles of the present disclosure comprise a pilot-operated proportional pressure control valve in the form of a hydraulic cartridge valve that includes means for reducing a pilot flow between a supply port and a tank port in which the pilot flow reducing means comprise a pilot flow control assembly in a pilot stage to significantly reduce the pilot flow between the supply port and the tank port when the valve is in a neutral position. In embodiments, a hydraulic cartridge valve comprises a pilot flow control assembly including a first control element and a second control element that define, when the spool is in the neutral position, a restricted pilot flow path along a pilot flow passage including a first orifice and a second orifice in serial relationship with each other. In embodiments, the second orifice has a cross-sectional area equal to or less than the cross-sectional area of the first orifice. 
     Embodiments of a pressure control valve constructed according to principles of the present disclosure can comprise a two-stage valve including a main stage and a pilot stage adapted to use pilot flow to control a main stage spool. The pilot flow is controlled by limiting the flow of oil through a pair of small orifices in series relationship with each other. The leakage control element is configured to be self-cleaning. When the coil is energized, the spool will move away from the pilot pin and flush out any contaminate that is trapped in the spool or orifice passages to the tank port. 
     Embodiments of a pressure control valve constructed according to principles of the present disclosure can have a wide variety of different forms, as will be appreciated by one skilled in the art. For example, in embodiments, a pressure control valve constructed according to principles of the present disclosure can be, for example, in the form of a pilot-operated proportional pressure control cartridge valve. In other embodiments, a hydraulic valve constructed according to principles of the present disclosure can comprise a pilot flow control assembly applied to other hydraulic valves that utilize pilot flow to control the main stage spool. 
     Embodiments of a hydraulic control system constructed in accordance with principles of the present disclosure can selectively operate an actuator (e.g., cylinder) using an embodiment of a pressure control valve constructed in accordance with principles of the present disclosure. Embodiments of a pressure control valve constructed in accordance with principles of the present disclosure can be used to control the pressure inside an actuator. The control pressure is proportional to the amount of current applied to the coil of the pressure control valve. The current can be variably adjusted across a prescribed range using a variable electric input. 
     Turning now to the Figures, there is shown in  FIG.  1    an embodiment of a hydraulic cartridge pressure control valve  25  constructed according to principles of the present disclosure. The illustrated valve comprises a pilot-operated proportional pressure control valve that includes a main stage  27  and a pilot stage  28 . The illustrated pressure control valve includes a body  30 , a spool  31 , a biasing element  32  in the form of a spring, a pilot flow valve assembly  34 , and means for restricting pilot flow through the pilot flow passage in the form of a pilot flow control assembly  35 . 
     In embodiments, the body  30  can have any configuration suitable for the intended application(s) of the pressure control valve  25 . In embodiments, the body  30  can be made from a plurality of components that are assembled together to define an axial bore  43  and a plurality of ports in communication with the axial bore  43 . In embodiments, the body  30  can be configured to facilitate the installation of the pressure control valve  25  in a hydraulic body, manifold or other suitable component. 
     In the illustrated embodiment, the body  30  includes a frame  38  and a cage  40 . In embodiments, the frame  38  and the cage  40  can be made using any suitable technique as will be appreciated by one skilled in the art. For example, in embodiments, the frame  38  can comprise a cold-forged frame that is machined to its final shape. The lower portion of the frame  38  interfaces with the cage  40  and is assembled by forming the end of the frame  38  over the cage  40 . In embodiments, the cage  40  can be mounted to the frame  38  using any suitable technique, such as by being threadedly engaged therewith as shown in  FIG.  1   . 
     The frame  38  includes a circular flange  41  configured to secure the valve  25  within a valve cavity by use of a mounting plate (not shown). In other embodiments, the body  30  can include an external threaded surface that is adapted to be threadedly engaged with a body, manifold, or other suitable component to allow the pressure control valve  25  to be used in a hydraulic circuit. 
     In the illustrated embodiment, the main stage  27  of the pressure control valve  25  comprises the cage  40 , the spool  31 , and the biasing member  32 . The cage  40  is hollow and is configured to be inserted into a cavity formed in a suitable housing such that the valve  25  is in fluid communication with a hydraulic circuit within which the valve  25  is intended to be used. 
     The cage  40  of the body  30  defines the axial bore  43 , a supply port 2 , a work port 3 , and a tank port 4 . Each of the supply port 2 , the work port 3 , and the tank port 4  are in fluid communication with the axial bore  43 . The cage  40  defines three rows of cross-holes  45 ,  46 ,  47  in communication with the axial bore  43  with the cross-holes  45 ,  46 ,  47  of each row being disposed in spaced relationship to each other around the radial circumference of the cage  40  and respectively defining the supply port 2 , the work port 3 , and the tank port 4 . It should be understood that the names used herein for the ports 2-4  defined by the cage  40  are used for convenient reference only and should not be construed to limit the operation of the ports 2-4  or the nature of the fluid flow (in either direction) through the ports 2-4  of the cage  40 . 
     The spool  31  is disposed within the axial bore  43  of the body  30  and is axially movable over a range of travel between a neutral position, as shown in  FIG.  1   , and a shifted position, as shown in  FIG.  6   . When the spool  31  is in the neutral position, the spool  31  prevents fluid flow between the supply port 2  and the work port 3  such that the supply port 2  and the work port 3  are in fluid isolation from each other and permits fluid flow between the work port 3  and the tank port 4  such that the work port 3  and the tank port 4  are in fluid communication with each other. When the spool  31  is in the shifted position, the spool  31  permits fluid flow between the supply port 2  and the work port 3  such that the supply port 2  and the work port 3  are in fluid communication with each other and prevents fluid flow between the work port 3  and the tank port 4  such that the work port 3  and the tank port 4  are in fluid isolation from each other. In embodiments, the pressure control valve  25  is configured such that the spool  31  has a range of travel including at least one intermediate position between the neutral position and the shifted position in which the work port 3  is in fluid communication with both the tank port 4  and the supply port 2 . 
     Referring to  FIGS.  1  and  2   , in embodiments, at least one of the spool  31  and the body  30  define a pilot flow passage  50  in fluid communication with the supply port 2  and the tank port 4  when the spool  31  is in the neutral position. In the illustrated embodiment, the spool  31  and the body  30  cooperate together to define the pilot flow passage  50 . The illustrative pilot flow passage  50  of the pressure control valve  25  is indicated by the arrows found in  FIGS.  1  and  2   . In embodiments, the pilot flow passage  50  is in communication with the supply port 2  and the tank port 4  over the entire range of travel of the spool  31 . In embodiments, the degree to which the pilot flow passage  50  is restricted can vary as a function of the position of the spool  31  over the range of travel between the neutral position and the shifted position. 
     For example, in the illustrated embodiment, the spool  31  defines a counterbore opening  52  which leads to the pilot flow passage  50 . As the spool  31  moves from the intermediate position shown in  FIG.  5    to the shifted position shown in  FIG.  6   , the counterbore opening  52  to the pilot flow passage  50  closes so that the pilot flow passage  50  becomes more and more restricted. 
     Referring to  FIG.  1   , the spool  31  includes a supply land  54  at a first end  55  having a supply groove  57  defined therein and a tank land  58  at a second end  59 . A work groove  64  is defined between the supply land  54  and the tank land  58 . The spool  31  defines a pilot flow passage portion  65  that extends between the supply groove and the open second end  59  of the spool  31 . The spool  31  defines a damping flow passage  67  having a damping orifice  69  therein and extending between the work groove  64  and the open first end  55  of the spool  31 . 
     The supply land  54  is configured to block the row of metering cross holes  45  comprising the supply port 2  from being in fluid communication with the work port 3  when the spool  31  is in the neutral position and to permit fluid flow therebetween when the spool  31  is in the shifted position (see  FIG.  6   ). The supply groove  57  is configured to fluidly connect the supply port 2  and the pilot flow passage  50  when the spool  31  is in the neutral position but not when the spool  31  is in the shifted position (see  FIG.  6   ). The supply land  54  is configured to block the row of metering cross holes  45  comprising the supply port 2  from being in fluid communication with pilot flow passage  50  when the spool  31  is in the shifted position (see  FIG.  6   ). 
     The work groove  64  is configured to fluidly connect the work port 3  and the tank port 4  when the spool  31  is in the neutral position but not when the spool  31  is in the shifted position (see  FIG.  6   ). Rather, the work groove  64  is configured to fluidly connect the work port 3  and the supply port 2  when the spool  31  is in the shifted position (see  FIG.  6   ). 
     The tank land  58  is configured to permit fluid flow between the respective rows of metering cross holes  47 ,  46  comprising the tank port 4  and the work port 3  when the spool  31  is in the neutral position. The tank land is configured to prevent the tank port 4  from being in fluid communication with the work port 3  and to prevent the pilot flow passage  50  from being in fluid communication with the tank port 4  when the spool  31  is in the shifted position (see  FIG.  6   ). 
     Referring to  FIG.  1   , in the illustrated embodiment, a plug  71  is disposed within the axial bore  43  of the body  30  and is secured to the cage  40  at a distal end  72  of the cage  40  to occlude the axial bore  43 , thereby occluding an axial port 1  of the body  30 . The plug  71  is secured to the cage  40  by a retaining ring  74 . The plug  71  and the spool  31  cooperate with the cage  40  of the body  30  to define a spring chamber  75  within the axial bore  43 . The work port 3  and the spring chamber  75  are in fluid communication with each other via the damping flow passage  67  of the spool  31  over the range of travel of the spool  31  between the neutral position and the shifted position (see  FIG.  6   ). 
     In the illustrated embodiment, the biasing element  32  comprises a spring. In other embodiments, any other structure and/or technique for biasing the spool  31  to the neutral position can be used. The spring  32  is operatively arranged with the spool  31  to bias the spool  31  to the neutral position, as shown in  FIG.  1   . The spring  32  is disposed within the spring chamber  75  such that opposing ends of the spring  32  respectively act against the plug  71  and the spool  31 . 
     The spring  32  provides a bias force to put the spool  31  in the neutral position when the coil  105  is de-energized, thereby blocking the supply port 2  from the work port 3 . This type of spool design is known as “closed-in-transition” or “positive overlap.” In embodiments, the closed-in-transition spool configuration is used in pressure reducing/relieving valve applications to help maintain the stability of the valve (reducing/inhibiting pressure oscillations during operation). In the illustrated embodiment, damping of the valve  25  is accomplished by controlling the flow of hydraulic fluid in and out of the spring chamber  75  via the damping orifice  69 . 
     A plurality of seal members 81-85 provided to help provide a sealing arrangement within the valve  25  and between the valve body and the structure to or into which the pressure control valve  25  is mounted. The seal members 81-83 provide sealing between the ports 2-4  and prevent external leakage. The seal members  84 ,  85  provide internal sealing within the valve  25 . In embodiments, the seal members 81-85 can have any suitable form and construction, such as suitable O-ring seals, and can be provided in any suitable number to provide hydraulic isolation and/or seals to inhibit leakage, as appropriate and as will be understood by one skilled in the art. 
     The pilot stage  28  of the pressure control valve  25  comprises the pilot flow valve assembly  34  and the pilot flow control assembly  35 . The pilot flow valve assembly  34  is configured to selectively occlude the pilot flow passage  50 . In the illustrated embodiment, the pilot flow valve assembly  34  is configured to selectively prevent pilot flow from the pilot flow passage  50  out the tank port 4 . 
     Referring to  FIG.  2   , in embodiments, the pilot flow valve assembly  34  includes a closure element  90 , a seat  91 , a push pin  92 , and an actuator  93  (see  FIG.  1   ). The closure element  90  is movable between an open position (as shown in  FIG.  2   ) and a closed position (as shown in  FIGS.  5  and  6   ). When the closure element  90  is in the open position, the pilot flow passage  50  is open. When the closure element  90  is in the closed position, the pilot flow passage  50  is occluded. In the illustrated embodiment, the closure element  90  of the pilot flow valve assembly  34  comprises a spherical ball. 
     Referring to  FIG.  2   , in the illustrated embodiment, the seat  91  is secured to the cage  40  of the body  30  by being threadedly engaged therewith. The seat  91  includes a first end  96  and a second end  97 . The seat  91  defines a through passage  98  that comprises a portion of the pilot flow passage  50  and that extends from the first end  96  to the second end  97  of the seat  91 . The ball  90  is adjacent the first end  96  of the seat  91 . In embodiments, one of a pair of control elements  101 ,  102  of the means for restricting pilot flow is secured to the seat  91  adjacent the second end  97  thereof. 
     In the illustrated embodiment, the push pin  92  is arranged with the ball  90 . The push pin  92  is axially movable in order to selectively place the ball  90  in sealing engagement with the seat  91 . 
     Referring to  FIG.  1   , in embodiments, the actuator  93  is configured to selectively move the closure element  90  of the pilot flow valve assembly  34  to the closed position. In embodiments, the actuator  93  can be any suitable mechanism configured to selectively move the closure element  90  of the pilot flow valve assembly  34  to the closed position. In the illustrated embodiment, the actuator  93  is mounted to the body  30 . In the illustrated embodiment, the actuator  93  is arranged with the push pin  92  and is configured to selectively move the push pin  92  to thereby seat the ball  90  against the seat  91  to occlude the through passage  98  of the seat  91  and thereby occlude the pilot flow passage  50 . 
     In the illustrated embodiment, the actuator  93  comprises a solenoid assembly  104  including a coil  105 , an armature  107 , and a pole piece  108 . The coil  105  is mounted to the frame  38  of the body  30  and is disposed around the armature  107 . The coil  105  can be mounted to the frame  38  using any suitable technique as will be familiar to one skilled in the art. In embodiments, the coil  105  is operably arranged with a source of electrical current (not shown) via an electrical connector  109  such that a controller (not shown) can selectively actuate the coil  105  by applying electrical current thereto. 
     The armature  107  is associated with the coil  105  such that operation of the actuator  93  by a controller can selectively move the armature  107 . The armature  107  is disposed within the axial bore  43  of the body  30  and is configured to move toward the pole piece  108  in response to an electrical current being applied to the coil  105 . The armature  107  is arranged with the push pin  92  such that the movement of the armature  107  toward the pole piece  108  moves the push pin  92  to thereby move the ball  90  to the closed position and into sealing arrangement with the seat  91 . In embodiments, the solenoid assembly  104  is configured such that, when coil  105  is energized, the push pin  92  moves the ball  90  in an amount proportional to the electrical current applied to the coil  105 . 
     In the illustrated embodiment, the pole piece  108  is part of the frame  38  and is configured to limit the movement of the armature  107  to a predetermined range of axial travel. In embodiments, the solenoid assembly  104  has a proportional characteristic where the magnetic attractive force between the frame  38  and the armature  107  is proportional to the current applied to the coil  105 . The solenoid force therefore remains constant over the stroke. In embodiments, a non-magnetic spacer can be arranged with the armature  107  to help prevent the armature  107  from latching to the polepiece  108 . 
     Referring to  FIG.  2   , in embodiments, the means for restricting pilot flow define a restriction  115  along the pilot flow passage  50 . The restriction  115  is configured to restrict the flow of hydraulic fluid through the pilot flow passage  50  in a variable manner as a function of the position of the spool  31 . In the illustrated embodiment, the restriction  115  is in the form of an orifice. 
     In embodiments, the means for restricting pilot flow define, when the spool  31  is in the neutral position, a restriction in the form of an orifice  115  in serial relationship with at least one other orifice  117  disposed along the pilot flow passage  50 . In embodiments, the orifice  115  defined by the means for restricting pilot flow has a cross-sectional area equal to or less than the cross-sectional area of at least one other orifice  117  disposed along the pilot flow passage  50  when the spool  31  is in the neutral position, as shown in  FIG.  2   . In embodiments, the size of the orifice  115  defined by the means for restricting pilot flow is variable as a function of the position of the spool  31 . 
     Referring to  FIG.  2   , in the illustrated embodiment, the means for restricting pilot flow include the pilot flow control assembly  35 . In embodiments, the pilot flow control assembly  35  is disposed in the pilot flow passage  50 . In the illustrated embodiment, the pilot flow control assembly  35  includes the first control element  101  and the second control element  102 . The leakage control elements  101 ,  102  are used to control the pilot flow leakage. The first control element  101  is secured to the spool  31 , and the second control element  102  is secured to the body  30  such that the first control element  101  is movable with respect to the second control element  102  upon axial movement of the spool  31  relative to the body  30 . The first control element  101  and the second control element  102  define, when the spool  31  is in the neutral position, a restricted pilot flow path  50 ′ along the pilot flow passage  50  including the first orifice  117  and the second orifice  115  in serial relationship with each other. The second orifice  115  has a cross-sectional area equal to or less than the cross-sectional area of the first orifice  117 . In the illustrated embodiment, the first control element  101  comprises a restriction member, and the second control element  102  comprises a pilot pin. 
     In embodiments, the leakage control elements  101 ,  102  are configured to significantly reduce the pilot flow with supply pressure applied with no current applied to the coil  105 . In the illustrated embodiment, the leakage control elements  101 ,  102  incorporate an offset feature  118  (see also  FIGS.  3  and  4   ) in the first control element  101  which is in the form of the restriction member, or orifice spacer, along with the second control element  102  in the form of the pilot pin that protrudes into the opening  140  of the orifice spacer. The interrelationship between the pilot pin  102  and the offset opening  118  of the restriction member  101  defines the second orifice  115  which comprises a very small open area through which hydraulic fluid can pass, thereby reducing the pilot flow. 
     Referring to  FIG.  2   , in the illustrated embodiment, the restriction member  101  is installed in the spool  31  via a spring ring  120 . The restriction member  101  defines a through passage  122  comprising a portion of the restricted pilot flow path  50 ′ including the orifice  117 . The orifice  117  in the restriction member  101  is fixed in that the size of the orifice  117  is constant over the range of travel of the spool  31  and the restriction member  101 . The restriction member  101  is disposed in the pilot flow passage  50  such that the pilot flow through the pilot flow passage  50  is directed through the fixed orifice  117  in the portion of the restricted pilot flow path  50 ′ defined by the restriction member  101 . 
     In embodiments, the pilot flow passage  50  can omit the fixed orifice  117  such that the pilot flow passage  50  includes only the variable orifice  115  of the means for restricting pilot flow. In embodiments, the pilot flow passage  50  can include one or more fixed orifices disposed along the pilot flow passage  50  and each in serial relationship with the variable orifice  115  of the means for restricting pilot flow. 
     The restriction member  101  is configured to control flow of hydraulic fluid into the pilot stage  28 . The size of the first orifice  117  controls the pilot flow leakage. 
     In the illustrated embodiment, the pilot pin  102  is secured to the seat  91  with a retaining ring  130 . In embodiments, the pilot pin  102  and the seat  91  can be combined into one part, thereby eliminating the need for the retaining ring  130 . The pilot pin  102  includes a base  132  and a pin portion  134 . The base is generally disc-shaped and defines a pair of passages  135 ,  136  therethrough in order to all pilot flow therethrough. The pin portion  134  of the pilot pin  102  has a conical distal end  138  which is arranged with an opening  140  of the through passage  122  of the restriction member  101  to define the second orifice  115  when the spool  31  is in the neutral position. The conical distal end  138  of the pilot pin  102  extends into the through passage  122  of the restriction member  101 . 
     In the illustrated embodiment, when the spool  31  is in the neutral position, the first control element  101  and the second control element  102  are in a first position with respect to each other and cooperate together to define the second orifice  115  therebetween. When the spool  31  is in the shifted position (see  FIG.  6   ), the first control element  101  and the second control element  102  are in a second position with respect to each other that is different from the first position such that a clearance  141  is defined therebetween that is different from the second orifice  115  in at least one of shape and size such that the restricted pilot flow path  50 ′ does not include the second orifice  115  when the spool  31  is in the shifted position. 
     Referring to  FIG.  1   , the spring  32  under the spool  31  is configured to keep the restriction member  101  seated against the pilot pin  102  in the neutral position. In embodiments, the pre-load force from the spring  32  is slightly greater than the force due to the inlet pressure acting over the restriction member  101  center hole diameter. In embodiments of the present disclosure, a small amount of pilot flow through the second orifice  115  is provided to prevent pressure from building in the pilot stage  28  that is greater than the spring force of the spring  32 , which would cause the spool  31  to self-shift out of the neutral position. 
     Referring to  FIG.  1   , the illustrated embodiment of the pressure control valve  25  is shown with the spool  31  in the neutral position. When the spool  31  is in the neutral position, the supply land  54  of the spool  31  prevents fluid flow between the supply port 2  and the work port 3 . The supply groove  57  permits fluid flow between the supply port 2  and the tank port 4  via the pilot flow passage  50 . The work groove  64  permits fluid flow between the work port 3  and the spring chamber via the damping flow passage  67 . The work groove  64  permits fluid flow between the work port 3  and the tank port 4 . 
     Referring to  FIG.  5   , the illustrated embodiment of the pressure control valve  25  is shown with the spool  31  in an intermediate position between the neutral position and the shifted position. When the spool  31  is in the illustrated intermediate position, the supply land  54  of the spool  31  prevents fluid flow between the supply port 2  and the work port 3 . The supply groove  57  permits fluid flow between the supply port 2  and the pilot chamber  143  defined between the first and second control elements  101 ,  102  via the pilot flow passage  50 . The work groove  64  permits fluid flow between the work port 3  and the spring chamber  75  via the damping flow passage  67 . The tank land  58  of the spool  31  prevents fluid flow between the work port 3  and the tank port 4 . 
     Referring to  FIG.  6   , the illustrated embodiment of the pressure control valve  25  is shown with the spool  31  in the shifted position. When the spool  31  is in the shifted position, the supply land  54  of the spool  31  prevents fluid flow between the supply port 2  and the work port 3  and prevents fluid flow between the supply port 2  and the pilot chamber  143  via the pilot flow passage  50 . The work groove  64  permits fluid flow between the supply port 2  and the work port 3  and between the work port 3  and the spring chamber  75  via the damping flow passage  67 . The tank land  58  of the spool  31  prevents fluid flow between the work port 3  and the tank port 4 . 
     Referring to  FIG.  1   , when the coil  105  is de-energized, the spool  31  of the embodiment of the valve  25  depicted in  FIG.  1    allows bidirectional flow between the work port 3  and the tank port 4  while blocking hydraulic fluid flow from the supply port 2  to the work port 3 . In this mode of the pressure control valve  25 , the supply port 2  is connected to the tank port 4 , which is known as “pilot flow” or leakage. Embodiments of a pressure control valve constructed according to principles of the present disclosure can operate to reduce the pilot flow by more than half, and by up to about 90% in other embodiments, of the pilot flow that would otherwise be present in the absence of means for reducing pilot flow constructed according to principles of the present disclosure, including the leakage control elements discussed herein. 
     Referring to  FIG.  5   , when the coil  105  is energized, the armature  107  becomes attracted to the frame  38  and pushes, via the push pin  92 , the ball  90  against the seat  91  which blocks the flow of hydraulic fluid through the pilot flow passage  50 . The pilot chamber  143 , the area between the seat  91  and the spool  31 , fills with hydraulic fluid and pressurizes the spool end  55  at the spring chamber  75 , thereby causing the spool  31  to move down and compress the spring  32 . 
     Referring to  FIG.  6   , once sufficient current is applied to the coil  105 , the spool  31  will compress the spring  32  below the spool  31  to the point where the spool  31  is in the shifted position, as shown in  FIG.  6   , and connects the supply port 2  to the work port 3 . When the coil  105  is energized such that the supply port 2  is connected to the work port 3 , pressure at the work port 3  is controlled proportionally to the amount of current applied to the coil  105 . If the pressure at the work port 3  exceeds the setting controlled by the coil  105 , the pressure is relieved to the tank port 4 . 
     In the illustrated embodiment, the leakage control elements  101 ,  102  are configured to be self-cleaning. When the coil  105  is energized, the spool  31  will move away from the pilot pin  102  and flush out any contamination that is trapped in the spool  31  or orifice passages to the tank port 4 . 
     Embodiments of a pressure control valve constructed in accordance with principles of the present disclosure can provide a reduction in pilot flow leakage relative to a pressure control valve that does not include means for reducing pilot flow following principles of the present disclosure. Embodiments of a pressure control valve constructed in accordance with principles of the present disclosure can include leakage control elements that reduce or eliminate the need for a secondary valve which reduces overall cost of the hydraulic control circuit. Embodiments of a pressure control valve constructed in accordance with principles of the present disclosure can provide desired performance characteristics that are primarily unaffected by the reduction in the de-energized pilot flow provided by the means for reducing pilot flow following principles of the present disclosure. 
     Referring to  FIGS.  7 - 15   , other embodiment of means for restricting pilot flow constructed in accordance with principles of the present disclosure are depicted therein. In particular, embodiments of first and second leakage control elements constructed according to principles of the present disclosure and comprising means for restricting pilot flow are shown in  FIGS.  7 - 15   . It should be understood that the means for restricting pilot flow can be carried out in other equivalent ways which will be appreciated by one skilled in the art. 
     Referring to  FIG.  7   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  201 ,  202  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  201  comprises a restriction member, and the second control element  202  comprises a pilot pin. The restriction member  201  defines a through passage  222  having an opening  240  that includes an offset hole  218 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  222  of the restriction member  201  comprises a portion of the restricted pilot flow path  250 ′ including the first orifice (not shown). 
     The pilot pin  202  includes a conical distal end  238 . The conical distal end  238  of the pilot pin  202  projects into the portion of the restricted pilot flow path  250 ′ defined by the restriction member  201  and cooperates with the offset hole  218  to define the second orifice  215  when the spool is in the neutral position. When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  201 ,  202  can move axially with respect to each other such that the conical distal end  238  of the pilot pin  202  is axially displaced relative to the offset hole  218  of the restriction member  201  to effectively remove the second orifice  215  from the pilot flow path. The restriction member  201  and the pilot pin  202  of  FIG.  7    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  102  of  FIG.  1    in other respects. 
     Referring to  FIG.  8   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  301 ,  302  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  301  comprises a restriction member, and the second control element  302  comprises a pilot pin. The restriction member  301  defines a through passage  322  having an opening  340  that includes a counterbore  344  with a diameter greater than that of the through passage  322 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  322  of the restriction member  301  comprises a portion of the restricted pilot flow path 350′ including the first orifice (not shown). 
     The pilot pin  302  includes a cylindrical distal end  339 . The cylindrical distal end  339  of the pilot pin  302  extends through the counterbore  344  into the through passage  322  of the restriction member  301  and cooperates therewith to define the second orifice  315  when the spool is in the neutral position. When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  301 ,  302  can move axially with respect to each other such that the cylindrical distal end  339  of the pilot pin  302  is axially displaced relative to the counterbore  344  of the restriction member  301  to effectively remove the second orifice  315  from the pilot flow path. The restriction member  301  and the pilot pin  302  of  FIG.  8    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  102  of  FIG.  1    in other respects. 
     Referring to  FIG.  9   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  401 ,  402  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  401  comprises a restriction member, and the second control element  402  comprises a pilot pin. The restriction member  401  defines a through passage  422  having an opening  440  that includes a counterbore  444  with a diameter greater than that of the through passage  422 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  422  of the restriction member  401  comprises a portion of the restricted pilot flow path  450 ′ including the first orifice (not shown). 
     The pilot pin  402  includes a conical distal end  438  that defines an intermediate notch  445 . The conical distal end  438  of the pilot pin  402  projects into the portion of the restricted pilot flow path  450 ′ defined by the counterbore  444  of the restriction member  401  such that the intermediate notch  445  of the pilot pin  402  cooperates with the opening  440  of the restriction member  401  to define the second orifice  415  when the spool is in the neutral position. When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  401 ,  402  can move axially with respect to each other such that the conical distal end  438  of the pilot pin  402  is axially displaced relative to the opening  440  of the restriction member  401  such that the notch  445  of the pilot pin  402  is no longer in close proximity to the counterbore  444  of the restriction member  401 , thereby effectively removing the second orifice  415  from the pilot flow path. The restriction member  401  and the pilot pin  402  of  FIG.  9    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  402  of  FIG.  1    in other respects. 
     Referring to  FIG.  10   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  501 ,  502  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In embodiments, one of the first control element and the second control element comprises a spherical surface, and the other of the first control element and the second control element comprises a restriction member. In the illustrated embodiment, the first leakage control element  501  comprises a restriction member, and the second control element  502  comprises a ball having a spherical exterior surface. The restriction member  501  defines a through passage  522  having an opening  540  that includes an offset hole  518 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  522  of the restriction member  501  comprises a portion of the restricted pilot flow path 550′ including the first orifice (not shown). 
     The spherical exterior surface of the ball  502  is arranged with the opening  540  of the through passage  522  of the restriction member  501  and cooperates with the offset hole  518  to define the second orifice  515  when the spool is in the neutral position. When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  501 ,  502  can move axially with respect to each other such that the ball  502  is axially displaced relative to the opening  540  of the restriction member  501  such that spherical exterior surface of the ball  502  is no longer in close proximity to the offset hole  518  of the restriction member  501 , thereby effectively removing the second orifice  515  from the pilot flow path. The restriction member  501  of  FIG.  10    can be similar in construction and function to the restriction member  101  of  FIG.  1    in other respects. 
     Referring to  FIG.  11   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  601 ,  602  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  601  comprises a restriction member, and the second control element  602  comprises a pilot pin. The restriction member  601  defines a through passage  622  having an opening  640 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  622  of the restriction member  601  comprises a portion of the restricted pilot flow path 650′ including the first orifice (not shown). 
     The pilot pin  602  includes a tapered distal end  637 . The tapered distal end  637  of the pilot pin  602  extends into the through passage  622  of the restriction member  601  and cooperates therewith to define the second orifice  615  when the spool is in the neutral position. When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  601 ,  602  can move axially with respect to each other such that the tapered distal end  637  of the pilot pin  602  is axially displaced relative to the opening  640  of the restriction member  601  such that the second orifice  615  is effectively removed from the pilot flow path. The restriction member  601  and the pilot pin  602  of  FIG.  11    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  102  of  FIG.  1    in other respects. 
     Referring to  FIGS.  12  and  13   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  701 ,  702  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  701  comprises a restriction member, and the second control element  702  comprises a pilot pin. 
     Referring to  FIG.  12   , the restriction member  701  defines a through passage  722  having an opening  740  that includes a tapered countersink surface  747  circumscribing the opening  740 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  722  of the restriction member  701  comprises a portion of the restricted pilot flow path 750′ including the first orifice (not shown). 
     The pilot pin  702  includes a base  732  defining a groove  733  and a cylindrical distal end  739  projecting from the base  732 . The cylindrical distal end  739  of the pilot pin  702  extends through the opening  740  of the restriction member  701  into the through passage  722  of the restriction member  701  when the spool is in the neutral position. The groove  733  of the pilot pin  702  cooperates with the tapered countersink surface  747  of the restriction member  701  to define the second orifice  715  when the spool is in the neutral position (see also,  FIG.  13   ). When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  701 ,  702  can move axially with respect to each other such that the cylindrical distal end  739  of the pilot pin  702  is axially displaced relative to the opening  740  of the restriction member  701  such that the groove  733  of the pilot pin  702  is no longer in close proximity to the tapered countersink surface  747  of the restriction member  701 , thereby effectively removing the second orifice  715  from the pilot flow path. The restriction member  701  and the pilot pin  702  of  FIGS.  12  and  13    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  102  of  FIG.  1    in other respects. 
     Referring to  FIG.  14   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  801 ,  802  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  801  comprises a restriction member, and the second control element  802  comprises a pilot pin. The restriction member  801  defines a through passage  822  having an opening  840  that includes a tapered countersink surface  847  circumscribing the opening  840 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  822  of the restriction member  801  comprises a portion of the restricted pilot flow path 850′ including the first orifice (not shown). 
     The pilot pin  802  includes a base  832  comprising a porous material and a cylindrical distal end  839  projecting from the base  832 . The cylindrical distal end  839  of the pilot pin  802  extends through the opening  840  of the restriction member  801  into the through passage  822  of the restriction member  801  when the spool is in the neutral position. The base  832  of the pilot pin  802  cooperates with the tapered countersink surface  847  of the restriction member  801  to define effectively the second orifice  815  through the base  832  when the spool is in the neutral position. In embodiments, the porosity of the base  832  can be adapted to provide an effective orifice  815  through the base  832  according to the intended application of the pressure control valve and the desired flow rate through the effective second orifice  815 . When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  801 ,  802  can move axially with respect to each other such that the cylindrical distal end  839  of the pilot pin  802  is axially displaced relative to the opening  840  of the restriction member  801  such that the base  832  of the pilot pin  802  is no longer in close proximity to the tapered countersink surface  847  of the restriction member  801 , thereby effectively removing the second orifice  815  from the pilot flow path. The restriction member  801  and the pilot pin  802  of  FIG.  14    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  102  of  FIG.  1    in other respects. 
     Referring to  FIG.  15   , an embodiment of means for restricting pilot flow comprising first and second leakage control elements  901 ,  902  are shown which are suitable for use in a pressure control valve constructed according to principles of the present disclosure. In the illustrated embodiment, the first leakage control element  901  comprises a restriction member, and the second control element  902  comprises a pilot pin. The restriction member  901  defines a through passage  922  having an opening  940  that includes a textured mating surface  948  and a tapered countersink surface  847  circumscribing the opening  940 . When installed in a pressure control valve constructed according to principles of the present disclosure, the through passage  922  of the restriction member  901  comprises a portion of the restricted pilot flow path 950′ including the first orifice (not shown). 
     The pilot pin  902  includes a base  932  having a textured mating surface  933  and a cylindrical distal end  939  projecting from the base  932 . The cylindrical distal end  939  of the pilot pin  902  extends through the opening  940  into the through passage  922  of the restriction member  901  and the textured mating surface  933  of the pilot pin  902  cooperates with the textured mating surface  948  of the restriction member  901  to define the second orifice  915  when the spool is in the neutral position. The variation in surface features provided by the textured mating surfaces  933 ,  948  can provide an effective second orifice  915  through which the hydraulic fluid can flow in a restricted manner. In embodiments, the textured surfaces  933 ,  948  can be varied and configured to provide a desired flow rate for the effective second orifice  915   based upon the intended application of the pressure control valve. When the spool is moved from the neutral position to the shifted position, the first and second leakage control elements  901 ,  902  can move axially with respect to each other such that the textured mating surface  933  of the pilot pin  902  is no longer in close proximity to the textured mating surface  948  and the tapered countersink surface  947  of the restriction member  901 , thereby effectively removing the second orifice  915  from the pilot flow path. The restriction member  901  and the pilot pin  902  of  FIG.  15    can be respectively similar in construction and function to the restriction member  101  and the pilot pin  102  of  FIG.  1    in other respects. 
     Referring to  FIGS.  16  and  17   , another embodiment of a hydraulic cartridge valve  1025  constructed in accordance with principles of the present disclosure is shown. The hydraulic cartridge valve  1025  is illustrated in a neutral position. Referring to  FIG.  16   , the illustrated valve  1025  comprises a pilot-operated proportional pressure control valve that includes a main stage  1027  and a pilot stage  1028 . The illustrated pressure control valve  1025  includes a body  1030 , a spool  1031 , a biasing element  1032  in the form of a spring, a pilot flow valve assembly  1034 , and means for restricting pilot flow through the pilot flow passage in the form of a pilot flow control assembly  1035 . 
     In the illustrated embodiment, the main stage  1027  of the pressure control valve  1025  comprises the cage  1040  of the body  1030 , the spool  1031 , and the biasing member  1032 . The cage  1040  is hollow and is configured to be inserted into a cavity formed in a suitable housing such that the valve  1025  is in fluid communication with a hydraulic circuit within which the valve is intended to be used. 
     The pilot stage  1028  of the pressure control valve  1025  comprises the pilot flow valve assembly  1034  and the pilot flow control assembly  1035 . The pilot flow valve assembly  1034  is configured to selectively occlude the pilot flow passage  1050  defined by the body  30 . In the illustrated embodiment, the pilot flow valve assembly  1034  is configured to selectively prevent pilot flow from the pilot flow passage  1050  out the tank port 4 . 
     Referring to  FIGS.  16  and  17   , the pilot flow control assembly  1035  includes the body  1030 , the spool  1031 , and a control element  1102  mounted to the body  1030 . The body  1030  includes an interior bore surface  1048 . The body  1030  defines the pilot flow passage  1050  and a pilot cross bore  1051  in fluid communication with the pilot flow passage  1050  (see  FIG.  17   ). The pilot cross bore  1051  is open to the interior bore surface  1048 . 
     Referring to  FIG.  17   , the spool  1031  includes a pilot land  1053 . The pilot land  1053  of the spool  1031  and the interior bore surface  1048  of the body  1030  define the restriction  1015  with the diameter of the pilot land being smaller than the diameter of the interior bore surface  1048  to define the restriction  1015  therebetween. The pilot land  1053  is disposed axially between the pilot cross bore  1051  and the control element  1102  when the spool  1031  is in the neutral position, as shown in  FIGS.  16  and  17   . The pilot cross bore  1051  is disposed axially between the pilot land  1053  and the first control element  1102  when the spool  1031  is in the shifted position, that is, the pilot land  1053  moves below the pilot cross bore  1051  to effectively remove the restriction  1015  from the pilot flow passage  1050  when the spool  1031  is in the shifted position such that the restriction  1015  is no longer part of the pilot flow passage  1050 . 
     The spool  1031  includes an exterior surface  1060 . The exterior surface  1060  of the spool  1031  defines an exterior groove  1061 . The exterior groove  1061  of the spool  1031  is in axial alignment with the pilot cross bore  1051  when the spool  1031  is in the neutral position. 
     The pressure control valve  1025  of  FIG.  16    is similar in construction and function to the pressure control valve  25  of  FIG.  1    in other respects, as will be appreciated by one skilled in the art. 
     Referring to  FIGS.  18  and  19   , another embodiment of a hydraulic cartridge valve  1225  constructed in accordance with principles of the present disclosure is shown. The hydraulic cartridge valve  1225  is illustrated in a neutral position. The illustrated valve  1225  comprises a pilot-operated proportional pressure control valve that includes a main stage  1227  and a pilot stage  1228 . The illustrated pressure control valve  1225  includes a body  1230 , a spool  1231 , a biasing element  1232  in the form of a spring, a pilot flow valve assembly  1234 , and means for restricting pilot flow through the pilot flow passage in the form of a pilot flow control assembly  1235 . 
     In the illustrated embodiment, the main stage  1227  of the pressure control valve  1225  comprises the cage  1240  of the body  1230 , the spool  1231 , and the biasing member  1232 . The cage  1240  is hollow and is configured to be inserted into a cavity formed in a suitable housing such that the valve  1225  is in fluid communication with a hydraulic circuit within which the valve  1225  is intended to be used. 
     The pilot stage  1228  of the pressure control valve  1225  comprises the pilot flow valve assembly  1234  and the pilot flow control assembly  1235 . The pilot flow valve assembly  1234  is configured to selectively occlude the pilot flow passage  1250  defined by the cage  1240  of the body  1230 . In the illustrated embodiment, the pilot flow valve assembly  1234  is configured to selectively prevent pilot flow from the pilot flow passage  1250  out the tank port 4 . 
     Referring to  FIGS.  18  and  19   , the pilot flow control assembly  1235  includes the body  1230 , the spool  1231 , and a control element  1302  mounted to the body  1230 . The body  1230  defines the pilot flow passage  1250 . The body  1230  includes an interior bore surface  1248  defining an interior groove  1249  (see  FIG.  19   ). The interior groove  1249  is in fluid communication with the pilot flow passage  1250  and is open to the interior bore surface  1248 . 
     The spool  1231  includes a pilot land  1253 . The pilot land  1253  of the spool  1231  and the interior bore surface  1248  of the body  1230  define the restriction  1215 . The pilot land  1253  is disposed axially between the interior groove  1249  of the body  1230  and the control element  1301  when the spool  1231  is in the neutral position. The interior groove  1249  of the body  1230  is disposed axially between the pilot land  1253  and the control element  1301  when the spool  1231  is in the shifted position such that the restriction  1215  is no longer part of the pilot flow passage  1250 . 
     The pressure control valve  1225  of  FIG.  18    is similar in construction and function to the pressure control valve  25  of  FIG.  1    in other respects, as will be appreciated by one skilled in the art. 
     Referring to  FIG.  20   , an embodiment of a hydraulic control system  1400  constructed according to principles of the present disclosure is shown. The illustrated hydraulic control system  1400  includes a pump  1401 , a tank  1402 , a pair of actuators  1403 ,  1404 , a pair of hydraulic cartridge valves  1425 ,  1426  constructed according to principles of the present disclosure, and a controller  1429  (also referred to as an electronic control unit (ECU). 
     In the illustrated embodiment, the pump  1401  is adapted to provide a source of pressurized fluid. The pump  1401  is adapted to receive a supply of fluid from the tank  1402  and to discharge a flow of fluid therefrom. The pump  1401  is in selective fluid communication with the pair of actuators  1403 ,  1404  via the pair of valves  1425 ,  1426 , respectively, to selectively deliver a flow of hydraulic fluid to the actuators  1403 ,  1404 . 
     The pump  1401  is in fluid communication with the tank  1402 , which is adapted to hold a reservoir of fluid. In embodiments, the tank  1402  can be in fluid communication with the pump  1401  via any suitable technique. For example, in embodiments, the pump  1401  is in fluid communication with the tank  1402  via a pump supply line  1410  to receive a return flow of hydraulic fluid from the tank  1402 , which in turn can be used by the pump  1401  to deliver the flow of hydraulic fluid to the actuators  1403 ,  1404 . 
     In embodiments, the pump  1401  can be any suitable pump that is acceptable for the intended application, as will be readily understood by one skilled in the art. In embodiments, the pump  1401  can be a fixed-displacement pump or a variable-displacement pump. 
     In embodiments, the tank  1402  is adapted to hold a reservoir of fluid. In embodiments, the tank  1402  can be any suitable tank known to those skilled in the art. In embodiments, the tank  1402  comprises a reservoir of hydraulic fluid which can be drawn into the pump  1401  in order to generate a flow of hydraulic fluid for the system. 
     In embodiments, each actuator  1403 ,  1404  is in selective fluid communication with the pump  1401 . In the illustrated embodiment, the actuators  1403 ,  1404  are in selective fluid communication with the pump  1401  and the tank  1402  via the pair of valves  1425 ,  1426 , respectively. In embodiments, the actuators  1403 ,  1404  are adapted to use hydraulic power to perform a mechanical work operation. In embodiments, each actuator  1403 ,  1404  can be any suitable actuator for use in a hydraulic control system compatible with a control valve constructed according to principles of the present disclosure. 
     In the illustrated embodiment, each of the pair of actuators  1403 ,  1404  comprises a transmission clutch control which have a similar construction and functionality. Each actuator  1403 ,  1404  defines a chamber  1410  therein adapted to receive pressurized fluid. A flow of hydraulic fluid into the chamber  1410  of the actuator  1403 ,  1404  can cause the actuator  1403 ,  1404  to operate once the pressure in the chamber  1410  overcomes a bias member  1411 . The bias member  1411  of the actuator  1403 ,  1404  is configured to urge the hydraulic fluid from the chamber  1410 . An actuator port  1412  of the actuators  1403 ,  1404  leading to the chamber  1410  is in fluid communication with a respective one of the pair of valves  1425 ,  1426  to selectively receive a supply flow of pressurized hydraulic fluid from the pump  1401  or to selectively discharge a discharge flow of hydraulic fluid from the chamber  1410  of the actuators  1403 ,  1404  to the tank  1402 . 
     In embodiments, each pressure control valve  1425 ,  1426  is in fluid communication with the pump  1401 , the tank  1402 , and the actuator  1403 ,  1404  with which the respective pressure control valve  1425 ,  1426  is associated. In embodiments, each pressure control valve  1425 ,  1426  is interposed between the pump  1401  and the respective actuator  1403 ,  1404  and between the respective actuator  1403 ,  1404  and the tank  1402 . 
     In the illustrated embodiment, the valves  1425 ,  1426  are each in electrical communication with the controller  1429  and in fluid communication with the pump  1401  and the tank  1402 . The pair of valves  1425 ,  1426  are respectively interposed between the pump  1401  and one of the pair of actuators  1403 ,  1404 . The valves  1425 ,  1426  are adapted to selectively direct the flow of fluid from the pump  1401  to the chamber  1410  of the respective actuator  1403 ,  1404  with which the valve  1425 ,  1426  is associated. The pair of valves  1425 ,  1426  are respectively interposed between one of the pair of actuators  1403 ,  1404  and the tank  1402 . The pair of valves  1425 ,  1426  are adapted to selectively direct a return flow of fluid from the chamber  1410  of the respective actuator  1403 ,  1404  with which the valve  1425 ,  1426  is associated to the tank  1402 . 
     In the illustrated embodiment, each of the valves  1425 ,  1426  comprises a valve substantially shown in  FIG.  1    and as described above. Each valve  1425 ,  1426  includes a body, a spool, a biasing element in the form of a spring, a pilot flow valve assembly, and a pilot flow control assembly as described above in connection with the embodiment of a valve shown in  FIG.  1   . The body defines an axial bore, a supply port 2 , a work port 3 , and a tank port 4 . Each of the supply port 2 , the work port 3 , and the tank port 4  are in fluid communication with the axial bore. The supply port 2  is in fluid communication with the pump  1401 . The work port 3  is in fluid communication with the chamber  1410  of the respective actuator  1403 ,  1404  with which the valve  1425 ,  1426  is associated. The tank port 4  is in fluid communication with the tank  1402 . 
     The spool is disposed within the axial bore of the body and axially movable over a range of travel between a neutral position and a shifted position. In the neutral position, the supply port 2  and the work port 3  are in fluid isolation from each other and the work port 3  and the tank port 4  are in fluid communication with each other to thereby fluidly connect the chamber  1410  of the respective actuator  1403 ,  1404  with which the valve  1425 ,  1426  is associated to the tank  1402 . In the shifted position, the supply port 2  and the work port 3  are in fluid communication with each other to thereby fluidly connect the pump  1401  to the chamber  1410  of the respective actuator  1403 ,  1404  with which the valve  1425 ,  1426  is associated and the work port 3  and the tank port 4  are in fluid isolation from each other. 
     The spool and the body cooperate together to define a pilot flow passage in fluid communication with the supply port 2  and the tank port 4  when the spool is in the neutral position. The spring is operatively arranged with the spool to bias the spool to the neutral position. 
     The valves  1425 ,  1426  can be similar in other respects to the valve of  FIG.  1   . For example, the pilot flow valve assembly and the pilot flow control assembly of the valves are substantially the same as the valve of  FIG.  1   . 
     The controller  1429  is in electrical communication with the pump  1401  and the valves  1425 ,  1426 . The controller  1429  is configured to selectively operate the actuators  1403 ,  1404  by controlling the hydraulic cartridge valves  1425 ,  1426  in response to a suitable input received by the controller  1429 , and as will be readily appreciated by one skilled in the art. 
     In embodiments, the controller  1429  is configured to selectively send a drive signal to the coil of one or both of the actuators  1425 ,  1426  in response to a predetermined input. The drive signal can comprise a variable electrical current. The controller  1429  can be configured to vary the electrical current passed through the coil of each of the valves  1425 ,  1426  based upon the input received by the controller  1429 . 
     In embodiments, the controller  1429  can be any suitable electronic control unit or units as will be readily familiar to one skilled in the art. For example, in embodiments, the controller  1429  can comprise a suitable, commercially available plug-in style, microprocessor based valve driver. In embodiments, the controller  1429  can includes a valve driver operably arranged with each valve coil to selectively operate the cartridge valves. 
     In embodiments, the controller  1429  is configured to receive an input indicating a desired operational characteristic. For example, in embodiments, the controller  1429  includes a suitable graphical user interface configured to allow an operator to enter a desired set point for the cartridge valve  12 . In embodiments, the controller  1429  can be in electrical communication with other components, such as, when the hydraulic control system  1400  is used as an on-board control mechanism for a mobile machine, for example. 
     It will be understood that, in other embodiments, the hydraulic control system  1400  can be configured to selectively and independently operate a plurality of hydraulic cartridge valves constructed according to principles of the present disclosure. It will be understood that, in embodiments, the hydraulic control system  1400  can include other and different components. 
     Embodiments of a hydraulic control system constructed according to principles of the present disclosure can be used to carry out a method of controlling a hydraulic actuator using an embodiment of a cartridge valve as described above. In embodiments, a method of controlling a hydraulic actuator following principles of the present disclosure can use any embodiment of a hydraulic cartridge valve and/or any embodiment of a hydraulic control system constructed according to principles discussed herein. 
     In one embodiment, when the valve is de-energized (neutral position) with pressure applied at the supply port 2 , hydraulic fluid flows from the supply port 2  to the tank port 4 . The hydraulic fluid can pass through a filter screen outside the cage of the valve, then through a second filter attached to the spool. After the hydraulic fluid is filtered, it flows through the drilled first orifice defined in the spool and passes through the pilot flow passage defined axially in the spool. The hydraulic fluid then passes through the leakage control elements and then passes by the ball and out the tank port 4  to the tank. The leakage control elements form a second orifice in the pilot flow passage in serial relationship with the first orifice to restrict the amount of pilot flow when the valve is in the de-energized state. 
     In embodiments, a controller can selectively operate the valve by directing a drive current through the coil of the valve. Once the valve is energized, the leakage control orifice is “de-activated” as the leakage control element mounted to the spool (e.g., a restriction member) moves axially away from the leakage control element mounted to the body (e.g., a pilot pin). The amount of additional restriction results in a trade-off between leakage reduction and response time of the valve. The pilot flow from the supply port 2  to the tank port 4  is reduced via the leakage control elements as compared to a valve not containing these leakage control elements. The work port 3  is connected to the tank port 4 . 
     As current is applied to the coil, a magnetic force is established between the armature and the frame pulling the armature toward the frame pole piece face. As the armature moves, so does the pilot pin which seats the ball, blocking the flow of hydraulic fluid past the ball. The pilot chamber (volume between the seat and the spool) begins to fill and pressurize, thereby causing the spool to move down and compress the spring. If sufficient current is applied to the coil, the spool will compress the spring to the point where the spool moves to the shifted position to fluidly connect the supply port 2  to the work port 3 . The amount of current required to build this pressure under the ball is called threshold current. As the ball begins to lift off the seat, the magnetic force of the proportional actuator regulates the pressure in the pilot chamber which regulates the position of the spool. Once the pressure in the work port 3  reaches the desired level, the spool will move between a reducing position and a relieving position. 
     In embodiments, the valve is used with a supply pressure that is at least one bar higher than the maximum reduced pressure at the work port 3 . Under such conditions, the bias element keeps the valve closed (blocking the supply and work port 3 ) when no current is applied to the coil. 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any nonclaimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.