Abstract:
A valve for a fluid circuit is disclosed. The valve has a main valve element with a first end and a second end. The main valve element is movable between a flow-passing and a flow-blocking position in response to fluid pressure exerted on the first and second ends. The valve also has a solenoid mechanism operatively associated with the main valve element to move the main valve element toward one of the flow-passing and the flow-blocking positions. The valve further has a main valve spring configured to bias the main valve element in opposition to movement caused by the solenoid mechanism. The valve additionally has a relief valve element configured to communicate a fluid with the first end of the main valve element in response to a fluid pressure to initiate movement of the main valve element.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/614,434, entitled “Hybrid Electronic/Pilot-Operated Line Relief,” which was filed on Sep. 29, 2004, the disclosure of which is hereby expressly incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present disclosure relates generally to a drain valve, and more particularly, to a drain valve that is both electronically and hydraulically actuated.  
       BACKGROUND  
       [0003]     Work machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy machinery use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are selectively fluidly connected to a pump on the work machine that provides pressurized fluid to chambers within the actuators, and to a tank to allow the pressurized fluid to drain from the actuators. A valve arrangement is typically fluidly connected between the actuators and the pump and tank to control a flow rate and direction of pressurized fluid to and from the chambers of the actuators.  
         [0004]     The portion of the valve arrangement connecting the actuator to the tank is called a drain valve. The drain valve typically includes a solenoid operated electronic flow controlling valve or a hydraulic pressure limiting valve. The electronic flow controlling valve has a valve element that is movable against a spring bias between a flow-passing and a flow-blocking position in response to an electronic signal to control a flow of pressurized fluid to an actuator. The hydraulic pressure limiting valve generally includes a valve element that is spring biased toward a flow-blocking position and movable toward a flow-passing position in response to a fluid pressure exerted against the valve element to limit a maximum pressure within the actuator.  
         [0005]     A system having one of the electronic flow controlling and hydraulic pressure limiting valves can be problematic, while a valve arrangement having both the electronic flow controlling and hydraulic pressure limiting valves can be large and expensive. For example, the hydraulic pressure limiting valve does not afford the controllability of the electronic flow controlling valve, while the electronic flow controlling valve can not afford pressure limiting functions during electrical failure or system shut down and is not as responsive as the hydraulic pressure limiting valve. One method of providing the benefits of both the electronic flow controlling and hydraulic pressure limiting valves is described in U.S. Pat. No. 5,878,647 (the &#39;647 patent) issued to Wilke et al. on Mar. 9, 1999. The &#39;647 patent describes a hydraulic circuit having two pairs of valves, a variable displacement pump, a reservoir tank, and a hydraulic actuator. One pair of the valves includes a head-end supply valve and a head-end return valve that connects a head end of the hydraulic actuator to either the variable displacement pump or the reservoir tank. The other pair of solenoid valves includes a rod-end supply valve and a rod-end return valve that connects a rod end of the hydraulic actuator to either the variable displacement pump or the reservoir tank. Each of the head and rod-end return valves includes a solenoid operated pilot valve element that selectively communicates fluid from the hydraulic actuator to a hydraulically operated valve element. When both the solenoid operated pilot valve element and the hydraulically operated valve element are in a flow-passing position, fluid from the hydraulic actuator is allowed to drain from the hydraulic actuator to the reservoir tank.  
         [0006]     Although the return valves of the hydraulic circuit described in the &#39;647 patent may provide some of the benefits associated with both electronic flow controlling and hydraulic pressure limiting valves, the return valves of the &#39;647 patent may still be problematic. For example, in the situation of electrical failure or system shut down, the return valves of the &#39;647 patent do not perform any pressure limiting functions. Further, because flow through the return valves can be completely blocked by high fluid pressures acting on the hydraulically operated valve element, the hydraulic circuit of the &#39;647 patent lacks control. In addition, excessive pressures within the hydraulic circuit of the &#39;647 patent tend to move the hydraulically operated valve element toward a flow-blocking position rather than a flow-passing position, thereby allowing the excessive pressures to increase even further.  
         [0007]     The disclosed valve is directed to overcoming one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0008]     In one aspect, the present disclosure is directed to a valve. The valve includes a main valve element with a first end and a second end. The main valve element is movable between a flow-passing and a flow-blocking position in response to fluid pressure exerted on the first and second ends. The valve also includes a solenoid mechanism operatively associated with the main valve element to move the main valve element toward one of the flow-passing and the flow-blocking positions. The valve further includes a main valve spring configured to bias the main valve element in opposition to movement caused by the solenoid mechanism. The valve additionally includes a relief valve element configured to communicate a fluid with the first end of the main valve element in response to a fluid pressure to initiate movement of the main valve element.  
         [0009]     In another aspect, the present disclosure is directed to a method of operating a valve. The method includes operating a relief valve element to selectively allow pressurized fluid to flow to an end of a main valve element, thereby moving the main valve element between a flow-passing and a flow-blocking position. The method also includes operating a solenoid to move the main valve element toward one of the flow-blocking and flow-passing positions in opposition to a spring bias. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a side-view diagrammatic illustration of a work machine according to an exemplary disclosed embodiment;  
         [0011]      FIG. 2  is a schematic illustration of hydraulic circuit for the work machine of  FIG. 1 ;  
         [0012]      FIG. 3  is a cross-sectional illustration of an exemplary disclosed drain valve for the hydraulic circuit of  FIG. 2 ; and  
         [0013]      FIG. 4  is a cross-sectional illustration of another exemplary disclosed drain valve for the hydraulic circuit of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 1  illustrates an exemplary work machine  10 . Work machine  10  may be a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, or any other industry known in the art. For example, work machine  10  may be an earth moving machine such as a dozer, a loader, a backhoe, an excavator, a motor grader, a dump truck, or any other earth moving machine. Work machine  10  may also include a generator set, a pump, a marine vessel, or any other suitable operation-performing work machine. Work machine  10  may include a frame  12 , at least one work implement  14 , and a hydraulic cylinder  16  connecting work implement  14  to frame  12 .  
         [0015]     Frame  12  may include any structural unit that supports movement of work machine  10 . Frame  12  may be, for example, a stationary base frame connecting a power source (not shown) to a traction device  18 , a movable frame member of a linkage system, or any other frame known in the art.  
         [0016]     Work implement  14  may include any device used in the performance of a task. For example, work implement  14  may include a blade, a bucket, a shovel, a ripper, a dump bed, a propelling device, or any other task-performing device known in the art. Work implement  14  may be connected to frame  12  via a direct pivot, via a linkage system with hydraulic cylinder  16  forming one member in the linkage system, or in any other appropriate manner. Work implement  14  may be configured to pivot, rotate, slide, swing, or move relative to frame  12  in any other manner known in the art.  
         [0017]     As illustrated in  FIG. 2 , hydraulic cylinder  16  may be one of various components within a hydraulic system  20  that cooperate to move work implement  14 . Hydraulic system  20  may include a primary source  22  of pressurized fluid, a head-end supply valve  24 , a head-end drain valve  26 , a rod-end supply valve  28 , a rod-end drain valve  30 , a tank  32 , and a pilot source  34  of pressurized fluid. It is contemplated that hydraulic system  20  may include additional and/or different components such as, for example, make up valves, pressure relief valves, pressure sensors, temperature sensors, position sensors, controllers, accumulators, and other components known in the art.  
         [0018]     Hydraulic cylinder  16  may include a tube  36  and a piston assembly  38  disposed within tube  36 . One of tube  36  and piston assembly  38  may be pivotally connected to frame  12 , while the other of tube  36  and piston assembly  38  may be pivotally connected to work implement  14 . It is contemplated that tube  36  and/or piston assembly  38  may alternately be fixedly connected to either frame  12  or work implement  14 . Hydraulic cylinder  16  may include a first chamber  40  and a second chamber  42  separated by piston assembly  38 . First and second chambers  40 ,  42  may be selectively supplied with a fluid pressurized by primary source  22  and fluidly connected with tank  32  to cause piston assembly  38  to displace within tube  36 , thereby changing the effective length of hydraulic cylinder  16 . The expansion and retraction of hydraulic cylinder  16  may function to assist in moving work implement  14 .  
         [0019]     Piston assembly  38  may include a piston  44  axially aligned with and disposed within tube  36 , and a piston rod  46  connectable to one of frame  12  and work implement  14  (referring to  FIG. 1 ). Piston  44  may include a first hydraulic surface  48  and a second hydraulic surface  50  opposite first hydraulic surface  48 . An imbalance of force caused by fluid pressure on first and second hydraulic surfaces  48 ,  50  may cause piston assembly  38  to axially move within tube  36 . For example, a force on first hydraulic surface  48  being greater than a force on second hydraulic surface  50  may cause piston assembly  38  to displace to increase the effective length of hydraulic cylinder  16 . Similarly, when a force on second hydraulic surface  50  is greater than a force on first hydraulic surface  48 , piston assembly  38  will retract within tube  36  to decrease the effective length of hydraulic cylinder  16 . A sealing member (not shown), such as an o-ring, may be connected to piston  44  to restrict a flow of fluid between an internal wall of tube  36  and an outer cylindrical surface of piston  44 .  
         [0020]     Primary source  22  may be configured to produce a flow of pressurized fluid and may include a pump such as, for example, a variable displacement pump, a fixed displacement pump, a variable flow pump, or any other source of pressurized fluid known in the art. Primary source  22  may be drivably connected to a power source (not shown) of work machine  10  by, for example, a countershaft (not shown), a belt (not shown), an electrical circuit (not shown), or in any other suitable manner. Primary source  22  may be dedicated to supplying pressurized fluid only to hydraulic system  20 , or alternately may supply pressurized fluid to multiple hydraulic systems within work machine  10 .  
         [0021]     Head-end supply valve  24  may be disposed between primary source  22  and first chamber  40  and configured to regulate a flow of pressurized fluid to first chamber  40 . Specifically, head-end supply valve  24  may include a two-position spring-biased valve element that is solenoid-actuated and configured to move between a first position at which fluid is allowed to flow into first chamber  40  and a second position at which fluid flow from first chamber  40  is blocked. It is contemplated that head-end supply valve  24  may include additional or different mechanisms such as, for example, a proportional valve element, one or more restricted orifices, a pilot valve element, a pressure relief valve element, or any other valve mechanisms known in the art. It is also contemplated that head-end supply valve  24  may alternately be hydraulically-actuated, mechanically-actuated, pneumatically-actuated, or actuated in any other suitable manner. It is further contemplated that head-end supply valve  24  may be configured to allow fluid from first chamber  40  to flow through head-end supply valve  24  during a regeneration event when a pressure within first chamber  40  exceeds a pressure of the fluid supplied by primary source  22 .  
         [0022]     Head-end drain valve  26  may be disposed between first chamber  40  and tank  32  and configured to regulate a flow of pressurized fluid from first chamber  40  to tank  32 . Specifically, head-end drain valve  26  may include a three-position spring-biased pilot valve element  52 , a two-position hydraulically-actuated spring-biased main valve element  54  that is mechanically connected to pilot valve element  52  by way of a spring  56  and fluidly connected to pilot valve element  52  by a fluid passageway  58 , and a hydraulically-actuated spring-biased pilot relief valve element  60  that is fluidly connected to main valve element  54  by way of a fluid passageway  62 . Pilot valve element  52  may be solenoid-actuated and configured to move between a first position at which fluid from pilot source  34  is allowed to act on pilot valve element  52  and main valve element  54  via fluid passageways  64 ,  66 , and  68 , a second position at which the fluid acting on pilot valve element  52  and main valve element  54  is allowed to drain to tank  32  via a drain passageway  70 , and a third position at which all fluid through pilot valve element  52  is blocked. Restricted orifices  72  and  74  may be disposed within fluid passageways  66  and  68 , respectively, to reduce pressure and/or flow oscillations. It is contemplated that restricted orifices  72  and  74  may be omitted, if desired. Main valve element  54  may be hydraulically-actuated and configured to move between a first position at which fluid from first chamber  40  is allowed to drain to tank  32  via fluid passageways  76  and  78  and a second position where fluid from first chamber  40  is blocked. Main valve element  54  may be biased via fluid within a passageway  80  in a direction opposite the direction caused by fluid within passageway  58 . A restricted orifice  82  may be disposed within a fluid passageway  84  that connects pilot source  34  to one end of main valve element  54 . Pilot relief valve element  60  may be biased via fluid from first chamber  40  toward a flow-passing position to thereby communicate pressurized fluid from first chamber  40  with fluid passageways  80  and  84 . A one-way pressure bypass valve  85  may also be included within head-end drain valve  26  to relieve pressures from between pilot valve element  52  and main valve element  54  during situations where pilot relief valve element  60  has initiated motion of main valve element  54 , but pilot valve element  52  is blocking fluid passageway  64  and drain passageway  70 .  
         [0023]     Rod-end supply valve  28  may be disposed between primary source  22  and second chamber  42  and configured to regulate a flow of pressurized fluid to second chamber  42 . Specifically, rod-end supply valve  28  may include a two-position spring-biased valve element that is solenoid-actuated and configured to move between a first position at which fluid is allowed to flow into second chamber  42  and a second position at which fluid is blocked from second chamber  42 . It is contemplated that rod-end supply valve  28  may include additional or different valve mechanisms such as, for example, a proportional valve element, one or more restricted orifices, a pilot valve element, a pressure relief valve element, or any other valve mechanism known in the art. It is also contemplated that rod-end supply valve  28  may alternately be hydraulically-actuated, mechanically-actuated, pneumatically-actuated, or actuated in any other suitable manner. It is further contemplated that rod-end supply valve  28  may be configured to allow fluid from second chamber  42  to flow through rod-end supply valve  28  during a regeneration event when a pressure within second chamber  42  exceeds a pressure of the fluid supplied by primary source  22 .  
         [0024]     Rod-end drain valve  30  may be disposed between second chamber  42  and tank  32  and configured to regulate a flow of pressurized fluid from second chamber  42  to tank  32 . Specifically, rod-end drain valve  30  may include a three-position spring-biased pilot valve element  86 , a two-position hydraulically-actuated spring-biased main valve element  88  that is mechanically connected to pilot valve element  86  by way of a spring  90  and fluidly connected to pilot valve element  86  via a fluid passageway  92 , and a hydraulically-actuated spring-biased pilot relief valve element  94  that is fluidly connected to main valve element  88  by way of fluid passageway  96 . Pilot valve element  86  may be solenoid-actuated and configured to move between a first position at which fluid from pilot source  34  is allowed to act on pilot valve element  86  and main valve element  88  via fluid passageways  98 ,  100 , and  102 , a second position at which the fluid acting on pilot valve element  86  and main valve element  88  is allowed to drain to tank  32  via a drain passageway  104 , and a third position at which all fluid through pilot valve element  86  is blocked. Restricted orifices  106  and  108  may be disposed within fluid passageways  100  and  102 , respectively, to reduce pressure and/or flow oscillations. It is contemplated that restricted orifices  106  and  108  may be omitted, if desired. Main valve element  88  may be hydraulically-actuated and configured to move between a first position at which fluid from second chamber  42  is allowed to drain to tank  32  via fluid passageways  110  and  112 , and a second position where fluid from second chamber  42  is blocked. Main valve element  88  may be biased via fluid within a passageway  114  in a direction opposite the direction caused by fluid within passageway  92 . A restricted orifice  116  may be disposed within a fluid passageway  118  that connects pilot source  34  to one end of main valve element  88 . Pilot relief valve element  94  may be biased via fluid from second chamber  42  toward a flow-passing position to thereby communicate pressurized fluid from second chamber  42  with fluid passageway  96 . A one-way pressure bypass valve  119  may also be included within rod-end drain valve  30  to relieve pressures from between pilot valve element  86  and main valve element  88  during situations where pilot relief valve element  94  has initiated motion of main valve element  88 , but pilot valve element  86  is blocking fluid passageway  98  and drain passageway  104 .  
         [0025]     Head-end and rod-end supply and drain valves  24 - 30  may be fluidly interconnected. In particular, head-end and rod-end supply valves  24 ,  28  may be connected in parallel to a common upstream fluid passageway  120 . Head-end supply and return valves  24 ,  26  may be connected in parallel to a common first chamber fluid passageway  122 . Rod-end supply and drain valves  28 ,  30  may be connected in parallel to a common second chamber fluid passageway  124 .  
         [0026]     Tank  32  may constitute a reservoir configured to hold a supply of fluid. The fluid may include, for example, a dedicated hydraulic oil, an engine lubrication oil, a transmission lubrication oil, or any other fluid known in the art. One or more hydraulic systems within work machine  10  may draw fluid from and return fluid to tank  32 . It is also contemplated that hydraulic system  20  may be connected to multiple separate fluid tanks.  
         [0027]     Pilot source  34  may be configured to produce a flow of pressurized fluid and may include a pump such as, for example, a variable displacement pump, a fixed displacement pump, a variable flow pump, or any other source of pressurized fluid known in the art. Pilot source  34  may be drivably connected to a power source (not shown) of work machine  10  by, for example, a countershaft (not shown), a belt (not shown), an electrical circuit (not shown), or in any other suitable manner. Pilot source  34  may be dedicated to supplying pressurized pilot fluid only to hydraulic system  20 , or alternatively may supply pressurized fluid to multiple hydraulic systems within work machine  10 . A pressure relief valve  125  may be associated with pilot source  34  to facilitate a substantially constant pressure within the fluid supplied by pilot source  34 .  
         [0028]      FIG. 2  also illustrates a control system  140  in communication with hydraulic system  20 . Control system  140  may include a controller  142 , a first pressure sensor  144 , and a second pressure sensor  146 . Controller  142  may be in communication with first pressure sensor  144 , second pressure sensor  146 , pilot valve element  52 , pilot valve element  86 , head-end supply valve  24 , and rod-end supply valve  28  via communication lines  148 ,  150 ,  152 ,  154 ,  156 , and  158 , respectively. Controller  144  may be configured to receive input from an operator indicative of a desired movement of hydraulic cylinder  16  and to selectively actuate pilot valve elements  52  and  86  and head and rod-end supply valves  24  and  26  in response to the input to achieve the desired movement. Controller  144  may further be configured to sense the pressure of the fluid within first and second chambers  40  and  42  and to actuate pilot valve elements  52  and  86  in response the pressure exceeding a predetermined pressure.  
         [0029]      FIGS. 3 and 4  illustrate alternate locations for pilot relief valve elements  60  and  94  within head and rod-end drain valves  26  and  30 . Because both head and rod-end drain valves  26  and  30  are substantially identical and for purposes of simplicity, the reference numbers for only head-end drain valve  26  will be used in the description of  FIGS. 3 and 4 .  
         [0030]     As illustrated in  FIG. 3 , head-end drain valve  26  may include a valve body  126  having a central bore  128 . Pilot valve element  52  may be disposed within central bore  128  and slidably movable between the flow-blocking position and the flow-passing position where fluid passageway  64  and drain passageway  70  are fluidly communicated. Main valve element  54  may also be disposed within central bore  128  and slidably movable between the flow-blocking position and the flow-passing position to fluidly communicate passageways  76  and  78 . Pilot relief valve element  60  may be disposed within and axially aligned with a bore  132  of main valve element  54  and slidably movable between the flow-blocking position and the flow-passing position to fluidly communicate fluid passageway  76  with fluid passageway  84  and one end of main valve element  54 .  
         [0031]     Similar to  FIG. 3 , head-end drain valve  26  of  FIG. 4  may include pilot valve element  52  and main valve element  54  disposed within central bore  128  of valve body  126  to selectively connect fluid passageway  64  to drain passageway  70  and fluid passageway  76  to fluid passageway  78 . However, in contrast to  FIG. 3 , pilot relief valve element  60  of  FIG. 4  is not located within a bore of main valve element  54 . Instead, pilot relief valve element  60  of  FIG. 4  may be disposed within a bore  134  that is radially removed from main valve element  54  and located within valve body  126 .  
       INDUSTRIAL APPLICABILITY  
       [0032]     The disclosed hydraulic system may be applicable to any work machine that includes a fluid actuator where the benefits of hydraulically actuated and electrically actuated drain valves are desired. The disclosed hydraulic system may provide precise control over fluid flow to the fluid actuator, high response pressure limiting, and fail safe pressure limiting for the components of the hydraulic system in a low-cost space-saving configuration. The operation of hydraulic system  20  will now be explained.  
         [0033]     As illustrated in  FIG. 2 , hydraulic cylinder  16  may be movable by fluid pressure in response to an operator input. Fluid may be pressurized by primary source  22  and selectively directed to head-end and rod-end supply valves  24  and  28 . In response to an operator input to either extend or retract piston assembly  38  relative to tube  36 , controller  142  may direct the pressurized fluid to the appropriate one of first and second chambers  40 ,  42  by causing one of head-end and rod-end supply valves  24  and  28  to move to the flow-passing position. Substantially simultaneously, controller  142  may actuate the appropriate one of main valve element  54  or  88  of head-end and rod-end drain valves  26 ,  30  to direct fluid from the appropriate one of the first and second chambers  40 ,  42  to tank  32 , thereby creating a force imbalance on piston  44  that causes piston assembly  38  to move. For example, if an extension of hydraulic cylinder  16  is requested, head-end supply valve  24  may be moved to the open position to direct pressurized fluid from primary source  22  to first chamber  40 . Substantially simultaneous to the directing of pressurized fluid to first chamber  40 , main valve element  88  of rod-end drain valve  30  may be moved to the open position to allow fluid from second chamber  42  to drain to tank  32 . If a retraction of hydraulic cylinder  16  is requested, rod-end supply valve  28  may be moved to the open position to direct pressurized fluid from primary source  22  to second chamber  42 . Substantially simultaneous to the directing of pressurized fluid to second chamber  42 , main valve element  54  of head-end drain valve  26  may be moved to the open position to allow fluid from first chamber  40  to drain to tank  32 .  
         [0034]     Movement of main valve elements  54  and  88  may be affected in at least two ways (because main valve element  88  functions substantially identical to main valve element  54  and for purposes of simplicity, only the movement with respect to main valve element  54  will be described). An electronic signal from controller  142  may be received via communication line  152  by the solenoid associated with head-end drain valve  26  that causes the solenoid to energize. Upon actuation of the solenoid, pilot valve mechanism  52  may be magnetically repelled away from the solenoid, thereby communicating cylinder bore  128  with drain passageway  70  via fluid passageway  66 , allowing the fluid within cylinder bore  128  to drain to tank  32 . Because the opposite end of main valve element  54  is simultaneously exposed to pressurized fluid from pilot source  34  via fluid passageway  84 , main valve element  54  may be urged toward pilot valve element  52  by an imbalance of force, thereby communicating fluid passageways  76  and  78  allowing fluid from first chamber  40  to drain to tank  32 . The signal from controller  142  causing the solenoid of head-end drain valve  26  may be generated in response to operator input or in response to a pressure within hydraulic cylinder  16  being above a predetermined pressure, as measured by pressure sensor  144 . Movement of main valve elements  54  and  88  may also be affected when excessive pressures within first chamber  40  cause pilot relief valve element  60  to move to the flow-passing position, allowing the excessive pressures of first chamber  40  to exert force on one end of main valve element  54 . Because the opposite end of main valve element  54  is simultaneously exposed to a lower fluid pressure from pilot source  34 , an imbalance of force on main valve element  54  is created that urges main valve element  54  towards pilot valve element  52 , again communicating fluid passageways  76  and  78  and allowing the fluid from first chamber  40  to drain to tank  32 . During movement of main valve element  54  initiated by movement of pilot relief valve element  60  toward the flow passing position, fluid may be allowed to exit central bore  128  past pressure bypass valve  85  to prevent hydraulic lock.  
         [0035]     Because the movement of main valve elements  54  and  88  may be affected electronically, hydraulic system  20  may be precisely controllable. Specifically, opening and closing pressures and flow rates of fluid in communication with main valve elements  54  and  88  may be closely tailored to accommodate a variety of different operating conditions. This tailoring may be software facilitated and implemented with an electronic controller (not shown) to provide system-wide optimization and improved efficiency.  
         [0036]     Because the movement of main valve elements  54  and  88  may also be affected hydraulically, hydraulic system  20  may be able to respond to rising fluid pressures and fluid pressure spikes quickly and may provide fail safe pressure relief for hydraulic system  20 . In particular, a hydraulically actuated valve mechanism may respond on the order of 5-15 μs, while an electronically actuated valve mechanism may respond much slower, typically on the order of about 100 μs. The increased responsiveness of the hydraulically actuated main valve elements  54  and  88  may help to prevent potentially damaging pressure fluctuations that an electronic-only system might not be able to avoid. Further, even in situations of electronic failure or during power system shutdown, the movement of pilot relief valve element  60  may still cause movement of main valve element  54  from the flow-blocking position to the flow-passing position, thereby providing fail safe protection for hydraulic system  20  that electronic-only valve configurations can not provide.  
         [0037]     In addition, because the electronic relief function and the hydraulic relief functions can be embodied into a single valve configuration rather than completely separate stand-alone valve mechanisms, both cost and space savings may be realized. Further space savings may be realized when pilot relief valve elements  60  and  94  are disposed within main valve elements  54  and  88 , rather than in separate bores within valve body  126 .  
         [0038]     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed electro-hydraulic valve. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed electro-hydraulic valve. For example, it is contemplated that the solenoid actuation of pilot valve elements  52  and  86  may alternatively include a pull-type actuation where energizing the solenoid attracts pilot valve elements  52  and  86  toward the solenoid rather than repelling. It is further contemplated that pilot valve elements  52  and  86  may be omitted, if desired, and main valve elements  54  and  88  directly acted upon by the solenoids. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.