Abstract:
A first control valve selectively connects a first port of a double acting actuator to either a pump supply line or a tank return line, and a second control valve selectively connects a second port of the actuator to either a pump supply line or a tank return line. A first pilot operated check valve restricts fluid flow to a direction only from the first control valve to the first workport unless a pilot pressure from the second port is sufficient to open the valve for the opposite flow direction. A second pilot operated check valve restricts fluid flow to a direction only from the first control valve to the second workport unless a pilot pressure from the first port is sufficient to open the valve for the opposite flow direction. A load sense circuit is incorporated to produce signal indicating the greater pressure at the actuator ports.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulic systems, and more particularly to valve assemblies for controlling the flow of hydraulic fluid to and from an actuator to produce bidirectional motion. 
     2. Description of the Related Art 
     Various types of mobile equipment are operated by a hydraulic system that drives an actuator, such as a hydraulic cylinder and piston arrangement, which receives pressurized fluid controlled by a hydraulic valve. A typical four-position control valve selectively applies the pressurized fluid to one of two cylinder chambers and drains the hydraulic fluid from the other chamber, thereby driving the actuator in one of two directions depending upon which chamber receives the pressurized fluid. Usually a proportional control valve is employed, which can be opened to varying degrees to control the rate of fluid flow to and from the associated actuator, thereby moving the element of the machine that is connected to the actuator at different speeds. 
     Mobile equipment often incorporate auxiliary hydraulic valves for optional or lower usage type functions. A relatively low flow control valve usually acceptable for these auxiliary functions. If electrohydraulic operation is required, simple on/off valve can be used. For example, direct acting solenoids often are utilized to shift conventional spools in a manner similar to that employed in manual valves. On/off cartridge valves also may be utilized for this purpose, but in applications that require a three-position, four-way valve arrangement, cartridge valves become relatively large and complex, so as to not be cost effective. 
     Conventional three-position spool valves, that are commonly used to control auxiliary functions, have a center or neutral position which blocks the flow of fluid from the pump, as well as blocking the connection of the workports to tank. In hydraulic circuits that provide load sensing to control the supply pressure from the pump, these spool valves also require a bleed connection in the neutral position to relieve the load sense pressure signal. For bidirectional operation, a load sense signal must be provided, regardless of the direction of the valve motion. This is often accomplished with a bridge type connection through which the workport pressure flows in both directions of valve operation. 
     Thus, it is desirable to duplicate the function of a three-position, four-way control valve with solenoid operated valves in an assembly which is as cost effective as possible. 
     SUMMARY OF THE INVENTION 
     A control valve assembly is provided for a hydraulic system having a pump supply line, a tank return line, and a double acting actuator. The control valve assembly has a first workport and a second workport for connection to the double acting actuator. A first control valve is connected to the pump supply line and the tank return line, one of which at a time is connected by different operating positions of the first control valve to a first common port. A second control valve also is connected to the pump supply line and the tank return line, one of which at a time is connected to a second common port in different operating positions of the second control valve. 
     A first pilot operated check valve is connected between the first common port and the first workport and has a free flow direction from the first common port to the first workport. The first pilot operated check valve has a pilot inlet connected to the second common port, wherein sufficient pressure at the pilot inlet opens the first pilot operated check valve to fluid flow from the first workport to the first common port. A second pilot operated check valve has another pilot inlet connected to the first common port, wherein sufficient pressure at the pilot inlet opens the second pilot operated check valve to fluid flow from the second workport to the second common port. 
     To drive the actuator in one direction, the first control valve is placed in the position in which the pump supply line is connected to the first common port and the second control valve is placed in the position in which the tank return line is connected to the second common port. The pressure at the first common port opens the first pilot operated check in the free flow direction so that fluid is supplied to the actuator via the first workport. The pressure at the first common port also is applied to the pilot inlet of the second pilot operated check valve and causes that check valve to open allowing fluid to drain to tank from the actuator via the second workport. 
     To drive the actuator in the opposite direction, the positions of the first and second control valves are reversed to apply fluid from the supply line to the second workport and drain fluid from the second workport to tank. Pressure at the second common port of the second control valve, when applied to the first pilot operated check valve opens that valve. 
     A load sense circuit preferably is provided to receive the pressures at the first and second common ports and produce a load sense signal corresponding to the greater of those pressures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a hydraulic system utilizing the present invention; and 
         FIG. 2  is a cross-sectional view through a valve assembly that implements the hydraulic system in FIG.  1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to  FIG. 1 , a hydraulic system  10  comprises a pump  12  which draws hydraulic fluid from a tank  14  and furnishes the fluid under pressure into a supply line  16 . The supply line  16  is connected by a valve assembly  18  to a bidirectional hydraulic actuator, such as a hydraulic cylinder  20 . The hydraulic cylinder  20  has first and second chambers  21  and  22  separated by a movable piston  26 . The valve assembly  18  selectively applies hydraulic fluid under pressure from the pump  12  to one of the chambers  21  or  22  and drains hydraulic fluid from the other chamber  22  or  21  to the tank  14  via a return line  24 . Whichever cylinder chamber  21  or  22  receives the pressurized fluid determines the direction that the piston  26  is driven. 
     The valve assembly  18  has a pair of two-position, three-way control valves  28  and  30  which have a spool that is operated by a solenoid  29  and  31 , respectively. Each control valve  28  and  30  selectively connects either the supply line  16  or the tank return line  24  to a common port  32  or  33  of the valve. When energized, the first solenoid  29  drives the first control valve  28  into a first position in which the supply line  16  is connected to the first common port  32  which is coupled to a first intermediate passage  34 . When the first solenoid  29  is de-energized, a spring biased the first control valve into a second position in which the first intermediate passage  34  is connected to the tank return line  24 . Similarly, the second control valve  30  has a first position in which the pump supply line  16  is connected via the second common port  33  to a second intermediate passage  35  of the valve assembly  18 . A spring biases the second control valve  30  into a second position where the tank return line  24  is connected to the second common port  33 . 
     The first intermediate passage  34  is coupled by a first pilot operated check valve  36  to a first workport  38  of the valve assembly  18 , which workport is connected to the first chamber  21  of the cylinder  20 . The first pilot operated check valve is oriented to have a free-flow direction from the first intermediate passage  34  to the first workport  38 . The flow in the opposite direction is normally blocked by the first pilot-operated check valve  36 , unless the valve receives a pilot signal from the second intermediate passage  35  which is sufficient to unseat the check valve, as will be described. A second pilot operated check valve  40  is connected between the second intermediate passage  35  and a second workport  42  of the valve assembly  18 , which in turn, is connected to the second chamber  22  of cylinder  20 . The free flow direction of the second pilot operated check valve  40  is oriented to permit flow from the second intermediate passage  35  to the second workport  42 . The second pilot operated check valve  40  blocks flow in the opposite direction unless it receives a sufficient pilot signal from the first intermediate passage  34 . 
     The two intermediate passages  34  and  35  also are connected by a load sense circuit  37  and specifically are coupled by a shuttle valve  44  to a load sense passage  46 . A load sense signal, corresponding to the greater of the two pressures at those intermediate passages, is produced in the load sense passage  46  and is used to control the output pressure of the pump  12 . The exemplary hydraulic system  10  uses a variable displacement hydraulic pump  12  with the load sense passage  46  connected to the control input of the pump. Alternatively, a fixed displacement pump could be employed along with a conventional unloader valve controlled by the load sense signal. 
       FIG. 2  illustrates a preferred embodiment of the physical structure for the valve assembly  18  and its components. Each of the first and second control valves  28  and  30  is located in a separate aperture  51  or  53  within the body  50  of the valve assembly. The supply line  16  and tank return  24  communicate with each of those apertures  51  and  52 , as do the first and second intermediate passages  34  and  35 . The solenoid operators  29  and  31  selectively position a spool of each control valve  28  and  30  to connect the first and second intermediate passages  34  and  35  to either the pump supply line  16  or the tank return line  24 . 
     The first and second intermediate passages  34  and  35  open into a check valve bore  53  into which the workports  36  and  40  also open. A pilot plunger  54 , which also forms the shuttle valve  44 , is slidably received a central portion of the check valve bore  53  and has longitudinal grooves in its surface extending from each end to one of two annular notches  59  and  60 , thereby allowing fluid to flow from either intermediate passage  34  or  35  into one of those notches. A central land  76  on the pilot plunger  54  between the two annular notches  59  and  60  tightly engages the inner surface of the check valve bore  53  when the pilot plunger is displaced left or right from the neutral position illustrated in FIG.  2 . In the neutral position both of the annular notches  59  and  60  open into the load sense passage  46  in the valve assembly body  50 . 
     The pilot plunger  54  engages both of the pilot operated check valves  36  and  40  located in opposite ends of the check valve bore  53 . The first pilot operated check valve  36  with a first poppet  56  that abuts a first seat formed in the check valve bore  53 . The first poppet  56  has a central aperture there through into which a pin  58  of the pilot plunger  54  extends. A first sphere  61  is received within the poppet aperture and is urged against a second seat within that aperture by a piston  62  which is biased by a spring  64  to place the first check  36  valve in the closed position. The second pilot operated check valve  40  has an identical structure comprising a second poppet  66  that engages a second third seat in the check valve bore  53 . The second poppet  66  has an aperture there through into which a second pin  68  of the pilot plunger  54  extends. A second sphere  70  is urged against a fourth seat in this second poppet&#39;s aperture by a piston  72  that is biased by a second spring  74 . In the neutral position of the pilot plunger  54  as illustrated in  FIG. 2 , the respective plunger pins  58  and  68  do not apply force to either check valve sphere  61  or  70 . 
       FIG. 2  illustrates the valve assembly  18  in the “neutral” position in which both control valves are biased by their springs to connect the respective intermediate passages  34  and  35  to the tank return line  24 . However, the pilot operated check valves  36  and  40  prevent flow of fluid from the hydraulic cylinder  20  to the intermediate passages, because both of those passages are substantially at tank pressure and the check valves are not pilot operated at this time. In the neutral position, pressure in the load sense passage  46  bleeds past the pilot plunger  54  to both intermediate passages  34  and  35  and on into the tank return passage  24 . 
     To operate the actuator  20 , one of the two solenoid valves  28  or  30  within the assembly  18  will be energized depending upon the desired direction of movement of the piston  26 . For example, the first solenoid actuated valve  28  is energized to extend the piston&#39;s rod from the cylinder  20 . Doing so connects the pump supply line  16  to the first intermediate passage  34  thereby applying pressurized fluid to a nose chamber  84  of the first pilot operated check valve  36 . Pressure from that fluid forces the first pilot operated check valve  36  to open in the free-flow direction and allows the fluid to flow to the first workport  38  and the first cylinder chamber  21 . 
     The pressure in the nose chamber  84  also shifts the pilot plunger  54  to the right, toward the second pilot operated check valve  40 . This motion forces the second plunger pin  68  against the second sphere  70  of the second pilot operated check valve  40 , thereby unseating that sphere. When the second sphere  70  is unseated, pressure within a rear chamber  86  of the second pilot operated check valve  40  is vented to tank which reduces the pressure within that chamber. A small transverse aperture  80  provides a path through the second check valve poppet  66  from the second workport  40  into a cavity between that poppet  66  and piston  71 , thereby applying the workport pressure to an annular surface on the piston. This causes the second check valve piston  71  to move away from engagement with the second sphere  70  so that the force from the plunger pin  68  also unseats the second check valve poppet  66 . This action opens a path into the second intermediate passage  35  through which fluid from the second workport  42  drains to the second control valve  30  and onward into the tank passage  24 . 
     As the second pilot operated check valve  35  opens fully, the pilot plunger  54  moves farther toward it (rightward in the drawing) and into a position where pressure from the first intermediate passage  34  is communicated through the plunger&#39;s longitudinal grooves and notch  59  into the load sense passage  46 . Thus, the pressure in the first workport  38  is applied to the load sense passage  46 . At the same time, the position of the plunger  54  is such that the land  76  engages the wall of the check valve bore  53  and blocks pressure in the second intermediate passage  35  from reaching the load sense passage  46 . 
     In order to move the cylinder piston  26  in the opposite direction within the cylinder  20 , pressurized hydraulic fluid must be applied to the second cylinder chamber  22  through the second workport  42 . To accomplish this, the second control valve  30  is activated to couple the pump supply line  16  to the second intermediate passage  35  while the first control valve  28  is de-energized. This action reverses the operation described previously with respect to activating the first control valve  28 . That is, pressure within the second intermediate passage  35  drives the pilot plunger  54  toward the first pilot operated check valve  36  (leftward in the drawings) which opens that check valve. This motion of the pilot plunger  54  also opens a path between the second intermediate passage  35  and the load sense passage  46  and blocks communication between the first intermediate passage  34  and the load sense passage. This generates a load sense signal from the pressure at the second intermediate passage  34 . 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.