Hydraulic cylinder piston with center flow bypass valve
BACKGROUND OF THE INVENTION 
     This invention relates to relief valves for pistons which serve to position equipment or levers. 
     In many equipment structures, including agricultural implements, hydraulic cylinders are used to raise and lower or otherwise move members of the machine and to retain the members in a position until the hydraulic cylinder is actuated to move the member to a new position. When hydraulic fluid is pumped into the hydraulic cylinder and the piston has traveled fully to the end of the cylinder, the hydraulic pump supplying fluid to the cylinder continues to apply pressure to the fluid and must be stopped to avoid an over pressure condition which stresses and damages the cylinder end cap and head gland. In some existing systems, timer mechanisms are operable with the hydraulic pump to shut the pump off after the estimated time it takes for the piston to travel to an end of the cylinder. At best, this only approximates efficient operation. 
     In other prior art devices, a transducer is provided on the hydraulic cylinder to sense when a predetermined pressure is attained in the fluid input line. The transducer either sends a signal to the pump control to stop pumping, or to a switch valve to bypass the input line so that over stress conditions do not occur. The use of a transducer system requires addition of control apparatus to the hydraulic system. 
     Another prior art means for avoiding an over stress condition in the cylinder is by means of a relief valve in the cylinder or in the input line. This apparatus causes high heat build up and stress on the hydraulic pump. 
     A relief valve for a hydraulic piston is used in equipment manufactured by the Marathon Equipment Co. for compaction equipment, wherein a bypass valve member is positioned within a bore through the hydraulic piston such that the valve is displaced when its leading end engages the head gland of the cylinder. When the valve is displaced, a central reduced diameter portion of the valve spool comes into registry with a pair of axial passageways which communicate with the edge of the piston. In this design the hydraulic fluid is routed around the outside of the valve spool in a manner similar to typical manual control valves. The design requires special machining operations in the piston to provide fluid passageways. The machining required to create the passageways adds considerable expense to the fabrication of the piston. This previous design depends on extremely tight spool-to-bore clearance to minimize leakage in the closed position. The tight clearance makes the valve vulnerable to malfunction if there are machining inaccuracies in the piston or valve. Also the valve function becomes very sensitive to minute particles of contamination that could wedge between the spool and bore. As the outside diameter of the spool becomes worn, the hydraulic fluid leakage will increase, reducing the efficiency of the cylinder. Further, this prior design valve only works when the piston is advanced in one direction and it is found to tend toward premature exhaustion. 
     Another prior art device comprises a spring loaded relief valve positioned through the piston allowing a passageway for fluid to escape from the advancing side of the piston to the following side when the valve is urged against its spring loading by engagement of the leading end of the valve with the cylinder end wall. As with the Marathon Equipment Co. design valve, the bore through the piston requires complicated machining to provide valve seats and spring engaging shoulders. This valve structure causes heat build up, operates only in one direction of movement of the piston, and is subject to premature wear. 
     SUMMARY OF THE INVENTION 
     The invention relates to a valve mounted in a hydraulic cylinder piston that allows hydraulic fluid flow through the piston at the end of either the extend or retract stroke. The valve is opened as it makes contact with either the end cap or head gland inside the cylinder. The valve is closed by pressure when hydraulic fluid flow is reversed to the cylinder after reaching the end of the stroke. The purpose of the valve is to relieve pressure-induced loading on the end cap or head gland of the cylinder and to minimize problems with metal fatigue. The hydraulic energy expended by the cylinder is utilized in moving a load and not wasted by deadheading the pump at the end of stroke. 
     The valve is slideably mounted in a close-fitting bore running through the piston. The valve bore is offset from and parallel with the piston center line. The valve is free to slide back and forth within the bore. 
     The valve spool consists of a tubular mid-section with closed ends and circular stops at each end. The tubular center section has cross ports at each end that run perpendicularly to the axis of the valve. The cross parts are situated as close to the end stops as possible. The cross ports intersect the hollow center of the spool and allow hydraulic oil to flow through the piston via the passageway in the center of the valve when the valve is in the open position. 
     The circular stops on the valve serve three functions: first, the stops limit the spool travel and prevent it from escaping from the bore. Secondly, the stops function as poppet valves to cut off flow of hydraulic fluid when they contact the surface of the piston. Thirdly, the stops act as a spear-type cushion when they enter a small hydraulic dash pot machined in the piston at each end of the bore where the spool is mounted. The cushion diminishes the shock as the valve shifts, and improves the long-term reliability of the valve. 
     This invention reduces the complexity and improves the performance of the valve. The new design does not require the drilling of hydraulic oil passageways within the piston. All flow is directed through the center of the valve spool. The elimination of the drilled fluid passageways provides a considerable cost savings. 
     It is an object of the invention to provide a relief valve for a hydraulic piston which is simple and inexpensive to manufacture and assemble. It is also an object of the invention to provide a relief valve for a hydraulic piston which allows minimal leakage of working fluid when the valve is closed. It is a further object of the invention to provide a relief valve for a hydraulic piston which cushions itself when changing state. It is also an object of the invention to provide a hydraulic piston with a relief valve which operates to alleviate stress on the end cap and head gland of the cylinder in which the piston is driven. It is a further object of the invention to provide a hydraulic cylinder and piston with a relief valve which is not susceptible to heat build up. Another object of the invention is to provide a relief valve, the stops of which wear together with their seating surfaces to create improved sealing as use continues. These and other objects will be apparent from examination of the detailed description which follows. 
    
    
     DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a cross sectional view in perspective of the preferred embodiment hydraulic piston with the center flow bypass valve in place. 
     FIG. 2 is a cross sectional view in perspective of the bypass valve of the preferred embodiment. 
     FIG. 3 is a partly cut away elevation of a hydraulic cylinder equipped with the preferred embodiment piston in its retract stroke, the piston valve shown in cross section, with the valve of the piston being in its first closed state with no working fluid passing through the valve. 
     FIG. 4 is a partly cut away elevation of the hydraulic cylinder of FIG. 3 showing the preferred embodiment hydraulic piston with relief valve in cross section, with the piston at the end of its retract stroke and with the valve being in its intermediate state with a passageway established for working fluid to pass through. 
     FIG. 5 is a partly cut away elevation of the hydraulic cylinder of FIG. 3 showing the preferred embodiment hydraulic piston with relief valve in cross section, the piston shown at the beginning of its extend stroke and with the valve being in its second closed state. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawing figures and in particular to FIGS. 1 and 2, the preferred embodiment hydraulic piston 7 is disclosed. The hydraulic piston 7 is provided with an axial opening 30 into which a piston rod (not shown) is receivable. Edge 32 of piston 7 is provided with channel 31 into which a sealing ring 33 (See FIG. 3) may be received. The hydraulic piston 7 is provided with a bore 10 therethrough which is axially parallel to the axis of the piston 7 and is spaced apart therefrom. Counterbores 6 and 16 are made on the opposing faces 21, 22 of the piston 7, the counterbores 6 and 16 being coaxial with the bore 10 through the piston 7 and of a fixed depth. A valve spool 12 of cylindrical configuration is receivable within the bore 10 of the piston 7 in a close fit yet slidable therewithin. The valve spool 12 is provided with an internal axial passageway 4 and with at least two radial ports 3 along the length of the spool 12, the ports 3 being communicative with the internal passageway 4 of the valve spool 12. The ports 3 are positioned such that they are generally symmetrically located along the valve spool 12 and spaced apart somewhat more than the length of the bore 10 between the counterbores 6 and 16. Each port is immediately adjacent to a stop member 2 or 2&#39;. In the preferred embodiment, ports 3 are formed by cross drilling the valve spool 12 resulting in pairs of diagonally opposed paired ports 3 near the ends of valve spool 12. 
     The valve spool 12 is provided with stops 2 and 2&#39; which are suitably fixed to the valve spool 12 and which serve to enclose the ends of the axial bore 4 through the valve spool 12. In the preferred embodiment, stop members 2 and 2&#39; are retained to spool 12 by threaded shaft 30. Each of stop members 2 and 2&#39; is cylindrical in shape and is sized such that each will freely fit within the counterbores 6 and 16 on the faces 21 and 22 of the piston 7. When stop 2&#39; is fully received in counterbore 6, the inner face 5 of stop 2&#39; will abut seat 9 of counterbore 6. Stop 2 is identically receivable in counterbore 16 upon seat 19 therein. The radial ports 3 of the valve spool 12 are disposed along valve spool 12 such that when one stop 2&#39; of the valve 1 is abutted with the seat 9 of counterbore 6 of first face 21 of piston 7, the port 3 nearest the abutting stop 2&#39; will be within the bore 10 through the piston 7, while the port 3 on the opposing end of the valve spool 12 will be in communication with the counterbore 16 of the other face of the piston 7. 
     Counterbores 6 and 16 serve as hydraulic dash pots for stop members 2&#39; and 2 respectively when either is urged toward bore 10. Stop members 2 and 2&#39; act like poppet valves in their interaction with counterbores 16 and 6, each preventing working fluid from passing through bore 10 when the stop member is fully seated in its respective counterbore. The presence of working fluid in counterbores 6 and 16 allows for a cushioning of stops 2 and 2&#39; when they seat in the counterbores 6 and 16. 
     Referring to FIGS. 3, 4, and 5 in particular, operation of the valve may be visualized. When working fluid is applied to the first face 21 of the hydraulic piston, the working fluid forces the relief valve 1 along the bore 10 through the piston 7 to a first state where the stop 2 of the relief valve 1 abuts the seat 9 of the counterbore 6 on the first face 21 of the piston 7. As the piston 7 approaches the end cap 24 of the cylinder, the stop 2 on the first end 14 of the valve spool 12 which protrudes from the second face 22 of the piston 7 engages the end cap 24 of the cylinder 25 and is urged through the bore 10 of the piston 7 toward the trailing first face 21 of the piston 7. As the valve spool 12 slides through the bore 10 toward the first face 21 of the piston 7 into an intermediate position, the radial ports 3 of the valve spool 12 reach alignment (See FIG. 4) with the counterbore 6 and 16 on faces 21 and 22 respectively, of the piston 7, and a passageway is created from the counterbore 6 on the first face 21 of the piston 7 through the radial port 3 on the valve spool 12 nearest the first face 21 of the piston 7 through the internal bore 4 of the valve spool 12 and through the radial port 3 on the first end 14 of the valve spool 12 into the counterbore 16 on the second face 22 of the piston 7. When the passageway is so created, working fluid is allowed to pass from the first face 21 of the piston 7, in the direction of arrows b, to the second face 22, thereby reducing pressure on the first face 21 of the piston 7. 
     When working fluid is directed to the cylinder 25 to reverse the movement of the piston 7 into an extend stroke, the roles of the leading and trailing faces of the piston 7 reverse. Pressure of the working fluid causes the relief valve 1 to rapidly slide from its position abutting the end cap 24 of the cylinder 25 into a second state where the valve 1 in engagement with the pressurized working fluid is urged into the bore 10 through the piston 7 until the stop 2 on first end 14 abuts the seat 19 within the counterbore 16 on the second face 22 of the piston 7 which has become the trailing face of the piston 7. When the valve 1 is so positioned, no passageway is provided through the valve 1 or the bore 10 in the piston 7 and the piston 7 is urged toward the head gland (not shown) opposing the end cap of the cylinder 25. As the piston 7 approaches the head gland, the end of the valve 1 protruding from the advancing first face 21 of the piston 7 engages the head gland of the cylinder 25 and is forced rearward relative to the piston&#39;s advance, to the intermediate position where the passageway through the bore 10 of the piston 7 by way of the internal bore 4 of the valve 1 is established and working fluid may pass to the first side of the piston 7 to reduce the pressure and avoid stress on the head gland. 
     It is found that the disclosed design requires far simpler machining operations than that required for prior art valve structures, and that heat build up and premature failure are avoided. It is also found that the stop 2 to seat 9 abutment creates a tight seat for the valve 1 and that cushioning by residually present working fluid is provided as the stop 2&#39; and seat 9 of counterbore 6 engage. The same dash pot function is similarly provided by stop 2 and seat 19 in counterbore 16. 
     It is to be understood that in the preferred embodiment of the valve 1 shown in FIGS. 1 and 2, the valve spool 12 is provided with diametrically opposing radial ports 3. The use of opposing ports simplifies the machining of the valve spool 12 and alleviates stresses between the bore 10 and the valve spool 12. 
     In FIGS. 3, 4, and 5 it can be seen that the stops 2 and 2&#39; of the valve 1 may be retained by use of a central stud 30 which passes through internal bore 4 of valve spool 12 and retains stops 2, and 2&#39; by screw threading or other securing means.