Motors: expansible chamber type – Working member position responsive motive fluid control – Bypassing between expanding chamber and closed or throttled...
Reexamination Certificate
1999-01-15
2001-01-09
Look, Edward K. (Department: 3745)
Motors: expansible chamber type
Working member position responsive motive fluid control
Bypassing between expanding chamber and closed or throttled...
C091S422000, C092S183000
Reexamination Certificate
active
06170383
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to relief valves for pistons which serve to position equipment or levers.
In many equipment structures, particularly compactors, hydraulic cylinders are used in high pressure bi-directional operation over thousands of cycles to extend and retract compaction members and to retain the members in 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 rod guide. 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 rod guide 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 art 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.
The center flow bypass valve of my U.S. Pat. No. 5,425,305 issued Jun. 20, 1995, illustrates a piston relief valve having a hollow tubular midsection with ends closed by caps which serve as stops. Ports are formed in the tubular sidewall adjacent the end caps. The valve opens as the piston approaches either the end cap of the cylinder or the rod guide of the cylinder.
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 rod guide 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 rod guide 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 slidably 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 cylindrical mid-section with one or more longitudinal grooves along it. The midsection has an axial threaded rod passing through it and extending from each end of the mid-section. An end cap is threaded onto each threaded rod. Each end cap is provided with a hollow cylindrical cup into which an end of the cylindrical mid-section is received. The longitudinal grooves allow hydraulic oil to flow through the piston when the valve is in the open position.
The end caps on the valve serve three functions: first, the end caps limit the spool travel and prevent it from escaping from the bore. Second, the end caps function as poppet valves to cut off flow of hydraulic fluid when they contact the surface of the piston. Third, the end caps act as a spear-type cushion when they enter a small two-stage 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.
Specifically each end cap is provided with a hollow cylindrical cup, the sidewall of which extends a short way along the midsection of the spool midsection. Each face of the piston is provided with a two-stage counterbore axially aligned with the bore provided for the relief valve spool mid-section. The outer bore of each two-stage counterbore is sized to receive an end cap of the valve spool. An inner smaller diameter counterbore is machined in the piston face such that the sidewall of the cup on the end cap will fit in the inner counterbore when the valve is closed. A small clearance is provided between the periphery of the end cap and the outer counterbore and similarly a small clearance is provided between the sidewall of the cup and the inner counterbore. Chamfers are provided on the outer edges of the end caps and the cups allow trapped hydraulic fluid to center the spool in the bore.
This invention reduces the complexity of the valve and improves its performance. The new design does not require the drilling of hydraulic oil passageways within the piston. All flow is directed along the exterior of the valve spool. The elimination of the drilled fluid passageways provides a considerable cost savings.
In a compactor environment, the relief valve must withstand high velocity seating in both the extend and retract strokes and must be reliable for hundreds of thousands of cycles. The use of dashpots formed by the inner and outer counterbores enables the valve to withstand bi-directional operations without metal fatigue because the two-stage counterbore provides a cushioning action as the valve end caps move toward their respective seats. The cushioning action of the dashpots prevents damage to the valve body in repeated extend and retract operations.
In order to minimize the size and cost of the valve and to maximize flow performance, a small threaded rod through the center is used to hold the valve parts together. The valve is required to function reliably with that delicate part for hundreds of thousands of cycles in a compactor. When the cylinder control valve is activated to extend or retract the cylinder, t
Energy Manufacturing Co., Inc.
Harms Allan L.
Lazo Thomas E.
Look Edward K.
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