Tractor hydraulic lift control valve

Claim 1

In a tractor assembly including an implement supporting hitch and a hydraulic lift system for controlling the height of the hitch, which lift system includes an oil sump, a hydraulic lift cylinder, and a fixed displacement pump; an improved control valve assembly for interconnecting the various parts of the hydraulic lift system comprising: a lift cylinder body having a control valve opening, a flow control valve opening and hydraulic lines to the pump, the lift cylinder and sump; a mechanically actuatable spool-type control valve; first bushing means mounted within said control valve opening and receiving for movement said spool-type control valve, said first bushing means having ports and passages for selectively hydraulically interconnecting said lines to the lift cylinder, the pump and the sump in accordance with the position of said spool-type control valve in said first bushing means, the interconnection of said lines of the lift cylinder and the pump including a plurality of oil metering holes which are progressively uncovered by movement of said spool-type control valve in a selected direction; a flow control valve; second bushing means mounted within said flow control valve opening, said second bushing means having ports and passages for hydraulically interconnecting said lines to the pump and to the sump in accordance with the position of said flow control valve in said second bushing means; a spring engaging one end of and biasing said flow control valve to a normal position within said second bushing means at which position said lines to the pump and the sump are not interconnected; a first hydraulic chamber at the unsprung end of said flow control valve; a second hydraulic chamber at the sprung end of said flow control valve; a first hydraulic line interconnecting said first hydraulic chamber and said line to the pump; and a second hydraulic line interconnecting said second hydraulic chamber and said line to the lift cylinder; whereby when said spool-type control valve is moved in said first bushing means to uncover progressively said oil metering holes thereby interconnecting said lines to the pump and the lift cylinder, the lower initial hydraulic pressure on said line to the lift cylinder is applied through said second hydraulic line to said second hydraulic chamber while the full pressure within said line to the pump is supplied by the first hydraulic line to the first hydraulic chamber thereby resulting in movement of said flow control valve against said spring to interconnect said lines to the pump and said line to the sump to permit diversion of oil directly to the sump from the pump during times when only a small amount oF oil is being metered through said oil metering holes to said line for the lift cylinder.