Hydraulic valves

Claim 1

A hydraulic control valve comprising a main valve having a main valve inlet port and a main valve outlet port and incorporating a fluid pressure actuated valve piston located in a matching valve cavity and having first and second ends, said valve piston cooperating with said main valve outlet port for forming a variable metering orifice to control flow from said inlet port to said outlet port, said valve piston having at least one end-to-end fluid duct such that both ends of the valve piston are exposed to the pressure at the inlet to the metering orifice, said valve piston ends having equal areas, said valve piston comprising a collar defining a first effective control surface area and a second effective control surface area equal to the first effective control surface area, a pilot stage including a pilot valve and variable flow setting means acting on said pilot valve, fluid control means through which a controlled proportion of fluid pressure acting in operation of the valve on said second effective control surface area is applied to said first effective control surface area of the valve piston thereby to control the variable metering orifice, flow sensing means between the inlet port and the outlet port for generating a flow-dependent feedback signal, and feedback signal transmission means coupled to the flow sensing means and to the pilot stage to transmit the flow-dependent feedback signal to the pilot stage to control the variable flow setting means acting on said pilot valve.