Pressure compensating hydraulic control system

A hydraulic valve assembly for feeding hydraulic fluid to a load from a pump, the pump having a load sensing input and producing an output pressure which at any time is the sum of input pressure at a pump input port and a constant margin pressure, the hydraulic system comprising:

• (a) a pressure compensating valve apparatus adapted to feed fluid from the pump to the load through a metering orifice and to provide a constant pressure drop across the metering orifice, the valve apparatus having a load sense circuit which communicates a first load-dependent pressure to an isolator and a second load-dependent pressure from the isolator to the metering orifice, the pressure drop across the metering orifice being the difference between the pump output pressure and the second load-dependent pressure;
• (b) the isolator comprising a reciprocally sliding spool in a bore defined by one or more bore surfaces, the spool having a plurality of lands and narrow portions which, with the one or more bore surfaces, define:
• an input chamber in communication with the load sense circuit so that the first load-dependent pressure produces an input force urging the spool in a first direction;
• a connecting chamber in communication with the pump output pressure and adapted to connect the pump output pressure to an isolator output port in a bore inner surface as the spool moves in the first direction and to disestablish that connection as the spool moves in a second direction opposite the first direction;
• a reservoir chamber in communication with the reservoir and adapted to establish communication between the isolator output port and the reservoir as the spool moves in the second direction and to disestablish that connection as the spool moves in the first direction;
• a feedback chamber in communication with the isolator output port through a feedback bore in the spool, the pressure in the feedback chamber producing a feedback force urging the spool in the second direction;
• wherein pump output pressure is communicated to the feedback chamber and urges the spool in the second direction and wherein continued movement in the second direction disestablishes the connection between the pump output pressure and the isolator output port and establishes a connection between the reservoir and the isolator output port and therefore the feedback chamber;
• whereby the spool tends at any time to an equilibrium position at which the second load-dependent pressure at the isolator output port is a function of the first load-dependent pressure;
• wherein the isolator output port is in communication with the pump input port and with the load sense circuit which communicates the second load-dependent pressure to the metering orifice of the pressure compensating valve apparatus;
• and
• (c) whereby the pump input port sees the second load-dependent pressure but does not receive fluid flow from the load sense circuit and whereby the constant pressure drop across the metering orifice of the pressure compensating valve assembly is the margin pressure.

In a hydraulic system for feeding hydraulic fluid from a pump through an array of pressure compensating hydraulic valve sections having one or more workports to a plurality of hydraulic actuators in communication with pressure in the workports, the pump being of the type which produces an output pressure which is a constant amount greater than the pump input pressure, the array being of the type in which the highest pressure of all the workports is sensed and transmitted to a pressure relief valve and to a pressure compensating valve in each valve section a load sense pressure equal to the lower of (a) the set point pressure of the pressure relief valve and (b) the highest workport pressure, and in which each pressure compensating valve provides the load sense pressure at one side of a metering orifice which sees on the other side the pump output pressure so that the pressure drop across the metering orifice is equal to the constant amount, the improvement comprising:

• in at least one valve section, a switching valve between the relief valve and the pressure compensating valve, the switching valve transmitting to the pressure compensating valve of said at least one valve section the higher of (a) the load sense pressure or (b) the highest workport pressure of said at least one valve section, whereby the pressure compensating valve will be held closed to prevent backflow whenever the pressure relief valve is open.