Hydraulic Four-way Control Valve Characteristic
The four-way valve is so-named because of the required number of hydraulic lines to the valve as shown by Fig. above. The four-way valve requires a minimum of four lines, a fluid supply, two control lines to an actuator, and at least one drain or return line. This type of valve may have two, three, or more lands on the valve spool. The valve has four metering edges which are aligned with the edges of the ports in the valve sleeve. As the valve spool is moved in relation to the sleeve, the orifice areas which are formed by the edges of the valve lands and the ports vary. These variable-area orifices work in pairs so that one opens to supply more fluid to one side of the piston, while the corresponding orifice closes .the passageway to the drain line. The other two orifices controlling the fluid to the other side of the piston perform the opposite function by closing off the supply pressure and opening the drain port, thus allowing the piston to move in one direction. The combined operation of these four variable orifices thereby provides controlled fluid flow and pressure to and from each side of the piston actuator, giving proportional control to the piston as a function of the movement of the valve spool.
When the valve spool is centered, all of the orifices are closed, thereby providing no flow from the valve. For this reason, this type of control valve is sometimes called a closed-center type of valve. For different positions of the valve spool, the flow from the valve to a piston actuator is given by Fig. 1.4. The given curve is for zero differential pressure across the piston, that is, PI equals P2. The shape of the flow curve is dependent on many factors-such as valve size, number of orifices and valve with constant.pressure source. their size and shape, the oil-supply pressure, and the coefficient of discharge which varies as a function of the Reynolds number for the given orifice flows.
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