Directional Control Valves• Used to direct the supply of oil to the actuator in a hydraulic system.• The valve body is drilled, honed and heat treated. The inlet and outlet ports are drilled and threaded.• The valve spools are heat treated, ground to size and polished, some chrome plated.• The body and spool are then mated in assembly.• After assembly the spool is the only moving part.
Valve Spool• The spool consists of lands and grooves.• The lands block oil flow through the valve body.• The grooves allow oil to flow around the spool and through the valve body.• The position of the spool when not activated is called the “normal” position.• When an “open center” valve is in the normal position, the supply of oil flows through the valve and back to the tank.• When a “closed center” valve is in the normal position the oil is blocked by the valves pool.
Open Center – Hold Position• In the hold position the pump oil flows into the valve body, around the spool and return to the tank.• The oil also flows to the load check valve. The passage behind the load check is filled with blocked oil. The blocked oil and the load check valve spring keep the load check valve closed.• The spool also blocks the oil in the line to the rod end and the head end of the cylinder.
Open Center – Raise Position• When the spool is moved to the raise position, the valve spool blocks the oil flow to the tank.• The oil is also open to the load check valve. The spool in addition connects the cylinder head end to the oil behind the load check valve and the rod end to the tank return. The load check valve prevents the oil in the head end of the cylinder from flowing into the pump passage. The blocked pump oil flows causes an increase in oil pressure.
Open Center – Raise Position• This increase in pump oil pressure overcomes the pressure behind the load check valve and unseats it. The pump oil then flows past the load check, passed the spool and to the head end of the cylinder.• The oil in the rod end of the cylinder flows past the spool and to the tank.
ISO Symbols• The basic valve symbol consists of one or more envelopes. The number of envelopes used represents the number of positions that the valve can be shifted.
Valve Ports• Here we show the valve ports for attaching the working lines. A valve with two ports is commonly referred to as a two-way valve. Don’t confuse this with a two-position valve.• Valves can have as many position and ports as needed. But most commonly valves have one to three positions and ports from two to six.• For example you can have a valve that has two positions and but controls three cylinders and such three ports.
Flow Path• The lines and arrows inside the envelopes are used to represent the flow paths and directions between ports.
Three Position Valve• Here we show thee ISO symbols of the three position valve. In the three position valve, the center position is the neutral or hold position. When the valve is not doing work, the valve is placed in the hold position.• The valve on the top is a closed center valve. When in the hold position the center spool blocks all oil flow.• The middle valve is a tandem center valve. When in the hold position ports A and B are blocked but the valve connects the pump (P) to the Tank (T).• The bottom valve is an open center valve. In hold the spool connects all ports to the tank.
Three Position, Six Way, Open Center, Manual Control Valve• This valve is now in the hold position.• The pump oil flows around the spool to the tank. The oil in the cylinder is blocked at the valve spool.
Check Valve• The purpose of the check valve is to readily permit oil flow in one direction but prevent oil flow in the opposite direction. The check valve is often referred as a one-way check valve.• Most consist of a spring and a tapered seat, but some are free floating.• For the valve on the left, when the pump oil overcomes the spring tension the seat opens and allows oil to flow.• The valve on the right, when the pressure on the implement side becomes less than the pump side the seat will open allowing oil flow.