Introduction to Directional Control Valves
Directional control valves are used to direct oil into separate circuits of a hydraulic system. The maximum flow capacity and the pressure drop through the valve are the first considerations. Directional control valves may be interfaced with the following controls: manual, hydraulic, pneumatic and electronic controls. These factors are mostly determined during the initial system design.
The directional control valve is used in order to direct the supply oil to the actuator in a hydraulic system.
The valve body is drilled and honed. Sometimes the bore is heat treated. The inlet and outlet ports are drilled and threaded. The valve spool is machined from high grade steel. Some valve spools are heat treated, ground to size, and polished. Some valve spools are chrome plated, ground to size,and polished. The valve body and the valve spool are then mated in assembly to the design specifications. When the valve body and the valve spool are assembled, the valve spool is the only part that moves.
Valve Spool
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| Illustration 1 | g01063152 |
The valve spool in Illustration 1 consists of lands and grooves. The spool lands block the oil flow through the valve body. The spool undercuts allow oil to flow around the spool and through the valve body.
The valve spool lands have lubrication and centering grooves to prevent the spool from sticking in the bore of the body. The valve spool lands have throttling slots to provide a smooth transition of oil flow as the oil passages in the body are opened and closed by movement of the spool.
The position of the spool when the spool is not activated, is called the NEUTRAL position or the HOLD position.
When an open-center valve is in the NEUTRAL position, the supply oil flows through the valve and back to the tank. When a closed-center valve is in the NEUTRAL position, the supply oil is blocked by the valve spool.
Open-center Directional Control Valve in HOLD Position
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| Illustration 2 | g01063157 |
Illustration 2 shows a cutaway diagram of a typical open-center directional control valve in the HOLD position.
When the directional control valve is in the HOLD position, the pump oil flows into the valve body. The pump oil will flow around the valve spool and return to the tank. The pump oil then returns to the tank. The pump oil also flows to the load check valve. The passage behind the load check valve is filled with blocked oil. The blocked oil and the load check valve spring keep the load check valve closed. The valve spool also blocks the oil in the line to the rod end and the head end of the cylinder.
Open-center Directional Control Valve in RAISE Position
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| Illustration 3 | g01063158 |
Illustration 3, shows the valve spool at the time when the spool is moved into the RAISE position.
When the valve spool is moved to the RAISE position, the valve spool blocks the pump oil flow through the center of the valve to the tank. However, pump oil flow is open to the load check valve. The valve spool also connects the cylinder head end to the oil behind the load check valve and the cylinder rod end to the tank passage. The load check valve prevents the oil in the head end of the cylinder from flowing into the pump oil passage. The blocked pump oil flow will cause an increase in the oil pressure.
Open-center Directional Control Valve, RAISE Position
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| Illustration 4 | g01063160 |
In Illustration 4 the increase in pump oil pressure overcomes the pressure behind the load check valve. The pump oil pressure unseats the load check valve. The pump oil flows past the load check valve and around the valve spool to the head end of the cylinder.
The oil in the rod end of the cylinder flows past the valve spool to the tank.
Directional Control Valve ISO Symbols
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| Illustration 5 | g01063163 |
The basic valve ISO symbol in Illustration 5 consists of one or more basic envelopes. The number of envelopes that are used represents the number of positions that the valve can be shifted.
Valve Port
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| Illustration 6 | g01063164 |
The valve ports for attaching working lines are shown in illustration 6. A valve with two ports is commonly referred to as a two-way valve. A two-way valve is not to be confused with a two-position valve that is shown in Illustration 5. Valves may have as many positions and ports as needed. However, most valve positions are in the range of one to three and valve ports in the range of two to six.
Flow Path
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| Illustration 7 | g01063165 |
In Illustration 7, the lines and arrows inside the envelopes are used basically to represent the flow paths and directions between ports.
Three-position Valve
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| Illustration 8 | g01063166 |
Illustration 8 shows three 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 design of the spool determines what purpose the center position will serve.
The ISO symbol at the top represents a closed-center valve. When the closed-center valve is in the HOLD position, the closed-center spool blocks all oil flow.
The ISO symbol in the middle represents a tandem-center valve. When in the HOLD position, the tandem-center valve blocks oil flow at (A ) and (B) but connects the pump to the tank.
The ISO symbol on the bottom represents an open-center valve. When in the HOLD position, the open-center valve connects all ports to the tank.
Three-position, Six-way, Open-center, Spring centered, Manual Controlled Valve
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| Illustration 9 | g01063167 |
Illustration 9, shows a three-position, six-way, open-center,spring centered, manual controlled valve in the HOLD position. The pump oil flows around the valve spool to the tank. The oil in the cylinder is blocked at the control valve spool.
Springs on both ends of the valve spool indicate that the valve is spring centered in the hold position.
When a spring is shown on only one end of a valve spool, the spool is normally pushed to the position next to the spring in the hold (without external control input) position.
The lines running full length on either side of the valve indicate that the valve is infinately variable. The flow through the valve can be in any two adjacent position at the same time. The major flow path is determined by the amount of external force placed on the spool.
Six-way Valve
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| Illustration 10 | g01063168 |
Illustration 10 shows a three-position, six-way, closed-center,pring centered, pilot controlled valve. In the HOLD position, all oil flow is blocked at the control valve spool.
Directional Control Valve Actuator
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| Illustration 11 | g01063169 |
Illustration 11 shows the ISO symbols for various directional control valve actuators.
Rotary Valve
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| Illustration 12 | g01063170 |
The rotary valve (Illustration 12) consists of a round stem with passages or channels. The channels in the stem connect with the ports in the valve body. Instead of the valve shifting to the right or to the left, the valve will rotate.
In the diagram on the left, the valve connects the pump to the rod end of the cylinder. The oil in the head end flows to the tank. When the valve is rotated 90 degrees, the pump is connected to the head end and the oil in the rod end flows to the tank.
The rotary valve that is shown is a four-way valve. However, rotary valves may also be two-way or three-way. The rotary valve is used in low pressure operations.