Directional Control Valves: Function & Principle of Operation

Directional control valves are used to regulate the flow rate of hydraulic fluid; they are also utilized in pneumatic systems for flow control. Directional control valve works by regulating the movement of the piston in the cylinder. It admits pressurized fluid to either end of the cylinder while providing a return path for fluid being squeezed out of the other end of the cylinder. An illustration of this kind of valve is shown in Figure 1.0 below.

Components of Directional Control Valve

The directional control valve consists of two main parts:

• Valve body
• Spool

These two main parts are shown in the figure below:

With a reference to the figure above, the valve body has four-fluid connection ports:

1. High-pressure fluid supply (from the pump)
2. Low-pressure return to the tank.
3. Connection to right end of the cylinder
4. Connection to left end of the cylinder

The spool is a solid-metal machined shaft that can slide back-and-forth in the valve body. The spool shown in Figure 1.0 above has two deep grooves that allow fluid to pass from one port in the valve body to another. The position of the spool within the valve body determines how fast, and in what direction, the piston will move. The spool is moved by a hand lever, an electrical actuator such as solenoid, or a small hydraulic or pneumatic actuator.

The Operation of Directional Control Valve

The Figure 1.0 above shows the valve with the spool centered. In this position, the fluid from the pump is completely blocked by the spool. The lines connected to the cylinder are also blocked; with the cylinder fluid thus trapped, the piston is locked in place.

Figure 1.1 below shows the same valve with the spool moved to the right of center.

Observe from Figure 1.1 above, that the right-hand groove is allowing the fluid to pass from the pump to the right end of the cylinder, causing the piston to move to left. The left-hand spool groove has moved into position to allow an escape route for the fluid being pushed out of the left end of the cylinder. This low-pressure fluid is allowed return to the tank.

Figure 1.2 shows the same valve with spool moved to the left of center.

With reference to Figure 1.2 above, the high-pressure fluid from pump is allowed to pass, via the left-hand groove, to the left end of the cylinder (causing the piston to move to the right). Now, the return fluid from the cylinder passes through the right-hand spool groove and then back to the tank.

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