The Fundamentals of Pneumatics
Part V - Directional Control
Valves and Actuators

This is the fifth in a series of articles covering the basic  principles of pneumatics. In this article, directional control valve and actuators will be discussed.

Directional Control Valves

A directional control valve is a device which connects, disconnects or changes the direction of air flow in a circuit.

The first thing that needs to be determined is the number of positions the valve has. Most valves have two positions, but some valves do have three positions. The number of positions a valve has is represented in its symbol by a series of squares. The symbol in Figure 1A is composed of two squares, which represent the two positions of this valve. The  symbol in Figure 1B is composed of three squares, which represent the three positions of this valve. It should be noted
that the squares can be drawn side by side or one on top of the other.

The second thing that needs to be determined is the number of ports the valve has. A port is an opening through which air can enter or exit a valve. The number of ports can be determined by examining the valve and counting them, or by looking at the valve's symbol.

Figure 2 shows the symbol for a 2-position, 2-way directional control valve. It is a 2-position valve because it consists of two squares. It is a 2-way valve because if you look at any one square it has two ports labeled 1 and 2. Generally speaking, if each square has two ports it's a 2-way valve, three ports is a 3-way valve and four ports is a 4-way valve. There are, however, a couple of exceptions to this rule, which will be discussed later.

In Figure 2, the bottom square shows ports 1 and 2 represented by a T symbol. This symbol is used to represent a port which is closed or blocked off. The top square shows ports 1 and 2 connected by a line with an arrow on it. The line is used to show that the two ports are connected. The arrow is added to one end of the line to indicate which direction the air flows through the valve.

One square indicates how the ports are connected when the valve is off or de-energized; the other square indicates how the ports are connected when the valve is on or energized.


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