The Fundamentals of
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
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
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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