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The Fundamentals of
Pneumatics This is the sixth in a
series of articles covering the basic principles of pneumatics. In the
previous article a simple 3-way directional control valves Two-position, 3-way directional control valves generally come in two different configurations, normally open and normally closed. Figure 1 shows the symbol for a normally open 2-position, 3-way, solenoid-activated, spring-return directional control valve. The two squares indicate this valve has two positions. Since the actuator symbol for a solenoid is attached to the top square, the top square shows us how the three ports are connected when the valve is activated. In its activated state, port 1 is blocked and port 2 is connected to port 3. Since the actuator symbol for a spring is attached to the bottom square, the bottom square shows us how the ports are connected when the valve is deactivated and the spring has returned it to its normal condition, with port 1 connected to port 2, and port 3 blocked. Since in its normal or deactivated state this valve allows air to go through it to an air cylinder, we say this valve is normally open. Figure 2 shows the symbol for a normally closed 2-position, 3-way, solenoid-activated, spring-return directional control valve. This valve is identical to the one in Figure 1 with one major exception; in its deactivated or normal position the in-coming air is blocked. It is important to remember that an open air valve allows air to flow through it, and a closed air valve blocks the flow of air. The terms open and closed have different meanings depending upon whether we are referring to pneumatics or electricity. Figures 3A and B show some typical 2-position, 3-way valves.
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