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Directional Control Valves: Two-Position or Three-Position
2-Position Valves
Used for Simple On/Off Control...
As the name suggests a two-position directional valve has two positions, and is always in one of two states.
With a simple two-positon, two port valve (symbol shown at right) - the valve will either be open or closed.
Schematic Symbol for
2 port, 2 position valve
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...Or for More Complex, Two-State, Actions
With a two-position, five port valve, the two output ports are always in opposite modes. When one is receiving inlet air, the other is connected to the exhaust port.
Air path in position one:
Inlet air flows to OUT1, while OUT2 exhausts
Air path in position two:
Inlet air flows to OUT2, while OUT1 exhausts
Complete Schematic Symbol
(more about circuit symbols)
3-Position Valves
When actuated, 3-position directional control valves function similarly to the above. However, a center or "neutral" position is provided that blocks all ports (center closed), or connects both output ports to the exhausts (center exhaust) when the valve is not being actuated.
Pressure Held (Center Closed) 3-Position Valves
Pressure held models are ideal for "inching" operations where you want the cylinder rod to move to a intermediate position and hold. Cylinder Switches can be used to sense the position of the piston/rod mid-stroke.
Pressure Released (Center Exhaust) 3-Position Valves
Need help understanding circuit symbols?
Here are links to the various pneumatic valves currently sold by AutomationDirect:
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