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3/2 air valves are used when the device to which the compressed air is being supplied is also one from which the compressed air must exit after the work is performed.
When inflating a lift bag to move a large mass, compressed air is pumped into it. When the lift is complete, in order to remove the bag, the air must be exhausted from it. The valve selected to inflate and allow the bag to deflate, is a 3/2 air valve.
Like the 2/2 air valve, the first number in the 3/2 air valve indicates the number of ports the valve will have, and the second number indicates the number of positions that the internals of the valve can have when it is actuated. Most valves have two positions, but there are some that have three. I will cover three position valves later.
Port 1 or A on the 3/2 air valve will be the supply port to which the compressed air supply will be plumbed. The port will be a female NPT or female metric threaded hole into which will be installed an appropriately sized fitting . The fitting thread size must match the port in the valve, and the other side of the fitting must correspond to the air line size that will be connected to that fitting.
Port 2, or it may be marked port B, is a female port from which an air line will be plumbed to the application using the appropriately sized fitting and air line.
Port 3, perhaps marked ‘E’ or even number ‘5’ is the exhaust port. It may also be a female port.
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If the valve is air-piloted, meaning that the external valve actuator is an air signal from another source, then there will be an air port on one or both ends of the valve. Usually these ports will be smaller than the working ports unless the working ports are 10-32 or 3 or 4 MM in size. Then the control air ports might be the same size. Look for these to be marked with two digit numbers, perhaps as 12 or 14.
Most 3/2 compressed air valves will be NC, or normally closed. When the valve is not actuated, in it's resting state, it is closed, and compressed air cannot pass through it. In this resting state the air path through the valve from the working port will be to exhaust.
There will be some cases where having air pass through the valve to the application when it is not actuated is necessary for that air circuit. That being the case a 3/2 NO (normally open) valve would be selected. Normally open means that when the valve is at rest. compressed air passes through it, and the exhaust port(s) are blocked.
All 3/2 valves have actuators that will operate or 'shift' the air valve when required. An external button, a toggle, a lever or perhaps a solenoid actuator would be the visible actuator.
Inside the 3/2 valve there will likely be an internal actuator – a spring - which will shift the valve to the off position when the external valve actuator is not being used.
When the valve is operated if it stays in it's last selected position until an operator changes it, then it is called a detented air valve. Detented means it will stay where it's put! Detented valves are useful when an operator needs to actuate a valve and then perform another operation while the detented valve continues to feed compressed air to the application.
The 3/2 air valve is commonly used to supply compressed air to single acting devices.
These are single acting air cylinders, air springs, diaphragms, anything that is configured to “automatically” let the air back out when it is necessary to do so.
For example, a single acting air cylinder may contain a spring that will drive the piston in one direction or the other when air is shut off. The use of a single acting air cylinder is reputed to save energy costs in some applications.
Another example of a device that is supplied compressed air by a 3/2 valve is the air spring. Picture a big rubber tire with a plate covering both sides of the hole. Into one of the plates is a female port to allow an air supply. Place this “tire” under something, and you have relatively low cost actuator able to supply tremendous lift as compressed air flows into it. When the lifting process is complete, the air flows back out, and the “tire” deflates. Neat!
And there is the big difference between the 2/2 and the 3/2 valve. With the 3/2, the air can get back out again.
When the external actuator is released, an internal spring shifts the valve to off. The air supply is blocked and no more compressed air can flow through the valve to the application.
In shifting to off, the internal pathways of the valve open the working port that is connected to the actuator to the exhaust pathway inside the valve. Air can now flow freely back down the line, through the valve to exhaust.
If the exhaust port on this valve is threaded, then it is common to have an exhaust flow control installed so that the exiting air can be metered to reduce it’s speed, and thereby, reduce the retract speed of the device.
If the exhaust port is threaded then, in the event that you actually needed a 2/2 air valve for your application, by threading a plug into the exhaust port, you have created a 2/2 valve.
Some manufacturers don’t even manufacture 2/2 valves any more. Their product line up starts with a 3/2 style knowing that the cost to manufacture the 2/2 version is close to the 3/2, and a simple plug makes the conversion. This strategy helps reduce inventory too. Not all manufacturers follow this example.
Last point on the 3/2 valve. Two of them can be used to operated a single double acting cylinder in the event that a 4/2 air valve isn’t available.
Here's how to draw a 3/2 valve.
Not sure what a 4/2 air valve is? Please click here.
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