Connecting Solenoid Valve To Pneumatic Cylinders

This page describes several different ways to connect a solenoid valve to a pneumatic cylinder.

Why bother?
Two-Way Valve, Spring Return
Three-Way Valve Method, Spring Return
Three-Way Valve Method, Powered Return
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Why bother?

In order to activate a pneumatic prop, you need a way to feed it a supply of compressed air. Assuming that you already have a compressor and a bit of hose, the problem is reduced to a valve that can be used to turn on the air flow.

But there is one additional problem. Let's consider a little thought experiment...

Think of a toy balloon as your air-activated prop. Just imagine the balloon painted with a monster face, and inflating it causes the monster to suddenly appear in front of the trick-or-treaters.

You connect the balloon to a compressor and a valve that can be used to send air into the balloon on command. Kids come up the walkway; you open the valve; air flows into the balloon, making it expand; kids run away in terror.

Now what?

I mean, now, how do you prepare for the next batch of kids? You have to let the air out of the balloon. This allows the balloon to deflate, resetting the scare for the next batch of kids.

But we didn't plan for that! All we have is a way to get air into the balloon. If we open the valve again, even more air will go into the balloon. We need a way to exhaust the air. There are numerous ways to do this.

Two-Way Valve Method, Spring Return

The cheapest method is to insert a tee into the air line between the valve and pneumatic effect.

The side branch of the tee goes to a cheap "bleeder" valve, or even a piece of tubing, crimped or crushed to slow the escape of air.
When the valve is triggered, air rushes into the prop and out the escape hole. But flow through the escape hole is slow enough so that there is sufficient air to activate the prop.
When the valve is turned off, air from the prop flows back and out the bleeder.
This approach is often used with a two-way valve, which has only two ports, in and out, and works like a simple on/off switch.

When power is applied, compressed air goes into the pneumatic cylinder, and escapes through the bleeder valve.

When power is removed, exhaust air exits through the bleeder.

When using a two-way valve, the faster you want the prop to reset, the wider you must open the bleeder, and the more air that is wasted when you activate the prop. This will also cause your compressor to cycle more frequently.

Three-Way Valve Method, Spring Return

I prefer the use of a three-way valve.

A three-way valve has three connections: a "normally open", "normally closed", and common. The valve has two pathways through the valve: it can connect the common to the normally open port, or the common to the normally closed port.
When the valve is activated, air flows from the normally closed port, through to the common, and into the prop.
When the valve is deactivated, air flow from the normally closed port stops, but the other port opens.
The air from the prop flows back, through the normally open port, and is exhausted.
By placing a flow control on the exhaust port of the two-way valve, you can control the speed with which the air escapes, and how fast the prop resets.

HAM is built around a three-way valve.

When power is applied, compressed air goes into the pneumatic cylinder, and only into the cylinder. No wasted air.

When power is removed, exhaust air exits through the other port. HAM puts a flow restrictor there to let you set how fast the prop resets. This does not cause any extra air loss, since the restrictor is used only for exhaust.

Note that just providing a way for the air from the prop to escape isn't always enough for the prop to reset fully, or in a timely fashion. If you open the stem on an air mattress, some of the air will indeed escape, but not all of it, and not rapidly.

That's one reason why some people like to use screen door closers as pneumatic cylinders - they contain a heavy spring that will automatically reset the prop, if you allow the air to escape.

Three-Way Valve Method, Powered Return

TBD

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