Warning - Don't Do This!

History and Resources

Here are some projects, like grave jumpers and body slingers, using pneumatic cylinders:

Safety

How a door closer works

Let's start with the concept of a triangle, made from two pencils and a ruler. Depending on the angle between the pencils, we'll need a different length of ruler.

A large angle requires a large third side on the triangle.

Medium angle.

A small angle requires a small third side on the triangle.

The point is simply that, when two sides of the triangle stay the same size, the third side changes in size as the angle changes.

Now, think of one pencil as your screen door, and the other as the door frame. The place where the pencils come together is the door hinge.

The angle measures how far the door is open. If you replaced the ruler with a gadget capable of changing its length, it would be able to automatically change the size of the third side of the triangle, thereby changing the angle between the door and frame, opening and closing the door.

The screen door closer is exactly such a gadget. You can pull open the door, stretching the closer to its full length. When you let go of the door, the closer slowly shrinks in length. This pulls the door closed, firmly, but without slamming.

Here's a closer, mounted on a screen door.

Now, let's take a look at how the closer works inside.

Cutaway view of a door closer.

Let's forget about the air vent for a minute.

• If the body of the closer is attached to something, you can pull out the piston.
• You are pulling against the internal spring, which is quite strong.
• As soon as you let up, the spring pulls the piston back into the cylinder.

Now, let's consider the air...

• Think of the air vent on the right of the closer as being a manually operated valve.
• Open the valve and pull on the piston.
• You are pulling against the internal spring.
• As the piston moves to the left, air enters through the valve to fill the space vacated by the moving piston.
• As soon as you let up, the spring pulls the piston back into the cylinder, and the air is driven back out.

• The spring attempts to push the piston back into position.
• As the piston returns, it encounters the air that was allowed into the cylinder when the valve was open.
• With the valve closed, the air has no place to go - it can't get out of the way of the spring-driven piston.
• The air will compress a little, but eventually equal the force of the spring.
• The piston will move a little and stop. It can't continue moving back until the air pressure is released.

That's the intended use of the screen door closer right there: you can stretch it out to its full length, and it slowly collapses back to resting length as air leaks out.

How the modification works

Well, what if, instead of the valve, you hooked up a hose and pumped air in and out of the cylinder? The piston would move in and out. Hook it up to something and you have animation.

And, to make it even simpler, most of these cylinders have an air valve in the form of a screw threaded into the body of the cylinder. If you unscrew that, you have a port into the cylinder, that can be used to pump air in and out.

What is the result?

• operates on 30-40 PSI
• has a reach of 6 inches
• automatically retracts when air pressure is removed

What kind of closer do I need?

Read through all of these directions and study all the pictures, and you'll be able to easily recognize a suitable door closer.

Conversion process

These are available in black, white, gold anodized, aluminum, and other colors.

The one on the top of the picture is a tad longer.

Keep the assorted hardware that comes with it. You might find it useful in mounting the converted cylinder.

Included in the hardware is a small wedge that fits on the piston, and is intended for those situations when you want to keep the door open.

Pull the piston out a couple of inches and use the wedge to hold it out.

We do this so that there is no chance of harming the bottom end of the piston during conversion.

Find the air adjustment valve. It should be on the end of the cylinder body. It just looks like a screw. If you're lucky, it will be indicated by markings for "+" and "-" or "faster" and "slower". Most are unmarked.

Here are two slighly different models of screen door closer. The one on the left has a bit more metal surrounding the screw.

Unscrew the screw!

The hole for the valve screw is in two parts. The outer part is wide enough for the head of the screw to go in. The inner part is narrow, and threaded to accept the screw.

By locating and removing the screw, you have actually done the hard part!

All you now need is a way to blow air into that hole, and let it escape later, and you're done!

We want to be able to make a clean, strong air connection to the closer. Just about anything that you can screw in, epoxy in, or solder in will work ... if it will take a little pressure.

We'll show just two of the numerous workable methods to finish this project.

One way to finish

I came up with this approach for use on closers that have very little metal around the air valve screw. In such cases, there might not be enough material to drill and tap.

As side benefits, it's a little cheaper, and has a built-in flexible air connection.

This is a barbed hose splicer, intended for use in splicing together two pieces of hose. They come in various sizes, as shown here.

The one on the bottom has been cut down on the left side, so that one end is only long enough to go clear through the bottom of the closer.

The simplest way to cut down the hose barb is with a pipe and tubing cutter. Failing that, use a hacksaw.

In either case, clean up the cut with a file or sandpaper.

The trimmed barb is only long enough to go clear through the bottom of the closer.

The inner threads in the hole are too small for the hose barb. The outer part of the hole is too large.

We'll drill out the inner part just enough to make a very tight fit with the cut-down hose barb.

Use caution in drilling. It's best to firmly clamp the closer while working on it.

Make sure the hole and the hose barb are clean and grease-free! Then, epoxy the barb into the hole.

If your epoxy is runny, or slow to set, make the hole a tight fit, jam in the barb, and then pour the epoxy between the barb and the inner wall of the hole.

If your epoxy is thick, or sets fast, roll the barb in the epoxy until it has built up a thick collar of glue, and shove that into the hole, while giving it a twist.

Or make up your own technique!

Don't use so much epoxy as to cause drips inside the cylinder. It's probably a good idea to hold the cylinder upright, with the barb at the bottom, until the epoxy sets.

I like to run a bead of epoxy around the junction, too. Don't be cheap! This is the part that gives strength to the connection!

Finish up with a length of clear vinyl tubing and a hose clamp. I told you this was easy!

Finish the other end with whatever suits you. In my case, that's a quick connector and little bits of hardware to hook it up to the vinyl tubing.

This vinyl tubing is rated up to 55 PSI. You weren't planning to go that high with a door closer, were you?

Another way to finish

I rather like standard pipe fittings, so I'll be attaching this 90-degree elbow to the cylinder. The threaded part that will be going into the door closer is 1/8-inch NPT (National Pipe Thread).

You can find these at the hardware store. Just ask for a

90-degree street elbow
1/8-inch NPT

The "street" part means one side is male, the other female. (A regular elbow is female on both sides and will require an additional nipple to hook up.)

Other parts: 1/8" close nipple, 1/8" to 1/4" reducer, and male quick connector.

The threads in the inner hole are too small for the 1/8" NPT elbow. And the outer part of the hole isn't threaded at all, and makes a tight fit with the elbow.

At this step, some folks just force the elbow into the outer hole. Others drill the hole much larger and epoxy it in. You have to be careful that you don't drill away too much metal; there isn't much there to begin with.

We like to drill the inner hole out a little, without damaging the outer hole, then tap the hole all the way through.

This requires a closer with a decent amount of metal around the screw hole.

Use caution in drilling. It's best to firmly clamp the closer while working on it.

Tap the hole all the way through (inner and outer) with a 1/8" NPT tap.

I take it slow and easy, and don't use cutting oil.

Then turn the closer upside down and shake it a lot. You want no trace of metal filings left in the cylinder!

Now, dry-fit the elbow to the tapped hole in the cylinder. If they don't screw together, run the tap through the hole again, and chase the threads on the elbow with a 1/8" NPT die. [I have seen elbows that claimed to be NPT, but didn't taper properly.]

Now screw the elbow fitting into the cylinder.

I like to mix up some epoxy and lubricate the threads with that before assembly, and run a bead around the junction.

With a properly drilled and tapped hole, matching threads on the elbow, and epoxy, the result is quite strong.

Put the rest of the parts together, using teflon tape on the joints.

We're done!

A third way to finish