I needed an electronically controlled pneumatic cylinder for an upcoming project that I'm currently designing. I looked into purchasing one, but it turns out that they're way beyond my budget. Looking through the garage got me thinking that I might be able to build one out of some plumbing bits and bobs, and hook it up to my compressor.

After hours of explaining exactly why this was so important, my wife agreed that I should do it. Well, actually, that didn't happen. I explained it to her, and she looked at me like I have two heads, and quietly accepted that I was going to do it anyway.

This is a preliminary build of the cylinder, just as a proof-of-concept. I built it to be able to be disassembled (nothing is permanently glued). I had thought that this might make the final product weaker, but it turned out to be stronger than I had anticipated. I may end up using this method to build the final one that matches the size requirements, so that I can re-use as many of the parts as possible if the need arises.

For this build, there are a few parts that are must-haves, but everything else was cobbled together from what I had kicking around in the garage. Feel free to modify, substitute, and improvise if you're going to build one yourself.

Step 1: Materials

There are some parts I ended up purchasing at the local hardware store, and a few that I had on-hand that are absolutely necessary for this build. I took a lot of parts from a small compressor that had died after many years of faithful service. It will live on in my heart, and in this cylinder.

Here's a short list. There are probably some things I neglected to write down, but they're the sort of things that you've likely got already.

Plumbing Parts:
- A length of 2" ABS pipe (I used 2 feet)
- A similar length of 1 1/2" ABS pipe (it's not pictured, but I cut a 2' piece off some leftover pipe I had in the garage)
- A 2" inspection end cap and plug (it's a threaded end cap that can be opened if necessary)
- A 1 1/2" end cap (not threaded)
- A 2" to 1 1/2" reducer
- A large O-ring (mine came from a pool hose fitting)
- Teflon tape
- A solenoid valve. I scavenged mine from a defunct dishwasher, but I suspect a sprinkler valve would work in a pinch). Make sure you keep the wiring harness connector for the valve. It will make your life easier.
- Various fittings to make things fit together
- Some hose clamps for good luck
- Compressor quick-connect fitting

Other parts:
- nylon zip ties
- grease
- electrical tape
- "Peel'n'Seal" type removable caulking (mine was Draft Attack)
- Small screw or bolt
- scrap power cord cut from an old appliance
- probably some other stuff that I just picked up off the workbench (like a short length of hose, a net to fit the small screw, and so on)

Tools:
- saw to cut pipe
- drill
- air compressor (the bigger the better, but a small one should be able to operate this too)
- sand paper
- solder
- heat shrink tubing (optional)
- pliers, vice-grips, and/or a bench vise
- file
- power bar with switch

Because I had most of the stuff I needed already, this project cost me in the neighbourhood of $15. If you're starting from scratch, I would guess that it would run around $50 with the valve and stuff, and much more if you need a compressor. If you don't have a compressor, you might want to get one. They're loads of fun!

Step 2: Make the Slip Fitting

At the heart of this cylinder is the 2" to 1 1/2" reducer. The smaller diameter pipe slides through the reducer to move into or out of the larger pipe. Generally, these reducers have a stop molded into them so that the smaller pipe can only go in so far. You're going to need to remove the stop to allow the pipe to slide smoothly.

Unfortunately, I did not manage to get any good photos of this process. I used a sanding drum attached to my hand drill to slowly sand out the lip until it was smooth. If you look closely at the photo, you can see the sanding marks.

If you don't have a drum sander attachment, you can use a piece of sandpaper on a small block or piece of dowel. You can even use a small diameter pipe if you've got one. Take your time, and check the fit often. Using a power tool, it took me almost an hour to get rid of the lip! Be persistent, and it'll eventually work.

Step 3: Assemble Outer Body

Now that the slip fitting is ready, we can put together the cylinder body. It's made up of the slip fitting on one end of the 2" diameter pipe, and the inspection fitting on the other end.

I used the removable caulking to glue these pieces together. I'm not sure how long I need the actual cylinder to be, so I thought that this would allow me to test the function of the cylinder, but if necessary, I can disassemble it and re-size the pipes to fit my application.

Chamfer the edges of the pipe ends (both inside and out) to remove any burrs. You don't want things sticking!

Apply a thin layer of caulking to the inside of the inspection fitting, and stick it on the end of the 2" pipe. Do the same with the slip fitting. Be very careful not to use too much! You only want enough to create a good seal, but it shouldn't ooze out in globs.

When you attach the parts, give them a little twist. This seats the part, and spreads the glue all around the joint, giving a good seal.

If you want, feel free to use ABS solvent glue to permanently attach the part. Make sure you use ABS glue, and not PVC (they're specific to their application).

Step 4: Add a Hole

We're going to need two holes in the outer body. One where air goes in, and the other where air goes out. The size of the holes depends on what fittings you're using. I used a piece from a blow-gun from my compressor, but you can use any sort of fitting, as long as the outer body is threaded, and it will allow air to pass through.

Drill a hole near the inspection end cap. Make the diameter slightly smaller than the fitting. We will thread the fitting in gently, tapping the hole as we go. You can always go back and make the hole larger, but it's a lot harder to make a hole smaller.

Once the hole is drilled, slowly turn the fitting into the hole. I used a pair of vice grips to firmly hold onto the fitting. You want to use the thread tapping technique of turning it in about 3/4 of a turn, then backing off a half turn. It's slow going, but the hole will be tapped with threads, and you won't strip it out. Be careful! You don't want to mess up this hole and have to start over!

If you do mess up the hole, take the inspection end cap off (you used the removable glue, right!?!) and cut off the end of the pipe with the funky hole. Re-glue the end cap and try again. I got mine on the first shot, but you might not be as lucky.

Step 5: Make Another Hole

The fitting from the previous step is where the air will enter the cylinder. Now we need some way for the air to escape.

Use your file to flatten one side of the long screw or bolt. I threaded a nut onto the bolt before this step to make it easier to hold on to. An added bonus is that when you remove the nut, it will clean up any threads that you messed up with the file.

Take a careful look at the photo, and notice that you don't want to flatten the screw along the entire length. Do it at an angle, to allow for adjustment. The idea is that the bolt will fill the hole entirely when it's completely threaded it, and the more you back off the screw, the larger the air passage will be. This allows fine-tuning the amount of air that escapes the cylinder, and adjusts the rate of return of the cylinder.

I ended up using the edge of the file to make a larger groove in the screw, because it was not bleeding off the air quickly enough. I would recommend using a screw that you have some duplicates of, just in case you need need to start again. It will be easy to make adjustments.

Drill and tap a second hole close to the first. Use the screw to tap the threads (if you tap the hole after you've filed the threads, it will be fairly simple to do). Remember, go in 3/4 of a turn, then back off 1/2 a turn, and it will all come out fine!

Step 6: Attach the Solenoid Valve

One of the pieces I needed to buy was an adapter that threaded into the solenoid, and had a barbed hose fitting on the other end. Thread this into the solenoid, and connect the output of the solenoid to the fitting on the cylinder body. Attach the input of the solenoid to the quick-connect fitting for the compressor.

To make these attachments, I used the hose that came off the dishwasher. I cut it in half, and used half for each side of the solenoid. You can hose clamp each connection, or use zip ties. I used zip ties where necessary, and even just pressure fit the hose in a couple places. If you're using the cylinder to move heavier loads, you'll probably want to go with hose clamps.

Finally, connect the wires from the harness of the solenoid to a disused appliance plug. This facilitates testing. If you're using a relay or circuit board, you can feel free to connect is however you please. Just be sure to use common sense and avoid touching any bare wires. The solenoid works on 110VAC, so it can be dangerous if you don't use your brain to keep yourself safe. As always, make the connections, use heat shrink or electrical tape to insulate them, and neaten things up. Now is the time to ensure there are no bare wires sticking out, because you haven't plugged it in yet! (It's not shown in the photo, but I taped down the bare ground wire later on)

Step 7: Prepare the Inner Cylinder

This is easy. Cut a length of 1 1/2" pipe, and glue the end cap on. That's it.

I used enough 1 1/2" pipe to completely retract into the 2" pipe. Feel free to modify.

Apply some grease to the outside of the narrow pipe, and slide it into the cylinder body.

Step 8: Make a Seal

I'm still playing with different ways of sealing the inner and outer cylinders. This is by far the most challenging part of this build.

Here's a short list of what didn't work (and why):

Putting an O-Ring between the outer cylinder and the reducer:
It just didn't seem to get a good seal, and it slows down the movement quite a lot. Add to this that whenever the inner cylinder comes completely out of the outer by accident, there's no way to put the o ring back without disassembling the cylinder. All in all, not a great solution (but it was what I had initially thought would work).

No O-Ring at all:
The cylinder worked fine without an o ring, but it proved difficult to tune the extension and retraction properly due to all the air bleeding around the seal. It was also probably really robbing a lot of power from the cylinder.

O-Ring on the inner cylinder, outside the reducer:
The o ring seals well when placed on the inner cylinder outside the outer cylinder. It keeps leakage to a minimum, and still allows fairly free movement. However, it only works one time, then the o ring moves up with the inner cylinder, and the seal is lost.

Electrical Tape as a boot:
I wrapped electrical tape around the end of the reducer, stretching it tight so that it would seal the gap. It didn't stick to the inner cylinder, due to the liberal application of grease. This was actually pretty effective, but it didn't last very long. Eventually the tape would roll onto itself or bunch in the small gap between inner and outer cylinders, leading to small leaks.

A piece of bicycle inner tube as a boot:
This was too tight to slide freely on the cylinder. Plus, it kept slipping off and blocking the cylinder from retracting.

My final solution involved sandwiching the o-ring between two layers of electrical tape. I wrapped a single layer around the end of the reducer, overlapping slightly onto the greased inner cylinder. The tape needs to be stretched fairly tight in order to make a smooth seal. I then rolled the o-ring down on top of this tape to hold it in place, and wrapped the o-ring and tape with another layer of tape.

So far, this seal is working well. It keeps the air in, allows smooth movement, and the o-ring stays put. It even wipes off any dust that accumulates on the extended inner cylinder as the unit retracts. Unless I can think of something better, this is probably how I will leave it.

Step 9: Test It Out

Congratulations! The cylinder is now ready for testing!

Please take note: this pneumatic cylinder has absolutely no safety stop device. If you're not careful, the inner cylinder will launch out of the unit at high speed! Don't put your face above the cylinder, or point it at any part of your body. Don't start testing at extremely high pressure. Don't act stupid.

Now that the don'ts are out of the way, try it out.

Connect your compressor to the quick connect fitting. Start off with the regulator on your compressor all the way down. Yes, all the way down. Really. Do it. Turn that thing off.

Now plug your power bar into the wall or your favourite extension cord. Make sure the power switch is in the off position. Plug in the solenoid.

1. Test the function of the solenoid. Turn on the power bar, then turn it off again. You should hear the solenoid activating (sometimes it'll hum or buzz slightly).

2. Turn the regulator up to 10 psi. Now place your hand on the end of the cylinder that extends, and place the other end on the floor. Switch on the power bar. Did the thing move? Chances are, the pressure isn't high enough. Slowly ramp up the pressure in subsequent tests until you get a feel for how it works. I've found that between 30 and 50 psi seems to work best.

3. Adjust the air bleed screw. Now that the extension is working, tune the retraction by adjusting the air bleed screw. Back off the screw to allow more air out faster, or tighten it up to slow down retraction. Adjust to your taste. This adjustment will change the characteristics of extension, so you may want to play with the pressure as well.

Step 10: You're Done!

I neatened things up by attaching the solenoid to the outer cylinder, and cleaned up the hanging hoses and wires. In the future, things will change a bit, and more permanent mounts will be used, but for now, it's finished!

There will need to be pivots at either end of the cylinder before it can be used in my upcoming project, and I will tackle that when the time comes. I have some things in mind that may or may not work.

I will also need to provide a mechanism for automatic retraction. I have an old bicycle inner tube just waiting for that purpose, and once it's set up, it should provide enough force to automate the retraction. Consistent retraction and a consistent load will allow me to tune the characteristics to suit the application.

Hope you enjoy it!

Step 11: Some Things I Learned

Here's what I learned throughout this project:

1. This thing is really springy!
Because there's such a large volume of air in it, and the air is compressible, it's actually pretty bouncy.

2. It's also pretty strong! At about 30 psi, a really really rough estimate is that it has about 72 square inches of area when retracted (and more when extended). That means it can push about 2500 pounds. Effectively, it's less than that due to air bleeding out,friction, and so on, but man oh man does that thing push when it wants to!

3. Friction is irritating! It has proven difficult to get smooth movement and a good seal at the same time. My best efforts have resulted in a compromise.

4. The solenoid needs pressure to close. The valve does not fully close right away - it uses some of the pressure pushing on it to close. That means that for a while after you shut off power, there is still air entering the cylinder. This makes tuning a challenge, but the problem becomes smaller as the pressure increases. A higher pressure combined with a larger opening in the air bleed screw reduces this problem by a bit, but it still persists. I've also lubed the solenoid, which did not prove particularly helpful.

5. If you're going to use a long piece of inner tube to automate retraction, secure it to both parts. Don't just loop it over the top. It will eventually slide off. Especially since you've probably been touching a lot of greasy parts and pieces. Take my word for it. Or take the circle indentation in the ceiling of my garage for it.

6. You could probably use different sizes or materials. I decided on ABS because it's what I had laying around, and I've seen many people use it in pressurized applications. PVC would probably be another good choice. I wouldn't recommend using central vacuum pipe, because it's thin walled, and might rupture. In a pinch, copper would probably work too (but it's gotten pricey over the last few years).

Please let me know by posting a comment if you build one, if you want to build one, or if you can think of any improvements. I'm always interested in tinkering with the design to see if I can't get it working more smoothly.