First Experience with Pneumatic Air Cylinders

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I have seen some cool effects that can achieved with pnuematics and I wanted to add them to my holiday displays. For Halloween, I wanted to add a lunging action to an existing static prop. Perfect use for a pnuematic air cylinder actuator.

I have a compressor. I have various fittings. There are lots of places to buy cylinders online. I've seen references to building cylinders out of storm door closers, underground pop-up sprinkers, and PVC pipe. How hard could it be? Hee.

I ordered a single acting air valve to play with because it was the cheapest I could find. Hee.

I looked at various websites and decided that I couldn't afford a real cylinder so I would build my own. I built one out of a screen door closer. After I finished it, I found a source for the fitting I should have used instead of drilling and J.B.Welding a improvised fitting. Grainger is a wonderful place.

A little background. The force that an air cylinder delivers is the pressure of the air (PSI) divided by the area of the face of the cylinder (inches2). Units cancel out leaving pounds force. The retraction force can also be calculated by subtracting the area of the face that the push rod takes up before the division step. So, a small cylinder can provide a lot of force by simply (hee) increasing the pressure.

A screen door closer has a built it spring return and a buffer to prevent hard slams at full extention. The spring return means I can get away with just a single acting valve. It also means a have to overcome the force of the spring on extension. It is a surprising amount of force. For this cylinder to be useful, I need to limit my force requirements to whatever the spring can pull back. I also have to think about how many PSI I'm will to risk putting into the cylinder. I was working about 60 psi and the cylinder was working very well but the throw wasn't that far. Only about 8 inches. Yes Chris, I can design and build a level mechanism to extend the throw, but that is more build and increased throw means a decreased level means more force which means higher psi. What is my PSI safety limit with jerryrigged equipment? Hmmm.

Okay, I can build whatever throw length I want into a custom made PVC air cylinder. 1" schedule 40 PVC fits inside 1.25" schedule 40 PVC with a few 100th's to spare. A wrap or 2 of electrical tape with a little bicycle grease will seal that nicely. Sched 40 is also very strong and contains a high PSI. Thinking about it more, I need a double acting cylinder because there will not be a retraction spring. I thought about how to rig that, and I couldn't come up with anything I liked. I settled on a design using steel rod and I attach a PVC end cap to as the plunger. I can't remember what PVC and fitting sizes I found worked tegether. I also looked at a neoprene washer sandwiched between smaller fender washers as the plunger. Nothing was quite right enough. I was also going to have to provide pressure fitting on both ends. I wanted to be able to disassemble it for repairs if need be. Disassmebly means the ends are screw on, not glue on. Stop. Sanity check time. I have no doubts about my ability to build this thing, but the BOM is adding up (PVC fitting are amazingly expensive for what they are). So is the construction time.

While I was researching home built PVC cylinders online, I keep running across lots of warnings against using PVC to contain air pressure. There were arguments about PVC degradation, OSHA directives, PVC manufacturer warnings, "I've done this for years with no problems", "its only plastic, what's the worst that could happen?", "I'm not using that much pressure", "Plastic  is elastic, it won't shatter", "cold plastic is brittle", air and water are different under pressure. I could agree with points on both sides but the last one made me think a bit. Water doesn't compress, air does. Compressed air can absorb and store a lot of energy without being obvious about it until it is very obvious about it. I've shattered platics for fun. Hmmm. I think my application would be safe enough but I don't have the experience to say for sure. I don't think this building a PVC cylinder is a good idea.

I thought about pop-up sprinklers but had not seen any tall ones. Also, based on experiences so far, I decided not to cross purpose theose either.

By this time I've seen references to suppliers of less expensive air cylinders with longer throws. I find a couple that I think will work nicely. I should say cheaper than I could build my own PVC cylinder. I end up buying a couple 3/4" by 14" throw cylinders from a surplus site @ $15 each + shipping. Not bad. They use uncommon mounting hardware. Not terrible but not good.

I manage to improvise a mount for one. When I apply air the first time, I am shocked. The cylinder is unbuffered and I haven't put a load on it. 40 PSI slams the rod out and back hard enough to make the whole mount lurch about a foot each direction. Wow! I worry a bit about the safety of what I'm doing, but just a bit. Even after just few cycles, the mount shows signs of bending, so I rework and strengthen the mount. I also add an adjustable exhaust limiter/mufflers to each exhaust of the valve so I can control the speed of the cylinder. Maybe ist not obvious, bu the speed of the cylinder is not a function of PSI but of air flow. The mufflers handle that.

This will be cool! I find a board and mount wheels on the bottom. I use a piece of aluminum tubing (scavenged cross brace from something) though a couple of eyebolts to guide the board. I mount guide but I just can't shim things right to make it line up. It's also loud and because it doesn't line up, it binds. Not good. Remembering that the deployment area is not that precise, I ditch the guide. That may cause problems later. I use an angle iron to mount the cylinder shaft to the board.

I test the setup. It works well enough. I add my payload and restest. It looks good. Some ToT's gonna scream!

Lessons learned:

  • Buy, don't build, an air cylinder. You can find them as cheap as you can build them and they are safer that way.
  • Buy 4 way 5 ported valves. More expensive but much more flexible.
  • Buy adjustable speed control mufflers.
  • Buy a lot of push connects. Much easier than anything else.
  • There is a hefty initial investment beyond the compressor, cylinder and valve when you add pneumatics.

-j

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