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Hubb's 2" piston

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Technician1002 » Mon Apr 26, 2010 11:50 am

The technique of cutting pipe and putting o rings in the resulting gap does work. My ABS Cellular Core test cannon used that technique to make the piston. The piston is 2 pipe caps shoved inside a piece of pipe. Rings were cut from the pipe and glued onto the pipe caps first, the o rings were added, then the caps with o rings and pvc rings were glued into each end of the pipe. To get the pipe and pvc rings to fit the caps, the pipe was heated to soften it to fit the parts together. Lesson learned, use sandpaper on a sheet of flat wood to polish all the flat surfaces that touch the o rings. I didn't sand this after cutting it with a saw. The o rings against the rough plastic leaks a little.

Notice that this piston is smaller in the middle than at the ends. The pipe was heated and shoved onto the PVC pipe caps so the rings and pipe expanded to fit. The pipe caps are facing each other top to top.

If memory hold true, the piston is made from a pair of 1 inch pipe caps inside 1-1/2 pipe to make a piston that fits inside the 2 inch ABS barrel in a QDV style valve. The valve is opened by pulling the rope. It is closed by pushing it into place with a rod. For safety at the church camp the rod becomes a safety. Without it, the valve can not be properly closed.

Image
This early QDV cannon does not use floating o rings. It can be hard to pull.

The full thread on this ABS test cannon cannon is here;
http://www.spudfiles.com/forums/abs-cel ... 17968.html
The cannon is still functional after all those years. It has not failed.
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Last edited by Technician1002 on Mon Apr 26, 2010 5:02 pm, edited 1 time in total.
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Unread postAuthor: clemsonguy1125 » Mon Apr 26, 2010 3:40 pm

Hubb wrote:If you can't find an o-ring that's the right size, then look for o-ring stock. It is found on a spool and is generally thicker than regular o-rings. Simply by a foot of it, then trim it to size with a razor and super glue it together

Where do you find o-ring stock, ebay only had 8 giant spools for 80 bucks
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Unread postAuthor: Gun Freak » Mon Apr 26, 2010 3:49 pm

@Hubb: Home depot didn't even have slip by thread tee's, neither did ace, and things like street fittings and big threaded bushings are rare. At least where I live.

I wa planning on using a pool noodle but when you said it needs to be big, I got scared :shock:

I think I got the o-ring grooves covered, I just put the extra bolt sticking out of my piston in my drill then spin it on the corner of my bench grinder. My o-ring is about the same ID as my coupling, I bought all of the hardware at once, at ace, so everything would be the right size. Your method seems like it would work, but I want to know if it is reliable before I try it.
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Unread postAuthor: Hubb » Mon Apr 26, 2010 7:36 pm

Your method seems like it would work, but I want to know if it is reliable before I try it.

Well, I've tried it with other stuff and it seems to work fine. I don't know why it wouldn't. Besides, it's back by Tech :D

And, um, no offense, but your hardware stores suck.
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Unread postAuthor: Gun Freak » Mon Apr 26, 2010 7:39 pm

None taken, not like I own them :D
They don't have all those fancy fittings that would make my launcher more compact so I just decided to order them online and I ended up saving money anyway.

I like the way my o-rings fit already, and my next one is going in the middle of the coupling anyway so it's a little too late to do it that way :wink:
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Unread postAuthor: Hubb » Mon Apr 26, 2010 7:45 pm

Gun Freak wrote:...I like the way my o-rings fit already, and my next one is going in the middle of the coupling anyway so it's a little too late to do it that way :wink:
If that is the case, then you could just use two pipe caps instead. It would probably make the build easier. But, from my experience, pipe caps tend to be just a tad smaller in diameter than the coupling. I first built one using two pipe caps and didn't have very much success. I've had no problems with the coupling pistons, though.

Hey, why not try and upgrade the piston? If it were me, I would begin by trying to make it lighter. I would, but I have since strayed away with coupling pistons.
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Unread postAuthor: Gun Freak » Mon Apr 26, 2010 7:47 pm

Well I have one o-ring in the rear and I'm putting one also in the middle. So two o-rings. But the cap idea seems ok too, wouldn't it be the same weight though? I seen a bunch of cannons with the same valve setup as mine with the same piston and a crappy bumper that do fine, so I'm sticking with it, although I may try a few different piston designs...
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