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Piston made with CAN!

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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Piston made with CAN!

Unread postAuthor: Rudesill Ballistics » Thu Sep 06, 2007 3:04 am

Well, after a year in the making. I've finally completed my piston valve.

I only have pictures of the piston to share, but I also have a diagram of the internals I made.

I don't have a barrel attached yet, but the kleenex test firing was promising.

The piston was made out of a hairspray can, silicone, a wood plug, a disk of foamie rubber, and three springs.
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Attachments
pistoncure1.jpg
Here is the piston curing, the golf ball is holding the foam sheet to the bottom of the can.
pistonopen2.gif
The diagram (Revised)
pistonopen2.gif (22.6 KiB) Viewed 714 times
Last edited by Rudesill Ballistics on Thu Sep 06, 2007 5:32 pm, edited 2 times in total.
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 06, 2007 3:09 am

Surely the piston is longer than you drew it in the diagram? Otherwise when it's sealed at the barrel then air can escape from the base, or is that your "equilisation hole"?
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Unread postAuthor: experament-u2 » Thu Sep 06, 2007 3:37 am

beautiful piston and great animation looks nice as
i like the 3 spring idea is this any better than having just one? probably balance the backwards force better.
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Unread postAuthor: dongfang » Thu Sep 06, 2007 7:00 am

Hi

That´s one great drawing.

I think you should totally redesign it, because .. ha, no, just kidding. Looks good.

I did the same thing with leaving a gap behind the piston when fully closed. I have never had any pressure on that valve yet, though.

I think the 3 springs are because a single central spring would stick out of the pilot hole, haha. A larger diameter spring is hard to find.

Regards
Soren
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Unread postAuthor: Rudesill Ballistics » Thu Sep 06, 2007 1:53 pm

The real thing is a little different than the diagram. The piston doesn't have a rear shock pad, and the cupped bottom has the foam disk. The barrel outlet is also not beveled.

BTW: I recommend InkScape to all those who wish to make great drawings like the one above.

The green area represents the non welded socket, three screws hold it on instead.

The only part that is airtight is the foam disk on the face of the piston, it is also pressurised from the tank, the pilot valve is the only thing on the back.

I've noticed a small gremlin in my design though, the diagram doesn't take into account the ridge in the threaded end cap. My father said he heard a slap when it was fired. The piston has only a small bit of clearance.

I'm going to have to trim that ridge down by about 3/4 of it's size.
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Unread postAuthor: Rudesill Ballistics » Thu Sep 06, 2007 4:59 pm

Ridge trimmed, now waiting for glue to dry on repaired piston.

I discovered that it "sneezed" some silicone out of the can, so I'm sealing it with some "Welder" glue. The foam disk popped up and folded over in an area that wasn't glued, so that's being fixed also.

I'm using a baseball this time, it's pressing the whole disk down.
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Unread postAuthor: iknowmy3tables » Thu Sep 06, 2007 5:44 pm

that diagram makes you piston valve look overly complicated, all the arrow that have no apearent purpose and the acuator pipe moving between slides
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Unread postAuthor: Rudesill Ballistics » Thu Sep 06, 2007 5:52 pm

The pipe has been fixed and the arrows represent pressure.
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Unread postAuthor: meatballs » Fri Sep 07, 2007 3:32 pm

my first piston was made out of a can too, but it was a lot simpler. it was a mandarin orange can, wrapped with a couple layers of athletic tape, with some packaging foam stuff glued on the front and a replacement toilet paper holder (the piece that goes through the roll) behind it for the spring. the problem was after a few shots at 50 psi the can began to crush, since it had pressure on the outside and not the inside once the pilot valve was opened.
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Unread postAuthor: Rudesill Ballistics » Sat Sep 08, 2007 12:13 am

^Ghetto, :D

After fixing the other parts today I noticed that it leaked out the back, through the only connector that wasn't solvent welded. I also needed to add an additional foam disk on the front. I'm using more of that Welder Contact Adhesive, so we'll see if that fixes the leakage.

Once it got past 40 PSI it was whistling, and inflation and leakage were equal. Even so, it still launched a paper towel eight feet from it's tiny five inch barrel port.
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Unread postAuthor: meatballs » Sat Sep 08, 2007 8:41 pm

if you're having trouble with the front sealing against the barrel port then try putting a piece of bike inner tube over the foam, with the inside of the inner tube facing the barrel port to make the seal, thats how i make the seal for my coaxial cannon and it works great. and the answer to most non-solvent welded leaks is lots and lots of teflon tape.
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Unread postAuthor: Rudesill Ballistics » Sat Sep 08, 2007 11:32 pm

Well that's the thing, I tried to use teflon and wrap a little gasket and it still leaked. Teflon's only good for threaded parts I guess.

The extra disk actually helped though, I guess because it was a thinner more dense foam. The thick foam is probably just bubbled with air to make it bigger anyways.
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Unread postAuthor: meatballs » Sun Sep 09, 2007 3:37 pm

if the foam works well, awesome, use it. is there any way to use some sort of o-ring for the non solvent welded part? and what part is leaking, is it coming out around the scrrews or between the socket and the fitting?
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Unread postAuthor: Rudesill Ballistics » Mon Sep 10, 2007 2:06 am

It's coming from the socket connection, although one screw was leaking, and I used teflon on that.

I guess I could try some sort of gasket or o-ring...
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Unread postAuthor: meatballs » Mon Sep 10, 2007 1:58 pm

can you try something like caulking the joint? you would have to recaulk every time you open it, but it might work. or you could try copious amounts of that thread seal paste stuff. or maybe you could seal it with hot glue, although that would make it a pain to open.
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