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Backwards 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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Backwards Piston

Unread postAuthor: PeteS » Fri Oct 04, 2013 4:56 pm

Is there any reason a piston in a coaxial should not be designed to be put in backwards from the way they usually are? By backwards I mean with the cupped end forward so the barrel extends into the piston.

The reason I am wondering is that I am trying to build a very short coaxial with the barrel being as long as possible for the short gun. By reversing the piston I figure that I can extend the barrel farther back in the launcher. It will also be easier to minimize the volume behind the piston.

The launcher will have an outer tube of 3" PVC, a piston turned down from a 2.5" end cap, and a 1-1/2" PVC barrel.
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Unread postAuthor: Labtecpower » Fri Oct 04, 2013 5:18 pm

I do not see any reason why it would have any disadvantages :)

For the sealing face you can maybe just use epoxy to attach a rubber sheet on the inside of the piston, no need for screws I think..
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Unread postAuthor: CannonCreations » Fri Oct 04, 2013 5:23 pm

yeah, you should be able to make a gun with a piston in that configuration, but you will actually need to increase the area behind the piston so that it can move back far enough. Also I dont know how much more barrel you could add with a piston like that, or if it would even make any difference. If you really want to cut down on the space behind the piston, cut it out completely, and instead you could try a diaphram. I have never personally used one as a valve for any of my cannons, but Im sure someone else here has
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Unread postAuthor: dart guy » Fri Oct 04, 2013 6:44 pm

gimme about 3 muinuts and i will post a piston idea
edit Image
the orings on the bigger part have 2 notches (not all the way around) and the black peice is a peice of rubber, the other orings are just for somethin to keep it on, hope you like my idea.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 04, 2013 7:24 pm

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Unread postAuthor: Technician1002 » Sat Oct 05, 2013 12:37 am

Way back, I saw a coax design that used a soup can in a 3 inch chamber with a rubber gasket bolted inside the can and covered with felt to adjust the fit. This can piston apparently worked well. I don't remember the barrel size. Think it was 1.5 inch or 2 inch.

I'm not going to try to find a thread that old. Not even sure it was on this site. Many have been on one of the early Spudgun Ring Exchange sites.
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Unread postAuthor: jrrdw » Sat Oct 05, 2013 6:33 am

jackssmirkingrevenge wrote:http://www.spudfiles.com/forums/cup-shape-piston-t19917.html


Wow I forgot about that L0L. Jeepkan had the better build in that thread (in hind sight).
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Unread postAuthor: PeteS » Sat Oct 05, 2013 9:51 am

A schematic sketch of what I am thinking:
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Unread postAuthor: Gun Freak » Sat Oct 05, 2013 11:06 pm

The good part about such a piston is that if you use a flexible material such as rubber, the piston acts as a check valve. With pressure inside of it, it will expand and not allow air flow into the pilot upon triggering. It can still let air by during filling.

Maybe you could try the idea out with one of those rubber clamp-on repair caps, or whatever they're called. Might need to reinforce the back with metal.
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Unread postAuthor: PeteS » Sun Oct 06, 2013 8:27 am

Gun Freak wrote:The good part about such a piston is that if you use a flexible material such as rubber, the piston acts as a check valve. With pressure inside of it, it will expand and not allow air flow into the pilot upon triggering. It can still let air by during filling.

Maybe you could try the idea out with one of those rubber clamp-on repair caps, or whatever they're called. Might need to reinforce the back with metal.


Interesting...
I may give that a try.

The current configuration may already be doing that to some extent because the piston is made from a 2-1/2" cap and the wall of the side are turned down pretty thin.

BTW: I test fired the launcher this morning and it works pretty well. No real testing just a shot at 60 PSI or so. It went something like 120 yards. I don't think that is too bad for such a short launcher (about 18" total length).
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