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1st Piston valve cannon help

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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1st Piston valve cannon help

Unread postAuthor: hatssold » Mon Jun 13, 2011 4:14 pm

Hello :D



I'm having trouble with my first piston cannon. It's a barrel-sealing piston.

I don't have pics, but this is basically it's design:

http://www.youtube.com/watch?v=WYyRp8fw_VA

The only thing I changed was the piston itself. I used the hot glue padding as a object for the air in the pilot to push against. I screwed that onto a bolt and added a rubber/felt/washer sealing face onto it. Otherwise, I used this design.


***On to the problem***

When I open the blowgun, all the air just goes through it. There is nothing coming out of the barrel.

Does my 2" piston require a bigger exhaust valve? Should I go with a metal ball valve?

Let me know if more information is required.
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Unread postAuthor: Gun Freak » Mon Jun 13, 2011 4:23 pm

Well you pretty much supplied the most important fact; the pilot is a blowgun. For a 2" valve, you are most likely going to need a bigger pilot and especially since you are using a hot glue piston. I would step up to at least a 1/2" ball valve. Metal or pvc, it doesnt matter... but metal is easier to open I think. Hope that helps.
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Unread postAuthor: hatssold » Mon Jun 13, 2011 7:08 pm

ah, will do! Since I'm on a limited budget, I just wanted to make sure.

Also since this is a barrel-sealing piston, my equalization leaks out through the sides of the piston. How much of a leakage is acceptable? How easily should my piston move through?
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Unread postAuthor: saefroch » Mon Jun 13, 2011 9:14 pm

Your piston shouldn't be too difficult to move, but with a large valve friction is almost negligible compared to the forces the piston will experience when opening.

The area of the gap between the piston and its housing must be less than the flow area of the pilot valve, or there will never be a drop in the pilot pressure. There is no definite "acceptable" leakage, but the tolerable level increases with a higher-flow pilot valve. Blowguns are one of the lowest-flow pilot valves.
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Unread postAuthor: hatssold » Tue Jun 14, 2011 7:48 am

Thank you both for the valuable information.

The cannon finally works! I got a nice 1/2" metal ball valve yesterday.

Unfortunately, the fitting between my 2" plug and 2" tee didn't seal properly. I got hasty and used a plug that was beat up. Will glue that today. Probably a good thing because I forgot to add in a decent bumper :lol:

thx
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Unread postAuthor: Gun Freak » Tue Jun 14, 2011 8:56 am

Sounds like your piston isn't serviceable?
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Unread postAuthor: hatssold » Tue Jun 14, 2011 10:14 am

That's my only regret from this cannon - I can service it only if I break a piece off. I'll make sure my next cannon is serviceable.

I tried finding a Tee with threading at all openings, but no luck. I hear some people used a lathe to make threading but I don't have one. Any suggestions???
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Unread postAuthor: Gun Freak » Tue Jun 14, 2011 10:23 am

Yes... put a female adapter on the back of your tee.
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