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Piston valve not sliding back?!?

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 valve not sliding back?!?

Unread postAuthor: lklaussen » Mon Jan 02, 2012 8:53 pm

So for my science fair project I decided to test something relevant to my FAVORITE hobby, so I needed a piston valve cannon. Today I got all the pieces to build it and assembled a 2" barrel sealing T with about 2 feet of 1 inch pipe for a barrel ( first picture is of whole cannon). The piston (second picture), is a 1 1/4 inch end cap with a piece of rubber gasket that seals GREAT bolted to it, and has some duct tape to get a better fit in the T. On the back threaded cap, there is a presta valve and a blow gun. When pumped, the chamber fills, the barrel seals, but when the blow gun is triggered, the piston doesn't slide back. (inside of cannon is picture three) The air from the chamber leaks out, and the piston just remains sealed to the barrel.

Please Help?!

EDIT - looks like the pictures didnt upload - ill look into that

EDIT - photos are at the link; its pretty obvious which one is what.

http://www.flickr.com/photos/73379367@N05/
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Unread postAuthor: Gun Freak » Mon Jan 02, 2012 9:23 pm

You're putting a lot of faith in that blowgun to pilot such a bad piston. You need 1) a bigger pilot valve or 2) a more airtight piston (check valve, o-ring, etc).
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Unread postAuthor: evanmcorleytv » Mon Jan 02, 2012 10:36 pm

Things that you need:

1. use a 1 inch sprinkler valve to pilot the piston, or a 3/4 inch QEV

2. Make/use a better piston!!

Not trying to be mean, just trying to help out!!
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Unread postAuthor: Gun Freak » Mon Jan 02, 2012 10:59 pm

Isn't that what I just said...? Except you were being overly specific on your pilot valve suggestions... It's not like those are the only 2 things that will work...
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Unread postAuthor: evanmcorleytv » Mon Jan 02, 2012 11:02 pm

Gun Freak wrote:Isn't that what I just said...? Except you were being overly specific on your pilot valve suggestions... It's not like those are the only 2 things that will work...


I didn't see your post..
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Unread postAuthor: Gun Freak » Mon Jan 02, 2012 11:21 pm

Ah ok my bad dude.
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Unread postAuthor: evanmcorleytv » Tue Jan 03, 2012 12:42 am

Gun Freak wrote:Ah ok my bad dude.


It's ok
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Unread postAuthor: mattyzip77 » Tue Jan 03, 2012 4:23 pm

did you lube the piston???? Also I would get rid of that piece of hose. If that dont work, its not going to. You need a faster pilot like said in above posts. good luck!!!
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Unread postAuthor: lklaussen » Tue Jan 03, 2012 8:35 pm

so today I took the blow gun out, and replaced it with a half inch ball valve. The cannon dumps about 80% of the air out of that, and the other 20 goes out a barrel. Its made to shoot paintballs at different pressures, barrel lengths etc.. and I built a chrono to record speed. i clocked the speed of the paintball in at a whopping 57.34 fps, not even enough to break it on a hard piece of pvc :(

The piston is greased with a thick waterproof grease, and seals fairly well.
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Unread postAuthor: jsefcik » Tue Jan 03, 2012 8:36 pm

lklaussen wrote:so today I took the blow gun out, and replaced it with a half inch ball valve. The cannon dumps about 80% of the air out of that, and the other 20 goes out a barrel. Its made to shoot paintballs at different pressures, barrel lengths etc.. and I built a chrono to record speed. i clocked the speed of the paintball in at a whopping 57.34 fps, not even enough to break it on a hard piece of pvc :(

The piston is greased with a thick waterproof grease, and seals fairly well.



is it sealng 100 percent against the sealing face when you pressurize it?
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Unread postAuthor: lklaussen » Tue Jan 03, 2012 8:48 pm

against the barrel? yes. there is a very small leak where I attached the ball valve, I drilled out my previous hole and pvc cement and hot glued it in place.
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Unread postAuthor: evanmcorleytv » Tue Jan 03, 2012 8:54 pm

The piston is your problem then..
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Unread postAuthor: iisthemuffin » Wed Jan 04, 2012 12:02 am

As Gun Freak said, you can make a piston with an o-ring to solve this issue. You can use a piston without an o-ring with good results. I have one on a cannon im working on and it works great. But it has to be tight enough

When using a piston without an o-ring it should fit fairly tight. with it inside the housing hold the housing so that the face of the piston is facing the ground. If the piston falls, its too loose.

The piston should only move when you apply pressure. You want it to move with little resistance if you push on it with your finger in the same scenario as above.

You want it loose enough for the air to move around it while filling, but tight enough so that when piloted the air is more likely to exit the barrel.
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Unread postAuthor: Technician1002 » Wed Jan 04, 2012 3:43 am

If you don't do an o ring, valve stem packing and grease may work. Anything to reduce the amount of air that can get past the psiton when you open the pilot is what is needed. Even a layer of felt glued on and soaked in grease works.
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Unread postAuthor: lklaussen » Wed Jan 04, 2012 9:56 am

Ok. Would a bigger pressure chamber help? It seems kind of small...
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