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Coaxial trouble

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 » Tue May 04, 2010 12:05 am

If the pilot area simply hisses out air, it indicates the pressure in the pilot area is remaining high holding the piston closed. The piston will remain closed if either/or the piston friction is too high (not likely) or the pressure in the pilot is too high. The pressure in the pilot is a balance between the air entering the area (too much) and the air venting out the trigger valve (too little). Start with trying to reduce greatly the amount of air that can go past the piston to the pilot area. If this can not be fixed, the only solution is to go massively large on the pilot valve size to simply blast out the pressure by brute size of the trigger valve. Sprinkler valves and ball valves make great trigger valves.

The Wiki on Piston valves has some information on the requirements of the pilot area pressure for a piston to unseat. This varies due to the ratio of the piston diameter vs the valve seat area and if it is a barrel sealer or chamber sealer. High performance barrel sealer (narrow ratio) piston valves are very picky about excess leakage holding the pilot area pressure up enough to hold it closed.

http://www.spudfiles.com/spud_wiki/inde ... ston_valve
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Unread postAuthor: Gaderelguitarist » Tue May 04, 2010 9:56 am

If you do fill it completely with hot glue, it's going to be a bit on the heavy side.
You might want to try to remove some of the hot glue in the center of the piston with a forstner bit.

Of course I still think my check valve idea is simpler :D
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Re: Coaxial trouble

Unread postAuthor: iknowmy3tables » Wed May 05, 2010 12:06 pm

Howzer wrote:The piston can be heard sliding freely when the launcher is shaken vertically.

sounds like a good hint that this isn't a matter of high fiction, but more likely it's caused by a pretty weak seal around the piston
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Unread postAuthor: MRR » Wed May 05, 2010 2:03 pm

My best coax piston design at the moment looks like this.

Image

It's light weight, doesn't need an equalization, no check valve, it's really fast and it doesn't bounce. It only needs a guide rod to work properly.
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