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coax piston 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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coax piston help

Unread postAuthor: daniel323 » Sat Feb 21, 2009 9:16 pm

sorry i am probably going to sound stupid but

on a coax piston gun- when you fill the chamber how does the pressure get past the piston when it seals up against the barrel, because isnt it a tight fit in the chamber? And when this pressure does get past the piston why doesnt the pressure (going back) actuate the piston?

sorry
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Unread postAuthor: dudeman508 » Sat Feb 21, 2009 9:23 pm

The piston is a tight fit but its not tight enought to be a complete seal so wene you fill air can still make it into the chamber but when the pilot valve is opened the piston it forced back by the air in the chamber and then the air in the chamber is sent out the barrel. you might have to experiment with different piston widths so you can get one thats tight enought but dosent seal.
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Unread postAuthor: daniel323 » Sat Feb 21, 2009 9:29 pm

ok thanks so the piston will only actuate when you pilot it (not from pressure in the chamber)?
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Re: coax piston help

Unread postAuthor: MrCrowley » Sat Feb 21, 2009 9:30 pm

daniel323 wrote:sorry i am probably going to sound stupid but

on a coax piston gun- when you fill the chamber how does the pressure get past the piston when it seals up against the barrel, because isnt it a tight fit in the chamber? And when this pressure does get past the piston why doesnt the pressure (going back) actuate the piston?

sorry

1. As the poster above said, the piston wont be an airtight seal inside the piston housing. It will look like there is no gap, but as long as the piston can be moved without too much force it will be enough for the air to get through and fill up the chamber.

If it takes too long to fill up the chamber, you can always drill a very small hole, known as an equalization hole, to allow air to past through the piston more easily.

2. Because when the air goes past the piston and fills up the chamber, the piston is moved forward and seals against the barrel. This means that on the front of the piston, the sealing face, the surface area is less then the rear of the piston because the front is sealed against the barrel.

Because the barrel sealed against the front of the piston takes up most of the piston's frontal exposed area, the piston wont move back because the rear of the piston is exposed more. More area = more force. Once this pressure behind the piston is exhausted, the front of the piston now has more force acting on it, forcing the piston back.

If you still don't understand, look at these animations:
http://www.spudfiles.com/forums/piston- ... t8157.html
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Unread postAuthor: daniel323 » Sat Feb 21, 2009 9:39 pm

ok...now i got it (i knew how they worked in a normal over under piston gun but now i understand both)

thanks
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