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Do piston valves have springs?

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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Do piston valves have springs?

Unread postAuthor: THEMOST » Thu Nov 09, 2006 9:22 pm

Ok I know I've been getting on everyones nerves by being nieve but I have an unfinished coaxle piston valve I'm building for a bb gun and I am stumped as to how these things work. I understand that when the exaust is released that the piston is sucked back and thus opens the valve. But I cant understand, if the pressure is equalized around the piston, how the heck the piston can keep a tight seal on the barrel unless there is a spring or magnets. Ive never heard anyone mention springs or show them and any piston valve diagrams. Will someone please tell me this last thing. Thanks. :?
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Unread postAuthor: SpudStuff » Thu Nov 09, 2006 9:41 pm

The inlet CV needs to be more than the CV around the piston. The piston should be a very good fit for a proper valve. You can use a very light spring if you want. You also need a good bumper.
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Unread postAuthor: THEMOST » Thu Nov 09, 2006 9:50 pm

What is CV? and What does the piston have to be a good fit with, the pipe it's iin or a good fit with the barrel seal?
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Unread postAuthor: boilingleadbath » Thu Nov 09, 2006 10:14 pm

Spudstuff, use proper grammar.
One doesn't say "the meter from you to me has to be smaller than the meter from you to your dog", so one should not use any unit (Cv, in this case) in that manner. Instead, use "flow" or similar word denoting this property.

...that said, spudstuff is saying that the flow of the pilot valve has to be bigger than the leakages around the piston - generaly not a problem.

The answear to your "how does it stay attatched to the barrel" question?
It's simple. The forces on each side of the piston are unballenced; there is no pressure inside the barrel to push backwards into the piston, decreasing the force on the muzzle side of the piston in relation to the breech side; hence the
force towards the sealing face.

Springs are sometimes used, but are not required unless you don't want to fill from behind the piston.
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Unread postAuthor: THEMOST » Fri Nov 10, 2006 7:01 am

Dude that's awsome! I get it now! So because there's less pressure on the chamber side since some of the area is taken up by the barrel it stays closed. :D
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Unread postAuthor: jrrdw » Fri Nov 10, 2006 8:23 am

No, your still confused, in short - all the air pressure is in the chamber and pilot. Non in the barrel. All the air wants to get out any way it can. The only way out is the barrel. The air is going to push anything in it's way out the barrel, (by design the only way out is the barrel). The sealing face of the piston is bigger round then the barrel, there for it pushs the piston against the barrel, sealing it up! Well maby not so short, but thats the jist of it.

BLB- I have a spring in mine and fill from behind the piston. Why do you say "Springs are sometimes used, but are not required unless you don't want to fill from behind the piston"?

Is it because of the shrader valve getting tangled in the spring?

I have my metal shrader valve from BCarms.com mounted in a coupler thats joining my pilot camber together, thick enough to keep it from getting tangled in my spring.
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Unread postAuthor: boilingleadbath » Fri Nov 10, 2006 11:49 am

What do you mean? He seems to understand it perfectly well.

As to my sentance, I was intending to say that 'a spring is not required unless you want to fill exclusivly from the chamber side'.

In other words, if you fill from the pilot side, a spring is not required (most of the time).

...I appologize for the double negative in my fist post.
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Unread postAuthor: THEMOST » Fri Nov 10, 2006 2:14 pm

Ya I get it. I was mabey saying it wrong. I meant to say that there is more surface area that is exposed to pressure on the front of the piston. Therefore the piston doesnt slide back withought being sucked back.
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Unread postAuthor: POS » Fri Nov 10, 2006 6:26 pm

you got it indeed
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Unread postAuthor: jrrdw » Fri Nov 10, 2006 6:46 pm

Yesssssss! Now thats understandable. Plus chamber pressure helps push the piston also. There should be some kinda gauge that measures the impack of the piston hitting the stops. Could possibly prolong the life piston if you could measure that and then adjust it. Wouldn't that be cool!
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