Piston valves explained visually [Update 3]

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Postby Freefall » Thu Nov 25, 2004 3:34 pm

Patduck, if the gun is filled from behind the piston, the incoming air pushes the piston forward to seal it. When it can't go forward anymore (because it's sealed against the barrel), the air leaks around or through to fill the chamber.
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Postby clide » Fri Nov 26, 2004 12:54 am

Thanks akb, as this is mainly a basic learning model I am not terribly worried about most of those things. But I am curious as to why it wouldn't be a jump. I'm not sure if this is a good reference point, but firing up GGDT with the default launcher would show that barrel pressure rises to about 96 psi (out of 100 chamber pressure) in about 2 ms (the projectile is in the barrel for 26 ms).
I just kept the pilot pressure decreasing because showing it increasing with respect to valve position would have just been too tricky to get everything timed where it looked right, and would have depended largely on the exhaust valve. (which is unrealisticly fast in this animation)
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Postby patduck2 » Fri Nov 26, 2004 1:42 am

Freefall, would the force of air from a schrader valve using a 12 volt pump be enought to slide the piston forward or would I need a spring to force it to seal? Also once the pilot valve is opened wouldnt the air from infront of the piston just go out of the back or would there be to much force and slide the piston?
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Postby Freefall » Fri Nov 26, 2004 2:23 am

If the schrader can flow air faster than the leak port, you'll be fine. If it's loose enough that it won't seal like that, it will probably slide under its own weight. Just tip it down as you fill it. If it still won't seat, use a small spring.

Once the pilot is opened, air typically does flow back through, but because the pilot is much larger than the equalization hole, the air leaves faster than it can come in, and the pressure drops. If the equalization hole is too big, the gun won't fire. This is actually a common problem for homemade piston or diaphragm valves.
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Postby rocketmn1990 » Fri Nov 26, 2004 1:48 pm

I dont understand how air goes past the piston and into the main chamber.
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Postby DrapeMyNape » Fri Nov 26, 2004 6:29 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Freefall
[br]Patduck, if the gun is filled from behind the piston, the incoming air pushes the piston forward to seal it. When it can't go forward anymore (because it's sealed against the barrel), the air leaks around or through to fill the chamber.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

That's how rocketman. The air molecules are substantially smaller than the gaps around the piston and some spots inside of it. We want it to be able to slide, and let some air around it, but not let too much past it when triggering it.
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Postby chewy » Fri Nov 26, 2004 8:27 pm

I finally got q.t. to work. Kudos Clide. This should help beginners a great deal, great job.
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Postby whifflingterror » Tue Dec 21, 2004 6:58 pm

AWESOME animation! I find it extremely helpful in getting my mind racing through ideas for pneumatic guns.:)
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Postby MonkeyDoo » Tue Dec 28, 2004 1:18 pm

For a piston valve where the air is dumped after the piston moves a certain distance, you have a tee with an inlet at the top, and then a capped end and an outlet. If the piston moves from outlet to cap, when it moves past the inlet then the air is dumped. That is one othher way a piston valve can work, correct?
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Postby BillyBobJoe » Tue Dec 28, 2004 1:40 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by MonkeyDoo
[br]For a piston valve where the air is dumped after the piston moves a certain distance, you have a tee with an inlet at the top, and then a capped end and an outlet. If the piston moves from outlet to cap, when it moves past the inlet then the air is dumped. That is one othher way a piston valve can work, correct?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

If you switched the inlet and the outlet so it the pressure in the chamber pushes the valve past the opening for the barrel, then yes, but I may have misunderstood your explaination.
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