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piston travel

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 travel

Unread postAuthor: no-limit » Sun May 25, 2008 10:30 pm

How far back should the piston go? Should it clear the opening to the chamber all the way?
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Unread postAuthor: daxspudder » Sun May 25, 2008 10:35 pm

Just 1/4 of the diameter of the chamber is all that is needed to be efficient.
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Unread postAuthor: no-limit » Sun May 25, 2008 10:39 pm

Getting tired here, so does that mean if I have a 2" T it should go back 1/2". Doesn't seem like enough, that's why I'm trying to clairfy it.
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Unread postAuthor: watto » Sun May 25, 2008 10:46 pm

1/4 of the barrel diameter is only for coaxials, if your using a tee make it a bit more, Ive got a 2" tee with a golf ball barrel and mine moves back 1.5cm
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Unread postAuthor: clide » Sun May 25, 2008 10:46 pm

1/4 of the seat diameter is all that is needed for a coaxial. For a tee valve, especially one with the seat close to the tee diameter, the 1/4 guideline may not apply. I would suggest 1/2 of the seat diameter if the seat is close to the diameter of the piston housing.

Edit: bah, watto beat me to it
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Unread postAuthor: no-limit » Mon May 26, 2008 12:19 am

1/4 of the barrel dia. is what is used for pilot volume, so it makes sense now for that to be the amount of travel for the piston. Doesn't seem to open enough.
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Unread postAuthor: starman » Mon May 26, 2008 1:20 am

no-limit wrote:1/4 of the barrel dia. is what is used for pilot volume, so it makes sense now for that to be the amount of travel for the piston. Doesn't seem to open enough.


But it is...do the area math and you'll see.
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Unread postAuthor: no-limit » Mon May 26, 2008 2:06 pm

If only I could!
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Unread postAuthor: SpudFarm » Mon May 26, 2008 2:09 pm

why don't you just make it 1/3 of the diameter on the barrel and stop thinking?
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Unread postAuthor: no-limit » Mon May 26, 2008 10:00 pm

Because 1+1+11. Or how about 1/2.
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Unread postAuthor: no-limit » Tue May 27, 2008 3:53 pm

Can someone explain the "area math" to me so I can try and do it.
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Unread postAuthor: SpudFarm » Tue May 27, 2008 3:55 pm

to put the theorie as i use it in aspect:
air comes rushing from 4 different main angles wich make one whole.

when i think of it GGDT has no "piston travel"
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Unread postAuthor: no-limit » Tue May 27, 2008 4:06 pm

Does that mean I shouldn't have any travel, lol.
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Unread postAuthor: clide » Tue May 27, 2008 4:52 pm

GGDT assumes the piston is fully open at 1/4 of the seat.

The math is fairly simple.
Your seat is (usually) a circle so its cross-sectional area is pi*r²

As the piston moves back imagine the area created where the air can move from the chamber to the barrel. You should be imagining a cylinder extending back from the pipe that the piston seats against. The surface area of the outside of a cylinder is 2*pi*r*h
The r would be the same in both cases and the h would be the travel of the piston.

To get the point where the flow through that cylinder equals the flow through the circular cross section you would simply set them equal to each other.
pi*r² = 2*pi*r*h

Then it is just a matter of solving for h to get
h = r/2

Since we deal with diameter much more often than radius just use r = d/2 to get:
h = d/4
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Unread postAuthor: no-limit » Wed May 28, 2008 11:32 pm

Thanks, I have a friend who can show me how to use this formula, I suck at math.
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