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Ideal piston/pilot design criteria

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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Ideal piston/pilot design criteria

Unread postAuthor: boyntonstu » Sat Oct 03, 2009 4:15 pm

Are these assumptions correct?

Piston should be as light as possible.

The piston should fit the pilot without leaking.

The piston sealing area should be as large as possible.

The pilot opening should be as large as possible.

The piston should move just far enough to allow unrestricted flow from the chamber to the barrel.

The pilot valve should open as fast as possible.

????

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Unread postAuthor: jackssmirkingrevenge » Sat Oct 03, 2009 4:19 pm

The piston should fit the pilot without leaking.

The piston sealing area should be as large as possible.


Can you clarify these? The others are pretty much spot on :)
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Unread postAuthor: psycix » Sat Oct 03, 2009 4:32 pm

Piston should be as light as possible.
Yes. Lighter pistons move faster, and they don't slam into the back as hard as heavier ones.

The piston should fit the pilot without leaking.
What do you mean with this?
It should fit in the tube it slides in as tight as possible, some blowby is allowed, but you should try to minimize it.

The piston sealing area should be as large as possible.
I do not completely understand with what you mean with sealing area.
For a fast-opening piston, the diameter of the piston should be close-ish to the porting diameter, though this requires a good pilot and piston fit.
When you are unsure about the fit of the piston in its sliding pipe and pilot valve, a larger diameter helps to make the piston valve more reliable. However, a larger diameter piston also makes more pilot volume.

The porting which leads to the barrel should have about the same diameter as the barrel.

The pilot opening should be as large as possible.
The pilot valve should open as fast as possible.
Yes, the pilot needs to be dumped as fast as you can. Larger and/or faster valves help.

The piston should move just far enough to allow unrestricted flow from the chamber to the barrel.
Yes, but to avoid restrictions due to bouncing or the sealing face flapping or whatever, add up a little distance, it doesn't hurt.
But yes, it should be minimized, so that the piston doesn't travel unnecessary distance. (Harder piston slam, and chamber volume expands)
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