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Which valve has better flow? and a question or 2 about UHMW

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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Unread postAuthor: ramses » Mon Nov 16, 2009 4:02 pm

So both valves have about the same flow? and no one has used both Oil filled UHMWPE and regular UHMWPE?

That means I have to ask another question? How does one calculate the yield pressure of a hollow piston? that is to say, instead of pressure forcing the walls of cylinder and applying tension, it is squeezing the walls in compression. A good way to think of this would be a coaxial barrel. I have heard that people have had barrels in coaxials crushed, even though they were operating under the rated pressure of the barrel.

Does anyone have a formula for this? It seems like it should be an adaptation of the internal pressure formula, but I can't find anything online, and I'm not a mechanical engineer.
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Unread postAuthor: Technician1002 » Mon Nov 16, 2009 8:08 pm

Both valves have about the same flow. Due to the way the ports can be rounded entering the valve the QDV may have an advantage and the barrel sealing piston does not have the ribs to align the piston and retain the o ring, so the barrel sealer may have an advantage.

The one valve is slightly faster but in relation to the overall launch this is minimal. The QDV with a running start wen the valve cracks open vs the dead stop at the start of the barrel sealer, the QDV has the advantage of a running start when the valve opens. It like comparing a running broad jump with a standing broad jump.

With heavy projectiles that wait for the valve to open, the difference is minimal. On light projectiles, the faster earlier opening does make a difference. Remember I designed the original 2 inch to launch a 4 inch foam ball in a competition and the marshmallow cannon to launch marshmallows in a faster than SOS competition. We have not had the chance to have that competition yet. It's 5 hours drive one way.
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Unread postAuthor: D_Hall » Mon Nov 16, 2009 9:11 pm

ramses wrote:Does anyone have a formula for this? It seems like it should be an adaptation of the internal pressure formula, but I can't find anything online, and I'm not a mechanical engineer.

It should be, but you'll need to use the general form equation and not the thin-walled approximation.
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Unread postAuthor: ramses » Tue Nov 17, 2009 5:25 pm

Unfortunately, I can't find that formula, either. Is using hoop stress calculated by this site in conjunction with the compressive strength for the material an acceptable approximation?
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Unread postAuthor: jhalek90 » Fri Nov 27, 2009 10:36 pm

UHMW the oil filled stuff, actually sucks. I used it as a plug once, but the problem with it is that its to soft.

It dose not really "break" just kinda dents and bends and twists.
and from my experiments, it wont hold threads worth a darn.

Not to good for a piston... use pine.. or some of Mcmaster's back plastic... i forget its name... but its in the plastic rod section... ultra light....ultra strong.

Hope i helped
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Unread postAuthor: spudtyrrant » Fri Nov 27, 2009 10:48 pm

jhalek90 wrote:UHMW the oil filled stuff, actually sucks. I used it as a plug once, but the problem with it is that its to soft.

It dose not really "break" just kinda dents and bends and twists.
and from my experiments, it wont hold threads worth a darn.

Not to good for a piston... use pine.. or some of Mcmaster's back plastic... i forget its name... but its in the plastic rod section... ultra light....ultra strong.

Hope i helped

the oil filled might suck but regular U.H.M.W works great, i have used it in many of my cannons, and i am not disappointed with its performance at all.
i would choose it before "pine" anyday
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Unread postAuthor: SpudBlaster15 » Sat Nov 28, 2009 1:14 am

spudtyrrant wrote:the oil filled might suck but regular U.H.M.W works great, i have used it in many of my cannons, and i am not disappointed with its performance at all.
i would choose it before "pine" anyday


Agreed, UHMWPE is awesome stuff. Somewhat of a pain to machine, but it has excellent toughness and flexibility, as well as high lubricity. Perfect for pistons is you have the tooling to work with it.
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