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mini coaxial

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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mini coaxial

Unread postAuthor: FordGtMan » Mon Nov 12, 2007 11:47 pm

Hi everyone, im new here so i have some questions...
Im halfway in the making of my mini coaxial and i have some presta valves laying around from some bike inner tubes my dad has. I have noticed that when the pump is taken off the valve, a lot of air rushes out, and i was wondering if this would pilot the piston going off when I take the pump head off. Would schrader valves work better for this setup? I have looked around and seen some like i am making and they all have schraders.
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Unread postAuthor: hi » Mon Nov 12, 2007 11:50 pm

if its a small cannon, yes. you could just buy a check valve and screw the presta into that, they are only a few dollers.
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Unread postAuthor: singularity » Mon Nov 12, 2007 11:52 pm

schrader and presta valves have very low flow and can only be effectively used as pilot valves when the pilot volume is very small. so yes if you have a really small cannon it will work but it would have much better preformance if you use a blow gun or ball valve
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Unread postAuthor: FordGtMan » Mon Nov 12, 2007 11:53 pm

lol :oops: Are these check valves at lets say at any home depot?
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Unread postAuthor: hi » Mon Nov 12, 2007 11:54 pm

ive never checked, but they might be.
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Unread postAuthor: FordGtMan » Mon Nov 12, 2007 11:56 pm

I am making this with 1/2 inch pvc and it will be about 6-8 inches long. How much pilot volume should i have?
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 12, 2007 11:58 pm

I use a shock pump to charge up my minis, the pump screws on directly to the schrader valve. What I do to avoid premature piloting is put an o-ring between the pump seal and the schrader, in such a way that the schrader's valve stem isn't actually pushed down by the pump's attachment. Air will still enter due to the pressure difference, but when I remove the pump no air leaks out because the schrader would be already closed.
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Unread postAuthor: FordGtMan » Tue Nov 13, 2007 12:04 am

Yeah, i might have to try that one since you JSR are the KING of all mini pneumatics! lol
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Unread postAuthor: Hubb » Tue Nov 13, 2007 12:07 am

A small check valve can be found at Home Depot. Also, as far as your question about pilot volume, is the 1/2" your barrel or your chamber and what size is the other pipe? Is it going to be a piston cannon or a diaphragm cannon?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 13, 2007 12:10 am

FordGtMan wrote:Yeah, i might have to try that one since you JSR are the KING of all mini pneumatics! lol


lol, I the downside of this job is that you get a mini-throne, mini-crown etc. :roll: :D
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Unread postAuthor: FordGtMan » Tue Nov 13, 2007 12:11 am

My barrel is going to be one of those 9/32 brass pipes from hobby shops and will be a hot glue piston that i already made.
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Unread postAuthor: Hubb » Tue Nov 13, 2007 12:25 am

Using another of those efficiency rules, you will want to shoot for about 7/100 of an inch of piston travel.* This would mean that your piston only needs to travel this much, which is hardly anything at all.

Assuming you are using 1/2" sch40, (ID of .602") your pilot volume should be .019 in^3.

I think.

D/4 Rule - (9/32) / 4 = 7/100
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Unread postAuthor: FordGtMan » Tue Nov 13, 2007 12:29 am

How would i measure all of this? Would it just be a rough estimate?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 13, 2007 12:33 am

Just make sure your piston only moves back 1/8 of an inch or so.
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Unread postAuthor: Hubb » Tue Nov 13, 2007 12:35 am

I apologize. I was kind of being an @$$. The piston does not need to travle very much, and 1/8" is easier to measure.

If you decide to build a larger piston valved launcher, you may want to keep the above in mind, however.
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