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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: wyz2285 » Tue Jan 31, 2012 5:47 pm

Nice technique. Do you know that's the idea temperature (more or less) needed for the o-ring to keep it's size? I also have some industrial o-rings that are designed for high temperature, probably they will be harder to mod.
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Unread postAuthor: G-man » Tue Jan 31, 2012 6:36 pm

No sorry, I don't have any idea.
All I can say is that I do this with a little torch, one of those who work with butane cartridges.
The "indicator" I use to know if it's enough, is the color of the steel.
But it's not accurate at all, it's really a matter of feeling, and tries.

Indeed, high temperatures o-rings are not appropriate for this methode I think.
I use basics o-rings, those that you found in generic hardware stores for taps.
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Unread postAuthor: inonickname » Fri Feb 03, 2012 7:16 am

G-man wrote:No sorry, I don't have any idea.
All I can say is that I do this with a little torch, one of those who work with butane cartridges.
The "indicator" I use to know if it's enough, is the color of the steel.
But it's not accurate at all, it's really a matter of feeling, and tries.

Indeed, high temperatures o-rings are not appropriate for this methode I think.
I use basics o-rings, those that you found in generic hardware stores for taps.


Oxide colours produced in steel are actually quite accurate, so if you described the exact colour we could figure out the right temperature to use.

Image

It's more for a tool steel variety, but probably close enough.
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Unread postAuthor: G-man » Fri Feb 03, 2012 1:06 pm

I'd say it's between blue and grey, so between 300°C and 427°C.
More 300 than 400 I think, because I only heat the tip of the steel tube.

Really interesting scale btw :)
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Unread postAuthor: sagthegreat » Mon Mar 05, 2012 5:46 pm

Here is another picture if things are still unclear
:idea:
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Unread postAuthor: sagthegreat » Mon Mar 05, 2012 5:50 pm

this method is kind of like urs gman, but its single shot...
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Unread postAuthor: killerbanjo » Mon Mar 05, 2012 6:52 pm

I dont think it will work well, you will have to pull the trigger very fast to get consistent power, for semi auto check out JSRs post in the how to section, you can make a semi auto out of a qev and a slide valve very cheaply, all you got to do then is make a blow forward bolt and your set.
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Unread postAuthor: Gun Freak » Mon Mar 05, 2012 7:06 pm

No it's a QDV... so the trigger only puts it in motion then the pressure opens it quickly.
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Unread postAuthor: killerbanjo » Mon Mar 05, 2012 7:23 pm

It wont pull it back though, theres nothing for the air to pull, it will just leak out the barrel
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Unread postAuthor: Gun Freak » Mon Mar 05, 2012 8:26 pm

Yes, that blue line in the back of the barrel is where the pressure acts on.
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Unread postAuthor: G-man » Tue Mar 06, 2012 11:37 am

You got the large lines, but this is not how it really works.
Firts, the QDV used is a "differential" one.
It is put in motion by the user, then a larger seat takes all the pressure and pulls the piston really fast.

And there's a space between the chamber and the hole in the barrel, to let the "blowback" system give a big acceleration to the piston.


Also, a simple QDV does not open by itself at all.
The only reason it will open fast, is if the projectile is heavy or if there's a big reduction in front of the QDV.
With a light projectile or a blank shot, it's almost as fast as opening a 1/4valve.
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Unread postAuthor: Technician1002 » Tue Mar 06, 2012 12:45 pm

Air has considerable mass. Without mass the airplanes could not fly. The mass of air in the valve is enough to pop open a QDV without a barrel attached.

There is no halfway open on my 2 inch QDV at 100 PSI. It is closed, or all the way open, even without a barrel attached. The mass of air exiting is enough to blow a impact air wrench cross the floor from 3 feet away. That is more than enough force to pop the piston open when triggered.

Without a barrel, projectile, etc, I think it is faster than a 1/4 turn valve.

Take a look for yourself.

From valve closed to full amplitude noise from the open valve is about 0.005 seconds. No projectile and no barrel.
Image

Sound file of the 3 gallon tank with the 2 inch QDV.
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Unread postAuthor: G-man » Tue Mar 06, 2012 5:43 pm

My mistake, I was talking about a QDV in the size of the launcher this topic is about.
You're right, with such a big QDV the air pushes the piston, even without barrel.
Even reminds me that I made a video 1year ago with a 3/4 one which shoots pretty well without barrel:
www.youtube.com/watch?v=UWvhl6VWSTk

But I once tried a 8mm QDV, and it definitly doesn't work well without something to restrain the pressure for a while.
I was shooting at 50bars:
-with steel bb, a nice bang and great amount of power
-with plastic bb, a fairly low "puf"
-blank firing, something like "psshhht"

With low diameter QDV it's really important to figure what you're about to shoot with it, and eventually design it so the pressure can push the piston.
Differential QDV works like a charm fort this :)
Barrel reduction is nice too, but larger seats means much higher stress for the piston.
(in the meantime, differential QDV means strong blowback...)
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