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PACa Mk.II, French airgun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Ragnarok » Tue Sep 21, 2010 8:43 pm

maverik94 wrote:How would a QDV have a faster opening time?

Because, as with a piston valve, when the full front face of the piston is exposed to the chamber pressure, there's a hell of a lot more than 30 pounds forcing it open - the "finger" is merely the triggering mechanism allowing it to move back and expose the front surface.

They're pretty much both the same, except that pneumatic forces start the opening process of one, the other a trigger rod pulling back on the piston. After that's started, both have the same opening force, pressure acting on the front of the piston, pushing it back (because, of course, pistons are not "sucked" back - pressure can ONLY push).

The difference however is that a QDV has no residual pressure behind it, which increases net pneumatic force to a degree.
However, I would argue that this has relatively little effect on opening time with a well designed piston valve and that this difference is ultimately already beyond the point of diminishing returns as far as performance.

Not to say that people shouldn't use QDVs as appropriate, but they shouldn't be touted as inherently superior to a piston valve. I'd choose between the two on their genuine, not imagined, merits.
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Unread postAuthor: killerbanjo » Wed Sep 22, 2010 5:14 pm

how did you get the spring out of the shrader valve? I have a couple of bike ones that ive cut and sanded the rubber off, but I can't find a way to remove the small spring on the inside :(

Image


On the left is the one with no rubber, on the right one with rubber (There from bike inner tubes)

Thanks in advance.
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Unread postAuthor: Gun Freak » Wed Sep 22, 2010 5:18 pm

I'd also like to know, I have a few spare schraders laying around that I might use to build a check valve. I have a tool that taps new threads and removes the core and all that but I don't know how to disassemble it.
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Unread postAuthor: POLAND_SPUD » Wed Sep 22, 2010 5:35 pm

AFAIK the inside of them is threaded too. You can remove the valve steam, or whatever you call it, and then remove the spring
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Unread postAuthor: Lockednloaded » Wed Sep 22, 2010 5:35 pm

usually the cores aren't removable on the cheap bike ones, but on threaded schraders, the core can be removed with needle nose tweezers
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Unread postAuthor: G-man » Thu Sep 23, 2010 3:26 am

I used a tool like this:

Image

Stuck the schradder valve in a vice, and unscrew the little block inside.
It may be hard to unscrew.
My valve wasn't a threaded one, it's a tubeless, exactly as killerbanjo's.
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Unread postAuthor: killerbanjo » Thu Sep 23, 2010 5:10 pm

Well I worked out how to take out the spring. Here is a little picture tutorial for you all

Here you can see the little cross section you need to grab onto and unscrew

Image

Unscrewing it with tweezers

Image

This valve i could not use as the spring was sheilded

Image

...So i used another that looked like this (Inside)

Image

I could not remove the spring so i cut it and pull it with a multitool
its un-clear but on the left hand side you can see the tip of the pliers

Image

And here it is without the spring, all done and dusted
Now just screw it back in the same way you got it out

Image


PS: When I tryed blowing and sucking (No jokes please :P) on the valve with no spring it worked fine but the valve did leake VERY VERY little when sucking (From the threaded end) so i guess it would leak a little when you stop pumping. But im sure once the pressure of the chamber get quite high it would seal the leak on its self.
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Unread postAuthor: G-man » Thu Sep 23, 2010 6:14 pm

It's totally normal that the valve leake, because of the very little surface of the piston. You cannot blow strongh enough to shut the valve as the spring will.

Don't worry, I made the same constatation on mine, but with only 1bar,
it's perfectly airtight ;)
You can add some oil too, it helps.

And because of the same principe, it's really easy to pump !
Just remember not to pump slowly, as without a return spring, the piston can stay open !
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Unread postAuthor: killerbanjo » Thu Sep 23, 2010 6:17 pm

Well thanks fot the tip but i got one more question that ill have to draw in paint. ill be right back :P
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Unread postAuthor: killerbanjo » Thu Sep 23, 2010 6:22 pm

Image
When you bull back the pump (Piston) the o-ring move forward but the piston would still be airtight in the piston housing, correct?

If so how do you move it backwards as wouldnt it cause a vacume?
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Unread postAuthor: G-man » Thu Sep 23, 2010 6:22 pm

The piston is not perfectly adjusted, there's a very little space between the tube and it, so the air can flow.
It may look little, but it's totally enough to delete the vacuum effect !

In fact, the piston is centered by the O-ring.
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Unread postAuthor: saefroch » Thu Sep 23, 2010 7:25 pm

The o-ring slides to the right when you pull back on the pump, and allows the pump to re-fill around the piston from the left.
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 24, 2010 12:41 am

killerbanjo wrote:Well I worked out how to take out the spring.


It works for that type but there are different types of schrader cores, most of them seem to be of the type without an exposed spring so they're not as easy to mod as you found out - but I find giving them a good stretching works well enough :)

Also,

Image

;)
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Unread postAuthor: Insomniac » Fri Sep 24, 2010 4:07 am

Well, I managed to remove the spring of my shrader core (the internal spring kind) without dissassembling the core itself...

It took me about 20 minutes with a pin, tweezers and pliers to poke around inside it enough to get the tip of the spring loose enough that the tweezers could grab the end, then they pulled it out far enough for the pliers to grab... With a bit of force the whole spring unwound and came out as a single piece of somewhat curly wire. A fiddly bastard of a task, but it IS doable.
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Unread postAuthor: killerbanjo » Fri Sep 24, 2010 10:15 am

jackssmirkingrevenge wrote:
Also,

Image

;)


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