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Can anyone tell me how this works?

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: G-man » Mon Jan 30, 2012 2:37 pm

It works really well with 8mm bb's, that makes a nice sound and the bb go kinda far.
But you're right, it won't work right with high pressures.
Except if you use a spring powered hammer, which is what I intend to use to trigger the 50bar version !

But this is definitely not the best way to do semi-auto.
The interesting thing is that differential QDV can open really fast, even if you dry fire it !
And even with low pressures.


You're half right: it's not direct, but indirect connection.
The trigger is attached to a moving part detached from the piston.
But indeed, if I keep my finger pressed, the QDV can not be sealed, and the pressure goes away.
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Unread postAuthor: warhead052 » Mon Jan 30, 2012 2:41 pm

G-man, can you post some more of your cannons here? They are all pretty impressive. I have seen your videos and they sound nice. Some details on them would be cool.
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Unread postAuthor: wyz2285 » Mon Jan 30, 2012 2:50 pm

The interesting thing is that differential QDV can open really fast, even if you dry fire it !
And even with low pressures.

I see, but there are other ways that can do the same, I don´t have a drillpress/lathe, so I can´t try this way :?
Except if you use a spring powered hammer, which is what I intend to use to trigger the 50bar version !

Care to be more clear? thanks
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Unread postAuthor: G-man » Mon Jan 30, 2012 2:58 pm

Well, why not, I always tell myself that I should come here more often :P
But I don't have really much to show for now...

Sure, there are many ways.
The possibilities are almost limitless, but I've choosen to use only QDVs and QDV derivatives on my launchers !
I really like this firing system :)

Well, that's not really complicated: at 50bars, it's really hard to open a QDV.
So I just want to build a hammer system (powered by a spring, triggered by a trigger group), which would strike the QDV.

It would only proceed to open the first seat.
Then the blowback system takes over.
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Unread postAuthor: wyz2285 » Mon Jan 30, 2012 3:26 pm

Well, that's not really complicated: at 50bars, it's really hard to open a QDV.
So I just want to build a hammer system (powered by a spring, triggered by a trigger group), which would strike the QDV.

It would only proceed to open the first seat.
Then the blowback system takes over.

What? the idea of a QDV is the piston "dump" when slightly pulled and the balance is broken. A little strick will open the valve fully, reclose it using a spring, it will have to be strong I think. I don´t see how it can be efficient. But I could be wrong, I´d like to see the final product :)
You should take a look at a gun called withoutloss, made by a french too. It uses the balance idea too, but his piston is the bullet.
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Unread postAuthor: G-man » Mon Jan 30, 2012 4:15 pm

Ah I see were's the problem, you're thinking in automatic.
The project I'm talking about will be manual, instead of the one in the video.
It was just a little try, I prefer manual airguns.

So I'm going to use a differential QDV, because of the opening speed, and I'm going to trigger it with a hammer instead of my fingers.
And I'd have to re-cock the hammer and open a filling valve to reset it.
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Unread postAuthor: wyz2285 » Mon Jan 30, 2012 6:04 pm

Ah ok, so it's a single shot, one shot per fill. I guess there is no problem then, good luck with it :).
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Unread postAuthor: POLAND_SPUD » Mon Jan 30, 2012 8:05 pm

damn I was wrong...

the valve is just like technician's QDV at first (so 100% balanced) but as soon as the trigger is pressed air from the chamber pilots the valve... but it's a positive piloting valve (unlike most piston valves - which use negative pressure differential to open)

it's still an interesting design

Isn't it cool that the freench come up with so many original ideas?
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Unread postAuthor: killerbanjo » Mon Jan 30, 2012 8:38 pm

I guess they do but they might look at some of the english ones and think... why didnt the french think of that!

Who knows :D

Also G-man where did you get the female gas piece on the bottom of the gun?
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Unread postAuthor: G-man » Tue Jan 31, 2012 7:50 am

Negative pressure could be interesting to trigger it, but I'm not used to this kind of thing ^^

Btw, I recommand to leave the bigger seal without and o-ring.
Why ?
Because if the smaller o-ring before it is not perfectly airtight, there's a major risk of auto-triggering.
A metal/metal sealing works fine for a blow-back (that's what is used on the vid).

The piece on the bottom is an airsoft refill valve.
I bought mine on Rsov:
http://www.rsov.com/product.php?langId= ... 0&cateId=0
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Unread postAuthor: jakethebeast » Tue Jan 31, 2012 1:31 pm

hmm, i have noticed that bicycle tire valves (not schrader) fit's over GreenGas can's valve like it was made for it :wink:
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Unread postAuthor: Technician1002 » Tue Jan 31, 2012 2:19 pm

He does call it a QDV valve so I think he may have seen my valve for inspiration. Why else would he call it a QDV? Anyway, it is a nice build and seems to work very well. I wonder if it uses floating rings?

If you check his other videos on the launcher (20 of them) he shows it being taken apart. It is a QDV in a T. I think his piston is too heavy as it is very long which gives it more of a chug instead of a pop.

He has a nice copper version with a built in pump. Nice builds.
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Unread postAuthor: G-man » Tue Jan 31, 2012 3:10 pm

This valve was from my imagination at first.
I found the principe by myself, started to give it a name on the french forum... and Al-xg told me that it was already used on spudfiles, and called "QDV" (damn !), so I adopted the name :)
I was so proud of my invention and... nope ! Already done ! :lol:

Yes, it uses floating rings.
I use a heating process which allow me to resize the o-ring diameter so it fits exactly in the tube.

What piston are you talking about ?
The one above: definitly.
It's almost 30cm long, mainly copper... There's friction everywhere and it's kinda heavy.

The one in the PACa Mk.II is not so heavy compared to the diameter of the barrel.
The flow is reduced from 20mm to 8mm, and I shoot heavy steel bb's, so the piston's recoil is really strong at 30bar.
I have to check for damages in the QDV from time to time, to ensure "safety".
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Unread postAuthor: wyz2285 » Tue Jan 31, 2012 3:54 pm

I use a heating process which allow me to resize the o-ring diameter so it fits exactly in the tube.

Really? :o I tried few times didn´t work very well, they still goes back to the original shape a little. Can you tell me a more about your methods? Thanks
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Unread postAuthor: G-man » Tue Jan 31, 2012 4:47 pm

Basically, here's all the process in one picture:

Image

I heat the tip of a little steel tube, which is gonna communicate the heat from the torch to the o-ring, without melting it.
The o-ring internal diameter is smaller than the steel tube external diameter of course, that's the way you can extend it.

Notice the piece of wood under the steel: its role is to prevent the heat to be absorbed by the vice.

All the heat that is not dissipated by the air goes to the o-ring, and forces it to keep a larger diameter.


The hard part is the heat dosage.
Too much heat: the o-ring will stay too large (and eventually will break)
Not enough heat: the o-ring will not be modified, and will go back to its original shape.

They always go back to their original shape a little, but not enough to pass under the wanted diameter, if well heaten. It reduces a little, but after it stays stable.
That's why it's essential to take a little margin to be sure that it will stay airtight in time.


I'm not saying that it is a perfect technic, it's kinda random and I don't know if it lasts "forever", but I've been doing this for more than a year, and I'm really satisfied with it :)
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