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Piston ideas

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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Piston ideas

Unread postAuthor: CpTn_lAw » Mon Apr 23, 2012 10:47 am

Hey guys,
I've been redesigning my old pneumatic piston rifles, and played with two designs, a 6mm coaxial (barrel) and a 3/4" barrel sealing piston in a Tee.
Now, I've been thinking, as I don't have commercial QEVs, how could I improve the overall performance of such valves?
I've drawn a few diagrams to show you, and hopefully you can think about them and post comments.
On the first diagram are two drawings:
The first one would benefit from the piston momentum to open much more rapidly. In the second one, the exact opposite happens, the sealing face enters the barrel, as opposed to the first where the barrel enters the piston.

Image


Now here is another diagram. This one is supposed to act as a one-way valve. When air enters, the rubber flexes and lets it in the chamber. When you activate your exhaust system, the sides get blocked by air pressure ensuring no air is lost behing the piston. Full force act on it and let is slide very quiclly back.

Image

What do you think?

(Forgot to put a legend in my drawing, bad ! self explanatory!!)
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Unread postAuthor: jackssmirkingrevenge » Mon Apr 23, 2012 11:35 am

I like the first one. Second one simply chokes flow.

Third one looks fairly standard :)
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Unread postAuthor: CpTn_lAw » Mon Apr 23, 2012 1:48 pm

How is the third one standard? It appears to me as a optimised one!
The first one of course you'd need to round the outer barrel, and flare the piston a little to ensure little wear.
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Unread postAuthor: jackssmirkingrevenge » Mon Apr 23, 2012 1:56 pm

Standard in the sense that it's exactly what you would find in a commercial QEV:

Image

Note that in this case it's all rubber, not backed up by a solid piston.
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Unread postAuthor: CpTn_lAw » Mon Apr 23, 2012 2:08 pm

Oh right i see... I didn't know that :D
I'll try eventually the first setup and compare how it performs with the Tee design. I need a new sealing surface, tennis ball rubber isn't all that good.
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Unread postAuthor: sagthegreat » Mon Apr 23, 2012 3:47 pm

How many psi could you take those cheap qev's tp
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Unread postAuthor: CpTn_lAw » Mon Apr 23, 2012 4:30 pm

The homemade ones, using epoxy piston, I have put up to 400 psi in them. The latest one with an almost airtight piston was really loud at such pressure.
Has someone ever tried the first design?
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Unread postAuthor: Technician1002 » Mon Apr 23, 2012 7:12 pm

In your first drawing, there are two problems with it. The concept of a running start for fast opening is negated by the increased mass of a much larger piston. This high mass piston will require a longer and much more substantial bumper to safely slow it back to a stop at the end of travel.

The much shorter and low mass pistons are a much better choice. By using a short fat chamber and a close ratio piston, a very short piston movement can provide a valve that can fully open in under 3 ms with very good flow.
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Unread postAuthor: CpTn_lAw » Tue Apr 24, 2012 3:03 am

That was a little bit what bumped me, I thought that the increased speed would be a good thing. Plus, there should be a way to make the piston out of some extra light yet very resistent material, wouldn't it be possible?
I am aware small pistons can open very fast, GGDT predicts 1-2 ms opening time for my small coaxial... but yet the sound is not as sharp as my Tee piston. with the Tee, when I pilot the piston valve, the sound is like with a burst disc, whereas my coaxial does quite a ball valve sound...
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 24, 2012 3:37 am

Sound schmound, ears are not a chrony ;)
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Unread postAuthor: CpTn_lAw » Tue Apr 24, 2012 4:19 am

Yeah I know that lol :D
The general feel of the valve gave me an impression of slow opening in the coaxial. However performance was great, about 1000 fps with a 0.24g BB at 400 psi.
It would be interesting to have a commercial QEV however i've been searching and couldn't find a retailer here in france.
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 24, 2012 7:34 am

CpTn_lAw wrote:It would be interesting to have a commercial QEV however i've been searching and couldn't find a retailer here in france.


surely you've seen this thread? http://www.spudfiles.com/forums/semi-au ... 23626.html

Besides, what's an accomplished hybrid builder like you doing indulging in such pnactivities :roll: :D
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Unread postAuthor: CpTn_lAw » Tue Apr 24, 2012 8:48 am

I had forgotten about that topic!
Still, I don't want to make a paypal account, and no France retailers sell these...

And i'm doing pneumatic activities again for several reasons:
1) Less noise
2) faster (ROF wise)
3) I don't have the pieces to mod my bike pump (quick disconnect § braided tubing)
4) I'm gathering all the money I can get to buy my self a GTX 680 (GPU)


Still, been able to remake the sealing face of my piston, no leaks up to 400 psi wich is my maximum.


And never forget : Never forget your roots!!! We were born in pneub section, risen into hybridists but from time to time it's good to come at your mamma's home!!!
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