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I finished a semi automatic piston valve. The power is not good (probably 100 fps at 100 psi) but the concept works and I think with a couple changes I think I could get near 300 fps. The huge potential in this design is simplicity. The bolt both fires and loads which I'm a big fan off due to reliability.
Most of the air dumps out the pilot for these reasons (tell me if this sounds right):
1. The piston weighs a ton because it has to move the whole barrel along with it. Not very efficient but a major part in simplicity. Need suggestions...
2. The piston's diameter is too close to the diameter of the barrel. This was discussed in some other thread. I think we decided that the a small difference in diameter means it's less likely that the piston will open but if it does, it will happen faster. This problem is fixable with a bigger piston.
When you fire, there's a huge delay between when the pilot dumps and the gun shoots. This could be solved with springs pulling the barrel back but I think there are deeper rooted problems. The next design will have a bigger piston unless you guys have some different suggestions.
thats very unique i look forward to seeing this thing develop in to a more powerful and efficient monster.
You could make a lightweight sleeve to connect to the piston instead of the whole barrel. The sleeve would slide over the barrel.
This would make it much lighter (faster opening time) and make the barrel stationary (better accuracy).
It is a good design. It's sort of like an open bolt launcher. Nice job.
do I get it right? the valve fires through those holes?
If it's true then my question is - are they big enough?
I think that hubb017 is right it will help but first check if the holes are not too small
The holes should be big enough. Right now, it's not flow constriction that's the problem. The problem is delay.
I think sleeving is the best solution but the goal is to keep it simple. I'll tinker around with a new design and try it out.
Does anyone think the piston should be bigger?
Definitely. I didn't mention making the piston larger because you did. Also, make it as light as possible. There really isn't much you can do about piston travel, due to the ammo and all, so that's about all I could suggest.
Seems a little familiar, n'est pas?
props to KIR for making it work, some videos would be lovely
Galfisk from the Spudtech forums built a similar mechanism with the bolt and piston linked.
Planning stages: http://www.spudfiles.com/spudtech_archi ... hp?t=11200
After it was built: http://www.spudfiles.com/spudtech_archi ... hp?t=11396
Unfortunately the pics are broken.
Copperboy made a larger version of basically the same mechanism Galfisk used. Pics work on this one.
http://www.spudfiles.com/forums/marble- ... 10142.html
As for taking so long to fire. The closer your seat is to the side of your piston, the better your piston fit needs to be in order to not leak a bunch of air from the chamber out of the pilot. So yeah making the piston bigger would be one way to fix it, but it may also be able to be fixed if you can figure out a way to put o-rings on the piston. Although given the extra weight and friction on the piston because of the bolt/barrel it may be better to just make the piston bigger.
Nice work on the gun.
<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult
I was wondering why I hadn't seen something similar. Apparently I'm not keep up with the Jone's these days.
The delay needs to go before I can say it works. Every once in a while I'll get a shot out of the blue that goes fast. It makes me very happy. I'll take some video tomorrow. JSR do you have any suggestions for improvements?
You definitely need a bigger diameter piston, also instead of moving the whole barrel, consider just moving a bolt as in the diagram above.
I'm assuming your diagram is in a closed position.
When it fires the bolt is back. Wouldn't that let air into the clip? Or would it have let out all the air by then?
I thought of that when I saw this thread.
In the diagram, it's theory works just like your design. As the piston moves back, the bolt moves with it. The magazine is sealed by the front of the bolt, allowing air to only exit the barrel.
As the man says:
I'd draw it quickly in the open position but I have four simultaneous assays distilling at 200ºC to look after maybe when I get home.
Yeah, I was going to say bigger piston and sleeve.
Since your piston is travelling the same distance as diameter(round shot)
But I was certain some more knowledgable members would pick up on this interesting concept.
Dang it's good to see JSR back!!!
Good to be here I can tell you at least now I have a PC at home I can browse through this forum without looking over my shoulder and pretending to work
Interesting to note that in spite of my hiatus, I'll still the top poster hehe
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