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auto piston v3.0

A place to ask general spud cannon related questions.
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Unread postAuthor: SPG » Mon Dec 31, 2007 8:28 am

Hotwired wrote:No, it might be hard to see but the bottom piston has been slimmed down so a positive pressure is still upwards (after the pressure overcomes the spring anyway).


OK still being dumb here, and desperately trying to remember my schoolboy physics from 23 years ago (that ages me), but surely that doesn't make any difference?

Can anyone explain to me the physics of why this moves?
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 31, 2007 8:33 am

The top piston is wider than the lower one, so the air is exerting more pressure upwards than it is downwards, so logically the piston moves up...
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Unread postAuthor: Skywalker » Mon Dec 31, 2007 9:53 am

JSR, I think I see what Hotwired is saying:

As your mod w/o the sliding seal begins a cycle, the rod starts to move upward (relative to your picture). The sliding seal at the top will be part-way open before the bottome seal engages and stops the airflow in from the supply. So then the sliding seal will stay cracked open, 'farting,' with the pressure in the firing chamber being kept up by the open fill valve.

That won't happen with the design with a sliding seal at both ends. Make the length of the rod right, and the two valves can't be open at once, so it won't hang there and leak.
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Unread postAuthor: SPG » Mon Dec 31, 2007 11:23 am

Ahhh, I see, all is clear (I found my old "Revise O-level Physics" book.

On JSR's latest version of this a way round the "farting" would be to have a spring between the two movable seals, so that the inlet could seal before the high pressure inside the chamber opened the outlet. Like so:
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Unread postAuthor: POLAND_SPUD » Mon Dec 31, 2007 12:52 pm

JSR, I think that 2 or 3 rounds per second can be easily achieved with a semiauto.... all you have to do is to pull the trigger fast enough.. :D

What caliber are you planning to use ? 1/2" would be quite powerful but 3 rounds per/second would waste a lot of air :shock:

Ok. I've managed to find the manufacturer of these valves.... you can find different models and some info here http://www.azpneumatica.com/azweb/ingl/integrus.htm
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Unread postAuthor: Hotwired » Mon Dec 31, 2007 6:18 pm

I bashed about in GIMP for a while to make some demos.

ImageImageImage
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Unread postAuthor: VH_man » Mon Dec 31, 2007 7:02 pm

JSR, what about that version of the eleectronic valve that took advantage of a larger surface area in the rear???

to me thats fail-safe......
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Unread postAuthor: MrCrowley » Mon Dec 31, 2007 7:19 pm

That third one of Hotwired's looks good.

But maybe a smaller area for the bottom piston so there's less resistance.
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Unread postAuthor: Hotwired » Mon Dec 31, 2007 7:22 pm

MrCrowley wrote:That third one of Hotwired's looks good.

That IS my one :lol:

The other two have issues and I animated them to demonstrate.

It's all just a demo diagram, certainly it might be best not to have such a small area difference for a prototype.
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Unread postAuthor: MrCrowley » Mon Dec 31, 2007 7:35 pm

Hotwired wrote:
MrCrowley wrote:That third one of Hotwired's looks good.

That IS my one :lol:

The other two have issues and I animated them to demonstrate.

It's all just a demo diagram, certainly it might be best not to have such a small area difference for a prototype.


Yeah thought so, I read back and managed to match up posted designs with the first two animations and I now I understand it all.

Yours seems to be perfect, bit of fiddling around to get it to be perfect may be necessary, but no reason why it can't work.
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Unread postAuthor: POLAND_SPUD » Mon Dec 31, 2007 7:44 pm

yeah it might work... but someone has to build this and check whether it really works....

Jsr could have build a working prototype using a motor that would move the valve, which would let air at adjustable intervalves.... but that would complicate the whole thing......

I still think that first it would be a better idea to build a powerful semiauto gun.... like ragnarok's HEAL but in semiauto and with detachable magazine.... :D
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Unread postAuthor: Hotwired » Mon Dec 31, 2007 8:28 pm

JSR wrote:There is however the difficult of lining up two concetric pistons for a friction free setup in the absence of a machine shop


To further simplify it, it can just all be setup in one tube. No enlarged chamber section, just one longish floating chamber between the pistons.

A reducer halfway down would allow for the different piston diameters, a tee or a tapped pipe wall for air in and out.

Pistons complete with connecting rod could be cast inside the same pipe it was to run in.


10h shift tomorrow :(

No. Make that today.

I'd better make the most of the 5h's of sleep opportunity I have left. Night.
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Unread postAuthor: SPG » Tue Jan 01, 2008 5:58 am

No I don't think the valve would lock up at that point, it might if you had it looking exactly like it is in my picture, with a loose tension spring, but that wouldn't actuallly be the idea. The simplest would be to use a long tension spring and bend it to form the whole of the connecting rod. Of course it could be modded with two tension springs pulling rather than the one pulling and the compression spring pushing.

The one concern I'd have with any version which relied on sliding pistons so much would be the sealing and the lubrication. To seal well it needs to be a tight fit, but too tight and it won't slide easily. That's why I prefered JSR's version (but with the spring in between).
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Unread postAuthor: Hotwired » Tue Jan 01, 2008 5:23 pm

Oh come on. Pour epoxy into a polished and greased copper tube with a couple of greased o-rings suitably placed and when it sets it'll be pretty hard to get a better fitting sealed piston without machining. Lubricate it as you would any sliding mechanism and it'll be fine.

With some modification extra springs could work but you need to know the strengths of the springs to match the pressure they have to work against.

I deliberately design for simplicity on the grounds of reliability. It's very hard to get more simple than a rod with a fixed piston on each end plus one spring. It also doesn't matter what pressure there is and will function with any strength spring that can push back the unpressurised pistons. But if you can get a stronger spring it will allow a higher pressure buildup before dumping the chamber.

Anyway, given my tuppence's worth of ideas to the JSR semi-auto foundation :)
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Unread postAuthor: Ragnarok » Tue Jan 01, 2008 6:23 pm

Whoops, forgot to make my diagram, but I'll get back on it - may nick something from Hotwired's design that I like.

People are forgetting what makes piston valves so blazingly fast, and gives them their performance...
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