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auto-piston v4.0 - the bullpup

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 02, 2008 1:51 pm

Not for want of trying :p

I'm with the externally cocked hammer at the moment.
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Unread postAuthor: SPG » Wed Jan 02, 2008 1:57 pm

Have I seen this?
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Unread postAuthor: Hotwired » Wed Jan 02, 2008 1:59 pm

Sounds like a reinvention of the back end of a revolver :P
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Unread postAuthor: POLAND_SPUD » Wed Jan 02, 2008 2:07 pm

wow SPG I think your design solves ale the problems JSR had.... it's perfect... becase the air ram that moves itself (and the bolt & barrel too) to load new round (and close the air inlet) actually vents itself after firing allowing the whole cycle to repeat... and it's not as complicated as my ideas that I had... I think JSR or you should try it...

it's the best design so far... and the one that makes sense
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 02, 2008 11:40 pm

SPG wrote:Have I seen this?


Image

pros: allows for external cocking (cue the dramatic *click CLICK* - many coolness factor points) and an easy trigger.

cons: probably doesn't offer as good a flow as the autopiston designs

... or is a stepped piston the solution? decisions, decisions...

How about this campers, do you think it will work without the need to block off the air inlet?
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Unread postAuthor: SPG » Thu Jan 03, 2008 5:51 am

I think I have the solution to calm Skywalker's worries and up the power a bit, and it is this:

Image

An oil filled damper, but not just any old oil filled damper it's a variable rate damper from Tamiya - oh yes it is and I just happen to have four of them lying around. Why is this the solution? Because it has tiny one way valves inside it which open on the compression stroke allowing oil to freely pass, but close on the extension stroke, slowing oil, and slowing the extension.

Now imagine this in the action I posted above.

As the pressure increases inside the chamber piston moves forwards and the damper compresses, the gas port then closes, but the piston/damper continues to compress under the pressure of gas in the now sealed chamber. The BB port opens and the BB begins to travel down the barrel. At this point the chamber pressure begins to decrease, and the piston begins its rearward travel. BUT this travel is greatly slowed by the damper now having to extend, so that the chamber can depressurise much more before the BB port closes.

See, more gas in the chamber gets used (so less gas is needed); the chamber vents better so the return spring force doesn't have to work against a partially pressurised chamber; and the system is more likely to fully cycle and not hang.

A damper.. simple hey?
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 03, 2008 6:37 am

Sounds like a plan, plus you'll still have enough left over for an R/C Robin Reliant :D
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Unread postAuthor: Skywalker » Thu Jan 03, 2008 9:02 am

Go for it, SPG! I personally dislike the idea of adding resistance, even controled resistance, to the action, since we are working with so little power to start with. But it might be just what you need.

@JSR: you know, that stepped piston might just about do it. You probably want to make sure the barrel entrance is a bit lower in your diagram, so that no air gets trapped when the piston starts to close. Also, you might find that the air flow has to be limited to a certain amount to prevent 'farting'; so maybe the fire rate will have an upper limit.

Strange: that thing is basically a pop-off valve, isn't it?
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 03, 2008 9:08 am

You probably want to make sure the barrel entrance is a bit lower in your diagram, so that no air gets trapped when the piston starts to close.


Yeah, the diagram was just a rough concept.

Also, you might find that the air flow has to be limited to a certain amount to prevent 'farting'; so maybe the fire rate will have an upper limit.


Not a problem, I don't want more than 2-3 rounds per second

Strange: that thing is basically a pop-off valve, isn't it?


Pretty much, I'm hoping it will open quick enough and stay open long enough for a decent shot at 100 psi for a 6mm barrel.

I'm settled on this design at the moment, my only dilema is whether or not to add the flow restriction at the air source to shut off the flow to allow sufficent pressure drop for the piston to reset.
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Unread postAuthor: Skywalker » Thu Jan 03, 2008 9:16 am

Isn't there any way to make it adjustable so you can tinker with it? I've seen some adjustable 'flow regulators' at a hardware store, I wonder if that'd work? You might not be able to find them in the UK, though; you know how that goes.

Or are you trying to figure whether to put the restriction before or after the trigger valve?
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 03, 2008 9:28 am

I would certainly like the spring pressure adjustable on the piston. What I meant was do I make it as per the diagram, or do I include the below modification?
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Unread postAuthor: SPG » Thu Jan 03, 2008 11:36 am

Skywalker wrote:Go for it, SPG! I personally dislike the idea of adding resistance, even controled resistance, to the action, since we are working with so little power to start with. But it might be just what you need


But are we working with little power. I'm talking about a gun at 110psi so the main return spring will have to be pretty strong. Also the point of a damper is that it allows the system to fully return, it just does it slowly. I'm now off to study Hooke's Law fully, in relation to pressure so I can work out the piston size I want in relation to the spring on one of those Tamiya shocks, after all I may as well use the spring too.
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Unread postAuthor: Skywalker » Thu Jan 03, 2008 12:46 pm

SPG: Ok, point taken.

JSR: Here's the way to look at it: think about a pop-off. You have an opening pressure, say 100psi, and a closing pressure, say 50psi. The valve will hang if the airflow from the supply (say @ 110psi) into the firing chamber is large enough to keep the firing chamber at 50psi with the barrel wide open. If the flow from the supply is less, then the piston will start closing, and once it plugs the small stepped passage, the remaining pressure on the piston will vent out the barrel, and the piston will close fully; assuming the piston hasn't covered the passage to the breach.

If you have a small valve that gets poked open by the stepped piston, then the flow rate during firing is always zero, so the piston will never hang. If you have a needle valve or similar, then you must keep it below a certain flow rate, probably less than your barrel permits at the same pressure (but depending on the design of the stepped piston). If you do that, the piston will never hang.

Looks to me like you can do without the 'poke-open.' It also looks like this design is a real winner.

I'd say that assuming your opening and supply pressures are about twice your closing pressure, then as long as the airflow from the source is limited to a bit less than the flow through your barrel, the gun will operate without that little valve.

Edit: for that matter, if you don't have to poke that valve open, you might get away with a face seal instead of a sliding seal to plug the smaller stepped opening, basically like Rag's original idea with a flow rate control.
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 03, 2008 2:55 pm

if you don't have to poke that valve open, you might get away with a face seal instead of a sliding seal to plug the smaller stepped opening, basically like Rag's original idea with a flow rate control.


My thoughts exactly, now to find enough alone time with my workbench *sigh*
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 04, 2008 3:52 am

this is how I envisage it in practice:
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