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Unreged co2 - lathe - Doubt about materials and wall thickne

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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Unreged co2 - lathe - Doubt about materials and wall thickne

Unread postAuthor: apocavorme » Sun Apr 22, 2012 8:41 pm

Hello spudgunners!
I would like to build a gun to use unregged co2, ggdt simulation bellow;

I have no access to metal pipes, metal fittings, cpvc, 6061 aluminum, just the 2 materials bellow:

-304L steel block.
-6351 aluminum block.

The price of 304L is 2X the price of aluminum.
I made a draw of the piston valve:

I´m afraid to use that formula( P = (2*S*T)/((O.D.-2*T)*SF)) to calculate wall thickness... this formula is meant to be used with pipes, and the geometry of a piston valve is diferent. I dont know how to use software like solidworks, so i can not simulate it, but i can draw it in auto cad.

So my question are:
-what is a safe wall thickness to be used with each material (something that its built and did not explode :) )

-how deep the threads needs to be (in the drawing it is 3mm, some places 2mm)

in the drawing i used 12mm wall in chamber and qev (cos it need to hold pressure) and 8mm in barrel and blow back cos it will hold pressure for a short time. These values are totaly arbitrary and massive. It is also missing the detent of the bullet and the detent of bolt.
Sorry for bad english and for bad pics... and thank you!
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Unread postAuthor: Lockednloaded » Sun Apr 22, 2012 8:50 pm

I don't think your valve will open very well as drawn, you need a stepped seat to get it to pop open effectively
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Unread postAuthor: apocavorme » Sun Apr 22, 2012 9:13 pm

you mean something like this?
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Unread postAuthor: Lockednloaded » Sun Apr 22, 2012 9:15 pm

much better, the air needs some surface are to push on to initially crack open the valve. On another note, you've got all the o-rings in your design, why not put a floating o-ring on the piston body?
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Re: Unreged co2 - lathe - Doubt about materials and wall thi

Unread postAuthor: velocity3x » Sun Apr 22, 2012 9:51 pm

apocavorme,
You might want to rethink your design. I think your piston O-ring will suffer explosive decompression using unregulated pressure the first time the gun is fired.
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Unread postAuthor: apocavorme » Sun Apr 22, 2012 11:11 pm

it will be much better if i could do this, but i ll have to build a check valve in the piston, or between the pilot and the chamber... the pieces will be small and the check valve seal will be a problem, cos i cant find a o-ring that small; if i use a piece of rubber, i think it will be a problem too, cut it prerfect and fit with a screw (the head need to be very small), and the rubber can find is not very good... when the hole thing is working, with a simple piston, the next step will be build this check-valved piston.

When i made the draw, i was inspired by this:
http://www.spudfiles.com/forums/viewtopic.php?t=160&
In the schematic draw, the seat is not stepped, but i dont know in fact if it is or not; i agree with your logic, there needs to be surface area to be the force, but what would happen if not? not open? slow open? once it begins to open the area exposed to pressure increases, so the force is big, and the open is not slow. but wil it open?

Thank you for comment!

-----

velocity3x

i dont understand, could you explain more?
i´m thinking in 1000psi, co2 at 80F.
thank you for the help!
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Unread postAuthor: Technician1002 » Sun Apr 22, 2012 11:18 pm

I second the current design having single shot o rings. Other worthy mention is the chamber size is too big due to the expected expansion of the gas. The extra volume is well into the range of diminishing returns.

Play with chamber volume vs power and gas consumption.

On your orignal drawing, the piston is stepped. The O ring moving seal is a smaller diameter than the piston in the Pilot. These are the two diameters used to calculate the forces on the piston. When the force from the chamber forcing the piston into the pilot is greater than the force pushing the pistion into the valve seat, it will open. Force is a function of pressure times area.

With the pilot under pressure the force pressing on the valve seat is high. As the pilot pressure is vented toward zero, the force is removed from the seat and depending on the ratio, a point is reached where the force pressing the piston into the pilot is greater then the force holding it closed. This only happens when the diameter of the seat is smaller than the diameter of the pilot end of the piston. If they are the same diameter, there will be no opening force. My valve requires a mechanical force to open it due to the 1:1 diameter ratio.
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Unread postAuthor: velocity3x » Sun Apr 22, 2012 11:25 pm

apocavorme wrote:velocity3x
i dont understand, could you explain more?
i´m thinking in 1000psi, co2 at 80F.
thank you for the help!


Here's a link explaining EXPLOSIVE DECOMPRESSION of orings: http://www.rlhudson.com/O-Ring%20Book/d ... uses5.html
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Unread postAuthor: apocavorme » Sun Apr 22, 2012 11:40 pm

Technician1002, so the original draw will open without problems?
About the chamber you mean use a volume that alow a bigger pressure drop in the chamber to save gas?
Sorry, my english is bad, i dont understand what you say about single shot orings; thanks for helping!

velocity3x
i read this about explosive deconpression, is that what you said?
http://www.rlhudson.com/O-Ring%20Book/d ... uses5.html
the problem is that when i decompress the gun firing it the o-ring will explode... so i have to use lower pressure or use polyurethane o-rings or put specific oils in it. will it solve the problem?
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Unread postAuthor: velocity3x » Mon Apr 23, 2012 12:38 am

apocavorme wrote: so i have to use lower pressure or use polyurethane o-rings or put specific oils in it. will it solve the problem?


It may slove the problem, but I doubt it. Co2 is the biggest offender for explosive decompression. Can you switch to nitrogen?
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Unread postAuthor: Crna Legija » Mon Apr 23, 2012 3:05 am

looks like you gonna machine that so id suggest go with a plastic piston something like Teflon,nylon and HDPE and a lapped seat it will diffidently seal that that pressure.
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Unread postAuthor: apocavorme » Mon Apr 23, 2012 9:55 am

i could use nitrogen, did not buy the tank yet; :)
i´ll search for teflon and hdpe to buy, nylon i know that i can find. a nylon piston without o ring will seal? will it stand the impact when the valve opens?

And what about wall thickness, i´m afraid... will it explode like a grenade? :)
Thank you all again for the help!
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Unread postAuthor: pneumaticcannons » Mon Apr 23, 2012 10:12 am

a nylon piston without o ring will seal?

If you square it next to perfectly it will with the help of a bit of lubrication :wink:
will it stand the impact when the valve opens?

*very* thick o-rings or even rubber washer will work as bumpers.
And what about wall thickness, i´m afraid... will it explode like a grenade?

You'll have to get the math aficionado's over here to figure that out for you :)DYI
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Unread postAuthor: Goats spudz » Mon Apr 23, 2012 1:09 pm

If you use a nylon piston, at high pressures it will seal but you will have to have perfectly flat sealing faces or 60 degree ones also if they dont seal you can lap them with lapping compound, About wall thickness my 3mm thick mild steel tube (Seamless) has taken 1500 psi of air and been hydro tested to 5000psi in the hose tester at work.
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Unread postAuthor: apocavorme » Thu Apr 26, 2012 1:46 pm

hi guys,
i found ptfe (teflon), nylon (polyamide), and did not find hdpe; i rethink the design to get more shots, so i´m thinking in 400 psi and 100cm^3 chamber (but i also want to use high pressures sometimes...) i think a 20oz co2 paintball tank could give me arround 90 shots at 400 psi; but will it seal with ptfe or nylon at that pressure? if i use a pump to put the air in chamber, a teflon or nylon piston could be used? (the pressuer will increase slowly)

I also discovered that can only buy a 3´´X 3´´ block of aluminum, so i will have to redesign the valve, wall thisckness will be 6mm...

An autocad model is under construction, when ready i ll post here!

Thank you all for help!
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