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Another burst disk cartridge

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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Another burst disk cartridge

Unread postAuthor: daccel » Sun Dec 07, 2008 11:56 pm

Inspired by jsr's idea http://www.spudfiles.com/forums/burst-disk-cartridge-idea-t10647.html, I thought I would start my own thread as I will hopefully be able to give it a go over Christmas.

I was perusing the plumbing isle today and realized flare fittings might make good burst disk holders so I picked one up. This is just thrown together with the limited parts I had on hand, but it appears to hold pressure with a disk made from a piece of coke can.

The only modification required is to drill out the cap to the barrel size and the other flare fitting to the barrel inside diameter. The little barrel there isn't the right size, I just used it to see how hard it was to puncture the disk.

Any input on the viability of the flare fittings?
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Unread postAuthor: mega_swordman » Mon Dec 08, 2008 12:15 am

I don't see much of a difference as far as performance goes. It is just a different fitting. However, I think this will work well for a burst disk holder because of its shape.

Also, concerning your last two pictures, are they of the burst disk?
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Unread postAuthor: daccel » Mon Dec 08, 2008 12:24 am

Yes, that's the burst disk, it's only about 3/8" wide. Just wanted to show how evenly it bent.
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Unread postAuthor: psycix » Mon Dec 08, 2008 11:29 am

Nicely done. (good camera btw)
Maybe a coke can is too thick for such a small diameter.
Tried it with aluminium foil yet?

How much pressure did it held?
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Unread postAuthor: daccel » Mon Dec 08, 2008 9:52 pm

I believe my bike pump goes up to ~160 psi, though the gauge broke off the first time I used it. You get what you pay for :roll:.

The can material may prove too tough, just thought I'd try as fiddling with a bunch of tiny pieces of foil wasn't appealing.

Edit: Some progress, see pictures. Pretty self explanatory. The caps aren't drilled out because I don't have a big enough bit on hand. The bushing will have a check valve inside. The fittings will be epoxied in.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 1:11 am

Looking very good, my one concern would be the excess weight if you come to making a self-cycling system.

Also, you definitely need a thinner burst disk, at 6mm I found that a single layer of simple adhesive tape will hold in excess of 350 psi.
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Unread postAuthor: daccel » Tue Dec 09, 2008 2:04 am

Good point. I was planning on self-cycling.

If I keep the length the same with no couplings or caps and replace the bushing with the (lighter) 1/4"f end of the flare fitting I cut off, it drops the weight from 213g to 114g.

Should have thought of that - I let aesthetics get the best of me :oops:. But no use in a pretty paperweight.

That would also allow for a series of couplings to form the breech. I was scratching my head over how to make one the right size.

Good to know on the burst disk material. At least that's easy to adjust once the rest is designed and built.

Edit: grammar
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Unread postAuthor: jimmy101 » Tue Dec 09, 2008 5:29 pm

Looking very promising, and elegant.

Like Jack said, as the diameter of the burst disk goes down the burst pressure will probably go up. Since most burst disk data is for diameters of 1 to 2 inches I think there is a good chance that something thin like Al foil or plastic wrap might work well for this small of a burst disk.

Wouldn't the burst pressure of a particular disk material scale as something like the ratio of the area to the diameter? That would be perhaps
(Pi)(r<sup>2</sup>)/2Pi(r) = r/2.

So 1/4"D disk would burst at a pressure about 8x higher than a 2"D of the same material?
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Unread postAuthor: daccel » Sat Dec 13, 2008 1:10 am

Sounds like the disk will have to be much thinner, but there should probably still be somewhat of a safety factor to prevent accidental firing.

I was going to work on the breach this weekend, so hopefully I can get some input on the layout pictured here. Specifically, how far forward or back relative to the magazine to position the ejection port.

Not asking for hand-holding, just trying to minimize the number of versions I have to try to get this to work. Still have to buy christmas presents :).
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Unread postAuthor: psycix » Sat Dec 13, 2008 2:21 pm

daccel wrote:but there should probably still be somewhat of a safety factor to prevent accidental firing.

What has a thin disk to do with accidental firing?
The only thing I can imagine is a thin disk being damaged faster, so it will be more prone to leaking and will have to be handled carefully.
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Unread postAuthor: Gippeto » Sat Dec 13, 2008 3:21 pm

For calculating burst pressure of a disk.

http://www.ansys.com/events/proceedings ... RS/227.pdf

Doesn't help much unless you have K.
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Unread postAuthor: daccel » Sat Dec 13, 2008 4:48 pm

psycix - I mean that because it isn't ruptured like other burst disks, it has to stay in a pressurized state for a period of time. If it's right on the threshold of bursting, disk or clamping inconsistency could cause one of the disks to prematurely fail. So it makes sense to go thicker than the bursting pressure by some amount, as long as it doesn't require too heavy a spring that prevents it from cycling.

Gippeto - Does that formula mean that there is a direct relationship between thickness and pressure? So if you doubled the thickness the pressure would double?
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Unread postAuthor: Gippeto » Sat Dec 13, 2008 5:14 pm

For a fixed diameter and tensile strength, yes.

Seems reasonable, I only wish k could be calculated. It seems to be a constant derived by experimentation.

If you ignore the fact that the disk will stretch and get thin in the middle, you should be able to calculate the approximate shear failure force using area (pi*d*thickness) and tensile strength in psi. Ff=ts*a

Then calculate the pressure at which the failure should occur. Pf=F/(pi*r*r)

Perhaps using yield strength in place of tensile strength would compensate for disk stretch, and give a reasonably accurate burst pressure?

Or average the two?

Edit:

My bad. Use shear strength in place of tensile strength. (Use 60% of the tensile strength.)

Some info on "typical" aluminum foil;

http://www.matweb.com/search/DataSheet. ... 30305a3ba6
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