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Shear discs?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Shear discs?

Unread postAuthor: D_Hall » Wed Jun 10, 2009 10:37 pm

I'm thinking that for a large bore gun burst discs can be problematic. It occurs to me however, that a solution may be a shear disc instead. More to the point... A disc with a thin permitter and thicker middle that forms the aft end of the projectile. The disc is held firmly at the breech end much as a burst disc would be.

When the gun comes up to pressure, instead of bursting a disc, this disc simply shears around the perimeter.

Make sense?

Anybody else ever try this?

Thoughts?
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Re: Shear discs?

Unread postAuthor: Technician1002 » Wed Jun 10, 2009 10:51 pm

D_Hall wrote:I'm thinking that for a large bore gun burst discs can be problematic. It occurs to me however, that a solution may be a shear disc instead. More to the point... A disc with a thin permitter and thicker middle that forms the aft end of the projectile. The disc is held firmly at the breech end much as a burst disc would be.

When the gun comes up to pressure, instead of bursting a disc, this disc simply shears around the perimeter.

Make sense?

Anybody else ever try this?

Thoughts?


It is used commercially in disposable tanks. They call them rupture disks.
Here is one on one of my cannons.
Image
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Unread postAuthor: D_Hall » Wed Jun 10, 2009 10:55 pm

Not the same thing. Or at least, not as I've ever seen them.

Or rather, in my experience, "burst disc" and "rupture disc" are different terms for the same thing. The only difference is the applications they're used in.
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Unread postAuthor: Gippeto » Wed Jun 10, 2009 11:54 pm

A very interesting idea Dave. The fail pressure should be much more predictable, especially on a "hobby" level of fabrication.

Are you leaning toward a one piece disc? I'm thinking that fabrication would be an issue.

Two piece discs might be made more easily. ie. epoxy a steel (near) bore sized disc to an aluminum disc. The aluminum should suffer a shear type failure.

Just some thoughts. :)
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Re: Shear discs?

Unread postAuthor: Moonbogg » Wed Jun 10, 2009 11:54 pm

D_Hall wrote:I'm thinking that for a large bore gun burst discs can be problematic. It occurs to me however, that a solution may be a shear disc instead. More to the point... A disc with a thin permitter and thicker middle that forms the aft end of the projectile. The disc is held firmly at the breech end much as a burst disc would be.

When the gun comes up to pressure, instead of bursting a disc, this disc simply shears around the perimeter.

Make sense?

Anybody else ever try this?

Thoughts?


You saw that patent to? Yeah it looks like it was designed by a spudder. I was actually thinking of the same thing recently. It would be great to eliminate burst discs and even piston valves by having the round do everytyhing. It would totally work IMO. The only thing is coming up with a good round. Maybe a pellet shaped round with the back end of the skirt oversized would work.
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Unread postAuthor: Gippeto » Wed Jun 10, 2009 11:55 pm

Patent??? What patent??

Link please. :)
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Re: Shear discs?

Unread postAuthor: jimmy101 » Thu Jun 11, 2009 12:23 pm

D_Hall wrote:I'm thinking that for a large bore gun burst discs can be problematic. It occurs to me however, that a solution may be a shear disc instead. More to the point... A disc with a thin permitter and thicker middle that forms the aft end of the projectile.

So the "burst disk" is part of the round, that what you are going for?

So, glue (weld, ...) a ring to the base of the round? Instead of thin perimeter and thicker middle (which would be hard to fabricate) the ring (held in the gun like a standard burst disk) is part of the round. When the pressure is high enough the joint between the ring and the round is broken.

Perhaps something along the lines of the knockouts in household wiring electrical boxes?
Image
(image stolen from http://www.wikihow.com/Remove-a-%22Knoc ... trical-Box )


Overall, kind of going for a high static friction, but low dynamic friction, on the ammo. And, simplifying the burst disk useage by including it as part of the round in a breach loader?

How about the basic shape of a bullet cartridge but with a big hole in base of the casing (where the cap would be). The slug is press fit into the casing. The friction between the casing and the slug is the "burst disk". Once the pressure is high enough the slug exits the casing and proceeds down the barrel against a much lower friction force. The casing would be reuseable just like a rifle casing and the casing rim would be what holds it stationary in the breach.
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Re: Shear discs?

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Freon Tank Rupture disk

Unread postAuthor: Technician1002 » Thu Jun 11, 2009 12:31 pm

I grabbed a camera to show the stamped metal breakaway thin rim on the QDV Cannon. Sorry I didn't get any inside the tank photos of the orifice.
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Disposable Freon tank rupture disk. On the inside of the tank is a second layer with about a 3/32 inch orifice to limit the burst energy. This is not designed to eject the disk, but retain it like a pop can pop top.
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Unread postAuthor: D_Hall » Thu Jun 11, 2009 10:07 pm

Gippeto wrote:A very interesting idea Dave. The fail pressure should be much more predictable, especially on a "hobby" level of fabrication.

Precisely my thoughts. Especially if one annealed the rim.

Are you leaning toward a one piece disc? I'm thinking that fabrication would be an issue.

Fabrication should be completely trivial.

- Start with an aluminum disc; we'll say 1/4" thick.
- Put disc on a rotary table.
- Put rotary table on a mill.
- Use mill to start a groove at the appropriate radius and the appropriate depth.
- Spin rotary table.

Voila, a 1/4" thick plate with a groove cut leaving (say) 1/64" of material around the periphery where you desire it to shear off.







As for the "patent" comment earlier.... No, I've not seen any such patent. And if push comes to shove, I recall having this conversation on this site (or spudtech) a couple years ago but I was too lazy to go searching for it and figured some conversation might be fun.
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Last edited by D_Hall on Thu Jun 11, 2009 11:41 pm, edited 1 time in total.
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Unread postAuthor: THUNDERLORD » Thu Jun 11, 2009 11:26 pm

I had a similar (smaller) idea before,
to buy all pull tab opening soup and canned stuff.
Then open the opposite end with a traditional can opener.

A piece of plywood could be cut to fit ID (maybe there's a hole saw for a drill that would work(?))
Then maybe drill a hole in the wood centered for a shrader.
Screws could go through the can into the wood and some glue to seal it.

No Idea what pressure they will pop open at though (Yet).

Also was wondering before about the top of 12 oz. soda cans too. 8)
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Unread postAuthor: D_Hall » Thu Jun 11, 2009 11:44 pm

Tech,

OK, that is interesting. They're just controlling the energy via an oriface. I'd never seen the backside of one of those so I always just figured they designed 'em a bit thicker than that.
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Unread postAuthor: Technician1002 » Thu Jun 11, 2009 11:49 pm

D_Hall wrote:Tech,

OK, that is interesting. They're just controlling the energy via an oriface. I'd never seen the backside of one of those so I always just figured they designed 'em a bit thicker than that.


As far as I can tell the burst disk is about as thick as the tank with a weakness stamped into the rim much like the pull tab on a pop can so it bursts first. Behind it the orifice is there I presume to keep the tank from becoming a missile when it lets go. It's my best guess. If I ever cut into one of these tanks again, I'll be sure to grab photos.

@ THUNDERBIRD
Also was wondering before about the top of 12 oz. soda cans too. Cool


There is a good possibility the entire top of the can may come off. It's just crimped into place. Attached is a photo of a can that hit a tree at high speed. The tab is intact, but the can separated from the top. The other photo is a can that was launched 300 yards onto a football field. It landed sideways and blew off the top. Again the tab is intact.
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Shasta can after a 300 yard flight.
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Unread postAuthor: THUNDERLORD » Fri Jun 12, 2009 12:22 am

...There is a good possibility the entire top of the can may come off. It's just crimped into place. Attached is a photo of a can that hit a tree at high speed. ...


Wow, 300 yd.s soda can! Wonder what launcher that was shot from?

Yeah, with the soup/ other can tops, I was thinking of the entire can being part of a cartridge.
But with the soda cans I was thinking for smaller diameter, and cut the tab out with aviation snips, then sandwich it between barrel and chamber is all.
Seems like too much work, but they do appear pretty hefty from the looks of those damage pics! 8)

EDIT: Another "one time at band camp story"...But when i was a kid at summer camp, we used to put canned stuff on the hot coals for a practical joke and leave...a few times it really was an UNpractical joke though.(dangerous) :twisted:
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Unread postAuthor: benstern » Sat Jun 13, 2009 1:15 am

Please tell me this is relation to your project at china lake!

PLEASE!!!

You have not given any info about it for a long time. Just tell me our hope for a huge hybrid are not gone with the wind.
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Unread postAuthor: D_Hall » Sat Jun 13, 2009 10:53 am

benstern wrote:Please tell me this is relation to your project at china lake!

It is.

You have not given any info about it for a long time. Just tell me our hope for a huge hybrid are not gone with the wind.

As of yesterday, my lead welder is guessing 3 more weeks until primary structures are completed. Of course, there's still a LOT of plumbing and such remaining....


I will say, however, that for fun I crawled through the length of the combustion chamber. Very odd sensation to realize WHERE you are. :)
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