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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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Unread postAuthor: D_Hall » Sun Aug 02, 2009 5:05 pm

Well, last week I made the decision: The system is going to use a shear disc and not a burst disc. Basically it looks like I'll be going with 6061 T6 machined to about 0.028" thick around the rupture line.

With this in mind, I've also instructed my crew to manufacture what I'm calling a "shear ring" for the breech. Basically, it's a glorified washer, but the purpose will be to provide a nice knife edge (even if it isn't particularly sharp) to back the shear disc and increase stresses in the machined shear groove.
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Re: Shear discs?

Unread postAuthor: Technician1002 » Mon Aug 03, 2009 5:38 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?


Disposable freon tanks use these, but have a section where the disk is retained instead of ejected completely. The freon tank on the inside has a flow restricting orifice, so when blown open the orifice is about 1/4 inch in diameter.

I retained this disk on my T Shirt cannon, but it's highly unlikely to pop at any of the pressures I am using.

The thin stamped ring at the edge of the disk is easy to see.
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Shear disk on the T shirt launcher. This is the original Freon tank over pressure protection device.
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Unread postAuthor: Hotwired » Mon Aug 03, 2009 6:24 pm

Burst disks for all kinds of applications have been using scored, stamped, laser cut etc patterns to assist rupture. Sometimes to prevent fragments from being created too.

Air bags, 'lytic capacitors, innumerable industrial safety valves. If it wasn't for the size of the bore there's probably one with what you need more or less off the shelf.

As for the shearing part you mentioned, a lot of industrial burst disks do that too. There are a set of knives on the lower pressure side which the burst disk flexes into on overpressure to initiate a rupture which any scoring patterns then exacerbate.

The only version I'm thinking of which doesn't even try to retain fragments would be a graphite burst disk and IIRC certain large bore pneumatic launchers used for impact testing use what are described as ceramic burst disks which are physically ruptured.

There wouldn't be a need to retain the fragments as petals for safety reasons but I'm not seeing any harm in multiple fragments either. The payload wouldn't be in direct contact with the blast.

I'm wondering why you'd let it go as a full bore disk.
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Unread postAuthor: D_Hall » Mon Aug 03, 2009 8:09 pm

Hotwired wrote:If it wasn't for the size of the bore there's probably one with what you need more or less off the shelf.

There are off the shelf solutions... And they're expensive (Those that I've found are about $500/ea.).

Moreover, if I use a home-grown solution I can alter it to burst at different pressures as my needs require. The use of a shear disc makes it easy... Just machine the groove a bit deeper! With an off the shelf part of traditional design, I don't really have that option. I'd have to maintain a large inventory of discs "just in case".
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