Launcher Failure Analysis/Log

Safety aspects of spud cannons.

Postby BillyBobJoe » Sat Sep 03, 2005 5:35 pm

Heh, I didn't even know it was on Spudfiles. Yep, thats a slide valve, recommended by Clide[:p]:D;)
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Postby Ian2accurate » Sat Sep 03, 2005 5:35 pm

I haven't had much failure. Just a few leaks and one incident from improper gluing...

It was a combustion. 15" x 4" chamber with cleanout plug for filling. 2" x 4" barrel. I didn't use any primer, and I probably didn't twist the fittings when I put them on. It was the first time I would show off my gun to someone. My grandad, and older cousin were there. I put in the propane, and pushed the BBQ igniter. Instantly the female adapter on the back of the gun blew clean off. It was loud like a gunshot. My ears rung and the little fuzz on my fleece sweater was slightly melted (as I was holding it at my hip level). I later realized the back (complete with cleanout plug screwed in) blew back about 5-10'. Everyone was actually fine, and the potato actually still fired out at about 3/4 speed.

I got the same crack like BBJoe from pumping up something in about 30<sup>o</sup>F. It just popped at about 20psi.
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Postby BillyBobJoe » Sat Sep 03, 2005 9:15 pm

Hmmm, after I took the pics of the piston cannon, I decided to salvage it for the brakeline barrel, so I started breaking it at the crack with by putting it in the vice. Heres a pic after a minute:

<img src="" alt="Image hosted by">

The female adapter cracked of quite violently, and with surprising ease - it was <b>extremely brittle</b>. Once I reached the pipe, the PVC became noticeably less brittle, and I actually had to twist of the section of the pipe as you can see in the picture. The endcap was not brittle like the female adapter was. I'm assuming this brittleness was the cause of failure, as it could not take the outward stress of the SCH. 80 threaded plug, so it cracked.
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Postby nunu » Tue Sep 27, 2005 4:00 am

My potato laucher has a général price level of 20 € for 6 month, what want to say every six month my potato launcher explode and i build a new one.
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Postby jgiglio » Sun Oct 16, 2005 8:08 pm


Looks like you may have excessive primer/glue there. I read once that excess glue will eventually weaken PVC. Maybe that was it?
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Postby srfnscubamunkey » Sat Oct 29, 2005 1:19 am

<img src="" alt="Image hosted by">

never fired pressurized it to around 35 psi, wasnt nsf-pw, good thing that i didnt do the test with 100 right off the bat. also bolts may have been to tight
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Postby Freefall » Wed Nov 16, 2005 2:37 pm

Failure posted for Mike32:

During attempted shot of a heavy projectile (steel bicycle peg) loaded behind loose gravel, the barrel ruptured violently. More details here: http://localhost/forums/viewtopic.php?t=11577

Mike32, please follow up with a summary and photo if available. I'll delete this reply when you do that.
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Postby SpudMonster » Sun Mar 26, 2006 6:01 pm

Temperature: Around 65 to 70 degrees farenheit

Pressure: 110 PSIG

Pipe/fittings: All NSF-PW

Launcher type: Piston-valved pneumatic

Probable cause of the event: Operating the piston at too high a pressure, old piston, piston weakened by a bolt right through the middle of the piston (cap).

I fired the cannon one time. It was very loud, and I was happy that the cannon was still working. I had pressurized it to around 110 PSI and thought to myself "I hope the valve holds up to the shock..." The sprinkler valve had a slight leak though, so I was a bit annoyed. I fired the cannon anyways, and then disassembled the sprinkler valve. It had a small PVC fragment on the diaphragm seat. I removed the fragment, cleaned the diaphragm a bit, and re-assembled the valve. Well, I attempted to re-pressurize the cannon, but the air kept leaking out of the barrel. I thought that the piston was notseating properly or that it was stuck, so I shot some WD-40 down the barrel and into the male quick-disconnect. I attempted to charge the gun AGAIN, but no dice. I decided to see what the hell was going on, so I removed the 3" male adapter. As I was moving the cannon around, I heard more PVC fragments rattling around inside the chamber. Not a good sign... When I removed the male adapter, the remains of the piston fell out.

All the pieces I could recover. Only the piston was harmed. Everything else was fine, and can definently be used again. ... C-062F.jpg

A close-up of the pieces. ... C-063F.jpg

A shot of the inside of the chamber with the piston removed. ... C-064F.jpg

The same shot, with flash. ... C-065F.jpg

The duct-tape seal around the piston itself. ... C-066F.jpg

The inside of the 2.5" duct-tape wrapped piston ... C-067F.jpg

Another view of the 2.5" piston ... C-068F.jpg

The 3" male adapter ... C-069F.jpg

A view of the 1.5" slip cap. Note that the gasket and face of the cap are gone. ... C-070F.jpg

A side view of the 1.5" cap ... C-071F.jpg

The other side of the 1.5" cap ... C-072F.jpg

The gasket and part of the 1.5" cap ... C-073F.jpg

The sealing face of the gasket ... C-074F.jpg
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