SpudFiles

You can drill out at the flush bushingand replace the small section. Here is how.
http://www.spudfiles.com/forums/viewtopic.php?t=6584&

If the barrel was unsupported it could be that stress was being put on the elbow from firing and possibly from lifting the cannon by the barrel(?)

Heres a mock-up made of recoloured copper pipe I do my designs with



The elbow is smaller than the chamber and also less solid than the tee so any flexing caused by movement of the barrel is going to be focused on it - the weakest part.

It could be that it had been weakened by that over time, or as you say it could have been defective.

ooops...

That must have heart your ears. At least I know now not to test out my chambers without a barrel.

But I am still not clear why NOT have the barrel puts MORE stress on the PVC. Can anyone explain?

EDIT: Sorry just read the above post thank you. BTW that explains alot then why strapping the front of your barrel to your chamber is more then just for looks- correct?

Well if I continue my theory then the elbow was already weakened and just needed a bit more shock to cause it to fail under pressure - caused by the recoil of the piston valve thumping back.

*edit: to the edit above: actually its not strapped down on mine, the barrel is clamped at the breech and is held straight by unfixed tees. The two designs aren't really comparable though.

I don't think the elbow was already weakened (note my nearly identical failure at the bottom of the last page) Anyone who has fired a high flow valve with a large chamber can tell you that the recoil without a barrel can be fairly significant.

My launcher was on a heavy cart and the launcher was fairly heavy itself and the cart still rolled back a few feet even with the failure. I think akb's post on the thread I linked to explains it pretty well
"Amazing. At 100 psi you are launching almost 0.4 pounds of air. If it was leaving at 1100 feet per second it would have 7,000 ft-lbs of energy. "

Basically it is a lot of air going very fast, so it creates a significant recoil.

I missed that link

Two identical incidents suggests there was nothing wrong with the elbows then and it's just design flaw with the cannons.

Torque from firing will focus on the corner of the elbow because the tee is more solid and is actually where the force is being applied and the chamber fitting is much larger and has no focusing point.

Corners in everything focus stress and even if they're rounded to spread that it's still a stress point.

Maybe I'm having problems with the paportions (sp)in your pictures.
but the short socket depth in the pic indicates a dwv fitting.

the term elbow indicates a dwv fitting.((90 for pressure).
?so I'm curious about your fitting?

No, these are all pressure rated fittings, and that fitting was brand new... there was probably about 6 shots before that happened.

They make pressure rated elbows.
Thats what it says on the box.
I find you completely full of utter bs.

I'd say its defective.

after 6 shots i don't think it would have had enough stess to break so quick.

I'm saying the design of the two more or less identical cannons it happened in was focusing the firing stress on the elbow and its unlikely that two cannons of the same design with the same crack failure just happened to have defective elbows.

The opposing force of the air leaving the piston valve is trying to go backwards into the tee. It recoils and the distance of straight pipe between the Tee and elbow acts as a lever. The elbow doesn't have the strength to support the internal pressure and this levering action so it cracks, most likely starting at the inner side of the elbow.

So if I always fired this gun with the barrel on I could prevent this from happening again?

Thatt's prolly your best bet...

Fixing the barrel to the chamber would be best.