pic: T Shirt Shooting Robot

You can hear the leak of the escaping air, and if you submerge it under water you see the bubbles. Our PVC has leaked before, and you can hear the air escaping. and I know that PVC with running water you don’t hear it, but you do see the water that it leaks. We had a rather bad gluing job done on an older version of our cannons and you can clearly hear the leak as well as see the pressure drop. So sorry, but i don’t think your correct… because i do know what it sounds like, and I have seen them in water

Pressure extered by a fluid is exerted evenly on all surfaces of its container. So when the valve is released and 120 psi of air are instantaneously in the pipe leading to the t-shirt, all of the PVC is experiencing 120 psi of force.

Ohhh ok, so your talking about the barrel then? I gotcha! that makes sense… thanks!

It’s physics and such. Let’s use some everyday examples…

You always have 14 psi pressing down on you, due to atmospheric pressure. But now, you decide to have some fun. You go into a room that is totally sealed, with an air compressor hose coming down to a smaller chamber that seals to your skin. You have gauges and things of that nature all over the room. The room gauges read zero to start–they’ve been calibrated so 14 psi is zero. You pressurize the smaller chamber to some higher pressure (say, 50 psi as the gauges measure). Now, something jerks the small chamber away from your skin. Instantly, all the pressure is released from that chamber into the other chamber, and air being air, it fills the entire space as fast as it can. The gauges now measure more air pressure than zero (how much more depends on the size of the room and the size of the small chamber). Instantly.

But you’ve rigged this room with a small air cannon, and it’s a small room, with an insane amount of pressure in the small chamber. Suddenly, the small chamber breaks, and all that air finds the cannon, which is loaded. The ammo moves under the pressure, which at the instant the chamber broke, was applied throughout the room, and shoots out, releasing all the pressure. The room returns to the normal air pressure of 14 psi, the gauges read zero, and you now have to reload and try again.

Does this help?

Yes sir! I wasn’t sure what part he was talking about, tank or barrel. Thank You

We built a launcher last summer out of spare parts and various scraps, and we managed to blow a 200 lb solid steel door open 90 degrees at about 110 psi. We found that the t-shirts work best when either taped or zip-tied (if you are going for distance). If you aren’t going for distance, then there is a way to fold the shirt so it opens when you launch it, making it much less dangerous. We have used it at a couple of tractor pulls, and we are planning to mount it on our robot from the 2006 season (the robot’s name is Tank, so we are planning on making the launcher rotate). To help with safety concerns, we found that it is best to keep the launcher and air storage tanks in separate locations, along with the hoses (if there are any). Hope the build goes well!

sigh … so much misinformation…

PVC leaks cannot be heard as they will cause the PVC to fail violently. The leaks that you describe are leaks at the bonds or joints, not in the PVC itself.

Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block … if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it’s original shape but rockets off of the surface it was on.

Now on to PVCs material properties:
PVC is rigid polymer. This allows it to flex while maintaining it’s strength, however once it is flexed too far it will crack and the PVC pieces will then (violently) attempt to return to their original shape. PVC is extruded which means that it’s wall thickness is not consistant. Also each time it is flexed it gets reduced in strength (each pressurization cycle flexes the PVC).

If the PVC pipe had pressurized liquid inside then when the PVC pipe fails all of the pressure within the pipe is released but does not violently explode because it is like the steel block and was not compressed.

If the PVC pipe had pressurized air inside then when the PVC pipe fails the air will violently rush out the failed spot which will continue to apply pressure to the now failing spot causing it to fail further. Pieces of PVC at this point can be ripped off and carried (or shot) by the escaping air.

In short, a failure of the PVC (not a bond or joint leak) will not be ‘heard’ until it violently fails. There will be little to no warning. The PVC is not rated for pressurized gas, and the shock loads that a cannon put on PVC are far beyond what it is designed to hold.

Now I’m not saying that yours WILL fail, but considering that you will expose the public to any failure you have, I would suggest that you take every precaution possible to ensure the publics safety. FIRST (and the FIRST teams) do not need the political ramifications from a publicity stunt gone bad.

P.S. @ Krunch dude. The chart you referenced is out of ASTM D1785 which specifically states that PVC is for liquids only and that those numbers do not reflect gas pressurization.Read Abstract here.

Here is our t-shirt cannon :slight_smile:

I know this thread is pretty much dead. None the less, I am the current president of Krunch 79 and we are still using the T-shirt cannon, completely unaltered. Just to get a gauge for how much we use this, we fire it off about 20 times per barrel each football game. So every week for a 3-4 month season for 2 years now. We have not had any problems with this robot, no leaks, noises, or explosions. I do understand the risks and effects of using weak PVC, but due to time and cost constraints we are holding off on the much need repairs until mid 2016. Please do not continue to educate me on the dangers of PVC as I am well aware off the dangers of using PVC. I also do not want any comments on why we waited 6 or so years to replace the PVC as I said we had time and money constraints. Just looking out for my fellow Krunch members, Gallo!