FIRST air cannon robots PVC??

[John Gutmann]

Originally Posted by KenWittlief

I would look into using a pnuematic cylinder, with a mechanical trigger

if you try to use a cylinder to launch stuff the airflow into the cylinder is too restricted by the small connectors and hoses

to fix that, you put a mechanical stop, about half way down the cylinders travel, that holds the pressurized piston back

to fire the launcher you pressurize the cylinder against the trigger/ release pin, then you pull the pin out of the way, allowing the cylinder to extend all the way

in effect you have a large pnuematic spring.

I think you will find this takes much less air, than blowing Tee shirts and stuff out the end of a PVC tube. Also the thing you are launching does not have to fit snugly into the tube, you could put just about anything in there.

The plus for safety is you can build this will all standard (approved) pnuematic parts and valves, you wont have to worry about pressurized PVC failing in a room with thousand of people around.

It can still be made to look like a cannon, in fact you could release CO2 or smoke to make it look more omminous. Only you would know whats really happening inside.

Or partially prussurizeon eend, and then create a vaccuum in the other.

[mizitchell]

Isn't ABS blastic rated for higher pressures?

[Tytus Gerrish]

I Have lot's of experience with pressurized gas in PVC. i have made it a habit to hydrotest my PVC devices before i use them. i tested an assembly witch included about 5'in length of 4" sch 40 PVC pipe to 400 PSI with no problems. Clearly with PVC they're are many factors that contribute to the strength of pressure vessels made of PVC such as surface area glued in joints, external damage, glue curing time, exposure to sunlight. as sutch every assembly unique and as a general safety precaution should be hydrotested. i have done such things like re-enforce PVC pressure cylinders with Fiberglass to hug comtainers and prevent them from expanding. I swear by this practice. before i started doing that. I had an accident with A pvc air tank exploding. 170PSi a 2" vessel burst at the joint of a reducer and the pipe. the whole cylinders disintrated and threw a 2" cap into my chest left me with a big painfull black and blue mark for about a month, I figured that lucky considdering all the other sharp pieces of PVC shrapnel that could have hit me. it probaly happened because the pipe wasant fully seated in the connector when the cement hardned. Let me tell you When that Happenes to you Once you learn to be carefull Real quick.

[ChrisH]

Originally Posted by Tytus Gerrish

i have made it a habit to hydrotest my PVC devices before i use them.

...

Clearly with PVC they're are many factors that contribute to the strength of pressure vessels made of PVC such as surface area glued in joints, external damage, exposure to sunlight. with makes every assembly unique and as a general safety precaution should be hydrotested.

Many pressure vessels are tested by over pressuring with a fluid, generally water, inside. It is definitely the right thing to do. The overpressures used are 400% or more of the rated working load. The exact number depends on the pressure, operating temperature, and service conditions.

Hydro testing is used because the pressure is released as soon as there is a leak. It only takes a small amount of fluid to leak out to release pressure so the failure is not likely to propogate too far. If you use gas pressure however, A small leak just results in a very small reduction in pressure. The bulk of the energy is still there waiting to rip things apart.

[Rickertsen2]

I might just be lucky and or stupid, but i have built a least a dozen pvc based projectile launchers some of which have been statically pressurized to pressures up to 350 psi. The only failure i ever experienced was a hairline crack in a combustion potato cannon leading from a screw screwed into the pvc as an igniter electrode. The cannon had been left out in the sun for some time. SUN DESTROYS PVC. I am not saying it safe, only that i have never had any serious failures.

[Gdeaver]

It's not that PVC can not handle the pressure, it's what happens when it fails. There is at least one plastic air pipe system on the market that is rated for air. A local automotive shop used PVC pipe for air and it failed. The shrapnel pattern was scary. Do not use PVC for pressurized gasses.

[Arkorobotics]

Air cannon + PVC + tennis ball = http://www.duke.edu/~jwc13/tennisball.html

I think these guys went up to 125 PSI with PVC tubes.

If you are really scared of it blowing up, you can always add a pop safety valve.

[mizitchell]

i thaught about the whole pop-saftey valve but the only thing is if its rated for 125 PSI and its filled to 124 PSI when it fails what good does it do you?

[Arkorobotics]

Originally Posted by mizitchell

i thaught about the whole pop-saftey valve but the only thing is if its rated for 125 PSI and its filled to 124 PSI when it fails what good does it do you?

When you say "it fails" do you mean it explodes? Why would it explode if you were to cap off the pressure lower than its maximum? If you were to know the maximum PSI of your tube, you can then buy a pop safety value 40 - 50 PSI less than the maximum. Therefore you can't fail, unless all the stress on the tube from charge and discharge weakens the material.

[ahecht]

The problem is that you can never know what the maximum the tube will withstand is. Hairline fractures, sun damage, dents, improper gluing, manufacturing defects, etc, can all lower the amount of pressure needed for a catastrophic result.

[NextPerception]

Out T-Shirt Canon Robot has never had any issues going up to 100 psi. when doing public demos however we never have to bring it above thirty psi because of the massive amount of air volume we can chug into the barrel, compliments of our specialty valve.