Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Would lamination (with carbon fiber or fiber glass) prevent fragmentation? Metal Mesh?

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

I may end up making my half inch thick lexan boxes after all, We probably won't be able to replace the PVC but i'm going to install bulletproof box around them for saftey. The dump tank on the gun however i'm not sure yet.
http://maikenmagic.com/?ref=gallery&...irt%20Launcher

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

I still don't see how that contributes to making the material fail. I can see how it would make the failure explosive, but it seems to me that if the gauge reads 100 psig, there is a net force on the tank walls of 100 lb/in^2, no? If not, how does the compression add a force that the gauge doesn't see? Don't pressure gauges just measure the deflection of a spring?

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

I think the real problem is the 'glassy' behaviour of PVC at room temperature. This is a characteristic of the polymer, and means it will fail in a brittle manner (fatigue and brittle fracture, neither of which would be non-catastrophic in a pressure vessel) rather than the ductile failure mode of a tank made of most metals or a different plastic.

I don't get why people choose to build a PVC air cannon after all the warnings that have come out over the years. There has to be a safer way of doing this.

EDIT: fatigue is based on the pretty much uncontrollable microcracks in your material, it could fail after 1 cycle or a million, and it is pretty much impossible to tell when it will fail.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Eye protection too, please. If this thing fails in the way people have been describing it may be the last thing your operators see!

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

You are both mistaken.

SDCantrell - Water will compress when you press upon it, just like every ordinary real material will. However, as John said, it compresses very little, and for that reason pretending it is incompressible is a useful approximation.

John - Water's compression has nothing (or at least very little) to do with whether a 1000psi waterjet can cut something. That pressurized water will fly out of a nozzle and cut things is a separate topic from how much or how little it's volume changes when it is put under pressure.

Blake

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

To be very blunt about this, can people please stop being unintelligent and just purchase something that is certified for air pressure instead of just assuming it will work for an air cannon?

Your robot for demo's doesn't need to come under any weight constraints so feel free to use a high capacity, industrial, CERTIFIED, tank.

ie: http://www.toolsoutlet.ca/osc/images...ank%20copy.png


Here's your testing: http://www.youtube.com/watch?v=Cal71EN5Gt8

Or you could be idiots like these guys and laugh about it: http://www.youtube.com/watch?v=AQo81...eature=related

Have fun with the shrapnel. ::ouch::

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

You commentary is unnecessary..we are having an intelligent discussion about why the dangers occur. I'm going to be getting our 1600psi firefighter air tank to use. soo thank you.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

The point everyone here is trying to make is better safe than sorry. Your PVC may be rated for pressure on the inside, but what happens when you've used your gun time and time again? You know, storage, moving it from place to place. What if you drop it? Chances are you'll make defects in the exterior, and it'll weaken as time goes on.

Your cannon has a chance of failure. What happens after failure is shrapnel, bits of plastic flying in every direction at ridiculous speeds.

By definition, liquid water isn't compressible (liquids have a definite volume). Since water doesn't violently expand or change shape when its depressurized, there is no chance that it'll take the plastic with it.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

A good way to win the Darwin Award?

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Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Last I checked, liquid water IS compressible, just very little. Heck, a solid steel block is compressible, you just need to apply enough pressure :P

Call me a bit of a n00b, but I'm not sure I see the difference between water pressure and liquid pressure, PSI is PSI, no matter what is CREATING the pressure on the walls of the PVC, the pressure is still there, and (assuming it is regulated) the same amount in both cases.
I understand that the PVC wouldn't be safe in this situation, I just don't really understand the difference between the ratings (liquid vs. air)...

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

It's how it fails that's the issue. Water, you get an incompressible (or close enough as to be easily approximated as such) fluid that, if it's under some pressure (as it might be in a sprinkler system), and it gets an outlet, it will leave in a reasonably orderly fashion, spraying out until somebody turns the water off.

Air, on the other hand, will try to all leave at once, causing further mayhem as it breaks whatever it's in further. If you have something that's rated for air, presumably it would have failure modes to safely release the air even if something breaks.

What we're concerned about is the "mayhem" as the PVC breaks. I've seen PVC that's fallen from a height of about 3.5' break (fortunately, no pressurized air inside).

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Just to clear things up- The water in your hose wants to leave the hose at a constant rate. For example, lets say 5 gallons a minute. Now, if you have a one inch diameter opening at one end of your hose, the water will come out relatively gently. If you shrink that opening (such as with your thumb), the water still wants to exit the hose at 5g/min, but because the opening is now smaller, it must exit faster, and thus with more force.


On a side note, our team operates our tshirt cannon off a 5-gal tank designed for air pressure (craftsman tank, I believe). It works fantastically well. PVC can not only fail in the piping itself, but it is also highly susceptible to end caps and joints failing.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

You should listen to Akash, he does know what he's talking about.

I'm glad to know that your getting a metal airtank It's alot easier to spend some money and be safe, than to pay a hospital to fix you.

By the way, a SCUBA tank would work fine, it holds more air at 3000psi, and is way more safe than any PVC storage tank you could practically come up with. Plus they have standard size fittings!

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

That's what we are going to use until we get a new tank, this one is rated 2560psi