Re: Most Powerful FIRST Team created T-Shirt Cannon
 

From a safety standpoint I would not want to use a material that has unknown fatigue characteristics and can fail brittley creating shrapnel. You should only use a well characterized material that has a well researched fatigue characteristics for which you can get a Goodman diagram. Cyclical loading application like this are far harsher an environment than you realize. Proper design would keep the loading either below the endurance limit of the material or take it out of service before the failure occurs. This is why all aircraft parts have service life limits to prevent a fatigue failure like Aloha Flight 243 from happening.
http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243

Fatigue causes parts to fail after continually being cycled and they don't even have to be cycled close to their yeild or ultimate strength for this to be an issue. Please keep this in mind when designing something like this.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Off-topic sorry, but it's odd that Peter used Aloha 243 as an example... the episode of MayDay(?) that Discovery was playing this morning was about Aloha 243. I would not want my PVC t-shirt cannon doing that right next to me. Or anywhere within direct line of sight of me really.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

When I was in undergrad that was the go to example of fatigue failure for all Stress Analysis/Strengths of Materials professors. I think the Minnesota bridge failure from a couple years ago might be replacing it though.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

The Barrel and Dump tank are going to be left the same due to the fact our engineers have made it indestuctable. But on the underside our 2 1.5 footlong 3inch airtanks are getting replaced with 2 1/2 gallon airtanks. Our mentors are still confused and i'm wondering if anyone has proof of this happening to pressure rated PVC not crap PVC PRESSURE RATED ONLY! They had never heard of it happeneing only low grade and the internet does not provide any either.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

What?? Pop Top Airlines being replaced by the Minnesota Disappearing Bridge? No way! Soda can airplanes are much cooler. Besides, that sucker landed safely. :ahh:

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

The argument here is "What is 'safe'?"

And the thing is... it varies. What I consider 'safe' to do on my own encompasses much, much more than what I consider 'safe' to do with my students.

Personally, I think that PVC cannons can be made safe enough for me to use, by myself... or with a few adult friends... away from students and the general public. But then again, I ride a motorbike, and bicycle to work in city traffic. Safe? Maybe not, but those are risks that as an individual adult that I'm willing to take.

But the risk of catastrophic failure is such that I no longer use PVC in compressed air applications with students. As slim as the risk may be, if a student were seriously injured and I were to be asked, "Were you aware of the potential for serious injury from this use of PVC?" and "Were you aware that PVC was not rated for compressed air applications?" and "Were you aware of safer options that could have achieved the same educational outcome?".... What would I say?

So in this context, rather than trying to argue over an absolute "safe" or "not safe" when using PVC for compressed air, I think it is sufficient to say that the potential for hazard does exist and that we should each evaluate that risk, and the steps required to mitigate that risk.

Be safe... whatever that means to you.

Jason


P.S. Thank you to the people who have provided references, etc. that have helped educate me as to the potential risk.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Carbon fiber (and I believe fiberglass too) fail catastrophically, that is to say that once they yield, they fail entirely. I know some high-pressure air tanks are wrapped in CF or fiberglass, but they have a METAL tank on the inside that will yield and (I am guessing here) bleed off the compressed gases more slowly.

Or hear... wear very good hearing protection. I can't hear so good out of one ear since a soda bottle rocket blew up near me. You know, the little water-and-compressed-air-propelled dealies you can make with a 2L soda bottle and some cardboard.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Not to put myself in this PVC argument, but I found this video interesting.

http://www.youtube.com/watch?v=9tg0CagbU9I

Though PVC is cheap, readily available, and easy to use I think there is a point to which it is better to use something else. Though that point depends on, as others have said, what is considered "safe".

-Tanner

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Once again I keep seeing people posting videos of PVC being shot at with arrows..and .22 caliber handguns no proof that this happens on it's own.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

I'm not saying the cause is the same, but the results are what matters. One crack/ding/hole/some sort of stress causes disintegration.

-Tanner

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Other than the multiple linked to videos, posts of people's first hand accounts, and OSHA safety expert warnings...

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

We learned about the Aloha plane in my undergrad ME design class, so its still around.


I've seen the aftermath first hand of a PVC cannon exploding. A member of our team while I was in high school constructed a PVC cannon on his own. We were in the parking lot at our school and he was launching some things off with it. He charged the tanks and was walking away from the group to fire a shot when he slipped and dropped the charged gun on the ground. The "storage tank" made from PVC EXPLODED, sending shrapnel everywhere, exactly as everyone is describing. The result was, him being lucky to be alive. He had a piece of PVC embedded in his skull and other parts of his body, and to this day (~4 years later) has a scar on his head that looks awfully similar to harry potters.


I'm not going to take a stance here as to "PVC is okay to use" or "PVC is not okay to use", but this first hand account may add some insight to people's decisions in the future. Sure, he dropped the cannon and it was his own fault, but anything can happen. Using the cannon is a lot more fun when no one is going to get hurt, regardless of how it happens. Theres a lot of energy stored inside a tank for a t-shirt cannon (look at it! you launched a tshirt across a football field!!) its important to be smart about it.


-Brando

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

I did a little internet searching on PVC material strength.

One thing that you need to know is that its rated for that PSI at a certain temperature - probably 20C. If the temperature is lower, the PVC becomes very brittle and loses its strength rapidly.

http://www.madisongroup.com/services...eanalysis.html

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Also, there is a high end too. You hook that up to a compressor, which gets hot and bad things begin to happen as well. Hot water is enough to cause issues which is why even in liquid service, only cold water is allowed.

Re: Most Powerful FIRST Team created T-Shirt Cannon
 

Food for thought: Has anybody else noticed that any time the Mythbusters are doing something with compressed gas of any sort, they use metal?

I wonder if there could be any connection there... Nah, can't be that PVC in compressed gas situations is dangerous, so insurance won't let them do it. That's hard to believe...[/:rolleyes:]