pic: T-Shirt Launcher, still needs a chasis

[MrForbes]

Read the warnings provided by PVC pipe manufacturers....

"WARNING: Do not use PVC pipe for pressurized air systems. Injury or death may result due to the catastrophic nature of pipe failure should failure occur. Rapid expansion of compressed air could propel shards of plastic throughout the area."

[Daniel_LaFleur]

Originally Posted by Mr. Freeman

Seems to me that if PVC rated at 400+ PSI fails at anything less than 400 PSI then it was either:
A) Not actually rated
B) Not properly used (exposed to conditions it wasn't rated for, etc.)

Apparently you did not read my previous post.

PVC is not rated for compressed gasses. It is only rated for liquids. The difference is not in the pressure itself (as 400 PSI is 400 PSI). The difference is in the failure effect. With liquid the PVC will split and release it's pressure quickly (as liquid is only compressed as far as the gas within the liquid allows) while the gas will rapidly expand and sunder the PVC causing it to expel the shards in all directions.

Whats worse is that PVC gives no (or very little) warning that it is about to fail and that the PVC item in question may have been operating "properly" for years (as was the case with the 1 liter volume I discribed earlier).

Oh, and another thing ... The forces on the barrel and breach are almost instantanious (sp?), whereas the t-shirt takes time to be expelled from the barrel ... thus the entire force of the air is applied to every surface (universal gas laws) within the cannon. So when you fire the cannon it puts almost 60 PSI (assuming thats the pressure you are using) on every portion of that cannon being used until the t-shirt leaves the barrel. Not only that but those forces are applied as a shock load which is about as bad as you can apply to PVC due to its brittleness properties.


as to another question in this thread:
And as far as making PVC safe for use with compressed air ... the only way you can do this is to ensure that the blast from a catastrophic failure (when it happends) is contained.

And lastly ... if someone doesn't want to listen and makes a pneumatic device out of PVC, well I personally don't care too much. If they want to hurt themselves thats fine. But when they state that they are going to use it for 'marketing' then I have to stand up and be heard, because those near the device may not fully understand the dangers that the device presents.

[ZInventor]

i just say if you're going to put it on a chassis anyways, give the drive strong motors, and make the whole think out of steel.

that's what i'm working on right now, is backpack version (with exoskeleton like legs to carry the incredible weight (over 150 lbs)

when it's done, it'll be a dual-wield (one for each arm) 10 or 12 shot revolver (total of 20-24 shots)...we'll see if i can make it work...

-Z

[Mr. Freeman]

Originally Posted by Daniel_LaFleur

Apparently you did not read my previous post.

PVC is not rated for compressed gasses. It is only rated for liquids. The difference is not in the pressure itself (as 400 PSI is 400 PSI). The difference is in the failure effect.

I did read your previous post. My question was not why the failure modes are different (I understand the difference in compressed gasses vs. liquids), but why it fails in the first place.

[ZInventor]

the reason it fails is that PVC was never rated to go from 0-60 psi and back to 0 in less than a second.

the instantaneous stresses are just too much.

think of it this way, your tires give you traction at 60MPH, but if you try to go from 0-60 too fast, you'll spin out...

i know, it's totally different forces at work, but the result is essentially the same.

it's MUCH harder to create instantaneous pressure with liquids, as they do not compress, and once given more volume, return to 0 without much trouble.

i've seen first-hand the problems with PVC AND ABS!!! IMHO, don't use plastics of any kind for air pressure, unless they are flexible (ie. Pneumatics tubing)

just use metal.

Just my $0.02...

-Z

[Beener]

I like the revolver design. How much does it weigh?
This is what we came up with: Vex Air Cannon

There is no PVC used anywhere on this robot.

[Bruceb]

Well it took me a while but I read this intire thread. What a great discussion. First let me say that I think I will build a sheet aluminum clamp to completely encase the pressure tank. Think of it like a hose clamp that is as wide as the tank is long. Make it from .050 aluminum and roll it to the exact diameter of the pressure tank then bolt the ends together every 2 inches. That should eliminate the potential failure of the pressure tank. The chambers are sealed on both ends by a spring loaded piece of 3 inch pvc. These seals open somewhat every time the gun is fired and I am sure they will release all the pressure very rapidly in the event the t-shirt refuses to go down the barrel. As was said before, if the gun is fired when the chambers are not properly indexed then the seals do not ....seal.. :>) So that is not a problem. Squirrel, this looks like your valve because I am far better at copying an idea than I am at coming up with my own. It is your valve as best as I could tell from the pictures. At least I think it was you. It is only a 3/4 in gate valve. I believe the small size of the valve actually limits the pressure buld up in the barrel quit a bit. As far as the big reducer at the back of the barrel not being pressure rated goes, that is not a problem either as it is not carrying ANY pressure. The one inch pvc that is connected to the union goes all the way through the reducer to the back of the chamber and is sealed at that end as well so no pressure can get into the reducer at all.

Why not build it all from something stronger(read that more expensive)? Budget is all I can say. If anybody wants to donate some 4 inch steel pipe and the necessary fittings I would be happy to use them. No, I'm not ignoring the safety concernes due to budget constraints. We built the thing with the aid of 2 GE engineers who don't see how the PVC can be a problem. That was before this thread got started and all the concerne got started so now I am trying to minimize the risk and concern using materials we have available.
Thanks for all you input. Keep it coming.

Bruce

[Tyler Hicks]

That is one of the coolest things I have ever seen......this gives me an idea!!!

[cbale2000]

One suggestion that I've heard before but I'm not sure has been mentioned here, if someone does plan on using PVC for a cannon, perhaps just build shielding around it. I seem to recall another launcher design that incorporated a larger diameter PVC tube around the main cannon that would block any flying parts in case of a catastrophic failure.

In the case of this design, maybe even just a polycarbonate box around the launcher so that onlookers can still see all the mechanisms but are protected?

[Tahti101]

cool....im new to robotics and my team has to build a t-shirt launcher..