pic: T-Shirt Launcher, still needs a chasis

[MrForbes]

I don't see how it could develop full pressure, one or both shirts will move as soon as pressure builds up to a few psi.

[Fe_Will]

For all of the posters concerned about safety:

Why don't you help this team find properly rated parts so this design can be safe, find a suitable alternative that serves the same function or, post a new design altogether...

Over half this thread is, " OMG PVC EXPLODES!!!!" It reminds me of the propeller threads...

Finding a solution > Whining about the problem

P.S. My team is working on a launcher of our own and we are excited about the PR opportunity it brings.

[Steigerwald]

I'm not very good with paint, but here is what I mean. Say the barrel is 1 and where the tshirts are is number 2. Assume that both tshirt tubes are filled and have something in them. The shirts would not be able to move. Assuming they are relatively packed in the barrel, they will let air through, but it will not be instant and it will expose the assembly to the full pressure.

[MrForbes]

Since neither tube is sealed at the back end where air enters, the air will all leak out immediately. It won't have to go thru the shirts.

[artdutra04]

Originally Posted by IceStorm

How could a T-Shirt ever Jam inside? And even if it did wouldn't the air still be able to pass through the T-Shirt.

You should always design things for the worst case scenario.

99.997% of the time there aren't any 100-year-storm hurricanes in New York City. Does that mean that the structural engineers there should ignore designing their buildings to withstand 150+ mph winds? Absolutely not!

Originally Posted by squirrel

I don't see how it could develop full pressure, one or both shirts will move as soon as pressure builds up to a few psi.

"will move" is not the same as "should move", the latter of which is more correct in assuming the worst case scenario. The engineers behind the Tacoma Narrows Bridge thought that wind would never pose a problem to bridge design.

The levees around New Orleans weren't designed well enough to protect against a direct hit of a Category Five hurricane. Engineers knew about the problem, but political leaders thought the risks of a direct strike weren't high enough to divert billions to rectify the problem as quickly as possible.

How about if someone told you that all car accident rollovers will always happen at only high speeds, and that it is impossible for someone to flip over a car going only 10 mph? And yet, it's been done.

The I-35 Bridge in Minnesota was known to be structurally deficient as far back as 1990 (along with design flaws), but again the powers-that-be deemed the risk to be low enough to carry on as is without spending money to fix the problem immediately.

PVC also has a design flaw in that it is very brittle, and several people here have posted in this thread with real life examples of what happens with PVC fails. The odds of an accident happening are low, just as the odds were low with New Orleans and the I-35 Bridge.

But ask yourself this: do you want to be the one responsible for making the decision of "don't worry, the risk of PVC exploding is low, so it's safe" if something does happen?

Originally Posted by Fe_Will

For all of the posters concerned about safety:

Why don't you help this team find properly rated parts so this design can be safe, find a suitable alternative that serves the same function or, post a new design altogether...

Over half this thread is, " OMG PVC EXPLODES!!!!" It reminds me of the propeller threads...

Finding a solution > Whining about the problem

If you search through this thread, and the dozens of similar threads here on ChiefDelphi over the past few years about air cannons, you'll find people suggesting plenty of alternative pipe materials, such as ABS, aluminum, steel, copper, brass, or polycarbonate. Of these, steel and copper fittings are extremely easy to get at your local Home Depot or Lowe's, and brass fittings can be found at most truck part stores in the Hydraulics section. All of these can be ordered from McMaster or similar online retailers and be at your school within a few days.

The reason people keep saying " OMG PVC EXPLODES!!!!" is because there are a lot of people don't believe the warnings, and keep insisting that there isn't any problems with PVC. Eventually I'll stop caring to convince these people (I haven't reached that point... yet), and if I see any thread that deals with PVC air cannons, I'll just avoid it like the plague. If these people are seriously convinced there is no danger with PVC, then nothing we write here will convince them otherwise until an accident happens, and all we can hope for is that no one gets injured in the process.

[MrForbes]

Thanks for your concern, Art, keep it up! I agree that pressurized PVC is a bad thing. I'm not so sure that designs that can't build pressure on the PVC part should be dismissed out of hand, on the vague supposition that "something" could go wrong. If there is a specific concern or scenario that could cause problems, please let us know. Bringing up unrelated cases of bad judgement by engineers or politicians doesn't really help us evaluate this case.

[Steigerwald]

Originally Posted by squirrel

Bringing up unrelated cases of bad judgement by engineers or politicians doesn't really help us evaluate this case.

I respectfully disagree, I believe that is exactly what is needed here. I think that people need to see that sometimes things that aren't expected happen.

I have seen that same argument that things don't have to be designed for the worst case scenario on our robot. Why do the construction properly the first time when you can just fix it in competition. Why over design something the first time when you can just make it work temporarily. It's things like that that make a huge difference.

[EricH]

Originally Posted by Steigerwald

I respectfully disagree, I believe that is exactly what is needed here. I think that people need to see that sometimes things that aren't expected happen.

I have seen that same argument that things don't have to be designed for the worst case scenario on our robot. Why do the construction properly the first time when you can just fix it in competition. Why over design something the first time when you can just make it work temporarily. It's things like that that make a huge difference.

Especially in elims, when that part that wasn't designed/built right fails suddenly because it took a load that it couldn't take.

Do it right the first time, and it'll keep going. Right, Mr. Lavery?

As for "How could a T-shirt jam?", most launched T-shirts are wrapped somehow for distance reasons. Usually, it's a rubber band or three. Guess what? Said rubber band or other wrapping has a much higher coefficient of friction than a T-shirt, and may stick in the barrel. The T-shirt then sticks too, and you suddenly have a jam. Now you have pressure that can't get the shirt out and can't go elsewhere. Your best bet in this case is to drain pressure immediately and use a stick to remove the shirt, but sometimes, it takes care of that for you, in spectacular fashion.

[Mr. Freeman]

Originally Posted by Steigerwald

I think that people need to see that sometimes things that aren't expected happen.

Yes, but there is a point at which it reaches absurdity. Bringing up cases of bridges collapsing doesn't help anyone determine failure modes of a potato cannon.
I do agree though that this cannon should be able to withstand a T-shirt getting stuck in the barrel. I do see this as actually being a possibility due to T-shirts being pressed into the barrel too tightly, rubber bands around them increasing friction with the barrel, etc. That said, air will move through the T-shirts. I don't know how fast, but it might not be as bad as being completely clogged.

Anyway, on the subject of PVC. Why do I keep hearing stories about pressure rated PVC exploding? Air/water shouldn't make any difference regarding what pressure it fails at, only what happens AFTER the PVC fails.

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.)

[gorrilla]

Originally Posted by Mr. Freeman

Anyway, on the subject of PVC. Why do I keep hearing stories about pressure rated PVC exploding? Air/water shouldn't make any difference regarding what pressure it fails at, only what happens AFTER the PVC fails.

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.)

actually,

since PVC is pressure rated for water only....

Air compresses differently than water(dont remember the exact science)....

which is why, PVC can(not will)-fail at much lower psi's than its rated for....

[MrForbes]

A compressed gas has far more energy stored in it than a liquid compressed to the same pressure. It took a lot more work to compress the gas, and it's volume changed considerably. Liquids are "incompressible", meaning the volume will change very little while the pressure increases greatly. It's this energy that causes problems with pressurized air in PVC, the plastic can shatter and sharp pieces of it will fly at high speed all over the place.

If you're making something that will hold pressurized air, make it out of something besides PVC.

[ubermeister]

Originally Posted by EricH

As for "How could a T-shirt jam?", most launched T-shirts are wrapped somehow for distance reasons. Usually, it's a rubber band or three. Guess what? Said rubber band or other wrapping has a much higher coefficient of friction than a T-shirt, and may stick in the barrel. The T-shirt then sticks too, and you suddenly have a jam. Now you have pressure that can't get the shirt out and can't go elsewhere. Your best bet in this case is to drain pressure immediately and use a stick to remove the shirt, but sometimes, it takes care of that for you, in spectacular fashion.

This means that your cloth T-shirt is able to hold more pressure than your welded 3/16" thick pressure-rated plastic. Just sayin...

I built an airgun (2, actually) and have had the projectile jam in the barrel. The air just leaked out of the poor seal between the movable barrel and its seat. And that seal is (I think) much better than this one. And that was at 120 PSI.

I'm just trying to say that you should be more worried about the chamber, and that your very awesome airgun does not need a complete redesign.

[XXShadowXX]

Originally Posted by artdutra04

Rule #1 when making air cannons is do not use PVC.

Rule #2 is DO NOT USE PVC.

There are huge safety concerns with PVC, because PVC IS NOT DESIGNED TO HOLD ANY KIND OF COMPRESSED GAS, AND HAS BEEN BANNED BY OSHA FOR USE WITH PRESSURIZED GAS IN INDUSTRY.

PVC is rated to 350 psi of air by UL;
I got this from a team mentor who builds PVC potato cannons, and has never had one explode at pressures as high as 150 psi (fired like a bazooka)

[Steigerwald]

Originally Posted by EricH

Especially in elims, when that part that wasn't designed/built right fails suddenly because it took a load that it couldn't take.

Do it right the first time, and it'll keep going. Right, Mr. Lavery?

As for "How could a T-shirt jam?", most launched T-shirts are wrapped somehow for distance reasons. Usually, it's a rubber band or three. Guess what? Said rubber band or other wrapping has a much higher coefficient of friction than a T-shirt, and may stick in the barrel. The T-shirt then sticks too, and you suddenly have a jam. Now you have pressure that can't get the shirt out and can't go elsewhere. Your best bet in this case is to drain pressure immediately and use a stick to remove the shirt, but sometimes, it takes care of that for you, in spectacular fashion.

Well put!

The fact is that on this cannon, the barrel assembly is not made properly. The fittings are not inserted and some are not pressure rated. The need to be replaced to be safe. Period.

You asked for a way to solve the problem instead of just complaining? Fine, here you go!
Plumbing Supply
McMaster

I'll put up some pictures of what I have done which would work perfectly for your application.

[Mr. Freeman]

Originally Posted by squirrel

A compressed gas has far more energy stored in it than a liquid compressed to the same pressure. It took a lot more work to compress the gas, and it's volume changed considerably. Liquids are "incompressible", meaning the volume will change very little while the pressure increases greatly. It's this energy that causes problems with pressurized air in PVC, the plastic can shatter and sharp pieces of it will fly at high speed all over the place.

Do you have a reference for this by any chance. I agree that compressed gas has more stored energy than a liquid under pressure, but it doesn't make sense that this would cause the PVC to fail, as long as the force exerted on the PVC remains the same in both cases.