pic: T-Shirt Launcher, still needs a chasis

[ahecht]

If you're concerned about weight of using metal, don't forget that only your pressure reservoir needs to be metal. If you store your air at 120psi (or higher) you can get away with quite a small reservior.

[Bruceb]

I knew we would get some warnings about the PVC. We have 2 GE engineers on our team and they don't see how this PVC can be a problem. We did not buy the cheap unrated PVC from Manards, this is all pressure tested and pressure rated PVC. The 3 in dia reservoir is rated at something like 320 psi. Don't remember the exact spec but it was WAY higher than the 60psi we intend to carry at max. I was thinking about wraping it in fiberglass cloth and epoxy just for safety but I realy don't think it is an issue. I can't help thinking that the accidents have been caused by using the unrated pipe or overpressurizing it. Any way. The chambers are gasketed at both ends with a spring loaded deal that seals both ends of the chamber. We do get a little leakage here but not much. I think at max elevation and 60 psi we could launch a rolled up t-shirt about 200 ft. Thats a guess as we have not tried it outdoors yet but it hits real far up the wall indoors.
Bruce

[IceStorm]

Before I really got involved with FRC too much we had actually built two different potatoes cannons that used air pressure. We were doing a very similar design only difference was that our barrel actually went down through the air tank but we were easily pushing 100 + psi into the 4 inch air tank with no long term issues. Again though we were always careful to make sure that we were using at a very minimum schedule 40 PVC and in most cases used SCH60 pvc so that we would have at least a 3x safety factor on our intended PSI and the max we could do.

So what also has me extremely curious is how do you fold/roll the Tshirts so that they come out of the barrel and stay somewhat together on the way to there intended target and not just unfold and act like a big sail/parachute and come right down to the ground?

[Bruceb]

Fold them in thirds the long way so they are about 10 inches wide then roll them up. Secret weapon.....rubber bands.
Bruce

[Bruceb]

Sorry, I misspoke a bit before. The cannon barrel is 3" pvc with a pressure rating of 260psi and the tank is 4" pvc with a pressure rating of 220 psi.
Bruce

[billbo911]

Originally Posted by Bruceb

Fold them in thirds the long way so they are about 10 inches wide then roll them up. Secret weapon.....rubber bands.
Bruce

This is exactly the folding method we used when I mentored 1147. One variation, we didn't use rubber bands, we used electrical tape. Rubber bands can come off in the chamber or barrel yielding unsatisfactory and unpredictable flight characteristics.

[Steigerwald]

I would like to point out that the components on that gun are not assembled correctly and they are not all pressure rated as you say. The components on the barrel are not assembled correctly. Those are also the components that are the most sensitive to blowing out because they receive a large pressure shock when the gun is fired.



You use a tapered bell reducer to go from the valve size to the barrel size. These components are not available in pressure ratings. While they are Schedule 40 components, that does not mean that they can be run at the same pressure as other schedule 40 parts. They take a large amount of force pushing them apart and because of that, they do not rate them for pressure.



Additionally, the components are not even pushed in all the way, this is very bad as the strength of the joint is based on the amount of material which is bonded together.

I would recommend you reconsider your design. I believe you are mistaken that this is acceptable for pressure. It is a great design and I would love to see it work, however, it is not safe and I would hate to see someone get hurt because of a explosion.

[MrForbes]

Interesting....I wonder what the actual maximum pressure is on the T shirt side of the valve? Mainly I wonder this because we have a similar situation, using PVC (although not with a reducer like that) on our T shirt cannon on the same side of a very similar valve.

[artdutra04]

Originally Posted by squirrel

Interesting....I wonder what the actual maximum pressure is on the T shirt side of the valve? Mainly I wonder this because we have a similar situation, using PVC (although not with a reducer like that) on our T shirt cannon on the same side of a very similar valve.

Let's say that an air cannon has a 120 PSI storage accumulator, and when the valve is opened, for some reason the tee shirt jams in the barrel. Since the 120 PSI just expanded to a slightly larger volume (between the valve and the tee shirt), the overall pressure will drop slightly, but will still remain very high. Regardless of whether it jams or not, all fittings in the entire shooter have to be able to withstand the maximum pressure in the accumulator, because all of the fittings near the barrel will see similar pressures in very short bursts.

[MrForbes]

Let's assume that I'm trying to figure out what the actual maximum pressure is, and that T shirts don't "jam" in the barrel...at most it takes about five pounds of force to move the shirt thru the barrel. Since the barrel bore has about 5 or so square inches of cross sectional area, the shirt will begin to move when the pressure in the barrel exceeds about one psi. The actual pressure probably depends on the mass of the shirt, and how fast it accelerates, how fast the valve opens, and all kinds of other interesting dynamic things that would be pretty hard to calculate.

[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.

Before I got involved with anything FRC my buddies and myself used to build air powered pottato cannons that used very similar parts. The only difference is that we had our air storage chamber wrap around our barrel and their was a bell reducer at the end of that with the barrel sticking out of it. The only issue that we ever encountered was once the glue let loose and the barrel slid out. This happened well before we ever hit our highest PSI level.

And speaking of PSI level the highest we ever put through the PVC was nearly 200 psi. Now in saying that we were using solenoids so that the cannon would be well away from us as we both pressurized and fired it so that if something did blow up we would be inside of the polebarn looking out a door or window.

[Steigerwald]

It really does not matter whether there is a clog in the barrel. Assume there is one by some random occurrence. That would mean that the bell reducer which is not pressure rated would be experiencing the full pressure. Not only that, but when you open the valve, there is a sudden change in the pressure which causes a shock to the assembly making it more likely to blow out.

Every component on a project like this one has to be rated to the absolute maximum pressure that is to be put in. If air flows through a component, there is a chance that it will be at high pressure. Possibly not under normal conditions, but you should always plan for the worst.

[MrForbes]

Originally Posted by Steigerwald

It really does not matter whether there is a clog in the barrel. Assume there is one by some random occurrence.

Such as?.....

[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.