pic: T-Shirt Launcher, still needs a chasis

[Fe_Will]

Since you have access to a CNC I recommend Using a Geneva Drive to index the chambers. We have some experience with one and it works well if manufactured to close tolerances and well lubed.

[reversed_rocker]

i'll talk to one of the mechanical guys and see if we can get some drawings out on the internet (they lock me in a dark closet and make me write code all day). As for the exploding part, the only part of the cannon that holds pressure is the air tank on the bottom of the barrel, i'll see if we can get that replaced with a safer material, all other valves, pipes, ect. are metal.

Video: http://www.youtube.com/watch?v=0cHf3iY4h9k

sorry the video isnt that great, we finished the cannon right before build season. the website isnt quite up to date either.

[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.

[Daniel_LaFleur]

Originally Posted by 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

I can tell you, first hand, that those warnings are for good reason. I have had a 1 liter, 400+ PSI pressure rated PVC volume explode less than 2 feet away from me while holding less than 100PSI air. I can tell you that I am truely lucky that I came away with no injury (to speak of ...).

This accident was not the result of unrated PVC or overpressurizing it, or even shock loading it (Like you are doing with the cannon). This was the nature of PVC and of how it fails. PVC gives little to no warning before it fails and when it fails it fails catastrophically.

While I Highly recommend that you replace the PVC, if you are dead set on using PVC then I would suggest wrapping all of the PVC parts (My suggestion would be Twaron over your idea of Fiberglass).

While building devices like this is cool and a great learning experiance, please, please, please don't ignore the safety concerns just because you don't think there is an issue. You need to Know all the safety issues, especially if it's going to be shown to the public.