My team is trying to design a t-shirt cannon robot, to build later this year, if and when we are safely allowed to do so. I have been reading a lot of the other posts on here, and on the internet, as well as searching home depot, and I cannot find where to get a piping that is both large enough to make a barrel (2 1/2" I.D.) and that is pressure rated high enough to be used.
If anyone has any suggestions with stores/websites or parts that meet my needs help would be greatly appreciated.
Copper is perfect but expensive. Muffler pipe is cheap. You could probably get a piece donated by your local muffler shop. They would probably weld on an adapter to get down to pipe thread. Gear heads love T-shirt cannons.
to everyone saying not to use PVC for a pressure chamber, yes I know that. OP was asking about how to make a barrel, not the chamber. ABS or some metal is much better for that, due to the exploding thing you all mentioned, but for a barrel that doesn’t need to hold pressure, PVC works and is cheap.
If your shirt should get stuck in the barrel somehow, what is now a pressure chamber?
In the split second between opening the valve and the departure of the T-shirt, the entire barrel is a pressure chamber. This is how T-shirt cannons (among other things) work. It’s a rapidly-expanding pressure chamber, to be sure, and shouldn’t get up much pressure (because the shirt slides easily), but if something does go wrong you’ve got a fair amount of air with no safe escape.
Now add in fatigue, because every time you do this, the barrel gets a dose of high-pressure air.
We found that you can vary the “seal” between a t shirt and the barrel pretty significantly by changing the number of folds you make before rolling the shirts. With that technique, you might find you can make commonly available aluminum or steel pipe / tube work.
Another option: you can fire stress balls instead of t shirts. They can be bought whole sale in a couple different diameters to fit whatever tube you can get, and they’re cheap enough to bring the “ammo” cost from ~$10 per shot down below $1. The downside is they are less appreciated by the crowd, so it can lead to litter.
I’ve got a related t-shirt cabin questions. I hope it isn’t thread hijacking to much to ask the group: We find that the valves can freeze up after a few shots. Does anyone else have this issue? Can this be solved with a different type of valve, or is the only solution to switch from CO2 to a nitrogen bottle? (Which would be even more expensive to fill).
Alternatively has anyone ever got a wheeled t-shirt shooter working? We had a prototype with 8" flywheels working ok in 2016 (not as powerful as the c02, but way easier to reload). It might be worth revisiting.
To be more specific: If the shirt is rolled too small/tight, air leaks around it. If the shirt is rolled too loose/large, friction between the pipe and shirt will be a problem. The goal is to balance these effects for maximum range. Even with a single barrel, If you have multiple shirt sizes to launch, you will need to find an optimal folding pattern for each size empirically.
We also use CO2 and the only time our valves (actually the regulator) freeze up is when temperatures outside are in the 30s and we fire more than five or six shots per touchdown/big play. Warm weather games amd events are no issue.
What pressure are you firing at? Do you have an accumulator?
We have a 1.5 gallon accumulator to get the liquid co2 into a gaseous form prior to launching. We then launch between 12 and 30 PSI.
Using PVC. Pressure is pressure. PCV has the pressure rating to handle the pressure as for as the pipe stress is concerned. BUT. PCV is intended to be covered. It is not rated for UV. Exposed to sunlight it will degrade. PBC is brittle. It gets hit and cracks, you will likely find out when you shoot a tee shirt out of it. The last is a CYA thing. If something ever happens, you are going to have to explain why you thought PVC was a good idea in the first place.
Pipe sizing. Sizing of round cylindrical objects is strange. The size given is generally a nominal related roughly to the ID, OD, and maybe King George’s thumb size.
What size do you need? We are running a 2-1/2 L copper tube. 2-5/8 OD 2.465 ID (0.080" wall) The only way a tee shirt fits is tightly rolled. Even then it is a tight fit. I would not go any smaller. (We tried). I think most use 3" nominal pipe which results in a much looser fit. You then have to roll the tee shirt much looser or use wadding.
You are going to have to assume that anything that will fit into the barrel will be shot out of it with as much pressure as you can get out of the regulator. Mentors are worse than the students on this. Sabots in particular are fun.
We also have an accumulator downstream of the regulator (made from about 2.5 ft worth of 2" diameter steel threaded pipe). We run the regulator between 20 and 60 psi depending on the venue. The freezing happens when we shoot a lot of shots in short succession (like you, but at warmer temperatures).
Hmm, might just be the different valves. We use a 1/4" tube to fill our accumulator and then feed the valve with a 1 inch Parker press lock hose. Maybe the small tube is able to shed the cold faster than a heavy pipe.
Our valve setup is odd though, we run a well check valve in reverse and force it open using a pnuematic cylinder to fire through a magazine fed 2.5 inch pipe.
Why not (scuba) air? It’s pretty cheap to get them filled. Sometimes firehouses will charge them for free for “educational” purposes. Ask; the worst they are likely to say is no.
In any case, any rapid expansion of gas will get cold because PV^γ is constant for adiabatic processes, PV/T is constant for processes that don’t change the number of particles, and γ>1 for all gasses. But I don’t ever recall our scuba valves actually freezing.
The only time we actually froze something with expanding air was air cannon related. At the end of a 2018 football game, we quickly drained several gallons of ~120 psi air for the drive back to the build site. A small ice stalagmite (perhaps 1 cm or 1/2" tall) formed on the running track. Curiously, the student who did this has a skin tone quite similar to Lucius/Frozone in The Incredibles; I don’t usually photo-shop, but I photo-shopped the lower half of his face into a render of Frozone doing a high speed frost run and posted it on the team slack. The image was used by several team members as PC wallpaper for a few weeks.