Re: T-shirt Launcher
 

well.... thats complicated. we have tried to get in contact with them on numerous occasions. I mean this town was practically built on them. Its across the street from my house and sits there dead. Basically the company wants to get rid of the branch in long island and has wanted to for a long time. I know Mr.McLeod from team 358 was one of their previous employees. Basically all that gets done there now is movies. They went from 10s of thousands of employees to hundreds. This thread alone has some valuable knowledge about what they used to do http://www.chiefdelphi.com/forums/sh...t=29295&page=2 but from my understanding they no longer support first much. I could be wrong but they havent ever replied to our phone calls or emails and even though the place is across the street there isnt exactly a front entrance. I know they support the PoBots another local team the town over but I assume they had a parent or faculty member who knew someone to get them in contact with the company. Its a shame because its our towns legacy but we would be without our biggest sponsor (cablevision) if they didnt sell some land to them a while back so I guess its not all bad.

Re: T-shirt Launcher
 

I see the liability. Thankfully or should I say unthankfully none of our mentors are enginners. Our lead mentor teaches tech our second lead is a programming and math teacher and our third mentor is the orchestra teacher. But... yea I see what your are saying our bot exploding in the middle of a crowded place isnt good for anyone and is especially bad if its at a real arena like madison square garden (im pretty sure we cant afford to fix that) I see the point and my final decision is no to pvc. However I will be on the lookout for tubing.

Re: T-shirt Launcher
 

http://www.northropgrumman.com/Corpo...uidelines.aspx

I just did a lot of surfing and found this...going to bookmark it and figure out what to do.

Re: T-shirt Launcher
 

Great idea! Good find.

Re: T-shirt Launcher
 

Okay, I have quite a few people disagreeing with me saying that the failure mode for PVC isn't water hammer, but no one presenting evidence. If you simply read the wikipedia entry on water hammer, you'll see that is it common with air too. With water it isn't explosive as it is with air because water isn't compressible. Even the engineers studying this airplane crash attributed it to water hammer.

And it is true that in a water-system you can introduce compressible trapped-air to prevent water hammer because you've introduced a 'spring' and suddenly you have a second-order differential equation-- that system won't be able to find 'reflection' to 'hammer' the end into a pressure spike.

And I've seen some other people saying that the barrel will "fail catastrophically" but it's not clear to me how even that can result in shrapnel from the back end of the PVC when the barrel-end of the system is OPEN. And again, I point out, I've done this and I've seen PVC fail using it for air pressure on a test system for train brakes. I didn't shrapnel, it just blew at a joint. I still maintain that the barrel won't introduce shrapnel, even in a catastrophic failure. It will blow a joint or split.

Also, when I blew up that accumulator tank that I was using, it was while quickly opening then CLOSING 1-inch dia piloted valves. I maintain you have to close a valve to get water hammer and with these systems, you never close a valve under high flow.

If I'm wrong...and I sometimes am...provide me with a logical explanation of why and how PVC--rated to usually 100+psi for water--fails with air.

Re: T-shirt Launcher
 

Also, if you don't want liability, don't build a t-shirt launcher. The purpose of this machine is to aim projectiles at the crowd.

Projectiles are dangerous and litigious.

Wow this has become a pretty big debate can't wait to see some evidence from both sides.

Ive seen pvc crack under pressure (tried to make long paralletes out of pvc and when I did handstand on it it cracked) thd crack was super clean and smooth.

But putting flex on it and air pressure through it are different. I kinda want to see a video of someone's pvc cannon exploding or an experience of that. If there's nothing I'll still try not to use pvc but won't go crazy if we need to.

Re: T-shirt Launcher
 

http://youtu.be/IVO4_hUvFsc?t=1m8s

Keep in mind that the forces involved in this video are far less than those that can be produced by the typical FRC robot.

Re: T-shirt Launcher
 

well of course it eventually exploded. It has a closed front. Our launcher barrel would obviously be open.

Also dropping a weight over and over again from 12 feet would damage anything.

Re: T-shirt Launcher
 

Please read This paying attention to the failure modes and summary.

here is another website stating not to use PVC with pressurized gasses.

and here's OSHA telling you not to do it.

Oh, and BTW,your cannon barrel is NOT open as you have a t-shirt inside that is acting as the restricting orifice that allows the pressure to build up behind it ... which is why the t-shirt flies out.

One last thing. I happen to be someone who has had a 1 liter volume, made of PVC, with 100 PSI inside of it blow up less than 2 feet from me. There was no warning of the impending explosion. The only reason I am here to talk about it is that it happened to blow out the opposite direction. I am one who knows what the failure mode is like (back then I didn't, and I'm lucky my ignorance wasn't fatal). So when I say don't use PVC for pressurized gasses I know what I'm talking about.

Re: T-shirt Launcher
 

The pyrotechnics industry also highly discourages using PVC pipe. When PVC fails, it shatters and creates splintered shrapnel which can seriously injur or kill someone. They use EPDM pipe or fiberglass for mortar tubes from 1.75" up to 12" diameter (for the big 4th of July shows). EPDM pipe is more expensive and a little harder to find that PVC pipe, but fails in a more ductile (vs. brittle) manner.

http://www.pyrouniverse.com/FAQ.htm#9 (this has some good examples of failed PVC pipe)

This is for information only. I don't encourage anyone to build a t-shirt canon with any plastic.

+1

Re: T-shirt Launcher
 

This is a little off to the side of the main conversation, but here is the T-shirt launcher we made last summer. It uses a balloon tank with a sprinkler valve and 2 Viair compressors. It has a water separator to keep moisture out of the tank (our last one rusted through the bottom). It also has a analog pressure sensor so I can set the pressure shutoff psi.

It runs off a cRIO-II. We will be putting addressable LEDs on soon.

http://youtu.be/H_IjhJFgQIY

Re: T-shirt Launcher
 

First of all any rigorous discussion on fluid dynamics that mentions water hammer and air in the same sentence gets points off. But this is CD and not rigorous. :0
Taking the second reference first. The "water hammer" reference was discounted by the NSTB. The accepted conclusion was aluminum has a fatigue life & the plane exceeded it by the high number of take off & landings relative to the relatively short flight miles of the air frame. See the Dehavilland Comet failures for another classic example of the fatigue of aluminum and stress risers which BTW effectively handed the commercial airplane market to the United States.

First example second. You should really read the article. Water hammer requires a quick closing valve on the exit of the fluid column. The pressure spike comes from the rapid deceleration of the fluid column. The spike is increased by the weight (directly related to the density) & the velocity of the column. It is decreased by the compressibility of the elements involved. So when you shoot a tee shirt, you might see actually see a vacuum in the barrel when the tee shirt exits the tube. Hard to predict what happens upstream of the valve without the specifics. I expect we see a vacuum spike in the accumulator of our cannon. it is a relatively small volume & we precharge it to 20-120 psi & fully discharge it on each shot. The cannon will constantly shoot a 70 yard field goal.

The reason why you get a large number of "PVC" works for me is that the pressure involved is well within the working pressure of PVC pipe. Failure is not a regular occurrence. The issue comes from the severity of the failure mode & the documentation from both the pipe manufacture & OSHA saying don't do it.

Really simple design and all we would have to buy is the barrel sprinkle valve and a water seperator(we are pretty close to the ocean and humidity is an issue) what barrel and tubing is that BTW.

We have 2 balloon time tanks that are filled but we can probably dump it all by making balloons for little kids.

Re: T-shirt Launcher
 

The tubing is just high density pvc hose. It's tied off at each end with hose clamps.

The barrel is just some 1/4" thick strong plastic type stuff. I don't know exactly what it is, but we have used the same barrel and hose for the last 10 years.