Air Cannon
 

Hi,

We have created a simple air cannon to launch T-Shirts and possibly confetti. I've seen pics on Chief Delphi of other teams that have done this and launched shirts at events such as football games. Before we present it to our principal, we were wondering what you did to prove its safety and convinced the school to allow it. Also it may help to show that other schools have successfully done it. Thanks!!

Re: Air Cannon
 

Well, we never got it approved through the school, we just did it. :)

Re: Air Cannon
 

Psch, your guys' cannon is dangerous. The high school from IRI has a few new holes in its ceiling :rolleyes:

Re: Air Cannon
 

Our school hasn't forced us to do a safety run. We just talked to the athletic director and he thought it was cool. Before the first game, he may require some demos though.
As for proving it's safe, I'd say do some test fires, maybe have people catching them up in the bleachers, prove they aren't getting hurt.

Re: Air Cannon
 

Is any part of it made out of PVC? If so, you may want to look into shielding it (in case of rupture) or using higher-PSI rated fittings. (PVC, though often labeled on the pipe itself as able to hold enough PSI to be considered "safe", is never supposed to be used for any sort of pressure application.)

Replacing the PVC with safer alternatives, such as air hoses and metal fittings rated for high pressure gas use with sufficient PSI rating (including a safety factor), would be a smart idea.

Re: Air Cannon
 

No PVC is used for stored pressure (that's in a craftsman pressure tank).
The barrel and a couple of fittings are PVC, but these will never be under high pressure as the barrel is open.
(Any further questions, comments criticisms, PM Mike Nawrot. It's his baby.)

Re: Air Cannon
 

I strongly suggest that you remake the barrel out of ABS plastic. Unless you have a burgoiuse amount, it should be pretty cheap (I got 5 feet of 2 inch dia ABS plastic for about 6$).

Even though the PVC will not be subjected to pressures over 60 (probably) for more than one second at most, if PVC fails it will shatter, whereas ABS will split or crack.

So, in terms of safety, Cracking > Shrapnel.

Re: Air Cannon
 

Believe it or not, simply painting the cannon very nicely can be enough to convince people of its safety, if you are having trouble with that. People are easily swayed by appearance.;)

Re: Air Cannon
 

MARS Team 1523 created a t-shirt launcher over the summer too and has also run into school approval issues. Initially it was approved for use as long as it was mounted to a robot. Now the high school will not allow the "launcher's" use without team liability insurance. (We've found that calling it a "launcher" rather than "shooter" or "air canon" garners greater support. The earlier post about painting it is also true)

As far as safety, we discussed the design with an ME, EE, & PhD Chemist. (MARS used heavy wall schedule 40 PVC (whole thing) and fill the launcher to 80 psi.) Here were their thoughts : Regarding PVC bursting, it is dependent on the pressure and size being used. Typically what will happen is a crack will occur and slowly propagate at lower pressures. At the pressures being used in our launcher, crack propagation causing more than a leak would probably only occur if a weakness was present (such as the PVC was poorly made - which could be said of any material being used) or if mistreated (pouring a solvent on it, using a sledge on it, etc.). Where care must occur is in the glue joints, which if not done properly could leak or, if very poorly done, come off rapidly.

With others in the CD community suggesting PVC explosion, shrapnel, etc, I'm curious if they still feel this is a concern with launchers built using designs similar to MARS (heavy wall schedule 40, 80 psi fill).

Re: Air Cannon
 

I'd still see it as an issue. Since the chance for failure is always present (and mostly the damage is microscopic but at those pressures you will magnify the problems) and PVC does tend to react in the way it does, it would really be your best bet to switch to ABS plastic.

Search on YouTube for Potato Cannon explosions. You'll notice all of those ones are PVC. That's why I built mine out of ABS. ;)

Re: Air Cannon
 

I don't think a total rejection of heavy wall PVC is warrented. Schedule 40 is rated for 200 psi and that's a conservative number (probable factor safety of 2). Household water pressures can run 70 psi (think of all the pvc used without a problem, i.e. water exploding from pipes) and I know of businesses using PVC tubing for compressed air running 120 psi.

Re: Air Cannon
 

You'll also notice that all proper usages for PVC are underground, and that's for a reason...

Re: Air Cannon
 

Probably because it weakens with long term exposure to UV light. A quick coat of paint should fix that right up. (assuming it is the correct type of paint, one that doesn't weaken the PVC)

Re: Air Cannon
 

It's actually against OSHA guidelines to use PVC for any kind of (non-buried) compressed gas use.

And easier way to think of the PVC vs. non PVC debate for air cannons is to compare acrylic to Lexan. We all know that acrylic may work for many uses on our robots, but we one needs to baby it in order to prevent it from cracking and shattering. If they used Lexan on the other hand, they don't have to worry as much about catastrophic shattering.

Re: Air Cannon
 

:D

Re: Air Cannon
 

We're using a freon tank on our robot now. We used a PVC tank for over 3 years and never had trouble with it, but the team, including myself, were waiting on it to blow like a bomb anytime. Since this robot's main purpose is to demonstrate around people (most of the time little children) we changed the tank to metal with a blowout hole in case something goes catastrophically wrong. We are still using PVC for the shooter, and regular flexible hose for delivering the air, but all the pressure is behind the valve and really never on the cannon. We've also limited the amount of time that the valve can remain on for. I think its like 1/10 of a second or something so it uses less air, and also reduces the chances for something to go wrong. I think we will limit the pressure to 100PSI even though the tank can handle 300PSI. I'm not sure how much pressure our sprinkler valve can handle, and I don't want to find out either :D. With 60PSI I can shoot a t-shirt over 300 feet over a tree with with a 1/10 of a second shoot. Here are some pictures of our setup:

Re: Air Cannon
 

I've decried the use of PVC tanks before, and I stand by that. I would never use PVC for storage of any more than atmospheric pressure. I've seen what it can do, and i know someone with the scar to prove it.
However, the odds of damage to the barrel of the cannon are fairly low as the air is bound to follow the path of least resistance. In a sealed tank, this would be any crack or weakness in the tank. In an open barrel or the pipe to an open barrel, this would be the opening in the barrel, with only the resistance of a light shirt.
Is there some risk of a leak, a crack, or a blowout? Yes. Would that risk exist with any material? Also yes, although perhaps it would be a smaller risk with heavier/stronger materials. Like everything, it's a case of acceptable risk.

Re: Air Cannon
 

The air cannon we built is pretty simple. We already had a sprinkler valve (which limits flow to 110PSI) and used PVC rated at 280PSI. I'll bring up changing to ABS. I uploaded a picture here. Thanks for the advice!

Re: Air Cannon
 

Definately change the air tank to ABS. What you (or whoever) was effectively holding was a bomb.

Re: Air Cannon
 

The comparison given is very helpful; I was unfamilar with OSHA's PVC use restrictions. ABS is clearly the better choice.

Since MARS can not meet the school's team liability insurance requirement, there are no current plans to remake or publicly use our launcher.

One additional safety consideration we had with our design was preventing someone being shot pointblank or directly, so we mounted it at an approx. 45 degree angle on our 2005 bot and discussed use of a remote enabler that an adult would control.

The launcher was a fun summer building project; liability issues, probably more than the build, have opened eyes about all the stumbling blocks encountered when trying something new.

Re: Air Cannon
 

Liability Insurance.
Thats a biggie. Is it worth $1500/yr or more for $1M policy to shoot a T-shirt into the stands? I think not!

I've seen industrial accidents happen with stored pressure. They ARE NOT PRETTY. Fortunately, nobody was seriously injured, but the insurance is there for a reason.

For tank safety, there is an aluminum-welded tank on ebay for about $16.00. This should solve that issue. Contact me if interested,for the link.

Of Course the kids-will-be-kids factor is always in play. At my high school, they fired an air cannon after touchdowns. It was located on the flat roof of the announcer's booth just to limit who had access.

Re: Air Cannon
 

When it comes to air cannons, there's one general rule of thumb I keep in mind: PVC+Pressure= Explosion and blood stains on the metal shop floor. Just don't do it. If you want a pressure tank without spending too much money, sears offers a $20, 5 gallon craftsman tank, rated for 135 psi. That's the equivalent of 13.6 feet of 3" PVC. That's a lot of air. Our cannon, which is based on this tank, shoots t shirts a good 200 feet, and we have a relatively bad barrel to tank ratio (there's a lot of wasted air, but having a massive barrel is more trouble than it's worth. We're working on it). The operating pressure is limited to 100 psi by the kit's auto-shut-off switch, and just in case, the relief valve is at 120 psi. The only PVC component that holds any pressure is the sprinkler valve, which is rated for 150 psi, so there's a decent safety factor. It's all mounted on a robot, so it's remotely operated. I'd consider it safe, but there's no such thing as too safe when it comes to pressurized air. So general rules:

1. A safety factor of 1 generally isn't good enough. It's like having exactly 0" clearance between moving parts. It works on paper, but that's it.

2. Nobody stands in front of the barrel. Everybody should be behind the cannon, except for those in the crowd, but by then, air resistance should have slowed the projectile enough to prevent it from hurting anyone.

3. Test everything countless times before using the cannon in any crowded places.

4. Having a safety relief valve is a good idea

5. PVC DOES NOT SAFELY HOLD PRESSURE. Even if it says it's rated for pressure, that's at standard operating conditions. PVC will shatter easier in the cold, and even if it's at the rated pressure, any sort of unusual stress or strain will cause it to explode, leaving nasty cuts on anyone in the blast radius. From what I hear, stitches on your face aren't fun.

Overall, accidents can be prevented with the use of common sense, and by reading the labels. Have fun, be safe, avoid hospital trips, medical bills, and law suits.

Re: Air Cannon
 

think about these things -

what happens when you launch at the wrong angle and shoot someone (maybe a small child) with the rocket er. I mean shirt, at close range?

Are you willing to stand in front of the launcher and take a direct hit to the stomach, leg or head? If not, why do you want to risk 'letting' some by-stander?

Most sporting events have returned to throwing shirts into the crowd - there is a good reason.

There is a hole in a ceiling tile at the top of a high school gym (40+ feet up) where a shirt hit. Suppose that had been someones face?

Re: Air Cannon
 

Actually this adds to that point. The sad thing was that the ceiling tile that broke from our cannon was just air. The shock wave from it did that at only 60psi. I know when we did it at IRI, we would make sure everyone in front of it almost to the second level was sitting down plus we would block the crowd off both ways. I still felt uneasy shooting where we did.

Re: Air Cannon
 

WOW! I'm glad I asked this question. I definitely didn't know parts of this were so dangerous. We are changing to ABS or a non-PVC tank and will do extensive safety tests.

Ours doesn't launch with lots of power. At a 45 degree angle and 110PSI, a shirt goes approximately the height of a 2 story house. Last Thursday, we tested it by first hitting a mostly empty trashcan, which it did not knock over (but did rock). One of the parents then volunteered to get hit at close range (10ft) and was fine. Then some kids got hit at catching distance and were fine. The lack of extreme power in our launcher may be due to the barrel being so much larger than the tank and the shirt not being too tight in the barrel (allowing air around it). We'll try to be careful and scrap the plans if it looks dangerous.

For those that do launch at football games, pep rallies, etc. what other safety precautions have you taken?

Re: Air Cannon
 

110psi = Lots of pressure. I wouldn't go above 60, or hell, anything above 1 bar with PVC (1 bar = average atmospheric pressure, 14.7psi).

Keep in mind, the short length of the barrel is why your launcher isn't "powerful". Firearms can acheive higher projectile speeds with longer barrels because of the physics involved, and the same thing is applied here. The longer barrel means that the projectile (in this case, a shirt) has pressure applied to it for a longer time. The short barrel means that the shirt gets a quick blast of pressure (which I'm sure some of it is lost because I'm assuming you don't put the shirts in a break-away casing that would create a better seal) before the air can escape the barrel and dissipate, no longer imparting it's energy upon the shirt.

Now, for a much safer system...

Use the FIRST air tanks and mount them to a dedicated launcher-bot (simple frame, tank drive, 2/4 air tanks, etc) that has a pneumatic system for an ARM instead of a cannon. Do the math and figure out the general trajectory of the different pressures (one bar at a time, ie 15psi, 30psi, 45, 60, 75, 90, so forth) as well as the boom ratios (1:2 ratio? 1:3? What can your pneumatics handle? How well does it work?) and such.

This way you know the trajectories it launches at, about how far and such, and it's as controlled as it can be every time.

-Otaku

Re: Air Cannon
 

As for the air cannon safety issue, the answer is pretty simple. Just don't wrap elastic bands around the tee shirts. If you just roll them up, if you shoot them at a high angle* shot (like a mortar), they will just open up in the air and flap and drift downward at a very safe speed. You'll only get about half the range of what you normally would, but it'll be a lot safer.

* Another safety benefit of shooting tee shirts this way is that with very high angle shots, it becomes almost impossible to hit anyone at point blank range. A cannon with a three foot barrel, held four feet above the ground, at 60 degrees above the horizontal, means the end of the barrel is six and a half feet above the ground. Unless you're Yao Ming, is a fairly safe launch position. ;)


The FRC-legal pneumatics are too puny for air cannons. You need at least 3/4" NPT fittings to get a flow rate good enough to shoot a tee-shirt a considerable distance; the biggest fittings in the KoP are only 1/4" NPT. (From being bored at a meeting experience, no matter how many tanks and hoses in parallel you add, if you limit yourself to only KoP components, don't expect to shoot much more than a ping pong ball.)

Re: Air Cannon
 

Uh oh... i was trying to design one of these air cannons on a dime using a 3" internal diameter barrel 12" long (abs plastic, .25" wall thickness) and was going to just use four of the 16cu in clippard air cylinders as air storage.... I was not however using standard IFI air solenoids, i got my hands on some used (free) Mac Valves with a Cv constant of 30, 1/4" npt.

With the hose included, if me and my Machinery's Handbook calculations were right, I should get an output rate from each clippard/valve pair of 38CFM, or 152CFM for all four pairs. If i switch that over from CFM to cubic inches/second it is 4377.6 cuin/sec. The volume of my barrel is 84.823cu in, so that should fill the barrel (and eject the tee shirt) in 0.0193 seconds... which is 35mph.

Was this velocity not high enough? It stands to reason if i double the cylinder/valve pairs i should be able to get this up to 70mph if it isnt. Or, is there another cheap alrternative (remember clippard cylinders are cheap and the valves i'm using are free (have 10))? What's the cheapest way to switch that much air on with one valve? Whats this I hear about irrigation valves?

Sorry, i'm new to the T-Shirt cannon (high flow pneumatics) business... :rolleyes: Any help would be much appreciated.

-q

Re: Air Cannon
 

I didn't mean for you to make an Air cannon with FRC parts. I meant to make a pneumatic catapult. Since it doesn't have to be FRC-legal, you could bump it up to 1/2" ID tubing if you need to (But I'm sure everybody has tons of the 1/4 stuff)

Re: Air Cannon
 

Well, yeah thats my problem. I have a lot of 1/4" fittings and such... however, I'm thinking I'm going to switch to a Craftsman 5gallon pressure tank instead of the clippard cylinders... so i'll probably be moving up to 1/2"

So.... does any body know where i can get a high flow solenoid for 1/2" for cheap?

thanks,

-q

Re: Air Cannon
 

You can get a one inch solenoid-operated lawn sprinkler valve for under $15. They can be retrofit with a pneumatic release for manual operation or if you wanted to use a 12 volt valve from the KOP to operate the larger valve. I believe the standard lawn sprinkler valves are nominally 24 volt.

Re: Air Cannon
 

What kind of ABS? The kind at our local Lowe's and Home Depot don't have pressure ratings? Is it just the generic one they have, or do we need a special type? Thanks!

Re: Air Cannon
 

Schedule 40, should say "SCH40" on it. Schedule 80 if you want to, as well.

Re: Air Cannon
 

An interesting article entitled "Kids, Do Try This at Home...We built a backyard cannon, and you can, too" appeared in the September 2007 issue of the Electrical Engineering magazine: Spectrum. Since so many of us enjoy this stuff, here's a link to the article online: http://www.spectrum.ieee.org/sep07/5483 Who wouldn't like to try some of the projects in the book cited. (Do pay careful attention to the safety issues it discusses before trying projects).

Re: Air Cannon
 

that was so classic

Re: Air Cannon
 

OK a air cannon how to make one quick and easy for about 50-75 USD you need

1. PVC pipe
2. Sprinkler valve
3. pneumatics tube
4. pneumatics fittings

take the sprinkler valve top off and drill a hole in the top of it in all the other holes plug up air tight. then let it dry for at lease 10 to 20 min. Make your cannon as big as you want but make sure you can fit it on the sprinkler valve. make sure one end has a cap on it and leave one end open for your barrel. drill a small hole in the end of the end cap and put a pneumatic hose fitting on it and connect you hose to the end of your gun. then screw you barrel and the end that you put the hose on to the valve then put a air trigger on the hose comming from the top of your valve and pump it up and you are good to go have fun and be safe my buddys put a hole in there wall :)

Re: Air Cannon
 

Please, for safeties sake. Do not use PVC for compressed gasses, it is not safe.

Re: Air Cannon
 

Before this thread goes badly downhill like the other one, take a look at this:

http://www.chiefdelphi.com/forums/sh...ht=Matt+Howard

Pay attention to the last page. :)

Re: Air Cannon
 

Hey all... I just wan to clear some stuff up about PVC, and there really isn't a better way to do that then to borrow Some words from a physics prof on a forum I frequent.

In short, as long as everything is set up correctly, and safety checked multiple times, there is very little that can go wrong, especially while operating at 100psi and under... a 2 to 1 safety factor on top of standard is really good.

I'm not saying this is a good Idea, but people in my local offroading club have been using PVC as air tanks for portable systems for years. They mount these on bottoms of bumpers, roll cages, and other places where they are almost sure to get hit, and HARD. When I went to my first meeting and saw that, I remembered threads like this from when I frequented CD and asked if they'd ever had any blowouts or problems associated with this, and I was surprised to find that there had only been one... And it had happened when the guy forgot to put PVC cement on an end cap.

Hows that for working in the real world?

As a side note, the reason he says to test the system with liquid is because at pressure, the main difference between water and air is that air will explosively expand, and water won't. This means that if you have a leak and fill it with water, you won't have any sort of explosion, and if it holds up for five minutes at 200 psi, you know you won't have any problems below 100.


Then again, It's pretty easy just to weld up a steel tank.

Re: Air Cannon
 

One of my good friends built a potato gun using PVC pipe as the air storage tank.

He was outside, with the tank pressurized and loaded, getting ready to launch one. He accidentally dropped it from a height of <3 feet...the tank exploded sending shards of PVC everywhere. One of those shards lodged itself into my friends forehead.

He's ok, but he still has a harry potter like scar on his head. I'm not advocating the use or non-use of PVC as a tank here...but a story like this is definitely something to consider. Safety is always imporant, especially when working with pressurized vessels...

Re: Air Cannon
 

How far are you planning on shooting. If you don't have to shoot far, a PVC tank is fine because you don't need a lot of pressure. I attached some data I took from out air gun in the hopes it will ease some minds. Sorry it is in a documents. I couldn't make it fit any other way.