pic: T Shirt Shooting Robot

I finally finished the robot. It was one of our old robots from 2002 that was pulled to pieces. We were able to get it up and running again, but this time, we’re adding a few cannons! The side panels will be custom made by another student. And hopefully after building this, we’ll be able to add lights like underglow. The goal is to shoot this off at our school football games, Especially the homecoming game.

There are:
-6 spikes and 4 victors.
-4 on-board compressors.
-An old basic controller.
-2 T Shirt cannons with sprinkler solenoids

If you have any comments or questions feel free to ask!

Any chance we will get to see this in action at the TNT event this year, or will we have to wait till the 2010 Florida regional?

I am going to quote artdutra04 from another thread on the reasons to not use PVC

If you want to read more about this topic, just search CD for “PVC cannon”

Well we’re hoping its ready for football season. So… I’m positive you’ll see this beast at TNT.

And we understand the hazards of PVC. But we glued everything and made sure that there are no leaks. The PVC is very heavy and was brand new when we glued it. So as of right now there is NO chance of it breaking. These cannons have been through a lot, and I know that a lot of PVC cannons have bursted… but because this is brand new PVC there shouldn’t be any chances of it shattering… just an end cap or reducer popping off. But we made sure that the glue was perfect and its definitely covered. Thank you for your concern. As of right now we’re looking for nice metal tubes in our closet for our 3rd Gen tanks.

I wasn’t going to post in this thread (I’ve posted enough about the dangers of PVC) but this post has enough poor information that I needed to post.

New vs. old PVC – Makes very little difference. PVC can fail on the 1st or on the 1000th shot, with no (or very little) warning.

Glueing PVC makes it stronger – Depending on the type of glue (or cement) this will (more than likely) weaken the PVC as it breaks up the polymer chains within the PVC. Typically bonding plastics to plastics weakens the strength due to the bonding process at the molecular level.

“there is NO chance of it breaking” – Never assume nothing can go wrong or something cannot break. Things can always break with enough force applied in the wrong (or right) directions. Instead plan on worst case and make it safe. Especially if it will be near the public.

I am very happy to hear that you are looking into metal tubes for your next generation t-shirt launcher.

Sorry to be so negative … but pressurized PVC and the public always brings out the ‘safety police’ in me.

I would set this aside as a proof of concept, and not fire it until the PVC has been replaced with a safer material.

The cost is really worth it, it shouldn’t be much work to replace them, and it could potentially save someone’s life (assuming extreme failure), prevent decent injury and/or making your team look very amateur.

We had it working with these cannons last year, but there were a couple design flaws including the cannons that we are going to fix now.

Yeah the PVC is not a good idea. The only way to make them PVC and not risk them shattering is if you fiberglass coat the PVC. Which is really cool if you’ve ever seen it done before, doesn’t come cheap though.

::safety:: To all of you out there who detest PVC it is meant for this. You are all underestimating the streingth of PVC. It is put in sewers and all over houses unless you are using paper thin piping or PVC that is not made to hold pressure it is actually better than metal ::safety::

The only thing I’m going to say to that is: You don’t know what you’re talking about, and you’re comparing apples to oranges.

PVC is NOT used within houses. It’s commonly used outside, underground, for irrigation systems. Note that irrigation systems are never used with air, unless they are being drained (and, in that case, they are left open at at least one point to release pressure). Also note that the ground will contain most catastrophic failures of reasonably sized PVC pipe. Within the house is always metal, often copper.

And no, PVC isn’t used for sewers, either. I would guess that either that’s ABS or it’s a specialized pipe plastic. Again, note that sewers are underground and are often not pressurized (at all).

Note: in all of the cases you cited, the PVC is underground and used for liquids, not gasses. A T-shirt cannon is above ground and used for compressed gasses, which is a completely different case. If you’re going to use parallel examples, then please use examples that are truly parallel.

To be fair I’ve seen quite a bit of PVC pipe used underground but in houses. Probably not what’s supposed to be used but it’s there. Regardless, a pressurized liquid and a pressurized gas are completely different forces (especially because gas firing tends to shock load), and PVC should never ever be used to hold any kind of pressurized air for any period of time.

PVC IS used for sewer. It can and will gain pressure while draining, but like stated above it does not compare to air. Draining from the house into the main city sewer system is where PVC is used. Many houses are piped with PVC as well for draining, but not water that comes into the house. The main sewer that provides for the city Liquid pressure is different, clay pipes were used and their being replaced with concrete and a special plastic, which is also PVC, just a REALLY high grade… With sprinkler and irrigation systems, only when the valves are opened to release will the pressure be at the minimum. Most irrigation systems have a storage tank to help irrigate faster once the valve is opened, and the pump fills water into that tank. While all the valves are closed there is at no point a release for the pressure and the water will pressurize in the PVC. I understand that this is “apples and oranges” but I wanted to make that clear. My irrigation system at home and on my grandparent’s farms up in Delaware use schedule 20 which is extremely thin walled. Now we have slowly replaced that over time. Now we use schedule 40 which can handle up to 200psi or 330psi depending on the diameter of the PVC.

PVC is rated on only straight lengths. Those lengths are rated on their diameter. 220psi is a good range because that is what 4” diameter schedule 40 PVC is rated at. Anything smaller, the psi it can handle will grow. Of course that rating is specifically for liquid, but it can also be used for air. Obviously those cannons will NOT withstand 220psi. Because of the end caps and joints, the stress levels are different. With normal cement the PVC is rated for about… 133psi. heavy duty cement is rated a lot higher. We will be launching approx… 80-100psi MAX. We WILL be shooting at a lot lower PSI, and we will change our barrel of our cannon to a smaller diameter to allow for more distance instead of raising the pressure. 80psi is a LOT better than 120psi. and after gluing the PVC together, we understand that the cement really did its job. We had our guys wearing gloves, because you get burned instantly if it gets on your hands. It even got onto the gloves and they still felt heat. Yes PVC is not the best option for this. But there was a table created to explain the Max Operating pressure of both schedule 40 and 80 PVC along with their Bursting Pressure. That’s the point where it’ll just EXPLODE into a million pieces. Anything above that rated pressure will raise the issue of stress levels on the end caps and reducers/increasers that we attached.


Now I can’t remember if we used Schedule 40 or Schedule 80 because I don’t have the cannons with me now, but we’ll assume that it was Schedule 40, just in case. The max operating pressure is 133psi for 4” diameter PVC. The bursting pressure is something we will never hit with normal competition compressors… So we won’t account for those. The pressure regulators we have on there will only allow up to 100psi. PVC is a very good material for pressure, especially for liquids. The reason why everyone has been fighting this is because if there is a crack, a hole, or terrible gluing job (like only half an end cap for ex.) the air will expand rapidly upon that initial failure. Liquids will usually leak and crack the PVC. We and 100% positive that we glued the tanks correctly and there are no existing leaks. The brand new PVC won’t pose a problem in itself because it hasn’t been exposed to the sun for long periods of time and its brand new. The joints are different, because they will be where the PVC will fail if at all. One of our lead mechanical students thought it would be cool to incase the cannons with a cover, like you see on battle ships and tank turrets. We decided that heavy duty lexan would do the trick (since we found a bunch after cleaning our closet) and the tanks will be covered by that, which should deflect the danger of a failure a bit. Its understood that it could still burst through our cover and pose danger, but we’re positive and confident in our design that there won’t be failures, and if at all the safety taken while running the cannons and robot will be an extreme priority. We load the cannons while the tanks are empty so there is no accidental launch. Launches take 10 seconds with a HUGE warning light to signal that it is about to launch. No one will be within touching range of the robot. We will be about 50-60ft away to allow for driver interaction through the old wireless radios. And with the casing around the tanks, a failure will be contained within the robot, extremely damaging the inside of the robot.

So safety has and will always be an important factor with this project and robot. Even though a lot of people are furious with the PVC, we’re confident that no catastrophic injuries will exist from the explosion of PVC, just the usual scratches from FIRST robotics.

If any of you guys have more comments please PM me personally. I feel that it would better serve its purpose and we can work out the thoughts. Because I know there is a danger with it, and there is a danger with using the machines and tools in our machine shops, and there’s a danger with the robots we build, cars we drive, bridges we cross, everything we use and touch, but I think some of you are ill-informed on what we are actually using and I want to make sure that you know what schedule PVC we’re using, what pressure, what adapters/end caps/reducers, because if someone were to think we’re using Schedule 20, they would think we’re stupid! But I would really appreciate your insights and wisdom for this project.

The reason why everyone has been fighting this is because if there is a crack, a hole, or terrible gluing job (like only half an end cap for ex.) the air will expand rapidly upon that initial failure.

Partially. The other fault is that it is nearly impossible to tell when PVC is on the brink of failure. That’s the main thing.

Normally with PVC you will hear the air leak, normally with anything with air pressure. The tank would have to be instantly filled up to pressure to cause it to explode. But if no one notices the air leak, and you continue to fill up (though it WOULD be slower) you can and probably will experience the failure.

I only have a few comments left.

  1. I hope nobody gets hurt; the change to metal cannot come too quickly for me.

  2. Again, do not assume that water and air pressures will behave the same in the same material.

  3. MURPHY’S LAW. Also note that this is a case where nothing can go wrong, so invariably it will go wrong all at once when it does. Also read “The Wonderful One-Hoss Shay” by Oliver Wendell Holmes, Sr., a poem about something that was constructed with no weak points.

  4. The cover also can’t go on fast enough for me. Again, safety. Remember the fan covering debates we had this year?

  5. Usually when a teenager is 100% positive about something, especially if it involves something dangerous, I head for somewhere safer.

Any time pressure is released, an air tank is instantly being applied pressure on all surfaces equal to the air pressure inside the reservoir. So this instant fill up… happens every time the gun is fired.

But if no one notices the air leak, and you continue to fill up (though it WOULD be slower) you can and probably will experience the failure.

Except PVC doesn’t have a “leak” warning. It fails instantly and catastrophically.

A leak warning would be hearing the noise. I’m almost positive that you will hear it, and we have on past tanks. You hear it on robots with pneumatics or anything with leaks. Its the exact same thing if there’s a leak in an inner tube or a track ball. You can even notice the pressure drop while testing at a low pressure (we watch our gauges and test before going up to full pressure). So I don’t believe that is correct. But I do believe that the instant failure is correct.

I’m also confused about that… What do you mean exactly? Because if the air is released, shouldn’t the pressure on the surfaces inside the reservoir be just a few psi (or whatever is left over. usually 0-5psi)? And how is it instantly filling up in the gun is firing? that’s an instant release… I think I’m confused here

Wait… Let me get this straight. You have never heard a leak in PVC. You have heard it on pneumatics or “anything [air-filled] with leaks”. (The edit is because you don’t exactly hear a water leak in the same way–or a steam leak.)

Am I correct?

Because if I am, you probably don’t know what an air leak in PVC sounds like. It’ll likely sound similar, yes… But it won’t be the same. You see, an FRC robot’s pneumatic system has two types of leaks. One is from a punctured tube, the other is from a poorly sealed valve. A bike tire or trackball sounds about the same as a puncture tube; a poorly-sealed valve will be hard to hear due to the slow nature of the leak. And in fact, I’d rather you had that than a leak in the PVC itself.

What a pneumatic tube, a bike tire, and a trackball have in common is that they are “soft” plastics–thin-walled, flexible, and somewhat “bang-resistant”–that is, they won’t make a big mess if they do burst. There are still FIRSTers who can tell you of the doublers in 2004, which, if mistreated, could go all at once on a slight hole. Being flexible, they can bend a bit out of the way. PVC pipe is not nearly as flexible, especially at the size you’re using. And if you do develop a leak, you’ve probably got about 1 shot to realize you do before something happens. You might not hear it, you might not notice a pressure drop, but something’s going to give and you hope it isn’t catastrophic. That’s why we’re pushing you guys to switch to a safer material, such as metal–PVC isn’t exactly “bang-resistant”. (Metal isn’t either, but it can sure take more pressure than PVC.)