13-07-2009 12:46

techtiger1

Re: pic: T Shirt Shooting Robot

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?

13-07-2009 17:59

jason701802

Re: pic: T Shirt Shooting Robot

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"

13-07-2009 18:24

gallo26

Re: pic: T Shirt Shooting Robot

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.

15-07-2009 12:23

Daniel_LaFleur

Re: pic: T Shirt Shooting Robot

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.

15-07-2009 16:29

AdamHeard

Re: pic: T Shirt Shooting Robot

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.

15-07-2009 18:25

Jacob Paikoff

Re: pic: T Shirt Shooting Robot

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.

16-07-2009 10:05

techtiger1

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 17:54

KRUNCH DUDE

Re: pic: T Shirt Shooting Robot

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

18-07-2009 18:03

EricH

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 18:52

Chris is me

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 20:32

KRUNCH DUDE

Re: pic: T Shirt Shooting Robot

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.

http://www.engineeringtoolbox.com/pv...res-d_796.html

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.

18-07-2009 20:36

gallo26

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 20:57

Chris is me

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 21:07

gallo26

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 21:12

EricH

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 21:55

Chris is me

Re: pic: T Shirt Shooting Robot

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.

Except PVC doesn't have a "leak" warning. It fails instantly and catastrophically.

18-07-2009 22:06

gallo26

Re: pic: T Shirt Shooting Robot

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.

18-07-2009 22:11

gallo26

Re: pic: T Shirt Shooting Robot

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

18-07-2009 22:34

EricH

Re: pic: T Shirt Shooting Robot

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.)

18-07-2009 22:39

gallo26

Re: pic: T Shirt Shooting Robot

You can hear the leak of the escaping air, and if you submerge it under water you see the bubbles. Our PVC has leaked before, and you can hear the air escaping. and I know that PVC with running water you don't hear it, but you do see the water that it leaks. We had a rather bad gluing job done on an older version of our cannons and you can clearly hear the leak as well as see the pressure drop. So sorry, but i don't think your correct.... because i do know what it sounds like, and I have seen them in water

18-07-2009 22:40

Chris is me

Re: pic: T Shirt Shooting Robot

Pressure extered by a fluid is exerted evenly on all surfaces of its container. So when the valve is released and 120 psi of air are instantaneously in the pipe leading to the t-shirt, all of the PVC is experiencing 120 psi of force.

18-07-2009 22:43

gallo26

Re: pic: T Shirt Shooting Robot

Ohhh ok, so your talking about the barrel then? I gotcha! that makes sense... thanks!

18-07-2009 22:44

EricH

Re: pic: T Shirt Shooting Robot

It's physics and such. Let's use some everyday examples...

You always have 14 psi pressing down on you, due to atmospheric pressure. But now, you decide to have some fun. You go into a room that is totally sealed, with an air compressor hose coming down to a smaller chamber that seals to your skin. You have gauges and things of that nature all over the room. The room gauges read zero to start--they've been calibrated so 14 psi is zero. You pressurize the smaller chamber to some higher pressure (say, 50 psi as the gauges measure). Now, something jerks the small chamber away from your skin. Instantly, all the pressure is released from that chamber into the other chamber, and air being air, it fills the entire space as fast as it can. The gauges now measure more air pressure than zero (how much more depends on the size of the room and the size of the small chamber). Instantly.

But you've rigged this room with a small air cannon, and it's a small room, with an insane amount of pressure in the small chamber. Suddenly, the small chamber breaks, and all that air finds the cannon, which is loaded. The ammo moves under the pressure, which at the instant the chamber broke, was applied throughout the room, and shoots out, releasing all the pressure. The room returns to the normal air pressure of 14 psi, the gauges read zero, and you now have to reload and try again.

Does this help?

18-07-2009 22:49

gallo26

Re: pic: T Shirt Shooting Robot

Yes sir! I wasn't sure what part he was talking about, tank or barrel. Thank You

18-07-2009 23:27

acidrain2012

Re: pic: T Shirt Shooting Robot

We built a launcher last summer out of spare parts and various scraps, and we managed to blow a 200 lb solid steel door open 90 degrees at about 110 psi. We found that the t-shirts work best when either taped or zip-tied (if you are going for distance). If you aren't going for distance, then there is a way to fold the shirt so it opens when you launch it, making it much less dangerous. We have used it at a couple of tractor pulls, and we are planning to mount it on our robot from the 2006 season (the robot's name is Tank, so we are planning on making the launcher rotate). To help with safety concerns, we found that it is best to keep the launcher and air storage tanks in separate locations, along with the hoses (if there are any). Hope the build goes well!

19-07-2009 10:20

Daniel_LaFleur

Re: pic: T Shirt Shooting Robot

*sigh* ... so much misinformation...


PVC leaks cannot be heard as they will cause the PVC to fail violently. The leaks that you describe are leaks at the bonds or joints, not in the PVC itself.

Lets consider PVCs pressure rating for a moment. PVC is only pressure rated for liquids (and for a very good reason):
Water is incompressable, much like a steel block ... if you push down on a steel block with 50 PSI it will not compress and if you release that force instantly the steel block will not move.
Air, on the other, hand is compressable, like a 50 pound spring. Apply 50 psi to it and the sprin compresses. release that force instantly and the spring not only goes back to it's original shape but rockets off of the surface it was on.

Now on to PVCs material properties:
PVC is rigid polymer. This allows it to flex while maintaining it's strength, however once it is flexed too far it will crack and the PVC pieces will then (violently) attempt to return to their original shape. PVC is extruded which means that it's wall thickness is not consistant. Also each time it is flexed it gets reduced in strength (each pressurization cycle flexes the PVC).

If the PVC pipe had pressurized liquid inside then when the PVC pipe fails all of the pressure within the pipe is released but does not violently explode because it is like the steel block and was not compressed.

If the PVC pipe had pressurized air inside then when the PVC pipe fails the air will violently rush out the failed spot which will continue to apply pressure to the now failing spot causing it to fail further. Pieces of PVC at this point can be ripped off and carried (or shot) by the escaping air.

In short, a failure of the PVC (not a bond or joint leak) will not be 'heard' until it violently fails. There will be little to no warning. The PVC is not rated for pressurized gas, and the shock loads that a cannon put on PVC are far beyond what it is designed to hold.

Now I'm not saying that yours WILL fail, but considering that you will expose the public to any failure you have, I would suggest that you take every precaution possible to ensure the publics safety. FIRST (and the FIRST teams) do not need the political ramifications from a publicity stunt gone bad.

P.S. @ Krunch dude. The chart you referenced is out of ASTM D1785 which specifically states that PVC is for liquids only and that those numbers do not reflect gas pressurization.Read Abstract here.

03-11-2009 09:37

robself705

Re: pic: T Shirt Shooting Robot

Here is our t-shirt cannon

http://www.youtube.com/watch?v=xKCwbomY82c

14-10-2015 19:21

Pierce

Re: pic: T Shirt Shooting Robot

I know this thread is pretty much dead. None the less, I am the current president of Krunch 79 and we are still using the T-shirt cannon, completely unaltered. Just to get a gauge for how much we use this, we fire it off about 20 times per barrel each football game. So every week for a 3-4 month season for 2 years now. We have not had any problems with this robot, no leaks, noises, or explosions. I do understand the risks and effects of using weak PVC, but due to time and cost constraints we are holding off on the much need repairs until mid 2016. Please do not continue to educate me on the dangers of PVC as I am well aware off the dangers of using PVC. I also do not want any comments on why we waited 6 or so years to replace the PVC as I said we had time and money constraints. Just looking out for my fellow Krunch members, Gallo!