This past week at the Lake Superior regional we had one of the plastic air tanks from Clippard explode on the field, sending pieces all the way up into the stands (at LSR the stands start about 10 ft above the field, with plenty of space between them and the field), and I heard there was a tank that blew in week 1 as well.
At LSR, we believe the failure was caused by damage to the tank, not a manufacturing defect (like a void in the plastic wall). The team used hose clamps to attach the tank to their robot - the screws from the clamps bit into the sides of the tank, and when reassembled the tank was slightly deformed from being squeezed by the clamps. As far as we can tell, the initial fracture point was right under the clamp, near the screw. Further, the tanks were mounted in a very exposed position, and took some hits from other robots.
I want to strongly caution teams to mount these tanks properly (with the provided plastic clips or through some other means that holds them securely without applying undue pressure on the sides of the tanks) and to protect them inside the frame perimeter. The tanks were designed for a much gentler use than we’re putting them through (mounting to a static mechanism that doesn’t take impacts or damage, and doesn’t need firm mounting to hold the tanks in place), and rough handling will eventually lead to failure!
I have witnessed this in person twice (both times in 2013) and thankfully nobody was hurt either time. But they probably could have been.
In one case, the problem was a student over-tightening a fitting to the plastic threads. In another case, we believe that Loctite was used on the fitting threads.
Last year I proposed a mandatory sleeve in this post. I think it would be a cheap, easy, and effective harm reduction measure (though it would certainly need to be well-tested)…and it would let teams color their air tanks however they see fit
I would definitely second the mention of being careful with your mounting. Any harsh mounting (hose clamps) that deforms the tank or otherwise marks it is changing the mechanics of the system rather dramatically.
The polymer used to make these tanks seems fairly brittle, that combined with the high stresses involved can lead to failure with little to no warning.
Makes me comforted to know that on Neutrino our air tanks are placed in our 3x3 6061 drive tubes so in the event of a failure it should all be contained.
Our team had our tanks on our intake a very vulnerable position. We were highly highly HIGHLY recommended by the inspectors (almost to the point of not passing) to cover our tanks. We simply added bumper material over both of the tanks. They still fit right in the same c claps they some with and they dont look all that bad. A couple zip ties and the bumper fabric and we were good to go. You can see in the video that our robot has two red tubes at the top of the intake and around 9 seconds in you can see why they highly recommended covering them up.
Here is the video. http://www.youtube.com/watch?v=nvAFLz-S9Rw&list=UUgokhIYOmu8USGMhWY2qgkw
If you are using these tanks make some covers or make sure they are in a very safe location.
A spandex sleeve (or even bumper material…) over each tank would be an elegant and easy way to safeguard folks from flying debris, and the porous nature of the fabric would prevent the air pressure from ‘exploding’ the sleeve.
1310 had an air tank explode in their pit and the pieces went right through the FRC Ball puncturing 2 layers of fabric and rubber without slowing it down. I doubt the bumper fabric would be able to contain it.
One of 1310’s tanks blew at GTR west. One of the mentors was tightening a fitting into the tank with the tank inflated to 120 psi try to eliminate a leak. The tank exploded in the mentors hands, (thankfully no injury’s) and destroyed the laptop that was below the tank, aswell as popping one of their balls. The shrapnel was sent flying all the way to the roof of the fieldhouse, and was quite a shock to those at the event.
PLEASE ensure that if you are using the tanks, that you do not over tighten the fittings. I highly recommend pipe dope (thread sealant paste) for the plastic tanks. It allows you to tighten the fittings less while maintaining a good seal.
something like this
The tanks are both a blessing and a curse, please treat them with care, as both inexperienced students and “experienced” mentors have fallen victim to these tanks.
No, no it wouldn’t. See other posts in this thread about shrapnel puncturing ball covers without issue.
I think I posted something similar last year… but biaxial kevlar sleeve or similar would be required to provide even modest protection. Multiple layers would be required IMO, and a good way to keep the ends of the sleeve closed.
Edit: what velocity air flow is needed to exert a significant pressure on a fabric? Said fabric doesn’t need to be air-tight to sustain significant loading in the event of a storage tank’s catastrophic failure. Not to mention cutting and tearing resistance to the sharp tank fragments.
Don’t do it. It’s seriously scary, especially when you’re 4ft away from it. I couldn’t hear from the ear facing the tank for about 15 minutes, and it hurt for the rest of the weekend. The shrapnel went flying into a pit about 30 ft. away, and killed our driver station computer.
Don’t do it.
So it seems that this has happened at least four(?) times in the last two years. If FIRST was serious about safety these would be outlawed.
Thankfully, the new (for 2014) Black Clippard Tanks that AM sells have a push to connect fitting that should (in theory) eliminate any failures that the connection point, or at the very least cause the hose to pop off before the tank fails. Otherwise, over tightening of threads, especially NPT threads into plastic is an issue that’s difficult to make 100% fool proof.
Assuming the proposed sleeves all are wrapped reasonably tightly around the tanks, it shouldn’t take much to contain a tank failure pretty well. In the instance you mention, it sounds like the tank burst and the pieces of shrapnel were allowed to accelerate to some speed (becoming projectiles) and then they punctured the ball. Assuming you’re containing the failure as it happens, the chances of getting projectile type shrapnel will be significantly lower (since they’ll be slowed as they contact/break through the sleeve) and if anything does mention to get past the sleeve, it will be traveling significantly slower than if the sleeve were not there.
We’re actually running two of the black clippard tanks on our robot now, and haven’t had any issues. Our mounting is fairly simple, we ran a strip of adhesive backed rubber down one of our metal structural members and then ziptied the tank to the member. The rubber does a pretty good job of holding the tank snug while also allowing it to have a bit of cushioning.
I realize what happened at GTR West and 1310’s exploding tank puncturing their ball and killing a laptop.
I can’t help but wonder, though, if even a bumper-fabric sleeve would eat a significant amount of the kinetic energy, when applied at the failure point (thereby partially inhibiting the extreme acceleration of the plastic shrapnel). Agreed that kevlar would be better, though.
I spoke to the mentor involved within a minute of the explosion. He told me that he had tightened the tank and was testing the tank when it exploded. I may be wrong but if he was tightening when it exploded he would have had shrapnel in his hands or body. There was enough force to blow the top off of a laptop and throw plastic everywhere and he had only a small minor cut on one of his hands. There are many “stories” going around about what happened. Please don’t pass around hearsay.
That said, air pressure is nothing to play around with. As we have seen an many posts about exploding tanks this year, safety MUST be taken when using pneumatics. Many people are trying to find a way around the Safety Rules by saying they are not rules. In FIRST we do not want to see people injured. We want FIRST to be a positive experience for everyone. Losing a part of your body or an eye is NOT a positive experience. I know, it won’t happen to you, but we don’t want to take a chance. Some rules are maybe a bit overboard but I would rather have that than have someone hurt because of lack of rules.