At the 2023 Western Ontario district event, a robot inspector said we could not mount a 10lb mass with 4 (four) 250 lb tensile rated zip ties. This is a factor of safety of 100x
A 600lb tensile load rated plumbing strap was offered as an alternative by the inspector. Links below to the zip ties and plumbing strap. This would work great, but would require holes to be drilled in the weight in question to use.
Is there a factor of safety that inspectors would consider safe?
I’m not trying to challenge the inspectors or be difficult, just looking to get zip ties that are correctly rated to add dead weight to underweight robots without requiring the drilling of holes in the COTS weight.
In 2019 we strapped a 1x1x12" steel ballast to our drivetrain using a combination of VHB (very high bond) 3M double sided tape and beefy zipties, did not require any holes in frame.
The VHB secures against shear/translation and the zipties prevent prying/peeling off the VHB. It was rock solid and we had no questions about it.
I’m an RI. Direction I have received directly from several LRIs (and one of the chief RIs) has been that zip ties are not considered secure for bumpers, batteries or ballast.
TBH, I get it. I’ve seen loads secured with too-few rivets to keep the load from moving along the axis of the rivet, or jankily placed threaded fasteners that I’d rather see some sturdy zip ties on that passed.
As Alex_Y notes below, consider dynamic loading. The bottom line is that if there is any wiggle room for your load (whether battery, bumper, ballast, or whatever), those dynamic loads multiply like crazy when FRC robots hit each other or a field element. There are more than a few times I’ve told teams amenable to advice that their mounts will pass, but that if I were a mentor on your team, I’d have you do something to keep the load from moving to reduce the shock loads.
Only under static loads and that’s against ultimate tensile. Under dynamic ladings with cornners stressing specific points, among several other factors, the loading is significantly higher than 10lbs and which will quickly eat away against the margin.
Photos of how you attached the weight would be helpful in this. Theres likely a degree of freedom not accounted for.
Last year, 7461 at Sundome attempted to add some ballast to their 3rd pick, 4104, using a cinderblock. While it was well secured vertically secured using zipties and rope, the inspectors had the concern that it would be able to slide out horizontally of the zipties/rope, and find its way onto the field. 4104 ultimately removed the ballast before the beginning of playoffs.
The issue with zip ties is that under collision, the moment of point load, specifically on the corner of the steel bar can be extremely high. The 10 lb weight can become effectively 88 pounds or so if 2 robots collide where both are travelling at 14 ft/sec, which is a relatively common speed among FRC robots. Several hits could wear through the plastic of a zip tie, after which it could become a projectile. 5672 was also looking to add several pieces of steel ballast in the 10 lb range at the Georgian event and I believe they used plumbing strapping to do so, although I only saw them acquire the said strapping and didn’t actually see it employed so this is somewhat speculative.
For reference, I have a climbing carabiner handy for reviewing that can withstand 24 kn of force, which is roughly 5400 lbs of force, for loads that are maybe a 400 lb human on the end of a nice soft-ish rounded rope (ie no sharp corners), which itself stretches to ameliorate some of the force so that it doesn’t all get loaded in one single moment.
If you have a belly pan on the robot, using the plumbing strapping with some holes drilled through and bolted with washers to distribute the load that would result in no holes in the frame rails, maybe that’s a possibility? Also, if you have either no belly pan or an aluminum one, replacing it with relatively heavy gauge steel before your next event is very worth considering.
I don’t know about zipties, but we have personal experience that rope can be a very bad idea – a kettle bell broke loose and completely trashed our robot.
I completely understand dynamic vs static loading, hence the significant factor of safety. Hence using such an overkill amount and strength of zipties.
I don’t have a picture of the weights in situ, but these are the weights in question. They were planned to be mounted around a 2x1 above the kop chassis, and butted up against another frame member. One weight on each side.
Ballast has been discussed on LRI calls this season, given the nature of the game and the relative light weight of most robots. The general direction we’ve gotten is that zip ties are not considered secure mounting for ballast, given the dynamic loading we see as robots drive around and run into each other - the static tensile strength ratings for the zip ties don’t represent this scenario. Metal fasteners are much preferred, with the final determination made by the LRI at each event.
We also should be considering more than just the fasteners used on the ballast. We also need to consider what the ballast is attached to. You can have some beefy bolts going through your ballast to hold it on, but if those are going into some thin wall tubing they likely won’t last.
For what it’s worth, I’ve seen 100lb zip ties pop off on the field when a robot hits the wall the wrong way, having to be replaced to keep the small polycarb panel on before the next match.
Thanks for clarifying, I was picturing raw steel blocks, the weights you’ve pictured with no sharp corners those are a lot less of a concern, remaining question would be the corners on the tubing it would have gone around. With multiple zip ties that could likely have survived a match, and replace zip ties before next match if any signs of abrasion showing.
The decision becomes “if these weights break free, we’ll likely be e-stopped” (and likely discussion with HR), which scenario is more likely, a tip because we need more weight low vs. possibility ballast breaks loose and we don’t finish the match.
While volunteering field side a week ago, I saw a relatively large (>10lb) steel bar come off a robot and get left on the CS. It had been attached using pop rivets. It was given to the LRI, who visited the team’s pit to return it, and discuss how it might be reattached more securely.
The zip ties holding our battery “tab” broke in a somewhat mild collision in playoffs. The battery moved enough to take out the CAN bus to a swerve module in what is likely our final match of the season (play-offs are a little more violent and losses are unforgiving).
