We had an interesting find that the drivetrain inside bolts in the center wheels had backed out and been cutting a groove on the inside plate. These go against the hub and thread into the pulley/wheel.
I didn’t pay close attention but expect they were assembled normally. And so it was a little surprising to find. But good thing we did.
We noticed the same thing on both sides of our chassis. Definitely something to add to the pre-match inspection checklist.
We hadn’t experienced this before so if was definitely concerning. I was told that the thread cutting screws this year are different, and so that was a concern that they might be at issue.
We decided that the cut was shallow enough to continue using the plate, less than half the plate thickness. And added epoxy with the bolts.
I posted here just to let people add this to their checklist. But also, @Nick_Lawrence, you probably want to see this. If you’ve had any similar issues show up. They seemed to think the threads were intact (first concern was they’d stripped them installing with drills). We’d switched back to 6" from 4" wheels, so I knew that was a potential issue.
We had to scrap 2 wheels after the bolts were tightened down too hard and stripped out completely. It was quite easy to go too far when using an impact driver. We determined the way to avoid this was to impact them down until just before the heads touched then use a ratchet or nut driver to go the rest of the way.
Thanks for the pic. Feedback like this is super important to us, especially when making hardware changes like we did on the AM14U5.
Do you happen to know how much drive time this robot has had? What sort of floors has it been run on? Do you know exactly what tools were used to assemble the wheel assemblies? Hand nut drivers, or a drill driver?
Sincerely,
Above 5 hours in drive time, but guessing just how many faux match times we may have run plus all the autonomous times. Could be up to 10 hours, but probably somewhere in the middle.
All on carpet field.
Assembled with drills, but iirc they stop short then used hand tools. This was what the students conveyed, but I wasn’t closely watching.
We started with 4" wheels and then went back to the kit 6" wheels, so it was partially taken apart in that process.
I may be able to provide more details after the season, like making sure no incorrect hardware slipped in, if I remember to.
Here’s another case. Sure wish we would have come across this thread earlier. We have probably the same number of hours and drive conditions as @ngreen stated they had. We are using Neo’s instead of CIMs, all on carpet driving, 123lb robot without bumpers/battery.
Our frame rail is cut clear through and we don’t have another one long enough to replace it. Our plan is to slip another 1/8" plate with bearing holes in it between the chassis rail and gearbox. This will move the gearbox 1/8" in from its current position and we are hoping the drive shaft will still be sufficiently long enough to reach the bearing on the outer frame rail. The alternative is to spend more time removing this inside frame rail and welding a patch into the hole, but that would take many more hours. We compete next week so we can’t afford to wait for shipment of a new frame rail.
I’m glad this happened before our competition rather than during it. Other than posting here, is there a way to notify other teams to check for this on their KOP chassis?
Hope your solution works. We also had NEOs with the 8.45 ratio on 6" wheels. Thanks for posting the screws.
This is a great case study in the difference between fine thread and coarse thread screws. The older fine thread screws have a lesser tendency to back out than the new coarse thread screws.
It still sucks for teams. Something I’m going to be looking out for with all the robots at my event next week, to hopefully help teams catch the issue early if it happens to them!
Hold on there a second.
You could also say this is a great case study in using the screw designed for the application.
The old screw is a thread forming screw intended to make coarse (10-24) threads . I’m assuming that the wheels were designed with the correct size hole for use with said fastener.
The screws supplied this year are sheet metal screws, intended to pierce a hole into sheet metal.
I was skeptical when I found those in this year’s AM14U iteration, but trusted that the engineers at AndyMark had thoroughly tested them and found them to be an acceptable substitute.
We had the same thing happen to us with similar drive time. We found it before it was too bad. We replaced the screws with longer ones with plastic epoxy for thread locker. They shouldn’t back out now.
Uh I feel like this is important enough for enough teams that it should be pinned.
After some investigation it appears that our failures may have been caused by overtightening. The student that assembled the wheels used an impact driver and inspection of the wheels noted heavily deformed or even partially torn out threads in some holes. We do not yet have enough time on the replacement wheels that were assembled with hand tools to be sure that they won’t also back out, but a visual inspection of the fasteners is going to take place after every match this weekend.
We had this happen during practice at home.
After we found that it had compromised one of the drive rails, we were able to swap that rail out with an old leftover (luckily) and reassemble with old screws (and replacement leftover wheels). This was also lucky – we had a huge bag of screws sitting around and a bunch of lightly-used HiGrips. It did also make us realize that we had out-of-spec hubs, and gave us a chance to replace the partially-worn center wheels before competition, so it wasn’t all bad. The opposite side’s inner rail was starting to show superficial scratches, so we didn’t swap it.
I was initially happy with the screws that came in the kit this year, since they had 1/4" heads, and thus didn’t require a specially-modified 5/16" socket, ground down to clear the pockets in the pulleys. However, after this incident…yeah
It’s a 4x Falcon drivetrain, stock 10.75:1, stock 6" wheels.
We first started driving the chassis around 2/20 for practice/testing on carpet (~2/3 size field), and one side of the robot stopped moving on 3/13. At that point, we had probably done about 15x 1v0 practices (match-length) and 8x 1v1 practices with heavy defense and ramming. The matches immediately before the failure became noticeable were 1v1 with T-boning. Earlier in the day, we had done some full-speed back-and-forth runs across the cable protector as well. Robot weighed about 102 lbs. for the latter 20 of those matches, plus bumpers at 10 lbs. and battery. Wheel assembly was originally done with hand tools: ratchets and sockets.
I have noticed in the past that the screws have plenty of thread engagement when used to hold the pulleys directly to the HiGrip wheels, but the addition of the hubs to this stackup reduces the thread engagement to the point that they are easy to strip out, even by hand. We stripped out 2 holes in a wheel on the hub side when making a new center wheel assembly (using old screws). One of those holes had been previously threaded with an old-style screw, and we did do the “backward-until-it-clicks” trick while threading it in.
Does anyone know where we can buy the old screws? I’d like to swap out some spares before our competition next week.
Screws continued backing out during our district event this weekend. Inspecting after every match allowed us to catch the screws backing out and snug them up each time, preventing further damage. The drivetrain will get a full teardown before the next event for a proper replacement and hopefully prevention of further issues
It may not be a good idea to use them in the same holes, since they will have to form new threads of a different pitch and geometry. If you choose to use epoxy to bond/strengthen what remains, my recommendation would be G/flex, as most epoxies don’t bond strongly to polycarbonate.
It may also be possible to thru-bolt instead, using locknuts on the spacer side of the wheel, but I have never done this for drivetrain wheels, only intake/shooter wheels.
Sometimes we have used longer wood screws for this.
But we so far haven’t had them back out again after epoxying in place through our first event.
