Hello, I’m a rookie member of my team. I’ve noticed a problem with our robot that we use for the off-season: after a few matches the bolt vibrate loose. Do other teams have this problem and/or how do you deal with it?
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Lots of good solutions!
By far the most common two solutions are Loctite or nylon lock nuts, depending on if the bolts go into tapped holes or are secured with a nut.
Loctite 243 is the most common, and acts like a glue between threads. Can be bought here or many other places. McMaster-Carr
Nylon lock nuts have a piece of plastic in the nut that adds extra friction and helps stop the bolts from loosening. Similar to loctite, loads of other places to buy them. Make sure you get the correct thread size. 10-32 and 8-32 are the most common in FRC. McMaster-Carr
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If you’re using the KOP chassis, you should look at this FRC Blog post: Guest Blog: Screws Loose? | FIRST
All great advice. I’ll add 1 more - Nylon patch bolts…
In general, Nylon lock nuts are the right solution when that is an option. We almost entirely avoid Jam nuts (ya know, normal nuts with no nylon insert). Remember, nylon lock nuts are 1 time use. If you take that bolt out, throw away the nut.
If you are running a standoff, like a churro rod, or Rev rounded hex shaft, then loctite is usually the best solution.
This is NOT the best solution when that standoff goes into polycarbonate (which our team tends to do a lot). Loctite eats the polycarbonate, which is bad (we had an intake disintegrate during inspection because of this…) In that case, we use a Nylon Patch bolt. Unfortunately, they are kind of expensive and also 1 time use.
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And for the really, really important connections (battery and main power electrical terminals)
I recommend Nord-Locks https://www.youtube.com/watch?v=IKwWu2w1gGk
#8
https://a.co/d/cLNtZnU
#10
https://a.co/d/dTsyfAD
1/4
https://a.co/d/2NARB9B
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While true for “real life”, I’ve found very few situations on FRC robots where I’d be uncomfortable reusing a nylon insert nut a few times. Certainly not good for bumpers that change every match, but fine for an intake that gets swapped out 1-2 times through the season. And, well, if you run out of new ones it’s better than a plain nut alone.
There’s a fun experiment to be had, using a digital torque wrench and measuring the resistance decease with repeated use. Some brands are much better than others, and it depends on the bolt material/finish.
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We too have reused them often and never seen an issue. Might be some cases where I wouldn’t, but generally, yes.
While I wouldn’t expect performance to be just as good after a removal, for FRC applications you can totally get away with tightening and loosening nylock nuts several times. I don’t know of any team that actually religiously throws away nylon hardware every time they need to change a bolt. You will feel the difference once a nylon nut is worn to the point where it won’t work.
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I doubt we are perfect.
But I think this is still and ideal practice, especially for 10-32 hardware. Each nut is 5 cents (at least the ones we get in McMaster when we are in a hurry). I’d really hate for a 5 cent nut to cause a problem on the robot.
In my mind, it’s a bad idea to put a used nut back in the box. Eventually, I think you would run into trouble.
My team likes to use nuts with a serrated washer built into it
How do they do with repeated loosening & tightening?
I’ve had great experiences with them. Depending on the size and variety they can wear out, but the failure modes are evenly matched, for lack of a better descriptor.
The outer serrations get rounded off and don’t grip the nut as well with repeated use, but the mechanism operates in a way that almost any grip at all is sufficient, as attempting to loosen the nut will increase the force acting on it, until that grip is broken by a significant torque.
Over time the inner wedges can wear down as well, but I’ve only seen this on a single unit. A 1/4-20 screw I torqued to 80in-lb probably 10-15 times. The nordlock still worked as designed, but the wear was very evident on the inner surfaces.
I think they’re overkill for many FRC applications (and can’t be used at all in some of those), but they’re excellent where thread locker or nylon isn’t the best option, like the inside of a transmission, or other hot and oily places.
Notably, if you have a very hard flanged nut with a totally flat bottom, nordlocks may not work. At least, not quite as designed.
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I couldn’t tell you. When we use them it’s normally in a place, we never have to go back to.
from what I’ve read as long as you do a visual inspection and the ridges and ramps are still intact you can reuse them.
In addition to everything said here (nylocks, loctite, and nord-locks).
Blind Rivets(aka pop rivets) are often used in many FRC fastening applications and they won’t come loose from vibration.
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Nylon Nuts are your friend, also blue Loctite for tapped holes. but do NOT use Loctite with polycarbonate or lexan. For that i would get sufficient preload in cases where they can be checked periodically or make use of nordlock washers or something of that sort.
We like to use 3M 4910 VHB tape between the parts we are riveting.
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Could you share your experience and process there? Seems almost it would act like a spring lock washer. Well, how those are supposed to work.
Our construction is plasma cut .063" thick aluminum attached to .75" square tubing. The VHB is between the parts. We have undersized holes in the sheet and match drill rivet holes.
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Is that something you compensate for in CAD? Is there a magic number for compressed VHB? Though I notice the assembly here is unaffected by the additional thickness. edit: except for the width of the “wheel well”
I’m totally trying this out the next chance I get; gussets starting to wobble a bit across the season eats at my soul.
Sorry for all the questions but I literally never though of putting something between riveted tube/plates.
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Could you ELI5 the advantage of putting the VHB between riveted parts?