Free spinning or Locked Screw Joints?

Hello everyone,

I was wanting to use screw joints for my drivebase, but was confused as to how the screws should be mounted. I’ve seen people say to keep nuts just loose enough to allow the screw to fairly spin while others suggest the screw to be locked. My intuition would think that a free spinning screw would lead to less friction, but if there’s a reason why it should be one or the other, please let me now.


From past experience, not at all. If you’re using the circular inserts, they cause a lot of friction (at least for me they do) and are not worth using, but are easier to put on than shafts. Shafts generally have lower friction when bearings are used properly, but take up much more space.

No, screw joints have less friction than shafts and are much easier to make and maintain (at least in my opinion).
Vex does have pretty bad tolerances on the green circle inserts, so I drill all of them out with an 11/64 bit. The newer gray ones are a lot better, and I haven’t tried the new bronze ones yet.

To answer the OP’s question, you want to lock the screw to the c-channel with a keps nut so that it cannot rotate, as this will add more friction.


I was wondering why you opted for a keps nut over something like a nylock. Is it because the teeth of a keps nut better ensure that the screw doesn’t rotate?

People use keps nuts instead of nylocks because it is very torturous to screw a nylock all the way down 2 inches of thread.

Use a wrench to make sure the keps is fully tightened to the metal.

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You can also use a second keps threaded the other way and tighten them into each other. If you don’t want to use a nylock, that’s probably a good idea. Maybe also threadlock?

You can practically build a new bot from the non-nylock nuts left on the field after a match.

How many nylocks I’ve seen on a field in all my time with robots? Zero.

Cordless screwdriver and nutdriver. Buy them. Extra long tips that will go through a 1x3 c-channel. They exist.


Where do you get them?

If you properly tighten your keps, they should never come apart. I’ve been running mostly keps on my bots for the entire season and the few times where I’ve had a keps fall off or come loose on my bot was because I didn’t tighten it enough. A wrench and screwdriver will suffice. If it doesn’t, do some push-ups.

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Budget brand driver… main thing is torque control to you don’t overtighten things.


These are good, but only go through 1x2.


These are 3" long, should go through 1x3 easily. You might have to spin them in the driver and sand down the tip just a little. Worth every penny.


And my favorite assembly/disassembly tool. Will speed up your work more than any other tool. This is a premium brand, but you can buy cheaper at wal-mart, harbor freight, etc.


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Continuing the discussion from Free spinning or Locked Screw Joints?:

I agree, nylocks >>> keps nuts 99% of the time.

But for multiple 2” long screw joints it’s very tedious to use nylocks there, unless you just hate your builders.

If anyone’s screw joint it falling apart on the field… i think they have bigger problems …

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The nut on a screw shaft must be absolutely tight (that answer the OPs question).

In general, nylocs are better. Use them by default.

One reason for using keps on screw shafts is that they take less space than nylocs, even less than the low profile RoboSource nylocs. Space on axles is scarce. Also, the opposite face of a keps nut is flat and provides an even balanced surface for spacers or washers that interact with it.


Thank you!
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