I'm trying to build a robotic arm, and I had the following built so that there was the minimum possible movement created by the gear wheel (that connects to the worm).

However, there is still play, where all the pieces are pushed together by the force of the gear wheel the worm drive is connected to... this leave a small (but significant) gap, allowing the cross axel to slide.

Is there any way to build this without the play, as it doesn't help when I'm trying to build an arm with a fairly high level of accuracy?

enter image description here

4 Answers 4


You could introduce an arbitrarily sized gap between the red and yellow bushes in the depicted design, thus forcing the other yellow bush and the black gear against the frame. If this gap collapses due to the forces generated by the worm screw, you could place thinner, non-LEGO pieces (washers, bits of paper, etc.) between them to act as spacers. This won't win any prizes for beauty or elegance, but it might just reduce slop to bearable levels. It would also increase friction.

  • Washer... brilliant!! As you mention, not exactly "orthodox" but hey, that's got to work! Will let you know how I get on :-)
    – freefaller
    Commented Jul 14, 2014 at 10:53
  • Perfect, works like a charm :-)
    – freefaller
    Commented Jul 14, 2014 at 12:07

There are 2 ways to get it tighter. First is to use the new worm gear part: http://rebrickable.com/parts/15457/Technic-Worm-Gear-2-Axle-Holes

The other is to use an axle with stop. Have the stop on the end which has the worm gear pulling it in. Not sure which end that is, so experimenting would be your best bet.

  • Thanks @tl8, I wasn't aware of that part, although not entire sure how it would be tighter, as the 1x3 part would just be replace 1x2 and two 1x0.5
    – freefaller
    Commented Jul 14, 2014 at 8:04
  • 3
    The 'old' worm gear is not exactly 2. It is more 1.9. Additionally, there is no friction on the gear so it slides a lot. The newer one is exactly 3 and has plenty of friction that reduces sliding.
    – tl8
    Commented Jul 15, 2014 at 0:11
  • Ahhh... that would definitely explain it. I will look at getting a few of the new ones, but for now zovits suggestion of a small washer is working perfectly for my current needs. Thanks for your time :-)
    – freefaller
    Commented Jul 15, 2014 at 6:47

Pre-tensioning or pre-loading, the way that most CNC machines work. In essence this means a constant force pushing one way, so that the slop doesn't matter. Often you can do this with gravity, and if slop at one (or two) when you rotate is acceptable then that may be all you need. If you do have variable-gravity parts that need pre-tensioning, try to make that dynamic so you're not fighting both gravity and preload on some movements.

It may be possible to work around some of that using strings'n'springs, so you have hidden counter-pulls on some heavy parts (the way old school drafting desks and anglepoise lamps do).

As you might gather from high-end metal work using springs or gravity for pre-load, there's no way to actually machine anything without some slop. Lego has a fair bit of slop by design, so you're going to find it even harder.

  • Yes, @Mσᶎ I think you're right, it's going to be pretty difficult to stop this. I guess I need to take the slop (wasn't aware of that term) into account
    – freefaller
    Commented Jul 14, 2014 at 8:05
  • 4
    one reason for my answer was to drop a bunch of those terms in so you can search for solutions more effectively :)
    – Móż
    Commented Jul 14, 2014 at 8:22

Use LEGO Rubber Bands as spacers.

enter image description here

LEGO has several sizes of rubber bands - some are quite small, and they are perfect for small spacers when doing builds like this. Just twist them a couple times and you're set.

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