MPCNC: Belt Tensioners

The GT2 / GT3 beltĀ specs call for 10-ish pounds of tension, but I don’t yet have a good feel for the actual MPCNC belt tension … and it’s hard to measure in-situ. So I picked up some spring tensioners and yanked one with a luggage scale:

GT2 Belt Tensioner - 4 kg

GT2 Belt Tensioner – 4 kg

You’re looking at 4 kg = 8-ish pounds of tension. When they’re relaxed, the arms sit at roughly right angles.

I installed them on the far end of the belts, although that’s a bit snug under the roller:

GT2 Belt Tensioner - installed

GT2 Belt Tensioner – installed

An endstop switch will eventually add some clearance and it’ll be all good.

Even though they’re neither linear nor precisely calibrated, they’ll serve as a reminder to check the tension every now and again.

Install them with the same casual disregard you reserve for fish hooks and you’ll emerge unscathed.



  1. #1 by Vedran on 2017-12-04 - 12:12

    Adding a spring to make the system rigid… just sounds wrong to me :) At best you’ll get negligible oscillations. Why not use the zip-tie that hold it to the printed block to set the tension? Belts will never get shorter on their own, so you never have to reverse it, just add a click or two each time they stretch. On the other hand, zip-ties are not all that rigid themselves :)

    I test belt tension by two different methods:
    1. by observing the belt geometry where it wraps around the pulley – if geometry changes when direction is reversed you definitely need more tension
    2. by plucking the belt and listening to the note it makes and then balancing both sides to the same tension

    • #2 by Ed on 2017-12-04 - 15:49

      It definitely doesn’t help with rigidity. In truth, though, the whole rest of the affair isn’t the most solid hunk of machinery you’ve ever felt, so the springs get lost in the roundoff.

      The cable ties in the front-left corner tension the belts, as recommended in the MPCNC instructions, so the tensioners serve as a visible indication I’ve got it about right. I should get used “what feels right” in a while, whereupon they can come off.

      As far as tensioning by listening to notes goes, that definitely won’t be me … [sigh]

      • #3 by Vedran on 2017-12-06 - 11:19

        Android app for guitar tuning… probably better then my sense of what some arbitrary note should sound like anyway :)

        • #4 by Ed on 2017-12-06 - 14:28

          The belts definitely sound deep bass notes!