Anyone else seeing their GT3 belts do this

I’ve noticed that on my 2 Y axis belts there is a spot that looks like the picture below. It’s 13" from the front on the right side and 16" on the left, I’ve not really done anything aggressive nor far enough back on the Y axis yet (that I’m aware of) for this to have been caused by the pulley skipping.

Anyone else seen this on their GT3 belts?

It looks like the spindle got bound up and the pulley kept turning and ate the belt. At least you know the pulley was on the shaft tightly and the belt was properly tensioned.

Weird, because I watch the machine like a hawk when I’m cutting and I’ve never seen nor heard any changes in the behavior…

Guess I’ll change the belts and watch it closer :wink:

I had the belt get caught by the edge of the smooth idler once ad it looked just like that. I had made some adjustments prior to that happening and my belt was too loose and got caught.

i recommend to get a belt that is GT2 belt, 9mm wide and stronger.
SDP/SI part number A26R51M080090 for $6.44 each.

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Will that belt directly replace the stock one with no changes? If so where did you get it?


Interested to see where you got the Z axis belt as well. I replaced my X and Y pulleys and belts with 6mm GT3 from openbuilds parts store.

Its still 6mm so the smooth idlers and clips at the ends stay the same.

This ?

looks like GT3 6mm…

Darryl bet me to it.
Looks like the “3M” series is the same as the GT3; they also have steel cord re-enforced GT3.
They have numerous widths available; just make sure your pulleys can handle the width.

That pulley also says it has no set screws, in addition to a pilot bore, so you’d also have to drill and tap set screws…

I think I’ll stick with the 6mm GT3 pulleys from OpenBuilds, but I may try their steel-reinforced belts next time, as long as they’re 6mm.

Replacing the Z closed loop belt with a different pitch belt can be tricky, due to:

  • the fixed center-to-center distance between the pulleys (maybe there is a sub-mm wiggle room)
  • and the introduction of new pulley diameter(s) where the belt sits

the new belt will need to have
teeth=(piradius_of_pulley_1 + piradius_of_pulley2 + 2*center_to_center_distance)/pitch

This was my hope initially when I saw the 4 slots on the Z plate, but it is the circular pocket that the motor’s circular protrusion fits that restricts adjustment to something just over a hair.

Actually, the diameter matters as it changes the two pi*r parts (red) below.

t: teeth total
r: radius (not the outer, the one where the belt sits)
d: center to center distance
p: belt pitch

If you go with different size (i.e. teeth #) pulleys, it gets more complicated. I started drawing that as well and after the first arctan, I thought to google for it… and indeed I did not need to reinvent the wheel. There’s plenty of calculators out there.

This one has a nice visual too. Just divide your belt length with the pitch in the end (3mm in our case).


After looking at the Z plate CAD, you are right about the 4mm range of adjustment. The pocket where the circular top of the motor sits is 38.5mm dia, and NEMA23’s top circular protrusion is 38.1mm dia. So there is indeed 4mm of forgiveness. All you need for the exercise now is the “belt diameter” of the GT3 pulley.

Btw, is this one 8mm bore or 1/4" ? It mentions both! Is it a copy paste error from the specs of the old 1/4" one? Looks like 8mm.

Continuing the monologue :slight_smile:

Short version:

SDP-SI 6R53M060060 ($6.75)
Misumi GBN1803GT-60 (4.76$)

Long version:

After finding a calculator specific for the kind of belting we are dealing with which already knows the pitch diameters in question, it’s pretty easy to pick a belt.

The center to center distance taken from the aforementioned CAD is 74.5−13.98=60.52mm. Taking the +/-2mm of adjustment range into account, when using two 20T GT3 pulleys, the minimum and maximum belt loops that correspond to 58.52~62.52mm are 177~185mm. Valid belts for this range are

59T, 177mm
60T, 180mm
61T, 183mm

So picking the 60T, 180mm makes most sense.

They just corrected it 1h ago, after I asked on their forum which of the two is correct.

SDP-SI 6R53M060060 ($6.75)
Misumi GBN1803GT-60 (4.76$)

Ok, here are the GT3 9mm width 60T (180mm length) versions of the above:

SDP-SI A 6R53M060090
Misumi GBN1803GT-90

Glad to help. Btw, I follow your build on the other thread. Good luck!