Build a belt drive that turns

Two pulleys and the belt that wraps them, all printable — and the three things that have to match.

A belt drive is the most satisfying thing on this list to watch working and the easiest to get subtly wrong, because three separate parts have to agree about a standard and nothing physical stops you mixing them up. All three generators take the standard by name for exactly that reason.

Pitch does not identify a belt. GT2-3M and HTD-3M are both 3 mm pitch and they are not interchangeable — the teeth are different heights, so a belt cut for one climbs out of the other. Choosing by name selects the whole row at once: pitch, tooth height and belt thickness. The same trap sits in the chain sizes, where #40 and #41 share a 12.7 mm pitch and different rollers. If the belt and the pulleys disagree on the name, the drive will look perfect on screen and skip on the bench.

Tooth count is the input, everywhere. A belt is sold by its tooth count, and pitch length is simply teeth × pitch, so the loop closes exactly with nothing rounded. The pulleys are sold by tooth count too, and the ratio between them *is* the reduction — the same relationship gears have, without the meshing constraint, because a belt is happy to span whatever distance you give it.

Centre distance is read, not chosen. Once the belt length and both pulleys are fixed, there is exactly one distance at which the loop is taut, and the row tells you what it is. Build the frame to that figure. Guess it instead and you get a belt that either will not go on or hangs slack, and the fix is a new belt rather than a new frame.

The belt back is the number that decides whether this prints. It is the thinnest continuous section of the loop — the material above the tooth roots — and on the finest profile it is 0.62 mm. A 0.4 mm nozzle lays that as one and a half lines, which is a belt that snaps the first time it is tensioned. This is the honest limit of a *printed* belt: go up a profile, print it in something flexible, or treat the printed loop as the thing that proves the geometry before you buy a rubber one at the same tooth count. The generator makes the same belt either way.

Flanges keep it on. A timing pulley can track a belt by friction alone right up until something is slightly misaligned, and then the belt walks off in seconds. Guide flanges on one pulley of the pair are the usual answer; on both is belt-and-braces and costs nothing but print time.

Let the bearings do the alignment. A print-in-place ball bearing comes off the plate turning, and two of them on a shaft hold it parallel far better than two printed holes will. Pick them by trade size and the bore, outside diameter and width come from the published table — so the day you would rather have a steel 608 in there, it drops straight into the same pocket.

1. Open the Generate panel and pick Pulley / sprocket (under Gears & drives). Leave it on Timing pulley, choose the GT2-2M profile, and set Teeth to 20 and the Bore to your shaft.
2. Duplicate it and set the copy to 40 teeth. Two tooth counts are the whole reduction: 20 driving 40 turns the output half as fast with twice the torque.
3. Pick Timing belt from the same group and set its Belt profile to GT2-2M — *the same name*, not just the same pitch — then its Pulley A teeth and Pulley B teeth to 20 and 40.
4. Set Belt teeth to the loop you want. The Pitch length row is teeth × pitch exactly, and the Centre distance row solves for where the two shafts then have to sit — 100 teeth of GT2-2M is a 200 mm belt, and across a 20- and a 40-tooth pulley that puts the shafts 69.7 mm apart.
5. Read the Belt back row before you print. At GT2-2M it is 0.62 mm, which is thinner than two passes of a 0.4 mm nozzle — GT2-3M more than doubles it, to 1.3 mm, and GT2-5M reaches 1.9 — go up a profile, or accept a belt that is a demonstration rather than a drive.
6. Pick Ball bearing from the same group for each shaft, by trade size — a 608 is the skateboard bearing in every parts drawer. It prints already assembled.
7. Measure the centre distance onto your frame, print, and check the belt runs without climbing the flanges before you load it.