The Belt That Lost Motion Every Direction Reversal: Backlash in a Belt Drive
A belt-driven positioning axis. Move forward: accurate. Move in reverse: the carriage didn’t move for the first 0.5 mm of commanded motion. The belt had stretched on the forward move. When the direction reversed, the slack in the belt had to be taken up before the carriage moved. This is backlash in a belt drive, and it’s not the same as belt stretch from over-tensioning.
What’s happening
A timing belt has two strands: the tight side and the slack side. When the axis moves forward, the tight side is on one end. The belt tension is balanced (both sides tensioned). But when the direction reverses, the tight and slack sides swap. For a brief moment, the belt is slack on the new tight side. The motor rotates, taking up the slack, but the carriage doesn’t move. That lost motion is the backlash.
For a properly tensioned belt, this lost motion is small (0.05-0.1 mm). For an under-tensioned belt, it can be 0.5 mm or more. The customer’s belt was under-tensioned.
How to measure it
Command the carriage forward by 1 mm. Then command it backward by 1 mm. Measure the actual carriage movement with a dial indicator. If the carriage only moves 0.5 mm back (instead of 1 mm), the lost 0.5 mm is the backlash. For our axis, we measured 0.4 mm of lost motion on reversal.
The fixes
| Fix | Backlash reduction | Cost |
|---|---|---|
| Retension the belt | 0.4 mm to 0.05 mm | Free |
| Use a double-sided belt (preloaded) | 0.05 mm to 0.01 mm | New belt |
| Switch to ball screw | Near zero (with preload) | New mechanics |
| Move always in one direction (homing offset) | Avoid backlash in program | Program change |
What we did
We retensioned the belt. The belt tension was 5 N (way too low; should be 15-20 N for a 6 mm GT2 belt). We adjusted the motor mount to increase tension to 18 N. The backlash dropped to 0.05 mm. The customer’s requirement was 0.1 mm. We met it.
For future builds where precision on reversal matters, we spec a preloaded belt (two belts, or a single belt with a tensioner that preloads both strands). Or we use a ball screw. A belt drive is fine for positioning in one direction, but for bidirectional precision, the belt will always have some reversal lost motion.
The program workaround
If you can’t eliminate the backlash, program around it. Always approach the target position from the same direction. Move past the target, then back up to it. The carriage always arrives from the same side, so the backlash is consistent and doesn’t affect accuracy. This is standard for belt-driven axes in pick-and-place applications.
The tension check
Belt tension should be checked with a tension gauge (not by feel). For a GT2 6 mm belt, the recommended tension is about 15-20 N. A gauge (like a Gates tension meter) measures the belt’s natural frequency, which correlates to tension. Set it to the manufacturer’s spec. A belt that’s too loose has backlash. A belt that’s too tight wears the bearings and the belt itself.
The backlash rule: Lost motion on reversal is belt backlash, not belt stretch. Retension the belt to spec. If you need bidirectional precision under 0.05 mm, use a ball screw. For belt drives, always approach targets from the same direction in the program. The belt that lost 0.4 mm on reversal was under-tensioned by a factor of 3.
