Design Review: GT2 Timing Belt on a Linear Axis
This is the review notes from a linear axis that started slipping under load. The belt was GT2, 6 mm wide, on a 20 tooth pulley, driven by a 400 W servo. At low speed it tracked fine. At high acceleration, the belt skipped teeth on the small motor pulley. The review caught what the BOM selection missed.
Review item 1: Belt width vs tooth load
Design intent: 6 mm GT2 belt, 20 tooth drive pulley, 100 mm/s belt speed, 5 kg load on the carriage.
Reviewer note: The belt’s maximum tooth shear force is about 18 N per mm width for GT2 at 20°C. A 6 mm belt handles 108 N. At 5 kg accelerating at 2 m/s², the belt tension is: F = m·a = 5 × 2 = 10 N. That’s well within 108 N. So why did it skip?
Because the belt was at 3000 rpm on the motor (3000 rpm × 20 tooth × 2 mm pitch / 60,000 = 2 m/s belt speed). The acceleration peak was higher than 2 m/s² — the servo’s S-curve hit 4 m/s² during the first 50 ms. That’s 20 N. Still under 108 N. But the belt tension was too low. A loose belt doesn’t transfer force through the teeth — it peels off the pulley.
Action: Increase belt tension to the manufacturer’s recommended value (about 5 N per mm width for GT2, so 30 N total). Re-tension at commissioning. The initial tension was 5 N — way too loose.
Review item 2: Pulley diameter and belt fatigue
Design intent: 20 tooth GT2 pulley (20 mm pitch diameter). Belt speed 2 m/s.
Reviewer note: The minimum pulley diameter for GT2 is about 14 teeth. 20 is fine. But the belt is bending around a 20 mm pulley at 2 m/s. That’s 2 m/s / (π × 0.02) = 32 belt bends per second. Over a year of 24/7 operation, that’s about a billion bends. GT2 belt life is rated at about 10⁷ bends. You’re at 100× the rated life.
Action: Either increase the pulley to 30 teeth (reduces bends/second by 50%), or switch to AT5 belt (thicker tooth, longer fatigue life on small pulleys). The belt that slipped wasn’t worn — it was running at the fatigue limit.
Review item 3: Tensioning method
Design intent: Fixed-center distance (motor and idler pulley on fixed positions). Belt tension set during assembly.
Reviewer note: A fixed-center belt stretches 0.5–1% over its life. After 6 months, the tension drops. A tensioner (spring-loaded idler, or adjustable motor mount) compensates for this. Without it, the belt gets looser over time and slips more.
Action: Add an adjustable motor mount (slotted holes) for re-tensioning, or a spring-loaded idler on the slack side. Mark the tension point on the mount so maintenance knows where to set it.
Review item 4: Belt speed and noise
Design intent: 2 m/s belt speed, 3000 rpm motor.
Reviewer note: GT2 at 2 m/s is quiet. But GT2’s tooth profile is trapezoidal — it meshes with noise at high speed. AT5 (curvilinear tooth) runs smoother and quieter at the same speed. If the machine is in a noise-sensitive area, AT5 is worth the upgrade.
Final spec after review
| Parameter | Original | After review |
|---|---|---|
| Belt type | GT2 6 mm | AT5 9 mm |
| Pulley teeth | 20 | 25 |
| Tension method | Fixed center | Slotted motor mount |
| Tension at setup | ~5 N | ~15 N (1.5 N/mm) |
| Max acceleration | 4 m/s² (peak) | 3 m/s² (derated for belt) |
The revised belt didn’t slip. The tensioner mount made re-tensioning a 2-minute job at PM. The tooth skip that started this review never came back.
The review takeaway: Belt selection isn’t just “will the teeth hold the torque.” It’s tension, fatigue life, tension compensation, and tooth profile. A belt that’s too loose slips at any torque. A belt on a too-small pulley wears out in months. Size the pulley, set the tension, and add a re-tensioning path.
