The Timing Belt That Slipped Under Load
We used a timing belt (GT2, 6 mm wide) to drive a linear axis. At low speed, it positioned fine. When we accelerated the axis (0.5 m/s²), the belt skipped — the drive pulley’s teeth jumped a tooth. The position was off by one belt pitch (2 mm). The problem: the belt was too narrow (6 mm) for the torque. The belt teeth couldn’t transmit the acceleration torque. We upsized to a 25 mm wide belt (same pitch). The tooth load dropped. No more skipping. The mistake was picking the belt width by eye (the cheapest 6 mm) instead of calculating the tooth load.
Timing belt drive sizing is about the tooth load and belt tension. Skip a tooth, and the axis loses position. This article covers the numbers.
Timing Belt Basics
A timing belt (notched belt) has teeth that mesh with the pulley. No slip (unlike a V-belt). It’s used for linear drives (article 71) where position matters.
Key parameters:
- Pitch: The tooth spacing. GT2 (2 mm), GT3 (3 mm), 8M (8 mm). Smaller pitch = finer positioning, lower torque.
- Width: The belt width (6, 16, 25, 50 mm). Wider = more torque capacity.
- Pulley teeth: The number of teeth on the drive pulley. More teeth = more tooth engagement.
Step 1: Calculate the Belt Tension
The belt transmits torque. The tension (T1, tight side) is:
T1 = (F_load + F_inertia) / 2 + T_preload
Where F_load is the external force, F_inertia is m × a (the acceleration force), and T_preload is the belt’s preload (tensioned by the tensioner, article 99).
For a moving load of m = 5 kg, a = 5 m/s², F_load = 20 N (friction): F_inertia = 25 N. Total = 45 N. The belt tension is about half this (two belt sides): T1 ≈ 25 N (plus preload).
Step 2: Tooth Load
The teeth transmit the torque. The tooth load (force per tooth) must be under the belt’s rating. The number of engaged teeth on the drive pulley is typically 15–20 (for a 20-tooth pulley).
The belt manufacturer rates the allowable force (N) for a given width and pitch. For a GT2 6 mm belt, the allowable force is about 50 N. For GT2 25 mm, it’s about 250 N.
For the axis above (T1 = 25 N), a 6 mm belt (50 N) should handle it. But at acceleration (spike), the force doubled to 50 N — right at the limit. Upsize to 16 mm (150 N allowable) for margin.
The timing belt rule: Size the belt width for the peak (acceleration) force, not the average. The belt that skipped was 6 mm at its limit. Upsize to 16–25 mm for acceleration spikes. Always add preload (tensioner) so the belt doesn’t jump teeth.
Step 3: Belt Tension (Preload)
The belt must be tensioned (preloaded) so the teeth stay engaged. Too loose, and the belt skips (jumps a tooth). Too tight, and the bearings wear (the belt pulls hard on the shafts).
For a GT2 belt, the preload is about 5–10 N per mm of width. For a 25 mm belt, preload = 125–250 N. The tensioner (article 99) sets this.
Avoid over-tensioning. A belt that’s too tight loads the drive bearings (and the linear guide). Check the tension with a belt tension gauge (or by deflecting the belt with a known force).
Step 4: Pulley Diameter and Speed
The belt speed depends on the pulley diameter and motor speed. A smaller pulley runs faster (more rpm for the same belt speed). But a smaller pulley bends the belt tighter (less belt life).
For GT2 belts, the minimum pulley diameter is about 20 teeth (40 mm pitch diameter). Don’t use a 10-tooth pulley on a GT2 belt — the belt bends sharply and fails early.
Step 5: Backlash and Positioning
A timing belt has some backlash (the tooth-to-pulley clearance). For GT2/GT3 (curved tooth), backlash is about 0.05 mm. For positioning, this is acceptable. For high-precision, use a preloaded belt (double-sided or tensioned to eliminate backlash).
| Belt Width | GT2 Allowable Force | Best For |
|---|---|---|
| 6 mm | ~50 N | Light axes, small loads |
| 16 mm | ~150 N | Medium axes, standard |
| 25 mm | ~250 N | Heavy acceleration, high speed |
| 50 mm | ~500 N | Heavy gantry, high force |
A Timing Belt Checklist
- What is the load mass and acceleration?
- Peak force = m × a + friction?
- What belt pitch? (GT2, GT3, 8M?)
- Belt width rated for peak force? (With margin?)
- Minimum pulley teeth? (Not too small?)
- Belt preload set? (Tensioner?)
- Is the tension measured? (Not over-tight?)
- Is there belt wear? (Check teeth?)
- Does the belt skip at acceleration? (Test?)
- Is the guard in place? (Belt doesn’t flail?)
The Bottom Line
Timing belt drive sizing is the peak force, not the average. The belt that skipped was 6 mm at its torque limit during acceleration. Size the width for m × a + friction, with margin. Set the preload with a tensioner (not too tight). Use a minimum pulley size. The belt that never skipped a tooth wasn’t the widest one — it was sized for the acceleration spike.
