The Belt That Lost Motion Every Direction Reversal
We used a timing belt (revisit article 59) for a linear axis. The belt was GT2 (2 mm pitch). On a unidirectional move (one direction), the axis was accurate. On a reversal (move left, then right), the axis lost motion — it didn’t move for the first 0.5 mm. The problem: the belt pitch (and tooth profile) had backlash. The GT2 belt (standard) has play between the belt tooth and the pulley tooth. On reversal, the play is taken up (the belt “walks” across the teeth). The axis doesn’t move until the play is gone. We switched to a HTD 5M belt (helical teeth, lower backlash) or a steel-reinforced belt. The lost motion dropped. The mistake was using a standard belt for a reversing (positioning) axis.
Timing belt pitch and backlash affects reversing accuracy. This article covers the selection.
Belt Pitch Types
A timing belt has a pitch (the tooth spacing). Common pitches:
- GT2 (2 mm): Fine pitch. Used in small, light axes (3D printers, small actuators). Has some backlash (the tooth profile isn’t zero-backlash).
- HTD (3M, 5M, 8M): Curved tooth (higher torque, lower backlash than GT). Used in automation (linear actuators). 5M is standard.
- AT5 / AT10 (arc tooth, steel cord): High precision (low backlash). Used in positioning axes. The tooth is trapped (less play).
Step 1: Backlash on Reversing
Backlash is the play between the belt and the pulley. On a reversal (the axis switches direction), the play is taken up (the belt “runs across” the tooth gap). The axis doesn’t move until the play is gone. For a positioning axis (that reverses), this lost motion is the backlash.
For a unidirectional axis (the belt only drives one way), backlash doesn’t matter (the belt is always tensioned in one direction). For a reversing axis (forward/back), backlash matters.
The belt backlash rule: For a reversing positioning axis, use a low-backlash belt (AT5/AT10, or a steel-reinforced belt). The axis that lost motion had a GT2 belt on a reversing axis. Use AT5 (curved tooth, trapped). Tension the belt (article 79) to reduce the play.
Step 2: Belt Tension (Article 79)
A slack belt has more backlash (the teeth “walk”). Tension the belt (article 79) to remove the slack. But don’t over-tension (it wears the bearings). A properly tensioned belt has minimal play. For a reversing axis, tension it tighter (to reduce the backlash).
Step 3: Pulley Diameter (Article 59)
A small pulley (few teeth) has more backlash (the belt bends sharply). Use a pulley with at least 20 teeth (for a 5M belt). A 20-tooth 5M pulley is Ø32 mm. A 12-tooth pulley (Ø19 mm) has more belt fatigue (and more play). Use the largest practical pulley (within the space).
Step 4: Steel Cord vs Fiberglass
The belt’s tension member (inside) is fiberglass (standard) or steel cord. A steel cord belt stretches less (lower backlash, more accurate). A fiberglass belt stretches (more backlash). For a positioning axis, use a steel cord belt. For a light conveyor (non-reversing), fiberglass is fine.
| Belt Type | Backlash | Best For |
|---|---|---|
| GT2 (2 mm, fiberglass) | Higher | Light, unidirectional |
| HTD 5M (fiberglass) | Medium | General automation |
| AT5 / AT10 (steel cord) | Low | Reversing positioning |
| Steel-reinforced (zero-backlash) | Very low | Precision axes |
A Belt Backlash Checklist
- Is the axis unidirectional or reversing?
- For reversing: low-backlash belt? (AT5?)
- Is the belt steel cord? (Less stretch?)
- Is the pulley ≥20 teeth? (Article 59?)
- Is the belt tensioned? (Article 79?)
- Is there lost motion on reversal? (Measure?)
- Is the axis accurate? (Repeat? Article 68?)
- Is the belt not over-tensioned? (Bearings?)
- Is the pulley aligned? (Article 59?)
- Is the belt the right width? (Torque? Article 59?)
The Bottom Line
Timing belt pitch and backlash — reversing axes need low-backlash belts. The axis that lost motion had a GT2 belt on a reversal. Use AT5/AT10 (steel cord) for positioning. Tension the belt (article 79). Use a ≥20-tooth pulley. The axis that reversed accurately wasn’t the cheapest belt — it was low-backlash.
