The Belt That Stretched and Lost Position
We used a timing belt to drive a linear axis: 1 m travel, 10 kg carriage, 1 m/s speed. On the bench, positioning was fine. After a week, the repeatability went from ±0.05 mm to ±0.5 mm. The belt had stretched. The problem: we used a standard timing belt without proper tensioning. The belt’s carbon tensile cord stretched slightly under load, and the carriage position shifted. We retensioned the belt, but it stretched again. We switched to a steel-reinforced belt and added a tensioner. The position held. The mistake was not accounting for belt elongation and not providing a tension adjustment.
Timing belt drive for linear axes is a cost-effective alternative to ball screws for long travels. But belt elongation, tension, and pulley size determine the accuracy. This article covers the selection.
Why Use a Timing Belt?
For long travels (over 1 m), a ball screw gets expensive (long screws are costly and have critical speed limits). A timing belt loop is cheaper and lighter. The belt runs between two pulleys at the ends of the axis. The carriage bolts to the belt. One pulley is driven by the motor.
Best for: Long travels (1–10 m), moderate accuracy (±0.1–0.5 mm), high speed (up to 5 m/s). Not for high-precision (the belt stretches).
Step 1: Belt Pitch and Pulley Size
Timing belts come in pitches (tooth spacing). Common pitches for linear axes:
- T5 (5 mm pitch): Light duty, small pulleys. For small axes.
- T10 (10 mm pitch): Medium duty. Standard for most linear axes.
- AT10 (10 mm pitch, arc tooth): Higher torque, less backlash. Better than T10 for positioning.
- HTD 8M (8 mm, curvilinear): High torque, high speed. For heavy axes.
The pulley size is the number of teeth × pitch. A 20-tooth T10 pulley has a pitch diameter of 20 × 10 / π = 64 mm. More teeth = larger pulley = less belt bending (longer life) but bigger envelope.
Step 2: Belt Length and Travel
The belt forms a loop around the two pulleys. The belt length is:
L_belt = 2 × travel + π × D_pulley
For 1 m travel and D = 64 mm: L = 2,000 + π × 64 = 2,000 + 201 = 2,201 mm. The belt is supplied in standard lengths. Pick the nearest standard length.
Step 3: Belt Tension (Critical)
The belt must be tensioned correctly. Too loose, and the belt skips teeth under load (loses position). Too tight, and the bearings and shaft wear prematurely.
Tensioning Methods
- Adjustable motor mount: The motor plate slides on slots. Move the motor away from the axis to tension the belt. Then lock it down.
- Tensioner idler: An idler pulley pushes against the belt (on the back side). Adjust the idler to tension.
- Fixed center distance (pre-tensioned belt): The belt length is slightly shorter than the loop distance. The belt is installed under light tension. No adjustment. For small axes.
Tension Check
The belt tension is checked by deflecting the belt mid-span with a known force. A 10 mm deflection under a specific force (from the belt manufacturer’s chart) is correct. Too loose = teeth skip. Too tight = bearing overload.
The belt drive rule: Use an adjustable mount or idler tensioner. Don’t rely on a fixed center distance. Re-tension after the first week (the belt beds in). Use steel or carbon tensile cords (not fiberglass) for low elongation. The belt that stretched lost position because it was a standard fiberglass-cord belt without a tensioner.
Step 4: Backlash and Positioning Accuracy
A timing belt has some backlash (lost motion when reversing). The belt teeth can roll off the pulley slightly. For AT or HTD (curvilinear tooth) belts, the backlash is lower than T (trapezoidal) belts.
- T5/T10 (trapezoidal): 1–3 mm backlash. For general transport.
- AT5/AT10 (arc tooth): 0.5–1 mm backlash. For positioning axes.
- HTD (curvilinear): 0.3–0.5 mm backlash. For precision positioning.
For bidirectional positioning (move left, then right), the backlash matters. Use AT or HTD belts. Or preload the belt (tension it so one side is always tight).
Step 5: Speed and Acceleration
Timing belts run at high speeds (up to 5–10 m/s). But at high acceleration, the belt’s elasticity causes oscillation. The carriage bounces slightly when stopping.
- Steel-reinforced belts: Low elongation. Better for high acceleration. Less bounce.
- Fiberglass belts: More elastic. Cheaper, but bounce at high accel.
For fast pick-and-place (2 m/s, 1 g accel), use steel-reinforced AT belts. For slow transport (0.5 m/s), fiberglass is fine.
Belt vs. Ball Screw: When to Use Which
| Criterion | Timing Belt | Ball Screw |
|---|---|---|
| Travel length | 1–10 m (long) | 0.3–2 m (short) |
| Accuracy | ±0.1–0.5 mm | ±0.01–0.05 mm |
| Speed | Up to 5–10 m/s | Up to 1–2 m/s (critical speed) |
| Cost (long travel) | Low | High (long screw) |
| Backlash | 0.3–3 mm | 0.005–0.02 mm (preloaded) |
| Maintenance | Re-tension, belt wear | Lubrication, nut wear |
Mounting the Carriage to the Belt
The carriage attaches to the belt. Standard method: clamp the belt ends in a belt clamp block on the carriage. One end of the belt is fixed to the left pulley end, runs to the carriage clamp, goes around the right pulley, and returns to the carriage clamp. The carriage moves as the belt loops.
Make sure the clamp grips the belt firmly (no slipping). A slipping belt means lost position. Use the manufacturer’s clamp (designed for the belt).
A Timing Belt Drive Checklist
- What travel length? (m)
- What speed? (m/s) and acceleration?
- What accuracy is needed? (mm repeatability)
- Pick belt pitch: T10/AT10 for general, HTD for precision.
- Pulley size: enough teeth (min 20) for belt life.
- Belt material: steel or carbon cord for low elongation?
- Is there a tension adjustment? (Sliding mount or idler)
- What backlash is acceptable? (AT/HTD for low backlash.)
- Is the carriage clamped to the belt? (No slipping.)
- Are the pulley shafts supported? (Bearing blocks at both ends?)
- Is there a belt cover? (Safety, debris.)
- Re-tension schedule? (First week, then quarterly?)
The Bottom Line
Timing belt drive for linear axes is the right choice for long travels at moderate accuracy. The belt that stretched and lost position wasn’t defective — it was a fiberglass-cord belt without proper tensioning. Use steel-reinforced AT or HTD belts, provide an adjustable tensioner, and re-tension after the first week. For high precision, use a ball screw. For long travel at speed, a timing belt wins. The axis that holds position over years wasn’t the tightest belt — it was the right belt with the right tension.
