The Belt That Cracked on the Small Pulley
We used a timing belt (article 59) on a small motor pulley (12 teeth, 5M pitch). The belt was 20 mm wide. On the bench, it ran. After a month, the belt cracked (the belt fabric split) at the small pulley. The problem: the pulley was too small for the belt. The belt bends sharply around a small pulley. The fabric (inside the belt) flexes at a tight radius. After thousands of cycles, it fatigues and cracks. We upsized the pulley (20 teeth instead of 12). The bend radius increased. The belt stopped cracking. The mistake was using a small pulley to save space (the belt needs a minimum pulley diameter).
Conveyor/timing pulley diameter is the minimum pulley size for the belt. This article covers the selection.
Why Pulley Size Matters
A belt bends around the pulley. The bend radius is the pulley radius. A small pulley = tight bend = the belt flexes sharply. The belt’s fabric (or cords) fatigues. The belt cracks (or delaminates). A large pulley = gentle bend = the belt lasts.
The Minimum Pulley Diameter
The belt manufacturer specifies a minimum pulley diameter (or minimum number of teeth). For a 5M timing belt:
- Minimum teeth: 12–16 (for a light load). For heavy, 20+.
- Minimum diameter (5M): 12 teeth = Ø19 mm. 20 teeth = Ø32 mm. 28 teeth = Ø45 mm.
For a 5M belt on a small motor (Ø19 mm pulley), the belt bends sharply. Use a gearbox (article 67) to reduce the speed (and the pulley can be larger), or use a smaller pitch belt (3M, smaller pulley for the same speed). But the smaller pitch belt has lower torque capacity.
The pulley diameter rule: Use a pulley with at least 16–20 teeth (for a 5M belt). The belt that cracked had a 12-tooth pulley (Ø19 mm). Upsize to 20 teeth (Ø32 mm). The bend radius is larger, the belt lasts. If space is tight, use a gearbox (the motor runs faster, the pulley is bigger).
Step 1: The Belt’s Minimum Bend Radius
Each belt has a minimum pulley diameter (from the manufacturer). For a rubber (neoprene) timing belt, it’s about 10–15× the belt thickness. For a polyurethane (steel cord) belt, it’s tighter. Check the belt catalog. The pulley must be at least this diameter.
Step 2: Conveyor Belt (Flat) Pulley
For a flat conveyor belt (article 57, 93), the pulley diameter must be large enough (the belt doesn’t bend sharply). A flat belt on a small pulley (Ø50 mm) cracks (the belt bends tight). For a fabric conveyor belt, use a pulley of at least Ø100 mm (or per the belt spec). For a thin PVC belt, Ø50 mm is OK. For a thick rubber belt, Ø150+.
Step 3: Crown (Tracking, Article 140)
The pulley is crowned (slightly larger in the middle) to track the belt (article 140). The crown is a small taper (0.5–1°). The crown doesn’t affect the bend radius (it’s a small feature). But it helps the belt track (doesn’t walk off, article 140).
Step 4: Bore and Keyway
The pulley mounts on a shaft (with a bore). The bore size (and keyway, article 102) matches the shaft. For a small pulley (Ø19 mm), the bore is small (Ø6–8 mm). For a larger pulley (Ø32 mm), the bore is Ø10–12 mm. Match the bore to the shaft (article 89, 102).
| Belt Type | Min Pulley | Notes |
|---|---|---|
| 5M timing (12 teeth) | Ø19 mm | Light, short life |
| 5M timing (20 teeth) | Ø32 mm | Standard (recommended) |
| 5M timing (28 teeth) | Ø45 mm | Heavy, long life |
| PVC conveyor (thin) | Ø50 mm | Light conveyor |
| Rubber conveyor (thick) | Ø100+ mm | Heavy, impact |
A Pulley Checklist
- What belt? (5M? PVC?)
- What is the min pulley diameter? (Catalog?)
- Is the pulley ≥16–20 teeth? (5M?)
- Does the belt crack? (Small pulley?)
- Is the space tight? (Use a gearbox? Article 67?)
- Is the pulley crowned? (Tracking? Article 140?)
- Does the bore match the shaft? (Article 102?)
- Is the keyway right? (Article 102?)
- Is the pulley aligned? (Article 59?)
- Is the belt tensioned? (Article 79?)
The Bottom Line
Conveyor/timing pulley diameter is the minimum bend radius. The belt that cracked had a 12-tooth pulley (too small). Use ≥16–20 teeth (5M) or the catalog minimum. If space is tight, use a gearbox (bigger pulley). The belt that lasted wasn’t the smallest pulley — it was large enough for the bend radius.
