Keywords: V-belt pulley, Imperial pulley, Metric pulley, taper bush, pulley bore, keyway standards, SPZ SPA SPB SPC
In power transmission systems, V-belt pulleys are widely used in industrial machinery, HVAC, conveyors, and agricultural equipment. However, pulley designations can be confusing because there are two main systems: Imperial (inch-based) and Metric (millimeter-based).
This article explains how to interpret pulley part numbers, convert between Imperial and Metric standards, and distinguish finished bore pulleys from taper bush pulleys.
1. Imperial vs. Metric Pulley Designations
Diameter Units
- Imperial: measured in inches (
") - Metric: measured in millimeters (
mm)
Groove Types
- Imperial: A, B, C, D, or 3V, 5V, 8V
- Metric: SPZ, SPA, SPB, SPC
Conversion Reference:
- A ≈ SPA/SPZ
- B ≈ SPB
- C ≈ SPC
- 3V ≈ SPZ
- 5V ≈ SPB
- 8V ≈ SPC
Bore and Keyway
- Imperial: shown in fractional inches
- Example: 1.1/2″ bore (≈ 38.1 mm), 3/8″ keyway (≈ 9.525 mm)
- Metric: shown in millimeters
- Example: 38 mm bore, 10 mm keyway
2. Groove Type and Size Conversion
| Imperial | Metric | Top Width (approx.) | Height (approx.) |
|---|---|---|---|
| A | SPA/SPZ | 13 mm / 10 mm | 8 mm |
| B | SPB | 17 mm | 11 mm |
| C | SPC | 22 mm | 14 mm |
| 3V | SPZ | 9.5 mm | 8 mm |
| 5V | SPB | 15.9 mm | 13.5 mm |
| 8V | SPC | 25.4 mm | 23 mm |
3. Diameter Conversion (Inch → Millimeter)
Formula: Inches × 25.4 = Millimeters
| Imperial | Metric |
|---|---|
| 5.5″ | 140 mm |
| 6″ | 152 mm |
| 8″ | 203 mm |
| 10″ | 254 mm |
4. Bore and Keyway Reference
| Bore (in) | Bore (mm) | Keyway (in) | Standard Keyway (mm) |
|---|---|---|---|
| 1″ | 25.4 mm | 1/4″ | 8 mm |
| 1.1/2″ | 38.1 mm | 3/8″ | 10 mm |
| 2″ | 50.8 mm | 1/2″ | 14 mm |
👉 Note: fractional keyways are rounded to the nearest ISO standard (e.g., 3/8″ = 9.525 mm → standardized as 10 mm).
📌 5. Example: Reading an Imperial Pulley
Imperial designation:
5.5″*2B1.1/2*3/8″**
Breakdown:
- Outside diameter = 5.5″ ≈ 140 mm
- Two grooves, B section
- Bore = 1.5″ ≈ 38 mm
- Keyway = 3/8″ ≈ 10 mm
Metric equivalent:
140-2SPB-38-10
6. Finished Bore vs. Taper Bush Type
Finished Bore Pulleys
- Include bore and keyway dimensions in the designation
- Examples:
5.5"*2B**1.1/2*3/8"(Imperial)140-2SPB-38-10(Metric)
Taper Bush Pulleys
- Do not include bore/keyway in the pulley designation
- Instead, they specify a bush number (e.g., 1210, 1610, 2012)
- Example:
180-2SPB-2012
7. Common Taper Bush Sizes
| Bush No. | Bore Range (mm) | Common Keyway (mm) |
|---|---|---|
| 1210 | 11 – 32 | 4 / 5 / 6 / 8 / 10 |
| 1610 | 14 – 42 | 5 / 6 / 8 / 10 / 12 |
| 2012 | 19 – 50 | 6 / 8 / 10 / 12 / 14 |
| 2517 | 25 – 65 | 8 / 10 / 12 / 14 / 16 |
| 3020 | 30 – 75 | 8 – 18 |
| 3525 | 35 – 90 | 10 – 22 |
| 4030 | 40 – 100 | 12 – 25 |
| 5040 | 50 – 125 | 14 – 32 |
Quick Identification:
- Fractional bore/keyway (e.g., 1.1/2, 3/8) → Imperial finished bore
- Metric bore/keyway (e.g., 38-10) → Metric finished bore
- Bush numbers (1210, 1610, 2012, etc.) → Taper bush type
Conclusion
- Finished bore pulleys: specify exact bore and keyway in either inches or millimeters.
- Taper bush pulleys: rely on bush numbers; bore and keyway are determined by the bush.
- Conversion rules:
- Inches × 25.4 → Millimeters
- Groove types: A/B/C ↔ SPZ/SPA/SPB/SPC
- Fractional keyways → nearest ISO standard sizes
By following these rules, engineers and buyers can confidently convert Imperial ↔ Metric ↔ Taper Bush pulley designations, ensuring accurate product selection and smooth equipment compatibility.
