Correct pulley sizing keeps belt speed within manufacturer limits and preserves drive ratio between motor and driven equipment. Whether you are replacing a worn pulley on a compressor or specifying a new drive for a conveyor, the same dimensions matter: profile, pitch or outside diameter, groove count, bore, and keyway. This guide walks through measurement, calculation, and what to send for a verified quote.
Key dimensions defined
Outside diameter (OD) is the overall pulley diameter measured at the outer edge. Pitch diameter is the effective diameter where the belt centreline runs — catalogues and speed calculations use pitch diameter, not OD.
Face width must accommodate the belt section and number of grooves without overhang. Groove count is the number of V-belts or ribs in multi-groove drives.
| Dimension | How to measure | Why it matters |
|---|---|---|
| OD | Calliper across outer edge | Rough sizing, space envelope |
| Pitch diameter | From catalogue or formula per profile | Speed ratio calculations |
| Face width | Across groove faces | Fits belt section + groove count |
| Groove count | Count grooves | Must match belt count |
| Bore | Internal diameter | Shaft fit, taper bush series |
Speed ratio calculation
Speed ratio = diameter of driver pulley ÷ diameter of driven pulley (using pitch diameters). Driven RPM = motor RPM × (driver pitch diameter ÷ driven pitch diameter).
Example: Motor 1450 RPM, driver pitch 150 mm, driven pitch 300 mm → driven speed = 1450 × (150/300) = 725 RPM.
When replacing one pulley only, match the existing ratio unless engineering has approved a change — altering ratio affects torque, belt tension, and equipment performance.
Belt speed check
Belt speed (m/s) ≈ π × pitch diameter (m) × RPM / 60. Most industrial V-belts have recommended maximum speeds depending on profile — exceeding limits causes heat and early belt failure.
If motor RPM is high and pulley diameter is small, belt speed may be excessive. In that case a larger pitch diameter or different profile may be required — share motor nameplate data with your supplier.
Centre distance and belt length
Centre distance between shaft centres affects belt wrap angle and required belt length. For replacements, measure the old belt length or count effective length with a flexible tape along the belt path.
For new layouts, standard formulae use pitch diameters, centre distance, and profile to calculate belt length — provide all three shaft positions if requesting engineering support.
Taper lock vs finished bore on sizing
Taper lock pulleys are sized by OD, profile, groove count, and bush series — bore is determined by the bush (e.g. 2012, 2517). Finished bore pulleys are ordered by exact bore and keyway.
Always confirm whether you are measuring OD or pitch diameter when comparing to catalogue tables — confusing the two is a common source of wrong pulley supply.
What to send for a quote
- Profile: SPZ, SPA, SPB, SPC, classical, HTD, poly-V
- OD or pitch diameter (mm) and groove count
- Bore type: pilot, finished bore + keyway, or taper lock series
- Motor RPM and driven equipment if ratio verification needed
- Photos of pulley marking and groove profile
- Quantity and brand preference if any
Common mistakes
- Ordering by OD when supplier catalogue lists pitch diameter
- Wrong groove count on multi-belt drives
- Ignoring belt profile when measuring groove width
- Changing one pulley size without checking belt length compatibility
- Specifying bore without keyway dimensions on finished bore units
RFQ checklist
- Profile and groove count
- Pitch diameter or OD (mm)
- Bore size and type (TL series or finished)
- Keyway dimensions if applicable
- Motor RPM / driven speed if sizing new drive