For a simple two-pulley drive, the first sizing relationship is based on the pitch/datum diameters, not the outside diameters. Ignoring that point is one of the easiest ways to end up with a drive speed that is close but wrong.
Speed-ratio relationship
Here n₁ is driver speed, n₂ is driven speed, D₁ is the driver datum diameter and D₂ is the driven datum diameter. Real belts have slip and elastic effects, so final speed should be checked against the belt supplier’s design method.
Example
A 1450 rpm motor using a 100 mm datum-diameter driver and a 280 mm driven pulley gives a theoretical driven speed of about 518 rpm. The catalogue should therefore be searched by P = 100 and P = 280, not by O.
| Input | Example |
|---|---|
| Driver speed | 1450 rpm |
| Driver datum diameter | 100 mm |
| Driven datum diameter | 280 mm |
| Theoretical driven speed | ≈ 518 rpm |
Do not stop at the ratio
Power, belt section, groove count, centre distance, wrap angle and shaft load still need to be checked. A ratio calculation can identify candidate diameters, but it does not rate the belt drive. Include power, speeds, centre distance and service conditions when requesting selection support.
Need help matching a pulley? Email [email protected] with the belt marking, shaft diameter and pulley measurements.