Pulley ratio
and belt length.
Begin with the pitch diameters of the drive and driven pulleys. A timing pulley can also be described by its tooth count and pitch.
Calculate driven RPM
For an ideal drive, multiply driver RPM by the driver pitch diameter divided by the driven pitch diameter. An 80 mm driver at 1,200 RPM turning a 160 mm driven pulley gives 600 RPM. On timing pulleys of the same pitch, the diameter ratio equals the tooth count ratio.
Driven RPM = driver RPM × driver pitch diameter ÷ driven pitch diameter
Estimate the belt speed and wrap
To find metres per second, multiply driver RPM by π and the driver pitch diameter in metres, then divide by 60. The example moves the belt at about 5.03 m/s. With 300 mm between the shafts, the smaller pulley has about 164.7° of wrap.
Find pitch length from two circles and two tangents
An uncrossed belt has two straight tangent runs and two wrapped arcs. For radii r₁ and r₂ and center distance C, each straight run is √(C² − (r₂ − r₁)²). The two wrap angles are π minus and plus 2 × asin((r₂ − r₁) ÷ C), in radians. Multiply each angle by its pulley radius and add the straight runs. The example needs about 982.33 mm of pitch length.
Start from a belt you already have
Enter its actual pitch length, or its tooth count and pitch for a timing belt, to solve for shaft spacing. Use the belt maker’s dimensions and engineering limits when choosing a belt and pulley combination. See the Gates light power and precision drive design manual for design references.