EngineerBenchPulley diameter and driven speed
A larger driven pulley turns more slowly than a smaller driver. With no slip, driven RPM equals driver RPM multiplied by driver pitch diameter divided by driven pitch diameter. Use this relationship to check a belt-driven spindle or choose a pulley for a required shaft speed.
Use pitch diameter, not the outside edge
The belt's effective running diameter determines the ratio. A pulley measured across its outer rim may not match its pitch diameter, particularly with a V-belt. Use the pulley or belt manufacturer's pitch dimensions when available.
Allowing for slip
The slip input reduces the calculated driven speed. It is an assumed loss, not a prediction of belt grip. The result does not establish the drive's power rating or required belt tension. Check the actual speed under load when matching a machine setting.
Comparing a speed change
A 100 mm driver and a 200 mm driven pulley give half the input speed in an ideal no-slip drive. At 1,400 input RPM, that is 700 output RPM. If the assumed slip is two percent, the model gives 686 RPM instead. Do not apply the slip allowance again after reading that result. A larger motor pulley increases driven speed; a larger driven pulley decreases it.
When replacing a pulley, record both the original and proposed pitch diameters. Compare the resulting speed with the driven machine's permitted range, and check whether the motor still has enough torque for the task. Pulley changes also alter belt wrap and the required belt length, neither of which is resolved by the ratio alone. If available diameters fall on either side of the desired speed, calculate both arrangements and choose using the machine requirements, rather than rounding a diameter and assuming the target speed is preserved.
Formula
N₂ = N₁ × D₁ / D₂ × (1 − slip). Belt speed = πD₁N₁ / 60.