Fans and energy
Enter motor speed and pulley diameters to calculate fan speed and compare drive changes.
Estimate driven fan RPM, open-belt pitch length and smaller-sheave wrap angle from actual pitch diameters, center distance and explicit slip.
An open belt drive relates shaft speeds through the sheaves’ pitch diameters. At a fixed driver speed, a larger driver or smaller driven sheave increases fan speed. Explicit slip reduces that ideal speed estimate; it does not supply a model of belt traction or mechanical efficiency.
Center distance and pitch geometry also determine approximate belt pitch length and wrap angles. Pitch diameter differs from outside diameter, and the calculated length is not a stock belt designation. Changing the speed ratio can substantially change fan power, so a geometric fit alone is insufficient grounds for a sheave change.
N_fan = N_driver × D_driver/D_driven × (1−slip); L ≈ 2C + π(D+d)/2 + (D−d)²/(4C)
N = shaft speed (RPM); D and d = large/small pitch diameters (in); C = shaft center distance (in); L = approximate pitch belt length (in). Slip is entered as percent and used as a fraction.
Treat fan RPM as a proposed drive result and pitch length as a preliminary geometry result. Check fan power and speed limits, then select an actual belt and take-up arrangement.
Inputs: 1,750 RPM driver, 4 in driver pitch diameter, 8 in driven pitch diameter, 20 in centers, zero slip.
Calculation: The diameter ratio is 0.5. Open-belt pitch length is approximately 40 + 18.8496 + 0.2 in.
Result: 875 RPM driven speed and about 59.050 in belt pitch length.
Use the manufacturer’s compatible pitch dimensions. Outside diameter does not necessarily locate the belt pitch line and can distort both the speed ratio and length estimate.
No. Wrap is geometric. Traction and capacity also depend on belt type, tension, condition, loading, and manufacturer drive requirements.