Belt-drive fan service
Motor Sheave Size Calculator
Estimate the driver pitch diameter required for a documented target fan speed.
Find a target motor sheave
Start from a fan speed supported by the fan curve, maximum-RPM limit, motor capacity, and field objective.
How to use this calculator
- Establish the current measured operating point and a supported target fan RPM.
- Enter motor RPM and the driven fan-sheave pitch diameter.
- Select the nearest available rated sheave, then recalculate actual speed and belt length.
Formula
Worked example
For a 1,750 RPM motor, 1,000 RPM target fan speed, and 8-inch fan sheave, the theoretical motor sheave pitch diameter is 4.571 inches.
Result interpretation
The result is not a part number. Available sheaves have defined belt sections, bores, groove counts, adjustment ranges, speed ratings, and pitch-diameter conventions. Any speed increase can raise static pressure and brake horsepower sharply; check the Fan Law Calculator before field adjustment.
Limitations and safety
Do not adjust a running drive. Follow lockout/tagout procedures and the equipment, fan, motor, belt, and sheave manufacturers’ instructions. A qualified technician must verify alignment, tension, guards, current, vibration, and final airflow.
Frequently asked questions
Can I install the exact diameter shown?
Only if a compatible rated sheave exists. Normally you choose an available pitch diameter and recalculate the resulting speed.
Why does motor current need to be checked?
For the same fan and system, required power changes approximately with the cube of speed. A modest speed increase can overload the motor.
Does an adjustable sheave use outside diameter?
Use the manufacturer’s effective or pitch diameter for the actual setting, not an assumed outside diameter.