Gearbox Sizing Basics: Power, Torque and Service Factor
June 22, 2026 · Acme Engineering Team
A practical introduction to sizing gearboxes for OEM applications, including how to read service factors and avoid the most common mistakes.
Getting gearbox sizing right on the first pass saves weeks in the development cycle. Here is the simplified method our engineers use with OEM customers.
Start from the driven load
Calculate the required output torque from the load and the operating duty cycle, not from the motor.
Apply the service factor
The service factor accounts for shock loads, starts per hour and ambient conditions. A conveyor with frequent starts needs a higher factor than a light fan.
Check the ratio and motor speed
Output speed = motor speed ÷ ratio. Confirm the selected ratio lands within your target speed window.
Verify the radial load
Many premature failures come from overhung loads on the output shaft. Check the radial load rating of your shortlist.
Common mistakes
- Sizing from the motor instead of the load
- Ignoring starts per hour
- Forgetting ambient temperature derating
Send us your load data and we’ll size a gearbox for you — usually within one working day.