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Motor & gearbox selection
Enter the absorbed power and the speeds at each end. The calculator returns the gearbox ratio, the torque at both shafts, and the design power to select the gearbox on once the service factor is applied.
Your figures
1450 for a 4-pole at 50 Hz, 2900 for 2-pole, 960 for 6-pole.
What the driven machine actually turns at.
1.0 uniform, 1.25 moderate shock, 1.5 heavy, 1.75+ for crushers and mills.
Result
Power
- Absorbed power
- 15 kW · 20.12 HP
- Design power
- 22.5 kW
- Motor frame
- 18.5 kW · 24.8 HP
Absorbed × 1.5 service factor — select the gearbox on this
Next IEC size above the duty with 15% margin
Speed & torque
- Gearbox ratio
- 32.22 : 1
- Motor shaft torque
- 98.8 N·m
- Output shaft torque
- 3,183 N·m
- Output torque with SF
- 4,775 N·m
1,450 rpm in, 45 rpm out
9550 × kW ÷ rpm
What the coupling, key and shaft have to carry
The figure to check a gearbox rating against
Torque rises as speed falls — a slow output shaft is the expensive end of a drive, not the motor.
Questions
What is a service factor for?
It covers the difference between the smooth power a motor nameplate implies and what the machine actually does to the drive — shock loads, starting torque, hours a day, reversals. Select the gearbox on absorbed power times the service factor, not on absorbed power.
Which service factor should I use?
Around 1.0 for a uniform load like a centrifugal pump, 1.25 for moderate shock such as a conveyor, 1.5 for heavy duty, and 1.75 or more for crushers, hammer mills and anything that can jam. When in doubt the gearbox supplier's table for your specific machine wins.
Why is output torque so much larger?
Because power is torque times speed. Reducing the speed forty to one multiplies the torque by forty, less the gearbox losses. That is why the slow end of a drive is the expensive end — the shaft, key, coupling and gearbox casing are all sized by that number, not by the motor.
Does it size the motor?
It suggests the next IEC frame above the absorbed power with a fifteen percent margin. Whether that frame is right also depends on starting torque, duty cycle, ambient temperature and altitude, none of which are considered here.