How to choose a gearmotor: ratio, torque and output speed

A gearmotor couples an electric motor to a gear reducer to lower speed and raise the torque available at the driven machine. Choosing it well means the right torque without oversizing. Here are the parameters that matter.

1. Output torque (M₂)

The starting figure: the torque, in Nm, the output shaft must deliver to the machine. If you know the required power and output speed, estimate it as M₂ ≈ 9550 × P₂(kW) / n₂(rpm). The catalogue rated torque must exceed the required one, multiplied by the service factor.

2. Output speed (n₂) and ratio (i)

The reduction ratio i = n₁/n₂ links motor speed (n₁) to output-shaft speed (n₂). Once you fix the target output speed, the ratio follows from the motor speed: with a 4-pole motor at 1400 rpm and n₂ = 70 rpm you need i ≈ 20.

3. Power and service factor

The motor is chosen for the power absorbed by the machine, but the reducer is verified through the service factor (fs): it accounts for load type, running hours and starts/hour. The rule is rated M₂ ≥ required M₂ × fs.

4. Reducer type and mounting position

In-line, parallel-shaft, bevel-helical or worm: the geometry depends on space, shaft orientation and required efficiency. Also fix the mounting position (B3, V5…) and the shaft type (solid, hollow, shrink-disc), because they affect lubrication and seals.

The Rossi series

The A series covers worm gear reducers; the E/ES series the coaxial (in-line) units; the H series parallel-shaft and bevel-helical; the EP series planetary units. Not sure? Use our selector starting from output torque and speed, or browse the Rossi catalogue.