Output torque and speed: how to size a gear reducer
Every reducer is chosen from two quantities at the output shaft: output torque (M₂) and output speed (n₂). Together they define what the drive must do.
Output speed (n₂)
The output-shaft speed in rpm. It comes from motor speed divided by the ratio: n₂ = n₁ / i. It answers: how fast must the machine turn?
Output torque (M₂)
The turning effort available at the machine, in Nm. It ties to power and speed: M₂ ≈ 9550 × P₂(kW) / n₂(rpm). For the same power, the more you cut speed, the more torque you get. It answers: with how much force?
Rated torque and service factor
The catalogue gives the reducer's rated torque M₂n. The check is: M₂n ≥ required M₂ × fs, where fs is the service factor covering load type and running hours. You must also verify the permissible radial and axial loads on the shaft.
Example
Need 60 rpm and 250 Nm on a conveyor (uniform load, fs = 1.25)? The torque to verify is 250 × 1.25 = 312 Nm; with a 1400 rpm motor the ratio is i ≈ 23. On our selector enter n₂ and M₂ and we show the Rossi reducers that cover that point.
Frequently asked questions
What is the difference between output torque and speed?
Output speed n₂ (rpm) is how fast the output shaft turns; output torque M₂ (Nm) is the turning effort available. They are linked by power: M₂ ≈ 9550 × P₂/n₂.
Why does reducing speed increase torque?
Because at constant power torque and speed are inversely proportional: the reducer lowers rpm and, net of losses, multiplies the torque available at the output.