Planetary Gearbox Torque Calculator: Ratio, Efficiency and Speed

Use this planetary gearbox torque calculator for a first estimate of transmitted output torque and speed. Enter the motor's operating torque, reduction ratio, gearbox efficiency and input speed. It assumes the motor drives the input of a reduction gearbox; it does not calculate allowable gearbox capacity or backdriving performance.

Calculate output torque and speed

The starting values are illustrative inputs, not the specifications of an ANDANTEX product.

Formulas and units

T_out = T_in × i × η
n_out = n_in / i

• T_in and T_out: input and output torque in Nm.
• i: reduction ratio, defined as input speed divided by output speed.
• η: gearbox efficiency as a decimal; 94% becomes 0.94.
• n_in and n_out: input and output speed in rpm.

Worked calculation

With an illustrative motor torque of 3 Nm, ratio of 10:1, efficiency of 94% and input speed of 3,000 rpm, output torque is 3 × 10 × 0.94 = 28.2 Nm. Output speed is 3,000 / 10 = 300 rpm. A larger ratio raises the estimated torque and lowers speed, subject to the selected gearbox's actual limits.

Calculated torque is not a gearbox load rating

The calculator estimates what the motor and ratio transmit. It does not determine the strength, bearing capacity, thermal rating or service life of a gearbox. Compare the result with the permitted output torque for the exact model, ratio and reduction stage. A motor capable of producing more torque can overload a smaller gearbox.

Enter the motor torque available at the operating speed. For a stepper motor, holding torque is not a substitute for its running torque-speed curve. For a servo axis, check continuous running and each acceleration or braking peak separately. Read rated torque versus peak torque before applying a peak result to a repeated cycle.

Choose the next selection check

Compare ratios in the planetary reduction-ratio guide. Browse the PLF planetary gearbox series or precision planetary gearbox range for candidate configurations. Check motor mounting dimensions, output bearing loads and thermal duty before approving a model.

For engineering review, send the motor model, required output speed, load cycle and installation drawing. Include peak durations, acceleration time and any radial or axial loads; the four calculator inputs alone are not a complete selection brief.

Formula reference: Bonsystems: motor and gearbox torque calculation.