Planetary (epicyclic) gear reducers: how they work and when to use them

The planetary (epicyclic) reducer offers the highest torque density for its size. It is coaxial, compact and capable of high ratios over multiple stages. Here is how it works and where it fits.

The gear train: sun, planets, ring

A central gear (sun) drives several planet gears on a carrier; the planets mesh with an internally toothed ring. In the typical layout the sun is the input, the ring is fixed and the planet carrier is the output.

Why it handles such high torque

The load is shared across several planets in parallel (usually 3): torque is spread over multiple contact points, so for the same size a planetary transmits far more torque than a single-gear train. The coaxial power flow keeps efficiency high (typically 96–98% per stage).

Ratios and stages

A single planetary stage usually covers ratios from about 3 to 10. For higher ratios you stack stages in series, staying coaxial and compact.

Precision versions for servomotors

Precision planetaries for servo-controlled axes focus on low backlash: standard values of a few arcmin, high-precision versions even below 1–3 arcmin. Low backlash means accurate positioning and good dynamic response. The Rossi range includes the EP planetary series and servo-axis solutions.

When to choose it

• You need high torque in little space → planetary
• You need positioning accuracy (robots, machine tools) → low-backlash precision planetary
• Medium torque with in-line shafts is enough → a coaxial may suffice

Compare models in the Rossi catalogue or start from the selector.