Planetary Gears

A planetary (epicyclic) gearset arranges a central sun gear, several orbiting planet gears on a carrier, and an outer ring gear with internal teeth, all meshed together and sharing one axis. Fixing any one of the three members and driving another produces a different gear ratio from the same set of parts.

MembersSun, planets, ring, carrier

AdvantageHuge ratio in a small housing

Used inFinal drives, TBM cutterheads, automatic transmissions

01Definition

02Physical Principle

Because load is shared across three or more planet gears instead of a single meshing pair, a planetary set transmits far more torque for its size and weight than an equivalent simple gear train — which is exactly why it lives inside the tight final-drive housing at a bulldozer's track sprocket, or inside a tunnel boring machine's cutterhead drive, where space is the scarcest resource on the machine.

03Mathematical Foundation

Ratio, ring fixed

GR = 1 + (N_ring / N_sun)

Driving the sun gear with the ring gear held stationary is the most common configuration in heavy-equipment final drives.

04Interactive Diagram — Planetary Gearbox Simulator

Choose which member is held fixed and watch the other two rotate at the resulting speed — the same three gears, three different outcomes.

Sun = 24 teeth, ring = 72 teeth throughout — only which member is held changes the outcome.

05Interesting Facts

The same physical planetary gearset can produce several different ratios just by changing which member is held stationary — this is exactly how a planetary automatic transmission shifts gears without a clutch pedal.
Because the planet gears orbit the sun gear the same way moons orbit a planet, the arrangement borrowed its name directly from astronomy.
A single-stage planetary set can realistically achieve ratios up to about 10:1; stacking two or three stages in series reaches into the hundreds, which is exactly what a large final drive needs.