Epicyclic Gear Train

If the axe of the shafts, on which gears are attached, move relative to a fixed axis, then the gear train is known as epicylic gear train.

Principle of operation
An Epicyclic gear box consists of two, three or even four Epicyclic or planetary gear sets. A simple gear set has a sun gear, about which planets turn round. These planet gears are carried by a carrier and a shaft and are also in mesh internally with a ring gear, which is also called annulus or internal gear sometimes.

Different torque ratios i.e. speed ratios are obtained by making anyone of the parts, viz. the sun gear, the planets and the annulus stationary. Similarly by locking two parts with each other, a solid drive i.e. direct gear is obtained.


An Epicyclic single-stage gear train consists of an internally toothed annular (ring) A with a Band brake encircling it. In the center of this gear is sun gear S, which forms part of the input shaft.

The sun gear and the annular gear are connected by a number of planet (pinion) gears P which are mounted on a carrier C and is integral with the output shaft. For transmission of torque, either the sun gear, the carrier, or the annular gear must be held stationary.

The situation is considered when only the annular gear is stationary. When the input sun gear shaft is driven keeping the annular gear band brake fixed, the planet gears simultaneously rotate around their axes and revolve around the input sun gear axis along the inner circumference of the annular gear. Consequently, the carrier and the output shaft, which support the planet-gear axes, also rotate, but slower than the input shaft.