An epicyclic gearbox consists of two, three or even four epicyclic or planetary gear sets.

A simple gear set has a sun gear, about which planets turn around.

These planet 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 that is speed ratios are obtained by making any one of the parts viz, the sun gear, the planets and the annulus stationery.

Similarly by locking two parts with each other, a solid drive that is direct gear is obtained.

 Determination of speed ratio of epicyclic gear trains :-

As described above an epicyclic gear train is one in which sum of the wheels have a bodily motion also that is their axes rotate about some other axis, in addition to any rotation which they possess about their own axis.

The most simple method to determine speed ratio (also called velocity ratio) is the algebraic method.

The principle on which this method is based may be stated as :-

"The gear ratio of a pair of mating gear wheels with respect to the link carrying the axes of the gears is always the same whether the link carrying the axis is fixed or moving."

In a simple epicyclic gear set there maybe six possible speeds four of which are forward and two reverse.

(i) Forward, fast output speed :- This is obtained when planet carrier is driving, ring is driven and the sun gear is held stationary.

(ii) Forward, very fast output speed :- This is obtained when planet is driving, sun gear is driven and the ring gear is held stationary.

(iii) Forward, slow output speed :- With ring gear driving, planet carrier driven and the sun gear stationary, this comes about.

(iv) Forward, very slow output speed :- This results when the sun gear is driving, planet carrier driven and the ring gear stationary.

(v) Reverse, slow output speed :- This is obtained when the sun gear is driving, ring gear is driven and the planet carrier is stationary.

(v) Reverse, fast output speed :- This condition is obtained with the ring gear driving, sun gear driven and the planet carrier stationery.

 Controls in the epicyclic gearbox :-

There are two controls that is brake and the clutch.

The break is in the form of a band that surrounds a drum attached to the gear (in case of sun gear) or the outer surface of the gear itself (in case of ring gear).

The clutch used is of multi plate type.

Both the brake and the clutch are applied by fluid pressure.

These are selected by hydraulic shift valves which are usually located in the bottom of the gearbox.

 Advantage of epicyclic gearbox :-

The advantage of epicyclic gearing is thus obvious.

All the gears are in constant mesh and to engage any desired gear on simply has to apply the particular brake or the clutch.

For this, the drive from the engine need not be disconnected as in case of ordinary crash type gearbox.

The gear changing operation thus becomes very easy with an epicyclic gear box, which makes it suitable for use in automatic transmission.