Epicyclic Trains

One or more of the wheels of the gear train are now carried on an arm which itself rotates about the axis of the main wheels.

The central main wheel is called the Sun wheel. The wheels carried by the arm are known as the Planet wheels and the outside internal wheel is called the Annulus. In some epicycilic gear trains the "arm" has a number of "limbs," each carrying a planet wheel. In these cases the "arm" may be referred to as the Spider.

There are a number of ways of analysing epicyclic gear trains. Three are described here:

Tabular Method

·         The first line of the table is obtained by fixing the arm and then giving one wheel (e.g. the wheel which is to be fixed later) one revolution.

·         Write down the corresponding revolutions of the other wheels from their number of teeth.

·         As all the wheels and the arm can be turned about the same axis, an equal number of revolutions may be added (or subtracted) from each wheel.

·         Any two lines may be multiplied (or divided) through by the same factor.

·         Any two lines may be combined. By these means the given conditions are satisfied and the resulting gear ratio can be determined.