The epicyclic gear trains are useful for transmitting high velocity ratios with gears of moderate size in a comparatively lesser space. The epicyclic gear trains are used in the back gear of lathe, differential gears of the automobiles, hoists, pulley blocks, wrist watches etc.

Tooth thickness = 1.5708 x module.

Train value = Speed of the last driven or follower/Speed of the first driver.

The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and in clocks (where the minute and hour hand shafts are co-axial.

The speed ratio and the train value, in a simple train of gears, is independent of the size and number of intermediate gears. These intermediate gears are called idle gears, as they do not effect the speed ratio or train value of the system.

In reverted gear train and last gear train is on the same axis. Such an arrangement has application on speed reducers clocks and machine tools.

In an epicyclic gear train, the axes of the shafts, over which the gears are mounted, may move relative to a fixed axis.

When the axes of the first gear (i.e. first driver) and the last gear (i.e. last driven or follower) are co-axial, then the gear train is known as reverted gear train.

For turning differential gears are used.

Working depth is twice of addendum and whole depth is sum of addendum and dedendum.