__Epicyclic gearing__

Epicyclic gearing is used here to increase output speed. The planet
gear carrier (green) is driven by an input torque. The sun gear (yellow)
provides the output torque, while the ring gear (red) is fixed. Note the red
marks both before and after the input drive is rotated 45°
clockwise.

Epicyclic gearing is used here to increase output speed. The
planet gear carrier (green) is driven by an input torque. The sun gear (yellow)
provides the output torque, while the ring gear (red) is fixed. Note the red
marks both before and after the input drive is rotated 45°
clockwise.

Epicyclic gearing or planetary gearing is a gear system that
consists of one or more outer gears, or planet gears, rotating about a central,
or sun gear. Typically, the planet gears are mounted on a movable arm or carrier
which itself may rotate relative to the sun gear. Epicyclic gearing systems may
also incorporate the use of an outer ring gear or annulus, which meshes with the
planet gears.__Gear ratio__

The carrier (green)
is held stationary while the sun gear (yellow) is used as input. The planet
gears (blue) turn in a ratio determined by the number of teeth in each gear.
Here, the ratio is -24/16, or -3/2; each planet gear turns at 3/2 the rate of
the sun gear, in the opposite direction.

Enlarge

The carrier (green) is
held stationary while the sun gear (yellow) is used as input. The planet gears
(blue) turn in a ratio determined by the number of teeth in each gear. Here, the
ratio is -24/16, or -3/2; each planet gear turns at 3/2 the rate of the sun
gear, in the opposite direction.

The gear ratio in an epicyclic gearing
system is somewhat non-intuitive, particularly because there are several ways in
which an input rotation can be converted into an output rotation. The three
basic components of the epicyclic gear are:

* Sun: The central gear

* Planet carrier: Holds one or more
peripheral planet gears, of the same size, meshed with the sun gear

*
Annulus: An outer ring with inward-facing teeth that mesh with the planet gear
or gears

In any epicyclic gearing system, one of these three basic
components is held stationary; one of the two remaining components is an input,
providing power to the system, while the last component is an output, receiving
power from the system. The ratio of input rotation to output rotation is
dependent upon the number of teeth in each gear, and upon which component is
held stationary.

One situation is when the planetary carrier is held
stationary, and the sun gear is used as input. In this case, the planetary gears
simply rotate about their own axes at a rate determined by the number of teeth
in each gear. If the sun gear has S teeth, and each planet gear has P teeth,
then the ratio is equal to -S/P. For instance, if the sun gear has 24 teeth, and
each planet has 16 teeth, then the ratio is -24/16, or -3/2; this means that one
clockwise turn of the sun gear produces 1.5 counterclockwise turns of the planet
gears.

This rotation of the planet gears can in turn drive the annulus,
in a corresponding ratio. If the annulus has A teeth, then the annulus will
rotate by P/A turns for each turn of the planet gears. For instance, if the
annulus has 64 teeth, and the planets 16, one clockwise turn of a planet gear
results in 16/64, or 1/4 clockwise turns of the annulus. Extending this case
from the one above:

* One turn of the sun gear results in -S/P turns of
the planets

* One turn of a planet gear results in P/A turns of the
annulus

So, with the planetary carrier locked, one turn of the sun gear
results in -S/A turns of the annulus.

The annulus may also be held fixed,
with input provided to the planetary gear carrier; output rotation is then
produced from the sun gear. This configuration will produce an increase in gear
ratio, equal to 1+A/S.

If the annulus is held stationary and the sun gear
is used as the input, the planet carrier will be the output. The gear ratio in
this case will be 1/(1+A/S). This is the lowest gear ratio attainable with an
epicyclic gear train. This type of gearing is sometimes used in tractors and
construction equipment to provide high torque to the drive wheels.

More
planet and sun gear units can be placed in series in the same ring gear housing
(where the output shaft of the first stage becomes the input shaft of the next
stage) providing a larger (or smaller) gear ratio. This is the way some
automatic transmissions work.

During World War II, a special variation of
epicyclic gearing was developed for portable radar gear, where a very high
reduction ratio in a small package was needed. This had two outer annular gears,
each half the thickness of the other gears. One of these two annular gears was
held fixed and had one fewer teeth than did the other. Therefore, several turns
of the "sun" gear made the "planet" gears complete a single revolution, which in
turn made the rotating annular gear rotate by a single tooth.