Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear.
Epicyclic gearing systems also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.
Planetary gears (or epicyclic gears) are typically classified as simple and compound planetary gears.
The three basic components of the epicyclic gear are: In many epicyclic gearing systems, 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.
In one arrangement, the planetary carrier (green) is held stationary, and the sun gear (yellow) is used as input.
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 each of the planet gear(s) about its axis.
Extending this case from the one above: So, with the planetary carrier locked, one turn of the sun gear results in 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.
These are all described by the equation: where n is the form factor of the planetary gear, defined by: If the annulus is held stationary and the sun gear is used as the input, the planet carrier will be the output.
More planet and sun gear units can be placed in series in the same annulus housing (where the output shaft of the first stage becomes the input shaft of the next stage) providing a larger (or smaller) gear ratio.
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.
Advantages of planetary gears over parallel axis gears include high power density, large reduction in a small volume, multiple kinematic combinations, pure torsional reactions, and coaxial shafting.
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