Main Page | See live article | Alphabetical index Epicyclic gearing used to increase output rotation. The planet gear carrier (green) is driven by an input rotation. The sun gear (yellow) provides the output rotation, while the annulus (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. The gear ratios between the input rotation and output rotation in the epicyclic gear train depends on what components are used to transfer rotation into and out of the system. The annulus may be held fixed, with input provided to the planetary gear carrier; output rotation is then produced from the sun gear. This configuration may produce an increase or decrease in gear ratio, depending on the relative sizes of the gears. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.