DIFFERENTIAL (or planetary gear)

The Differential, also called a planetary (or epicyclic) gear train, is a gear system that offers, with a minimum of components, a wide range of transmission ratios and the ability to combine or distribute movements. It generally consists of two concentric, superimposed wheels and one or more small pinions, the planet gears, which roll between them while rotating around a central axis. Depending on which element is driven and which is held fixed, different ratios are obtained, or two inputs are added into one output. In watchmaking, the Differential is used in particular to add movements together — for example to display a Power reserve from the Winding and Letting down of the Barrel, or to combine two energy sources —, as well as in certain constant force constructions. Compact and versatile, this mechanism illustrates the ingenuity of gear trains and allows for elaborate functions without multiplying components.