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A cycloidal gear is a toothed gear with a cycloidal profile. Such gears are used in mechanical clocks and watches, rather than the involute gear form used for most other gears. Cycloidal gears have advantages over involute gears in such applications in being able to be produced flat (making them easier to polish, and thereby reduce friction), and having fewer points of contact (both reducing friction and wear).
Their gear tooth profile is based on the epicycloid and hypocycloid curves, which are the curves generated by a circle rolling around the outside and inside of another circle, respectively.