Editing Gear (section)

==== Parallel and crossed axes ====

However, two constant velocity tooth profiles are the most commonly used in modern times for gears with parallel or crossed axes, based on the ''[[cycloid gear|cycloid]]'' and ''[[involute gear|involute]]'' curves. 

Cycloidal gears were more common until the late 1800s. Since then, the involute has largely superseded it, particularly in drive train applications. The cycloid is in some ways the more interesting and flexible shape; however the involute has two advantages: it is easier to manufacture, and it permits the center-to-center spacing of the gears to vary over some range without ruining the constancy of the velocity ratio. Cycloidal gears only work properly if the center spacing is exactly right. Cycloidal gears are still commonly used in mechanical clocks.