A planetary gearbox cycloidal drive or cycloidal rate reducer is a system for reducing the speed of an input shaft by a particular ratio. Cycloidal rate reducers are capable of fairly high ratios in small sizes. 
The input shaft drives an eccentric bearing that subsequently drives the cycloidal disc in an eccentric, cycloidal movement. The perimeter of the disc is geared to a stationary ring gear and has a group of result shaft pins or rollers placed through the facial skin of the disc. These output shaft pins straight drive the output shaft as the cycloidal disc rotates. The radial movement of the disc is not translated to the output shaft.