A gear bearing is a type of rolling-element bearing similar to an epicyclic gear. This is the lowest gear ratio attainable with an epicyclic gear train. In 1992, Lepelletier proposed three degrees of freedom epicyclic gear mechanisms. The sun and planet gear is a simple epicyclic gear. Some epicyclic gear trains employ two planetary gears which mesh with each other. Torque multipliers typically employ an epicyclic gear train having one or more stages.