Before the invention of involute gears, cycloidal gears represented the main gears, and were used in many applications. They are part of the non-involute family and are used to transmit a rotation from a driving shaft to a driven shaft.
Accurate cycloidal gears are extremely difficult to be manufactured because their teeth do not have straight edges. The face of the cycloidal tooth is composed by an epicycloid outline and hypocycloid flank, which are connected at the circle of the pitch. One of the main design feature of these gears is that the center distance is extremely sensitive.
Since the rotation of the driven shaft is not uniform, cycloidal gears are used to transmit slow rotations. Moreover, their teeth are much stronger than the one of involute gears, since their flanks are not radial. Another advantage of these gears is that they have less sliding action.