Abstract: A primary transmission has a first eccentric disc coupled in a fixed driving connection to a drive shaft extending into the transmission housing. An internal ring gear is mounted in the housing concentrically to the drive shaft. A first spur gear is supported on the first eccentric disc as a first drive member and meshes with the internal ring gear. The first spur gear has at least one tooth less than the internal ring gear. The diameter of the circle defined by the end of the first spur gear teeth is smaller than the diameter of the circle defined by the base of the internal ring gear teeth. The first eccentric disc has an eccentric throw that corresponds to half a difference of the circle diameters. A driven disc as a second drive member is concentric to the drive shaft and is torque-transmittingly connected to the first spur gear. One of the drive members has at least one axially projecting pin and the other has at least one receiving bore.