Abstract: A reduction starter employing a planetary gear type reduction gear mechanism having an outer ring gear displaceable radially of a common axis of a central sun gear and an output shaft carrying planet gears for revolution about the common axis. The outer ring gear is formed by an internally toothed ring gear elastically deformable to assure uniform distribution of load torque to all planet gears. The tooth root bending stress of the ring gear is substantially equalized to the outer periphery bending stress thereof to improve the load performance of the planetary gear type reduction gear mechanism and thus reduce the size and weight of the starter. The outer ring gear has an outer rim of a radial thickness t and a plurality of radially inwardly extending teeth each having a height h. The rim thickness t is determined to'fall with a range of from +.ltoreq.0.8 h to +.gtoreq.0.4 h.

Abstract: A reduction starter employing a planetary gear type reduction gear mechanism having an outer sun gear displaceable in directions perpendicular to a common axis of a central sun gear and an output shaft carrying planet gears for revolution about the common axis. The outer sun gear is formed by an internally toothed ring gear elastically deformable to take up an offset, greater than a predetermined value, of any of the planet gears due to irregular mounting thereof, for thereby avoiding local or uneven contact between gear teeth and assuring a uniform distribution of load torque to all the planet gears.