Patent ID: 8734283

Claim:
A speed reduction mechanism, comprising: a rotary shaft that rotates around a first axis, and that has an eccentric portion of which a central axis is a second axis that is offset from the first axis; an input member that is arranged radially outward of the rotary shaft, and that is formed of an external gear mechanism having a center hole of which a central axis is a third axis and a plurality of through-holes arranged at equal intervals around the third axis and having a pitch circle of which a central axis coincides with the third axis, and provided with a bearing interposed between an inner periphery of the input member, which defines the center hole, and an outer periphery of the eccentric portion; a cylindrical housing that has a rotation force applying member formed of an internal gear mechanism that is arranged so as to be fitted to the input member in a radial direction of the input member, that is in mesh with the input member, that has teeth the number of which is larger than the number of teeth of the external gear mechanism, and that has a pitch circle of which a central axis is a fourth axis; and output members that receive rotation force applied to the input member by the rotation force applying member of the housing and output the rotation force, and that are passed through the respective through-holes, wherein when the bearing includes an outer ring and an inner ring and the outer ring is fitted to the inner periphery of the input member, which defines the center hole, with a clearance in a radial direction of the rotary shaft and the inner ring is fitted to the eccentric portion with a clearance in the radial direction of the rotary shaft, a size between the second axis and the third axis is set to a size that is smaller than or equal to half of a size obtained by adding a diameter difference between an outside diameter of the bearing and an inside diameter of the input member, which defines the center hole, a diameter difference between an inside diameter of the bearing and an outside diameter of the eccentric portion and an operating clearance of a radial internal clearance of the bearing in a state where the input member has been moved to contact the housing on a line perpendicular to the second axis and the fourth axis.