Patent ID: 8272282

Claim:
A starter motor comprising: an electric motor generating a rotational power; a planetary gear speed reduction device that reduces a rotation speed of the electric motor and outputs the reduced rotation speed; a shock absorbing device that limits a rotation of an internal gear in the planetary gear speed reduction device by a friction force, and absorbs a shock applied from an internal combustion engine by rotating the internal gear against the friction when the internal gear receives the shock caused by a load torque over a predetermined level, the shock absorbing device comprising: a cylindrical casing comprising an annular bottom flange, a cylindrical wall part, and an upper part, the annular bottom flange being opposite to the upper part and the cylindrical wall part extending between the annular bottom flange and the upper part, the upper part being formed by bending a distal end of the cylindrical wall part toward an inside of the cylindrical casing; a plurality of rotatable disks placed in an inner periphery of the cylindrical casing, each of the plurality of rotatable disks having a radial inner surface having a first diameter, wherein the internal gear is formed on said radial inner surface; a plurality of fixed disks fixedly placed in an inner periphery of the cylindrical casing so that each rotatable disk is sandwiched between a pair of adjacent fixed disks; a frustoconical disk spring comprising: a first end having an inner diameter, wherein said inner diameter is smaller than said first diameter; and a second end, opposite the first end, the second end having an outer diameter larger than the inner diameter and larger than the first diameter; wherein the first end of the disk spring abuts the annular bottom flange of the cylindrical casing and the second end of the disk spring abuts a disk stack structure composed of the rotatable disks and the fixed disks stacked alternately; and wherein the upper part of the cylindrical casing is caulked toward the annular bottom flange to fix the disk stack structure against the second end of the disk spring, and the disk spring generates an elastic force against the disk stack structure; and wherein the inner diameter and outer diameter of the disk spring are selected so as to provide a spring constant that provides a stable spring load to the disk stack structure in the cylindrical case.