Patent ID: 7819041

Claim:
A variable transmission ratio apparatus comprising: a differential gear mechanism that adds rotation resulting from a motor drive to rotation from an input shaft resulting from a steering operation and transmits the added rotation to an output shaft; a housing accommodating the differential gear mechanism and a motor comprising the motor drive, and fixed to a non-rotating portion; and a lock device that locks the housing and a rotation shaft of the motor to prohibit relative rotation therebetween; wherein the differential gear mechanism is formed by a strain wave gear mechanism including: a pair of circular splines arranged next to each other, the circular splines being concentric with each other, wherein one of the circular splines is connected to the output shaft and the other one of the circular splines is connected to the input shaft; a tubular flexible spline arranged at an inner side of the circular splines and a concentric with the circular splines so as to be partially mated with each of the circular splines; and a wave generator that rotates a mated portion of the flexible spline when driven by the motor; wherein the one of the circular splines that is connected to the output shaft has a greater number of teeth than the other one of the circular splines that is connected to the input shaft.