Transmission ratio variable device

A transmission ratio variable device that adds motor-driven rotation to rotation of an input shaft by using a differential mechanism, and transmits resultant rotation to an output shaft, characterized in that:

• a nutation gear mechanism is employed as the differential mechanism, and the nutation gear mechanism includes a first gear that rotates together with the input shaft, a fourth gear that rotates together with the output shaft, and a nutation gear that includes a second gear that meshes with the first gear and a third gear that meshes with the fourth gear, the nutation gear rotating about an axis that is inclined to an axis of the first and fourth gears;
• one of the input shaft and the output shaft has a shaft portion projecting outside a housing that houses the nutation gear mechanism, and a communicating hole, through which an outside and an inside of the housing communicate with each other, is formed in the shaft portion;
• and
• the transmission ratio variable device includes preload applying means including a preload adjusting plug that is screwed to one of the input shaft and the output shaft and is able to be operated from outside through the communicating hole, the preload applying means applying an axial preload to the nutation gear mechanism in accordance with a position of the preload adjusting plug.