Transmission ratio varying mechanism

A transmission ratio varying mechanism that is joined to a first steering shaft that rotates integrally with a steering handle and to a second steering shaft that is disposed coaxially with the first steering shaft and rotates integrally with a steering member that steers steered wheels, with the transmission ratio varying mechanism performing transmission of rotation between the first steering shaft and the second steering shaft and being capable of varying the transmission ratio of that rotation, the transmission ratio varying mechanism comprising:

• an eccentric rotary plate that is disposed facing the first steering shaft and the second steering shaft and is eccentrically rotatable with respect to these steering shafts;
• a carrier that rotatably supports the eccentric rotary plate and is rotatably disposed coaxially with respect to the first steering shaft and the second steering shaft;
• and
• carrier driving means that causes the carrier to rotate,
• wherein
• the eccentric rotary plate includes, in its surface facing the first steering shaft, a first cycloidal groove configured on the basis of a cycloidal curve and includes, in its surface facing the second steering shaft, a second cycloidal groove configured on the basis of a cycloidal curve,
• the first steering shaft includes, in its surface facing the eccentric rotary plate, a third cycloidal groove configured on the basis of a cycloidal curve,
• the second steering shaft includes, in its surface facing the eccentric rotary plate, a fourth cycloidal groove configured on the basis of a cycloidal curve,
• the first cycloidal groove and the second cycloidal groove are formed concentrically with an axial center of the eccentric rotary plate,
• the third cycloidal groove and the fourth cycloidal groove are formed concentrically with an axial center of the first steering shaft and the second steering shaft,
• transmission of the rotation of the first steering shaft and the eccentric rotary plate is performed via a first rotation transmitting ball group that engages with the third cycloidal groove and the first cycloidal groove,
• transmission of the rotation of the eccentric rotary plate and the second steering shaft is performed via a second rotation transmitting ball group that engages with the second cycloidal groove and the fourth cycloidal groove,
• the first to fourth cycloidal grooves are configured to vary the transmission ratio of the rotation between the first steering shaft and the eccentric rotary plate and the transmission ratio of the rotation between the second steering shaft and the eccentric rotary plate, and
• the carrier driving means causes the carrier to rotate, whereby the transmission ratio varying mechanism varies the transmission ratio of the rotation between the first steering shaft and the second steering shaft.