Geared speed reducing unit

A geared speed reducing unit, comprising:

• a first rotary shaft and a second rotary shaft arranged coaxially with each other;
• a first gear that is arranged around a common rotational axis of the first rotary shaft and the second rotary shaft and that is not allowed to rotate;
• a second gear that is arranged around the common rotational axis to be rotated integrally with the second rotary shaft;
• a third gear that meshes with the first gear;
• a fourth gear that is arranged coaxially with the third gear to be meshed with the second gear and to be rotated integrally with the third gear;
• and
• an eccentric carrier that is arranged around the common rotational axis to support the third gear and the fourth gear such that the third gear and the fourth gear rotate around an eccentric axis offset from the common rotational axis and revolve around the common rotational axis,
• wherein the geared speed reducing unit is adapted to reduce a rotational speed of the second rotary shaft lower than a rotational speed of the first rotary shaft,
• the first gear includes a first sun gear as an external gear,
• the second gear includes a second sun gear as an external gear,
• the third gear includes a first ring gear as an internal gear,
• the fourth gear includes a second ring gear as an internal gear,
• the eccentric carrier supports outer circumferential surfaces of the first ring gear and the second ring gear such that the first ring gear and the second ring gear are rotated by a torque delivered through the first rotary shaft around the eccentric axis while revolving around the common rotational axis, and
• a gear ratio between the first sun gear and the first ring gear and a gear ratio between the second sun gear and the second ring gear are set to different values.