Power transmission mechanism

A power transmission mechanism, comprising:

• a reference gear member arranged on a center shaft;
• a first carrier member arranged rotatably about the center shaft of the reference gear member;
• a second carrier member arranged rotatably about the center shaft of the reference gear member;
• a first planetary gear member rotatably arranged to the first carrier member via a first shaft and meshed with the reference gear member;
• and
• a second planetary gear member rotatably arranged to the second carrier member via a second shaft and meshed with the first planetary gear member;
• wherein the second carrier member is configured to be relatively rotatable with respect to the first carrier member in a predetermined range,
• the first carrier member and the second carrier member are arranged concentrically about the reference gear member,
• the reference gear member, the first planetary gear member, and the second planetary gear member are arranged such that a line connecting the center shaft of the reference gear member and the first shaft of the first planetary gear member and a line connecting the center shaft of the reference gear member and the second shaft of the second planetary gear member form a specific angle,
• rotation in one direction is prevented when the first carrier member and the second carrier member are relatively rotated in a direction to decrease the specific angle, and
• a rotation prevention release means is provided to release a prevention state of the rotation when the first carrier member and the second carrier member are relatively rotated in a direction to increase the specific angle.