Patent ID: 8009974

Claim:
A power transmission, comprising: a rotation shaft having a thread formed on a periphery thereof; a nut member including a resilient part that straddles the rotation shaft and has a thread engaged with the thread of the rotation shaft, and the nut member moving synchronously with the rotation of the rotation shaft; and a guide member including a regulating part and a regulation releasing part, wherein the regulating part restricts the width of the resilient part from expanding so as to allow the nut member to move when the regulating part is in contact with the resilient part, the regulation releasing part allows the width of the resilient part to expand such that the nut member and the rotation shaft disengage from each other under a predetermined condition, the regulating part and the regulation releasing part are configured to lie at different positions in a direction parallel with the rotation shaft, and the nut member moves in a first direction and disengages from the rotation shaft at the regulation releasing part when the rotation shaft rotates in a first rotational direction and moves in a second direction opposite to the first direction when the rotation shaft rotates in a second rotational direction opposite to the first rotational direction.