Patent ID: 8903276

Claim:
An arm unit, comprising: a first member; a rotation shaft; a second member expandably connected to the first member via the rotation shaft; a spring member configured to bias the first member and the second member to a direction of expanding the first member and the second member; and a displacement adjusting device configured to displace the spring member at an amount of displacement less than an amount of displacement between the first member and the second member around the rotation shaft, wherein the displacement adjusting device comprises: a first gear secured to the first member placing the rotation shaft as a center; a second gear engaging the first gear, the second gear being freely rotatable relative to the second member; a first contact point portion located at the first member, the first contact point portion being in contact with one arm of the spring member; and a second contact point portion located at an apart position from a rotational center position of the second gear, the second contact point portion being in contact with another arm of the spring member.