Patent ID: 7121947

Claim:
A coupling device comprising: a pulley which rotates when a torque is applied thereto; a driven rotation member; a hub through which the torque applied to the pulley is transmitted to the driven rotation member, the hub including an outer peripheral portion fixed to the pulley, an inner peripheral portion fixed to the driven rotation member, and an elastic member connecting the outer peripheral portion and the inner peripheral portion, the inner peripheral portion including a wall portion and a contact portion; the wall portion of the inner peripheral portion having a first thread through which the wall portion of the inner peripheral portion is connected to a second thread of the driven rotation member; and a component disposed between the contact portion of the inner peripheral portion and an axial tip end surface of the driven rotation member; wherein: the component has flat faces in the axial direction on which the contact portion of the inner peripheral portion and the axial tip end surface of the driven rotation member come in contact, respectively, the component providing friction contacts with both the contact portion of the inner peripheral portion and the axial tip end surface of the driven rotation member; a contact surface pressure of the contact portion of the inner peripheral member and one flat face of the component and a contact surface pressure of the tip end surface of the driven rotation member and the other flat face of the component increases when the hub fastens the first and second threads by the transmitted torque; the inner peripheral portion includes a breakable portion connected to the wall portion and the contact portion; the breakable portion breaks when the transmitted torque is larger than a value; and a sectional area of the breakable portion is smaller than a sectional area of the wall portion.