Pulley structure

A pulley structure comprising:

• a cylindrical outer rotating body around which a belt is to be wound;
• an inner rotating body provided on a radial inside of the outer rotating body and relatively rotatable with respect to the outer rotating body around a common rotation axis with the outer rotating body;
• and
• a coil spring provided between the outer rotating body and the inner rotating body and compressed in an axial direction along the common rotation axis,
• wherein the coil spring is configured to be engaged with the outer rotating body and the inner rotating body due to torsional deformation in a diameter expansion or diameter reduction direction to transmit torque between the outer rotating body and the inner rotating body, and is configured to come into a disengaged state in which the coil spring slides with respect to the outer rotating body or the inner rotating body due to torsional deformation in a direction opposite to the direction when the torque is transmitted, so as to block the transmission of torque between the outer rotating body and the inner rotating body, and
• wherein the coil spring has a number of windings within a range of M-0.669 or more and less than M, where M is a natural number.