COIL SPRING

A coiled spring has reduced hysteresis loss due to friction resistance with a member, and resistance to contact between the spirals of the wire can be improved. An arc-spring 41 is disposed in a clutch unit 1. In order to reduce hysteresis loss, a wear-resistant layer having a friction coefficient µ of less than 0.15 and a hardness of Hv1000 or more is formed on the surface of the arc-spring 41. The wear-resistant layer is a MoS 2 layer (molybdenum disulfide layer), an electroless NiP layer (electroless nickel-phosphorus layer), a DLC layer (diamond like carbon layer), or a TiN layer (titanium nitride layer). The wear resistance can be improved in the wear-resistant layer excluding the MoS 2 layer. The DLC layer should be formed on the surface of the coiled spring as shown in Figs. 2A and 2B , which requires a high wear resistance, because the hardness of the DLC layer is approximately Hv3000.