According to the present invention, with regard to a plurality of notch portions, a plurality of tilt faces are provided for a respective one of the notch portions. As a result, the present invention is capable of lowering a facial pressure of a respective one of a plurality of tilt faces of one notch portion more remarkably than a facial pressure of one tilt face of one notch portion. As a result, the present invention is capable of reducing a frictional wear and tear of a tilt face without a need to use any other component.