A seat surface of a flat head bolt and a seat surface of a front plate are in contact with each other such that a tightening axial tension acts at an axial tension action point in an axial cross-section. A normal vector is perpendicular to the seat surface of the front plate and passes through the axial tension action point in the axial cross-section. An axial tension reach point is an intersection point between the normal vector and a shoe front surface in the axial cross-section. The normal vector intersects the shoe front surface at the axial tension reach point in the axial cross-section, and the axial tension reach point is included in a range of a shoe part.