A coupling system for coupling a curved object to a central shaft, the curved object includes an inner surface facing the central shaft and an outer surface. The system includes a coupling member for coupling the curved object to the central shaft; at least one pocket integrally formed on a curved surface of the curved object and defined as a depression with respect to at least one of the inner and outer surfaces. The pocket includes: a flat portion having an aperture for holding the coupling member; and a flange portion which merges continuously from the flat portion to the curved object and is shaped such that the flat portion is disposed perpendicularly to the coupling member; a fastening member for fastening the coupling member to the flat portion. The thickness of the flat portion and flange portion is the thickness of the curved object.