Patent attributes
Each of the first hinge portions includes a first central planar portion, and first both-sides planar portion, and each of second hinge portions includes a second central planar portion, and second both-sides planar portion which is arranged on the same plane as the first both-sides planar portion. At least one of each first hinge portion and each second hinge portion includes/include an axial step portion which is provided between the first central planar portion and the first both-sides planar portion and/or between the second central planar portion and the second both-sides planar portion over the entire length of the nodal ring in the longitudinal direction and allows/allow the first central planar portion to be arranged to an outer peripheral side than the second central planar portion by a distance corresponding to a wall thickness of the nodal ring.
