Patent attributes
A recessed part formed on a side surface includes a dynamic pressure part extending in the circumferential direction to converge on the side surface, and an introduction part extending from the dynamic pressure to open the dynamic pressure part toward the inner periphery side. One or two inner peripheral wall parts are provided for each of the recessed parts. The inner peripheral wall part is a portion defined on the inner periphery side of the corresponding recessed part by the dynamic pressure part and the introduction part. The inner peripheral wall part includes an inner peripheral wall surface. The inner peripheral wall surface extends toward the introduction part in the circumferential direction with inclining with respect to the side surface such that the inner peripheral wall surface is depressed from the side surface.
