Abstract: Oil grooves (40) formed in at least one of surfaces of a ring-shaped member (30) each have a longitudinal direction that forms an angle falling within a range of from 40 degrees to 75 degrees with respect to a radial direction of the ring-shaped member, and are arranged at equal intervals in a circumferential direction of the ring-shaped member (30). The oil grooves (40) include communicating oil grooves (40A) and non-communicating oil grooves (40B) formed to extend to the vicinity of an outer peripheral end (34), and are arranged with periodic regularity in the circumferential direction of the ring-shaped member (30). In addition, a communicating oil groove area ratio expressed by Expression: “S1/(S1+S2)” falls within a range of from 0.15 to 0.85, where S1 represents a sum of plane areas of the communicating oil grooves (40A), S2 represents a sum of the non-communicating oil grooves (40B).

Abstract: Oil grooves (40) formed in at least one of surfaces of a ring-shaped member (30) each have a longitudinal direction that forms an angle falling within a range of from 40 degrees to 75 degrees with respect to a radial direction of the ring-shaped member, and are arranged at equal intervals in a circumferential direction of the ring-shaped member (30). The oil grooves (40) include communicating oil grooves (40A) and non-communicating oil grooves (40B) formed to extend to the vicinity of an outer peripheral end (34), and are arranged with periodic regularity in the circumferential direction of the ring-shaped member (30). In addition, a communicating oil groove area ratio expressed by Expression: “S1/(S1+S2)” falls within a range of from 0.15 to 0.85, where S1 represents a sum of plane areas of the communicating oil grooves (40A), S2 represents a sum of the non-communicating oil grooves (40B).