FIGS. 8A to 8D are diagrams useful in describing a thrust bearing shown in FIG. 1, in which FIG. 8A is a plan view illustrating a split bearing member the outer circumferential surface of which is formed to have four engaged portions spaced equidistantly in the circumferential direction, FIG. 8B a side view of the split bearing member shown in FIG. 8A, FIG. 8C a plan view of the split bearing memb