1) Three materials: alloy copper-basis, aluminum-basis and resin-basis 2) The ball retaining rings can effectively prevent the steel balls from shedding off 3) With the tight fits between steel balls and holes or axles, the ball retaining rings have very high fit precisions, thus are the ideal substitutes of original retaining rolls Inner packing: 1pr/polybag Outer packing: 50prs/ctn