US Patent 7963852 Spline shaft

A spline shaft that can suppress occurrence of stick slip by measures other than just simply making the surface roughness smaller. The spline shaft includes a first shaft with an outer-peripheral spline formed, and a second shaft with an inner-peripheral spline, which engages with the outer-peripheral spline peripherally and that is made slidable axially, formed. And, at least either one of the outer-peripheral spline and the inner-peripheral spline includes a substrate whose surface is formed to a predetermined surface roughness, and an amorphous carbon-system hard thin film, which coats and forms the surface of substrate.