US Patent 8141448 Ball spline

First shoulder portions are formed on the outer periphery of the shaft such that the first spline grooves are interposed between the first shoulder portions adjacent to each other in a circumferential direction of the shaft. Second shoulder portions are formed on the inner periphery of the outer tube such that the second spline grooves are interposed between the second shoulder portions adjacent to each other in a circumferential direction of the outer tube. Main passages of spline-purpose ball circulation passages are formed with the first and second shoulder portions. Clearance are formed respectively between the first and second shoulder portions and the spline-purpose balls in a rotational direction of the screw shaft and the spline-purpose outer tube. The spline-purpose balls disposed in the main passages bear a clockwise torque and a counterclockwise torque transmitted between the screw shaft and the spline-purpose outer tube.