Patent ID: 8141448

Claim:
A ball spline comprising: a shaft comprising a plurality of axially-extending linear first spline grooves formed in an outer peripheral surface thereof; an outer tube having the shaft passed therethrough and including second spline grooves corresponding to the first spline grooves of the shaft; and a plurality of balls disposed between the shaft and the outer tube, wherein ball circulation passages in which the balls are disposed are formed between the shaft and the outer tube by a plurality of main passages and a plurality of return passages communicating with opposite end portions of the main passages, wherein first shoulder portions are formed on the outer periphery of the shaft such that the first spline grooves of the shaft are interposed between the first shoulder portions adjacent to each other in a circumferential direction of the shaft so as to border both sides of the first spline grooves, wherein second shoulder portions are formed on the inner periphery of the outer tube such that the second spline grooves of the outer tube are interposed between the second shoulder portions adjacent to each other in a circumferential direction of the outer tube so as to border both sides of the second spline grooves, wherein the first and second shoulder portions and the balls are arranged such that clearances are formed respectively between the first shoulder portions and the balls and between the second shoulder portions and the balls in a rotational direction of the shaft and the outer-tube, and the balls disposed in all of the main passages bear a clockwise torque and a counterclockwise torque transmitted between the shaft and the outer tube, and wherein a cross section of the first spline grooves comprises a radius of curvature of 53% or more, and a cross section of the second spline grooves of the outer tube comprises a radius of curvature of 53% or more.