With the THK high torque type Ball Spline, the spline shaft has three crests positioned equidistantly at 120 degree, and along both sides of each crest, two rows of balls (six rows in total) are arranged so as to hold the crest. The raceways are precision ground into R-shaped grooves whose diameters are approximate to the ball diameter. When a torque is generated from the spline shaft or the spline nut, the three rows of balls on the load-bearing side evenly receive the torque, and the center of rotation is automatically determined. When the rotation reverses, the remaining three rows of balls on the unloaded side receive the torque.