spline shaft

planetary gearbox splines are ridges or tooth[1][2][3] on a drive shaft that mesh with grooves in a mating piece and transfer torque to it, maintaining the angular correspondence between them.

For instance, a gear mounted on a shaft might use a male spline on the shaft that fits the feminine spline on the gear. The splines on the pictured drive shaft match with the female splines in the center of the clutch plate, as the smooth suggestion of the axle is usually supported in the pilot bearing in the flywheel. An alternative solution to splines is usually a keyway and important, though splines give a longer fatigue life.[2]