Ball Splines - Design Overview

A ball spline consists of an inner race and mating outer race containing axial races and ball bearings.  Ball Splines supply nearly friction-free linear motion while simultaneously transmitting torsional loads. Recirculating ball bearings reduce friction providing predictable life and smooth operation. Similar to a ball screw, the ball bearings provide the only physical contact between the inner and outer race. However, in a ball spline, the path of the ball bearings is straight, not helical, which allows for a highly efficient coupling device suitable for a variety of applications involving axial load transfer and/or torque transmittal.

Similar to ball screws the balls in the spline outer race are not retained.  If the outer race is removed from the inner race, without being transferred directly to an arbor, the balls will fall out.