Ball spline

Ball splines (Ball Spline bearings) are a special type of linear motion bearing that are used to provide nearly frictionless linear motion while allowing the member to transmit torque simultaneously. There are grooves ground along the length of the shaft (thus forming splines) for the recirculating ground balls to run inside. The outer shell that houses the balls is called a nut rather than a bushing, but is not a nut in the traditional sense—it is not free to rotate about the shaft, but is free to travel up and down the shaft.