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 ball bearings 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. for a shaft travel of any significant length the nut will have channels that recirculate the balls, operating in the same way as a ball screw.