A ball screw is a mechanical assembly composed of a screw and a nut linked by ball bearings that roll in the threads of the screw and the nut. The low rolling friction of the ball bearings makes ball screws highly efficient devices for converting rotary motion into linear motion. The ball bearings, screw threads, and nut threads are made of hardened steel which gives ball screws high load capacities. Commonly there are wipers at each end of a nut to prevent the ingress of debris.  There must be a ball return system built into the nut. The ball return system recirculates the ball bearings in the nut or else the balls would exit the nut out the end as it traveled along the screw.  The primary ball return system used by PBC Linear is a multiliner return system. The multiliner ball return system guides turns of balls back to the same threads creating ball circuits within the threads.