Below is a list of Ball Screw Assembly manufacturers from Monrovia, California. A ball screw assembly is the combination of a threaded screw with an internally threaded nut that is filled with ball bearings that travel in matching helical grooves. Ball screw assemblies are often simply called ball screws and are used to convert the rotational motion of the nut into horizontal motion up and down the screw. They are commonly used with linear slides and linear actuators to change torque into thrust when precise linear positioning is critical.