A rolled ball screw assembly is metallic, usually steel, and it is made of a threaded rod called the screw in addition to an internally threaded nut whose helical grooves match those of the screw. Inside the grooves and contained within the nut are many small ball bearings typically made of chrome steel. The balls provide smooth motion down the screw as they circulate through the nut. Deflectors or return systems retain the balls and keep them circulating through the nut.
Rolled ball screws are up to 90% efficient when used with a motor. They are fairly precise with accuracies of several thousandths of an inch per foot and may be custom made. Rolled ball screws are used by many industries for the precise control they offer including the aerospace, computer, electronic, automotive and medical industries.
Ball screws are also frequently used in manufacturing processes and are common in robots, automated assembly lines, material handling equipment, conveyors, machine tools, fly by wire control, precision assembly equipment and more.
There are three main ways that the threads on ball screws are fabricated: through grinding, milling and rolling. Milled screws are the result of machining that uses tools to cut and remove selected material. Ground screws are first machined to their gross shape then case hardened and ground on machines with abrasive materials that spin at rapid speeds. This process allows the resulting ball screws to withstand high temperatures that would otherwise distort the shape and efficiency of the screw.
Ground screws are the most precise option but are also more expensive. Rolled ball screws are made from blank metal workpieces that are cold rolled into the screw shaft. Rolled ball screws are made by using movable and fixed forming dies. A servo maintains the correct pressure and compensates for any changes in screw diameter as the dynamically positioned movable die forms the screw.
The linear axis of the screw, however, cannot be guaranteed past certain points; the rod may not be perfectly cylindrical. Because there is no grinding, machining costs decrease, which makes rolled screws an affordable choice for many manufacturers. After the rolled ball screw shaft is fabricated, the nut must be added to complete the assembly.
The screw and nut do not twist together like a nut and bolt do; instead, the balls spin around the grooves between the threading and allow the nut to slide.