Ground Ball Screws

Ground Ball Screw

A ground ball screw assembly is metallic, usually steel, and made of a threaded rod called the screw as well as 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. Ground ball screws are up to 90% efficient when used with a motor. They can also be very precise; ground screws are typically manufactured with 0.0005 inch per foot lead accuracy where the lead refers to the axial distance a screw travels during one revolution. Ground 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 rolling, milling and grinding. Rolled thread ball screws are cold rolled from a blank and are inexpensive but less precise, though still mechanically efficient. Milled threading is the result of machining that uses tools to cut and remove selected material, carving it away. Ground threads are the result of a three step process. First, the steel is machined to its gross shape. It is 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. Once the screw is manufactured, a corresponding nut is added. The screw and nut do not twist together like a nut and bolt because the balls inside the nut housing make contact with the grooves in the nut and screw threading, spinning inside the nut as the nut rotates up and down the screw. Various deflectors and return methods are used to recirculate the ball bearings because they enable the ball screw to function. In an external ball return system, the balls are returned to the opposite end of the nut by a return tube that curves around the outside diameter of the nut. The internal ball return system employs the same idea but without using exterior tubes.