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09.11.2017 | Machinery | News | Onlineartikel

Improved Ball Screw with Long Service Life

Nadine Klein

Ball screws with preloaded nuts are usually exposed to very high loads. A too high preload leads to great friction and wear, while too little tension dos not allow the thread to work precisely enough. The University of Stuttgart solves this problem with additional spring elements.

Ball screws have less friction, high positioning accuracy and less wear compared to conventional screw drives. Nevertheless, more than one third of all machine tool failures can be attributed to fatigue in ball screws. In order to ensure the required accuracy and rigidity of a ball screw, a preload is set using so-called double nuts for example. Despite the preload, however, great axial forces can create an overload state in which one of the two nut halves is completely relieved. As a result, excessive sliding and breakdown of the lubricant film can lead to premature destruction of the ball screw. In order to avoid this, high preload values are usually set, but this leads to more friction and thus greater wear. This, in turn, affects the accuracy and service life of the ball screw.

A constant preload can be created, for example, by having a spring element between the two nut halves, as is the case with some alternative double nut systems. However, due to the great elasticity of the spring element, such systems cannot be used for applications that are highly accurate and dynamic. At the University of Stuttgart, Dr.-Ing. Siegfried Frey has developed a solution that should make it possible to ensure great rigidity and thus a high degree of accuracy and a long service life, despite the minimal preload force. Additional elements are introduced within an annular groove, for example, between the two nut halves of a symmetrical double nut. These are located in the contact-type flanged joint between the nut body and mounting flange and have no function when at rest. At high axial loads, however, the preloaded elements emerge from the contact-type flanged joint and generate a residual preload force in the relieved nut half before these are relieved completely, thus preventing excessive sliding movements according to the researchers.

Not detrimental to rigidity and accuracy

The invention should make it simple and inexpensive to design ball screws with a much lower preload force without being detrimental to the rigidity and accuracy of the overall system. The solution should be suitable for a wide range of ball screws, especially for machine tools with stringent requirements. Depending on the application and type of load, it can thus increase the service life of a ball screw many times over.

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