At EMO 2019, NSK launched two interesting novelties in the field of precision ball screws. The former is a new series of ball screws in which the nut and shaft can be freely combined, a design which makes it easier for both dealers and end users to manage their stock. The latter series offers higher durability when the component is subjected to high loads.
by Elena Magistretti
As high precision drive elements, ball screws are always designed to customer specifications and adapted to a specific task. The manufacturer then supplies individual assemblies of nuts and shafts that have been matched in the factory to give the required preload.
At EMO, NSK introduced a new series which does not require using an individually matched nut-shaft combination. Instead, the nut and shaft can be freely interchanged, following the principle of random matching.
Based on the NSK BSS series for high speed low noise applications, NSK’s interchangeable balls screws are available in accuracy class C7. They come in shaft diameters from 15 to 32 mm, with pitches from 5 to 20 mm. The dimensions of the flange and mounting holes, as well as the tolerance classes, meet the requirements of DIN 69051 (ISO 3408). Optionally, the nut bodies can be equipped with an NSK K1 lubrication unit.
Thanks to their inherent design, the units are shipped with very little axial play as standard. If necessary, a desired preload can be generated by changing the size of the balls at a NSK dealer or directly at NSK.
Suitable for handling, pick&place, MRO and other applications
The new series, combining performance with an entry level price, is ideal for applications that include pick-and-place systems, as well as automated transfer units for assembly, handling and internal transport. Many other uses are also possible.
For users, the free interchange of nut and shaft has the advantage that the required ball screw can simply be taken or ordered from stock, and then cut to length either in-house or by an authorised NSK dealer. The result is greater flexibility for the user and the rapid availability of ball screws in individual dimensions, whenever they are needed for special machines or prototypes, for example.
Higher durability in presence of high loads
Another innovation introduced at EMO is a series of ball screws which, among other things, achieves a service life that is around three times longer when subjected to high loads. Of enormous benefit to machine tool builders and users, this longevity is made possible by a specially devised surface texture that promotes improved oil film formation.
Like all mechanical components, ball screws are subject to wear. One of the constant development goals of NSK’s research is to continuously reduce friction, and thus increase service life.
Before commencing development, NSK noted that typical damage patterns on the surface of ball screws can be traced back to inadequate lubrication, which leads to direct metal-to-metal contact between the shaft and nut.
The shafts of precision ball screws are precision ground, and their surface texture appears under a microscope as a strip. Unfortunately, oil can easily run off these micro-strips, soon leading to inadequate lubrication.
On the basis of these findings, NSK set about developing a new surface treatment that produces a flat texture with micro-recesses. Even the smallest oil deposits can settle in these recesses and prevent inadequate lubrication, permanently and effectively. The nut thus moves constantly on an oil film, reducing wear on the ball screw, especially during slow and short strokes.
Compared with standard NSK ball screws, service life is increased by a factor of three. In addition, the new surface texture reduces break-away and stick-slip effects, even during the aforementioned slow and short movements.
The new series is suitable for use in machine tools that perform high-precision machining operations, such as milling centres for mould and die production, spark erosion machines, grinding centres, and other similarly challenging applications.
NSK’s ball screws featuring the new surface texture will be available in pitches from 10 to 50, in accuracies of C5 and better.