Linear Motion Control - 2008-07-02
Durable and noiseless operation at the highest speeds – these are the essential requirements of spindle bearings. Deckel Maho Gildemeister (DMG) equips its main spindles with NSK bearings from the ROBUST series which clearly have higher service life and capacity and thus give DMG a distinct competitive edge. As a world-wide leading company for rolling bearings and linear technology, NSK has been building spindle bearings for machine tools for decades. The machine spindles of DMG machining centres, lathes and milling machines are key positions that determine the capacity of the complete machine unit. Comprehensive tests and practical experience have shown DMG that the ROBUST technology for NSK spindle bearings offers distinct advantages.
Even if the long-standing trend for constantly higher speeds in main spindles has stabilised at a high level, now the machine manufacturers and endusers are focussing on longer service life and operational reliability and, as such, in particular on spindle bearings.
Bearings from the ROBUST series which have been developed according to the most innovative perspectives with state-of-the-art development and simulation tools are being taken into account.
A main factor here was optimisation to the least possible heat generation during operation to achieve maximum speed ranges. Spindle bearing failure is generally caused by overheating during operation and indeed at high speeds. Besides, during grease lubrication at lower temperatures, the grease lifespan is extended and subsequently the lifespan of the spindles is as well to a considerable extent. Bearing geometry and materials are thus especially adapted to this optimum. For this purpose, NSK uses the so-called Z steel as standard material. This shows distinctly fewer non-metallic inclusions, oxides and other impurities compared to conventional vacuum degassed steel and hence offers a 1.8x higher fatigue lifespan. As a unique selling position, NSK also has the option of highly pure SHX steel as material for the bearing rings. This material is especially heat resistance due to a special heat treatment procedure by NSK. With regard to this feature, it hardly differs from the M50 steel, from which bearings for jet engines are manufactured, where temperatures of up to 300°C develop. This heat resistance, the low friction and permissible higher surface pressure make the SHX steel an optimum material for spindle bearings at maximum speeds and place it above other metals with regard to capacity of resistance to wear.
It Depends on the Cage
A very important functional element in the bearing is the cage. With phenolic, PEEK or polyamide cages, NSK has a wide range to choose from, the advantages of which can be utilised depending on the application. The classic model here is the outer ring guided phenolic cage. This tried-and-tested solution offers stable running behaviour even at high speeds. At speeds up to 1.4 million n * dm, the ball-guided polyamide cage has its advantages. Thus, operational noises and friction are reduced and the free space in the bearing interiors is greater, so as to provide more space for lubrication grease. These features also result in a lower run-in period for the bearing.
For extremely high-quality applications, we also have an outer ring guided PEEK cage (PEEK=Polyetheretherketone). This material is highly temperature-resistant, extraordinarily wearproof and comes with high resistance, viscosity and fatigue resistance. Its dimensions are stable and only deforms slightly, even at high speeds. As a result, considerably higher speeds can be achieved than with the phenolic cage.
ROBUST Series Offers Many Options
The most frequently used bearing type in main spindles is the angular contact ball bearing. The ROBUST series offers the contact angle of 18° and 25°. The 15° contact angle, usual in the past and used widely for standard spindle bearings, has proved to be less appropriate for operation at high speeds in practice. As a result of the extensive research at NSK, a contact angle of 18° was derived as an optimum compromise between excellent speed suitability and "robust" operation. The angular contact ball bearings are also available in a sealed and greased design.
Non contact seals are used which lead to no increase in the bearing friction. The vital advantage here lies, on the one hand, on the handling, since the bearing itself must not be washed and greased. This drastically reduces the danger of impurities and incorrect greasing and simultaneously increases reliability during operation. Due to the metal-cladding inside the bearing, lubrication grease losses are also reduced and the grease lifespan increases by about 50%. The high speed bearings of the ROBUST series include single-row cylindrical roller bearings in addition to angular contact ball bearings.
Even here, the bearing construction was optimally adapted to the requirements for operation at maximum speeds. In addition to the standard design made of roller bearing steel, NSK is the only manufacturer to offer a range with rings and rollers made of SHX steel. Thus, it is possible to achieve in speed ranges, which are otherwise only reserved for bearings with extremely expensive ceramic rollers. With that, crucial cost savings are achieved for sophisticated applications. In the Robust cylindrical roller bearings, a carbon fibre reinforced PEEK cage is used as standard. This not only offers typical material advantages, but also prevents wear caused by metallic particles that occur due to friction on the otherwise conventional brass cages.
The many advantages and extensive tests in advance led DMG, even some years ago, to go with ROBUST technology. The use of ROBUST bearings has paid rich dividends for DMG and its customers with regard to speed, lifespan and operating safety.