NSK’s new ROBUST system of ultra high speed bearings meets the requirements of the machine tool industry for ever higher spindle speeds (up to 3.5 x 106 dmN1), enabling a wider range of machining operations - from rough machining to finishing - to be performed on a single machine tool. The Super Precision system of bearings also optimises acceleration and deceleration times to minimise production losses due to tool-changes, and provides smooth running at all times, even when subjected to the severe thermal effects that occur during rapid accelerations to required spindle speeds.
The Robust bearing system is designed to satisfy all the demanding operational conditions of the latest motorised machine tool spindles. As such it is characterised by its high speed operation, low heat generation, high resistance to seizure and excellent running stability under severe thermal conditions.
Included in the ROBUST range are ultra high-speed angular contact bearings, high-speed angular contact thrust bearings and high performance cylindrical roller bearings. All of these bearings meet the widest range of operational demands with a graduated construction which, in its ultimate form, combines the high resistance to seizure and wear of NSK’s new SHX steel technology, with high speed/temperature qualities of PEEK (Poly Ether Ether Ketone) cage material and the operating speed and inherent stiffness of hybrid silicon nitride ball material.
The advantages of a graduated construction mean that both manufacturers and rebuilders of machine tool spindles can select bearings that meet their requirements both in terms of performance and cost. For example: at the entry level of the ROBUST range users can specify competitive all steel bearings that offer a 20% higher speed capability than conventional precision angular contact bearings. The material of manufacture of these bearings is NSK’s long life Z steel. Specially developed by NSK, this material has a reduced amount of non-metallic inclusions, oxides and other inclusions.
As a result, it is an ideal bearing steel, providing significantly extended service life (up to 1.8 times longer) when compared to bearings manufactured using conventionally degassed steel.
For higher speed operation the advantages of Z steel are complemented by the addition of silicon nitride balls. The resulting ceramic hybrid bearings offer many excellent performance characteristics, such as higher heat resistance, extended life, lighter weight –hence lower centrifugal forces, lower thermal expansion, higher rigidity and lower friction and noise. The effectiveness of this design can be judged from the fact that it can employ grease lubrication at speeds(» 1.3 x 106 dmN) where conventional angular contact bearings require oil/air lubrication.
Where the operating demands are for speeds above 1.8 x 106 dmN, the ROBUST specification evolves further to include SHX steel. With grease lubrication this design of bearing can achieve operating speeds of up to 2.1 x 106 dmN (i.e. over 21,000rpm for a 70mm bore bearing) and exceed 3.0 x 106 dmN using oil/air lubrication. Crucial to achieving high levels of reliability at these speeds is NSK’s SHX steel, a new heat and seizure resistant material that is highly suited for use in machine tool spindles, which must run at high speeds with a minimum supply of lubricant. The heat resistance of SHX is nearly equal to that of M50 steel, a material used for bearings on the main shafts of jet engines where temperatures reach 300oC, while its wear resistance, resistance to seizure and durability are all superior.
SHX is also a key element in the specification of the highest-level ROBUST bearings, which are designed for operation at peak spindle speeds up to 3.5 x 106 dmN . The design of the Robust bearings at this level is essentially a synergy of material and lubrication technology, the latter NSK’s exclusive SpinshotÔ lubrication system.
While previously oil jet lubrication has only been possible at speeds exceeding 2 x 106 dmN , the SpinshotÔ lubrication system enables the ultra-high speed ROBUST Series bearings to sustain such speeds with a mixture of air and oil. The use of oil/air instead of oil jets reduces overall costs by allowing the structure of the spindle to be simplified. It also cuts down on the consumption of oil, an added environmental benefit.
New Global Super Precision Bearing Catalogue
Full details of the new ROBUST bearing system and NSK’s other Super Precision range of bearing products are contained in a new Global Catalogue just produced by the company. The new publication covers what is probably the largest range of Super Precision bearings available from any one manufacturer. It runs to 210 pages and contains seven easy-to-use sections, plus a host of illustrations and tables dealing with everything from mounting to pre-load constants to lubrication types, and tolerances for shaft and housing bore diameters. The layout of the new catalogue has been designed to support spindle designers by simplifying the task of finding product and application information. The catalogue also contains a section on spindle bearing selection with helpful explanations of spindle design.
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