NSK Ltd. (Japan; Tokyo: 6471; 5251R9N7) has expanded its high-speed and ultra-high-speed machine tool bearing line with hybrid ceramic bearings, under the Robust brand. Hybrid ceramic bearings combine steel races with hybrid silicon nitride rolling elements; the Robust line currently covers angular contact ball bearings and cylindrical roller bearings.
The inners and outers of Robust bearings are machined from NSK's proprietary SHX steel, which is specifically engineered for use with ceramic rolling elements. Most other hybrid ceramic bearings use standard 52100 bearing steel, engineered for steel rolling elements.
By high-speed, NSK indicates continuous operating speeds of up to 3.3 x 106 dmn, which is equivalent to 47,100 RPM for a 70mm bore bearing. (1 dmn is bearing PCD expressed in mm x spindle speed in RPM).
Although NSK offers no specifics, SHX is a low-inclusion steel, with races reportedly heat treated to a Rc hardness higher than standard 52100 high-carbon chrome bearing steel races, allowing them to take further advantage of the ceramic rolling element characteristics. NSK indicates this allows higher operating speeds suited to high-precision machine tools, while keeping operating temperatures down and helping to eliminate brinelling and skidding during acceleration and deceleration. In hybrid ceramic applications, SHX offers fatigue strength up to four times that of 52100.
In addition to the ceramic rolling elements, Robust bearings generally employ PEEK (polyetheretherketone) cages.
Robust bearings come in H, X, and XE ball bearings, and RXH cylindricals. The H series bearings use NSK's Z steel with ceramic balls, can be greased, and are designed for use in high-speed motorized machine tool spindles. The X series aims at higher speed applications, using phenolic cages and SHX steel, and can run with minimal lubrication. XE series are essentially X series which can be lubricated by air/oil mist injection.
RXH series hybrid ceramic bearings with PEEK cages are a relatively new application; high-speed cylindrical roller bearings which can be fitted to both ends of next-generation high-speed motorized spindle shafts. Normally, the bearings at each end of a spindle shaft have a critical fitup compromise, allowing for heat-induced axial and radial shaft runout, but not enough to set up harmonic vibrations or raceway fretting. The RXH hybrid ceramic cylindrical roller bearings support the necessary spindle speeds and can be firmly located to eliminate radial movement, while still allowing the shaft to move axially.
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