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.

NSK hybrid Robust series with SHX steel
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.