Novel ultra-high-speed hybrid angular-contact bearings

The superior performance characteristics of the new Robust series of hybrid bearings mean that they are well suited to high-speed machining operations where conventional angular-contact bearings would be beyond their operating limits. The new bearings satisfy the exacting demands of the latest generations of motorised machine tool spindles for smooth running - even during periods of rapid acceleration, where their higher levels of temperature tolerance, low heat generation and resistance to seizure help to improve bearing life.

Included in the Robust hybrid range are high-speed and ultra-high-speed angular-contact ball bearings, and high-performance cylindrical roller bearings. All of these bearings meet a wide range of operational demands with a graduated construction that, in its ultimate form, combines the high resistance to seizure and wear of NSK's new SHX steel technology with the high-speed and temperature qualities of PEEK (polyetheretherketone) cage material and the operating speed and inherent stiffness of hybrid silicon nitride ball material.

The Robust hybrid series of angular-contact ball bearings is available in three types: H, X and XE. The base unit in the series is the hybrid H type with silicon nitride balls and bearings rings manufactured from NSK's long-life Z steel. Specially developed by NSK, this material has fewer non-metallic oxide inclusions. As a result, it is an excellent bearing steel, providing significantly extended service life (up to 1.8 times longer) when compared to bearings manufactured using conventionally degassed steel rings.

Spindle applications

Robust H series bearings are designed for use in motorised and non-motorised spindles and are claimed to be capable of speeds 25 per cent higher than conventional precision angular-contact ball bearings. The effectiveness of their design can be judged from the fact that the H type can be lubricated with grease at speeds where conventional bearings require oil/air lubrication (~1.4 x 10^6 dmn).

Moving further up the performance scale, Robust X type hybrid ceramic bearings have the same design as H types; however, they are equipped with special high-strength outer-ring guided phenolic cages and have rings made of SHX material, a new heat- and seizure-resistant steel developed by NSK. SHX steel is highly suitable for use in machine tool spindles that must run at high speeds with a minimum supply of lubricant. Importantly, its heat resistance is nearly equal to that of M50 steel, a material that is used widely in bearings for jet engines, while its wear resistance, resistance to seizure and durability are all superior. In user terms, this means that the X type bearings can be used at even higher speeds than the H type, up to 2.0 x 10^6 dmn) with grease lubrication

SHX is also a key element in the specification of the highest-level Robust hybrid bearings, which are designed for operation at peak spindle speeds up to 3.5 x 10^6dmn. The design of the Robust bearings at this level is essentially a synergy of material, SHX steel, ceramic rolling elements, high-temperature PEEK cages, and lubrication technology - this being NSK's exclusive Spinshot lubrication system.

While previously oil jet lubrication has only been possible at speeds exceeding 2 x 10^6 dmn, the Spinshot lubrication system enables the ultra-high-speed Robust XE series of hybrid bearings to sustain such speeds with a mixture of oil and air. 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, which is an added environmental benefit.

Completing the Robust hybrid range of bearings is the RXH series of cylindrical roller bearings (CRBs), a new ultra-high-speed bearing line that is designed to facilitate the development of the next generation of high-speed machine tool spindles with integral motors.

Benefits for machine tool builders

The development of the series RXH hybrid CRB brings advantages for machine tool builders, machine tool refurbishers and spindle builders. Most machine tool spindles comprise a fixed/located set of bearings at the front of the spindle (ie the work end) and a pair of angular-contact bearings at the rear. These bearings are allowed to float axially in the bearing housing to accommodate thermal and axial expansion of the spindle shaft during operation. One of the problems with this design is that the pair of bearings must be able to slide in the axial direction; if the fit is too close, they may not slide due to radial expansion of the bearings; if the fit is too slack, then vibration could occur and fretting corrosion on both the bearing and bore of the housing.

Despite its drawbacks, the arrangement of fixed and floating bearings has been the one most commonly used by spindle builders to allow for axial expansion of spindle shafts. The reason for this is that the alternative, of using a roller bearing, would drastically compromise the speed performance of the spindle. However, the launch of NSK's RXH series CRB means that speed is no longer compromised by the use of rollers. Moreover, the use of rollers allows 'perfect' axial movement of the shaft, while the outer ring can be a location fit, thereby eliminating the possibility of vibration and fretting.

The RXH bearings are manufactured from SHX steel and employ ceramic rollers and high-temperature PEEK cages. As a result, they offer higher speed operation up to 2.5 x 10^6 dmn, optimum resistance to seizure, and three to four times longer wear life compared to machine tool grade CRBs that are manufactured from standard bearing steel SUJ2.

New super-precision bearing catalogue

Full details of the new Robust hybrid bearings and NSK's complete range of super-precision bearings are contained in a new Global Catalogue. The publication covers what is possibly the largest range of super-precision bearings available from any one manufacturer. The layout of the new catalogue has been designed to support spindle designers by simplifying the task of finding product and application information. To this end, it contains a section on spindle bearing selection, with helpful explanations of spindle design.

Copies of the publication are available from NSK's marketing department by calling +44 (0)1636 643031.