Hybrid bearing facilitates next generation of machine tools

A hybrid bearing from NSK is the response to machine tool makers demanding high spindle speed and ultra smooth running even during periods of rapid acceleration. According to NSK it also produces low heat and provides seizure resistance and improved bearing life.

The company produced the Robust bearings by combining SHX steel – a heat and seizure resistive steel – with high temperature PEEK cages and a ceramic rolling element.
The Robust H series bearings are designed for use in motorised and nonmotorised spindles and NSK claims the series is capable of speeds 25% higher than conventional precision angular contact ball bearings. Additionally, the H type can be lubricated with grease at speeds where conventional bearings require oil and air lubrication.

The 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. SHX steel is suited for use in machine tool spindles that must run at high speeds with a minimum supply of lubricant. Importantly, its heat resistance is almost equal to that of M50 steel.

Completing the hybrid range of bearings is the RXH series of cylindrical roller bearings (CRB), a high speed bearing line designed to facilitate the development of the next generation of high speed machine tool spindles with integral motors.
The RXH hybrid CRB series has been designed 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 and a pair of angular contact bearings at the rear.

These bearings are allowed to slide 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, it may not slide due to radial expansion of the bearings and 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 sliding bearings has been the one most commonly adopted by spindle builders to allow for axial expansion of spindle shafts. The reason for this is that the alternative - 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. Additionally, 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.

Author
Justin Cunningham

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