Bearings to provide greater load-carrying capacity

Paul Boughton
Developments and upgrades in four core technologies - tribology, material engineering, analysis technology and mechatronics - are providing the platform for product developments in motion and control products.

This technology platform provides the basis for the design of NSK's rolling bearings, automotive components and precision parts. In addition, by integrating these advanced technologies, NSK is able to respond quickly and effectively to customer requirements across all key sectors. This is evidenced by recent product innovations in the key pumps, compressors and wind turbine sectors.

NSK has worked with the global pump industry to meet operating demands for improved bearing life, as a means of reducing failures in centrifugal pumps - the most widely used units in process industries. The result of this collaboration is the HPS (High Performance Standard) range of Angular Contact Ball Bearings. The basic dynamic load rating of these bearings has been improved by 20 per cent increasing bearing L10 life(1) by up to 90 per cent. This allows for longer maintenance intervals, and can enable a reduction in bearing size for new pump designs.

The optimal design characteristics that provide longer life operation on the HPS series are also key to the higher speed operation of the bearings; permissible speed being increased by 15-20 per cent compared with conventional angular contact ball bearings. The major benefit of this increase for pump users is reduced operating temperatures, enabling the bearings to operate in a higher range of high-speed pumping applications.

The same development processes that have led to improved reliability and performance in pumping systems have also been employed to produce a new range of high capacity bearings - the ELCOMP series - that enable screw compressors to achieve market demands for longer trouble free life and extended intervals between overhauls.

Key to the performance of the ELCOMP bearings is NSKs patented L-PPS cage material. L-PPS (linear polyphenylene sulphide) is a high performance plastic especially developed for screw compressors). This material, in combination with rolling element guided cage, enables the ELCOMP bearings to provide greater load-carrying capacity, and have much improved lubrication capability. The bearings also offer excellent thermal (temperatures up to 190°C), and chemical stability - the strength and chemical stability of the L-PPS cage changing little, even when used with compressor oil, refrigerator oil or ammonia gas.

Cages manufactured from L-PPS are standard on both the new types of cylindrical roller and angular contact bearings that NSK has developed for use in screw compressors. In both types of bearings, substantial improvements in load capacity have been achieved by redesigning the bearing interior to take full advantage of the L-PPS plastic cages. The result is substantial improvements in fatigue life for the two types of bearings.

In addition to delivering innovative solutions in the pumps and compressors sectors, NSK has also focussed its design platform on meeting the key demands of the wind energy industry to reduce weight and improve equipment reliability and wear life. As a major supplier to the industry, NSK has undertaken extensive FEM analysis to produce compact cylindrical roller bearings that save weight, and absorb uneven load distribution, delivering optimised life in a patented new multi-stage planetary/helical unit that extends the development of wind energy plants into the multi-megawatt area, ie up to 5megawatt and above.

An essential advantage of the new differential gearbox is its slim outer diameter, with only a slightly increased total length compared to existing - but lower capacity - megawatt gearboxes. Consequently, although offering increased plant power, the new unit delivers a 15 per cent weight reduction compared with today's gearbox concepts.

Torque reduction in the new gearboxes is undertaken in two planetary stages, using NSK's EM series CRB in the planetary gear wheels. Importantly for standardisation and cost savings, these bearings correspond closely to the original sizes of bearings used in smaller megawatt class gearboxes.

The use of paired EM series cylinder roller bearings, with one-piece solid brass cages, has proven to be the optimum bearing solution for this application. The EM bearings are able to also absorb uneven load distribution through load point displacement in the planet meshing.

EM series bearings are also used in the helical gear stage of the differential gearbox. This bearing arrangement is already tried and tested in gears with megawatt class gearboxes and is used in the differential unit with only minor modifications. As a result, it offers major advantages for manufacturers of wind energy plant, as the majority of components used in the new gearbox are already produced in large numbers, and with consistently high quality.

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Tony Synnott is UK Engineering Manager, NSK UK Ltd, Newark, Notts, UK.

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