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Product category: Bearings
News Release from: NSK UK | Subject: Bearings for the railway industry
Edited by the Engineeringtalk Editorial Team on 11 November 2003

Bullet train bearings advance railway
technology

With research showing that trains are the ideal form of transport for journeys up to 1000km, it is not surprising that many countries around the world are investing in their railway systems.

With research showing that trains are the ideal form of transport for journeys up to 1000km, it is not surprising that many countries around the world are investing in their railway systems This investment is not confined to infrastructure

It is also focused on rolling stock developments, but here with demanding new requirements: trains must operate at higher speeds, be lighter in weight and travel greater distances without maintenance.

Key components in ensuring reliability over extended operating periods are rolling element bearings.

These products are used extensively in the axle boxes of all the various types of rolling stock around the world.

Unfortunately for bearing manufacturers the word "various" is particularly operative as, historically, train-operating companies around the world have adopted different bearing designs for their axle units.

This policy has resulted in a profusion of bearing types, each offering its own specific benefits in the design for which it is employed.

Although this situation means considerable investment for bearing manufacturers serving the railway industry, one company, NSK, is regularly bringing new, more efficient and cost effective designs to the market.

These new bearing designs make use of new materials, manufacturing and lubrication technologies developed by NSK, many of which were pioneered for use on the highly successful Japanese Bullet series of trains.

Whatever the configuration employed, rolling stock axle bearings are routinely subject to radial impact loads caused by rail joints, points and, sometimes, wheel flats, as well as to the static and dynamic radial loads imposed by the vehicle weight.

They are also liable to receive axial loads generated by lateral movement as trains run on curved rails or in a snaking motion.

The sum of these loads is a complex combination that acts upon the axle bearings.

As a result, axle bearings must be designed not only on the basis of the dimensional requirements of the axle journal and bearing box geometry, but also with these complex load conditions in mind.

In addition, as axle bearings play a critical role in the safety of railroad operation, they are periodically disassembled for inspection.

For this reason, simple and dependable procedures for disassembly, inspection and re-assembly are also important design factors.

Finally, to improve operating efficiency, axle bearings must offer longer inspection intervals, simplified maintenance procedures and increased integration of bearing components and adjacent parts.

To meet these needs, unitised bearings with advanced sealing devices have been introduced by NSK and are now widely used in modern rolling stock.

At the present time the axle bearings manufactured by NSK fall into six categories based upon the bearing type and the configuration of seal employed.

To ensure sufficient load-carrying capacity, all of the types are usually manufactured in double-row configurations, they include: cylindrical roller bearings combined with ball bearings; cylindrical roller bearings with ribs; RCT bearings (sealed-clean rotating end cap tapered roller bearings); RCC bearings (sealed-clean rotating end cap cylindrical roller bearings); spherical roller bearings; and tapered roller bearings.

One of the major focus areas of NSK's continuing development of axle box bearings is the Bullet series of high-speed trains.

As the speed of Bullet trains has increased over the last four decades, the type and size of axle box bearings has also had to change to ensure improved running performance and reduced weight.

For the railway axle bearings of the earlier O-Series Bullet trains, NSK supplied double row cylindrical roller bearings combined with ball bearings.

In this arrangement the cylindrical roller bearing sustained radial load in the vertical direction while axial load during running is sustained by ball bearings.

For the later (1992) 300-Series of trains (capable of 270km/h) this configuration was changed, cylindrical roller bearings with ribs being adopted for the first time for Bullet trains.

The objective of this change was to reduce the number of parts (ie the ball-bearings) in the axle assembly and its overall size and weight.

This was possible due to a new NSK design in which axial loads are borne by ribs of the inner and outer rings of the cylindrical roller bearing and by the end faces of the bearing rollers.

The configuration evolved once again with the latest 500 Series of trains, which are capable of speeds up to 300km/h.

Now, the demands for lighter, more maintenance free trains mean that NSK's rotating end cap (RCT) type double row tapered roller bearings are used.

In these sealed-type axle bearings, specially designed and improved wear-resistant oil seals of low torque are directly mounted on both ends of the double row outer rings and long-life special grease is packed into the bearings.

The latest version of the 500 series train is particularly significant because it heralded a departure, not only in the type of bearing used but also in the lubrication.

The railway axle bearings of Bullet trains of generations older than the 500-Series had an oil bath lubrication system necessitating an oil sump at the bottom of the axle box.

This meant that the axle box was large.

For the 500 series trains the axle box was simplified by the adoption of grease lubrication, enabling the weight of bearings and devices - including the axle box itself - to be greatly reduced.

The effectiveness of the RCT sealed type double row tapered roller bearings has been proven on Bullet trains over distances up to 900,00km with no maintenance.

On more conventional trains, the bearings have run for more than 900,000km, also without maintenance.

The introduction of a new long-life grease, which has been jointly developed by NSK and a grease manufacturer, should double this life figure, the tapered roller bearings aiming to achieve 1,800,000km with zero maintenance.

Another solution developed by NSK to address the problem of severe operating conditions, and the demands for prolonged intervals between maintenance, is the RCC rotating end cap-type cylindrical roller bearing.

Equipped with specially designed and wear resistant oil seals and packed with grease, these bearings are easier to disassemble and assemble than RCT types, enabling the train operator to save time - and reduce costs - through simplified inspection and maintenance.

In addition, the RCC bearings offer extended life operation, no maintenance being required for 5 years, or 1,500,000km when they are used on suburban trains.

The bearing developments made by NSK to satisfy the extremely testing operating conditions encountered in the operation of Bullet trains offer substantial cost saving benefits to train operating companies generally.

In the UK the predominant bearings used in axle boxes are tapered roller bearings, the focus of much design improvement by NSK aimed at providing train operating companies with greater levels of operating performance and longer maintenance free running.

With speeds for Bullet trains expected to reach 350km/h and those for conventional trains to increase to between 160 and 200km/h, manufacturers of rolling stock will need to adopt the latest developments in bearing technology to remain competitive in a growing global market.

In addition, as the demand for rail travel continues to grow and passenger miles increase further, the pressure on train operators to purchase trains that operate for longer without maintenance will grow accordingly.

This, in turn, will fuel demands for more reliable bearings from manufacturers such as NSK, who have a proven record in delivering the technology upon which many of the future improvements in train performance will be based. Request a free brochure from NSK UK ...

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