Historically, train operators around the world have adopted many
different types of customised bearings for rolling stock axle units,
resulting in a profusion of styles that are not easily transferred from
one operator to another. Over a number of years, NSK has attempted
rationalisation by introducing specific designs for the sector, designs
that have subsequently been improved by the use of new materials,
manufacturing methods and lubrication technologies
Whatever the configuration employed, rolling stock axle bearings are
routinely subject to radial impact loads caused by rail joints, points
and 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 when running on curved track. In
addition to their need to cope with these loads, axle bearings must be
easy to dismantle and re-assemble (for vital safety inspections), have
effective sealing, offer low weight advantages and compatibility with
axle journal and bearing box geometries.
NSK has developed unitised bearings with advanced sealing devices for the
railway industry, and these fall into six categories based on bearing
type and seal configuration. They are almost always manufactured in
double-row configurations, and include cylindrical roller bearings
combined with ball bearings, and cylindrical roller bearings with ribs.
Additional formats include sealed-clean rotating end-cap tapered and
cylindrical roller bearings, spherical roller bearings and tapered roller
bearings.
Some of the company's major developments in this area are axle box
bearings for the famed Bullet train, which (hard to believe) has been
providing high-speed rail travel in Japan for more than four decades.
Over this time, the speed of these trains has increased and the type and
size of axle box bearings has also had to change. For example, the early
O-Series Bullet trains were equipped with NSK double-row cylindrical
roller bearings combined with ball bearings, whereas for the later
300-Series, cylindrical roller bearings with ribs were adopted for the
first time. This configuration reduced the number of parts in the axle
assembly and reduced its overall size and weight.
The latest 500-Series trains, which are capable of speeds of up to
300km/h use NSK's rotating end-cap type double-row tapered roller (RCT)
bearings. In these sealed-type axle bearings the low-torque,
wear-resistant oil seals are directly mounted on both ends of the double
row outer rings, with special long-life grease being packed into the
bearings. This type of lubrication is a departure from that used on
previous versions of the Bullet Train, which included an oil sump beneath
the axle box. Grease lubrication allowed a significant decrease in the
size and weight of the 500-Series axle box.
RCT sealed type double-row tapered roller bearings have now been proven
on Bullet Trains over distances of up to 900,000km without maintenance.
On more conventional trains, these bearings have run for greater
distances, also without maintenance. Indeed, a new grease, developed
jointly by the grease supplier and NSK, is expected to double this life
figure to 1.8 million kilometres without maintenance. The rotating
end-cap cylindrical roller bearing (RCC), equipped with wear-resistant
oil seals and packed with grease, is easier to disassemble and
re-assemble, compared with RCT types, and also offers extended life - no
maintenance being required for five years (or 1.5 million kilometres)
when used on suburban rolling stock.
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