Wheel/Rail Contact
Steel wheels rolling on steel rails is the principal characteristic that distinguishes railways from other
forms of transport (Figure 34.3). Wheel and rail meet at a contact patch that is small (typically about
100 mm
2
) and carries the full wheel load through which all steering, traction, and braking forces are
transmitted. This contact patch sees a severe working environment. Stresses normal to the plane of contact
can reach values several times the wheel or rail tensile strength, and sometimes shear stresses in the plane
of contact can exceed the shear yield stress. Rapid temperature rises, caused by relative slip between the
wheel and rail, can reach several hundred degrees Celsius in routine operation, and over 1000°C in
extreme circumstances.
These stress and temperature conditions inevitably lead to wear, deformation, and damage to the
wheels and rails; and a major goal of railroads is to arrange service conditions and maintenance procedures
to minimize deterioration and hence extend component life. This is important because rails —
and to a lesser extent wheels — constitute a large part of a railroad’s asset base. For example, there are
about 1.4 million freight vehicles and some 25,000 locomotives in service in North America, which give
a total population of about 13 million wheels. North American railroads also own over 170,000 miles of
track, which equates to about 35 million tons of steel rail. Railways have more money invested in rail
than in any other asset, excepting land and perhaps bridges. Extending the life of these components, and
especially that of the rail, has a major impact on railroad profitability.
An understanding of the tribology of the wheel/rail system is essential if wheel/rail life is to be extended.
This system is complex, and its behavior depends on interactions between the materials (wheel, rail, and
any third body introduced, such as lubricant/debris mixtures) and the stress-temperature environment
(among other things, a function of vehicle weight, vehicle/track interaction, wheel/rail profiles, wheel/rail
adhesion, and speed).