Interface - The Journal of Wheel/Rail Interaction

RAIL GRINDING

Target Profiles for Rail Grinding:
A Never Ending Story
By Anders Frick and Dr. Wolfgang Schoech • July, 2008

Rail maintenance work is a process of removing metal from the rail head at the right time and at the right place. At first, rail grinding was performed to remove surface irregularities and defects — a process that required significant metal removal. During this curative process, the transverse rail profile was of secondary importance. Railways, today, have taken a preventive approach to rail maintenance — an approach in which thin layers of metal are removed at regular intervals and the transverse profile is kept within tight tolerances in order to optimize wheel-to-rail contact.

The question is: What is the best profile to be applied?

As rails are installed at different locations and as traffic characteristics change, one single profile cannot suit all conditions. Experience has shown that even new rail profiles, as-rolled and installed in track, do not provide ideal contact conditions for the wheels. Consequently, grinding worn rails to their original profile does not always improve wheel/rail contact conditions. Instead, a number of rail profiles have been developed to improve wheel/rail interaction and extend the life of rail.

In addition to supporting wheel loads, rail must also guide the vehicles. An effective contact geometry between wheel and rail is required to center wheelsets in tangent track and to promote curving through effective rolling radius difference in curves. Even when the contact geometry is good, however, conical wheel treads always create creep and resulting wear in the contact zone. The wheel tread tends to wear to a hollow shape while the rail profile tends to flatten, causing the contact geometry to deteriorate.

A variety of rail and wheel profiles have been developed over time in Europe, though standardization designed to ensure interoperability between different networks has reduced the number of wheel and rail profiles currently in use. The S 1002 wheel profile is now fairly standard, for example, as are the rail profiles standardized by UIC. Their geometry features a well-balanced compromise for average track situations and normal operational conditions in tangent track and curves.

“Profiling” in Grinding Practice
When rail grinding started roughly 50 years ago, corrugation removal was the primary task; the transverse profile was not considered to be of great importance. With the introduction of production tolerances for the longitudinal and transverse profiles, railways and their grinding contractors usually aimed to re-establish the profile of the various as-new rail profile designs that were in use.

As grinding practices showed that it was possible to produce individual railhead shapes within tight tolerances, new target profiles that were different from the as-rolled profiles were introduced. So long as a profile can be produced with the existing rail grinding equipment, the shape of the profile does not matter to the grinding contractor. Grinding technology, which applies the flat end of the rotating grinding wheels to the rail surface, provides an almost unlimited number of potential target profiles. Grinders are now equipped with systems that continuously measure the transverse profile to ensure that the specified tolerances are met, and the desired profile is achieved.

The German railways (DB AG) made the first attempt to reduce the number of target profiles for grinding in the late 1980s. DB AG introduced a single target profile derived from the UIC 60 profile inclined at 1:40. This slightly more convex profile is now ground everywhere, regardless of the installed rail type, to ensure that all wheels run on profiles of the same shape (at least on lines that are regularly ground).

These ground profiles also overcome the effect of different rail inclinations (or cant), as some railways started and stayed with a 1:20 inclination while others changed to 1:40. The original UIC 60, now named 60E1, is inclined at 1:20; the modified German profile (60E2), now standardized, is inclined at 1:40. While the inclination varies for the two rail sections, the profiles provide virtually the same contact conditions. Changing from one profile to the other does not require a change in fastening systems, allowing rail grinding to be performed at much less cost.

In addition to being affected by wear, rail life is also affected by surface fatigue. Heavy loads and an increased number of loading cycles play an important role in fatigue, as does the size of the contact zone. Gauge corner fatigue, normally called headchecks, appears on the high rails of curves with big radii (see Figure 1). Sometimes they develop in tangent track, too.

Headcheck development is favored, when parameters, such as high axle loads, high train speeds, and high traction forces during acceleration and braking, are at work. In order to combat rolling contact fatigue (RCF), the gauge corner is systematically undercut and the top layer of rail steel that is showing signs of fatigue is removed.

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