Maintenance is a vital part of the system approach: appropriate and timely maintenance forms an integral part of the system approach that needs to be adopted by railways if they are to meet the demanding challenges being put upon them

International Railway Journal, Sept, 2005 by Jay Jaiswal

THE primary mechanisms that deteriorate both the rail and wheel surfaces are wear, plastic deformation, and metal fatigue. They result in rolling contact fatigue (RCF) cracks, loss of profile, and the development of corrugations. The effects are exacerbated by increases in dynamic forces resulting either from incompatible rail-wheel profiles or track irregularity.

Regular rail profile grinding is a proven means of control for these problems. However, the growing demand for train paths is reducing the time available for track maintenance and particularly the operation of grinding trains. Clearly, the technological challenge is to increase grinding productivity. It is also necessary to examine the functionality provided by the current grinding systems and establish whether some of them could be better fulfilled through material selection.

There are two factors that need to be considered: control of rail profile and control of RCF crack initiation and growth.

Considerable work has been done to develop an anti-RCF rail profile. One such profile is the 60E2 being adopted by a number of European railways. Figure 1 shows a comparison of this profile with the standard 60E1 profile while Figure 2 emphasises the magnitude of this difference. Ground profiles have also been optimised to provide gauge corner relief although the magnitude of this relief is often larger than that shown for the 60E2 profile.

[FIGURES 1-2 OMITTED]

The crown profile of the rail is achieved most efficiently and accurately through the hot rolling and roller straightening process using precisely shaped rolls. More importantly, increasing the longevity of the desired profile requires the resistance to wear and plastic deformation to be designed into the material properties of the rail steel rather than repeated correction through in-track rail grinding.

RCF cracks are initiated through the response of the material to the imposed stresses. Therefore it should be possible to increase the period to crack initiation by using more RCF resistant materials. But this needs to go hand-in-hand with the management of the rail-wheel profiles and dynamic interaction to reduce stress.

Although a range of steels with much higher resistance to RCF initiation is available (Figure 3), freedom from RCF cracks in the most susceptible stretches of track requires grinding to remove damaged material and the incipient cracks. This is equivalent to redressing the wear balance to achieve a "Magic wear rate" (this refers to the combined influence of both natural wear and that enforced through grinding). The synergy of optimum rail metallurgy, good track engineering, and non-intrusive high-speed grinding is illustrated in Figure 4.

[FIGURES 3-4 OMITTED]

When restoring the worn rail profile, it is important to consider the profile with reference to RCF crack initiation. The small gauge corner relief apparent in Figure 2 is designed to avoid contact with the wheel and thereby prevent any existing cracks from growing.

On a new rail, the profile reduces the conicity and thereby the forces. Based on very extensive failure investigations of RCF-affected rails from a wide range of track conditions in Britain and regular monitoring of several sites, Corus has established that RCF cracks initiate at a location between ~25 to 30mm from the active side of head of the rail or ~6 to 11mm from the centre of the rail head. As Figure 2 shows, the magnitude of relief available at this position is so tiny that it is likely to be removed by only small amounts of vertical wear or by almost negligible rail roll under dynamic loading. Hence, provided the forces generated are high enough, RCF cracks will eventually initiate at these locations necessitating grinding to remove fatigued material and restore the crown profile and slight gauge corner relief.

A high-speed grinding technique being developed by Stahlberg Roensch, Germany, has significant potential to increase rail life and track availability. Commercial operation of the unit on German track is planned for the last quarter of this year. The unpowered cylindrical grinding stones are arranged at an angle to the longitudinal axis of the rail and rotate at high speed as the vehicle moves (see photo above). The speed of operation makes the process almost non-intrusive to the normal maintenance operation and the small metal removal is sufficient to remove incipient cracks and damaged layers. Frequent removal of such a small amount also helps to remove minor surface irregularities. Corus has also developed a model to predict the frequency of grinding based on track and traffic characteristics.

[ILLUSTRATION OMITTED]

Following the Hatfield derailment in Britain in 2000, Corus helped to establish the original relationship between the surface length and vertical depths of RCF cracks (Figure 5) which was later populated with more data (Figure 6) from sample examinations. This relationship was employed as a criterion for rail renewal based on crack lengths determined by visual inspection. There appears to be considerably more spread than in the original data indicating the influence of other factors.