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.
Other railways have investigated the use of special profiles and have come up with similar solutions. The infrastructure company of Dutch Railways (ProRail) has developed a particular Anti-Headcheck-Profile with 1-mm gauge-corner relief, compared to the standard UIC 54 profile (see Figure 2 right).
French Railways (SNCF) have defined two new particular target profiles. An anti-headcheck “preventive” profile with limited gauge corner relief is applied where headchecks are not yet visible. A “corrective” profile with considerable gauge corner relief is ground where headchecks are clearly visible. Headchecks often cannot be completely removed, as the cracks are deeper than the prescribed metal to be removed (see Figure 2 left).
After an extensive test program, the German Railways incorporated a standard target profile, which permits only negative production tolerances (that are equal to moderate gauge-corner undercutting), into its grinding specifications.
Specific target profiles ensure that the equivalent conicity is kept within low limits in order to maintain vehicle stability at higher speeds. The German Railways introduced special gauge-widening profiles. The Austrian railways (ÖBB) developed a so-called “convex” rail head profile with a 22-mm gauge corner radius in order to combine the effects of low conicity and reduced surface fatigue development.
Wheel/rail interaction specialists can define ideal target profiles for grinding, knowing the feasibility of producing them within tight tolerances at low extra costs. The current focus is on the development of optimized target profiles for:
• Specific wheels (wear-adapted profiles).
• Fatigue reduction (anti-headcheck profiles).
• Running behavior (gauge-widening, equivalent-conicity profiles).
• Wear reduction (asymmetric profiles).
Profiles for Heavy Haul
Sweden’s 473-km “Malmbanan” heavy-haul line, which connects the city of Luleå on the Baltic Sea and the Norwegian city of Narvik, located on the Atlantic coast (see Figure 3), is characterized by small radius curves and steep gradients. The electrified line handles 30 million gross tonnes per year of mixed passenger and freight traffic, predominantly 30-tonne axle load iron ore cars. Temperatures on the line range from -40 ºC to +25 ºC. The resulting stresses in the rails vary from high tensile stresses during winter, increasing the risk of crack propagation and rail breakage, to compressive stresses in the summer.
Head checking, spalling and shelling defects were quite common in the 1990s. Earlier these phenomena were considered “unavoidable,” and rails were frequently changed. Today, Banverket’s maintenance strategy on Malmbanan includes yearly maintenance grinding (including rail head re-profiling) and extensive rail lubrication in curves of less than 600 meter in radii.
When rail profiling was introduced (using a planing machine) in 1987, asymmetric profiles were applied over distances of 200 meters in order to check their effect with regard to wear reduction. At about the same time, spalling and shelling started to appear on the gauge corner of the high rails. With that, the focus of rail profiling changed toward surface fatigue, and so-called “wear-adapted” profiles emerged. Tests continued using a grinding train, and “gauge-corner relief” became a leading principle in the continued development of grinding profiles.
These experiences led to the development of a profile that was more specifically adapted to the fairly hollow-worn wheels of the ore trains. In 1994, a new (MB1) profile was developed. At first, this new profile was only ground in selected curves at the high rails, while the low rails and tangents still were ground to the standard BV50 profile (see Figure 4, left). In this profile design, the gauge corner area is much lower in order to accommodate the hollow worn wheels. The field corner side remains unchanged. The initial results showed reduced wear and delayed appearance of RCF defects.
A five-year grinding project was initiated on Malmbanan in 1997. Test curves were ground once a year; profiles at 60 locations were selected for evaluation of the transverse profile. As the contact path of the MB1 profile is better adapted to the hollow-worn wheels, the amount of RCF defects, such as head-checking, spalling and shelling, decreased. But the old RCF defects could not be removed entirely by grinding. However, with the use of the MB1 profile, an unloading of the gauge corner was achieved, resulting in a decreased growth rate of existing RCF defects. In 2000, the MB1 profile was introduced as the standard profile on all high rails, and the BV50 profile was restricted to the low rails and tangents.
Damage to a series of older rails made it necessary to develop a profile that could prevent emergency rail renewal. It was also determined that the MB1 profile was not the optimal profile in some curves, and the head-checks became too large before the following grinding program took place. Experience on other railways indicated that more pronounced gauge-corner grinding could reduce further wheel contact at the damaged contact zone on the rail.
Another profile (MB3) (see Figure 4, right) was developed to shift the contact band farther to the field side and thereby optimize gauge corner relief. The application of this profile had the desired effect and the rails could be kept longer in track. The MB1 profile is now standard on all rails (tangents, low and high rails in curves), except for specific, “problematic” high rails to which the MB3 profile is applied.
An economic evaluation of the heavy haul grinding program between 1990 and 2005 (shown in Figure 5) indicates that rail maintenance costs were significantly reduced after the introduction of the rail grinding program. The grinding program soon provided a “pay-back” in the reduced need for rail renewal.
Rails in turnouts are ground for the same reasons as rails in open track. Switches at Malmbanan were first ground in 1994. The results were encouraging, but it took some time to establish a strategic rail grinding policy for turnouts. Today, almost every switch on Malmbanan is ground every year (30 MGT). Initially, the target profile was the “standard” BV50 profile with an inclination of 1:30, instead of the installed vertical profile. However, as in open track, this standard profile resulted in unacceptable wear and the rapid occurrence of RCF defects. Consequently, another profile (MB 4) (see Figure 4, left, on prior page) was developed. This profile has a reduced gauge corner relief compared to the MB1 profile.
The reason for not grinding directly to MB1 profile was due to the reduced production capacity of the switch grinder in service at that time. To grind the MB1 profile from a deformed profile would have been very time consuming. Grinding in steps was the answer to the problem. In 2007, some switches were ground to the target MB1profile for test purposes. These switches are monitored closely, and the MB1 profile is expected to become the standard for turnouts.
On Malmbanan, the upper part of the switch blade point is in many cases exposed to cracking due to a local overload when the wheels are moving from the stock rail to the switch blade. Due to wear, both natural and artificial by repeated grinding, the stock rail is lowered over time with respect to the tip of the switch blade. To minimize the risk of cracking of the critical upper part, the switch blade is lifted 6 mm before grinding (see Figure 6). Because of the metal removal by grinding from the top portion of the switch blade, the transition point from the blade to stock rail is moved toward the frog.
In Sweden, the annual grinding budget was earlier decided by each region or local hub. In many cases, the grinding budget was reduced due to economic measures or postponed until the following year. In 2001, a central funding source was established, with each region reporting its grinding needs to the head office in order to “jointly” determine maintenance priorities. Rail grinding has since become a higher priority maintenance procedure on Malmbanan and other lines at Banverket.
Today, planning of the grinding operation is managed in a “turnkey” fashion. Spark Trade AB, a Swedish company, is subcontracted by the grinding company to plan, execute and ensure quality control of the daily work, including the organization of pilots, road guards and responsibility for all necessary service arrangements.
At present, grinding on the Malmbanan ore line is performed once a year. All curves are ground and switches are ground each year at 30-MGT intervals. Tangent track is ground every third year, at 90-MGT intervals. The development of grinding profiles on Malmbanan has been performed in close cooperation between Banverket, the transportation company MTAB (responsible for the vehicles) and the grinding contractor. This cooperation has been characterized by open discussion between the three parties, aimed at improving the contact between wheel and rail.
Future grinding work of the ore line will focus on increasing the 30-tonne axle loads and 30-MGT annual tonnage. The MB1 and MB3 profiles will be used on open track and the MB1 and MB4 profiles will be used in turnouts. A new research project will be launched on Malmbanan this year to identify the presence and number of RCF defects through the use of manually operated Eddy-Current equipment. Depending on the results, the grinding strategy — grinding cycles, metal-removal rates, optimized profile designs — may be modified. A complete track renewal between Kiruna and the Norwegian border will be completed by 2009. All of the existing 50E3 rail will be replaced by 60E1 rail. The new rail will be immediately profiled to the appropriate profiles MB1 or MB3 profiles.
Banverket also continues to investigate lubrication techniques and the development of improved grades of rail steels, such as bainitic steels and steel grades with higher carbon content, which should provide improved fatigue properties and wear resistance. These and ongoing improvements to rail grinding practices continue to contribute to improving overall life cycle costs on the Malmbanan and other railway lines.
Anders Frick is Senior Metallurgist, Banverket, Swedish Rail Administration, Track Engineering. Dr. Wolfgang Schoech is Manager of External Affairs, Speno International SA
(1) Nilsson P., Wheel/Rail Interaction – From theory to practice, 6th International Conference on contact Mechanics and Wear or Rail/Wheel Systems – CM2003, Gothenburg