Interface - The Journal of Wheel/Rail Interaction

WHEEL/RAIL INTERFACE

Applying Quality Concepts to the Wheel/Rail Interface
(part 1 of 3 – continued)

NS monitored the rail profiles at several sites of a test route during the six-sigma project. During one six-month period, the gauge corner of several new high rails wore heavily, particularly in the first two months following grinding (see Figure 3 for an example). Using the new low rail template and pre-inspection, NS was able to create conformal contact for 78% of the typical wheelsets at this site. However, within two months, only 50% of the wheelsets had conformal contact; and within six months, only 28% of the wheelsets had conformal contact.

During the same six-month period, NS observed very little wear on an 18-year-old high rail with similar curvature at a different location (see Figure 4). This rail profile never produced conformal contact with more than 10% of the typical wheelsets. The high gauge-face angle on this rail caused heavy two-point contact, however, which generates poor steering and high lateral forces and caused tie plate cutting and loose spikes. Grinding cannot eliminate this problem. It can only be addressed by preventing the formation of the gauge face angle on new rail, by transposing an older rail, or by implementing top-of-rail lubrication to limit the creep forces.

Based on this information, NS reviewed data that had been collected on a different coal route as part of a lubrication study. Figure 5 shows the metal loss in the high rail gauge shoulder over time. The arrows indicate each time that this rail was ground. The red arrows indicate that the rail was ground in a corrective mode. The Figure shows a dramatic jump in wear immediately after each grind, followed by a return to a steady-state wear rate after a short period of time. The green arrow indicates a grind in which the contractor was directed to limit metal removal in the gauge shoulder to 0.010 inches of depth. There was no jump in wear after this grind. If NS had been able to prevent the rapid wear that followed the corrective grinds, the rail would have had only about half of the total accumulated metal loss that it had when it was removed from service. NS determined that it was removing too much metal using corrective grinding and was, in fact, removing the work-hardened layer of the rail. As a result, the as-ground profiles were not stable.

In 2006, NS set out to stabilize the rail profiles on the system. It started by prioritizing selected routes and determined a time-based grinding frequency based on prior experience. It also began using a rail profile measuring system to perform pre-grind inspections and to automatically select the grinding pattern that would provide the best finished profile that would maintain the rail’s work-hardened layer. As the profiles have begun to stabilize, NS has begun to evaluate the finished grind quality indices as a measure of determining the appropriate grinding frequency.

Figure 6 indicates that the mean pass mile (PM) to track mile (TM) has declined since NS initiated the six sigma project. The decrease between 2004 and 2005 was due to the introduction of a new low rail template. The reduction between 2005 and 2006 was due to the introduction of a profile stabilization program. The span — a measure of how much variation there is in the process — dramatically declined last year in response to the stabilization program. While NS is now grinding efficiently, it still must ensure that the finished rail profiles provide the best operating conditions.

Stephen S. Woody is Manager, Track Inspection & Development, Norfolk Southern

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