Interface - The Journal of Wheel/Rail Interaction

CONTACT MECHANICS AND RAIL STRESSES

Understanding the effects of track gauge, wheel/rail geometry and friction on stresses at the wheel/rail interface (continued)

Effect of Inadequate Gauge Restraint

Figure 7a shows the wheel contact with the low rail at the head end of a front-end-only powered train where the gap between the wheel flange and the gauge side of the head of the low rail is acceptable. Contrast this with the image in Figure 7b obtained from the tail end of the same train, where the wheel flange has drifted farther from the head of the low rail due to progressive gauge widening. In this case, the wheel flange is up to 2 inches away from the side of the railhead and is riding on the false flange, providing a smaller radius of contact with the railhead. The resulting high intensity contact stresses (see Figure 8) caused spalling of the rail within less than one month after rail grinding, leading to the need for more grinding and rail wastage. Installation of elastic fasteners provided the restraint necessary to prevent gauge widening under trains.

On the high rail, if the angle of the grinding stone used at the gauge corner is shallow, a sharp ridge can be left behind. The lack of elastic fasteners will allow the rail to rotate and show the high point on the gauge face. The resulting contact stresses will be as shown in Figure 9. Both of these issues would combine to cause shelling at the gauge corner of the high rail. These shells quickly turn transverse, leading to a detail fracture (shown in Figure 10). Note the depth of the shell before it became a transverse fatigue defect. To correct such a defect would necessitate removing more than 1/4 inch of material from the gauge face of the high rail. Such a severe corrective measure would significantly shorten the usable rail life. Hence, restraint of the rails against gauge widening is paramount for curve rail life extension.

Wheel and Rail Radius and Friction
How does the radii of the wheel and rail and friction between them relate to the relief interval before the initiation of contact fatigue? Figure 11 shows the effect of low-rail crown radius and friction coefficient on maximum stresses in the rail head. The analyses were performed using a 33-kip wheel load and a new 1:20 wheel profile. As can be seen, maximum shear stresses increase with a reduction in rail crown radius and an increase in friction coefficient. The same thing happens to the wheel, for which shelling from contact fatigue is shown in Figure 12. Table 1 shows these stresses when converted into the period before fatigue initiation (based on the fatigue strength of the rail material). Reducing the friction coefficient to 0.2 provided a relief period of 12.4 mgt of life before the initiation of rolling contact fatigue. The relief period can be as low as 2 mgt with a friction coefficient of 0.4.

While low friction is better, it can drive the damage underground and affect the adhesion capability of the locomotive. You do not want to stall trains, burn the rail or consume unnecessary fuel. A friction coefficient below 0.35, while good for reducing rail stresses, would affect the adhesion of the locomotives’ wheels and the ability of the locomotives to pull the train. Assuming a coefficient of friction of 0.35 at the top of rail (obtained through the use of TOR friction modifiers) and the average wheel loading we can determine the appropriate interval between rail grinding cycles. The key is to determine the appropriate intervals for a given line, based on wheel loads, gross tonnage, wheel and rail profiles, and lubrication conditions, before the initiation of fatigue damage. A grinding plan designed in this manner will optimize rail life.

Jude O. Igwemezie, Ph.D., P.Eng., is President of Applied Rail Research Technologies Inc.

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