Interface - The Journal of Wheel/Rail Interaction

RAIL STRESSES

Understanding Stresses in Rails - Part 1 of 2 (continued)

One-point conformal contact conditions at the gauge corner of high-rail occur when the vertical and lateral loads are applied at the same point. This gives rise to very large stresses (see Figure 8 on prior page) and leads to the formation of gauge corner shells and detail fractures (see Figure 9). The one benefit of conformal contact is that it provides rolling radii difference on curves by allowing the wheel on the high rail to ride close to the throat (with a bigger radius), and the wheel on the low rail to ride on the field side of the wheel tread (with a smaller radius). This creates the same effect as rolling a cone. This condition improves steering around curves and reduces the lateral forces generated during curving. But, it increases wear at the gauge corner of the rail. The magnitude of the contact stress (371 ksi) is three times the yield strength of premium rail (typically 120 ksi).

The wheel/rail contact point on the low rail, which always sees one-point contact, is also important. The wheel/rail contact point should occur as close to the center of the wheel running tread as possible (as shown in Figure 10). Good track gauge and a stiff truck system are required for this to occur.

When the track gauge is wide or the truck warping resistance is weak, the wheel contacts the rail at the field side of the wheel where the false flange is formed (see Figure 11). False flange contact will also occur if the flange that is riding the high rail is thin and the entire axle is displaced towards the high rail. Rail damage from false flange contact is shown in Figure 12.

Two-point wheel/rail contact occurs only on the high rail, and is more desirable because it reduces the concentration of stress at the gauge corner of the rail. In this case, the wheel tread makes contact on top of the rail while the flange makes contact with the gauge face of the same rail head. This form of contact is achieved by grinding off the material at the gauge corner of the rail head. Two-point contact usually leads to truck warping. It can also generate high lateral forces, which, however, can be dramatically reduced with proper lubrication of the top of the low-rail.

Figure 13 shows typical Von Mises stress in the head of the high rail from two-point contact with the wheel. (Loadings similar to those shown in Figure 8 were used to obtain the stress in Figure 13.) As can be seen, the maximum stress is reduced by 33%. So, while each type of contact has its merits and demerits, two–point contact produces lesser stresses and reduced high rail damage.

Dr. Jude Igwemezie is President of Applied Rail Research Technologies (ARRT), Inc. .

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