Interface - The Journal of Wheel/Rail Interaction

WHEEL SHELLING

Implementing an Anti-Shelling Wheel Profile (continued)

Rolling Contact Fatigue
Due to the severity of the problem, the Railway Association of Canada (RAC) created a committee to address this issue, and Canadian National (CN), Canadian Pacific (CP) and QCM were the prime participants and sponsors of the committee’s work. The committee determined that rolling contact fatigue (RCF) damage was the primary cause of shelling on the Canadian railroads; martensite was a lesser contributor to the problem.

RCF damage occurs when the wheel (or rail) steel is overstressed. The damage occurs in the form of cracks that initiate at or below the surface. These cracks grow and branch together under the surface until the steel above them no longer has sufficient strength to bear the imposed loads. The surface steel can then fall out of the wheel, revealing a shell. The physical conditions which must be present for RCF to develop, and probable shelling causes associated with those conditions are shown in Table 1.

A shakedown diagram, shown in Figure 2, can be used to determine if a particular contact condition will result in RCF damage. The diagram is a plot of the load factor (contact stress divided by the shear strength of the steel) vs. traction coefficient (vector sum of the creep forces in the contact patch divided by the normal contact load). If the point representing the contact condition falls under or to the left (in the green area) of the shakedown limit (represented by the thick black line), no damage will occur. If it falls on the other side (in the red area), an increment of RCF damage will accrue at that location on the tread.

In the summer of 1996, the RAC sponsored the development of a new wheel profile based on the positive results obtained at QCM, and the understanding of the main cause of the shelling. The profile was installed on a portion of the wheels on the CN and CP grain and coal fleets, as they were captive, high-mileage fleets, and results could be quickly obtained.

A total of 732 transverse wheel profiles from both car types were measured in order to obtain a good distribution of the worn shapes that were in operation at both railroads. Using CSTT’s proprietary techniques and software, the profiles were aligned to determine the necessary flange angle, and to create the proper flange root geometry. The tread slope was again maintained at 1:20, although it was shortened compared to the AAR1B profile. The field side of the wheel had a tread roll-off, as per the QCM-Heumann profile. This profile was also designed as a wide-flange wheel, so it could be installed as a new wheel on the fleets.

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