Interface - The Journal of Wheel/Rail Interaction

WHEEL RAIL INTERFACE

Pre-engineering the Wheel/Rail Interface
By Rob Caldwell • October 2010

The wheel/rail interface is a complex system – one that benefits greatly from careful pre-engineering. There are significant benefits from properly addressing the wheel/rail interface, and serious consequences from allowing the interface to govern itself. While there are examples of both pre- and post-engineering scenarios, there are more instances of the latter.

fig 1

Pre-engineering means making informed decisions based on engineering analysis during the design phase, when changes can be made economically. Solid engineering and planning during the design phase is far more cost effective than reactive maintenance and problem solving after the system is up and running.

The specific goal of pre-engineering is to reduce downstream costs by taking into account and eliminating or minimizing common issues that may arise over the life of a system. Infant mortality is one such issue that can be eliminated. This includes premature wheel or rail replacement due to unusually rapid gauge-face or flange wear, surface damage or corrugation development, wheel flats from sliding, rapid tread hollowing — anything that fails soon after entering service. Infant mortality is most common when vehicles and track are separately designed and specified. Proper pre-engineering minimizes future service reliability issues, such as shops and maintenance-of-way equipment that may not be able to handle a sudden influx of vehicles or miles of track that need repair, and thus bottleneck the system. The potential for noise and ride quality issues must also be considered during the design phase so that costs associated with mitigating them do not come as a surprise. From inception, the system must be designed with the understanding that certain ongoing preventive measures, such as grinding and friction management, will be necessary. Their costs should be factored in during design.

While proper engineering will reduce wear in the wheel/rail interface, wear cannot be eliminated. There are different wear mechanisms at work, but the most problematic for transit systems tends to be adhesive wear between the wheel flange and the gauge corner/face of the rail. Adhesive wear occurs as the asperities between the apparently smooth wheel and rail surfaces weld together under contact then are sheared and torn apart, leaving behind a rough surface and often rail "dandruff" on the tie plates (see Figure 1).

Wear rate reductions of 8 - 20 times are readily achievable with gauge-face lubrication, making it the most effective treatment for gauge-face wear. Use of head-hardened rail can reduce wear rates by 2 - 3 times, compared to standard carbon rail. Proper wheel/rail profile design and the use of flexible bogies can also provide comparable reductions to the damage done by wear. In combination, the effects are synergistic and it is possible to virtually eliminate measurable gauge-face and wheel-flange wear. These solutions are also effective in minimizing rolling contact fatigue.

Rolling Contact Fatigue
Rolling contact fatigue (RCF) can occur on wheels and rail because of the very high contact stresses and shearing action that occurs at the wheel/rail contact. For each contact cycle where the traction stresses exceed the strength of the material, a small increment of fatigue damage occurs in the near-surface material. Repeated overstressing leads to the initiation and propagation of surface cracks that can grow into the material and link together, causing a shell to break out of the surface.

Figures 2a and 2b show examples of RCF on rails from passenger systems that National Research of Canada's Centre for Surface Transportation Technology (CSTT) has investigated. Figure 2a is from Railtrack (now Network Rail) in England. Rolling contact fatigue on the gauge corner of the rail was the root cause of the Hatfield derailment, in which four people were killed and 70 were injured. Figure 2b shows a shelled rail from a very light axle load system in South America. These photos illustrate that RCF is not confined to freight railroads.

The wheel/rail interface is subject to a great deal of stress. Minimizing stress is not a simple process. Curiously, wheel/rail contact stresses are not overly sensitive to the load between the wheel and the rail. Halving the load reduces the stress by about 26%. Doubling the wheel radius produces a similar effect. Consequently, modifying wheel load and diameter are neither effective nor practical solutions for controlling contact stresses. Wheel/rail contact stresses can be effectively controlled through careful selection of the transverse wheel and rail radii where these bodies come into contact. For example, a wheel tread could be an arc or a taper and depending on the transverse radius of the top of the rail, the contact stress could be damaging to standard carbon rail that is often used in tangent track. A potential solution to this problem could be to install higher hardness steel on tangent track. A more cost effective solution, especially if there is significant wear life remaining in the rail may be reshaping the rail head to reduce the contact stress to a level that is suitable for standard carbon rail. Acceptable contact stress varies for different rail metallurgies, which adds complexity to the task of engineering optimum profiles.

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