Interface - The Journal of Wheel/Rail Interaction

INTERFACE OPTIMIZATION

Tools and Techniques for Optimizing the Wheel/Rail Interface (continued)

Task 3. What to do with all the Data
While the ability to record digital wheel and rail measurements is good in and of itself, the information can also be used to analyze the performance of typical wheels on a railway’s established rail profile. Closed-loop systems, such as transit systems or mine-to-port operations, have the ability to control both wheel and the rail profiles. Typical freight operations, which handle free-interchange cars, must work with an average worn profile of cars operating on the system.

In either case, the goal of wheel/rail profile optimization is to maintain wheel and rail profiles to a shape that will maximize the life of both assets by reducing the contact stresses and wear that develops at the wheel/rail interface. In the process of minimizing wear, the risk of derailment, related to contact fatigue cracking at the gauge corner or top of rail, and shelling on the surface of the wheel tread, is also reduced. Both of these “surface” failure modes can lead to subsurface cracking of either the wheel or rail and, ultimately, to catastrophic failure.

While lubrication can reduce rail gauge-face wear, some level of wear is beneficial in removing incipient surface cracks. This “magic wear rate” can be achieved through optimal wheel/rail interaction, or more typically through controlled artificial wear generated by periodic rail grinding.

A significant by-product of minimizing wheel/rail contact stresses it that wheelset steering in curves is greatly improved. When the wheel is centered on the rail correctly, the maximum steering moment is achieved. This reduces the risk of derailment due to wheel climb or rail rollover. Harmonic activity between the wheel and rail, which can lead to rail corrugations or wheelset hunting at higher speeds, is also reduced when wheel/rail profiles are in sync.

An optimized wheel/rail profile system will allow wheelsets to shift laterally in curves and develop rolling radius difference as the wheel moves across the rail head. Approximately 3 - 4 mm of rolling radius difference should be developed when the wheel flange is in contact with the rail in curves. Track gauge and the wheelset back-to-back dimensions also factor into rolling radii analysis (see Figure 9).

Software programs, which can process thousands of wheel profiles, can be used to perform statistical analysis of existing profile conditions, and to determine the average worn wheel profile. They can also be used to analyze specific profile conditions when investigating a derailment or excessive wheel or rail wear condition. More advanced analysis makes use of vehicle dynamic simulation programs such as VAMPIRE®, which was developed by British Rail in the 1970s and has become a standard tool for performing vehicle dynamics analysis and wheel/rail optimization studies.

Programs have been developed to integrate digital data from rail and wheel measurement systems to create the “wheel/rail geometry” file (see Figure 10) that VAMPIRE® uses to calculate the wheelset steering and creep forces (which are generally based on Hertzian contact theory and Kalker coefficients). With this information, the model calculates the amount of energy consumed at the flange and tread surface for a given wheel/rail profile combination, as well as the lateral and longitudinal wheelset creep forces, which indicate the wheelset’s steering efficiency.

Using information from simulation models, parametric studies can be performed that sequentially analyze how small changes in the wheel or rail profile shape effect energy consumption, wear and steering at the contact patch. The optimum combination can be developed for any given operating environment, whether transit, high-speed passenger, or heavy-haul freight.

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