Interface - The Journal of Wheel/Rail Interaction

WHEEL RAIL INTERACTION ON TRANSITS

Engineering the Wheel/Rail Interface for Rail Transit (continued)

Moorhead was quick to point out that the industry is short on answers. It has, however, begun asking pertinent questions, such as:
• Why does short-wavelength corrugation occur only in tangent track?
• Is it unique to light rail, or can it also develop on heavy rail systems?
• Could it be caused by low floor LRVs with independently rotating wheels at the center trucks?
• Is it somehow related to vehicles with AC-propulsion and/or Bochum-type wheels?
• Is it prevalent where traction and braking are at or near adhesion limits?
• Is it related to wheel/rail contact geometry and vehicle hunting characteristics?
• Does it occur only where railhead grinding has been performed?

The investigation is ongoing, with no primary suspects identified at this time. But based on field observations on different properties, it seems clear that the dynamics of resiliently mounted wheels, both powered and independently rotating wheels, warrant additional scrutiny, Moorhead said. High tangential forces near the slip-limit and rail/wheel perturbations may result in wheel/tire micro slip as the resilient element unwinds (see Figure 4). The effects of AC-propulsion synchronization when running mismatched wheel and truck sets also warrants investigation, he said. Until the source of the problem is identified, short-wavelength corrugation will only get worse, and will likely appear on more systems.

Stress State Reduction
The wear that shapes wheel and rail profiles has a profound effect on curving performance and the dynamic stability of bogies, said Joe Kalousek, Senior Consultant, Advanced Rail Management. “Reducing the spread between worn and unworn profiles, and in particular reducing the concavity of worn wheels, can significantly improve curving and ride quality of bogies, decrease wheel/rail damage and increase wheel and rail lifespan.”

Track curvature and bogie suspension systems influence not only the shape of worn wheels and rails, but also govern the shape of new wheel and rail profiles that are optimal to a specific mass transit, high-speed or heavy-haul system, Kalousek said.

While the magnitude may differ, the forces that cause wheel wear (see Figure 5) on rail transit systems are the same as those occurring on freight systems. And while the damage may appear to be different, it is caused by the same mechanism, he said. As a result, wear prevention and reduction techniques, like friction management, optimized wheel/rail profiles, rail grinding and other preventive maintenance, are needed on rail transit systems, just as they are on heavy-haul railroads.

Ultrasonic Testing
While broken wheel failures are not common in LRT and commuter operation, cracked wheels represent a significant problem in the railway industry, overall, generating an annual maintenance cost of at least $24 million, said David Giragosian, Vice President of Engineering, Dapco Industries, Inc. And the problem continues to grow. Conservative estimates indicate that the number of wheels with cracks grows at 5% more cracks per year; other estimates place the growth rate at 12% to 15%.

Internal shattered rim cracks account for 99% of broken wheel failures. The remaining 1% is caused by tread and flange cracks, Giragosian said. Until recently, visual inspection was the only method by which wheel cracks could be detected and evaluated. Thermal cracks too shallow to see often escape detection, allowing a crack to propagate and, potentially, lead to wheel failure. State-of-the-art non-destruction ultrasonic testing techniques now provide a complement to visual inspection, and are able to detect thermal cracks in rolling inspections at 5 mph.

Track Measurement

Accurate and detailed track measurement is another essential part of effective system maintenance, said Paul Mittermayer, Managing Director of the Bureau of Applied Mechanics and Mathematics. Data collected by track measurement systems typically includes flange radius, rail head, gauge, cant, and corrugations, among others, can be used, not only to find flaws, but to evaluate and model wear over time, and to plan maintenance cycles (see Figure 6). The goal of these and other technologies and maintenance practices addressed at the seminar is to better optimize and maintain the wheel/rail interface.

Jeff Tuzik is Editorial Assistant, Interface Journal

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