TTCI R&D, RAILWAY AGE JUNE 2020 ISSUE: The AAR-2A, a new standard wheel profile recently implemented for use in North American freight railway operations, is expected to provide improved service performance for wheelsets. This assessment is based on the analysis, simulation and testing conducted by Transportation Technology Center, Inc. (TTCI).
The new AAR-2A wheel profile was designed to produce a nearly conformal contact condition with a typical rail on the high side of a curve to provide benefits such as improved steering and minimized rolling resistance, wear, fuel consumption and surface damage. Compared to the older AAR-1B profile, the AAR-2A produces a more-even distribution of wear across the wheel flange and tread such that the profile shape does not change substantially during the service life of the wheel.
Analytical evaluation, computer simulation and service testing were used over the course of multiple years to ensure that the design goals of the AAR-2A profile were met. Each method of evaluation showed improved curve performance of the AAR-2A profile compared to the AAR-1B profile. Starting with turned wheels, implementation of the AAR-2A profile has been under way since 2016, and it became the primary standard profile in North American freight operations in January 2020.
Although service-worn freight car wheel and rail profiles show a wide distribution in gross shapes, they generally wear to similar shapes in the wheel flange root and high rail gage corner. The annual replacement rates of wheels and rails strongly imply that the majority of wheels and rails in service at any time will exhibit worn profiles that can potentially diverge substantially from their as-manufactured shapes. Rail grinding is used to control rail profile shapes, but grinding templates are not designed to restore the rail to the as-manufactured shape.
Development of the AAR-2A wheel profile started with the collection of measured service-worn wheel and rail profiles to aid in the investigation of the distributions and patterns of worn wheel and rail profile shapes. Curves were fit to match the most commonly worn shapes of the high rail gage corners and wheel flange roots. In comparison to the AAR-1B profile, the AAR-2A profile contains more material in the flange fillet to reduce and/or eliminate the strong two-point contact that occurs when the wheel makes flange contact with a typically worn high rail in a curve. Eliminating this two-point contact reduces the initial high wheel flange wear experienced in the first 25,000 to 100,000 miles of service, depending on the railcar types and service routes. Although the flange thickness was revised during the evaluation period, the shape of the flange root area remained consistent. Other critical features of the AAR-2A profile include a 75-degree maximum flange angle and a 1:20 wheel tread slope.
Dynamic performance was initially assessed via simulations with the new wheel profile contacting representative rail profiles through curved and tangent tracks. Limited on-track testing was performed under controlled conditions before starting revenue service tests involving more than 200 cars. Revenue service testing showed that the AAR-2A wheels maintained largely the same flange root shape as they experienced service wear while AAR-1B wheels experienced substantial change in the flange root shape.
The AAR-2A wheels also showed reduced overall wear in every case. When new, the AAR-2A wheels held advantages over the AAR-1B wheels in terms of gage spreading force, though this benefit gradually decreased as the profiles converged through natural wear to become more similar in shape. Despite higher conicity of the AAR-2A profile, high-speed stability was not shown to be a problem.