Development of a Standard for New Passenger Car Wheel Designs

The American Public Transportation Association (APTA) is seeking to develop specifications to ensure that wheels used in transit and commuter applications perform safely under the service conditions to which they are exposed. To this end, a design standard has been conceived to ensure that new wheel designs proposed for such applications are not susceptible to fatigue cracking in the wheel plate and hub. Historically, the Association of American Railroads (AAR) Standard S-660 has been applied in the industry for the purposes of qualifying wheel designs for use in passenger applications. The standard stipulates particular loads to apply in a simple finite element analysis of the new wheel design. The basis for approval is an empirical comparison (by an independent third party) of the results with those in a database of previous analysis results of other qualified wheels. The proposed “S-660 equivalent” design standard is envisioned to be self-qualifying, in that results of the analysis will directly determine whether the wheel design will perform safely in service; a review or approval body will not be required. The new standard is needed to overcome limitations embodied in the current wheel qualification process, namely, the assumption of purely elastic material behavior, the omission of residual stresses due to manufacturing, and the use of comparative approval criteria. The Union Internationale des Chemins de Fer (UIC) introduced a wheel design requirement based on finite element analysis, the results of which are subjected to a fatigue criterion in order to achieve acceptance of the wheel design. As in the current S-660 methodology, a set of thermal and mechanical loads are prescribed. This methodology is essentially self-qualifying as the results of the analysis (obtained following a prescribed procedure) determine whether the wheel design will perform safely in service. The proposed design standard is envisioned to be a combination of the current S-660 analysis requirements and the fatigue calculation-based approach of the UIC. The task force developing the standard is still resolving the specific details of the thermal and mechanical loading requirements. This paper explores the underlying methodology behind the developing standard. A finite element calculation forms the basis of the qualification procedure. Initial (asmanufactured) residual stresses present in a new wheel are determined. Mechanical and thermal loading representative of passenger operations are applied. The analysis yields three characteristic stress distributions: as-manufactured, mechanical, and thermal. The Sines criterion, with temperature-dependent material fatigue properties obtained from testing, is applied to infer whether the candidate wheel design is fatigue-prone. Results are presented for a wheel design currently in transit/commuter service. The APTA committee is currently investigating the thermal and mechanical load levels to be prescribed in the proposed standard.