Wheel Spalling: Simulation of High Speed Wheel Slip

Abstract

The Wheel Defect Prevention Research Consortium (WDPRC) conducted an analysis of the possibility of wheel spall creation under revenue service conditions when a car traverses perturbed and/or lubricated track with the brakes applied. When the brake retarding force acting on a wheelset is greater than the wheelset vertical load multiplied by the wheel/rail coefficient of friction (COF), the wheelset rotational speed will begin to decrease, because the braking force has exceeded the available wheel/rail traction. Due to its large rotational inertia, the wheelset will not immediately stop rotating. As the wheelset slows rotationally, a relative motion (slip) between the contact patch of the wheel and the rail will be introduced due to the continued forward motion of the vehicle. Any sliding action generates heat in the contact patch. If sufficient heat is generated, martensite can form and spalling problems can be initiated. However, as long as the wheelset is rotating, the contact patch is cooled by continually moving circumferentially around the wheel and the tread surface temperature is limited. A NUCARS® multi-body computer simulation model was used to determine wheel normal forces at a variety of speeds across perturbed track. The wheel slip rate was then calculated for each discreet output time step of the NUCARS model. The resulting wheel tread temperature due to the wheel slip was calculated. The predicted contact patch temperature was compared to the austenitic transformation temperature to form a prediction about whether or not martensite would be created. Based on the results of this analysis, it does appear to be possible to create martensite on the wheels of loaded cars under heavy braking while traversing track surface irregularities. However, most operating conditions would not provide the required conditions and this is probably not a major source of spalling.