Brake Shoes and Thermal Mechanical Shelling

The Wheel Defect Prevention Research Consortium (WDPRC) conducted a review of existing brake shoe tests to explore the combinations of brake shoe force, time, and wheel/brake shoe coefficient of friction (COF) needed to achieve the temperatures of concern for thermal mechanical shelling (TMS). Data was found involving normally expected brake shoe forces applied to a dynamometer wheel and a variety of test parameters and procedures. All temperatures were measured using a sliding thermocouple pressed lightly against the wheel tread. All testing reported involved new brake shoes that may have different COF properties from used brake shoes. Tests included brake shoe tests conducted in accordance with the Association of American Railroads (AAR) M-926 specification, constant horsepower tests, and simulated stuck brake tests. Tread conditioning brake shoes were investigated, as well as high friction composition brake shoes. Findings from these tests include the following: • Tread conditioning brake shoes produced lower wheel tread temperatures compared to high-friction composition brake shoes. The magnitude of this difference varied depending on the test conditions; • In the dynamometer test most closely simulating revenue service conditions, 37 kW (50 hp) was required to heat wheels above 316°C (600°F); • Different models of high-friction composition brake shoes produced similar wheel temperatures under the parameters of the AAR M-926 tests. The range of average wheel temperatures produced by different models of high-friction composition brake shoes was only about 28°C (50°F) at any point during the test; • Individual brake shoes of the same model usually produced consistent results with respect to wheel temperature. Two exceptions to this statement were observed in the AAR M-926 test results. In a laboratory dynamometer test involving service damaged wheels, the WDPRC was unable to produce any measurable reduction in wheel tread defect size through the application of tread conditioning brake shoes. However, four service trials of tread conditioning brake shoes showed potential benefits in using tread conditioning brake shoes to reduce the number of wheelsets removed for tread damage.