The surface profile wear and the resulting third-body layer accumulation are investigated on the Virginia Tech-Federal Railroad Administration Roller Rig (VT-FRA Roller Rig), using a high-precision 3D laser profiler by Keyence. It is observed that the wheel surface gradually wears and the fine worn material accumulates on the running surfaces. It is further observed that with the progression of the wear and accumulation of a powder-like material at running surface, a preocular change in wheel-rail traction occurs. The first step in exploring the physics of the changes in traction is quantifying, precisely, the micron-sized surface finish changes and wear material accumulation. This paper provides the process of choosing a suitable surface profiler for the VT-FRA roller rig adaptation, and the results of a series of tests under various wheel load and contact configurations.
A baseline experiment is conducted to evaluate the wheel wear and the accumulation of the naturally-generated third body layer (the worn material) on the wheel’s running surface. Before each series of experiments, the wheel and roller running surfaces were polished and thoroughly cleaned to make sure that they are perfectly smooth (finished) and free of any debris or dust. The wheel surface profile is measured in the perfectly cleaned condition using the 3D laser scanner. Time controlled experiments are run with different wheel loads, % slippage, and angle of attack (AoA) for 500 seconds. For each run, the change with surface finish and accumulated third body layer build up is measured.
It is consistently observed that the traction forces increase gradually with time until they reach a stable level, presented by a plateau in the traction-slippage curves. The time to reach the plateau is directly related to wheel’s dynamic conditions including load, %slippage, and AOA. For some of the conditions, such as wheel load, the relationship is linear and for others (such as AOA) it is nonlinear. Concentrating on the analysis of the baseline results, it is observed that the change in surface finish and the third body layer that naturally accumulates at the running surface — hence, referred to as “natural third body layer” or “NTBL” — is directly correlated and somewhat linearly with NTBL and change in the running surface from smooth to rough. The correlation is most noticeable for the NTBL. With increasing NTBL, traction increases rapidly until the running surface is saturated with the accumulated wear material, presumably a ferrous oxide. There is far weaker correlation between the change in surface finish and traction variations. It is noticed that the surface finish continues to change beyond the time that the traction forces reach their plateau, and its effect remains far more gradual than NTBL.