Understanding the impact of emulated initial rail roughness on traction performance under constant tribomachine testing conditions

This study used a twin-disc test under locomotive traction tribological conditions to investigate how initial surface roughness influences the traction coefficient at the wheel-rail interface. We propose functions to describe the evolution of disc surface roughness. The findings show that the initial roughness of the rail disc significantly impacted both traction coefficients and wear. Roughness values were selected based on measurements from a Western Australian freight line where tracks are subject to high tractive and dynamic braking efforts. Specifically, the wheel disc roughness was set at Ra = 1 μm for all samples, while the rail disc roughness values were set at Ra = 1 μm, 5 μm, and 10 μm. Experimental results revealed significantly different traction curves for each wheel-rail combination. Compared to the smoother 1 μm rail disc, the rougher discs (5 μm and 10 μm) exhibited a slower, linear increase in the traction coefficient before reaching a steady state. Notably, the 10 μm rail disc resulted in longer running-in periods, lower overall traction coefficients, and higher wear rates. This is attributed to the fact that contact asperities deformed plastically and were more easily removed during rolling/sliding. As a result, traction coefficients became similar for all tests following the running-in period.