The wear measurement of journal bearing coatings that are poor reflectors using ultrasound analytical signals

In journal bearings, the soft coatings are designed to facilitate running in and served as a low friction sacrificial layer in case of lubricant starvation. Ultrasound methods have shown the feasibility for in-situ monitoring of the coatings acoustically dissimilar to the bearing shell substrate. However, some coating materials, notably bronze alloys, always show a low acoustic reflection at coating-substrate interface. This paper proposes a “Reflection Package” analysis method based on Hilbert transform and analytical signal to measure the thickness and wear of coatings with low acoustic reflection interface. Finite element simulations were performed to study ultrasound behaviour in this case. For experimental validation, the journal bearing shells with 250 μm bronze-lead alloy (Cu-Pb) coating were subjected to ultrasound inspection under various conditions, including dry-running, excessive dry-running and in-situ condition. The wear depth was obtained by comparing the thickness before and after operation and the dynamic wear progression was demonstrated for these in-situ tests. Compared to microscopic results from sectioned bearings, ultrasound results were generally accurate to within ±15 μm. This study provides a method for measuring bearing wear during real-time operation, especially when the acoustic impedance of the coating is similar to that of the bearing steel. It broadens the ultrasound wear measurement of various materials and has the potential to be applied to the safety monitoring of rotating components in important or inaccessible structures.