Acoustic emissions from oil lubricated metal on metal sliding contacts

Abstract

This study investigates the condition monitoring of oil lubricated metal on metal sliding contacts using various sensing techniques including acoustic emissions. The worn surfaces are examined and wear mechanisms are explored and linked with the on-line sensor signals. This study tackles the condition monitoring problems from the tribological point of view, ie at a micro-scale. Experimental work was carried out on a ball-on-disc tribometer using an En31 bearing steel ball and disc lubricated by a mineral base oil, Shell Vitrea 460. The base oil was used to eliminate the formation of possible additive films and to promote the onset of scuffing. The sliding speed was kept at 6 m/s under maximum Hertzian contact pressures of 3 GPa. An oil starvation strategy was also applied to promote adhesive failure. The tests generated multiple contact wear mechanisms and enabled correlations between AE signals and wear mechanisms to be identified. Tests were stopped at various stages to enable surface examination using scanning electron microscopy (SEM). The results show that AE gives an early indication of adhesive wear of the lubricated metal contacts using r.m.s signals. Snapshots of AE waveforms also show that burst signals changed to continuous signals when the contact changed from fully lubricated to starved conditions. AE energy shifts within the broad frequency band of 300 kHz and 1 MHz give early indications of scuffing.