Pitting of Rolling and Sliding Steel Discs by fire-Resistant Fluids

Abstract

In many industries lubrication by mineral oils is not possible due to the risk of ignition and fire-resistant fluid shave, therefore, been developed as a substitute. A major disadvantage of the use of such fluids however, is their relatively poor performance in high-stress rolling-contact bearings where components suffer fatigue cracking and pitting at an earlier stage than would be expected with mineral oils. In this investigation factors affecting the cracking and pitting of EN31 steel in the presence of water-glycol based fire-resistant fluids have been studied using an Amsler2-disc machine. A stribeck curve was plotted for the system and it is shown that pitting damage is dependant on the region of the curve in which the system is operated, pitting life being a minimum in the mixed-elastohydrodynamic region. Wear rates were found to be inversely proportional to the fluid film thickness. A good inverse correlation was found between the pitting life and initial surface roughness to lubricant film thickness, D, up to a value of D= 22. Above this value an increase in life was observed. The introduction of decanoic anti-wear additive increased pitting life between two and seven times. Other experiments are described using the Unisteel and Rolling four-ball machines in which a similar trend was observed. Finally, based on the observations made by optical and Scanning Electron Microscopy (SEM) a mechanism for crack initiation, propagation and subsequent pit formation in aqueous lubricants is proposed.