A finite-element model for a paste lubricated steel wire vs cast iron contact

A finite-element model for a paste lubricated steel wire vs cast iron contact was developed. The modelled contact included two surfaces with engineering topography and a paste lubricant. The paste lubricant consisted of a synthetic transmission oil, binder and solid manganese oxide particles. The purpose of adding solid particles in the lubricant was to increase friction without increasing wear of the surfaces. According to numerical simulations and experiments, the solid particles in the contact increased friction from a level of 0.03 for grease lubrication to a level of 0.05-0.22 for paste lubrication, by forming a thick shear-resistant pad-like layer between the sliding surfaces. The effects of particle size distribution and surface roughness were studied in order to find an optimal paste lubricant composition. The experimentally used and characterized cast iron surface had a surface roughness of R-a = 0.92 mu m but the computational simulations showed that the optimal surface roughness with regard to the overall stress distribution in the contact and friction generation would be roughly 25% higher. The coefficient of friction for the contact lubricated with manganese tetroxide paste was calculated by the finite-element model to be 0.08 and this was validated by twin-disc experiments. The highest coefficient of friction value, 0.22, was achieved by increasing the surface roughness by 25% from the originally characterized surface. In addition, further development of the paste characteristics in terms of morphology and composition were found to have a prominent effect on friction while not compromising its surface wear protection characteristics.