EHD Friction Properties of ISO VG 320 Gear Oils with Smooth and Rough Surfaces

The elastohydrodynamic (EHD) friction properties of seven ISO VG 320 gear oils including three polyalphaolefins (PAOs), three polyglycols (PG) and a mineral oil have been investigated in rolling/sliding conditions at six different temperatures and three roughnesses. Film thickness, Stribeck and traction curves have been generated using a ball-on-disc tribometer. Film thickness results are in agreement with previous work that it is primarily controlled by pressure-viscosity coefficient and viscosity of lubricants. The results with smooth surface show that all oils experience significant shear heating leading to friction reduction at higher strain rates or lambda ratios but only PGs reach limiting friction whereas mineral oil and PAOs do not. Friction curves obtained at different temperatures and roughnesses enable simulating an extensive range of lubrication regimes and allow isothermal friction correction for shear heating. Stribeck curves with rough surfaces show an increase in friction in the lambda range of 0.5–3.5, where asperity separation varies from partial to full–indicating that roughness effects can be expected even under full film condition. This increase in friction is attributed to formation of a micro-EHD region, and is seen only with mineral oil and PAOs whereas not with PGs. The results also highlight how EHD friction properties of different family of fluids could be influenced by roughness effects, and the possible mechanisms are discussed.