Abstract
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.
Highlights
Many engineering components like rolling bearings, gears and cams operate in elastohydrodynamic (EHD) lubrication condition
By comparing the limiting friction of the seven gear oils tested at the six test temperatures (Fig. 9), it can be seen that mineral oil has higher limiting friction than PAOs and PGs at all temperatures studied
It should be noted that the limiting friction for mineral oil and PAOs slightly increases with the test temperature up to 50 °C and eventually drops at temperatures above 60 °C (Fig. 9) while for PGs it always drops with increasing temperature. These results suggest that the measured friction coefficients of mineral oil and PAOs started decreasing due just to temperature rise even before the limiting friction was reached for the test temperatures from 25 °C to 50 °C while PGs reached the limiting friction at all temperatures studied
Summary
Many engineering components like rolling bearings, gears and cams operate in elastohydrodynamic (EHD) lubrication condition. The efficiency can generally be improved either by using optimized surfaces (e.g. surfaces with lower surface roughness, coatings) and/or by reducing frictional losses. The latter can be achieved by use of lubricants with low viscosity, low pressure-viscosity coefficient and low shear strength [1, 2]. This drives the current trend towards the use of less viscous lubricants [3,4,5]. It is important to gain a better understanding of the effects of roughness on friction in mixed lubrication
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