Experimental investigation of the influence of lubricant additives on rheology of elastohydrodynamic films

Abstract

An experimental investigation was conducted to study the influence of additives on lubricant rheology in elastrohydrodynamic lubrication (ehl) and micro-ehl as found in applications such as gears and rolling element bearings. The lubricants were subjected to very high pressures which acted globally over the apparent area of contact in ehl and to even higher pressures which acted locally at asperity contacts in micro-ehl. For ehl, calculations were described for evaluating the effective film thickness parameter (theoretical central film thickness divided by measured composite RMS surface roughness). This parameter determined an approximate point of transition to micro-ehl. As well, an analytical procedure was developed for determining the limiting shear stress, which was considered an important parameter in high pressure rheology. For micro-ehl, a calculation procedure, based on measured surface roughness parameters and a modified Greenwood and Williamson type model, was developed to evaluate the real area of contact.

A side-slip disc machine was used to measure friction forces which sheared elastrohydrodynamic lubricant films as slip-roll ratio increased from zero. Major disc machine modifications included adding devices to measure surface temperature, using an electrical resistance circuit to measure cross film voltage drop, and providing an automatic data acquisition system. Lubricant additives, in the general categories of friction modifier, anti-wear, and extreme pressure, were studied in disc machine experiments involving rough and smooth disc surfaces.

Friction force rose rapidly to achieve a maximum value at a slip-roll ratio of about 5%. In ehl, with the smooth discs, small but distinct and repeatable differences in the friction forces were caused by the additives. These differences permitted evaluation of limiting shear stress expressions which were functions of pressure for each lubricant. In micro-ehl, with a rough disc surface, the friction force was 2-3 times higher than occurred with smooth disc surfaces and the additives had significant influence on friction forces. The measured friction force divided by the estimated real area of contact gave an alternative evaluation of the limiting shear stress, which agreed quite closely with the limiting shear stress predictions based on expressions derived from testing in ehl. This agreement suggested that friction measurements under ehl could be used to evaluate and explore the influences of additives on the limiting shear stress. Thus, the lubricant chemistry could be designed to give an optimal limiting shear stress using disc machine measurements, under conditions of ehl, for guidance.