The influence of macro and micro geometry on the generation of adverse contact conditions associated with micropitting in gear tooth contacts

This paper presents the results of an investigation into the influence of both macro (tip relief) and micro (surface roughness) geometry on the generation of adverse conditions, such as high contact pressures, thin lubricant films or direct metallic asperity contact, which are known to be factors linked to the occurrence of micropitting in heavily loaded gear tooth contacts. Results are shown of a transient elastohydrodynamic (EHL) analysis of the conditions at the contact of two pairs of helical gears operating with different gear ratios. The effects of the variation of contact geometry, kinematics and tooth loading during the meshing cycle are taken into account together with both tip relief and axial crowning profile modifications. The form of tip relief profile adopted is shown to be highly influential in determining the peak contact pressures and thinnest oil films within the contacts. These conditions are experienced at all axial positions on the tooth flanks at the locations of the start of tip relief profile modifications. If the tip relief profile introduces a slope discontinuity to the gear profile then this can cause a significant stress concentration and very poor film forming behaviour. A range of tip relief profiles are investigated, from linear tip relief to fully parabolic forms. Further work is reported which investigates the level of direct metallic contact occurring between surface asperities in disk machine tests, which are designed to replicate heavily loaded rolling/sliding EHL contacts typical of aerospace gearing practice. The Electrical Contact Resistance (ECR) technique was implemented in these experiments to study realistic mixed lubrication conditions, and is found to be capable of identifying individual asperity contact events on a repeatable basis between two surfaces. The technique is then used to investigate the variation of mean ECR measurements with operating conditions for ground surfaces in contact, and in addition detailed high frequency measurements of contact resistance are shown to clearly demonstrate the influence of thermal effects on the occurrence of metallic contact