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a b s t r a c tElectrical contact resistance (ECR) techniques are widely used to study mixed-elastohydrodynamic lubrication conditions, where direct asperity contact takes place in conditions of very thin lubricant films.This paper presents the use of the ECR technique to study realistic mixed-EHL contacts, identifying the high frequency variation of instantaneous contact resistance on a repeatable basis between two superfinished surfaces. The variation of mean ECR measurements with operating conditions for ground surfaces in contact is investigated, and it is shown that they are strongly related to the lubricant film thickness and lambda ratio. Thermal effects are considered and shown to be highly influential on both the mean and instantaneous contact resistance. The influence of load on contact resistance is also investigated.

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The paper presents the results of a thermal analysis of a set of disk experiments carried out by Patching et al. to investigate scuffing. The experiments used crowned steel disks at 76-mm centers with maximum Hertzian contact pressures of up to 1.7 GPa. Experimental measurements of contact friction were used as the basis for a thermal analysis of the disks and their associated support shafts. Temperatures measured by embedded thermocouples 3.2 mm below the running tracks of the disks were used to determine the heat partition between the faster and slower running disks in order to match the experimental with calculated temperatures. This partition was found to vary approximately as a function of the product of sliding speed and surface temperature difference. A transient (flash) temperature analysis of one of the experiments was also carried out. This shows large differences between the disk transient surface temperatures. These surface temperature distributions were compared with those obtained from corresponding elastohydrodynamic lubrication (EHL) analyses using two different non-Newtonian lubricant formulations. The EHL analyses show that the heat partition obtained depends on the form of non-Newtonian behavior assumed, and that to achieve the same partition as is evident in the experiment a limiting shear stress formulation is necessary. It is suggested that the combination of heat transfer and EHL analysis presented in the paper could be used as a sensitive tool for distinguishing between different non-Newtonian lubricant models under realistic engineering loads and with high sliding speeds.

Micropitting is a serious form of erosive wear, which can occur on the teeth of transmission gears. It is associated with roughness effects and surface fatigue and has become a particular problem in the speed-increasing gearboxes of wind turbines. This paper reviews the contributions which the authors have made towards an understanding of the basic mechanism of micropitting in gears based on analysis of the contact mechanics and elastohydrodynamic lubrication (EHL) of gear tooth surfaces under realistic operating conditions. Results are presented which demonstrate the crucial influence of EHL film thickness in relation to roughness (the ‘Λ ratio’) on predicted contact and near-surface fatigue. The important effect of plastic deformation, which takes place during the initial stage of running-in of gears, has also been investigated, and it has been shown that significant residual effects occur, which can contribute to the early formation of surface-initiated cracks.