Influence of Defined Water Contents in Gear Lubricants on the Pitting Performance of Case-Carburized Gears
Authors: Engelhardt Christian , Witzig Jochen , Tobie Thomas , Stahl Karsten 
Published: November 2017

In various gear applications, water contaminated lubricants are a well known problem leading to a significantly decreased lubricant performance under certain circumstances. In FVA-research projects 488/I and 488/II (FVA-Forschungsvereinigung Antriebstechnik: German research association for transmission technology) the influence of a defined water content ranging from 1 000 ppm to 20 000 ppm in typical gear lubricants on the pitting load carrying capacity of case carburized gears was investigated. Regarding the test series with a relatively high water content from 5 000 to 20 000 ppm, a generally negative influence was observed. This influence was differently distinctive for the investigated base oil types. Especially non-polar lubricants were affected negatively. Subsequently, non-polar lubricants, such as polyalphaolefines and mineral oils, were chosen for further investigations as the water content was also lowered on more practice-relevant levels ranging from 1 000 to 2 900 ppm. The water contamination still influenced the pitting load carrying capacity in the upper range with water contents resulting in a relative humidity significantly higher than 100% (absolute water contents from 2250 to 2900 ppm). At the level of 1 000 ppm (rel. humidity approx. ≥ 100%), no significant influence was detected. Thus, it was possible to derive critical threshold limits for the water contamination of non-polar lubricants in practical applications, as from which, the water contamination may affect the pitting load capacity. The water-induced reduction of the pitting load carrying capacity seems to be primarily dependent on the blended additives. The used non-polar base oil type only had a subordinated impact. Based on the results of theoretical investigations, it was possible to derive and expand a general model of damaging mechanisms of water contamination in lubricants, which is split into: interaction lubricant-water, chemical-material-technological influence and tribological influence. In short, it describes the possible impact of water contamination on the lubricant, the material and lubricating-film properties. The results of additional measurements indicate some damaging mechanisms to be dominant under the herein performed test conditions. In detail, decomposition effects of additive components and an increased oil aging effect were identified to be crucial factors, while the influence on the material properties like the measured residual stresses in the gear material seemed to be subordinated.
Undissolved water is supposed to affect the pitting load carrying capacity more negatively than dissolved water. Therefore, the relative humidity is recommended as a suitable characteristic of lubricants to evaluate, whether measured water contents in practical gearbox applications may lead to a reduced pitting performance. Based on the results of the experimental and theoretical investigations a threshold limit for the relative humidity of 100 % appears to be reasonable, given that the additive system of the lubricants is not affected negatively.