Influence of water contamination in gear lubricants on wear and micro-pitting performance of case carburized gears

Abstract

Purpose Water can alter the performance of modern gear lubricants by influencing the flank load carrying capacity of gears significantly. The purpose of this paper is to investigate the influence of water contaminations in different kinds of base oils on the micro-pitting and wear performance of case carburized gears. Design/methodology/approach Concerning micro-pitting and wear, tests, based mostly on the following standardized tests, are performed on a Forschungsstelle fuer zahnraeder und getriebebau (FZG)-back-to-back gear test rig: micro-pitting short test Graufleckenkurztest (GFKT) according to DGMK 575 (screening test), micro-pitting test Graufleckentest (GT) according to FVA 54/7 (load stage test and endurance test) and Slow-speed wear test according to DGMK 377. To investigate the effect of water on the gear load carrying capacity dependent on different types of base oils, two polyglycol oils (PG1 and PG2), a polyalphaolefin oil, a mineral oil and an ester oil E are used. Each of these oils are common wind turbine gear oils with a viscosity ISO VG-220. Additionally, a manual transmission fluid with a viscosity of society of automotive engineers (SAE) 75W-85 is tested. Findings Considering the micro-pitting and wear performance, a significant decrease caused by water contaminations could not be detected. Regarding pitting damages, a generally negative influence was observed. This influence was differently distinctive for different base oil types. Especially non-polar lubricants seem to be affected negatively. The documented damages of the tooth flanks confirm this observation. While typical pitting damages appeared in test runs with polar lubricants, the disruption in test runs with non-polar lubricants was more extensive. Based on the experimental investigations, a general model of the damaging mechanisms of water contaminations in lubricants was derived. It is split into three partitions: interaction lubricant–water (effect of water on the molecular structure of base oils and additives), chemical-material-technological (especially corrosive reactions) and tribological influence (effect of water droplets in the contact zone). It has to be considered that the additive package of lubricants affects the influence of water contaminations on the flank load carrying capacity distinctively. An influence of water on the micro-pitting and wear performance in other than the given lubricants cannot be excluded. Originality/value While former research work was focused more on the effects of water in mineral oils, investigations concerning different types of base oils as well as different types of damages were carried out within this research project.