Micropitting performance of nitrided steel gears lubricated with mineral and ester oils

Abstract

This study compares the gear micropitting performance of high pressure nitriding (HPN) steel gears, lubricated with three different gear oils: a standard mineral lubricant, containing a special micropitting additive package, and two biodegradable esters with low toxicity additivation. The physical, chemical and wear properties of the three lubricants were determined, as well as their biodegradability and toxicity characteristics. The gear material and the corresponding heat treatment are presented. Gear simulation tests were performed in a Falex machine, using a roller–disc geometry, in order to evaluate the lubricant temperature and friction coefficient corresponding to each gear oil. Gear micropitting tests were performed on the FZG machine, using type C gears, and lubricant samples were collected during the tests for wear particle analysis. Post-test analysis included the mass loss measurement of the gear (pinion and wheel), the ferrometric analysis of the lubricant samples and the teeth flank roughness measurement below and above the pitch line. The teeth flanks were inspected using scanning electron microscopy (SEM) and surface topography measurements to assess the number and depth of micropits. Metallurgical cuts were done to observe the size and depth of micropits as well as contact fatigue crack initiation and propagation. The ester lubricants show better micropitting performance than the mineral oil, confirming the potential of environmental friendly fluids as high-performance gear oils.