Abstract
Environmental issues are leading to a growing interest in bio-lubricants, which can have similar or even better performance than mineral and synthetic oils. In this work the biodegradability and toxicity characteristics of ester-based gear oils are discussed and their physical, chemical and wear properties are compared with those of a commercial mineral oil. Power loss tests were performed on the FZG test rig, using carburized (C) and high pressure nitrided (N) gears and two different lubricants. The operating temperatures of the oil and of the FZG gearbox wall were measured for different values of the input torque and speed. At the end of the tests, the gear mass loss and the oil viscosity were measured, and the tooth flanks were inspected, looking for typical surface failure mechanisms. An energetic model of the FZG test gearbox was developed, taking into account the power loss mechanisms inside the gearbox and the heat flow mechanisms from the gearbox to the surrounding environment. Using this model it was possible to quantify the influence of lubricant formulation and gear materials on the average friction coefficient between gear teeth. Gear micropitting tests were performed on the FZG test rig, using carburized (C) and high pressure nitrided (N) gears and two different oils. Post test analysis included the mass loss measurement of the gear and the ferrometric analysis of lubricant samples. The tooth flanks were inspected using surface topography measurements to assess the number and depth of micro-pits. Metallurgical cuts were done to observe contact fatigue crack initiation and propagation using scanning electron microscopy. If carefully formulated, biodegradable low-toxicity ester-based gear oils, beside their environmental advantages, enhance gear performance when compared with the mineral lubricant providing (i) lower friction coefficients and lower operating temperatures of gearboxes and (ii) less tooth flank micropits and shallower micropits.
