Gear power loss performance of biodegradable low-toxicity ester-based oils

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 power loss performance of two biodegradable, low toxicity, ester-based gear oils was evaluated and compared with a commercial mineral oil and an ester-based fluid containing zinc dialkyl dithio phosphate (ZDDP) additives. Power loss tests were performed on the FZG test rig, using type C carburized gears and the different lubricants under evaluation. The operating temperatures of the oil and of the FZG gearbox wall were measured for different values of the input torque and speed and lubricant samples were periodically collected and analysed by direct reading ferrography. At the end of the tests, the gear mass loss and the oil viscosity were measured, the teeth flanks were inspected, looking for typical surface failure mechanisms, and the tooth flank surface roughness was measured. An energetic model of the FZG test gearbox, which took into account the power loss mech-anisms inside the gearbox and the heat flow mechanisms from the gearbox to the surrounding environment, was used to calculate the friction losses between the gear teeth, knowing the oil, gearbox wall, and room temperatures, in steady-state conditions. Using this model, it was poss-ible to analyse the influence of lubricant formulation on the average friction coefficient between gear teeth. The results obtained showed that one of the biodegradable, low-toxicity, ester-based gear oils generated significantly lower mass loss than all the other lubricants, presenting almost no degradation, even when operating at very high temperature for long periods. These bio-lubricants generated friction coefficients between gear teeth up to 27 per cent smaller than the commercial mineral oil, promoted significant reductions of the power loss inside the FZG gearbox, increasing the overall efficiency up to 0.25 per cent.