Friction coefficient in FZG gears lubricated with industrial gear oils: Biodegradable ester vs. mineral oil

Two industrial gear oils, a reference paraffinic mineral oil with a special additive package for extra protection against micropitting and a biodegradable non-toxic ester, were characterized in terms of their physical properties, wear properties and chemical contents and compared in terms of their power dissipation in gear applications [Höhn BR, Michaelis K, Döbereiner R. Load carrying capacity properties of fast biodegradable gear lubricants. J STLE Lubr Eng 1999; Höhn BR, Michaelis K, Doleschel A. Frictional behavior of synthetic gear lubricants. Tribology research: from model experiment to industrial problem. Elsevier 2001; Martins R, Seabra J, Seyfert Ch, Luther R, Igartua A, Brito A. Power Loss in FZG gears lubricated with industrial gear oils: biodegradable ester vs. mineral oil. Proceedings of the 31th Leeds-Lyon symposium on tribology. Elsevier; to be published; Weck M, Hurasky-Schonwerth O, Bugiel Ch. Service behaviour of PVD-coated gearing lubricated with biodegradable synthetic ester oils. VDI-Berichte Nr.1665 2002.]. The viscosity–temperature behaviors are compared to describe the feasible operating temperature range.Standard tests with the Four-Ball machine and the FZG test rig [Winter H, Michaelis K. FZG gear test rig—desciption and possibilities. In: Coordinate European Council second international symposium on the performance evaluation of automotive fuels and lubricants; 1985.] characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants.Power loss gear tests are performed on the FZG test rig using type C gears, for wide ranges of the applied torque and input speed, in order to compare the energetic performance of the two industrial gear oils. Lubricant samples are collected during and at the end of the gear tests [Hunt TM. Handbook of wear debris analysis and particle detection in liquids. UK: Elsevier Science; 1993.] and are analyzed by Direct Reading Ferrography (DR3) in order to evaluate and compare the wear particles concentration indexes of both lubricants.An energetic model of the FZG test gearbox is developed, integrating the mechanisms of power dissipation and heat evacuation, in order to determine its operating equilibrium temperature. An optimization routine allows the evaluation of the friction coefficient between the gear teeth for each lubricant tested, correlating experimental and model results.For each lubricant and for the operating conditions considered, a correction expression is presented in order to adjust the friction coefficient proposed by Höhn et al. [Höhn BR, Michaelis K, Vollmer T. Thermal rating of gear drives: balance between power loss and heat dissipation. AGMA Technical Paper; October 1996. pp 12. ISBN: 1-55589-675-8.] to the friction coefficient exhibited by these lubricants. The influence of each lubricant on the friction coefficient between the gear teeth is discussed taking into consideration the operating torque and speed and the stabilized operating temperature.