Thermo-elastohydrodynamics of hypoid gears with formulated lubricants

One of the main design criteria currently used by the automotive industry us that of designing fuel efficient vehicles. Strict regulations imposed within the European Union and the United States, dictate progressively lower limits of greenhouse gases emissions, either via imposed taxation or, in extreme cases via refraining the distribution of certain vehicle models. Friction itself is one of the sources responsible for increased fuel consumption. Consequently, its understanding on a full scale system level is of essential importance in employing improved design methodologies and fuel efficient lubricants. The present study focuses its attention on the theoretical prediction of the power losses and the conjunctional efficiency of hypoid gear pairs which are located in the differential units of modern cars. Particular attention is paid on the effect of the lubricant formulation on the efficiency performance of the system. This is realised through the rheological characterisation of the lubricating oil under well controlled laboratory conditions, such as those appearing in viscometers. A total of 6 gear lubricants of the same viscosity grade (SAE 75W-90), blended with the same additive pack are examined. The key difference between each lubricant is on the type and the concentration of the Viscosity Modifier (VM). The viscosity of each fluid is characterised for high temperature, high pressure and high shear rate, conditions usually encountered in the Elastohydrodynamic (EHD) conjunctions of highly loaded hypoid gears. The conjunctional efficiency of three different hypoid gear pair geometries is examined under the influence of different lubricant formulation. The actual contact geometry of each gear-set is captured through a quasi-static Finite Element (FE) procedure known as Tooth Contact Analysis (TCA). The torsional gear dynamics of the gear pair are also captured through a 4 Degree of Freedom (DoF) lumped parameter model, highlighting the impact of the inertial properties of the system on its efficiency performance.