An analytical friction model is presented, predicting the coefficient of friction in elastohydrodynamic (EHD) contacts. Three fully formulated SAE 75W-90 axle lubricants are examined. The effect of inlet shear heating (ISH) and starvation is accounted for in the developed friction model. The film thickness and the predicted friction are compared with experimental measurements obtained through optical interferometry and use of a mini traction machine. The results indicate the significant contribution of ISH and starvation on both the film thickness and coefficient of friction. A strong interaction between those two phenomena is also demonstrated, along with their individual and combined contribution on the EHD friction.
A tribo-dynamics model, predicting the conjunctional inefficiency and dynamic response of automotive hypoid gear pairs is presented. A dynamics model is coupled with an analytical friction model (viscous and boundary). The temperature rise at the centre of the conjunction is accounted for through use of thermal network model and Time Temperature Superposition (TTS) method, as well as the time varying geometry of the meshing gear teeth. Newtonian and non-Newtonian lubricant shear behaviour are both considered Surface topography measurements of a run-in pinions are obtained. Inefficiency calculations are performed for typical automotive drive cycle snapshots. Precisely measured lubricant shear characterstics for lubricants different blended viscosity modifiers and evolving surface topography are used in the study of transmission inefficiency. The integrated thermal-tribodynamic analysis is shown to distinguish between different viscosity modifier types, an approach not hitherto reported in literature.
Film thickness and sub-surface stress distribution in a highly loaded automotive differential hypoid gear pair are examined. A 4-Degree of Freedom (DoF) torsional gear dynamics model, taking into account the torsional stiffness of the pinion and the gear shafts, is used in order to evaluate the contact load, the surface velocities and the contact radii of curvature of the mating teeth during a full meshing cycle. The torsional gear dynamics model takes into account both the geometric non-linearities of the system (backlash non-linearity) as well as the time varying properties (contact radii, meshing stiffness) and the internal excitations caused by geometrical imperfections of the teeth pair (static transmission error). The input torque used for the study of the film thickness and the sub-surface stress distribution corresponds to the region after the main resonance, where no teeth separation occurs. The contact conditions predicted by the gear dynamics are used as the input for the elastohydrodynamic elliptical point contact analysis. The lubricant film thickness and the corresponding pressure and surface traction distributions are obtained quasi-statically using the output load of the dynamic gear pair model. The variation of the induced sub-surface stress field is determined throughout a meshing cycle. Based on the sub-surface reversing orthogonal shear stresses, marginal differences occur when the viscous shear on the conjunctional surfaces are taken into account, which are mainly influenced by the applied pressure distribution. The numerical prediction of lubricant film thickness agrees reasonably well with that predicted using the well-established extrapolated oil film thickness formulae reported in the literature.
The main objective of the current work is to determine a relationship between the top and bump foil's geometry and load-carrying capacity in a journal compliant generation I air foil bearing, as well as determining the effect of the thermohydrodynamic phenomena in the performance of the air foil bearing (AFB). Static and steady-state operation is assumed throughout the analysis. A finite element model is adopted in order to investigate the operational characteristics of the specific bearing. Bump foil's elastic behavior is modeled using two node linear link spring elements. During the hydrodynamic analysis, incompressible viscous steady state Navier-Stokes equations are numerically solved, due to the low bearing compressibility number. During the thermohydrodynamic analysis, compressible, viscous, steady-state Navier-Stokes equations were solved, coupled with the energy equation. The material used during the structural analysis is Inconel X750, and it is assumed that it has linear and elastic behavior. Constant ambient pressure is applied at the free faces of the fluid as well as no slip condition at the surface of the fluid that faces the top foil. Computational fluid dynamics (CFD) and structural models are solved separately. At the beginning of the analysis, the CFD problem is solved with the assumption that the top foil has not yet been deformed. After the solution of the CFD problem, the pressure distribution at the surface of the fluid that faces the top foil is applied at the top foil and then the structural problem is solved. Consequently, the deflections of the top foil are applied on the corresponding surface of the CFD model and the algorithm continues until convergence is obtained. As soon as the converged solution for the pressure distribution is obtained, numerical integration is performed along the surface of the bearing in order to calculate its load-carrying capacity. Static bearing performance characteristics, such as pressure distribution, bump foil deflection, and load capacity are calculated and presented. Furthermore, fluid film thickness, top foil deflection, and fluid pressure are investigated as functions of the bearing angle as well as loadcany ing capacity as a function of the bump and top foil stiffness. The same procedure is repeated for the thermohydrodynamic analysis. Moreover, in order to estimate the heat flux from the top foil to the bump foil channel as a function of the top foil temperature, a simple finite element model of the bump foil-cooling channel is constructed.
The dynamics of an automotive differential hypoid gear pair is investigated. The gear pair model is a 4 degree-of-freedom torsional model, including the torsional deflections of the supporting shafts of the pinion and the gear. It also includes the dynamic transmission error of the mating teeth pairs. The variations in teeth contact stiffness/contact, principal radii of contact and static transmission error are determined during the meshing cycle, using the CALYX software. The equations of motion are solved using a numerical integration scheme. A preliminary parametric study is presented, enabling identification of different periodic responses as the vehicle cruising speed alters.
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