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Tribological behavior of lubricated, load-bearing contacts in combined rolling and sliding has been a major research topic. Physics of such contacts are governed by the elastohydrodynamic lubrication (EHL) principles where the lubricant, material, surface roughness, and operating conditions related parameters define collectively how well the contact is lubricated toward long-cycle performance of them in terms of wear, pitting, scuffing and power loss. Currently available EHL models are not fully capable of representing various real-life lubricated contacts such as gears and rolling element bearings since they are subject to numerous limiting conditions and assumptions. In this research study, a general-purpose EHL formulation is proposed for such complex contacts having the following features: (i) the contact does not conform to Hertzian distributions and shapes (line or elliptical); (ii) surface velocities are neither collinear nor remain uniform across the contact zone, resulting in spatially varying rolling and sliding velocity profiles along each instantaneous contact zone; and (iii) basic contact parameters such as rolling and sliding velocities, normal force, and radii of curvature are all time-varying parameters. This generalized EHL formulation (i) accounts for actual, measured surface roughness profiles in a deterministic and transient manner, (ii) captures the rheology of the lubricant accurately including an exact treatment of the non-Newtonian effects, and (iii) employs several efficient discretization and numerical solution techniques to achieve better accuracy and robustness while reducing the sensitivity of the EHL solution on grid density. The proposed model is first applied to various basic contact problems including ball-on-disk, roller-on-disk, and cone-on-disk contacts. While the normal force, spatial velocity distributions and the radii of curvature are time-independent, the relative matching of surface roughness profiles and orientation of rotational axes are shown to result in unique EHL contact conditions. The model for the ball-on-disk contact, with the assumption of small contact circle, shows clearly the sole impact of non-collinear surface velocities, while the models for roller-on-disk and cone-on-disk contacts quantify effects of different levels of velocity and/or radius of curvature variations across the contact along with the influence of roughness lay directions in relation to the surface velocities. As a product example, the EHL formulations are customized and coupled with a gear load distribution model to capture complex contact conditions exhibited by helical gears. Transient effects due to both surface roughness profiles of contacting tooth surfaces and movement of the contact lines along the tooth surfaces are captured in this model. While the surface velocities at each contact point are collinear, they show significant variations along the contact line at each instant in addition to the radius of curvature. The helical gear EHL model results reveal that the variations to the contact conditions with non-uniformity of surface velocities, curvatures and contact pressure across the contact zone as well as the variation of contact parameters with the movement of the contact line impact the lubrication conditions significantly. Further, the level of asperity interactions is shown to be significant, justifying the necessity to use a mixed EHL formulation. Parametric studies on the influence of operating parameters are performed to show that these asperity interactions are amplified at low-speed, high-torque and high-oil temperature (low-viscosity) conditions. Further, the helical EHL model is extended to predict mechanical power loss. Parametric studies are performed on the influence of the operating conditions and profile modifications on power loss including its rolling shear, sliding shear and asperity shear components. Finally, helical gear power loss experiments from an earlier study are simulated to assess the accuracy of the proposed helical EHL model. |
