Two-dimensional generalized non-Newtonian EHL lubrication: Shear rate-based solution versus shear stress-based solution

Lubricant behaves non-Newtonian at high shear stress and high shear rate. The non-Newtonian shear behavior of oil such as shear-thinning, viscoelasticity, and limiting shear stress could have influences on almost all characteristics of an elastohydrodynamic lubrication (EHL) contact, that is, the central film thickness, the coefficient of friction, and the temperature rise in the lubricating film. For example, for lubricants of large molecular weight or of polymer blended ones, there can be inlet shear-thinning, which would reduce the EHL film thickness. For the EHL traction in a rolling/sliding EHL contact, it cannot be reasonably predicted without the consideration of non-Newtonian rheology. In EHL numerical studies, the non-Newtonian properties and the constitutive equations are expressed by the concept of generalized viscosity [Formula: see text], which can be either a function of shear rate [Formula: see text] or a function of shear stress [Formula: see text]. In this way, a non-Newtonian lubrication problem could be solved as a generalized Newtonian problem based on solvers for a Newtonian EHL problem. According to the function of the generalized viscosity [Formula: see text], numerical solutions can be classified into shear rate-based ones and shear stress-based ones. In this work, these two kinds of numerical solutions are revisited. And their efficiency is compared for a two-dimensional (2D) non-Newtonian point contact EHL problem (here 2D means non-Newtonian flow in both the x and y directions). Results show that the shear rate-based numerical solution has a higher efficiency than the shear stress-based one. The shear rate-based 2D generalized Newtonian method is more suitable to analyze multiple EHL contacts in angular contact ball bearings and gears with complex 2D flow and/or transient EHL lubrication problems.

On the Relation between Friction Increase and Grease Thickener Entraining on a Border of Mixed EHL Lubrication

This paper deals with an experimental study of film thickness and friction of commercial-grade grease and its base oil in a highly loaded contact. In-situ measurements were conducted for two surface textures on a ball-on-disc optical tribometer at the border of mixed lubrication. At high speeds, the film thickness and the friction of grease correspond with the base oil, while, the thickener enters the contact area and locally affects the film thickness and friction at low speeds. It was found out that the thickener starts to enter the contact area approximately at the same speed when the base oil friction increases on Stribeck curve but without direct solid to solid contact. It indicates that both effects can have the same origin. Change of lubricant flow in contact inlet area was discussed as a possible explanation.

The Effect of Surface Defect on EHL Point Contact

In this paper, the lubrication of three-dimensional EHL point contact with surface defect was solved, the lubrication film shape and film pressure distribution were presented and the characteristics of EHL point contact with defect were analyzed, the effect of surface defect on EHL point contact was studied. The results were presented that convex defect could lead to film pressure peak increasing, there was obvious micro-EHL effect, concave defect could lead to film pressure hollow and peak, film pressure peak was also increasing. The results also shown that convex and concave defects would have negative impact on EHL lubrication, there was great influence of surface defect on EHL lubrication pressure, the significant fluctuation of local film pressure was caused by surface defect, and generating greater pressure peak, maximum film pressure was rapid growth with the defect size increasing, so any kind defect should be to avoid in EHL lubrication.

Soft EHL Lubrication of Complex Multiphase Fluids

The lubrication properties of a series of multiphase water-based fluids of complex rheology and microstructure, including o/w emulsions, have been studied in a rolling-sliding steel ball-on-elastomer flat contact. The results show that friction curves of Newtonian fluids made over a wide range of entrainment speeds and viscosity can be used to identify the prevailing mechanisms of lubrication for more complex fluids and, for emulsions, to show the predominant film-forming phase.