In elasto-hydrodynamic lubrication (EHL) condition, traction property of oil films changes from linear region to non-linear region with increase in slide roll ratio. This means that the oil films behave as non-Newtonian fluid. In fact, as shear rate increases, shear stress acting to the oil films becomes small compared with that of Newtonian fluid. Namely, viscosity of the oil films decreases with increase in the shear rate. The fluid with such viscosity property is referred to as shear-thinning fluid. Until now, in order to evaluate the traction property in the EHL condition, constitutive equations of the shear thinning models have been incorporated to dominant equations of the EHL theory, and many numerical analyses have been performed. In addition, it has been reported that the oil films of some synthetic oils by the EHL oil film thickness calculating equations are evaluated thicker than the results of the EHL numerical analyses and the experiments. In the present review paper, the typical shear thinning models such as EYRING model and CARREAU-YASUDA model are introduced. Subsequently, the results of the EHL numerical analyses applying the shear-thinning models are explained.

When solving the Reynolds equation for hydrodynamic lubrication, boundary conditions at the inlet and the outlet of lubricated area is necessary and important. There are many kinds of boundary conditions for the outlet end of pressure profile. We have to choose an adequate one by considering operating conditions, geometries and so on. Moreover, we have few choice for the boundary condition for inlet. To avoid the problems of boundary conditions, particle-based method can be applied to hydrodynamic lubrication analysis. Particle-based method can be used to numerically solve continuum dynamics with large deformation of free surface flow. With the method, the oil film profile presents itself as a consequence of fluid flow. Explicit boundary conditions at the inlet and the outlet are not necessary. The basic concept of the particle-based method, focusing on the smoothed particle hydrodynamics method is described in this report. And its applicability to hydrodynamic lubrication analysis is also discussed.

This article reviews some of the research on numerical analysis of elastohydrodynamic grease lubrication and on relevant models of grease aging and bleeding, aiming at a better understanding of the underlying phenomena. Several degradation models have been developed to describe the influence of shear and temperature on the reduction of the yield stress. The feed-loss balance model, which is generally applied to rolling bearings, is also applied to seals to predict the film thickness decay in an axial lip contact. Grease EHL theory that takes grease as a non-Newtonian fluid has been developed. A numerical analysis is recently performed to predict the grease film thickness at low speeds, which can be significantly larger than the base oil film thickness due to increase in the yield stress, which is due to increase in the thickener concentration. The model is detailed to clarify the mechanism of the increase in the thickener concentration.

In situ observation methods of grease lubrication from micro to macro were introduced. The latest researches on the behavior of thickener and the phenomenon of thickening at very low speeds were introduced, with particular emphasis on in situ observation by infrared spectroscopy. In addition, several methods of in situ observation, which has not yet been applied to grease lubrication but is expected to be used in the future, were also mentioned.

This paper describes rheological measuring techniques in elastohydrodynamic lubrication (EHL) contact using in-situ observation systems with fluorescence method. One of the important issues facing the study of EHL mechanism is understanding how rheological properties change through the EHL contact. It has been reported that the viscosity profile and the state of lubricant, such as solidification and glass transition, in the lubrication film were obtained by using new type of measuring system shown in this paper. In this article, previous researches regarding to rheological properties in the EHL contact and the technique for measuring film thickness using the fluorescence method are also described.

This report introduces recent trends of author's research on rough estimation of lubricant high-pressure viscosity by simple observation of its solidification pressure to be grasped quantitatively in predicting film thickness and traction force of elastohydrodynamic lubrication (EHL). Employing a DAC (diamond-anvil pressure cell), solidification pressure Pd for several oils was empirically determined by observing the onset pressure of the aluminum small sphere deformation in the pressure chamber (lubricant) of the DAC. By substituting Pd and solidification viscosity 10⁷ for the Barus equation, pressure-viscosity coefficient αPd at Pd was obtained only at room temperature (24°C). αPds at up to 150°C were roughly estimated by applying the recently proposed pressure temperature superposition graph. They were compared with Blok's pressure-viscosity coefficients α*s calculated from the published viscosity data at up to 150°C. The ratios α*/ αPd at 24°C to 150°C were almost independent of the kind of oils and temperature. Utilizing these ratios, simple high-pressure experiments with the DAC only at room temperature facilitate rough estimation of pressure-viscosity coefficients of lubricants at high temperatures up to 150°C.

The high pressure properties of lubricants are important for the lubrication analysis of metal forming, bearings and gears. Therefore, it is necessary to measure the high pressure viscosity using a high pressure viscometer and calculate the pressure viscosity coefficient using the Barus equation. Recent years, it was found that high pressure viscosity and pressure viscosity coefficient were derived from intrinsic constants of the extended Barus equation and the extended Dowson-Higginson density equation. Therefore, the data of the intrinsic constants of various lubricants are accumulated, and the estimation equation of high pressure viscosity and pressure viscosity coefficient by multiple regression analysis using physical properties and structure as explanatory variables was introduced. Here, how to select the explanatory variables in creating the multiple regression equation was an important point. Therefore, this commentary will introduce the selection method, as well as the latest trends in theoretical analysis and measurement technologies of EHL oil film and future prospects.

Deterioration of friction and wear due to particulate matter is one of the common lubrication problems. Lubrication failure due to soot generated by combustion in internal combustion engines frequently becomes a problem. In particular, abnormal wear of each part of the engine due to exhaust gas measures called EGR (Exhaust Gas Recirculation) is well known. The papers analyzed from different viewpoints are introduced in two explanations. This explanation is the second part of two explanations. This explanation summarizes the following contents. The wear mechanism between the piston ring and the cylinder liner is mainly abrasive wear. This was confirmed by elemental analysis and surface analysis using an electron microscope. Even if soot by EGR enters, the mechanism of wear does not change. Abrasive grains are carbides in cast iron. Soot functions as a grinding or cutting oil and promotes abrasive wear. When the shape of carbide in the metal structure is changed, the increase in wear due to soot disappears.

The life of rolling bearings can be limited by sudden increase in friction torque due to exhaustion of lubricant after long-term operations. With the purpose of developing grease having long “service life”, research has been conducted with angular contact ball bearings lubricated with urea greases, and the present First Report describes experimental work. The life was determined on FAG-FE 9 rolling bearing grease testers, and considerable difference in the life was found among the greases with different type thickeners. Observation of bearings by interrupting the run inferred the following scenario. In a short churning period, grease was re-distributed to form a lump on the front face of the outer race and thin layers on some other parts of the bearing. The lump served as an oil reservoir, and the oil bled from it infiltrated through the thin layer on the outer race to lubricate the raceways. A part of the oil was then carried by the balls to lubricate the sliding interface between the cage and the balls and, when this lubrication became insufficient, scuffing initiated to limit the life.

The preceding First Report showed that the service life of urea grease in angular contact ball bearings, determined on FAG FE9 rolling bearing grease testers, considerably differed depending on the type of thickeners. Further, it reasoned that insufficient “Feed”of the base oil to the bearing raceway lead to seizure limiting the lubrication life on the basis of the results obtained by tracing infiltration of the base oil through re-distributed grease in the bearing. In order to provide a logical rationale for these findings, the present Second Report analyzes the “Feed”behavior. The equivalent radius characterizing the infiltration rate of the base oil through grease with different thickener concentration is determined on a newly developed test apparatus using paper filter as the medium. A chart is introduced to show the change in the equivalent radius of grease during an operation and the limiting condition leading to the failure is discussed. Finally, the means to make relative comparison of the service life of grease without conducting bearing tests is proposed.