Recent trends of measurement technology and theoretical analysis concerning hydrodynamic lubrication are reviewed. As for measurement technology, recent development on direct measurement technique of lubricating oil film behavior, fluorescence, interferometry and photochromism are introduced and their characteristics are compared each other. Also recent ultrasonic sensor and thin film sensor methods are introduced. Recent analytical approach concerning hydrodynamic and mixed lubrication are also reviewed, cavitation and starvation models, CFD, particle method, flow factor models and contact models of rough surfaces, and so on. Development on precise contact and mixed lubrication models, and construction of multi-scale simulation on hydrodynamic lubrication are highly expected in the near future.

This paper introduces a new visualization method using photochromism for movement of lubricating oil film. A photochromic dye is dissolved in oil and an arbitrary spot of oil is illuminated with ultraviolet light at an arbitrary timing, which makes a marker with relatively long lifetime in the oil film via a photochromic reaction. The color density of the colored oil is quantified based on the absorbance, and it can be confirmed that the color density is proportional to the oil film thickness. This method is used to visualize the movement of oil film on the piston lands of an optical engine. Some characteristic results are introduced in this paper to validate the usefulness of a photochromic visualization method.

This article describes two-phase flow visualization experiments and CFD analysis of oil film bearings in the author's previous work. So far, many researchers have been studying the oil-gas boundary of fluid bearings because of its importance. The authors also performed oil-gas two-phase flow visualization and CFD analysis, focusing on bearing clearance two-phase flow. In addition, X-ray CT was used to visualize the double oil lubrication membrane on the floating bush bearings. From the above research results, new information on the lubrication flow of the bearing clearance regarding the bearing characteristics was obtained.

In the case of ultrasonic methods, it is unnecessary to drill a hole leading to the lubricated surface for mounting the probe and to deposit a sensor film on the lubricated surface. The required process for the measurements is to simply place of an ultrasonic probe behind the lubricated surface (e.g. the behind of the piston ring or journal bearing). In this explanation, we will introduce examples of simple lubrication evaluation technologies to estimate oil film thickness and oil film rupture state, including the eddy current method possible to measure oil film thickness without influence of film rupture.

Minimum clamping force on metal pushing belt is effective in decreasing oil pump loss in order to improve efficiency of Continuously Variable Transmission (CVT). In contrast the belt slip on a pulley occurs credibility problem. It is important to clear the contact and slip behavior between the pulley and elements which are components of the belt for satisfying both of lower belt clamping force and credibility simultaneously. The contact pressure is important for the setting of clamping force, but it could not be measured in operation. Therefore, the thin film sensor which was applicable to contact pressure in boundary lubrication was developed and measured contact pressure during operation of the CVT. As a result, the contact pressure became clear and this senor is utilized in reliability improvement of the CVT. This paper discusses thin-film measurement of contact pressure on CVT.

This report reviews the latest trends in the numerical simulation of hydrodynamic lubrication. The hydrodynamic lubrication theory has been well developed since Professor Osborne Reynolds first derived a sophisticated governing equation for the lubricant oil flow in a lubricated area by applying suitable assumptions to the Navier-Stokes equations. Owing to the efforts of many researchers, precise numerical simulations of hydrodynamic lubrication can generally be used for the design of lubricated areas. Recently, coupling the numerical simulations of hydrodynamic lubrication with structural movements and deformations has garnered significant attention; this approach is termed as fluid-structure interaction (FSI) analysis, and it is used for addressing problems related to actual machine components. The latest research on FSI analysis in the field of tribology is described herein.

This report shows an overview of the computational models for fluid lubrication and solid contact, which are commonly used in the analysis of mixed lubrication conditions where the effect of surface roughness cannot be neglected, and the surface property parameters that significantly affect the prediction accuracy of these models. In addition, as examples of mixed lubrication analysis for real parts, pistons of swash plate compressors, piston rings of engines, and hydraulic dampers of automatic transmissions are introduced. It is shown that it is possible to perform mixed lubrication analysis for various sliding parts by applying mechanical motions and external forces according to the characteristics of each product based on the coupled calculation of fluid lubrication and solid contact models.

Smoothing surface roughness by the use of additives is one of the promising approaches to reduce friction loss and wear under boundary lubrication. Effect of phosphorus anti-wear agents on controlling surface roughness is focused in this study. Zinc dithiophosphate (ZnDTP), generally used as an anti-wear agent for engine oil, formed rough surface by forming inhomogeneous phosphorus and sulfur films. In contrast, a particular neutral phosphorus agent that reduced surface roughness and prevented wear was found, which forms homogeneous phosphoric acid film. An acidic phosphorus agent also reduced surface roughness and reduced friction and promote low shear stress molybdenum disulfide film formation with Molybdenum dithiocarbamate (MoDTC), but it was accompanied with a large amount of wear. Use of the neutral and the acid phosphorus agents together realized low friction, reduced wear, and very smooth surface by forming reaction films composed of sulfur and phosphorus type films. The study indicates that forming a homogeneous phosphoric film of low shear strength by the use of specific phosphorous agents is one of a promising approach to establish and sustainable boundary film of low friction and low wear.

In the extended Dowson-Higginson density equation in the second report, it was found that the reciprocal of the density increase ratio, 1/(ρ_pt/ρ_0t－1), is proportional to the reciprocal of the pressure temperature, 1/PT. However, it was difficult to understand the physical meaning of the proportional relationship between these two reciprocals. Therefore, in this report, we examined whether a linear equation could be constructed for the relationship between the dimensionless density ρ_pt/ρ_0t and the pressure temperature product PT. As a result, it was found that the dimensionless density 6th power (ρ_pt/ρ_0t)⁶ has a linear relationship with the pressure temperature product PT. We derived the linear equation (ρ_pt/ρ_0t)⁶=εPT+1. Considering the physical meaning of the derived equation, the dimensionless density cube (ρ_pt/ρ_0t)³ squared (=density substitute function square) corresponds to the pressure temperature product PT. For that reason, it can be understood that the linearization was caused by the dimensionless density 6th power (ρ_pt/ρ_0t)⁶. This is similar to the linearization of the extended Barus equation in the first report. Since the dimensionless density ρ_pt /ρ_0t is unitless and the value itself is one-dimensional, the three-dimensionalization is required to express the characteristics as a substitute function of density. It was consistent with what we assumed to be.