Minimum Film Thickness Ratio Research Articles

Three-dimensional mixed lubrication analysis of spur gears with machined roughness

The timing gears of the ship internal combustion engine usually undergo dynamic loading conditions, and often work in a mixed lubrication state. In this study, by considering the three-dimensional (3D) machined roughness, an integrated model with the analysis of mixed lubrication state and dynamic-meshing performance for the spur gear is developed. The model is verified by comparing its results with the predictions using the typical line-contact EHL formulas. The machined roughness apparently affects the local 3D distributions of the film pressure and thickness. The shaved surfaces have the minimum film thickness ratio and the worst lubrication state, while the polished surfaces can improve the lubrication performance of the timing gear pairs greatly. Under the conditions of heavy-load and low-speed, almost all of the Hertzian contact zone appears to be in contact with 0 film thickness, and the local maximum pressure may lead to serious stress concentration, film breakdown, and excessive wear, etc. By optimizing the gear module and pressure angle, the loading capacity of the timing gear is promoted and the contact area gets reduced, the lubrication state improves significantly from mixed lubrication to full film lubrication.

The effects of surface texture in reciprocating contacts – An experimental study

The effect of surface texture on friction has been investigated experimentally for a convergent-divergent bearing, operating under different lubrication regimes. This was achieved using recently developed test apparatus that closely simulates an automotive piston ring-liner contact, by sliding patterned plane fused silica surfaces against a convex steel pad. Textured patterns, consisting of pockets of varying shapes and orientations, were assessed on their ability to reduce frictional losses, and results compared with those from a reference smooth surface. These tests were performed under a range of lubricant viscosities and applied normal loads in order to vary lambda values (i.e. the ratio of minimum film thickness to composite roughness) and hence highlight the beneficial or detrimental effects of texture in boundary, mixed and full film regimes. In the boundary and mixed regimes – the regimes where surface texture was beneficial – grooves normal to the sliding direction were the most effective patterns among those investigated, reducing friction by up to 62%. The results suggest that pockets act to increase fluid entrainment and hence reduce any asperity contact that is present. However, pockets at reversal were shown to increase friction dramatically. In the full film regime, when a liquid film fully separates sliding surfaces, texture was shown to increase friction compared to the smooth case. In addition to this, the experimental setup allowed cavitation in the ring-liner pairing to be imaged. These results may suggest that the increase in full film friction due to surface texture may be due to the interaction of pockets with the cavitated region. Furthermore, imaging results confirm previous research in that the number of cavitation streamers increase as the film thickness decreases.

Research on the Performance of Space Liquid Lubrication System with Oil-Storage

The gravity oil feed method dose not suit for microgravity environment as the same way dose for ground lubrication system. It is the key problem to be solved that how to supply the oil to friction surface in space liquid lubrication system. It analyzed the lubricating manner. A space liquid lubrication system was designed based on a principle of using deformation energy to supply oil. In the analysis of its basic structure and operating principle, the film calculation mode was established based on the elastohydrodynamic lubrication theory. The lubricating performance was simulated by ANSYS finite element analysis software in microgravity. The calculation results show that: in the microgravity, it could achieve elastohydrodynamic lubrication on friction surface in this lubrication system. When the oil supply hole diameter is 2mm, the film-thickness ratio changes between 1.91 and 4.28. It belongs to elastohydrodynamic lubrication. The hole diameter decreases, the film thickness reduces. The minimum film-thickness ratio is 0.93 when the temperature changes widely from -50°C~+80°C. It belongs to boundary lubrication state.

A Thin Film Flow Rupture Model for Newtonian and Viscoelastic Fluids

The problem of specifying adequate boundary conditions in two-dimensional steady incompressible flow between a cylinder and a plane for Newtonian fluids has been studied for many years. Unfortunately, Newtonian boundary conditions cannot be readily transposed to the case of viscoelastic fluids. In the present paper we consider the case of the second-order Rivlin-Ericksen fluid in prescribing suitable boundary conditions. The crux of the issue is that there are at least three possible stresses to be considered: (1) the transverse normal stress σy, (2) the longitudinal normal stress σx, and (3) the mechanical pressure p. The former, σy, is constant across the film and this variable is the one which carries load and measured by an experimental “pressure” tap. In the process of developing the viscoelastic model, a Newtonian model is developed which uses the Birkhoff-Hays separation conditions, considers surface tension as the Coyne-Elrod model but invokes the thin film assumption. The equations of motion are satisfied along and across the separated interface. Variation of the transverse normal stress profile with a minimum film thickness ratio, a surface tension parameter and Deborah number is shown. An earlier paper [6] shows that inconsistencies arise in applying simple rupture models (Gumbel or Swift-Stieber) to viscoelastic fluids. A relatively complicated model of this type is necessary to portray the basic behavior of film rupture in viscoelastic fluids.