Mixed Lubrication Regime Research Articles

Mutual Effect of Groove Size and Anisotropy of Cylinder Liner Honed Textures on Engine Performances

The cylinder liner surface texture, widely generated by the honing technique, contributes a lot on engine functional performances (friction, oil consumption, running-in, wear etc.). In order to improve these functional performances, different honing processes are being developed. These different honing processes generate surfaces with various texture features characteristics (roughness, valleys depth, valley width, cross hatch angle, etc.). This paper addresses a comparison of ring-pack friction for cylinder texture with different cross-hatch angles and valley sizes. It takes in consideration the mutual effect of valley depth and honing angle. A numerical model is developed to predict friction within the cylinder ring-pack system in mixed lubrication regime and a morphological method is used to characterize groove depth. The results show the effect of different honing variables (rotation speed, stroke speed and indentation pressure) on cylinder bore surface textures and hydrodynamic friction of the ring-pack system.

Design principles for the area density of dimple patterns

Increasing interest has been paid to surface texturing in order to improve tribological properties of sliding surfaces. Currently, the patterns of micro-dimples have attracted more attention since such closed texture cells are supposed to generate additional hydrodynamic pressure easily. It has been proven that the area density of the dimple pattern is a critical parameter for hydrodynamic pressure generation. However, the optimal values of area density obtained by theoretical models are usually different to that obtained from experiments, which show material dependence. In order to understand the design principles of the area density for mixed lubrication regime, a brief review on the studies related to the area density issue of dimple patterns is carried out; the phenomena during contacting and sliding are analyzed numerically in this paper. It is found that the stress concentration and deformation will occur on the contacting and sliding area, and influence the tribological properties of the textured surface significantly depending on the area density and material properties. These negative influences should be considered carefully during surface texture design.

Efficiency of laser surface texturing in the reduction of friction under mixed lubrication

The tribological behavior of steel sliding pairs with dimples ranging from 15 to 800µm in diameter was characterized in the mixed lubrication regime in a pin-on-disk experiment. The pin was textured, keeping the total dimpled area constant at 10% and the depth-to-diameter ratio at 0.1. Polyalphaolefin (PAO) was used as a model lubricant. Experiments were carried out under unidirectional sliding conditions at 50 and 100°C. At constant depth-to-diameter ratio the results showed a significant non-linear dependence of the friction coefficient on the texture diameter, sliding speed and the oil temperature (viscosity). A friction reduction of up to 80% was possible with the optimal diameter for certain sliding speeds. The dimple diameters leading to the highest friction reduction significantly depend on the oil temperature. By reducing the oil temperature from 100 to 50°C the dimple diameter resulting in the highest friction reduction changed from 40 to 200µm.

Influence of Surface Roughness on the Tribological Behavior of Gear Oils in Steel–Steel Contacts: Part II—Mixed Friction Properties

Hypoid gears are known to be commonly used in rear-wheel-drive automobiles and they typically operate under high sliding and high contact pressure conditions. Providing efficient lubrication and necessary surface finish to such gear contacts is important in order to achieve a significant gain in energy and torque efficiencies. This requires knowledge of the behavior of gear oils under different operating conditions, especially their abilities to work under high contact pressure and sliding conditions. In light of this and in view of optimizing friction in transmission systems, this article explores the influence of surface roughness on friction in a mixed lubrication regime. In addition to friction–speed measurements and Stribeck curves described in a companion article (Vengudusamy, et al. (1)), this article explores the friction response of tribofilm-generated surfaces with base oil and highlights the significance of such experiments.

Minimize friction of lubricated laser-microtextured-surfaces by tuning microholes depth

We have investigated the friction properties of lubricated laser micro-textured surfaces. The micro-texture consists of a square lattice of micro-holes whose diameter, depth and spacing are controlled during the laser texturing process. All surfaces have the same texture area density, but different diameters and depths of the micro-holes. We measure the coefficient of friction on a range of sliding velocities from the mixed lubrication regime to the hydrodynamic regime. We find that the depth and the diameter of the micro-holes have a huge influence in determining the amount of friction reduction at the interface. Interestingly experiments also show that optimal micro-hole depth values, minimizing the friction in the hydrodynamic regime, are remarkably effective also in the mixed lubrication regime.

Effect of Temperature on Lubricated Steel/Steel Systems With or Without Fatty Acids Additives Using an Oscillating Dynamic Tribometer

The friction behavior of steel/steel contacts lubricated with glycerol, 150NS and PAO4 is investigated using a new technique. Then, three additives are used in PAO4 lubricant: oleic, linoleic and stearic acids. The experiments are performed at different temperatures. Moreover, nearly all the systems operate in either the mixed or boundary lubrication regime. The experimental methodology is able, with a high accuracy and without any force transducer, two contributions of friction: μ0, velocity-independent contribution and μ1, velocity-dependent one. This study shows that glycerol is the most effective in reducing friction among the three pure lubricants. In addition, only glycerol affects the viscous damping of the system at some operating conditions. Using the additives in PAO4, the friction is reduced. More precisely, the unsaturated fatty acids are more significant in reducing the friction than saturated acids. Moreover, the viscous damping of the apparatus is slightly affected as additives are used in PAO4.

A Mathematical Model for the Mixed Lubrication of Non-Conformable Contacts With Asperity Friction, Plastic Deformation, Flash Temperature, and Tribo-Chemistry

A mathematical model is presented in this paper for rolling-sliding contacts operating in a mixed regime of elastohydrodynamic lubrication and boundary lubrication. The model is based on the framework of Johnson et al. (1972, “A Simple Theory of Asperity Contacts in Elastohydrodynamic Lubrication,” Wear, 19, pp. 91–108). It incorporates into this framework a number of important asperity-level variables including asperity friction, friction-induced plastic flow, flash temperature, and boundary-film tribo-chemistry. The model yields a number of variables useful for the assessment of the state of the mixed lubrication. They include the load sharing between fluid and asperities, area of asperity contacts, and fraction area of asperity contacts undergoing plastic flow along with experimentally measurable variables such as the traction coefficient, friction power intensity, and temperature of the overall contact. The model is limited to mixed-lubrication problems in which the load is mainly carried by the fluid pressure and the total area of asperity contacts is a small percentage of the Hertz area. Further development is possible to formulate a model into a wider mixed-lubrication regime using some modeling concepts developed in this paper in conjunction with other modeling techniques.

A numerical study on the performance of straight bevel gears operating under mixed lubrication regime

A model has been developed for predicting the film thickness and friction coefficient under the mixed-lubrication regime in straight bevel gears. Each pair of straight bevel gear teeth is replaced with multiple pairs of spur gear teeth using the Tredgold approximation, and the transmitted load and radii of curvature are accordingly evaluated. The bevel gears' performance is predicted by employing the load-sharing concept with the consideration of elastic, elasto-plastic and plastic deformation for asperities. The effect of parameters such as load, roughness, hardness, and rolling speed on the performance of the gear system is investigated. It has been shown that under the reported operating conditions, increasing the surface roughness to values higher than 0.5μm results in a shift of lubrication regime from elastohydrodynamic regime to mixed-elastohydrodynamic regime.

In-cycle and life-time friction transience in piston ring–liner conjunction under mixed regime of lubrication

The piston ring/cylinder liner conjunction can experience various regimes of lubrication during piston strokes inside the engine cylinder. In the current engines, the nature of lubrication usually remains hydrodynamic at mid-stroke, while a mixed regime of lubrication may be experienced at and near reversals. The direct contact between the tips of some of the asperities of opposing surfaces leads to mixed (partial) regime of lubrication. A model proposed by Greenwood and Tripp can be used to predict asperity-level contribution to the total piston friction. At the same time, Reynolds equation can be employed to predict the portion of load carried by the lubricant trapped between the asperities. Friction between the asperity tips is usually proportional to the load that they support, stated in terms of a proportionality factor, that is, coefficient of friction. The surfaces are usually furnished with hard wear-resistant coatings and in parts by solid lubricants. Both the piston rings and cylinder liner surfaces are usually coated. These coatings change the friction characteristics of the counterfaces because of their surface topography as well as material mechanical properties. Atomic force microscope is used to obtain surface topographical parameters in contact tapping mode. The corresponding surface topographical parameters are obtained from representative regional areas of the contacting solid surfaces, using a Talysurf. The combination of topography and coating characteristics are used to develop the necessary parameters for a boundary friction model. A numerical model of the top compression ring to cylinder liner is developed based on mixed-hydrodynamic regime of lubrication. The results for friction and the effect of coating on the power loss and wear of the conjunction are discussed in this article.

J1640104 ランダムナノストライプ表面の形成と摩擦特性([J164-01]マイクロ・ナノ理工学:nmからmmまでの表面制御とその応用(1))

Surface pattern is expected the following effects in friction, such as generating dynamic pressure, supplying and preserving lubricate on oil and trapping wear particles. One of the authors has developed the nanostripe surface having nano-scale groove array that was formed by depositing two kinds of materials on micro-scale slope array and by polishing the surface. This report describes a method of fabricating irregular nano-scale grooves formed on a substrate having random asperities. The random asperities were fabricated on a stainless steel substrate by shot blast processing. Then friction tests were carried out using pin-on-plate type friction tester driven by a PZT actuator with magnification mechanism. The result showed that the friction coefficient of random nanostripe surface was lower than that of untextured surface in the boundary to mixed lubrication regime.

Influence of temperature on the friction performance of gear oils in rolling-sliding and pure sliding contacts

The friction behaviour of five different gear oils in rolling–sliding and pure sliding contacts and how temperature influences their friction properties were investigated. It is found that increasing temperature decreases boundary friction with gear oils that contain friction modifiers while not for other gear oils, at all contact pressures investigated. In mixed lubrication region, temperature decreases friction at low contact pressures while increases friction at high contact pressures. The effect of slide–roll ratio on friction is significant in boundary lubrication region especially at higher temperature while less significant in mixed lubrication region at both low and high temperatures. The ranking of gear oils for friction in boundary and mixed lubrication regimes is similar both in rolling–sliding and pure sliding contacts, regardless of temperature. Copyright © 2013 John Wiley & Sons, Ltd.

Influence on friction behavior of micro-texturing under lubricated non-conformal contact

Laser surface texturing (LST) is a well-known surface engineering process used to create arranged dimples on surface using a material ablation process. In this process, a well arranged micro-dimple can produce significant improvements of engineering components in aspects such as load carrying capacity, wear resistance, wetting characteristics and reduction of friction coefficient. In the present work, we investigated the effect of multi-dimple LST steel surfaces on tribological properties under non-conforming contact. The multi-dimple textured surfaces with some specific formula arrays were fabricated by laser ablation process on steel disc by combining patterns of circles and triangles, and circles and squares, having different dimple densities to optimize the surface texturing effect on tribological performance. The tribological test of multi-dimple textured surface was performed by a ball-on-flat unidirectional tribometer in lubricated condition, and the results compared with that of a single-dimple and untextured (polished and ground) surfaces. The results indicate that the surface with multi-dimple textured patterns had better friction performance than the untextured and single-dimple textured surfaces. The beneficial effect of multi-dimple textured patterns was associated with transition of lubrication from boundary to mixed lubrication regime.

Transmission efficiency and noise, vibration and harshness refinement of differential hypoid gear pairs

This article presents a combined multi-body dynamics and lubricated contact mechanics model of vehicular differential hypoid gear pairs, demonstrating the transient nature of transmission efficiency and noise, vibration and harshness performance under various driving conditions. The contact of differential hypoid gears is subjected to mixed thermo-elastohydrodynamic regime of lubrication. The coefficient of friction is obtained using an analytical approach for non-Newtonian lubricant shear and supplemented by boundary interactions for thin films. Additionally, road data and aerodynamic effects are used in the form of resisting torque applied to the output side of the gear pair. Sinusoidal engine torque variation is also included to represent engine order torsional input resident on the pinion gear. Analysis results are presented for New European Driving Cycle transience from low-speed city driving condition in second gear to steady-state cruising in fourth gear for a light truck. It is shown that the New European Driving Cycle captures the transmission efficiency characteristics of the differential hypoid gear pair under worst case scenario, with its underlying implications for fuel efficiency and emissions. However, it fails to address the other key attribute, being the noise, vibration and harshness performance. In the case of hypoid gears, the resultant noise, vibration and harshness characteristics can be particularly annoying. It is concluded that broader transient manoeuvres encompassing New European Driving Cycle are required for assessment, in order to obtain a balanced approach for transmission efficiency and noise, vibration and harshness performance. This approach is undertaken in this article, which is not hitherto reported in the literature.

Research of oil film thickness model and surface quality in cold rolling copper foil

Rolled copper foil is widely used in high frequency and speed transmission of fine line printed circuit board, because of its high strength, good toughness and high density. In this paper, a theoretical model for copper foil rolling in mixed lubrication regime was developed on the basis of the average volume flow model and asperity flattening model. A more accurate relation for the variation of the lubricant viscosity with pressure and temperature was considered. The cold rolled copper foil experiment was carried on with different viscosity of rolling oil and pass reduction. The effects of rolling oil viscosity and pass reduction on lubricant pressure, contact area ratio and film thickness ratio were studied. The calculation results agree well with the measured data from copper foil rolling experiment. For obtaining higher surface quality, the rolling oil viscosity is about 10 mm2/s, and the pass reduction is about 30%. Copyright © 2013 John Wiley & Sons, Ltd.

Numerical Simulations of Asperity Crushing Using Boundary Conditions Encountered in Cold-Rolling

The general framework of this paper is in the field of numerical simulation of asperity crushing. Different material forming processes, such as strip-rolling and deep drawing, imply mixed lubrication. In this lubrication regime, two types of contact are present at the same time: a direct contact between the two solids at the asperity level and also valleys filled with pressurized oil. Theses contact conditions have a large influence on friction and wear taking place during the upsetting process. As this mixed type of contact is not yet fully understood from the physics point of view, numerical models are essential to achieve a better understanding. For example, semi-analytical asperity crushing models have been developed by Wilson&Sheu [1] and Sutcliffe [2] to take into account the influence of bulk plastic deformations on asperity crushing. The finite element method has also been used to model asperity crushing. Ike&Makinouchi [3] studied the behavior of 2D triangular-shaped asperities under different boundary conditions. Krozekwa et al. [4] modeled 3D triangular asperities behavior, for various bulk strain directions. More recently, Lu et al. [5] compared experimental results of pyramid-shaped asperity and ridge-shaped asperity crushing with finite element simulation results. As in the three former references mentioned above, it has been decided, to study the interaction between a rigid plane and a simplified geometry asperity without lubricant. In this article, numerical asperity crushing results obtained with Metafor[6], a home made large strains software, will be presented. Those results will illustrate the influence of boundary conditions, contact pressure, large bulk strain and geometry of asperities on the evolution of the contact area. As the asperity crushing behaviour is known to be very sensitive to the boundary conditions, in this article, we will also present results using boundary conditions from a cold rolling model named MetaLub. MetaLub [7-8] is a software developed at the University of Liege in partnership with ArcelorMittal R&D center. It iteratively solves the equations resulting from the discretisation using the slab method of the strip coupled to a mixed lubrication model at the interface. This lubrication model takes into account the evolution of the oil film thickness as well as the asperity crushing along the roll bite. We will compare the evolution of the relative contact area obtained with MetaLub to the results obtained with finite elements simulations using the same boundary conditions. [1] Wilson, W.R.D and Sheu, S. Real area of contact and boundary friction in metal forming. Int. J. Mech. Sci. 1988, 30(7), 475-489. [2] Sutcliffe, M.P.F Surface asperity deformation in metal forming processes. Int. J. Mech. Sci., 1988, 30(11), 847-868. [3] Ike, H. and Makinouchi, A. Effect of lateral tension and compression on plane strain flattening processes of surface asperities lying over a plastically deformable bulk. Wear, 1990, 140, 17-38. [4] Korzekwa, D.A., Dawson, P.R. and Wilson W.R.D., Surface asperity deformation during sheet forming. Int. J. Mech. Sci., 1992, 34(7), 521-539. [5] Lu, C., Wei, D., Jiang, Z., and Tieu, K., Experimental and theoretical investigation of the asperity flattening process under large bulk strain, Proc. Inst. Mech. Eng. J. 222 (2008), 271–278. [6] LTAS-MN2L. ULg. http://metafor.ltas.ulg.ac.be/. [7] Stéphany, A., Contribution à l’étude numérique de la lubrification en régime mixte en laminage à froid. PhD dissertation (in French), Université de Liège (2008) [8] Carretta, Y., Stephany, A., Legrand, N., Laugier, M., and Ponthot, J.-P., MetaLub – A slab method software for the numerical simulation of mixed lubrication regime. Application to cold rolling. In Proceedings of the 4th International Conference on Tribology In Manufacturing Processes (ICTMP), 2010,799-808.

A Parametric Study on Fatigue Life for Mixed Elastohydrodynamic Lubrication Point Contacts

The lubrication characteristics and fatigue life are numerically analyzed under full film and mixed lubrication regimes, in which the three-dimensional sinusoidal surfaces with changeable wavelengths in x and y directions are used, the geometry changes of the contact areas are described by the various ellipticity, and the non-Newtonian flow of lubricant is described by the sinh-law rheology model. The results show that the influences of characteristic shear stress, wavelength ratio, and ellipticity on lubrication characteristics and fatigue life are remarkable. The effect of surface topography on lubrication characteristics has a close relationship with speed. Increasing the ellipticity and decreasing wavelength ratio and characteristic shear stress can prolong the fatigue life.

Pleural mesothelium lubrication after hyaluronidase, neuraminidase or pronase treatment

Coefficient of kinetic friction (μ) of pleural mesothelium has been found to increase markedly after mesothelial blotting and rewetting. This increase disappeared after addition of a solution with hyaluronan or sialomucin, though previous morphological studies showed that only sialomucin occurs in mesothelial glycocalyx. In this research we investigated whether μ of rabbit pleural mesothelium increased after hyaluronidase, neuraminidase or pronase treatment. Hyaluronidase and neuraminidase did not increase μ, though neuraminidase cleaved sialic acid from mesothelial glycocalyx of diaphragm specimens, and removed hystochemical stain of sialic acid from glycocalyx. Sialomucin treated with neuraminidase lowered μ of blotted mesothelium, though less than untreated sialomucin; this feature plus lubrication provided by other molecules could explain why μ did not increase after neuraminidase. Short pronase treatment (in order to affect only glycocalyx proteins) increased μ; this increase was removed by hyaluronan or sialomucin. After pronase treatment μ decreased with increase in sliding velocity, indicating a regime of mixed lubrication, as in blotted mesothelium.

The Failure and Damage Mechanisms Under Friction of Copper in the EHL and Mixed EHL Regions

Recently, friction and wear behaviour of Copper (Cu)–steel pairs rubbed under different lubrication conditions were studied. The Stribeck curve was used to identify the different regimes of friction of copper with different virgin grain sizes: the elasto-hydrodynamic lubrication (EHL), mixed lubrication and boundary lubrication (BL) regimes. The aim of this work is the detailed analysis of the damage evolution under friction in the EHL and mixed EHL regions. The effects of load on the friction and the wear and damage mechanisms have been studied. The surfaces of Cu samples before and after friction have been analysed using SEM and AFM techniques as well as roughness and hardness measurements. It was shown that the mechanisms of damage and failure of Cu samples under friction in the EHL region are similar to the damage and failure mechanisms in Very High Cycle Fatigue. Friction in the EHL region is accompanied by initiation and coalescence of pores and microcracks. The effects of the coalescence of pores and microcracks observed in the EHL region are enlarged with a subsequent loading in the mixed EHL region. The effect of the loading rate on the transition from the EHL to BL region has been studied. It was shown that decreasing the loading rate increases significantly the load of the transition from the EHL to BL region. The pore and microcrack coalescence remain as the dominant damage mechanism under friction in the mixed EHL region with the low loading rate, while a lot of ploughing tracks and large delaminated regions appeared on the surface of Cu sample after friction with the higher loading rate.

Mutual influence of crosshatch angle and superficial roughness of honed surfaces on friction in ring-pack tribo-system

The cylinder bore surface texture, widely produced by the honing technique, is an essential factor for a good engine performance (friction, oil consumption, running-in, wear etc.). This explains the improvement and development of various new honing techniques. These different honing processes generate surfaces with various texture features characteristics (roughness, valleys depth, crosshatch angle, etc.).This paper addresses a comparison of ring-pack friction for cylinder surfaces produced by plateau honing and helical slide honing. It takes in consideration the mutual effect of superficial plateau roughness amplitude and honing angle. A numerical model is developed to predict friction within the cylinder ring-pack system in mixed lubrication regime. The results show the effectiveness of helical slide honed surface texture in comparison to plateau honed bore surfaces.

속도의 영향에 따른 3차원 거친 표면의 혼합윤활해석

Dept. of Mechanical Engineering, Pusan National University(Received September 5, 2012 ; Revised November 11, 2012 ; Accepted November 12, 2012)Abstract − This study examined the effects of surface roughness in the mixed lubrication regime of smooth andrough surfaces for roller bearings. The average flow model was adopted for interaction between the flow rhe-ology of the lubricant and the surface roughness. The average Reynolds equation and related flow factor thatdescribes the coupled effects of surface roughness and flow rheology, the viscosity–pressure and density–pressureequations, the elastic deformation equation, and the force balance equation were solved simultaneously. Theresults showed that the effects of surface roughness on the film thickness and pressure distribution should be con-sidered, especially in elastohydrodynamic lubrication contact problems. Keywords − average Reynolds equation (평균레이놀즈방정식), flow factor (유동계수), surface roughness(표면거칠기), roller bearing (롤러베어링), dimensionless film parameter (유막계수)