Friction Reduction in Lubricated Rough Contacts: Numerical and Experimental Studies

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Zhiqiang Liu ◽  
Arup Gangopadhyay
Keyword(s):  
Friction Coefficient ◽  
Ambient Pressure ◽  
Classical Model ◽  
Experimental Studies ◽  
Elastohydrodynamic Lubrication ◽  
Contact Problems ◽  
Friction Reduction ◽  
Asperity Contact ◽  
Oil Viscosity ◽  
Lubrication Regimes

Combining the contact model of elastic-layered solid with the concept of asperity contact in elastohydrodynamic lubrication (EHL), a mixed-lubrication model is presented to predict friction coefficient over rough surfaces with/without an elastic-layered medium under entire lubrication regimes. Solution of contact problems for elastic-layered solids is presented based upon the classical model of Greenwood and Williamson (GW) in conjunction with Chen and Engel's analysis. The effects of the Young's modulus ratio of the layer to substrate and the thickness of the layer on the elastic real area of contact and contact load for a fixed dimensionless separation are studied using the proposed method, which is used for the asperities having contact with an elastic coating. Coefficient of friction with elastic-layered solids in boundary lubrication is calculated in terms of Rabinowicz's findings and elastic-layered solutions of Gupta and Walowit. The effect of rough contacts with an elastic layer on friction coefficient in lubrication regimes has been analyzed. Variations in plasticity index ψ significantly affect friction coefficients in boundary and mixed lubrications. For a large value of ψ, the degree of plastic contact exhibits a stronger dependence of the mean separation or film thickness than the roughness, and for a small value of ψ, the opposite result is true. The effect of governing parameters, such as inlet oil viscosity at ambient pressure, pressure–viscosity coefficient, combined surface roughness, and El/E2 on friction coefficient, has been investigated. Simulations are shown to be in good agreement with the experimental friction data.

The Effect of Triangle-Shaped Surface Textures on the Performance of the Lubricated Point-Contacts

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
Wen-zhong Wang ◽  
Zhixiang Huang ◽  
Dian Shen ◽  
Lingjia Kong ◽  
Shanshan Li
Keyword(s):  
Friction Coefficient ◽  
Electrical Contact ◽  
Tribological Performance ◽  
Operating Conditions ◽  
Friction Reduction ◽  
Point Contacts ◽  
Electrical Contact Resistance ◽  
Coverage Ratio ◽  
Lubrication Regimes ◽  
Ratio Size

It has been recognized that purposefully designed surface texturing can contribute to the improvement of tribological performance of elements and friction reduction. However, its optimal parameters may depend on the operating conditions. This paper investigated the effect of a triangle-shaped dimples array on the tribological performance of the lubricated point-contacts under different lubrication regimes, based on the rotational sliding experiment of a patterned steel disk against smooth steel balls. The dimples arrays were produced by laser process and characterized by the 3D profilometer. A series of tests were conducted with different dimple parameters including depth, coverage ratio, size, and direction. Stribecklike curves were obtained to depict the transition of lubrication regimes, and the electrical contact resistance was utilized to qualitatively characterize the lubrication status. The test results showed that the dimples arrays with different sizes, depths and coverage ratios had a distinct effect on the friction behaviors. Compared with the nontextured surfaces, when the dimple depth decreased from 30μm to zero with fixed coverage ratio and size, the friction coefficient firstly decreased, and then increased. The friction coefficient finally approached that of the nontextured surface, during which the lowest value appeared at the dimple depth of approximately 10∼15μm. The coverage ratio of texture showed the similar effect on the friction coefficient. Usually, the coverage ratio of approximately 10% resulted in the lowest friction coefficient. The dimple size and direction also had obvious effects on the friction coefficient. Thus, it can be concluded that there exists a set of optimal values for the dimple depth, coverage ratio, size, and direction to realize the friction reduction.

Experimental and numerical studies of positive texture effect on friction reduction of sliding contact under mixed lubrication

2020 ◽  
pp. 135065012093091 ◽  
Author(s):  
P Venkateswara Babu ◽  
Syed Ismail ◽  
B Satish Ben
Keyword(s):  
Numerical Study ◽  
Mixed Lubrication ◽  
Friction Reduction ◽  
Asperity Contact ◽  
Textured Surfaces ◽  
Area Density ◽  
Surface Textures ◽  
Lubrication Regimes ◽  
Numerical Predictions ◽  
Pin On Disc

Textured surfaces have been remarkable in improving the frictional performance of sliding contacts, particularly at instances such as boundary or mixed lubrication regimes. This article reports the results of an experimental and numerical study carried out by introducing surface textures in the form of protrusions to investigate its effects on friction performance under a mixed lubrication regime. The surface textures produced by the chemical etching process are tested on the pin on disc test rig by varying area density and height of the texture. In numerical simulation, the modified Reynolds equation (Patir–Cheng flow model) and asperity contact model (Greenwood–Tripp model) are solved for hydrodynamic and asperity pressures, respectively. The results indicate that the experimental measurements are qualitatively in good agreement with numerical predictions. Furthermore, the simulations are performed for different texture shapes by varying texture area density, height, and sliding velocity. The results depict that a maximum friction reduction of 87% with elliptical textures compared to the un-textured case.

Effects of Lubricant Viscosity on Ring/Liner Friction in Advanced Reciprocating Engine Systems

2006 ◽  
Author(s):  
Rosalind Takata ◽  
Victor W. Wong
Keyword(s):  
Shear Rate ◽  
Combustion Engine ◽  
Current Model ◽  
Friction Reduction ◽  
Asperity Contact ◽  
Oil Viscosity ◽  
Hydrodynamic Friction ◽  
Piston Speed ◽  
Ring Pack ◽  
Lubricant Viscosity

The piston ring-pack contributes a large portion of the mechanical losses in an internal combustion engine. In this study, the effects of lubricant viscosity are evaluated with the goal of reducing these mechanical losses. Oil viscosity affects friction directly in the hydrodynamic regime, where hydrodynamic friction increases with viscosity. It also influences boundary friction indirectly via oil film thickness — higher viscosity causes oil films to be thicker, which reduces asperity contact. At the optimum viscosity (the viscosity at which minimum friction losses are incurred) there is a balance between these hydrodynamic and boundary effects. As piston speed, ring loading, and other parameters change during the engine cycle, the optimum oil viscosity also changes. If the variation of viscosity could be controlled during the cycle, it could be maintained at an optimum at all times. In this study, several theoretical and realistic cases were studied to quantify the friction benefit that could be obtained if this were possible. Idealized cases with low viscosity near mid-stroke (to reduce hydrodynamic friction) and high viscosity near end-strokes (to reduce boundary contact) were considered, as were several more realistic cases based on temperature and shear-rate dependencies. It was found that, for the oil control ring studied, the effect on friction of keeping viscosity high near end-strokes is very small, and does not provide a substantial benefit (in terms of friction) over allowing viscosity to vary naturally with temperature and shear rate. Two mechanisms lead to the relatively small size of the friction benefit: the contribution to total cycle ring friction from the dead-center area is small, because of low piston speeds there; and any reduction in asperity contact due to increased viscosity is accompanied by an increase in hydrodynamic friction, which cancels out some of the benefit. Oil viscosity near mid-stroke, where most of the ring/liner friction is generated, is the dominant viscosity that controls the overall friction losses for the ring. Although its contribution to friction reduction is not large, maintaining high lubricant viscosity near dead-centers can lead to a reduction in wear in that region, because asperity contact decreases. For the ring-pack studied, a friction reduction of ∼7% is predicted when viscosity is reduced in the mid-stroke region (based on OCR effects alone). If end-stroke viscosity is also kept high, the end-stroke regions, where current engines experience the most wear, will see a reduction in asperity contact (although there will still be a slight wear increase in the mid-stroke). An end-stroke wear reduction of up to 25% is predicted by the current model.

Improvements in the mixed elastohydrodynamic lubrication and in the efficiency of hypoid gears

2019 ◽  
Vol 234 (6) ◽  
pp. 795-810 ◽  
Author(s):  
Vilmos V Simon
Keyword(s):  
Gear Tooth ◽  
Numerical Procedure ◽  
Elastohydrodynamic Lubrication ◽  
Equation System ◽  
Density Variation ◽  
Asperity Contact ◽  
Oil Viscosity ◽  
Hypoid Gears ◽  
The Real ◽  
Transient Nature

In this paper, the influence of the manufacturing parameters on the conditions of mixed elastohydrodynamic lubrication is investigated. On the basis of the obtained results, recommendations are formulated to improve the mixed EHL and the efficiency of face-milled hypoid gears. A full numerical analysis of the mixed EHL in hypoid gears is applied. The equation system and the numerical procedure are unified for a full coverage of all the lubrication regions, including the full film, mixed, and boundary lubrication. In the hydrodynamically lubricated areas, the calculation method employed is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace's equations. In the asperity contact areas, the Reynolds equation is reduced to an expression equivalent to the mathematical description of dry contact problem. The real geometry and kinematics of the gear pair based on the manufacturing procedure are applied; thus, the exact geometrical separation of the mating tooth surfaces is included in the oil film shape, and the real velocities of these surfaces are used in the Reynolds and energy equations. The transient nature of gear tooth mesh is included. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The non-Newtonian behaviour of the lubricant is considered. Using this model, the pressures, film thickness, temperatures, and power losses in the mixed lubrication regime are predicted. The effectiveness of the presented method is demonstrated by using hypoid gear examples.

On the Behavior of Friction in Lubricated Point Contact With Provision for Surface Roughness

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
H. Sojoudi ◽  
M. M. Khonsari
Keyword(s):  
Surface Roughness ◽  
Friction Coefficient ◽  
Numerical Solutions ◽  
Point Contact ◽  
Elastohydrodynamic Lubrication ◽  
Hydrodynamic Pressure ◽  
Operating Conditions ◽  
Asperity Contact ◽  
Type Curve ◽  
Lift Off

This paper presents a simple approach to predict the behavior of friction coefficient in the sliding lubricated point contact. Based on the load-sharing concept, the total applied load is supported by the combination of hydrodynamic film and asperity contact. The asperity contact load is determined in terms of maximum Hertzian pressure in the point contact while the fluid hydrodynamic pressure is calculated through adapting the available numerical solutions of elastohydrodynamic lubrication (EHL) film thickness formula for smooth surfaces. The simulations presented cover the entire lubrication regime including full-film EHL, mixed-lubrication, and boundary-lubrication. The results of friction, when plotted as a function of the sum velocity, result in the familiar Stribeck-type curve. The simulations are verified by comparing the results with published experimental data. A parametric study is conducted to investigate the influence of operating condition on the behavior of friction coefficient. A series of simulations is performed under various operating conditions to explore the behavior of lift-off speed. An equation is proposed to predict the lift-off speed in sliding lubricated point contact, which takes into account the surface roughness.

Piston ring-cylinder liner tribology investigation in mixed lubrication regime: part I-correlation with bench experiment

2015 ◽  
Vol 67 (6) ◽  
pp. 520-530 ◽  
Author(s):  
Lin Ba ◽  
Zhenpeng He ◽  
Lingyan Guo ◽  
Young Chiang ◽  
Guichang Zhang ◽  
...  
Keyword(s):  
Friction Coefficient ◽  
Sudden Change ◽  
Mixed Lubrication ◽  
Friction Reduction ◽  
Asperity Contact ◽  
Film Rupture ◽  
Frictional Loss ◽  
Content Type ◽  
Oil Film ◽  
Ring Sample

Purpose – The purpose of this paper is to improve the environment and save energy, friction reduction, lower oil consumption and emissions demand that are the chief objectives of the automotive industry. The piston system is the largest frictional loss source, which accounts for about 40 per cent of the total frictional loss in engine. In this paper, the reciprocating tribometer, which is updated, was used to evaluate the friction and wear performances. Design/methodology/approach – An alternate method is introduced to investigate the effect of reciprocating speed, normal load, oil pump speed and ring sample and oil temperature on friction coefficient with the ring/liner of a typical inline diesel engine. The orthogonal experiment is designed to identify the factors that dominate wear behavior. To understand the correlations between friction coefficients and wear well, different friction coefficient results were compared and explained by oil film build-up and asperity contact theory, such as the friction coefficient over a long period and averaged the friction coefficient over one revolution. Findings – The friction coefficient changes little but fluctuates with a small amplitude in the stable stage. The sudden change of frequency, load and stroke will lead to the oil film rupture. The identification for the factors that dominates the wear loss is ranged as F (ring sample) > , E (oil sample) > , B (stroke) > , D (temperature) > , A (load) > , G (liner) > and C (frequency). Originality/value – This paper develops and verifies a methodology capable of mimicking the real engine behavior at boundary and mixed lubrication regimes which can minimize frictional losses, wear, reduce much work for the experiment and reduce the cost. The originality of the work is well qualified, as very few papers on a similar analysis have been published, such as: The friction coefficient values fluctuating in the whole stage may be caused by the vibration of the system; suddenly, boundary alternation may help the oil film to form the lubrication; and weight loss mainly comes from the contribution of the friction coefficient value fluctuation. The paper also found that the statistics can gain more information from less experiment time based on a design of experiment.

A new asperity interaction model considering transient flash temperature and inhomogeneous distribution of oil viscosity

2016 ◽  
Vol 231 (3) ◽  
pp. 347-356
Author(s):  
Jun Liu ◽  
Zhinan Zhang ◽  
Bo Zhao ◽  
Youbai Xie
Keyword(s):  
Friction Coefficient ◽  
Boundary Lubrication ◽  
Interaction Model ◽  
Asperity Contact ◽  
Inhomogeneous Distribution ◽  
Sliding Velocity ◽  
Flash Temperature ◽  
Oil Viscosity ◽  
Temperature Characteristics ◽  
Simulation Results

A momentary contact of asperities will generate transient flash temperature phenomena in boundary lubrication. According to the viscosity–temperature characteristics of lubricant, the inhomogeneous distribution of temperature during the process of asperity contact would cause the inhomogeneous distribution of oil viscosity. This paper proposes a coupled Eulerian–Lagrangian based approach to analyze the influence of inhomogeneous distribution of viscosity on the friction coefficient of an asperity junction. The asperity interaction in boundary lubrication with different sliding velocities and different degrees of overlap of the undeformed surfaces were taken into account. Simulation results showed that the friction coefficient is proportional to the overlap and inversely proportional to sliding velocity. The results also showed that the influence of inhomogeneous distribution of oil viscosity on friction coefficient in boundary lubrication is limited.

Analysis of the Piston Ring/Liner Oil Film Development During Warm-Up for an SI-Engine

2000 ◽  
Vol 123 (1) ◽  
pp. 109-116 ◽  
Author(s):  
K. Froelund ◽  
J. Schramm ◽  
T. Tian ◽  
V. Wong ◽  
S. Hochgreb
Keyword(s):  
Piston Ring ◽  
Hydrodynamic Lubrication ◽  
Spark Ignition Engine ◽  
Asperity Contact ◽  
Oil Viscosity ◽  
Warm Up ◽  
Oil Film ◽  
Lubrication Regimes ◽  
Ring Pack ◽  
Film Development

A one-dimensional ring-pack lubrication model developed at MIT is applied to simulate the oil film behavior during the warm-up period of a Kohler spark ignition engine. This is done by making assumptions for the evolution of the oil temperatures during warm-up and that the oil control ring during downstrokes is fully flooded. The ring-pack lubrication model includes features such as three different lubrication regimes, i.e., pure hydrodynamic lubrication, boundary lubrication and pure asperity contact, nonsteady wetting of both inlet and outlet of the piston ring, capability to use all ring face profiles that can be approximated by piece-wise polynomials, and, finally, the ability to model the rheology of multigrade oils. Not surprisingly, the simulations show that by far the most important parameter is the temperature dependence of the oil viscosity.

Experimental Investigation of Lubrication Regimes of a Water-Lubricated Bearing in the Propulsion Train of a Marine Vessel

2020 ◽  
Vol 143 (4) ◽  
Author(s):  
Dov Avishai ◽  
Groper Morel
Keyword(s):  
Experimental Investigation ◽  
Friction Coefficient ◽  
Hydrodynamic Lubrication ◽  
Low Cost ◽  
Pollution Prevention ◽  
Elastohydrodynamic Lubrication ◽  
Sliding Bearings ◽  
Naval Vessel ◽  
Lubrication Regimes ◽  
Lubrication Regime

Abstract Sliding bearings, operating in a full hydrodynamic lubrication regime, exhibit a low friction coefficient and extended life. In recent years, with the increase in environmental awareness and pollution prevention, attention is being directed to oil spills, which pollute the environment. This is extremely prominent in ships and submarines whose propeller shafts are typically supported by oil-lubricated sliding bearings. To reduce pollution risk and also to obtain a simpler and low-cost maintenance system, the propeller shafts of numerous modern marine vessels are supported by water-lubricated bearings. An experimental investigation into the lubrication regime of a water-lubricated bearing in the propulsion train of a naval vessel is presented. A test rig was designed and built to allow testing of a scaled water-lubricated composite bearing supporting a naval vessel propeller shaft. Experimental results quantifying the effect of the rotational speed on the operating eccentricity, the friction coefficient, and the bearing’s lubrication regimes are presented. The experimentally obtained results are compared with an elastohydrodynamic lubrication (EHL) model solved by employing comsol multiphysics modeling software, and the differences are addressed. Finally, conclusions that may assist in better understanding the operation profile of the bearing and thus improving the vessel’s operability are presented.

Friction Reduction Opportunities in Combustion Engine Crank Train Bearings

2015 ◽  
Author(s):  
Rainer Aufischer ◽  
Rick Walker ◽  
Martin Offenbecher ◽  
Oliver Feng ◽  
Gunther Hager
Keyword(s):  
Friction Coefficient ◽  
Internal Friction ◽  
Combustion Engine ◽  
Operating Conditions ◽  
Friction Reduction ◽  
Viscosity Reduction ◽  
Oil Viscosity ◽  
Advantages And Disadvantages ◽  
Mixed Friction ◽  
Further Development

The development of combustion engines is heavily influenced by environmental regulations and efficiency. Since the environmental regulation have influenced engine design already with special combustion system and exhaust gas treatments, efficiency and the greenhouse gas CO2 has become a major issue for further development. CO2 emissions and fuel efficiency are linked and are directly influenced by the internal friction of the combustion engine. One major part of this internal friction is coming from the crank train bearings. Since we have to consider different operating conditions for the crank train bearings like hydrodynamic and mixed friction (hydrodynamic in combination with boundary contact), working principles as well as different engine operating conditions like full load, idle, start stop etc. different measures need to be employed for a friction reduced crank train. The optimal dimensioning of the bearings in combination with oil viscosity reduction are already known to a certain extent. Nevertheless they result in changes of bearing loads and may in consequence increase the share of boundary friction. Therefore, only looking on these two optimization steps is not enough. In addition the friction coefficient between bearing and shaft as well as the interaction between bearing surface and lubricant need to be addressed to reduce friction loss. In order to gain a complete picture, influences and the interaction of • geometric properties and bearing dimensions, • friction coefficient of bearings in combination with crankshaft materials, • oil formulation, viscosity and their interaction with engine application and duty cycle as well as • losses caused by the lubrication system design and components are investigated and analyzed based on simulation and testing. At first the different steps are investigated individually and secondly combinations and interactions are derived on basis of parameters derived on tribological tests and material data. Oil viscosity as major driver during hydrodynamic operation but also the influence of additive packages during mixed friction is roughly estimated on basis of tribological investigations. Since the overall friction system and its optimization are very complex, an example for a truck engine in different applications shows advantages and disadvantages of the different approaches. Also border lines given by operational risk and improvement limits are explained. The improvement options given by bearing materials and special coatings are explained in combination with different engines and engine applications. Further development activities, ways of collaboration between engine manufacturer and bearing supplier and an outlook on up-coming bearing system are completing the picture for a holistic approach on friction reduction in crank train bearings.

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