Friction In Mixed Lubrication Research Articles

Study on real-time thermal–mechanical–frictional coupling characteristics of ball bearings based on the inverse thermal network method

The real-time thermal–mechanical–frictional coupling characteristics of bearings are critical to the accuracy, reliability, and life of entire machines. To obtain the real-time dynamic characteristics of ball bearings, a novel model to calculate point contact dynamic friction in mixed lubrication was firstly presented in this work. The model of time-varying thermal contact resistance under fit between the ring and the ball, between the ring and the housing, and between the ring and the shaft was established using the fractal theory and the heat transfer theory. Furthermore, an inverse thermal network method with time-varying thermal contact resistance was presented. Using these models, the real-time thermal–mechanical–frictional coupling characteristics of ball bearings were obtained. The effectiveness of the presented models was verified by experiment and comparison.

Mixed lubrication friction model including surface texture effects for sheet metal forming

In deep drawing processes, mixed lubrication friction regime is common in which the friction condition is governed by solid–solid asperity contacts and lubricant pressure. In this study, a friction model in the mixed lubrication regime is developed that accounts for the effect of the surface topographies of sheet and tool on the lubricant pressure distribution. The overall friction due to solid–solid asperity contacts and lubricant pressure is determined using a coupled hydrodynamic and boundary friction models. The model is utilized in an in-house FE code (DiekA) for deep drawing simulations. In the FE simulations, the lubricant pressure is determined by solving the average Reynolds equation. The flow factors required in the average Reynolds equation are determined separately using measured tool and sheet surface topographies. Cross-die experiments are performed at different lubricant amounts to validate the friction model at a component level. The results show that punch force vs. displacement and strain field from experiments and FE simulations (using the new friction model) correlate very well.

Dynamic rolling force modeling of cold rolling strip based on mixed lubrication friction

Dynamic rolling force modeling of cold rolling strip based on mixed lubrication friction

An experimental framework for determining wear in porous journal bearings operated in the mixed lubrication regime

Abstract We present an experimental and analysis workflow to characterize the tribological behavior and the wear resistance of porous journal bearing systems operating at high loads and small rotational speeds. Our approach consists of a laser-instrumented tribometer that allows parallel testing of five bearings, an experimental procedure that is optimized for producing sufficient wear during mixed lubrication operation while maintaining realistic operating conditions, as well as several methods to visualize and quantify bearing wear. The simultaneous testing of five bearings prevents outliers from distorting the results and yields a statistical estimation of the performance variations between nominally equivalent tribosystems. We showcase our approach by analyzing the influence of the bearing material and its porosity on mixed-lubrication friction and wear.

Texture-induced effects causing reduction of friction in mixed lubrication for twin land oil control rings

Piston rings are responsible for a major part of the frictional losses in a heavy-duty diesel engine. Such losses can be reduced by applying texture, such as dimples, on the cylinder liner surface. This paper investigates the effect of such texture on the friction between a land of the oil control ring and a textured cylinder liner via numerical simulation. A simulation model considering inertia and mixed lubrication together with a mass-conserving cavitation model is developed. The model is used to determine the dimple parameters that yield the lowest amount of friction for a specific oil control ring of a heavy-duty diesel engine.

Influence of surface form deviations on friction in mixed lubrication

Abstract The focus of this study is on the effect of form deviations of a partial journal bearing's sliding surface on the friction force in mixed lubrication conditions. The measured friction varied considerably when the bearing was rotated in the opposite direction at the same speed and normal load. This unexpected observation motivated the presented simulation study. The overall form deviations of the surface of the test bearing were measured with white light interferometry and used as simulation input. The two-scale simulation approach considers the effect of the surface roughness on a microscopic and the global bearing geometry with the surface form deviations on a macroscopic scale. Simulation results show that the surface form deviations can have a noticeable effect on the pressure distribution of the lubricant and hence on the size of the asperity contact area which leads to the differences in friction. The influence of the lubricant viscosity, the clearance gap and the surface roughness were analysed in a parametric simulation study. Results show that the clearance and the surface roughness control the impact of the surface form deviations on the friction force. All in all, the results show the necessity to include all scales of surface form deviations in the simulation of journal bearings in a mixed lubrication regime.

Transition Between Mixed Lubrication and Elastohydrodynamic Lubrication with Randomly Rough Surfaces

We are interested in understanding the effect of roughness on friction in mixed lubrication (ML) and elastohydrodynamic lubrication (EHL), notably in the case of realistic surfaces. Rolling–sliding experiments in piezoviscous base oil are conducted between rough steels obtained with different machining, polishing and coating processes in order to represent realistic surface conditions found typically in a car engine. The Reynolds number based on the maximum lubricant film thickness in our experiments is below \(R_{e}=4\cdot 10^{-3}\). Our experiments at moderate pressure (\({<}1\) GPa) and speeds (mm/s \(\rightarrow\) m/s) allow to reach EHL for all surface roughnesses which range from the nanometre up to the micron. These surfaces have a complex and multi-scale texture and require a statistical description. Using the most raw and simple filter on large topographic measurements, we correlate the most probable standard deviation of surfaces to friction during Stribeck experiments occuring both in ML and EHL without measurable wear. In order to predict the conditions leading to high friction in ML, we propose a friction-based definition of ML and describe the role played by pressure and roughness on the onset of ML.

Multi-scale friction modeling for sheet metal forming: The mixed lubrication regime

A mixed lubrication friction model is presented to accurately account for friction in sheet metal forming FE simulations. The advanced friction model comprises a coupling between a hydrodynamic friction model and a boundary lubrication friction model, based on the lubricant film thickness. Mixed lubrication interface elements are introduced to solve the governing differential equations. The interface elements have been implemented in FE forming software. Two deep-drawing applications are discussed to demonstrate the performance of the friction model. Results show friction coefficients that vary in space and time, and depend on external process settings like the amount and type of lubricant. A comparison with experimentally obtained punch-force displacement diagrams is made to prove the enhanced predictive capabilities of FE forming simulations.

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

Improving knowledge of the tribological behavior of gear oils under different operating conditions is essential to be able to design efficient lubricants for gear contacts. This article examines how surface roughness influences the friction properties of gear oils in boundary-lubricated steel–steel contacts at different temperatures and contact pressures. Stribeck curves were generated for smoother and rougher substrates. Friction in the boundary lubrication regime from Stribeck curves and the evolution of friction during prolonged rubbing were used to evaluate the abilities of additives to adsorb on steel surfaces, form tribofilms, and withstand severe operating conditions. In general, smoother substrates produce lower friction but higher wear than rougher substrates for the oils and operating conditions investigated in this study. Surface roughness has significant effects on additive adsorption and tribofilm formation at low contact pressures and negligible effects at high contact pressures. In addition, whether or not other surface-active additives present in gear oils affect the typical low friction response of friction modifiers (FMs) was investigated. The results show that severe operating conditions (pure sliding and high contact pressure) could cause disruption of FM films with some gear oils and hence do not reduce friction. In a forthcoming companion article, Part II, these Stribeck curves are used to understand the response of different gear oils in mixed lubrication with varying substrate roughnesses and to understand how tribofilms influence friction in mixed lubrication by evaluating tribofilm-generated surfaces with base oil (Vengudusamy, et al. (1)).

A low-viscosity ionic liquid demonstrating superior lubricating performance from mixed to boundary lubrication

This study reports on an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as a promising engine lubricant candidate . This IL has a relatively low viscosity of 4.7cSt (similar to 0W-10 racing engine oil), a low viscosity–pressure (V–P) coefficient of 6.5GPa−1 at 100°C, a high viscosity index of 159, and a high onset decomposition temperature of 472°C. The ionic liquid delivers substantially lower friction in mixed lubrication (ML) and provides better scuffing protection in boundary lubrication (BL) than those of hydrocarbon lubricants with similar viscosities. The low ML friction is possibly attributed to its low V–P coefficient and a hypothetical multi-layer boundary film at the interface. In BL, the tribochemical reactions between the IL and the contact surfaces to form a protective tribo-film are believed to be responsible for the IL's superior anti-scuffing behavior. Microstructural examination and chemical composition analysis were conducted on the wear scars from both the top surface and the cross section to study the wear mode and characterize the tribo-film and near-surface structure.

Effect of water spray on friction and wear behaviour of noncommercial and commercial brake pad materials

The reduction in the friction coefficient to unacceptable levels for frictional brake pad applications in a rainy and humidly environment is considered as a serious problem as it influences the safety of the vehicle. Thus, in the present work, particular emphasize were given to the effect of water spray on the level and behaviour of friction and wear characteristics of frictional brake pad (FBP) materials. Four different non-commercial FBP materials (NF1, NF2, NF4, and NF5) were developed and evaluated along with other two chosen commercial FBP materials (CMA and CMB) using a small-scale tribo-tester of pad-on-disc type. The results confirmed that in spite of spraying water to the disc, no evidence of HD water film could be observed. Hence the friction behaviour was influenced by factors other than HD film. The values of friction coefficient obtained under wet condition were in the range of dry friction, mixed and boundary lubrication friction. For instant, some values of friction coefficient for NF1 &NF4 were less than 0.05 which was in the range of friction in mixed lubrication. Regardless of the type of brake pad materials, the results indicated that spraying water eliminates the establishment of the transfer layer at the friction interface and suppresses the temperature rise in the disc and so the formation of the char or other reaction products such as oxides. Besides, wear rate of all FBP materials behaved linearly with contact pressure. Qualitative assessment of the SEM morphologies of brake pad surfaces confirmed that tribofilms were hardly formed in wet braking. In addition, all brake pad surfaces showed formation of contact plateaus “patches” and disintegrations of various sizes and locations depending on the braking condition. Furthermore, the removal of material was associated with either mechanical crushing action performed by entrapped wear debris or due to disintegration of plateaus which were accelerated by spraying the water.

Friction Reduction in Mixed Lubrication

Minimization of frictional losses in the drivetrain of heavy-duty vehicles is important from both consumer satisfaction and environmental perspectives. Approaches to friction reduction in these components can be evaluated using simulation-based investigations. However, nearly all drivetrain components operate in the mixed lubrication regime which is difficult to model because both hydrodynamic lubrication and surface contact are significant and therefore, the total friction consists of hydrodynamic friction due to lubricant shearing and boundary film friction at asperity contact locations. Recent advances in simulation methods for mixed elastohydrodynamic lubrication (EHL) have enabled improved virtual design tools, such as those developed by Zhu and Hu and further improved by Liu et al. Here, these simulation tools are used to evaluate friction reduction and predict the effects on a mixed EHL interface under severe operating conditions. Three practical means of friction reduction are discussed based on the experimentally validated mixed lubrication friction model and its predictions made for representative, sample cases.

Origins of the friction and wear properties of antiwear additives

Experimental techniques have been developed to measure the friction, antiwear film-forming and wear properties of lubricants in rolling–sliding contact. Friction measurements show that zinc dialkyldithiophosphates (ZDDPs) and also some other phosphorus-based additives increase friction in mixed lubrication. Film thickness measurements show that this increase in friction correlates with the thickness of antiwear film. They also reveal some of the drivers of antiwear film formation and removal. A novel wear tester is described which enables the mild wear resulting from ZDDP-containing oils to be monitored. Copyright © 2006 John Wiley & Sons, Ltd.

A frictional study of uncoated EBT steel sheets in a bending under tension friction test

A factorial design was used to investigate the frictional response of different uncoated EBT surface topographies. The test included three different topographies (Sibetex® materials), two lubricants, two contact pressures, and two sliding speeds. The steel sheet strips were subjected to a Bending Under Tension (BUT) friction test. Our findings indicate that the most significant factor, affecting friction, is lubricant viscosity, followed by sliding speed, contact pressure, and surface topography. For measurement of the sheet surface topography, a 3D stylus-instrument was used and the ball-filtering technique was applied. The surface topography was measured in the contact zone on the test samples. The results show that the surface topography is a significant factor, influencing friction in mixed lubrication. The larger the number of isolated lubricant pockets, the smaller is the friction coefficient. These isolated pockets identified here are located in the topmost area and are imprints from the rolls before the first textured rolls.