Mixed Lubrication Regime Research Articles

Friction Induced Vibration in Windscreen Wiper Contacts

This research is aimed at understanding the mechanisms that give rise to friction induced noise in automotive windscreen wipers, with a focus on frequencies between 500 and 3500 Hz. To study this phenomenon, experimental friction, sound, and high-speed video measurements are combined with finite element modeling of a rubber wiper/glass contact. In agreement with previous research, simultaneous sound and friction measurements showed that wiper noise in this frequency range results from the negative damping effect caused by the dependence of friction on speed in the mixed lubrication regime. Furthermore, during sliding, the friction induced noise recorded by the microphone occurred in one of two frequency ranges (close to 1000 Hz and between 2000 and 2500 Hz). These coincided closely with the eigen-frequencies of first two bending modes, predicted by finite element modeling. Experimental observations also showed the wiper to be oscillating backward and forward without any torsional motion and that the thickness of the glass had no effect on the emitted noise. These observations highlight how friction induced noise—although caused by conditions within contact—has characteristics that are determined by the structure of the excited component. A number of additional findings are made. Most importantly, both experiment and finite element modeling showed that the presence of water in contact with the wiper modulates the frequency and amplitude of the emitted noise by effectively adding mass to the vibrating system. While this is occurring, Faraday-like standing waves are observed in the water. In addition to this, friction induced vibration is shown only to occur for glass surfaces with intermediate surface energies, which is possibly due to high contact angles preventing water reaching the contact. Based on the understanding gained, a number of suggestions are made regarding means of reducing windscreen wiper noise.

Research of Lubrication Model and Surface Quality in Cold-Rolling Copper Alloy Using O/W Emulsions

Abstract A theoretical model for cold-rolling copper alloy in mixed lubrication regime was developed based on the average volume flow model and asperity flattening model. The developed scheme was able to calculate oil concentration λ d at any point within work zone, lubricant pressure, contact area ratio and film thickness ratio with different oil viscosity, pass reduction and the initial oil concentration λ ds . According to the lubrication model, for getting good lubrication and cooling effect of emulsions in rolling process, the rolling oil viscosity was selected in the range from 40 mm 2 /s to 50 mm 2 /s, the pass reduction about 30% and the initial oil concentration λ ds of emulsion in the range from 0.02 to 0.05. For verifying the lubrication model, the last nonseizure load was tested by the four-ball tests to confirm oil viscosity and initial oil concentration, and the rolled copper alloy surface topographies were analyzed to confirm pass reduction. The results show that the calculation data agree well with the measured data from experiment. They provide technical references for selecting rolling process parameters and emulsions.

A Mixed Thermoelastohydrodynamic Lubrication Analysis of Mechanical Face Seals by a Multiscale Approach

This article presents an improvement to a previous multiscale approach used to model the mixed lubrication regime in a mechanical face seal. The physical mechanisms considered by the improved model are the surface roughness effects on the fluid film lubrication, the thermal deformations, and the heat transfer in the seal rings due to viscous and dry friction. The developed numerical model determines the pressure distribution by taking into account the effect of cavitation and contact asperity between the surfaces. Heat dissipation, heat transfer, and deformations are computed from the heat dissipated at the seal interface by a finite element technique. The multiscale model significantly reduces computation time while maintaining the accuracy of the results. The results obtained through a parametric study show that there are different operating zones where the lubricating film thickness is controlled by the roughness height or by the thermal effect.

Tribological performance of tin-based overlay plated engine bearing materials

A block-on-ring test setup is employed to investigate tribological performance of Sn-based overlay plated bearing materials under mixed and boundary lubrication condition. Pb-containing bearing material is also studied as a reference. Sn-based and Pb-based overlays have shown similar transition in friction when rotational speed is varied. Under relatively longer test duration, Sn-based overlays exhibit comparable friction and wear properties with that of Pb-based overlay. It takes longer time to obtain steady-state friction for Sn-based overlay than Pb-based overlay. Wear behavior of tested samples are also similar except for tests in the mixed lubrication regime where Sn-based overlays show better wear resistance. In the Sn-based overlays main wear mechanisms are adhesive and abrasive wear leading to exposure of the Ni interlayer.

Friction and wear characteristics of different Pb-free bearing materials in mixed and boundary lubrication regimes

Traditional bearing materials contain different amounts of lead (Pb) because of its friction reducing properties. However, in view of the negative health and environmental impact of Pb, there is growing emphasis on restricting the usage of Pb in engine bearings. Owing to this, new bearing materials that provide at least comparable tribological performance to that of Pb containing alloys are being developed and some new Pb-free materials are being already used in engine bearing applications. It is, however, still unclear how these new engine bearing materials would perform in mixed and boundary lubricated conditions. In this study, a block-on-ring test setup was employed to investigate the tribological performance of several bi-metal and multi-layer Pb-free bearing materials with different compositions of bearing lining and overlay plating. Pb-containing bearing material was also studied as a reference material. Friction and wear properties of these bearing materials were investigated and their wear mechanisms under lubricated conditions have been analysed. Bearing material with Polyamide-Imide based overlay containing graphite and MoS2 exhibited better friction and wear properties than Pb-based and Al–Sn based materials. Pb-containing bearing material shows higher wear of material and Al–Sn based material has shown higher friction compared with the other test materials.

Specific dissipated friction power distributions of machined carburized martensitic steel surfaces during running-in

Due to new design specifications, current tribological systems are subjected to numerous modifications of their operating conditions such as lubrication regime, higher contact pressures, and longer operating times at lower maintenance costs. Thus the mixed lubrication regime might become most abundant. In order to still maintain ultra-mild sliding wear an approach is needed, which combines the contact conditions, surface topography, and near surface properties. One approach is to shorten the running-in phase e.g. by choosing well-aimed topographical parameters for optimized incipient contact conditions. In this study milled, ground, milled and finished, and polished samples were tested in a self-mating reciprocating ball-on-plane sliding wear tests while the tangential as well as the normal forces were measured simultaneously. After predefined test cycles 3D surface topographies as well as the wear rates were measured by means of confocal white light microscopy (CWLM). From all these data the specific dissipated friction power (SDFP), the contact temperatures are computed by means of a three dimensional elastic – ideal plastic contact model as well as the lubrication characteristics.This contribution shows the SDFP and relates it to the incipient topographies as well as their alterations during run-in. Considering the SDFP being distributed over the sliding domain the affected area (≈real contact area) can be evaluated as well. Now the SDFP can be related to the affected area and the marked differences of the running-in behavior even under ultra-mild sliding wear conditions become visible.

A mixed lubrication model for studying tribological behaviors of surface texturing

An analytical numerical model is specifically developed to understand the tribological behavior of textured surface in mixed lubrication regime. The model employs the average flow Reynolds equation and an elasto-plastic contact model, established on the basis of finite element analysis, to calculate (i) the hydrodynamic pressure in full film region and (ii) the contact pressure in contact region, respectively. Fast Fourier transform (FFT) method is used to compute the elastic deformation of substrate. It is able to simulate Stribeck curve for the textured surface. Predictions demonstrate surface texturing reducing friction coefficient. Studies on the tribological influence of texture shape, depth, distance and diameter are also performed.

Tribological behavior of PEEK-based materials under mixed and boundary lubrication conditions

In modern industries, more and more mechanical components are exposed to mixed and even boundary lubrication conditions, inducing fast wear and even scuffing of the motion systems. In order to enhance the lifetime and reliability of the motion systems, replacing metal–metal friction pairs by metal–polymer ones can be one of the most effective approaches. The present work focuses on tribological behavior of pure polyetheretherketone (PEEK) and a formulated PEEK composite lubricated with diesel and engine oil. It was demonstrated that in mixed and boundary lubrication regimes the structure of PEEK materials affect significantly the tribological performance. Formation of a tribofilm on the surface of metallic counterbody plays an important role on the tribological behavior of the PEEK-based materials.

Wear predictions for cams in line contacts based on multidisciplinary simulation

Purpose – This paper aims to present the model and method involving multi-body system dynamic analysis, finite element quasi-statics contact analysis and numerical calculation of elastohydrodynamic lubrication (EHL), according to the cam wear prediction using Archard’s model. Cam–follower kinematic pairs always work under wear because of concentrated contacts. Given that a cam and follower contact often operates in the mixed or boundary lubrication regime, simulation of cam wear is a multidisciplinary problem including kinematic considerations, dynamic load and stress calculations and elastohydrodynamic film thickness evaluations. Design/methodology/approach – Multi-body system dynamic analysis, finite element quasi-statics contact analysis and numerical calculation of EHL are applied to obtain the dynamic loads, the time histories of contact pressure and the oil film thicknesses in cam–follower conjunctions to predict cam wear quantitatively. Findings – The wear depth of the cam in the valve train of a heavy-load diesel engine is calculated, which is in good agreement with the measured value in the practical test. The results show that the cam–tappet pair operates under a mixed lubrication or boundary lubrication, and the wear depths on both sides of the cam nose are extremely great. The wear of these points can be decreased significantly by modifying the local cam profile to enlarge the radii of curvature. Originality/value – The main value of this work lies in the model and method involving multi-body system dynamic analysis, finite element quasi-statics contact analysis and numerical calculation of EHL, which can give good prediction for the wear of cam.

Ultrasmooth submicrometer carbon spheres as lubricant additives for friction and wear reduction.

Ultrasmooth submicrometer carbon spheres are demonstrated as an efficient additive for improving the tribological performance of lubricating oils. Carbon spheres with ultrasmooth surfaces are fabricated by ultrasound assisted polymerization of resorcinol and formaldehyde followed by controlled heat treatment. The tribological behavior of the new lubricant mixture is investigated in the boundary and mixed lubrication regimes using a pin-on-disk apparatus and cylinder-on-disk tribometer, respectively. The new lubricant composition containing 3 wt % carbon spheres suspended in a reference SAE 5W30 engine oil exhibited a substantial reduction in friction and wear (10-25%) compared to the neat oil, without change in the viscosity. Microscopic and spectroscopic investigation of the carbon spheres after the tribological experiments illustrated their excellent mechanical and chemical stability. The significantly better tribological performance of the hybrid lubricant is attributed to the perfectly spherical shape and ultrasmooth surface of carbon sphere additive filling the gap between surfaces and acting as a nanoscale ball bearing.

Roller bearing dynamics under transient thermal-mixed non-Newtonian elastohydrodynamic regime of lubrication

The paper describes a combined tribodynamics analysis (dynamics and contact tribology) of cylindrical roller bearings of a heavy duty truck transmission under high applied loads. The dynamic analysis provides the transient variations in contact load. It also determines the vibration spectrum of the bearing as well as that of contact dynamics. It is shown that with sufficient preloading and/or interference fitting a widely spread loaded region results, which reduces bearing-induced vibration. The transient tribological analysis, including thermal analysis with a novel and realistic lubricant inlet boundary condition demonstrates that non-Newtonian mixed elastohydrodynamic regime of lubrication is prevalent, but with reduced friction compared with unrealistic dry Coulombic friction, which is often assumed in literature.

A Parametric Study on the Sub-Surface Stresses in Spur Gears under Mixed-lubrication Regime using Load-Sharing and Finite Element Simulation

Gears as key elements in power transmission systems are widely used in industry. Based on the manufacturing process, the surface of gears compared to other mechanical components such as roller bearings might be rougher and thus the effect of surface roughness in their performance should be given full consideration. One of the most important parameters in the gears' performance is the maximum shear stress that occurs below the surface, which is believed to be the key factor in surface fatigue and pitting. In this research, a model that employs the load-sharing concept to predict the friction coefficient and film thickness for each point along the line of action is developed. The sub-surface stress field that is generated below the tooth surface is calculated based on the friction coefficient. The predicted sub-surface stresses are compared to the results obtained from the commercial finite element software ABAQUS. An acceptable agreement is observed in comparing the results from the two methods. Finally, a parametric study has been conducted to study the effect of load, velocity, viscosity and surface hardness on the calculated maximum sub-surface shear stress and its location.

Effect of cylinder liner wear on the mixed lubrication regime of TLOCRs

A full-scale mixed lubrication regime model was applied to the contact between a parallel, flat rigid surface and the honed cylinder liner topography of internal combustion engines. The hydrodynamic and asperity contact effects are taken into account simultaneously in the same solution framework. The hydrodynamic phenomena are described in terms of the Reynolds equation and the fluid-film cavitation is modelled by using the mass-conserving p−θ Elrod–Adams approach. The modified Reynolds equation is solved numerically adopting a robust extended Successive-Over-Relaxation (SOR) algorithm based on a finite volume discretisation method. The eventual asperity interactions are determined according to the Hertzian contact theory with maximum elastic pressure limited by the hardness of the softer material. Numerical simulations were carried out in order to investigate the hydrodynamic load carrying capacity and the associated contact pressures produced for a Twin Land Oil Control Ring (TLOCR) sliding against a honed bore. Actual 3D roughness measurements taken at the same point position of the honed liner surface before and after a 100h dynamometer engine test were used as case study. The ring face was assumed unchanged to focus only on the bore surface modifications. Significant differences were observed in the magnitude of the average hydrodynamic and asperity contact pressures, as well as in the hydrodynamic shear stresses responsible for the viscous dissipations. The asperity contact pressures decreased on the worn surface due to the reduction in the asperity interactions caused by the removal of the highest summits due to wear. On the other hand, the smoothing of the cylinder liner promoted the raising in the hydrodynamic pressures and shear stresses.

Estimation of coefficient of friction for a mechanical system with combined rolling–sliding contact using vibration measurements

A new dynamic experiment is proposed to estimate the coefficient of friction for a mechanical system with a combined rolling–sliding contact under a mixed lubrication regime. The experiment is designed and instrumented based on an analogous contact mechanics model, taking into consideration the constraints to ensure no impact and no sliding velocity reversal. The system consists of a cam (rotating with a constant speed) having a point contact with a follower that oscillates about a frictionless pivot, while maintaining contact with the cam with the help of a well-designed translational spring. The viscous damping elements for contact are identified for two different lubricants from an impulse test using the half-power bandwidth method. Dynamic responses (with the cam providing an input to the system) are measured in terms of the follower acceleration and the reaction forces at the follower pivot. A frequency domain based signal processing technique is proposed to estimate the coefficient of friction using the complex-valued Fourier amplitudes of the measured forces and acceleration. The coefficient of friction is estimated for the mechanical system with different surface roughnesses using two lubricants; these are also compared with similar values for both dry and lubricated cases as reported in the literature. An empirical relationship for the coefficient of friction is suggested based on a prior model under a mixed lubrication regime. Possible sources of errors in the estimation procedure are identified and quantified.

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.

A study on the effect of starvation in mixed elastohydrodynamic lubrication

The effect of starvation in mixed elastohydrodynamic lubrication (EHL) regime is studied. Numerical simulations are conducted for both line and point (elliptical) contacts with the consideration of the surface roughness. The degree of starvation is linked directly to the reduction in the lubricant mass flow rate. Results are presented to gain insight on the influence of starvation on the film thickness as well as the interaction between the surface asperities. Extensive sets of simulation results are used to quantify the effect of starvation in the EHL of rough surfaces. Expressions are developed to predict the percentage of the load carried by the surface asperities (asperity load ratio) as well as the reduction of the central and minimum film thickness in the starved mixed EHL.

Tribocorrosion of passive metals in the mixed lubrication regime: theoretical model and application to metal-on-metal artificial hip joints

Wear is a key problem limiting the lifetime and performance of total hip arthroplasty. The synovial fluid, acting as lubricant, can alleviate the wear of implants but also introduces corrosion. The interaction of mechanical wear and electrochemical corrosion, nominally tribocorrosion, has been proposed as one of the crucial degradation mechanisms of implants and recently saw significant progress in mechanistic understanding and modeling. This study presents a predictive wear model based on a mechanistic approach of tribocorrosion and lubrication and taking into account clinical relevant parameters such as normal load, velocity and clearance. Predicted wear rate corresponds well to the running-in wear rate from simulator experimental results and allows to identify the dominating wear mechanism and to evaluate the influence of several parameters.

Smoothness and plateauness contributions to the running-in friction and wear of stratified helical slide and plateau honed cylinder liners

Cylinder liner surface has a great influence on frictional and wear performances of combustion engines during the running-in period. Two surface texture anisotropies produced by plateau honing (PH) and helical slide honing (HSH) processes (which consist of 50° and 130° cross-hatched grooves, respectively) are commonly used in automotive industry for thermal combustion engine cylinder liners. They are generated by a three stages process. The first stage, rough honing, removes enough material to obtain the desired cylindricity. The second step, finish honing, generates the honed texture which consists of grooves with a specific cross-hatch angle. The third stage permits to reduce the surface peaks and therefore allows varying plateau superficial roughness amplitude.This paper is devoted to studying the influence of respectively smoothness and plateauness on honed surface wear and friction performances during running-in. For that, HSH and PH textures are generated using different final honing stage durations in order to obtain different levels of surface peak clipping. Then, friction, wear and surface topography evolution were analyzed during running-in tests on a reciprocating ring-liner tribometer under mixed lubrication regime. The results show that the superficial surface roughness generated by helical slide honing has a very low contribution into friction. This is promising for the honing process optimization, in which the third stage can be significantly reduced or avoided.

Hydrodynamic Performance Produced by Nanotexturing in Submicrometer Clearance With Surface Roughness

Surface texturing is a promising way to expand the hydrodynamic lubrication regime and thereby modify the tribological properties of sliding surfaces. Spiral-groove textures in particular have attracted much attention over the past several decades because they produce a thicker lubrication film in the gap. However, no research has been reported on the effect of periodic texturing with a several 100 nm depth on hydrodynamic performance in submicrometer clearance with surface roughness. The purpose of the study reported here was to investigate the effect of such nanotexturing on hydrodynamic performance. This was done by conducting ring-on-disk friction tests, focusing on the existence of surface roughness in the narrow clearance. The samples were rings with various degrees of surface roughness and disks with spiral-groove textures produced by femtosecond laser processing. The friction coefficients experimentally obtained were plotted as a Stribeck curve and compared with a theoretical one calculated using a Reynolds equation formulated from two physical models, the Patir–Cheng average flow model and a sinusoidal wave model. The results showed that surface roughness did not affect the friction coefficient in the hydrodynamic lubrication regime. However, the hydrodynamic lubrication regime gradually shrank with an increase in surface roughness, and mild transitions to the mixed lubrication regime were observed at higher rotational speeds. The minimum clearances reached at the transition speed were almost the same, about 200–300 nm, for all experiments regardless of surface roughness.

Preparation of the electroless Ni–P and Ni–Cu–P coatings on engine cylinder and their tribological behaviors under bio-oil lubricated conditions

Two electroless Ni–P and Ni–Cu–P coatings on engine cylinder were prepared with chemical plating nickel technology. The coatings were characterized by Energy Dispersive X-ray spectrometer (EDX), Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD). The friction and wear behaviors of cylinders with these two coatings were compared with those of the cylinder without coating under bio-oil lubricated conditions. It was found that the cylinders after coating had significant improvement of tribological properties; in addition, the Ni–Cu–P coating had higher wear resistance and lower friction coefficient than those of Ni–P coating. Under the bio-oil lubricated conditions, the evolution of friction coefficient with sliding speed agreed with the classical Stribeck curves, namely, there was a transition from boundary lubrication (BL) to mixed lubrication (ML) to hydrodynamic lubrication (HL) as sliding speed increased. The lower friction coefficient in ML regimes than that in BL regimes depended on both of the adsorbed oil film and the tribo-film on the worn surfaces. The better tribological performances of Ni–Cu–P coating than those of Ni–P coating were ascribed to the formation of a tribo-film with CuO. Therefore, the Ni–Cu–P coating showed a very potential for preventing engine cylinders from the corrosiveness of bio-oil.