Investigation of the strain distribution with lubrication during the deep drawing process

A lubrication/friction model can be implemented in FEM codes to predict the contact area ratio, friction coefficient and strain distribution in lubricated deep drawing process. In the lubrication analysis, the surface roughness effect on lubrication flow is included by using Wilson and Marsault's average Reynolds equation that is appropriated for mixed lubrication with severe asperity contact. With regard to the asperity contact theory, the well-known flattening effect is considered. Friction is expressed in terms of variables such as lubricant film thickness, sheet roughness, lubricant viscosity, interface pressure, sliding speed, and strain rate. The proposed lubrication/friction model combined with a finite element code of deep drawing process to predict the contact area ratio, friction coefficient and strain distribution. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

Roughness and Lubricant Effect on 3D Atomic Asperity Contact

Large-scale molecular dynamics simulations were performed to study the sliding process of rough surfaces with and without lubricant. In the dry contact, a linear relationship has been observed between the load and the contact area for surfaces with large root mean square (RMS) roughness. However, it becomes nonlinear when the RMS is small. In the presence of adhesion, small roughness results in a large friction force when the surfaces are flattened and the contact area reaches 60 %. In order to confirm this observation, nonadhesive models have been established with an observation that under the combined influence from roughness and adhesion, the contact area plays a crucial role to determine whether the dry sliding is under the domination of roughness or adhesion. In the lubricated sliding, an increase in friction force has been found for the partially lubricated condition because the asperity contact still accounts for a great deal of resisting force. Besides, the lubricant exerts a comparable resisting force to the sliding.     

An Interfacial Friction Law for a Circular EHL Contact Under Free Sliding Oscillating Motion

The friction response of a lubricated interface under free sliding oscillating motion is investigated as a function of the contact pressure and the rheology of the lubricant in terms of viscosity and piezoviscosity. For loaded contacts, both velocity dependent friction, referred to as viscous damping, and friction independent of the instantaneous sliding velocity contribute to the energy dissipation. Viscous damping mainly corresponds to the dissipation in the lubricant meniscus surrounding the contact, while dissipation within the confined lubricated interface is mainly independent of the instantaneous sliding velocity. The friction coefficient independent of the instantaneous sliding velocity falls on a master curve for the wide range of tested operating conditions and lubricant rheological properties. The master curve is a logarithmic function of a dimensionless parameter corresponding to the ratio of the viscosity of the confined lubricant to the product of the pressure and a characteristic time. The physical meaning of this latter and the friction law are discussed considering the confined interface as a viscoelastic fluid or a non-Newtonian Eyring fluid.     

Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Abstract

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.     

表面粗糙度对滑动电接触磨损率的影响

在电气化铁路弓网系统中,磨损率是衡量列车运行状态与接触导线使用状态的重要指标。为了充分模拟弓网系统中磨损率情况,利用自行搭建的滑动电接触摩擦磨损试验机对滑板和接触导线进行摩擦磨损试验,分析滑板表面粗糙度、法向压力、接触电流与运行速度对磨损率的影响。得出结论:滑板磨损率随滑板初始表面粗糙度、接触电流、法向压力、运行速度的增加而增加,而高载荷下粗糙度对于磨损率的影响降低;滑板摩擦从磨合期进入稳定摩擦期存在一个临界表面粗糙度,当滑板初始表面粗糙度值等于临界粗糙度值时,其磨损率最低;不同初始表面粗糙度的滑板在跑合期内磨损过程不同,在稳定摩擦期内磨损过程趋于一致,且摩擦试验后滑板表面粗糙度也接近。     

A numerical model for mixed lubrication taking into account surface topography,tangential adhesion effects and plastic deformations

The present paper focuses on numerical investigations of mixed lubrication phenomena. Generally minimized for sliding systems, this type of lubricated contact is essential to ensure the original basic function of a lubricated friction system. As analyses of such a contact are hardly possible with experimental facilities, the finite element method was chosen to model the occurring friction phenomena at the micro-scale. A three-dimensional model is implemented considering two lubricated real rough bodies, Bowden and Tabor adhesion model and heat generation occurring in the solid–solid contact. Present analysis gives an overview on how surface roughness and machining direction influence the friction behavior as well as the contact and fluid pressure fields in mixed lubricated systems. First results are in accordance with the literature but therefore require the use of a statistical approach to deliver quantitative results in adequation with real systems.     

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACT

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.     

The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.     

Sliding wear and friction of electroplated and clad connector contact materials: effect of surface roughness

A modeling study with bench apparatus was conducted with connector contact materials consisting of electroplated hard gold and gold flashed palladium on nickel underplatings mated to a clad noble metal. The clad metal contact was the ‘rider’, i.e., had the smaller surface involved in sliding compared to the plated ‘flat’ surface. This configuration is consistent with good engineering practice in commercial products. It was found that although a lubricant could reduce friction significantly and virtually eliminate adhesive transfer of metal, a requirement for negligible wear, i.e., the absence of loose particles, was that the plated surface be very smooth. If this contact was rough, such as might occur due to it having a nodular nickel underplate, tool marks, or burrsm abrasive wear of the cladding occurred in proportion to the magnitude of roughness. However, when both contacts were plated, lubricated wear was low and surface roughness was not a significant factor. Unlubricated plated contacts were severely with high friction by the prow formation adhesive wear mechanism regardless of their surface roughness.     

Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate

Surface texture of harder mating surfaces plays an important role during sliding against softer materials and hence the importance of characterizing the surfaces in terms of roughness parameters. In the present investigation, basic studies were conducted using inclined pin-on-plate sliding tester to understand the surface texture effect of hard surfaces on coefficient of friction and transfer layer formation. A tribological couple made of a super purity aluminium pin against steel plate was used in the tests. Two surface parameters of steel plates, namely roughness and texture, were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture and are independent of surface roughness (R_a). Among the various surface roughness parameters, the average or the mean slope of the profile was found to explain the variations best. Under lubricated conditions, stick–slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick–slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.     

Textured surfaces in sliding boundary lubricated contacts – mechanisms,possibilities and limitations

Abstract

The mechanisms of friction and wear in boundary lubrication are complex with influences from the surface roughness and hardness of surfaces, the lubricant and the wear products. Introduction of a texture on either surface can influence several important parameters. Wear particles can be collected or produced by the surface texture. A lubricating film can suffer or gain and the lubrication regime might change. This paper presents an overview of the tribological effects and important parameters of textured surfaces in sliding boundary lubricated contact, based on the experience of the authors and on published results. Examples of successful and less successful textured contacts are given and some recommendations regarding size, orientation and textured area fraction are presented.     

Influence of testing method on the contact pressure distribution and its effect on coefficient of friction in polymeric bearings

Influence of contact pressure distribution on the coefficient of friction was investigated for some polymeric bearings in dry and lubricated conditions. Reciprocal pin on plate sliding test and Soda pendulum type sliding tests were carried out. It was found that the contact pressure distribution had a significant effect on the coefficient of friction in lubricated condition. The coefficient of friction became smaller when the pin was sliding with the lower contact pressure side ahead. The coefficient of friction in dry condition does not depend on sliding direction regardless of contact pressure distribution. Furthermore a parametric equation was derived to calculate contact pressure in polymeric journal bearings. The accuracy of the equation was confirmed by Finite Element Method (FEM).     

Transient thermo‐elastohydrodynamic lubrication of gear teeth

During a gear mesh cycle, load, rolling and sliding velocities, curvature, and temperature change rapidly. In this paper, a transient, thermo‐elastohydrodynamic lubrication model is presented that has been used to study the lubrication parameters at 250 contact points along the path of a contact. The working flanks were assumed to be smooth. The line load was calculated at every contact point, using the finite‐element method. Dynamic loading was not considered. The steady‐state temperature field of the working flanks was used to obtain the surface temperature at the inlet of each contact point, determined by an experimental‐analytical method presented previously. In this model, the lubricant is considered to follow the Ree‐Eyring constitutive equation. The influences of both pressure and temperature on density, thermal conductivity, and specific heat of the lubricant are also taken into account. Results are presented for an FZG type A gear pair lubricated with an FVA No. 4 reference oil; these include film shape, pressure and temperature distributions, as well as friction coefficient.     

Analysis of rough line contacts operating under mixed elastohydrodynamic lubrication conditions

Heavily loaded machine elements, such as gears, usually operate in the mixed lubrication regime. Surface roughness has a significant effect on the pressure distribution, the subsurface stress field, and the friction coefficient. Based on the superposition of a dry rough and a fully flooded smooth contact, a mixed lubrication model has been developed. The roughness profile is assumed to be known. Surface deformation is calculated by taking into account the pressure distribution that is built up by asperity contacts, asperity interactions, and lubricant flow. Thermal and sliding effects are incorporated into the analysis. Non‐Newtonian lubricant behaviour is considered by using a power‐law rheological model. The pressure distribution, subsurface stress field, and friction coefficient were calculated from the model at several points along the contact path for an FZG type C gear pair. It was shown that a significant part of the load is carried by the contacting asperities. The position of the maximum shear stress is very close to the surface.     

Hydrodynamic analysis of partial film lubrication in the cold rolling process

Lubrication in cold rolling plays an important part for process feasibility and process quality. The hydrodynamic process of lubrication is very complicated and affected by many material and process parameters. This paper examined partial lubrication in the cold rolling process. The average flow Reynolds equation for rolling lubrication was 2set, which considered the pressure?Cviscosity and average flow effects. Lubricating factors such as sidling, surface waviness, lubricant viscosity, surface roughness, and reduction ratio were investigated. The results of the lubrication equation show that sliding, lubricant viscosity, and surfaces roughness affect the values of rolling friction. Surface waviness and reduction ratio also influence both rolling pressure and rolling friction.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

Temperature Analysis in Lubricated Simple Sliding Rough Contacts

A temperature analysis of dry sliding fully plastic contact is extended to calculate the asperity temperatures between a sliding lubricated rigid smooth plane and a stationary elastic rough surface. First, surface roughness is generated numerically to have a Gaussian height distribution and a bilinear autocorrelation function. Lai and Cheng's elastic rough contact computer program is then used to determine the asperity contact loads and geometries of real contact areas. Assuming different frictional coefficients for shearing the lubricant film at the noncontact areas, shearing the surface film at the asperity contacts and shearing the oxide film as the asperity temperature exceeds a critical temperature, asperity temperature distributions can be calculated. Eight cases in Durkee and Cheng's scuffing tests of lubricated simple sliding rough contacts are simulated by using 20 computer-generated rough surfaces. The results show that scuffing is correlated to high-temperature asperities which are above the material-softening temperature.     

Tribological behaviours of PA/UHMWPE blend under dry and lubricating condition

Dry-sliding and lubricated friction and wear behaviours of polyamide (PA) and ultra-high molecular weight polyethylene (UHMWPE) blend were studied using a pin-on-disc method (polymer pin sliding against a stainless steel disc) at room environment. The tribological performance of PA and UHMWPE were also investigated for the purpose of comparison. The worn surfaces were examined using a scanning electron microscope (SEM) and optical microscope. It was observed that PA specimen demonstrated highest friction coefficient, UHMWPE the lowest in both dry-sliding and lubricated sliding test. The friction of PA could be sufficiently decreased by blending with UHMWPE. Statistical analysis suggested the relationship between the wear volume loss and the sliding distance could be expressed by a linear model for dry-sliding, while a logarithmic model was determined for lubricated sliding. The difference in wear modes between both sliding series suggested that there was change in the mode of material removal process. The lower wear rate in lubricated sliding was attributed to the elastohydrodynamic or partial elastohydrodynamic lubrication through the development of a continuous lubricant film between the polymer and the counterface, while the high wear rate of the specimens, in dry-sliding test, was mainly caused by fatigue process due to the repeated action of tearing and crack-propagation.     

Effect of Lubricant Contamination on Gear Wear

In this work the degradation of spur gears lubricated in a severe dusty environment studied. An experiment was carried out, which simulates the operation of gears in contaminated media such as the Sahara desert, quarries, or mines. The present study gives the effect of the presence of solid bodies in lubricants during surface contact. This paper shows that the use of a lubricant contaminated by very fine sand particles, leads to significant wear in the first few operating cycles, in zones with a high rate of sliding. This wear is more significant at the gear tooth root than at the tip. In order to understand the wear phenomenon, Scanning Electronic Microscopy (SEM) images were taken. The presence of contaminants, leads to an increase in friction, and therefore raises the temperature; consequently the roughness increases. The roughness values become more significant with the presence of particles smaller than 40 μm in the lubricant.