La tribologie est définie comme la science qui étudie les phénomènes de frottement, d’usure et de lubrification. Elle intervient à chaque fois que deux surfaces sont en mouvement relatif l’une par rapport à l’autre. On la rencontre donc principalement dans les moteurs, les instruments de coupe, les assemblages mécaniques etc. Le frottement et l’usure sont les causes principales des pertes de performances et de durabilité des systèmes. L’usure coute 3 % du PIB en France et on estime que 10 % à 30 % de l’énergie produite dans le monde est consommée par frottement, c’est dire l’importance que porte le monde industriel à cette science.C'est la lubrification qui a pour rôle de minimiser le frottement et de protéger les surfaces de l'usure. Il existe différents régimes de lubrification, cependant les travaux effectués au cours de cette thèse de doctorat s’intéressent exclusivement au régime de lubrification limite. Dans ce régime de lubrification, l’épaisseur du film de lubrifiant est du même ordre de grandeur que la hauteur des aspérités des surfaces en contact et le frottement et l'usure des surfaces peuvent être très sévères. La stratégie mise en place pour lutter contre ces phénomènes consiste à ajouter des additifs à l'huile lubrifiante, qui formeront au cours du frottement un film protecteur (le tribofilm). Il apparait donc primordial de bien caractériser ces tribofilms afin de mieux comprendre les mécanismes de réduction de frottement et de l’usure, qui restent encore à ce jour mal connus.Les nanotechnologies permettent aujourd'hui d'apporter un nouvel éclairage sur cette problématique et nous les avons mises à profit pour étudier le comportement mécanique des tribofilms à l’échelle nanométrique.L’objectif de ce travail est de comprendre les processus de réduction de frottement observés à l’échelle macroscopique lorsque des particules de composés lamellaires sont associées à un liquide de faible viscosité. Les tribofilms étudiés sont formés à partir de trois composés lamellaires différents, le graphite, le thiophosphate de nickel NiPS3 et le bisulfure de molybdène MoS2. Pour ces trois composés, deux types de tribofilms ont été étudiés : ceux formés à partir des particules seules (les tribofilms secs) et ceux formés en présence de liquide (les tribofilms éther).L’analyse des propriétés tribologiques mesurées à macro et nano échelle, nous ont permis de mieux comprendre la contribution des effets individuels et collectifs sur les mécanismes réducteurs de frottement. Ces expériences ont permis de mettre en évidence que la réduction du coefficient de frottement observée à macro-échelle n'est pas due à la nanostructuration des tribofilms ni à une baisse des interactions entre les feuillets mais plutôt à un effet collectif des particules et du liquide. / Tribology is the science of friction phenomena, wear and lubrication. It takes part every time two surfaces are in relative motion. It performs meanly in engine and mechanical systems. Friction and wear are the mean reasons of loose of performance and durability of mechanical systems. Wear costs 3% of the GDP in France and about 10% to 30% of the produced energy in the world is consumed by friction.The aim of lubrication is to reduce friction and protect the surface against wear. There are four different types of lubrication regimes but this study is focused on the boundary regime. In this regime, lubricant thickness is of the same order of the magnitude of the sliding surfaces roughness, leading to a high-level of friction and wear. Additives are added to the lubricating base oil, in order to build a protecting tribofilm. Characterization of the tribofilms appears essential to better understand the friction and wear reduction mechanisms.Nanotechnology allows to bring a new insight on this issue through the study of the mechanical behavior of the tribofilms at nanoscale.The aim of this work is to better understand the process of the friction reduction observed at macroscale when lamellar particles are associated to a low viscosity liquid. The tribofilms are formed from three different lamellar compounds, graphite, nickel thiophosphate NiPS3 and molybdenum disulfide MoS2. Two different kinds of tribofilms are studied: those formed with the solids particles only (dry tribofilms) and those formed in presence of liquid (wet tribofilms).The analyses of the tribological properties measured at nanoscale allow us to better understand the contribution of the individual and collective effects on friction reduction mechanims. We show that the reduction of the friction coefficient observed at macroscale is not related to the surface nanostructuration of the tribofilms neither to the decrease of the interaction energy between the sliding surfaces, but rather to collective effects of the liquid and the particles in the contact.

Elastohydrodynamically lubricated (EHL) contacts are fundamental for severalrotating machine elements. For example gears, rolling element bearingsand lubricated chain drives work due to the principle of EHL. All of these machineelements require maintenance and, as condition based maintenance hasincreased in industry, the need for monitoring techniques has also increased. Inorder to avoid incorrect condition indications, since the 60’s researchers haveimproved signal processing of existing monitoring tools and developed newtechniques as a complement to these existing tools. In the past two decadesacoustic emission has been identified as a new complementary tool for monitoringof rolling element bearings and investigated intensively by several researchgroups. However, most of the investigations were carried out at lowrotational speeds. Furthermore, most of the investigations used simple signalprocessing methods like activation counts (AC) or trend analysis of the rootmean square signal (RMS). One reason for using simple experimental conditionsand signal processing methods is the complexity of a rolling elementbearing itself. A rolling element bearing consists of several EHL contacts andeach contact has different operational conditions (film thickness, slide to rollratio, contact pressure, entrainment speed). The measured signal is the summationof all EHL contacts. This complexity is one reason why the high frequencyemissions of an EHL contact are still not fully understood. Therefore, an investigationof the acoustic emission of a single EHL contact was here carriedout within the framework of a PhD project.In this thesis simplified experiments were used to represent either a single EHLcontact or elements of an EHL contact. Both acoustic emissions of tensile testsand ball impacts on a solid plate were studied and analyzed with respect totheir significance for EHL contacts. For all investigations carried out in thisthesis an absolute calibration method developed by McLaskey and Glaser wasused. This calibration method was validated for boundary restricted systems,where a good agreement for zero frequencies was found, however, unsatisfying agreement was discovered for resonances of a boundary restricted system. Theinvestigation found elastic waves in boundary restricted systems consist of twofundamental types. Zero frequencies will be enhanced for cases were excitationsource and elastic wave are independent, while an interaction of sourceand elastic wave results in a pure resonance problem.Furthermore, the time dependency of acoustic emission signals was investigated.As mentioned previously most existing investigations are carried outat low and constant rotational speed. The dependency of acoustic emissionsignals and speed is reported in literature as well as difficulties with acousticemission measurements at elevated rotational speeds. By using ball impactswith different ball sizes and tensile tests with different displacement speeds thetime dependency was analyzed with respect to excitation time (contact time ofball impact) and event frequency (amount of dislocation movement and planeslip movements in a certain time frame). Thereby an indirect quadratic proportionalitybetween acoustic emission amplitude and contact time was found.This proportional relationship is also valid for RMS signals with short averagingwindows if system damping is low. For event frequency and RMS signalsthe results of the tensile tests suggest a direct proportional relationship.Furthermore, Hertzian and EHL contact impacts were studied and compared.Thereby it was observed that the overall amplitude of the signal increases forEHL contacts in comparison to Hertzian contacts. In addition the third zerofrequency disappears, which is most likely due to cavitation effects. Furthermore,the results show a shift of the first and second zero frequency towardshigher frequencies, which is caused by the localised deformation of EHL contactsas a result of the solidification of the lubricant. This behaviour of zerofrequencies was in line with simulation results. However, the agreement betweensimulation and measurement for the location of zero frequencies and thesignal amplitude was not satisfying. This mismatch was most likely caused bythe assumption of the global contact force acting at a single point, causing aperfect elastic deformation in the simulation. Additionally, for the findings regardingzero frequencies, a change in the excitation of resonances above thefirst zero frequency in boundary restricted systems was also found, comparingHertzian and EHL impacts.Finally, full scale tests on a complete rolling element bearing were carried outduring the PhD project to validate findings of the single contact experiments.Magnetite contaminated rolling element bearings and their acoustic emissionsignals were investigated with respect to the use of sulfur additives, contaminationand rotational speed. These tests were executed at varying speed forsingle measurements and constant speed for continuous measurement recording. The results of the full scale tests showed good agreement with previousresults of the component tests, such as bouncing ball and tensile tests. Transientforces are the main source of signals for well lubricated rolling elementbearings or bearings at high rotational speed, while acoustic emission signalsof contaminated bearings at low rotational speed were dominated by plasticdeformation signals. Furthermore, it was found that sulfur additives reducethe plastic deformation signal by up to 70% in comparison to contaminatedbearings lubricated with plain grease.

Modelling and simulation in tribology, and more specifically of friction in lubricated contacts, has gained increasing attention over the past years. In a lubricated contact, the dissipation of energy is due to the relative motion of the mating surfaces and arise due to direct contact as well as shearing of the lubricant film. The presence of a thin lubricant film is crucial for the operation of various machine elements, e.g., for the concentrated contact between the rolling element and the raceway in a bearing. The contact in this type of applications is typically exhibiting substantial elastic deformation which together with hydrodynamics governs the formation of the lubricant film. Therefore, these type of contacts are said to operate in the Elastohydrodynamic lubrication (EHL) regime. An elastohydrodynamically lubricated contact can be classified as line, circular or elliptical. The line contacts can also represent a truncated ellipse or be of finite length. The line contact that appears between two cylindrically shaped bodies of infinite length does of course not exist in reality. It does, however, constitute an important type of simplification of the contact in real applications where the contact length perpendicular to direction of motion is comparatively large. The reason for this is that it permits a 2D-model for the flow and there are analytical solutions, at least in the most elementary cases. The circular and the fully elliptical contacts are more complicated. The case where the surfaces are fully separated by the lubricant film has, however, been addressed by many researchers and there are quite a few papers reporting numerical predictions validated by experimental data. The finite length line and truncated elliptical contact are even more challenging, but these are also the only physically reasonable models for EHL contacts exhibiting edge effects, created by profiled rollers or in cases where the contact ellipse becomes larger than the physical size of the contacting elements. This thesis presents the development of a fully coupled model that can be used to predict the pressure build-up and lubricant film formation in finite length line contacts. More precisely, in EHL contacts where the rolling element have profiled edges (fillets) and the surface of the counteracting element is wider than the roller, e.g., in a typical cam-roller follower contact. Hereafter this type of contact will be referred to as a 'finite EHL line contact'. The numerical analyses, conducted with the present model, were designed so that generic knowledge about friction in cam-roller follower applications would be generated, but also to provide for the development of a semi-analytical for fast and efficient estimation of friction. There are quite a few parameters that affect the friction in EHL contacts and it is already a challenge to include the most basic ones in the model. The most advanced and sophisticated models are very complex with millions of degrees of freedom and are, therefore, not yet feasible to conduct parametric studies with. The extreme conditions associated with EHL, i.e., nm thin films, with phase transition from liquid to solid, GPa pressure, temperature increase with considerable implications on lubricant flow and surface chemistry, etc., makes it even more difficult to model these systems. When modelling a cam-roller follower application, which typically results in a finite length EHL line contact, the size and geometry of the contacting parts further complicates modeling of EHL. The main objective, with this thesis project, was to design a low degree of freedom model that can be employed in a multibody dynamics model, to estimate EHL friction in milliseconds and yet capturing the most important features of a lubricated contact including edge effects. This resulted in a semi-analytical low degree of freedom (LDOF) model taking thermal effects into account and that include lubricant shear thinning and compressibility, in order to estimate the viscosity and volume of the lubricant. In addition, this LDOF model was extended to perform friction prediction covering the mixed lubrication regime where colliding asperities partially influence shearing of the lubricant. The extended, mixed lubrication LDOF, model was utilized to perform friction predictions covering a range of operating conditions, which were also covered in an experimental investigation using a ball-on-disk test device. The results turned out to compare well, suggesting that the model established in this project, is a usable tool that can be employed when designing lubricated devices and that it constitutes a suitable foundation for further developments.

Today, with increasing demands on industry to reduce energy consumption and emissions, the strive to increase the efficiency of machine components is maybe bigger than ever. This PhD thesis focus on friction in elastohydrodynamic lubrication (EHL), found in, among others, gears, bearings and cam followers. Friction in such contacts is governed by a complex interaction of material, surface and lubricant parameters as well as operating conditions. In this work, experimental studies have been conducted that show how friction varies over a wide range of running conditions when changing parameters like lubricant viscosity, base oil type, surface roughness and lubricant temperature. These measurements have also been used to predict the friction behaviour in a real gear application. Numerical modeling of elastohydrodynamic (EHD) friction and film thickness are important for increased understanding of the field of EHL. Due to the high pressure and shear normally found in EHD contacts it is crucial that appropriate rheological models are used. An investigation has been carried out in order to assess the friction prediction capabilities of some of the most well founded rheological models. A numerical model was used to predict friction coefficients through the use of lubricant transport properties. Experiments were then performed that matches the predicted results rather well, and the deviations are discussed. The numerical model in combination with experimental measurements are used to investigate the friction reducing effect of diamond like carbon (DLC) coatings in EHL. A new mechanism of friction reduction through thermal insulation is proposed as an alternative to the current hypothesis of solid-liquid slip. These findings opens up for new families of coatings where thermal properties are in focus that may be both cheaper, and more effective in reducing friction in certain applications than DLC coatings of today. / Godkänd; 2014; 20140825 (marbjo); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Marcus Björling Ämne: Maskinelement/Machine Elements Avhandling: Friction in Elastohydrodynamic Lubrication Opponent: Professor H P Evans, Cardiff School of Engineering, Cardiff University, Cardiff, Wales, UK Ordförande: Professor Roland Larsson, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Torsdag den 2 oktober 2014, kl 09.00 Plats: E231, Luleå tekniska universitet

Lubricating grease is commonly applied to lubricate e.g. rolling bearings, sealsand gears. Grease has some clear advantages over lubricating oil: it is a semisolidmaterial, which prevents it from flowing/ leaking out from the bearingsystem and gives it sealing properties, and it also protects the system fromcontaminants and corrosion. Due to its consistency, lubricating grease has manyadditional advantages over lubricating oil: it does not require pumps, filters andsumps. However, the rheology of grease makes it more difficult to measure andstudy its flow dynamics. This study focuses on the influence of rheology ongrease flow in different geometries involving a straight channel with restrictions,concentric cylinder geometry, and free-surface flow on a rotating disc.To better understand grease flow in bearings and seals, two types of flowrestrictions were applied into the straight channel in order to simulate the flowof grease near a seal pocket. In the case of a single restriction, the horizontaldistance required for the velocity profile to fully develop is approximately thesame as the height of the channel. In the corner before and after the restriction,the velocities are very low and part of the grease is stationary. For the channelwith two flow restrictions, this effect is even more pronounced in the narrowspace between the restrictions. Clearly, a large part of the grease is not moving.This condition particularly applies in the case of a low-pressure gradient andwhere high-consistency grease is used. In practice this means that grease may belocally trapped and consequently old/contaminated grease will remain in theseal pockets.A configuration comprising a rotating shaft and two narrow gap sealing-likerestrictions (also called Double Restriction Seal, DRS) was designed to simulatea sealing contact. Two different gap heights in the DRS have been used tocompare the grease flow. It is shown that partially yielded grease flow isdetected in the large gap geometry and fully yielded grease flow in the small gapgeometry. For the small gap geometry, it is shown that three distinct grease flowregions are present: a slip layer close to the stationary wall, a bulk flow layer,and a slip layer near the rotating shaft. The shear thinning behaviour of thegrease and its wall slip effects have been determined and discussed.Free-surface flow of grease occurs in a variety of situations such as during relubrication and inside a rolling element bearing which is filled to about 30%with grease in order to prevent heavy churning. Here the reflow of lubricant tothe bearing races is a key point in the lubricant film build-up, and centrifugalforces have a direct impact on the amount of available grease. Understanding ofthe free-surface flow behaviour of grease is hence important for theunderstanding of the lubrication mechanism. Adhesion and mass loss aremeasured for greases with different rheology on different surfaces andtemperatures. It is shown that the critical speed at which the grease starts tomove is mostly determined by grease type, yield stress and temperature ratherthan surface material. A developed analytical model covers a stationary analysisof the flow resulting in solutions for the velocity profile of the grease as well as asolution for the thickness of the viscous layer remaining on the disc. / Godkänd; 2014; 20141017 (jinlit); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Jinxia Li Ämne: Maskinelement/Machine Elements Avhandling: Lubricating Grease Experiments and Modellingo f Wall-Bounded and Free-Surface Flours Opponent: Professor Arto Lehtovaara, University of Technology, Tampere, Finland Ordförande: Professor Erik Höglund, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag den 19 december 2014, kl 09.00 Plats: E632, Luleå tekniska universitet

Godkänd; 2013; 20130529 (kimber); Tillkännagivande disputation 2013-09-11 Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Kim Berglund Ämne: Maskinelement/Machine Elements Avhandling: Predicting Wet Clutch Service Life Performance Opponent: Professor Homer Rahnejat, School of Mechanical and Manufacturing Engineering, Loughborough University, Leicestershire, UK Ordförande: Bitr professor Pär Marklund, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag den 4 oktober 2013, kl 09.00 Plats: D770, Luleå tekniska universitet

Fuel efficiency is one of the most important areas of automotive vehicle research and development today, with rising fuel costs, energy security and environmental concerns being at the forefront of customers and legislators minds. Heavy Duty Diesel Engines (HDDE) are the primary source of mechanical power generation in today’s trucks and buses and this is likely to continue for the foreseeable future. In the 2011 European Commission White Paper on transport, a reduction of at least 60% of greenhouse gas emissions from transport by 2050, with respect to 1990 levels, was called for. The report concludes that acting on vehicles’ efficiency through new engines, materials and design will help in the reduction of oil dependence, the competitiveness of Europe’s automotive industry as well as health benefits, especially improved air quality in cities. Therefore, the efficiency and frictional losses in a vehicles powertrain are areas of great interest. This thesis focuses on the Piston Ring to Cylinder Liner (PRCL) contact and the potential for improving its performance through the specification of an optimised cylinder liner surface texture. The PRCL contact is one of the biggest contributors to mechanical losses in a HDDE and so there is potential for large performance gains to be achieved through optimisation of this contact. This research has led to the development of a simulation tool capable of calculating the friction, lubrication regime, oil consumption risk and wear that occurs in the full ring-pack of a HDDE. Furthermore, the tool allows for the evaluation of the relative performance of different cylinder liner surface topographies. A mixed lubrication model, incorporating flow factors calculated using the homogenization technique, has been implemented to allow all regimes of lubrication to be considered. A mass-conserving cavitation algorithm, formulated as a Linear Complimentarity Problem, enables lubricant cavitation, fully-flooded or starved inlet conditions and the quantity of lubricant deposited on the cylinder liner surface to be modelled. The simulation tool is validated with both reciprocating bench tests and full single cylinder fired engine tests. The reciprocating bench tests measured both friction and film thickness and both showed good correlation with the predictions from the simulation tool. Simulations and experiments were conducted on four different cylinder liner variants and both ranked the frictional performance of the cylinder liner variants in the same order. A parametric study of honing depth, spacing and angle was undertaken using the developed simulation tool and the influence of these parameters on lubricant film thickness, friction, wear and oil consumption was investigated. The thesis concludes that a reduction in specific fuel consumption is achieveable through the optimisation of cylinder liner texture and outlines how this might be achieved. / Godkänd; 2013; 20131110 (spencer); Tillkännagivande disputation 2013-11-22 Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Andrew Spencer Ämne: Maskinelement/Machine Elements Avhandling: A Simulation Tool for Optimising Combustion Engine Cylinder Liner Surface Texture Opponent: Professor Martin Priest, School of Mechanical Engineering, The University of Leeds, UK Ordförande: Professor Roland Larsson, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag den 13 december 2013, kl 10. Plats: E632, Luleå tekniska universitet

A seal is a commonly used machine element whose function is to preventthe flow of a fluid from a high to a low pressure region. Metal-to-metalseals, in particular, are used whenever extreme conditions prevent theuse of less expensive rubber seals. Situations where such extreme condi-tions may be encountered are found, for example, in oil wells and nuclearpower plants. In such applications, the failure of a metal-to-metal sealcan become catastrophic, as it might mean the leakage of hazardousfluids to the environment. In order to minimize the risk, it is critical tounderstand the mechanisms controlling the seal’s performance and,if possible, be able to predict capability to prevent leakage on before-hand. Not surprisingly, the surface topography plays a crucial role hereand therefore requires careful consideration when conducting studies ofthis kind. Indeed, it has been shown that even very small details in thetopography (of size of the order of micrometres) can have a large effecton the performance of the whole seal (of size of the order of centimetresor larger). Another complicating factor is the topography’s stochas-tic nature, which makes even the identification of the relevant detailschallenging. Modelling is, in this context, a desirable approach, as itprovides the possibility to easily zoom in those fine details as well asisolate individual parameters. Moreover, it can provide for a predictionon the expected leakage.This work focuses primarily on the development of a model suitablefor studying the mechanisms controlling the performance of metal-to-metal seals and to enable prediction of leakage. To accomplish this, amodel that follows a two-scale approach is proposed. More precisely,the small details in the topography are considered in a local problemconnected to a highly resolved local-scale domain, while the componentlevel features are considered in a global problem allowing for a coarsegrid discretisation of the corresponding global-scale domain. During the present work it was also found that realistic results can only be obtainedif the model explicitly considers the surface topography’s stochastic na-ture. The model was first developed for liquids and was based on theassumption of incompressible and iso-viscous flow. Further work, withthe objective to enable studies of more complex type of flow situations,resulted in a versatile transformation translating results for incompress-ible and iso-viscous to compressible and piezo-viscous fluids and viceversa. This means that, the flow of gases and other more complex fluidscan be studied by combining the model for the simplistic incompressibleand iso-viscous flow with this newfound transformation.Using the model developed, the sealing performance of metal-to-metal seals during load cycling, i.e., by gradually increasing the load toa certain value and then releasing it again, is studied. The scope of thisstudy is to assess how the plastic deformation that the metal surfacesundergoes during the loading phase can affect the leakage during unload-ing. It is shown that this results in a change of the original topographythat may lead to a better sealing performance during unloading. Themain result obtained is, however, that given the right conditions, the ap-plied load can be released considerably (even down to half of the reachedbefore starting the unloading) with only a small increase in leakage asa result. This shows the seal’s capability to prevent leakage evenif an unexpected reduction of load occurs and is therefore is a valuabledescription of the robustness of the seal.

This study was intended to establish which polymer material is best suited as the material for bushes used in high force small oscillating angular contact applications as typically found in a large sugar mill coupling. A Tribometer as well as a designed and manufactured Bearing Testing Machine, which simulates and accelerates the wear caused in the intended application, was used to determine the wear rates and coefficients of friction for seven polymer materials. Materials were selected for their superior mechanical and thermal properties. The Tribometer was fitted with a linear reciprocating rig, so as to simulate the small oscillating movement prevalent in the intended application. Each Tribometer test ran for a duration of two hours with a frequency of 33.1 Herts and a normal loading of 40N. Bearing Testing Machine tests were done under similar constraints but with 30kN loading present on the surface of a bushes inner race. These tests had a longer duration of 24 or 48 hours or until intense wearing occurred. Initially both testing methods will be analysed independently of each other in order to try and identify a relationship between the wear rate and coefficient of friction for the different polymers. The different testing methods’ results will be compared to each other in order to establish if a correlation between the materials tested could be found.

Different aspects of Elastohydrodynamic Lubrication (EHL) are studied. For smooth surfaces, a novel approach which solves separately the inlet and outlet regions by using the fracture mechanics equations, is proposed to solve EHL line contacts for shape and pressure. For rough surfaces, the full EHL geometry is reduced to an infinitely long contact with known mean film thickness and pressure; so real-roughness steady state analyses are carried out by considering the separate Fourier components of roughness and pressures, transient analysis by applying general finite difference methods. The subsurface stresses under micro-EHL are also calculated and given in form of a probability rather than a specific value and location. Initially, full-geometry EHL line contacts of smooth surfaces are studied. The spike of pressures is assumed to be singular and the idea is to start with an original Hertzian pressure distribution, then the edges of this pressure are truncated and the effects calculated via linear fracture mechanics; after this, the removed pressures are replaced by the converged inlet and outlet pressures, previously obtained by iterating the Reynolds and fracture mechanics equations. It is found that the outlet pressures follow a modified logarithmic function and therefore the exit bump in the shape joins the parallel film zone with a finite value of slope, unlike the Greenwood extension of Grubin's theory. From a set of solutions, the behaviour of the pressure spike as a function of two dimensionless numbers is followed. Comparisons with results from full numerical solutions are shown, giving good agreement. The scheme is later extended to consider compressibility and the Roelands viscosity law. After reducing full EHL geometry, the effects of real and wavy roughness in microEHL of Newtonian and Eyring fluids with or without compressibility are studied. Steady state analyses of real roughness show that only the high frequency components remain after deformation. By linearizing the Reynolds- Eyring equation an analytical solution is obtained and a criterion for the deformation of the roughness in EHL is given; from this, it is shown that the deformation is very much dependent on the ratio λ/ħ, obtaining little deformation for low values of it. Transient analyses of roughness in lubrication are also carried out considering the infinitely long contact. It is found that the transient pressure and film distributions are made of two parts: a) the well known steady state solution, plus b) a complementary function depending only on the modulation of film and pressures from the inlet. It is shown that the conclusions outlined for some authors (e.g. Venner and Lubrecht) about pressures travelling with the velocity of the roughness but shape with the average velocity of the lubricant, are only a particular case of a more general understanding. It is now believed that there is no a real physical damping in the transient shape.