As características tribológicas do poli-éter-éter-cetona (PEEK) contendo de 0% a 40% de fibra de carbono, sob deslizamento em ambiente lubrificado com óleo, foram estudadas para diferentes acabamentos superficiais da contrapeça. Os quatro acabamentos escolhidos, comumente usados na indústria automobilística, foram o torneamento, a retífica, o brunimento e o polimento. O sistema de teste usado foi o tri-pino-disco, com o pino em PEEK e a contrapeça em aço, sendo totalmente imersos no óleo ATF Dexron VI. Alguns parâmetros de teste foram mantidos constantes como a pressão aparente de 2 MPa, a velocidade linear de deslizamento de 2 m/s e a temperatura do óleo em 85°C. Analisou-se o efeito do tempo de deslizamento na taxa de desgaste e no atrito para o PEEK com 30% de fibra de carbono contra disco acabado por torneamento, estudou-se o efeito do acabamento do disco no atrito e desgaste do PEEK natural e do PEEK com 30% de fibra de carbono, avaliou-se o efeito da adição da fibra de carbono na taxa de desgaste e atrito em acabamento torneado e foi definido o regime de lubrificação para os acabamentos de polimento e torneamento para o PEEK natural e para os acabamentos de polimento, brunimento, retífica e torneamento para o PEEK com 30% de fibra de carbono nas condições de 2 MPa e 2 m/s. Constatou-se que o regime permanente de desgaste ocorreu a partir de 60 minutos após a estabilização da taxa de desgaste e coeficiente de atrito. O atrito e o desgaste do PEEK e seus compósitos se mostraram sensíveis ao acabamento da contrapeça, pois este foi o principal fator que definiu os regimes de lubrificação. Nas condições de teste supracitadas o polimento operou em regime hidrodinâmico, o torneamento e a retífica em regime limítrofe, e o brunimento em regime misto. Foi levantada a curva de Stribeck para o PEEK com 30% de fibra de carbono deslizando contra o disco com acabamento de brunimento para a faixa de pressão de 1 MPa a 14 MPa. Nestas condições foram observados os três principais regimes de lubrificação, tornando possível gerar um mapa que relaciona o regime de lubrificação com pressão, velocidade e coeficiente de atrito. / The sliding tribological properties of poly-ether-ether-ketone (PEEK) containing from 0 to 40% of carbon fiber were studied within oil lubricated environment for different surface finishing of the counterpart. Four different finishing processes commonly used in the automotive industry were chosen for this study: turning, grinding, honing and polishing. The test system used was three pin-on-disc with pins made of PEEK and counterpart made of steel, they were fully immersed in the Dexron VI ATF oil. Some test parameters were held constant as the apparent pressure of 2 MPa, the linear velocity of 2 m/s and the oil temperature at 85 °C. It was analyzed the effect of the sliding time on friction and wear of PEEK with 30% of carbon fiber on turning disk, studied the effect of disc finishing on friction and wear of natural PEEK and PEEK with 30% of carbon fiber, evaluated the effect of carbon fiber on wear rate and friction for turning counterbody finishing and the lubrication regime was for polishing and turning finishes for natural PEEK and polishing, honing, grinding and turning for PEEK with 30% of carbon fiber under 2 MPa and 2 m/s. It was found that steady state of wear occurred from 60 minutes of test after wear rate and friction coefficient stabilization. The friction and wear of PEEK and its composites were sensitive to the counterbody finishing; it was the main factor which defined the lubrication regimes. In the above test conditions polishing operated in hydrodynamic regime, turning and grinding on boundary condition and honing in mixed regime. The Stribeck curve was raised for PEEK with 30% carbon fiber by sliding against the honing finishing disk under 1 MPa to 14 MPa of apparent pressure range. Under these conditions the three main lubrication regimes were observed and it was possible to create a map that relates lubrication regime with pressure, speed and coefficient of friction.

Yes / Advanced engineering polymers of the Polyaryletherketone (PAEK) family with carbon fibre reinforcement are finding application in engineering systems as tribological bearing surfaces under severe operating conditions that cyclically move the polymer into and beyond the glass transition temperature region. To support such an application, the friction in high speed and low load PAEK-steel sliding contacts was investigated both unlubricated and lubricated with a trinonyl trimellitate ester, a base fluid for high temperature industrial lubricants. Four polymers in the PAEK family, PEEK, PEK, PEKEKK and PEKK, with 30%wt of carbon fibre whiskers were tested against an AISI 4140 steel disc. When unlubricated, low friction depended upon the formation of a PAEK transfer film on the steel disc and when this became unstable in the glass transition region the friction increased to much higher levels with associated polymer surface damage. Frictional heating due to the high sliding speed dominated the differences in glass transition behaviour between the four PAEKs. When lubricated, the lubricant film controlled friction and there was no significant effect of the glass transition of any of the PAEKs. The irreversible nature of the glass transition in PAEKs in such tribological applications, due to high surface damage at high temperature, means that it is essential to ensure effective lubrication in both fluid film and boundary lubrication. / Innovate UK, Knowledge Transfer Partnership

L’étude du frottement dans les contacts lubrifiés fortement chargés est un sujet complexe. En effet, le frottement dépend fortement du comportement rhéologique du lubrifiant dans le centre du contact et, celui-ci n'est pas bien connu à pression et taux de cisaillement élevé. Diverses méthodes expérimentales ont été développées pour expliquer le comportement plateau dans les courbes de frottement, associé à l’existence d’une contrainte limite de cisaillement, mais aucune d'elles n'a fourni une image complète des mécanismes réels impliqués. Dans la continuité de ces efforts, des études sont présentées dans ce mémoire. Le premier défi dans ce travail est d'effectuer des mesures de frottement dans des conditions isothermes nominales, ce qui signifie que même si des effets thermiques doivent se produire dans toute mesure de frottement, il est possible de les minimiser et de rendre les résultats insensibles à une faible dissipation d'énergie dans le volume expérimental d'intérêt. La minimisation de l’échauffement du lubrifiant aide à se focaliser sur l’origine mécanique de la contrainte limite de cisaillement et de mieux caractériser sa dépendance à la pression et à la température. C'est pourquoi, tout d'abord, une série d'expériences a été réalisée sur deux lubrifiants, un diester pur (benzyl benzoate) et une huile minérale de turbine (Shell T9) avec des vitesses d'entraînement variables. Cela nous permet d'abord d'observer directement l'influence des effets thermiques sur les valeurs de la contrainte limite de cisaillement et ensuite, de déterminer les conditions expérimentales qui limitent ces effets tout en assurant un régime en film complet. Le deuxième objectif est de caractériser le comportement en frottement des lubrifiants sous conditions isothermes nominales et sur une large gamme de pression (jusqu'à 3 GPa) et de température (jusqu’à 80°C) afin d'établir un nouveau modèle découplé permettant de décrire la dépendance à la température et à la pression de la contrainte limite sous fortes charges. Enfin, l'étude se focalise sur la compréhension du comportement microscopique des lubrifiants dans des conditions extrêmes de cisaillement et de pression. Des mesures in situ de spectroscopie Raman et Brillouin ont été également effectuées sous conditions statiques, afin d'étudier le changement de phase du lubrifiant. / Friction in highly loaded lubricated contacts is a complex issue. Indeed, it highly depends on the lubricant rheological behaviour in the Hertzian region, which is not well known under such high pressure and high shear stress. Various experimental methods have been developed to explain the plateau-like behaviour in friction curves referred to as the limiting shear stress (LSS), but none of them provided a full picture of the real mechanisms involved. In a continuation of these efforts, some investigations are presented in this manuscript. The first challenge in this work is to carry out friction measurements under nominal isothermal conditions, meaning that even if thermal effects must occur in any friction measurement, it is possible to minimize them and to make the results almost insensitive to a weak energy dissipation within the experimental volume of interest. Minimizing shear heating of the lubricant help us to focus on the mechanical origin of the LSS and to better characterize its dependence to pressure and temperature. That’s why, first of all, a series of experiments was performed on two lubricants, a pure diester fluid (benzyl benzoate), and a commercial turbine mineral oil (Shell T9) with varying entrainment velocities. This allow us first to directly observe the influence of the lubricant shear heating on the LSS values and then to determine the experimental conditions which limit this thermal effect while ensuring a full film regime. The second objective is to characterize the frictional behavior of both lubricants under nominal isothermal conditions and over a wide range of pressure (up to 3 GPa) and temperature (up to 80°C) in order to establish a new uncoupled model to describe the temperature and pressure dependence of the limiting shear stress under highly loaded conditions. Finally, the study focuses on the understanding of the microscopic behavior of lubricants under extreme shear and pressure conditions. In situ Raman and Brillouin spectroscopy investigations were also conducted under static conditions, in order to study the lubricant phase changes under various pressure and temperature conditions.

Aluminium silicon alloy has been found to be advantageous in many automobile components like pistons, cylinders, brakes and clutches. The main objective in using these alloys is to obtain lightweight and low friction at a reasonable cost without sacrificing reliability and durability. Out of all the tribological components piston skirts, piston rings and cylinder liners, have to face the most hostile of environments in an internal combustion engine. Wear mechanism of these components have been identified as abrasion, scuffing and corrosion. Narrowing down the line of interest, cylinder wear is more important than ring wear to both the engine manufacturer and the user, as cylinders are more expensive to replace than piston rings. Wear of piston ring and cylinder combination have been studied using a wide range of techniques. It is difficult to predict the tribological performance of these parts in an engine, even with the most well designed laboratory tests, due to chemical, thermal and mechanical complexities in the operating environment. Therefore, a good correlation is sought from the wear behaviour of test bed engines and laboratory tests. This should form the basis of further development particularly in terms of efficiency, weight eduction and wear life improvement of the components. Many ASTM bench-wear tests are used to study wear, some of the common tests being ball-on-disc and pin-on-disc testing. From these tests, a large database of wear information can be achieved and they offer rapid and low cost means of comparison. The only drawback is that the real components are not tested. However, since the bench tests can never simulate the engine environment completely, engine tests are always required for final verification. This thesis work reports preliminary studies of machining damage and wear in actual engine bore to set a bench mark, followed by a set of unidirectional sliding bench tests to study the wear of aluminium alloy under lubricated conditions, to classify the different wear regimes in boundary lubrication zone under different pressure conditions, and to study the effect of a surface modification technique, etching, which improves wear properties. The investigation is divided into four parts. 1. Study of subsurface damage in an actual cylinder surface as introduced by prior machining and actual worn case: A study of the microstructure of bores, processed through a range of machining variables; feed and speed, are investigated in this part of the thesis. This work suggests that the first step of rough machining may be responsible for the microstructure of the finished bore even though subsequent processing steps are intended to remove all prior damages. This also includes some observations of worn surface of an actually run engine, locating the various worn spots and studying the cause of this damage 2. Bench wear test in pin-on-disc under dry and lubricated condition with varying load and lubricant: After setting a benchmark on wear in engine using actual worn cylinder bore, a set of bench tests were carried out on aluminium alloy. Here, steel pins are slid on aluminium silicon alloy discs in the boundary lubrication regime in the presence of one drop of oil. The effect of pure hexadecane and engine oil containing additives on friction and wear are analysed and the data are discussed in terms of the formation of a mechanically mixed layer at the interface. 3. Ultra-mild Wear in Lubricated Tribology of an Aluminium Alloy: To study the different wear regimes in boundary lubrication zone, flat faces of cylindrical steel pins were slid on an eutectic aluminium silicon alloy under lubricated condition in the 1-100 MPa mean contact pressure range and 0.2 m/s sliding speed. Two transitions in wear rate were observed, at 10 MPa and 70 MPa. The wear rate in the 1-10 MPa regime was found to be very small and within the measuring instrument resolution and also insensitive to contact pressure. The regime is designated ultra-mild wear. Lack of plastic flow, minimal fragmentation of silicon particles and the presence of undistorted voids on the fractured and unfractured silicon particles in the subsurface suggest that the state of stress in the near surface region is elastic. Contact mechanical calculations demonstrate that at contact pressures less that 13.7 MPa the system is likely to shakedown to an elastic state. 4.Ball-on-disc wear tests for etched and unetched samples: In the fourth part of the thesis, comparative studies have been done between the as polished and chemically treated samples. Formation of grooves in a ball-on-disc experiment is observed on etched and unetched flats as a function of normal load and sliding distance. The groove is initially formed by plastic flow, and then expanded by micro-abrasion as the ball continues to slide on the groove. However etching causes surface hardening of the alloy, but, more importantly, creates a surface topology that reduces the peak contact pressure, which inhibits further plastic flow in the subsurface.

The majority of the vessels in the commercial fleet utilize oil lubricated stern tubes. Unfortunately, this brings about a risk of oil leaking from the stern tube into the marine environment if the stern tube seal would become worn or damaged by foreign materials. Previous studies concluded that, on average, 2.6 litres of oil per day leak out from the stern tube of ships. This essay has investigated the causes that could increase the leakage rate from the stern tube by reviewing literature, interviewing experts, and sending out surveys with questions regarding the subject. The answers that were received painted a clear picture that it is impossible to get a perfect seal on a stern tube. The causes that could influence the leakage rate were design related, such as vibrations, the rotational speed of the propeller shaft, radial and axial movements of the propeller shaft, as well as external causes such as the quality of water and foreign materials, for example, fishing lines and nets. The question whether water lubricated stern tubes were a viable alternative compared to oil lubricated stern tubes was also investigated. The result was that the bearing on a water lubricated stern tube did not have as long lifespan as an oil lubricated bearing. / Majoriteten av fartygen i handelsflottan använder oljesmorda propellerhylsor för att smörja propelleraxeln. Tyvärr medför det en risk att olja kan läcka ut från hylsan om tätningarna skulle bli utslitna eller skadas av främmande föremål. Tidigare forskning har visat att cirka 2,6 liter olja per dag läcker ut genomsnittligen från fartyg i handelsflottan. Denna uppsats har utrett vad som kan påverka läckagemängden från en propellerhylsa genom att utnyttja intervjuer och enkätsvar. Svaren som framkom var att det är mer eller mindre omöjligt att få en perfekt tätning på en propellerhylsa. Saker som kan påverka läckagemängden var designfenomen som vibrationer, axiella- och radiella rörelser och varvtal på propelleraxeln, samt yttre påverkan som vattenkvalité och främmande föremål som fiskelinor och nät. I uppsatsen jämförs även vattensmorda propellerhylsor med fokus på livstid, kostnad och underhåll, med en oljesmord propellerhylsa. En slutsats är att det vattensmorda lagret inte har samma livstid som det oljesmorda.

A eficiência dos sistemas mecânicos é influenciada diretamente pelo coeficiente de atrito local, sendo este relacionado com os regimes de lubrificação. O coeficiente de atrito pode ser estudado usando o método de elementos finitos (MEF) com a finalidade de substituir ensaios experimentais e reduzir tempo de análise. Esta dissertação teve como objetivo desenvolver uma metodologia para avaliar o coeficiente de atrito em um par tribológico e seu efeito na eficiência por simulação numérica. Para esta modelagem foi definido um sistema composto por um par de engrenagens helicoidais. Em uma primeira análise, foi empregado o programa AVL Excite Power Unit, que utiliza a ferramenta MBD (Multi-Body Dynamic - Dinâmica de Multi-Corpos), possibilitando a simulação da transmissão com seus principais elementos em um modelo 1D, que leva em consideração a teoria da lubrificação elastohidrodinâmica (EHL). Também foi utilizado o programa Abaqus®, da Dassault Systèmes, para a criação de um modelo 3D, que considera o par engrenado e sendo necessário o desenvolvimento de uma subrotina para a aplicação do modelo de lubrificação da literatura. Em todas as simulações foram considerados condições de operação de um banco de provas e três diferentes acabamentos superficiais dos dentes das engrenagens em função do processo de fabricação: fresamento, retificação e polimento. Além da rugosidade, avaliou-se o efeito da velocidade angular nas perdas do sistema. O modelo 3D foi comparado com o modelo 1D e com resultados de eficiência de ensaios de banco de prova. O modelo 3D permitiu avaliar o coeficiente de atrito local na linha de contato do dente engrenado. Maiores velocidades angulares aumentaram a espessura de específica de filme, diminuindo assim o coeficiente de atrito e promovendo uma melhoria na eficiência do sistema. Os menores valores de coeficiente de atrito estão na região do diâmetro primitivo, onde o SRR é nulo. A velocidade angular influenciou mais do que a rugosidade. As análises dos modelos 1D e 3D indicaram que o modelo 1D superestima a eficiência da transmissão, enquanto que o modelo 3D apresentou resultados compatíveis com o banco de provas. / The efficiency of mechanical systems is influenced directly by the local friction coefficient, which is related to the lubrication regimes. The coefficient of friction can be studied using the finite element method (FEM) to replace experimental tests and reduce analysis time. This dissertation had the objective to develop a methodology to evaluate the coefficient of friction in a tribological pair and the effect on the efficiency by numerical simulation. In this model, a system composed of a pair of helical gears was defined. In a first analysis, the AVL Excite Power Unit software was used, which uses the tool MBD (Multi-Body Dynamic), enabling the simulation of the transmission with the main elements in a 1D model, which takes into account the theory of elastohydrodynamic lubrication (EHL). The Abaqus® software was used to create a 3D model, which considers the pair of helical gear and a developed subroutine for application of the literature lubrication model. In all simulations, the conditions of operation of a rig tests and three different surface finishes of the gear teeth were considered, depending on the manufacturing process: milling, grinding and polishing. Besides the roughness, the effect of angular velocity on the losses of the system was evaluated. The 3D model was compared with the 1D model and with the results of rig tests efficiency. The 3D model allowed to evaluate the coefficient of local friction in the contact line of the tooth. Higher angular velocities increased the film specific thickness, thus reducing the coefficient of friction and promoting an improvement in the efficiency of the system. The lowest values of coefficient of friction are in the region of the primitive diameter, there the SRR is zero. The angular velocity influenced more than the roughness. The analyzes of the 1D and 3D models indicated that the 1D model overestimates the transmission efficiency, while the 3D model presented results compatible with the rig tests.

Grease-lubricated journal bearings are widely used in heavy duty applications, such as construction equipment, agriculture- and forest machines. The main purpose of the grease-lubricated journal bearing is solely to create sustainable rotation of a given application. However, purpose seldom decides complexity of journal bearing design. Depending on application, parameters such as Load , Material  and Lubrication immensely increase complexity of the design. Tribology is a highly interdisciplinary subject, which requires knowledge concerning physics, chemistry, metallurgy and solid mechanics. Tribological aspects of a design are frequently regarded as irrelevant. Tribological issues are commonly enlightened in combination with structural design weaknesses. The main aim of the thesis was to analyze two cylinder joints found in a Komatsu Forest 951 crane and establish root cause to the structural and tribological issues. Outline of the approach was divided into three main targets: 1. Investigate mechanical and tribological aspects of the crane design. 2. Understand how these aspects influence the life time of the bearings. 3. Summarize the analysis and suggest improvements based on the discovery. Fundamental structural and tribological design guidelines regarding grease-lubricated journal bearings have been presented in this thesis. Damaged components such as cylinders, pins and journal bearings have been examined at Komatsu Forest’s factory in Umeå. Further, a scanning electron microscopy study has been done at Luleå University of Technology, to determine predominant wear mechanisms in the harvester crane joints. A non-linear finite element model of the crane has been designed to represent pressure distribution in the contact between bearing and pin. The finite element analysis gives a good approximation of the contact but leaves room for further refinements. Temperatures, in the contacts, have not been identified and will be measured outside the time frame of the master’s thesis. Design changes, with respect to discovered structural and tribological issues have been suggested. The suggested improvements can potentially increase the life time of lift cylinders, pins and journal bearings.

The aim of this master’s thesis is to develop measuring methods for evaluation of the lubrication regime in a rolling bearing. The first part of thesis maps several measuring methods according to which the lubrication mode can be evaluated. Based on the assessment of this part, electrical monitoring methods were selected. For application of these methods was constructed testing device with rolling bearing. Specific measuring chains based on selected methods were designed and verified by using initial tests on a ball-on-disk tribometer. According to the results of these initial measurements was selected and applied suitable measuring chain to the selected test device. In the end, there was a successful measuring the lubrication parameter on the rolling bearing. Additionally, other tests dealing with the influence of temperature and type of oil on the measurement itself were performed.

Indiana University-Purdue University Indianapolis (IUPUI) / This thesis concentrates on the study of hybrid gas journal bearings (bearings with externally pressurized mass addition). It differs from most work in that it goes back to “basics” to explore the hydrodynamic phenomena in the bearing gap. The thesis compares geometrically identical bearings with 2 configurations of external pressurization, porous liners where mass-addition compensation is varied by varying the liner’s permeability, and bushings with 2 rows of 6 feedholes where the mass-addition compensation is varied by the feedhole diameter. Experimentally, prototype bearings with mass-addition compensation that spans 2 orders of magnitude with differing clearances are built and their aerostatic properties and mass addition characteristics are thoroughly tested. The fundamental equations for compressible, laminar, Poiseuille flow are used to suggest how the mass flow “compensation” should be mathematically modeled. This is back-checked against the experimental mass flow measurements and is used to determine a mass-addition compensation parameter (called Kmeas) for each prototype bushing. In so doing, the methodology of modeling and measuring the mass addition in a hybrid gas bearing is re-examined and an innovative, practical, and simple method is found that makes it possible to make an “apples-to-apples” comparison between different configurations of external pressurization. This mass addition model is used in conjunction with the Reynolds equation to perform theory-based numerical analysis of virtual hybrid gas journal bearings (CFD experiments). The first CFD experiments performed use virtual bearings modeled to be identical to the experimental prototypes and replicate the experimental work. The results are compared and the CFD model is validated. The ontological significance of appropriate dimensionless similitude parameters is re-examined and a, previously lacking, complete set of similitude factors is found for hybrid bearings. A new practical method is developed to study in unprecedented detail the aerostatic component of the hybrid bearings. It is used to definitively compare the feedhole bearings to the porous liner bearings. The hydrostatic bearing efficiency (HBE) is defined and it is determined that the maximum achievable hydrostatic bearing efficiency (MAHBE) is determined solely by the bearing’s mass addition configuration. The MAHBE of the porous liner bearings is determined to be over 5 times that of the feedhole bearings. The method also presents a means to tune the Kmeas to the clearance to achieve the MAHBE as well as giving a complete mapping of the hitherto misunderstood complex shapes of aerostatic load versus radial deflection curves. This method also rediscovers the obscure phenomenon of static instability which is called in this thesis the “near surface effect” and appears to be the first work to present a practical method to predict the range of static instability and quantify its resultant stiffness fall-off. It determines that porous liner type bearings are not subject to the phenomenon which appears for feedhole type bearings when the clearance exceeds a critical value relative to its mass-addition compensation. The standing pressure waves of hydrostatic and hybrid bearings with the 2 configurations of external pressurization as well as a geometrically identical hydrodynamic bearing are studied in detail under the methodology of the “CFD microscope”. This method is used to characterize and identify the development, growth, and movement of the pressure wave extrema with increased hydrodynamic action (either increasing speed or increasing eccentricity). This method is also used to determine the “cause” of the “near surface effect”. A gedanken experiment is performed based on these results which indicates that a bearing with a “stronger aerostatic strength” component should be more stable than one with a low aerostatic strength component. Numerical instability “speed limits” are found that are also related to the hydrostatic strength of the bearing. The local conditions in the standing waves are characterized in terms of their local Mach number, Knudsen number, Reynolds number, and Taylor Number. It is concluded that low eccentricity bearing whirl can be attributed to the off load-line orientation of the bearing load force caused by the overlay of the hydrodynamic bearing standing wave onto the hydrostatic bearing wave of the hybrid bearing, whereas it is hypothesized that aperiodic and random self-excited vibration which occurs at high eccentricity, as reported in the literature, is probably due to shock waves, turbulence, near surface effect, and slip at local areas of the standing wave.

Lubricated friction issues are central to all hydraulic actuation systems undergoing motion and any in-depth understanding of the nature of lubricated friction will advance future component design. The classic friction models of hydraulic actuation systems under steady state conditions and their dependency on velocity and temperature have been studied extensively over the past years. A model which is commonly employed to represent the characteristics of friction is that of Stribeck in which the dependency of the friction force is based on velocity alone. However, experimentally, it has been found that lubricated friction is dependent on acceleration. Thus, the Stribeck model can be considered as a subset of a dynamic friction model in which acceleration is zero. Thus, it can be concluded that the Stribeck model is best applied to cases when the change rate of the velocities is very small. This thesis considers the dependency of lubricated friction on acceleration when pressure and temperature changes are relatively constant. As such, the basic hypothesis for this study was proposed as follows: “Lubricated friction in hydraulic actuation systems is not only a function of velocity, but is also a function of both velocity and acceleration”. In this thesis several terms are defined which facilitate the description under which friction models are developed. For example, the term non-steady state friction is used to account for the effect of acceleration on lubricated friction force while in motion. Further, the lubricated friction models are divided into two groups: steady state friction models and non-steady state friction models. Nonlinear friction modeling and measuring methods are reviewed in this dissertation. This review also includes nonlinear lubricated friction modeling in hydraulic actuation systems. A conclusion from this review was that limited research has been done in documenting and explicitly demonstrating the role of acceleration on lubricated friction. The research first introduced a methodology to experimentally measure friction as a function of acceleration and to demonstrate this dependency in the form of a three dimensional graph. A novel technique to experimentally obtain data for the lubricated friction model was introduced. This allowed the lubricated friction forces to be measured as a function of velocity in a continuous manner, but with acceleration being held constant as a family parameter. Two different valve controlled hydraulic actuation systems (VCHAS) were studied under a wide variety of accelerations at constant temperature and pressure. To enable repeatable data collection for the different friction conditions and to accommodate for the effect of hysteresis, a periodic parabolic displacement waveform was chosen which enabled the acceleration to be a family parameter. The second phase of the research introduced a method of representing the data (lubricated friction model) in a lookup table form. The relationship of lubricated friction (in this work, pressure differential, ΔP across the actuator) as a function of velocity and acceleration was presented in a unique semi-empirical 2D lookup table (2D LUT). Limitations of this experimental approach were identified, but the dependency on acceleration was clearly established. The last phase of the study implemented this 2D LUT model into a practical software model of an actuator and demonstrated its accuracy when compared to its experimental counterpart. The semi-empirical model (2D LUT) was experimentally verified by implementing the semi-empirical and Stribeck models into a real time simulation of an actuator and by comparing the experimental outputs against simulated outputs for a common sinusoidal input. A sinusoidal actuator displacement input was chosen to test the simulations as it was not used in the collection of the original data. The output of the simulation was compared to the experimental results and it was evident that for the range in which data could be collected in developing the model, the proposed 2D LUT model predicted an output that was superior to a model which used a standard Stribeck model. It was concluded that the semi-empirical model could be integrated into a simulation environment and predict outputs in a superior fashion when compared to the Stribeck friction model. Thus it was concluded that the stated hypothesis is consistent with the experimental evidence shown by all hydraulic actuators considered. Further, it was also observed that the traditional Stribeck form (steady state dynamic friction) does change with increasing acceleration to the point that the standard breakaway friction almost disappears. It is evident that the 2D LUT is a viable tool for modeling the non-steady state friction of hydraulic actuation systems. The semi-empirical 2D LUT model so developed is a more global representation of hydraulic actuator lubricated friction. In this research, only linear hydraulic actuators were considered; however, the novel nonlinear semi-empirical 2D LUT lubricated friction model can be applied to any actuator (linear and rotary) and provides a new way in which the dynamic friction can be viewed and modeled.