In industry today there are increasing demands not only on product performance, but also on environmental performance. In striving to develop high performance environmentally adapted products, optimization of product life is a central issue. The success of design optimization relies on an understanding of the degradation process and of the associated degradation mechanisms. A high level of costs is often associated with downtime of machinery caused by service and by replacement of machine components. Knowledge of ageing mechanisms facilitates evaluation of the remaining useful life, thus optimizing performance during the entire service life of components. In this study the ageing process of wet clutches has been investigated. Wet clutches are used in automatic transmissions and limited slip differentials in private vehicles and are designed to transfer torque. A wet clutch consists of a clutch pack submerged in lubricant. Separator and friction discs are alternately positioned in the clutch pack. The separator discs are connected to the input shaft and the friction discs are connected to the output shaft. When the clutch pack is pushed together friction is generated between the friction and separator discs. Torque transfer is thus created in the interface between contacting surfaces and torque transfer characteristics are determined by the interaction between lubricant and contacting surfaces. The investigations performed in this thesis have been designed to increase the understanding of wet clutch ageing and failure. Tests have been performed from full scale to model tests. Full scale testing describes the actual system which means that all significant degradation mechanisms are present. The disadvantage with full scale testing is that it is difficult to separate and isolate degradation mechanisms. In small scale model tests it is possible to isolate the degradation mechanisms and hence also what effects they have. Correlation of results from full scale to small scale can in turn increase the understanding of which degradation mechanisms are important for the system and how they influence the wet clutch system. The wet clutch lubricant and the contacting surfaces provide the friction characteristics of the clutch. Therefore, this work has focused on how lubricant degradation affects friction characteristics and hence wet clutch performance. Results in this study show that friction levels increase as lubricant degradation proceeds. In accelerated wet clutch test rig ageing, results in this study indicate that high temperatures in the interface between contacting surfaces greatly influence wet clutch degradation.

Wear is a consequence of nature which becomes costly if uncontrolled. Basic wear protection is provided by lubrication which will decrease the severity of the contact between asperities. If the conditions of a contact are such that there can be no hydrodynamic lift off by the oil and most of the contact occurs in between such asperities, the protection is provided by chemically reacted layers, sometimes as thin as just a few nanometers.In such cases where wear is governed by the most basic wear mechanisms, analytical models and numerical simulation tools have been developed and used to predict the extent of wear. Few of these models concider the interplay between contact mechanics and wear mechanisms. Wear modelling must keep improving.The goal for this work is to examine the predictive efficiency of current models and initiate construction of reliable models for the chemical growth of wear reducing layers. To achieve this, numerical simulations of contact mechanics are used in Paper A to calculate the wear of contact surfaces and in Paper B as a basis for conditions of chemical growth.The contact mechanics model is based on a solution to Boussinesq’s problem applied to equations for the potential energy by Kalker. The method takes the contact’s surface topographies and substrate material properties as input and outputs elastic and plastic deformation, contact pressure and contact area. The numerical implementation is efficiently evaluated by means of FFT-accelerated techinques. The wear is usually treated as a linear function of contact pressure and in this case the Archard wear equation constitute a feasible approximation. This equation is implemented in the present contact mechanics model to approximately predict the extent of wear, in boundary lubricated contacts, by means of numerical simulations.The chemistry of lubricant additives is discussed. Using chemical theory for adsorption as by Arrhenius, the molecular perspective of antiwear additives is explored. Mechanical properties of tribochemical antiwear layers are taken into account in the developed method. The results in Paper A from wear simulations and comparison with an experiment shows the usefulness of wear equations of geometrical contact mechanics. The chemical model in Paper B for tribofilm growth is applied to rough surfaces allowing comparison of the synergy between contact mechanics and chemistry fordifferent surface contacts. The results show how tribofilms grow on rough or smooth surfaces. The model can be used to compare chemical acitivity for different surface designs.

Wet clutches are machine components using friction to transfer torque and providing interruptible connection between rotating shafts in different automobile applications including automatic transmissions. Like any friction generating machine components, wet clutches are susceptible to continuous wear and degradation during sliding. This regular deterioration process as well as the choice of operating conditions, ultimately change the overall system performance during operation due to resultant change in the system parameters. The first part of this thesis summarizes some of the notable studies on the wet clutch tribological performance and clarifies goals of the investigation.Previously, plenty of experimental studies on wet clutches have been reported but still some effects regarding the water contamination problem and the influence of mechanical design factors are not covered thoroughly. The thesis aims to experimentally analyse these two different aspects for improving wet clutch performance regarding frictional characteristics and reliability. These two investigations are focused on wet clutches in automatic transmission applications. For evaluating friction behaviour during a long clutch engagement period, suitable test equipment is designed where standard paper based friction plates and steel separator plates are tested with commercially available ATF. To investigate a clutch operated in a controlled environment is one of main the research objectives. The vital concern while designing the test rig is to monitor the clutch parameters for achieving the desired operating conditions for individual tests. Instead of using a multiple clutch plate configuration, as in real applications, a single friction and reaction plate arrangement is considered to simplify the analyses.An experimental study on wet clutch frictional behaviour under water contaminated lubrication condition reveals the change in the friction level for a water contaminated lubricant. It was shown that the friction level increased for the addition of water in the system. This is not a desirable clutch frictional behaviour for maintaining frictional stability. The increase of friction for added water was influenced by the water amount but not by the water exposure time. The test results also showed a higher change in the separator plate's roughness parameter (Ra) for water contaminated systems compared to an uncontaminated wet clutch. In the second part of this thesis, the influence of the clutch’s output shaft’s stiffness and inertia on the clutch system‘s friction reliability is experimentally evaluated. Test results show that the choice of these design factors can provide different outcome concerning clutch frictional performance and shudder sensitivity. Shudder tendency is seen to be increased for decreased torsion shaft stiffness. High frictional losses and clutch degradation are observed for systems with less inertia.

A radial piston hydraulic motor has several components involving sliding or rolling interfaces which may encounter tribological problems. Seizure is one of them and it results in complete stoppage of motion between the relatively moving components and has serious consequences in terms of the operation of a hydraulic motor. Extreme running conditions, like low viscosity and high pressure and speed can cause the rupture of lubricating films and this may lead to the seizure of hydraulic motors. The occurrence of seizure often leads to undesirable damage to the motor components and unscheduled maintenance and can prove to be highly expensive. The work reported in this thesis has focused on investigating the mechanisms of seizure and developing suitable tribo test methods for simulating the sliding interfaces of the hydraulic motor. Extensive motor tests have been performed at low viscosity, high pressure and high speed. The results of this work enabled in describing the seizure progression, namely break down of lubricant film, particle entrapment and thermal expansion. Further, tests have been carried out in a rotary tribometer by using a thrust washer conformal test configuration. In this, the influence of area ratio has been studied and this configuration appears to simulate the piston-cylinder contact effectively during boundary lubricated conditions.

Elasto-hydrodynamic lubricated (EHL) contacts can be found in various machine elements or systems, like rolling element bearings, cam followers or gear transmissions. The service life of these elements and systems are depending to some extent on the performance of EHL contacts. Today most tribological contacts are lubricated with the same type of lubricant throughout the entire service life. However operating conditions can change over the components service life and the contacts will therefore require different lubricant properties. In order to expend the service life of the component, the lubrication of the tribological contacts has to be optimized based on the current operating conditions. A future vision is to develop machine elements which can adopt to the actual operating condition, so called triboactive systems. A first step of necessary research in order to develop such systems is presented in this work.In order to enable operation dependent lubrication the mechanism of monitoring techniques and their interaction with different operating conditions have to be investigated. In this work the effect of surface topography, slide to roll ratio and additives on the running-in and the monitoring by contact impedance were studied. Characteristic dependences between the surface parameter Rq and the contact capacitance and between the surface parameter Rz and the contact resistance were found. Further tests with iron oxide (FE3O4) contaminated bearings, monitored by vibration and acoustic emission were carried out. Premature failure due to iron oxide contamination is the most common problem for rolling element bearings operating in mining environment. Thereby the effect of iron oxide contamination on the vibration and acoustic emission monitoring for two different types of greases were investigated. It was found that a simple RMS analysis of the vibration and acoustic emission signals enables the detection of improvements of contaminated contacts by lubrication. Both vibration and acoustic emission from the investigated bearings were reduced by adding extreme pressure additives (EP) to the contaminated contacts.Monitoring of the lubrication condition is necessary to generate information about the current performance of the tribological contact. However, in order to improve the performance of tribological contacts by changes of the lubricant or additives, the effect of such additives on the lubrication condition and the performance of the tribological contact need to be studied more in detail. The presented running-in tests in this work showed that EP additives are only favourable in the very first stage of running-in. The advantage of EP additives for running-in increases with increased surface roughness and increased slide to roll ratio.Another advantage of EP additives was observed during the tests with iron oxide contaminants. The use of EP additives reduced the acoustic emissions of the tribological contact by 70% and reduced the increase of surface roughness of the raceways by as much as 60%. Furthermore the tests indicate a lower wear rate for contaminated EHL contacts lubricated with greases containing EP additives in comparison to plain grease without EP additives, in case of iron oxide contaminated EHL contacts.

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The effect of greenhouse gas emissions contributing to the global warming is today becoming an increasingly important problem worldwide and has led to increased efforts being made on improving tribological performance of interacting surfaces in mechanical systems. Due to increasingly stringent CO2regulations, a reduced fuel consumption has become a key area of interest for the automotive industry where low cost, low emission solutions are continuously developed and where low friction alternatives to machine elements currently in use are evaluated. Crankshaft roller bearings have been shown to reduce the mechanical friction in internal combustion engines compared to the plain (sliding) bearings used today, further leading to a reduced fuel consumption and thereby reduced CO2emissions. However, the transition from plain (sliding) bearings into crankshaft roller bearings means new challenges with e.g. increased noise, vibration and harshness (NVH) levels and reduced durability of the bearings. Therefore, in order to optimise the crankshaft roller bearings that operate under the highly transient conditions in the engine, an increased understanding of the tribological system is required. Research related to elastohydrodynamic lubrication (EHL) has led to the possibility to improve friction performance and durability of machine elements where lubricated non-conformal contact geometries interact. Traditionally, simplifications of the contacting geometries and the assumption of steady-state conditions have often been applied to the EHL analysis. The purpose of this work has thus been to develop a simulation model based on previous work done in the field and further utilise the model to simulate the contact on a detailed level, incorporating transient effects and the influence of oil behaviour using state-of-the-art modelling. The influence of the piezoviscous response and the compressibility-pressure behaviour of the lubricant on the sub-surface stress field were studied, showing that stiff lubricants may lead to increased stress concentrations in the vicinity of the surface, which may further influence the durability of the bearing. It was also seen that highly transient loading conditions applied to the contact initiate oscillations in the lubricated system, affecting pressure, film thickness and sub-surface stresses over time. These findings further elucidate the importance of including non-steady behaviour while analysing highly transient lubricating conditions of EHL contacts. By considering and optimising the aforementioned effects during design of crankshaft roller bearings, an improved NVH performance and an increased durability of the crankshaft roller bearing may be achieved.

Wind power is the fastest-growing form of green energy production in Europe, today accounting for 15% of the total power demand with 100.000 turbines installed. This tremendous development relied on a massive technological undertaking that must be continued, and even accelerated in order to meet the European Commission’s environmental goals for 2050. Currently, more active individual control of the rotor blades, turning the blade into and out of the wind, has proven its ability to reduce structural loads on the blades and other components significantly, therefore paving the road towards strong cost reductions. To allow for such adjustment, the rotor blades are connected to the rotor hub via pitch bearings. However, these new structural load reduction control strategies force the pitch bearings into a much more demanding operation condition. More frequent positioning activity and often in the form of smaller oscillating motions, when compared to traditional pitch control. This leading to an increased risk of wear damage of the pitch bearing that could fully incapacitate the blade control. At which point the safe regulation of the turbine can no longer be guaranteed and catastrophic failure, such as the loss of a rotor blade, is possible. This project pertains to the design a bearing test rig that can be used to test rolling element bearings with contact conditions that emulate those found in pitch bearings. A novel frameless motor-driven concept is proposed. The concept is aimed towards preventing unnecessary damage of non-test bearings and improving the dynamic performance of the test rig for a given motor capacity. One further objective of the project involved using an existing KTH single contact test rig to study the friction behavior of different lubricants when minute reciprocal tangential displacements are imposed. / Vindkraft är idag det snabbast växande området för grön elproduktion i Europa och står med 100 000 installerade turbiner för 15% av den totala elförsörjningen. Denna otroliga utvecklingen har berott på en massiv teknologisk insats som måste fortsätta. För att nå Europakommissionens miljömål för 2050 måste expansionen av grön elproduktion och vindkraft till och med trappas upp. Nyligen har en mer aktiv individuell reglering av rotorbladen, vilket möjliggör att bladen kan styras in- och ut ur vinden, visat sig kunna reducera lasterna på blad och andra komponenter avsevärt, vilket därmed möjliggör stora kostnadsreduceringar. Dessa justeringar möjliggörs genom att rotorbladen ansluter hubben via ett rotorbladslager. Dessa nya lastreducerande reglerstrategier tvingar dock lagren att arbeta under högre belastning jämfört med traditionell reglering av rotorbladens lutningsvinkel. Det här sker genom mer frekvent positionering och ofta som små oscillerande rörelser, vilket leder till en högre risk för slitage på rotorbladslagren, som i sin tur kan leda till förlust av rotorbladsregleringen. När så sker kan inte längre en säker reglering av turbinen garanteras och katastrofala fel är möjliga, så som förlust av rotorblad. Det här projektet avser att utarbeta en design för en lagerprovningsrigg som kan användas för att testa rullager med kontaktvillkor som efterliknar de som återfinns i rotorbladslagren. Ett nytt koncept,m som är baserat på en ramlös motor, presenteras. Konceptet avser att förhindra onödigt slitage hos testriggens motorlager och förbättra de dynamiska egenskaperna för en given motorkapacitet. Projektet innefattar även en studie av friktionsbeteendet hos olika smörjmedel under små upprepande tangentiella rörelser, som utförts med en befintlig testrigg på KTH.

The journal bearing is a critical machine element typically used to support rotating motion in high speed machinery. Through the generation of a hydrodynamic pressure in its thin lubricant film,which is usually in the order of 10-100μmthick depending on the diameter of the journal itself, the bearing is able to withstand large loads, both statically and dynamically, while having a very low rate of wear. It is of course essential that these components provide for a safe operation with as little wear and frictional losses as possible and it is therefore of great interest to develop simulation models of constantly increasing accuracy. Typical relevant quantities when designing a bearing are the load carrying capacity, metal/oil temperature, minimum film thickness, stiffness, damping and power loss. Classical lubrication theory builds upon the Navier-Stokes equations which, with the thin film approximation, can be reduced to a single equation which governs the hydrodynamic pressure build up in the lubricant. Since the problem now has been reduced to solving a single non linear partial differential equation in 2 dimensions, a significant advantage in terms of simulation time compared to the full set of Navier-Stokes equations can be enjoyed with an, in most cases, insignificant error of approximation. However, with time, as the need for bearings capable of operating at higher loads,speeds and with new designs involving more complex geometries, such as, for example, textured surfaces, the applicability of classic thin film theory should not be taken for granted, especially not when there is an increasing amount of turbulence involved. The purpose of the work contained in this thesis is to develop and asses the performance of a state of the art 3D TEHD model using the commercial finite element multi physics software COMSOL Multiphysics. Of special interest is the assessment of the Menter Shear Stress Transport (SST) turbulence model, which is a widely used, standard, 2-equation RANS eddy viscosity model, in predicting characteristic values for a bearing operating in the transition range between laminar and turbulent flow. A comparative study is carried out where the present model is benchmarked against experimental data on a large 4 pad tilting pad journal bearing. The present model is also compared to one of the classic models based on thin film theory. The present model is also used to study the influence of the geometry that constitutes the leading edge groove in a tilting pad journal bearing on the turbulence levels. Finally the possibility of using a shear thinning lubricant for reducing the bearing power loss is investigated. The calculations were all performed using the resources of the super computer cluster at HPC2N at Ume ̊a University. The results clearly show the inadequacy of the SST turbulence model when performing calculations on a bearing operating in the transition range between laminar and turbulent flow. Moreover, the model predicts slightly higher average values of turbulence in a leading edge grooved bearing compared to a conventional one, yet a higher maximum value in the latter.

Axial piston machines are operating at high pressure and varying speeds, which requires high reliability of the components. The machine components are separated by a fluid film, but sometimes this film gets penetrated by the surface asperities causing the machine to operate with metal-to-metal contact. In order to improve the design and predict the operating conditions might numerical tools be used. The goal with this thesis was to describe how the tribological contacts in Parkers machines can be modelled, considering oil and surface roughness. The so called Luleå Mixed Lubrication Model have therefore been investigated and it has been described how the model can be used in Parkers machines. The model uses a two-scale method to include the influence of real surface topographies of the components, it gives information about how the surface roughness affect the lubrication conditions. Different options of softwares have been investigated and compared to get an understanding of what possibilities and shortcomings they might have when it comes to this kind of simulations. Results show that all interesting contacts can't be treated the same, therefore must each contact be investigated separetly even though the Luleå Mixed Lubrication Model is used in all cases.