Wet clutches are used in a variety of different machinery. Wet clutches and brakes are frequently used to distribute torque in vehicle drive-trains. The clutches can be located in e.g. automatic transmissions or limited slip differentials. Their frictional behavior is of great importance for the overall vehicle behavior and has to be thoroughly investigated when designing new wet clutch applications. Frictional behavior is normally investigated in test rigs where complete friction discs are tested under similar working conditions as for the clutches in the drive-train. However, it is today possible to simulate the clutch behavior and not limited ourselves to only use testings as a help in the design of the wet clutches. This is an advantage because it is possible to simulate behavior that is not possible to measure in test rigs. Another advantage is the faster and more cost efficient design process than when all tests are carried out in a laboratory.The torque transferred by the clutch during engagement can be roughly divided into full film torque and boundary lubrication torque. Full film torque originates from the part of the engagement where the friction discs are completely separated by a lubricant film and the friction surfaces are not in contact, whereas boundary lubrication torque occurs when the lubricant film is so thin that the surfaces of the friction discs are in direct contact. The distribution between these two types of torque is different for different types of wet clutches and engagements. When the clutch works in full film regime, it is possible to simulate the friction quite well, whereas the friction in the boundary regime is much more difficult to model and simulate, since it is very additive dependent.The most common use of wet clutches is in automatic transmissions for vehicles. Hence, most research in wet clutch testing and simulations are performed on wet clutches suitable for these applications. Here, the wet clutch is often used to brake a rotating shaft to standstill and the total engagement will have a duration of fractions of a second. During most of the engagement the clutch will be working in full film lubrication.In this investigation, the focus is on wet clutches working under limited slip conditions, i.e. the clutches will have a limited slip during a long duration without any lock-up. During this kind of engagement the clutch will work mainly in boundary lubrication and generate much heat. Such clutches can be found in, e.g. limited slip differentials.The optimum when designing a new wet clutch would be to simulate the clutch during the whole engagement without having to do any measurements in the laboratory. This, however, is not yet possible but an efficient way to design clutches can be obtained by combining fast friction measurements with efficient computer simulations. In this work, a simple friction measurement technique for wet clutches working in boundary lubrication is developed, based on pin on disc measurements. These friction measurements are combined with a temperature simulation of a wet clutch, where the lubricant cooling flow, dependent of the surface groove pattern, is simulated. This method makes it possible to allow a wet clutch working in boundary lubrication to be optimized for given working conditions regarding lubricant, friction material and surface groove pattern. The simulations are validated by measured data from a test rig in which whole friction discs are investigated with the same working conditions. / Godkänd; 2006; 20061204 (parmar)

The lubricants of the future have to be more environmentally adapted, have a higher level of performance, and lower total life cycle cost (LCC) than commonly used lubricants today. Since we live on a planet with finite resources, we have to think about coming generations and work for a sustainable development in the field of tribology. To be able to formulate those lubricants, the properties of the base fluids have to be well known. Base fluid properties that influence the formulated lubricant performance could be divided in to three different groups. These groups are; physical, chemical and film formation properties. In this study are properties from all of these groups investigated to improve the understanding on there influence on base fluid overall performance. There are more or less environmentally adapted base fluids available for formulation of lubricants. They could be divided in to different groups, mineral, semi synthetic and synthetic fluids. Synthetic fluids could be of different type: PAO (poly alpha olefins), Synthetic ester, Poly glycols and others. The most interesting group for formulation of environmentally adapted lubricants are the synthetic esters. In this thesis the properties for a large number of environmentally adapted ester base fluids are studied in detail. The tested properties relate to the macroscopic / molecular behaviour and include: viscosity-temperature-pressure-effects, thermal conductivity, and heat capacity per unit volume. The film formation capability in EHD contacts is also studied. Different connections between the molecular structure and the performance for the fluids are discussed. As an example, it is found that a large number of carboxylate groups in the ester molecule are improving the thermal properties, and thereby a thicker lubricating film could be maintained in highly loaded, high slip contacts. / Godkänd; 2004; 20070128 (ysko)

This work investigated the wear mechanisms of turbocharger components in heavy duty diesel engines. By understanding the wear mechanisms that are occurring in turbochargers the life time of the turbocharger components can be improved. For better understanding, as to why the components are worn out, an analysis of several turbocharger components and tribological tests were carried out.   The contact surfaces between turbocharger and sealing rings were analysed. Surfaces are analysed by several methods, including chemical composition of the surface. Influence of different parameters such as contact pressure, sliding velocity, temperature, and distance on friction and wear behaviour are established by performing tribological tests. Pin on disc sliding wear tests were carried out at both room temperature and at high temperature of 300˚C. The pin and the disc, that were used during the tribological tests, were made of the same materials that are used in turbocharger components.   Analysis of pins and discs from tribo-tests and turbocharger components (turbine sealing rings and shafts) show abrasive and adhesive wear on the worn surfaces of the components, from both the tribological tests and the turbochargers respectively. An increase of the temperature resulted in a reduced friction due to the formation of oxide layers on the sliding surfaces of pins and discs. In the turbocharger, traces of lubricant between the sliding surfaces was found, in addition to plastic deformations on the worn surfaces of the shafts and adhesive and abrasive wear on both worn surfaces that were in contact with each other. On the other hand, on the worn surfaces of the turbine sealing rings some cracks were observed that were perpendicular to the sliding direction, which indicates presence of a fatigue process. The presence of fatigue cracks is probably due to the way the trucks were operated, the increase and decrease of temperature and pressure in the turbocharger is probably the cause of these cracks.   To minimize the wear mechanisms that occurs in turbocharger components such as sealing rings and shafts, there are some parameters such as contact pressure between the sliding surfaces of the components that can be minimized. By minimizing the contact pressure between the sliding surfaces, the lifetime of turbocharger can be improved. A better surface finishing and geometry of the contacting surfaces can also improve the sealing rings and shafts lifetime. To minimize the relaxation of turbine sealing rings, materials that can better keep the mechanical properties of the sealing rings at high temperatures must be used.

The following report conducted with collaboration of the University of Halmstad and ABSandvik Coromant.The focus of the project is characterizing the surface topography of different surface treatmentvariants before and after chemical vapor deposition (CVD).As a part of improving the knowledge about the surface area characterization and accomplisha better knowledge and understanding about surfaces and its relation to wear of uncoatedWC/Co cutting tools The project initiated in February 2016 and end date was set to May2016.The methodology used in this thesis based on the statistical analysis of surface topographicalmeasurements obtained from interferometer and SEM by using Digital-Surf-MountainsMapsoftware.The finding from this thesis showed that Mean and Standard deviation method, Spearman’scorrelation analysis and Standard deviation error bar followed by ANOVA and T-test areeffective and useful when comparing between different variants.The thesis resulted in a measurement approach for characterizing different surfacetopographies using interferometer and SEM together with statistical analysis.Keywords: 3D-Surfaces Texture, CVD coating inserts, Interferometer, Spearman’s correlation andANOVA & T-test.

Godkänd; 1984; 20070424 (ysko)

The work presented in this thesis concerns computer simulations of lubrication processes, and the main part deals with simulations in the elastohydrodynamic lubrication (EHL) regime. The thesis summarises the work performed in the five papers referred to as Paper A, B, C, D and E. The aim is to give the reader a more explanatory description of the investigations performed in the papers and of the physical processes present in EHL. Lubrication is a sub-area of tribology, which is the science of interacting bodies in relative motion, two other sub-areas being wear and friction. Lubrication is commonly referred to as a way of reducing friction and protecting the surfaces from wear. Typical devices where EHL is present are machine components. Examples of these are bearings, cams and gears. The lubricant can in such an application have many different tasks. The ultimate goal is that the surfaces in motion should be separated by a fluid film, thus reducing the friction and wear. That leads to low frictional losses and long operating life for the machine components. This goal is, however, not always fulfilled, and to protect the surfaces from wear when the lubricating film collapses, there are additives added to the lubricant. Commonly, lubricants contain of a number of additives, but these are not in focus in this thesis. Common to many EHL-applications, especially machine components, are thin lubricating films and high fluid pressures. The high pressures result in elastic deformation of the contacting bodies. The lubricating films in such applications are very thin, often in the range 0.1-1 10^-6m with pressures ranging from 0.5-3 GPa. The contact diameter is approximately 1 mm and the time a fluid element needs to pass through the contact is approximately 0.1 ms. The altering geometrical scales and rapid changes in the physical variables, such as pressure, viscosity and temperature etc., make numerical simulations to a challenging task. The variables of primary interest in the numerical simulations are: film thickness, pressure, temperature and friction. The film thickness is an important variable that gives information as to whether the surfaces are separated by the lubricating film. It is the lifting force generated by the hydrodynamic pressure that governs the separation of the surfaces in motion. However, even if a lubricating film is present, EHL machine components deteriorate when they have been in service for a long time. It is then that the cycling in pressure and temperature leads to fatigue of the surfaces, so that the level of these variables is also of importance. The friction that has developed in the EHL-contacts leads to a loss of energy, which increases the temperature in the conjunctions. Friction is therefore important not only for the efficiency, but also when thermal aspects have to be considered. The physical processes present in EHL are inter-disciplinary, closely related to other fields of science such as fluid mechanics, solid mechanics, and rheology. In almost all numerical simulations of lubrication performed today, the hydrodynamics are modelled by an equation referred to as the Reynolds equation. This equation is derived from a simplified form of the momentum equations, which are combined with the continuity equation; and the result is a Poisson equation for the fluid pressure. The assumptions made when deriving this equation limit the size of the computational or spatial domain, and the equation cannot predict pressure variations across the lubricating fluid film. In the work presented in this thesis, an extended approach, where the technique is based on CFD (computational fluid dynamics), is used to simulate the lubricant flow. The extended approach is here based on more complete forms of the equations of momentum, continuity and energy and the above degeneracy will be removed. That implies, if such an approach works, that it should now be possible to simulate the lubricant flow under conditions where the Reynolds equation is not valid. So far, only few attempts have been made to use the CFD-technique. From the preceding discussion of rapid changes in accordance with elastic deformation of the contacting surfaces, a great deal of work has been carried out to modify the numerical algorithm in the CFD-software to fit EHL-problems. The CFD- software used throughout the work in this thesis is CFX4 (2003). / Godkänd; 2004; 20061030 (haneit)

To make the hydro power application more environmentally adapted new EALs such as synthetic esters will replace the existing mineral oils. This study aims to exploit the EALs potential in boundary lubrication, especially in slow sliding journal bearings with shaft oscillation as those supporting the Kaplan turbine blades. In order to evaluate the performance of different lubricated environmentally adapted journal bearings a test rig was built. Many test rig parameters were set to simulate water turbine conditions. Tests were performed with different bearing and shaft materials and different lubricants suited for water turbine application, EALs, water with and the present used mineral oil reference. Additional lubricated tests in oscillating motion were conducted with a roller on disc and a pin on disc. All tests were performed with oscillating motion. The results showed that synthetic esters affect the contact material to a larger extent than the tested mineral oil. The synthetic esters were also sensitive to the selection of bearing and shaft material. Highly alloyed bronzes were more prone to corrosive wear. A coated shaft showed excellent performance with a low alloyed tin bronze. The comparison of self lubricated bearings with water lubrication to an EAL lubricated tin bronze on stainless steel showed large performance differences between the water lubricated materials. They showed both better and worse friction and wear results than the EAL lubricated tin bronze. The oscillating pin on disc can generate similar contact mechanisms as in a journal bearing. The most important conclusion of this work is that it is possible to find self lubricated journal bearings better suited for shaft oscillation than EAL lubricated bronze bearings in water turbine application. Water lubricated design of the bearings are the most environmentally adapted solution. EALs such as the tested synthetic esters have a promising future in the hydraulic systems controlling the turbines and can under certain boundary lubrication conditions with bronze show excellent performance. / Godkänd; 2004; 20061026 (haneit)

Rolling bearings contain seals to keep lubricant inside and contaminants outside the bearing system. These systems are often lubricated with grease; the grease acts as a lubricant for the bearing and seal and improves the sealing efficiency. In this thesis, the influence of lubricating grease on bearing seal performance is studied. Rheological properties of the grease, i.e. shear stress and normal stress difference, are evaluated and related to the lubricating and sealing performance of the sealing system. This includes the seal, grease and counterface. The grease velocity profile in the seal pocket in-between two sealing lips is dependent on the rheological properties of the grease. The velocity profile in a wide pocket is evaluated using a 1-dimensional model based on the Herschel-Bulkley model. The velocity profile in a narrow pocket, where the influence of the side walls on the velocity profile is significant, is measured using micro particle image velocimetry. Subsequently, the radial migration of contaminants into the seal pocket is modelled and related to the sealing function of the grease. Additionally, also migration in the axial direction is found in the vicinity of the sealing contact. Experimental results show that contaminant particles in different greases consistently migrate either away from the sealing contact or towards the sealing contact, also when the pumping rate of the seal can be neglected. Lubrication of the seal lip contact is dependent on several grease properties. A lubricant film in the sealing contact may be built up as in oil lubricated seals but normal stress differences in the grease within the vicinity of the contact may result in an additional lift force. The grease, which is being sheared in the vicinity of the contact, will also contribute to the frictional torque. It is important to maintain a lubricant film in the sealing contact to minimize friction and wear. Here the replenishment of oil separated from the grease, also referred to as oil bleed, is of crucial importance. A model is presented to predict this oil bleed based on oil flow through the porous grease thickener microstructure. The model is applied to an axial sealing contact and a prediction of the film thickness as a function of time is made. The work presented in the thesis gives a significant contribution to a better understanding of the influence of lubricating grease on the sealing system performance and seal lubrication conditions. / Godkänd; 2011; 20110824 (andbra); DISPUTATION Ämnesområde: Maskinelement/Machine Elements Opponent: Georgia Power Distinguished Professor Richard F Salant, Georgia Institute of Technology, Atlanta, USA Ordförande: Professor Braham Prakash, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet, Luleå Tid: Torsdag den 22 september 2011, kl 09.30 Plats: E231, Luleå tekniska universitet

In recent years, electronically controlled automotive transmission systems, where wet clutches are used as intelligent differentials, have appeared in the market. These applications impose great demands on the transmission fluids and friction materials used as well as on controllability and vibration preventive (anti-shudder) properties of the clutch systems. This thesis focuses on transmission fluids used in wet clutches in all- wheel drive systems. The investigated all-wheel drive system, featuring a wet multi-plate clutch with a sintered brass based friction material, is described. A comprehensive literature review section outlines the state-of-art in this field and gives an insight into many of the problems commonly experienced in this type of application. Different methods used to investigate the function of wet clutch transmission fluids are also presented. Test equipment designed during this thesis work in order to determine the frictional characteristics of transmission fluids is described. This equipment measures friction torque, normal load, oil temperatures and the actual temperature experienced by the fluid in the contact zone. Base oil type and viscosity have been found not to significantly influence friction characteristics of a wet-plate clutch , indicating that the torque is primarily transmitted by asperity contacts rather than fluid films. Oil additives, on the other hand, have a considerable influence on friction, again leading to the conclusion that tribolayers on contacting asperities rather than fluid films govern friction. From these observations it can be concluded that the lubrication regime under the conditions studied are boundary lubrication, moving into mixed lubrication at high velocities and low temperatures. Results show the influence of several operating parameters on the frictional behavior of the clutch. Temperature is shown to have significant influence on the friction of transmission fluids which decrease with increasing temperature. It is therefore necessary to measure the true temperatures in the clutch contact in order to obtain realistic measurements of friction. A method which excludes the influence of temperature on measured friction data have been developed and verified. The influence of clutch disc pressure on friction is quite moderate compared to the influence of temperature and sliding velocity. The influence of velocity on friction is governed by the transmission fluid and friction material used in the clutch. The friction-velocity relationship is a good indicator of the fluid’s ability to suppress friction induced vibrations. It is, however, important to measure the friction-velocity relationship at constant temperature, or to compensate the relationship for temperature effects. A successful method to develop transmission fluids has been developed. Formulated fluids allow good anti-shudder properties to be combined with good lubrication performance for other machine elements present in the transmission. Interactions between different additives must be considered which can, in many cases, completely alter the friction characteristics since additives compete for the same adsorption surface. Extreme pressure additives have been found to be particularly troublesome when used in combination with other additives as far as their ability to maintain good anti-shudder properties is concerned. Based on the knowledge of clutch performance obtained from the research presented in this thesis, a model to predict transmitted clutch torque has been developed. This accurately determines the transferred torque from the current operating conditions and the thermal history of the clutch. It can be concluded that thermal effects have a significant influence on the torque transferred by the clutch, and it is therefore necessary to have a thermal model of the clutch combined with a temperature dependant boundary friction model based on empirical friction data for the friction material/transmission fluid combination of interest. / På senare tid har elektroniskt styrda drivlinor blivit allt vanligare i personbilar. I dessa transmissionssystem används ofta våta kopplingar för att överföra önskade vridmoment mellan olika axlar. För att uppnå lättkontrollerad och vibrationsfri momentöverföring ställs höga krav på de använda transmissionsoljorna och friktionsmaterialen. Denna avhandling behandlar främst transmissionsoljor för våta kopplingar i fyrhjulsdriftsystem. En beskrivning av det undersökta fyrhjulsdriftsystemet och den ingående flerlamellkopplingen ges. Dessutom beskrivs de friktionslameller av sintermässing som används i denna tillämpning. En omfattande litteraturstudie som syftar till att identifiera forskningsfronten inom området presenteras och ger en insikt i vilka problem som måste lösas i denna typ av applikationer. I anknytning till detta beskrivs även olika metoder som används för att undersöka funktionen hos våta kopplingar. För att undersöka friktionsegenskaperna hos olika transmissionsoljor har en testutrustning konstruerats. Denna utrustning mäter överfört vridmoment, pålagd kraft, oljetemperatur och kontakttemperatur i kopplingen. Både konstruktionen och typiska testcykler beskrivs detaljerat. Mätningar visar att basoljetyp och viskositet endast har en liten inverkan på friktionsegenskaperna (momentöverföringen), vilket tyder på att friktionen uppkommer i asperitetskontakter snarare än genom viskösa effekter i en oljefilm. Oljeadditiv har däremot kraftig inverkan på friktionen, även detta tyder på att friktionen genereras av additivskikt på yttoppar snarare än av en oljefilm. Sammantaget visar detta att den smörjregim som råder under undersökta förhållanden är gränsskiktssmörjning som går in i blandfilmssmörjning vid höga hastigheter och låga temperaturer. Resultat visar även hur en mängd olika parametrar påverkar friktionsegenskaperna i kopplingen. Studerade parametrar inkluderar temperatur, glidhastighet, tryck och oljesammansättning. Temperaturen har betydande inverkan på friktionsegenskaperna hos transmissionsoljor. Friktionen minskar med ökande temperatur. Därför är det nödvändigt att mäta den korrekta temperaturen i kopplingen för att få relevanta mätvärden på friktionen. En metod för att presentera mätdata på ett sätt där temperaturens inverkan elimineras har utvecklats och visat sig fungera väl. Friktionspåverkan från den pålagda normalkraften är liten jämfört med inverkan från temperatur och glidhastighet. Glidhastighetens påverkan på friktionen bestäms av den aktuella kombinationen av smörjolja och friktionsmaterial. Sambandet mellan friktion och glidhastighet ger en god bild av smörjmedlets vibrationsdämpande förmåga. Det är dock viktigt att detta samband mäts vid konstant temperatur eller att mätdata kompenseras på ett lämpligt sätt. En framgångsrik metod för att utveckla transmissionsoljor har utvecklats. Formulerade oljor har visat att det är möjligt att kombinera vibrationsdämpande egenskaper med goda smörjegenskaper med avseende på andra maskinelement i transmissionen. Växelverkan mellan olika additiv måste beaktas och kan i många fall leda till kraftiga förändringar i friktionsbeteendet eftersom alla additiv konkurrerar om att fästa på samma metallyta. Speciellt extremkontaktstrycksadditiv (EP) har visat sig vara svåra att kombinera med andra additivtyper. Baserat på den nyvunna kunskapen om kopplingars momentöverföring har en teoretisk modell utvecklats. Denna modell kan förutsäga vilket överfört moment kopplingen genererar under givna förhållanden. Eftersom temperatureffekter har stor inverkan på friktionen är det nödvändigt att ha en termisk modell av kopplingen som kan kopplas till en temperaturberoende friktionsmodell. Friktionsmodellen är baserad på empiriska friktionsdata för aktuell kombination av friktionsmaterial och smörjmedel. / Godkänd; 2005; 20061001 (ysko)

Fermionic matter is ubiquitous in nature, from the electrons in metals and semiconductors or the neutrons in the inner crust of neutron stars, to gases of fermionic atoms, like 40K or 6Li that can be created and studied under laboratory conditions. It is especially interesting to study these systems at very low temperatures, where we enter the world of quantum mechanical phenomena. Due to the Fermi-Dirac statistics, a dilute system of spin-polarised fermions exhibits no interactions and can be viewed as an ideal Fermi gas. However, interactions play a crucial role for fermions of several spin species. This thesis addresses several questions concerning interacting Fermi gases, in particular the transition between the normal and the superfluid phase and dynamical properties at higher temperatures. First we will look at the unitary Fermi gas: a two-component system of fermions interacting with divergent scattering length. This system is particularly interesting as it exhibits universal behaviour. Due to the strong interactions perturbation theory is inapplicable and no exact theoretical description is available. I will describe the Determinant Diagrammatic Monte Carlo algorithm with which the unitary Fermi gas can be studied from first principles. This algorithm fails in the presence of a spin imbalance (unequal number of particles in the two components) due to a sign problem. I will show how to apply reweighting techniques to generalise the algorithm to the imbalanced case, and present results for the critical temperature and other thermodynamic observables at the critical point, namely the chemical potential, the energy per particle and the contact density. These are the first numerical results for the imbalanced unitary Fermi gas at finite temperature. I will also show how temperatures beyond the critical point can be accessed and present results for the equation of state and the temperature dependence of the contact density. At sufficiently high temperatures a semiclassical description captures all relevant physical features of the system. The dynamics of an interacting Fermi gas can then be studied via a numerical simulation of the Boltzmann equation. I will describe such a numerical setup and apply it to study the collision of two spin-polarised fermionic clouds. When the two components are separated in an elongated harmonic trap and then released, they collide and for sufficiently strong interactions can bounce off each other several times. I will discuss the different types of the qualitative behaviour, show how they can be interpreted in terms of the equilibrium properties of the system, and explain how they relate to the coupling between different excitation modes. I will also demonstrate how transport coefficients, for instance the spin drag, can be extracted from the numerical data.