The ageing population has become more active and live longer, these patients require hip replacement surgery at a younger age. Artificial hip implants, consisting of the acetabular cup and femoral head, affect the lives of many people, and the longevity of these implants pose significant concerns (rarely longer than 17 years). To help understand the lubricating performance of such a system, a hip joint model was built based on the Reynolds equation; the model developed simulated hydrodynamic lubrication. A steady-state angular rotation model was built whereby it was concluded that such motion would not support any load due to the anti-symmetric nature of the resultant pressure distribution (anti-symmetric about the axis of rotation). The pressure distribution from the steady-state rotation simulation contained a pressure source and sink which converged to the centre of the cup and whose pressure value increased in magnitude, as the eccentricity ratio increased. Infeasible results were obtained when the intermediary pressure constraint, allowing only positive pressure values, was implemented. The results obtained were not representative of the problem and it is recommended that this constraint not be implemented. The transient walking cycle model showed that a fluid with viscosity of 0.0015Pa.s is not sufficient to support a load in the walking cycle under conditions representative of hydrodynamic lubrication. Increasing the fluid viscosity promoted better results in the hydrodynamic model. Increasing the femoral head radius and decreasing the radial clearance between the components also improves the possibility of hydrodynamic lubrication. It is recommended that the model should be extended to investigate elasto-hydrodynamic lubrication. If possible, the effects of a boundary lubrication model should be investigated, as it is believed to be a major contribution to the lubrication of hip joints. / Dissertation (MEng)--University of Pretoria, 2008. / Chemical Engineering / unrestricted

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)

This thesis concerns a study of the performance of an industrial low-speed high-torque hydrostatic drive system. This type of hydrostatic transmission is commonly used in continuous operation in a wide range of heavy-duty drive applications. In many applications the transmissions have to compete with e.g. electromechanical drives, such as DC or AC electric motors combined with gearboxes. In such situations, energy efficiency is a key selection criterion in that even a small increase in the efficiency of high power industrial drives would give substantial savings. Apart from efficiency, lifetime and reliability requirements are important parameters for industrial drive systems, as unplanned stops in industrial working processes can be very costly. The work presented in this thesis is primarily focused on analysing the efficiency behaviour in the transmission, both on the system level and on the component level. Attention has also been paid to lifetime issues, with special emphasis on wear occurring in a sliding contact in a radial piston hydraulic motor. In Paper A the distribution of power losses in a variable axial piston swash plate pump is investigated. The pump under study is commonly used in stationary industrial hydrostatic transmission systems. The churning losses in the pump have been estimated experimentally by measurements in a test rig. The leakage flow and the power losses in the contacts between the piston and the cylinder and between the slipper and the swash plate respectively were simulated with the help of the simulation tool CASPAR. For the pump studied, the churning losses are significant under the operating conditions typically occurring in industrial drive applications. The simulation results indicate that the leakage to the pump casing mainly originates from the gaps between the pistons and their respective cylinders and between the slippers and swash plate. The aim of Paper B is to study two sliding contacts inside a radial piston hydraulic motor and investigate their influence on the torque and power losses. Moreover, it is investigated whether and when a change in the lubrication regime can be expected in these contacts. This is accomplished by a combination of experimental and theoretical studies, with a special focus on two lubricated sliding contacts: the distributor valve contact and the piston/cam roller contact. The theoretical analysis of the contacts indicates, among other things, that the piston/cam roller contact can enter the mixed lubrication regime at low motor speeds. At low running speeds, an increased wear rate has been noted in the contact between the cylinder bore and the piston skirt in a radial piston hydraulic motor. Paper C describes a comparative investigation into different hydraulic fluids' friction properties and wear protection abilities. To simulate the contact between the cylinder bore and the piston skirt in the hydraulic motor, tests were performed in a reciprocating test rig where the contact geometry was of the cylinder-plate type. In the model test a synthetic ester aimed at meeting the conditions in water turbine applications received a top ranking regarding both friction and wear protection properties. / Godkänd; 2003; 20070217 (ysko)

No description available.

Friction and wear mitigation is typically accomplished by introducing a shear accommodating layer (e.g., a thin film of liquid) between surfaces in sliding and/or rolling contacts. When the operating conditions are beyond the liquid realm, attention turns to solid coatings. Solid lubricants have been widely used in governmental and industrial applications for mitigation of wear and friction (tribological properties). Conventional examples of solid lubricants are MoS2, WS2, h-BN, and graphite; however, these and some others mostly perform best only for a limited range of operating conditions, e.g. ambient air versus dry nitrogen and room temperature versus high temperatures. Conversely, lubricious oxides have been studied lately as good potential candidates for solid lubricants because they are thermodynamically stable and environmentally robust. Oxide surfaces are generally inert and typically do not form strong adhesive bonds like metals/alloys in tribological contacts. Typical of these oxides is ZnO. The interest in ZnO is due to its potential for utility in a variety of applications. To this end, nanolaminates of ZnO, Al2O3, ZrO2 thin films have been deposited at varying sequences and thicknesses on silicon substrates and high temperature (M50) bearing steels by atomic layer deposition (ALD). The top lubricious, nanocrystalline ZnO layer was structurally-engineered to achieve low surface energy {0002}-orientated grain that provided low sliding friction coefficients (0.2 to 0.3), wear factors (range of 10-7 to 10-8 mm3/Nm) and good rolling contact fatigue resistance. The Al2O3 was intentionally made amorphous to achieve the {0002} preferred orientation while {101}-orientated tetragonal ZrO2 acted as a high toughness/load bearing layer. It was determined that the ZnO defective structure (oxygen sub-stoichiometric with growth stacking faults) aided in shear accommodation by re-orientating the nanocrystalline grains where they realigned to create new friction-reducing surfaces. Specifically, high resolution transmission electron microscopy (HRTEM) inside the wear surfaces revealed in an increase in both partial dislocation and basal stacking fault densities through intrafilm shear/slip of partial dislocations on the (0002) planes via a dislocation glide mechanism. This shear accommodation mode mitigated friction and prevented brittle fracture classically observed in higher friction microcrystalline and single crystal ZnO that has potential broad implications to other defective nanocrystalline ceramics. Overall, this work has demonstrated that environmentally-robust, lubricious ALD nanolaminates of ZnO/Al2O3/ZrO2 are good candidates for providing low friction and wear interfaces in moving mechanical assembles, such as fully assembled rolling element bearings and microelectromechanical systems (MEMS) that require thin (~10-200 nm), uniform and conformal films.

Increasing interest in productivity, safety and environmental issues have highlighted the area of maintenance and reliability. The increasing cost of maintenance covers both preservation and sustainable exploitation of resources and awareness in maintaining equipment in a way to ensure return on investment both in the short and long run. The information obtained from condition monitoring of existing turbine, plant, rails and pumps can provide an important basis for dimensioning of future systems and components. The main objective of this research work is to develop and apply methods for efficient condition monitoring, and hence reduce maintenance costs and provide a framework for development and implementation of computer based decision tools. Furthermore, methods enabling existing process data and cost effective transducers to be used together with modern data analysis and diagnostic tools for condition monitoring of complex mechanical systems have been examined and prototypes developed. The areas of investigation covered in this work are hydropower turbines, rails and the main cooling pumps in a nuclear power-plant. The interest in diagnostics for hydropower turbines was driven by the obvious risk of contamination of water by oil leaks and expensive refurbishments caused by wear of the Kaplan turbine vane bearings. The intrest in risk analysis was motivated by Vattenfall's intrest in gaining knowledge about the state of all turbines in the company. The aim of this project was to develop a generic model of hydropower turbine behavior using physics-based models based on material properties, load tolerances, etc.. An important question was whether it was possible to predict the wear rate and plan predictive replacement or maintenance. A systematic approach to find failure modes, their effects, their causes and consequences in combination with Fault Tree Analysis was needed. The objective of this project was to examine a systematic approach to map failure modes and their causes in an hydropower turbine. We have restricted the study to turbine units of the Kaplan, Francis and tube types. The objective of the study concerning rail track was to develop methods and equipment for detection of surface damage in rail track rail in addition to the present system of practice of visual examination. The equipment developed has to be used to obtain objective statistical data for evaluating maintenance methods and efforts. We have restricted the study to spalling and headchecks on the rail head surface and running edge. The method developed enables measurements of different types of surface damage such as spalling and shelling to be made with inductive transducers sensitive to the distance to the measured object. The assumption here is that the damage being detected is characterized by the absence of material from rail surface. The main object of condition monitoring of the cooling pumps was to be able to detect bearing wear in order to be able to plan and carry out restoration well ahead of breakdown or bearing seizure. The study was restricted to the main cooling pump motor and its main bearings. Condition monitoring of the pumps was done using a method based on current measurements. Analysis of the currents on the main cooling pump of the power plant proved that it is possible to monitor the condition of the pump in spite of the presence of electronic frequency converters which distorts the signal. / <p>Godkänd; 2002; 20070222 (ysko)</p>

This licentiate thesis deals with a hybrid experimental and numerical method, the so-called Hybrid Technique, to study film thickness and pressure distribution and to investigate the behaviour and properties of lubricants in highly loaded EHL conjunctions. The Hybrid Technique combines experimental studies and advanced numerical calculations. In the experimental part of the hybrid approach Optical interferometry and a Ball &amp; Disc system are applied to obtain the colour intensity images of the EHL contact. The images that captured by a colour video system are then subjected for image analysis to evaluate the film thickness maps from the colour interferograms. The pressure distribution is obtained numerically by solving the equation of elastic deformation. Using the Hybrid Technique the pressure and lubricant film thickness in the contact are obtained without having made any assumptions regarding rheological behaviour of the lubricant. This makes the described approach to a very useful tool in investigations of EHL conjunctions under extreme conditions or in the study of multi-phase lubricants. A detailed description of the Hybrid Technique is given. Method is tested out on oil lubricated and grease lubricated contacts. Results are presented and discussed. / <p>Godkänd; 1999; 20070403 (ysko)</p>