Elastohydrodynamic lubrication (EHL) allows transfer of power and forces in gears and rolling bearings without surface-to-surface contact and is the basis for a continuously variable transmission studied in this report. Previous research constructed models and derived solution methods, but often lacked full explanations of the approach and was usually applied to limited and specific cases. This report precisely develops the numerical solution of EHL contact and includes the more general cases of elliptical contacts and side-slip. The model and numerical method are validated on known benchmark cases and test results. Side-slip is investigated and the results shown in this report. Finally, the model is used to determine the film thickness and pressure of a contact patch under identical conditions to that in a physical drive developed by Fallbrook Technologies in Austin, TX. A minimum film thickness of 0.8978 [mu]m is found, setting a benchmark for the maximum allowable surface roughness values to prevent surface-to-surface contact. Additionally, under normal drive conditions the film thickness to surface roughness ratio is in the range of ideal values for maximum life. / text

This diploma thesis is an experimental study of grease lubrication in a starved elastohydrodynamic lubricated (EHL) contact. Lubricating film thickness is measured with tribological experimental device ball-on-disc using optical interferometry and high speed camera. The experiment is aimed at a comparison of results acquired by ball-on-disc device and full scale axial bearing. In addition, this paper also investigates the influence of number of the rolling elements (overrolling time) on the resulting film thickness.

<p>Rolling contact fatigue (RCF) is the primary mode of failure in tribological contacts like rolling-element bearings (REBs), gears, and cam-follower systems. RCF processes have a crack initiation phase followed by a propagation and coalescence phase, resulting in spalls that lead to catastrophic failure. Crack initiation is a highly localized process that is strongly influenced by the inhomogeneity of the material microstructure. Therefore, a microstructure-sensitive model is required to simulate the damage evolution and failure due to RCF loading. This document presents the development of a microstructure-based finite element (FE) framework for RCF, which accounts for the inhomogeneity of bearing steel microstructure by using an explicit definition of polycrystal topology and material anisotropy. The granular topology of the bearing steel microstructure is described using randomly generated Voronoi tessellations. A cubic elastic material definition with a random spatial orientation is specified for each Voronoi grain to simulate the material anisotropy. The Voronoi grains generated using this approach were used to model the critically stressed microstructural volume in RCF loading. A domain size study was conducted to estimate the minimum number of grains that need to be contained by the critically stressed volume such that the macroscopic material response of the polycrystalline aggregate matches the linear elastic material properties of bearing steel. The estimated critically stressed volume was then embedded into a semi-infinite domain for the FE simulation of RCF line contact loading. The RCF domains developed were then subjected to a moving Hertzian pressure over the surface to simulate a bearing load cycle. A boundary averaging scheme was used to estimate the effective stresses along the grain boundaries of the Voronoi cells. Due to the anisotropy of the polycrystalline material, local stress concentrations occur at the grain boundaries as compared to isotropic models. The resolved grain boundary stresses were used to predict critical locations for RCF crack initiation, which closely match observations from RCF bench test data. Since RCF failures typically exhibit subsurface locations for the first crack initiation, the model uses the critical resolved shear stress (RSS) reversal along the grain boundaries and the corresponding subsurface location of the maxima as the driving parameters for RCF fatigue failures. The parameters from the model were fit into a Weibull distribution to estimate the stochasticity in initiation life. The Weibull predictions corroborate well with experimentally measured RCF life scatter. The framework was then extended using a coupled damage mechanics - cohesive element method (DM-CEM) to individually model the crack initiation and propagation phases in RCF. An explicit definition of the grain boundaries was incorporated using cohesive elements. Damage is initiated at the grain boundaries by degradation of the cohesive elements and the rate of damage/degradation is used to characterize the evolution of fatigue life. The rate of damage was calculated at each grain boundary using a fatigue damage law based on the RSS reversal parameter. The model is able to simulate the crack initiation and the propagation/ coalescence phases in RCF, with distinct life estimates for each phase. This model framework is further extended to investigate the effects of lubrication conditions in RCF by integrating an elastohydrodynamic lubrication (EHL) model to simulate the pressure load with the DM-CEM model. Further improvements to the fatigue life predictions using the DM-CEM model are made by coupling it with a crystal plasticity (CP) based submodel approach to predict the crack initiation life in RCF. CP-based metrics are used to correlate the microplasticity developed under RCF loading with the formation of fatigue micro-cracks and the corresponding initiation life estimations. The resulting final spall patterns and RCF life estimates were found to match well with experimental data available in the open literature.</p> <p><br></p>

No description available.

Rolling contacts are utilized in many technical applications, both in bearings and in the contact between gear teeth. These components are often highly loaded, which makes them susceptible to suffer from rolling contact fatigue. This work focuses on the rolling contact fatigue mechanism of pitting. In order to attain a better understanding of why pitting initiates and grows, detailed simulations of rolling contacts have been performed. In particular the contact between two gears in a truck retarder was here used as a case study. The investigated contact experienced elastohydrodynamic lubrication conditions since the load was high enough to causes the surfaces in contact to deform and the viscosity of the lubricant to increase significantly. In Paper A it was investigated if surface irregularities in the size of the surface roughness are large enough to cause surface initiated fatigue. The investigation focused on the pitch line since small surface initiated pits were found here even though there was no slip present. Since there were pits present at the pitch line, it is important that the theories of pitting can explain the development of pits also in the absence of slip. The conclusion of the work was that surface irregularities of the size of normal surface roughness are enough to cause surface initiated fatigue at the pitch line. In Paper B it was investigated why pits are more likely to initiate in the dedendum of pinion gears than in the addendum. In both areas slip is present but in different directions. In the dedendum the friction from slip is against the rolling direction which enhances the risk for pitting. The investigation was performed by studying the effect of the temperature rise in the contact caused by the slip. The conclusion drawn was that the temperature rise in the contact explained why pitting was more common in the dedendum than in the addendum. / Rullande kontakter förekommer i många applikationer, till exempel i lager och mellan kugghjulständer. Både lager och kugghjul utsätts ofta för höga laster vilket gör att dess ytor löper stor risk att utmattas, vilket kallas rullande kontaktutmattning. Denna studie fokuserar på pitting, även kallat spalling, vilket är en typ av rullande kontaktutmattning där en utmattninsspricka växer fram som får delar av ytan att ramla av. För att få en bättre förståelse varför pittingskador uppkommer har noggranna simuleringar utförts av rullande kontakter. Kontakten mellan två tänder på kugghjul i en lastbilsretarder har används som underlag då många pittingskador påträffats på dem.  För att minska friktionen och nötningen i kontakten mellan kuggtänderna användes smörjmedel. De höga lasterna lastbilsretardern utsattes för deformerade kuggarnas ytor elastiskt samtidigt de kraftigt ökade viskositeten hos smörjmedlet. Dessa förhållanden gör att kontakten kallas för elastohydrodynamiskt smord, vilket på engelska förkortas till EHL. I Artikel A undersöktes om små ytojämnheter kan orsaka ytinitierade pittingskador. Eftersom skadan påträffats i friktionslösa kontakter, så som vid rullcirkeln på de undersökta kugghjulen, är det viktigt att teorierna som förklarar uppkomsten inte är beroende av friktion. Undersökningen fokuserade därför på förhållandena vid rullcirkeln.  Slutsatsen från arbetet var att små ytojämnheter, av samma storleksordning som ytojämnheterna på de undersökta kugghjulen, är tillräckligt stora för att orsaka utmattningsskador. I Artikel B undersöktes varför det är vanligare att pitts initieras i dedendum än addendum på drivande kugghjul. Kontakten på båda sidorna om rullcirkeln slirar svagt åt olika håll. Att kontakten slirar skapar friktion som är motriktad rullriktningen i dedendum vilket ökar risken för pittingskador. För att undersöka varför dessa förhållanden ökar risken för skador fördjupades analysen av kontakten genom att inkludera temperaturfältet. Simuleringarna visade att temperaturen ökar genom kontakten vilket orsakar en asymmetrisk spänningsfördelning. Denna asymmetriska spänningsfördelning gör att ytojämnheter i dedendum är troligare att orsaka skador än ytojämnheter i addendum. / <p>QC 20180213</p>

The elastohydrodynamic lubrication regime (EHD) is found in many machine elements, such as rolling element bearings, gears, cam/tappet, where a combination of hydrodynamic effect, elastic deformation of the surfaces and an increase of the lubricant’s viscosity with pressure create a continuous lubricant film which is capable of supporting pressures of the order of tens of thousands of atmospheres. One of the most important features of these films is their thickness, as this determines whether the bounding surfaces are completely separated, thus avoiding premature wear and failure of the contact. Consequently for many years scientists were interested in finding methods for measuring the lubricant film thickness in elastohydrodynamic conditions. One of the most versatile and widely used techniques for measuring lubricant film thickness in EHD contacts is the optical interferometry method. Apart from numerous advantages, this method has the limitation in the fact that one of the contacting surfaces must be transparent, usually glass or sapphire, thus it does not replicate real conditions found in machine elements contacts. On the other hand, the other group of methods used for studying the behaviour of elastohydrodynamic films includes a variety of electrical methods. Historically, these appeared before the optical methods, but gradually lost importance with the success of the later. Most capacitive, resistive, inductance methods developed so far use specially designed sensors for monitoring the lubricant film thickness. In the case of electrical techniques, both elements of the contact are metallic, which means that these can be used for measuring film thickness in real machine elements. One of the main disadvantages of electrical methods though, is the difficulty with which the calibration of various electrical quantities, against lubricant film thickness is obtained. This thesis describes the work carried out by the author on the application of a capacitive method for studying lubrication of elastohydrodynamic contacts. The novelty of the method used consists in the calibration of the capacitance of the contact with optical interferometry. This project started from the premises that a thicker Chromium layer will supply the phase change needed to precisely measure the lubricant film thickness by eliminating the fragile silica layer, and it has been shown that an increase in Cr thickness results in a increase in reflection of the glass–Cr interface making the resulting images hard to process. Modifications to the existing experimental rig were carried out in order to apply/collect an electrical signal from both the disc and the ball. Signal collection from the disc was quite straightforward and a graphite brush paired with a copper nut was used, as this is the oldest method of collecting/applying and electrical signal from a rotating element. Collecting an electrical signal from the ball presented quite a challenge as the ball is submerged in oil. A number of brushes was designed, made and tested and the one that provided the most stable results chosen. For calibration purposes a base oil and two additives were chosen, the additives were chosen in such a way that the improvement made to the lubrication process to be very different from one additive to the other. The chosen additives were a Viscosity Index Improver [VII] and an Organic Friction Modifier [OFM]. The VII is used by many researchers in order to obtain multigrade lubricants using the same base oil by varying its percentage in the mix. The OFM is used to provide protection between the two contacting bodies when EHD film fails and EHD lubrication is replaced by mixed lubrication by forming a boundary layer on the contacting surfaces. Optical measurements were carried out on the base oil and the two resulting lubricants from the additive mixes using the Ultra Thin Film Interferometry [UTFI] method. The measurements were used as a benchmark against which the capacitive measurements were calibrated. Tests were conducted in a number of controlled conditions for speed, temperature, load and sliding conditions. Results showed that the highest influence on the lubrication process was given by the speed, an increase in speed results in an increase in optically measured film thickness and a decrease in electrically measured film thickness. Phenomenon explained by a large amount of lubricant pushed into the contact. Another parameter that influenced the results quite significantly was temperature, a rise in temperature supplies a decrease in optically measured film thickness and an increase in capacitive measured film thickness which was explained by lubricant viscosity dropping with a rise in temperature. Three different sliding conditions were employed and a small drop in optically measured film thickness followed by a small rise in electrically measured film thickness was recorded due to a local increase in contact temperature when sliding was employed. The capacitive method developed in this project is precise enough to accurately measure lubricant film thickness down to 100nm; a model for thicknesses lower that 100nm was proposed Results from the optical and capacitive methods were compared and a good correlation was found, indicating that the developed capacitive method can be used as a tool for measuring metal on metal contacts without further calibration.

Wear of railroad rolling stock and rails costs millions of dollars annually in all rail systems throughout the world. The rail industry has attempted to address flange wear using rail curve lubricants and presently use a variety of lubricants and lubricant applicators. The choice of lubricant and applicator is currently based on considerations that do not address the wear problem directly. This research quantified rail curve lubricant performance through laboratory simulation. The effects of lubricants in the wheel/rail contact were investigated. Rail curve lubricant performance was measured with a laboratory rail/wheel simulator for the purpose of optimising the choice of lubricant. New methods for measurement of rail curve lubricant performance have been presented. These performance measurements are total absorbed energy, the energy absorbed in the lubricant film instead of being utilised for wear processes; total distance slid, the sliding distance or accumulated strain achieved prior to development of a set tractive force limit; half life of lubricant, the time taken for a lubricant to lose half of its sliding performance; and apparent viscosity, a measure of the lubricity presented with respect to accumulated strain. The rail/wheel simulator used in this research consists of two dissimilar wheels (disks) rotating in contact with one another simulating a conformal gauge corner contact. The first wheel, a simulated rail, is driven by an electric motor which then drives the second wheel, a simulated railroad wheel, through the contact. Hydraulic braking on the railroad wheel is used to simulate the rolling/sliding conditions. The variables of the simulated contact that are controlled with this equipment are normal force, input wheel speed, slip ratio between samples, sample geometries and material properties, and lubricant types. Rail curve lubricants were laboratory tested to define their properties using the ASTM and other appropriate standards. The performance differences measured using ASTM standards based tests were susceptible to repeatability problems and did not represent the contact as accurately as the rail/wheel simulator. This laboratory simulator was used to gather data in lubricated and unlubricated conditions for the purpose of providing lubricant performance measurements. These measurements were presented and the tested lubricants were ranked conclusively using three industrially relevant performance criteria. Total sliding distance and total absorbed energy measurements of the rail curve lubricants displayed clear differences in lubricant performance for both of these criteria. Total sliding distance is equivalent to the number of axles in the field situation, while total absorbed energy is the energy unavailable for wear processes of rails and wheels. Lubricants designed using these measurements will increase lubricant performance with respect to these performance criteria which in turn will reduce wear to both rails and wheels. Measurement of the apparent viscosity of rail curve lubricants, using the rail/wheel simulator, displayed changes in rheological characteristics with respect to accumulated strain. Apparent viscosity is a measure of the shear stress transmitted from the wheels to the rails. Designing a rail curve lubricant after analysing measurements taken from the rail/wheel simulator will assist in identifying lubricant properties to reduce the wear producing shear stresses generated in a rail wheel contact. Decay of lubricant performance was measured for three different rail curve lubricants under simulated conditions. The research found appreciable and quantifiable differences between lubricants. Industrial application of the findings will improve positioning of lubrication systems, improve choice of lubricants and predict effective lubrication distance from the lubricant application point. Using the new methods of lubricant performance measurement developed in this thesis, the objective of this research, to quantify rail curve lubricant performance through laboratory simulation, has been achieved.

No description available.

This diploma thesis is concerned with simulation of elastohydrodynamic lubrication of point contacts using the finite element method. The first part of the thesis focuses on the study of the issue and introduces equations for model creation and numerical methods which can be used for EHD calculation. The most suitable solution approach has been chosen from the overview, namely the Full system approach. The software we chose for applying the method was COMSOL Multyphysics. The following part of the thesis deals with model’s creation and gives its elaborate description. We introduce models for calculation of line and point EHD lubrication for newtonian lubricants and, last but not least, we also present a model for calculation of point contact EHD lubrication for non-newtonian lubricants. The next part of the thesis then verifies all the models. That is achieved by comparing the calculated results to results from different papers. The conclusive part of the thesis then examines the matches of acquired results to different prediction relationships and experiments.

In the thesis lubrication and wear at metal/HDPE contacts was addressed. In particular this type of contact occurs in artificial joint replacements. Wear of HDPE was recognized as a major factor limiting device performance. In the thesis, fully implicit fully coupled numerical approach was developed to simulate lubrication and wear. Approach allows solving stationary and transient problems for rough surfaces in a wide range of parameters. Wear coefficients were estimated from experimental data. Wear particles formed in wear process were investigated. Particles were found to be approximately 100 nm in diameter and spherical in shape. Considering theoretical solutions, it was concluded that debris may play a role of third-body abrasive wear particles. In the summary section, some discussion was provided on the topic of theoretical modeling of friction and wear and recommendations for future research were formulated.