1,721,292 research outputs found
Effect of macroscopic wear on friction in lubricated concentrated contacts
Full text linkWear is considered one of the main challenges in the twenty-first century for engineers and designers of mechanical systems. The objective is to understand the wear mechanisms and to seek new solutions - materials, lubricants, additives - to extend the lifetime of components, enable scheduled maintenance and replacement intervals. Research conducted during the past decades shows that wear can be minimized but not eliminated from systems operation due to a large number of parameters which are influencing the evolution of this phenomenon: load, velocity, temperature, type of lubrication as well as the surface roughness. Depending on operation conditions, the occurrence of wear leads to a change in the macro contact geometry of the components. In time, this will affect the functioning of the components, for example: high friction for brakes, clutches and transmissions or low friction for cylinder-piston contact, cam-follower and gears. In this thesis the influence of macroscopic wear on friction in lubricated sliding concentrated contacts is investigated. Experimental wear and friction tests were conducted on different types of lubricated contacts: line, point and elliptical. These tests have shown a change in contact geometry and in operating regime of the systems, i.e. friction level. To understand this, a relation between wear, contact geometry and minimum film thickness is made. In a lubricated contact three zones are distinguished: inlet, contact and outlet zone. The inlet zone dictates the formation of the minimum film thickness between contacting surfaces. When wear is present the contact geometry changes, leading to a modification in pressure distribution. Changes in separation due to wear are modeled based on hydrodynamic theory and are incorporated in a deterministic mixed lubrication friction model. Using this model the transition between the lubrication regimes, as shown in a Stribeck curve, can be predicted. The experimentally obtained results are in agreement with the theoretical simulations. It is shown that increased wear leads to a decreased friction level resulting from the occurrence of the hydrodynamic effects due to a reduced contact pressure. This causes a change in the operating regime of the system. In the one case this changed regime can be considered to be the main cause of failure in a system, whereas in another system it means smooth operation after a period of running-in. The theoretical results are presented in a generalized Stribeck curve. From this, it is possible to select the parameters such that the components of a lubricated system operate in the preferred regime in order to control friction and minimize wear
Mixed Lubricated Line Contacts
Full text linkThe present work deals with friction in mixed lubricated line contacts. Components in systems are becoming smaller and due to, for instance power transmitted, partial contact may occur. In industrial applications, friction between the moving contacting surfaces cannot be avoided, therefore it is essential that an engineer is able to predict friction
The transition to adhesive wear of lubricated concentrated contacts
Full text linkValidation measurements are carried out by means of pin-on-disk measurements, in which the pin consists of a cylindrical element, resulting in a line contact situation in contact with the disk. For these measurements a special high load pin-on-disk tribotester has been developed. From the experiments can be concluded that the measured transitions are well predicted by the model which is developed in this thesis, in contrast to the calculation models known from literature
Friction and wear of zirconia and alumina ceramics doped with CuO
Full text linkIn this thesis, a wear model is developed that relates the properties of the materials and the operating conditions to the type of wear (mild or severe wear) experienced by dry sliding ceramic systems. The wear model is verified experimentally, and hence, with this model, one can determine wheather a dry sliding ceramic system will experience mild wear or severe wear
Design of an anti head check profile based on stress relief
Full text linkHead Checking (HC) is a major type of Rolling Contact Fatigue (RCF) in railway rails across the globe. It mainly occurs on curved tracks in the rail shoulder of the gauge side and at the gauge corner because of the large lateral force. The related track radii are between 500 – 3000 m. It initiates from the surface due to high surface shear stresses arising at wheel-rail contact. HC has severe economic consequences as well as on the safety of railway operations. The serious accident caused by HC at Hatfield in the United Kingdom in October 2000 raised awareness to treat it seriously. The yearly total HC treatment-related cost was about 50 million euros in the Netherlands when the occurrence of HC was at its highest. Although a number of treatment methods for HC are possible, it is concluded that preventing or retarding HC initiation by optimal rail profile design is the most effective in terms of implementability, cost and time span. This thesis therefore aims at the design of an anti-HC profile of rails, based on a fundamental understanding of the mechanical mechanism of HC initiation. To such end, an investigation has been carried out on the quantitative relationship between HC occurrences, contact geometry, stresses and microslip. HC initiation has been reproduced under controlled laboratory conditions on a full-scale wheel-rail test rig. At the same time, HC initiation has been monitored in the field under service conditions. Using a non-Hertzian rolling contact solution method, it is found that HC initiation location tends to be at a distance 7 – 12 mm from the gauge face, where the surface shear stress is the highest as a result of the large geometrical spin in the wheel-rail contact. The optimization is therefore focused on the gauge part of the profile, with the objective of relieving the maximum shear stress. As the 54E1 rail is predominantly used on the Dutch railway network, the optimization is performed on it. After a statistical analysis of the AHC performance of the 54E1 and 46E3 profiles, it is concluded that an undercut of the 54E1 profile at the gauge corner, with the maximum undercut at about 9 mm from the gauge face, should achieve the objective. Together with a number of constraints arising from the existing 54E1 profile, from vehicle running performance, track structure and contact mechanics, an optimal Anti Head Checking 54E1 (AHC 54E1) profile is designed. This designed profile has shown its merits: By avoiding contact in the HC-prone part of the rail, the maximum surface shear stress is greatly reduced, mainly owing to the decrease of spin in the contact. A monitored field test shows that the AHC 54E1 profile can largely delay the HC formation and once HC arises, it also decreases the crack growth by a factor of half. The AHC profile changes due to wear, so that it has to be restored with cyclic grinding to maintain its effectiveness. Large-scale application on the Dutch railway network shows that HC in 2008 was reduced by about 70% with respect to 2004 when HC was the most widespread. At the same time, no negative influence of the AHC 54E1 on the running performance of the trains has been reported, either from the monitored site or from the large-scale application. As a result, the AHC 54E1 profile has been normalized as a standard European rail profile named 54E5 at 1:40, see prEN 13674-1, June, 2009. Recommendations for further research and development are made at the end of the thesis
Running-in of rolling contacts
Full text linkThis thesis deals with running-in of the pure rolling contact situation operating in the boundary lubrication regime, so that normal plastic deformation due to the contact between asperities is the main aspect. The change of the surface topography during the running-in process and the run-in surfaces are predicted locally. The main theme concerns the elastic-plastic asperity contact model. An asperity micro-contact model was considered due to its analytical nature and a well-ploughed study. A new single elliptic elastic-plastic asperity contact model has been proposed and demonstrated as the best prediction to the experimental results among the other models. Plastic deformation appears when the applied load is removed; therefore, the developed asperity contact model was extended to the unloading case
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Mild wear modeling in the boundary lubrication regime
Full text linkCurrently, the increasing demand for smaller and more efficient systems is increasing the stress put on interacting components. This forces components to operate in the boundary lubrication regime. In this lubrication regime, the normal load put on the components is no longer carried by the lubricant but rather by the interacting asperities, and by doing so solid-solid contact is inevitable. This increases the specific wear seen in these types of systems shortening the lifetime of components and increasing maintenance intervals. This decreases the operational times significantly. Therefore, it is of great importance to get a clear understanding of the concept of corrosive wear under these specific conditions. In this thesis three different aspects of wear are discussed namely: the transition from mild to severe wear, running-in and the steady state mild wear. The first is modeled using a thermal threshold originating from Blok’s hypothesis that the transition to adhesive wear is caused by transcending a predefined critical temperature. The model discussed in the current work is based on a numerical thermal model combined with an elastic-plastic contact solver, which are both using the DC-FFT algorithms combined with CGM iterative schemes. In this way the model is able to incorporate mild wear into the thermal and contact calculations while keeping the computational times within a reasonable range. The model is validated through an experimentally determined transition diagram. Running-in of surfaces is modeled using the hypothesis that an additive rich oil is able to protect the contacting elements from metal to metal contact therefore, the growth rate should be the same or greater than the layer removal rate. This hypothesis is combined with a wear model based on a maximum equivalent strain assumption. This states that for material to be removed both an equivalent plastic strain threshold should be met and that the volume including this strain should reach the surface. To be able to compute the plastic strain, a Semi-Analytical-Contact solver is developed based on a local friction model. The mild wear model is based on the dynamic chemical balance at the surface. Through mechanical removal the balance is disturbed and the system will restore the balance through chemical reactions between the base material and additives present in the oil. Since the chemical reaction layers are very thin compared to contact regions, it can be assumed that it has only a limited effect on contact conditions. Using this hypothesis, a model is presented to determine the removal rate of the chemical reaction layer and thus the intensity of corrosive wear. The validation of this model is done using model systems. This thesis is divided into two parts: the first part is a summary of theory presented in the appended papers presented in the second part. This way the reader is able to keep a clear view on the overall goal of the research by reading the first part while the details are discussed in the second part
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