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  • Dahlén, Leon (författare)
  • Numerical and experimental study of performance of a hydraulic motor
  • 2003
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>The aim of this study was to develop an understanding of the fluid properties that influence the efficiency of hydraulic systems in a steady-state, especially components in hydrostatic transmission systems under different running conditions. The aim has also been to investigate and model the sources of losses in hydraulic machines, in order to estimate the losses, taking fluid properties into account. Finally, the technique of optimization has been introduced in order to improve the efficiency of a distributor valve in a radial piston hydraulic motor, Marathon M200. From an experimental field-test, which was performed on a belt conveyer using a hydrostatic transmission system, the overall efficiency of the hydrostatic transmission was compared when using a mineral oil, Shell Tellus TX 68, a synthetic fluid, Mobil SHC 526, and a vegetable fluid, Binol Hydrap II. The experimental field-test showed that vegetable and synthetic fluids improve the efficiency compared to mineral oil. The three fluids have the same viscosity but experimental tests showed that the temperature ()- and pressure() -viscosity coefficient differs between them. It was also found that the pressure-viscosity coefficient () of mineral oil was higher compared to vegetable and synthetic fluids. The proposed steady-state model gives greater accuracy regarding overall efficiency than the Wilson model, when examining fluid properties that differ in other aspects than their viscosity. The study has focused on losses in lubricated sliding contacts within a radial piston hydraulic motor, Marathon M200, and an analysis of the losses has been made using the finite element method (FEM). A FEM software package, Solvia, which takes into account fluid properties, such as temperature- and pressure- viscosity coefficient, heat conduction and specific heat, has been used to simulate the behaviour and to estimate the losses in tribological contact. This approach of simulation has been applied to two different tribological contact within the hydraulic motor: to a journal bearing contact and to a hydrostatic annular multi-recess plane thrust bearing. By using an FEM software package linked to an optimization algorithm, the losses in the tribological contact in a distributor valve were reduced significantly. The study shows that the optimized geometry of the distributor valve in the motor can successfully be improved, with regard to losses, by small changes in the geometry. Combining an FEM software package with the optimization routine offers an effective tool for designers to simulate and improve the efficiency of a hydraulic unit.</p>
  • Isaksson, Per (författare)
  • On crack growth under compressive stresses
  • 2001
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>This thesis concerns fractures subjected to compressive stresses. In the four papers appended fracture behavior in brittle as well as ductile materials is studied. In the first paper, an expression for the mode II stress intensity factor at a straight extended kink has been calculated under the condition that crack opening is suppressed during crack growth. The expression has been found as a function of the mode II stress intensity factor K2 at the parent crack, the direction and length of the kink, and the difference between the remote compressive normal stresses perpendicular to, and parallel with, the plane of the parent crack. Crack growth directions have been suggested based on the result. At a sufficiently high non-isotropic compressive normal stress, so that the crack remains closed, the crack will propagate along a curved path maximizing the mode II stress intensity factor. Only at an isotropic compressive normal stress will the crack continue straight ahead in its original plane without directional change. By analyzing experimental crack growth patterns in paper two, the conclusion is that crack paths experimentally observed indicate that mode II crack growth under compression in some brittle materials follow a propagation path described by a function y=gx^b. In fact, the agreement between the experiments and the propagation path prescribed by the model, in which b=3/2, is astonishingly good since b was found in the interval [1.43-1.58] in all the experiments studied. Further, the investigation of the curvature parameter g has revealed that g also agree with the simplified model, even though not as good as the exponent b. However, the experimentally observed g differs in general less than 15% from the theoretical value predicted by the analytical model discussed in paper I. In paper three, a directional crack growth criterion in a compressed elastic perfectly-plastic material is considered. A slip-line solution is derived for evaluation of the stresses at the crack tip, which considers hydrostatic pressure and friction between the crack surfaces. Based upon the slip-line solution a projection stress based model is discussed for prediction of the direction of initiated crack growth. The opening displacement of an extended kink has been examined in paper four, using a finite element procedure. The conclusion is that an over-critical pressure in the plastic zone surrounding the crack tip suppresses crack opening regardless the direction of crack growth. The only possibility seems to be shear mode crack growth, which occur straight ahead in the crack plane if the crack is assumed to follow the plane of maximum shear stress. At a sub-critical hydrostatic pressure, or lower friction between the crack surfaces, the crack can extend via a kink subjected to local opening mode. An expression for the critical value determining fracture mode has been found as a function of hydrostatic pressure and friction between the crack surfaces assuming the fracture process to be predominantly controlled by local tensile stresses at the crack tip. The crack growth directions predicted by the projection stress based criterion in paper three are comparable with the directions maximizing the opening displacement of an extended kink computed in paper four.</p>
  • Juter, Kristina (författare)
  • Limits of functions : university students' concept development
  • 2006
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>Avhandlingen består av sex artiklar som föregås av en kappa. Kappan sammanfattar de teorier som använts i artiklarna, empiridelen samt resultat och slutsatser. Fem av artiklarna är publicerade i tidskrifter eller accepterade för publikation i tidskrifter och den återstående är accepterad för publicering i en konferensrapport.</p><p>Gränsvärden är väldigt centrala i matematisk analys och därför viktiga att förstå för studenter som ska syssla med matematik. Avhandlingen handlar om studenter som läser sin första matematiktermin på universitetsnivå. Två studentgrupper har undersökts med avseende på hur de hanterar gränsvärden av funktioner under 10 respektive 20 veckor. Utländsk litteratur har visat att detta begrepps komplexitet försvårar studenternas inlärning. Undersökningen genomfördes för att ytterligare klargöra hur studenterna resonerar kring gränsvärden, speciellt i Sverige där liknande undersökningar inte gjorts tidigare.</p><p>Undersökningen genomfördes år 2002 med en grupp på våren och en på hösten. Studenterna fick tre enkäter med gränsvärdesproblem och attitydfrågor om matematik. Femton studenter från den andra gruppen intervjuades vid två tillfällen vardera. Dessutom gjordes fältanteckningar. De olika sätten att samla in data gav en bred bild av verksamheten i klassrummen och föreläsningssalen samt hur studenterna resonerade i olika situationer.</p><p>Undersökningen visade att kontinuiteten i studenternas inlärningssituation brutits på flera nivåer och skapat stora problem för studenterna. Det fanns till exempel klyftor mellan gymnasiets hantering av gränsvärden och universitetets hantering av gränsvärden och mellan teoretisk hantering av gränsvärden och problemlösning. Studenterna var bra på att lösa standardproblem, men de hade svårigheter att koppla den formella teorin till problemen de löste och att lösa problem som var formulerade något annorlunda än studenterna var vana vid. Resultaten visar på att det behövs större variation i studenternas inlärning så de blir varse om sina egna uppfattningar och kan korrigera det som är felaktigt. Många studenter tyckte själva att de hade kontroll över begreppet trots att undersökningen visade att de inte hade det.</p>
  • Johansson Falck, Marlene (författare)
  • Technology, Language and Thought Extensions of Meaning in the English Lexicon
  • 2005
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>In this thesis, the relationship between technological innovation and the development of language and thought is analysed. For this purpose, three different fields of technology are investigated: 1) the steam engine, 2) electricity, and 3) motor vehicles, roads and ways. They have all either played an extremely important part in people’s lives, or they are still essential to us. The overall aim is to find out in what ways these inventions and discoveries have helped people to develop abstract thinking and given speakers of English new possibilities to express themselves. Questions being asked are a) if the correlations in experience between the inventions and other domains have motivated new conceptual mappings? b) if the experiences that they provide people with may be used to re-experience certain conceptual mappings, and hence make them more deeply entrenched in people’s minds? and c) if the uses of them as cognitive tools have resulted in meaning extension in the English lexicon?</p> <p>The study is based on metaphoric and metonymic phrases collected from a number of different dictionaries. In the material a large number of metaphorical and metonymic expressions including terms connected to the inventions and discoveries that are part of this thesis are found. As is clear from the expressions, the steam engine, electricity, motor vehicles, roads and ways have all provided us with ample tools for structuring our thoughts, and for conveying our thoughts to others. Primarily, it seems to be the different functions of the discoveries and inventions, or the effects that they have on other objects that have motivated the mappings. In addition to analysing the cognitive role of the inventions that are part of this thesis, some general conclusions concerning the relationship between language, thought and world are suggested.</p>
  • Fors, Vaike, 1969- (författare)
  • The Missing Link in Learning in Science Centres
  • 2006
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>Science centres have been identified as an important resource in encouraging teenagers to choose higher education in science and technology. This is of interest to society, since there seems to be a problem in getting sufficient numbers to do so. And accomplishing this is sometimes described as a fatal question for a nation’s future prosperity and development. Still, there is an international trend where teenagers fail to visit science centres.</p><p> </p><p>Through research, little is known about what is interesting or useful to the public, as well as how to reach those who are ‘unengaged’. Considering teenagers as exponents for what distinguishes today’s society makes their apparent unwillingness to participate in science centres interesting to study with regards to what culture, history and ideology these centres were initially produced. Hence, from this point of view, what is missing in science centres that would make them interesting for the young people of today?</p><p> </p><p>Many studies of learning in science centres have come to focus on visitors who visit voluntarily and how well the embedded messages in the exhibits have been acknowledged by these visitors. This study focuses instead on teenagers who are reluctant to participate in science centres, with their perspective of science centres as the point of departure, specifically what kind of social activities are formed in their encounters with science centre exhibits. This encounter is regarded as an encounter between the two different practices of the science centre and the teenagers. The applied theoretical perspective is mainly assembled from socio-cultural theories of learning.</p><p> </p><p>This research is a microanalytic study of five teenagers who were equipped with video cameras and asked to film a visit to the local science centre, Teknikens Hus. The films were later discussed in a focus-group interview consisting of the teenagers and the researcher. Visual ethnography provided the theoretical framework for this research design.</p><p> </p><p>The results showed that the teenagers want to use exhibits to have the authority of interpretations and the possibilities to contribute to the meaning of the activity. At the same time, they want to use the exhibits in a way that the activities become places for developing social identity. To negotiate the meaning of the exhibits there is a need for an openness that may be constrained by too inflexible and limiting exhibit designs. This pattern is described as two different forms of participation in the exhibits; <em>ignoring</em> or <em>extending</em> the intended meaning of the exhibits. Meaningfulness also demands a closeness created by connections between the exhibit and the user’s personal experiences. This pattern is described as two different ways in which the teenagers identified the exhibits; exhibits which they <em>dissociated</em> from or to which they had an <em>ongoing</em> relationship. Providing a space for negotiation seems crucial to inviting teenagers into opportunities of meaningful experiences, even more significant than any specific physical feature in the exhibit.</p><p> </p><p>The teenagers’ agenda, in which forming practices where they can express themselves and contribute to the meaning seem to be very important, appears not to be greatly enabled by science centre exhibits. In this situation they learn <em>to</em> <em>not </em>participate. Science and technology represented in this matter show a ‘ready-made’ world that they cannot change. The missing link in learning in science centres is here described as the part of the meaning making process where the teenagers get to re-negotiate the meaning of the activities in the centre and use the exhibits as tools to accomplish this.</p>
  • Gao, Chuansi (författare)
  • Slip and fall risk on ice and snow : identification, evaluation and prevention
  • 2004
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Slip and fall accidents and associated injuries on ice and snow are prevalent among outdoor workers and the general public in winter in many regions of the world. To understand and tackle this multi-factorial problem, a multidisciplinary approach was used to identify and evaluate slip and fall risks, and to propose recommendations for prevention of slips and falls on icy and snowy surfaces. Objectives were to present a systems perspective of slip and fall accidents and related risk factors; to evaluate the integration of slip resistance, thermal insulation, and usability of footwear by subjective trials. Further, to study the interaction mechanisms between footwear soling material and ice; to investigate and describe the relationship of ice and snow living experience and participation in winter sports with slips and falls; and to explain the role and mechanisms of lower extremity muscles in maintaining gait dynamic balance on inclined icy surface. The following methods were applied: 1) a systematic analysis of multiple risk factors through literature review; 2) direct observation; 3) videotaping; 4) subjective evaluations of slipperiness, thermal comfort and usability using rating scales; 5) a tribological study of the Coefficient of Friction (COF); 6) a questionnaire survey to identify and evaluate risk factors related to slips and falls; 7) a biomechanical study of human locomotion on ice. A systematic analysis of slips and falls on icy and snowy surfaces made it possible to use a multi-faceted approach to evaluation and prevention. The slip resistance, thermal insulation and usability of footwear tested were not suitably integrated, indicating the need of improving anti-slip properties and usability of footwear for use in cold environments. The artificially abrasive wear of soling materials improved slip resistance on hard ice, but not on melting ice. When comparing hard ice, melting ice and lubricated steel surfaces, the highest slip resistance was observed on hard ice, then on lubricated steel plate. Melting ice was most slippery. Curling footwear with crepe rubber soling showed significantly higher COF than other types. The footwear tested included winter footwear, professional footwear, safety footwear, and footwear considered to be slip resistant by manufacturers did not provide adequate protection against slips and falls on melting ice. Thus, additional measures should be taken to reduce slip and fall risk on melting ice. Slip and fall events declined with increased living experience in cold environments and winter sports participation. Biomechanical studies showed both increased and decreased magnitudes of gait muscle activities (EMG) at different gait phases and a generating of cautious gait strategies with an anticipated icy surface. These findings provided insights into how the proactive kinetic and kinematic adjustments of gait are achieved and the control mechanisms of stabilizing gait and posture through certain muscle activities when ascending on an icy slope. The mechanisms identified may be applied to develop intervention, rehabilitation and training techniques, which prevent slip and fall risks and associated injuries on icy surfaces and to improve performance in human locomotion, e.g., for winter sports. Measurement methods of slipperiness on ice and snow, related standards, task-related factors, slip resistant footwear, and intervention programs should be further studied and developed.
  • Vinberg, Stig (författare)
  • Health and Performance in Small Enterprises : Studies of Organizational Detminants and Change Strategy
  • 2006
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • <p>The present thesis focuses on relations between work organizational factors and outcomes related to health and performance, and how these factors and outcomes are related to change strategies in small enterprises. Reasons for this work are the growing interest in entrepreneurship, small business development, statements to the effect that small enterprises lack adequate resources and competence needed for successful management of workplace change processes and fragmented small business research about mentioned relations. The thesis comprises four empirical studies with a total sample of 118 small enterprises and 50 small public workplaces that include a total of 1206 co-workers and leaders, and one more theoretically based study. Data was collected by the use of questionnaires, structured interviews and by register sources about economical outputs. Correlation analyses presented in Paper II showed strong or rather strong relationships between the outcome indicators related to health and performance. The relations between these outcomes and indicators of assumed determinant organizational factors resulted in a rather large number of relationships (Paper I, II and V). A general result is that there are more strong relationships between determinants and performance than between determinants and health. However, there are strong relations between, on the one hand, leadership indicators and team spirit and, on the other hand, health. Structural analyses in two studies presented in Paper I and II resulted in the identification of six components. Thus, it was possible to group indicators into larger “bundles” which have similarities to some results for larger enterprises. In one study (Paper II), analyses using multidimensional scaling resulted in a grouping of enterprises with high positions versus enterprises with low positions on two main dimensions. Results in one study (Paper IV) showed that studied micro-enterprises attached less priority to goals related to workplace health and work organization compared to the studied public workplaces. The micro- enterprises attached more priority to goals connected with the physical working environment, production and quality development. In another study (Paper V) different change strategy approaches were studied with a longitudinal design. The results concerning changes of determinants and outcomes, after versus before workplace related interventions indicates that change processes with a broad learning strategy and high top management involvement can apply to small enterprises, and help to improve their health and performance. This is partly in line with the discussion in Paper III about the need of integrated planning and participative approaches in workplace change processes. The thesis findings point at the importance of organizational factors and health related aspects for small business development.</p>
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