| 231. |
- Åhrström, Bert-Olof
(författare)
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Friction in highly pressurized lubricants and its relation to thermo-physical properties
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The objective of this work was to increase the understanding of which lubricant properties control transient friction, and during what environmental conditions friction is minimized. The experiments have been conducted during conditions relevant to an elastohydrodynamic contact, with pressures extending up to 2.5 GPa, initial temperatures between 20-80°C in the lubricant and loading durations of approximately 200-400 ms. A major revision of an existing work on the relation between pressure and dilatation, for mineral- and synthetic oils, using a modified split Hopkinson pressure bar was undertaken. A quadratic expression with lubricant specific constants, describing the pressure-dilatation relationship without an asymptotic behavior, was presented. A thermal study of isothermal and adiabatic effects on the result was added to the report. One significant observation was that the difference between adiabatic- and isothermal dynamic behavior of the compressed lubricants was small. The proposed idea was that, in practical calculations, it is unnecessary to distinguish between adiabatic and isothermal material data for dynamic calculations. Time-dependent processes on a molecular level must explain the difference found. A new type of experimental set-up was developed, utilizing the flexural response to an exerted load and dynamic beam theory, enabling simultaneous measurement of transverse and normal forces, and thus friction coefficient, as a function of time or pressure. The reproducibility of the load exertion was found to be low but the accuracy of the measurements were encouraging. Differences in friction histories between lubricated and dry contacts were found. Improvement of the developed friction coefficient measurement device was made to enhance reproducibility and accuracy. The experimental set-up was in many ways inverted and a new theory was developed, encompassing both rotational inertia and shear deformation in the energy of motion consideration. Reproducibility between experiments, along with accuracy, went from low to excellent. Five different lubricants were tested and frictional data, as a function of time, were presented. Several phenomena concerning friction were observed and discussed. Two interesting observations were that: Friction increases with increasing pressure during each load cycle (and decreases again as the loading is removed). Friction decreases with increasing maximum pressure of the loading pulse. This apparent contradiction inspired further work in the field and indications suggested that the solution might be embedded in the thermodynamic properties of the lubricants. Enhanced environmental control, added to the set-up, enabled initial temperature increase up to 60°C above ambient. Focus on thermodynamic properties was rewarded with a number of observations and a suitable explanation to the previously described contradiction was formed. It was found that increased coefficient of heat conduction lowered the friction coefficient, as did an increase in specific heat capacity. Both effects strive to reduce the temperature in the contact (by different means) but the tendency is clear: colder contact temperature, lower friction. Further support through the finding that increased initial temperature correspondingly increased friction was acknowledged. The main conclusion was that solid friction is lower than fluid friction during the prevailing test conditions and that a numerical analysis is considered necessary. Frictional data for 12 different lubricants and model hydrocarbons with different physical- and thermodynamic properties were presented. A numerical study of the thermal, shear stress and velocity gradient profiles along with friction coefficients for a paraffinic mineral oil, subjected to a transient pressure pulse in-between two infinite, rigid plates with relative motion was undertaken. Two rheological models were used: one nonlinear viscous and one nonlinear viscoelastic (Ree-Eyring). It was concluded that a Ree-Eyring rheological model together with pressure and temperature dependent viscosity and heat conduction can be used to qualitatively simulate transient rheology experiments. A localized temperature increase occurs in the lubricant film. It is associated with a viscosity decrease and a velocity gradient increase and a “slip plane” is formed. The slip plane will be more pronounced as the pressure pulse peak becomes higher until eventually all motion is localized to that region. The observed contradiction in friction is not viscosity related. The simulation indicated that the friction coefficient increased with increasing pressure, for the rheological model used, in total contradiction to measured data on a real EHL contact. It was consequently concluded that that the frictional properties of the localized shear bands, occurring in the solidified lubricant, were the origin of the detected anomaly and refined rheological models are necessary to bring crisp clarity to the matter.
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| 232. |
- Åström, Peter
(författare)
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Simulation of manufacturing processes in product development
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the manufacture of aero engine components, three key manufacturing processes can be identified. These are welding, heat treatment and mechanical cutting. Depending on what is required in terms of mechanical- and geometrical properties in the final product these processes are typically combined in a certain sequence. It is consistently these three processes, and the modelling and simulation of them using the finite element method, that are of concern to the work presented in this thesis. Product development phases such as the detailed design phase and the process planning phase are typically simulation intensive in aerospace industry. Normally, both more extensive component and manufacturing process data are available at these stages compared to in the previous concept development phase. It is however becoming increasingly important to enable predictions of manufacturability to be made early in the product development process. Changes made in the manufacturing process chart after a product offer is released will result in an increase in cost and a decrease in profit of that component. The aim of the work presented here is thus to develop strategies and computational technologies suitable for simulation of manufacturing processes in early stages of product development. An example of a computational technology developed in this work is the method of element deactivation used to simulate distortion effects of mechanical cutting. Within the area of simulation of manufacturing processes most research work and most publications focus on one individual process being simulated. This has been of great importance for the evolution of computational methods and analysis tools used both in industry and academia. However, when the focus of using simulations often is to determine process parameters in order to obtain desired product properties in the final product, it is essential to include the entire manufacturing process sequence in these simulations. In order to enable a sequence of manufacturing processes to be simulated, engineering information data need to be transferred between simulations of the individual processes. A system for engineering information data exchange in the context of manufacturing simulations is presented in this work. An approach commonly used for concept generation is knowledge based engineering (KBE). In KBE-systems, rules are implemented that embodies the enterprises’ product- and process knowledge. In this thesis, an integrated system where mechanical cutting simulations are performed within a knowledge based engineering software framework is presented. The thesis also deals with friction modelling in the rotary friction welding process and what is appropriate, in terms of computational matters, when attempting to simulate friction welding in a concept development context. The main theme in the thesis is thus to develop strategies, methods and computational technologies suitable for simulation of manufacturing processes such as rotary friction welding and mechanical cutting in early stages of product development.
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| 233. |
- Ödberg, Christer
(författare)
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Skogsavverkningsrätt
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An analysis from civil law point of view with special reference on one hand to the parties´ rights and obligations and on the other hand to the treatment of cer-tain third party conflicts. An agreement between a landowner and another subject of law meaning that the latter is entitled to fell trees on the landowner’s property is according to Swedish law regarded as right to use the property, a usufruct. The Swedish Real Estate Law, however, does not pay much attention to the question of the parties’ rights and obligations regarding this kind of usufruct. Since this kind of agreement has also got important similarities with purchase, it has in Swedish legal writing been a common point that the Swedish Sale of Goods Act ought to be, either directly or analogically, applicable at least as long as the question concerns the parties rights and obligations. The dissertation examines some of several unclear or uncertain questions concerning the parties´ rights and obligations and especially such questions that from a practical point of view seem to be of interest in the line of business. A few examples of questions of this kind that are discussed are if and when the risk is handed over to the feller, if the feller has to pay compensation even if the felling has not been carried out, if the feller is entitled to make claims against the landowner if there is faults or shortage or the land-owner’s corresponding possibilities if the feller breaches the contract. In this connection special questions concerning the forming of standard contracts in this field are observed. The dissertation also deals with different kinds of third party conflicts concerning rights to fell trees. In this case there is no doubt that the rules of the Real Property Code are applicable, but the special circumstances seem to make the application troublesome, especially when the conflict primarily has reference to the ownership of the timber in-stead of the right to use the property. Such conflicts where the creditors of the feller or the landowner are involved are also examined. The main purpose is to find appropriate answers to the different questions according to Swedish law, but the answers are often uncertain. In several questions the relevant ju-ridical sources are not admitting certain conclusions. These difficulties have more or less enforced an approach to the subject characterised by recommendations and sugges-tions de lege ferenda.
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| 234. |
- Öhman, Micael
(författare)
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The measurement of compression wood and other wood features and the prediction of their impact on wood products
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wood is a complex and highly variable biological material formed to give the tree best possible conditions for sustaining life. Thus every piece of wood is possessed of unique qualities. The great challenge in the process of manufacturing wood products has always been to select pieces of wood with properties that fulfil requirements for the product. The importance of selecting the right piece of wood will increase along with demands from customers for products with specific properties, such as moisture content, warp, strength, biological and aesthetic features. In order to supply customers with the products they request, a considerably improved selection of the raw material is needed. The earlier an accurate selection can be done, the better. To improve this selection or pregrading process, knowledge of the relationships between different features and different aspects of quality, as well as methods for measuring external and internal features, must be developed. The main objective of this work was to contribute to improved predictability of the quality of dried sawn products using the features both of logs and of sawn green products as input. This work was divided into two parts: Part one focused on the possibility of learning how to predict the quality grades of centre planks by manually inspecting longitudinal radiograph images (LRI) that depict the density variation within a log. In a survey respondents were interviewed regarding their interpretations of the density related features visible in the LRIs of Scots pine logs (Pinus silvestris L). The purpose was to be able to use these interpretations in predicting the final quality of planks sawn from the logs. The LRIs were reconstructed with the aid of an X-ray CT scanner. Part two focused on the relationship between compression wood (CW) in foremost butt logs of Norway spruce (Picea abies (L.) Karst) and the warp of the sawn products, on how to detect CW and on how to predict warp. The logs used in the study were chosen among logs delivered to and sawn at commercial sawmills located in the northern part of Sweden in order to assure that conditions in the study match those extant in commercial sawmills. The most important findings in this thesis are: Longitudinal radiograph images of the density variation within a log can be a powerful aid in manual grading of logs with respect to the quality of the resulting sawn products. The shape of the sawn, but not dried, centre planks is an indicator of both the amount and distribution of compression wood. Basing the cutting of planks on their shapes while still green can considerably increase the total length of acceptably straight dried products. This improvement is achieved through the elimination of compression wood.
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| 235. |
- Öhrling, Kerstin
(författare)
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Being in the space for teaching-and-learning : the meaning of preceptorship in nurse education
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this study was to illuminate student nurses' and nurses' lived experience of preceptorship and develop an understanding of the meaning of being a preceptor and of teaching and learning in a preceptor-preceptee relationship during student nurses' clinical education on hospital wards. A phenomenological approach was applied to study the experiences of those who are living in an ongoing experience of preceptorship. Seventeen student nurses and the seventeen nurses acting as their personal preceptor volunteered to participate. To illuminate their experience of preceptorship, tape recorded narrative interviews were conducted, transcribed verbatim and analysed by using a phenomenological-hermeneutic method. According to the findings, the student nurses disclosed the meaning of being precepted in themes related with their being directed towards increased competence and with the feelings and bodily sensations that accompanied the learning, a learning that occurred when performing practical nursing actions. The student nurses learning was facilitated by preceptors who created a space for their learning and in that space provided concrete illustrations, exercised control and managed to find the peace and quite needed for reflection. The meaning of being a preceptor revealed a balancing to find solutions to the problem of interrupting the time for nursing when including the student in one's daily work, as well as increased awareness of the processes of learning. In good examples of preceptoring the meaning of preceptoring was understood as the preceptors sheltering and facilitating the student nurses when learning by the use of teaching strategies. The meaning of preceptorship was compared with a didactic model, where the student-preceptor relationship formed the basis of the processes of teaching-and-learning, which embraced intentions, acting, lived-experience and reflections. Further, it is demonstrated that preceptorship supports the student nurses and the preceptors increasing their competence in nursing, parallel with the student nurses reducing their dependence on preceptor support.
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| 236. |
- Ökvist, Lena Sundqvist
(författare)
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Co-injection of basic fluxes or BF flue dust with PC into a BF charged with 100% pellets : effects on slag formation and coal combustion
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Based on 100% pellets operation at BF No. 3 at SSAB Tunnplåt in Luleå a new pellet with CaO/SiO2=1 was developed during early nineties. The pellet showed good results in metallurgical laboratory test but caused slag formation problems in the bosh. A high basicity slag was formed during interaction with basic fluxes and its melting point was increased when the slag was finally reduced. By injection of basic fluxes, the slag formation problems in the bosh can be avoided. Without a sinter plant, dusts have to be recycled in a cold bonded briquette. Injection of some of BF flue dust would improve the properties of the briquette and may be beneficial for the BF process. Based on top charging or tuyere-injection of basic fluxes and the injection of BF flue dust studies have been done. The studies carried out will clarify some phenomena of slag formation and the effect of co- injection on coal combustion in the blast furnace, including the effect of: 1) basic fluxes on slag formation in the blast furnace, if they are top- charged; 2) basic fluxes on slag formation in the blast furnace, if they are tuyere injected; 3) chemical composition and metallurgical properties of pellet and fluxes on slag formation; 4) reduction conditions on (reduction temperature, reducing gas composition) on bosh slag formation; 5 co- injection of BOF slag or BF flue dust on coal combustion efficiency and BF performance. From the results, it can be concluded that the interaction between pellets and fluxes starts when softening and melting starts in the cohesive layer. By choosing fluxes of a high melting point the dissolution of them in the bosh slag can be delayed and the excessive basicity of the bosh slag can be limited. A basic flux with a low melting point will easily dissolve into the bosh slag causing its basicity to increase, which is beneficial to S refining when acid pellets are used. The formation of a bosh slag of excessive basicity (which causes BF operation disturbances, because its high melting point, increases further during reduction) can be avoided by tuyere injection of basic fluxes with the coal. The positive effect is greatest when a fluxed pellet of basicity B21 and with a high Fe content is used as ferrous burden. In this case, the slag amount can be significantly decreased. BOF slag has suitable high-temperature properties for use as a basic flux in combination with e.g., olivine pellets. It has a low softening and melting temperature, does not shrink, is slowly reduced in the BF shaft and its properties are almost unaffected by the partial reduction occurring in the shaft of the BF. BOF slag absorbs a low content of alkalies, which causes volume increase at elevated temperatures, when the basicity is still high. The results indicate that co-injection of BOF slag or BF flue dust with PC has positive effects on the BF operation. The injection of BOF slag decreases the Si content in HM by an increased basicity of the slag formed during combustion leading to a decreased activity of SiO2 in the slag and by an increased FeO content in the tuyere slag that will be reduced by SiFe in the metal at the tuyere level, if the SiFe is initially high. The consumption of reducing agents can be reduced because of a decreased Si content in HM and a decreased slag amount. The permeability is improved by prohibiting the formation of a shell at the raceway end because of improved melting properties of tuyere slag caused by an increased basicity and FeO content, consumption of coal and coke fines by FeO and improved radial transport of tuyere slag. The injection of BOF slag improves the bosh slag because an excessive basicity caused by top-charged fluxes added for neutralisation of coal and coke ashes and non-uniform slag formation caused by uneven distribution of top-charged fluxes are avoided. The productivity increases because of the possibility to decrease the slag amount. The injection of BOF slag can be done without negative effects on combustion efficiency by using very finely ground BOF slag and because of improved total consumption of coal and coke fines by direct reduction in the coke bed. The injection of BF flue dust decreases the Si content in HM by an increased FeO content of the tuyere slag that will be reduced by SiFe in the metal at the tuyere level, if the SiFe is initially high. The consumption of coal and coke is decreased because of a high of C content in BF flue dust and the decreased Si content in HM. The permeability is improved by prohibiting the formation of a shell at the raceway end because of consumption of coal and coke fines by FeO and improved melting properties of tuyere slag promoting the radial transport of the tuyere slag. The injection of BF flue dust can be done without negative effects on the total consumption of coal and coke fines by direct reduction in the coke bed. Injected BF flue dust supplies oxygen for combustion, but because of endothermic reactions as reduction of hematite and calcination of limestone occurring in the BF flue dust at the same temperatures as release and disintegration of VM, it will have a negative effect on the combustion of HV coal. SSAB Luleå and SSAB Oxelösund have started up projects aiming to make tuyere- injection of BF flue dust part of the standard BF operation.
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