• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2001.11a
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• pp.7-7
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• 2001

The increasing interest in light weight materials coupled to the need for cost -effective processing have combined to create a significant opportunity for aluminum P/M. particularly in the automotive industry in order to reduce fuel emissions and improve fuel economy at affordable prices. Additional potential markets for Al PIM parts include hand tools. Where moving parts against gravity represents a challenge; and office machinery, where reciprocating forces are important. Aluminum PIM adds light weight, high compressibility. low sintering temperatures. easy machinability and good corrosion resistance to all advantages of conventional iron bm;ed P/rv1. Current commercial alloys are pre-mixed of either the AI-Si-Mg or AL-Cu-Mg-Si type and contain 1.5% ethylene bis-stearamide as an internal lubricant. The powder is compacted in closed dies at pressure of 200-500Mpa and sintered in nitrogen at temperatures between $580~630^{\circ}C$ in continuous muffle furnace. For some applications no further processing is required. although most applications require one or more secondary operations such as sizing and finishing. These sccondary operations improve the dimension. properties or appearance of the finished part. Aluminum is often considered difficult to sinter because of the presence of a stable surface oxide film. Removal of the oxide in iron and copper based is usually achieved through the use of reducing atmospheres. such as hydrogen or dissociated ammonia. In aluminum. this occurs in the solid st,lte through the partial reduction of the aluminum by magncsium to form spinel. This exposcs the underlying metal and facilitates sintering. It has recently been shown that < 0.2% Mg is all that is required. It is noteworthy that most aluminum pre-mixes contain at least 0.5% Mg. The sintering of aluminum alloys can be further enhanced by selective microalloying. Just 100ppm pf tin chnnges the liquid phase sintering kinetics of the 2xxx alloys to produce a tensile strength of 375Mpa. an increilse of nearly 20% over the unmodified alloy. The ductility is unnffected. A similar but different effect occurs by the addition of 100 ppm of Pb to 7xxx alloys. The lend changes the wetting characteristics of the sintering liquid which serves to increase the tensile strength to 440 Mpa. a 40% increase over unmodified aIloys. Current research is predominantly aimed at the development of metal matrix composites. which have a high specific modulus. good wear resistance and a tailorable coefficient of thermal expnnsion. By controlling particle clustering and by engineering the ceramic/matrix interface in order to enhance sintering. very attractive properties can be achicved in the ns-sintered state. I\t an ils-sintered density ilpproaching 99%. these new experimental alloys hnve a modulus of 130 Gpa and an ultimate tensile strength of 212 Mpa in the T4 temper. In contest. unreinforcecl aluminum has a modulus of just 70 Gpa.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.2
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• pp.634-643
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• 2020

In this study, the rheological and tribological properties of urea grease were studied according to the type and amount of polytetrafluoroethylene (PTFE) powders added to the urea grease, which is the most widely used among solid lubricants, to develop an optimal lubrication system. Urea grease was synthesized using 4,4'-methylenebis(phenyl isocyanate)(MDI), oleylamine, and cyclohexylamine, and PTFE powders prepared by dispersion or suspension polymerization process were then added. The basic rheological and tribological properties of the prepared greases were compared. The worked penetration numbers of urea grease decreased with increasing amount of PTFE powders, but both PTFE powders caused no significant changes in heat resistance and copper corrosion resistance. The shear viscosity increased with increasing PTFE powder content, and the dispersion-type PTFE powder was more effective in increasing the viscosity. In the value of the loss coefficient = 1, the shear stress was higher for the grease containing PTFE powders than the non-PTFE added grease, and the dispersion-type PTFE-added grease showed higher viscosity than the suspension-type PTFE-added grease. Finally, urea grease was found to have a low-performance improvement in terms of wear reduction effects by adding PTFE powders, but the load-bearing performance was up to 2.5 times higher for the dispersion-type PTFE and five times higher for the suspension-type PTFE.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.11
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• pp.697-704
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• 2017

This study was carried out to investigate and alleviate the vibration problem of commercial high-speed trains. First, the measurement of the carbody vibration was performed, in order to determine the vibration level of the high-speed train. The measurement result showed that the vibration level of the driver cab was higher than that of the passenger car and that the vibration became bigger toward the trailing end of the train. The vertical vibration of the driver cab and passenger car was larger than the transverse vibration, and the maximum value of the vibration in the ballast section was larger than that in the concrete section. A dynamic analysis was carried out to improve the vibration of the KTX-Sancheon train. The results of the analysis showed that it is necessary to reduce the vibration of the driver cab and both ends of the passenger cars. To reduce the vibration of the driver cab, it was recommended that the stiffness of the secondary coil spring be reduced and the damping coefficient of the secondary vertical damper be increased. It was found that the failure of the suspension system could be the origin of the vibration problem of the high-speed train. The proper management of wheel wear plays an important role in the improvement of the operation efficiency and reduction of the carbody vibration of high-speed trains, and research is underway to change the present wheel profile to increase the mileage between wheel turning.

• KEPCO Journal on Electric Power and Energy
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• v.3 no.2
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• pp.79-88
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• 2017

In this paper, the wind tunnel test for the measurement of aerodynamic characteristics of transmission conductors with asymmetric sections is described. A single conductor model and bundled conductor models with ice accreted shapes are tested both in steady and turbulent flow, and the aerodynamic coefficients are acquired. Transmission conductor galloping is a kind of wind-induced vibration which is characterized by primarily vertical oscillation with a very low frequency and a high amplitude. It is well known that transmission conductor galloping is generally caused by moderately strong, steady winds when a transmission conductor has an asymmetric cross-section shaped by accreted ice. Galloping should be considered from the design stage of overhead lines because it can cause severe wear and fatigue damage to attachments as well as transmission conductors. It is reported that there have been normally 20 events of galloping per year in Korea, which may be followed by serious consequences in the electric power system. Therefore, this research is performed to measure aerodynamic characteristics of ice accreted transmission conductors to understand and control transmission conductor galloping so that it would help to prevent unexpected failures and reduce the maintenance costs caused by galloping.

• Applied Chemistry for Engineering
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• v.10 no.3
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• pp.419-426
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• 1999

In this study, the reaction rate constant, activation energy, total crosslinking density, elastic constant, cure properties ($t_5,\;t_{90}$), modulus, and abrasion resistance of rubber compounds were investigated as a function of cure temperatures, cure systems and reinforcing filler loadings. Reaction rate constants showed strong dependence on thc carbon black loading, cure temperature and cure system, and increased sharply with increasing the reaction temperatures. The lowest activation energy was obtained in the efficient cure (EC) system which corresponds to the high level of sulfur to accelerator ratio, and the activation energy was decreased with decreasing the carbon black loadings. The change of carbon black loadings directly affects the modulus and abrasion resistance, but the change of cure system showed various effects on the rubber compounds. Increased carbon black loadings showed the high modulus, improved abrasion resistance and short scorch time but decrease in crosslinking density and elastic constant. Higher crosslinking density and elastic constant were shown in the EC cure system regardless of carbon black loadings, but scorch timc ($t_5$) was not affected by the change of the ratio of sulfur to accelerator. Rapid optimum cure time ($t_{90}$) were showen in the EC cure system. Also, the equivalent cure curve coefficient of rubber compound was 0.96 for conventional cure (CC) system, and 0.94 for semi-efficient cure (SEC) and EC system regardless carbon black loadings. As regarding the abrasion resistance, wear volume showed the logarithmic increase for the loaded weight.

• The Journal of Korean Academy of Prosthodontics
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• v.44 no.2
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• pp.229-242
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• 2006

Statement of problem: In order to increase preload with reducing the friction coefficient, abutment screws coated with pure gold and Teflon as dry lubricant coatings have been introduced. But the reported data indicate that if screw repeated tightening and loosening cycle, an efficiency of increasing preload was decreased by screw surface wearing off. Purpose: This study was to evaluate the influence of tungsten carbide/carbon coating, which has superior hardness and frictional wear resistance, on the preload of abutment screws and the stability of coating surface after repeated closures. Material and method: The rotational values of abutment screws and the compressive forces between abutment and fixture were measured in implant systems with three different joint connections, one external butt joint and two internal cones. Moreover the stability and the alteration of coating surface were examined by comparison of the compressive force and the removable torque values during 10 consecutive trials, observation with scanning electron microscope and analyzed the elemental composition with energy dispersive x-ray spectroscopy Results and conclusion: 1. Application of coating resulted in significant increase of compressive force in all implant systems(P<.05). The increasing rate of compressive force by coating in external butt joint was gloater than those in internal cones (P<.05). 2. Coated screw showed the significant additional rotation compared to non-coated screw in all implant systems (P<.05). There were no significant differences in the increasing rate of rotation among implant systems (P>.05). 3. Removable torque values were greater with non-coated screw than that with coated screw (P<.05). 4. Coated screw showed insignificant variations in the compressive forces during 10 consecutive trials(P>.05) 5. After repeated trials, the surface layer of coated screw was maintained relatively well. However surface wearing and irregular titanium fragments were found in non-coated screw.

• Applied Chemistry for Engineering
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• v.34 no.4
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• pp.434-443
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• 2023

We have investigated the performance improvement of grease by synthesizing calcium sulfonate grease as an alternative to lithium grease, which is widely used globally. Since the composition ratio of the grease, when manufactured, is usually 50% base oil and 50% thickener, using grease as a lubricant in a cryogenic environment is not encouraged due to its inferior low-temperature performance. In this study, we have synthesized three types of calcium sulfonate grease with paraffin oil and PAO-based base oil and thickener. Furthermore, lithium grease was synthesized via saponification with PAO-based base oil, lithium hydroxide, 12-hydroxystearic acid, and sebacic acid. We have measured low-temperature characteristics using a rheometer and low-temperature torque meter, and tribology characteristics were obtained using a four-ball lubricant tester and schwingung reibung verschleiß (SRV). As a result, the flow point of the calcium sulfonate grease synthesized with a PAO-based base oil and thickener was found to be -40℃, overcoming the existing calcium sulfonate grease's low-temperature limitation. Moreover, the synthesized calcium sulfonate grease showed low-temperature performance similar to that of lithium grease synthesized with PAO base oil, but superior anti-wear, extreme pressure, coefficient of friction, heat resistance, adhesion, and corrosion resistance. It is thus expected to commercially replace the existing lithium grease.

• Child Health Nursing Research
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• v.8 no.1
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• pp.32-43
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• 2002

The purpose of this study was to provide basic data on elementary school children's lifestyle and to contribute to developing on the health education program in elementary schools. The subjects were 1,412 children in 4 elementary schools in Gangwon-Do and Chonrabuk-Do area. Data collection was done from September to November 2001 by questionnaire and school health documents. The questionnaire corrected for the purpose of this study which had been developed by Bronson School of Nursing(1991), 'Lifestyle Questionnaire for School-age Children'. The questionnaire consists of 3 categories; 'Activities that promote health', 'Injury prevention', 'Feeling'. Cronbach coefficient alpha for the 29 items was .68. The data analyzed to obtain frequency, mean, percentage, t-test, ANOVA and Pearson correlation coefficient by SPSS Win program. The results of this study were as follows. 1. Females(50.2%) of gender, 6th grade(24.2%) of grade, nuclear family(82.0%) of family type, beyond college graduate(54.5%) of father's school career, under high school graduate(58.1%) of mother's school career, first of birth order(47.1%) were majority. Mean of father's age was 41.2 and mother's age was 38.1. 2. The mean of lifestyle was 66.4, feeling was 73.3, activities that promote health was 60.3 and injury prevention was 64.0. The highest degree of activities that promote health was 「I eat fruits」and injury prevention was 「I look both ways when crossing streets」and feeling was 「I enjoy my family」. The lowest degree of activities that promote health was 「I visit the dentist every tear」 and injury prevention was 「I wear a helmet when I go on bike trips」 and feeling was 「I think it is okay to cry」. 3. There were significant differences in lifestyle of gender(t=4.309, p=.000), grade(F=6.299, p=.000), father's age(t=2.534, p=.011), father's education(t=-4.933, p=.000), mother's education(t=-3.360, p=.001), birth order (t=5.363, p=.000). There were significant differences in activities that promote health of gender(t=-2.462, P=.014), grade(F=4.893, p=.000), father's education(t=-4.480, p=.000), birth order(t=4.343, p=.000), in injury prevention of gender(t=-4.452, p=.000), grade(F=8.636, p=.000), father's age(t=3.386, p=.001), mother's age(t=2.059, p=.040), father's education(t=-6.051, p=.000), mother's education(t=-5.173, p=.000), birth order(t=4.417, p=.000) and in feeling of gender (t=-3.285, p=.001), grade(F=7.526, p=.000), mother's age(t=-3.268, p=.001). 4. Activities that promote health was positively correlated with injury prevention(r=.432, p=.000), feeling(r=.210, p=.000), lifestyle (r=.785, p=.000). Injury prevention was positively correlated with feeling(r=.256, p=.000), lifestyle(r=.854, p=.000) also feeling was positively correlated with lifestyle(r=.504, p=.000). These findings suggest the need to develop nursing strategy to promote elementary school children's health. Because helmet use score in injury prevention marked the lowest score, it is necessary to encourage helmet use when planning injury prevention and health promotion.