• Tribology and Lubricants
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• v.33 no.5
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• pp.220-227
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• 2017

In this work, carbon nanotube and nano-size alumina particle are exploited as additive for lubrication experiment. We used pin-on-disk type tribometer to investigate the tribological characteristics of lubricants with respect to additives and rotational speed. We conducted more than 15 trials of tribotests for two hours for each specimen to obtain stable and accurate frictional force and to create measurable wear track on the substrate. We conducted tests at the boundary/mixed lubrication regime to evaluate the influence of additives on the tribological characteristics. We found that the friction coefficient decreased as the rotational speed increased and as additives were added. In particular, the reduction of friction by adding additives was more significant at low rotational speed than at high rotational speed. We speculate that the additives helped to separate and protect the two contacting surfaces at low speed, while the influence of additives was not significant at high speed since sufficiently thick lubricant film was formed. The wear of the substrate was also reduced by adding additives to the lubricant. However, in contrast to friction, the amount of wear at high rotational speed was less when alumina particles were added to the lubricant than the amount of wear at low speed. We speculate that the increased wear at low rotational speed is as a result of the intermittent abrasive wear caused by alumina particles with uneven shape, while the reduced wear at high speed is as a result of sufficient film thickness which prevented the abrasive wear.

• Journal of Magnetics
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• v.16 no.2
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• pp.145-149
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• 2011

The coercivity $H_c$ of $Nd_2Fe_{14}B$ magnets and $Nd_2Fe_{14}B/(Nd_{0.7}Dy_{0.3})_2Fe_{14}B$ composite magnets were calculated by computer simulation based on the micromagnetic theory under assumptions that $Nd_2Fe_{14}B$ and $(Nd_{0.7}Dy_{0.3})_2Fe_{14}B$ grains have magnetically deteriorated layers on their surfaces and diffusion of Dy from $(Nd_{0.7}Dy_{0.3})_2Fe_{14}B$ grains to $Nd_2Fe_{14}B$ ones through the contacting boundaries recovers the magnetic anisotropy of the deteriorated layers of $Nd_2Fe_{14}B$ grains. $H_c$ of $Nd_2Fe_{14}B/(Nd_{0.7}Dy_{0.3})_2Fe_{14}B$ composite magnets increased by the diffusion of Dy from $(Nd_{0.7}Dy_{0.3})_2Fe_{14}B$ grains to $Nd_2Fe_{14}B$ ones and the resultant recovery of the anisotropy field of deteriorated layers of $Nd_2Fe_{14}B$ grains. The $H_c$ vs fraction of $(Nd_{0.7}Dy_{0.3})_2Fe_{14}B$ grains curve were convex for the magnets with the degree of alignment between 0.94 and 0.99, which suggests that the above composite magnets have larger $H_c$ values than the alloy-magnets with the same Dy content, and that we can save the consumption of Dy by using these composite magnets.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.11
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• pp.1147-1156
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• 2011

A contact-type area-varying capacitive displacement sensor, or CLECDiS, can measure displacements over millimeter ranges with nanometer resolution. However, a small changes in the contact condition due to the surface profile or friction, which are inherent characteristics of contact-type sensors, lead to significant distortion of the output signal. Therefore, ensuring reliable contact conditions during CLECDiS measurements is the most important area to be improved in their actual use. Herein, in order to design an instrument for ensuring reliable contact conditions, the contact condition is analyzed by characterizing the signal distortion, observing the pressure distribution between the contacting surfaces, and measuring the motional errors of the sensor using a laser Doppler vibrometer (LDV). The manufactured instrument enables a CLECDiS to be used in an ultraprecise positioning system with improved reliability.

• Bulletin of the Society of Naval Architects of Korea
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• v.9 no.2
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• pp.43-48
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• 1972

Inertia friction welding, a relatively recent innovation in the art of joining materials, is a forge-welding process that releases kinetic energy stored in the flywheel as frictional heat when two parts are rubbed together under the right conditions. In a comparatively short time, the process has become a reliable method for joining ferrous, and dissimilar metals. The process is based on thrusting one part, attached to a flywheel and rotating at a relatively high speed, against a stationary part. The contacting surfaces, heated to plastic temperatures, are forged together to produce a reliable, high-strength weld. Welds are made with little or no workpiece preparation and without filler metal or fluxes. However, In order to obtain a good weld, the determination of the optimum weld parameters is an important problem. Especially, because the amount of the flywheel mass will be determined according to the initial rotating velocity values at the constant thrust load, the initial rotating velocity is an important factor to affect a weld character of the inertia-welded IN713C-SAE8630, which is used for the wheel-shafts of turbine rotors or turbochargers, exhausting valves, etc. In this paper, the effects of initial rotational velocity on a weld character of inertia-welded IN713C-SAE8630 was studied through considerations of weld parameters determination, micro-structural observations and tensile tests. The results are as the following: 1) As initial rotating velocity was reduced to 267 FPM, cracks and carbide stringers were completely eliminated in the micro-structure of welded zone. 2) As initial rotating velocity was reduced and flywheel mass was increased correspondingly, the maximum welding temperatures were decreased and the plastic working in the weld zone was increased. 3) As initial rotating velocity was progressively decreased and carbides were decreased, the tensile strengths were increased. 4) And also the fracture location moved out of the weld zone and the tensile tests produced, the failures only in the cast superalloy IN713C which do not extend into the weld area. 5) The proper initial rotating velocity could be determined as about 250 thru 350 FPM for the better weld character.

• Tribology and Lubricants
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• v.31 no.6
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• pp.281-286
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• 2015

Automotive transmission systems are assembled with a large number of gears and shafts, and rolling bearings are used to ensure their smooth operation. Gear oil in the gear box contains solid particles such as wear debris from contacting gears and metallic chips. This particle-enriched lubricating oil can cause premature failure of the rolling bearings. Research aimed at improving the service life of these rolling bearings has been confined mainly to design and lubrication of the inner/outer rings and the rolling elements. In this paper, we redesigned the shape of the cage pocket of a deep groove ball bearing to reduce the premature failure due to particle contamination. Test bearings are assembled with this new cage design containing a hole punched in the cage pocket. Endurance tests are carried out using the contaminated lubricating oil with miracle grid as hard particle. The duration and damaged bearing component shapes are compared for two different cages. The B₁₀ life of bearing with new cage is increased by about 66% compared to the conventional cage. This is because the hard particles can be easily discharged through the pocket hole without staying for a long time in the lubrication regions. This greatly decreases abrasive wear and dents on the highly stressed ball bearing surfaces. Therefore, the cage design of this study, containing a pocket hole, can significantly delay the premature failure of rolling bearings and improve the endurance life.

• Tribology and Lubricants
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• v.38 no.5
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• pp.199-204
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• 2022

This paper presents a numerical investigation of the influence of water molecule thickness on frictional behavior at the nanoscale using molecular dynamics simulation. Three different models, comprising water thin films of various thicknesses, were built, and indentation and sliding simulations were performed using the models. Various normal loads were applied by indenting the Si tip on the water film for the sliding simulation to evaluate the interplay between the water thin film thickness and the normal load. The results of the simulations showed that the friction force generally increased with respect to the normal load and thickness of the water thin film. The friction coefficient varied with respect to the normal load and the water film thickness. The friction coefficient was the smallest under a moderate normal force and increased with decreasing or increasing normal loads. As the water film became thicker, the contact area between the tip and water film became larger. Under well-lubricated conditions, the friction force was proportional to the contact area regardless of the water film thickness. As the normal force increased above a critical condition, the water molecules beneath the Si tip spread out; thus, the film could not provide lubrication. Consequently, the substrate was permanently deformed by direct contact with the Si tip, while the friction force and friction coefficient significantly increased. The results suggest that a thin water film can effectively reduce friction under relatively low normal load and contact pressure conditions. In addition, the contact area between the contacting surfaces dominates the friction force.

• Journal of the Korean Society for Nondestructive Testing
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• v.25 no.2
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• pp.95-102
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• 2005

There are many tube and pipeline in nuclear power plant under high temperature and high pressure. Erosion and corrosion defects were expected on these tube and pipe-line by environmental and mechanical factors. These erosion and corrosion defects ran be evaluated by ultrasonic technique. In these study, Scanning Laser Source(SLS) technique was applied to detect defect and construct image. This technique also makes detection possible on rough and curved surfaces such as tube and pipe-line by scanning. Conventional ultrasonic scanning technique requires immersion of specimen or water jet for transferring ultrasonic wave between transducer and specimen. However, this SLS technique does not need contacting and couplant to generate surface wave and to get flaw images. Therefore, this SLS technique has several advantages, for complicated production inspection, non-contact, remote from specimen, and high resolution. In this study, SLS images were obtained with various conditions of generation laser ultrasound and receiving in order to enhance detectability of flaws on the tube. Stress corrosion cracks were produced on tube and images of stress corrosion cracks were constructed by using SLS technique.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.35 no.2
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• pp.481-491
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• 2015

Since the crack in the surface of the concrete acts as the main reason influencing the life span of the structure, regular inspections and maintenance are required. The sealing required for maintenance of the concrete surface is a method of repairing the crack in the surface in the beginning, and is effective in preventing additional cracks and expansion that occurs with time. However, sealing on large sized structures such as tall buildings or bottom parts of bridges are difficult to ensure safety of the workers due to inadequate working environments. Due to this reason, the importance of the need for sealing automation for the maintenance of large sized concrete structures is emerging. This study proposes two control methods to apply robot systems to the sealing of cracks on the bottom parts of concrete bridges. First is the method of automatically tracking the trajectory of cracks. The robot gets the trajectory of the cracks using video information obtained from cameras. Comparing the previous several points and new point, the next point can be estimated. Thus, the trajectory of the crack can be tracked automatically. The other method is sealing by maintaining steady force to the contacting surface. The concrete surface exposed to an external environment for a long time gets an irregular roughness. If robots are able to carry out sealing while maintaining a steady contact force on these rough surfaces, complete equal sealing can be maintained. In order to maintain this equal force, a force control method using impedance is proposed. This paper introduces two developed control methods to apply to sealing robots, and conducts a Lab Test and Field Test after applying to a robot. Based on the test results, opinions on the possibilities of field application of the robot applied with the control methods are presented.

• Analytical Science and Technology
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• v.28 no.4
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• pp.288-298
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• 2015

A distribution study of glitter was conducted from a local university building. The potential recipient surfaces chosen were the 1,000 chairs kept in 16 separate classrooms of the building. The surface of chairs contacting with buttocks and back of users were tape lifted with commercial adhesive tapes, and the surface of adhesive tapes were examined with a stereomicroscope to locate the glitters. 12 glitter particles were found from 11 chairs kept in 16 classrooms of a building. This result indicates that the possibility of glitter finding from Korean violent crime scene is high. The 12 glitters found from the chairs were analyzed with a stereomicroscope and an FTIR spectrophotometer to find the matching glitter. The stereomicroscope was used for the observation of the shapes, dimensions, colors and holograms of each glitter, and the FTIR spectrophotometer was used for the analysis of the chemical composition of the coating material of each glitter. The examination results revealed that 8 glitters out of 12 glitters were not sharing the common origin each other. Also, two glitters found from two separate classroom chairs share common microscopic and FTIR spectroscopic characteristics which indicating two glitters share common origin. The record of the students who had used the classrooms was traced and the authors could find that the classrooms where the same glitters were found were shared by a group of students who listen to the same class. This result implies that the path of the involved people should be considered while assessing the value of trace evidence.

• Journal of the Korean earth science society
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• v.26 no.6
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• pp.524-540
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• 2005

This study has focused on the amount of evaporites and geochemical characteritics of evaporites from the acid mine drainage and on the variation of constituents in acid mine drainage during evaporation. The various colors of evaporites are frequently observed at the rock surfaces contacting acid mine drainage. In order to produce evaporites in the laboratory, acid mine drainages were sampled from the abandoned mine areas (GTa, GTb, GH and GB) and air-dried at room temperature. During the evaporation of acid mine drainages, TDS, EC values and the concentrations of major and minor ions increased, whereas ER and DO values decreased with time. The concentration of Fe increased gradually with evaporation time in the GTb and GB, whereas GH founded in one day but rapidly not detected in the other day after due to removal of Fe by formation-precipitation of amorphous Fe hydroxide. The amounts of the evaporites were produced in amounts of 4 g (GTa), 5 g (GB), 15 g (GH), and 24 g (GTb) from 4 liter of acid mine drainage after 80 days of the evaporation, respectively. In linear analysis from the products with the parameters which are the EC, TDS, salinity, ER, DO and pH contents in field, the determination coefficients were 0.98, 0.99, 0.98, 0.88, 0.89, and 0.25 respectively. If we measure the parameters in field, it would be easy to estimate the amount of evaporites in acid mine drainage. Gypsum and epsomite were identified in all of the evaporites by x-ray powder diffraction studies. Evaporite (GTb) was heated at 52, 65, 70, 95, 150, 250, and 350oC for one hour in electrical furnaces. Gypsum, $CaSO_4\cdot1/2H_2O$ and kieserite were identified in the heated evaporite by XRD. With increased heating temperature, the intensity of the peak at $7.66/AA$ (diagnostic peak of gypsum), the peak at 5.59A ($CaSO_4{\cdot}1/2H_2O)$ and the peak at $4.83{\AA}$ (kieserite) decreased in x-ray diffraction due to dehydration. In the SEM and EDS analysis for the evaporite, gypsum of well-crystallized, radiating cluster of fibrous, acicular, and columnar shapes were observed in all samples. Ca was not detected in the EDS analysis of the flower structures of GTb. Because of that, the evaporite with flower structures is thought to be eposmite.