• Tribology and Lubricants
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• v.30 no.6
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• pp.330-336
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• 2014

Sintered metallic brake pads and low alloy heat resistance steel disks are applied to mechanical brake systems in high energy moving machines that are associated with recently developed 200km/h trains. This has led to the speed-up of conventional urban rapid transit. In this study, we use a lab-scale dynamometer to investigate the effects of the composition of friction materials on the tribological characteristics of sintered metallic brake pads and low alloy heat resistance steel under dry sliding conditions. We conduct test under a continuous pressure of 5.5 MPa at various speeds. To determine the optimal composition of friction materials for 200 km/h train, we test and the evaluate frictional characteristics such as friction coefficients, friction stability, wear rate, and the temperature of friction material, which depend on the relative composition of the Cu-Sn and Fe components. The results clearly demonstrate that the average friction coefficient is lower for all speed conditions, when a large quantity of iron power is added. The specimen of 25 wt% iron powder that was added decreased the wear of the friction materials and the roughness of the disc surface. However when 35 wt% iron powder was added, the disc roughness and the wear rate of friction materials increased By increasing the amount of iron powder, the surface roughness, and temperature of the friction materials increased, so the average friction coefficients decreased. An oxidation layer of $Fe_2O_3$ was formed on both friction surfaces.

• Tribology and Lubricants
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• v.11 no.4
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• pp.28-34
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• 1995

PTFE has good mechanical and chemical stability at wide temperature range, and more over, shows a low value of friction coefficient. On the other hand, it shows cold flow and high wear rate. However, these short comings can be overcome by adding various fillers. In this experiment, PTFE and polyimide powder were mixed into composite and its tribological characteristics was investigated. 100% polyimide was also tested for comparison. The countefface material was a stainless steel (SUS304). Friction and wear tester of ring-on-block type was used at room temperature and under atmosphere. After the wear test, the worn surfaces were examined by optical microscope. The test results show that PTFE-polyimide composite generates. the wear transfer film on both sides of the friction surfaces, and, the friction coefficient and the wear rates are relatively low. 100% polyimide generated little wear transfer films, showed high friction and wear rates, and also showed some problems of vibration and noise. It even damaged the stainless steel countefface. It was concluded that 100% polyimide does not generate transfer film well because its shear resistanbe is high and it stickslips, thus, friction coefficients and wear rates are high. In case of PTFE-polyimide composite, on the other hand, transfer film containing sufficient PTFE adheres and remains on both wear surfaces well enough because PTFE has low shear resistance. Polyimide particles in the composite were proved to be able to bear normal load and does not show stick-slip because they are covered with transfer film containing much PTFE.

• The Journal of Engineering Geology
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• v.30 no.1
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• pp.85-101
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• 2020

The estimation of strength parameters is very important for the stability analysis of rock slopes. Various methods for the determination of strength parameters were suggested by various researchers. The number of methods used for the estimation of strength parameters in the stability analysis of rock slopes were investigated based on literature reviews. The frequency of the method determining strength parameters were investigated with respect to failure types. The cohesion and friction angles of the rock and discontinuities are presented with RMR values. The cohesion shows wider range of values relative to those of friction angles according to current studies. Even though RMR does not show any correlation with cohesion values, RMR and the friction angle of the rock clearly shows a positive relationship. Proper methods should be utilized for the determination of strength parameters with consideration for failure types and be proved through literature reviews. The credibility of determining strength parameters is expected to improve if strength parameters data are accumulated from the back analysis performed for failed local rock slopes.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.39 no.6
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• pp.9-19
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• 2002

An NAC(Natural Admittance Control) system design is presented for interaction controller that achieves high-performance and guarantees stability. The NAC can be classified as a particular flavor of impedance control similar to control schemes that have velocity compensator and force compensator. The NAC significantly improves the response characteristics when Coulomb friction is presented in One-link Robot System and guarantees stability when robot contacts with environment. Pragmatic rules for NAC synthesis are derived. It shows method to choose a target impedance for realizable force compensator. Important parameters are found experimentally. It is demonstrated, by the experimental result, that NAC algorithm is successful in rejecting Coulomb friction through velocity compensator and guarantees stability through force compensator. We implement an experimental set-up consisting of environment-generated one-link robot system and DSP system for controller development. We apply the natural admittance controller to the One-link robot system, and show the good performance on desired force control in case of contacting with arbitrary environment.

• Journal of Advanced Marine Engineering and Technology
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• v.8 no.1
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• pp.85-99
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• 1984

The stability of the two-phase flow in a heated channel is of great importance in the design and operation of the boilers and light water nuclear reactors, because it can cause flow oscillations and lead to a violation of thermal limits with resultant overheating of the channels and cladding. This paper presents a systematic evaluation to the variation effects of the basic four (4) dimensionless parameters in a homogeneous equilibrium model. The flow stability is examined on the ground of static characteristic curves. The complicated transfer function of flow dynamics which gives consideration to the transport lag of density wave is derived, and the transient flow stability is analysed by applying the Nyquist stability criterion in control engineering. The analysis results summed up as follows 1. The coolant flow becomes stable in large friction number and specific flow, while it is unstabale in small friction number and flow. 2. Large phase-change number and Froude number destabilize the two-phase flow, but small numbers stabilize it. The effect to variation of phase-change number is more dominant compared with Froude number. 3. The dynamic analysis is required to hold the sufficient safety of heated channels since only static results does not keep it. The special attention could be payed in the design and operation of heat engines, because the unstaable region exists within the stable boundary at small and middle phase-change number and Froude number.

• Geomechanics and Engineering
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• v.12 no.6
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• pp.1021-1046
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• 2017

The assessment of slope stability is an essential task in geotechnical engineering. In this paper, a three-dimensional (3D) finite element analysis (FEA) was employed to investigate the performance of different shear pin arrangements to increase the stability of a soil block resting on an inclined plane with a low-interface friction plane. In the numerical models, the soil block was modeled by volume elements with linear elastic perfectly plastic material in a drained condition, while the shear pins were modeled by volume elements with linear elastic material. Interface elements were used along the bedding plane (bedding interface element) and around the shear pins (shear pin interface element) to simulate the soil-structure interaction. Bedding interface elements were used to capture the shear sliding of the soil on the low-interface friction plane while shear pin interface elements were used to model the shear bonding of the soil around the pins. A failure analysis was performed by means of the gravity loading method. The results of the 3D FEA with the numerical models were compared to those with the physical models for all cases. The effects of the number of shear pins, the shear pin locations, the different shear pin arrangements, the thickness and the width of the soil block and the associated failure mechanisms were discussed.

• Journal of the Korean Society of Hazard Mitigation
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• v.10 no.3
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• pp.91-100
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• 2010

In this paper a sensitivity analysis about effects of influencing factors on the stability of soil cut and embankment slopes in field was performed. Slope stability analysis of slopes in field was carried out with dry, rainy and seismic conditions. As results of analyzing the sensitivity of factors for the dry and rainy conditions, effect of cohesion, internal friction angle and unit weight of soil on the stability of cut slope is more critical in the dry condition than in the rainy condition. However, their effects on the stability of embankment slope for both conditions are similar to each other. The horizontal seismic coefficient does also affect the stability within the similar range of values irrespective of dry or rainy conditions. Cohesion and internal friction angle are more dominant factors influencing the slope stability irrespective of dry or rainy conditions than unit weight of soil and the horizontal seismic coefficient.

• Journal of the Korean Geotechnical Society
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• v.21 no.5
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• pp.75-82
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• 2005

Shear strength parameters obtained from field tests are important factors in the analysis of slope stability. In this paper, sensitivity analysis was performed to evaluate the effect of input parameters on the analysis of slope stability. The input parameters selected for sensitivity analysis were slope angle, cohesion, and friction angle. Monte-Carlo Simulation method was to estimate input parameters for sensitivity analysis in slope stability, and the limit equilibrium method was used to calculate the factor of safety of slope stability. A rock slope, failed in the field, was used for the sensitivity analysis of input parameters in the analysis of slope stability. The result of analysis shows that the factor of safety of the rock slope was a little low. From partial correlation coefficient (PCC) of input parameters from the sensitivity analysis, slope stability was dependant mainly on cohesion and slope angle. The effect of friction angle was smaller than those of cohesion and slope angle on slope stability.

• Journal of the Korean Geotechnical Society
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• v.21 no.5
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• pp.123-131
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• 2005

Recently, construction activities in reclaimed onshore areas increase in our country In this case, the stability evaluation of the piles for negative skin friction is an important factor for the design of pile foundation in soft grounds. Nevertheless, the design of piles for negative skin friction (or downdrag forces) is probably poorly understood by many geotechnical engineers. It is mainly because only the bearing capacity aspect is taken into account for the downdrag evaluation of piles in most of design specifications. However, the problems fur negative skin friction of piles are mostly related with settlement rather than bearing capacity Meanwhile, LRFD (Load Resistance Factor Design) approach considers both ultimate limit state in terms of bearing capacity and serviceability limit state in terms of settlements. This paper proposes LRFD approach for the downdrag evaluation of piles and compares this approach to traditional design approach. And also a case history is analyzed. Through the analysis some suggestions to solve the problems for the design of piles for negative skin friction are suggested.

• Journal of the Korean GEO-environmental Society
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• v.13 no.8
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• pp.65-73
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• 2012

Soil-rock interface, mainly founded in Granite region of Korea, is known as one of the important factor of the slope failure at the rainfall due to smaller shear strength than soil itself. However, research of the effect on slope stability by soil-rock interfaces is insufficient. Therefore, a series of direct shear tests were performed in order to investigate the effect of soil-rock interface on slope stability. The method of tests is to get sand itself and sand-artificial rock interface shear strength from different grain size of sands and artificial rock samples. The results of tests show that the friction angle of interface depends primarily on particle size and surface roughness. Interface friction angle ratio ${\mu}(={\delta}/{\Phi})$ is in the range of 0.75 ~ 0.96, this results indicate that interface friction angle is smaller than sand itself.