• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.45 no.11
• /
• pp.967-974
• /
• 2017

Reaction wheel shows nonlinear torque response on low-speed region due to friction. Thus precise satellite attitude control on this region is hard to achieve. Previous research tries to solve this problem, by compensating friction or applying dither command. However, due to difficulties of drag torque modeling or frequent zero wheel speed crossing, these methods are not suitable to apply on the real satellite attitude control. To solve this problem, we propose the attitude controller gain adjustment method based on the attitude error.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.13 no.7
• /
• pp.588-594
• /
• 2001

The purpose of this study is to investigate the thermal and hydrodynamic characteristics of the channel in corrugated plate type heat exchangers numerically. Numerical work has been conducted using the Reynolds Stress Model(RSM) by utilizing the commercial finite-volume code, FLUENT. Based on this model, the dependence of heat transfer and friction factor on geometrical parameters have been investigated. It is found that larger corrugation angle give higher values of heat transfer coefficients and friction factors. As the reynolds number increases, the heat transfer coefficient also increases. It is also observed that the heat transfer coefficient reaches maximum while the friction factor stays relatively low at same corrugation angle. Through the analysis, it is found that the optimum corrugation angle for the heat exchanger performance exists. It is noted that the flow repulsions at the contact point of the two fluid streams make the low mixing more active for larger corrugation angle and high reynolds number.

• Tribology and Lubricants
• /
• v.24 no.5
• /
• pp.234-240
• /
• 2008

It is well known that surface texturing improves the tribological properties of mechanical components for enhancing hydro-dynamic effect or serving as a micro reservoir. There are not, however, enough researches to reveal the effects of surface texturing on the tribological properties under grease lubrication which is used in lubricating many mechanical elements. In the present study, therefore, the effects of surface texturing on the tribological properties are investigated under grease lubrication based on an experimental approach. The results show that surface texturing decreases friction coefficient. It is found that the friction coefficient can be decreased by controlling the diameter and density of micro-dimple. The diameter of dimple is more effective under high load and low speed than otherwise. And, the density of dimple is effective under low load and high speed.

• Transactions of Materials Processing
• /
• v.27 no.2
• /
• pp.107-114
• /
• 2018

Although the application of high strength steel is increasing to cope with the various updated regulations of automobiles, high strength steel sheets are difficult to formulate due to the high tensile strength and low elongation of those materials. In this study, the slide motion was controlled using a servo press in order to improve the formability of the process of manufacturing ultra-high strength steel of above 1.2GPa. Also, the friction characteristics of the slid motion were investigated through a high speed friction test. The slide motion was optimized by adjusting the number of steps, the rising start position and the rise height of the slide. At the same time, it is noted that the optimal slide motion increased the forming depth by about 40%. From the results of the high speed friction test, the application of the slide motion reduces friction resistance, thereby improving friction characteristics and improving formability.

• Geomechanics and Engineering
• /
• v.10 no.3
• /
• pp.315-324
• /
• 2016

Frictional forces between soil and structural elements are of vital importance for the foundation engineering. Although numerous studies were performed about the soil-structure interaction in recent years, the approximate relations proposed in the first half of the 20th century are still used to determine the frictional forces. Throughout history, wood was often used as friction piles. Steel has started to be used in the last century. Today, alternatively these materials, FRP (fiber-reinforced polymer) piles are used extensively due to they can serve for long years under harsh environmental conditions. In this study, various ratios of low plasticity clays (CL) were added to the sand soil and compacted to standard Proctor density. Thus, soils with various internal friction angles (${\phi}$) were obtained. The skin friction angles (${\delta}$) of these soils with FRP, which is a composite material, steel (st37) and wood (pine) were determined by performing interface shear tests (IST). Based on the data obtained from the test results, a chart was proposed, which engineers can use in pile design. By means of this chart, the skin friction angles of the soils, of which only the internal friction angles are known, with FRP, steel and wood materials can be determined easily.

• Tribology and Lubricants
• /
• v.31 no.6
• /
• pp.287-293
• /
• 2015

Laser surface texturing (LST), a surface engineering modification, has been considered as one of the new processes used to improve tribological characteristics of materials by creating artificially patterned microstructure on the contact surface of mechanical components. In LST technology, the laser is optimized to obtain or manufacture the dimples with maximum precision. The micro-dimples reduce the coefficients of friction and also improve the wear resistance of materials. This study investigates the effect of dimple density is investigated. For this purpose, a ball-on-disc type tester is used with AISI 52100 bearing steel as the test material. Discs are textured with a 5% and 10% dimple density. Experimental work is performed with normal loads of 5 N, 10 N, and 15 N under a fixed speed of 150 rpm at room temperature. The effect of the textured surface is compared to that of the untextured one. Experimental results show that the textured surface yields lower friction coefficients compared to those of untextured surfaces. Specifically, the 10% dimple density textured surface shows better friction reduction behavior than the 5% dimple density textured sample, and has an 18% improvement in friction reduction compared with the untextured samples. Microscopic observation using a scanning electron microscope (SEM) shows that the major friction mechanisms of the AISI 52100 bearing steel are adhesion, plastic deformation, and ploughing.

• Structural Engineering and Mechanics
• /
• v.22 no.1
• /
• pp.85-105
• /
• 2006

Control of structural response due to seismic excitation in a manner of coupling adjacent buildings has been actively developed, and most attention focused on those buildings of similar height. However, with the rapid development of some modern cities, multi-story buildings constructed with an auxiliary low-rise podium structure to provide extra functions to the complex become a growing construction scheme. Being inspired by the positively examined coupling control approach for buildings with similar height, this paper aims to provide a comprehensive analytical study on control effectiveness of using friction dampers to link the two buildings with significant height difference to supplement the recent experimental investigation carried out by the writers. The analytical model of a coupled building system is first developed with passive friction dampers being modeled as Coulomb friction. To highlight potential advantage of coupling the main building and podium structure with control devices that provide a lower degree of coupling, the inherent demerit of rigid-coupled configuration is then evaluated. Extensive parametric studies are finally performed. The concerned parameters influencing the design of optimal friction force and control efficiency include variety of earthquake excitation and differences in floor mass, story number as well as number of dampers installed between the two buildings. In general, the feasibility of interaction control approach applied to the complex structure for vibration reduction due to seismic excitation is supported by positive results.

• International Journal of Naval Architecture and Ocean Engineering
• /
• v.13 no.1
• /
• pp.746-757
• /
• 2021

To investigate the mechanism of friction-induced vibration and noise of ship water lubricated stern bearings, a two-degree-of-freedom (2-DOF) nonlinear self-excited vibration model is established. The novelty of this work lies in the detailed analysis of influence of different parameters on the stability and nonlinear vibration characteristics of the system, which provides a theoretical basis for the various friction vibration and noise phenomenon and has a very important directive meaning for low noise design of water lubricated stern bearings. The results reveal that the change of any parameter, such as rotating speed of shaft, contact pressure, friction coefficient, system damping and stiffness, has an important influence on the stability and nonlinear response of the system. The vibration amplitudes of the system increase as (a) rotating speed of shaft, contact pressure, and the ratio of static friction coefficient to dynamic friction coefficient increase and (b) the transmission damping between motor and shaft decreases. The frequency spectrum of the system is modulated by the first mode natural frequency, which is continuous multi-harmonics of the first mode natural frequency. The response of the system presents a quasi-periodic motion.

• Tribology and Lubricants
• /
• v.20 no.5
• /
• pp.237-244
• /
• 2004

Micro/nano tribological characteristics of PTFE coating films were experimentally studied. PTFE (polytetrafluoroethylene) modified polyethylene and low molecular weight PTFE were used as a coating materials. These films were deposited on Si-wafer (100) by IBAD (ion beam assisted deposition) method. The Ar ion beam sputtering was performed to change the surface topography of films using a hollow cathode ion gun under different Ar ion dose conditions in a vacuum chamber. Micro/nano tribological characteristics, water wetting angles and roughness were measured with a micro tribotester, SPM (scanning probe microscope), contact anglemeter and profilometer, respectively. The durability of the films were measured with macro tribotester. Results showed that the PTFE coating surfaces were converted to hydrophobic. The water contact angle of coated surfaces and surface roughness increased with the coating thickness. Adhesion and friction in micro and nano scale were governed by magnitude of normal load in soft material such as PTFE films. As the increase of sputtering time on low molecular weight PTFE films, the surface roughness was increased and nano adhesion and friction were decreased. The nano tribological characteristics of surfaces are mainly improved by chemical modification such as PTFE coating and given a synergy effect by the physical modification such as topographic modification.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.20 no.8
• /
• pp.706-710
• /
• 2007

Ti-containing amorphous carbon (a-C:Ti) films shows attractive mechanical properties such as low friction coefficient, good adhesion to various substrate and high wear resistance. The incorporation of titanium in a-C films is able to improve the electrical conductivity, friction coefficient and adhesion to various substrates. In this study, a-C:Ti films were depositied on Si wafer by closed-field unbalanced magnetron (CFUBM) sputtering system composed two targets of carbon and titanium. The tribological properties of a-C:Ti films were investigated with the increase of DC bias voltage from 0 V to - 200 V. The hardness and elastic modulus of films increase with the increase of DC bias voltage and the maximum hardness shows 21 GPa. Also, the coefficient of friction exhibites as low as 0.07 in the ambient. In the result, the a-C:Ti film obtained by CFUBM sputtering method improved the tribological properties with the increase of DC bias volatage.