• 대한토목학회논문집
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• 제38권5호
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• pp.627-634
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• 2018

본 연구에서는 바람의 영향을 받는 박스형 콘크리트 거더교에 대한 풍력계수를 산정하기 위하여 전산유체해석(CFD)를 수행하였다. 방음벽이 없는 교량 단면에 대한 항력계수, 양력계수 및 비틀림모멘트계수를 산정하였고, 이 풍력계수 값들을 다양한 높이의 방음벽을 갖는 교량 단면에 대한 풍력계수 값들과 비교하였다. 전산유체해석에서 풍력계수들을 산정할 때 전단응력수송(SST) $k-{\omega}$ 난류 모델을 적용하였고, 마찰 항력계수가 전체 항력계수에 미치는 기여도를 조사하였다. 연구 결과, 바람이 수평으로 불 때 항력계수는 방음벽의 높이가 커질수록 증가하였고, 마찰 항력의 기여도는 교량 단면에 방음벽이 없을 때 가장 높았다. 따라서 교량설계에서 풍력을 산정할 때 방음벽의 높이의 영향을 고려할 필요가 있으며, 벽면 마찰력은 교량에 작용하는 풍력을 산정할 때 중요한 역할을 하였다.

• Structural Engineering and Mechanics
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• 제48권6호
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• pp.809-822
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• 2013

Base isolation is widely used in seismic resisting buildings due to its low construction cost, high reliability, mature theory and convenient usage. However, it is difficult to design the isolation layer in high-rise buildings using the available bearings because high-rise buildings are characterized with long period, low horizontal stiffness, and complex re-distribution of the internal forces under earthquake loads etc. In this paper, a simple and innovative isolation bearing, named Teflon-based lead rubber isolation bearing, is developed to address the mentioned problems. The Teflon-based lead rubber isolation bearing consists of friction material and lead rubber isolation bearing. Hence, it integrates advantages of friction bearings and lead rubber isolation bearings so that improves the stability of base isolation system. An experimental study was conducted to validate the effectiveness of this new bearing. The effects of vertical loading, displacement amplitude and loading frequency on the force-displacement relationship and energy dissipation capacity of the Teflon-based lead rubber isolation bearing were studied. An analytical model was also proposed to predict the force-displacement relationship of the new bearing. Comparison of analytical and experimental results showed that the analytical model can accurately predict the force-displacement relationship and elastic shear deflection of the Teflon-based lead rubber isolation bearings.

• 복식문화연구
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• 제21권6호
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• pp.967-973
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• 2013

During high speed sewing, the needle thread is exposed to dynamic loading, short strike loading, inertia forces, friction, rubbing, force of check spring, bending, pressure, friction, impact, shock and thermal influence. The dynamic thread loading/tension alters throughout the stitch formation cycle and along its passage through the machine. The greatest tensile force occurs at the moment of stitch stretching, when the take up lever pulls for required thread length through the tension regulator. These stresses act on the thread repeatedly and the thread passes 50-80 times through the fabric, the needle eye and the bobbin case mechanism, before getting incorporated into the seam, which result in upto 40% loss in tensile strength of the sewing thread. This damage in the sewing thread adversely affects its processing and functional performance. In this paper, the contribution of dynamic loading, passage through needle and fabric, and bobbin thread interaction in the loss in tensile properties has been studied. It is observed that the loss in tensile properties occurs mainly due to the bobbin thread interaction. Dynamic loading due to the action of take up lever also causes substantial loss in tenacity and breaking elongation of cotton threads.

• 제어로봇시스템학회논문지
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• 제5권4호
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• pp.454-461
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• 1999

A concrete floor trowel machine, developed in the U.S in 1990's, consists of only two rotary trowels, and doesn't need any other mechanism for motion such as wheels. When the machine flattens a concrete floor with its rotary trowels, the machine can move in any direction by utilizing the unbalanced friction forces occurring between the rotary wheels and the floor when the trowels are tilted in appropriate directions. In order to automate the trowels machine, this paper proposed the self-propulsive concrete finishing trowel robot which has twin trowels. For the control of the robot, this paper discussed the following. Firstly, the dynamics model of the driving frictional force applied on each trowel from the floor is derived. Secondly, the relationship between the driving force for the robot and the control variable of the robot is derived. Finally, the basic motion of the robot are realized by using the obtained relationship. This paper figures out how the concrete floor finishing robot with tow trowels moves and will contribute to realizing it.

• 한국정밀공학회지
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• 제34권3호
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• pp.185-189
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• 2017

Recently, titanium alloys have been widely used in aerospace, biomedical engineering, and military industries due to their high strength to weight ratio and corrosion resistance. However, it is well known that titanium alloys are difficult-to-cut materials because of a poor machinability characteristic caused by low thermal conductivity, chemical reactivity with all tool materials at high temperature, and high hardness. To improve the machinability of titanium alloys, cryogenic cooling with LN2 (Liquid Nitrogen) and nanofluid MQL (Minimum Quantity Lubrication) technologies have been studied while turning a Ti-6Al-4V alloy. For the analysis of turning process characteristics, the cutting force, the coefficient of friction, and the surface roughness are measured and analyzed according to varying lubrication and cooling conditions. The experimental results show that combined cryogenic cooling and nanofluid MQL significantly reduces the cutting forces, coefficients of friction and surface roughness when compared to wet condition during the turning process of Ti-6Al-4V.

• International Journal of Automotive Technology
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• 제3권4호
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• pp.165-170
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• 2002

For a vehicle Anti-lock Braking System (ABS), the control target is to maintain friction coefficients within maximum range to ensure minimum stopping distance and vehicle stability. But in order to achieve a directionally stable maneuver, tire side forces must be considered along with the braking friction. Focusing on combined braking and turning operation conditions, this paper presents a new control scheme for an ABS controller design, which calculates optimal target wheel slip ratio on-line based on vehicle dynamic states and prevailing road condition. A fuzzy logic approach is applied to maintain the optimal target slip ratio so that the best compromise between braking deceleration, stopping distance and direction stability performances can be obtained for the vehicle. The scheme is implemented using an 8-DOF nonlinear vehicle model and simulation tests were carried out in different conditions. The simulation results show that the proposed scheme is robust and effective. Compared with a fixed-slip ratio scheme, the stopping distance can be decreased with satisfactory directional control performance meanwhile.

• 한국정밀공학회지
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• 제25권10호
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• pp.137-143
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• 2008

The objective of this study was to characterize the role of synovial fluid and hyaluronan in the frictional response of bovine articular cartilage. Seven cylindrical cartilage specimens were harvested from three fresh bovine humoral heads (4-6 months old). Reciprocal sliding motion (1mm/s) was provided by a custom-made friction testing apparatus with a normal load of 22.3 N. From the measured time-dependent normal and frictional forces, the minimum and maximum frictional coefficients were calculated. Synovial fluid reduced the minimum frictional coefficient by ${\sim}75$ % and maximum frictional coefficient by ${\sim}11%$, while the reduction of the minimum and maximum frictional coefficients with hyaluronan was ${\sim}42%$ and ${\sim}24%$, respectively. To the best of our knowledge, this experimental study investigates the first such comparisons of frictional response of articular cartilage with and without synovial fluid and hyaluronan, and provides significant insights into their role in the articular cartilage friction and lubrication.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2006년도 추계학술대회논문집
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• pp.893-897
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• 2006

Structures with additional frictional damping system have strong nonlinearity that the dynamic behavior is highly affected. by the relative magnitude between frictional force and excitation load. In this study, normalized response spectra of the structures with non-dimensional friction force are obtained through nonlinear time history analyses of the mass-normalized single degree of freedom systems using 20 ground motion data recorded on rock site. The variation of the control performance of frictional damping system is investigated in terms of the dynamic load and the structural natural period, of which effects were not considered in the previous studies. Least square curve fitting equations are presented for describing those normalized response spectrum and optimal non-dimensional friction forces are obtained for controlling the peak displacement and absolute acceleration of the structure based on the derivative of the curve fitted design spectrum.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2004년도 추계학술대회 논문집
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• pp.681-685
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• 2004

A media-feeding (or media-transport) system is a key component in daily consumer systems such as printers, copiers and ATM's. The role of the media-transport system is to feed a medium, which is usually in the form of a thin film, to the main process in a uniform and repeatable manner. Even small slippage between the media and the feeding rollers could significantly degrade the performance of the entire system. The slippage between the medium and the feeding rollers is determined by many parameters which include the friction coefficient between the feeding rollers and the medium material, the angular velocity of the feeding rollers, and the normal force applied by feeding rollers on the medium. This paper investigates the effect of the normal force and the angular velocity of feeding rollers on the slippage of the medium. Authors have constructed a test bed for experiments, which consists of a feeding module and various measuring devices. Using regular paper as media being fed, the authors experimentally measured the slippage of the medium under various normal forces and angular velocities of driving feeding roller. Also the authors developed a novel two-dimensional simulation model for the media-transport system. The paper medium is modeled as a set of multiple rigid bodies interconnected by revolute joints and rotational springs and dampers. Simulations were executed using a multi-body dynamic analysis tool called RecurDy $n^{ⓡ}$. The slippage obtained by the simulation is compared to experimental results.ults.

• 터널과지하공간
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• 제15권4호
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• pp.305-315
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• 2005

불연속 변형해석은 에너지 최소화원리를 이용하여 전체 행렬을 구성하는 유한요소법과 개별블록의 대변위를 모사할 수 있는 개별요소법의 장점을 모두 가지고 있는 해석법이다. 이 연구에서는 불연속 변형해석에서 해석결과에 문제점이 있는 접촉문제를 개선하는 시도로써 불연속 변형해석에서 정의하는 접촉 중 선-선 접촉을 해석하는 새로운 기법을 적용하였다. 이를 위해 접촉쌍의 개념을 이용하여 새로운 7 가지의 선-선 접촉의 상태를 정의하고 이에 따른 새로운 전단거동 결정 방법과 개폐수렴반복해석에서의 수렴조건, 접촉찾기 등의 세부알고리듬을 제시하였다. 또한 블록간의 접촉에 의해 작용하는 마찰력을 기존의 점하중 형태에서 선하중의 형태로 개선하는 식을 유도하여 새 알고리듬에 적용하였다.