• Tribology and Lubricants
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• 제19권4호
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• pp.230-236
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• 2003

In applications such as MEMS and NEMS devices, the adhesion force and contact load may be of the same order of magnitude and the static friction coefficient can be very large. Such large coefficient may result in unacceptable and possibly catastrophic adhesion, stiction, friction and wear. To obtain the static friction coefficient of contacting real surfaces without the assumption of an empirical coefficient value, numerical simulations of the contact load, tangential force, and adhesion force are preformed. The surfaces in dry contact are statistically modeled by a collection of spherical asperities with Gaussian height distribution. The asperity micro-contact model utilized in calculation (the ZMC model), considers the transition from elastic deformation to fully plastic flow of the contacting asperity. The force approach of the modified DMT model using the Lennard-Jones attractive potential is applied to characterize the intermolecular forces. The effect of the surface topography on the static friction coefficient is investigated for cases rough, intermediate, smooth, and very smooth, respectively. Results of the static friction coefficient versus the external force are presented for a wide range of plasticity index and surface energy, respectively. Compared with those obtained by the GW and CEB models, the ZMC model is more complete in calculating the static friction coefficient of rough surfaces.

• 한국안광학회지
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• 제12권3호
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• pp.27-33
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• 2007

구면과 비구면 R.G.P 렌즈 착용실태 조사를 인천 경기지역 안경사 374명을 대상으로 착용비율, 관리법, 예비검사법등 여러가지 문항을 설정하여 조사 하였다. 대부분의 안경사들은 비구면 R.G.P 렌즈를 구면보다 7:3의 비율로 많이 판매하고 있었으며 R.G.P 렌즈 핏팅시 반드시 행하여야할 예비검사 항목 Schirmer test, Tear Break up time test, Slit lamp에 의한 Fluorescein검사, Topography에 의한 각막 지형도 검사를 시행하는지 조사하였으나 대부분 시행하지 않고 있었다. 또한 R.G.P 렌즈 핏팅시 가장 많이 나타나는 증상으로는 가벼운 이물감, 충혈, 시력 흐림, 건조감 순으로 나타났다. 본 실태 조사는 이상적인 R.G.P 렌즈를 핏팅을 위하여 반드시 행하여야 할 예비검사 항목을 나열하였으며, 그 중요성을 일깨우고자 하는데 목적이 있다.

• International Journal of Automotive Technology
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• 제8권6호
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• pp.799-806
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• 2007

One method for improving the torque capacity of the CVT is to use a split-powered CVT(SPCVT) to reduce the power transmitted into a continuously variable unit(CVU). A variable bridge SPCVT with two planetary gear units(PGUs), which are composed of a sun gear, a ring gear, and carrier and planetary gears, can minimize the power to the CVU. However, a SPCVT with a conventional CVT should possess a dual mode, which would allow the conventional CVT to be used at high speeds and an additional gear train to be used at low speeds. The inner-spherical CVU(ISCVU) with an inner and outer spherical contact mechanism developed in this study can cover the range from low to high speeds. The rated power and the overall speed ratios were 100 kW and $0.09{\sim}0.36$, respectively. Power efficiency was numerically calculated to be over 90% over the speed ratio range of $0.1{\sim}0.29$. The maximum shear stress at the two contact areas of the rotor pairs, the minimum life and the overall size were estimated to be 700 MPa, 276 kh and $350{\times}350{\times}400mm^3$, respectively. This study shows that an ISCVU and a variable bridge type PGU can realize the SPCVT with a single mode for a vehicle.

• 한국자동차공학회논문집
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• 제13권5호
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• pp.134-141
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• 2005

Semi-Spherical Continuously Variable Transmission(SS-CVT) is the CVT which transmits power by rolling friction between dry metals, and its outstanding advantage is the ability of realizations of neutral, forward and reverse states without relying on any other devices. A spherical shaped variator transmits power and also changes gear ratio where keeping contact with a output disk. The frictional direction to transmit power and the other direction to change gear ratio are perpendicular to each other, so we can describe that two dimensional rolling friction is occurred in the contact point between the variator and the disk. Since magnitudes or relations of the two forces are completely unknown, they were analyzed by the experiments in this study. We determined the variables related with the friction phenomena by the dimensional analysis, and manufactured the SS-CVT test bench which could measure the related variables. Finally, the empirical formula for evaluating the two friction forces was proposed based on the experimental results.

• 한국안광학회지
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• 제17권3호
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• pp.255-264
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• 2012

목적: 본 연구에서는 형상 및 난시도가 상이한 각막에 구면 및 비구면 RGP 렌즈를 피팅 상태를 달리하여 착용시켰을 때 각막과 렌즈와의 간극에 미치는 영향을 알아보아 적절한 RGP 렌즈의 선택에 도움이 되고자 하였다. 방법: 직난시를 가진 $(25.12{\pm}3.52)$세의 90안에 구면과 비구면 RGP 렌즈를 각기 다른 피팅 상태로 착용시키고 플루레신 염색의 정량적인 분석을 위하여 중심부, 중간주변부 및 주변부로 나눠 면적을 계산하여 각막과 콘택트렌즈의 간극을 비교하였다. 결과: 양호한 피팅 시 중심부와 주변부 면적은 렌즈 디자인에 따른 유의한 차이가 있었으나 스팁 혹은 플랫하게 피팅되었을 때의 중심부 면적은 각막형상과 렌즈 디자인에 따른 차이는 없었다. 각막 형상에 따라 분석하였을 때 양호한 피팅과 플랫 피팅에서는 두 렌즈 디자인 모두에서 중심부와 주변부 면적이 각막 형상에 따라 통계 적으로 유의하게 달라졌으나, 스팁 피팅에서는 차이가 없었다. 각막 난시도에 따라 분석하였을 때 각막형상에 상관없이 각막난시가 증가함에 따라 일정하게 중심부 및 주변부의 면적이 변하는 양호한 피팅의 경우와는 달리 스팁 피팅과 플랫한 피팅의 경우는 원형 각막과 대칭나비형 각막에서의 면적 변화 양상이 상이하였다. 결론: 본 연구에서 는 RGP 렌즈를 스팁이나 플랫하게 피팅 되었을 경우 양호한 피팅시 보다 각막난시나 각막 형상에 따른 렌즈와 각막과의 간극 변화가 더 커지며 이러한 결과는 구면 RGP 렌즈와 비구면 RGP 렌즈 모두에서 공통적으로 나타나는 것임을 알 수 있었다.

• 한국안광학회지
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• 제20권4호
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• pp.455-462
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• 2015

목적: 친수성 콘택트렌즈 정점굴절력 측정 방식중 하나인 wet cell 측정법을 활용한 국산화 측정 홀더와 프로그램 개발에 관한 연구를 위해 현재 사용되어지고 있는 Poster wet cell 홀더와 함수율별 굴절력의 측정값을 비교 분석하였다. 방법: 안경원에서 사용 중인 기존 제품과 새롭게 개발된 국산제품에 식염수를 채워 넣고 소프트콘택트렌즈의 굴절력을 측정하였다. 실험에 사용한 친수성 콘택트렌즈는 안경원에서 많이 사용되고 있는 국내산 렌즈를 사용하였으며, 함수율별과, 구면굴절력별로 분류하여 각각 실험하였다. 결과: wet cell 방식으로 콘택트렌즈 굴절력 측정 시 새롭게 개발한 홀더의 경우 자동렌즈미터를 사용하여 굴절력을 측정한 결과 -3.00 D에서 함수율 38%는 -3.01 D, 함수율 45%는 -3.00 D, 함수율 58%는 -2.98 D로 측정되었으며, Poster wet cell 홀더를 사용하여 렌즈를 측정한 결과 -3.00 D에서 함수율 38%는 -3.60 D, 45%는 -3.06 D, 58%는 -2.46 D로 각각 측정되었다. 결론 : 고함수율에서 굴절력 측정값은 wet cell 방식의 두 홀더 모두 낮게 나타났으며, 새롭게 개발된 홀더와 프로그램을 통한 자동렌즈미터의 측정값이 기존의 Poster soft contact lens wet cell 측정방식에 비해 더 정확한 굴절력 측정 결과를 나타내었다.

• Tribology and Lubricants
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• 제25권4호
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• pp.225-230
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• 2009

Seals are very useful machine components in protection of leakage of lubricant or working fluid, and incoming of debris from outside. Various elastomer are widely used as sealing materials and the shaft surfaces are generally coated with high hardness material after heat treatment. It is generally known that the foreign debris and wear particles get stuck into sealing surface, the steel shaft surface can be damaged and worn by mainly abrasive wear. In this paper, using MARC, contact analysis are conducted to show the hard coated steel shaft surface can be fatigue failed by very small elastic particle intervened between seal and steel surface. Variations of contact and von-Mises stress distributions and contact half-widths with interference and coating thickness are presented. The maximum von-Mises stress occurs always in the coating layer or between coated layer/substrate interface. Therefore the coated sealing surface can be fatigued and then failed by very small particles. The results can be used in design of sealing surface and further studies are required.

• 한국임상보건과학회지
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• 제1권1호
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• pp.1-7
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• 2013

Purpose. To analysis the prevalence of the myopia and corneal astigmatism in Korea women university students. Methods. From August 2011 to December 2012, one hundreds subjects were performed in refraction test using the Auto-Keratometry. Results. The mean age of the 100 subjects (200 eyes) was $21.23{\pm}2.34$. The mean spherical refractive power was -$1.78{\pm}1.65$(OD) and -$1.83{\pm}1.67$(OS) Diopter. The mean astigmatism power was $1.22{\pm}0.96$ (OD) and $1.27{\pm}0.91$ (OS). The mean corneal astigmatism was $1.44{\pm}0.81$(OD) and $1.55{\pm}0.93$(OS). Corneal astigmatism was between 0.25 D and 1.25 D in 67.7% of eyes, 1.25 D or higher in 27.5% eyes, and less than 0.25 D in 4.8% of eyes. Astigmatism was with the rule in 65%, against the rule in 31.5%, and oblique in 3.5%. There was a statistical significance between right eye and left eye in the spherical equivalent power(p=0.002). Also there was a statistical significance between spherical power and refractive astigmatism in OD(p=0.006) and OS(0.003) and a statistical significance between corneal astigmatism and refractive astigmatism in OS(p=0.0003). However, there was not a statistical significance between spherical power and corneal astigmatism in OD(p=0.08) and OS(0.1) and a statistical significance between corneal astigmatism and refractive astigmatism in OS(p=0.48). Conclusions. In this study, these results suggested that the analysis of the refractive myopia and corneal astigmatism can provide the visual correct and useful diagnosis information for the eyewear dispensing, contact lens fitting and corneal refraction surgery.

• 대한기계학회논문집A
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• 제34권2호
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• pp.161-166
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• 2010

본 논문에서는 시일 설치면에서의 마멸발생기구를 정확하게 조사하기 위한 연구의 일환으로 미세입자가 시일과 스틸면 사이에 존재하는 경우의 접촉문제를 비선형문제해석 프로그램인 MARC를 사용하여 해석하였다. 이 결과, 시일의 재질은 스틸면에서의 응력분포와 변형형상에 아주 큰 영향을 미쳤다. 특히, PTFE와 같이 탄성계수가 높은 시일인 경우에 스틸 표면은 국부적으로 항복상태에 도달할 뿐만 아니라 시일을 제거한 후에도 영구변형과 함께 상당한 크기의 압축/인장 잔류응력이 존재하였다. 따라서, 시일과 스틸면 사이에 경질입자가 존재할 경우에는 연삭마멸과 함께 피로마멸이 발생할 수 있음을 확인하였으며, 다양한 설계변수에 대한 추가연구가 요구된다.

• Journal of Mechanical Science and Technology
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• 제17권11호
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• pp.1638-1649
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• 2003

A study is made on the characterization of damage tolerance by spherical indentation in hardly coated layer structure with modest elastic modulus mismatch. A hard silicon nitride is prepared for the coating material and silicon nitride with 5wt% of boron nitride composites for underlayer. Hot pressing to eliminate the effect of interface delamination during the fracture makes strong interfacial bonding. The elastic modulus mismatch between the layers is not only large enough to suppress the surface crack initiation from the coating layer but sufficiently small to prevent the initiation of radial crack from the interface. The strength degradation of the layer structure after sphere contact indentation does not significantly occur, while the degradation of silicon nitride-boron nitride composite is critical at a high load and high number of contacts.