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김옥현,이희복,이민기,홍정희,곽용운,Kim, Ock-Hyun,Lee, Hee-Bok,Lee, Min-Ki,Hong, Jeong-Hui,Kwak, Yong-Woon 한국생산제조학회 2010 한국생산제조학회지 Vol.10 No.6
Magnetic fluid seal is characterized by its simple design, low friction and being dustless. Those advantages are deduced from the fact that the sealing element is not a solid such as rubber or plastic but it is a fluid. Those are critical for application to a rotating shaft which is inserted into a vacuum chamber where high level of vacuum and cleanness are required. For the reason the magnetic fluid seal has become a standard for vacuum chambers for semiconductor and LCD processing. It should be noted that its sealing performance is sensitive to temperature. If necessary, water cooling should be considered. Thus anticipation of the temperature distribution of the magnetic fluid seal is important before applying it. In this paper an FEM analysis of the heat transfer has been executed and compared with experimental results. An overall convective heat transfer coefficient has been adopted for the analysis, which results in satisfactory consistency of the theoretical and experimental results.
김옥현(Ock Hyun Kim),이규호(Kyu Ho Lee) 한국생산제조학회 2012 한국생산제조학회지 Vol.21 No.3
Air bearing is characterized by its extremely low friction and cleanliness such that it is widely used especially for spindles with ultra-high rotational speed at several tens of thousands rpm. This paper contributes to design of a static radial air bearing suggesting numerical analysis to anticipate its performances. The numerical analysis is an iteration method based on finite difference formulation of the Reynolds equation. A prototype air bearing has been designed and manufactured. Its load capacity has been measured and compared with the numerical solutions. The result shows good consistency between the experiment and theory, which informs that the numerical analysis can be used as an useful tool to anticipate the performances. Effects of design variables on the bearing performance have been examined by Taguchis experimental methods using orthogonal array. Number of holes for supplying pressurized air, clearance between shaft and bearing, the hole diameter and bearing length are chosen for the design variables. The result shows that the clearance and the bearing length are the most influential variables while the others can be considered as almost negligible.
김옥현 ( Ock Hyun Kim ),이규호 ( Kyu Ho Lee ) 충북대학교 산업과학기술연구소 2011 산업과학기술연구 논문집 Vol.25 No.2
This paper contributes to design of a static air bearing. Performance of the air bearing is characterized by its load capacity and consumed air flow rate. Performance of the air bearing has been estimated theoreticallyby a numerical analysis based on the Reynolds equation. For the numerical analysis finite difference formulation of the Reynolds equation has been provided. A prototype air bearing has been designed and fabricated. Its performance of load capacity has been tested and compared with the numerical solutions. The result matches well with each other, which convinces that the numerical approach can be used as an efficient tool to anticipate air bearing performances. Effects of design parameters on the bearing performance have been analyzed by Taguchi’s experimental methods. It is shown that the clearance is the most influential parameter and the next is the number of holes.
김옥현(Ock Hyun Kim) 한국기계가공학회 2009 한국기계가공학회지 Vol.8 No.1
Magneto-Rheological(MR) fluid composes of a base fluid and ferromagnetic particles less than tens of micrometer size dispersed in the fluid. It is called as a smart material because its rheological properties are changable by a magnetic field. Its important applications are active devices such as controllable dampers and controllable clutches. The merit of those products is that their functional characteristics are controllable such that they enable active control strategies. This paper proposes an idea for machine tool applications of the MR fluid clutch as a safety device for power transmission. FEM has been used for magnetic field analyses and the results are compared with some former experiments. Some design syntheses of the MR clutches are suggested and hopefully considered that it may be an effective safety device for power transmission of machine tools.
김옥현(Ock-Hyun Kim),이민기(Min-Ki Lee) 한국기계가공학회 2011 한국기계가공학회지 Vol.10 No.2
Magnetic seal uses a magnetic fluid to seal a gap between a rotating shaft and housing. It is distinguished from other kinds of seals from the fact that solid contact does not occur in the seal. This implies that it is free from solid rubbing thus dustless and provides a clean circumstance. That is the reason why the magnetic seal is used exclusively for most of vacuum chambers in semiconductor process where dustless clean circumstance is critical. A magnetic seal has been developed of which design parameters are determined based on published data, and an air pressure test has been done to examine its sealing capability. Effects of some design parameters have been studied through FEM analysis. The results show some notable aspects of design parameters and provide suggestions for developing the seals. Regarding the sealing capacity of the magnetic seal the factor to match the theoretical value with the actual one was found to be 0.4~0.7, which means still there is some discrepancy between theory and actual.
광학면 연마기의 OMM을 위한 Hartmann Test 방법 연구
김옥현(Ock-Hyun Kim),이응석(Eung-Suk Lee),오창진(Chang-Jin Oh),김용관(Yong-Kwan Kim) Korean Society for Precision Engineering 2004 한국정밀공학회지 Vol.21 No.1
Recently, aspheric optical lenses and mirrors, which are harder to manufacture and measure than the conventional spherical ones, are widely used, particularly in electronic fabrication process. Generally, interferometric optical method is used for the measurement of spherical optical surface. However, the interferometric method for aspheric surface measurement is difficult because it needs a precise null corrector and strict environmental conditions such as constant temperature, humidity and vibrations. We have been studied on the manufacturing of aspheric optics to improve the surface profile accuracy and productivity using a corrective polishing process. For the corrective polishing, a practical method of On-Machine Measurement (OMM) is required. For this purpose, an optical OMM system has been studied using the Shach-Hartmann test, which is very robust to the practical polishing environment. The wavefront has been reconstructed from the measured data using the primary aberration polynomial function by the least squares fitting. The measured result of the OMM system shows that the maximum deviation is less than 200 nm for the one of commercial Fizeau interferometer Wyko 6000.
김옥현 ( Ock Hyun Kim ),문사현 ( Sa Hyun Moon ) 충북대학교 산업과학기술연구소 2012 산업과학기술연구 논문집 Vol.26 No.2
This paper presents a study for developing a shock absorber of which the orifice is being changed according to the oil pressure. The orifice is widen when the oil pressure is high and becomes narrow when it is low, thus the damping force is less sensitive to the velocity than the case of a shock absorber with a fixed orifice. It is well known that the damping force can be written as C ·ua. For fluid film damping the force is proportional to velocity, i.e., a: = 1, and for orifice damping it is proportional to the square of velocity i.e., a = 2. The shock absorber proposed in this paper should show different relationships between the damping force and velocity because the orifice is being changed according to the induced oil pressure. Theoretical and Experimental study have been done to verify the damping characteristics of the shock absorber. Effect of some major design parameters of the shock absorber has been also examined to relate the design parameters to the damping characteristics.
김옥현(Ock-Hyun Kim),이희복(Hee-Bok Lee),이민기(Min-Ki Lee),홍정희(Jeong-Hui Hong),곽용운(Yong-Woon Kwak) 한국생산제조학회 2010 한국생산제조학회지 Vol.19 No.3
Magnetic fluid seal is characterized by its simple design, low friction and being dustless. Those advantages are deduced from the fact that the sealing element is not a solid such as rubber or plastic but it is a fluid. Those are critical for application to a rotating shaft which is inserted into a vacuum chamber where high level of vacuum and cleanness are required. For the reason the magnetic fluid seal has become a standard for vacuum chambers for semiconductor and LCD processing. It should be noted that its sealing performance is sensitive to temperature. If necessary, water cooling should be considered. Thus anticipation of the temperature distribution of the magnetic fluid seal is important before applying it. In this paper an FEM analysis of the heat transfer has been executed and compared with experimental results. An overall convective heat transfer coefficient has been adopted for the analysis, which results in satisfactory consistency of the theoretical and experimental results.