• Proceedings of the Korean Society of Precision Engineering Conference
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• 1993.10a
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• pp.177-180
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• 1993

최근 항공,자동차산업을 필두로 해서 가전산업에 이르는 많은 민수산업에서 경량화를 목표로한 연질금속의 소형 가공물이 급격히 증가하고 있고, 특수조건을 겨냥한 세라믹과 복합소재와 같은 신소재의 개발 및 적용이 활빌히 일어나고 있다. 이와 같은 추세는 생산성 향상과 가공면의 고품위화를 동시에 만족하면서 진행되어야 하므로 고속,고능률 가공시스템이 필수적으로 요구된다. 본 연구에서는 베어링의 예압량,회전속도,열팽창이 베어링의 내.외륜 지름변화에 어떤 영향을 미치는지 분석하고 베어링 제조사에서 추천하는 조립공차가 고속주축의 작동조건에 어느정도 타당한지를 분석하였다.

• Journal of the Korean Society for Precision Engineering
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• v.26 no.2
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• pp.109-117
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• 2009

This paper presents a linear air bearing stage with compensated motion errors by active control of preloads generated by magnetic actuators with combination of permanent and electromagnets. A 1-axis linear stage motorized with a linear motor with 240mm of travel range is built for verifying this design concept and tested its performances. The three motions of the table are controlled with four magnetic actuators driven by current amplifiers and a DSP based digital controller. Three motion errors were measured combined method with laser interferometer and two-probe method with $0.085{\mu}m$ of repeatability for straightness error. The measured motion errors were modeled as functions of the stage position, and compensation were carried out with feedforward control because the characteristics of the motion control with magnetic actuators are linear and independent for each degree-of-freedoms. As the results, the errors were reduced from $1.09{\mu}m$ to $0.11{\mu}m$ for the vertical motion, from 9.42 sec to 0.18 sec for the pitch motion and from 2.42 sec to 0.18 sec for roll motion.

• Tribology and Lubricants
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• v.32 no.1
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• pp.1-8
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• 2016

This paper focuses on the thermal deformation induced preload change in the tilting pad journal bearing, using a three-dimensional (3D) thermo-hydro-dynamic (THD) approach. Preload is considered as a critical factor in designing the tilting pad journal bearing. The initial preload measured under nil external load and nil thermal gradient is influenced by two factors, namely, the thermal deformation and elastic deformation. Thermal deformation is due to a temperature distribution in the bearing pads, whereas the elastic deformation is due to fluid forces acting on the pads. This study focuses on the changes induced in preload and film clearance due to thermal deformation. The generalized Reynolds equation is used to evaluate the force of the fluid and the 3D energy equation is used to calculate the temperature of the lubricant. The abovementioned equations are combined by establishing a relationship between viscosity and temperature. The heat transfer within the bearing pads, the lubricant, and the spinning journal is calculated using the heat flux boundary condition. The 3D Finite Element Method (FEM) is used in modeling the (1) heat conduction in the spinning journal and bearing pads, (2) thermal gradient induced thermal distortion of the spinning journal and pads, and (3) viscous shearing, and heat conduction and convection in a thin film. This evaluation method has an increased fidelity, and it can prove to be a cost-effective tool that can be used by designers to predict the dynamic behavior of a bearing.

• Tribology and Lubricants
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• v.30 no.3
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• pp.131-138
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• 2014

This study investigates the effects of the slopes of the rotational axis and bearing preloads on the natural frequencies and onset speeds of the instability of a rotor supported on gas foil bearings (GFBs). The predictive model for the rotating system consists of a rigid rotor supported on two gas foil journal bearings (GFJBs) and a pair of gas foil thrust bearings (GFTBs). Each GFJB supports approximately half the rotor weight. As the slope of the rotational axis increases from $0^{\circ}$(horizontal rotor operation) to $90^{\circ}$(vertical rotor operation), the applied load on the GFJB owing to the rotor weight decreases. The predictions show that the natural frequency and onset speed of instability decrease significantly with an increase in the slope of the rotational axis. In a parametric study, the nominal radial clearance and preload for the GFJB were changed. In general, a decrease in the nominal radial clearance lead to an increase in the natural frequency and onset speed of instability. For constant assembly clearance, the decrease in the preload changed the natural frequency and onset speed of instability with insignificant improvements in the rotordynamic stability. The present predictions can be used as design guidelines for GFBs for oil-free high-speed rotating machinery with improved rotordynamic performance.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.3
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• pp.231-237
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• 2007

The torque ripple of the hydraulic pumps for the Electro-hydrostatic Actuators can disturb the cylinder position control under slewing speed operation condition. In principle, the periodic change of the reaction torque generated by a piston type pump is highly dependent on the waveform of its cylinder chamber pressure. In case of uni-directional pumps operating at constant speed, the transient overshoot and rising slope of the cylinder pressure can be adjusted by the precompression angle and notch shape of their valve plates. Therefore, the influence of the valve plate geometry on the torque ripple magnitude of a bent-axis type piston pump for EHA application was investigated in this study. The results showed that any improvement of the torque ripple of such a bi-directional pump can not be achieved by modifying the valve plate geometry, regardless of its operation speed.

• Transactions of Materials Processing
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• v.12 no.2
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• pp.116-122
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• 2003

In the Prestressed die design for cold working, many constraining conditions should be considered to insure the die safety and to improve the dimension accountancy products. Among the constraining conditions, yielding conditions, diameter ratios and interferences between rings are very important. . In this paper, therefore, flexible tolerance method was used in order to search the optimum values of design variables. The maximum inner pressure is used as objective function in this numerical analysis. In the design Process, it was also involved the safety factor to the yield strength of each ring by considering the allowable tensile or compressive hoop stress in each ring. The proposed technique has been applied to the die design of backward extrusion process, and it's analytical results have been compared with that of the conventional design method.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1991.04a
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• pp.99-103
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• 1991

본 논고에서는 Radial 및 Axial 하중을 동시에 받을 수 있고, 예압에 의해 주축강성을 증대시킬수 있으며, Ball수의 증가로 부하능력을 향상시킨 초정 밀.초고속용 Angular ball bearing을 조합 사용하고, 고속회전에 지금까지 개 발된 ball bearing 윤활방법 중 가장 효과적인 간헐적인 oil 공급과 air 압력 으로, 계속 ball을 냉각, 윤활시켜주는 air oil 윤활법의 채용과, 국내 공작기 계 제조회사들이 주로 사용하는 일본 FAMUC AC Spindle motor(FAMUC-H type : 8000-15000rpm)를 사용해서 초고속 주축의 최적 설계조건을 제시하기 위한 기초연구 단계로 MT40-12000rpm 급의 Machining center용 Cartridge type의 Spindle unit개발을 통해서 주축설계에 따른 문제점과 연구용 주축제작의 생산 기술적 문제점 및 진동특성을 검토 하고자 한다.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.19-22
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• 2003

In this study, the design of prestressed die for spur gear forging have been investigated. The stress concentration at notch of the die insert is very important in the design of die for the forging of spur gear such as non-axisymmetric geometry. In the previous study, the flexible tolerance method was used in order to search the optimal value of design variables considering the constrain conditions. In the design process, it was also involved the safety factor to the yield strength of each ring by considering allowable tensile or compressive hoop stress in each ring. Using this technique, the die deign for spur gear forging has been successfully performed without yielding of the die after shrink fitting and during forging.

• The KSFM Journal of Fluid Machinery
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• v.2 no.2 s.3
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• pp.27-31
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• 1999

This paper describes the shaft vibration phenomena measured on a pump-turbine of a pumped storage power plant. The pump-turbine runs at a rotational speed of 450 rpm (7.5 Hz). The power output (load) of the pump-turbine is varied from 100 to 300 MW in the generating mode. The magnitude of the shaft vibration highly depends on the power load. The vibration magnitude of the shaft is very high in the middle load zone from 170 to 210 MW, elsewhere the vibration is low. From nitration spectra, it is shown that the frequency of major nitration in that load zone is 2.5 Hz which is approximately $34\%$ of the shaft rotating speed in Hz. This frequency component does not occur below and above that load zone. This subsynchronous vibration is caused by the flow induced disturbance due to spiral vortex flow downstream of the pump-turbine runner. Furthermore, the shaft vibration is highly decreased due to an increased bearing preload.

• Journal of the Korean Society for Precision Engineering
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• v.14 no.3
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• pp.35-40
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• 1997

The traction drive has advantages of having high efficiency and transmitting the power without blacklash. However, when high ratio of speed reduction is desired, excessively large size is required. In this study, a new type of traction drive is invented, designed and manufactured so that stable speed reduction mech- anisms are available by adopting a cross roller type drive. It has a simple structure, but produces high speed reduction ratio. Power loss is observed, and also, driving torque and torque transfer efficiency are calculated. Pre-loads are needed in order that the traction drive transfers power without slipping, and the spacer is enlarged due to the pre-loading. After all, the key point of pre-load mechanism is that the spacer's diameter becomes larger as pre-loads are applied.