• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2002년도 춘계학술대회논문집
• /
• pp.166-174
• /
• 2002

This paper presents an analytical method to Investigate the stability of a hydrodynamic journal bearing with rotating herringbone grooves. The dynamic coefficients of the hydrodynamic journal bearing are calculated using the FEM and the perturbation method. The linear equations of motion can be represented as a parametrically excited system because the dynamic coefficients have time-varying components due to the rotating grooves, even in the steady state. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving Hill's infinite determinant of these algebraic equations. The validity of this research is proved by the comparison of the stability chart with the time response of the whirl radius obtained from the equations of motion. This research shows that the instability of the hydrodynamic journal bearing with rotating herringbone grooves increases with increasing eccentricity and with decreasing groove number, which play the major roles in increasing the average and variation of stiffness coefficients, respectively. It also shows that a high rotational speed is another source of instability by increasing the stiffness coefficients without changing the damping coefficients.

• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2002년도 춘계학술대회논문집
• /
• pp.181-189
• /
• 2002

This research presents an analytical model to investigate the stability due to the ball bearing waviness in a rotating system supported by two ball bearings. The stiffness of a ball bearing changes periodically due to the waviness in the rolling elements as the rotor rotates, and it can be calculated by differentiating the nonlinear contact forces. The linearized equations of motion can be represented as a parametrically excited system in the form of Mathieu's equation, because the stiffness coefficients have time-varying components due to the waviness. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as the simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving the Hill's infinite determinant of these algebraic equations. The validity of this research is proved by comparing the stability chart with the time responses of the vibration model suggested by prior researches. This research shows that the waviness in the rolling elements of a ball bearing generates the time-varying component of the stiffness coefficient, whose frequency is called the frequency of the parametric excitation. It also shows that the instability takes place from the positions in which the ratio of the natural frequency to the frequency of the parametric excitation corresponds to i/2 (i= 1,2,3..).

• 한국전산구조공학회:학술대회논문집
• /
• 한국전산구조공학회 2004년도 봄 학술발표회 논문집
• /
• pp.185-192
• /
• 2004

The goal of this paper is to determine the accurate effective length factor(K factor) for buckling design of plane frames and to point out the practical limitations of the alignment chart which provides the approximate effective length factor. At present, the most general method to obtain K factors is to use the alignment chart which is given in the form of nomograph in LRFD-AISC specification commentaries. However it should be realized that various simplifications and assumptions were used in obtaining the alignment chart. Therefore, a simple but effective method to obtain accurate K-factors through the stability analysis of plane frames is developed in this study. To demonstrate the accuracy and effectiveness of the present scheme, K-factors by system buckling analysis of frames are calculated and compared with those calculated by the alignment chart.

• Geomechanics and Engineering
• /
• 제16권3호
• /
• pp.321-329
• /
• 2018

Classic braced walls use struts and wales to minimize ground movements induced by deep excavation. However, the installation of struts and wales is a time-consuming process and confines the work space. To secure a work space around the retaining structure, an anchoring system works in conjunction with a braced wall. However, anchoring cannot perform well when the shear strength of soil is low. In such a case, innovative retaining systems are required in excavation. This study proposes an innovative earth-retaining wall that uses in situ soil confined in dual sheet piles as a structural component. A numerical study was conducted to evaluate the stability of the proposed structure in cohesionless dry soil and establish a design chart. The displacement and factor of safety of the structural member were monitored and evaluated. According to the results, an increase in the clearance distance increases the depth of safe excavation. For a conservative design to secure the stability of the earth-retaining structure in cohesionless dry soil, the clearance distance should exceed 2 m, and the embedded depth should exceed 40% of the wall height. The results suggest that the proposed method can be used for 14 m of excavation without any internal support structure. The design chart can be used for the preliminary design of an earth-retaining structure using in situ soil with dual steel sheet piles in cohesionless dry soil.

• 지질공학
• /
• 제19권3호
• /
• pp.261-268
• /
• 2009

터널 굴착 중 수행되는 계측은 구성 지반의 거동을 가시화하여 안정성을 평가할 수 있는 중요한 정보를 제공한다. 일반적으로 계측은 터널 굴착면 형성 초기에 집중적으로 수행되며, 시간이 경과할수록 계측횟수를 줄여 나간다. 하지만 이러한 계측 기준 및 관리 지침은 획일적이며 뚜렷한 정량적 기준이 부족한 실정이다. 본 연구에서는 굴착 초기 발생하는 변위의 안정화 시점의 정량적 판단과 안정화 이후의 미세한 변위 변동 특성 분석 및 이상거동 평가를 위해 통계 관리도 기법을 적용하였다. 계측변위에 대한 이동범위(MR) 관리도 및 누적합(CUSUM) 관리도를 작성하여 변위 평가 가능성을 검토하였다.

• Steel and Composite Structures
• /
• 제6권2호
• /
• pp.87-101
• /
• 2006

The deformation and dynamic behavior mechanism of submerged shell-like lattice structures with membranes are in principle of a non-conservative nature as circulatory system under hydrostatic pressure and disturbance forces of various types, existing in a marine environment. This paper deals with a characteristic analysis on quasi-periodic and chaotic behavior of a circular arch under follower forces with small disturbances. The stability region chart of the disturbed equilibrium in an excitation field was calculated numerically. Then, the periodic and chaotic behaviors of a circular arch were investigated by executing the time histories of motion, power spectrum, phase plane portraits and the Poincare section. According to the results of these studies, the state of a dynamic aspect scenario of a circular arch could be shifted from one of quasi-oscillatory motion to one of chaotic motion. Moreover, the correlation dimension of fractal dynamics was calculated corresponding to stochastic behaviors of a circular arch. This research indicates the possibility of making use of the correlation dimension as a stability index.

• 한국환경과학회지
• /
• 제28권12호
• /
• pp.1133-1145
• /
• 2019

Successful launch requires state-of-the-art launch vehicle technology and constant test operations, However, the meteorological threat to the launch vehicle flight trajectory is also an important factor for launch success. Atmospheric stability above the Naro Space Center at the this time is very important, especially because the initial flight operation can determine the success of the launch. Moreover, during the flight of launch vehicle with rapid pressure and thrust into the atmosphere, convection activity in the atmosphere may create environmental conditions that cause severe weather threats such as thunderstorms. Hence, studies of atmospheric instability characteristics over the Naro Space Center are a necessary part of successful launch missions. Therefore, the main aims of this study were to (1) verify the atmospheric stability index and convection activity characteristics over the Naro Space Center using radiosonde data observed from 2007 to 2018 by the Naro Space Center, (2) analyze changes in the atmospheric stability index according to monthly and seasonal changes, and (3) assess how the calculated atmospheric stability index is related to actual thunderstorm occurrence using statistical analysis. Additionally, we aimed to investigate the atmospheric characteristics above the Naro Space Center through the distribution chart of the atmospheric stability index during summer, when convection activity is highest. Finally, we assessed the relationship between lightning occurrence and unstable atmospheric conditions, through predictability analysis performed using the lightning observation data of the Korea Meteorological Administration.

• 한국지반신소재학회논문집
• /
• 제15권4호
• /
• pp.17-23
• /
• 2016

본 연구에서는 터널을 대상으로 비대칭 분기부 암반 필러의 거동을 3차원 수치해석 방법으로 검토하였다. 터널 비대칭 분기부의 경우 필러부의 안정성을 확보하는 것이 본 터널의 안정성을 좌우하는 매우 중요한 요소라 할 수 있다. 암반 필러의 거동에 영향을 미치는 매개변수로 암반 필러의 이격거리, 토피고, 암반등급 변화에 따른 안전율을 평가하였다. 터널 비대칭 분기부 암반 필러의 이격거리가 증가할수록 토피고와 암반등급에 따른 안전율 변화 곡선은 비선형 거동을 보였으며, 터널 비대칭 분기부에서 암반 필러의 거동 특성을 최소 안전율을 기준으로 안전율 도표로서 제안하였다.

• 대한기계학회논문집A
• /
• 제26권12호
• /
• pp.2647-2655
• /
• 2002

This research presents an analytical model to investigate the stability due to the ball bearing waviness i n a rotating system supported by two ball bearings. The stiffness of a ball bearing changes periodically due to the waviness in the rolling elements as the rotor rotates, and it can be calculated by differentiating the nonlinear contact forces. The linearized equations of motion can be represented as a parametrically excited system in the form of Mathieu's equation, because the stiffness coefficients have time -varying components due to the waviness. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as the simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving the Hill's infinite determinant of these algebraic equations. The validity of this research is proved by comparing the stability chart with the time responses of the vibration model suggested by prior researches. This research shows that the waviness in the rolling elements of a ball bearing generates the time-varying component of the stiffness coefficient, whose frequency is called the frequency of the parametric excitation. It also shows that the instability takes place from the positions in which the ratio of the natural frequency to the frequency of the parametric excitation corresponds to i/2 (i=1,2,3..).

• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 1996년도 추계학술대회 논문집
• /
• pp.648-652
• /
• 1996

Grinding Center(GC)는 머시닝센터가 갖는 드릴링, 태핑 등의 절삭가공의 수행은 물론 주축의 고속화, 이송계의 고정밀화를 토대로 연삭숫돌의 절삭성 회복과 다듬질면의 거칠기를 확보하기 위한 기계로서 Dressing/Truing장치, 기상계측 장치, CNC기능을 구비하여 테이블과 공구간에 3차원의 상대운동을 시킴으로써 평면연삭, 내면연삭, 홈연삭, 캠연삭 등의 복잡한 형상의 연삭가공에 대해서도 공정을 집약화 할 수 있는 기계이다.(중략)