Replacement problems can be classed as either deterministic of stochastic. Deterministic problems are those in which the timing and the outcome of the replacement action are assumed to be known with certainty. Before proceeding with development of replacement models it is important to note that preventive replacement actions, that is, ones taken before equipment reaches a failed state, require two necessary conditions: (1) The total cost of the replacement must be greater after failure than before. (2) The failure rate of the equipment must be increasing. Equipment is subject to failure. On failure, one of two possible actions can be taken : repair or complete replacement of the failed equipment. In this paper, we proposed optimal overhaul, repair, replacement maintenance model with two-state.

Companies adopted a program called Six Sigma, in order to make fundamental changes in the way the company operated to fulfill customers' expectations. Six Sigma reduces the occurrence of defects. This approach derives the overall process of selection the right projects based on their potential to improve performance metrics and selection and training the right people to get the business results. However, in the course of Six Sigma process steps, companies are in the face of problems. This study is to solve the problems using TP(Thinking Process) of TOC(Theory of Constraints). TOC is methodology for solving key problem in system which is called Constraints. Nowadays, its application is going to be wide and its concept is being implemented. In this paper, it is showed possibility of application TOC to Six Sigma.

본 연구에서는 현재 일반가정이나 영업소에서 사용중인 액화석유가스 용기용밸브의 문제점을 제시하고, L.P 가스사고를 예방할 수 있는 LPG용 과류밸브의 과류차단장치를 개발제작함에 있어 고압가스 안전관리기준 통합고시 제12장 4절 과류차단형 액화석유가스 용기용밸브에서 규정한 과류차단 성능시험을 실시하는데 필요한 여러 가지 변수를 선정하여 차단장치의 형상에 따른 최적의 차단 유량 범위를 찾고자 실험을 실시하였으며, 보다 궁극적으로는 일반 용기용밸브가 가진 문제점을 조속히 해결하고 진정한 의미의 안전밸브를 개발하는데 목적이 있다.