Research On Credibility Dynamic Model Of The Machining Center Based On The Life Cycle

Research on credibility dynamic model of the machiningcenterbased on the Life cycle1. IntroductionAs an essential attribute of products, reliability is an important index for evaluating product quality and competition focus in world product market. Now it has become an indispensable basic element for the industrial fields, such as engineer design, enterprise management, management decision, products operation and maintenance etc. and it is also an important basis and a measurement index to improve economic benefit and management level. However, for a long time, the research on reliability is confined to the discussion on the basis of failure data, and the study on the product maintainability and the relationship between them are seldom involved. Through long practices, it has been found that it is not a better idea only to enhance reliaibity from the aspect of decreasing the life cycle cost, the relationship between reliability and maintainability must be considered to get a better result. Therefore, only based on the study and evaluate the quality index of product reliability and maintainability, to research the dependence between them using generalized reliability analysis model, which is also called availability model, is the best way to evaluate the level of product reliability and quality, more reasonably and more effectively.In this thesis, a detailed analysis has been made from the aspects of failure laws and maintainability index through the study on a certain series of machine center as research objects on the basis of gathered the failure data. Furthermore, availability model has been established based on the research on the interdependence between the reliability and maintainability. Comprehensive evaluation of failure law and maintenance rate of machine center been also carried out to provide the quantitative basis and theoretical guidance for improving the quality and availability level of CNC system made in China.2. The reliability of machine center & the evaluation of reliability Importance critical subsystemsAccording to the large quantity of failure data collected, the failure frequency of machine center at different cumulated operation times is plotted, and then the failure distribution law of the series of machine center is generally studied. It's found that the failure data covered the failure occurred during earlier failure period and also the random failure period. To establish the model of failure rate of this series of CNC equipments, the sectional Weibull distribution model is used to build the life cycle distribution laws of earlier failure period and the random failure period and their sectional relationship of the whole machine and its all components. The reliability function of the whole machine system is as follows:The failure function of the whole machine system is determined as follow:The coefficient k in reliability function serves as a weight to change the position of reliability curve of earlier failure period in order to smooth connect with that of random failure period.According to the "Reliability Test Standard of Machine Center" developed by the institute, the preliminary analysis of failure position of the whole machine system and failure model has been carried out, as seen in table A, and then to investigate the weakest link of reliability. It's found that about 78.3% of all failure of the whole machine system took place in sis subsystems, including tool system etc, which is considered to be the critical subsystems. Life cycle distribution laws of the critical subsystems have been studied.Based on the aforementioned RBD series model, related reliability importance of subsystems is quantitative evaluation using Leemis method as follows:In above equal, R_s(t) is the reliability of the whole system while R_i(t) is thereliability of the subsystems.According to the model of reliability and its importance, quantitative relationship between the whole system and the critical subsystems has been researched. The effect of life cycle characteristics of critical subsystems on the reliability of the whole machine center and the interdependence between them are also studied in order to provide theoretical support to analyze the critical subsystems and improve product quality to some extent.3. The analysis of maintainability of machine center To enhance the performance of the machine center, not only good functions but also the conveniences of maintainability are required. The repairable system switches between the normal state and the fault state. Good maintainability is a useful measure to improve the accuracy of the machine center, which can exert the processing capacity of machine center in the best way. The maintainability of the system is measured by the maintenance time, shorter time leads to better maintainability. According to analysis results of maintenance time, lognormal distribution is used to build the distribution law of the maintenance time of the machine center. Maintenance time distribution function is as follow:Maintainability probability function:The estimation value of MTTR :In order to study the impact of subsystems on the machine center in further way, the classification of composed subsystems is studied, and tree model is founded to be more suitable. The importance of the maintainability of key subsystems on the whole system is also established as follow:The relationship between the whole system and the composed critical subsystems is studied in detail, and the effect of the importance of critical subsystems on the whole system in different periods is also investigated.4. The analysis of credibility of the machine centerTo study the interdependence between the reliability and maintainability and build unified evaluation index of product quality, which is called system credibility model, are the objective to analyze them. It will be great helpful to analyze and improve the system credibility level through optimized compute method of system credibility. Giving that the maintainability data of the collect test data of machine center is not sufficient, the research on credibility in this thesis is on the basis of the reliability and maintainability without considering the factor of the supportability. The credibility has been studied with the results of reliability and maintainability, and the concept and connotation of the credibility of the machine center are also exhaustive analyzed. The credibility model has been established through a series of theoretical derivation with the combination of reliability and maintainability. Such credibility indices as inherentcredibility , stability availability, average availability are also analyzed and computed.The model of inherent availability:The model of average availability:The model of stability availability:As the research on the reliability and the maintainability, the availabilities of subsystems have been computed and the mathematics model of the credibility importance effect of subsystems on the whole machine system is also built in order to investigate the availability dependence between subsystems and the whole machine system.5. ConclusionSo far, there are seldom reports about the reliability and maintainability of the whole machine system based on the data of failure and maintainability of machine center. Especially there is almost neither mature or systematic evaluation model nor theory about the research on the credibility of the interdependence between them. According to the collected data of the failure and maintainability of machine center, the reliability, maintainability and credibility of the whole machine are systematic evaluated, meanwhile the structure relationship between the critical subsystems and the whole machine system has been preliminarily analyzed. In order to analyze and evaluate the product quality and enhance system structure, reliability importance model and maintainability importance model are established more systematically. These efforts provide a more integrated foundation to improve the reliability level, and also have great significant meanings to the further theoretical research and the system model analysis.