Research On Maintenance Decision Method Of NC Machine Tools Considering Resource Constraints And Production Modes

As strategic equipment,NC machine tools are important symbols to measure a country’s industrial level and manufacturing capacity.The comprehensive level of NC machine tools is not only reflected in the aspects of compound functions,high speed and precision,but also in their ability to maintain long-term reliable operation,which is a key concern for host manufacturing and user enterprises.As an effective means to ensure the long-term stable operation of equipment,maintenance is an important guarantee for improving the general reliability of equipment.With the development and upgrading of the industry,the changing working conditions,limited maintenance resources,and different production modes bring more challenges to maintenance decisions.In view of the fact that the influence of resources and production has not been fully considered in the current research,this paper has carried out in-depth and systematic research on the maintenance decisions of NC machine tools from the perspectives of "maintenance resources" and "production mode",such as dynamic working conditions,insufficient maintenance resources,and different production modes.The main research contents and innovative achievements of this article are summarized as follows:(1)The reliability modeling method which considers dynamic working conditions for NC machine tools is proposed.Aiming at the disadvantages of the traditional model which neglect the working conditions,the relationships between reliability,time,and working conditions of unrepairable systems are analyzed from the perspective of cumulative damage.Then the cumulative damage model of repairable systems is established by combining it with stochastic process theory.Based on the equality of cumulative damage,the relationship of failure intensity in different working conditions of repairable systems is studied.To deal with the shortcomings of the traditional proportional intensity model,which can not be applied in dynamic working conditions,an improved proportional intensity model is proposed which considers the historical cumulative effects of operating conditions.In response to the difficulty of parameter estimation,an approximate parameter estimation method is proposed by combining the partial likelihood estimation and the maximum likelihood estimation methods.The improved proportional intensity model is analyzed and illustrated through some examples,and compared with the traditional proportional intensity model.(2)The method to determine the maintenance priority of NC machine tool components based on time-varying FMEA is proposed.To address the issue of insufficient maintenance resources,the risk-based method for determining the maintenance priority of components is determined.To overcome the shortcomings of traditional method,the time-varying FMEA method which considers the influence of time and working conditions is proposed with greatly improving in weighting and evaluation.In terms of weighting,expert domain weight,and risk factor combination weight are proposed.In terms of evaluation,risk factors are divided into subjective and objective types.In the subjective risk factors,the fuzziness and uncertainty of intuitionistic linguistic evaluations from experts are dealt with by using the interval-valued spherical fuzzy theory,and the consistency of judgment is used as the acceptance standard.In the objective risk factor,the failure intensity which contains time and working condition information is taken as the judgment basis of the occurrence,and the corresponding mapping transformation method is proposed.The improved indicators are used to determine the maintenance priority of components.The new method is analyzed and explained through the actual case,and compared with the ordinary FMEA.(3)The dynamic group maintenance decision method suitable for the NC machine tools used in mass production mode is proposed.Based on the characteristics of mass production mode in the production rhythm and working conditions,the dynamic group maintenance decision method which applies the relevance of component is proposed.The working conditions transfer and merging method is proposed to reduce the difficulty in modeling and calculation.Aiming at the disadvantage that static group maintenance can’t respond to the dynamic changes of maintenance information,the maintenance decision model,which takes the grouping structure as the optimization variable and the maintenance benefit as the objective function is established based on rolling horizon approach.To deal with the problem of insufficient maintenance resources,a short-term resource limitation adjustment method is proposed to give priority to the maintenance effect of high-priority components.Based on the actual failure data of NC machine tools,the proposed maintenance decision method is analyzed and verified under reasonable scenario assumptions.(4)The dynamic maintenance decision method of NC machine tools suitable for customized production mode is proposed.Aiming at the deficiency that the traditional method has not fully considered the characteristics of customized production mode,the dynamic maintenance decision method is proposed.Aiming at the difference of cumulative damage caused by dynamic working conditions,an imperfect maintenance model in dynamic working conditions is constructed based on the approach of damage reduction.Then,the quantitative relationships between maintenance effect and maintenance resources,maintenance effect and cumulative damage are considered.To ensure the timely delivery of orders,the maintenance decision method is built with the cumulative failure intensity difference in the order production process as the optimization objective,with the maintenance resources obtained by each component as the decision variable,with the current total maintenance resources as the constraint condition.At the same time,the maintenance effect of high-priority components should be ensured to be better.To solve the problem that the maintenance scheme falls into local optimum caused by the continuous renewal of orders,the process of execution judgment and optimization scope determination of maintenance activities is put forward.Based on the actual failure data of NC machine tools,the proposed method is analyzed and verified under reasonable scenario assumptions.