| CNC machine tools are very important equipment for manufacturing,they are also the core of advance manufacturing technology.China is a manufacturing country,the goal of research and development of CNC machine tools in China is to invent a machine which is upscale,high-speed,precision and multi-axes synchronous machining.However,because of the low reliability of CNC machine tools which are made in China,high-end CNC machine tools can only rely on importing.Therefore,by studying the reliability of machine tool is not only beneficial to meet the needs of China's manufacturing industry of numerical control equipment,but also can promote the comprehensive competitiveness.And one of the most key to improving reliability is reliability allocation in a reasonable way.In this paper,the H series of CNC machine tools was object of study and reallocated based on the failure data.The whole machine was divided into six sub-systems according to the function of CNC machine tools principle of relationship.And the failure data were classified based on the concept of each subsystem.Then the principle of fault data processing was put forward and according to this principle,the failure data collected in the factory was screened and eliminated.After screening,the preferred distribution model was got based on failure statistics by Minitab software.And for the failure data of small sample,maximum likelihood estimation method and parameter bias correction method were used to estimate the functions of reliability for the whole system as well as the subsystems,and they were accorded with D test.And the reliability function of each subsystem will be used to estimate the MTBF and the basis for calculating of dynamic effect.Fuzzy comprehensive allocation method was used to allocate this type CNC machine tool.And in order to getting influence degree of sub-systems to machine under kinds of situations,distribution factors and the weight of each factors were considered first,the impact factors considered by reliable allocation were studied,and the significant impact factors of reliable allocation were central effect degree,dynamic importance index,maintainability,complexity of subsystems,economy and manufacturing technical level.Failure correlation was considered between each system failure,when central effect degree was calculated;When dynamic importance index was calculated,the influence of the reliability of the subsystem to the whole machine reliability was considered based on the structure of series system;When maintainability was calculated,maintainability index and correlation among these indexes were considered;As for complexity of subsystems,the shortage of traditional computation complexity was unconsidering the failure interaction,so,expert scoring is used to calculate for each subsystem complexity;And as for economy and manufacturing technical level,the score of each subsystem was combined of interval analysis method and expert grading method.Using fuzzy comprehensive distribution method of CNC machine tools,besides of allocation factor,it still needs to consider the weight of each allocation factors.Combined with analytic hierarchy process,interval analysis and expert scoring method,the model of distribution factors weights were established.And the interval of weights were calculated by IEM method.Finally,combined with allocation and weights,the value of reliable index was allocated in subsystems.Based on allocation of the above theory and using MATLAB GUI as software development platform,the general software of reliability allocation which considering failure correlation of CNC machine tools was developed.And the efficiency of the designing was improved by this software. |
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