Research On Reliability Assessment Methods Of Domestic CNC Lathes

CNC lathes are important equipment in machining,which are widely used in the manufacture of various products.The improvements of related technologies have promoted the development of Chinese manufacturing industry.On the basis of reasonable cost,reliability of domestic CNC lathes determines to a large extent the efficiency,accuracy of machining and the lifetime of CNC lathes themselves.In order to analyze the reliability of CNC lathes,HTC 2050i and ETC36 series CNC lathes are taken as the research objects.Failure analysis is conducted by using corresponding failure data,failure modes are prioritized through fuzzy ranking and comparison by grey theory.Besides,reliability allocation is conducted considering failure information and FMECA results.Failure correlation is also considered when allocating reliability index to subsystems.Finally,an effective method for MTBF estimation is proposed,which is of great use in mechanical engineering.The main content of this paper is summarized as follows:(1)With respect to FMEA of CNC lathes,contributions have been made for the two types of domestic CNC lathes in this paper such as subsystem division,reliability block diagram making,potential failure modes analysis and FMEA form making.(2)In view of the risk prioritization of failure modes of the spindle system of CNC lathes,spindle systems of ETC36 series CNC lathes are taken as the research object.Based on the traditional RPN analysis in which severity,occurrence and detection are seen as the three variables,the average maintenance time of failure modes is introduced as another variable.Fuzzy set theory and expert rating method are applied to calculate the fuzzy numbers of failure modes,to solve a level sets,to make defuzzification,and finally risk rankings of all the failure modes are obtained.The results are compared and verified by the analytical results of grey theory.It shows that the results from the two methods are almost the same,and the method proposed in this section can reduce subjectivity in risk prioritization of failure modes.(3)For reliability allocation of CNC lathes,criticality is first considered in this section.Severity is transformed into exponential type,and cost needed for reducing failure rates of subsystems is taken into account.Thus,modified criticality is obtained.Besides modified criticality,failure frequency ratio,failure stopping time ratio,reliability influence degree and structure complexity are regarded as the other four factors for reliability allocation,which are all based on objective information.In this section,CNC lathes are seen as series systems,and through establishing intermediate matrices and vectors such as ratio matrix,comprehensive allocation matrix and weight vector,the allocated results for each subsystem are finally achieved.(4)As to the failure correlation among subsystems of CNC lathes,Copula theory is applied in this section.Reliability model of subsystems with failure correlation is established,and then the reliability model of the whole system is obtained.Based on the model,target value of reliability index is set,and a reliability allocation method is proposed.Furthermore,cubic transformed function of severity is considered to get modified criticality values,and a specific reliability allocation method is proposed considering comprehensive factors.Through two real cases,the significance of the proposed method in reducing cost of design and manufacture is illustrated.(5)With regard to calculation criteria for MTBF of CNC lathes,the differences between each two adjacent calculated values of MTBF are calculated based on field failure data of CNC lathes.Estimation criterion is given to obtain the cut-off point of experiment and the minimum experimental size correspondingly.The error between the results from the proposed method in this section and from the traditional method is reasonable and acceptable.However,the estimate times or the failure times of sample CNC lathes in the test process can be controlled effectively,and MTBF of the system can be obtained based on the minimum experimental size,which is significant in cost saving,so that the proposed method can provide theoretical support for evaluation of reliability index in mechanical engineering.