Research On Preventive Maintenance Strategy Of CNC Lathe Based On Fault Rate Correlations

For manufacturing enterprise,CNC lathe is core equipment.Safety and reliability of the CNC lathe has been playing an important role on keeping the core competitiveness of enterprise.It is the key that developing scientific and rational maintenance strategies can ensure the CNC lathe in normal operation.Adopting inappropriate strategies can make enterprises suffer huge losses,reduce enterprise economic effect,even impedes the development of enterprise.CNC lathe is a complex system incorporating the mechanics,electronics and hydraulics.Due to the complex structures of CNC system,its components faults have correlations.Because of scale effect of complex system,high rate of whole machine failure which increased exponentially can be leaded by even a extremely tiny fault rate,causing the increased maintenance costs and shut down loss,even suffering serious consequences.Therefore,taking CNC lathe as a research object,the preventive maintenance strategy based on the fault correlations analysis is studied in this paper.In the first place,fault correlations of CNC lathe components are analyzed.Based on CNC lathes are divided into some components,correlative fault data of CNC lathe components is collected and analyzed,which lists in table.By using the DEMATEL method,the integrated fault influence matrix is built,which reflects the interrelation and influence degree between CNC lathe components.Based on that,structure relationship model between correlative fault components is established by applying ISM method,and then through presenting the process of the failure propagation,the fault transmission and hierarchy are further clear.The next,the replacement strategy of CNC lathe components is formulated,based on the fault process model.For accurately estimating the stock of spare parts and replacement time,the fault process model of system components is established,considering the correlation between the fault times of the components.The fault time sequence of the system components is modified by Johnson method.And the trend and stability of the fault data are tested.the fault process model of system components is established.Then,the components maintainable time and spare part quantity can be calculated by the actual operation data of CNC lathe.Once again,the best preventive maintenance intervals time of CNC lathe machine is obtained on the basis of fault correlations analysis.The reliability modeling of CNC lathe components is established.The research of CNC lathe components can be divided into two categories,which are the fault source components and non-fault source components.Because the fault source components have little data,the established reliability model is modified by using deviation correction method and the goodness of fit is tested by D test method,then,the fault source components reliability modification model is built.In order to improve the reliability of calculation results,it is necessary to modify the fault rank by Johnson method,after that,parameter estimation is used by least square method,then,correlation test is examined by correlation coefficient method and hypotheses test is used by D test,the non-fault source components reliability model is established based on the assumption of independence of fault.Copula function as connection function is introduced,for developing the joint reliability function of fault dependent component.The parameters estimation of Copula function is estimated by calculating the Kendall rank correlation coefficient.Then,the non-fault source components of reliability model are established based on the fault correlations.And comparing with the reliability model which is established based on the assumption of independence of fault,the reliability model which is build based on the fault correlations is more rational and effective.And then the correlation coefficient is calculated between the fault interaction components.The fault rate of CNC lathe is deduced,through calculating the fault rate of every component.The best preventive maintenance intervals time is derived from the perspective of work availability.According to the maintenance cost data collected in enterprise,from the perspective of maintenance cost,the best preventive maintenance intervals time is obtained by simulation analysis.At last,based on the evaluation of availability for CNC lathe,the preventive maintenance schemes decision-making model is constructed.The CNC lathe condition is evaluated through the set of evaluation assessment index.Different availability states correspond to different maintenance schemes.In order to making the assessment results both subjective and objective,the subjective weight is calculated by fuzzy analytic hierarchy process,the objective weight is determined by adopting entropy method,on this basis,the comprehensive weights are decided by the comprehensive weight combination method.CNC lathe condition is evaluated by introducing the Gray cluster theory.The appropriate preventive maintenance schemes are selected,according to the results of the evaluation of the CNC lathe availability.The CNC lathe preventive maintenance strategy based on the fault correlations analysis is proposed combining with the fault characteristic.It provides a useful reference for the maintenance management of CNC lathe.