| As the machine tool with high-tech content,CNC machine tool is the backbone of equipment manufacturing industry.Heavy-duty CNC machine tool is the high-end equipment equipped with multiple systems and multiple technologies,the technical level and quality of which provide important support for national economy and strong guarantee for national defense security.However,the serious reliability problems exposed in domestic heavy-duty CNC machine tools not only affect their market share,but also relate to the ability of equipment manufacturing.To bear the heavy load,the heavy-duty machine tool is normally designed by using static pressure supporting and hydraulic driving,and equipped with complex hydraulic system.Refer to the field failure data,it is shown that the failure proportion and economic losses are mainly caused by the hydraulic system of heavy-duty CNC machine tool,especially by oil contamination.Therefore,the study of the oil contamination has both theoretical and practical significance in enhancing the reliability of heavy-duty CNC machine tool.Many reliability problems of national machine tools are result from the little attention during the leapfrog development of mechanical industry.The reliability study of heavy-duty CNC machine tool is inadequate on account of the lack of national experts,the existing technology to use for reference,as well as samples and so on.The relationship between the oil contamination and the reliability of heavy-duty CNC machine tool,which can be used as the reference for the optimal design,condition monitoring,fault early warming and fault diagnosis,was neglected for its edge position.For these reasons,this article studies the reliability of heavy-duty CNC machine tool for the oil contamination,taking the heavy-duty horizontal lathe and heavy gantry boring and milling machine tool as example.The main contents of this article are as follows.(1)Because the lack in research foundation,hydraulic system is firstly proved to be the weak link of the heavy-duty CNC machine tool.Aiming at the one-sidedness when evaluating the weak degree of subsystem as well as the few consideration of the maintenance cost and time when judging the weak link by using Failure Mode Effect and Critically Analysis(FMECA),this article divides the heavy-duty CNC machine tool into several subsystems based on structural analysis,and the failure data of 55 heavy-duty CNC machine tools from different manufacturers under different working conditions are collected by using the established collection norms.Based on the collected failure data,FMECA as well as the statistical analysis of maintenance cost and time for each subsystem are carried out to integrate into FMECA information.Indexes are screened by comprehensively considering the subjective weight,the relevance weight and the information weight.And then,Cognition Best Worst Method(CBWM)is used to calculate the weight of impact factor vector.After that,the weakness of each subsystem is ranked by Technique for Order Preference by Similarity to an Ideal Solution(TOPSIS)and the hydraulic system is finally determined as the weakest subsystem in the example.(2)Oil contamination is further proved to be the main reason for the failure of hydraulic system in heavy-duty CNC machine tool by using D-S evidence theory and interval rough number.Failure modes of hydraulic system are summarized according to the study in the failure data of hydraulic system.Meanwhile the fault tree is established and the minimum cut set is divided by Boolean operation.The importance degree of the probability of bottom event is selected as the evaluation index of the fault reason.Considering that the probability of bottom event cannot be obtained,the comprehensive occurrence degree composed of the subjective occurrence degree and the objective occurrence degree of bottom event is defined as an alternative to calculate the participatory importance degree,while the objective occurrence degree is obtained based on the statistical frequency of the failure data with known fault reasons and the uncertain interval of fault reasons gotten from the failure data with unknown fault reasons after being processed by D-S evidence theory.The subjective occurrence degree is obtained based on the score of interval rough number for the possible degree of failure reasons that have not occurred,maintenance personnel and users.The expected value of comprehensive score is calculated.The importance degree of bottom event is ranked by using the interval number ranking method based on Boolean matrix,after the importance degree of bottom event has been obtained.The results indicate that oil contamination is the fault reason with the highest importance degree.(3)To establish the quantitative relationship between the oil contamination and the reliability of heavy-duty CNC machine tool,the trend change test of oil contamination,the correlation test of oil contamination and the environment contamination and the oil contamination test of failure position have been carried out.After the time domain analysis on test data of oil sample,the dimensional and non-dimensional parameters of particle numbers are obtained.Through the analysis by Q-Q plot and K-S test on dimensional parameters,it can be seen that the degradation data of contaminated particles obey the normal distribution.The relationship between environmental factors and dimensionless parameters is analyzed by using correlation coefficient method,and the change value of oil contamination is hardly dependent of temperature,flow rate and pressure.Hydraulic component is summarized into three categories,pipeline,valve and filter.The blockage of pipeline and valve is studied by the particle number about 5μm.The wear of pipeline and valve is studied by the particle number larger than 15μm.The blockage of filter is studied by particles with the similar filtering accuracy.The threshold of the particle number is set according to the 20/17 level of ISO4406 standard.Reliability models of various components for a single failure mode are established based on the degradation path and the distribution of degradation amount.Considering that the intermediate threshold is not a deterministic value,the stress-strength interference model is applied to modeling again,where the strength function is obtained by converting the test data produced from the oil contamination test of failure position.Similar equipment method is used for pipeline and valve,and the distribution function of particle size is used in conversion.Finally,the reliability model under multiple failure modes is fused with the reliability model of each component under the single failure mode by competing failure model.These three reliability models reflect the quantitative relationship between the oil contamination and the reliability of heavy-duty CNC machine tools.(4)The established model is applied to the optimal design of the hydraulic system,and the conceptual design of hydraulic system is proposed.Taking the branch as the research object,the control model of oil contamination is established,and the least square method and maximum likelihood estimation(MLE)are used to estimate the unknown parameters in the model.The model is tested based on the test data.The global reliability model of hydraulic system is established combining with the reliability model of hydraulic component and the control model of oil contamination.After analyzing the size,the structure,the maintenance cost and the oil pressure that need to be considered in designing,the optimization model is established to minimize the average maintenance cost under the constraints of structural factors,MTBF of system and the oil pressure,with size structure and oil change period as optimization variables.The model is solved by particle swarm optimization(PSO),and the sensitivity analysis of parameters in the model is analyzed.Among these,the period of oil change provides a reference for the formulation of new instructions for heavy-duty CNC machine tool.(5)In order to assess the application effect of research results,a new method to calculate the growth rate of reliability is proposed by introducing the concept of net growth rate to deal with the issue that the difference of reliability growth before and after is not taken into account when calculating the reliability growth rate of products.Based on MTBF of the whole machine before reliability growth,the net growth rate model of the whole machine is established.The reliability relationship between the whole machine and subsystems is established through reliability diagrams.The net growth rate superposition model of subsystems is established based on test data and product information.When processing the product information,the network analysis model of factors affecting the reliability of equipment is established comprehensively considering the product structure,design,manufacturing,assembly and using environment.The comprehensive score reflecting the difference of reliability growth before and after is obtained,as well as the reliability change rate due to the difference of equipment themselves.The interval number is introduced into the network analysis model to consider the fuzziness of subjective judgment.The feasibility of the method and the effectiveness of reliability growth technology for hydraulic system of heavy-duty CNC machine tool are verified by an example.In this article,the theoretical system of the reliability research for the heavy-duty CNC machine tool is established for oil contamination,including the reliability analysis,modeling,design and evaluation.This system provides the method and support for the later study on fault diagnosis,early warning,and other researches of heavy-duty CNC machine tool for oil contamination. |
PDF Full Text Request