Reliability Allocation Of NC Lathe Based On Fuzzy Theory

NC machine tools are very important equipment for manufacturing, they are also the core of advance manufacturing technology with the growing of air space, mechanical engineering, vessels etc. The electronic information, biology, the rise of the high-tech industries give out more needs to the development of NC machine tool, now more accurate ,efficient machine tools are needed very much. High-tech NC machine tools are the basis for fulfill industrial manufacturing and trades, many countries are trying to make high-tech machine tools. China is a manufacturing countries, the development of its manufacturing equipment is not only meet the need of its own industry development, but also can improve the overall competitive, with profound strategic significance.This paper studied 12 sets a series of heavy vertical NC lathes, gathered reliability data and analysis these data,some failure model analyze had been done,and we can get the risk data of every sub-systems; According to the data fitting of whole machine and sub-system, we can get the reliability function of them; at the same time the three reliability importance index will be calculated, compare there of them, some useful conclusions will be obtained. According to the above conclusions and take the whole machine's reliability influence factors into consideration, and give out a reliability allocation decision of this type of machine tool.This paper make a failure mode analysis on sub-systems and point out the order of sub-system. First of all, gathered the failure data of one type of machine tool, and the failure mode analysis have been made, the whole machine was divided into 10 sub-systems, they are electrical system, tool carrier, spindle system, CNC system, spindle system, hydraulic system, lubricate system, collecting and transferring system, external interface and other else. To target each subsystem failure patterns of risk preference for a solution as a model of risk preference, in the solution of the process, the risk factors O S D are given different weights, and the weights are expressed as fuzzy numbers. This method can effectively overcome the defects of traditional RPN method, and overcome the subjective of evaluations made by experts. De-fuzzy the fuzzy number and we will get a real value, and sort the sub-system according to these real values.We preliminary processed the on-site data failure data, according to the classification of subsystems in the second chapter, and then calculated reliability functions of whole system and subsystems respectively. Next, we chose the best distrubution model through comparing with the fit correlation index of distribution functions, and comfirmed that both whole system and subsystems would obey the weibull distribution. Finally, we used a chart Examination methods and D Examination methods to test the fit of model. Calculating the eliability functions of whole system and subsystems would be benefit for reliability importance analysis in the fourth chapter, so in Chapter 4 we could figure out the MTBF (Mean Time between Failures) of whole system and subsystems.In order to figure out the reliability importance of subsystems, we focus on three importance indicators of reliability importance, including failure number importance index, failure downtime importance index and dynamic importance index. Based on the failure data needed by above three indicators, we computed values of this three indicators for 10 subsystems, drawn up the curve graphs of dynamic importance of 10 subsystems, and then calculate the value of importance as T=1000. Comparing the failure number importance and failure downtime importance of 10 subsystems with the values of importance as T=1000, we found out the key subsystems of this type NC machine tools, including hydraulic system(D), servo system(F), CNC system (NC), electrical system (V), the transmission (S), worktable (T).In order to allocate the reliable allocation for this type NC machine tool, we first studied the impact factors consided by reliable allocation, and found out the significant impact factors of reliable allocation are importance of subsystems, risk value of subsystems, c omplexity of subsystems, mean of maintenance time, level of technology, environmental conditions and cost. Then based on these impact factors, we taken on secondary processing for given data, and constructed a reliable allocation model. Third, we use expert evaluation method to deal with the impact factors which could not be supported by relative reliable date, and put date into reliable allocation model to figure out the values of reliable index which should be allocated in reliable design of next NC machine tool. Fourth, comparing with the reliabilities before allocation and after allocation, we found that the mamium difference between the actual value and target value would be existed in beam, lubrication system and the hydraulic system, so we should focus on the impovement in design for these three systems. We also found that, in CNC system, electrical system and server system, the differences between actual value and target value would be large too. Because most of the failure of CNC system are fault operation of operator, the cause of most electrical system failure are damage of components, so the operator should get more training and should improve the Standardization Management of the purchase components according to quality management system, the unqualified supplier should be correct once the components which are undesirable were found. Manufacturers should make a further improvement on confirming the law of quality acceptance clauses and the acceptance method, and at the same time reinforce the work concerning on debugging and detection.The reliability of subsystem is not the higher the better, and when the reliability is improved a little high, it also need a large amount costs of geometric progression, and every subsystem have its own optimum value, over this value will not get the optimum economic returns. Considering it is the first level of design of this type of NC lathe, the reliability can only implement to subsystem, with the development of technology and the gathering of failure data, we can divide the reliability to components.