Research On Multiple-Stress Reliability Accelerated Model Of Spindle System In Machining Center

The technology of CNC machine tools and its key functional assembly units is an important force to drive the transformation and upgrading of the manufacturing industry,which directly reflects the manufacturing level of a country.The spindle system is a key functional assembly unit of CNC machine tools.It is a complex electromechanical system affected by various loads during the working process,and results in difficult maintenance.At present,the gap of accuracy and speed of rotation between the domestical spindles and the international advanced level is shrinking,but there is still a large gap in reliability level.Reliability testing is an important fundamental work to improve the reliability of spindle.In order to shorten the test cycle and improve the test efficiency,it is necessary to carry out the reliability accelerated test and establish a multiple-stress reliability accelerated model for the spindle system,which describes the relationship of reliability level of the system and accelerated load.The accelerated model can be used to provide a basis for the rapid reliability evaluation,improvement design and the optimization of the test plan.With the support of the National Natural Science Foundation of China and the National Science and Technology Major Project,this paper takes the domestic spindle system of machining centers as the research object,and carries out the research of reliability multiple-stress accelerated model.According to FAST analysis of the spindle system,the main reliability indexes that characterizes the reliability level of the system are determined as the output variables of reliability accelerated model.According to the main failure mechanism of the important parts of the system,the sensitivity load of the system reliability is determined.A method for load impact evaluation considering the influence of load and the importance of components is proposed,and the main input variables of the reliability accelerated model are determined.The reliability level of spindle system is verified to be accelerated by the certain range of load.The load range of the system is counted by empirical formula,which is applicable to the established accelerated model is determined.According to the result of constant stress accelerated test result,an accelerated model considering a single stress of the spindle system is established.On this basis,a multiple stress accelerated model considering the difference of fault types of the spindle system is established.The model can predict and evaluate the reliability level of the spindle system within the applicable load range well.The main research work of this paper is as follows:(1)Reliability analysis of the spindle system based on FTA.The reliability characteristic indexes of the system is the output variable of the accelerated model.The functional analysis system technology is used to analyze the function of the spindle system and its componnents.The main reliability characteristics of the reliability level of the system are determined,including the temperature,vibration and power supply near the bearing.The sensitive load which affects system reliability is used as the input variable of the accelerated model.By FTA method,several importance indicators and downtime,the important components which affect system reliability are determined.According to the main failure mechanism of important components,the main sensitive load of the spindle system reliability level is determined.A quantitative evaluation method that considers the influence of the load and the load influence of the component importance is proposed,and the main sensitive load influence order of the system reliability level is determined:cutting force,spindle speed,cutting power,ambient temperature,the use of cutting fluid and number of tool changes.The key load is cutting force,spindle speed.The results of analysis and sorting are in line with actual engineering experience.(2)The verification of the accelerated characteristic of spindle system and load analysis.The premise of establishing an accelerated model is that the research object can be accelerated within a certain load range.The fault distribution of the research object has consistency and regularity,and the fault mechanism remains consistent.Firstly,according to the distribution function of the MTBF of the spindle system,the consistency and regularity of the fault distribution are verified.According to the characteristics of the test data,the consistency of the fault mechanism of the spindle system is verified by Bartlett statistic.According to the recorded process information and cutting force experience.The range of load levels are calculated by formula,at which the system can be accelerated and the range of model extrapolated is determined.(3)Research on a single-stress reliability accelerated model of spindle system.Firstly,according to the fault interval distribution of the spindle system and the load range when the system has accelerating,the constant stress accelerated test of the spindle system is designed and carried out.The acceleration stress is selected as the system reliability.The critical sensitive load cutting force and speed are carried on the system separately.The fault time and the vibration signal of the spindle are analyzed.According to the relative variation law of the amplitude of the vibration signal,the fault time of the system is predicted.A single stress accelerated model of the spindle speed and the system fault time is established.As the reliability characteristic of the spindle system,the single stress accelerated model of the cutting force and the reliability level of spindle is established.The two accelerated models are verified by statistical methods.The established single stress model can describe the relationship between system reliability level and single accelerated stress well,and the estimated parameter results are in line with engineering experience.(4)Research on the multiple-stress reliability accelerated model of the spindle system.Firstly,according to the influence of sensitive load on the main failure mechanism of the spindle system,an accelerated model of a single sensitive load is established.Assuming the load effects are independent of each other,the system reliability multiple accelerated model is proposed by using the AFT model,and the main reliability of the system is described.The stepwise regression method and the p-value test method is used by selected significant sensitive loads,which are the number of tool changes and the use of cutting fluid.The coefficient of significant stress is estimated.Although the multiple stress accelerated model can pass statistical tests,the statistically significant load determined by the model is quite different from the significant load of the actual spindle system.(5)A multiple-stress accelerated model for the reliability of the spindle system considering the type of fault is proposed and established.Firstly,based on the Bayesian decision theory and the nature of the competitive failure model,considering the fuzziness of the fault and the competing failure,a fault classification method for the spindle system is proposed.The system fault is reasonably divided into catastrophic and degradated.The generalized fault number is obtained.The AFT model of the two types of faults and the accelerated model of the sensitive load are established respectively.The stepwise regression method and the p-value test method are used to determine the significant stresses of the accelerated model of the two types of faults.Based on the relationship between the fault interval of two types of faults and the system reliability level,a multiple stress accelerated model of the spindle system reliability is proposed.The form of the multiple stress accelerated model is reasonable,and the significant stress is in line with the engineering experience.The parameter estimation result is similar to the parameter estimation corresponding to the single stress accelerated model,which is consistent with the analysis results of the main failure mechanism of the system.The accelerated model of the spindle system reliability is verified by the field reliability test data.The results show that the model can predict the reliability level of the spindle system within the extrapolated load range well.