Reliability Evaluation And Optimization Of The Servo Turret

To follow the developing trend of Computerized Numerical Control (CNC) machine tools with high-speed, high efficiency, high-precision, flexible and complex processing capacity, the fundamental task is upgrading supporting capacities of high-end CNC systems and functional components. As for CNC turret, the development of servo turret indicates that this kind of functional component has entered into mid-high-range products. Among all the key technical indicators, the reliability level of servo turret implicates the overall performance, operational stability and efficiency of the matched CNC machine tools. Therefore, it is utmost urgent to research and improve the reliability of servo turret.In this dissertation, a systematical research of qualitative and quantitative evaluation of turret reliability, reliability-based optimization design, and reliability test was carried out under the support of special fund project for major scientific and technological achievements in "high-end" CNC machine tools and basic manufacturing equipment. In course of this research, an advanced, integral and practical solutions suitable for effectively assessing and improving the reliability of turret was formed, through the comprehensive use of reliability technology system, in addition to the proposed Multi-State Dynamic Fault Tree (MDFT) analysis method, multi-state system failure-frequency analysis method, reliability test in cutting conditions, and PCM-PHM integrated model in operational reliability assesment. Detailed research contents and major achievements of this dissertation are listed as follows:(1) Failure Mode and Effects Analysis (FMEA) of the turret. In system planning and prototyping stage, gathering relevant reliability data and information can effectively improve the solution of FMEA. Therefore, statistical analysis of failure components and failure modes were carried out based on failure data of hydraulic turret, which has approximate structure with servo turret. Models to calculate the first failure moments of the whole system and its subsystems are established through distribution fitting and testing methods based on failure data containing censored data.10 sub-systems are divided based on independent functional flow in series/parallel /conversion rules. And then, similarities of each similar module and the whole system comparing to the hydraulic turret are etimated. As for each sub-system, possible failure modes and failure causes of the servo turret are analyzed. Combined with effects analysis, prevention and compensation measures are proposed.(2) Quantitative evaluation of turret reliability indexes. Considering the multi-state properties of turret, MDFT mathematical modeling method is proposed. Firstly, Linear Algebra Representation (LAR) method is presented to describe the state space and state transformation. Then, operation rules of all static and dynamic logic gates are established based on state transition ideas. Since the MDFT model has modular characteristic, Monte Carlo simulation is adopted to calculate system reliability and subsystem’s importance, which is supported by generating analog clock with variable steps. Using above methods, lifetime of system in special reliability is evaluated, as well as the critical sub-system.(3) Reliability Analysis and Optimization Design of key structures. In the improving design stage, state function of cutter sub-system of multi-failure modes is established based on stress-strength interference model, and reliability analysis is performed by integrating using of moment method and Monte Carlo simulation. Then, improved design for satisfying the predetermined reliability value is optimized through reliability sensitivity analysis, which is used to identify sensitive structural parameters. Multi-state system failure-frequency reliability analysis method is proposed, while nested Monte Carlo simulation is introduced to calculate the reliability of the system frequency. And then, sensitivity and sensitive direction of stochastic structural parameters are identified by using correlation analysis methods. Also, dynamic model of servo turret is established by Transfer Matrix Method of Multi-body System (MS-TMM). Then, the reliability of the system frequency has been significantly improved by lightweight design, which is supported by the proposed multi-state system failure-frequency analysis method.(4) Specification of the reliability development test in cutting conditions. In the prototype trial phase, reliability test in cutting conditions is proposed to obtain a reasonable assessment of turret reliability in the actual dynamic load. Firstly, the overall design ideas of test is systematically expounded, and the research platform for testing turret’s comprehensive performance and reliability is introduced, which is built based on functional requirements. Then, the mission profile compilation method based on typical test workpiece of various materials is proposed, while the manufacturing process of workpiece is determined by the cutting force testing system. Also, the log table for recording reliability test and the table for turret failure analysis are designed. Finally, comprehensive performance test procedures (including tests for static stiffness, repeating positioning accuracy, noise and acceleration frequency response function) and processing methods are developed.(5) Reliability Assessment based on intergrated operational information of servo turret. Regarding the competing failure modes, in which independent catastrophic and degradation failure modes are coexist, the PCM-PHM integrated model is proposed to estimate turret’s operational reliability, which is built on comprehensive utilization of environmental covariates, response covariates and historical data. The advantages of the proposed method are conclued as follows: firstly, influences of loading conditions are both cosidered in different types of failure modes; and secondly, multiple response covariates are utilized in establishing the reliability model of degradation failure modes. And then, the proposed method is applied to the reliability assessment of the test prototype which is participated in reliability development test. Testing load distribution model is established based on counting peak load steps, and distribution fitting and testing methods. Comprehensive response covariate characteristic value is established on the basis of correlation analysis and index model, while turret’s failure rate function is obtained by multivariate linear regression method. Finally, reliability function is established and reliable lifetime of the servo turret is estimated.A series of conclusions refined from above research have been applied to the actual design and manufacture of the servo turret. Currently, the mid-high-range functional component with independent intellectual property has been mass produced and also been supported in CNC lathes, whose reliability level has been generally recognized by the customers.