The Research Of Preload Automatic Control Technology On NC Machine Tool Spindle Unit

With the successful application of high speed and ultra-high speed machining technology,huge economic and social benefit have been produced because of its many advantages.A good performance of high-grade CNC machine tools is the premise of development and application of high speed machining technology.High-speed motorized spindle is the core part of the NC machine tool,which invloves many technologies such as high speed motor drive and control technology,precision bearing technology,precision motorized spindle bearing lubrication and cooling technology,high-speed spindle bearing preload technology,precision machining and precision assembly technology and high-speed spindle dynamic balancing technology.The bearing preload technology is necessary for high performance of motorized spindle.The preload has important influences on dynamic performance of spindle unit.If the bearing preload can change depending on the different conditions,high dynamic performance of the spindle unit can be obtained,and the service life of the bearing can be greatly extended.Preload can effectively improve the stiffness of shaft system and reduce the shafting vibration and temperature rise,thus the full potential of shafting can be reached.Positioning preload and constant pressure preload can not meet the demanding of modern machining.Therefore,taking the griding motorized spindle as objective research,the automatic control technology for bearing preload was analyzed in this thesis.The main studies are as follows:Firstly,as the key factor affecting the performances of the spindle,the preload has important effect on the temperature rise of the rotating shaft and the stiffness,vibration and service life of bearing.The minimum preload needed to prevent from gyroscopic motion for different rotating speeds,the influences of preload on stiffness and temperature rise were studied.Secondly,the static characteristic,the output characteristic under different loads,temperature characteristic,creep property and hysteresis characteristics for piezoelectric ceramics drive were investigated experimentally.The relation between the output force of piezoelectric ceramics and voltage under different loads conditions were fitted with by the software Excel for open loop state.Thirdly,the piezoelectric ceramics were used to apply preload on the bearing,and the related preload devices were designed for experimental analysis.The experimental process included the design of the preload devices,the selection of the experimental apparatuses,the construction of the experimental platform and the temperature measurement of the spindle for different preloads and speeds.The relation for discribing the effect of the preload of the bearing on the temperature and vrbration was determined.Through these studies in the thesis,the influence of bearing preload on stiffness and the temperature rise of the shafting were determined.The nonlinear curves of the output force of piezoelectric ceramics and voltage under different load were fitted for open loop state ring state.The output force can be determined according to the input voltage of the piezoelectric ceramic actuator and external loads.Bearing preload devices based on piezoelectric ceramics were designed and the related experiments were performed.The influence of preload and speed on temperature and vibration were analyzed based on the above relation.The mathematical model for controling the preload automatically were established,which achieved automatic control for the preload of the spindle bearing.