Study On Dynamic Characteristics Of Ball Bearing-Rotor System In Ultra High Speed Spindles

Electric spindle technology has been developing along with the developments and demands of modern manufacturing technology such as high-speed cutting and high-speed CNC machine tools. As the power and speed of electric spindles is higher and higher, the electric spindles with ultra high speed are used more and more in many machine tools. In these ultra high-speed electric spindles, most of the spindle bearings reached the limit speed allowed by their designs, and they are lubricated by outer oil-providing methods, including oil-air, oil-mist and so on. Besides, the working speed of the spindles almost achieves the first critical speed of the rotor system. Thus, the problems of rotor system dynamics in ultra high speed electric spindles should be studied and the rotor dynamic characteristics, such as the natural frequencies, critical speeds, unbalance vibration should be accurately calculated and reasonably designed.Based on the analysis of the dynamic states and the dynamic stiffness of spindle bearings running in ultra high speed, the analyzing, modeling, numerical calculating method and it's programming about the dynamics of the ball bearing-rotor system are studie in this paper.1. Based on quasi-static and quasi-dynamic analysing models of the ball bearing, the dynamic states of the spindle bearings are analyzed and the bearing dynamic stiffness is calculated by using of numerical analysis method under the ultra high speed condition. Also, combining with a concrete bearing as an example, the factors, such as the interior structures, the exterior working conditions of the bearing which affecting its supporting stiffness are studied synthetically. The results show that in the case of ultra high speed, the inertia effects of the centrifugal forces and the gyroscopic moments of the ball rolling elements in the bearing are so great that the parameters of the bearing dynamic states and its dynamic stiffness are significantly different from that in the case of lower or high speed, and some of parameters will change to several folds. The effects arising from ball inertia forces have exceeded that coming from external loads. The dynamic characteristics of the rotor systems of the ultra high speed electric spindles will be greatly different from that in lower or high speed spindles.2. The main characters and existing problems about rotor system dynamics have been analyzed for the ultra high speed electric spindles which supported by angular contact ball bearing. Combined with the analytical calculation of the spindle bearing dynamic states and its dynamic stiffness, the mathematical model of the ball bearing-rotor system dynamics for the ultra high speed electric spindles is built up with the finite element method, in which the effects of the supporting characters of spindle bearings are taken into account, and the numerical calcualting method and programs are developed synchronously.3. Taking an ultra high speed electric spindle as an example, the factors which affecting the dynamic characteristics of the rotor system, such as the speed, spindle bearings, structures of the rotor system and so on are studied systemically. The results show that the vibration in ultra high speed electric spindles is mainly unbalance response and resonance when the operational speed is equal to the critical speed of the rotor. The dynamic stiffness of spindle bearings affects the characteristics of the rotor dynamics obviously while running in ultra high speed. For the reason of the nonlinearity of supporting characteristics of spindle bearings, the characteristics of the rotor system dynamics in an ultra high speed electric spindle are also nonlinear, so the natural frequencies of the rotor system are generally not the critical frequencies. The extended length outside the shaft, the mass of the parts added to the end outside the shaft and the diameter of the motor's rotor greatly affects the first natural frequency and the first critical speed of the rotor system in an ultra high speed electric spindle.4. Some experiments about the rotor dynamics of ultra high speed electric spindles have been carried out by this paper. It is showed, influenced by the nonlinearity of the dynamic stiffness of spindle bearings, that the real critical speeds of the rotor system can not be obtained through the experiments in the conditions of static or lower speed for the ultra high speed electric spindles, and it has to combine with the dynamic experiments operating in ultra high speed.