| High-speed machining is a kind of advanced technology which represents thedevelopment trend of modern manufacturing. High-speed machine tools are the basicequipments to realize high-speed machining. The motorized spindle is the keyfunction part of high-speed machine tools for driving the cutting tool into rotating, toachieve high-efficiency and precision machining. Heat generated by bearing frictionand motor losses of the motorized spindle will lead to thermal deformation. Thethermal deformation makes the dynamic behavior of the motorized spindledeteriorate/worsen, is a problem which is required to be solved for achieving highspeed. Therefore, it is necessary to make a study of the dynamic characteristics of themotorized spindle considering the angular contact ball bearings thermal displacement.On the basis of the temperature field when the heat of the motorized spindlebalanced and bearings thermal displacement being analyzed, the quasi-dynamic modelfor angular contact ball bearings considering thermal displacement is created. Thenumerical method of dynamic and thermal behavior of the bearing is proposed. Thecreated model and the proposed method are used to simulate the effect of thermaldisplacement on bearing parameters and stiffness. The result is shown that contactangles between the rolling ball and the inner and the outer rings are closer to initialcontact angles due to the thermal displacement, respectively. Also the bearingstiffness increases. The above phenomena of the lock-ring preload are more obviousthan them of the constant preload.Based on the above study, the dynamic and thermal model of high-speedmotorized spindles is established. The established model consists of the shaft modelbased on Timoshenko beam theory, the stiffness model of the bearing consideringthermal displacement and the model of an HSK cutting tool system. Because the finiteelement method is more suitable for the model than other methods, it is adopted. Theestablished model and the finite element method are used to research dynamiccharacteristics of a30kW/24krpm motorized spindle for the machining center and a10kW/30krpm motorized spindle for internal cylindrical grinding. The thermalbalance temperature field, bearing stiffness, natural frequency and mode of vibrationfor the two motorized spindle mentioned above are analyzed. It is indicated that boththe first critical speed of the former and one of the latter are much higher than their maximum speeds to avoid resonance and improve system stability.The established dynamic and thermal model is used to analyze the factors whichinfluence static and dynamic characteristics of a motorized spindle with differentpreload methods consider bearing thermal displacement. The result is shown that thefirst critical speed of a spindle system is high when the preload of bearings withdouble-O configuration is big, and the bearing span, the arbor length and the mass ofgrinding wheel are small. And the first critical speed of a spindle system is obviouslyaffected by heat displacement when preloads of bearings with double-X, face to faceand back to back configuration are small, and the bearing span is big. It is alsoshowed that compared to the constant preload, the lock-ring preload makes bearingfriction torque and wear increase. Furthermore, the temperature rise is obvious. Therotational accuracy and the life are reduced. Therefore, the lock-ring preload is notsuitable for high speed. To meet the engineering application requirements ofhigh-speed, the constant preload should be adopted. Based on the above research, thetwo high–speed and high power motorized spindles are developed. The one is50kW/20krpm, and the other is18kW/40krpm. The result is shown that the stiffnessand the first critical speed of the two developed motorized spindles all meet thetechnical requirements. Their design is reasonable.In order to validate the analysis method for dynamic characteristics of themotorized spindle considering bearing thermal displacement, a10kW/30krpmmotorized spindle for internal cylindrical grinding is experimentally studied. It isshown that the calculated values of spindle stiffness, temperature rise and naturalfrequency are in agreement with the measured ones, respectively. The effectiveness ofthe established thermal and dynamic model for the motorized spindle is verified.The work offers important theories and effective approaches for analyzing staticand dynamic characteristics of the motorized spindle and its design optimization. |
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