Investigation Of Bearing Preload And Its Influence On Rotational Performance Of High-speed Motorized Spindles

The machining accuracy of the high precision machine tools is highly dependent on the performance of the motorized spindle.The bearing preload which provides the necessary stiffness for the high-speed spindle system is one of the important factors affecting the performance of the high-speed motorized spindle.Choosing the proper bearing axial preload can not only provide the required rigidity for the motorized spindle but also reduce the vibration caused by high-speed rotation and avoid excessive heat generation during the machining process.Thus,the research and analysis of bearing preload status and its relationship with the dynamic characteristics of motorized spindle is of great significance to improve the spindle rotation accuracy and dynamic performance of high-precision CNC machine tools.This thesis firstly studied the contact characteristics between the angular contact ball bearing rolling elements and the raceways with the variation of axial preload.Based on the bearing basic geometric relationship and Hertz contact theory the contact deformation and stiffness of angular contact ball bearings under different preloading conditions are studied.A new method was proposed to determine the contact resistance under different axial preload of bearings and related experiments were carried out.The experimental results showed that the contact resistance decreases non-linearly with the increase of bearing axial preload and have a “L” shaped curve relationship.For specific bearing,when the axial preload is greater than a certain value,the contact area does not increase significantly with the increase of the axial preload.Secondly,the influence of the axial preload on the rotational accuracy of the shaft bearing system in a back to back angular contact ball bearing configuration is studied.A test setup was designed considering the fit between the bearing inner ring and the shaft.The change of bearing outer ring radial runout along with the axial preload was measured.The experiment result showed that with the increasing of bearing axial preload the bearing outer ring radial runout value first drops then rises and again drops in a fluctuating manner making a ‘down-up-down' shape curve.In the initial stage of preloading,the radial clearance decreased with the increase of axial preload,which resulted in the decreasing of radial runout.However,due to end surface parallelism error of the outer ring spacer between the two angular contact ball bearings the contacting circumference surface of the back to back mounted bearings have a partial arc contact with the outer spacer.Thus,the bearing outer rings overturn azimuthally,resulting in the increase of the radial runout of the bearing outer rings.With the further increase of the axial preload,the entire circumference of the bearing outer ring received the influence of the axial preload and the contact area between the rolling elements and the raceways increased,which resulted in decreasing the bearing radial runout.The result presented that there is a local minimum radial runout phenomenon during the shaft bearing axial preloading process which need to be avoided.And the global minimum value which appears after the first fluctuation of the radial runout with the increasing of bearing axial preload should be adopted.The result also indicated that during the axial preloading process the smaller is the fit tolerance between the bearing inner ring and the shaft,the smaller is the radial runout fluctuation amplitude.Finally,the relationship between the spindle bearing axial preload and the dynamic characteristics of the spindle are studied.A grinding machine spindle with maximum operating speed of 60000 rpm was taken as the research object.The modal analysis of the spindle shaft is carried out using the FEM simulation and experimental validation were conducted to determine first mode shape frequency.The rigid nature of the shaft is evaluated by comparing the first mode shape frequency with the maximum operation speed frequency of the spindle.An experiment model of the motorized spindle was established for modal analysis to study the natural frequency and mode shapes of the motorized spindle under different preload conditions.Through the experimental modal analysis in axial and radial direction,the dynamic characteristics of the motorized spindle under different preload conditions are analyzed.Based on the harmonic frequency analysis of the vibration signal of motorized spindle,the preload condition without shaft bearing looseness is determined.On the basis of experimental and theoretical analysis,the optimization parameters of axial preload for high-speed motorized spindle were put forward.This thesis systematically studied the influence of preload on static rotational accuracy and dynamic vibration characteristics of rotating shaft and bearing of the motorized spindle.The minimum preload of a single bearing is obtained by studying the non-linear variation law of the axial preload and contact state of a single bearing.By studying the trend of radial runout of the bearing outer rings during the preloading process of the bearing pair,a global optimization method is provided for preloading of the spindle bearing pair.Through modal analysis,the influence of axial preload on the natural frequencies and mode shapes of spindle were presented.On the basis of vibration amplitude and frequency spectrum analysis,the range of axial preload of high-speed motorized spindle is determined.The optimum axial preload is determined based on the above three recommended range,which have a direct impact on the dynamic and static rotational performance of the spindle.