Study On Thermal Characteristics Of High-speed Motorized Spindle And Its Effect And Dynamic Characteristics Of Ball Bearing-rotor System

High speed machining (HSM) is a promising technology for drastically increasing productivity and decreasing production costs. High-speed motorized spindle is strategically critical to implementation of HSM, its dynamic characteristics play a very important role in machining since they affect dimension accuracy, surface finish, tool life, bearing life and noise. However, the built-in motor introduces large amounts of heat into motorized spindle, thermal displacement of bearing induced by temperature rise affects the dynamic characteristics of motorized spindle. Therefore, the thermal characteristics of motorized spindle are explored in the dissertation. On the basis of it, thermal displacements produced in rings and rolling element are investigated. Effects of thermal displacement in angular contact ball bearing on dynamic characteristics of bearing and on dynamic characteristics of bearing-rotor system are investigated. Details and conclusions of studies are as follows.1) Based on the the heat transfer theory, using the finite element method the temperature field of motorized spindle are analyzed and confirmed by the experiments. The results show that the temperature in bearing, rotor and shaft are high, by enhancing the cooling of water jacket and oil-air lubrication system temperature rise of outer ring can be controlled effectively, but temperature rise of inner ring and shaft can not be effectively controlled.The experiments confirm that simulation can accurately reproduce the process of temperature rise of high-speed motorized spindle, the finite element model and its boundary condition are reasonable.2) Based on the elastic theory, the calculating formulas of thermal displacement in rings is inferred. The Results show that thermal displacement of inner ring is highest, the next is outer ring, the lowest is rolling element.3) Based on the Hertz contact theory,the dynamics and raceway control theory of the rolling bearing, in consideration of thermal displacement in rings and rolling element, the numerical analysis method and its corresponding program for calculating the dynamic characteristics of the spindle bearings are given.Also, combining with a concrete bearing as an example, the dynamic characteristics of the angular contact ball bearings are studied synthctically.Thc results shows that thermal displacements of rings and rolling element reduce the difference of the contact angle between inner and outer raceways, decrease centrifugal force and increase gyroscopic moment, increase contact deformation, contact stress, contact load, contact stiffness and contact ellipse parameter, decrease spin-roll ratio between rolling element and inner raceway, decrease the life of bearing; Also, thermal displacements not only compensate bearing radial stiffness softening in high-speed condition but also greatly increase bearing stiffness, the effect of bearing thermal displacement on its stiffness depend on raceway radius, contact angular, material of rolling element, interface, preload and so on. The experiments confirm that considering of thermal displacement in rings and rolling element the calculation value of bearing stiffness is more suitable for the practice.4) Based on the dynamics theory of the bearing-rotor system, considering of bearing thermal displacement, the solving method for the dynamic characteristics of the bearing-rotor system are investigated.The results show that,bearing stiffness increase induced by thermal displacement increase the first natural frequency and decrease unbalance response, its effect on the first natural frequency increase but its effect on unbalance response decrease with the increase of rotational speed. Also, the effect of bearing thermal displacement on bearing-rotor dynamic characteristics depend on preload,bearing span, rotor structure dimension and so on.It is shown that in high-speed condition thermal displacement in rings and rolling element has become a leading factor which affects the dynamic characteristics of spindle bearing, considering its influence the proposed mothed for calculating dynamic characteristics of bearing-rotor system is more suitable for the practice.