Research On Auto-eliminating Clearance Auxiliary Bearing In Active Magnetic Bearing System

As one of the indispensable accessories in active magnetic bearing systems(AMBs), auxiliary bearing(AB) is a very important measure to protect the rotor and stator systems from damage during the destruction of the AMBs. Traditionally, an ordinary ball bearing is used as AB. However, in most case, the ball bearing cannot bear the following giant impact forces and centrifugal force produced by the rotor’s whirl motion when the rotor dropping on this type of traditional AB. To cope with the defects of traditional AB, a new type of auto-eliminating clearing auxiliary bearing(ACAB) is proposed to use as AB in AMBs, and its mechanical structure and working principles are introduced, as well as a series of relative researches are carried out.First, the finite element model of rotor is established. Based on this model, initial two order natural frequencies and vibration model shapes are calculated under the following three cases: no ball bearing mounted on the rotor, one ball bearing directly mounted on the one end of the rotor, two ball bearings separately mounted on the each end of the rotor. Also, the relative experiments are executed. The results show that the ball bearing could be deposed as a rigid disc when using the finite element mothed to calculate the rotor’s natural frequencies. Besides, each order of the rotor’s natural frequencies is reduced when ball bearings are directly mounted on this rotor. Second, the influences of different structural parameters on the maximum swing angle of the supports are kinematically analyzed and experimentally verified. The results indicate that, compared to the structure that the link rod located at the inner side of the revolute joint 1, support in which the link is placed in the outer side of the revolute joint 1 has a greater swing angle. When the requirements of the structure size and other factors is satisfied, reducing the distance between revolute joints 1 and 2 and the distance from revolute joint 1 to the center of swing device, appropriately increasing the clearance in revolute joint 2 could increase the support’s maximum swing angle. Following that, the static studies on the support and ball bearing are conducted. The results show that, after the clearance is eliminated, the support could not stably maintain the outer race(OR) of the ball bearing when the radial force arm of the support at the contact point doesn’t satisfy certain conditions. Moreover, the influences of different shapes of the support surface on the support’s swing angle and radial force arm at the location of the support eliminating the clearance are analysed. According to these results, some reasonable shapes of the support surface are proposed.The dynamic responses of an AMB rotor dropping on the ACAB are detailed analysed. The rotor dynamic models before and after AMBs failure, support model of active magnetic bearing, ball force model, impact model between the OR and supports, vibration model of single support as well as support’s swing model are respectively established. The influences of different parameters on the dynamic responses are simulated based on MATLAB and experimental verified on a 5- degree of freedom AMBs test rig. The results indicate that, when the structural parameters of the ACAB are appropriately choosed, the ACAB is able to eliminate the protective clearance in about 0.03 seconds after rotor drop. During the process of contact, the ball bearing suffers several times of impact force. Upon the clearance is eliminated completely, the ball bearing only need to support the centrifugal force and gavity of the rotor. However, large elastic deformation forces are existed between the OR and each support. Since those large forces, it is advisable to use the medium and heavy series ball bearings in actual applications. And the support’s materials need to choose quenched and tempered steel or carburizing steel with strengthening processing surface. To ensure the ACAB could work successfully, the radial force arm at the point that the support contact with the OR after clearance eliminated should be as small as possible. Moderate parameters, such as the lubrication coefficient of the ball bearing, the friction coefficient between the OR and supports, as well as the friction coefficient between supports and revolute joints, have significiant influences on the ACAB’s work performances. Large rotor unbalance would make the impact force acted on the ball bearing significantly increase. Fewer supports have no significant influence on the force that the ball bearing suffered but increase the elastic deformation forces between the OR and supports, even possibly cause the vibration of the rotor to arrive at the clearance between the rotor and AMBs. The installation angle of ACAB hardly affects the ACAB’s work performance. Compared to the traditional auxiliary bearing, ACAB could efficiently reduce the impact forces that the ball bearing suffered after the rotor drop. And the relative decrease ratio could be up to about 70%. Furthermore, the impossibility of incurring full-clearance barkwark whirl which would happen in traditional AB cases is eliminated. Thus, the life of the AB in AMBs is certainly prolonged and the reliability of AMBs is improved.