Influence of interference on bearing preload considering frictional properties

Abstract: Bearings are a necessary part of mechanism, and have been widely used in various mechanical systems, such as agricultural machinery, engineering machinery, precision machinery, aerospace machinery. The preload of bearings affects bearing stiffness, rotating precision and service life. Meanwhile, precise determination of bearing preload is the guarantee for the high-precision bearing support system. Related determining preloads have already been analyzed, but the literatures don't consider the interception of friction at fitting area. Space bearing works in space environment, and it is essential to accurately determine the preload. If the initial preload is not selected appropriately or accurately, the changes of preload induced by alternating temperature may lead to some failures. Therefore, it is necessary to accurately determine the bearing preload to ensure the accuracy and reliability of the shaft system. Focused on applying the accurate axial preload on the bearing of spacecraft mechanism, based on statics and elastic mechanics, and also under the premise of considering the assembly dimension and friction characteristics, the relationship between the bearing preload and the tightening torque is studied. Using the relationship of locknut tightening torque and axial force, the axial force applied on the bearing is got. According to the relationship between friction and friction characteristics as well as assembly dimension, the analysis on the interception of the friction on the axial force is done. Based on the bearing force balance equations, the actual bearing preload is computed. This paper studies the influence of friction characteristics and assembly dimensions on the preload, and reveals the relationship between assembly preload and tightening torque under different assembly conditions. In most researches, the bearing preload equals the force generated by the locknut. Theoretical analysis in this paper shows that the preload is the force required for the ball deformation. It is influenced not only by the tightening torque, but also by the interception of the friction in the region of the fitting bearing with the shaft and the housing. The friction coefficient can be measured by the relation between tightening torque and axial force, which is obtained by experiment. According to the geometry parameters of the bearing 71807C and the locknut M30×1.5, as well as the physical property parameters of the spindle and bearing, the force-interference factor and force-torque coefficient can be concluded. Finally, a more precise calculation formula for the preload is drawn. The influences of the tightening torque, the interference of friction and the friction properties on the bearing preload are analyzed by the bearing preload test system. It can verify the validity of the theory. It can meet the engineering requirements, although the formula can be used to approximately calculate the preload. Through analyzing the preload of the bearings 71807C, the results show that under the same tightening torque, when shrink range increases by 0.5 μm, there is 123 N axial force intercepted; when the friction coefficient increases by 0.05, the interception ratio increases by 13%. The larger the amount of interference fitting is, the greater of axial force is intercepted by the friction between the bearing and the spindle and the larger the tightening torque corresponding to the bearing preload is. Compared with heating furnace assembly, the oil bath heating can reduce the friction coefficient of contact area, and the bearing gets larger preload corresponding to the same axial force. The experiment proves that the theoretical analysis can accurately determine the preload, improve the quality of assembly and accurately exert the preload. The above-mentioned theoretical formula can be generally popularized to determine more accurate bearing preload. The study has established the theoretical model for preload measurement and can more accurately determine the preload using this model. It provides theoretical guidance and reference for the design and assembly of the bearing preload.