Study On Load Distribution And Loading Ability Of Heavy Screw-nut Transmission Pair

Screw-transmission is one of the most important mechanical-transmission manners, applied widely in many machines and equipments. Ac-servo motor driving is the new developing trend for forming equipments. Precision and heavy screw-nut with high efficiency is one of the key technologies for it. Non-uniform load distribution and stress concentration occur due to the special spiral structure of the threads. Therefore the load concentrates in the two ends of the spiral are in high level. As a result, the abrasion of the thread surface is increased, damaging transmission precision. Threads might even be broken in extreme conditions. The service life of the screw pair would be shortened. In order to increase transmission efficiency and loading ability, addition to developing new materials, another important approach is to change nut structure to improve load distribution and reduce stress concentration. Investigating the load distribution, discussing the regularities of the load distribution , as well as making measures to increasing bearing ability are significantly important.The rapid development of computer technology and Finite Element Method (FEM) provide a new approach to investigate the load distribution of the screw pair. At the present time, commercial FEM software is quite mature for different engineering applications. The thesis applied FEM to simulate the working situation of the screw pair, in order to investigate the load distribution.For better calculation efficiency, complex space spiral structure of the threads was simplified to 2D or 3D axisymmetric FEM model. The FEM analysis was compared with classical theoretical calculation and physical experiment. The comparisons prove that FEM is suitable to study the load distribution in screw pair. 2D FEM model can reach the normal engineering requirement, according to the FEM simulation results.According to the designed work condition of the 80 ton screw press, the basic structure of the screw pair was designed. Based on that basic structure, the effects of basic structure (such as nut diameter, taper, stage, and elastic modulus, as well as thread type) on the load distribution were analyzed by FEM. The regulations of the load distribution were discussed by analyzing those factors. According to those factors, the thesis suggests some measures to improve the load distribution in order to increase the bearing ability. Staged and tapered nuts could improve the load distribution. For regular cylinder nut, the outer diameter has effect on load distribution and the suitable ratio of nut diameter to lead screw diameter would be 2. The modulus of nuts should be smaller than that of lead screw and the optimum nut modulus would be 1/2 of the lead screw. The saw-tooth thread is better than trapezoidal inThe study not only set up the base for improving traditional screw transmission, but laid a foundation for developing new precision and heavy screw-nut with high efficiency as well, making contribution to development of AC servo driven forming equipments.