Research On Several Key Problems In The Forming Processing Of Stranded Wire Helical Springs

Stranded wire helical springs (multi-strand spring for short) is a cylindrically helical spring rolled by cable wire, which usually consists of 2 to 7 carbon steel wires of 0.4 to 3 mm in radius. It has two kinds of structures in accordance with whether it has central wire or not. The helix of strand is opposite in direction to that of the coils of helical compressing spring, while trenching spring and the coil spring have the same helical direction. Compared with the conventional single spring, the multi-stranded coil spring has strong rigidity and good effect of absorbing vibration, and therefore it has significant applying perspective in vibration-absorbing systems of the automobile instrumentations, discharge systems of weapons, aero-engine and other fields.So far, research findings on properties of multi-strand coil spring have been made in static stress strain. However, researches seldom cover the damping effect generated by frictions of strand and wire when the coil spring deforms, which is largely dependent on the number of shares in the formation of coil spring. With widely studies on formation methods and other relative research findings of multi-strand coil spring, the present thesis tentatively concentrates on theory of rebounce by unloaded multi-strand coil spring, helical experiment and numerical simulation, which is expected to guide the production of springs and improve the quality of the finished products.The present thesis makes efforts to detailed studies on formation of multi-strand coil spring from the following aspects:Firstly, steel wires of multi-strand coil spring are all in second helical form. Through transformation of space coordinates, mathematical model of curve has been established to explore the space of central line of steel wires in cable and in spring concerning compressing coil spring and trenching coil spring. It also defines multiples of steel wire helix, and applies two deductive methods to establish mathematical foundations for theoretical study of differential geometry of multi-strand coil spring.Secondly, the existing theory often simplifies multi-stranded coil spring as the parallel and overlay effects of ordinary cylindrical coil spring. Actually, multi-strand coil spring takes a second helix in its formation, while the ordinary cylindrical coil spring only takes one helical rotation. Therefore, the exiting simplified study method is not sufficiently precise. Based on studies of mathematical model of curve for center line of single steel wire and differential geometry of space curve theory, the present thesis gives expressions of curvature and helical ratio of the central line of steel wire as well as differential parameters for comparison with ordinary cylindrical coil spring.Thirdly, the dynamic performance of multi-strand coil spring is hot issue for study. Based on the principle of differential geometry, the present thesis deduces the expressions of curvature and helical ratio of multi-strand coil spring, and analyzes the stress strains to loads by compressing and trenching springs. It also analyzes deformation of multi-strand coil spring under speed load by finite element simulation method, and distribution of load on strands, etc. On this basis, deformation of loaded single-strand spring and steel wire is studied to provide research foundation for multi-strand coil spring.Fourthly, a large number of experiments have been made on winding formation of three-strand coil spring and multi-strand coil spring with three-strand interior and nine-strand exterior. Experiments mainly take into consideration elements such as the tension of steel wire, rotation moment of spring and cable, loading speed and wire diameter which have influence on the springback after unloading the spring. The analysis of springback is after axial and radial direction.Fifthly, the present thesis makes simulative analysis on winding formation and on springback after unloading of coil spring on basis of ABAQUS quasi-static pattern. It studies elastic-plasticity of steel wire through simulative analysis of performance of cross-section and surface loaded. It is proved to be effective by comparison of finite element numerical simulation with experiment result of coil spring winding experiment.