Research On Impact Characteristic And Damage Mechanism Of Stranded-Wire Helical Spring

Stranded-wire helical spring is a cylindrical helical spring rolled by a strand, which usually consists of 2 to 7 carbon steel wires of 0.4 to 3 mm in radius. The spring has two kinds of structures in accordance with whether it has central wire or not. The helical direction of the compression spring is opposite to that of the strand, while it has the same direction for the extension spring. Compared with conventional single helical springs, it has the unique characteristic of better damping and vibration reduction. Therefore, it is a critical part in aero engines and automatic weapons. In addition, the spring is widely used in vibratory equipments, such as vibration sieves and vibration breaking plants, high precision table-boards and steady transport vehicles, substituting traditional single springs or rubber springs.So far, the static response and formation of stranded-wire helical spring have been widely researched. However, researchers seldom cover the impact performance and wear mechanism, so the analysis of dynamic response and wear mechanism among spring wires is the most important aspect for the study of the spring. The super performance of stranded-wire helical spring mainly results from the damping effect generated by frictions of the strand and the wire when the spring mainly bears impact load during to-and-fro movement. The present thesis further perfects correlation theory of stranded-wire helical spring through the study of impact performance and wear mechanism of the spring, and concentrates on the theory of theoretical geometric model and corresponding numerical simulation when stranded-wire helical spring mainly bears impact load, the development of testing device, torsional fretting wear mechanism and FEM numerical calculation among spring wires.The present thesis makes efforts to detailed studies on the impact performance and wear mechanism of the spring from the following aspects:Firstly, the performance of stranded-wire helical spring is basically determined by its twist angle and wire diameter. The numerical methods raised in the existing simplified theory that the minimum distance between two adjacent wires is equal to wire diameter and the strand cross-section is elliptical tangent are not sufficient. An application software adapting newly established mathematic model about the strand cross-section is developed to calculate the precise twist angel and diameter of strands of arbitrary wires. Meanwhile, based on the mathematical model of curve for centre line of spring wire, the present thesis establishes the three dimensional entity model of stranded-wire helical spring of closed ends, adapting helical scan function of CATIA and PRO/E respectively, which provides the significant theoretical principle and calculation model for following dynamic design theory and numerical impact simulation.Secondly, the dynamic performance of stranded-wire spring is hot issue for study. Based on the three dimensional entity model of stranded-wire spring of closed ends and the motion analysis of the spring, this paper sets up a dynamic vibration equation considering the damping effect. The high-speed impact response analysis by using the implicit dynamic function of ABAQUS gives a significant reference for the optimum structure design of stranded-wire spring.Thirdly, a high-speed, non-contact and multi-channels device for testing the dynamic parameters of the spring is developed. The displacement, velocity and acceleration of every particle in the spring can be directly calculated with high precise by using the mentioned testing system, which offers a detection platform for the analysis of internal modification of stranded-wire spring under high-speed impact.Fourthly, the wear on the local area of the steel wires'surface is due to the torsional fretting on the working process of the stranded-wire helical spring. Two mathematical models to calculate the normal contact force among the wires and the angular displacement respectively are established at first when the stranded spring bore impact load. With the experimental parameters obtained from the models, the torsional fretting test adopting cylinder - cylinder contact mode, which authentically stimulates the torsional fretting among the wires on the working process of the spring, is realized successfully on a newly developed fretting tester. The running behavior and the failure mechanism of the spring wires under the torsional fretting conditions are systematically researched.At last, based on the experimental research, the distribution of stress and relative slip in the contact area through the torsional fretting numerical calculation of spring wires under different test conditions, cycles and friction coefficients is obtained. Combining the mathematical wear model adapting the modified Archard equation and the numerical simulation, the wear depth is able to be accurately calculated under different work conditions and cycles. Comparison the numerical simulation with the experimental results can provide significant reference for the further study of wear mechanism of stranded-wire helical spring under torsional fretting condition.