A Helical Compression Spring Fracture under Impact Velocity in Pump Operation by Theoretical Study and FEA

Helical compression springs have been used widely in various industries from automotive, aerospace and heavy-duty industries to electronics and medical devices. In the automotive industry, they appear in many places such as suspension, valvetrain, etc., especially in the discharge check valve of Gasoline Direct Injection (GDI) pump, which is the subject of study due to a recent fracture in lab testing. A theoretical study is conducted first to establish equation governing spring dynamic motion under impact velocity, which can be in high magnitude with surging shock wave along spring axis. A new spring shock wave equation is developed for spring axial motion coupled with coil torsional effect. This newly derived shock wave equation has a broad term than the classic spring formula found in engineering book. In the paper, it proves that the classic spring shock wave equation is only a special case for the new general wave equation discovered. Also, a formula on spring shock wave propagation speed is presented with a discussion compared to spring classic formula. Next, a FEA tool is employed to study the spring system under transient impact velocity, the spring critical stress and fracture location is obtained along with a fatigue life assessment, back up by the part fracture photo as well as fatigue life cycles observed in testing. They are in good agreement.