PurposeThe timing gear drive mechanism serves as the fundamental transmission component of a diesel engine, encompassing a sophisticated elastic mechanical system comprising gears, multi-branch drive shafts, and various loads. The increasing demand for reduced vibration in modern diesel engines poses greater challenges to timing gear systems which must now withstand more extreme loads and operating conditions. This paper presents a coupled analytical model that integrates three-dimensional mixed lubrication and multi-branch shaft system vibration for the timing gear system of marine diesel engines. Additionally, it investigates broken tooth failure occurrence using an improved method and proposes an optimization scheme.MethodsThis paper focuses on the marine 20 V diesel engine timing gear and conducts research on the vibration characteristics of its shaft system, as well as optimization of broken teeth failure while considering the influence of three-dimensional mixed lubrication. Firstly, a 3D mixed lubrication analytical model is established and the validation of lubricant film stiffness and friction excitation is conducted. Based on this, the gear train is modeled with lumped-parameterized bending-torsion coupling dynamics, incorporating time-varying oil film stiffness and friction excitation. The vibration characteristics of the shaft system are analyzed by establishing an analytical model, which takes into account the comprehensive excitation from both internal and external factors in the multi-branch transmission shaft system. The accuracy of this model is then verified through actual diesel engine testing experiments. The conventional tooth root bending stress load spectrum is finally corrected, and the faulty gear is calibrated. Furthermore, a vibration optimization design scheme is proposed based on practical engineering experience.ResultsThe accuracy of the coupled lubrication model developed in this study has been validated through comparisons with literature results and equivalent simulation tests. Furthermore, the precision of the vibration model has been confirmed through real-machine testing for both free and forced vibrations. Notably, during both calculation and experimentation, it was observed that the peak vibration energy at the oil pump shaft exceeded that at the flywheel end by a factor of 5.2; these poor vibration characteristics were identified as a primary cause of frequent tooth breakage in timing gears located near the oil pump shaft position. By improving the bending stress algorithm, we conducted a calibration of the bending fatigue strength for the timing gear that frequently experiences tooth breakage faults during diesel engine durability tests. The resulting minimum safety coefficient of 1.31 falls within the range of general reliability; however, it does indicate a susceptibility to tooth breakage faults. To address this issue in practical engineering applications, we propose an optimization scheme involving the addition of a vibration damper. This scheme increases the minimum safety factor to 1.64, effectively preventing occurrences of tooth breakage.ConclusionThe study results unveil the failure mechanism of broken teeth in timing gears from a dynamics perspective, offering theoretical guidance for accurate prediction of tooth root bending stress and performance optimization as well as providing theoretical support for vibration response analysis of diesel gear shaft systems and vibration and noise reduction.
