To study the dynamic characteristics of spur gears with crack faults, it is essential to establish the meshing stiffness model accurately. In this paper, the time-varying meshing stiffness (TVMS) model and load-sharing ratio (LSR) model of spur gears were established by the potential energy method, and the correctness of the TVMS model was verified by KISSsoft. The influence of gear crack depth and crack angle on TVMS and LSR was analyzed by introducing gear crack into the TVMS model. The multi-degree of freedom dynamics model of spur gears with centralized parameters was established. The time-domain characteristics of vibration of different crack fault systems were discussed, and statistical indicators were introduced to quantitatively evaluate the gear crack faults. The results show that TVMS decreases due to crack propagation, and the vibration impact increases with crack propagation. The sensitivity of crack depth propagation to gear fault characteristics is higher than that of crack angles; the statistical indicators can be used to monitor the propagation of early crack faults. The research results can provide a reference for the research of gearbox dynamic characteristics and fault diagnosis.

Background: Gearboxes are used in many industrial fields. Due to uncontrollable external factors and inevitable fa-tigue damage, gearboxes are likely to fail. It has become a major research trend to collect dynamic sig-nals of gearboxes and develop effective fault diagnosis methods for gearboxes. Objective: The purpose of this paper is to study the dynamic characteristics of herringbone gear trans-mission(HGT) under different friction coefficients and pitting faults. Methods: A virtual prototype model of the system with different friction coefficients and pitting faults was established jointly with KISSsoft, SolidWorks, and ADAMS. The contact stiffness, penetration co-efficient, damping, and other factors were considered in the model. The contact force and angular accel-eration signals of the Driven gear are collected and analyzed by simulation. Results: The increase of friction coefficient results in the increase of contact force and instantaneous angular acceleration. The modulation side frequency band on both sides of the frequency becomes dense, and the side frequency band in the frequency domain of angular acceleration becomes sparse. When pitting failure exists, all dynamic signals show fault characteristics. Conclusion: This work provided a theoretical basis for fault diagnosis and gear dynamics research, Due to many nonlinear factors, the fault diagnosis of the HGT system is relatively difficult. The research results can quickly diagnose the fault of the HGT system, which makes up for the shortcomings in the study of the fault diagnosis of the HGT system.