The early initiation and propagation of gear cracks are highly concealed, posing a severe challenge to fault diagnosis. This paper focuses on a spatial crack penetrating the tooth tip. A dynamic model of a cracked idler gear system, predicated on a slicing-based mesh stiffness calculation, is developed to explore the mechanism by which crack propagation affects system vibration. Key findings indicate that vibration acceleration signals measured near the crack initiation site exhibit higher sensitivity to crack propagation, making them more suitable for fault feature extraction. This work establishes a theoretical foundation and offers methodological guidance for the early diagnosis and condition monitoring of cracks in gearboxes.

tooth-tip penetrating crack,mesh stiffness,dynamic model,dynamic characteristics