Gear cracks are mainly divided into root cracks and indexing circle cracks. In 2009, chaari et al. Proposed for the first time that the crack propagates uniformly along the whole tooth width direction, that is, the so-called “through crack”. In 2011, Chen et al. Considered the actual situation of crack propagation, and proposed that the crack propagates unevenly along the tooth width direction, namely the so-called “parabola type” or “quasi parabolic type”. On the basis of Chen’s “parabola” crack model, Mohamed et al. Considered that the crack propagation path of tooth root is along the direction of crack depth and tooth width at the same time. In 2015, Ma et al. Put forward two kinds of cracks in different directions according to different root crack propagation paths, one is along the tooth direction, the other is along the flange direction. In addition, Lewicki et al. Also proposed that the crack propagation at the root of the tooth is carried out according to the flange thickness coefficient (the ratio of the flange thickness to the tooth height), and the value of the flange thickness coefficient determines the crack propagation direction. When the value is large, the crack propagation is along the gear tooth direction, otherwise, it is along the flange direction.
In conclusion, there are three types of root crack propagation paths
① Along the tooth width (tooth) direction expansion;
② It extends along the rim direction;
③ At the same time along the tooth width (tooth) direction and the direction of the flange.
At present, the research is mainly focused on the cracks on spur and helical gears, but there is little research on the crack propagation path of other types of gears, which needs to be further explored.