# An Advanced DEM-FEM Method for Herringbone Gear in Shot Peening
## 5 Conclusions  
Through the simulation of shot peening for herringbone gear, the following important conclusions are drawn.  
1. During the shot peening process of herringbone gears, the number of shot impacts along the tooth profile direction exhibits significant variation. The closer to the tooth tip, the fewer the shot impacts. Under the shot peening process parameters used in this study, the number of impacts at the gear root is three times that at the tooth tip. This characteristic is independent of the gear's orientation.
2. The closer to the gear tooth root, the higher the prevalence of low-velocity shots during the shot peening process. This is primarily due to the confined space at the gear tooth root, which facilitates secondary peening of the shots. In contrast, the distribution trends of shot impact velocities at the pitch circle and tooth tip positions do not show significant differences. Therefore, more attention should be paid to the position of the tooth root during the shot peening process.
3. In this study, the calculated surface roughness Sa values for the tooth tip, tooth root, and pitch circle positions indicate that the roughness parameter Sa is smallest at the gear root and largest near the pitch circle. This suggests that, although the gear root experiences a higher number of impacts, most shots have lower velocities. The combination of low velocity and frequent impacts results in lower surface roughness. However, the residual stresses at different tooth profile positions do not exhibit significant differences.
4. Under the same shot peening duration, increasing the shot diameter and velocity will significantly affect surface roughness and residual stress. Larger shot diameters and higher velocities will deteriorate surface quality and reduce the value of surface residual compressive stress, but they will introduce deeper residual compressive stress. Actual requirements should be considered when selecting process parameters.
5. The hardness of the shots will also significantly influence the shot peening effect. If shots with a hardness lower than the surface hardness of the herringbone gears are used, the level of residual compressive stress introduced will be significantly lower compared to using high-hardness shots. However, the surface quality after shot peening with low-hardness shots will be significantly better than that before shot peening.  
Figure 28: Effect of shot hardness on residual stress (**a**) S110; (**b**) S170.  
**Author Contributions:** D.Z. and L.L.; methodology, H.Z.; software, J.Z.; validation, W.S. and J.T.; formal analysis, D.Z.; investigation, J.T.; resources, W.S.; data curation, D.Z.; writing--original draft preparation, L.L.; writing--review and editing, L.L.; visualization, D.Z.; supervision, W.S.; project administration, J.Z.; funding acquisition. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by The National Key Research and Development Program of China, grant number 2023 YFB3406404 and The National Natural Science Foundation of China, grant number U22B2086.  
**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
**Conflicts of Interest:** Author [PERSON] was employed by the company Aero Engine Corporation of China. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.