## 6 Conclusions  
1. A straw guide structure with an adjustable spiral angle and its threshing device was designed, and the mathematical model of material movement on the straw guide board was analyzed. The model analysis shows that show that the axial velocity of the straw particle has an inverse parabola relationship with the spiral angle of the straw guide, and the increase in straw moisture and the decrease in spiral angle have the same effect on decreases the axial velocity of straw particles in some situations. This may help improve the threshing effect but may increase grain damage and the content of impurities. Therefore, adjusting the spiral angle of the straw guide board can be considered to cope with the influence of straw humidity on the threshing effect.
2. Design-Expert was used to design and analyze a three-factor and three-level response surface test for the drum rotation speed, operating speed, and spiral angle of the straw guide board. The results show that the drum rotation speed, operating speed, and spiral angle of the straw guide board have significant effects on the three performance indexes, the _PLR_, _PIR_, and _PBR_. Among them, the order of single factor effect on the _PLR_ is operating speed \(>\) drum rotation speed \(>\) spiral angle of straw guide board, the order of single factor effect on the _PIR_ is drum rotation speed \(>\) operating speed \(>\) spiral angle of straw guide board, and the order of single factor effect on the _PBR_ is drum rotation speed \(>\) spiral angle of straw guide board \(>\) operating speed. For the _PLR_, the order of influence of the interaction terms is operating speed \(\times\) spiral angle of straw guide board \(>\) drum rotation speed \(\times\) spiral angle of straw guide board \(>\) drum rotation speed \(\times\) operating speed; for the _PIR_, the order of influence of the interaction terms is drum rotation speed \(\times\) operating speed \(>\) operating speed \(\times\) spiral angle of straw guide board \(>\) drum rotation speed \(\times\) spiral angle of straw guide board; and for the _PBR_, the order of influence of the interaction terms is operating speed \(\times\) spiral angle of straw guide board \(>\) drum rotation speed \(\times\) spiral angle of straw guide board \(>\) drum rotation speed \(\times\) operating speed.
3. Using the optimization function of the Design-Expert software, the optimized drum rotation speed (\(A\)), operating speed (\(B\)), and spiral angle of straw guide board (\(C\)) were set at 550 r/min, 3 km/h, and 28\({}^{\circ}\), respectively. The results were as follows: _PLR_ = 1.26%, _PIR_ = 0.73%, and _PBR_ = 0.61%. The absolute error between the experimental and predicted values was very small; however, the verified values of the optimized field experiment were 8.31%, 50.04%, and 60.30% lower than the average values of the original 17 treatments field experiment, respectively. The results show that the response surface test model and the optimized parameters are valid, and the optimized harvester has better operation quality; in particular, the _PIR_ and _PBR_ can be improved greatly.  
Conceptualization, L.W.; Methodology, L.W., D.C., X.X. and Y.W.; Analysis, L.W., D.C., X.X. and Y.W.; Writing--original draft, D.C. and X.X.; Funding Acquisition, L.W.; All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge the Jiangxi Provincial Department of Finance and Jiangxi Provincial Department of Agriculture and Rural \"Green Ecological Agricultural Machinery and Tools Promotion\" (Gan Cainong 2022 No. 18) for providing support to this research effort.