## 3 Results and Discussion
### Grain Damage Rate and Threshing Efficiency  
The study shows that excessive threshing may result in lower threshing loss but higher grain loss; provided the threshing efficiency is guaranteed, decreasing the collision of grains and the frequency for being hit will decrease the damage rate of the grains greatly [27; 28]. It can be seen from Figure 9 that with five different threshing gaps, with the increase of forward speed and feed rate, the grain damage rate would increase gradually. The damage rate with concave adjustment type of 30 mm was the lowest and that with concave adjustment type of 30 mm took second place. Besides, the rising trends in both cases are relatively stable. The damage rate with initial gap of 20 mm was on the medium level, and that with threshing drum adjustment type of 10 mm was the highest, which was higher than the damage rate with concave adjustment type of 10 mm. Too small a threshing gap may result in the sharp increase of damage rate in both cases when the forward speed is within the range of 1.2-1.4 m/s. With the increase of forward speed, the threshing efficiency would decrease gradually. The threshing efficiency with threshing drum adjustment type of 30 mm was the lowest and that with concave adjustment type of 30 mm took second place. Besides, the decreasing trends in both cases were relatively stable. The threshing efficiency with an initial gap of 20 mm was on the medium level, and that with threshing drum adjustment type of 10 mm was the highest. The threshing efficiency with concave adjustment type of 10 mm took second place. Small threshing gap may result in the sharp decrease of threshing efficiency in both cases when the forward speed is within the range of 1.2-1.4 m/s. It can be seen from the curve growth trend of Figures 8 and 9 that when the speed is between 0.8-1.0m/s, the increase in entrainment loss rate and damage rate are lower, the threshing efficiency is higher, and the harvest efficiency is the best at this time. Among all the five threshing gaps, the one with threshing drum adjustment type of 30 mm was larger than that with concave adjustment type of 30 mm. The threshing gap with threshing drum adjustment type of 10 mm was smaller than that with concave adjustment type of 10 mm. With the decrease of threshing gap, the grain would suffer increasingly stronger friction and squeezing, therefore the damage rate of grain would increase, and the threshing efficiency would be higher gradually. With the increase of forward speed, the feed rate would increase and the thickness of the material layer within the thresher device would be higher. The quantity of the material was high, and the threshing efficiency of the grain would decrease gradually. In addition, the impact, friction, brushing, and squeezing to the grain became stronger, which would lead to increasingly higher grain damage rate. When the threshing gap was increased, the grain damage rate with threshing drum adjustment type was lower than that with concave adjustment type. In the meantime, the threshing efficiency with threshing drum adjustment type was lower than that with concave adjustment type, but the difference was not big. When the threshing gap was decreased, the grain damage rate with threshing drum adjustment type of 10 mm was higher than that with concave adjustment type of 10 mm. Figure 9: Diagram for the variation of grain damage rate and threshing efficiency at different forward speeds under various threshing gaps, the black — and - are initial gap of 20 mm, the red — and - are threshing drum adjustment type of 10 mm, the blue — and - are concave adjustment type of 30 mm, the pink — and - are concave adjustment type of 10 mm, the green — and - are threshing drum adjustment type of 30 mm. adjustment type was higher than that with concave adjustment type. In addition, the difference was relatively small under low or medium feed rate. However, the threshing efficiency with threshing drum adjustment type was far higher than that with concave adjustment type.