## 1 Introduction
After that, open the threshing device from the side or the top and change the relative position between the connecting base at the end of each threshing rack of the drum and the supporting wheel one by one. In addition, the connecting bases at the end of each threshing rack and the supporting wheel should be adjusted and fixed at the same time. Apparently, this operation is not only complex but also time and labor consuming. Moreover, the drum diameter can only be adjusted to several fixed values and no stepless adjustment is available, which make it unsuitable for real-time adjustment during field operation [13; 14]. Based on the comparison in terms of operation performance, concave plate type adjustment is used by most combine harvesters for changing threshing gap due to its simplicity for operation in the harvesting process [15]. The difference between the two adjustment methods lies not only in the way of operation but also in the effect, which is more obvious, on changing threshing gap. The concave plate type adjustment is only available to change the gap between the bottom of the concave grid and the drum instead of the threshing gaps on both sides of the drum. In addition, the moved concave grid is not concentric with the threshing drum, which may result in uneven threshing gaps within the thresher device. On the other hand, the drum type adjustment will only change the diameter of the drum and leave the positions of the drum and the concave grid unchanged; besides, the concave grid in the thresher device is still concentric with the drum, which results in even threshing gaps. [PERSON] has studied the impacts brought to the threshing performance for corn by Archimedes' spiral type concave grid and circular arc concave grid in the tangential flow threshing device, respectively. The research shows that for relatively even threshing gap (Archimedes' spiral type), the damage rate, and separation loss rate for corn are relatively low [16], however, no research is implemented on axial flow threshing drum. [PERSON] et al. found that threshing drum adjustment type (even threshing gap) was available to effectively protect the thresher device from being blocked at the time of fluctuation of feed rate. The comparison in terms of threshing performance shows that the threshing drum adjustment type has a better prospect in application [17]. To solve the problem that the diameter of the threshing drum is difficult to adjust and real-time adjustment is unavailable during the operation of combine harvester, the research on the development of a variable-diameter threshing drum was described in this study. The variable-diameter threshing drum was available for convenient and rapid drum diameter adjustment during the operation of combine harvester by including diameter adjustment device and electronic control self-locking device in the existing threshing drum of combine harvester. In addition, an experiment was implemented to compare and analyze the differences in terms of impact brought to threshing performance by both adjustment types. The optimal way to prevent the thresher device from being blocked and improve the adaptability of harvesting by the combine harvester was explored.