## Materials and Methods
### Design of Variable-Diameter Drum
#### 2.2.2 Diameter Adjustment Device  
It can be seen from Figure 1a that in the traditional drum diameter adjustment, the position of the connecting base at the end of each threshing rack should be changed one by one. Therefore, the operation is time and labor consuming. Several hole locations are provided at the end of each connecting base and adjustment in steps is available for drum diameter only. Overall and rapid changes of the relative positions between each threshing rack and the supporting wheel by diameter adjustment device is available, thus achieving the stepless adjustment of diameter. As the core parts of the diameter adjustment device, constant speed screw disc, and matching claws were designed according to Archimedes spiral principle. As shown in Figure 3a, the inner end face of the constant speed spiral plate is equipped with plane threads, the driving thread pitch of the threads is T = 8 mm, the spiral diameter is D = 150 mm. The thread teeth are embedded in one end plane of the six claws. The overall structure of the device is shown in Figure 3b, which consists of constant speed spiral plate, limit disc, claws, connecting rod, sun wheel, planet wheel, and spline sleeve. The constant speed spiral disc and sun wheel are jointly installed on the spline sleeve, the spline hub is installed on the drum shaft, and the limit disc is fixed on the support plates, where six claws are connected through each of  
Figure 2: 3D diagram of variable-diameter threshing drum, 1. Feed wheel, 2. Diameter adjustment device, 3. Electronic control self-locking device, 4. Threshing rack, 5. Drum main shaft, 6. Sleeve. the six I-shaped grooves, respectively. One end connected with the claw engages with the plane thread in the constant speed spiral disc, the other end is welded with the connecting rod, and the connecting rod is welded with the threshing tooth rod. Due to the complexity of the working conditions of threshing device in the combine harvester, in order to ensure the operation stability of variable-diameter threshing drum and prevent the radial displacement of claw caused by the force of threshing tooth rod and avoid the consequent rotation of constant speed spiral disc, force analysis was carried out for the constant speed spiral disc and claw in the device for diameter regulation. The plane threads on the constant speed spiral disc are simplified to single-tooth plane threads for force analysis. The claw is taken as the research object. As shown in Figure 4a, it is assumed that the centrifugal force \(F\) acting on the claw is concentrated on the meshing points on the spiral, the counter force \(N\) is perpendicular to the tangent of the meshing points, and the inclination angle of the tangent line \(\lambda\) is the helix one. For further force analysis of meshing point, as shown in Figure 4b, when the claw moves downwards under centrifugal force, the friction \(F_{f}\) goes upwards. In the figure, \(R\) is the resultant force of normal reaction \(N\) and friction force \(F_{f}\), \(F_{t}\) is the horizontal force component of \(R\), friction angle \(\rho\) is the angle between \(R\) and \(N\), and the angle between \(R\) and centrifugal force \(F\) is \((\lambda-\rho)\). \[F_{t}=Ftan(\lambda-\rho) \tag{7}\]  
In the formula, \(F_{t}\) is the circumferential force driving the rotation of the constant speed spiral disc; \(F\) is the radial force driving the movement of the connecting claw; Thread lead angle \(\lambda=\arctan\left(\frac{T}{nD}\right)\), \(T\): screw pitch of thread transmission, \(D\): thread diameter; \(\rho\) is the friction angle of thread transmission, \(\rho=arctan\,f\), and \(f\) is the friction coefficient. Formula (7) shows that if \(\lambda<\rho\), then \(F_{t}\) is negative. As long as the condition \(\lambda<\rho\)_is_ met, regardless of magnitude of the centrifugal force \(F\), the claw will not move radially under its action, and self-locking can be formed. The lowest friction coefficient of steel is \(f=0.05\), and the lowest friction angle \(\rho\) is \(3^{\circ}\).