## 3 Design of Main Working Parts
### Rotary Tillage Soil-Covering Device  
A rotary tillage knife can throw soil from the lateral side [22; 23] and, while in operation, the tractor transfers power to the gearbox to change the speed of the power and drive the rotary tillage soil-covering device, so the rotary tillage knife can break soil on the seedling soil-covering belt, ditch, and throw soil on both sides of the seedbed to complete the seedling soil-covering operation. In order to ensure that soil on both sides of the seedbed is uniform in the process of rotary tillage, the rotary tillage knife on the cutter head adopts the external installation method, and the rotary tillage knife at both ends of the cutter head bends outwards. The rotary tillage knife arrangement is shown in Figure 9.  
3.3.1. Analysis of Soil-Covering Amount of Rotary Tillage Knife  
The rotating direction of the rotary tillage knife is the direction of soil throwing [24]. The process of the rotary tillage knife throwing soil is shown in Figure 10. The advance speed of the machine is \(V_{m}\), the rotary tillage knife rotates at an angular speed \(\omega_{1}\), and the soil is thrown up by the rotary tillage knife to position and cover the seedlings. In order to ensure that the thickness of covering soil on seedlings is 40-80 mm, it is necessary to analyze the soil-covering amount of the rotary tillage knife on the seedbed. Firstly, the soil-cutting area of a single rotary tillage knife in a single rotation period is analyzed [25]. In order to establish the parameter equation of the soil disturbed by the rotary tillage knife, the area of soil (\(S\)) cut by a single rotary tillage knife in a certain cycle is determined at first, then the thickness of soil (\(T\)) cut by a single rotary tillage knife in a certain cycle is obtained, and finally the area of soil (\(V\)) disturbed by the rotary tillage knife is obtained. A rectangular coordinate system was established as shown in Figure 11. The motion curve of the blade end of the adjacent rotary tillage knife in the same direction in a certain cycle is established. The blade pivot (\(O_{1}\)) is taken as the center, the forward direction of the machine is the \(x\)-axis, and the vertical downward axis is the \(y\)-axis. The absolute motion of the rotary tillage knife's end is the combination of rotation motion of the blade pivot and forward motion of the machine, and the motion process of the end of the rotary tillage knife meets the requirements of trochoid [26]. Figure 10. Schematic diagram of rotary tillage knife throwing soil. 1. Seedlings. 2. Seedling soil-covering belt. 3. Ridges. 4. Soil particles. 5. Seedbed. 6. Rotary tillage knife. Figure 9. Schematic diagram of rotary tillage knife arrangement. As shown in Figure 11, since the area enclosed by points \(P_{2}\), \(P_{3}\), and \(P_{4}\) is torn apart by the cutter teeth during high-speed rotation, the soil-cutting area \(S\) is enclosed by points \(P_{1}\), \(P_{3}\), \(P_{4}\), and \(P_{5}\). The soil-cutting area of a single rotary tillage knife in a single rotation period is expressed as  
\[S=\!\!\int_{0}^{h_{a}}dy\!\int_{x_{1(y)}}^{x_{2(y)}}dx \tag{13}\]  
where:  
\(x_{1}(y)\) is the equation of the motion curve of the first rotary tillage cutter end;  
\(x_{2}(y)\) is the equation of the motion curve of the second rotary tillage cutter end in the same period. The equation of the motion curve of the first rotary tillage cutter end is  
\[\left\{ \begin{array}{l}x=R\cos(\omega_{1}t)+V_{m}t\\ y=R\sin(\omega_{1}t)\end{array} \right. \tag{14}\]  
If the number of blades in the same direction is 3, then the lag soil-covering angle of the second rotary tillage knife in the same direction is \(\frac{2\pi}{3}\), and the equation of motion curve of the second rotary tillage cutter end in the same direction is  
\[\left\{ \begin{array}{l}x=R\cos(\omega_{1}t-\frac{2\pi}{3})+V_{m}t+x_{0}\\ y=R\sin(\omega_{1}t-\frac{2\pi}{3})\end{array} \right.