# Design and Experimental Study of the General Mechanical Pneumatic Combined Seed Metering Device
## 3 Key Component Parameter Design and Analysis
### Seed Cleaning Process Analysis  
After the seeds were put into the hole under the combined action of gravity, drum friction, and side seed pressure, they rotated with the drum to the cleaning air nozzle. When there were multiple seeds in the hole, the airflow removed the bottom seeds. They were pressed to the bottom of the hole, and the excess seeds were released from the hole and returned to the seed filling box or seed collection box. During this process, the seeds passed through three areas, i.e., the seed filling area, the transport area, and the seed cleaning area. The seeds entered the transport area after filling the seeds smoothly in the filling area. When in the transport area, it was ensured that the seeds were not thrown out of the hole by the centrifugal force of the feeding drum. Now, under the critical state where the seeds at the bottom of the hole in the conveying zone were thrown out by the centrifugal force of the seeding drum in Figure 3, there was no force between the seeds in the same hole, and the seeds on the upper part of the hole were thrown out, and the seed at the bottom of the hole. Suppose the seeds are rigid body, spherical, and of the same size. The center of mass was the origin, and the kinematics equations were established along the tangential and normal directions of the seed movement in the hole:  
\[\left\{ \begin{array}{c}Gcos\alpha=F_{r1}+F_{n1}sin\frac{\theta}{2}\\ F_{n1}cos\frac{\theta}{2}=Gsin\alpha\end{array} \right\} \tag{4}\]  
where  
\[\left\{ \begin{array}{c}F_{r1}=m\omega^{2}r\\ G=mg\end{array} \right\} \tag{5}\]  
where \(G\) denotes gravity, N; \(F_{r1}\) represents the centrifugal force applied to the seed, N; \(F_{n1}\) is the supporting force of the seed filling hole sidewall to the seed, N; \(\alpha\) expresses the angle between the opposite direction of the gravity and the centrifugal force \(F_{r}\) applied to the seed, (\({}^{\circ}\)); \(\theta\) is the number of hole cone angles, (\({}^{\circ}\)); \(r\) denotes the distance from the center of mass of the seed to the rotation center of the drum, m; \(\omega\) is the rotational angular velocity of the feeding drum, rad/s; \(m\) represents the mass of the seed, kg; and \(g\) is the acceleration of gravity, 9.8 N/kg. Figure 2: Sketch map of seed filling holes. (**a**) Small seed filling hole; (**b**) Big seed filling hole. Combining Equations (4) and (5) gave the critical condition that the seeds were thrown out by the centrifugal force of the drum in the conveying area as:  
\[\omega\leq\sqrt{\frac{g\Big{(}cos\alpha-tan\frac{\theta}{2}sin\alpha\Big{)}}{r}} \tag{6}\]  
where \(\theta\) was selected in this study to not exceed 50\({}^{\circ}\); due to the continuous rotation of the seed feeding drum, \(\alpha\) changed constantly. The range of \(\alpha\) in the conveying area in this study is [\(\pi\)/12, \(\pi\)/4], and r is approximately equal to the seed sending. The radius of the drum, substituting into Equation (6), was obtained. In practical work, the critical condition to ensure that the seeds were not thrown out by the centrifugal force of the drum in the conveying area was \(\omega\leq\) 12.6 rad/s. The traveling speed of the tractor in the field was generally 2-10 km/h. Subsequently, the angular velocity of the feeding drum was 0.69-3.64 rad/s to meet the field metering, i.e., when the feeding drum was working normally, the seeds in the transport area were not thrown out the hole. After the seeds passed through the transport area smoothly, they entered the seed cleaning area. To ensure the effect of precise seeding, the cleaning airflow allowed the excess seeds to be cleaned out of the hole normally. Moreover, a seed was pressed to the bottom of the hole; the pressed seed was the research object, and the force on the seed is illustrated in Figure 4:  
Figure 4: The force map of the seed pressed to the bottom of the hole. Figure 3: Sketch map of seed stress during transportation.