# Design and Evaluation of Vacuum Central Drum Seed Metering Device
## 3 Key Component Parameter Design and Analysis
### Determination of Seeding Drum Parameters  
The inner and outer drum and the ventilation housing are installed coaxially. As indicated in Figure 3, the fixed ventilation housing (7) is attached to the inner drum (4) and the outer drum (1), and the inner cavity of the ventilation housing (7) is divided into a negative pressure zone (5) and a positive pressure zone (6). The suction hole of the seeding drum rotates to the negative pressure zone (5) sucks the rice seeds that are in the seed box, while the surplus rice seeds fall into the seed box under the action of the seed cleaning box. The sucked rice seeds enter the positive pressure zone (6) with the rotation of the drum, falling into the seeding tube due to self-weight and positive pressure, and then by self-weight and positive pressure circulation, the rice seeds fall into the sowing groove that is dug by the ditch-digging and ridge-forming base slab. The circular velocity of the drum is the basis for determining the drum diameter, and the rotation speed of the seeding drum has certain numerical relationships with the forward velocity of the metering device [26]. The seeding quantity of the metering device per second is:  
\[Q_{m}=\frac{V_{g}}{D+\Delta L} \tag{2}\]  
The seeding quantity of the metering device per second can be represented by the forward velocity \(V_{m}\) and the hole distance \(T\):  
\[Q_{m}=\frac{V_{m}}{T} \tag{3}\] \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cultivar** & **Average Length/mm** & **Average Width/mm** & **Average Thickness/mm** \\ \hline Ruanhuayou1179 & 9.82 & 2.28 & 1.82 \\ Shunliangyou1179 & 9.72 & 2.64 & 2.04 \\ Jingliangyou 1212 & 8.48 & 2.42 & 1.85 \\ Teyou338 & 8.22 & 2.87 & 2.14 \\ Wuyou1179 & 8.62 & 2.56 & 1.99 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The average triaxial size of six varieties hybrid rice seeds. Figure 3: The main structure of the seeding drum. 1. the outer drum, 2. the outer suction holes, 3. The inner suction holes, 4. the inner drum, 5. the negative pressure zone, 6. the positive pressure zone, and 7. the ventilation housing. Eliminate \(Q_{m}\) in Equations (2) and (3):  
\[V_{g}=\frac{V_{m}(D+\Delta L)}{T} \tag{4}\]  
Therefore, the diameters of the drum can be obtained as:  
\[D_{g}=\frac{\pm 60V_{m}(D+L)}{\pi NT} \tag{5}\]  
The number of suction holes is:  
\[Z=\frac{\pi D_{g}K}{D+\Delta L} \tag{6}\]  
The circumferential distance between two adjacent holes is represented by \(D+\Delta L\), which should be longer than the maximum size of two rice seeds, namely:  
\[D\,+\Delta L\,\geq\,2\,lmax \tag{7}\]  
It can be seen from Equations (3) and (4) that when the forward velocity of the seeder \(V_{m}\) and the hole distance is constant, the diameter of the drum is inversely proportional to the rotation rate. The decrease of the rotation rate can prolong the seed-filling time, but as the diameter increases, the required air flow must be increased, and then air leakage loss also rises. Reducing the diameter can decrease the height of the seed layer in the seed box and influence the seed-filling performance, and the material of the drum should be taken into consideration as well. The forward velocity of the direct-seeding seeder in the paddy field is generally 0.5\(\sim\)1.5 m/s, therefore, the curve length between two holes \(D+\Delta L\geq\) 0.02 m. According to the ratio of output speed of the riding type rice tractor head and the rotation rate of the gear box, the rotation rate being 0\(\sim\)60 rpm corresponds to a forward velocity of the direct-seeding seeder being 0\(\sim\)1.0 m/s, and the sowing hole distance of hybrid rice is generally between 0.14 and 0.18 m. The diameter of the drum can be estimated from Equation (5), and by taking factors such as the transmission system of existing hybrid rice direct-seeding seeders into overall consideration, it is appropriate to choose an external diameter as 135 mm.