2.2.1. Spring-Finger

During sectional harvesting, potatoes are excavated and laid out to dry in the field; the distribution is haphazard and irregular, as shown in Figure 3. Potatoes that are excavated and mixed with clods of mud, roots and stems, etc., are exposed on the ground as bright potatoes; meanwhile, others are completely or partially buried in the ground and are known as dark potato [14]. Potatoes are excavated and distributed either on the surface or at a depth of 30–50 mm from the surface [15]. The depth of the pickup tines should be designed to pick up as many potatoes as possible, ensuring that dark potatoes can also be picked up without difficulty, and avoiding soil congestion and severe wear on the tines as a result of going too deep into the soil. Based on the above distribution of potatoes after excavation, the embedded depth should be limited to between 60 and 80 mm to meet the potato pickup requirement. In addition, the mechanical characteristics of the spring-finger should also be taken into account during the design process, as the ground is uneven and mixed with debris in the field due to the constraints of the picking operation.

**Figure 3.** Potato distribution status: (1) dark potato (2) soil block (3) stone block (4) potato (5) pickup device.

The design of the angle of entry of the pickup tines is a function of the pickup effect and is based on the need to pick up potatoes smoothly while reducing wounded potato to the potatoes and not digging up too much soil, in order to reduce the forces on the pickup tines [16]. The potato is analysed for forces, as shown in Figure 4, and the relationship (1) is detailed.

$$\begin{cases} F\_N \sin \alpha + F\_I \cos \alpha - F - f \cos \alpha = 0 \\ F\_N \cos \alpha - F\_I \sin \alpha - \text{mg} + f \sin \alpha = 0 \end{cases} \tag{1}$$

where *F* is the resistance of the potato in the soil, N; *m* is the mass of the potato, kg; *FN* is the support force of the potato with the spring-finger, N; *f* is the friction force, N; and α is the angle between *F* and *f* , ◦; *FJ* is the centripetal force on the potato, N; *g* is the acceleration of gravity, N/kg.

The relevant parameters are brought into the above equation to calculate the pickup spring-finger entry angle *α* ≥ 53◦. This range ensures that potatoes enter the picker spring-finger smoothly, minimising the wounded potato rate and avoiding the high picking resistance caused by a large angle of entry, thus increasing efficiency and reducing consumption. After determining the lug depth and lug angle, the maximum length that the spring-finger can extend out of the picker is initially determined to be 200 mm, the total length of the spring-finger is 260 mm, the bending angle of the spring-finger *δ* is 120◦, and the material chosen is 65 Mn. The structure of the spring-finger is shown in Figure 5.

**Figure 4.** Potato force analysis diagram.

**Figure 5.** Schematic diagram of the spring-finger structure: (**a**) axonometric drawings. (**b**) side view.

The design of the pickup spacing of the spring-finger should be based on the physical characteristics of the potatoes and their pickup characteristics. The potatoes grown in China are excavated and harvested and then laid on the surface. According to their distribution on the surface, the potatoes are excavated and laid out in an orderly manner, and the grouping state is irregular.
