Once the seedlings have achieved a horizontal posture and are about to be released from the seedling transport belt, soil covering of the seedling roots is required while ensuring that the seedling tips remain exposed above the soil surface, referred to as the “soil covering and exposing seedling tips”. To achieve this operation, an active screw soil covering and tip exposure mechanism has been designed.
The active screw soil covering and exposing seedling tip mechanism mainly consists of a driving motor, connecting plate, screw mechanism, transmission shaft, chain case, and position adjustment plate, as shown in
Figure 17. The motor drives the screw mechanism to perform the soil covering and exposes the seedling tips via chain transmission. The position of the screw mechanism can be adjusted both vertically and horizontally using the adjustment plate.
Parameter Design of the Screw Mechanism
The soil covering process requires soil particles to move outward along the axial direction, meaning that the velocity vector of the particles must move from the inside toward the outside (with a friction angle relative to the normal of the screw surface) [
14,
15,
16,
17]. The conditions for this are as follows:
In the formula, is the screw angle (°) and is the friction angle between the soil and the steel plate (°). Since the screw angle varies at different points along the blade, with the maximum screw angle at the minimum radius, it is sufficient for the screw angle at the inner diameter to meet the conditions.
The theoretical volume of soil moved by the screw mechanism in the ditch can be considered as the volume of the furrow created, which can be modeled as a rectangular prism. The volume of soil required to cover the ditch can be expressed as
where
is the working width (m);
is the forward speed (m·s
−1); and
is the tillage depth (m). Based on the agronomic requirements for the sweet potato seedling transplanter and to improve transplanting efficiency, the operational speed of the tractor is set to be between 0.1 and 0.4 m·s
−1. With a ditch width of 50 mm and a ditch depth of 80 mm, the volume of soil required to cover the ditch is calculated to be approximately 4 × 10
−4~1.6 × 10
−3 m
3·s
−1.
Referencing the design methods for open screw conveyors, to ensure stable soil movement and prevent clogging, the screw covering mechanism must meet the following requirements:
The conveying capacity of the screw mechanism must exceed the soil input rate, or sludging may occur.
The soil must not exhibit vertical jumping or rolling perpendicular to the direction of conveyance.
The soil must be conveyed axially, meaning that both axial forces and axial velocity must be greater than 0.
Based on the design principles of screw conveyors, the formulas for the design parameters of the screw mechanism are as follows:
In the formula, is the amount of mud transported, m3·s−1; is the comprehensive characteristic coefficient of the mud; is the filling coefficient; is the slope coefficient; is the bulk density of the conveyed material, t·m−3; and is the comprehensive characteristic coefficient of the material.
Since the left and right soil covering screws are arranged symmetrically, the mud transport capacity of each screw mechanism should be half of the total soil required to fill the seedling ditch, and thus is set to range between 2 × 10−4 and 0.8 × 10−3 m3·s−1. The topsoil for sweet potato seedling transplanting has a certain fluidity, referring to the design standards for screw conveyors, and the estimated comprehensive characteristic coefficient for the soil is taken as 0.0415. The filling factor is set to 0.4; since the spirals are positioned horizontally, the inclination coefficient is set to 1.0; the comprehensive characteristic coefficient for the soil is taken as 75; and the bulk density of the soil is set at 1.8 t·m−3.
From Equation (19), the outer diameter of the screw is calculated to be mm. Considering the overall mass of the machine and its compatibility with the tractor, the outer diameter is determined to be . From Equation (20), the screw shaft diameter is determined to be between 20 and 35 mm. To minimize the entanglement of grass on the shaft and to enhance soil transport capacity, the screw shaft diameter is chosen as mm. The pitch from Equation (22) is determined to be in the range of 50 to 220 mm. To ensure smooth soil transport within the screw and to prevent mud accumulation, the soil transport amount of each pitch section must exceed the combined mud input from the current and previous pitch sections. Therefore, this study adopts a variable-pitch, constant-diameter screw, where the pitch increases along the helices towards the furrow, with the largest pitch closest to the furrow. Based on the structure and configuration dimensions of the machine, the maximum operational rotational speed from Equation (21) is calculated to be 237 rpm.