Design and Experiment of a Soil-Covering and -Pressing Device for Planters

Abstract

In response to the practical production challenges posed by the unreliable operation of the V-shaped squeezing soil-covering and -pressing device (VCP) for planters under clay soil conditions in Northeast China, incomplete seed furrow closure, and severe soil adhesion on pressing wheels, this study proposes a device with star-toothed concave discs for soil-covering and -pressing (STCP) with the aim of enhancing the soil-covering quality of planters. The main working principles of STCP were expounded, and its main structural and installation parameters were determined and designed. Based on bionics, with the dung beetle’s protruding head structure as the research object and UHMWPE as the material, an optimized protuberance-type bionic pressing wheel was designed. A Box–Behnken experiment was conducted by taking the width of the compression wheel, the spring deformation, and the installation angle as experimental factors, as well as the weight of the soil adhered to the surface of the pressing wheel (SW) and the soil compactness (SC) as the evaluation indicators. The optimal structural parameters of the pressing device were determined as follows: the width of the pressing wheel was 60.57 mm, the spring deformation was 55.19 mm, and the installation angle was 10.70°. The field comparison tests of soil covering performance showed that the star-tooth concave disc soil-covering device can effectively solve the problem of seed “hanging” and “drying”. The average covered soil weight of the star-tooth concave disc soil-covering device was 241.46 g, and the average covered soil weight of VCP was 223.56 g. Compared with VCP, the average covered soil weight of STCP increased by 8.01%. The variation coefficient of covered soil weight after the operation of the star-tooth concave disc soil-covering device was 3.71%, which was more uniform than VCP. The field comparison tests of soil-covering thickness showed that the uniformity of soil-covering thickness can be significantly improved by adding a star-tooth concave disc soil-covering device to VCP. The comparative tests of soil anti-adhesive showed that the convex hull type pressing wheels optimized by bionics had better soil anti-adhesive performance, and the soil adhesion weight was reduced by 43.68% compared with VCP. The field comparative tests of seedling emergence showed that the seedling emergence rate after STCP operation was 3.9% higher than that of VCP.

1. Introduction

Soil covering and pressing are key steps in precision seeding operations [1]. The soil-covering operation should ensure that the seeds in the planting furrow are tightly covered and achieve the goals of appropriate soil thickness and uniform soil covering. The functions of pressing work are to increase soil hardness, closely connect the seeds to the surrounding moist soil, and play roles in increasing, preserving, and supplying soil moisture [2]. The quality of soil-covering and -pressing operations is related to the growth environment of the seeds, which affects their germination and emergence, thereby affecting crop growth and yield [3,4]. Therefore, conducting research on the technology and devices of soil-covering and -pressing for planters is of great significance for promoting high and stable grain production and increasing the income of farmers. The V-shaped squeezing soil-covering and -pressing device (VCP), also known as the standard rubber closing wheel or closing wheel, is installed in a “V” shape from the left and right sides, and it can complete both the soil-covering and -pressing operations at once. Currently, VCPs are most widely used in corn precision seeders. When working under clay conditions, VCPs sometimes only close the upper soil surface of the seed furrow, so the seeds in the furrow cannot come into good contact with the soil, thereby forming voids above the seeds, causing seed “hanging”. In severe cases, the soil in the furrow cannot be closed, causing seed “drying”, which affects seed germination and emergence. In addition, as a working component in direct contact with the soil, the pressing wheel exhibits severe soil adhesion under clay soil conditions, resulting in high work resistance, high energy consumption, decreased sowing quality, and even an inability to operate normally. Related studies have shown [5] that soil adhesion issues lead to a 5–10% decrease in the emergence rate of seeding machinery. Therefore, it is necessary to design and develop a reliable and anti-adhesive soil-covering and -pressing device with which to improve the quality of sowing.

In recent years, scholars have carried out a lot of research on structural improvements for anti-adhesion and drag reduction in soil-covering and -compaction devices. In order to improve seed furrow closure, reduce sidewall compaction, and improve soil–seed contact under no-till conditions, farms and companies such as John Deere, Dawn Equipment Co., May Wes Manufacturing, Martin Industries, Yeater Manufacturing Inc. have developed different forms of V-shaped soil-covering and -pressing devices, namely cast-iron closing wheels, curved closing wheels, poly-star closing wheels, spiked closing wheels, and cast iron paddle closing wheels [6,7]. A recent study in Wisconsin found that aftermarket closing wheels increased corn stand counts by 2% across multiple sites under no-till conditions [8]. RFM AG from Australia has developed a spring coil closing wheel system. It can squeeze out the air near the seeds, promote contact between the seeds and the soil, and provide a good growth environment for the seeds. At the same time, during the operation process, it can automatically detach from the soil, adapting to both clay and sandy soil environments [9]. Schaffer Manufacturing designed the Furrow V Closer, which is better at closing the seed trench ahead of the press wheels in heavy, wet soils. It does not run in the seed trench, but rather to the side of the seed V opening, making it easier to place starter fertilizer and/or chemicals in the furrow with the seed [10]. Zhang et al. [11] thought that soil-covering devices could not effectively break up large soil blocks, thus resulting in the formation of voids; therefore, a broken bionic soil-covering device was designed. Chen et al. [12] designed a three-way adjustable V-shaped soil-covering and compaction device with the functions of covering soil and pressing, while also reducing adhesion, in order to improve the soil-covering, -pressing, and anti-adhesive performance of the 2BMFJ series no-tillage straw-mulching precision seeder in stubble fields. In order to simplify the structure of the machine and solve the problems of poor stability for soil covering and repression in the process of traditional sowing operations, Guo et al. [13] designed a soil-covering and -repressing device, composed of two soil-covering and repression wheels with a conical structure, which had both soil-covering and repression functions. Jia et al. [14] designed an elastic, soil-covered pressing wheel that could carry out profiling operations relative to the seeder in order to solve the problem of easy adhesion in the process of traditional pressing wheel operations. Hou [15] designed an elastic spiral soil-covering press, with active soil conveying and adhesion force resistance, in order to solve the problem of soil adhesion and disturbance on the surface of the soil-contacting working parts of the 2BMFJ series no-tillage straw-covering precision seeder during spring sowing. Zhang et al. [16,17] designed a rib-type bionic pressing roller based on the bionic prototype of the dung beetle and the excellent viscosity reduction characteristics of its ventral geometric structure. This kind of pressing roller has the advantages of low working resistance and less soil adhesion compared with traditional pressing rollers. Although scholars have carried out a lot of research, there are some differences in soil status and tillage methods in different regions. Under the conditions of heavy clay soil in Northeast China, existing soil-covering and -pressing devices still have weak adaptability, and actual production still faces the problems of seed “hanging” and “drying” by loose seed furrow closure, as well as soil adhesion problems with compaction wheels. Therefore, further research is needed.

Based on the abovementioned problems, this study proposes a star-toothed concave disc covering and pressing device (STCP) for clay working conditions in Northeast China, and it designs the structure of the covering and pressing parts to determine their main structural parameters. Aiming at the serious soil adhesion problem with the pressing wheel, the anti-adhesion optimization design of the proposed pressing wheel was carried out based on the bionic principle, and its structural parameters were optimized. The working performance of the covering and pressing device was verified through field experiments. The key novel aspects and contributions of the proposed STCP compared to existing designs like VCP are as follows. (1) The star-toothed concave disc can effectively cut the soil on both sides of the seed furrow, improving soil looseness and fluidity, thus better squeezing the soil to the center of the seed furrow. This solves the problems of seed “hanging” and “drying”. (2) The biomimetic optimized protuberance-type pressing wheel, made of UHMWPE material, has better wear resistance and anti-adhesion properties, effectively reducing soil adhesion on the pressing wheel surface. The design can significantly improve the working performance of soil-covering and -pressing devices for precision planters under clay soil conditions in Northeast China, which is important for improving corn sowing quality. The research results are expected to provide a reference for the future design of soil-covering and -pressing devices for planters.

2. Materials and Methods

2.1. The Overall Structure and Working Principles

2.1.1. Overall Structure

The structure of the STCP is shown in Figure 1; it is mainly composed of the side installation plate of the soil-covering discs, the fixing frame, the spring, the star-toothed concave soil-covering discs, the screw, the pressing wheel installation frame, the pressing wheel, and so on. The fixing frame and the planter unit frame are fixed with bolts, and the side of the covering plate is hinged with the planter unit frame. The installation frame of the pressing wheel is hinged and installed at the rear of the side plate on the soil-covering disc installation.

One end of the adjusting screw for the pressing wheel is hinged with the crossbeam between the two soil-covering disc side installation plates, and the other end is connected to the pressing wheel installation frame. By simultaneously adjusting the effective working length of the spring and the position of the screw, it is possible to ensure the depth of soil penetration while adjusting the pressure of the pressing wheel on the seed furrow soil by changing the subsidence of the pressing wheel relative to the ground. The pressing wheel is installed in a V-shaped manner. There are installation holes at different positions on the shaft of the pressing wheel, and the spacing between the pressing wheels on either side can be adjusted.

2.1.2. Working Principles

During operation, on the one hand, the left and right soil-covering discs rely on the friction between the soil-covering disc and the soil on either side of the seed furrow for passive rotary motion; on the other hand, their motion is linear with the unit forward. Because the soil on both sides of the seed furrow is compacted by the double disc opener and the depth-limiting wheel, the soil is dense and the fluidity is poor. The soil-covering process is first forced to cut the seed furrow soil with the star-shaped teeth on the soil-covering discs. In the process of continuous advancement of the unit, the star-toothed concave soil-covering disc continuously cuts the seed furrow soil. The originally dense seed furrow soil is broken by the device, which lowers the cohesive force between it and the soil and enhances its fluidity. Secondly, when the star-shaped teeth complete the soil cutting, the soil on either side of the seed furrow is squeezed to the center of the seed furrow under the action of the convex arc surface of the left and right soil-covering discs. Because the soil of the seed furrow becomes fine, it is easy to move the soil to the center of the seed furrow under the extrusion of the soil-covering disc, thus realizing the soil-covering operation. Finally, the soil of the seed furrow is compacted by the pressing wheel to achieve the best soil firmness so that the seeds and soil in the seed furrow are fully in contact, and a good growth environment for seed germination and emergence is thus provided.

2.2. Design of Key Components

Design of Soil-Covering Device

The structure of the star-toothed concave disc soil-covering device is shown in Figure 2, which is mainly composed of the star-toothed concave discs, side plates installed on the soil-covering disc, a depth adjustment spring, a screw, sleeves, a fixing frame, bearing seats, etc. The left and right star-toothed concave soil-covering discs are connected to the side installation plate through bolts and have a certain angle β with the vertical plane. The installation spacing L of the left and right soil-covering discs can be changed by replacing shaft sleeves of different lengths. In order to enhance the strength of the soil-covering device, the middle of each side plate is fixed with reinforcing bars. By adjusting the effective working length of the screw (i.e., adjusting the compression of the spring), the working depth H of the soil-covering disc is adjusted. As the core component of the soil-covering device, the structure of the soil-covering disc is shown in Figure 3. The main structural parameters of the soil-covering disc were determined based on previous research [18], as shown in

Table 1. Main technical parameters of the star-toothed concave soil-covering disc.

Parameter Name	Symbol	Unit	Value
Disc diameter	D	mm	400
The maximum outer diameter of the mounting flange disc	d	mm	100
Concave radius	R	mm	500
Disc thickness	δ	mm	4
Disc edge angle	i	(°)	15
The height of star-teeth	h1	mm	50
Tooth tip width	s1	mm	8
Tooth root tangent arc radius	R2, R3	mm	20
tooth number	n	-	12

. The installation parameters were determined as β = 25°, H = 77 mm, and L = 197 mm.

2.3. Design of Key Parameters of Pressing Device

2.3.1. Pressing Wheel Diameter

The diameter of the pressing wheel D_p has a significant relationship with the working performance, which directly affects the pressing effect and slip rate. When the forward speed and load are constant, but the diameter of the pressing wheel is too small, the pressing time is short, and the soil is not sufficiently suppressed. At the same time, the slip rate during operation is greater, and the phenomenon of soil blockage is serious. If the diameter of the pressing wheel is too large, it will affect the stability of the machine and reduce the slip rate during operation, but also increase the rolling resistance during operation, so the diameter of the pressing wheel should be within a reasonable range (generally 200 mm–500 mm) [12]. On the premise of ensuring working stability and working performance, combined with agronomic requirements, the diameter of the pressing wheel, D_p = 320 mm, was selected [13,19].

2.3.2. Pressing Wheel Width

The width of the pressing wheel is a critical determinant of pressing operation quality. Under a fixed load, if the width of the pressing wheel is too narrow, it can lead to excessive localized suppression strength and significant subsidence. Conversely, if the compaction wheel is too wide, the suppression strength may be insufficient and fail to meet the growth requirements of the crops. The primary function of the pressing wheel is to compress the soil displaced toward the seed furrow, ensuring thorough contact with the seeds and enhancing the seeds’ growing environment, promoting germination and increasing the seedling emergence rate. Consequently, the width of the pressing wheel is determined from both the cutting width of the soil-covering disc and the contour curve of the seed furrow after the operation of the soil-covering disc. As shown in Figure 4, the actual working width of the two pressing wheels is denoted as B, determined as follows:

$$B=2B_1\cos {\gamma }_p+B_2$$

(1)

where B₁ is the width of the pressing wheel, mm; B₂ is the distance between the left and right pressing wheels, mm; and ${\gamma }_p$ is the angle between the axis of the pressing wheel and the horizontal plane (i.e., the installation angle of the pressing wheel) in degrees.

According to the operational requirements, the actual working width B should approximate the cutting width, l₂ = 150 mm, of the soil-covering disc at the surface. Referring to the results found in the literature [13,20], the distance between the left and right pressing wheels B₂ = 10 mm. The soil-covering disc has a working depth of H, and the star-toothed tip cutting width is l₁ = 107 mm. In order to select the optimal width of the pressing wheel and consider the comprehensiveness of the test, the width of the pressing wheel, B₁ = 50~80 mm, was selected.

2.3.3. Installation Angle of the Pressing Wheel

The installation angle ${\gamma }_p$ of the pressing wheel affects the degree of soil compression by the wheel. With a fixed width B₁, a greater installation angle results in a smaller effective contact area between the pressing wheel and the soil, thereby increasing the pressure exerted on the soil by the pressing wheel; the opposite is true for a smaller angle. Therefore, a suitable installation angle is needed in order for the pressing wheel to meet the agronomic requirements for soil compaction. Additionally, since the pressing wheel directly contacts the soil after covering, the installation angle is constrained by the contour line of the seed furrow post-covering, i.e., the tangent angle ${\gamma }_s$ of the seed furrow cross-section contour line to the horizontal plane affects the installation angle ${\gamma }_p$. As shown in Figure 4b, if the installation angle ${\gamma }_p$ differs significantly from ${\gamma }_s$, it may lead to excessive local soil compaction and uneven soil firmness, thus affecting the compression effect. Taking into account the installation angle β of the soil-covering disc, an installation angle of 10° to 20° was chosen.

2.3.4. Calculation of Compression Strength

In order to study the adjustment principle of the compaction force for the pressing device, it is necessary to conduct a force analysis on the overall structure of the STCP.

As shown in Figure 5, the frame of the STCP was fixed to the frame of the planter unit, and the soil-covering disc side installation plate was hinged at point O with the frame of the planter unit. According to the theory of torque balance, the torque at the hinge point O can be calculated as follows:

$$\left\{\begin{array}{l}{F}_k{L}_1+G{L}_3=F_c{L}_2+F_p{L}_4+F_{cf}\left(h_1+\frac{H}{2}\right)+F_{pf}{h}_1\\ F_{pf}=f_1{F}_p\\ F_{cf}=f_2{F}_c\\ F_k=k\mathsf{\Delta }x\end{array}\right.$$

(2)

where L₁ is the distance from the spring force to the hinge point O, mm; L₂ is the distance from the vertical force on the soil-covering disc to the hinge point O, mm; L₃ is the distance from the gravity of the soil-covering and compression device to the hinge point O, mm; L₄ is the distance from the vertical force on the pressing wheel to the hinge point O, mm; h₁ is the distance from the ground to the hinge point O, mm; F_x is the horizontal force on the hinge point O, N; F_y is the vertical force on the hinge point O, N; F_c is the vertical resistance against the soil-covering disc, N; F_cf is the horizontal resistance against the soil-covering disc, N; F_p is the vertical resistance against the pressing wheel, N; F_pf is the horizontal resistance against the pressing wheel, N; f₁ is the rolling friction coefficient between the pressing wheel and the soil; f₂ is the rolling friction coefficient between the soil-covering disc and the soil; k is the spring stiffness coefficient, N/mm; and $\mathsf{\Delta }x$ is the spring deformation, mm.

Simplification yields the vertical reaction force F_p from the soil on the compression wheel as follows:

$$F_p=\frac{k\mathsf{\Delta }x{L}_1+G{L}_3-F_c\left(L_2+f_2{h}_1+\frac{f_{2}H}{2}\right)}{f_1{h}_1+L_4}$$

(3)

Since the compression force of the pressing wheel on the soil $F_p^{\prime }$ and the vertical reaction force F_p from the soil on the pressing wheel are action and reaction forces, Formula (3) shows that the compression force of the pressing wheel on the soil is related to the spring stiffness k and deformation Δx. Hence, for a given spring, adjusting the effective working length of the spring changes the compression force.

For a rigid wheel moving on a non-rigid surface (Figure 6), its contact area can be regarded as rectangular, represented by the product of the width of the rigid wheel and the length of the radial contact arc [13].

Since the designed pressing wheel is symmetrical, the contact area S is defined as follows:

$$S=2B_1{l}_{AC}=\frac{2B_1\pi D_p}{360}{\cos }^{-1}\left(\frac{D_p-2Z_0}{D_p}\right)$$

(4)

where l_AC is the length of the radial contact arc between the pressing wheel and the soil, mm, and Z₀ is the sinkage of the pressing wheel relative to the ground, mm.

Thus, the compression strength p is as follows:

$$p=\frac{F_p^{\prime }\cos {\gamma }_p}{S}$$

(5)

Substitute $F_p^{\prime }=F_p$ into Formula 5 to obtain the following:

$$p=\frac{360\left[k\mathsf{\Delta }x{L}_1+G{L}_3-F_c\left(L_2+f_2{h}_1+\frac{f_2}{2}H\right)\right]\cos {\gamma }_p}{2\left(f_1{h}_1+L_4\right)B_1\pi D_p{\cos }^{-1}\left(\frac{D_p-2Z_0}{D_p}\right)}$$

(6)

According to Formula (6), the compression strength of the pressing wheel is related to k, Δx, ${\gamma }_p$, and B₁. Therefore, it is necessary to optimize the structural parameters of the pressing wheel.

2.3.5. Selection of the Spring

According to Formula (3), the compression force of the pressing wheel originates from that of the spring, which should therefore be designed and selected accordingly. The stiffness coefficient of the spring k can be calculated using the following formula:

$$k=\frac{K_k{d}_k}{8C_k^3{N}_k}$$

(7)

$$C_k=\frac{D_k}{d_k}$$

(8)

where K_k is the shear modulus of the spring, kg/mm²; d_k is the diameter of the spring wire, mm; and C_k is the spring index. N_k is the number of effective coils of the spring, and D_k is the diameter of the spring, mm.

Based on engineering experience and the Mechanical Design Manual, D_k = 35 mm, d_k = 6 mm, the spring pitch is 13.4 mm, the material is carbon spring steel, and K_k = 79,000 kg/mm². According to Formulas (7) and (8), C_k = 5.83, k = 22.1. Considering the spatial installation dimensions of the spring, the selected free length of the spring was 170 mm.

With the structural dimensions known, as shown in Figure 5, H = 77.65 mm, L₁ = 93 mm, L₂ = 190 mm, L₃ = 300 mm, L₄ = 566 mm, and h₁ = 298 mm; taking f₁ = f₂ = 0.125, k = 22.1, F_c = 48 N, G = 295 N, ${\gamma }_p$ = 10°~20°, and B₁ = 50 mm~70 mm.

According to the agronomic requirements, the optimal soil compactness after corn seeding compression is 30~50 kPa [18]. Referencing the literature [13] and taking the middle value of compression strength, p = 40 kPa, and the soil sinking amount Z₀ = 8 mm for calculation, the spring deformation was determined to be between 22.93 mm and 63.93 mm.

2.3.6. Setting of the Compression Load

For the operability of the experimental process, the setting of the compression force was transformed into the sinking amount of the pressing wheel. To simplify the calculation, when the non-rigid surface bears a load, if the sinking of the wheel is not great, the calculation of the sinking amount can be simplified to the following [21]:

$$Z_0=\frac{6F_p^{\prime }}{5K{B}_0{D}_p^{1/2}}$$

(9)

$$K={\alpha }_0\left(1+0.27B_0\right)$$

(10)

where K is the soil characteristic parameter, N/cm², and ${\alpha }_0$ is a parameter related to soil properties.

According to Formulas (9) and (10), the soil sinking amount Z₀ is related to the soil characteristics, the structural parameters of the pressing wheel, and the compression force $F_p^{\prime }$ of the pressing wheel on the soil. Under the given conditions of the soil characteristics and pressing wheel structure, the compression force of the pressing wheel on the soil is positively correlated with the soil sinking amount Z₀. Based on the results of previous research [22,23], the quadratic equation fitting the compression force of the pressing wheel on the soil and the soil sinking amount Z₀ is as shown in Formula (11), with R² = 0.9979, indicating a good fit.

$$Z_0=-0.00001{F_p^{\prime }}^2+0.042F_p^{\prime }-1.292$$

(11)

Therefore, the soil sinking amount under the corresponding spring deformation can be calculated using Formula (11). After calculation, when the spring deformation is 20 mm, 42 mm, and 64 mm, the soil sinking amount is 6.58 mm, 9.37 mm, and 12.06 mm, respectively.

2.4. Biomimetic Optimization Design and Testing of the Pressing Wheel

2.4.1. Design of the Biomimetic Pressing Wheel

When soil-engaging components operate in moist and clay soil, a significant amount of soil adheres to their surfaces, significantly affecting the operational resistance, energy consumption, work quality, and lifespan of the machinery [24]. Through millions of years of evolution, soil-inhabiting animals have developed skills such as wear resistance, drag reduction, and strong digging capabilities as adaptations to underground life. These desirable characteristics provide a natural template and efficient optimization pathway for the anti-adhesion design of soil-engaging components. Research indicates that the geometrical structure of the bodily surfaces of soil-inhabiting animals is one of the main reasons for their unimpeded activity in moist and clay soil environments [25,26,27,28].

The dung beetle (Geotrupidae) is a typical soil-inhabiting animal. In the living environment of nature, it is often found in clay soil and feces. The protrusions on the surface of the dung beetle create a non-smooth surface in contact with clay soil or dung, allowing it to move freely with minimal soil or dung adhesion force. This study selects the protrusion structure on the head of the dung beetle (Figure 7a [29]), most relevant to its behavior of rolling dung balls, as the object under study. As shown in Figure 7b, a protrusion-type non-smooth pressing wheel surface was designed.

The bionic design of the protrusion-type pressing wheel is shown in Figure 7b. It consists of a reinforced ring surface, a spoke, a bearing sleeve, and a biomimetic wheel surface with protrusions. Based on the key structural parameters of the compression wheel design, the diameter of the compression wheel (D_p) was set at 320 mm, with the design radius of the compression wheel taken as the distance from the center of the wheel’s surface to half of the protrusions’ height. Studies have confirmed that, compared to steel materials, soil-engaging components of agricultural machinery made of UHMWPE can significantly reduce the adhesion of soil to the components [14,16,30,31,32,33,34]. Therefore, UHMWPE was selected as the material for the biomimetic compression wheel surface. To enhance the strength of the biomimetic wheel surface, a reinforcement ring with a surface made of Q235, with a diameter of 285 mm and thickness of 3 mm, was designed. The width of the reinforcing ring surface was consistent with the width of the biomimetic wheel surface. During installation, the outer side of the reinforcement ring surface was bonded to the inner side of the UHMWPE biomimetic wheel surface using AB glue. According to the research results of [30,35,36], the design for the protrusion-type pressing wheel includes protrusions with a height (h_t) of 5 mm, a base diameter (R_t) of 20 mm, and a surface area ratio of 45%. The surface area ratio is the ratio of the protrusions’ surface area to the whole ring’s surface area. Considering the narrow width of the pressing wheel, the arrangement of the protrusions was not considered to impact its operational performance.

2.4.2. Box–Behnken Experimental Design

(1) Experimental factors and evaluation indicators

The width of the pressing wheel, the amount of spring deformation, and the installation angle were determined as experimental factors based on the structural design. In selecting evaluation indicators for this experiment, it was considered that, in addition to exhibiting good anti-adhesion effects, the operation of the pressing wheel must also meet the soil compactness requirements for crop growth. Therefore, the soil weight adhered to the surface of the pressing wheel (SW) and the soil compactness (SC) after the pressing operation were selected as the evaluation indexes, and a Box–Behnken experiment was carried out in order to optimize the structural parameters of the pressing wheel.

(2) Experimental design

The Box–Behnken experiment was further conducted using Design-Expert 8.0.6 software, with the experimental factors coded as shown in

Table 2. Test factors and coding.

Code	Factors
	B1/mm	$\mathit{\Delta }\mathit{x}$/mm	${\mathit{\gamma }}_{\mathit{p}}$/(°)
−1	50	20	10
0	65	42	15
+1	80	64	20

.

(3) Experimental apparatus and method

The experiment was carried out at the National Key Laboratory of Agricultural Equipment and Technology of the Chinese Academy of Agricultural Mechanization Sciences Group Co., Ltd., in Beijing China. The soil type was clay (22.6% sand, 29.2% silt, and 48.2% clay). According to the experimental design, six pairs of protrusion-type biomimetic pressing wheels were processed, and the experimental apparatus is shown in Figure 8.

During the experiment, the moisture content in the soil bin was controlled at (15% ± 2%). Soil moisture content was measured by evenly watering the entire soil bin using its water sprinkling system and then letting it rest for 24 h to evenly distribute the moisture content. Soil samples were taken at six evenly spaced points along the length of the test area, with a sampling depth of 0 to 15 cm. The drying temperature was set at 105 °C, and the soil moisture contents of these points were determined using an LHS-20A rapid moisture meter (measurement accuracy: 1 mg/0.01%) to calculate the average soil moisture content. Watering, resting, and measuring were repeated to achieve the experimental conditions.

After the soil-covering and -compression test, the SW was scraped off and weighed. Each group of experiments was repeated three times, and the average value of SW was taken as the final experimental result.

The FK-JSD1 soil compaction meter (measurement depth: 0–450 mm; measurement range: 0–100 kg; measurement accuracy: 0.1 kg) produced by Shandong Fang Ke Instrument Co., Ltd. was used to measure the SC at 0–50 mm above the seed. After each test, 5 test points were evenly taken in the lengthwise direction of the stable operation stage to calculate the average SC. Each group of experiments was repeated three times.

3. Results and Discussion

3.1. Box–Behnken Experimental Results and Analysis

Using the Design-Expert 8.0.6 software, 17 sets of experiments were designed, and the experimental scheme and results are shown in

Table 3. Experiment scheme and results.

No	Factor			Y1/g	Y2/kPa
	x1	x2	x3
1	−1	−1	0	18.32	39.25
2	1	−1	0	20.11	31.46
3	−1	1	0	28.22	42.32
4	1	1	0	35.45	41.21
5	−1	0	−1	17.77	38.54
6	1	0	−1	22.42	34.88
7	−1	0	1	28.66	40.45
8	1	0	1	33.14	38.12
9	0	−1	−1	16.03	34.33
10	0	1	−1	24.57	45.64
11	0	−1	1	21.15	39.74
12	0	1	1	31.45	42.31
13	0	0	0	16.26	38.25
14	0	0	0	14.37	37.63
15	0	0	0	16.55	39.57
16	0	0	0	13.45	38.86
17	0	0	0	15.57	39.28

. x₁, x₂, and x₃ represent the coded values for the width of the pressing wheel B₁, the spring deformation $\mathsf{\Delta }x$, and the installation angle ${\gamma }_p$, respectively. Meanwhile, Y₁ and Y₂ represent the SW and SC after the seeding operation, respectively.

The experimental results were subjected to multivariate regression analysis using the Design-Expert 8.0.6 software, resulting in the regression model for the SW and its variance analysis, as shown in

Table 4. SW analysis of variance results.

Source	Sum of Square	Degree of Freedom	Mean Square	F	p
Model	767.31	9	85.26	25.40	0.0002 **
x1	41.18	1	41.18	12.27	0.0100 **
x2	242.88	1	242.88	72.37	<0.0001 **
x3	141.20	1	141.20	42.08	0.0003 **
x1x2	7.40	1	7.40	2.20	0.1812
x1x3	0.01	1	0.01	0.00	0.9643
x2x3	0.77	1	0.77	0.23	0.6456
x12	164.01	1	164.01	48.87	0.0002 **
x22	68.85	1	68.85	20.52	0.0027 **
x32	67.92	1	67.92	20.24	0.0028 **
Residual	23.49	7	3.36
Lack of fit	16.67	3	5.56	3.26	0.142
Pure error	6.83	4	1.71
Total	790.80	16

Note: p ≤ 0.01 is highly significant, marked as **.

.

From Table 4, it is evident that the regression model for the SW is highly significant (p < 0.01), with a non-significant lack of fit (p > 0.05), a coefficient of variation of 8.34%, a determination coefficient R² = 0.9703, and an adjusted determination coefficient R²_adj = 0.9321, indicating that the model fits well and is reliable. The data in Table 4 were fitted through quadratic multiple regression, and the quadratic regression equation between each factor and the SW was obtained as follows:

$$\begin{array}{l}{Y}_1=15.24+2.27x_1+5.51x_2+4.2x_3+1.36x_1{x}_2-0.042x_1{x}_3+0.44x_2{x}_3\\ +6.24x_1^2+4.04x_2^2+4.02x_3^2\end{array}$$

(12)

The analysis of variance shows that x₁, x₂, and x₃ had highly significant effects on the SW, the interaction term had no significant effect on the SW, and x₁², x₂², and x₃² had highly significant effects on the SW. By comparing the F values, it can be seen that the influence of each factor on SW, from great to small, was as follows: x₂ > x₃ > x₁.

The SC regression model’s analysis of variance results are shown in

Table 5. SC analysis of variance results.

Source	Sum of Square	Degree of Freedom	Mean Square	F	p
Model	168.23	9	18.69	27.02	0.0001 **
x1	27.71	1	27.71	40.07	0.0004 **
x2	89.11	1	89.11	128.83	<0.0001 **
x3	6.53	1	6.53	9.45	0.0180 *
x1x2	11.16	1	11.16	16.13	0.0051 **
x1x3	0.44	1	0.44	0.64	0.4502
x2x3	19.10	1	19.10	27.61	0.0012 **
x12	7.48	1	7.48	10.81	0.0133 *
x22	5.81	1	5.81	8.40	0.0230 *
x32	1.58	1	1.58	2.28	0.1746
Residual	4.84	7	0.69
Lack of fit	2.38	3	0.79	1.29	0.3933
Pure error	2.46	4	0.62
Total	173.07	16

Note: p ≤ 0.01 is highly significant, marked as **; 0.01 < p ≤ 0.05 is significant, marked as *.

.

From Table 5, it is evident that the regression model for the SC is highly significant (p < 0.01), with a non-significant lack of fit (p > 0.05), a coefficient of variation of 2.14%, a determination coefficient R² = 0.972, and an adjusted determination coefficient R²_adj = 0.9361, indicating that the model fits well and is reliable. The data in Table 5 were fitted through quadratic multiple regression, and the quadratic regression equation between each factor and the SC was obtained as follows:

$$\begin{array}{l}{Y}_1=15.24+2.27x_1+5.51x_2+4.2x_3+1.36x_1{x}_2-0.042x_1{x}_3+0.44x_2{x}_3\\ +6.24x_1^2+4.04x_2^2+4.02x_3^2\end{array}$$

(13)

The analysis of variance showed that x₁ and x₂ had highly significant effects on the SC, x₃ had a significant effect on the SC, x₁x₂ and x₂x₃ had highly significant effects on the SC, x₁x₃ had no significant effect on the SC, x₁² and x₂² had significant effects on the SC, and x₃² had no significant effect on the SC. By comparing the F values, it can be seen that the influence of each factor on the SC, from great to small, was as follows: x₂ > x₁ > x₃.

In order to analyze the influence of the interaction term of each factor on the SC, the influence of the interaction between the width of the pressing wheel and the deformation of the spring on the SC was obtained under the condition that the installation angle was 15°, as shown in Figure 9a. When the width of the pressing wheel was constant, the SC increased with the increase in the spring deformation. When the spring deformation was constant, the SC decreased with the increase in the width of the pressing wheel.

Under the condition that the width of the pressing wheel is 65 mm, the influence of the interaction between the spring deformation and the installation angle on the SC was obtained, as shown in Figure 9b. When the spring deformation was constant, the SC increased with the increase in the installation angle. When the installation angle was constant, the SC increased with the increase in spring deformation.

The reason for this analysis is that, according to the calculation Formula (3) of the pressing wheel compression force, it can be inferred that adjusting the effective working length of the spring, that is, changing the spring deformation, can change the compression force. The greater the deformation of the spring, the greater the compression force of the pressing wheel on the soil, the better the compaction effect on the seed furrow soil, and the greater the SC. Therefore, within the test range, when the width of the pressing wheel and the installation angle were fixed, the SC increased with the increase in spring deformation. When the spring deformation and installation angle were constant, according to the calculation Formula (6) of the compression strength, the compression strength was inversely proportional to the width of the pressing wheel. The greater the width of the pressing wheel, the greater the contact area between the pressing wheel and the soil, the lower the compression strength, the weaker the compaction effect on the soil, and the lower the SC. Therefore, within the test range, the SC decreased with the increase in the width of the pressing wheel. When the width of the pressing wheel and the deformation of the spring were constant, with the increase in the installation angle, the squeezing effect of the pressing wheel on the soil was enhanced, the compaction effect was increased, and the SC was increased. Therefore, within the test range, SC increased with the increase in the installation angle.

3.2. Optimization of Parameters

To explore the best combination of experimental factors, with the minimum SW and the maximum SC as the optimization objectives within the target range, and taking into account the boundary conditions of each experimental factor, the established regression model was used for both optimization and solution. The objective function and constraints were as follows:

$$\left\{\begin{array}{l}\min Y_1\left(B_1,\mathsf{\Delta }x,{\gamma }_p\right)\\ \max Y_2\left(B_1,\mathsf{\Delta }x,{\gamma }_p\right)\\ \mathrm{s}.\mathrm{t}.\left\{\begin{array}{l}50\mathrm{mm}\le B_1\le 80\mathrm{mm}\\ 20\mathrm{mm}\le \mathsf{\Delta }x\le 64\mathrm{mm}\\ 10°\le {\gamma }_p\le 20°\end{array}\right.\end{array}\right.$$

(14)

Using the Design-Expert 8.0.6 software for optimization and solution, the optimal results were obtained when the pressing wheel width was 60.57 mm, the spring deformation was 55.19 mm, and the installation angle was 10.70°, resulting in an SW of 18.75 g and an SC of 42.17 kPa.

In order to verify and analyze the operation performance of STCP, the operation effects of STCP and VCP were compared and analyzed. The soil-covering performance test, soil-covering thickness verification test, anti-adhesive performance test of pressing wheel, and seedling emergence verification test were carried out.

3.3. Field Verification Test of Soil-Covering Performance

3.3.1. Test Conditions and Methods

In order to verify the performance of the star-tooth concave disc soil-covering device in the field, field tests were carried out in Gannan County, Qiqihar City, Heilongjiang Province, China in May 2023. The previous crop in the experimental field was corn. After the corn harvest, corn straw was recycled. There was corn stubble and a small amount of straw on the surface. The average height of corn stubble was 15.6 cm, and the average straw coverage was 0.73 kg/m². The soil in the experimental field was clay (16.76% sand, 26.73% silt, and 56.51% clay). The average moisture content of 0~10 cm soil was 27.64%, the average soil compactness of 0~10 cm was 11.47 kg/cm², and the average soil compactness of 0~20 cm was 23.78 kg/cm².

According to the design results, a star-tooth concave disc soil-covering device was processed and trial-produced, and it was installed on the 2BMY-2 corn no-tillage precision planter produced by China Academy of Agricultural Mechanization Sciences Group Co., Ltd. During the test, the VCP on the left side of the 2BMY-2 corn no-tillage precision planter was replaced with a star-tooth concave disc soil-covering device for soil covering. The comparative tests between STCP and VCP were conducted side-by-side in the same field under the same operating conditions. We believed that soil properties and stubble conditions were quite uniform on short test strips and would not affect the test results. The test device and process are shown in Figure 10. The operating speed of the planter was 8 km/h. After the soil-covering operation was completed, the seed cross-section was planned, and the selection of the seed was random. By observing whether there was a gap around the seed, we can judge whether the seed had a “hanging” phenomenon, and by observing whether the seed furrow was closed strictly, we can judge whether the seed had a “drying” phenomenon.

When measuring the weight of the covered soil, we randomly delineated a cross-section of the furrow after the soil-covering operation and included a seed in this cross-section. We then measured the cross-section to find the centerline of the furrow, aligned the centerline of the soil sampler (35 mm lateral × 80 mm vertical × 100 mm longitudinal) with the centerline of the furrow cross-section, and pressed the sampler vertically down into the soil to a depth equal to the seed depth. The soil was removed from the soil sampler and weighed with an electronic scale. The test was repeated five times and the mean value was taken.

3.3.2. Test Results

(1) Comparative test results of seed “hanging” and “drying” phenomena

The cross-section of the seed is shown in Figure 11. It can be seen from the seed cross-section in the figure that on one side of the installation of VCP, the seed “hanging” phenomenon caused by the lax closure of the seed furrow will occur intermittently. However, on the one side of the star-tooth concave disc soil-covering device, the soil on both sides of the seed furrow was squeezed better, the seed furrow was completely closed, and there was no seed “hanging” phenomenon.

The field operation effect after the soil covering operation is shown in Figure 10. On one side of the installation of VCP, the seed “drying” phenomenon appeared intermittently. However, the soil on both sides of the seed furrow was well squeezed to the center of the seed furrow on one side of the star-tooth concave disc soil-covering device, and there was no phenomenon of seed “drying”.

(2) Comparative test results of the weight of the covered soil

The seed “hanging” and “drying” were caused by the fact that the soil particles were not completely filled with the seed furrow. Therefore, within the test range, the greater the weight of the covered soil in the seed furrow, the more soil particles entering the seed furrow, and the better the quality of the soil covering operation. Based on this, the weight of the covered soil of the seed furrow was selected to evaluate the operation performance of the soil-covering device. The measurement results of the weight of the covered soil are shown in

Table 6. Measurement results of the weight of the covered soil.

Test Number	Ws/g	Wv/g
1	247.91	210.65
2	230.75	235.73
3	231.35	241.44
4	244.24	214.35
5	253.05	237.58
Average value	241.46	223.56
Coefficient of variation	3.71%	5.25%

. W_s represents the measurement results of the average covered soil weight after the operation of the star tooth concave disc soil-covering device. W_v represents the measurement results of the average covered soil weight after the operation of VCP.

According to the comparative test results in Table 6, it can be seen that the average covered soil weight of the star tooth concave disc soil-covering device was 241.46 g, and the average covered soil weight of VCP was 223.56 g, an increase of 8.01%. By comparing the coefficient of variation of covered soil weight, it can be seen that the soil was more uniform after the operation of the star-tooth concave disc soil-covering device, and the coefficient of variation of covered soil weight was 3.71%.

3.4. Comparative Test of Soil-Covering Thickness

3.4.1. Test Methods

After the soil-covering operation of the planter, if the weight of the covered soil is too large, the thickness of the covering soil will be too large after being pressed, and the sowing depth will be too deep. If the weight of the covered soil is too small, the thickness of the covering soil will be small after being pressed, and the sowing depth will be too shallow. Excessively deep or shallow sowing depths will affect the soil environment for seed growth and affect germination and emergence. According to the local corn planting agronomy, the sowing depth should be 40~50 mm. In order to verify whether the weight of the covered soil under the optimal parameter combination meets the requirements of agronomic production after pressing, field comparative tests were carried out. During the test, a star-tooth concave disc soil-covering device was installed in front of the VCP on the left side of the corn no-tillage precision planter, and the pressure of the two rows of VCP was adjusted to be consistent. After the sowing operation was completed, five seeds each in the left and right rows were randomly selected. The cross sections of the seeds in the furrow were gouged, and the sowing depth of the seeds was measured.

3.4.2. Test Results

The measurement results of the soil-covering thickness are shown in

Table 7. Measurement results of the soil-covering thickness.

Test Number	Hs/mm	Hv/mm
1	48.3	46.5
2	43.5	30.8
3	51.2	41.6
4	46.4	47.2
5	44.7	28.6
Average value	46.8	38.9
Coefficient of variation	5.82%	20.08%

. H_s represents the measurement results of the soil-covering thickness after the operation of the “star-tooth concave disc soil-covering device + VCP”. H_V represents the measurement results of the soil-covering thickness after the operation of the VCP.

According to the test results, the measurement results of the soil-covering thickness after the operation of the “star-tooth concave disc soil covering device + VCP” and VCP were 46.8 mm and 38.9 mm, respectively, and the coefficients of variation were 5.82% and 20.08%, respectively.

Under clay conditions, after the installation of the star-tooth concave disc soil-covering device, the thickness of the covering soil meets the agricultural requirements, while the thickness of the covering soil of VCP is shallow. By comparing the coefficient of variation, it can be seen that the uniformity of the thickness of the covering soil can be obviously improved after the installation of the star-tooth concave disc soil-covering device.

Combined with the test results of soil covering performance, the reasons for the shallow soil covering thickness and poor consistency of VCP were analyzed.

In some places, the phenomenon of lax closure of seed furrows and soil adhesion of VCP’s pressing wheels appeared, resulting in the small weight of covered soil above the seeds, the shallow soil-covering thickness, and the poor uniformity of soil covering operation.

3.5. Anti-Adhesive Performance Test of Pressing Wheels

3.5.1. Test Conditions and Methods

In order to verify the operation performance of STCP and the anti-adhesive effect of the pressing wheels, field tests were carried out in the experimental field of Jilin Agricultural University in China in April 2024. The previous crop in the experimental field was corn, and the corn stubble was not broken. The surface was covered with corn straw, and the average straw coverage was 1.36 kg/m². The soil in the experimental field was clay (26.58% sand, 28.77% silt, and 44.65% clay). The average moisture content at 0~10 cm of soil was (16.2 ± 0.63)%. The average soil compaction at 0~10 cm was (13.93 ± 2.44) kg/cm². The average soil compaction at 0~20 cm was (26.52 ± 3.37) kg/cm².

The test device is a 2BMY-4 corn precision planter produced by China Academy of Agricultural Mechanization Science Group Co., Ltd., Beijing, China, as shown in Figure 12. During the test, the VCP of one row of the planter was replaced with STCP. The pressing wheels of STCP were convex hull-type pressing wheels optimized as abovementioned, and the material was UHMWPE. The seeding speed was 8 km/h. According to the optimization results of the soil bin test of the pressing device, the spring deformation was adjusted to 55 mm. After the sowing operation was completed, the soil adhered to the surface of the pressing wheels was collected, weighed, and recorded with an electronic scale. The working distance of the planter was not less than 20 m at a time, repeated in three groups, and the average weight of the adhesive soil was calculated.

3.5.2. Test Results

The measurement results of the soil-adhesion weight of the pressing wheels are shown in

Table 8. Measurement results of the soil adhesion weight of the pressing wheels.

Test Number	Qs/g	Qv/g
1	17.6	32.1
2	24.4	36.7
3	19.5	40.5
Average value	20.5	36.4

. Q_s represents the measurement results of the soil adhesion weight of the convex hull-pressing wheels in STCP. Q_v represents the measurement results of the soil adhesion weight of the pressing wheels in VCP.

According to the test results, the mean value of Q_s was 20.5 g, and the mean value of Q_v was 36.4 g. Compared with Q_v, Q_s decreased by 43.68%. In other words, the pressing wheels optimized by bionics have better soil anti-adhesive performance.

3.6. Validation Test of Seedling Emergence

In order to compare the performance of STCP and VCP, the field verification tests were carried out with the emergence rate of corn seeds as the evaluation index. The test conditions and equipment are the same as in Section 3.5.1. The corn seed variety Tianyu 108 was selected, and the seeds were coated. When measuring the seedling emergence rate, from the first day of seedling emergence in the test field, a 50 m counting area was randomly selected in the test area to count the number of seedlings. The measurement time was 5:00 pm every day. The end time node of the test is that the emergence rate in the test area does not change. Three counting areas were randomly selected in the test field to calculate the average value of the emergence rate.

The measurement results of the emergence rate are shown in

Table 9. Measurement results of emergence rate.

Test Number	Es/%	Ev/%
1	97.5	92.0
2	95.5	92.5
3	97.0	94.0
Average value	96.7	92.8

. E_s represents the measurement results of seed emergence rate after STCP operation. E_V represents the measurement results of seed emergence rate after VCP operation.

According to the measurement results of seedling emergence rate, the seedling emergence rate of corn after STCP operation was 96.7%, the seedling emergence rate of corn after VCP operation was 92.8%, and the seedling emergence rate increased by 3.9%. The operation effect of STCP was better. The reason for the increase in seedling emergence rate was analyzed. Under the same operating conditions, compared with VCP, STCP solved the phenomenon of seed “hanging” and ”drying”, effectively improved the contact between soil and seeds, and provided a good growth environment for seed germination and seedling emergence.

4. Conclusions

(1) In response to the practical issues encountered when using soil-covering and -pressing devices under clay conditions in Northeast China, such as unreliable operation, incomplete seed furrow closure, and significant soil adhesion to the pressing wheel, a star-toothed concave disc soil-covering and -pressing device was proposed, and its main structural parameters were designed;

(2) Utilizing biomimetic principles and focusing on the dung beetle’s protruding head structure, a protuberance-type biomimetic pressing wheel was developed. A Box–Behnken experiment was carried out to optimize the structural parameters of the pressing wheel, with the width of the pressing wheel, spring deformation, and installation angle as the test factors and the SW and SC values as the evaluation indexes. The optimal parameter combination is as follows: when the width of the pressing wheel was 60.57 mm, the spring deformation was 55.19 mm, the installation angle was 10.70°, the SW was 18.75 g, and the SC was 42.17 kPa;

(3) The field comparison tests of soil covering performance showed that the star-tooth concave disc soil-covering device can effectively solve the problem of seed “hanging” and “drying”. The average covered soil weight of the star-tooth concave disc soil-covering device was 241.46 g, and the average covered soil weight of VCP was 223.56 g. Compared with VCP, the average covered soil weight of STCP increased by 8.01%. The variation coefficient of covered soil weight after the operation of the star-tooth concave disc soil-covering device was 3.71%, which was more uniform than VCP. The field comparison tests of soil-covering thickness showed that the uniformity of soil-covering thickness can be significantly improved by adding a star-tooth concave disc soil-covering device to VCP. The comparative tests of the soil anti-adhesive showed that the convex hull-type pressing wheels optimized by bionics had better soil anti-adhesive performance, and the soil adhesion weight was reduced by 43.68% compared with VCP. The field comparative tests of seedling emergence showed that the seedling emergence rate after STCP operation was 3.9% higher than that of VCP;

(4) This study provides a reference for the research and design of planter soil-covering and -pressing devices in Northeast China. In the future, discrete element modern simulation technology can be used to further optimize the structure of soil-covering discs, such as disc diameter and number of teeth. Further research could be conducted on the intelligent control of compression force in soil-covering and -pressing devices. In addition, adaptability tests and seedling emergence tests for corn and other crop seeds can be carried out continuously for many years, and the operational performance of the soil-covering and -pressing device in the manuscript can be continuously observed.