Design and Experiment of the Belt-Tooth Residual Film Recovery Machine

Abstract

To address poor film pickup, incomplete soil–film separation, and high soil content in conventional residual film recovery machines, this study designed a belt-tooth type residual film recovery machine. Its core component integrates flexible belts with nail-teeth, providing both overload protection and efficient conveying. EDEM simulations compared film pickup performance across tooth profiles, identifying an optimal structure. Based on the kinematics and mechanical properties of residual film, a film removal mechanism and packing device were designed, incorporating partitioned packing belts to reduce soil content rate in the collected film. Using Box–Behnken experimental design, response surface methodology analyzed the effects of machine forward speed, film-lifting tooth penetration depth, and pickup belt inclination angle. Key findings show: forward speed, belt angle, and tooth depth (descending order) primarily influence recovery rate; while tooth depth, belt angle, and forward speed primarily affect soil content rate. Multi-objective optimization in Design-Expert determined optimal parameters: 5.2 km/h speed, 44 mm tooth depth, and 75° belt angle. Field validation achieved a 90.15% recovery rate and 5.86% soil content rate. Relative errors below 2.73% confirmed the regression model’s reliability. Compared with common models, the recovery rate has increased slightly, while the soil content rate has decreased by more than 4%, meeting the technical requirements for resource recovery of residual plastic film.

1. Introduction

Plastic film mulching technology has been widely used in farmland cultivation, offering benefits such as soil temperature elevation, moisture retention, disease and pest prevention, and weed suppression, effectively improving crop yields. It has become the primary planting model for cotton production in northwest China, with a 100% film coverage rate in Xinjiang cotton fields [1]. However, most films are made of non-degradable polyethylene, and their long-term retention in soil causes severe ecological issues [2]. In developed countries, agricultural films typically have thicknesses ranging from 0.015–0.250 mm, featuring advantages such as high tensile strength, tear resistance, and minimal damage during recovery, enabling roll-based recycling [3,4,5]. In China, agricultural films characterized by thinner thickness (<0.015 mm) and lower tensile strength are predominantly used, rendering direct rolling-based recycling impractical. Consequently, recovery primarily relies on a film-picking mechanism [6,7,8,9,10]. To date, multiple machine types have been developed, categorized by core functional components, with mainstream models including spring-tooth type, chain-tooth type, drum type, and chain-harrow type systems.

The spring-tooth type and chain-harrow type feature elongated tooth profiles. During the film collection operation, the spring-teeth undergo deformation due to soil resistance. Upon exiting the soil, the elastic potential energy stored in the teeth is released, resulting in a sudden increase in velocity. This rapid motion tends to tear the plastic film into strips, thereby compromising the effectiveness of the collection process [11,12,13,14]. Drum and chain-tooth types generally employ hook-shaped or cylindrical teeth with diameters exceeding 8 mm, reducing deformation. For example, the 11SM-1.5 residual film recovery machine developed by Xinjiang Academy of Agricultural Sciences adopts a drum type pickup device that loosens soil beneath films with lifting teeth before collecting them [15]. However, drum type machines poorly separate films from impurities, resulting in high impurity content in recovered films. Jiang Deli’s team designed a nail-tooth-chain plate type residual film recovery machine, utilizing a friction-driven nail-tooth-chain plate for pickup with overload protection [16]. Gou Haixiao’s team developed a two-order pin-tooth chain plate type residual film recovery machine, incorporating a secondary film-conveying mechanism to reduce impurity content [17]. The chain-tooth residual film recovery machine exhibits relatively better impurity removal performance; however, its chain structure is complex, featuring protruding components such as chain plates and pins, which are prone to entanglement and accumulation of weeds, long stalks, or bundled residual film, leading to blockages. Additionally, the gaps between chain links are susceptible to debris intrusion, accelerating wear, shortening the service life of the chain, and making maintenance difficult. Most existing models integrate straw crushing and returning with film recovery, but no effective machine exists for recovering residual films after straw has already been crushed and returned.

To address these issues, our research group designed a belt-tooth type residual film recovery machine specifically for fields where straw has been crushed and returned. It employs a flexible belt as the film-picking component, providing overload protection. Theoretical analysis of the nail-tooth film pickup process was conducted, and EDEM simulations were used to select the optimal tooth shape for pickup performance. Based on motion and force analysis of residual films, a rational film removal and packing mechanism was designed. Field experiments and quadratic regression orthogonal composite experimental optimization were performed to determine optimal operational parameters, which were subsequently validated.

2. Materials and Methods

2.1. Overall Structure

The belt-tooth type residual film recovery machine primarily consists of a traction frame, transmission system, film-lifting teeth, pickup and conveying device, film removal device, packing device, walking wheels, depth-limiting wheels, and machine frame. Power is transmitted via chain drives to the gearbox, film removal roller, and packing roller, while the pickup roller is driven by the packing roller through a belt transmission. The overall structure is shown in Figure 1.

2.2. Working Principle

The belt-tooth type residual film recovery machine is connected to a tractor via a towing mechanism, with its transmission system powered by the tractor’s output shaft. During operation, the tractor propels the machine forward, causing the film-lifting tooth to penetrate the soil beneath the residual film at a specific angle. As the machine advances, the teeth loosen the sub-film soil and lift the residual film from the surface. Power from the output shaft is transmitted through the gearbox to the packing roller, which drives the belt transmission. As the nail-tooth on the pickup roller performs an arc motion, it lifts the residual film raised by the film-lifting tooth until the film is fully detached from the soil, completing the pickup process. During the film pickup process, if the nail-teeth encounter stones or large obstructive objects, the resistance acting on the teeth increases sharply and is transmitted to the belt. If this resistance exceeds the maximum static friction between the belt and the pickup roller, relative slippage occurs between the belt and the roller. This slippage continues until the pickup roller passes over the obstacle, thereby achieving an overload protection effect. The film is conveyed along the belt toward the film removal roller. During this upward transport, soil and impurities adhering to the film detach due to gravity overpowering their adhesion to the film. At the film removal roller, the outer diameter of the residual-film-stripping ring exceeds the trajectory of the nail-tooth tips. The rings lift the film off the teeth and utilize friction to transport it backward until the film disengages from the rings and falls into the packing box under gravity, achieving film removal. When the residual film drops into the V-shaped packing box, the packing belts drive the film’s movement via friction, causing it to tumble within the box and gradually form a cylindrical film bundle, thereby completing the packing process. After the residual film recovery operation concludes, the hydraulic device opens the packing box to discharge the film bundle.

2.3. Technical Parameters

Table 1. Main technical specifications.

Parameter	Value
Machine dimensions (L × W × H)/m	3.85 × 3.18 × 1.88
Working width/m	2.05
Suspension mode	Towed type
Working speed/(km·h−1)	4~8

lists the main technical specifications of the belt-tooth type residual film recovery machine.

2.4. Key Component Design

2.4.1. Film-Lifting Device

The film-lifting device consists of edge-film-lifting teeth, standard film-lifting teeth, crossbeams, and fixing plates. During operation, the teeth penetrate the soil beneath the residual film, loosening the soil and lifting the residual film upward to facilitate subsequent pickup. The cotton field adopts a one-film-six-row planting pattern, with 100 mm spacing between two cotton stalks in the same row group and 660 mm spacing between adjacent row groups. The film width is 1950 mm, and the edge film burial depth is 60–70 mm [18]. To avoid root stubble interference, the spacing between the film-lifting teeth is designed as 150 mm, as shown in Figure 2a.

Based on field surveys, the surface height variation in cotton fields ranges from 25–40 mm. Therefore, the penetration depth of the standard film-lifting teeth is set to ≥40 mm, while the edge teeth penetration depth is ≥70 mm. When the teeth penetrate the soil, they exert normal stress (σ_n) and tangential stress (τ) on the soil (Figure 2b). According to Coulomb’s friction law, the critical slip condition occurs when the tangential stress reaches its maximum value [19]:

$$\tau ={\mu }_s{\sigma }_n+c$$

(1)

where:

μ_s is the soil internal friction coefficient ${\mu }_s=\tan \varphi$ (ϕ denotes the friction angle between the soil and the tooth surface);

c is the soil cohesion.

The angle α denotes the inclination between the tooth surface and horizontal direction, as observed in Figure 2b.

$$\tan \alpha ={\displaystyle \frac{\tau }{{\sigma }_n}}$$

(2)

By combining Equations (1) and (2), the penetration angle α of the film-lifting tooth can be derived as:

$$\alpha >\arctan \left(\tan \varphi +{\displaystyle \frac{c}{{\sigma }_n}}\right)$$

(3)

Here, φ is the soil internal friction angle, taken as 25° [20]. For sandy soils, soil cohesion c can be neglected. To prevent soil from slipping upward along the tooth surface, the penetration angle α must be designed to be no less than 25°. According to reference [21], the optimal penetration angle for film-lifting performance is 35°. Therefore, the penetration angle α is designed as 35°.

2.4.2. Pickup and Conveying Device

The pickup and conveying device is a critical working component of the belt-tooth type residual film recovery machine, primarily composed of nail-teeth, belts, pickup roller, film removal wheel, and tension rollers, as shown in Figure 3. The nail-teeth lift the residual film and transport it upward along the belt to the top of the packing box. The conveying section is inclined at a specific angle to the ground. To ensure thorough separation of the film–impurity mixture during transportation, the effective conveying distance of the device is designed as 1630 mm, with a working width of 2050 mm.

• Determination of Pickup Device Speed

The pickup device is a key working component of the belt-tooth type residual film recovery machine. The pickup roller is fixed to the frame via self-aligning roller bearings at both ends, with annular grooves machined on its surface to enhance belt adhesion. Its power is provided by the film removal roller via belt transmission. The pickup roller diameter D is designed as 0.2 m. The relationship between the roller rotational speed n (r/min) and the belt linear velocity v_b (m/s) is:

$$v_b={\displaystyle \frac{\pi Dn}{60}}$$

(4)

According to the literature [22], increasing the nail-tooth penetration depth beyond 50 mm has minimal impact on the recovery rate. The film-lifting teeth can raise the soil by 30–40 mm. To avoid soil accumulation caused by excessively low pickup roller height, the nail-tooth length is designed as 60 mm, with an initial penetration depth of 20 mm.

The belts are arranged in pairs within the gaps between the film-lifting teeth. During operation, the film-lifting teeth elevate the soil–film layer to a predetermined height, forming a loose soil–film mixture. The pickup roller drives the belts, enabling the nail-tooth to penetrate the soil at a specific speed for film pickup.

Let the ratio of belt linear velocity to machine forward speed be λ. When λ < 1, residual film leakage is likely, and the film cannot remain taut during transportation, leading to high impurity content. When λ > 1, tension between the nail-teeth enhances friction on the residual film, improving pickup efficiency. Additionally, the film lies flatter on the belt, facilitating impurity detachment [23]. According to the literature [24], when the pickup speed ratio λ exceeds 1.3, the nail-teeth tend to tear the residual film, resulting in a decreased pickup rate. Based on preliminary testing, the optimal value of λ was determined to be 1.1. Considering that excessively high forward speeds can lead to reduced stability of the working components’ penetration depth, a forward speed range of 4–8 km/h was preset to ensure both operational efficiency and stable working conditions. Through calculation, the corresponding belt linear velocity was determined to be 4.4–8.8 km/h, with the pickup roller speed ranging from 117 to 233 r/min.

• Analysis of the Pickup Process.

The nail-tooth is vertically fixed to the belt surface. Two belts are grouped and distributed on both sides of the film-lifting teeth, as shown in Figure 4a. The belt width is 40 mm. Based on the arrangement of the film-lifting teeth, 26 belts are required, with an axial spacing d of 75 mm between belt centerlines.

Figure 4b illustrates the nail-tooth film pickup process. Point P is the tangent point between the belt and the pickup roller, and θ is the angle between line OP and the x-axis. Let R denote the distance from the tooth tip to the center O of the pickup roller. The trajectory equation of the nail-tooth tip during the circular motion of the belt around the pickup roller is expressed as:

$$\left\{\begin{array}{c}x=v_{m}t+R\cos (\theta +\omega t)\\ y=R\sin (\theta +\omega t)\end{array}\right.$$

(5)

During the film pickup process, the residual film is subjected to the supporting force F_N1 from the nail-tooth, the frictional force F_f1 exerted by the nail-tooth, and its own gravitational force m₁g. Let β be the angle between the extension line of F_N1 and m₁g. According to D’Alembert’s principle, the equilibrium equation is:

$$\left\{\begin{array}{c}{F}_{N1}-m_{1}g\cos \beta -m_1{\displaystyle \frac{dv}{dt}}\cos \beta =0\\ F_{f1}-m_{1}g\sin \beta -m_1{\displaystyle \frac{dv}{dt}}\sin \beta =0\end{array}\right.$$

(6)

Equation (6) indicates that, as the linear velocity v at the nail-tooth tip increases, the frictional force F_f1 becomes larger, making the residual film less prone to detachment from the nail-tooth. However, excessively high linear velocity may tear the film, compromising pickup performance. The linear speed of the spike-teeth maintains a fixed ratio to the forward speed of the implement; therefore, optimal pickup efficiency of the spike-teeth can only be achieved within an appropriate range of forward operating speeds.

2.4.3. Film Removal Device

The film removal process is accomplished through the coordinated operation of the film removal roller, residual film stripping rings, and the belt nail-teeth. The stripping rings are welded onto the film removal roller in an annular guide rail structure, with an inclined inner surface. Two opposing stripping rings form a V-shaped structure. To ensure tight contact between the belt side and the stripping ring surface, the inner inclination angle of the stripping rings is slightly smaller than the belt’s inclination angle. When the residual film is transported to the film removal roller, the outer edge of the stripping rings exceeds the length of the nail-tooth. During belt conveyance, the film is gradually lifted by the outer edge of the stripping rings, detaching from the nail-tooth tips, and finally falls into the packing box under gravity. This design effectively reduces issues such as film backflow, entanglement, or blockage during the removal process.

From the analysis of Figure 5b, the condition for successful film removal is:

$$2F_l·\cos \delta \ge F_m$$

(7)

F_l: Transverse tension acting on the residual film (N);

δ: Angle between the transverse tension and the vertical direction (°);

F_m: Frictional force acting on the residual film (N).

where:

$$F_m={\mu }_1{F}_{\mathrm{e}}$$

(8)

μ₁: Friction coefficient between the residual film and nail-tooth, taken as 0.4;

F_e: Elastic force of the residual film (N).

Based on preliminary experimental measurements, residual film strips with a coverage duration exceeding 180 days, thickness of 0.01 mm, width of 10 mm, and length of 150 mm exhibited an elastic deformation force range of 0–0.9 N; thus, F_e was taken as 0.9 N. Substituting Equation (8) into Equation (7) yields:

$$2F_l·\cos \delta \ge 0.36$$

(9)

When the residual film moves toward the nail-tooth tip, F_l gradually decreases and δ increases. Since cos δ monotonically decreases over (0, 2π), the residual film can detach smoothly when the force at the tooth tip satisfies Equation (9). The preset numerical height difference between the nail-tooth tip and the outer edge of the film-stripping ring is 15 mm, with δ set to 51°. Under these conditions, the residual film deformation at the tooth tip is 5.32 mm, and the nominal strain at film fracture is 8.3% [24]. The residual film stress F_l at this stage is 1.0 N. Calculating 2F_l · cos δ yields 1.48 N, meeting the theoretical requirements for film removal. Thus, R₁ is designed as 225 mm and R₂ as 210 mm.

Figure 5. Film removal device and its schematic diagram. (a) Residual film transport trajectory diagram, (b) Force diagram of the residual film detachment process from the nail-tooth, (c) Simplified force diagram of residual film at the film-removal wheel. Note: In (a), R₁ denotes the radius of the stripping ring and R₂ represents the arc radius traced by the nail-tooth tips on the film removal roller.

Figure 5. Film removal device and its schematic diagram. (a) Residual film transport trajectory diagram, (b) Force diagram of the residual film detachment process from the nail-tooth, (c) Simplified force diagram of residual film at the film-removal wheel. Note: In (a), R₁ denotes the radius of the stripping ring and R₂ represents the arc radius traced by the nail-tooth tips on the film removal roller.

Figure 5c illustrates the force analysis diagram of the residual film at the film-stripping ring. Point A marks the position where the nail-tooth tip coincides with the stripping ring. From this point onward, the residual film detaches from the nail-tooth, and the friction from the stripping ring drives the film backward. The forces acting on the residual film include:

F_f2: Friction force exerted by the stripping ring (N);

F_N2: Normal force exerted by the stripping ring (N);

G₂: Gravitational force of the residual film (N);

F_c: Centrifugal force (N).

Applying D’Alembert’s principle, the force equilibrium equations are:

$$\left\{\begin{array}{c}{F}_{f2}-\mathrm{m}{a}_2-G_2·\cos {\psi }_2=0\\ F_{N2}-F_c-G_2·\sin {\psi }_2=0\end{array}\right.$$

(10)

$$F_{f2}={\mu }_2{F}_{N2}$$

(11)

$$F_c={\displaystyle \frac{\mathrm{m}}{R_1}}{({\displaystyle \frac{dx}{dt}})}^2$$

(12)

$$G_2={\mathrm{m}}_{2}g$$

(13)

The simultaneous Equations (10)–(13) give:

$${\psi }_2=\arccos \left({\displaystyle \frac{{\mu }_2{v}^2}{g{R}_1\sqrt{1+{{\mu }_2}^2}}}\right)-\arctan {\mu }_2$$

(14)

μ₂: Friction coefficient between the film-stripping ring and the residual film, taken as 0.4 [25].

The angle ψ₁ can be adjusted by altering the position of the front tension roller. Analysis of Figure 5c shows that, as ψ₁ decreases, point A gradually moves upward and to the right along the outer contour of the stripping ring, causing ψ₂ to increase. This reduces the gravitational component that the friction force must overcome, making it easier for the residual film to be transported to the rear packing box.

Given the minimum belt linear speed of 4.4 km/h (1.94 m/s at point A), substituting into Equation (14) yields the minimum ψ₂ of 28.7° required for smooth film transport. Using the graphical method, the corresponding ψ₁ is determined as 40°. Thus, the maximum design value for ψ₁ is set to 40°.

Additionally, decreasing ψ₁ increases the angle σ between the pickup and conveying belt section and the horizontal line. The graphical method confirms that, when ψ₁ is 40°, the corresponding angle σ is 68°.

2.4.4. Packing Device

The packing device primarily consists of side panels, a lower packing conveyor belt, an upper packing conveyor belt, a hydraulic device, and a frame. Its structural diagram is shown in Figure 6. The lower and upper conveyor belts are joined at one end, forming a V-shaped internal surface. The packing roller is powered by the tractor’s output shaft via chain transmission. Residual film inherently exhibits adsorption and entanglement properties [26]. After entering the packing box, the residual film is driven by the friction of the lower packing belt to the bottom of the V-shaped section, pressing it against the upper packing belt. The friction between the upper belt and the film provides instantaneous acceleration along the belt surface, causing the film to flip backward. Repeating this process forms an initial core of the film bundle. As residual film continuously enters the packing box, the bundle diameter increases, achieving the film-rolling and bundling effect. After completing the film collection, the hydraulic device lifts the upper packing belt to open the film box for discharge.

To increase the surface friction coefficient of the packing belts, anti-slip belts with regular concave–convex patterns are selected. The packing belts are designed as strips, separated by spacer plates with a thickness of 10 mm. This design offers two advantages: first, in the event of damage, only the affected strip needs to be replaced, thereby reducing maintenance costs; second, during the packing process, any soil clods that were not initially separated can fall through the gaps between the belt strips, enabling a secondary film–soil separation.

When the film bundle tumbles inside the packing box, it is subjected to five primary forces:

f₁: Frictional force from the lower packing belt (N);

N₁: Normal force from the lower packing belt (N);

f₂: Frictional force from the upper packing belt (N);

N₂: Normal force from the upper packing belt (N);

G_b: Gravitational force of the film bundle (N).

The force analysis is illustrated in Figure 7.

The force equilibrium equations for the film bundle inside the packing box are:

$$\left\{\begin{array}{c}{N}_1·\sin {\gamma }_1+f_1·\cos {\gamma }_1-N_2\cos {\gamma }_2-f_2·\sin {\gamma }_2=0\\ N_1·\cos {\gamma }_1+f_2·\cos {\gamma }_2-N_2\sin {\gamma }_2-f_1·\sin {\gamma }_1=G_b\end{array}\right.$$

(15)

where: $f_1={\mu }_3{N}_1$, $f_2={\mu }_3{N}_2$.

From Equation (15), we obtain:

$$N_1={\displaystyle \frac{G(\cos {\gamma }_2+{\mu }_3\sin {\gamma }_2)}{(1+{{\mu }_3}^2)\cos ({\gamma }_1+{\gamma }_2)}}$$

(16)

$$N_2={\displaystyle \frac{G(\sin {\gamma }_1+{\mu }_3\cos {\gamma }_1)}{(1+{{\mu }_3}^2)\cos ({\gamma }_1+{\gamma }_2)}}$$

(17)

The static friction coefficient μ₃ between the packing belts and the residual film is measured to be approximately 0.572. When N₁ and N₂ increase, f₁ and f₂ also increase, facilitating the formation of a compact film bundle. To ensure sufficient space inside the film box, γ₃ is set to 45°, with γ₁ + γ₂ = 45°. To determine the values of γ₁ and γ₂, function plots were generated for the non-constant terms in the numerator of Equations (16) and (17), as shown in Figure 8. As shown in Figure 8, N₁ first increases and then decreases with γ₂, remaining in the increasing phase when γ₂ is less than 22.5°. N₂ increases with the rise in γ₁. To maximize both N₁ and N₂ within the allowable range, and taking into account the effective impurity-leakage length of the lower packing belt, both γ₁ and γ₂ are set to 22.5°.

The linear speed of the packing belt significantly impacts the packing effectiveness. If the packing belt speed is lower than the belt conveying speed, residual film will accumulate at the bottom of the packing box and fail to bundle promptly. If the packing belt speed exceeds the belt conveying speed, the residual film undergoes elastic deformation due to stretching, resulting in tighter bundling. However, excessive speed may fracture the film, and fragmented film pieces struggle to adhere to the bundle, degrading packing quality. Under conditions where no sliding friction occurs between the packing belt and residual film, the speed ratio can be regarded as the stretching multiple of the film. The linear speed ratio between the nail-tooth belt and packing belt is designed as 1:1.2, ensuring effective stretching of the residual film without rupture [27].

3. Results and Analysis

3.1. Simulation Experiment on Nail-Tooth Film Pickup Performance

3.1.1. Simulation Model Establishment

The pickup operation of the nail-tooth primarily relies on contact interaction between the tooth tip and residual film; consequently, the morphological characteristics of the tooth tip significantly influence retrieval efficacy. Drawing upon established designs from existing residual film recovery machines, three distinct tooth profiles were engineered as illustrated in Figure 9.

Figure 9a depicts a cylindrical tooth, which is commonly employed in chain type residual film recovery machines. Figure 9b presents a wedge-shaped tooth with a rounded tip design, which increases the contact area between the tooth tip and the residual film. This configuration is beneficial for enhancing the frictional force between the nail-tooth and the film while minimizing the risk of film puncture. The tooth body is designed to be thinner; however, its geometric profile improves bending resistance. Figure 9c shows a hook-shaped tooth inspired by the J-type structure described in reference [28]. This design modifies the SP₁ tooth by reshaping the tip into a hooked form. The sharp tip allows for concentrated stress penetration into the soil, while the gradually widening tooth body enhances structural stability. To avoid adverse effects on the film removal process, the curvature of the hook must be limited. Excessive curvature or a sharpened tooth crest may penetrate the film, leading to tearing under high-speed conditions and subsequently reducing pickup efficiency.

To identify the profile of nail-tooth that is optimal for residual film retrieval, EDEM-based simulation analysis of the film pickup process was conducted, implementing comparative simulations for all three profiles under identical operational conditions.

The actual thickness of the residual film is approximately 0.01 mm. Constructing a film model in EDEM 2023.1 simulation software using real-scale particles would require an excessively large particle count, making the simulation impractical. To ensure the feasibility of the simulation while considering practical factors such as soil particles and fallen cotton impurities adhered to the film surface—which increase the weight that the nail tooth must overcome during pickup—the residual film was modeled using spherical particles with a radius of 0.05 mm and a contact radius of 1.6 times the particle radius. Based on preliminary five-point sampling measurements, the mass ratio between the residual film and the surface impurities was determined to be 1:30, and the combined mass closely approximated the mass of the simulated residual film.

Given the ductility and flexibility of the residual film, the Hertz–Mindlin with bonding contact model was applied for film–film interactions. Soil particles were modeled as spheres with a radius of 4 mm. Due to the presence of moisture capillary forces and van der Waals forces between soil particles, cohesive effects needed to be simulated, and thus the Hertz–Mindlin with JKR model was used for soil–soil interactions. To accurately simulate the adhesion behavior of soil particles on the surface of the residual film, the Hertz–Mindlin (no slip) model was applied for soil–film interactions. The nail-tooth material was set as high-manganese steel. Given the adhesive properties of the residual film, the Hertz–Mindlin with JKR model was used for nail-tooth–film interactions, while the Hertz–Mindlin (no slip) model was applied for nail-tooth–soil interactions to ensure complete transmission of tangential forces during soil shearing resistance when the nail-tooth penetrates the soil. Relevant material and contact parameters were obtained from previous measurement studies and references [29,30,31,32], as detailed in

Table 2. Material Parameters and Contact Parameters.

Parameter	Value
Poisson’s ratio of the nail-tooth	0.3
Shear modulus of the nail-tooth/Pa	7.9 × 1010
Density of the nail-tooth/(kg/m3)	7865
Poisson’s ratio of the residual film	0.42
Shear modulus of the residual film/Pa	8.3 × 108
Density of the residual film/(kg/m3)	910
Poisson’s ratio of soil	0.32
Shear modulus of soil/Pa	4.5 × 106
Density of soil/(kg/m3)	1500
Residual film–nail-tooth JKR surface energy (J/m2)	7
Soil–soil JKR surface energy (J/m2)	7.8
Coefficient of restitution (residual film–residual film)	0.2
Static friction coefficient (residual film–residual film)	0.4
Dynamic friction coefficient (residual film–residual film)	0.3
Coefficient of restitution (residual film–nail-tooth)	0.2
Static friction coefficient (residual film–nail-tooth)	0.4
Dynamic friction coefficient (residual film–nail-tooth)	0.22
Coefficient of restitution (residual film–soil)	0.31
Static friction coefficient (residual film–soil)	0.4
Dynamic friction coefficient (residual film–soil)	0.28
Normal stiffness per unit area (residual film–residual film)/(N/m3)	5 × 107
Shear stiffness per unit area (residual film–residual film)/(N/m3)	5 × 107
Normal strength (residual film–residual film)/Pa	1.5 × 106
Shear strength (residual film–residual film)/Pa	1.5 × 106
Bonding scale parameter (residual film–residual film)	1.6

.

A virtual soil bin with dimensions (length × width × height) of 800 mm × 170 mm × 100 mm was constructed, and a virtual residual film with dimensions (length × width) of 600 mm × 150 mm was laid flat on the soil bin surface, as shown in Figure 10.

3.1.2. Simulation Results and Analysis

After generating the virtual soil bin and residual film, three-dimensional models of pickup rollers equipped with different nail-tooth profiles were created in SolidWorks 2024 and imported into EDEM software. Based on the relationship between the machine’s forward speed (v_m) and the pickup roller’s rotational speed (n), three speed conditions were set:

Low speed: v_m = 4 km/h, n = 117 r/min;

Medium speed: v_m = 6 km/h, n = 175 r/min;

High speed: v_m = 8 km/h, n = 233 r/min.

Each tooth profile was simulated once under the three speed conditions. Comparative analysis was conducted to evaluate the film pickup effectiveness of the nail-tooth under varying profiles, forward speeds, and roller rotational speeds [33]. The simulation results are shown in Figure 11.

The simulation results show that:

The SP₁ type nail-tooth can lift the residual film to a certain height under low-speed conditions (4 km/h, 117 r/min), but film slippage occurs. At medium and high speeds (6/8 km/h, 175/233 r/min), effective pickup is achieved with no film tearing.

The SP₂ type nail-tooth achieves stable and effective pickup across all three speed conditions, with no slippage or tearing.

The SP₃ type nail-tooth effectively achieves film pickup under all three speed conditions, with no slippage of the film on the nail-tooth observed. However, under high-speed conditions (6 km/h, 233 r/min), tearing of the residual film occurs, with 87 bond fractures recorded. A tear of approximately 40 mm forms on the film, extending from the initial contact point with the tooth tip to the area traversed by the tooth tip.

Conclusion: The SP₂ type nail-tooth exhibits the most stable pickup performance without causing film damage, making it the optimal choice.

3.2. Whole Machine Performance Test

3.2.1. Test Materials

In November 2024, field trials for residual film pickup performance of the belt-tooth type residual film recovery machine were conducted in cotton fields in Yuli County, Bayingolin Mongol Autonomous Prefecture, Xinjiang. The test area covered 10 hectares of flat terrain with drip irrigation tapes already removed. The soil was sandy loam with a moisture content of 17.56–21.23%. The previous crop was cotton, planted in a “660 mm + 100 mm” pattern (row spacing). The residual film had a thickness of 0.01 mm and a width of 2050 mm, with a coverage duration of over 180 days. Fragmented cotton stalks, fallen cotton, and soil were present on the surface of the film.

3.2.2. Test Method

Field performance tests of the pickup device were conducted according to the Chinese National Standard [34]. The primary evaluation metrics were residual film recovery rate and soil content rate. Test instruments and equipment included: John Deere 1204 wheeled tractor, a portable soil moisture meter, stopwatches, tape measures, a balance scale, shovels. Figure 12 shows the field performance test of the belt-tooth residual film recovery machine.

• Test Factors.

The preset forward speed of the machine was set to 4~8 km/h. Three operational speeds of 4, 6, and 8 km/h were selected according to the tractor’s gear settings and engine speed. To maintain a belt linear velocity-to-machine speed ratio (λ) of 1.1, the pickup roller speed was adjusted to 117, 175, and 234 r/min by replacing the sprocket teeth at the drive roller. According to the analysis in Section 3.2, the penetration depth of the standard film-lifting teeth was set to 40~50 mm, while the edge-film-lifting teeth penetrated 30 mm deeper than the standard teeth. The angle of the pickup belt relative to the horizontal plane significantly influences residual film lifting and film–soil separation. As the angle σ increases, the gravitational component of residual film or soil particles along the slope increases, affecting both recovery rate and soil content rate. Based on Section 3.3 the minimum slope angle σ was set to 68°, adjustable within 70°~80° by adjusting the position of the belt tension roller.

The test factors selected were:

X₁: Machine forward speed (km/h);

X₂: Penetration depth of film-lifting teeth (mm);

X₃: Angle of pickup belt relative to horizontal plane (°).

The coding of test factors is shown in

Table 3. Test factor levels.

Levels	Factors
	Machine Forward Speed X1/(km/h)	Penetration Depth of Film-Lifting Teeth X2/(mm)	Angle of Pickup Belt Relative to Horizontal Plane X3/(°)
−1	4	40	70
0	6	45	75
1	8	50	80

.

• Evaluation Metrics

The test area measured 100 m in length and 2.05 m in width. Before testing, five-point random sampling was conducted: each sampling point covered a rectangular area of 5 m × 2.05 m, where surface residual film was collected. The average mass of film collected from these five points was recorded as m₀. After testing, five-point sampling was repeated to collect remaining surface residual film, with its average mass measured as m₁. The mass of the film bundle (m₂) in the collection box was measured post-operation. Subsequently, soil within the film bundle (excluding fragmented cotton stalks and other impurities) was manually separated and weighed (m₃). The calculation formulas for the residual film recovery rate and the soil content rate in the film are as follows:

$${\mathrm{Y}}_1=1-{\displaystyle \frac{{\mathrm{m}}_1}{{\mathrm{m}}_0}}\times 100\%$$

(18)

Y₁—Residual film recovery rate, (%);

m₀—Avg. surface film mass pre-operation, (g);

m₁—Avg. surface film mass post-operation, (g).

$${\mathrm{Y}}_2={\displaystyle \frac{{\mathrm{m}}_2}{{\mathrm{m}}_3}}\times 100\%$$

(19)

Y₂—Soil content rate, (%);

m₂—Film bundle mass in packing box, (g);

m₃—Soil mass in film bundle, (g).

Figure 13 shows the ground surface effect diagram after operation of the belt-tooth residual film recovery machine.

• Test Results and Analysis

Based on the Box–Behnken experimental design principle, a three-factor, three-level orthogonal experiment was conducted [35,36], comprising 17 sets of regression orthogonal combination tests. Each test was repeated three times, and the average of the three measurements was taken as the final experimental result. The experimental results are presented in

Table 4. Design Scheme and Results of Quadratic Orthogonal Test.

Test No.	X1	X2	X3	Y1/%	Y2/%
1	−1	−1	0	87.86	6.24
2	1	−1	0	83.06	7.89
3	−1	1	0	87.56	8.62
4	1	1	0	85.26	10.05
5	−1	0	−1	89.52	7.52
6	1	0	−1	85.68	9.31
7	−1	0	1	87.85	6.02
8	1	0	1	84.35	7.45
9	0	−1	−1	88.15	7.78
10	0	1	−1	88.25	10.36
11	0	−1	1	85.3	6.36
12	0	1	1	88.27	8.12
13	0	0	0	89.87	6.35
14	0	0	0	89.58	6.24
15	0	0	0	90.75	6.38
16	0	0	0	90.69	6.27
17	0	0	0	91.05	6.31

, where X₁, X₂, X₃ represent the coded values of the factors.

3.3. Regression Model Establishment and Validation

Experimental results were analyzed using Design-Expert 13 software. A p-value < 0.01 indicates a highly significant impact of parameters on the model, while p < 0.05 indicates a significant impact. The analysis of variance (ANOVA) for the quadratic regression model and significance tests of regression coefficients are shown in

Table 5. Analysis of variance.

Source of Variation	Residual Film Recovery Rate Y1				Soil Content Rate Y2
	DOF	Sum of Squares	F1	P1	DOF	Sum of Squares	F2	P2
Models	9	9.83	39.00	<0.0001 **	9	3.55	784.32	<0.0001 **
X1	1	26.06	103.45	<0.0001 **	1	4.96	1095.54	<0.0001 **
X2	1	3.09	12.26	0.0100 **	1	9.86	2176.58	<0.0001 **
X3	1	4.25	16.86	0.0045 **	1	6.16	1360.26	<0.0001 **
X1 X2	1	1.56	6.20	0.0416 *	1	0.0121	2.67	0.1461
X1 X3	1	0.0289	0.1147	0.7448	1	0.0324	7.15	0.0318 *
X2 X3	1	2.06	8.17	0.0244 *	1	0.1681	37.12	0.0005 **
X12	1	27.33	108.48	<0.0001 **	1	1.81	398.89	<0.0001 **
X22	1	15.28	60.66	0.0001 **	1	6.42	1418.11	<0.0001 **
X32	1	4.13	16.39	0.0049 **	1	1.57	345.97	<0.0001 **
Residual	7	0.2519			7	0.0045
Lack of fit	3	0.0606	0.1534	0.9223	3	0.0062	1.92	0.2682
Pure error	4	0.3954			4	0.0032
R2			0.9804				0.9990
R2adj			0.9553				0.9977
C.V.%			0.5715				0.8989

Note: ** denotes highly significant difference (p < 0.01); * denotes significant difference (0.01 < p < 0.05).

.

Analysis of Table 5 indicates that, for the quadratic regression models established for the residual film recovery rate and soil content rate, both P₁ and P₂ values are less than 0.01, denoting statistical significance; meanwhile, the lack-of-fit terms (P₁ and P₂) exceed 0.05, confirming their non-significance. Furthermore, the R² values for the two models are 0.9804 (recovery rate) and 0.9990 (soil content rate), with coefficients of variation (C.V.) of 0.5715% and 0.8989%, respectively, demonstrating that the models explain over 98.04% and 99.90% of response variability. The high correlation between predicted and actual values, minimal experimental error, and congruence of the quadratic regression equations with empirical data confirm that the models accurately characterize the relationships between residual film recovery rate Y₁, soil content rate Y₂, and operational parameters X₁/X₂/X₃, thereby demonstrating robust predictive capability for performance test outcomes.

For the residual film recovery rate: the primary items X₁ (machine forward speed), X₂ (penetration depth of film-lifting teeth), and X₃ (angle of pickup belt relative to horizontal plane) of the model have a significant impact on the residual film recovery rate. The influence of quadratic terms X₁², X₂², and X₃² is very significant. Interaction terms X₁X₂ and X₂X₃ have a very significant impact, while X₁X₃ is not significant. For the soil-containing rate of film: the primary terms X₁, X₂, and X₃ of the model have a significant impact on the soil content rate. The influence of quadratic terms X₁², X₂², and X₃² is very significant. Interaction term X_1×3 has a significant impact, X₂X₃ has a very significant impact, and X₁X₂ has no significant impact. A quadratic regression model of residual film recovery rate Y₁ and film soil-containing rate Y₂ expressed by coded values was obtained after eliminating insignificant items.

$$\begin{array}{l}{\mathrm{Y}}_1=90.39-1.8{\mathrm{X}}_1+0.6213{\mathrm{X}}_2-0.7288{\mathrm{X}}_3+0.625{\mathrm{X}}_1{\mathrm{X}}_2+0.085{\mathrm{X}}_1{\mathrm{X}}_3+0.7175{\mathrm{X}}_2{\mathrm{X}}_3\\ -2.55{{\mathrm{X}}_1}^2-1.91{{\mathrm{X}}_2}^2-0.9903{{\mathrm{X}}_3}^2\end{array}$$

(20)

$$\begin{array}{l}{\mathrm{Y}}_2=6.31+0.7875{\mathrm{X}}_1+1.11{\mathrm{X}}_2-0.8775{\mathrm{X}}_3-0.055{\mathrm{X}}_1{\mathrm{X}}_2-0.09{\mathrm{X}}_1{\mathrm{X}}_3-0.205{\mathrm{X}}_2{\mathrm{X}}_3\\ +0.655{{\mathrm{X}}_1}^2+1.24{{\mathrm{X}}_2}^2+0.61{{\mathrm{X}}_3}^2\end{array}$$

(21)

Equation (20) indicates that the factors affecting residual film recovery rate in descending order are: machine forward speed, angle of the pickup belt relative to the horizontal plane, and penetration depth of film-lifting teeth. Equation (21) shows that the factors affecting soil content rate in descending order are: penetration depth of film-lifting teeth, angle of the pickup belt relative to the horizontal plane, and machine forward speed.

3.4. Analysis on Influence Rule of Each Interaction Factor on Performance

Based on the results in Table 5, the interaction effects of machine forward speed X₁, penetration depth of film-lifting teeth X₂, and pickup belt angle relative to the horizontal plane X₃ on residual film recovery rate Y₁ and soil content rate Y₂ were examined, with response surface plots generated using Design-Expert software.

3.4.1. Analysis of Interactive Factors on Residual Film Recovery Rate

Based on Table 5, the interaction terms X₁X₂ and X₂X₃ have significant effects on the residual film recovery rate Y₁, with the corresponding response surfaces illustrated in Figure 14.

As shown in Figure 14a, with the progressive increase in the forward speed of the machine (X₁), the residual film recovery rate (Y₁) initially increases slowly and then gradually decreases. As the penetration depth of the film-lifting teeth (X₂) increases, Y₁ first increases and then tends to stabilize. This behavior can be attributed to the fact that, when the forward speed of the machine increases, the linear speed of the spike-teeth also increases, enhancing the friction between the tooth tips and the residual film, making film pickup easier. However, if the linear speed of the spike-teeth becomes too high, it may tear the residual film, making it difficult to collect and thus reducing the recovery rate. Given the surface unevenness in cotton fields, increasing the penetration depth of the film-lifting teeth helps lift more residual film to a height favorable for pickup by the spike-teeth, thereby improving recovery efficiency. However, as the ground undulations exist within a limited range, the positive effect of further increasing the penetration depth diminishes beyond a certain point.

As shown in Figure 14b, with the gradual increase in the penetration depth of the film-lifting teeth, the residual film recovery rate initially rises and then gradually declines. Similarly, as the angle between the pickup belt and the horizontal plane increases, the recovery rate first increases slowly and then decreases. Regarding the influence of the belt inclination angle, the initial increase in recovery rate can likely be attributed to a moderate increase in angle σ, which leads to a reduction in angle ψ₁. This reduction helps mitigate the back-transfer phenomenon, where residual film fails to detach from the nail-teeth during the film removal process. However, as angle σ continues to increase, the gravitational component of the residual film acting in the direction along the belt also increases. This results in partial slippage of the film from the belt surface, ultimately leading to a decrease in the recovery rate.

3.4.2. Impact Analysis of Interactive Factors on Soil Content Rate

As shown in Table 5, the interaction term X₁X₃ has a significant effect on the soil content rate Y₂, while the interaction term X₂X₃ exhibits a highly significant effect on Y₂. The corresponding response surfaces are presented in Figure 15.

As shown in Figure 15a, when the forward speed of the machine increases, the belt’s linear speed also increases. As a result, the soil attached to the residual film does not have sufficient time to fall off during the conveyor belt transport and is instead carried into the packing box, leading to an increase in soil content. However, when the inclination angle between the pickup belt and the horizontal plane increases, the gravitational component of the soil acting along the belt direction also increases, making it easier for the soil to fall off. This, in turn, reduces the soil content in the recovered film.

As shown in Figure 15b, with the increase in the penetration depth of the film-lifting teeth, the soil content in the recovered film gradually increases. Conversely, as the angle between the pickup belt and the horizontal plane increases, the soil content initially decreases and then tends to stabilize. Regarding the influence of the penetration depth, a greater depth results in a larger volume of contact between the spring-teeth and the soil. Consequently, more soil is lifted along with the residual film and transported to the packing box, leading to an increase in soil content.

3.4.3. Optimization of Target Parameters and Experimental Verification

Utilizing the constraint optimization solving module in Design-Expert, the optimal parameter combination satisfying the constraints for maximizing residual film recovery rate Y₁ and minimizing soil content rate Y₂ was determined as: machine forward speed X₁ = 5.18 km/h, penetration depth of film-lifting teeth X₂ = 44.4 mm, angle of pickup belt relative to horizontal plane X₃ = 74.8°, with corresponding residual film recovery rate of 90.67% and soil content rate of 6.02%.

To verify the reliability of the model predictions, validation tests were conducted using the optimal parameters for the belt-tooth residual film recovery machine. Due to practical constraints related to the tractor’s operating conditions and power system, the forward speed of the machine was set to 5.2 km/h. As values such as 0.4 mm and 0.8° are difficult to control precisely during actual adjustments, rounding was applied, resulting in a film-lifting teeth penetration depth of 44 mm and a pickup belt angle of 75° relative to the horizontal plane. Each test was repeated three times, and the average of the results was taken as the final outcome. The results are presented in

Table 6. Comparison of predicted and experimentally validated values for test indicators.

Item	Residual Film Recovery Rate/%	Soil Content Rate/%
Predicted Value	90.67%	6.02%
Experimental Value	90.15 ± 0.82%	5.86 ± 0.35%
Relative Error	0.58%	2.73%

.

As shown in Table 6, under the optimal parameter combination, the belt-tooth residual film recovery machine achieved an average residual film recovery rate of 90.15% and an average soil content rate of 5.86%. The relative error between the experimental and optimized values was 0.58% for the recovery rate and 2.73% for the soil content rate. The experimental results closely matched the model predictions, confirming the reliability of the second-order response model. Compared with commonly used machines such as the drum type residual film recovery machine [15,37] (with a recovery rate of 86.1–88.2%) and the chain-tooth type recovery machine [38] (with a recovery rate of 89.54%), the average recovery rate increased by approximately 1%. In comparison to earlier experimental models of residual film recovery machines equipped with integrated packing belts, the soil content in the recovered film bundles was reduced by over 4%, demonstrating a significantly improved film–soil separation effect during the recovery process.

4. Discussion

The belt-tooth residual film recovery machine utilizes belt transmission to drive the pickup roller for film collection, demonstrating a significant advantage in environmental adaptability compared to chain-driven systems. In near-ground operations for residual film recovery, chain drives are susceptible to contamination by soil, fallen cotton, and other debris infiltrating the chain links. To prevent link seizure, frequent cleaning of sediment between the chain links and regular lubrication maintenance are required, thereby increasing operational costs and system complexity. In contrast, the smooth surface of belt transmission effectively avoids these issues. With a more simplified structure, belt transmission offers competitive advantages in terms of manufacturing and maintenance costs.

However, ensuring the long-term reliable operation of the belt-tooth residual film recovery machine hinges on the service life of its core transmission component—the belt. The typical failure mode of the belt is progressive fatigue fracture, which, while relatively predictable and suitable for scheduled replacement, often occurs during machine operation. Therefore, real-time monitoring of belt operational status is crucial. The literature [39] has demonstrated that the operating condition of chain drives can be assessed in real time using localized temperature variation and vibration signals. Building upon this foundation, our future research will focus on belt condition monitoring technologies, aiming to develop effective detection methods and provide a more robust foundation for the reliability design and validation of belt-tooth residual film recovery machines.

5. Conclusions

(1)

A belt-tooth residual film recovery machine was designed, featuring nail-tooth components mounted on belts driven by belt transmission to rotate the pickup roller. To ensure pickup reliability, EDEM software was employed to conduct comparative simulations on the film retrieval efficacy of different tooth profiles, leading to the selection of an optimal tooth structure with superior pickup performance. The film removal mechanism was analytically optimized, determining critical parameters and establishing a belt slope conducive to residual film detachment from tooth tips. Through mechanical analysis of film bundles within the packing box, the inclination angle of packing belts was determined, while partitioned packing belts were engineered to reduce soil content in recovered film bundles.

(2)

Integrating the Box–Behnken experimental design principle, a three-factor, three-level response surface methodology was employed to conduct residual film recovery performance tests on the belt-tooth residual film recovery machine; response surface analysis revealed that factors influencing residual film recovery rate in descending order of significance were machine forward speed, angle of the pickup belt relative to the horizontal plane, and penetration depth of film-lifting teeth, while factors affecting soil content rate in descending order were penetration depth of film-lifting teeth, angle of the pickup belt relative to the horizontal plane, and machine forward speed.

(3)

Utilizing the optimization module of Design-Expert software with residual film recovery rate and film soil content rate as optimization objectives, and taking into account practical operational conditions, the optimal working parameters were determined as follows: a machine forward speed of 5.2 km/h, a penetration depth of the film-lifting teeth of 44 mm, and a pickup belt angle of 75° relative to the horizontal plane. Field validation tests indicated that, under these optimized parameters, the residual film recovery rate reached 90.15%, while the soil content rate was 5.86%. The overall machine performance was excellent, showing a slight improvement in recovery rate and a reduction of more than 4% in soil content compared to commonly used models.