Design and Test of a Traction Side-Pull Square Straw Bale Pick-Up-and-Stack Truck

Abstract

To address the issues of limited one-time loading capacity, single functionality, and low automation level in existing square straw bale pickers, a large automated square straw bale pick-up-and-stack truck that integrates picking, stacking, transporting, and bundling functions has been developed, combining the technical advantages of one-time field removal and storage of bales. We innovatively designed a side-pulling traction mechanism that can realize the rapid transition between the transporting state and working state of the machine; a picking device that can complete the continuous action of forking, lifting, turning positioning, and de-forking; and a bundling device that can realize the adjustment of the attitude of square straw bales. Response surface tests were conducted on the prototype to determine the key structural and operational parameters, using the bundle completion rate and regular bale rate as evaluation indicators. Regression and significance tests were performed on the machine’s forward speed, chassis frame offset, and the ground clearance of the fork tine to determine the influence and priority of these factors on the evaluation indicators. Through multi-objective function optimization of the regression model, the optimal parameter combination was found to be a machine forward speed of 15.5 km/h, a chassis frame offset of 2126 mm, and a fork tine ground clearance of 225 mm, resulting in a bundle completion rate of 98.85% and a regular bale rate of 96.96%. Subsequent field tests with the optimized parameters showed that at a machine forward speed of 15.5 km/h, a chassis frame offset of 2126 mm, and a fork tine ground clearance of 225 mm, the bundle completion rate was 98.37% and the regular bale rate was 95.83%, meeting the relevant design requirements. This study can provide a reference for the design and development of straw collection and storage machinery.

1. Introduction

China is not only a major producer of grain but also a leading producer of crop residues [1]. In recent years, with the increase in grain output, the volume of crop residues has also risen steadily, making their management a pressing issue [2,3]. Traditional field burning severely impacts the environment, leading to farmland degradation and posing risks to human health. Technological advancements have highlighted the potential value of straw as a resource [4,5]. Collecting and baling straw for removal from the field eliminates the risk of burning at the source, while also enhancing straw utilization, achieving both environmental and economic benefits [6]. As a result, the high-value utilization of straw through field removal has become an inevitable trend [7].

Currently, mechanized straw baling technology in China is becoming increasingly advanced [8]. However, the integrated off-field processing technology for straw is still underdeveloped, which restricts the improvement of efficiency in straw removal operations [9].

Agricultural technology in the developed countries of Europe and North America is highly advanced, with most production processes being fully mechanized and automated. The Model 1037 bale wagon from Case New Holland (Racine, WI, USA) utilizes a hydraulic system to automate bale picking, bundling, transport, and unloading operations, offering a high degree of automation [10]. The fully automated bale stacker from Arcusin company (Madrid, Spain) can be used for a variety of crops. It features a floating pick-up system that automatically steers, and when a bale is detected, the conveyor chain automatically activates to complete the loading process. It can also adjust according to bale size, enhancing its operational adaptability. The square bale picker–stacker produced by the Groupe Anderson (OH, USA) uses a mechanical arm to grab and lift scattered square bales from the field onto the stacking platform. Once the stacking platform is full, the unit is towed by a tractor to a designated location for unloading. Large-scale foreign pick-up equipment is primarily used in regions that produce commercial hay. While these machines offer high loading efficiency, their complex structures and high costs, along with their poor compatibility with China’s forage cropping systems and field conditions, have significantly limited their domestic adoption [11].

Since the 1960s, China’s research institutes have been conducting studies on related machinery, which have improved the operational efficiency of equipment to some extent. The Hohhot Branch of the Chinese Academy of Agricultural Mechanization Sciences (Hohhot, China) developed the 9JK-2.7 small square bale picker, the 9JCK-15 small square bale collector, and the 9DCK-15 small square bale stacker. The 9JK-2.7 bale picker uses ground-wheel-driven chains to lift bales from the ground to a certain height, which the bales automatically roll onto the loading truck for manual stacking. The 9JCK-15 bale collector attaches to a tractor-pulled baler, which stacks baled hay neatly in the collector. Afterward, a hydraulic cylinder-operated unloading mechanism evenly lays the collected bales on the ground. The 9DCK-15 bale stacker is used to grab and lift bale groups collected by the 9JCK-15 bale collector for loading and stacking. Additionally, the CYQ-A small square bale picker, designed by Shandong Chuangyi Agriculture and Animal Husbandry Machinery Co., Ltd. (Dezhou, China), operates by picking up bales from the ground with a primary pick-up device, then conveying them via a lift chain to a central steering mechanism that rotates the bales 90 degrees. The bales then enter a secondary pick-up mechanism for loading. This machine requires manual stacking and supporting transport equipment. To enhance automation, the Shandong Academy of Agricultural Machinery Sciences designed and developed the 9JF-3 automatic bale picker and stacker, which features an automatic control system and hydraulic system for picking, compressing, and tying bales efficiently. The equipment has a maximum load capacity of 3000 kg and can be paired with large transport vehicles, significantly improving operational efficiency [12]. To ensure operational safety, a study about the tipping and unloading mechanism was conducted. The hydraulic cylinder bearing beam was optimized through finite element analysis of the frame, combined with theoretical design, simulation, and field trials [13]. Currently, products such as balers, rakes, and press balers in China meet the basic requirements for domestic mechanized field operations [14,15,16]. However, the problem of lacking equipment which can finish integrated operations for picking, stacking, and transporting bales has remained.

To address these challenges, including limited material loading, single-function operation, and low automation levels in existing square bale pick-up machines, a large-scale automated bale pick-up-and-stack machine, integrating the functions of bale picking, stacking, and bundling, was developed. We innovatively designed a side-pulling traction mechanism that can realize the rapid transition between the transporting state and working state of the machine, a picking device that can complete the continuous action of forking, lifting, turning positioning, and de-forking, and a bundling device that can realize the adjustment of the attitude of square straw bales. This paper details the overall structural design and working principles, followed by the structural design and parameter determination of key components. Theoretical analysis was conducted on the pick-up device, side-pull traction mechanism, and tipping bundling device. Field tests were carried out to determine the optimal working and structural parameters of the machine, aiming to achieve low-cost, low-intensity, and high-efficiency straw removal from the field.

2. Materials and Methods

2.1. The Overall Structure and Working Principles of the Square Straw Bale Pick-Up-and-Stack Truck

2.1.1. Overall Structure

The overall structure of the square straw bale pick-up-and-stack truck is shown in Figure 1. It mainly consists of a chassis frame, side-pull traction mechanism, anti-drift lifting device, pick-up device, stacking device, tipping and bundling device, and hydraulic system.

The machine combines tractor towing with hydraulic transmission, allowing for the rapid and stable completion of all processes, including bale pick-up, stacking, transport, unloading, and bundling. It eliminates the need for multiple devices working in tandem and enables the efficient collection and storage of scattered bales from the field in a single operation, without the need for manual stacking. The machine features a compact structure and operates smoothly and efficiently.

2.1.2. Working Principle

Before operation, the hydraulic cylinder of the anti-drift lifting device extends, allowing the anti-drift wheels to touch the ground, ensuring stability during machine operation. The hydraulic cylinder of the side-pull traction mechanism extends, positioning the tractor to tow the pick-up-and-stacking machine from the side, which can prevent the tractor and the machine from directly running over the bales.

During operation, the tractor tows the pick-up-and-stack truck forward, with the hydraulic system providing power to the hydraulic cylinders of the various components. When picking up square straw bales, the hydraulic cylinder of the pick-up device is contracted, and the fork tines insert into the bale. The hydraulic cylinder of the small pick-up arm extends, lifting the bale to the large pick-up arm. Once two bales are on the large arm, its hydraulic cylinder extends, working with the small arm and fork tines to flip and position the two bales upright on the bundling platform. The hydraulic cylinder, articulated between the chassis frame and the pick-up device, contracts, pushing the bales backward to the stacking device. The stacking device compresses the bales using the contraction of the hydraulic cylinder, adjusting their orientation and arranging them neatly. The fork tines repeat the process until 14 bales (2 layers) are stacked on the bundling platform. At this point, the hydraulic cylinder of the side-pull traction mechanism is contracted, switching the tractor from side traction to a forward transport mode. The anti-drift wheels are lifted off the ground, allowing the entire load of bales to be transported to the designated location. The unloading hydraulic cylinder extends, tipping the bundling platform 90 degrees to complete the unloading process. The main parameters of this pick-up-and-stack truck are shown in Table 1.

2.2. Key Component Design

2.2.1. Side-Pull Traction Mechanism

To adapt to different operating conditions, the square straw bale pick-up-and-stack truck features a side-pull traction mechanism that was designed based on the theory of swing link mechanisms. This mechanism is welded to the chassis frame, enabling rapid transitions between the transporting and working states. As shown in Figure 2, the mechanism mainly consists of a drawbar, side-pull rod, pull rod, sliding bar, and hydraulic cylinder.

Before pick-up operations, the hydraulic cylinder of the side-pull traction mechanism extends, causing the pull rod to move forward along the sliding bar. The pull rod exerts forward thrust on the drawbar through the pin shaft, creating a forward motion tendency in the drawbar. While the tractor remains stationary, the pull rod, through the side-pull rod, positions the chassis frame laterally relative to the tractor, converting the pick-up-and-stack machine to its working state, as shown in Figure 3a, facilitating bale pick-up operations. After the pick-up is completed, the controller commands the solenoid valve to retract the hydraulic cylinder of the side-pull traction mechanism, moving the pull rod backward along the sliding bar. This generates a backward pulling force on the drawbar and acts on the chassis frame through the side-pull rod, converting the pick-up-and-stack machine to its transport state, as shown in Figure 3b, facilitating bale transport operations.

The motion process and force analysis of the side-pull traction mechanism are shown in Figure 4 (position 1 represents the side-pull state, and position 2 represents the straightened state).

A Cartesian coordinate system was established with the forward direction of the pick-up-and-stacking truck as the x₁-axis to analyze the straightening process of the side-pull traction mechanism. Point a represents the tractor’s towing point. F₁ and $F_1^{\prime }$ represent the action and reaction forces. At the moment when the side-pull traction mechanism begins to realign from the side position, F₁ = $F_1^{\prime }$. At the moment of starting to straighten, the resultant force at the pin shaft point O₁ is zero, which leads to the following equation.

(1)$$\begin{gathered} F_2=F·\mathit{cos}{\theta }_1 \\ {F}_1^{\prime }=F·\mathit{sin}{\theta }_1 \\ {F}_1=F_1^{\prime } \end{gathered}$$

where θ₁ is the acute angle between the forward direction of the machine and the drawbar, (°); F is the hydraulic cylinder’s retraction generating a backward pulling force, N; F₂ is the pulling force of the drawbar on the pin shaft at point O₁, N; $F_1^{\prime }$ is the resultant force of the hydraulic cylinder’s pulling force and the drawbar’s pulling force on the pin shaft at point O₁, N; and F₁ is the supporting force of the pull rod on the pin shaft at point O₁, N.

As can be seen from Equation (1), when the working pressure of the hydraulic cylinder is determined, the resultant force $F_1^{\prime }$ of the hydraulic cylinder’s pulling force and the drawbar’s pulling force on the pin shaft at point O₁ increases with the increase in θ₁, and the offset of the chassis frame also increases with the increase in θ₁. Therefore, the resultant force $F_1^{\prime }$ increases as the offset of the chassis frame increases. The chassis frame is directly connected to the pick-up device, exhibiting synchronized offset characteristics. During pick-up operations, if the chassis frame offset is too large or too small, it increases the relative offset of the pick-up device to the bale, resulting in a “fork misalignment” phenomenon, which affects the efficiency of bale pick-up. Additionally, the excessive offset of the chassis frame increases energy loss and wheel wear during the offset process.

The chassis frame ensures the stable movement of the entire machine and plays a critical role in connecting various components and coordinating the safe operation of the machine. The upper part is connected to the tipping and bundling device and the pick-up device, while the lower part is connected to the wheels. Through the tipping and bundling device, it is also connected to the stacking device and must bear the weight of some components as well as the total weight of the bales. Increasing the distance between the left and right wheels of the chassis frame can enhance the machine’s stability during operation [17]. Considering the assembly relationships with various components, operational coordination, and stability, the chassis frame length was set at 5800 mm and the width at 1165 mm.

2.2.2. Tipping and Bundling Device

The tipping and bundling device is a key component of the pick-up-and-stack truck, responsible for ensuring that the loaded square straw bales are unloaded neatly and smoothly onto the ground while preventing safety incidents during the flipping process. A tipping and bundling device designed in this study is shown in Figure 5. It consists mainly of a stacking device, bundling platform, bundling platform hydraulic cylinder, and rear bale fork. The tipping and bundling device is hinged to the chassis frame via a pin shaft, and the double-eye hydraulic cylinder connects both the tipping and bundling device and the chassis frame at its ends. During platform flipping for unloading, the rear bale fork supports the square straw bales, allowing them to drop vertically onto the ground from the truck bed.

When the square straw bale pick-up-and-stack truck transports the material to its destination, the bundling platform hydraulic cylinder extends, working in conjunction with the rear bale fork for bundling. As the platform’s flipping angle increases to 90°, the hydraulic cylinder locks in place, and the tractor pulls the machine forward, allowing the rear bale fork to be withdrawn from the base of the upright bale stack. After unloading is completed, the hydraulic cylinder retracts, and the bundling platform returns to the initial position.

The movement process and force analysis of the tipping and bundling device are shown in Figure 6. A Cartesian coordinate system was established, with the horizontal direction as the x₂-axis and the vertical direction as the y₂-axis.

G represents the combined center of gravity of the square straw bale and platform, ω₂ is the angular velocity of the bundling platform, θ₂ is the angle between the bundling platform and the chassis frame, O₂ is the center point of the pin shaft connecting the bundling platform and the chassis frame, A is the connection point between the chassis frame and the hydraulic cylinder, and B is the connection point between the bundling platform and the hydraulic cylinder piston rod.

Let a = O₂A, b = O₂B, and c = AB. In △O₂AB, the cosine theorem can be applied to obtain the following equation.

(2)$c^2=a^2+b^2-2\mathit{ab}·\mathit{cos}{\theta }_2$

The movement speed v₂ of the hydraulic cylinder piston rod can be expressed by the following equation.

(3)$v_2={\displaystyle \frac{\mathit{dc}}{\mathit{dt}}}={\displaystyle \frac{Q}{\pi ({\frac{D_1}{2})}^2}}={\displaystyle \frac{4Q}{\pi {D_1}^2}}$

where D₁ is the inner diameter of the bundling platform hydraulic cylinder; and Q is the oil inflow into the hydraulic cylinder. This primarily addresses the case of uniform platform rotation, where brief speed fluctuations occur at the beginning and end of the platform’s rotation.

Before the center of mass G of the bundling platform and the hay bales crosses the O₂y₂-axis, the rodless chamber of the hydraulic cylinder acts as the working chamber, with the load exerting a pushing force that provides the primary torque. As the hydraulic cylinder extends, the platform’s tilting angle gradually increases, and once the center of mass G crosses the O₂y₂-axis, the rod chamber becomes the working chamber, and the load applies a pulling force, providing resistance to cushion the descent speed of the platform and hay bales. When the platform operates stably, the net torque from all acting forces in the system equals zero. Assuming the positive direction of the platform’s rotation is clockwise, with point O₂ as the moment center, and neglecting the frictional and inertial forces in the system, the torque balance equation can be established under two conditions as follows.

Before G crosses the O₂y₂-axis, the following equilibrium equation can be established.

(4)$\left\{\begin{array}{c}{M}_p-M_g=0\\ F_{T1}=p_1·{\displaystyle \frac{\pi {D_1}^2}{4}}\\ F_{T1}·asin\delta =M_g\end{array}\right.$

After G crosses the O₂y₂-axis, the following equilibrium equation can be established.

(5)$\left\{\begin{array}{c}-Mp+Mg=0\\ F_{T2}=p_2·{\displaystyle \frac{\pi ({D_1}^2-d^2)}{4}}\\ F_{T2}·asin\delta =M_g\end{array}\right.$

where M_p is the torque generated by the hydraulic cylinder of the bundling platform; M_g is the torque generated by the combined weight of the tipping and bundling device and the bales; p is the operating pressure of the hydraulic cylinder, N; d is the diameter of the hydraulic cylinder piston rod, m; δ is the angle between the axis of the hydraulic cylinder and the chassis frame, (°); and F_T is the hydraulic force (thrust) acting on the piston, N.

The working pressure of the hydraulic cylinder must meet the following equation.

(6)$\left\{\begin{array}{c}{F}_T=Sp={\displaystyle \frac{\pi }{4}}{D_1}^{2}p\gg F_{\max }\\ p=(0.6 ~ 0.7){ p}_n\end{array}\right.$

where S is the piston area, m²; and p_n is the rated pressure of the hydraulic cylinder, also known as nominal pressure, MPa. The rated pressure series for hydraulic cylinders are specified according to the national standard GB/T 7937-2008 [18].

Analysis shows that the working pressure of the hydraulic cylinder decreases as the angle δ increases, with the maximum working pressure occurring when the bundling platform begins to rotate. The efficiency and performance of the hydraulic system are directly influenced by its working pressure, so selecting an appropriate pressure is crucial for improving mechanical efficiency and cost-effectiveness. In practical agricultural machinery applications, medium-pressure systems are typically used, with working pressures ranging from 7 to 21 MPa. Considering the total mass of the hay bales when fully loaded (≤6000 kg), along with the mass of the bundling platform and stacking device, two sets of double-earring hydraulic cylinders were selected to operate simultaneously [19,20], with a working pressure of 17 MPa.

The tipping and bundling device must be able to load 14 large bales, each measuring 1900 mm × 875 mm × 1200 mm, with a maximum total weight of 5500 kg when fully loaded. In accordance with the Regulations on the Administration of Over-limit Transport Vehicles on Highways, which outlines the requirements for the dimensions of transport vehicles [21], the bundling platform was designed with a length of 6820 mm and a width of 1990 mm, and the hydraulic cylinder connection plate was positioned 2350 mm from the rear of the vehicle. The entire structure was welded using Q345 material.

2.2.3. Pick-Up Device

• (1). Design and principle of the pick-up device

To collect, lift, and flip square bales scattered in the field and position them onto the bundling platform for proper arrangement, the square straw bale pick-up device was designed based on the operational principle of an excavator’s digging device as shown in Figure 7. The main components included the fork tines, small pick-up arms, pick-up auxiliary wheels, large pick-up arms, straw bale support frame, large arm auxiliary support structures, straw bale guard plates, and hydraulic cylinders. The fork tines were bolted to the fork arms, the large pick-up arm was hinged to the small pick-up arm through a pin, the straw bale support frame and large arm auxiliary support structures were welded to the large pick-up arm, and the pick-up device was connected to the chassis frame via pull rods.

Before operation, the hydraulic cylinder of the pick-up device retracts, causing the fork arm to lower into the pick-up position. During operation, the fork tines penetrate the square straw bale, and the hydraulic cylinder of the small pick-up arm extends, rotating the small pick-up arm under the push force of the hydraulic cylinder, lifting and flipping the straw bale onto the large pick-up arm. Under the force of gravity, the straw bale slides down along the pick-up auxiliary wheel to the straw bale guard plate. When approaching the next straw bale, the fork arm returns to the pick-up position, and the process is repeated to collect the bale. Once two bales are on the large pick-up arm, the hydraulic cylinder of the pick-up device extends, the fork tines penetrate the straw bale, and the fork tines, fork arm, small pick-up arm, and large pick-up arm work together to flip and position the two straw bales upright on the bundling platform. The hydraulic cylinder hinged to the pull rod and the chassis frame retracts, causing the pick-up device to slide backward along the sliding bar. Under the action of the hydraulic cylinder, the pull rod moves backward along the sliding bar, pushing the straw bales towards the stacking device.

• (2). Analysis of the pick-up force of the pick-up device

To analyze the theoretical pick-up force of the pick-up device, a rectangular coordinate system was established as shown in Figure 8, labeling the large pick-up arm (MK), the large arm hydraulic cylinder (NC), the small pick-up arm (KH), the small arm hydraulic cylinder (DE), the fork arm, and other moving parts. The height of the fork tines from the ground is denoted as h. The self-weight of the components, energy transmission efficiency, and return pressure in the hydraulic cylinders were ignored. The pick-up arms and hydraulic cylinders were considered as two-force members. The pick-up force acts directly on the fork tines, and based on the principles of moment balance and force equilibrium, the theoretical pick-up forces of both the large and small arms were derived [22].

During the pick-up operation, the fork tines penetrate the straw bale, and the hydraulic cylinder’s extension and retraction complete the lifting action to achieve the pick-up task. During this process, the resistance of the fork tines penetrating the straw bale, the friction between the fork tines and the straw bale, and the weight of the straw bale constitute the resistance faced by the small pick-up arm during operation. The resultant force is the theoretical pick-up force of the small pick-up arm, and the force acting on the small pick-up arm varies under different working conditions. To analyze the theoretical pick-up force of the small pick-up arm, as shown in Figure 9, there is no relative rotation between the small pick-up arm and the fork arm when the fork tines penetrate the straw bale. Therefore, the small pick-up arm and the fork arm were considered as a whole for analysis. The thrust exerted by the small pick-up arm hydraulic cylinder on the entire structure causes it to rotate around point K.

The values of h₁ and h₂ are obtained from Figure 9. Taking point K as the moment center, the following equilibrium equation can be established.

(7)$\left\{\begin{array}{c}\sum M_K=0\\ F_{c3}\times h_1+F_3\times h_2=0\\ F_{c3}={\displaystyle \frac{F_3\times h_2}{h_1}}\end{array}\right.$

where F_c3 is the theoretical picking force of the small pick-up arm, N; F₃ is the thrust force of the small arm hydraulic cylinder, N; h₁ is the moment arm of the theoretical picking force of the small pick-up arm about point K, m; and h₂ is the moment arm of the thrust force of the small arm hydraulic cylinder about point K, m.

The maximum force output by the small pick-up arm hydraulic cylinder can be determined by the following equation.

(8)$F_3={\displaystyle \frac{\pi · P{D_2}^2}{4}}$

Based on experience, the inner diameter of the small pick-up arm hydraulic cylinder was set as D₂ = 100 mm, and the working pressure P was set as 17 MPa. By substituting these values into Equations (7) and (8), the theoretical pick-up force of the pick-up arm can be calculated.

The thrust of the large pick-up arm hydraulic cylinder acts on the large pick-up arm, driving the small pick-up arm and fork arm to rotate simultaneously around point M. A force analysis of the theoretical pick-up force on the large arm was conducted, as shown in Figure 10.

The values of h₄, h₅ and h₆ were obtained from Figure 10. Taking point M as the moment center, the following equilibrium equation can be established.

(9)$\left\{\begin{array}{c}\sum M_M=0\\ F_4\times h_4+F_{c4}\times h_{3 }=0\\ h_3=\sqrt{{h_5}^2+{h_6}^2-2h_5{h}_6·\cos {\theta }_3}\end{array}\right.$

The maximum force output by the large pick-up arm hydraulic cylinder can be determined by the following equation.

(10)$\left\{\begin{array}{c}{F}_{c4}={\displaystyle \frac{F_{4 }\times h_4}{h_3}}\\ F_4={\displaystyle \frac{\pi ·P{D_3}^2}{4}}\end{array}\right.$

where F_c4 is the theoretical pick-up force of the large pick-up arm, N; F₄ is the thrust of the large pick-up arm hydraulic cylinder, N; h₃ is the moment arm of the theoretical pick-up force of the large pick-up arm relative to point M, m; h₄ is the moment arm of the hydraulic cylinder thrust relative to point M, m; h₅ is the distance from the hinge point M to the hinge point K on the large pick-up arm, m; h₆ is the distance from the hinge point K on the large arm to the tip of the fork tines, m; and θ₃ is the angle between the line connecting point K to the tip of the fork tines and the large pick-up arm, (°).

Based on practical experience, the diameter of the large arm hydraulic cylinder was selected as D₃ = 80 mm, with the working pressure P set at 17 MPa. By substituting these values into Equations (9) and (10), the theoretical pick-up force of the large arm can be obtained.

Taking into account the weight and shape of a single square straw bale, in order to ensure that the pick-up device provides a stable pick-up performance and reliable subsequent stacking while maintaining safe and efficient operation during the straw bale forking process to prevent bale drop [23], the dimensions were set as follows: the length of the large pick-up arm MK was 2100 mm, the length of the small pick-up arm KH was 920 mm, the fork arm was 470 mm, and the fork tines were 1100 mm long. The fork tines were made of 65Mn steel, with a small end diameter of 12 mm and a large end diameter of 35 mm, and the distance between the two fork tines was 960 mm. Comprehensive analysis shows that as the hydraulic cylinder of the pick-up device extends, the angle between the line connecting point K to the tip of the fork tine and the large pick-up arm increases, while the theoretical pick-up force of the large arm decreases. Additionally, as θ₃ increases, the height of the fork tines from the ground increases. If the fork tines are too high off the ground, it will cause a significant displacement relative to the center of the bale, leading to uneven force distribution on the bale. This can result in reduced straw bale compaction after pick-up, negatively affecting operation efficiency.

2.2.4. Hydraulic System of the Square Straw Bale Pick-Up-and-Stack Truck

To improve operational efficiency, a hydraulic system for the square straw bale pick-up-and-stack truck was designed. This system mainly includes a three-position four-way solenoid directional valve, hydraulic cylinders, hydraulic pump, hydraulic lock, and an integrated control handle.

The traditional mechanical control method uses levers to achieve operations, but it cannot ensure the accuracy and completeness of the actions. This machine adopts hydraulic control to replace the traditional mechanical control mode, improving the flexibility of mechanical actions and overcoming the shortcomings of mechanical control. The working principle of the hydraulic system for the square straw bale pick-up-and-stack truck is shown in Figure 11. The hydraulic system’s rated working pressure was kept within 17 MPa. The high-pressure oil output from the hydraulic pump was directed by the solenoid directional valve to control the oil circuit’s flow direction. High-pressure oil drives the hydraulic cylinders to perform operations such as the flipping of the pick-up device, compression of the stacking device, flipping of the bundling platform, movement of the side-pull traction mechanism, and movement of the anti-drift lifting wheel.

2.3. Pick-Up Performance Test of the Square Straw Bale Pick-Up-and-Stack Truck

2.3.1. Test Factors

The preliminary tests revealed that the main factors affecting the overall operational efficiency of the machine include the machine’s forward speed, the offset of the chassis frame, the ground clearance of the pick-up device’s fork tines, and the ground smoothness during bundling. Wheat straw bales with a moisture content of 14% to 16%, naturally dried, were selected for the pick-up-and-stacking tests [24]. Based on the actual operating conditions of the bundling process, the tests were conducted on loamy soil with good smoothness. The final experimental factors were determined to be the machine’s forward speed, the chassis frame offset, and the ground clearance of the fork tines.

A higher forward speed of the machine allows the fork to penetrate deeper, effectively reducing the impact of the fork tines on the shape of the straw bale during pick-up, resulting in better straw bale pick-up performance. If the forward speed is too slow, the fork tines will penetrate too shallowly, making pick-up difficult. To ensure operational efficiency, the forward speed should not be less than 14 km/h. Considering the average field operating speed of the baler, the forward speeds selected for the tests are 14, 15, and 16 km/h.

A tractor with power output suitable for the square straw bale pick-up-and-stack truck was selected for the operation. Studies show that tractors with an output power of 120 kW generally have a maximum width ranging from 2000 mm to 2400 mm. Based on the tractor’s dimensions and the size of the large square straw bales, combined with previous tests, the chassis frame offset was set at 2050, 2100, and 2150 mm.

When the hydraulic cylinder of the pick-up device retracts, the height of the fork tines above the ground also affects the shape of the straw bale during pick-up. Based on the force analysis of the pick-up device and previous tests, the height of the fully retracted tines above the ground was set at 175, 200, and 225 mm.

2.3.2. Test Materials and Conditions

The experiment was conducted at the straw bale picking test base of Jining Shengdi Agricultural and Forestry Machinery Co., Ltd., located in Guocang Town, Wenshang County, Jining City, Shandong Province, China. The test used wheat straw bales tied by an AGCO Massey Ferguson MF 2270XD baler, with bale dimensions of 1900 mm × 900 mm × 1200 mm. The weight of a single straw bale ranged from 300 to 400 kg. The test site is shown in Figure 12.

2.3.3. Test Methods

According to GB/T 25423-2023, rectangular bale baler, the bundle completion rate S_k after pick-up and the regular bale rate S_g were selected as the test evaluation indicators [25,26]. A bale was considered a complete bundle when it was picked up and stacked on the bundling platform without falling apart. After 2 h of continuous operation, the total number of straw bales picked up and the number of scattered bales were recorded. The bundle completion rate can be expressed by the following equation.

(11)$S_k={\displaystyle \frac{I_d-I_{\mathrm{ss}}}{I_d}}\times 100\%$

where I_d is the cumulative number of picked straw bales; and I_ss is the cumulative number of scattered straw bales.

To measure the length of the four long edges along the length of the straw bale, the difference between the maximum and minimum values should not exceed 10% of the average length of the long edges. Bales meeting this criterion were classified as regular; otherwise, they were considered irregular. S_g is an assessment of the dimensions of individual bales. The regular bale rate can be expressed by the following equation.

(12)$S_g={\displaystyle \frac{I_{\mathit{gc}}-I_{\mathit{gb}}}{I_{\mathit{gc}}}}\times 100\%$

where I_gc is the cumulative number of tested straw bales; and I_gb is the cumulative number of irregular straw bales.

3. Results and Discussion

3.1. Optimization of the Experiment

3.1.1. Establishment of the Regression Model

The working parameters were analyzed using a three-factor, three-level Box–Behnken response surface method in Design-Expert10.0 software. Each test was repeated three times, resulting in a total of 17 test groups, with 12 groups serving as factorial points and 5 as center points. The center point tests were repeated multiple times to estimate the experimental error. The factor and level codes are shown in Table 2, and the test scheme and results are presented in Table 3. A, B, and C represent the coded values for the machine’s forward speed, chassis frame offset, and ground clearance of the fork tines, respectively.

3.1.2. Analysis of Variance

Using Design-Expert software, variance analysis was performed on the experimental results. A second-order polynomial response surface regression model was obtained, with each factor as the independent variable and the bundle completion rate (S_k) and regular bale rate (S_g) as the response functions.

• (1). Bundle completion rate

Variance analysis was conducted on the test results of the bundle completion rate, as shown in Table 4. The results indicate that the overall test results were highly significant (p < 0.01). The machine’s forward speed, chassis frame offset, and ground clearance of the fork tines all had highly significant effects on the bundle completion rate. The interaction term between the chassis frame offset (B) and ground clearance of the fork tines (C) had a significant impact on the bundle completion rate, and the quadratic terms of each factor had highly significant effects. Other terms had no significant impact on the response value. The order of the factors’ influence on the response value was A, B, and C. After removing the insignificant factors, the regression equation for the influence of each factor level on the bundle completion rate was obtained as follows.

(13)$S_k= 94.59 + 3.21A+ 2.12B+ 1.52C-1.33BC-6.26A^2-4.05B^2+ 3.63C^2$

The lack-of-fit test for variance showed p = 0.1229, which was not significant (p > 0.05), indicating that the experimental analysis results were reasonable and that the regression equation had a high degree of fit. The model’s coefficient of determination (R²) was 0.9806 before removing the non-significant factors and 0.9705 afterward, indicating that the model could explain more than 97% of the evaluation metrics. Therefore, the working parameters of the square straw bale pick-up-and-stack truck can be optimized using this model.

• (2). Regular bale rate

The overall test results for the regular bale rate were highly significant (p < 0.01). The machine’s forward speed, chassis frame offset, and ground clearance of the fork tines all had significant effects on the regular bale rate. The interaction between the machine’s forward speed (A) and the ground clearance of the fork tines (C), as well as the quadratic term of the machine’s forward speed (A), had a highly significant effect on the regular bale rate. The interaction between the chassis frame offset (B) and tine ground clearance (C) had a significant effect, while the other terms had no significant impact on the response variable.

The order of influence of each factor on the response value was A, C, B. After removing the insignificant factors, the regression equation for the influence of each factor level on the regular bale rate was obtained as follows.

(14)$S_g=95.25 + 3.06A+ 1.15B-1.61C+ 1.40\mathit{AC}+ 0.98\mathit{BC}-1.44A^2$

The lack-of-fit test for variance showed p = 0.0539, which is not significant (p > 0.05), indicating that the experimental analysis results are reasonable and that the regression equation has a high degree of fit. The model’s coefficient of determination (R²) was 0.9777 before removing the non-significant factors and 0.9544 afterward, indicating that the model could explain more than 95% of the evaluation metrics. Therefore, the working parameters of the square straw bale pick-up-and-stack truck can be optimized using this model.

3.1.3. Response Surface Analysis

The experimental results were analyzed using Design-Expert software, and the response surfaces for the interaction effects of the machine’s forward speed, chassis frame offset, and ground clearance of the fork tines on the bundle completion rate and regular bale rate were obtained, as shown in Figure 13.

At a machine forward speed of 15 km/h, the effect of the interaction between the chassis frame offset and ground clearance of the fork tines on the bundle completion rate is shown in Figure 13a. The bundle completion rate initially increases and then decreases with the increase in chassis frame offset, while it initially decreases and then increases with the increase in fork tine ground clearance. In scenarios with a higher bundle completion rate, the optimal range for fork tine ground clearance is 175~187.5 mm and 212.5~225 mm, while the optimal range for chassis frame offset is 2075~2125 mm.

When the chassis frame offset is 2100 mm, the interaction between the machine’s forward speed and the ground clearance of the fork tines affects the regular bale rate, as shown in Figure 13b. The regular bale rate is positively correlated with the machine’s forward speed and negatively correlated with the ground clearance of the fork tines. When the machine’s forward speed is 15 km/h, the interaction between the chassis frame offset and ground clearance of the fork tines affects the regular bale rate, as shown in Figure 13c. The regular bale rate is positively correlated with chassis frame offset and negatively correlated with the ground clearance of the fork tines.

3.1.4. Parameter Optimization

To achieve optimal operating performance parameters, the regression model was optimized and solved. Based on the actual working conditions and operational requirements, objective functions and constraints were selected. The objective functions and constraints are as follows.

(15)$\left\{\begin{array}{l}max{S}_k(A,B,C)\\ max{S}_g(A,B,C)\\ s.t.\left\{ \begin{array}{c}-1\ll A\ll 1\\ -1\ll B\ll 1\\ -1\ll C\ll 1\end{array}\right.\end{array}\right.$

The optimal parameter combination was determined to be a machine forward speed of 15.5 km/h, chassis frame offset of 2126 mm, and fork tine ground clearance of 225 mm. Under these conditions, the bundle completion rate was 98.85%, and the regular bale rate was 96.96%.

3.2. Field Test Validation

To verify the reliability of the optimized parameters, field tests were conducted with the following set parameters: a machine forward speed of 15.5 km/h, chassis frame offset of 2126 mm, and fork tine ground clearance of 225 mm. The tests were performed five times, and the average values obtained were a bundle completion rate of 98.37% and a regular bale rate of 95.83%. The test results were very close to the predicted values, confirming the reliability of the model. The machine meets the requirements for straw bale collection operations.

3.3. Discussion and Analysis

The bale pick-up-and-stack truck designed in this paper integrates picking, stacking, transporting, and bundling functions. Compared to domestic bale pickers, it eliminates the need for manual stacking and supporting transport vehicles, allowing for unloading without the collaboration of multiple machines. This enhancement improves the efficiency of bale removal from the field, lowers costs, and ensures the safety of the operators. The machine is fully functional, comparable to large foreign machinery, and is suitable for China’s forage planting models.

4. Conclusions

(1) A trailed square straw bale pick-up-and-stack truck was designed, with the entire machine operating based on a hydraulic system to complete the tasks of picking, stacking, transporting, unloading, and bundling square straw bales. The machine primarily consists of key components such as the picking device, chassis frame, and tipping and bundling device. It offers the technical advantage of neatly stacking field straw bales at the destination in a single operation, enabling the rapid collection of square straw bales and maximizing the potential benefits of the straw bales.

(2) The Box–Behnken response surface methodology was used to analyze the effects of the machine’s forward speed, chassis frame offset, and the ground clearance of the fork tines on the bundle completion rate and regular bale rate. An optimization model was established, and the accuracy of the model and optimization results were verified through experiments. The influence of the experimental factors on the bundle completion rate, from greatest to least, was as follows: machine forward speed, chassis frame offset, and fork tine ground clearance. For the regular bale rate, the order of influence was the machine forward speed, fork tine ground clearance, and chassis frame offset. The optimal parameter combination was determined to be a machine forward speed of 15.5 km/h, chassis frame offset of 2126 mm, and fork tine ground clearance of 225 mm. Field tests showed that, under this combination, the bundle completion rate was 98.37%, and the regular bale rate was 95.83%, meeting the operational requirements for straw bale picking, with overall good machine performance.

Author Contributions

Conceptualization, Z.L. and S.W.; methodology, Y.Z. and P.D.; validation, Y.Z. and X.L.; investigation, P.D. and Z.L.; data curation, Y.Z. and S.L.; writing—original draft preparation, Y.Z.; writing—review and editing, P.D.; visualization, Y.Z.; supervision, Z.L.; project administration, P.D.; funding acquisition, P.D. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available during the substantive examination process of relevant patents.

Acknowledgments

The authors would like to appreciate the technical editor and anonymous reviewers for their constructive comments and suggestions on this study.

Conflicts of Interest

Author Zetan Lu was employed by the company Jining Shengdi Agricultural and Forestry Machinery Co. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Overall structure diagram of square bale pick-up palletizer, where 1 is the side-pull traction mechanism; 2 is the large pick-up arm; 3 is the chassis frame; 4 is the stacking device; 5 is the tipping and bundling device; 6 is the rear fork stop; 7 is the anti-drift lifting wheel; 8 is the pick-up arm support mechanism; 9 is the pick-up auxiliary wheel; 10 is the hydraulic hose guard; 11 is the small pick-up arm; and 12 is the fork tines.

Figure 2. Structure diagram of side-pull traction mechanism, where 1 is the handle; 2 is the drawbar; 3 is the side-pull rod; 4 is the pin shaft; 5 is the front fork sliding bar fixing plate; 6 is the sliding bar; 7 is the pull rod; 8 is the shaft sleeve; and 9 is the hydraulic cylinder.

Figure 3. Diagram of pick-up-and-stack truck in different working conditions, where 1 is the tractor; 2 is the drawbar; 3 is the square straw bale; 4 is the pick-up device; 5 is the side-pull traction mechanism; and 6 is the chassis frame. (a) Working state of the square straw bale pick-up-and-stack truck. (b) Transport state of the square straw bale pick-up-and-stack truck.

Figure 4. Movement process and force analysis diagram of the side-pull traction mechanism.

Figure 5. Structure diagram of tipping and bundling device, where 1 is the stacking device; 2 is the bundling platform; 3 is the hydraulic cylinder connecting plate; 4 is the pin shaft; 5 is the rear bale fork; and 6 is the bundling platform hydraulic cylinder.

Figure 6. Movement process and force analysis diagram of the tipping and bundling device.

Figure 7. Structure diagram of pick-up device, where 1 is the fork tines; 2 is the small pick-up arm; 3 is the fork arm; 4 is the pick-up auxiliary wheel; 5 is the large pick-up arm; 6 is the straw bale support frame; 7 is the large arm auxiliary support structure; 8 is the large arm hydraulic cylinder; 9 is the straw bale guard plate; and 10 is the pull rod connection plate.

Figure 8. Pick-up device marking diagram, where the green arrow indicates the rectangular coordinate system.

Figure 9. Theoretical pick-up force analysis diagram of the small pick-up arm, where the green arrow indicates the direction of the action force.

Figure 10. Theoretical pick-up force analysis diagram of the large pick-up arm, where the green arrow indicates the direction of the action force, the purple arrow indicates the θ3.

Figure 11. Hydraulic system principle diagram of square straw bale pick-up-and-stack truck, where 1 is the pressure gauge; 2 is the flow divider and relief valve; 3 is the oil pump; 4 is the oil tank; 5 is the small arm hydraulic cylinder; 6 is the large arm hydraulic cylinder; 7 is the anti-drift lifting device hydraulic cylinder; 8 is the pick-up device pushing hydraulic cylinder; 9 is the side-pulling traction mechanism hydraulic cylinder; 10 is the right stacking device hydraulic cylinder; 11 is the left stacking device hydraulic cylinder; 12 is the bundling platform hydraulic cylinder; 13 is the hydraulic lock; and 14 is the solenoid directional valve.

Figure 12. Field tests of square straw bale pick-up-and-stack truck. (a) Diagram of the process of picking up square straw bales. (b) Diagram of the process of stacking square straw bales. (c) Diagram of the process of bundling square straw bales.

Figure 13. Response surfaces of interaction factors to indexes. (a) the fork tine ground clearance and the chassis frame offset on the bundle completion rate; (b) the fork tine ground clearance and the machine’s forward speed on the regular bale rate; (c) the fork tine ground clearance and the chassis frame offset on the regular bale rate.

Table 1

The main parameters of the pick-up-and-stack truck.

Item	Value
Overall size (L × W × H mm)	9500 × 2160 × 3220
Total machine weight (kg)	5000
Tractor power (kw)	110
Maximum towing speed (km·h−1)	25
Maximum load capacity (kg)	9000
Square straw bale size (L × W × H mm)	1900 × 900 × 1200
Number of straw bales per load	14

Table 2

Experiment factors and codes.

Coding	Factor Level
	Machine’s ForwardSpeed A/(km·h−1)	Chassis Frame Offset B/(mm)	Fork Tine Ground Clearance C/(mm)
1	14	2050	175
0	15	2100	200
−1	16	2150	225

Table 3

Test scheme and results.

No.	Factor Level			Sk/%	Sg/%
	A	B	C
1	−1	−1	0	78.83	88.4
2	1	−1	0	86.12	95.27
3	−1	1	0	83.39	93.36
4	1	1	0	88.79	97.14
5	−1	0	−1	85.42	93.92
6	1	0	−1	93.82	98.02
7	−1	0	1	91.99	87.33
8	1	0	1	96.61	97.03
9	0	−1	−1	89.71	96.56
10	0	1	−1	97.23	96.98
11	0	−1	1	93.77	91.97
12	0	1	1	95.97	96.29
13	0	0	0	94.26	94.98
14	0	0	0	94.09	95.74
15	0	0	0	93.76	95.13
16	0	0	0	95.51	94.66
17	0	0	0	95.31	94.94

Table 4

Analysis of variance of performance index.

PerformanceIndex	Source	Sum of Squares	Degree ofFreedom	MeanSquare	F Value	Significance Level p
Bundle completion rate	Model	433.86	9	48.21	39.33	<0.0001 **
	A	82.63	1	82.63	67.41	<0.0001 **
	B	35.91	1	35.91	29.30	0.0010 **
	C	18.48	1	18.48	15.08	0.0060 **
	AB	0.89	1	0.89	0.73	0.4216
	AC	3.57	1	3.57	2.91	0.1316
	BC	7.08	1	7.08	5.77	0.0473 *
	A 2	164.83	1	164.83	134.47	<0.0001 **
	B 2	68.95	1	68.95	56.25	0.0001 **
	C 2	55.50	1	55.50	45.28	0.0003 **
	Residual	8.58	7	1.23
	Lack of fit	6.17	3	2.06	3.41	0.1335
	Pure error	2.41	4	0.60
	Total	442.44	16
Regular bale rate	Model	135.81	9	15.09	34.17	<0.0001 **
	A	74.73	1	74.73	169.22	<0.0001 **
	B	16.73	1	16.73	37.89	0.0005 **
	C	20.67	1	20.67	46.81	0.0002 **
	AB	2.39	1	2.39	5.41	0.0530
	AC	7.84	1	7.84	17.75	0.0040 **
	BC	3.80	1	3.80	8.61	0.0219 *
	A 2	8.99	1	8.99	20.36	0.0028 **
	B 2	0.031	1	0.031	0.071	0.7977
	C 2	0.84	1	0.84	1.90	0.2107
	Residual	3.09	7	0.44
	Lack of fit	2.45	3	0.82	5.07	0.0754
	Pure error	0.64	4	0.16
	Total	138.90	16

Note: ** indicates highly significant (p < 0.01); * indicates significant (0.01 ≤ p < 0.05).

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