Design and dynamic analysis of the profiling mechanism for suspended mowing and flattening machines in hilly and mountainous areas

Abstract

Aiming at poor profiling effect and uneven stubble caused by complex terrain of the suspended mower and flattening machine while harvesting alfalfa in hilly and mountainous areas, a profiling mechanism for a suspended mowing and flattening machine was designed, and the profiling performance of the mechanism on undulating, sloping, potholed and raised roads was simulation analyzed by applying multi-body dynamics software RecurDyn. The results show that by coordinating the force of the suspension spring adjustment device and the profiling device, the profiling mechanism can control the ground pressure of the cutting table within 2000 N, achieving adaptive adjustment on slopes of ± 30°, undulating roads of 250 mm, and raised roads with depressions or protrusions of ± 50 mm. The key technical parameters of profiling springs and suspension springs had been precisely calculated, the maximum working load of the profiling spring is 9014 N, and that of the suspension spring is 10,290 N, meeting the allowable shear stress requirements for Class III loads. The effectiveness of the profiling mechanism was verified by field experiments, and the cutting height was 63.2 mm and the flattening rate was 95.1%. The research results can provide theoretical and technical support for the design and optimization of forage harvesting machinery.

Introduction

Alfalfa (Medicago Sativa L.) is a kind of perennial herb with perennial roots, it is known as the “king of forage” because of its high yield, excellent palatability, high nutritional value and suitable growth in sandy soil with loose soil^1,2,3, and it is an ideal forage source for animal husbandry⁴. The hilly and mountainous terrain is complex with significant variations in slope, but it is widely distributed in China and is an important area for the development of animal husbandry and grassland industries^5,6. Timely harvesting of forage grass is a crucial step in ensuring its yield and nutritional quality⁷. As the core equipment for modern grass harvesting, the mowing and flattening machine crushes the tough stems to make them consistent with the dryness of the leaves and completes multiple processes at one time, reducing labor intensity and time costs significantly⁸. The mowing and flattening machine have become a standard configuration for large-scale alfalfa production bases and modern livestock farms, it plays an irreplaceable role in the production of high-quality alfalfa hay in arid and semi-arid regions. However, the existing equipment has insufficient compatibility with hilly and mountainous areas, with problems such as poor passability, uneven cutting height, and high risk of the cutting blade penetrating the soil^9,10. Therefore, developing specialized mowing and flattening machines that can adapt to hilly and mountainous areas is an urgent need to improve the mechanization level of mountain grass industry and ensure the sustainable development of the grass industry.

The terrain adaptability of the mowing and flattening machine is an important factor to achieve high quality and efficient alfalfa harvest in hilly and mountainous areas. The miniaturization of the entire machine, the installation of sensors, the adoption of a tracked chassis, and the addition of profiling mechanisms are main technical approaches to achieving terrain adaptation at present^11,12,13. The profiling mechanism can adaptively adjust the position and orientation of the cutting table according to terrain relief through elastic elements and linkage mechanisms, which is one of the key technologies to improve cost-effectiveness. The profiling mechanism can convert the gravity of the cutting table into a regulating driving force, effectively controlling the ground pressure and thereby solving problems such as uneven stubble and cutting blades penetrating the soil. Therefore, it is necessary to study the profiling mechanism of mowing and flattening machines in hilly and mountainous areas to improve the quality of harvesting operations.

Existing research has investigated the profiling mechanisms of different agricultural machinery for various working conditions and targets. Liao et al.¹⁴ developed an automatic lawn mower with an image recognition function, and the mower exhibited excellent terrain adaptability through the cooperation of the network recognition system and the tracked chassis. Nie et al.¹⁵ developed a low-loss soybean header based on synchronous profiling and achieved precise control of the cutting device with ground undulation and solved the issues of poor adaptability and high loss in intercropping soybean harvesting. Tian et al.¹⁶ designed a self-propelled alfalfa harvester that can simulate the adaptability of the profiling mechanism to the terrain by adjusting the cutting Angle, achieving precise control of the stubble height. Ma et al.¹⁷ designed a bionic cutter for the common problem of straw damage during alfalfa harvesting and analyzed its cutting performance, significantly improving the flatness of the cutting surface. Zhou et al.¹⁸ proposed the coverage trajectory planning problem on three-dimensional terrain, as well as precise and heuristic algorithms, achieving safe and efficient path planning for the automatic lawn mower on three-dimensional terrain and slope adaptation. However, the planting conditions of alfalfa in hilly and mountainous areas mainly feature large variations in slope, uneven surface, scattered plots and irregular shapes, these conditions are significantly different from other crops in terms of termination conditions and mechanical harvesting requirements. Therefore, the application of the existing profiling mechanisms in alfalfa harvesting equipment for hilly and mountainous areas has problems such as parameter mismatch, inaccurate ground pressure control, and limited universality and adaptability. Thus, the research on profiling mechanisms for hilly and mountainous area lawn mowers and flattening machines with adaptive slope functions remains a technical challenge to be solved. However, in the existing research on alfalfa harvesting profiling mechanisms, Zhu et al.¹⁹ studied the hydraulic profiling system of suspended lawn mowers, and their research results showed that the ground pressure of the cutting table during hydraulic profiling operations can fluctuate within the range of 1700 ~ 2500 N, with a maximum value close to 3000 N, and the uniformity of stubble retention is poor with a flattening rate of less than 92%. This indicates that the existing hydraulic profiling system has the disadvantages of inaccurate grounding pressure control and large fluctuations, which can easily lead to uneven stubble retention and the risk of cutting blades entering the soil. Compared with the above hydraulic profiling scheme, this study aims to design a profiling mechanism based on the synergistic effect of mechanical springs, with the core goal of achieving more accurate and stable control of ground pressure (stabilizing it within 2000 N), and this mechanism can reduce the fluctuation range of ground pressure to 300 ~ 1500 N, while improving the uniformity of stubble retention and flattening rate (≥ 95%), thus solving the technical problems of existing profiling mechanisms in hilly and mountainous areas.

To achieve equipment profiling during mechanized forage harvesting in hilly and mountainous areas, this study adopted a combination of theoretical analysis, simulation, and experimental methods to design a profiling mechanism featuring the collaborative operation of suspension springs and profiling devices based on the agronomic requirements for mechanized alfalfa harvesting in such areas. The dynamic characteristics of profiling mechanism were analyzed via RecurDyn simulation, and field tests were conducted for verification. The research provided technical solutions and theoretical support for the development of profiling mechanisms for high-performance mowing and flattening machines in hilly and mountainous areas.

Materials and methods

Overall machine structure

As shown in Fig. 1, the structure of the suspended lawn mowing and flattening machine for hilly and mountainous areas mainly consists of a suspension frame, a profiling mechanism, a flattening device with adjustable preload force, and a rotary cutting device. The suspended lawn mower and flattening machine for hilly and mountainous areas is equipped with tractor power, which can be folded and lifted, and is easy to transfer and turn around.

Fig. 1

Structure of the suspended lawn mowing and flattening machine: (1) Suspension frame; (2) Profiling mechanism; (3) Adjustable preload force flattening device; (4) Rotary cutting device..

Profiling mechanism design

Structural composition of the profiling mechanism

Through field measurements and terrain data analysis of alfalfa planting areas in hilly and mountainous areas of Gansu Province, it was found that the slope of typical work areas is mostly concentrated between 20° and 35°, and the height difference of surface undulations is generally within the range of 200 ~ 300 mm. The height of local potholes and protrusions is about ± 50 mm. In order to adapt to the characteristics of large surface undulations and significant changes in slope during alfalfa harvesting in hilly and mountainous areas, the design of this copying mechanism needs to meet the following requirements: (1) maintain the cutting table in contact with the ground on a ± 30° slope; (2) Realize adaptive adjustment on 250 mm undulating road surface; (3) Stable operation on uneven/raised road surfaces with a tolerance of ± 50 mm. Therefore, the institution adopts a synergistic effect of a copying device and a suspension spring adjustment device, and achieves the goal of a cutting table grounding pressure of ≤ 2000 N through spring force distribution and control. As shown in Fig. 2, The profiling mechanism mainly consists of a suspension frame, profiling device, suspension spring adjustment device, crossbeam, transmission, cutting table bracket, and profiling spring. Both the profiling device and the suspension spring adjustment device play a crucial role in profiling operations.

Fig. 2

Structure of the profiling mechanism: (1) Profiling device; (2) Suspension spring adjustment device; (3) Suspension frame; (4) Crossbeam; (5) Transmission; (6) Cutting table bracket; (7) Profiling spring. (a) Structure diagram of profiling device, (b) 3D renderings.

As shown in Fig. 3, the profiling device consists of an elastic split pin, a pin shaft, a profiling spring, a spring seat, a lock nut, an adjustment bolt, and a spherical washer. The left end of the profiling device is connected to the cutting table, and the right end is connected to the suspension frame. The thread length of the adjusting bolt is 600 mm, and it is connected to the suspension frame with a spherical washer as the support. The profiling device can automatically adjust the overhang length when the cutting table rises or falls. A slide groove with the same parameters as the profiling spring is cast on the spring seat 1 at the left end of the device. It can be fitted with the profiling spring through a helical connection, achieving the structural simplicity of the profiling device. To achieve the problem of both adjusting the length with bolts and fixing the profiling spring, a conical surface with built-in threaded spring seat 2 and a profiling spring with a closed end were designed.

Fig. 3

Structure of the profiling device: (1) Elastic split pin; (2) Pin shaft; (3) Profiling device; (4) Spring seat 1; (5) Spring seat 2; (6) Lock nut; (7) Adjusting bolt; (8) Spherical washer.

As shown in Fig. 4, the suspension spring adjustment device is composed of a spring seat, an elastic cylindrical pin, a suspension spring, an adjustment rod, a locating pin, an elastic split pin and a suspension connecting rod. The right end of the suspension spring adjustment device is connected to the suspension frame and the left end is connected to the crossbeam, forming a triangular fixation. Among them, the spring seat is externally cast with a slide groove with the same parameters as the profiling spring, which can be fitted with the profiling spring through a spiral connection method. The suspension connection rod is externally consistent with the spring seat and has multiple holes, which can be adjusted according to different working environments to adapt to different undulating road surfaces. A sliding groove with a limit sliding function is provided at the left end of the adjusting rod.

Fig. 4

Structure of the suspension spring adjustment device: (1) Spring seat; (2) Elastic cylindrical pin; (3) Suspension spring; (4) Adjustment rod; (5) Locating pin; (6) Elastic split pin; (7) Suspension connecting rod.

Force analysis of the profiling mechanism

The profiling mechanism needs to adapt to the operation on slopes and undulating road surfaces in hilly and mountainous areas. Its slope self-adaptation performance is mainly achieved through the profiling device, which allows the cutting table to rotate around the hinge point, thereby maintaining the contact between the cutting table and the terrain on a ± 30° slope. The gravity of the lawn mowing and flattening device needs to be balanced to the suspension frame when the lawn mowing and flattening machine performing the rear suspension profiling operation. The profiling device and the suspension spring adjustment device are under force at this time and can only carry out the profiling operation when they are balanced in force. The profiling mechanism needs to adapt to operations on slopes and undulating roads in hilly and mountainous areas. The force conditions during the operation process are shown in Fig. 5.

When the profiling mechanism is in operation, the gravity of the cutting table is overcome by forces F₁ and F₂. To ensure that the machine can perform the profiling function, a force analysis is required:

Fig. 5

Simplified diagram of force analysis for the profiling mechanism.

The main displacement variable of the suspension spring adjustment device is in the Y-axis direction, so it is suitable for adjustment during operations on undulating road surfaces. When working at high or low-lying areas, the up and down adjustment range is H_r=−150 to 150 mm. Therefore, the adjustment is divided into three levels: At −150 mm, 0 mm, and 150 mm. To achieve a better profiling effect, when working at 0 mm, the gravitational force must be completely overcome. When working at −150 mm and 150 mm, the distance between the force arms of the suspension spring adjustment device can be adjusted using positioning pins to change the force on the spring, thereby adapting to different terrains. The main displacement of the floating device is the rotation around point o, so it is suitable for adjustment during operations on slopes. When the floating device operates on a slope, the adjustment range α of the cutting table is ± 30°. Therefore, the adjustment range is divided into three levels: −30°, 0°, 30°. To achieve a better profiling effect, the thread adjustment is set at the end of the profiling device. When operating on different slopes, the entire machine needs to be stopped first, and the force on the profiling spring is controlled by changing the deformation of the spring through the adjusting nut, thereby reducing the force exerted by the cutting table on the ground. Based on the principle of static equilibrium, the force analysis of the cutting table was carried out. The torque balance equation of the cutting table rotating around o point is:

$$\left\{ \begin{gathered} {F_1}{l_3}\cos {\alpha _2}={G_1}{l_2} \hfill \\ {F_2}{l_4}\sin {\alpha _1}={G_2}\left( {{l_4}+{l_1}+{l_2}} \right)+{G_3}{l_5} \hfill \\ \end{gathered} \right.$$

(1)

In the formula, F₁ is the working load of the profiling spring, N; F₂ is the working load of the suspension spring adjustment device, N; G₁ is the gravity borne by the profiling device, N; G₂ is the gravity borne by the suspension spring adjustment device, N; G₃ is the weight of the right-end connection components of the suspension spring adjustment device (pulley assembly, protective cover, coupling, crossbeam, etc.), N; l₁ is the distance from the right-end connection point of the suspension spring adjustment device to the rotational center of the cutting table, mm; l₂ is the distance between the cutting table rotation center and the cutting table center of gravity, mm; l₃ is the distance between the cutting table rotation center and the right-side connection point of the profiling spring, mm; l₄ is the distance between the horizontal beam rotation center and the right-end connection point of the suspension spring adjustment device, mm; l₅ is the distance between the center of gravity of the right-end connection component of the suspension spring adjustment device and the horizontal beam rotation center, mm; α₁ is the angle between the suspension spring adjustment device and the crossbeam, °; α₂ is the Angle between the force arm l₄ of the suspension spring adjustment device and the point of rotation perpendicular to the rotation point of the profiling spring to the vertical direction of the profiling spring, °; α₃ is the angle between l₃ and the horizontal direction, °. The equation establishes the relationship between the force of the imitation spring and the force of the suspension spring adjustment device, where F₁ and F₂ are the internal forces generated by the mechanism to achieve force balance, and their magnitude depends on the external load and the geometric parameters of the mechanism, rather than directly equal to the weight of a component.

Through the above analysis and calculation, the working loads of the profiling spring and the suspension spring adjustment device can be obtained as Eq. (2):

$$\left\{ \begin{gathered} {F_1}=\frac{{{G_1}{l_2}}}{{{l_3}\cos {\alpha _2}}} \hfill \\ {F_2}=\frac{{{G_2}\left( {{l_4}+{l_1}+{l_2}} \right)+{G_3}{l_5}}}{{{l_4}\sin {\alpha _1}}} \hfill \\ \end{gathered} \right.$$

(2)

From the force analysis, it can be known that the gravity of the cutting table is borne by two parts when the profiling mechanism performs profiling operations: the suspension spring adjustment device and the profiling device. Relevant studies have shown that during hydraulic profiling operations, the ground pressure of the cutting table fluctuates within the range of 1700 to 2500 N, with a maximum value approaching 3000 N¹⁹. In order to achieve a better profiling effect for the entire machine, this study presets the gravity of the cutting table(G) is 3796 N, the ground pressure of the cutting table is 1000 N, and the gravity of the profiling device is 2796 N. In order to ensure the consistency of the two materials of the spring, it is determined that the weight of the cutting table borne by the suspension spring adjustment device is 600 N and that of the right end connecting piece is 1470 N, while the profiling device bears 2196 N. The relevant data can be obtained through measurement: l₁ = 384 mm, l₂ = 891 mm, l₃ = 265 mm, l₄ = 861 mm, G = 3796 N, α₁ = 23°, α₂ = 24°, α₃ = 49°, β = 13°, G₁ = 2196 N, G₂ = 600 N, G₃ = 1470 N. Substituting Eq. (1) yields F₁ = 8082 N and F₂ = 6475 N. When the suspension spring adjustment device fully bears the weight of the cutting table or the profile device fully bears the weight of the cutting table, the maximum working loads F_1max = 9014 N; F_2max = 10,290 N are obtained. When the suspension spring adjustment device does not bear the weight of the cutting table or the profiling device does not bear the weight of the cutting table, the minimum working load F_1min = 6474 N, F_2max = 6610 N is obtained. The calculation of F₁ and F₂ not only takes into account the weight of the cutting table but also the gravity distribution of other components. The calculated F_1max = 9014 N and F_2max = 10,290 N were compared with the maximum force on the spring in the RecurDyn simulation (9050 N and 10320 N), and the error was less than 0.5%, verifying the validity of the calculation.

Parametric design of profiling springs

The tension spring is adopted by the suspension spring adjustment device and the profiling device to provide tension for the profiling operation of the cutting table. The springs selected for the suspension spring adjustment device and profiling device are oil-quenched and tempered silicon-manganese alloy spring steel wire of type C according to GB/T 1222–2016 and GB/T 1239.6–1992²⁰: Shear modulus S = 7.8 × 10⁴ MPa, ultimate tensile strength σ_b = 1569 N/mm², the allowable shear stress [τ] = 0.44σ_b = 647 MPa is taken according to Class III load, and its main parameters are as follows:

(1) Spring curvature coefficient

$$K=\frac{{4C - 1}}{{4C - 4}}+\frac{{0.615}}{C}$$

(3)

In the formula, K is the spring curvature coefficient; C is the spring winding ratio, also known as the spring index. According to GB/T 1239.6–1992, it is generally taken as $C=\frac{D}{d} \approx 4\sim 8$.

In the formula, D is the mean diameter of the spring, mm; d is the diameter of the spring wire, mm.

(2) Spring wire diameter

$$d \geqslant 1.6\sqrt {\frac{{{F_{\hbox{max} }}KC}}{{\left[ \tau \right]}}}$$

(4)

In the formula, [τ] is the allowable stress of the spring, MPa, which is selected based on the spring material and load type.

(3) Spring stiffness

$$k=\frac{{{F_{\hbox{max} }}}}{\lambda }$$

(5)

In the formula, λ is the maximum elongation of the spring during operation, mm.

(4) Effective number of spring coils

$$n=\frac{{Sd}}{{8{C^3}k}}$$

(6)

In the formula, n is the effective number of coils of the spring; S is the shear modulus, MPa.

(5) Based on the above calculation results, the following spring parameters can be calculated:

$$\left\{ \begin{gathered} D=Cd \hfill \\ {D_1}=D - d \hfill \\ {D_2}=D+d \hfill \\ H={H_0}+{X_0} \hfill \\ H=nd - {X_1} \hfill \\ \partial =\arctan \frac{p}{{\pi D}} \hfill \\ L=\pi Dn+2\pi D \hfill \\ \end{gathered} \right.$$

(7)

In the formula, D is the mean diameter of the spring, mm; p is the pitch, mm; D₁ is the inner diameter of the spring, mm; D₂ is the outer diameter of the spring, mm; H is the total length of the spring, mm; H₀ is the free length of the spring, mm; X₀ is the installation length, mm; X₁ is the length of the flattened ends, mm; ∂ is the spring pitch angle, L is the spring unfolded length, mm.

(6) Verification of fatigue strength.

According to the spring winding ratio C, refer to Fig. 7.1-4 in the mechanical design manual to obtain the curvature coefficient K value.

$$\left\{ \begin{gathered} {\tau _{\hbox{min} }}=\frac{{8KDF}}{{\pi {d^3}}} \hfill \\ {\tau _{\hbox{max} }}=\frac{{8KD{F_{\hbox{max} }}}}{{\pi {d^3}}} \hfill \\ \end{gathered} \right.$$

(8)

From the working shear stress, it can be obtained:

$$\gamma =\frac{{{\tau _{\hbox{min} }}}}{{{\tau _{\hbox{max} }}}},\:\frac{{{\tau _{\hbox{max} }}}}{{{\sigma _b}}}$$

(9)

The parameters of the profiling spring are determined according to the above calculation process, as shown in Table 1.

Table 1 Main parameters of the profiling spring.

According to the calculation results, using the mechanical design manual diagram, γ = 0.64 and 0.72 and τ_max/σ_b = 0.55, 0.60 intersect at 6 × 10⁴, 10⁴ cycles, it is indicated that the fatigue life N of this spring complies with the number of cycles, thus the spring meets the design requirements.

Results and discussion

Virtual prototype construction

A three-dimensional model was constructed using SolidWorks to study the profiling effect of the suspended lawn mower and flattening machine on various road surfaces in hilly and mountainous areas, this comprehensive model included the suspension frame, safety device, profiling device, suspension spring adjustment device, and hydraulic lifting device. The research obtained the simulation working conditions of the virtual prototype, a sinusoidal waveform was used to simulate the undulating road surfaces of hilly mountainous areas. In SolidWorks, four consecutive sinusoidal waveform undulating road surfaces were constructed^21,22, each with a total length of 10,000 mm, a horizontal road surface total length of 2000 mm, a road surface width of 2000 mm, and a peak height of 250 mm. A 30° slope road surface was constructed to simulate a hilly mountain road surface, with the road surface length matching the undulating road surface length. Potholes and bumps with a height of ± 50 mm were created to simulate potholes and bumps on hilly terrain^23,24, the setting of these simulation conditions ensures that subsequent simulation analysis can directly and effectively verify whether the designed copying mechanism meets the predetermined operational requirements. Export the 3D model assembled in SolidWorks in the X-T format and import it into RecurDyn.

In the Professional module of RecurDyn, the slip pair and drive were set for the suspension frame to realize linear motion according to the actual connection relationship, the rotation pair was set between the suspension frame and the beam, and the key parts of the cutting table and the beam. The spring attribute was defined in the profile mechanism and the contact constraint was set between the cutting table and the road. Add contact constraints between the cutting table and the undulating road surface and set the dynamic friction coefficient to 0.45 and the static friction coefficient to 0.4²⁵. After the constraints were set, the mass properties of each component were defined, the safe forward speed of the entire machine was set to 0.5 m/s, the simulation duration is 23 s, and the step size is 200. Before conducting dynamic simulation, the established virtual prototype model was verified. The attitude and reaction force of the model in static equilibrium under the action of gravity were examined, and it was confirmed that they were consistent with the theoretical analysis. In addition, the static deformation of the spring obtained from simulation was compared with the theoretical calculation value. Before conducting dynamic simulation, the virtual prototype model was double validated: Static balance validation: the static posture of the header under gravity was consistent with the theoretical analysis, and the theoretical calculation value of the reaction force at the rotating hinge point was 1863 N, the simulation value was 1901 N, and the relative error was 2.04%; Spring static deformation verification: The theoretical value of static deformation of the imitation spring is 103.6 mm, the simulation value is 105.2 mm, and the relative error is 1.54%; The theoretical value of static deformation of the suspension spring is 44.1 mm, the simulation value is 43.2 mm, and the relative error is 2.04%. The relative error of all validation indicators is less than 3%, which is within the acceptable range of engineering simulation (≤ 5%), proving the rationality of model construction and parameter settings, and laying the foundation for the reliability of dynamic simulation results. The virtual prototype during RecurDyn calculation is shown in Fig. 6.

Fig. 6

RecurDyn simulation model of a virtual prototype. (a) Undulating road surface simulation model, (b) Simulation model of slope road surface, (c) Simulation model of potholed/raised road surface.

Analysis of simulation results of undulating road surface

The dynamic response curves curves of the vertical displacement of the cutting table, the height of the cutting knife from the ground, the force of the profiling spring, and the deformation of the profiling spring during virtual prototype operation are presented in Fig. 7.

Fig. 7

Virtual prototype undulating road surface simulation data curve. (a) Vertical displacement curves of cutting table, (b) The curves of the cutting knife’s height from the ground, (c) Profiling the force curves of a spring, (d) Deformation curves of profiling spring.

As shown in Fig. 7(a), the cutting table can be adjusted vertically between 210 ~ 480 mm with horizontal displacement changes. The displacement changes rapidly at the troughs of the waveform road surface, and the displacement at the peaks can reach the design value. As shown in Fig. 7(b), the cutting table has a smaller ground clearance at the trough and a larger ground clearance at the peak. Therefore, the upper suspension point should be tightened to tilt the cutting table and reduce the ground clearance when working on waveform road surface.

The force condition and deformation of the profiling spring are important factors affecting the profiling operation. As shown in Fig. 7(c), the profiling spring and suspension spring are compressed when the entire machine passes over an undulating road surface, the profiling springs and suspension springs are stretched when the entire machine passes over a road surface with troughs. Since the up-and-down fluctuations of the cutting table are supported by suspension springs, waveform changes occur with the vibration of the cutting table. The rotation of the cutting table is supported by profiling springs, so as the waveform road surface moves forward, the spring force also changes with the degree of undulation of the waveform road surface. As shown in Fig. 7(d), the calculation results of the spring parameters show that the stiffness coefficient of the profiled spring is greater than that of the suspension spring, so the deformation of the profiling spring during operation is always greater than that of the suspension spring, which satisfies the calculation results. In summary, the profiling device meets practical requirements when operating on waveform road surface.

The right monitoring point is set at the sliding palm near one end of the suspension bracket to verify the balance of the cutting table during the forward movement of the entire machine and monitor the contact force between the cutting table and the road surface under the action of the suspension springs²⁶. The left monitoring point is set at the sliding palm far from one end of the suspension frame to monitor the contact force between the cutting table and the road surface under the action of the profiling spring. The simulation results are shown in Fig. 8. When the entire machine passes through the waveform road surface, the contact force of the road surface changes significantly with time. The contact forces at the left and right monitoring points are mainly concentrated within the range of 0 to 1500 N, and the values between the two monitoring points are basically the same. The results show that the designed profiling device can pass through a 250 mm waveform road surface, proving that the entire machine has good passability on undulating road surfaces in hilly and mountainous areas.

Fig. 8

Road surface contact force curves.

Analysis of simulation results of slope road surface

To verify the slope adaptive performance of the profiling mechanism, a simulation analysis was conducted on a 30° slope pavement. The dynamic response curve of the profiling spring force and the dynamic response curve of the contact force between the cutting table and the slope road surface during the virtual prototype operation are shown in Fig. 9.

Fig. 9

Virtual prototype slope road surface simulation data curves. (a) Profiling the force curves of spring, (b) Road surface contact force curves.

As can be seen from Fig. 9(a), when the cutting table is operating on a sloping road surface, the change in the suspension spring force is small, approaching 5000 N. The spring force changes continuously as the cutting table moves forward. It can thus be concluded that when working on slope road surface, it is the profiling spring that perform the profiling function, while the suspension springs only provide support.

As shown in Fig. 9(b), when the left monitoring point is operating stably, the contact force varies between 600 and 700 N. When the right monitoring point is operating stably, the contact force varies between 100 and 200 N. The contact forces at the left and right monitoring points are mainly concentrated within the range of 0 to 1500 N, meeting the requirements of sloping road surface operations.

Analysis of simulation results of potholes and Raised road surfaces

The vertical displacement of the cutting table, the change curve of the contact force between the cutting table and the potholed or raised road surface, the force of the profiling spring, and the deformation curve of the profiling spring during the operation of the virtual prototype are shown in Fig. 10.

Fig. 10

Virtual prototype pothole road surface simulation data curves.

As shown in Fig. 10(a), the cutting table vertical displacement varies between 430 and 435 mm in response to horizontal displacement, enabling vertical displacement adjustment. Displacement variation peaks at pothole road surface depressions, conforming to design requirements. As shown in Fig. 10(b), when the machine passes over a pothole road surface, the ground contact force changes significantly over time. The ground contact force of the cutting table is mainly concentrated in the range of 300 to 450 N. The results indicate that the designed profiling device can pass over a pothole road surface with a depth of −50 mm, proving that the machine has good passability in hilly and mountainous areas.

As shown in Fig. 10(c), when the entire machine passes over a pothole road surface, the profiling spring and suspension spring are stretched. Since the suspension spring supports the up-and-down movement of the cutting table, it generates waveform changes with the vibration of the cutting table. The profiling spring support the rotational movement of the cutting table, and as the machine moves forward over pothole road surface, the spring force varies with the degree of depression in the terrain. According to the above spring parameter calculation, the stiffness coefficient of the profiling spring is greater than that of the suspension spring, so the deformation of the profiling spring is always greater than that of the suspension spring during operation, as shown in Fig. 10(d), which meets the calculation results. In summary, the profiling spring meets practical requirements when working on pothole road surface.

Fig. 11

Virtual prototype bumpy road simulation data curves. (a) Vertical displacement curve of the cutting table, (b) Road surface contact force curve, (c) Profiling the force curves of spring, (d) Deformation curves of the profiling spring.

As shown in Fig. 11(a), the change of the cutting table in the vertical direction along with the horizontal displacement is between 415 mm and 445 mm. With the change of the road level, the cutting table can realize the displacement adjustment in the vertical direction. As shown in Fig. 11(b), when the whole machine passes through a raised road surface, the contact force of the road surface changes significantly with time, and when the cutting platform quickly passes through a raised road surface with a height of 50 mm, due to the sudden change in terrain, there is a brief peak in the contact force (the maximum instantaneous value is close to 10000 N), which lasts for a very short time, corresponding to the instantaneous process of the cutting platform and the top of the raised impact contact. In the remaining stable operation stages, the average contact force remains stable within the range of 300 ~ 450 N, meeting the design requirement of grounding pressure ≤ 2000 N. The results indicate that the designed profile device can pass through a 50 mm raised road surface, proving that the overall machine has good passability on raised road surface in hilly and mountainous areas.

As can be seen from Fig. 11(c), when working on a raised road surface, the change in the force of the suspension spring is relatively small, approaching 1400 N. When passing over raised road surface, the profiling spring force changes significantly and reaches its minimum value. It can thus be concluded that when the machine is operating on a raised road surface, the profiling spring springs perform a profiling spring function, while the suspension springs only perform a supporting function. From the calculation of spring parameters, it can be known that the stiffness coefficient of the profiling spring is greater than that of the suspension spring. Therefore, during operation, the deformation of the profiling spring is always greater than that of the suspension spring, as shown in Fig. 11(d), which meets the calculation results. In summary, the spring meets the actual requirements when working on the raised road surface.

The influence of operation speed on the performance of profiling mechanisms

The forward speed of the entire machine on a level road surface is set to three gears: 1.8 km/h, 3.6 km/h, and 7.2 km/h. The virtual prototype used in RecurDyn calculations is shown in Fig. 12.

Fig. 12

Level road profiling RecurDyn virtual prototype simulation model.

The curves showing the changes in the profiling spring force and the contact force between the cutting table and the road surface during the virtual prototype operation are shown in Fig. 13.

Fig. 13

Simulation data curves for virtual prototypes at different speeds

As shown in Figs. 13(a), (b), and (c), at the initial stage of operation, the force of the profiling spring has a distinct peak at different speeds. This is attributed to factors such as the initial contact between the profiling mechanism and the road surface, which causes the spring to experience a significant instantaneous force. As the simulation time increases, the force of the profiling spring gradually stabilizes. Since the suspension spring supports the up and down fluctuations of the cutting table, it generates waveform changes with the vibration of the cutting table. The profiling spring provides support for the rotation of the cutting table. Comparing different speeds (1.8 km/h, 3.6 km/h, 7.2 km/h), the force of the profiling spring in the stable stage is relatively higher as the speed increases. It is explained that an increase in speed will cause the profiling spring to bear greater force during operation, possibly because the cutting table responds more violently to road surface undulations at high speeds, resulting in greater spring deformation and increased force.

The monitoring points set at the cutting table are used to verify the balance of the cutting table during the forward movement of the entire machine and to monitor the contact force of the cutting table on the road surface under the action of the profiling spring. The simulation results are shown in Fig. 13(d). The contact force between the cutting table and the road surface also fluctuates to some extent during operation at different speeds, but the overall fluctuation amplitude is relatively smaller than that of the profiling spring force curve. The change in speed affected the magnitude of the contact force between the cutting table and the road surface, the change in contact force becomes more significant as the speed increases.

Test conditions

The actual operational performance of the contouring mechanism of the suspended grass cutting and flattening machine for hilly and mountainous areas was tested and verified in June 2025 in the alfalfa field located in Huining County, Baiyin City, Gansu Province. The subject of the experiment was alfalfa, and the experimental equipment included a Dongfeng Hong 904 tractor, a tape measure, a speed measuring device, etc. The lawn mowing and flattening machine was connected to the tractor through three-point suspension. The scene of the lawn mowing and flattening test is shown in Fig. 14.

Fig. 14

Field test of suspended lawn mowing and flattening machine in hilly and mountainous areas.

Test methods

According to GB/T 21,899 − 2008 “Lawn Mowing and Flattening Machine”²⁷, the cut height and flattening rate were selected as the test performance evaluation indicators of the suspended lawn mowing and flattening machine. During the field experiment, after the prototype passed through the initial stable zone of 20000 mm, five test samples were randomly selected along the forward direction in the stable zone. The actual stubble height, the mass of harvested alfalfa, and the mass of flattened alfalfa were measured respectively, and the evaluation index flattening rate G_p was calculated. The test results were taken as the average of ten measured indicators. The calculation formula is as follows:

$$Gp=\frac{{{M_0}}}{M} \times 100\%$$

(10)

In the formula, G_p is the flattening rate, %; M₀ is the flattened alfalfa mass, g/m²; M is the actual harvested alfalfa mass, g/m².

Results analysis

The results of the cutting height and flattening rate tests are shown in Table 2.

Table 2 Performance test results.

Table 3 Technical requirements for lawn mowing and flattening machines.

During the field experiment, the lawn mowing and flattening machine had good passability on different road surfaces and achieved a good profiling operation effect. The average cutting height was 63.2 mm and the flattening rate was 95.1%. As shown in Table 3, the developed lawn mowing and flattening machine has good adaptability to harvesting operations in hilly and mountainous areas and meets the technical requirements.

Conclusions

A profiling mechanism for lawn mowing and flattening machines was designed to operate in complex terrains of hilly and mountainous areas, which incorporates a suspension spring adjustment device and profiling device working in coordination. Main research findings are as follows:

(1) The designed profiling mechanism achieved precise control of the ground pressure of the cutting table at ≤ 2000 N through mechanical model analysis and optimization of spring parameters, achieved the adaptive adjustment requirements for slopes and undulating road surfaces.

(2) The results of the multi-body dynamics simulation show that under various working conditions such as a sinusoidal wave-tilted road surface (peak height of 250 mm), ± 30° slope, and ± 50 mm potholes and raised road surfaces, the vertical displacement range of the cutting table is 210 to 480 mm. The contact force at the left and right monitoring points remains stable within 0 to 1500 N, and the deformation of the profiling spring is always greater than that of the suspension spring. The rationality of the design and parameter matching of the mechanism has been verified. When the forward speed of the machine increased from 1.8 km/h to 7.2 km/h, the stable value of the profiling spring force also increased. However, the contact force still fluctuated within the safe range (< 1500 N), indicating that the mechanism has good stability under medium and low speed conditions.

(3) The field test results show that the average cutting height of the profiling mechanism during operation is 63.2 mm, and the flattening rate is 95.1%, meeting the technical requirements. This proves that lawn mowing and flattening machine has good passability, profiling accuracy and operation quality in hilly and mountainous areas.

(4) This study verified the effectiveness of the designed copying mechanism through simulation and experimentation, but there are still certain limitations in the work. Although dynamic simulation can effectively predict the macroscopic dynamic behavior of mechanisms, it has not yet been experimentally compared and verified with physical prototypes on dynamic signals such as vibration and dynamic stress; The optimization of spring parameters is mainly based on static force balance and allowable stress, and in the future, multi-objective optimization can be carried out by combining fatigue life and vibration characteristics; This study validated the final operational effectiveness of the mechanism in field experiments, but did not quantitatively test its dynamic adaptability under extremely complex road conditions. Subsequent research will focus on experimental vibration testing, fatigue optimization of key components, and reliability in a wider range of terrains to further enhance the comprehensive performance and engineering applicability of the mechanism.