1. Introduction
Peanut is one of the most important cash and oil crops, and it occupies a paramount position in global oil production and trade [
1,
2]. With the restructuring of China’s planting sector, the planting area of peanuts has increased significantly, accelerating the development of scale production [
3,
4]. Therefore, it is imperative to realize an efficient mechanized peanut harvest. The design theory and method of key components of peanut harvesting are inadequate, and the performance parameters of shell picking systems cannot completely adapt to the production conditions in China [
5,
6]. In particular, effective research methods for gaining insight and analyzing local physical phenomena, such as contact displacement and impact force in the peanut harvester shell picking process, are still lacking [
6]. The research and development (R&D) technology reserve of and investment in peanut harvesting machinery are insufficient because the equipment accuracy is not high. Reduction of the damage rate in shell picking and transportation has always been critical in developing peanut mechanization harvesting technology [
7]. The loss caused by collision damage accounts for approximately 15% of the annual total loss. The impact damage to peanuts during harvesting, processing, and transporting is a crucial factor for maintaining the quality and economic value of peanuts [
6,
8,
9]. Most of the peanut harvesting is carried out in two steps in harvesting operations in China. This first involves digging out the peanuts from the soil to dry them using a peanut digger, then the dried peanuts are collected and harvested using a combine. The types of peanut picking are divided into axial flow and tangential flow, which have similar peanut pod picking characteristics. As shown in
Figure 1 (type of tangential flow for peanut picking), peanut seedlings and kernels are fed from the pick-up area and are rotated using a picking drum by grasping and driving via picking spring teeth. Peanut pods break away from the vine and fall through a concave screen to the selected parts due to active forces (strike force, carding-pulling force, and comb-pulling force) from the picking teeth, constraining forces (friction and impact force) from the concave screen, and squeezing and rubbing forces between plants. In addition, peanut plants are discharged from combines, thus completing the fruit picking operation [
10,
11,
12,
13]. The entire process can be considered a process of picking via continuous collisions and contact between the spring teeth, concave screen, and peanut plants. The peanut shells are mainly damaged by contact forces of various forms, such as collision and extrusion, between the peanut pod, vine, and fruit picking mechanism [
12,
13]. However, so far only studies of peanut impact damage have been undertaken and no friction collision content has been included [
7,
10]. In addition, no study investigating the frictional characteristics of peanut harvesting has been reported.
Since the 19th century, research on the characteristics of grain friction has been ongoing, mainly focusing on the highly mechanized production of rice, wheat, corn, soybean, and other staple crops [
14,
15,
16]. There are few studies on the frictional mechanical properties of grains in other countries. However, the effects of collision and friction contact characteristics have not been considered. The reported experimental data are unsuitable for direct application in the production practice of mechanized peanut harvesting in China due to several differences in grain varieties [
17]. While studying the collision and contact mechanisms for harvesting other crops, Lizhang et al. investigated the impact characteristics of threshing materials from the perspective of threshing damage of corn and wheat and reviewed the research progress of the threshing damage of rice [
15,
18]. They established the displacement history and maximum pressure distribution equations between rice seeds and threshing elements from the perspective of contact mechanics. In addition, they obtained the critical velocity between rice and the threshing element when rice cracked or broke. Moreover, the influence of moisture content on the impact damage of rapeseed and wheat has been studied [
19,
20]. Horabik et al. studied the influence of rapeseed on the recovery coefficient of different impact materials based on the viscoelastic Hertz contact model [
21]. Dintwa et al. studied the effect of the viscoelastic coefficient of the collision of two apples on their kinetic energy loss [
22]. Stropek et al. measured the deformation of apples due to impact through high-speed projection [
16]. They used high-speed cameras, mechanical sensors, and two independent measurement systems to evaluate and estimate the dynamic behavior of the impact on a rigid plate and study the effect of different types of material packaging on the impact damage. The above studies mainly focused on the contact damage theory of various crops, such as corn, soybean, wheat, and apple [
15,
16,
19,
20,
22,
23]. However, the mechanism of the impact damage of peanut harvesting has not yet been studied. The physical properties of the impact of peanuts in different directions have also been mentioned in many studies using a universal testing machine, but the friction coefficient during impact contact has been ignored [
24]. Several methods for measuring the coefficient of friction of peanuts have been proposed in other studies, such as the parallel wall method, shear box method, inclined surface method, etc. [
25,
26]. The coefficient of friction used in many studies of peanut harvesting equipment design is usually measured using the inclined surface method [
11,
12,
13]. The coefficient of friction measured by this method is simply the coefficient of sliding friction or rolling friction. However, the contact created between the peanut pod and the picking part is friction with impact during the picking process.
In this study, a peanut-picking impact-friction tester was designed to study the impact and friction characteristics of peanuts from harvesting mechanical parts during peanut harvesting. The parameters of shape, size, and friction characteristics of peanuts with different moisture contents were measured using the peanut-picking impact-friction tester. The variations in friction parameters with the moisture content of peanut pods, contact linear velocity, and invasion depth were analyzed. The difference between impact-friction characteristics of peanuts and different contact materials was also investigated. This study provides a reference for the design of peanut harvesting equipment, research on the damage characteristics of biological impact-friction, and a method of parameter measurement.
2. Materials and Methods
2.1. Specimens of Peanut and Pin
Figure 2 depicts peanut and pin specimens. The peanut specimen is “Dabaisha”, which is a typical peanut variety from the main peanut-producing areas in China. The specimens were collected randomly during peanut maturation. The parameters of moisture content, surface morphology, and weight were calibrated after collecting the specimens.
The two-step process for peanut harvesting is the main harvest method in China. Peanut plants are dug out of the ground for drying first, then collected for peanut picking and cleaning (separation of plant and pod) by combine, thus completing the peanut harvesting.
Full-feed peanut harvesting involves digging, drying, and picking peanuts using peanut combines. The average moisture contents of peanuts for two, three, and four days were 33.8%, 24.5%, and 16.7%, respectively. Therefore, the moisture content of the peanut for testing was set according to the three intervals of 14~16%, 24~26%, and 34~36%. To obtain peanut specimens with different moisture contents, the specimens with the required moisture content were prepared following the drying method of Yan et al. [
27] and the moisture content measurement standard of the national standard [
28]. Hiscan XM micro-computed tomography (Micro-CT) (Suzhou Hiscan Information Technology Co., Ltd., Suzhou, China) was employed to record the three-dimensional (3D) scanning of peanut pods, which was used to compare the surface damage characteristics of peanuts after the experiment. The X-ray tube settings were 60 kV and 133 μA, and images were acquired at 70 μm resolution. A rotation step of 0.5° through a 360° angular range with a 50 ms exposure per step was used. A scanning white-light interferometer (SWLI) (Taylor Hobson, Leicester, UK) was used to observe the surface morphology of the contact area of the pin. A 0.25 mm cutoff with a 10 × objective, 0.3 numerical apertures, and 1 × scanning speed in the XYZ mode (512 × 512 resolution) were used.
The peanut specimens were cleaned with a brush before the experiment to prevent the soil carried on the surface of the peanuts from contributing to contact processes such as impact and friction. All specimens were weighed and labeled after preparation, placed in double-layered sealed bags, and stored in a refrigerator at 26 °C for future use. A schematic of a peanut specimen used in the test is shown in
Figure 2a. Mechanical picking contact part (pin) specimens were made of Q235A steel, 6061 aluminum alloy, and PVC. The size of the pin specimens was 10 mm, and they were processed according to the diameter of the peanut fruit picking spring tooth. To effectively measure the collision and friction contact during fruit picking, the contact end of the pin specimens was processed as a semicircle with a 10 mm diameter. A schematic of a pin specimen used in the test is shown in
Figure 2b.
2.2. Peanut-Picking Impact-Friction Tester
In this study, a peanut-picking impact-friction tester was designed to examine the peanut-picking mechanism of peanut harvesting under different conditions.
Figure 3 shows a schematic and photograph of the tester. The main working parts are the control computer, 1000 W Servo motor (DELTA, ECM-B3M-E21310RS1, Taipei, Taiwan), motor drive (ASD-B3-1021-L, Taipei, Taiwan, dynamic torque sensor (MRN-01) (range: 0 ± 20 Nm; accuracy: <±0.5%), support bearing seat, picking rotary disk, picking spring teeth, peanut specimens, peanut fixed platform, 3D force sensor (SZOBTE, China (CL-TR5S) X: 5/200 N, Y: 5/200 N, Z: 5/200 N), lifting platform, and other parts. The driving shaft was driven by a servomotor to achieve positive and negative rotations and stable speed output under fixed torque to ensure the accuracy of test conditions. The driving shaft transmits power to the rotary disk through the torque sensor, and the connecting part is connected by coupling. The rotary disk was held in place by a bearing support to avoid vibrations or eccentricity due to the impact force in the test process. The peanut specimen was fixed on the platform, and its height could be adjusted by lifting. The power output by the motor drives the pin to rotate and contact with the fixed peanut to produce impact-friction. The fixing claw was coated with silica gel (thickness: 2 mm) to keep the surface of peanut intact when the peanut specimen was being fixed. The dynamic torque sensor was connected to the driving shaft to measure the spindle torque (
T). A 3D force sensor was mounted under the specimen holder to measure the variation in triaxial force (
Fx,
Fy,
Fz). The control and parameter settings of the experiment were realized using Labview programming. Dynamic curves of coefficients of friction and period changes can be identified in the display. The coefficient of friction (
μ) was calculated using the measured force, as follows:
where
r denotes the turning radius and
Fx,
Fy, and
Fz are the applied load forces in the
x-,
y-, and
z-directions, respectively.
2.3. Test Conditions and Methods
In this study, the main exposure conditions of peanuts during harvesting were considered. In particular, the variation characteristics of the surface of the peanut shell after impact-friction were investigated under three conditions: moisture content, invasion depth, and contact material type. The test bench’s speed of 1500 rpm was calculated according to the contact linear velocity (9.8 m/s) of the peanut-picking drum rotating at 274 rpm. Therefore, the contact linear velocity was set to 5, 10, and 15 m/s. Because the moisture content of peanuts at the time of harvest was between 15% and 35%, the moisture level was set to 15%, 25%, and 35% in the experiment. Invasion depths of 1, 2, and 3 mm were set for the impact-friction test. Three types of contact materials (Q235A steel, 6061 aluminum alloy, and PVC) were selected to be processed into pin specimens and compared with peanut in the experiment. All single-factor tests were performed with median values, which were set as 10 m/s, 25%, 2 mm, and Q235A steel.
Table 1 lists the details of the conditions for the impact-friction test.
A multifactor orthogonal test was performed to investigate the three factors (contact linear velocity, moisture content, and invasion depth), which significantly influence the coefficient of friction and wear loss of the peanut pods. The effects of the three factors and three horizontal conditions on the coefficient of friction and wear loss were investigated based on the Box–Behnken experimental design principle [
29]. The experimental scheme included 17 experiments, comprising 12 analysis factors and 5 zero errors. The test data were analyzed by quadratic polynomial regression using Design-Expert software (STAT-Ease Inc., Minneapolis, MN, USA). The correlation and interaction effects among different factors were analyzed using response surface analysis.
The pin specimens were ultrasonically cleaned thoroughly with acetone before each test. The soil on the surface of the peanut was cleaned with high-pressure air currents to ensure the consistency of the surface of the peanut specimens in each test. To investigate the surface wear morphology of pin specimens after collision friction, optical microscope (OM) and SWLI micrographs are shown to identify the wear mechanisms involved.
2.4. Surface Contact Analysis of Impact-Friction
The mechanical properties of peanuts are related to the moisture content; peanuts are brittle and plastic when the moisture content is low and high, respectively. To simplify the theory, the peanut specimen was regarded as a brittle material because it was harvested after drying until its moisture content was low.
The collision and friction processes between the peanut and picking part can be divided into two stages: elastic deformation and damage. The stress distribution in the contact zone during the elastic deformation stage can be derived from the quasistatic Hertz theory [
30,
31]. When the elastic deformation reaches the maximum deformation, the peanut is damaged, forming a stress crack or breakage [
21,
30]. A schematic of the contact and wear areas is shown in
Figure 4. The coefficient elliptic equations are as follows:
where
A and
B denote the coefficients of the elliptic equations of the contact area of the pin and peanut specimens, respectively, and
R1 and
R2 denote the radius of the pin and peanut specimens, respectively.
The dimensions of the contact surface can be expressed as follows:
where
a and
b denote the long and short axes of the ellipse contact surface, respectively.
n1 and
n2 denote the coefficient of Hertzian contact stress deformation [
32].
F represents the normal load,
E1 and
E2 denote Young’s modulus of the pin and peanut specimens, respectively, and
and
denote Poisson’s ratio of the pin and peanut specimens, respectively.
The types of forces mainly affecting peanuts during fruit picking are impact force and friction force. The maximum compressive stress (
) [
30] of peanut shell due to collision is
The center of the two objects is close to the displacement due to elastic deformation during the collision and friction between the peanut and picking part. The contact relative displacement (
δ) is estimated as follows:
The maximum shear stress (
) due to friction is calculated as follows:
Suppose that the yield compressive stress of a peanut shell is , the peanut shell is destroyed under compression when , which is the condition of peanut shell impact failure.
When is greater than the adhesion between the pin and peanut shell or the yield shear stress () of the peanut shell, the peanut shell peels or rubs off under the action of friction, which is the condition of peanut shell friction damage.
The of peanut shells increases with increasing yield compressive stress of peanut shells. Therefore, the extent to which impact or friction damages the peanut shell depends on the ratio of .
4. Conclusions
The impact-friction and wear characteristics of peanut and pin specimens were investigated using a peanut-picking impact-friction tester in this study. The effects of various factors on peanut impact-friction and wear characteristics were evaluated using an orthogonal test with three factors and levels. The picking parts of different materials have a certain influence on the peanut impact-friction. The hardness, strength, plasticity, and toughness of the materials lead to differences in the coefficient of friction. The relationship between the friction coefficient of peanuts and different materials is PVC (about 0.19) > 6061 aluminum alloy (about 0.18) > Q235A steel (about 0.17). From the point of view of friction coefficient alone, the Q235A steel is suitable for peanut picking parts.
The surface tissue composition and moisture of peanut are involved in biological friction behavior and are also the direct factors affecting friction and wear. In the process of peanut shell and pin contact friction, an increase in the contact linear velocity accelerates the appearance of contact surface damage and makes the friction pair reach a stable contact state as soon as possible. The invasion depth increases the contact area between the pin and peanut shell, accelerating the brittle damage of the peanut shell’s contact surface and shedding of the fiber tissue. It also makes peanut shells show different coefficients of friction during the surface tissue shedding. Moisture content is the most significant factor affecting the friction coefficient of peanuts. The moisture in the peanut shell also plays a role in the friction process, affecting the change in the coefficient of friction. The order of influence of the contact linear velocity, invasion depth, moisture content, and other factors on the coefficient of friction of peanuts is as follows: invasion depth > moisture content > contact linear velocity. The friction coefficient of peanut is between 0.15 and 0.21 when the moisture content of peanut is in the range 15–35%. The moisture content of peanuts is a key factor affecting the friction coefficient. The most prominent influence on wear loss is the invasion depth. The range of friction coefficient (0.182~0.187) can be used as a critical point for peanut shell damage in peanut harvesting equipment and simulation analysis. Therefore, a coefficient of friction below 0.182 is helpful for the efficiency of peanut picking. In this study, the biotribological characteristics of peanut and pin were evaluated via impact-friction tests under different conditions. The relevant content and results of this study can provide references for the study of the biotribological characteristics of agricultural crops and the design of peanut harvesting or hulling equipment, and also provide a new method for the impact-friction test that is similar to the peanut picking operation.