1. Introduction
Rice is the staple food source for almost half of the world’s population. Asia accounts for 90% of the world’s rice production and consumption [
1,
2,
3,
4]. As the world’s first country to cultivate rice, China has the largest rice yield and unit yield. Rice accounts for approximately 65% of China’s food consumption, meeting the basic survival needs of its population [
5,
6]. However, due to population growth, social development, and climate change, improving the unit yield of rice has become a key focus of rice production research [
7,
8,
9]. China has always adhered to the strategy of grain storage and the adoption of new agricultural technology to ensure national food security [
10,
11,
12,
13]. High-yielding rice was first planted in 1996 and has become increasingly popular. For example, the yield per ha of the “Liangyou 293” and “Nanjing Jinggu” rice varieties has reached 12,636.6 kg [
14] and 13,013.8 kg [
15], respectively. Furthermore, the unit yield of rice has increased substantially. For example, high-yielding rice cultivation areas covered 2.55 million ha and 8.65 million ha in 2005 and 2013, respectively. They increased to 0.098 billion ha by the end of 2018 due to China’s promotion of high-yielding rice [
16,
17].
The characteristics of high-yielding rice and ordinary rice are very distinct. For example, the former has a higher unit yield, larger panicles, more grains, denser growth, greener stems, lusher leaves, higher water content, and stronger disease resistance than the latter [
18,
19,
20,
21,
22]. These characteristics of high-yielding rice significantly impact key processes of traditional rice combine harvesting, including threshing separation, cleaning, and grain transportation. For example, the combine harvester cannot efficiently separate the grains of high-yielding rice during threshing, with difficulties in separating and cleaning high-yielding rice with green stems, lush leaves, and high-water content [
23,
24,
25]. Moreover, static friction is a key physical property of high-yielding rice. Particle motion occurs during the threshing separation, cleaning, and grain transportation [
26,
27], resulting in complex friction. The characteristics of the material’s static friction are closely related to particle motion. Therefore, research on the static friction characteristics of high-yielding rice is necessary for choosing appropriate harvesting techniques, technical parameters, and optimizing the design of combine harvesters [
28].
Friction is a highly complex characteristic and depends on the friction location, surface micromorphology, contact state, material, environment, time, and other factors. The material is the most important factor affecting friction [
29,
30]. Compared to pure industrial materials, the frictional characteristics of biomaterials are strongly affected by time and space; they are highly variable. Rice grains and stems are biomaterials; the static friction between these components and the surface of the harvester’s steel plate are substantially affected by the field environment and the harvest time [
31]. However, the operators of combine harvesters often ignore the adverse effects of the field environment and harvest time to maximize outputs, resulting in blockages of the combine harvester and increases in losses and the crushing rate. These problems have adverse effects on machine usage and reduce the quality of the rice harvest.
Knowledge of the static friction characteristics is crucial for harvesting and machine design optimization. Several studies have attempted to determine the static friction and related parameters of rice to provide information for the design of rice production machinery [
32,
33,
34]. Bhattacharya investigated the physical characteristics of rice, revealing that the friction coefficient of rice increased with an increase in humidity [
35]. Yuan et al. observed that the harvest season and rice varieties affected the physical characteristics of rice, including friction [
36]. Yu measured the friction characteristics and other parameters of Indica rice with different water contents and concluded that the static friction (and other parameters) of this rice variety was related to changes in the moisture content [
37]. Zhang et al. studied the friction of rice and other crops on a galvanized surface and proposed an empirical model of friction and relative displacement based on experimental data [
38]. Kayode et al. tested the friction and other characteristics of rice and other crops with different moisture contents [
39]. Kumar et al. analyzed the changes in friction and other characteristics of the rice variety “Gurjari” and its processed products during tablet pressing [
40]. Previous studies have determined the static friction and other characteristic parameters of rice in the laboratory and investigated the effects of different factors on the friction characteristics of rice. This information is crucial for rice harvesting and processing. However, most studies that measured friction and other characteristics did not focus on high-yielding rice and did not compare laboratory data with field measurements during the rice harvest. Laboratory data may contain errors and lags in measuring friction and other parameters. In addition, combine harvester designers and operators are more concerned with friction characteristics in the field because this information provides reliable references for the design and optimization of rice production machinery. Field research on static friction and other characteristics of high-yielding rice varieties is limited, although it is required for the optimal design of combine harvesters and the development of standards in agricultural operations.
Therefore, we conduct an experimental study on the static friction characteristics of grains and stems of high-yielding rice during the harvest. We statistically analyze the effect and significance of the spatial location, harvest time, and external temperature on the static friction characteristics of rice grains and stems. The results provide a basis for parameter selection, design optimization of high-yielding rice combine harvesters, and quantitative data to standardized combine harvesting. Our work provides new insights in combine harvesting operations of high-yielding rice for researchers, designers, and farmworkers.
4. Discussion
The
p-values of the static friction coefficients of the grains and stems of the high-yielding rice variety “Nanjing Jinggu” are smaller than 0.05 in the X-direction and larger than 0.05 in the Y-direction of the experimental field (
Table 4), indicating that there are significant differences in the coefficients in the X-direction (
Table 5) and no significant differences in the Y-direction. Multiple canals are located around the test field, and the distance between the rice plants and the water source is different in the X-direction. Thus, the distance from the water source affects the static friction characteristics of rice grains and stems.
The static friction characteristics of rice are crucial for harvesting and machine design optimization. Particle motion occurs during the separation, cleaning, and transportation of the grains. The motion and location of the material particles differ in the working components of the harvester, resulting in complex friction. The
p-values of the friction coefficients between the steel plate and the rice grains and stems were 0.122 and 0.000, respectively (
Table 6). The contact configurations of the rice grains had no significant effect on the static friction coefficients, whereas those of the rice stems significantly affected the static friction coefficients. The difference in the static friction coefficient of the rice stems for different contact configurations was 9%. This result shows that the contact characteristics between the rice stems and the working parts of the combine significantly influence the static friction characteristics.
The
p-values of the rice grains and stems for different measurement times and temperatures are all 0.000 (
Table 7), indicating significant effects of these factors on the static friction coefficient of rice grains and stems. The combine harvester works continuously for a long time to maximize economic benefits, while the adverse effects of the harvest time, temperature, and other factors on the rice harvest are not considered. The static friction coefficients of rice grains and stems show a negligible impact on the harvesting operation and do not affect the smooth flow of materials in the combine. The optimum harvesting period is 10:11 a.m.–3:30 p.m. and the optimum temperature range is 16.5–21 °C (
Figure 8). The quantitative results obtained in this study can be utilized to improve the design of combine harvester and to optimize farm operations.
5. Conclusions
This study analyzed the effects of the spatial configurations of rice grains and stems, harvest time, and temperature on the static friction characteristics of rice grains and stems of high-yielding rice. Significant differences were observed in the average static friction coefficients of rice grains and stems in the X-direction of the experimental field, indicating that the spatial location affected the static friction characteristics of rice grains and stems and the harvest quality of high-yielding rice.
The contact configuration between the rice grains and the steel plate surface had no significant effect on the static friction characteristics. In contrast, the contact configuration between the rice stems and the steel plate significantly affected the static friction characteristics. Therefore, the flow of materials in the combine is affected.
The harvest time and temperature had significant influences on the static friction characteristics of rice grains and stems. The static friction coefficient of rice grains and stems decreased sharply, then stabilized, and sharply increased during the harvest period (from morning to evening). The coefficient decreased with a decrease in the temperature. The optimum harvest conditions were 10:11 a.m.–3:30 p.m. with a temperature above 16.5 °C. Quantifying the effect of the temperature and harvest time on the static friction characteristics of rice provides data support to standardize the harvesting operation and information for the design and optimization of high-yielding rice combine harvesters. The data can also guide research and be used to train combine harvester and farm operators.
In a future study, we will analyze the effects of various factors on the static friction characteristics using a larger test site, a larger sample size, and a longer measurement period. In addition, this study still has some limitations. The high-yielding rice is limited to a single variety, and the research object is relatively simple. Due to the strong regionality of rice planting, there are some differences in its physical characteristics, so the research region is relatively limited.