2.4.2. Test Factors and Levels

The test factors were the speed *n* of the toothed roller, ground clearance *α*, and cogging angle *θ*. According to the results of calculations, the speed of the V-shaped toothed roller varied between 176 and 500 r/min; thus, the test speeds were preliminarily determined to be 200, 300, and 400 r/min. The results of the theoretical analysis indicated that operating the toothed roller at a higher ground clearance yields better performance; thus, *α* was assigned values of −20 mm (i.e., 20 mm into the soil), 20 mm, and 60 mm in this study. Based on the optimized above-described structural design, the cogging angle *θ* should range between 25◦ and 38.6◦. However, to account for the coefficient of friction between the cotton stalk and steel plate and to minimize processing difficulty, the maximum cogging angle was set to 40◦ for the field test. In the case of a certain thrust, the smaller the cogging angle, the greater the clamping force on the cotton stalk. However, there are two uncertainties: 1. Too small an angle leads to too long tooth height, which easily causes the insufficient strength of tooth plate structure; 2. Excessive clamping force will easily cause the cotton stalks to fall off easily. Thus, the cogging angles implemented in the field test were 25◦, 32.5◦, and 40◦. The test levels are summarized in Table 2. The L9 (34) orthogonal

experiment was designed to have nine test groups. The experiment was repeated three times for each group, and the mean value was calculated.


**Table 2.** Factors and levels of orthogonal experiment.

#### 2.4.3. Test Methods

The test was carried out in accordance with the standard GB/T8097-2008 [26]. To ensure stable operation of the equipment and minimize error, the puller machine was allowed to adjust its operating posture by traveling a distance of 20 m before data were collected. To facilitate the process of data collection, two rows of cotton stalks were harvested each time. The data collection region was 30 m long, corresponding to approximately 200 cotton stalks. The evaluation index for the test was the removal rate; it was determined as follows:

$$y = \frac{M\_b}{M\_z} \times 100\% \tag{16}$$

In the formula, *y*—the removal rate of cotton stalks, %; *Mb*—the number of cotton stalks pulled out, plant;

*Mz*—the sum of cotton stalks, plants.

#### **3. Results and Discussion**

The test plan and results were shown in Table 3 (A, B, and C are the ground clearance, the rotation speed, and the cogging angle, respectively).


**Table 3.** Test results and analysis.

According to the results of numerical analysis for the range *R* for each factor in Table 2, it can be seen that the order of significance in terms of the influence of each factor on the evaluation index (i.e., the removal rate) was *A* > *B* > *C*. The ground clearance most significantly influenced the removal rate, followed by the rotational speed, and the cogging angle. To optimize the combination of these factors to obtain the highest cotton stalk removal rate, the removal rate was plotted as a function of the averaged test factor results presented in Table 2; the resulting graph is shown in Figure 14. It can be ascertained from

Figure 12 that the combination of factors that yielded the highest removal rate was *A*1*B*2*C*1, i.e., the combination of a ground clearance of −20 mm, a rotational speed of 300 r/min, and cogging angle of 25◦.

**Figure 14.** Relationship between factor level and removal rate.

Minitab software was used as a platform to analyze the variance of the results of the orthogonal experiment and quantify the respective influences of the three factors (i.e., cogging angle, rotational speed, and ground clearance) on the removal rate; the results are summarized in Table 4.

**Table 4.** Analysis of variance for orthogonal experiment results.


Note: *p* < 0.01 (highly significant, \*\*); *p* < 0.05 (significant, \*). Based on the analysis of variance results presented in Table 3, it can be ascertained on the basis of FA > FB > FC that A had an extremely significant influence on the removal rate, B had a significant influence, and C had minimal influence. Such results are consistent with the range analysis results. In addition, it can be also ascertained from Table 3 that the sum of squares error was much less than that of factor A, factor B, or factor C, indicating that the correlations between test factors did not considerably affect the evaluation index.

Compared with the knife-roller type stalk pulling operation equipment, it has the advantages of not entering the soil and reducing energy consumption on the basis of nonalignment operation. Tests have proved that in the Binzhou area, cotton stalks with larger diameters can be effectively pulled out. However, since this type of stalk-pulling device has many V-shaped structures on its stalk-pulling rollers, once cotton stalks are entangled, the cotton stalks will be damaged. The damaged cotton stalk cannot fall off, which leads to its working failure; secondly, this device has been tested in a densely planted cotton area. When the average root diameter of cotton stalks is about 10 mm, the stalk-pulling effect is very poor, so this device still needs further in-depth research. The most foreign equipment introduced in the literature is the double-roller type stalk pulling equipment. Our team has used the domestically produced double-roller equipment to conduct experiments in the Binzhou area, and the effect is not ideal. The variety and planting mode of cotton stalks have a great influence on the mechanical properties of cotton stalks. Up to now, there is no stalk-pulling equipment that can adapt to cotton stalks in different regions. Therefore, further research is needed to solve the issue of stalk-pulling technology.
