3.5.1. Steepest Ascent Test of the Phloem

Based on the significant factors affecting the stacking angle obtained from the phloem Plackett–Burman test, the phloem–phloem static friction coefficient (*X*5) and phloem– phloem rolling friction coefficient (*X*6) were taken as the independent variables to carry out the steepest ascent test of the phloem. The test results are shown in Table 11. The results show that, as the values of *X*<sup>5</sup> and *X*<sup>6</sup> increased, the relative error between the simulated stacking angle and the physical stacking angle first decreased and then increased. The minimum relative error was obtained when the fifth level was selected, indicating the existence of the optimal value range for the fifth level. Therefore, the stacking angle test result of level 5 was taken as the center point, and the stacking angle test results of levels 4 and 6 were taken as the low and high levels for the subsequent response surface design. The optimization ranges of the phloem–phloem static friction coefficient (*X*5) and rolling friction coefficient (*X*6) were determined to be 0.35–0.52 and 0.053–0.084, respectively.


**Table 11.** Results of the steepest ascent test of the phloem.
