2.2.2. Coefficient of Friction

Measurement of the Static Friction Coefficient

In this study, we mainly measured the static friction coefficients of phloem–Q235A steel, xylem–Q235A steel, and phloem–xylem. The static friction coefficient is the ratio of the maximum static friction force applied to the object to the normal pressure. It is usually measured by the incline plane method [34,35]. The ramie stalk with a high degree of roundness was selected, and the phloem was peeled with a cutter knife to create the test sample. A self-made inclinometer and a digital angle measuring instrument (accuracy of 0.05◦) were used to measure and calculate the required static friction coefficient, as shown in Figure 3.

**Figure 3.** Measurement of the static friction coefficient: (**a**) test equipment; (**b**) sliding angle.

The test object was placed on a steel plate when the static friction coefficients of the ramie stalks for phloem–Q235A steel and the xylem–Q235A steel were measured. The handle of the inclinometer was shaken to raise the incline angle, and the sliding angle was recorded when a sliding trend occurred. When measuring the static friction coefficient of the phloem–xylem, the phloem was attached to the steel plate, and the xylem was axially placed above it for the test. The sliding angle was recorded, and the coefficient of static friction was calculated. The calculation of the static friction coefficient is shown in Equation (3):

$$\mu\_1 = \frac{f}{F\_2} = \frac{mgsin a}{mgcos a} = \tan a \tag{3}$$

where *μ*<sup>1</sup> is the static friction coefficient, *f* is the static friction force between the object and the inclined plane, N; *F*<sup>2</sup> is the force perpendicular to the inclined plane, N; *m* is the measured material mass, g; and *α* is the inclination angle, ◦.
