## 3 Results and Discussion
### Influence of Free Surfaces on the Performance of Cutting Head
#### 3.4.3 Free Surfaces at the Top and Rear Side of Rock Mass  
When free surfaces appear on both the top and rear side of the rock mass, as shown in Figure 3d, the deformation of the rock mass during the clockwise cutting process of the drum is shown in Figure 20. Compared to the boundary conditions shown in Figures 16 and 18, the deformation of the rock mass in the _y-positive_ direction increases in Figure 20, which means that the squeezing force between the top rock mass and the drum is reduced in this working condition, helping reduce the cutting force of the drum; However, the decreased deformation of the rock mass in the \(z\) positive direction occurs in the Figure 20 and it means that little interval between the end plate of drum and rock mass, which might add extra friction on the end plate of drum. Therefore, the cutting force on the shearer drum might increase or decrease when compared to the working conditions in Figures 16 and 18, which will be studied in the following section.  
Figure 21 shows the forces of the cutting head under the clockwise and anti-clockwise cutting directions. According to the figure, it can be found that the influence of the rotation direction of the cutting head on the cutting forces is not significantly different; As for the mean cutting force, the mean cutting force in the clockwise cutting direction is about 1994 kN; When the cutting head rotates anti-clockwise, the mean cutting force of the cutting head is about 1952 kN. Compared to the forces in the anti-clockwise rotation, the forces in the clockwise cutting direction increased by 42 kN, an increase of approximately 2.15%.  
Figure 19: Rock cutting force in the _y_-direction if the rear side of the rock mass is a free surface.  
Figure 20: Deformation of rock mass if the top and rear side of rock mass are free surfaces. (**a**) boundary condition of rock mass; (**b**) deformation in _y_-direction; (**c**) deformation in _z_-direction.  
3.4.4. Comparisons of Cutting Force in Conditions of Different Free Surfaces on Rock Mass  
According to Figures 17, 19 and 21, the mean cutting force of the shearer drum under different boundary conditions could be obtained in Figure 22. Figure 22 shows the cutting forces in the \(y\)-direction under different constraints on rock mass and different rotating directions of the cutting head.  
According to Figure 22, the minimum mean cutting force of the shearer drum occurs in the boundary condition as shown in Figure 18a where the rear side of the rock mass is free and the cutting force in this boundary condition is about 1870 and 1822 kN. Significantly, the number of free surfaces of rock mass in Figure 20a is four, larger than that in Figure 18a, but the mean cutting force in Figure 20a is 1994 and 1953 kN, still larger than that in Figure 18a. Therefore, the amount of free surfaces on rock mass is not the more the better and the best boundary condition to improve the cutting performance is to create a free surface on rock mass that is close to the end plate of the shearer drum as shown in Figure 3c. According to [PERSON] et al. [19; 20], saw blade and water jet assistant technology provide the possibility for excavation of free surface on the rock mass. Therefore, it is applicable to reduce the cutting force on the shearer drum by improving the boundary condition of the rock mass.