## 4 Shearer Pick Forces during Coal-Rock Cutting
### Load Spectrums of the Pick
Meanwhile, he monitored the differences in the vibration and noise under different coal-rock occurrence conditions. [PERSON] [76] believed that the existing dynamic analysis of the shearer drum assumed the coal seam homogeneity, but did not consider the discontinuous characteristics of the coal seam at coal-rock interface and its interaction with the pick and drum. Especially, mechanical mutation behaviors and stress distribution states have not been investigated in cases of the discontinuous impact and friction between the pick and the coal seam. When the pick and drum are subjected to more severe impact and friction while cutting the coal seam at the coal-rock interface, it exhibits significant discontinuity. That makes it difficult to effectively control and predict the motion law and dynamic behavior of the drum, and to ensure the reliability of the shearer [77, 78, 79, 80, 81]. [PERSON] et al. [82] proposed the possibilities of increasing the efficiency of separating coal from the massif by improving the pick alignment schemes of shearer cutting drums. Given the existing structural limitations of the radial extension of the picks and the thickness of the cuts, an increase in efficiency was achieved by using combined series-group schemes for arranging picks, including energy-efficient cutting, paired and group with a larger cross-sectional area of the cuts and, therefore, with a larger particle size of the separated coal fragments, reducing dust yield and specific energy consumption. During the coal-rock cutting, especially the cutting of semi-coal, rock and hard rock, non-uniformities of the physical and mechanical properties of coal rock induce the random vibration load of the shearer pick [83, 84]. In addition, a great deal of vibration (vibrations of motor and traction transmission gearbox [85], vibration due to unevenness of mining face floor [86] and impacts (meshing impact between pin row and pin teeth of the shearer traction wheel [87], impacts due to changing meshing stiffness and manufacture error of transmission gear) is transmitted to the pick, which accelerates the random vibration of the pick. Meanwhile, the random vibration of the shearer pick is affected by structural parameters, kinematic parameters and other factors as shown in Table 4.  
In order to quantitatively demonstrate random vibration characteristics of the pick during coal-rock cutting, it is of great significance to obtain the load spectrum of the pick. Up to now, lots of researchers have investigated the load spectrums of shear picks through theoretical analyses, field tests (Table 5) and computer simulations. During theoretical analyses of shearer pick force, Soviet scholars in the 1960s calculated the cutting resis \begin{table}
\begin{tabular}{c c c} \hline \hline
**Structural Parameters of Shearer**[88] & **Kinematic Parameters of Shearer**[89, 90] & **Other Factors**[88, 89, 90] \\ \hline Pick type & Rotational speed of drum & Cutting conditions (straight cutting, oblique cutting) \\ Pick configuration & Traction speed & Cutting process \\ Geometrical parameters of drum & Matching relationship between drum & Joint development of coal rock \\  & speed and traction speed & Coal-rock hardness \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Influence factors of shearer pick vibration. tance of the wedge pick based on rock mechanics. [PERSON] [60] obtained the formula derivation of cutting resistance and normal force. [PERSON] [91] derived the formula for pick cutting force. [PERSON] [92] calculated the cutting resistance based on fractal theory. [PERSON] [77] established the mathematical model of drum cutting resistance based on single pick cutting resistance. [PERSON] [93] calculated the cutting resistance based on the reconstruction algorithm. [PERSON] [94] calculated the cutting force based on dynamics theory. [PERSON] [95] obtained the ranges of cutting force based on the Taylor formula and Tikhonov regularization.