## 5 The Coal-Rock Cutting Mechanism
### Pick Failure Mechanisms  
Although the ability of existing shearer drums to cut hard coal has been improved, most shearer drums cannot cut rocks. Meanwhile, the ability to cut thin coal seams, small faults and coal seams with dirt bands is still insufficient. The shearer drums cannot work stably in coal seams with faults and sandstone bands [144; 145]. Especially when cutting the coal seam at the coal-rock interface, the pick will suffer from excessive wear and fatigue failure due to the long-term large impact load, resulting in broken and missing teeth. That not only reduces cutting performance, but also brings harm to the normal operation of the cutter. The cutting picks on the shearer operate in a complex and stressful environment, i.e., the rotation and advancement rate from the machine side as well as geomechanical conditions of the coal seam. They transfer the energy from the shearer to the coal body, thus causing stress concentration in the rock and ultimately leading to chip formation and coal fragmentation. Therefore, the failure of the picks results in the extraction stoppage, and thereby causes the extra operational costs and production loss [146; 147; 148; 74]. In addition, failed picks may pose an increased risk to personnel and production if not replaced in time, for example, causing frictional ignition in underground coal mining production [149]. [PERSON] et al. [44] thought a number of parameters, such as pick geometry, pick body material, mounting configuration, depth of cut and rock properties, influence the pick performance. During coal rock cutting, the main failure modes of the shearer pick are pick wear, alloy head shedding, pick body bending and pick body fracture. It is found that the pick wear accounts for 45-50% of pick failure [94; 150; 151]. Pick wear mechanisms are abrasive wear, fatigue wear and corrosion wear [152], of which abrasive wear dominates. The generally used pick material is alloy steel (Table 6) [153; 154; 155]. In order to obtain a high wear-resistant pick material, the carburizing process is employed [80]. In fact, the pick wear degree is affected by occurrence conditions of coal rock seams, pick layout (installation angle, pick number, et al. ), pick structural parameters (diameter, tip angle and cone angle) and the kinematic parameters of the shearer. Another important factor affecting the pick service life is pick body fracture, which involves the crack initiation, propagation and final rupture [152]. However, the reasons for pick fracture may be residual stress-induced pick defects during manufacturing [156], crack initiation and propagation due to instantaneous pick force exceeding the ultimate strength of the pick material [157; 158; 159; 160; 161], and the improper operation by the shearer operator [158]. In addition, the premature failure of the pick will be accelerated due to a mismatch between pick type and coal rock type, high dust concentration at the working face [162; 163] and a wet corrosive medium [164]. Further, [PERSON] et al. [165] investigated failures of coal shearer picks in the Parvadeh coal mine in Iran, as well as the main mechanisms and causes for pick failures. The reliability of picks is critical to the productivity of the shearer [166]. [PERSON] et al., believed that pick body wear, breakage of the pick tip, slimming of pick body, and separation and removal of the pick tip, are the most frequent failures and wear modes of studied picks. In order to improve the cutting performance and reduce wear failure of the pick, we should pay more attention to the selection of structural parameters and layout of the pick. [PERSON] and [PERSON] [167] proposed the relationship between the manufacturing system stiffness, and the machining accuracy and quality, and developed a system of mathematical models. Meanwhile, large coal block spalling and energy consumption during coal rock cutting should be fully considered [88; 168].