# A Study on the Effects of Hob Temperature on the Rock-Breaking Characteristics of Sandstone Strata
## 4 The Establishment and Analysis of the Rock Breaking Model of Hobbing
The impact of heat on specific energy con  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & **Temperature** & \(P\) & \(S\) & **Confining** & **Mean Normal** & **Specific Energy,** \\  & **T (\({}^{\circ}\)C)** & **(mm)** & **(mm)** & **Pressure, (MPa)** & **Force, (\(MNF\) (KN)** & **SE (MJ/m\({}^{3}\))** \\ \hline
1 & 25.0 & 4.0 & 60.0 & 0.0 & 198.1 & 24.14 \\
2 & 25.0 & 5.0 & 70.0 & 5.0 & 303.7 & 27.13 \\
3 & 25.0 & 6.0 & 80.0 &  
sumption for rock breaking is reflected in the efficiency of rock breaking. On one hand, the increase in temperature directly leads to thermal damage on the rock at the cutting surface, resulting in fragmentation and fracture. On the other hand, the rise in temperature also causes the crushed rock debris to expand, thereby exerting pressure on the cutter. At a temperature of 600 \({}^{\circ}\)C, there are evident cracks on the cutting surface, and the rock debris on the cutter's cutting surface is not pressed against the cutter, resulting in optimal fragmentation efficiency. When the S/P is 14, the specific energy of rock breaking is the smallest and the rock-breaking efficiency is the highest. An increase in confining pressure will make the rock-breaking ratio increase and will make the rock-breaking efficiency decrease. It can be seen from Figure 14 below that as the temperature rises, both sides of the particles with the rock-breaking process flake; at this time, the hob tip force based on the received particle contact force decreases and decreases, but after 600 \({}^{\circ}\)C, the tip force goes back up. This is because as the temperature continues to rise, the linear strain of the particles increases and the extrusion between the particles increases. At this time, the particles on both sides affected by the temperature will squeeze the hob, increasing the force on the hob. The effect of this phenomenon is not obvious when the temperature is low because the force of breaking rock at this time mainly comes from the horizontal and vertical dumping load of the hob, and the temperature on both sides of the spalled rock and particles is basically  
Figure 13: Specific energy (\(SE\)) versus penetration, tool tip distance, confining pressure, and temperature. (**a**) SE versus different temperatures. (**b**) SE versus different spacing patterns. (**c**) SE versus different penetration degrees. (**d**) SE versus different confining pressure conditions. not transferred over. The temperature change in the particles on both sides is not obvious and the volume deformation that occurs is low. The tip force in Figure 14 also depends on the slag formation and the fracture form. The crack forms and the cutting mouth has no obvious expansion slag, so the hob receives the minimum tip force and the lowest wear on the hob. When exposed to heat and confining pressure, and when the normal tip force is the smallest, the tip distance (\(S\)) is taken as 70 mm and the rock-breaking efficiency is also the highest under this tip distance. When exposed to heat and circumferential pressure, and when the tip force is the smallest, the tip distance/penetration (S/P) is taken as 14. The rock-breaking ratio at this tip distance is also the lowest. When exposed to heat and surrounding pressure conditions, the tool tip force increases with an increase in confining pressure, and with an increase in confining pressure, hob tilting becomes difficult. Therefore, when exposed to heat and confining pressure, the optimal knife tip distance for rock breaking with needle sandstone hobbing is 70 mm and the optimal \(S/P\) value is 14.