# A Study on the Effects of Hob Temperature on the Rock-Breaking Characteristics of Sandstone Strata
## 5 Conclusions  
1. The study determined the mechanical properties of sandstone at various temperatures through indoor experiments. Mechanical and thermodynamic parameters in PFC3D were calibrated, leading to the development of a comprehensive numerical simulation of rock breaking that incorporates hob temperature. This research revealed the variation in tip force and specific energy in rock breaking under different temperature conditions, confining pressures, tip distances, and penetration depths. The analysis of crack distribution and particle arrangement showed that an increase in temperature reduces the temperature transfer between particles, initially decreasing the hob tip force due to rock flaking. As temperatures rise, the expansion of the rock body on either side of the hob results in an increase in hob tip force, which intensifies with increasing surrounding pressure conditions. When the tip range is 70 mm, the minimum tip force is 372 KN. It can be seen from Figure 14 that the tip force decreases with an increase in the tip range, and the minimum tip force is reached when the tip range is 70 mm. When the S/P value is 14, the lowest tip force is 370 KN. With an increase in S/P, the tip force first decreases and then increases, which is consistent with the changing trend of the tip distance, indicating that an optimal tip distance and an optimal S/P reduce the force on the tip, thus reducing the wear of the hob. Comparing fracture mechanisms and development in rocks under varying pressures  
Figure 17: Variations in particle contact force with different temperatures at a confining pressure of 5 MPa: (**a**) 25 \({}^{\circ}\)C; (**b**) 300 \({}^{\circ}\)C; (**c**) 600 \({}^{\circ}\)C; and (**d**) 900 \({}^{\circ}\)C.  
and temperatures indicated a more stable temperature transfer and a steady increase in contact force in rock hobs under constrained pressure conditions. 2. The influence of the knife tip force, S/P values, and circumference pressure conditions on hob tip distances is summarized above, and the optimal S/P and S values in different cases are obtained. As can be seen from Figure 13, the specific energy of rock breaking with temperatures of 25 \({}^{\circ}\)C and 600 \({}^{\circ}\)C is minimum, and the influence of heat on rock breaking efficiency is mainly reflected in two aspects. First, the increase in temperature will make the cutting rock mass suffer heat damage and crushing. See Figure 15 for detailed microscopic damage analysis. At this time, the rock breaking efficiency increases. In addition, an increase in temperature will cause the cutting surface to expand the rock block and squeeze the hob, which will wear the hob and reduce the cutting efficiency, as shown with b and d in Figure 15.
3. A microscopic examination of contact force variations with temperature changes in the rock surrounding the breakage site, both with and without confining pressure, was conducted. In the absence of confining pressure, increasing hob temperatures predominantly affect particles in the vertical direction, aligning with the hob's normal direction. Without constraint pressure, the circumferential force is not limited by rock expansion, making the hob's force direction predominant. Under confining pressure, however, the hobbing force increases incrementally in all directions, consistent with the temperature force. Conceptualization, X.D. ; Methodology, H.-D.T. ; Software, F.-Y.T. ; Validation, X.-J.W. ; Formal analysis, S.-R.W. ; Investigation, S.-M.L. ; Resources, B.P. ; Writing--original draft, Y.-G.P. ; Writing--review & editing, Y.-L.C. All authors have read and agreed to the published version of the manuscript. Not applicable. The data that support the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available due to privacy. Author [PERSON] was employed by the company Shanghai Construction Engineering Group Engineering Research Institute.