## 3 Analysis and Optimization of Friction Flash Temperature of Gears Tooth Surface
### Micro-Geometry Modification Method to Reduce Tooth Surface Flash Temperature  
Since the mechanical system will produce elastic deformation after being loaded, the gear will not mesh at the theoretical contact position, but will cause eccentric load and stress concentration, as shown in Figure 6. This will lead to failure modes such as early pitting, scuffing, and tooth fracture, among others, and can also cause NVH problems such as meshing impact and gear whine in the transmission system [31]. The emergence of gear modification technology is to solve the above problems. Gear modification refers to the small micron-level modification of the tooth surface. The modified tooth surface deviates from the theoretical tooth surface, becoming no longer the theoretical involute helical surface. For open helical surfaces, gear modification includes profile modification, lead modification, triangular modification, twist modification, and topological modification. Tooth surface modification can improve the transmission accuracy and stability of gears and eliminate phenomena such as eccentric load and stress concentration, thereby increasing the service life and improving the load capacity of gears. For continuous operation of high-power heavy machinery such as drum shearers, gear modification can also reduce the contact temperature of the meshing tooth, which can effectively improve the anti-scuffing ability of the gear and can also reduce meshing power loss, improving the efficiency of the machine. As shown in Figure 6, a pair of meshing tooth surfaces undergoes both rolling and sliding during the meshing process. The pitch position is pure rolling, and the other positions have relative sliding speeds. The farther away from the pitch, the greater the relative sliding speed \(v_{s}\). The maximum value area of \(v_{s}\) is close to the tooth tip or root, so the friction power loss in this area is larger. The theory tooth surface often causes edge contact due to elastic deformation near the tooth tip, root, or both ends of the tooth face. If the contact load in these areas is greater, the friction loss will be greater, which will produce a higher flash temperature and cause suffing failure. The principle of tooth modification with the purpose of reducing flash temperature and improving efficiency is to use a variety of modification forms to remove some trace amounts of material from the tooth surface, thereby avoiding heavy load contact in the area around the tooth tip and root during the meshing process. This will reduce the friction power loss in areas with the higher relative sliding speeds, reduce the tooth surface flash temperature, and avoid tooth surface scuffing failure. Based on the meshing misalignment results analyzed in Section 2.3, comprehensive modifications to the tooth surface of each gear are carried out, including tooth tip and root relief, tooth profile crowing and slop, tooth lead crowing, slop, and end relief. Through these means, the modified gears can meet the requirement that the contact pattern is located in the middle of the tooth surface under small and medium loads and the tooth surface full contact under full load. The tooth surface flash temperature is positively related to the local load and relative sliding speed \(v_{s}\), and \(v_{s}\) is related to the gear macro geometry, so it cannot be changed through micro-modification. However, the contact load at the position where the relative sliding speed is higher can be minimized through micro-modification, so that the flash temperature of the tooth surface can be reduced. Taking gear G17 as an example, the micro-geometry parameters of comprehensive modification are shown in Table 2, and the optimization topological surface of the tooth is shown in Figure 7.  
First, apply the LTCA method to calculate the tooth surface load distribution and stress distribution after tooth modification. Second, iteratively calculate and evaluate whether the modification parameters are optimal according to the load distribution state.