## 6 Conclusions  
We aimed to develop a simulation model which can replace the fatigue life testing of conventional actual gears for the evaluation of the fatigue life of a tractor by using dynamic field load data. To this end, a simulation model of a spiral bevel gear was developed using commercial gear analysis software. To apply dynamic load conditions to the simulation model, an axle torque measurement system was developed and the dynamic field load data of the tractor under major agricultural operations performing in the field were measured. The fatigue life evaluation using the developed simulation model is based on Miner cumulative damage rule. To evaluate the fatigue life of the spiral bevel gear, the damage sum was calculated using RFC, SWT, and S-N curves, based on torque data measured in the field. The fatigue life for moldboard plow tillage, subsoiler tillage, rotary tillage, and baler operation were 13,599, 285, 278,884, and 525,977 h, respectively. In addition, the fatigue life of the tractor under subsoiler tillage was 0.104 times that of the service life, while the fatigue life under baler operation was 192 times that of the service life. The difference of fatigue life under these two operations was approximately 1846 times. The results of this study show that the life of a spiral bevel gear may differ, depending on which agricultural operation the user mainly uses, and that the maximum life of the spiral bevel gear may also differ by 1846 times or more. The fatigue life of a spiral bevel gear according to its agricultural operation type was significantly different, indicating that the life of the spiral bevel gear might be significantly different depending on which operation type the user mainly performs. Therefore, tractor designers must design the transmission in consideration of the working conditions (e.g., implement type) under which the tractor will be mainly used. In addition, the fatigue life of the spiral bevel gear was found to be excessively low during subsoiler tillage, which may mean that the combination of the used subsoiler and the tractor is not appropriate. In this case, the manufacturer of the subsoiler should recommend the user to use a tractor with a higher horsepower. The developed simulation model can be used for fatigue life evaluation according to design conditions when designing a transmission,  
Figure 11: Comparison between fatigue life analysis results by simulation and obtained fatigue life from a previous study with three samples for each load condition: (**a**) case 1 and (**b**) case 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Cases**} & \multicolumn{5}{c}{**Fatigue Life (h)**} \\ \cline{2-6}  & \multirow{2}{*}{**Simulation**} & \multicolumn{5}{c}{**Previous Study Based on ALT [41]**} \\ \cline{3-6}  & & **Sample 1** & **Sample 2** & **Sample 3** & **Average** \\ \hline
1 & 197 & 123 & 273 & 157 & 184 \\
2 & 43,315 & 26,330 & 58,453 & 33,702 & 39,495 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Fatigue life of spiral bevel gear, according to the simulation results and a previous study based on ALT. by reflecting various working environment conditions. In particular, if a database constructed using field dynamic load data according to various tractor working conditions was combined with the developed simulation model in this study, it can be used for designing optimal tractor transmissions, thereby securing original technology, improving design reliability, and reducing costs. Despite these contributions, this study calculated the torque of the spiral bevel gear by using an equation applying only the gear ratio and power transmission efficiency based on the data measured on the wheel. This may be difficult to sufficiently reflect the dynamic effect of the final reduction gear. According to the dynamic characteristics of the gear, when the torsional stiffness of the drive shaft of the gear increases, the vibration average of the gear and the stress amplitude of the gear root increase, and the reliability of the gear decreases [31].