## 1 Introduction
Most of the previous studies did not reflect the actual dynamic field load conditions and, thus, simulation-based performance evaluations reflecting actual conditions are required. Gear breakage frequently occurs in the form of fatigue breakage, which occurs due to the repeated action of a dynamic load lower than the allowable load [13]. As the tractor is operated under soil conditions, the load generated on the tractor during field operations has the characteristics of irregular fluctuations in size and direction [14; 15], and certain soil types (e.g., reclaimed land) cause high loads. In addition, the load on the tractor varies depending on the various working conditions, such as attached implement type [16; 17] and gear stages [3; 18]. This load directly affects the fatigue life and, so, the fatigue life of the tractor may vary depending on the user's work characteristics. Therefore, fatigue life evaluation using dynamic field load data of major agricultural operations is necessary. However, to date, most fatigue life tests are conducted with the engine-rated load as the input condition [8]. Therefore, fatigue life evaluation using dynamic field load data of major agricultural operations, as measured through actual field experiments, is necessary. Research on the load measured through field experiments has mainly consisted of basic statistical analyses [19], creating the load spectrum [18; 20; 21; 22; 23], relative severity [3; 18; 24; 25], and damage evaluation [26; 27]. [PERSON] et al. [18] measured the load data of a 78 kW tractor, according to the disk plow and rotary tillage, and evaluated the relative severity for evaluating the transmission fatigue life by analyzing the load spectrum. [PERSON] et al. [27] analyzed the influence of key factors on the load dynamic characteristics and fatigue damage of the powertrain. They reported that the harmonic engine torque and vibration characteristics of the powertrain system had a great influence on the fatigue damage of the shaft parts, and that the average engine torque had an effect on the fatigue damage of the gear parts. However, the above studies only allowed for the relative comparison of torque, load spectrum, and severity according to conditions such as major agricultural work, number of stages, soil, and so on, and were not applied to actual durability tests. In conclusion, studies evaluating the fatigue life of a simulation-based tractor considering actual field dynamic loads are insufficient. Therefore, it is necessary to evaluate the fatigue life based on simulations utilizing dynamic field load condition data measured for each agricultural operation through field experiments. The purpose of this study is to develop a simulation model of a tractor transmission, in order to evaluate the fatigue life of an agricultural tractor transmission spiral level gear using dynamic field load data measured in the field. The novelty of this study is that the fatigue life evaluation can be reproduced and repeated using the simulation model and dynamic field load data which fluctuates according to various conditions (e.g., soil conditions, gear stage, attached implements, and so on) without additional experimental equipment. This can contribute to securing original technology for the optimal design and durability evaluation of tractors. The major objectives of this study are as follows: (1) analysis of dynamic field load data for each agricultural operation measured through field experiments; (2) developing a simulation model of a transmission including the spiral level gear based on commercial software; (3) evaluation of the fatigue life of the spiral level gear for each agricultural operation.