1. Introduction
The global tractor market is expected to gradually increase from USD 45.3 billion in 2017 to USD 73.9 billion in 2027, with a combined annual growth rate (CAGR) of 5.5% [
1]. The tractor production in Korea was 57,369 units in 2019, accounting for about 68% of major production of agricultural machinery, and their utilization rate was 98.7%, which was the highest among agricultural machinery [
2]. In particular, the import of agricultural machinery in Korea increased by about 156% from USD 85 million in 2015 to USD 133 million in 2018, mostly in the form of high horsepower tractors (75 kW or higher), which are difficult to produce in Korea [
3]. In view of this trend, the demand for high zzzhorsepower tractors has steadily increased; however, there is still a lack of original technology for developing high horsepower tractors in Korea. Therefore, it is necessary to use original technology to design and evaluate high horsepower tractors in Korea.
Agricultural tractors have irregular and fluctuating loads depending on agricultural operations (e.g., plow tillage, rotary tillage, baler operation, and loader operation), so it is important to ensure the durability of tractor components [
4]. Transmission is a key component of a tractor for proper gear selection, and it accounts for over 30% of the tractor’s total price [
5]. In particular, among the various gear pairs of the transmission, the gears of the main and range shift are mainly used during driving and agricultural operation. As a result, failure is likely to occur in the main and range shift gears. If the gear is damaged during agricultural operations, it affects the entire transmission and may cause tractor failure and accidents [
6]. Therefore, it is necessary to analyze the durability of tractor transmissions. Additionally, following the occurrence of gear failure, the strength of the damaged gear must be improved. There are methods that can be used to change gear specifications, such as tooth, width, module, and pressure angle, in order to increase gear strength [
7]. However, changing gear specifications affects the transmission structure, which may incur high costs. On the other hand, if the material and heat treatment of the broken gear are changed, there is no need to change the structure of the transmission, so this strategy can be used to improve the strength of broken gears [
8].
Recently, various studies have been conducted to ensure durability of tractor transmissions. The durability of tractor transmission is mainly evaluated using field test, dynamometer test, and simulation analysis. A field test was conducted to evaluate the fatigue life of a tractor using field data [
2]; in order to measure the dynamic load of the tractor according to various agricultural operations, a tractor load measurement system was developed, and a field test was performed for major agricultural operations. Fatigue life was calculated using rainflow counting (RFC), the Smith-Watson-Topper (SWT) model, and S-N curves based on torque data measured in the field. The results showed that the fatigue life values under moldboard plow tillage, subsoiler tillage, rotary tillage, and baler operation were 13,599, 285, 278,884, and 525,977 h, respectively. The results of tractor gear fatigue life were significantly different according to agricultural operations, and it was suggested that the simulation model developed in this study could be used for evaluating fatigue life using dynamic field data. A dynamometer test of transmission was conducted to analyze the types of gear tooth wear using a gear monitoring system [
9]. Gear tooth wear is an inevitable phenomenon that has a significant influence on gear dynamic features. Gear vibration analysis has been used to diagnose gear tooth defects, but gear wear monitoring techniques are not well established. Accordingly, a gear wear monitoring system was developed, and gear performance with reference to gear wear was analyzed. Two averaged logarithmic ratios (ALR) were defined with fixed and moving references to provide complementary information for gear wear monitoring. Since a fixed reference was utilized in the definition of the ALR, it reflected the cumulated wear effects on a gear state. Increases in ALR values indicated that the gear state was further deviating from its reference condition. Accordingly, it was judged that the proposed indicators and gear monitoring system can prevent gear failure in advance.
A simulation analysis was conducted to evaluate the durability and performance of a tractor transmission [
10]. In order to evaluate the performance of a hydro-mechanical transmission (HMT), a test bench was installed based on the engine of the HMT platform, and a simulation model of HMT was developed using software for analyzing gear-train. To compare the results of the simulation, a bench test using the platform was performed according to the gear stages. The similarities between the measured and simulated data were analyzed using a t-test; no significant differences for the axle torque, rotational speed, and power transmission efficiency were observed.
In conclusion, there have been insufficient studies evaluating the fatigue life of simulation-based tractors while considering actual field dynamic loads. Therefore, it is necessary to optimize tractor transmission gears based on simulations utilizing dynamic field load condition data measured for each agricultural operation through field experiments and dynamometer tests. In Korea, tractor companies design and mass-produce transmissions for agricultural tractors, and it is difficult to check and repair damaged transmissions when they break down in the field [
11]. However, simulation analysis allows for easy identification of and improvements to such failures.
Therefore, the purposes of this study were to verify the type of gear failure for an 86 kW class agricultural tractor through a field test, to analyze the strength of the damaged gear by changing gear materials using simulation analysis, and to verify the durability of the transmission built with the selected gear materials through dynamometer tests. The novelty of this study is that a field test was performed using a tractor under development, and a gear material that reached the target lifespan was selected through simulation analyses of the damaged gear, as verified through dynamo tests. This study can contribute to the development of original technology for the optimal design and durability evaluation of agricultural tractors. The major objectives of this study were as follows: (1) analyze gear failure type for the 86 kW class agricultural tractor through a field test; (2) compare the strength and life of the damaged gear with different gear materials using commercial software; (3) verify the modified the tractor transmission through dynamometer tests.
4. Discussion
In this study, the gear failure types of the 86 kW class agricultural tractor transmission were analyzed with a field test, and the damaged gears were changed using simulation analysis. Finally, the transmission was modified following a dynamometer test to verify that the changed material met the target life.
Table 8 shows a comparison of the service life values for the modified range shift gears between the simulation analysis and the dynamometer test, which were both performed as accelerated life tests. Accordingly, we calculated the equivalent life in order to confirm whether the damaged gears reached the target life. The service life of the range shift A gear was found to be 312.6 h with the simulation analysis, and it was found to be 250 h or more with the dynamometer test. The service life of the range shift B gear was found to be infinite with simulation analysis and more than 250 h with the dynamometer test. The equivalent life of the range shift A gear was found to be 3632 h with simulation analysis and 2641 h or more with the dynamometer test, and the equivalent life of the range shift B gear was found to be infinite with simulation analysis and more than 5642 h with the dynamometer test.
The life of a tractor transmission is usually determined based on the life of the lowest gear. Accordingly, the life value of the 86 kW class agricultural tractor transmission studied here was determined by the life value of the range shift A gear, which was more than 2641 h and thus reached the target life time of 2496 h or more, considering the tractor’s service life and annual working hours. Therefore, it was judged that the durability of the tractor transmission was ensured by changing the material of the damaged gear during the dynamometer test. The simulation model of the 86 kW class agricultural tractor developed in this study is expected to be used for future optimal design and cost reduction.
5. Conclusions
This research was conducted to ensure the durability of a tractor transmission as a basic study of the development and optimal design of tractor transmissions. A field test was conducted using an 86 kW agricultural tractor for plow and rotary tillage, which are typical agricultural operations. The plow and rotary tillage were performed at the F10 (6.71 km/h) and F8 (4.12 km/h) gear stages, respectively. The field operation was completed after around 107 h due to transmission noise and operational problems during a forward and reverse movement. As a result of disassembling the transmission, it was found that the range shift A and B gears were damaged. For the range shift A gear, it was judged that plastic deformation occurred due to low contact stress, and for the range shift B gear, the bending stress was low, so gear tooth breakage occurred.
The simulation results showed that the contact safety factor of the range shift A gear was the lowest, and the bending safety factor of the range shift B gear was the lowest. Accordingly, it was observed that the simulation and field test results presented similar trends. In order to ensure the durability of the transmission, four materials of alloy steel for machine structural use, such as SCr420, SNCM220, SCM822, and SNC815, were selected, and the safety factor and service life values of the damaged range shift gears with different gear materials were analyzed.
The simulation analysis results demonstrated that the SCM822 material reached the design target life and was selected as the new gear material. The materials of the range shift A and B gears were changed to SCM822, and an axle dynamometer test was performed to verify the improvement of the modified transmission. After conducting the axle dynamometer test, the transmission was disassembled, and it was confirmed that the range shift A and B gears were in normal condition. Therefore, it was determined that the durability of the transmission was ensured because it reached its target life. However, a drawback of this study is that the axle dynamo test was only performed once for the transmission that reflected the changed gear material due to issues such as test time, test cost, and limited transmission samples. Accordingly, it was judged that additional dynamo tests are necessary to verify the target life of the tractor transmission.
Finally, in this study, we have proposed a method for improving the strength and service life of damaged gears. In the future, our proposed transmission simulation model for 86 kW class agricultural tractors is expected to be utilized for the development of tractor transmissions, cost reduction, and optimal design.