Abstract: The static strength design theory used in the design phase of planetary gearboxes fails to reflect the dynamic loads they bear during operation, leading to numerous failures in actual use. To fundamentally solve this problem, it is necessary to determine the dynamic loads borne by the planetary gearbox under different mission profiles. Traditional load determination methods have significant limitations. With the widespread application of computer simulation technology in various industries, this paper conducts driving simulation tests under different mission profiles and performs dynamic simulations on the planetary gearbox based on these tests. The dynamic loads borne by each component of the planetary gearbox under different mission profiles are obtained. The paper also analyzes, from the perspective of fatigue damage accumulation theory, why fatigue failure still occurs even when the maximum stress under alternating loads is much lower than the design maximum stress. Taking the planetary carrier as an example, this paper provides the dynamic loads borne by it under different mission profiles, providing important load reference data for fatigue strength calculation and life prediction. Keywords: Dynamic simulation, dynamic load, virtual prototype, planetary carrier Introduction Planetary gearboxes are widely used in modern automobiles, tanks, self-propelled artillery, engineering machinery, and tracked vehicles. Compared with ordinary speed reduction mechanisms, planetary reducers have advantages such as compact structure, small size, light weight, large transmission ratio, high transmission efficiency, and large load capacity[1]. According to the survey, the planetary reducer in a certain tracked vehicle has many cracks and fractures during use, which seriously affects the reliability and safety of the entire system. Moreover, it is understood that most users only ensure the reliability of the planetary reducer from the perspective of maintenance, that is, when a fault occurs, the entire machine is disassembled for maintenance, which seriously affects the completion of work and production tasks. However, this cannot fundamentally solve the problem of the recurrence of such faults from the design itself. According to the survey of planetary reducer design units, when planetary reducers are designed and researched, they lack accurate load data and only use static strength theory for design based on the designer's experience and accumulated data. The safety factor of the designed parts is generally too large. However, the static strength design theory cannot reflect the dynamic load that the planetary reducer bears during the working process. Therefore, it is impossible to accurately determine the life of the parts in the design stage. According to statistics, more than 70% of the parts on automobiles are fatigued due to dynamic loads[1]. Therefore, in the design and strength calculation stage, in addition to adopting advanced design methods and accurate calculation formulas, determining the actual load of the designed parts is a basic and important task. Since the magnitude and characteristics of dynamic loads are affected by many factors such as road conditions, usage conditions, and structural parameters of the entire machine and its components, it is a very complex task to study and determine the load of the running system. In my country, the research on determining the random load of the vehicle running system is still in an immature stage, so there is little data accumulation in this area. The traditional load determination methods include experimental measurement and mathematical analysis [1]. These methods use the method of measuring loads under all driving conditions. Theoretically, the data obtained is relatively reliable, but the test cycle is long, the data processing work is heavy, the cost is high, the effect is slow, and it requires close cooperation of huge human and material resources. This leads to many users putting the machine into production without conducting these tests, resulting in many failures and unpredictable lifespan during use. With the development of multibody dynamics and computer hardware and software and the emergence of virtual prototype technology, it has become possible to conduct dynamic simulation of the overall mechanical system [4]. Based on driving simulation experiments, this paper provides the dynamic loads borne by various components of a planetary reducer on various driving roads. Based on this, a virtual prototype model of the planetary reducer was established on the MSC.ADAMS platform. Based on the driving simulation test, the dynamic simulation of the planetary reducer was carried out to provide the dynamic loads borne by each component of the planetary reducer under different driving road surfaces, providing reliable load data for failure cause analysis and fatigue strength design and fatigue test of components [2]. 1 Dynamic simulation analysis flowchart based on driving simulation test With the development of computer technology and the improvement of modeling methods and simulation level, virtual prototype technology has provided a new means for the performance analysis and evaluation of mechanical systems. Engineering researchers can use virtual prototype technology to build virtual prototypes of mechanical systems, simulate their motion and force under real environmental conditions, and analyze and evaluate the overall dynamic characteristics of mechanical systems. At present, virtual prototype technology has been successfully applied to mechanical manufacturing, aviation and aerospace fields, and has achieved gratifying results [2][4]. We combine virtual prototype technology with dynamic simulation analysis to establish a virtual prototype model of planetary reducer. In conjunction with the driving simulation test of tracked vehicle, we establish road surface spectrum under different driving conditions, determine the selection of vehicle ground mechanical model, and give appropriate physical parameters. This can more realistically reflect the internal force of planetary reducer, greatly improve the accuracy and reliability of dynamic analysis of tracked vehicle, accelerate the test process of new products and improved products, and reduce development costs. At the same time, due to the unparalleled convenience and flexibility of simulation technology, this research method breaks through the traditional analysis mode and can conduct tests under various extreme conditions that physical prototypes cannot perform. This is of great significance for improving the test conditions of tracked vehicles, perfecting test methods, reducing test costs, improving test efficiency, and improving development level. The flowchart of the application of dynamic simulation analysis of planetary reducer based on driving simulation test to the redesign of planetary reducer is shown in Figure 1. 2 Establishment of virtual prototype of planetary reducer 2.1 Working principle of planetary reducer The planetary reducer introduced in this paper adopts 2K-H(A) type planetary reducer device (as shown in Figure 2). This mechanism is usually called unit planetary gear mechanism or simply planetary gear set [3]. The rotational speed equation of the 2K-H(A) type planetary gear set shown in Figure 2 is as follows: The 2K-H(A) type planetary gear set has three basic components: sun gear a, internal gear b, and planet carrier. If the specific details of the input and braking components are known, the transmission ratio calculation formula can be derived. The planetary reducer discussed in this paper is a transmission form in which the internal gear b is braked, i.e., n=0, the sun gear a is input and the planet carrier is output (see Figure 2). Its kinematic equation is then obtained. The formula for calculating its transmission ratio is 2.2 Planetary reducer solid modeling According to the flowchart shown in Figure 1, the three-dimensional solid model of each component of the planetary reducer discussed in this paper should be established first, and each component should be assembled to form each level of sub-assembly and finally the overall assembly modeling should be performed. The following issues should be noted in the process: (1) Model simplification Although many small parts in the planetary reducer model cannot be ignored, it is unrealistic to consider each component. For example, some sealed components, which do not need to be considered in dynamic simulation, can be omitted in the modeling process to reduce the size of the model and save time in dynamic simulation. When establishing the model, the focus is on vulnerable parts and difficult-to-repair parts. (2) Determination of physical characteristics and parameters of the three-dimensional solid model The physical characteristics and parameters of the established three-dimensional solid model of the planetary reducer are verified to ensure that the three-dimensional solid model is consistent with the physical characteristics of the real components, such as mass, center of mass position, and moment of inertia. Figure 3 shows the solid model of each component of the planetary reducer established using Pro/E. The established model was checked against the design drawings one by one according to the above precautions. The structure and physical characteristics are well matched. 2.3 Establishment and verification of virtual prototype On the MSC.ADAMS platform, the necessary constraints were applied to the established three-dimensional solid model according to the working principle of the planetary reducer. The established virtual prototype model is shown in Figure 4 (the internal gear is specially treated to be semi-transparent, so as to make the internal structure clearer). The established virtual prototype was verified in the ADAMS platform [4] and the following information was obtained: VERIFY MODEL: MPRO-model 0 Gmebler Count (approximate degrees of freedom) 8 Moving Parts (not including ground) 3 Revolute Joints 5 Fixed Joints 1 Motions 2 Gears 0 Degrees of Freedom for MPRO-model There are no redundant constraint equations. Model verified successfully. The model verification result shows that there are no redundant constraints, and the model verification is successful, proving that the qualitative analysis of the established virtual prototype of the planetary reducer is correct. In order to increase the accuracy of the established model, this paper also verified the transmission ratio of the established virtual prototype model, which can confirm that the virtual prototype has extremely high credibility. The transmission ratio of the planetary reducer studied in this paper can be obtained by using equation (3). The kinematic simulation of the established virtual prototype under the condition of no drive is performed. The transmission ratio of the three-dimensional solid model prototype of the virtual planetary gear is determined to be 3.5786 by the ratio of the angular velocity of the motion input and output. It is almost the same as the designed transmission ratio. Therefore, it can be considered that the established virtual prototype is accurate and can replace the physical prototype for testing. 3 Driving simulation test based on MSC . ATV ArV system is a toolkit based on ADAMS software. It is a tracked vehicle toolkit developed by MDI (Mechanical Dynamics Inc.) AB in the United States. As an ideal tool for analyzing various dynamic performances of military or commercial tracked vehicles, it has the characteristics of multiple tracked systems in one model, full 3D capability, different topological structures, full dynamic tracked models, and soft and hard soil road surface interfaces. Based on the ArV driving simulation system, the performance prediction, fatigue analysis and system optimization design of tracked vehicle systems can be performed [2]. Figure 5 shows a simulation diagram of the tracked vehicle chassis on a certain level of road surface in ArV. Since the ArV toolkit is a dedicated toolkit for tracked/tire vehicles in ADAMS, it can study the dynamic performance of vehicle models under various road surfaces, different vehicle speeds and usage conditions; it is an ideal tool for analyzing various dynamic performances of military or commercial tracked/tire vehicles; driving simulation calculation can obtain a large amount of data closely related to the structural design and dynamic performance of tracked vehicles, which can provide an effective technical approach for the final realization of virtual manufacturing, optimization design and performance prediction of tracked vehicles. 4 Dynamic simulation based on driving simulation test 4.1 Verification of torsional strength of planetary carrier critical section under static load This paper uses the planetary carrier, a typical component in the planetary reducer, as a special case for dynamic simulation analysis. According to the design calculation manual of the planetary reducer of the tracked vehicle, the static load strength design condition is: on a 30° side slope, the adhesion force between the track and the soil is calculated, and the torque on one side is calculated as 0.6G of the vehicle weight. At this time, the calculated torque is known that the material of the planetary carrier is 45CrNi, σ[sub]s[/sub]=8000kg/cm[sup]2[/sup]. According to the traditional design theory, the critical end face of the planetary carrier is the shaft diameter of the planetary carrier. Thus, the torsional stress of the critical section of the planetary carrier is obtained from the known calculated torque and known material properties. The allowable torsional stress[5] is the torsional stress under static load. It can be seen from the comparison between the stress under static load and the allowable stress that the safety margin is large, and the critical section of the shaft diameter of the planetary carrier should meet the strength design requirements. However, this is not the case in reality. During use, the planetary carrier shaft diameter of the planetary reducer exhibits fatigue failure phenomena such as cracks and fractures. Since static strength design cannot fully reflect the characteristics of materials under dynamic loads, and since tracked vehicles mostly bear alternating dynamic loads during operation, it is necessary to analyze and calculate the fatigue strength of the planetary reducer under dynamic loads and conduct dynamic simulation analysis of the critical cross-sections of the planetary carrier. 4.2 Dynamic Simulation Research of Planetary Carrier Based on Driving Simulation Tests Due to the complex and varied working surfaces of tracked vehicles, various heavy machinery is subjected to alternating dynamic loads during operation. Due to current research limitations, most mechanical products do not undergo life prediction under dynamic loads, resulting in a significant difference between the actual lifespan and the design lifespan during actual use. To solve this problem, studying alternating dynamic loads under different mission profiles is crucial for redesign, optimization, and lifespan prediction. Due to limitations in testing methods, test time, and funding, and the widespread application of computer simulation in various industries, simulation analysis of each component under dynamic loads based on driving simulation tests has become an effective means of solving the above problems. The example of the planetary carrier in the planetary reducer demonstrates the feasibility of this method. This paper presents dynamic simulations of the planetary reducer under different road surfaces and driving speeds based on driving simulation tests. However, due to space limitations, only the driving simulation results for 3rd, 4th, and 5th gears on a certain experimental track are given, and the alternating loads borne by the planetary carrier at different gears are calculated. Figure 6 shows the alternating load borne by the planetary carrier shaft diameter when in 3rd gear. The maximum absolute value of the alternating load obtained from Figure 6 is used to calculate the maximum torsional stress (maximum torque 5233 N•m) at the critical section of the planetary carrier in a certain road section. Figure 7 shows the alternating load borne by the planetary carrier shaft diameter when in 4th gear on a certain experimental track. The maximum absolute value of the alternating load obtained from Figure 7 is used to calculate the maximum torsional stress (maximum torque 8763 N•m) at the critical section of the planetary carrier in a certain road section. Figure 8 shows the alternating load borne by the planetary carrier shaft diameter when in 5th gear on a certain experimental track. The maximum absolute value of the alternating load obtained from Figure 8 is used to calculate... The maximum torsional stress (taking the maximum torque as 10707 N·m) at the critical section of the planetary gear carrier in a certain section is compared with the allowable torsional stress at the critical section of the planetary gear carrier under static load by the simulation results of the maximum stress at the critical section of the planetary gear carrier under alternating loads in 3rd, 4th, and 5th gears. It can be seen that the maximum stress during operation in 3rd, 4th, and 5th gears is much smaller than the allowable torsional stress at the critical section of the self-propelled artillery planetary gear carrier under design. Under such a large safety margin, the main reason for the failure of planetary gear reducer components is that the planetary gear reducer mainly bears an irregular alternating load. The main reason for the failure of the planetary gear carrier is the accumulation of fatigue damage caused by a series of alternating loads during operation. Since the mechanism of fatigue damage evolution is very complex, the fatigue cumulative damage theory has not yet been well solved. The Miner linear fatigue cumulative damage theory is widely used in engineering and can predict the mean fatigue life relatively well. Miner's theory definition includes three aspects: 1) damage caused by one cycle; 2) damage caused by n cycles under constant amplitude load; 3) critical fatigue damage D[sub]CR[/sub] If it is a constant amplitude cyclic load, fatigue failure obviously occurs when the number of cycles is equal to its fatigue life N, that is, n=N. From equation (12), we get D[sub]CR[/sub]=1. For the case of level two or very few levels of loading, the critical damage value D[sub]CR[/sub] of the test specimen failure deviates greatly from 1. For random load, the critical damage value D[sub]CR[/sub] of the test specimen failure is near 1[6]. Based on Miner's definition of fatigue damage and failure criteria, it can be concluded that during the operation of a planetary reducer, the changing working profile of the system leads to significant dynamic stress and strain in the planetary reducer components. The critical section of the planetary frame is subjected to alternating loads. Although the values ​​of these loads may not be large enough to cause sudden component failure, due to their frequent action, the accumulation of fatigue damage from these variable amplitude alternating loads on the planetary frame reaches its critical fatigue damage D[sub]CR[/sub]. This results in the phenomenon that the planetary frame suddenly fails even when the stress on the critical section is far below the design "safe" stress. Therefore, determining the alternating dynamic loads borne by the planetary reducer during operation and conducting dynamic analysis of the various components of the planetary reducer under dynamic loads is of great practical significance. 5. Conclusion and Outlook This paper addresses the issue of a significant discrepancy between the actual service life and the design life of planetary reducers under static strength design theory. Based on driving simulation tests, a dynamic simulation of a planetary reducer for a tracked vehicle was conducted to obtain the alternating dynamic loads it bears under different working profiles, providing important load reference data for fatigue strength calculation and service life prediction. This paper presents a driving simulation test conducted at a test site. Combined with a virtual prototype of the planetary gear reducer, the dynamic simulation of the alternating loads borne by the planetary carrier under different gear positions was performed. The paper also analyzes, from the perspective of fatigue damage accumulation theory, why fatigue failure still occurs even when the maximum stress under alternating loads is much smaller than the design maximum stress. The dynamic simulation analysis based on the driving simulation test studied in this paper is of great significance for the study of life prediction and structural optimization under alternating loads. References : 1. Jilin University of Technology, Automotive Teaching and Research Section, ed., *Automotive Design*, Beijing: Machinery Industry Press, 1983. 2. Wu Dalin. Research on Driving Simulation Test of Self-Propelled Artillery [Master's Thesis]. Shijiazhuang: Ordnance Engineering College, 2005. 3. Rao Zhengang. Design Research of Planetary Gear Transmission, *Transmission Technology*, 1999, 39-45. 4. Zheng Jianrong. *Introduction and Improvement of ADAMS Virtual Prototype Technology*, Beijing: Machinery Industry Press, 2002(1): 1-10. 5. Wang Yizhi, Li Shuhan. Engineering Mechanics, Chongqing: Chongqing University Press, 1-5 (2) 6 Yao Weixing. Structural Fatigue Life Analysis. Beijing: National Defense Industry Press, 2003 (1)