## 1 Introduction  
As an indispensable core piece of equipment in agricultural production systems, the combine harvester plays a critical role in key operations such as grain harvesting, threshing, and cleaning. Its efficient and stable operation is directly linked to agricultural productivity and profitability [1, 2, 3, 4, 5]. However, during field operations, combine harvesters are subjected to complex and severe vibration problems [6, 7, 8]. This phenomenon stems primarily from the dynamic excitation generated by various internal working components--such as the engine, cutting header, threshing cylinder, and vibrating sieve--operating at high speeds or in periodic cycles. It is further compounded by external impacts arising from complex field conditions, including uneven ground and variations in crop feed rate [9, 10, 11, 12]. Thesemulti-source, dynamic excitations are ultimately transmitted to the chassis frame, which serves as the core load-bearing structure of the entire machine. Severe vibration exerts multiple adverse effects on the performance and lifespan of a combine harvester. It not only accelerates the structural fatigue of the chassis frame and its connected components, leading to premature failure and consequently reducing the machine's overall reliability and service life, but also increases maintenance costs and downtime [13; 14; 15; 16; 17; 18]. Furthermore, vibration significantly impairs operational efficiency and quality by, for instance, causing instability in the cutting and cleaning processes and reducing operational precision [19; 20]. More critically, the vibration transmitted to the operator's cabin severely degrades the driver's comfort and health, thereby compromising operational safety [21; 22; 23]. Therefore, precisely identifying excitation sources, quantifying their transfer paths, and understanding the chassis frame's vibration response are critical for developing targeted vibration reduction strategies. Such efforts are essential for enhancing machine performance, extending service life, and improving operator comfort and safety [24; 25]. To address the vibration problems in mechanical systems, researchers worldwide have conducted extensive studies. In the analysis of structural mechanical properties, the Finite Element Method (FEM) has been widely applied to the static and modal analysis of agricultural machinery chassis to assess their strength, stiffness, and natural frequencies, thereby providing a foundation for structural optimization [26; 27; 28]. However, these studies often focus on single operating conditions or static analysis and thus fail to comprehensively capture the complex vibration behavior under multi-source dynamic excitation. Recent research on combine harvesters has explored these complex dynamics using various approaches. For instance, studies [29] have used advanced signal processing techniques like the low-pass filter-EEMD combination to extract vibration signals from specific components under different working conditions and analyzed non-stationary vibration characteristics caused by the machine's time-varying mass during harvesting [30]. Other works have focused on identifying the overall vibration response under multisource excitation [31] or on optimizing the topology of individual components like the header to mitigate vibration [32]. While these valuable studies successfully identify dominant vibration frequencies and characterize the system's response, they often do not quantitatively diagnose the entire vibration transmission system. Specifically, there is a research gap in systematically decoupling the contributions of individual excitation sources and precisely quantifying how much each specific transmission path contributes to the vibration at a target point on the chassis. This precise decoupling and quantitative analysis of multi-source coupling effects and their corresponding transfer paths represent a common challenge and a research hotspot in the design and optimization of modern complex engineering systems [33].