## 4 Conclusions  
1. This study developed and validated a robust diagnostic framework based on an improved Operational Transfer Path Analysis (OTPA) to decouple and quantify multi-source vibrations in a combine harvester chassis. The primary contribution of this work lies in demonstrating that an SVD-enhanced OTPA methodology can effectively address the challenge of highly correlated vibration sources inherent in such complex agricultural machinery, providing clear, actionable insights for structural optimization. Future research should be conducted to further enhance the vibration control of the combine harvester and to gain a more comprehensive understanding of the dynamic characteristics of the chassis frame. For example, detailed experimental modal analysis should be performed to clearly determine the critical modal damping ratio. This will provide a more precise basis for selecting and designing vibration suppression solutions.
2. The analysis quantitatively identified the 'cutting header \(\rightarrow\) hydraulic cylinder \(\rightarrow\) chassis frame' path as the most critical contributor to vertical vibration (117.6 dB). Furthermore, a coupled resonance between the threshing cylinder and the engine's second-order harmonic was found to be the main driver of lateral vibration. These findings are broadly consistent with related research that identifies the engine and primary working units as major vibration sources [45]. However, our work extends these findings by providing quantitative detail on the specific transmission paths and coupling phenomena, thereby offering more precise targets for engineering interventions.
3. From a methodological standpoint, the use of SVD to regularize the input matrix proved essential for achieving stable and reliable transfer functions, overcoming a common limitation of traditional OTPA, as noted in the literature [42]. The high fidelity of the reconstructed signal against the measured signal validates the effectiveness of this approach. However, minor discrepancies persist, likely due to unmodeled nonlinearities or background noise, indicating a limitation of the linear system assumption. This highlights that while the improved OTPA is a powerful diagnostic tool, its application requires careful sensor placement and an awareness of potential non-linear effects, a challenge common in all TPA-based methods [41].  
[PERSON] conceived the project, consulted the literature and collected the data, wrote the manuscript, and prepared the figures. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] revised the manuscript. All authors have read and agreed to the published version of the manuscript.  
This research work was supported by the Taizhou Science and Technology Support Programme (Agriculture) Project: Key Technology and Equipment for Efficient Harvesting of Pea Seedlings Growing in Disorder in the Field (TN202315); the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX25_4246); the Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX25_2455); and the Key Laboratory of Modern Agricultural Equipment and Technology of Ministry of Education, Jiangsu University (MAET202326).  
Not applicable.  
Data Availability Statement: The data used to support the findings of this study are available from the corresponding author upon request.  
The authors declare no conflicts of interest.