## 3 Results and Discussion
### Lateral Vibration (Z-direction):  
Figure 17 presents the lateral contribution contour plot for chassis frame measurement point 1. For the lateral vibration at this point, the threshing cylinder and the engine are the primary contributing components, and the contour plot clearly shows significant influence regions from these two sources within specific frequency ranges. The PSD analysis results indicate that the main contribution peak frequency corresponds to the theoretical excitation frequency of the threshing cylinder at 58 Hz. This validates the direct impact of the threshing cylinder, as a high-speed rotating component, on the lateral vibration of the chassis frame, with the lateral forces generated by its imbalance or tooth impacts being a significant cause.  
Figure 16: Contribution contour plots from each excitation source for chassis frame measurement point 1 (X-direction).  
Figure 17: \(Cont\).  
Furthermore, this frequency is also close to the second-order harmonic of the engine's fundamental excitation frequency (29.3 Hz), which is approximately 58.6 Hz. This second-order harmonic resonance effect implies that the excitation frequency of the threshing cylinder is coupled with a harmonic frequency of the engine, leading to a marked intensification of the chassis frame's lateral vibration, especially when the threshing cylinder and engine operate concurrently. This resonance phenomenon suggests that in designing for lateral vibration reduction, it is crucial not only to consider the characteristics of individual excitation sources but also to comprehensively analyze the potential frequency coupling relationships between different sources to avoid or suppress harmonic resonance.  
(3) Longitudinal Vibration (Y-direction):  
The longitudinal vibration at measurement point 1 is primarily influenced by contributions from the cutting header, engine, and vibrating screen, as shown in Figure 18. The contribution contour plot indicates that all three excitation sources contribute to the longitudinal vibration at different frequencies, although their distribution characteristics vary. The PSD analysis reveals that the primary peak frequencies are 29 Hz and 58.6 Hz, corresponding to the excitation frequencies of the engine and the threshing cylinder, respectively, which indicates that these components play a significant role in longitudinal excitation. It is noteworthy that the special cantilevered structure of the cutting header's conveyor trough, due to its own structural characteristics or its connection properties with the frame, may produce an amplification effect under longitudinal excitation. This could significantly increase the effect of the engine's contribution to longitudinal vibration, making it a more prominent factor in this direction.  
The contributions of each excitation source and their primary excitation frequencies to measurement point 1 are summarized in Table 5.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Direction** & **Excitation Source (s)** &
\begin{tabular}{c} **Excitation Frequency** \\ **(Hz)** \\ \end{tabular} \\ \hline
1 & Vertical (X-direction) & Cutting header, Engine & 120, 29.3 \\
2 & Lateral (Z-direction) & Threshing cylinder, Engine & 58, 29.3 \\
3 &
\begin{tabular}{c} Longitudinal \\ (Y-direction) \\ \end{tabular} & Threshing cylinder & 58 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summary of primary excitation sources and frequencies for triaxial vibration at measurement point 1.  
Figure 17: Contribution contour plots from each excitation source for chassis frame measurement point 1 (Z-direction).