## 2 Materials and Methods
### Modified Top Cover with 8 Linked Guide Vanes  
To improve flow regulation and adaptability of the axial threshing-separation system under varying crop conditions, the factory rotor top cover--equipped with only six adjustable guide vanes and two fixed front vanes--was redesigned (Figure 3). The modified guide vane assembly allows all eight guide vanes to be linked and adjusted simultaneously via a mechanical synchronization system. This design change is intended to eliminate discontinuities in material flow, reducing internal resistance zones and ensuring consistent crop handling throughout the entire length of the rotor housing. The standard configuration of the combine harvester includes:  
* Two fixed guide vanes at the front section (positions 0.2 m and 0.4 m), set at a constant angle of 20\({}^{\circ}\). * Six adjustable guide vanes at the rear (positions 0.6 m to 1.6 m), controlled by an electric actuator integrated with the combine system. This partial adjustability leads to a flow mismatch along the rotor, especially under high-yield, high-MOG (material other than grain) conditions. Specifically, when rear vane angles are reduced to increase residence time for better separation, the fixed front vanes continue to accelerate crop flow. This results in an unbalanced axial progression rate, where the material accumulates in the middle rotor segments. Such accumulation can elevate rotor torque demand, increase grain damage due to over-threshing, and impair uniform separation. To solve this, the top cover from the combine was fully removed with guide vanes (Figure 4). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Scenario A: Rear Vane at 10\({}^{\circ}\)** & **Scenario B: Rear Vane at 35\({}^{\circ}\)** \\ \hline Front 2 vane angle & 20\({}^{\circ}\) (fixed) & 20\({}^{\circ}\) (fixed) \\ Rear 6 vane angle & 10\({}^{\circ}\) & 35\({}^{\circ}\) \\ Total material rotations & 5.19 & 2.38 \\ Average material flow speed & 0.56 m s\({}^{-1}\) & 4.07 m s\({}^{-1}\) \\ Residence time & Longer & Shorter \\ Separation intensity & Higher & Lower \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Theoretical material flow summary of scenarios A and B.  
Figure 3: Schematic of guide vanes of modified study object: 1. Rotor threshing rotation; 2. Guide vanes; 3. Guide vane linkage; 4. Variable guide vane angle 10–35\({}^{\circ}\); 5. Guide vane movement direction. The redesigned top cover was developed using SolidWorks 2024 Computer-Aided Design (CAD) software (Dassault Systems, Waltham, MA, USA) [25]. This CAD drawing served as the basis for both component fabrication and installation layout. Based on the CAD model, all needed dimensions were extracted (Figure 5). Once validated digitally, the top cover was physically modified and built according to the drawings, ensuring tight tolerances and compatibility with the combine's original mounting points (Figure 6), and installed back to the combine (Figure 7). The modified top cover design was developed and fabricated with the following key features:  
* Eight mechanically linked guide vanes, covering the full range of the top cover (positions 0.2 m to 1.6 m). * Single actuator control, enabling a unified response to automated or manual vane angle adjustment commands. * Mechanical linkage system, composed of synchronized arms, pivot joints, and connection rods that transmit angular motion from the actuator to each vane. This configuration ensures that all vanes adjust their inclination simultaneously and proportionally, regardless of position. As a result, the guide vane system now provides a consistent flow profile, minimizing the potential for abrupt transitions that disrupt material movement along the rotor helical path. The linkage system is mounted to the top cover frame and interfaces with the internal guide vane shafts through connecting brackets (Figure 6). This mechanical system is compatible with the existing combine mounting structure and does not interfere with grain handling or cleaning systems.