## 4 Fault Simulation and Analysis  
### Fault Simulation  
The clutch \(C_{2}\) was taken as the research object of fault simulation. Six failure modes were created in the laboratory, including the normal mode T\({}_{1}\), piston jamming mode T\({}_{2}\), seal ring damaging mode T\({}_{3}\), oil way blocking mode T\({}_{4}\), oil leaking mode T\({}_{5}\), and T\({}_{3}\times\) T\({}_{4}\) combined failure mode T\({}_{6}\).  
For failure mode T\({}_{1}\), no change was needed to the hydraulic oil circuit; for failure mode T\({}_{2}\), the axial clearance between the clutch piston and the hydraulic cylinder could be filled with sandpaper before the solenoid valve was energized to make the piston unable to move; for failure mode T\({}_{3}\), the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multicolumn{2}{c}{**Limited Flow**} & \multicolumn{1}{c}{**Pump**} & \multicolumn{1}{c}{**Shift Timing**} & \multicolumn{1}{c}{**Engine Speed**} & \multicolumn{1}{c}{**Load**} \\  & **(I/min)** & **Displacement** & **Sequence (s)** & **(r/min)** & **(N\_m)** \\ \hline
1 & 4 & 1 & 0 & 900 & 150 \\
2 & 4 & 1 & 0.15 & 1100 & 300 \\
3 & 5 & 0.7 & 0 & 900 & 300 \\
4 & 5 & 0.7 & 0.15 & 1100 & 150 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test scheme of the power shift.  
Figure 10: Test verification of the transmission model.  
normal sealing ring at the rotary joint could be replaced by the damaged one; for failure mode T\({}_{4}\), the opening degree of speed control valve should be reduced; for failure mode T\({}_{5}\), the pipe joint should be loosened; for failure mode T\({}_{6}\), the damaged sealing ring and the smaller opening of the speed control valve could be used for simulation at the same time.  
The oil pressure response of the engaged clutch C\({}_{2}\) under different failure modes is shown in Figure 11. It should be noted that the shape of the pressure curve in failure mode T\({}_{3}\)-T\({}_{5}\) was related to the severity of the fault; therefore, different fault pressure curves were selected for calculation and analysis. Take failure mode T\({}_{3}\) as an example: pressure T\({}_{3}\)\({}^{\times 1}\) corresponds to the serious damage of sealing ring, pressure T\({}_{3}\)\({}^{\times 2}\) to the moderate damage of sealing ring, and pressure T\({}_{3}\)\({}^{\times 3}\) to the slight damage of sealing ring.  
Compared with the normal mode T\({}_{1}\), the failure mode T\({}_{2}\) had the fastest rise speed of oil pressure, but this was caused by the piston being stuck and unable to move. If the stuck position was close to the initial position of piston movement (the situation simulated in this study), the clutch could not be engaged all the time. On the contrary, if the stuck position was close to the end stop of piston movement, the clutch would always be in a half engaged state, and the clutch would be burnt out. Similar situations occurred in the laboratory, as shown in Figure 12. Although the failure modes T\({}_{3}\) and T\({}_{4}\) changed the shape of oil pressure curve to some extent, it could still achieve stable engagement pressure within 1.5 s. The failure mode T\({}_{5}\) not only caused the clutch to fail to reach the expected working pressure and reduced the load driving ability of the tractor, but it also delayed the engagement time of the clutch to a large extent, which may lead to power interruption. Although the failure mode T\({}_{6}\) was the test result of the combination of the failure modes T\({}_{3}\) and T\({}_{4}\), the shape of its oil pressure curve was completely different from the two failure modes, and its lower pressure could not drive the piston action.  
Figure 11: Oil pressure curve of clutch C\({}_{2}\) in different failure modes.  
Figure 12: Clutch damaged due to failure mode T\({}_{2}\).