## 4 Results and Discussion
### Dynamic Response of the Hinged Joint between the Canopy and the Caving Shield  
The impact dynamic load needs to be transmitted downward to other components and floor strata for the hydraulic support. The hinged pin shafts connecting adjacent components play a key role in the load transfer process. The stress condition of hinged pin shafts affects the underground safety support. Figure 1b shows the typical failure modes of the hinged pin shafts. Therefore, to analyze the influence of different forms of movement instability on the force of the hinged pin shafts, the load of the hinged joint between the canopy and the caving shield under different instability forms is compared, as shown in Figure 12. Load curve of the hinged joint between the canopy and the caving shield.  
Figure 11: The vertical displacement and stress distribution of the canopy: (**a**) Sliding instability at the coal wall, (**b**) Rotary instability at the coal wall, (**c**) Rotary instability at the roof cutting.  
As shown in Figure 12. Load curve of the hinged joint between the canopy and the caving shield., under the sliding instability of the upper immediate roof at the coal wall, the load of the hinged joint is in a damped oscillation state. The peak value of the load is 4488 kN, and the stable value is 2600 kN. In the event of rotating instability of the upper immediate roof, the load of the hinged joint grows rapidly and varies violently, but the different positions of the rotary instability lead to the change of the peak and stable value of the load. Compared with the rotary instability at the coal wall, under the rotary instability at the roof cutting, the peak value of the load is 1.25 times higher, which is 13,560 kN. However, the stable value of the load is the smallest of the three instability forms, at only 1300 kN. On the contrary, in the case of rotary instability at the coal wall, the stable value of the load is the largest, which is 6950 kN. This is because the cutting line is close to the hinged joint between the canopy and caving shield, and the hinged joint will continue to receive the impact dynamic load. At the same time, because the unstable rock block at the roof cutting is also close to the goal, this will cause the impact energy to be absorbed by the gangue in the goaf, so in the case of the rotary instability at the roof cutting, the fluctuation gradient of the load is the largest. In summary, different forms of movement instability of the upper immediate roof cause the hinged joint load to fluctuate. The impact dynamic load of the canopy is affected by the crushing state of the top coal, resulting in severe stress on the hinged pin shafts and altering the peak and steady value of the load.  
Therefore, to meet the requirements of load transfer and ensure the safety and reliability of the hydraulic support, the pin shaft should use high-strength materials and have sufficient toughness to avoid bending fractures due to impact dynamic load. In addition, the buffer energy absorption structure can be used to weaken the impact load of the pin shaft caused by the roof instability and ensure the stability between the bearing components and the support attitude of the hydraulic support.