## 4 Discussion
### Test Process Analysis  
The results of static loading tests indicated that when the inflation pressure of the hydro-pneumatic spring was around 7 MPa, there was no leakage observed in either of the two hydro-pneumatic springs with seal ring compression ratios of 10% and 15% under static loading conditions.  
Through dynamic loading tests, it was found that the internal pressure of the hydro-pneumatic spring reached approximately 23 MPa at maximum working stroke. The hydro-pneumatic spring with a seal ring compression ratio of 10% experienced hydraulic fluid leakage during the initial reciprocating motion of the hydro-pneumatic spring and stopped the follow-up trial. The hydro-pneumatic spring with a seal ring compression ratio of 15%, on the other hand, did not exhibit significant leakage in the first 4000 cycles of reciprocating motion testing. However, at approximately 4600 cycles, the seal ring began to leak oil. As shown in Figure 16, at this point, the lip of the step seal on the seal ring had been completely worn flat, presenting a sloped condition, and the seal ring had lost its sealing capability entirely.  
Building upon the analysis in Section 4.1, when the maximum stress is concentrated on the contact surface between the step seal ring and the piston rod, in the practical operation of the hydro-pneumatic spring, as the piston rod reciprocates, there is repeated friction between the piston rod and the step seal ring. The greater the stress on the step seal ring, the greater the frictional wear it receives. A Figure 15 depicts the wear conditions of the step seal ring under different durations of use. Due to the prolonged reciprocating friction between the step seal ring and the piston rod, the contact portion of the step seal ring with the piston rod experiences severe wear, eventually resulting in the flattening of the step seal ring lip, causing the seal ring to lose its sealing performance [22]. The maximum cross-sectional thickness of the step seal ring after testing with five sets of seal rings is shown in Figure 16. The step seal rings suffered varying degrees of wear and deformation. The two seal rings with the most severe wear had their step seal ring lips completely worn flat, presenting an overall sloped condition. Although there was no oil leakage at 4000 cycles, by this point, the lip of the step seal ring had been completely flattened. At this stage, the contact surface width was excessive, and even if it could still maintain a seal under this pressure impulse, the resulting friction would limit the damping characteristics of the hydro-pneumatic spring and impact its performance.  
The tests, which included both static and dynamic loading tests, verified the sealing performance of the hydro-pneumatic spring seal rings under two different compression ratios. To a certain extent, these tests confirmed the reliability of the simulations. The seal rings with compression ratios of 10% and 15% exhibited good sealing performance under static loading conditions. However, when subjected to impact pressure, the seal with a compression ratio of 10% experienced a change in its relative position, leading to leaks during subsequent impacts and a loss of sealing performance. On the other hand, when the seal ring had a compression ratio of 15%, it maintained good sealing performance even under impact pressure, ensuring that the seal surface did not leak. In subsequent sealing structure design and sealing ring selection, the appropriate sealing ring compression rate should be selected, and the wear resistance of the stepped ring should be improved to achieve the purpose of improving the service life of the sealing ring.