Abstract: Under extremely small space, the pressure control performance of the hydraulic system can be improved by using a two-stage relief valve with a balanced piston pilot valve. In order to address the absence of a transfer function model for an integrated two-stage relief valve in the aerospace industry, the fundamental equation of a two-stage relief valve with a balanced piston pilot valve is established. The balancing piston separates a chamber in front of the pilot poppet, which is coupled with the pilot poppet to form a mass-spring vibration system with high resonant frequency. Additionally, an annular clearance is formed between the balancing piston and the pilot valve body, which is equivalent to an energy storage element, and its break frequency will decrease with the decrease of the clearance. When the break-frequency of the energy storage element is much lower than the natural frequency of the pilot valve mass-spring vibration system, the stability of the pilot valve will be guaranteed, thus improving the stability of the whole valve. The design requirement for the balanced piston is determined by the correlation between the annular clearance value and the stability of the relief valve. Finally, the correctness of the theoretical analysis is verified by experiments.

Key words: double stage relief valve, balance piston type pilot valve, stability, frequency characteristics, transfer function