Although the middle position of the O-type 3-position-4-way directional valve with the spool structure is not the working position of the valve, the performance of the middle position is also one of the considerations in the system design. The leakage characteristics of the middle position affect the correctness of the final action of the external load. Leakage effects of the middle position on the action of load under 3 common working conditions were analysed. The results showed that the position of the load actuator moved towards the load direction because of the leakage. The larger the leakage, the faster the response, and the faster the moving speed; the different gaps between different valve ports aggravated the previous phenomenon. The use of a pair of pilot-operated check valves in series and a parallel bypass valve at the inlet and outlet of the actuator could effectively improve the actual cut-off effect of the directional valve.
A superimposed hydraulic back pressure valve with adaptive back pressure was designed. The back pressure of the valve changes as the diving depth of underwater vehicles changes, and is always higher than outboard environmental pressure by an adjustable value, preferred 0.5MPa, to avoid leakage of environmental medium to the inboard hydraulic system. The valve was designed in a pilot-operated style with a flat valve port of main valve. Variable damping plugs were used to easily regulate the dynamic performance of the valve. And an isolation device was adopted to isolate the contact between the inboard and outboard media. The static mathematic models of the valve were established and simulations were carried out. The simulation results show the valve has good pressure stability and flow adaptability, and its back pressure changes with the change of outboard pressure well.
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