This paper is on the development of a Direct Drive servo Valve (DDV) with flow force compensated spool. In valves with spool, flow force is caused by the unbalance of pressure exerted on each land of spool when high-speed flow passes through very narrow orifices. A simple method for flow force compensation using a stepped spool is presented in this paper. It is easy to manufacture the stepped spool of the presented method because the shape of it is simple. The method has another merit that the size of valve need not be increased. Actuating force required for driving the spool can be decreased through the compensation of flow force. The effectiveness of the proposed method is predicted through CFD analysis. The results of the CFD analysis are also utilized for the optimization of step shape. Prototypes of flow force compensated DDV are manufactured and the measurements of flow force are carried out. The measured effectiveness of flow force compensation is very similar to that from the CFD analysis. Performance test results of prototype DDV satisfy required specifications.

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