Abstract: The pre-stage asymmetry is the main factor causing the zero deviation of the jet-pipe servo valve. In view of the uneven size of two receiving holes, the misalignment of jet-pipe and the receiver and the asymmetry of the receiving hole center, a mathematical model of the jet-pipe pre-stage was established. A modified model based on definite integral was designed to deal with the asymmetry of the receiving hole center. Considering the asymmetry of the receiving area of the pre-stage, the quantitative analysis method and model of the pressure characteristics and the zero deviation value were established. The results show that the pre-stage processing, assembly and environmental effects will cause the asymmetry of the geometric structure and directly cause the zero deviation of the servo valve. The initial assembly error between the jet-pipe and the receiver, the initial error of the radius of the recei-ving hole will seriously cause the servo valve to produce a zero deviation. The servo valve produces a positive zero deviation, when the right receiving hole is smaller than the left receiving hole, the jet-pipe has an offset error to the right, and the angle between the axis of the right receiving hole and the vertical is smaller than the angle between the left receiving hole and the vertical. The experimental results are in accordance with the theoretical results. In the processing and assembly process of the servo valve, the symmetry of the pre-stage structure should be guaranteed as much as possible.

Key words: jet-pipe servo valve, pre-stage, asymmetry, mathematical model, zero deviation