This paper presents research aimed at understanding the effects of geometrical imperfections and tolerances upon the internal leakage occurring around null in the main stages of servovalves. Specifically, a two-dimensional (2D) computational fluid dynamic analysis was used to predict the direct leakage flow as a function of the overlap and clearance between the spool and bushing sleeve, as well as the roundness on the edges of the spool and bushing sleeve. Predictions of direct leakage flow against edge overlap, which have general validity, are provided in the paper for three selected values of the pressure drop. For different values of the pressure drop, analytical correlations can be applied using the data retrieved from these graphs. The analysis shows that the leakage flow is highly affected by the above-mentioned geometrical parameters. As expected, for given values of overlap and radial clearance, the greater the roundness of the edges caused by manufacturing processes or wear, the higher the leakage flow. For low leakage and hence low power loss requirement, the radii on the spool and bushing sleeve as well as the clearance must be maintained as low as possible. In addition, it is well-known that overlap between the spool and its bushing sleeve can help to reduce the leakage flow at null, and the effect of edge roundness on this reduction is now revealed.

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