Hydraulic servomotors are widely used to vibrate heavy inertia loads. In these heavy duty operations, positive pressure condition in actuators must be considered for smooth operation, low noise, and the prevention from probable cavitation damage.
In the theoretical analysis on the positive pressure condition, published so for the maximum amplitude of the sinusoidal input displacement of spools was discussed in detail, but attention was little paid to output, for instance driving force of the servomotors.
In this paper, the effects of asymmetrical underlap in spool valves and other factors on the driving force of the servomotors are discussed, and it is demonstrated that the maximum permissible output force when the positive pressure condition is satisfied is largely affected by the asymmetry of the underlap.

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