It is well-known that the axial flow force exerted onto a spool is caused by the fluid flowing through the spool valve. This force has not only a large influence onto the driving force of the spool, but also sometimes causes self-excited owll. lation of the spool valve. Therefore, it is important to have full knowledge of this force in designing a spool valve.
To the author's knowledge, there have been few published papers showing the experimental results for this force, except in steady state.
In this reserch, the frequency responses of the variation of the valve opening with both the axial flow force and the flow rate flowing through the spool valve are obtained experimentally. Moreover, it is shown that the experimental results of the above mentioned axial flow force coincide well with the calculated ones obtained from a generalized formulation of the momentum theory derived recently by Ikebe and Ohuchi.

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