The nozzle-flapper valve has been widely used as a preamplifier in electro-hydraulic servovalves by virtue of the characteristics of high sensitivity, broad bandwidth and a relatively simple construction.
However, when we increase the supply pressure, we often experience very high frequency self-excited oscillation of the nozzle-flapper valves, and also observe hysteresis phenomena such that two stability limits exist according to the struting and stopping of the oscillation.
To the authors, knowledge, there have been very few published papers about the self-excited oscillation of the nozzleflapper valve, and no researches has been made on the hysteresis of stability limit.
Therefore, the purpose of this research is to clarify the causes of the self-excited oscillation and the hysteresis of stability limit.
From the results of the experiments carried out the following became clear: 1) the magnitude of the discharge coefficient was strongly dependent of the flow rate through the orifice or the nozzle: 2) consequently, the nonlinearity between the flapper displacement and the jet force acting on the flapper was saturation (soft) or anti-saturation (hard) owing to the choice of the operating (equilibrium) point of the valve decided by the nozzle-flapper distance and the supply pressure.
As the result of the above mentioned, it was shown theoretically as well as experimentally that the jet force strongly. contributes to the self-excited oscillation of the valve and that the anti-saturation characteristic of the jet force results in the hysteresis of stability limit.

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This paper deals with the cavitation characteristics in poppet valves, especially as regards the pressure distribution along the valve seat and valve chamber surface, the thrust and transverse vibration of various valves which can be obtained by changing the combinations of the valve face angle 2a, the valve seat width s and valve chamber diameter D.
The results obtained are as follows: 1) In case of s/d = 1, 2 (d: inlet diameter of the valve) the pressure of the valve seat surface indicates the minimum value just behind the upstream edge of the valve seat or just before edge of the valve seat and is slightly higher than those in the middle region.
2) In case of D/d= 6, 10 for each combination of s/d and 2α, upstream pressure along the valve chamber is lower than downstream one.
3) In case of a valve with one combination of s/d = 0.25, 0.5 and a valve face angle of 2α = 30°, 60° in cavitating flow, there is little difference between thrust coefficient f obtained from the pressure distribution along the valve face and thrust coefficient f_m, calculated by the momentum theory, for all the values of the sectional area A(H) of the annular passage way of the valve and upstream P₁, but in case of s/d =1, 2, there is a small difference between f and f_m
4) The stability for transverse vibration of the poppet valve in cavitating flow is affected by various parameters; s/d, 2α, P₁ and A(H).
Namely, in case of a large A(H), lower P₁, indicates better stability against valve vibrations for each s/d.
Furthermore, as the value of s/d becomes larger, the range of A(H), which is able to stabilize the valve's vibrations, becomes larger under the condition where P₁ is constant.

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