It is well known that we can obtain a large change of back pressure by a slight displacement of a flapper in nozzleflapper valves. By virture of the above mentioned characteristics, nozzle-flapper vaives are widely used as pre-amplifiers in electro-hydraulic servovalves.
The static characteristics of the nozzle-flapper valves have been established and detailed results have been published. However there have been few published works on the dynamic characteristics of the valves, especially in a high-frequency region such as more than several hundreds Hz.
Recently, the band-width of the electro-hydraulic control system has been required to be broader than 100Hz and we have often experienced very high frequency self-excited oscillation of nozzle-flapper valves whose cause has not been clarified.
Therefore, the purpose of this research is to clarify the dynamic behaviors of the nozzle-flapper valves in a very high frequency region theoretically and experimentally.
From the results of the present work, it becomes clear that the dynamic characteristics of the nozzle-flapper valves can be given by a relatively simple linear transfer function and the effect of flapper dynamic motion must be taken into consideration. Moreover, it is also clarified that the deliberate use of the inertia effect of the oil column in upstream restriction can improve the dynamic characteristics of the valve.

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