In this paper, caviation phenomena occanring at a restrictor in oil hydraulic system were investipted. As a result, the following conclusions were obtained.
(1) In case of bell mouth choke which has a rounded entrance and in case of orifice plate, the cavitation occurs in the downstream apart from the outlet edge of the restrictor. On the other hand, in case of sharp edge choke, the cavitation occurs at the inside of the choke.
(2) In case of sharp edge choke, the pressore at a place where the cavitation begins to be observed is much lower than the atmospheric pressure. But in case of bell month choke, the pressure is very high.
(3) In case of bell mouth choke and of crifice plate, cavitation does not affect the discharge coefficient. But in case of sharp edge choke, the discharge coefficient is saffected by cavitation
(4) In case of bell mouth choke, the critical outlet pressure at the beginning of cavitation for a certain value of inlet pressure becomes larger when the diameter of the choke is larger, the length of the choke is shorter, and the oil temperature is higher.
(5) When a baffle plate is placed just behind the outlet end of bell mouth choke, noise doe to the cavitation is reduced.

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In a previous paper, pressure response and movement of pneumatic cylinder driven by a pair of pulses were analyzed. In this paper, we study dispersion of piston displacement driven by the same method and consider a method to decrease the dispersion.
The dispersion of displacement is considered mainly to be dependent on the dispersion of friction. Assuming that the dispersion of friction is a normal distribution, the dispersion of displacement is calculated by a strict solution of the basic equation as well as by a linear approximation. They are verified by experimental results.
The theoretical and experimental results show that the coefficient of variation of the piston displacement is almost constant regardless of changing the mean displacement, while the standard deviation increases proportional to the mean displacement. The error of the feedback positioning system is related to the dispersion of piston displacement by the final pair of pulses. It is shown that this dispersion can be decreased by using a higher supply pressure.

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