There are three types of rotary hydraulic motors; the axial-plunger-type, the vane-type and the gear type.
Among them, the gear type is usually considered that its performance is not so satisfactory as.others. But the gear-type has the advantage of simplicity and low cost.
In order to investigate its practicability, a gear motor was built experimentally and applied to a torque amplifier.
As a conclusion, it was confirmed that a properly designed gear motor can be used as a component: of a control system.

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The surge tank of a hydraulic power station is designed to absorb the surges in the pressure tunnel at the load-trip of the generator.
Recently automatic frequency control has been studied in the electric power systems, but before practice this method, it is necessary to re-investigate the capacity of the surge tanks of hydraulic power stations.
In this paper, the author gives a way to estimate how much control gain can be expected from a hydraulic power station with a simple surge tank, when controlled by this method.
By using an analog computer he calculates the variation of the water level in the surge tank under resonance with sinusoidal load change when the generator-load is cut-off. This is done in accordance with the statistical calculation of the water level variation, using the RMS-error method developed by Phillips.

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It was described in the previous papers that the time constant of the D. C. shunt type servomotor driven by the grid controlled thyratron amplifier was several times larger than that driven by the battery source. Therefore, in the actual servosystem, the velocity feedback by a tachometer is generally available for improving system performance.
As shown in Figure 15, the electronic servosystem by the grid controlled thyratron amplifier should be analyzed as a sampled-data control system, the sampling period of which is determined by the line frequency applied to the anode of the thyratron. In this paper, it is assumed that the thyratron amplifier is a sample-data system with zero-order hold circuit and a sampler as shown in Figure 1.
Under this assumption, the inditial responses of the velocity servosystem as shown in Figure 1 and of the positional servosystem as shown in Figure 7, are analyzed theoretically and experimentally. As the result of these analysis, it has revealed the following;
(1) The servomotor with thyratron amplifier should be treated as a sampled-data control element.
(2) The stability of inditial response calculated by this treatment is worse than that calculated by treating this system as a linear system shown in Figure 6 (See Figure 2 and 13).
(3) The experimental response is less stable than the calculated response because of nonlinearity in elements; saturation, dead zone, and others (See Figure 5 and 13)

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