Hydrostatic bearings are designed on the assumption of fluid film lubrication. However, during severe operating conditions, for instance, the bearing/seal parts of hydraulic equipment, they are usually operated in the mixed lubrication range. A mixed lubrication model of the parallel film thickness is applied to circular hydrostatic thrust bearings. The characteristics of friction, flow rate and power losses on the fluid and mixed lubrication ranges are calculated and the effects of surface roughness, eccentric loads (moment loads), supply pressure and speed of rotation are clarified numerically. By delivering the concept of the hydrostatic balance ratio, the performances of the hydrostatic bearings on the wide region from the fluid film to the mixed lubrication are expressed effectively. The conclusions are as follows : 1. a larger the roughness, larger the coefficient of friction and power losses; 2. the larger the supply pressure, the larger the coefficient of friction and power losses on the mixed lubrication range; 3. the larger the speed of rotation, the larger the coefficient of friction and power losses, and the effects of roughness become remarkable in the region of the high speed of rotation, and 4. the minimum power losses are given in that the balance ratio is close to unity.

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The purpose of the present study is to investigate two-dimensional flow in a valve chamber of a spool valve commonly used for hydraulic power applications when the valve is instantly opened and closed repeatedly.
A two-dimensional model of the spool valve was designed to view the jet flow in a valve chamber using the hydrogen bubble method. The spool is directly moved by an oil hydraulic actuator to open and close the valve repeatedly. The illuminated flow fields were recorded with a video system using a CCD B/W camera. A digital-image processing technique was employed for yielding quantitative flow field information. The time variation of the jet angle was obtained by a image-processing technique.
A numerical simulation by a source distribution method and discrete-vortex method was used to analyze the two-dimensional jet flow being issued from the orifice of the spool valve. A prediction of the jet flow pattern, pressure distribution and force was undertaken. Furthermore, the behavior of the jet flow was researched by comparing the numerical flow visual results with the experimental flow visual ones.
As a result, this study proved that the present calculation method is very useful in analyzing the unsteady jet flow issuing from the orifice of a spool valve. A deeper understanding of the behavior of the jet flow and fluid properties for repeated spool-valve opening and closing has been realized by this experimental and numerical flow visual examination.

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Cavitation erosion is one of the most serious problems as regards hydraulic equipment. Vibratory and cavitating jet methods are the major test methods of cavitation erosion. The merits of the former are a compact apparatus and convenient to exchange test fluids. Therefore, they are in wide use. However, it is impossible to test brittle and coated specimens. Also, the rise in the temperature of fluids and thermal distortion of the vibrating horn are the demerits. Cavitation used in the latter is almost same as that in valves and restrictors in the hydraulic systems. The experimental results are in good agreement with those of field operations. However, high-pressure pumps and tanks are required and the total size of the apparatus is too large. In this study, we tested six types of metal specimens (made of aluminium alloy, superduralumin, high-strength brass, stainless steel, carbon steel and chrome-molybdenum steel), and clarified experimentally relationships of the test results between the vibratory and cavitating jet methods. The conclusions are as follows. 1) The eroded volumes of aluminium alloy are largest, and those of superduralumin, those of high-strength brass, in order, and the order agree of well with both test methods. 2) The surfaces are eroded uniformly and by ring-shaped for the vibratory and the cavitating jet methods, respectively, and these behaviors are independent on the materials of specimens. The eroded surfaces due to vibration is finer than those due to the cavitating jet. 3) Fatigue fracture, plastic deformation and striation are observed. Mainly, striation and plastic deformation are observed for the vibratory and cavitating jet methods, respectively. 4) The ratios of eroded volumes by the vibratory to cavitating jet methods become constants as the time proceeds, and the ratios are almost independent regarding the materials.

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Ever since fast-switching solenoid valves were accepted as a powerful device for interfacing hydraulic systems with electronics, mechatronics technology for hydraulic actuators has developed rapidly. Such actuator development, however, has been mainly concentrated on linear motion actuators, as a result, the development of rotary-type actuators has still yet to reach a sufficient level.
A rotary-type actuator acts as an actuator by converting hydraulic energy to mechanical rotational motion within a specified angle, having a wide necessary applications. This study deals with a vane-type rotary actuator since it is superior as regards its high torque/inertia ratio and small size. By constructing a systemin which the actuator is operated by two 3-way solenoid valves, a differential-pulse width modulation method has been adopted as the method for digital control for the system. The solenoid valves in this system act as converters of the electronic-pulse signals to hydraulic ones. An arbitrary pulse width of the pressure difference across both sides of the actuator is obtained by adjusting the switching time of each valve.
The dynamic characteristics of the system were investigated by way of experiments and digital simulations. Thus it was clarified that the actuator, operated by differential-PWM, shows good linearity as a control element, achieving accurate positioning. Consequently, the availability of the system is well confirmed. Furthermore, it was recognized that the mathematical model used in this simulation might serve as a comparatively simplified model for describing the dynamics of this system.

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