To increase the operating speed of machines which use hydraulics, it is necessary to increase the flow rate of hydraulic systems. As flow rate becomes larger, it becomes increasingly important at the design stage to take into account fluid transients in the fluid lines. In particular, column separation in the low-pressure return line generates a high surge pressure causing an undesirable noise and possible damage to components.
A number of studies have already been made of this phenomena. In the actual development of hydraulic systems however, there are problems in which the check valves are installed in the return line, or one of a variety of valve stroke patterns is used. These problems have not been sufficiently dealt with in the previous studies.
To analyze fluid transient phenomena in the fluid lines, a simulation program which combines the method of characteristics with numerical integration was developed. The results of the calculations with the program were verified by a comparison with the experimental results. Using this simulation program, the effect of various parameters on the surge pressure in the low-pressure return line investigated. These parameters include line dimensions, void ratio, check valve position, pressure loss, and timing of the valve control. Furthermore, to reduce the surge pressure, the use of two check valves mounted in a circuit was proposed. The effect of this proposed idea was verified with the numerical method and by experiments.

View full abstract

If the airflow from source is controlled directly with a high-speed switching valve, and the pressure in the downstream can be operated as desired. The pressure regulator, using the high-speed switching valves, is shown in this paper. The dynamic characteristics of the regulator depend on the response of the high-speed valve, then it exceeds that of a conventional diaphragm type pressure regulator. To control the pressure, the valve turns on or off the airflow very frequently according to the load conditions. In this operation, both the load flow rate and the load volume are estimated by measuring the pressure in load and the turning on/off durations are decided to keep the minimum fluctuating condition of the output pressure. Then, output pressure precision of regulator is tuned to the optimal condition at various loads, automatically. The effect of automatically tuning is shown theoretically and experimentally in this paper.

View full abstract

Principal factors that cause a decrease in the efficiency at startup and in low-speed operation of swash-plate type axial piston motors include behavior of sliding parts between i) slipper and swash-plate, ii) piston and cylinder bore and iii) valve plate and cylinder. Especially, the sliding part between ii) piston and cylinder bore has an influence on the behaviors of the slipper and valve plate.
Several studies has been made on the sliding characteristics between piston and cylinder bore in swash-plate type axial piston pumps and motors. However, in those studies, each part of the motor and pump was assumed to be rigid. Therefore, this study has been made to clarify the friction characteristics between the piston and cylinder bore at startup and low-speed operation in swash-plate type axial piston motors, regarding the elasticity of the piston and cylinder. In the simulation analysis, using the finite element method, the stress distribution and deformation of the piston and cylinder, the static frictional force between the piston and cylinder bore with a change in supply pressure, piston tube thickness and static friction coefficient were obtained. As the verification of the simulation analysis and examination in low-speed operation, the static frictional force and the dynamic frictional force between the piston and cylinder bore with a change in the supply pressure and piston tube thickness were measured.

View full abstract