Eizo Urata

Frequency response of a water hydraulic servovalve

A servovalve for use with clear water is designed and constructed. The spool of the valve is supported by hydrostatic bearings, and the bearing fluid is used in its nozzle flapper system. The frequency response of the valve is measured and verified that it has a bandwidth greater than 30 Hz. A linearized analysis is made to explain the dynamic characteristics of the valve.

Development of a Direct Pressure-Sensing Water Hydraulic Relief Valve

Abstract A balanced-piston-type water hydraulic relief valve working at a rated pressure of 14 MPa is developed. The design focuses on preventing cavitation and improving the static characteristics and stability. To prevent cavitation, the main valve has two throttles of nearly equal dimensions in series. As static characteristics, pressure override and hysteresis are considered. To reduce the pressure override, the supply pressure is directly led to the pilot valve. To reduce the hysteresis, the main valve is supported by hydrostatic bearings, which eliminate Coulomb friction in the main valve. To improve the stability of the main valve, a viscous damper is attached to the main valve besides inserting a damping orifice between the main- and the pilot valves. Simulation of static- and dynamic characteristics are carried out to determine the valve dimensions. The experimental results for a flow rate of up to 20 litres/min showed that the measured pressure override and hysteresis are about 1 % and 0.1 % of the pre-set pressure, respectively. The developed relief valve did not radiate cavitation noise in the range (maximum pressure 14 MPa) of the experiments.

Development of a Water Hydraulic Pressure-Compensated Flow Control Valve

Abstract This paper submits a new design of a pressure-compensated flow control valve for water hydraulics. The operating pressure difference range and flowrate range of the developed valve are 1–14 MPa and 2.7–17 l/min, respectively. A pressure-compensator valve and a metering valve are connected in series, placing the former at the upstream side. The major features of the designed valve are that the pressure-compensator valve has two throttles to prevent cavitation, a ring for flow force compensation, and a viscous damper to stabilise the motion of the valve. Valve dimensions were determined based on dynamic and static analysis. An experimental study was carried out for a produced valve. No cavitation noise was observed for operating pressures up to 14 MPa. Mounting the flow force-compensating ring reduced flowrate variation from 8% to 4% of reference flowrate, while it increased hysteresis from 0.5% to 2.5% of reference flowrate.

One-Degree-Of-Freedom Model for Torque-Motor Dynamics

Abstract This paper shows a simplified mathematical model for a torque-motor used in servovalves. The torque-motor treated in this paper is a commonly used prototype, in which an armature and flapper assembly is supported by a flexure tube. Combining mathematical expressions for mechanical and electromagnetic elements, transfer functions between variables are given. To represent the mechanical structure, a one-degree-of-freedom vibration system is assumed. This assumption simplifies the model, although restricts the frequency range in which the model can be applied. The analysis includes the influence of eddy currents in the yokes and armature. A first-order delay relating coil current to armature torque is used to represent the influence. Experiments verified that the presented mathematical model is valid up to the fundamental natural frequency of the vibration system.

Lateral forces on tapered pistons – An overview

The purpose of this article is to contribute to a more comprehensive overview of the lateral force on tapered pistons. Numerical analyses are carried out to evaluate the lateral force on a piston; the flow rate through the clearance is also calculated. The range of parameters, namely, length-to-diameter ratio, the taper and the eccentricity, covers almost all the applications that appear in fluid power engineering. The pressure distribution obtained by difference equations is verified using an iteration convergence test. The lateral force is obtained applying Simpson’s integral to the obtained pressure. Using the dimensionless lateral forces obtained using finite mesh sizes, an evaluation of the dimensionless lateral force for an infinitely fine mesh size is made. The numerical results coincide well with experiments reported in the literature. An approximate function is shown by expressing the dimensionless force with an accuracy of about 3% for a region of the taper and the length/diameter ratio appearing frequently in practice. Four types of difference equations that are derived from the laminar flow equation are compared with respect to necessary iteration numbers and accuracy of the results.

Cavitation Erosion in High-Water-Content Fluids.

Cavitation erosion is studied for oil in water emulsions, aqueous solutions of surfactants and polymers, and polymer-added emulsions. A magnetostrictive vibrator with an open beaker is used as the test apparatus. Weight loss of an aluminium alloy specimen is measured after exposure to cavitation. The surface of the test specimen after exposure to cavitation is observed with SEM. Cavitation erosion characteristics of high-water-content fluids depend strongly on the constituent materials. Emulsified liquid paraffin with higher density suppresses cavitation erosion more effectively, and no saturation is observed up to 11 percent volumetric density. Polymer solutions can suppress erosion when the molecular weight of the polymer exceeds a certain value. Erosion suppression of mixtures of emulsion and polymer solution becomes lower or higher than those of the original constituents. The apperance of the eroded surface also varies widely with the fluids.