Hydraulic or pneumatic motors are replacing electromagnetic servomotors in many applications. It is important for engineers and designers to select adequate actuators in given servo applications. In a previous paper, Nakano had predicted the performance of future electric or hydraulic motors after that there have been many advances in actuators. In this paper the performance of hydraulic and pneumatic motors, and electric AC servo and DC motors has been calculated, surveyed, and evaluated on the basis of specifications listed for them in current catalogs and nonpublic data. We selected 765 different kinds of electric motors and 404 different kinds of fluid power motors available in the market. Power density, torque–inertia ratio, power rate, and power rate density were selected as the performance indexes for comparison. Rated power and torque were found to be nearly proportional to motor mass and moment of inertia, respectively. The electromagnetic motors have developed high performance with large rated torque and a smaller moment of inertia. The newly developed small-size hydraulic motor was also included in the performance index, and its characteristics were plotted. The compact size of fluid power actuators has great potential for power rating or quick response.
Until the 1970s, hydraulic actuators were widely used in many mechanical systems; however, recently, electric motors have become mainstream by virtue of their improved performance, and hydraulic motors have largely been replaced by electric motors in many applications. Although this trend is expected to continue into the future, it is important to comprehensively evaluate which motor is most suitable when designing mechanical systems. This paper presents the results of a survey of the performance of electric and hydraulic servo motors and aims to provide quantitative data that can be used as a reference for selecting appropriate motors. We surveyed AC, AC direct, brushless DC, and brushed DC electric motors and swash plate-type axial piston, bent axis-type axial piston, crank-type radial piston, and multistroke-type radial piston hydraulic motors. Performance data were collected from catalogs and nonpublic data. We compared and evaluated the characteristics of these diverse servo motors using indexes such as torque, rotating speed, output power, power density, and power rate.
Hydraulic systems have high-power density because its oil transmitting power has high rigidity. However, when air bubbles are mixed into oil, they reduce oil stiffness and decrease system efficiency. This study mitigates this problem by removing air bubbles from the oil using an active bubble elimination device that uses a swirl flow to eliminate air bubbles from a hydraulic fluid. We focus on the relationship between the change in the bulk modulus and elimination of air bubbles from the hydraulic fluid and experimentally measure the bulk modulus of the hydraulic oil with and without air bubbles. Moreover, to clarify the relationship between the amount of air bubbles and the effective bulk modulus of oil, we propose a mathematical model of the bulk modulus of oil containing air bubbles. The experimental results indicate that the effective bulk modulus of oil increases by eliminating the air bubbles in oil, and the curve of the bulk modulus with the bubble eliminator turned off has a small hysteresis depending on whether it is pressurized or depressurized. We investigate the calculation method of the effective bulk modulus by considering the amount of air bubbles and the amount of air being dissolved and released. Finally, we confirm that the effective bulk modulus calculated using the mathematical model agrees well with the experimental results. We conclude that the volume of air contained in the oil and the differences due to the process of dissolving and releasing air significantly influence the bulk modulus of the hydraulic fluid.
Air bubbles in working oil affect the stiffness and efficiency of hydraulic systems; thus it is important for technical issues that air bubbles be actively eliminated from the hydraulic oil. A bubble eliminator is a device that uses a swirl flow to remove air bubbles. The shape of the device affects bubble elimination performance, so the selection of shape is the most important parameter in increasing the performance of the device. The purpose of this study is to design a bubble eliminator with an optimal shape. This paper discusses the validity of numerical simulation by comparing, using various diameters of the vent port, the numerical results with the results of the experimental flow visualization. Moreover, we focus on the length of the inlet tube and tapered tube of the bubble eliminator and establish a method of selecting them.
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