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.

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