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S the hydraulic motor, in most applications, absorbs only a small proportion of the available power for acceleration and the balance for controlling the load, it is well suited for automatic control systems requiring fast response and accurate control. The hydraulic motor is capable of delivering or absorbing a large amount of power within a small, light package, and can be readily adapted to a wide range of installations. Because of its light weight, it requires less mechanical support than other types of equal rating. As part of the control system, the speed of the motor can be easily regulated by variable displacement pumps or metering valves. Using these components in combination with lowpower electrical torque motors and pilot valves, remotecontrolled systems can be readily obtained with large power amplification. In addition, the high sensitivity in the lower power ranges affords a high degree of accuracy. The amount of torque that the hydraulic motor will develop is proportional to its displacement and the fluid pressure applied or developed. This torque is independent of the speed at which the motor is rotating. The limit on the torque available for a given size motor depends, in turn, upon the allowable pressure in the system. While the pressure limit is usually set by mechanical design, it is sometimes limited to control the maximum torque developed in the system. At present, hydraulic motors are manufactured to deliver from 15 to 24,000 in.-lb at 1000 psi. Motors in both aircraft and industrial designs are available for pressures up to 5000 psi. Some design information of particular interest to servo system design engineers is given in Table I. The ratings listed are theoretical and refer to the motors operated at no-load. Certain items such as acceleration and time constant will, of course, be modified by the characteristics of the load. Selection of the correct hydraulic motor for a particular application should be carefully made if optimum response is to be obtained. General rules can be followed, but the final choice must depend upon detailed calculations. The motor, however, must be capable of meeting these three basic conditions : (1) The speed range (ratio of the maximum to the minimum speed) that the motor is capable of must be greater than that required by the load. (2) The maximum allowable speed of the hydraulic motor must be greater than the maximum speed required of the load multiplied by the gear ratio. (3) The motor must supply more torque than the sum of that required by the load (as reflected through the gear train) and that required to accelerate the hydraulic motor. Speed range is the first operating characteristic that should be considered. Hydraulic motors have a minimum smooth speed of about 10 rpm, regardless of motor size. Hydraulic motor speed range is then the ratio of the maximum rated speed of the motor divided by 10 rpm. For standard production aircraft type motors, this ratio varies from 910:1 for the smallest to 312:1 for the largest. In many applications, components can be designed to reduce the minimum speed and thereby extend the speed range by a considerable margin over these figures. The speed range, therefore, determines the maximum motor sizes that can be used for a particular application. If the horsepower requirements are greater than that of the motor selected on the basis of speed range, then two or three motors must be used in parallel. The limiting 10 rpm speed also regulates the minimum gear ration that can be used. For example, if a slow speed of 0.36 deg/sec or 0.06 rev/min is required of the load, the gear ratio for this application is 10/0.06 or 167. Speed range and slow, smooth speed thus determine the maximum motor size and the minimum gear ratio. Actual r^draulic motor size will be determined from the horsepower, acceleration, and speed requirements of the load. Generally, the horsepower controls the minimum motor size while the speed determines the maximum that can be used. The selection of a hydraulic motor for a particular application can be made by the use of the graphs shown in Fig. 1. To use the graphs, the horsepower required by the load and the ratio of the required load acceleration to load velocity must be known. By entering the graph with the horsepower, the displacement can be determined where the horsepower line intersects with a point on the selected acceleration/ velocity curve. Anj^ motor which has a displacement greater than this amount is suitable for the application, providing it meets the speed range requirements of the load. Size of the hydraulic motor for a particular application can be selected by the use of the graphs and the speed range criteria. Where two or more motors are suitable, further calculation should be made to select the one which gives the best response. Experience has shown, however, that the smallest size motor will generally give the maximum response. |
