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Positive-Displacement Hydraulic Drive

a mechanism for the transmission of mechanical energy and the conversion of one type of motion into another by the hydrostatic pressure of a fluid. Such drives may be distinguished kinematically as reciprocating, oscillating, and rotary types. The beginning of the industrial application of positive-displacement hydraulic drives dates to 1795, when the hydraulic press was invented. At the turn of the 20th century the drive was being used on naval vessels for rotating gun turrets, and by 1920-30 its applications grew to include metal-cutting tools.

The positive-displacement hydraulic drive consists of a positive-displacement pump (the driving member), a positive-displacement hydraulic engine, a tank for the working fluid, and main tubing. A hydraulic accumulator or other source of hydrostatic pressure is used at times instead of pumps. The pressure fluid (mineral oil or synthetic fluid) is taken by the pump into its working chamber and forced into the working chamber of the hydraulic engine or cylinder. The use of a positive-displacement hydraulic drive assures smooth and continuous regulation of speeds with a small persistence lag and automatic protection against overload. The self-lubricating characteristic of the drive contributes to the length of its useful life. Complex kinematic drive systems are assembled from standard production units. The compactness of positive-displacement hydraulic drives is achieved by their operation at pressures up to 35 meganewtons per sq m (MN/m²), or 350 kilograms-force per sq cm (kgf/cm²); hydraulic presses operate at pressures up to 70 MN/m² (700 kgf/cm²). The power of the units reaches 3,000 kilowatts, and their control range attains 1:1000. A positive-displacement hydraulic transmission is a major component of positive-displacement hydraulic drives used for various machines.

According to the type of control, positive-displacement hydraulic drives are classified as positive-displacement, step-controlled, and throttle-controlled. In rotary drives with positive-displacement control (see Figure 1) the fluid is forced from the working chambers of the controlled positive-displacement pump by the ejector pistons into the working chambers of the hydraulic engine. The spent fluid then flows out of the hydraulic engine and into the tank, from which it is again sucked up by the pump. The engine speed is controlled by changing the size of the working chambers of the pump and engine by means of worm gears, which are hand-operated by wheels. This changes the angle of the washer and consequently the stroke length of the ejector pistons.

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Positive-Displacement Hydraulic Drive

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