Hydraulic Machine Drive
Hydraulic Machine Drive
a combined source of energy and means of transformation and delivery of energy by a fluid to a driven machine. The primary purpose of hydraulic machine drive is to provide the required relationship of speed to load in the driven machine. In a number of applications other advantages of hydraulic machine drive can be utilized; its use permits the rational arrangement of equipment, a more complete exploitation of power from the prime mover, the reduction of system shock loads, and so on. The drive may be powered by electric motor, heat engine, fluid under pressure, or other source of energy; correspondingly, the hydraulic machine drive will be termed hydroelectric drive, gas or steam turbohydrodrive, and so on. Another classification is based on the form of hydrotransmission— that is, on the nature of transformation and delivery of energy. This classification includes the positive-displacement (static-pressure), dynamic, or mixed drive.
Positive-displacement hydraulic machine drive permits highly accurate maintenance or change of speed of machines operating under a freely changing loading. It also allows machines to perform tracking operations, exactly reproducing the assigned program for rotational or reciprocal motion and amplifying the simultaneous controlling operation. Positive-displacement hydraulic machine drive is most widely used in metal-cutting machine tools; presses; systems of heat engines and hydroturbines, for automatic control and regulation; and control systems of aircraft, ships, and heavy motor vehicles. Less often it is used as the principal drive in automobile and crane propulsion systems.
Dynamic hydraulic machine drive is suitable for rotary motions only. In drives of this type the speed of the drive shaft automatically changes with change in loading, which makes these drives especially suitable for transport applications; these drives can change the speed of the vehicle automatically with changes in resistance to motion. Dynamic hydraulic machine drive is used in ships for propeller drives. It is also found in stationary-system applications, such as in feeder pumps of steam power plants, in mine-shaft lifts, and in ventilators. In such cases the dynamic form of drive makes programmed changes in the speed of the driven equipment similar to the performance of the positive-displacement type of hydraulic machine drive.
An example of mixed hydraulic machine drive is the energy storage mechanism of stamping presses, in which the energy of an electromotive source is stored in a centrifugal pump that delivers the pressure fluid to the hydraulic cylinder operating the moving parts of the press. Other combinations are also possible. For instance, in starting a gas turbine, the energy of compressed gas in the hydraulic accumulator is transmitted to the hydraulic turbine driving fluid, which in turn spins up and thus starts the thermal power plant.
A schematic diagram of a hydraulic system in a passenger car might include dynamic hydraulic transmission (hydraulic drive), a positive-displacement hydraulic system for operation of clutches and band brakes, and a system for the maintenance of fluid in the turbine drive. A direct or reduction transmission is assembled with a distributor—a positive-displacement hydraulic drive—together with a system of cranks.
Positive-displacement hydraulic machine drives are built with power up to 5,000 kilowatts; however, a very large number of these units are of 5-15 kilowatt capacity. Their speed is usually in the 1,000 rpm range, although speeds up to 18,000 rpm are found in aircraft applications. Dynamic types of hydraulic machine drives have speeds up to 35,000 rpm (although there are units with speeds down to 300 rpm). There are no practical limitations on the magnitude of power: installations of 18,000 kilowatts and more exist, although most of the units have been constructed for automotive use in power sizes up to 400 kilowatts.