hydraulics

 

branch of engineering concerned mainly with moving liquids. The term is applied commonly to the study of the mechanical properties of water, other liquids, and even gases when the effects of compressibility are small. Hydraulics can be divided into two areas, hydrostatics and hydrokinetics. Hydrostatics, the consideration of liquids at rest, involves problems of buoyancy and flotation, pressure on dams and submerged devices, and hydraulic presses. The relative incompressibility of liquids is one of its basic principles. Hydrodynamics, the study of liquids in motion, is concerned with such matters as friction and turbulence generated in pipes by flowing liquids, the flow of water over weirs and through nozzles, and the use of hydraulic pressure in machinery.

Hydraulics is a topic of science and engineering dealing with the mechanical properties of liquids. Hydraulics is part of the more general discipline of fluid power. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. Hydraulic topics range through most science and engineering disciplines, and cover concepts such as pipe flow, dam design, fluid control circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement, river channel behavior and erosion.

The word "hydraulics" originates from the Greek word ὑδραυλικός (hydraulikos) which in turn originates from ὕδραυλος meaning water organ which in turn comes from ὕδωρ (water) and αὐλός (pipe).

History

The earliest masters of this art were Ctesibius (flourished c. 270 BC) and Heron of Alexandria (c. 10–70 AD) in the Greek-Hellenized West. In ancient China there was Sunshu Ao (6th century BC), Ximen Bao (5th century BC), Du Shi (circa 31 AD), Zhang Heng (78 - 139 AD), and Ma Jun (200 - 265 AD), while medieval China had Su Song (1020 - 1101 AD) and Shen Kuo (1031 - 1095). The ancient engineers focused on sacral and novelty uses of hydraulics, rather than practical applications. In ancient Sri Lanka, the Sinhalese used hydraulics in many applications, in the ancient kingdoms of Anuradhapura and Polonnaruwa. The discovery of the principle of the valve tower, or valve pit, for regulating the escape of water is credited to Sinhalese ingenuity more than 2,000 years ago. By the first century A.D, several large-scale irrigation works had been completed. Macro- and micro-hydraulics to provide for domestic horticultural and agricultural needs, surface drainage and erosion control, ornamental and recreational water courses and retaining structures and also cooling systems were in place in Sigiriya, Sri Lanka.

In 1619 Benedetto Castelli (1578–1643), a student of Galileo Galilei, published the book Della Misura dell'Acque Correnti or "On the Measurement of Running Waters", one of the foundations of modern hydrodynamics. He served as a chief consultant to the Pope on hydraulic projects, i.e., management of rivers in the Papal States, beginning in 1626.^[1]

Blaise Pascal (1623–1662) study of fluid hydrodynamics and hydrostatics centered on the principles of hydraulic fluids. His inventions include the hydraulic press, which multiplied a smaller force acting on a smaller area into the application of a larger force totaled over a larger area, transmitted through the same pressure (or same change of pressure) at both locations. Pascal's law or principle states that for an incompressible fluid at rest, the difference in pressure is proportional to the difference in height and this difference remains the same whether or not the overall pressure of the fluid is changed by applying an external force. This implies that by increasing the pressure at any point in a confined fluid, there is an equal increase at every other point in the container, i.e., any change in pressure applied at any point of the fluid is transmitted undiminished throughout the fluids.

Hydrostatic power transmission

A hydrostatic power transmission system makes use of fluid under pressure to drive a mechanical load. In this sense, hydrostatic means that energy transfer is brought about by fluid flow and pressure, but not from the kinetic energy of the flow (the latter would be characteristic of a hydrodynamic drive, such as a fluid coupling or torque converter).

A basic hydrostatic power transmission system consists of a positive displacement pump driven by the prime mover, a positive displacement hydraulic motor, interconnecting piping (which may be a combination of steel tubing, actual pipe and hoses), and a reservoir. Additional components, such as valves and filters, are often part of such a system, the former to provide control, and the latter to protect precision machined parts from damage due to oil-borne contaminants.

Motion is transmitted by the pump drawing oil from the reservoir, pumping it into the motor, with the discharge returning to the reservoir. The flow of oil causes the motor to rotate at a speed that is proportional to the pump speed. Any resistance to motor rotation will cause system pressure to rise due to the use of the positive displacement pump, which will translate as torque at the motor.

The maximum torque that can be exerted by the motor is determined by the maximum pressure in the system, as well as the ratio between the displacement of the pump and the displacement of the motor, displacement being expressed in cubic inches or cubic centimeters per revolution. For example, a pump specified as displacing 10 cubic inches per revolution will (in theory) pump exactly 10 cubic inches of oil for each revolution (the actual output will be lower due to internal leakage in the pump). If said pump is mated with a motor that displaces 20 cubic inches per revolution, the drive ratio will be 2:1 and the motor will run at one half the speed of the pump, but develop approximately twice the torque applied to the pump. Hence hydrostatic power transmission behaves in a fashion similar to that of a purely mechanical equivalent of gears and shafts.

The principal advantage of hydrostatic power transmission is the flexibility of pump and motor positioning within the equipment, since the only connection between the pump and motor is through the piping, which can be routed in whatever fashion is convenient for the machine designer. Hydrostatic transmission also has the advantage of high capacity relative to component size.

Hydrostatic power transmission is widely used in industrial machinery and earthmoving equipment, and has found some application in transportation.

The main disadvantage of hydrostatic drive is its inefficiency relative to geared transmissions. This inefficiency stems from internal leakage, viscous and inertial fluid flow losses and mechanical friction losses, all of which result in power being wasted as heat.

See also

• Hydraulic press
• Hydraulic machinery
• Hydraulic drive system
• Hydraulic fluid
• Rotary power

References

1. ^ Benedetto Castelli (1578-1643), The Galileo Project

External links

• International Association of Hydraulic Engineering and Research (IAHR)
• National Fluid Power Association (NFPA)
• Pascal's Principle and Hydraulics
• The principle of hydraulics
• More than 300, freely available, published research articles on hydraulic engineering and related topics by Professor Hubert Chanson, Department of Civil Engineering The University of Queensland

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