Hydraulics

Hydraulics, application of fluid mechanics to engineering devices involving liquids, usually water or oil. Hydraulics deals with such problems as the flow of fluids through pipes or in open channels and the design of storage dams, pumps, and water turbines. With other devices it deals with the control or use of liquids, such as nozzles, valves, jets, and flowmeters.

Two of the most important applications of hydraulics are in the design of hydraulic actuators and hydraulic presses; these are based on Pascal's law, which states that the pressure exerted on a liquid is the same in all directions. Because force equals pressure multiplied by area, forces can be greatly amplified by having liquid enclosed between two movable pistons of different area. If, for instance, one piston has a unit area of 1 and the other a unit area of 10, a unit force of 1 applied to the smaller piston, corresponding to a pressure of 1 per unit area, will result in a force of 10 per unit area on the larger piston. This mechanical advantage can be used in such hydraulic actuators as the brake on a motor car, where the relatively small force applied at the pedal is greatly multiplied to produce a large force at the brake shoe. The control flaps of aircraft are actuated by similar hydraulic systems. Hydraulic jacks and lifts are used for raising vehicles in service stations and for lifting heavy loads in the construction industry. Hydraulic presses, which were invented by the British engineer Joseph Bramah in 1796, are employed to shape, extrude, or stamp metals and to test materials under high pressures. Units developing a force of more than 4 million kg (9 million lb) have been developed for shaping whole aircraft sections.