HYDRAULIC TRANSMISSION

device employing a liquid to transmit and modify linear or rotary motion and linear or turning force (torque). There are two main types of hydraulic power transmission systems: hydrokinetic, such as the hydraulic coupling and the hydraulic torque converter, which use the kinetic energy of the liquid; and hydrostatic, which use the pressure energy of the liquid. The hydraulic coupling is a device that links two rotatable shafts. It consists of a vaned impeller on the drive shaft facing a similarly vaned runner on the driven shaft, both impeller and runner being enclosed in a casing containing a liquid, usually oil (see figure). If there is no resistance to the turning of the driven shaft, rotation of the drive shaft will cause the driven shaft to rotate at the same speed. A load applied to the driven shaft will slow it down, and a torque, or turning moment, that has the same magnitude on both shafts will be developed. In a properly designed hydraulic coupling, under normal loading conditions, the speed of the driven shaft is about 3 percent less than the speed of the drive shaft. By means of a scoop tube, the quantity of liquid in a coupling and the speed of the driven shaft can be varied. Since there is no mechanical connection between the impeller and the runner, a hydraulic coupling does not transmit shocks and vibrations. The hydraulic torque converter is similar to the hydraulic coupling, with the addition of a stationary vaned member interposed between the runner and the impeller. All three elements are enclosed in a casing containing a liquid, usually oil. The effect of the stationary member is to make the torque, or turning moment, on the driven shaft greater than the torque on the drive shaft. When the driven shaft is stopped (stalled), the torque on it is a maximum and may be as much as 3.5 times the drive-shaft torque. A hydraulic torque converter acts like an infinitely variable speed transmission, delivering its higher torques when the output speed is low. In automatic transmissions for automobiles, it can be used as a partial or total substitute for a gearbox and clutch. Hydraulic transmissions of the hydrostatic type are combinations of hydraulic pumps and motors and are used extensively for machine tools, farm machinery, coal-mining machinery, and printing presses. The motor and pump can be widely separated and connected by piping. Such a system, using pressurized water, was built in London in 1882 and is still used to drive machinery to lift bridges and operate hoists. hydraulics branch of science concerned with the practical applications of fluids, primarily liquids, in motion. It is related to fluid mechanics (q.v.), which in large part provides its theoretical foundation. Hydraulics deals with such matters as the flow of liquids in pipes, rivers, and channels and their confinement by dams and tanks. Some of its principles apply also to gases, usually in cases in which variations in density are relatively small. Consequently, the scope of hydraulics extends to such mechanical devices as fans and gas turbines and to pneumatic control systems. Liquids in motion or under pressure did useful work for man for many centuries before French scientist-philosopher Blaise Pascal and Swiss physicist Daniel Bernoulli formulated the laws on which modern hydraulic-power technology is based. Pascal's law, formulated in about 1650, states that pressure in a liquid is transmitted equally in all directions; i.e, when water is made to fill a closed container, the application of pressure at any point will be transmitted to all sides of the container. In the hydraulic press, Pascal's law is used to gain an increase in force; a small force applied to a small piston in a small cylinder is transmitted through a tube to a large cylinder, where it presses equally against all sides of the cylinder, including the large piston. Bernoulli's law, formulated about a century later, states that energy in a fluid is due to elevation, motion, and pressure, and if there are no losses due to friction and no work done, the sum of the energies remains constant. Thus, velocity energy, deriving from motion, can be partly converted to pressure energy by enlarging the cross section of a pipe, which slows down the flow but increases the area against which the fluid is pressing. Until the 19th century it was not possible to develop velocities and pressures much greater than those provided by nature, but the invention of pumps brought a vast potential for application of the discoveries of Pascal and Bernoulli. In 1882 the city of London built a hydraulic system that delivered pressurized water through street mains to drive machinery in factories. In 1906 an important advance in hydraulic techniques was made when an oil hydraulic system was installed to raise and control the guns of the USS Virginia. In the 1920s, self-contained hydraulic units consisting of a pump, controls, and motor were developed, opening the way to applications in machine tools, automobiles, farm and earth-moving machinery, locomotives, ships, airplanes, and spacecraft. In hydraulic-power systems there are five elements: the driver, the pump, the control valves, the motor, and the load. The driver may be an electric motor or an engine of any type. The pump acts mainly to increase pressure. The motor may be a counterpart of the pump, transforming hydraulic input into mechanical output. Motors may produce either rotary or reciprocating motion in the load. The growth of fluid-power technology since World War II has been phenomenal. In the operation and control of machine tools, farm machinery, construction machinery, and mining machinery, fluid power can compete successfully with mechanical and electrical systems (see fluidics). Its chief advantages are flexibility and the ability to multiply forces efficiently; it also provides fast and accurate response to controls. Fluid power can provide a force of a few ounces or one of thousands of tons. Hydraulic-power systems have become one of the major energy-transmission technologies utilized by all phases of industrial, agricultural, and defense activity. Modern aircraft, for example, use hydraulic systems to activate their controls and to operate landing gears and brakes. Virtually all missiles, as well as their ground-support equipment, utilize fluid power. Automobiles use hydraulic-power systems in their transmissions, brakes, and steering mechanisms. Mass production and its offspring, automation, in many industries have their foundations in the utilization of fluid-power systems.