jack, mechanical device
jack, mechanical device used to multiply a relatively small applied force so that it can lift and support heavy loads, or sometimes, move massive objects into a desired position. The lever jack, often used in lifting automobiles, has a lever combined with a ratchet; the lever is used to lift the load a small distance and the ratchet prevents the load from falling back while the lever is reset so that the process can be repeated. In the screw jack the load is moved or lifted by the turning of a screw; the pitch of the screw threads is arranged so that friction is sufficient to hold the load in place when the torque applied to the screw is released. In yet another form of jack a hydraulic device is used. See hydraulic machine hydraulic machine, machine that derives its power from the motion or pressure of water or some other liquid. Hydraulic EnginesWater falling from one level to a lower one is used to drive machines like the water wheel and the turbine.
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jack, in zoology
jack: see pompano pompano , common name for fishes of the genus Trachinotus, and for Palometus simillimus, members of a large and important family of mackerellike fishes, abundant in warm seas around the world...... Click the link for more information. ; tuna tuna or tunny, game and food fishes, the largest members of the family Scombridae (mackerel family) and closely related to the albacore and bonito. They have streamlined bodies with two fins, and five or more finlets on the back.
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jack
In practical mechanics, portable hand-operated device for raising heavy weights through short distances, exerting great pressures, or holding assembled work firmly in position. The ratio of the load to the amount of force applied to the handle can be made quite high by using a gear or screw to regulate the upward extension. A ratchet allows a heavy weight to be raised in short successive stages. Though limited by the requirements of portability and ease of manual operation, jacks may lift, or exert a force of, several tons. A familiar example is the automobile jack, used to raise one end of a car to change a tire.
jack
Any of more than 150 species of fishes (family Carangidae, order Perciformes) found in temperate and tropical portions of the Atlantic, Pacific, and Indian oceans and occasionally in fresh or brackish water. Though body size and shape vary greatly, many species have small scales that create a smooth appearance, a laterally compressed body, rows of large spiky scales along the side near the tail fin, and a deeply forked tail. Many have a bluish green, silvery, or yellowish sheen. Jacks are important commercially and are favoured sport fishes. See also amberjack.
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jack [jak]
Jack
a naval flag that is raised daily on the bow of first-class and second-class ships only when they are at anchor. It is flown with the ensign (usually from 8 A.M. until sunset). The jack of warships of the Soviet Union is a red flag on which is depicted a five-pointed star in white outline with the hammer and sickle in the center.
Jack
a mechanism for lifting heavy piece loads during repair, assembly, or loading and unloading operations. Jacks are characterized by small dimensions and light weight (usually not more than 1 percent of the lifting capacity), low speed (0.01-0.25 m/min), and a lifting height of 0.15-1.0 m. However, special-purpose jacks may have a lifting capacity of several hundred tons and may lift loads to a height of several meters. A jack can raise loads smoothly, stop them accurately, and hold them at a given height. They are classified according to the type of drive as manual and electric and according to the principle of operation and their design features as rack-and-pinion, screw, and hydraulic.
The main part of a rack-and-pinion jack is the load-carrying rack, with a load-supporting pan for the load and with the lower end (foot) bent at a right angle in order to lift loads with a low-slung bearing surface. According to the type of transmission mechanism, rack-and-pinion jacks are divided into rack-and-lever and gear types. In rack-and-lever jacks the rack is advanced by a rocking drive lever; in the gear types it is advanced by means of a pinion rotated by a drive crank. Jacks with a load capacity of up to 6 tons have a one-step transmission, those with a capacity of 6-15 tons have a two-step transmission, and those with a capacity of more than 15 tons have a three-step transmission. Loads that have been lifted by the jack are held up by locking devices. The efficiency of a rack-and-pinion jack with a single-gear drive is 0.85; with a double-gear drive, it is 0.7.
The main part of a screw jack is a screw with a swiveling load-supporting pan; the screw is rotated by a crank. Jacks having sliding carriages that are also equipped with a screw are used to move loads horizontally. The load is held in position by the self-braking action of the screw, thus ensuring a high degree of operating safety. The load capacity of screw jacks usually does not exceed 20 tons, but special jacks are made with capacities of 100 tons and higher and with lifts of up to 2 m. The efficiency of screw jacks does not exceed 0.3-0.4.
Hydraulic jacks may have either periodic action with manual drive or continuous action with mechanical drive. In the periodic-action type, a plunger that supports the load is raised by a working fluid, which is forced into the lower cavity of the housing by a piston pump operated by a hand lever and equipped with intake and delivery valves. The load is lowered by letting the working fluid pass from the housing into the pump reservoir. In the continuous-action type the working fluid is forced into the space above the piston, so that it is not the piston but the housing—together with the load, reservoir, and pump—that is raised. The housing and piston are interconnected with return springs. If the cylinder is supported from below in its extreme upper position and the bypass valve is opened, the piston will rise under the action of the spring forces, and the fluid will flow into the reservoir. Supports are then put under the piston again, and the operating cycle is repeated without shifting the jack. Hydraulic jacks combine the advantages of the screw and rack-and-pinion types. They have high efficiency (0.75-0.80), smooth motion, accurate stopping, automatic braking, compactness, and large load capacity (up to several hundred tons). Their drawbacks are low speed and small lift height per operating cycle.
REFERENCES
Kifer, L. G., and I. I. Abramovich. Gruzopod”emnye mashiny, 2nd ed., part 1. Moscow, 1956.Mashinostroenie: Entsiklopedicheskii spravochnik, vol. 9. Moscow, 1949. Pages 857-63.
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