A jack is a mechanical lifting device. Most often, the term "jack" denotes equipment used to apply great forces or lift heavy loads. There are also smaller, more delicate jacks used as part of the action of keyboard instruments. Of the jacks of the first category (heavy lifting), a mechanical jack employs a screw thread. A hydraulic jack uses hydraulic power. The most common function of a jack is as a car jack, floor jack or garage jack, which lift vehicles so that maintenance can be performed. Jacks are usually rated for a maximum lifting capacity (for example, 1.5 tons or 3 tons). Industrial jacks can be rated for many tons of load.
Etymology The personal name Jack came in the 16th century to be used as a colloquial word for 'a man (of low status)' (much as in the modern usage 'jack of all trades, master of none'). From here, the word was 'applied to things which in some way take the place of a lad or man, or save human labour'. The first attestation in the Oxford English Dictionary of jack in the sense 'a machine, usually portable, for lifting heavy weights by force acting from below' is from 1679, referring to 'an Engine used for the removing and commodious placing of great Timber.'
Jackscrew
Scissor jack
A scissor jack uses the mechanical advantage of a leadscrew and 4-bar linkage to allow a human to lift a vehicle by manual force alone. They are inexpensive and are common in manufacturer-supplied breakdown kits. The jack shown (see image) is made for a modern vehicle and the notch fits into a jack-up point on a unibody. Earlier versions have a platform to lift on a vehicle's frame or axle. A well-engineered jack employs an acme thread for efficient winding without the jack losing its position when under load. Electrically operated car scissor jacks are powered by 12-volt electricity supplied directly from the car's cigarette lighter receptacle. The electrical energy is used to power these car jacks to raise and lower automatically. Electric jacks require less effort from the motorist for operation.
House jack
A house jack (also called a screw jack) is a mechanical device primarily used to lift buildings from their foundations for repairs or relocation. After employing a series of jacks, wood cribbing temporarily supports the structure. This process is repeated until the desired height is reached. The house jack can be used for raising carrying beams that have settled, or for installing new structural beams. On top of the jack is a cast-iron circular pad upon which the jacking post rests. The pad moves independently of the house jack so that it does not turn as the acme-threaded rod is turned with a metal rod. This piece tilts very slightly, but not enough to render the post dangerously out of plumb.
Hydraulic jack
In 1838, William Joseph Curtis filed a patent in Britain for a hydraulic jack. In 1859, inventor Richard Dudgeon was granted a patent for a "portable hydraulic press" — the hydraulic jack. This jack proved to be vastly superior to the screw jacks in use at the time. Hydraulic jacks are typically used for shop work, rather than as an emergency jack to be carried with the vehicle. Use of jacks not designed for a specific vehicle requires more than the usual care in selecting ground conditions, the jacking point on a vehicle, and ensuring stability when the jack is extended. Hydraulic jacks are often used to lift elevators in low and medium-rise buildings. A hydraulic jack uses a liquid, which is incompressible, that is forced into a cylinder by a pump plunger. Most often, the liquid of choice is oil, since it is self-lubricating and stable. When the plunger pulls back, it draws oil out of the reservoir through a suction check valve into the pump chamber. When the plunger moves forward, it pushes the oil through a discharge check valve into the cylinder. The suction valve ball is within the chamber and opens with each draw of the plunger. The discharge valve ball is outside the chamber and opens when the oil is pushed into the cylinder. At this point, the suction ball within the chamber is forced shut, and oil pressure builds in the cylinder.
Floor jack
Within a floor jack (a.k.a. trolley jack), a horizontal piston pushes on the short end of a bellcrank. The long arm provides the vertical motion to a lifting pad, kept horizontal with a horizontal linkage. Floor jacks usually include casters and wheels, allowing compensation for the arc taken by the lifting pad, so may only be used on a hard, smooth, level floor. This mechanism provides a low profile when collapsed, for easy maneuvering underneath the vehicle, while allowing considerable extension.
Bottle jack
A bottle jack (a.k.a. whiskey jack) resembles a bottle in shape, having a cylindrical body and a neck. Within is a vertical lifting ram with a support pad of some kind fixed to the top. The jack may be hydraulic or work by screw action. In the hydraulic version, the hydraulic ram emerges from the body vertically by hydraulic pressure provided by a pump either on the baseplate or at a remote location via a pressure hose. With a single-action piston, the lift range is somewhat limited, so its use for lifting vehicles is limited to those with a relatively high clearance. For lifting structures such as houses, the hydraulic interconnection of multiple vertical jacks through valves enables the even distribution of forces while enabling close control of the lift. The screw version of the bottle jack works by turning a large nut running on the threaded vertical ram at the neck of the body. The nut has gear teeth and is generally turned by a bevel gear spigotted to the body, the bevel gear being turned manually by a jack handle fitting into a square socket. The ram may have a second screwed ram within it, which doubles the lifting range telescopically. Bottle jacks have a capacity of up to 50 tons and may be used to lift a variety of objects. Typical uses include the repair of automobiles and house foundations. Larger, heavy-duty models may be known as a barrel jack. This type of jack is best used for short vertical lifts. Blocks may be used to repeat the operation when a greater amount of elevation is required.
Pneumatic jack
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