Jack (device)

A jack is a mechanical lifting device used to apply great forces or lift heavy loads. A mechanical jack employs a screw thread for lifting heavy equipment. A hydraulic jack uses hydraulic power. The most common form is a car jack, floor jack or garage jack, which lifts 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. And are very useful lifting things that a normal human, for example, a car, cannot.

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.'

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 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.

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.

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.

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.