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For other uses, see jack or carjacking.

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A jack is mechanical device used to lift heavy loads or apply great forces. Jacks employ a screw thread or hydraulic cylinder to apply very high linear forces.

[edit] Mechanical jack

Jackscrews are integral to one of the simplest kinds of car jacks still used.

Crew member prepares a NASCAR jack

A mechanical jack is a device which lifts heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Car jacks usually use mechanical advantage to allow a human to lift a vehicle by manual force alone. More powerful jacks use hydraulic power to provide more lift over greater distances. Mechanical jacks are usually rated for a maximum lifting capacity (for example, 1.5 tons or 3 tons). The jack shown at the right is made for a modern vehicle and the notch fits into a hard point on a unit body. Earlier versions have a platform to lift on the vehicles' frame or axle.

[edit] Hydraulic jack

A hydraulic jack uses a fluid, which is incompressible, that is forced into a cylinder by a pump plunger. Oil is used 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.

[edit] House jack

Threaded rod 7" fully extended

2.5 ton house jack that stands 24 inches from top to bottom fully threaded out.

A House jack is a mechanical device primarily used to lift houses from their foundation. A series of jacks are used and then wood cribbing temporarily supports the structure. This process is repeated until the desired height is reached. The house jack can be used for jacking carrying beams that have settled or for installing new structural beams. On the top of the jack is a cast iron circular pad that the 4" X 4" post is resting on. This pad moves independently of the house jack so that it does not turn as the acme-threaded rod is turned up with a metal rod. This piece tilts very slightly but not enough to render the post dangerously out of plumb.

[edit] Strand jack

A strand jack is a sophisticated hydraulic jack that grips and lifts steel cables; often used in concert, strand jacks can lift hundreds of tons and are used in engineering and construction.

[edit] Smoke jack

Smoke Jack from A Treatise of Mechanics (1826)

A smoke jack is a jack which gets its energy from a column of rising air or smoke. It is typically used to turn a spit in a cooking fire. It is considered an early example of a steam turbine and steam engine.

Smoke-jacks were first described by Taqi al-Din in his Al-Turuq al-samiyya fi al-alat al-ruhaniyya (The Sublime Methods of Spiritual Machines, 1551)^[1] and by John Wilkins in his Mathematical Magick (1648).

[edit] References