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A jack is a mechanical device used as a lifting device to lift heavy loads or apply great forces. A mechanical jack employ a screw thread for lifting heavy equipment. The most common form is a car jack, floor jack or garage jack which lifts vehicles so that maintenance can be performed. Mechanical jacks are usually rated for a maximum lifting capacity (for example, 1.5 tons or 3 tons). More powerful jacks use hydraulic power to provide more lift over greater distances and can be rated for many tons of load.
Scissor car jacks usually use mechanical advantage to allow a human to lift a vehicle by manual force alone. The jack shown at the right is made for a modern vehicle and the notch fits into a hard 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. A series of jacks is 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 jacking post rests on. This 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 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 the vehicle, and to ensure 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 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.
In a floor jack (aka 'trolley jack') a horizontal piston pushes on the short end of a bellcrank, with the long arm providing the vertical motion to a lifting pad, kept horizontal with a horizontal linkage. Floor jacks usually include castors and wheels, allowing compensation for the arc taken by the lifting pad. This mechanism provide a low profile when collapsed, for easy maneuvering underneath the vehicle, while allowing considerable extension.
A bottle jack or whiskey jack is a hydraulic jack which resembles a bottle in shape, having a cylindrical body and a neck, from which the hydraulic ram emerges. In a bottle jack the piston is vertical and directly supports a bearing pad that contacts the object being lifted. With a single action piston the lift is somewhat less than twice the collapsed height of the jack, making it suitable only for vehicles 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.
They 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.
A pneumatic jack is a hydraulic jack that is actuated by compressed air - for example, air from a compressor - instead of human work. This eliminates the need for the user to actuate the hydraulic mechanism, saving effort and potentially increasing speed. Sometimes, such jacks are also able to be operated by the normal hydraulic actuation method, thereby retaining functionality, even if a source of compressed air is not available.
A strand jack is a specialized hydraulic jack that grips steel cables; often used in concert, strand jacks can lift hundreds of tons and are used in engineering and construction.
The farm jack, invented in 1905 comprises a steel beam with a series of equally spaced holes along its length, and a hand operated mechanism which can be moved from one end of the beam to the other through the use of a pair of climbing pins. Typical sizes for the farm jack are 4 feet (1.2 m), 5 feet (1.5 m)and 6 feet (1.8 m) referring to the length of the beam.
The jack's versatility stems from its use for such applications as lifting, winching, clamping, pulling and pushing. It is this versatility, along with the long travel it offers and its relative portability, which make the farm jack so popular with off-road drivers.
Suspension jack or Lift Kits
Lift kits are sometimes mistaken as a type of jack, owing mainly to the slang phrase "jacked up", referring to an automobile (most often a truck or SUV) in which the frame and all attached components have been lifted away from the axles and drive members. It should be noted that suspension or body lift kits are not actually jacks in an engineering sense (ie, they are neither tools nor lifting equipment).
Safety Standards for Jacks
National and international standards have been developed to standardize the safety and performance requirements for jacks and other lifting devices. Selection of the standard is an agreement between the purchaser and the manufacturer, and has some significance in the design of the jack. In the United States, ASME has developed the Safety Standard for Portable Automotive Lifting Devices, last revised in 2009, including requirements for hydraulic hand jacks, transmission jacks, emergency tire changing jacks, service jacks, fork lift jacks, and other lifting devices. 
- Electric Scissor Jacks, JackMaster. "Electric Scissor Jacks". Retrieved 8 February 2014.
- William Cox (July 2001), "Light Talk on Heavy Jacks", Old-House Journal: 37
- Brian S. Elliott (2006), "Air-Over-Hydraulic Jacks", Compressed air operations manual, McGraw-Hill Professional, pp. 56–58, ISBN 978-0-07-147526-6
- George William Sutcliffe (1895), Steam power and mill work principles and modern practice, Whittaker & Co., p. 828, "The bottle-jack is exceedingly firm and safe for short vertical lifts, but is not convenient for pushing in a horizontal or oblique direction."
- John Norman (2009), Fire Department Special Operations, Fire Engineering Books, p. 51, ISBN 978-1-59370-193-2