Jack (device)

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

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.

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).



Jackscrews are integral to the scissor jack, one of the simplest kinds of car jack still used.

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.

Due to the tremendous improvement in technology, many innovations has been made to the scissor car jacks. Modern car jacks functions by 12 volts electricity supplied directly from the car's cigarette lighter receptacle. The electrical energy is used to power up these car jacks to raise and lower itself automatically. This jack is conventionally known as an electric scissor jack, which is both a time and energy saver. Electric scissor jacks are significantly more efficient than classic scissor jacks in terms of the usability. Less manpower will be needed to operate these modern electric scissor jacks.[1]

House jack[edit]

Threaded rod, 7 inches fully extended
2.5-ton house jack that stands 24 inches from top to bottom fully threaded out

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 4-inch × 4-inch 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.

Hydraulic jack[edit]

Trolley jack lifting a car

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.

Bottle jack[edit]

A bottle jack or whiskey jack[2] 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.[3]

This type of jack is best used for short vertical lifts.[4] Blocks may be used to repeat the operation when a greater amount of elevation is required.[5]

Pneumatic jack[edit]

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.

Strand jack[edit]

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.

Farm jack[edit]

A farm jack mounted to the back of a Land Rover

The farm jack (also known by the brand names HANDYMAN[6] jack or HI-LIFT[7] jack) is a versatile mechanical tool that can be put to a wide range of uses. Originally invented some time around 1905 by P. J. Harrah and sold as the Automatic Combination Tool, the basic design has remained largely unchanged to this day.

The farm jack is characterized by rugged, simple construction. It 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, 5 feet and 6 feet, 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.

See also[edit]


  1. ^ Electric Scissor Jacks, JackMaster. "Electric Scissor Jacks". Retrieved 8 February 2014. 
  2. ^ William Cox (July 2001), "Light Talk on Heavy Jacks", Old-House Journal: 37 
  3. ^ Brian S. Elliott (2006), "Air-Over-Hydraulic Jacks", Compressed air operations manual, McGraw-Hill Professional, pp. 56–58, ISBN 978-0-07-147526-6 
  4. ^ 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." 
  5. ^ John Norman (2009), Fire Department Special Operations, Fire Engineering Books, p. 51, ISBN 978-1-59370-193-2 
  6. ^ U.S. Trademark Registration Serial Nos. 740,563 and 2,781,640
  7. ^ U.S. Trademark Registration Serial No. 804,605