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.
More powerful jacks use hydraulic power to provide more lift over greater distances and
can be rated for many tons of load.
Jackscrew Vehicle
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.
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.
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 jack 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 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 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.
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 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 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 (Most commonly referred to as a Hi-Lift Jack)
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, 5 feetand 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.
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 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.
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.
References