A jackscrew is a type of jack that is operated by turning a leadscrew. In the
form of a screw jack it is commonly used to lift moderately heavy weights, such
as vehicles. More commonly it is used as an adjustable support for heavy loads,
such as the foundations of houses, or large vehicles. These can support a
heavy load, but not lift it. Advantages
An advantage of jackscrews over some other types of jack is that they are
self-locking, which means when the rotational force on the screw is
removed, it will remain motionless where it was left and will not rotate
backwards, regardless of how much load it is supporting. This makes them
inherently safer than hydraulic jacks, for example, which will move backwards
under load if the force on the hydraulic actuator is accidentally released.
Mechanical advantage The mechanical advantage of a screw
jack, the ratio of the force the jack exerts on the load to the input force on
the lever, ignoring friction. This derives from two factors, the simple
lever advantage of a long operating handle and also the advantage of the
inclined plane of the leadscrew. Overall it is
where is the force the jack exerts on the load
is the rotational force exerted on the handle of the jack
is the length of the jack handle, from the screw axis to where the force is
applied is the lead of the screw.
However, most screw jacks have large amounts of friction which increase the
input force necessary, so the actual mechanical advantage is often only 30%
to 50% of this figure. = Limitations=
Screw jacks are limited in their lifting capacity. Increasing load increases
friction within the screw threads. A fine pitch thread, which would increase
the advantage of the screw, also reduces the size and strength of the threads.
Longer operating levers soon reach a point where the lever will simply bend
at their inner end. Screw jacks have now largely been
replaced by hydraulic jacks. This was encouraged in 1858 when jacks by the
Tangye company to Bramah's hydraulic press concept were applied to the
successful launching of Brunel's SS Great Britain, after two failed attempts
by other means. The maximum mechanical advantage possible for a hydraulic jack
is not limited by the limitations on screw jacks and can be far greater.
After WWII, improvements to the grinding of hydraulic rams and the use of O ring
seals reduced the price of low-cost hydraulic jacks and they became
widespread for use with domestic cars. Screw jacks still remain for minimal
cost applications, such as the little-used tyre-changing jacks supplied
with cars. Applications
A jackscrew's threads must support heavy loads. In the most heavy-duty
applications, such as screw jacks, a square thread or buttress thread is
used, because it has the lowest friction. In other application such as
actuators, an Acme thread is used, although it has higher friction.
The large area of sliding contact between the screw threads means
jackscrews have high friction and low efficiency as power transmission
linkages, around 30%–50%. So they are not often used for continuous
transmission of high power, but more often in intermittent positioning
applications. The ball screw is a more advanced type
of leadscrew that uses a recirculating-ball nut to minimize
friction and prolong the life of the screw threads. The thread profile of
such screws is approximately semicircular to properly mate with the
bearing balls. The disadvantage to this type of screw is that it is not
self-locking. Ball screws are prevalent in powered leadscrew actuators.
Jackscrews form vital components in equipment. For instance, the failure of
a jackscrew on a McDonnell Douglas MD80 airliner due to a lack of grease
resulted in the crash of Alaska Airlines Flight 261 off the coast of California
in 2000. The jackscrew figured prominently in the
classic novel Robinson Crusoe. It was also featured in a recent History
Channel program as the saving tool of the Pilgrims' voyage – the main
crossbeam, a key structural component of their small ship, cracked during a
severe storm. A farmer's jackscrew secured the damage until landfall.
In electronic connectors The term jackscrew is also used for the
captive screws that draw the two parts of D-subminiature electrical connectors
together and hold them mated. When unscrewed, they allow the connector
halves to be taken apart. These small jackscrews may have ordinary screw heads
or extended heads that allow the user's fingers to turn the jackscrew.
Furthermore, the head sometimes has an internal female thread, with the male
externally threaded screw shaft extending from that. The threaded-head
type can be used to panel-mount one connector and provide a means to attach
the mating connector to the first connector.
See also Acrow prop
Ball screw Leadscrew
Roller screw References
"Jackscrew". Collier's New Encyclopedia. 1921.