Jackscrew
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A jackscrew, or screw jack, is a type of
Description
A screw jack consists of a heavy-duty vertical
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
Mechanical advantage
The
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.
The screw jack consists of two simple machines in series; the long operating handle serves as a lever whose output force turns the screw. So the mechanical advantage is increased by a longer handle as well as a finer screw thread. 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 speed of which the jack can operate. Using a longer operating lever soon reaches the point where the lever will simply bend at its inner end.
Screw jacks have now largely been replaced by
Applications
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.[citation needed]
In heavy-duty applications, such as screw jacks, a square thread or buttress thread is used, because it has the lowest friction and wear.
Industrial and technical applications
In technical applications, such as
The
Aviation
Jackscrews are also used extensively in aviation to raise and lower
The failure of a jackscrew on a Yakovlev Yak-42 airliner due to design flaws resulted in the crash of Aeroflot Flight 8641 in 1982.
The failure of a jackscrew on a McDonnell Douglas MD-80, resulting from a lack of grease, brought down Alaska Airlines Flight 261 in 2000.
The destruction of a jackscrew on a
Machinist's jacks
A machinist's jack is a miniature screw jack used to support protruding parts of a workpiece or to balance clamping forces on that workpiece during machining operations. Aside from their size, these frequently look no different from the screw jacks used to lift buildings off their foundations.[1][2] Machinist's jacks can be as simple as a threaded spacer with a bolt in it to serve as a jackscrew.[3]
In electronic connectors
The term jackscrew is also used for the captive screws that draw the two parts of some
The idea of incorporating jack screws into electrical connectors was not considered novel in the late 1950s and early 1960s. Some patents from that era show pairs of jackscrews on opposite sides of a multi-pin connector.[4][5] Another shows a single central jackscrew.[6] These patents mention the phrase "jack screw" incidentally, without asserting a claim to the idea.
Jack screws may have either male or female threads, and on some connectors, the genders of the screws as well as various alignment pins may be mixed in order to prevent the wrong connector from being connected to the wrong socket.[7]
See also
- Acrow prop
- Ball screw
- Leadscrew
- Roller screw
References
- ^ By Heinz K. Flack and Georg Möllerke, ed., Machine Jack, Illustrated Engineering Dictionary, Springer, 1997; Page 349.
- ^ S. L. Walton, Drill Press Vise Doubles as Machinist.s Jack, Popular Science, Vol. 188, No. 1 (Jan. 1966); page 127.
- ^ Mike's Workshop, Machinist Jacks, retrieved July 1, 2021.
- ^ Angilo M. Di Monte, Electrical Connector with Resiliantly Mounted Removable Contacts, U.S. patent 3,028,574, granted Apr. 3, 1962.
- ^ Lincoln E. Roberts, Electrical Connecting Device, U.S. patent 3,178,669, granted Apr 13, 1965.
- ^ Ronald G. Hartwell, Electrical Connector, U.S. patent 3,173,734, granted Mar. 16, 1965.
- ^ Douglas W. Jones, Repairing Electrical Connectors, last modified July 2017, retrieved June 2021.
- Reynolds, Francis J., ed. (1921). Collier's New Encyclopedia. New York: P. F. Collier & Son Company. .