Screw is an inclined
plane wrapped in a spiral around a shaft. The screw is one of the six simple
machines developed in ancient times. The other five are the lever, the wheel
and axle, the pulley, the inclined plane, and the wedge. See Machine (Six
simple machines) .
A screw consists of two main parts—the body
and the thread. The body of a screw may be a cone or a cylinder. The
center line of the body is called the axis. The thread is the inclined
plane that sticks out from the body. The distance between two adjacent
(neighboring) crests of the thread is known as the pitch.
Uses. Screws have many practical applications, especially as
fasteners. The most common are the wood screw and machine screw.
When rotated, such screws can be made to move into, or out of, an object. As the
screw rotates one full turn, it travels a distance equal to its pitch. Most wood
screws and machine screws have a slotted or recessed head into which a
screwdriver is placed in order to turn the screw. Screws come in various sizes
and shapes. They are made of steel, copper, aluminum, and other metals that are
easy to form.
In addition to holding materials together securely, screws
perform many other functions. Screws open and close nearly all vises. The
screw's ability to overcome resistance with relatively little force makes it
ideal for use in boring and drilling tools. A jackscrew combines a screw
with a lever. This device raises heavy loads without requiring great effort.
Jackscrews can lift automobiles and other heavy objects, even houses.
Many screws help produce motion. A marine screw propeller pushes
water backward, causing the boat or ship to move forward. Similarly, an airplane
propeller is an airscrew that pushes back air and makes the plane move
forward. See Propeller .
The screw also performs important
operations in delicate and complex machinery. Differential screws enable
objects to be spaced apart with great precision. The rotation of special screws
in timing devices causes switches to turn on or off at certain times. The
micrometer uses the revolutions of extremely fine threads to measure
small dimensions with great accuracy (see Micrometer ).
History. Some historians claim a Greek philosopher and
mathematician named Archytas, who lived about 400 B.C., invented the screw.
The ancient Greeks used screws for various purposes. The Greek mathematician and
inventor Archimedes supposedly developed a machine that uses a screw to raise
water (see Archimedean screw ). The ancient Greeks also developed the
screw press, a device consisting of two flat surfaces connected with
screws. Tightening the screws brings the surfaces together and puts increasing
pressure on whatever is placed between the surfaces. The Greeks used the screw
press to squeeze juice from grapes to make wine and from olives to produce oil.
For many centuries, screws were made with simple hand tools. But during
the 1500's, engineers developed the screw-cutting lathe, a machine that
permitted more efficient and precise production of wooden and metal screws.
Screws then began to replace nails and pegs as fasteners in joining hinges and
other metal items to wood and in holding together parts of locks, watches, and
other articles. Advancements in techniques for cutting finer, more exact threads
led to the adoption in the mid-1800's of the first standardized screw thread.
Screws with points that could be easily twisted through wood also appeared
during the 1800's. Previously, all screws had flat ends and could be inserted
only in specially drilled holes.
______________ Contributor: • W. David Lewis, Ph.D., Distinguished University Professor, Auburn
University.
How to cite this article: To
cite this article in a footnote, World Book
recommends the following format:
W. David Lewis, "Screw,"
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