A SCREW, or bolt,
is a type of fastener characterized
by a helical ridge, known as an external thread or
just thread, wrapped around a cylinder. Some Screw threads
are designed to mate with a complementary thread, known as an internal
thread, often in the form of a nut or an object that has the
internal thread formed into it. Other Screw threads are designed to cut a helical
groove in a softer material as the Screw is
inserted. The most common uses of Screw are to hold
objects together and to position objects.
A Screw will almost always have a head, (a set Screw is an example of a Screw without a head) which is a specially formed section on one end of the Screw that allows it to be turned, or driven. Common tools for driving Screw include Screwdriversand wrenches. The head is usually larger than the body of the Screw, which keeps the Screw from being driven deeper than the length of the Screw and to provide a bearing surface. There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set Screw often have a head smaller than the outer diameter of the Screw; J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the Screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded.The distance between each thread is called the "pitch".
The majority of Screw are tightened by clockwise rotation, which is termed a right-hand thread; a common mnemonic device for remembering this when working with Screw or bolts is "righty-tighty, lefty-loosey." Screw with left-hand threads are used in exceptional cases. For example, when the Screw will be subject to counterclockwise torque (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. The left side pedal of a bicycle has a left-hand thread.
More
generally, Screw may mean any helical device,
such as a clamp, a micrometer, a ship's propeller or
an Archimedes' Screw water pump.
TIPE SCREW
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