Screwdriver
From Wikipedia, the free encyclopedia
The screwdriver is a device specifically designed to insert and tighten, or to loosen and remove, screws. The screwdriver comprises a head or tip which engages with a screw, a mechanism to apply torque by rotating the tip, and some way to position and support the screwdriver. A typical hand screwdriver comprises an approximately cylindrical handle of a size and shape to be held by a human hand, and an axial shaft fixed to the handle, the tip of which is shaped to fit a particular type of screw. The handle and shaft allow the screwdriver to be positioned and supported and, when rotated, to apply torque. Screwdrivers are made in a variety of shapes, and the tip can be rotated manually or by an electric or other motor.
A screw has a head with a contour such that an appropriate screwdriver tip can be engaged in it in such a way that the application of sufficient torque to the screwdriver will cause the screw to rotate.
There are many types of screw heads, of which the most common are the slotted, Phillips, PoziDriv/SupaDriv (crosspoint), Robertson, TORX, and Allen (hex).
Screwdrivers come in a large variety of sizes to match those of screws, from tiny jeweler's screwdrivers up. It is important to use a screwdriver that is the right size and type for the screw used, or it is likely that the screw will be damaged in the process of tightening it. This is less important for PoziDriv and SupaDriv, which are designed specifically to be more tolerant of size mismatch. When tightening a screw with force, it is important to press the head hard into the screw, again to avoid damaging the screw.
Some manual screwdrivers have a ratchet action whereby the screwdriver blade is locked to the handle for clockwise rotation, but uncoupled for counterclockwise rotation when set for tightening screws; and vice versa for loosening.
Many screwdriver designs have a handle with detachable head (the part of the screwdriver which engages with the screw), called bits, allowing a set of one handle and several heads to be used for a variety of screw sizes and types. This kind of design has allowed the development of electrically powered screwdrivers, which, as the name suggests, use an electric motor to rotate the bit. Some drills can also be fitted with screwdriver heads.
Manual screw drivers with a spiral ratchet mechanism to turn pressure (linear motion) into rotational motion also exist, and predate electric screwdrivers. The user pushes the handle toward the workpiece, causing a pawl in a spiral groove to rotate the shank and the removable bit. The ratchet can be set to rotate left or right with each push, or can be locked so that the tool can be used like a conventional screwdriver. Once very popular, these spiral ratchet drivers, using proprietary bits, have been largely discontinued by manufacturers such as Stanley, although one can still find them at vintage tool auctions. Companies such as Lara now offer a modernized version that uses standard 1/4-inch hex shank power tool bits. Since a variety of drill bits are available in this format, it allows the tool to do double duty as a push drill.
Many modern electrical appliances, if they use screws at all, use screws with heads of non-standard shape in an attempt to prevent users of the device from easily disassembling them— see the article on screws for details. TORX is one such pattern that has become very widespread, with suitable screwdrivers widely available in electronics stores. Other patterns of security screws are less common, though sets of security heads are, again, readily available.
While screwdrivers are designed for the above functions, they are commonly also used as improvised substitutes for pry bars, levers, and hole punches, as well as other tools.
[edit] See also
[edit] Further reading
- Witold Rybczynski, One Good Turn: A Natural History of the Screwdriver and the Screw, Harper Flamingo Canada, Toronto, 2000, ISBN 0-00-200031-8
[edit] External links
- Henry F. Phillips — Phillips Head Screwdriver
