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This article refers to the threaded fastener. Screw can also refer to screw propellers, often referred to as the "ship's screws". Also see Archimedes' screw for a type of pump containing a screw as
its impeller.
A screw is a shaft with a helical groove formed on its surface. Its
main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force.
Threaded Fastener
A screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a
helical ridge or thread formed on it. The thread mates with a complementary helix in
the material. The material may be manufactured with the mating helix, or the screw may create it when first "driven" in. The head
is specially shaped to allow a screwdriver to grip the screw when driving it
in. It also stops the screw from passing right through the material being fastened and provides compression.
Screws can normally be removed and re-inserted without reducing their effectiveness. This may make them preferable in some
applications to nails, which are frequently unusable after
being removed.
A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads
are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread.
Bolt
A bolt is a screw that passes right through the workpiece and is fastened by a nut on the other side. This is a very common way of holding together temporary and permanent constructions.
It avoids the need for making a threaded hole to fit the screw. An unthreaded hole is known as a clear hole.
A small bolt is called a machine screw.
The thread on a bolt often occupies only part of the shaft, the remainder of the shaft being clear. A bolt whose shaft is
threaded along its whole length is called a set screw.
Other Fastening Methods
When screws and bolts cannot be used, riveting, welding, soldering, brazing
and gluing are all alternatives.
Mechanical Analysis
A screw is a specialized application of the wedge or inclined plane. It contains a wedge, wound around an interior cylinder or
shaft, that either fits into a corresponding plane in a nut, or forms a corresponding plane in the wood or metal as it is
inserted. The technical analysis (see also statics, dynamics) to determine the pitch, thread shape or cross
section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to
predict wedge behavior. Wedges are discussed in the article on simple
machines.
Tensile Strength
Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear
forces. For example, when two overlapping metal bars joined by a
bolt are likely to be pulled apart longitudinally, the bolt must be tight enough that the friction between the two bars can overcome the longitudinal force. If the bars slip then the bolt may be sheared
in half, or friction between the bars (called fretting) may weaken them. For this type of application, high-tensile
steel bolts are used and these should be tightened with a torque wrench.
High-tensile bolts are usually in the form of hexagonal cap screws with an ISO strength
rating (called property class) stamped on the head. The strength ratings most often used are 8.8 and 12.9. The number
before the point is the ultimate tensile strength in
N/mm2 (or MPa) divided by 100. This is the stress at which the bolt will fail, i.e. break in half.
The number after the point is the yield strength as a percentage of the ultimate tensile strength, divided by 10. Yield
strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the
force is removed) of 0.2%.
Mild steel bolts have a 4.6 rating. High-tensile bolts have an 8.8 rating or above.
Types of Screw
- Cap screw has a convex head, usually hexagonal, designed to be driven by a spanner or wrench.
- Wood screw has a tapered shaft allowing it to penetrate undrilled wood.
- Machine screw has a cylindrical shaft and fits into a nut or a tapped hole, a small bolt.
- Self-tapping screw has a cylindrical shaft and a sharp thread that cuts its own hole, often used in sheet metal or
plastic.
- Drywall screw is a specialized self-tapping screw with a cylindrical shaft that has proved to have uses far beyond
its original application.
- Set screw has no head, and is designed to be inserted flush with or below the surface of the workpiece.
- Dowel screw is a wood-screw with two pointed ends and no head, used for making hidden joints between two pieces of
wood.
Shapes of Screw Head
- Pan head: disc with chamfered outer edge.
- Button or dome head: cylindrical with a rounded top.
- Round: dome-shaped, commonly used for machine screws.
- Truss: lower-profile dome designed to prevent tampering.
- Flat or Countersunk: conical, with flat outer face and tapering inner face allowing it to sink into the
material, very common for wood screws.
- Oval: countersunk with a rounded top.
- Cheese head: disc with cylindrical outer edge.
- Mirror screw head: countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for
attaching mirrors.
Types of Screw Drive
Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The
most common screw drives are the slotted and Phillips; hex, Robertson, and torx are also common in some applications. More exotic
screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be
serviced by the home repairperson.
- Slot head has a single slot, and is driven by a flat-bladed screwdriver. The slotted screw is common in woodworking
applications, but is not often seen in applications where a power driver would be used, due to the tendency of a power driver to
slip out of the head and potentially damage the surrounding material.
- Cross-head, or Phillips screw has a "+"-shaped slot and is driven by a cross-head screwdriver, designed
originally for use with mechanical screwing machines. The Phillips screw drive has slightly rounded corners in the tool recess,
and was designed so the driver will slip out, or cam out, under strain to prevent over-tightening. The Phillips Screw
Company was founded in Oregon in 1933 by Henry Phillips, who bought the design from J. P. Thompson. Phillips was unable to
manufacture the design, so he passed the patent to the American Screw Company, who were the first to manufacture it.
- Pozidriv is patented, similar to cross-head but designed not to slip, or cam out. It has four additional points of
contact, and does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in
Pozidriv screws, but Pozidriv screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Pozidriv
was jointly patented by the Phillips Screw Company and American Screw Company.
- Torx is a star-shaped or splined bit with six rounded points.
- Hexagonal or hex screw head has a hexagonal hole and is driven by a hexagonal wrench, sometimes called an Allen key,
or by a power tool with a hexagonal bit.
- Robertson drive head has a square hole and is driven by a special power-tool bit or screwdriver (this is a low-cost
version of the hex head for domestic use).
- Tri-Wing screws have a triangular slotted configuration. They are for instance used by Nintendo on its Gameboys to discourage home repair.
- Torq-Set is an uncommon screw drive that may be confused with Phillips; however, the four legs of the contact area
are offset in this drive type.
- Spanner drive uses two round holes opposite each other, and is designed to prevent tampering.
Many screw drives, including Phillips, Torx, and Hexagonal, are also manufactured in tamper-resistant form. These typically
have a pin protruding in the center of the bit, necessitating a special tool for extraction. The slotted screw drive also comes
in a tamper-resistant one-way design with sloped edges; the screw can be driven in, but the bit slips out in the reverse
direction.
Tools Used
The hand tool used to drive in most screws is called a screwdriver. A
power tool that does the same job is a power screwdriver; power drills may also be used with screw-driving attachments. The hand
tool for driving cap screws and other types is called a spanner (UK usage) or wrench (US usage).
Screw Measurements
There are many systems for specifying the dimensions of screws, but in Europe the ISO metric system has displaced the many
older systems. See also: Unified Thread Standard
Metric Screws
The diameter of a metric screw is usually specified in millimetres (mm) prefixed by the capital letter M, as in "M5" for a 5mm
diameter screw. The diameter of a screw is the outer diameter of the thread, which is approximately equal to the diameter of the
shaft before a thread was cut in it.
ISO Metric screw threads are available in coarse and fine versions. The coarse thread is by far the more common. Fine metric
threads are sometimes found in electronic equipment made in the Far East.
Non-metric Screws
Before the metric system was common, many engineering companies had their own standard screw sizes. The first person to create
a standard (in about 1841) was the English engineer Sir Joseph Whitworth. Whitworth screw sizes are still used, both for repairing old machinery and where a
coarser thread than the metric fastener thread is required. This system had two thread sizes: coarse (BSW) and fine (BSF). The
thread angle was 55°.
A later standard in the UK was the BA system, named after the
British Association for Advancement of Science. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used.
While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 1mm pitch. These
are still the most common threads in some niche applications. Certain types of fine machinery, such as moving-coil meters, tend
to have BA threads wherever they are manufactured.
The USA has its own system, usually called SAE, for Society of Automotive Engineers. Screws are
described as 4-40, 6-32, 8-32, 10-32, 10-24, etc. (for numeric sizes, odd numbers are rare), or 1/4"-20, 1/4"-28, etc. (for inch
unit sizes), with the first number giving shaft diameter (numeric or inches) and the second number being threads per
inch. These screws are sometimes found outside the USA in personal computers based on the IBM PC specification. There must be millions of PCs outside the USA that have metric screws
jammed into non-metric holes!
External links
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