screw is a shaft with a helical groove or
thread 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.
It can also be defined as an inclined plane
wrapped around a shaft.
A screw is a shaft with a helical groove or
thread 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.
It can also be defined as an inclined plane
wrapped around a shaft.
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 is
essentially an inclined plane wrapped around
a shaft. The thread mates with a
complementary helix in the material. The
material may be manufactured with the mating
helix (taps and dies), or the screw may
create it when first driven in (a
self-tapping screw). The head is specially
shaped to allow a screwdriver or wrench
(British English: spanner) 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
reinserted without reducing their
effectiveness. They have greater holding
power than nails and permit disassembly and
A screw that may be 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. Examples include
rotating items such as the left hand
grinding wheel on a bench grinder or the
left hand pedal on a bicycle (both looking
towards the equipment).
Threaded fasteners are traditionally made by
a cutting action such as taps and dies
provide, however recent advances in tooling
allows them to be made by rolling the blank
(a section of rod) between two specially
machined dies. The thread form and shape of
the fastener are squeezed onto the blank.
This method work hardens the threads and
saves material. A rolled thread is obvious
after manufacture because the outside
diameter of the thread is greater than the
diameter of the blank material. Bicycle
spokes, which are just very long thin bolts,
always use rolled threads for strength.
 Differentiation between bolt and
A capscrew with a nut, washer and locknut.
A screw, by definition, is not a bolt. A
bolt is designed such that a nut (or other
turning device) is required for operation. A
bolt is not designed to be turned. What most
people refer to as a bolt is in fact a 'cap
screw', which is designed to be turned (or
screwed). Cap screws may, or may not be used
with nuts. The distinction is subtle, but
significant in the design of the fastener.
See also the article on the bolt
 Other fastening methods
When screws and bolts cannot be used,
nailing, riveting, roll pins, pinned shafts,
welding, soldering, brazing, gluing, and
duct tape (taping) are some alternatives.
Another alternate is the threaded insert.
Examples include HeliCoil  and Keensert
 Materials and strength
Screws and bolts are made in a wide range of
materials, with steel being perhaps the most
common, in many varieties. Where great
resistance to weather or corrosion is
required, stainless steel, titanium, brass
or bronze may be used, or a coating such as
brass, zinc or chromium applied.
Electrolytic action from dissimilar metals
can be prevented with aluminium screws for
double-glazing tracks, for example. Some
types of plastic, such as nylon or Teflon,
can be threaded and used for fastening
requiring moderate strength and great
resistance to corrosion or for the purpose
of electrical insulation. Even porcelain and
glass can have molded screw threads that are
used successfully in applications such as
electrical line insulators and canning jars.
The same type of screw or bolt can be made
in many different grades of material. For
critical high-tensile-strength applications,
low-grade bolts may fail, resulting in
damage or injury. On SAE-standard bolts, a
distinctive pattern of marking is impressed
on the heads to allow inspection and
validation of the strength of the bolt.
However, low-cost counterfeit fasteners may
be found with actual strength far less than
indicated by the markings. Such inferior
fasteners are a danger to life and property
when used in aircraft, automobiles, heavy
trucks, and similar critical applications.
 Mechanical analysis
Rotating screw and fixed trough
A screw is a specialized application of the
wedge or inclined plane. It contains a
wedge, wound around a cylinder or shaft,
that either fits into a corresponding
inclined plane in a nut, or forms a
corresponding inclined 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.
Critical applications of screws and bolts
will specify a torque that must be applied
when tightening. The main concept is to
stretch the bolt, and compress the parts
being held together, creating a spring-like
assembly. The stretch introduced to the bolt
is called a preload. When external forces
try to separate the parts, the bolt sees no
strain unless the preload force is exceeded.
As long as the preload is never exceeded,
the bolt or nut will never come loose
(assuming the full strength of the bolt is
used). If the full strength of the bolt is
not used (e.g., a steel bolt threaded into
aluminum threads), then a thread-locking
adhesive may be used.
If the preload is exceeded during normal
use, the joint will eventually fail. The
preload is calculated as a percentage of the
bolt's yield tensile strength, or the
strength of the threads it goes into, or the
compressive strength of the clamped layers
(plates, washers, gaskets), whichever is
 Tensile strength
Rusty hexagonal bolt heads
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 so 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 bolt and
slipping bars may erode and weaken the bolt
(called fretting). For this type of
application, high-strength steel bolts are
used and these should be tightened with a
High-strength bolts usually have a hexagonal
head with an ISO strength rating (called
property class) stamped on the head. The
property classes most often used are 8.8 and
10.9. The number before the point is the
tensile ultimate strength in MPa divided by
100. The number after the point is 10 times
the ratio of tensile yield strength to
tensile ultimate strength. For example, a
property class 5.8 bolt has a nominal
(minimum) tensile ultimate strength of 500
MPa, and a tensile yield strength of 0.8
times tensile ultimate strength or 0.8(500)
= 400 MPa.
Tensile yield strength is M10, property
class 8.8 bolt can very safely hold a static
tensile load of about 15 kN.
 Types of screws and bolts
Combination flanged-hex/Phillips-head screw
used in computers
· A hex cap screw has a protruding hexagonal
head, designed to be driven by a spanner or
· A socket cap screw has a hexagonal
recessed drive, usually with a cylindrical
head, but can also be found with a rounded
button head or a countersunk flat head.
Socket cap screws can be torqued more
tightly than other drives without stripping,
and they are usually made from a high
strength steel alloy.
· A Wood screw has a tapered shaft, allowing
it to penetrate undrilled wood, or a wall
plug in brickwork etc.
· A Machine screw has a cylindrical shaft,
threaded its entire length, and fits into a
nut or a tapped hole.
· Self-tapping screws or thread cutting
screws have sharp threads that cut into a
material such as sheet metal or plastic.
They are sometimes notched at the tip to aid
in chip removal during thread cutting.
· A Self-drilling screw is similar to a
self-tapping screw, but has a drill-shaped
point to cut through the material without
· Thread rolling screws have a lobed
(usually triangular) cross section. They
form threads by pushing outward during
installation. They may have tapping threads
or machine threads.
· A Drywall screw is a specialized
self-tapping screw designed to bind drywall
to wood or metal studs, but it has proven to
be a versatile construction fastener.
· A Set screw, used to prevent loosening due
to vibration, is available with thumb screw,
square head, Hex head (inset socket) and,
most commonly, headless (a grub screw in UK
parlance, designed to be inserted flush with
or below the surface of the work piece).
Alternatively defined to be a screw whose
thread reaches the head (if any), as opposed
to a bolt.
· Dowel screw is a wood screw with two
pointed ends and no head, used for making
hidden joints between two pieces of wood.
· A stud is similar to a bolt but without
the head. Studs are threaded on both ends.
In some cases the entire length of the stud
is threaded, while in other cases there will
be an unthreaded section in the middle. It
may be anchored in concrete, for example,
with only the threads on one end exposed.
(See also: screw anchor, wedge anchor.)
· A carriage bolt or coach bolt has a domed
or countersunk head, and the shaft is topped
by a short square section under the head.
The rib neck carriage bolt has several
longitudinal ribs instead of the square
section, to grip into a metal part being
· A stove bolt is similar to a carriage
bolt, but usually used in metal. It requires
a square hole in the metal being bolted to
prevent the bolt from turning.
· The Superbolt, or Multi-Jackbolt Tensioner
is an alternative type of fastener that
retrofits or replaces existing nuts, bolts,
or studs. Tension in the bolt is developed
by torquing individual jackbolts which are
threaded through the body of the nut and
push against a hardened washer. Installation
and removal of any size tensioner is
achieved with hand tools, which can be
advantageous when dealing with large
diameter bolting applications.
· Acme screw form has threads that are
stronger and broader than standard V-profile
threads, making them much better for load
carrying, linear actuating (also see ball
screws), and quick threading. It is often
used for vise screws.
· A Shoulder Screw is a screw used for
revolute joints in mechanisms and linkages.
The main shank of the screw is smooth and
works as a suitable bearing surface. At the
end of the cylindrical bearing surface is a
reduced diameter thread.
 Shapes of screw head
(a) pan, (b) button, (c) round, (d) truss,
(e) flat, (f) oval
· pan head: a low disc with chamfered outer
· button or dome head: cylindrical with a
· round head: dome-shaped, commonly used for
· truss head: lower-profile dome designed to
· flat head or countersunk: conical, with
flat outer face and tapering inner face
allowing it to sink into the material.
· oval or raised head: countersunk with a
· bugle head: similar to countersunk, but
there is a smooth progression from the shaft
to the angle of the head, similar to the
bell of a bugle.
· cheese head: disc with cylindrical outer
edge, height approximately half the head
· fillister head: cylindrical, but with a
slightly convex top surface.
· socket head: cylindrical, relatively high,
with different types of sockets (hex,
square, torx, etc.).
· mirror screw head: countersunk head with a
tapped hole to receive a separate screw-in
chrome-plated cover, used for attaching
· headless (set or grub screw): has either a
socket or slot in one end for driving.
Some varieties of screw are manufactured
with a break-away head, which snaps off when
adequate torque is applied. This prevents
tampering and disassembly and also provides
an easily-inspectable joint to guarantee
 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. Some types of
drive are intended for automatic assembly in
mass-production of such items as
automobiles. 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 repair person.
· 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
Phillips vs. Frearson
BNAE driver bit
· Cross-head, cross-point, or cruciform has
a "+"-shaped slot and is driven by a
cross-head screwdriver, designed originally
for use with mechanical screwing machines.
There are five types:
o 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 high torque to prevent
over-tightening. The Phillips Screw Company
was founded in Oregon in 1933 by Henry F.
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 was the first to
o A Reed & Prince or Frearson screw drive is
similar to a Phillips but has a more pointed
75° V shape. Its advantage over the Phillips
drive is one driver or bit fits all screw
sizes. It is found mainly in marine hardware
and requires a special screw driver or bit
to work properly. The tool recess is a
perfect cross, unlike the Phillips head,
which is designed to cam out. It was
developed by an English inventor named
Frearson in the 19th century and produced
from the late 1930s to the mid-1970s by the
former Reed & Prince Manufacturing Company
of Worcester, Massachusetts a company which
traces its origins to Kingston,
Massachusetts in 1882, and was liquidated in
1990 with the sale of company assets.
o A JIS (Japanese Industrial Standard) head,
commonly found in Japanese equipment, looks
like a Phillips screw, but is designed not
to cam out and will, therefore, be damaged
by a Phillips screwdriver if it is too
tight. The standard number is JIS B
o French Recess, also called BNAE NFL22-070
for Bureau de Normalisation de
l'Aéronautique et de l'Espace, a French
o 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 "hexalobular" drive
with six rounded points. It was designed to
permit increased torque transfer from the
driver to the bit compared to other drive
systems. TORX is very popular in the
automotive and electronics industries due to
resistance to cam out and extended bit life,
as well as reduced operator fatigue by
minimizing the need to bear down on the
drive tool to prevent cam out. TORX screws
were found in early Apple Macintosh
computers, to discourage home repairs. TORX
PLUS is an improved version of TORX which
extends tool life even further and permits
greater torque transfer compared to TORX. A
tamper-resistant TORX head has a small pin
inside the recess. The tamper-resistant TORX
is also made in a 5 lobed variant. These
TORX configurations are commonly used in
correctional facilities, public facilities
and government schools.
· TTAP is an improved "hexalobular" drive
for without wobbling and stable stick-fit.
TTAP is backward convertible with generic
Hex socket screws
· Hexagonal (hex) socket head has a
hexagonal hole and is driven by a Hex
Wrench, sometimes called an Allen key or Hex
key, or by a power tool with a hexagonal
bit. Tamper-resistant versions with a pin in
the recess are available. Hex sockets are
increasingly used for modern bicycle parts
because hex wrenches are very light and
easily carried tools. They are also
frequently used for self-assembled furniture
(e.g. from Ikea).
· Robertson head, invented in 1908 by P.L.
Robertson, has a square hole and is driven
by a special power-tool bit or screwdriver.
The screw is designed to maximize torque
transferred from the driver, and will not
slip, or cam out. It is possible to hold a
Robertson screw on a driver bit horizontally
or even pendant, due to a slight wedge fit.
Commonly found in Canada in carpentry and
woodworking applications and in
Canadian-manufactured electrical wiring
items such as receptacles and switch boxes.
· Square-drive head is an American clone of
the Robertson that has a square hole without
taper. Due to the lack of taper, the hole
must be oversize relative to the
screwdriver, and is much more likely to
strip than the Robertson.
· Tri-Wing head has a triangular slotted
configuration. They were used by Nintendo on
several consoles and accessories, including
the Game Boy, Wii, and Wii Remote, and on
some Nokia phones and chargers to discourage
· Torq-Set or offset cruciform 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. Commonly seen in
elevators in the United States.
· Clutch Type A or standard clutch head
resembles a bow tie. These were common in GM
automobiles of the 1940s and '50s,
particularly for body panels.
· Clutch Type G head resembles a butterfly.
This type of screw head is commonly used in
the manufacture of mobile homes and
 Combination drives
Some screws have heads designed to
accommodate more than one kind of driver.
The most common of these is a combination of
a slotted and Phillips head. Because of its
prevalence, there are now drivers made
specifically for this kind of screw head.
Other combinations are a Phillips and
Robertson, a Robertson and a slotted, and a
triple-drive screw which can take a slotted,
Phillips or a Robertson. The Recex drive
system claims it offers the combined
non-slip convenience of a Robertson drive
during production assembly and Phillips for
after market serviceability. Combination
head screws are becoming more and more
 Tamper-resistant screws
Tamper-resistant TORX driver
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 screw
head, necessitating a special tool for
extraction. However, the bits for many
tamper-resistant screw heads are now readily
available from hardware stores, tool
suppliers and through the Internet. What is
more, there are many commonly used
techniques to extract tamper resistant
screws without the correct driver — for
example, the use of an alternative driver
than can gain enough purchase to turn the
screw, modifying the head to accept an
alternative driver or forming ones own
driver by melting an object into the head to
mould a driver. Thus, these special screws
offer only modest security.
One-way slotted screw
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.
There are specialty fastener companies that
make unusual, proprietary head designs,
featuring matching drivers available only
from them, and only supplied to registered
 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. Where the holding
power of the screwed joint is critical,
torque-measuring and torque-limiting
screwdrivers are used to ensure sufficient
and not excessive force is developed by the
screw. The hand tool for driving cap screws
and other types is called a spanner (UK
usage) or wrench (US usage).
The knock-off bolt is a high security screw
that is extremely difficult to remove. It is
comprised of a counter-sunk flat head screw,
with a thin shaft and hex head protruding
from the flat head. The hex head is used to
drive the bolt into the countersunk hole,
then the wrench or hammer is used to knock
the shaft and hex head off of the flat head,
leaving only a smooth screw head exposed.
Removal is facilitated by drilling a small
hole part way into the outer part of the
head and using a punch and hammer at a sharp
angle in an anti-clockwise direction. This
type of screw is used primarily in prison
 Mechanics of use
When driving in a screw, especially when the
screw has been removed and is being placed
again, the threads can become misaligned and
damage, or strip, the threading of the hole.
To avoid this, slight pressure is applied
and the screw is driven in reverse, until
the leading edges of the helices pass each
other, at which point a slight click will be
felt (and sometimes heard.) When this
happens, the screw will often assume a more
aligned position with respect to the hole.
Immediately after the 'click', the screw may
be driven in without damage to the
threading. This technique is useful for
re-seating screws in wood and plastic, and
for assuring the proper fit when screwing
down plates and covers where alignment is
 Thread standards
See also: Screw thread
There are many systems for specifying the
dimensions of screws, but in much of the
world the ISO metric screw thread preferred
series has displaced the many older systems.
Other relatively common systems include the
British Standard Whitworth, BA system
(British Association), and the SAE Unified
 ISO metric screw thread
The basic principles of the ISO metric screw
thread are defined in international standard
ISO 68-1 and preferred combinations of
diameter and pitch are listed in ISO 261.
The smaller subset of diameter and pitch
combinations commonly used in screws, nuts
and bolts is given in ISO 262. The most
commonly used pitch value for each diameter
is known as the "coarse pitch". For some
diameters, one or two additional "fine
pitch" variants are also specified, for
special applications such as threads in
thin-walled pipes. ISO metric screw threads
are designated by the letter M followed by
the major diameter of the thread in
millimeters, e.g. "M8". If the thread does
not use the normal "coarse pitch" (e.g.,
1.25 mm in the case of M8), then the pitch
in millimeters is also appended with a
multiplication sign, e.g. "M8×1" if the
screw thread has an outer diameter of 8 mm
and advances by 1 mm per 360° rotation.
The nominal diameter of a metric screw is
the outer diameter of the thread. The tapped
hole (or nut) into which the screw fits, has
an internal diameter which is the size of
the screw minus the pitch of the thread.
Thus, an M6 screw, which has a pitch of 1
mm, is made by threading a 6 mm shaft, and
the nut or threaded hole is made by tapping
threads in a 5 mm hole.
Metric hexagon bolts, screws and nuts are
specified, for example, in British Standard
BS 4190 (general purpose screws) and BS 3692
(precision screws). The following table
lists the relationship given in these
standards between the thread size and the
maximal width across the hexagonal flats
In addition, the following non-preferred
intermediate sizes are specified:
ISO metric thread
wrench size (mm)
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. Whitworth
became British Standard Whitworth,
abbreviated to BSW (BS 84:1956) and the
British Standard Fine (BSF) thread was
introduced in 1908 because the Whitworth
thread was a bit coarse for some
applications. The thread angle was 55° and a
depth and pitch of thread that varied with
the diameter of the thread (i.e., the bigger
the bolt, the coarser the thread). The
spanner size is determined by the size of
the bolt not distance between the flats. The
most common use of a Whitworth pitch
nowadays is the standard photographic tripod
thread, which for small cameras is 1/4"
Whitworth (20 tpi) and for medium/large
format cameras is 3/8" Whitworth (16 tpi).
 British Association screw threads
A later standard established in the United
Kingdom 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 1 mm
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.
 Unified Thread Standard
The United States has its own system,
usually called the Unified Thread Standard,
which is also extensively used in Canada and
in most other countries around the world. At
least 85% of the world's fasteners are
dimensioned to Unified thread dimensions,
and the biggest selection of fastener sizes
and materials are found supplied in this
standard (Source: World Fastener Review,
Industrial Press, 2006). A version of this
standard, called SAE for the Society of
Automotive Engineers, was used in the
American automobile industry. The SAE is
still associated with inch-based fasteners
by the public, even though the U.S. auto
industry (and other heavy industries relying
on SAE) have gradually converted to ISO
preferred series fasteners for some
assemblies from the 1970s onward, because
global parts sourcing and product marketing
favor international standardization.
However, all automobiles sold in around the
world contain both metric (engine
assemblies) and Imperial fasteners (for
example, lug nuts, oxygen sensors, internal
electrical assemblies, body fasteners,
lamps, steering, brake and suspension
Machine screws are described as 0-80, 2-56,
3-48, 4-40, 5-40, 6-32, 8-32, 10-32, 10-24,
etc. up to size 16. The first number can be
translated to a diameter, the second is the
number of threads per inch. There is a
coarse thread and a fine thread for each
size, the fine thread being preferred in
thin materials or when its slightly greater
strength is desired.
The numbering system follows a roughly
logarithmic series where an increase in each
screw number size approximately doubles the
tensile strength of the screw and is given
by Dia = (#N X .013") + .060" Using this
formula a #5 screw has a major diameter of
.125" (1/8"), a #10 screw has a diameter of
.190" (or 3/16" in practical terms), etc.
The formula applies for screw thread numbers
#0 and higher, but does NOT apply to smaller
Unified miniature screw thread series.
Typically screws smaller than size #0 are
supplied in the Unified Miniature Series.
The formula for number sizes smaller than
size #0 is given by Dia = .060" - (#zerosize
X .013). So a #00 screw is .047" dia, #000
is .034" dia, etc.
The number series of machine screws once
included odd numbers (7, 9, etc.) and
extended up to #16 or more. Standardization
efforts in the late 19th and the early part
of the 20th century reduced the range of
sizes considerably. Now, it is less common
to see machine screws larger than #14, or
odd number sizes other than #1, #3 and #5.
Even though #14 and #16 screws are still
available, they are not as common as sizes
#0 thru #12.
Sizes 1/4" diameter and larger are
designated as 1/4"-20, 1/4"-28, etc. the
first number giving the diameter in inches
and the second number being threads per
inch. Most thread sizes are available in UNC
or UC (Unified Coarse Thread, example
1/4"-20) or UNF (example 1/4"-28 UNF or UF).
Other thread systems include Acme thread
form, BSP (British standard pipe thread
which exists in a taper and non taper
variant; used for other purposes as well)
and BSC (British Standard Cycle) a 26tpi
thread form, CEI (Cycle Engineers Institute,
used on bicycles in Britain and possibly
elsewhere), British Standard Brass a fixed
pitch 26tpi thread, NPT (National pipe
thread) and NPTF (pipe threads), and PG
(German: "Panzer-Gewinde"), used in thin
plate metal, such as for switches and
nipples in electrical equipment housings.