A screw is a type of fastener, in some ways similar to a bolt (see Differentiation between bolt and screw below), typically made of metal, and characterized by a helical ridge, known as a male thread (external thread). Screws are used to fasten materials by digging in and wedging into a material when turned, while the thread cuts grooves in the fastened material that may help pull fastened materials together and prevent pull-out. There are many screws for a variety of materials; those commonly fastened by screws include wood, sheet metal, and plastic.

An assortment of screws
A bolt and a screw


A screw is a combination of simple machines—it is in essence an inclined plane wrapped around a central shaft, but the inclined plane (thread) also comes to a sharp edge around the outside, which acts a wedge as it pushes into the fastened material, and the shaft and helix also form a wedge in the form of the point. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut, or 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 screws are to hold objects together and to position objects.

Wood screw with legend
A wood screw: a) head; b) not threaded shank; c) threaded shank; d) tip.

A screw will usually have a head on one end that contains a specially formed shape that allows it to be turned, or driven, with a tool. Common tools for driving screws include screwdrivers and 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 screws 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.[1] The distance between each thread is called the "pitch".

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread; a common mnemonic device for remembering this when working with screws or bolts is "righty-tighty, lefty-loosey". If the fingers of the right hand are curled around a right-hand thread, it will move in the direction of the thumb when turned in the same direction as the fingers are curled. Screws with left-hand threads are used in exceptional cases, where loads would tend to loosen a right handed fastener, or when non-interchangeability with right-hand fasteners is required. 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.

Differentiation between bolt and screw

Bolt (PSF)
A carriage bolt with a square nut
DIN6914 UNI5587
A structural bolt with a hex nut and washer

There is no universally accepted distinction between a screw and a bolt. A simple distinction that is often true, although not always, is that a bolt passes through a substrate and takes a nut on the other side, whereas a screw takes no nut because it threads directly into the substrate (a screw screws into something, a bolt bolts several things together). So, as a general rule, when buying a packet of "screws" nuts would not be expected to be included, but bolts are often sold with matching nuts. Part of the confusion over this is likely due to regional or dialectical differences. Machinery's Handbook describes the distinction as follows:

A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut. A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head. An externally threaded fastener which is prevented from being turned during assembly and which can be tightened or released only by torquing a nut is a bolt. (Example: round head bolts, track bolts, plow bolts.) An externally threaded fastener that has thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. (Example: wood screws, tapping screws.)[2]

This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw[3][4] and bolt.[5][6][7]

The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction, and usage variations.[8] Some of these issues are discussed below:

Wood screws

Early wood screws were made by hand, with a series of files, chisels, and other cutting tools, and these can be spotted easily by noting the irregular spacing and shape of the threads, as well as file marks remaining on the head of the screw and in the area between threads. Many of these screws had a blunt end, completely lacking the sharp tapered point on nearly all modern wood screws.[9] Eventually, lathes were used to manufacture wood screws, with the earliest patent being recorded in 1760 in England.[9] During the 1850s swaging tools were developed to provide a more uniform and consistent thread. Screws made with these tools have rounded valleys with sharp and rough threads.[10][11] Some wood screws were made with cutting dies as early as the late 1700s (possibly even before 1678 when the book content was first published in parts).[12]

Once screw turning machines were in common use, most commercially available wood screws were produced with this method. These cut wood screws are almost invariably tapered, and even when the tapered shank is not obvious, they can be discerned because the threads do not extend past the diameter of the shank. Such screws are best installed after drilling a pilot hole with a tapered drill bit. The majority of modern wood screws, except for those made of brass, are formed on thread rolling machines. These screws have a constant diameter, threads with a larger diameter than the shank, and are stronger because the rolling process does not cut the grain of the metal.

Machine screws

ASME standards specify a variety of "Machine Screws"[13] in diameters ranging up to 0.75 in (19.05 mm). These fasteners are often used with nuts but also often driven into tapped holes (without nuts). They might be considered a screw or a bolt based on the Machinery's Handbook distinction. In practice, they tend to be mostly available in smaller sizes and the smaller sizes are referred to as screws or less ambiguously as machine screws, although some kinds of machine screw can be referred to as stove bolts.

Hex cap screws

ASME standard B18.2.1-1996 specifies Hex Cap Screws whose size range is 0.25–3 in (6.35–76.20 mm) in diameter. These fasteners are very similar to hex bolts. They differ mostly in that they are manufactured to tighter tolerances than the corresponding bolts. Machinery's Handbook refers parenthetically to these fasteners as "Finished Hex Bolts".[14] Reasonably, these fasteners might be referred to as bolts, but based on the US government document Distinguishing Bolts from Screws, the US government might classify them as screws because of the tighter tolerance.[15] In 1991 responding to an influx of counterfeit fasteners Congress passed PL 101-592[16] "Fastener Quality Act" This resulted in the rewriting of specifications by the ASME B18 committee. B18.2.1[17] was re-written and as a result they eliminated the "Finished Hex Bolts" and renamed them the "Hex Cap Screw"—a term that had existed in common usage long before, but was now also being codified as an official name for the ASME B18 standard.

Lug bolts and head bolts

These terms refer to fasteners that are designed to be threaded into a tapped hole that is in part of the assembly and so based on the Machinery's Handbook distinction they would be screws. Here common terms are at variance with Machinery's Handbook distinction.[18][19]

Lag screw

Lag screws aka lag bolts 001
Lag screws, also called lag bolts

Lag screws (US) or coach screws (UK, Australia, and New Zealand) (also referred to as lag bolts or coach bolts, although this is a misnomer) are large wood screws. Square-headed and hex-headed lag screws are covered by ASME B18.2.1 standards, and the head is typically an external hex. A typical lag screw can range in diameter from 14 to 1 14 in (6.35 to 31.75 mm), and lengths from 14 to 6 in (6.35 to 152.40 mm) or longer, with the coarse threads of a wood-screw or sheet-metal-screw threadform (but larger).

The materials are usually carbon steel substrate with a coating of zinc galvanization (for corrosion resistance). The zinc coating may be bright (electroplated), yellow (electroplated), or dull gray hot-dip galvanized. Lag screws are used to lag together lumber framing, to lag machinery feet to wood floors, and for other heavy carpentry applications. The attributive modifier lag came from an early principal use of such fasteners: the fastening of lags such as barrel staves and other similar parts.[20]

These fasteners are "screws" according to the Machinery's Handbook criteria, and the obsolescent term "lag bolt" has been replaced by "lag screw" in the Handbook.[21] However, to many tradesmen, they are "bolts", because they are large, with hex or square heads.

United States government standards

The federal government of the United States made an effort to formalize the difference between a bolt and a screw, because different tariffs apply to each.[22] The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction between screws and bolts for some threaded fasteners. The document also reflects (although it probably did not originate) significant confusion of terminology usage that differs between the legal/statutory/regulatory community and the fastener industry. The legal/statutory/regulatory wording uses the terms "coarse" and "fine" to refer to the tightness of the tolerance range, referring basically to "high-quality" or "low-quality", but this is a poor choice of terms, because those terms in the fastener industry have a different meaning (referring to the steepness of the helix's lead).

Historical issue

Old USS and SAE standards defined cap screws as fasteners with shanks that were threaded to the head and bolts as fasteners with shanks that were partially unthreaded.[23] The relationship of this rule to the idea that a bolt by definition takes a nut is clear (because the unthreaded section of the shank, which is called the grip, was expected to pass through the substrate without threading into it). This is now an obsolete distinction, although large bolts still often have unthreaded sections of shank.

Although there is no reason to consider this definition obsolete, because it is far from clear that "a bolt by definition takes a nut". Using a coach "bolt" as an example (and it has been a 'bolt' for a very long time). It was not originally intended to receive a nut, but did have a shank. Its purpose was not to pass through the entire substrate but only one piece of it, while the threaded portion bit into the other in order to draw, and clamp the materials together. The 'carriage' bolt was derived from this and was employed more to speed up manufacturing than achieve a different function. The carriage bolt passes through both pieces of materials and employs a nut to provide the clamping force. Both are still, however, bolts.

Controlled vocabulary versus natural language

The distinctions above are enforced in the controlled vocabulary of standards organizations. Nevertheless, there are sometimes differences between the controlled vocabulary and the natural language use of the words by machinists, auto mechanics and others. These differences reflect linguistic evolution shaped by the changing of technology over centuries. The words bolt and screw have both existed since before today's modern mix of fastener types existed, and the natural usage of those words has evolved retronymously in response to the technological change. (That is, the use of words as names for objects changes as the objects change.) Non-threaded fasteners predominated until the advent of practical, inexpensive screw-cutting in the early 19th century. The basic meaning of the word screw has long involved the idea of a helical screw thread, but the Archimedes screw and the screw gimlet (like a corkscrew) preceded the fastener.

The word bolt is also a very old word, and it was used for centuries to refer to metal rods that passed through the substrate to be fastened on the other side, often via nonthreaded means (clinching, forge welding, pinning, wedging, etc.). The connection of this sense to the sense of a door bolt or the crossbow bolt is apparent. In the 19th century, bolts fastened via screw threads were often called screw bolts in contradistinction to clench bolts.

In common usage, the distinction (not rigorous) is often that screws are smaller than bolts, and that screws are generally tapered while bolts are not. For example, cylinder head bolts are called "bolts" (at least in North American usage) despite the fact that by some definitions they ought to be called "screws". Their size and their similarity to a bolt that would take a nut seem linguistically to overrule any other factors in this natural word choice proclivity.

Other distinctions

Bolts have been defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as ISO metric screw thread M, MJ, Unified Thread Standard UN, UNR, and UNJ) such that they can accept a non-tapered nut. Screws are then defined as headed, externally threaded fasteners that do not meet the above definition of bolts. These definitions of screw and bolt eliminate the ambiguity of the Machinery's handbook distinction. And it is for that reason, perhaps, that some people favor them. However, they are neither compliant with common usage of the two words nor are they compliant with formal specifications.

A possible distinction is that a screw is designed to cut its own thread; it has no need for access from or exposure to the opposite side of the component being fastened to. This definition of screw is further reinforced by the consideration of the developments of fasteners such as Tek Screws, with either round or hex heads, for roof cladding, self-drilling and self-tapping screws for various metal fastening applications, roof batten screws to reinforce the connection between the roof batten and the rafter, decking screws etc. On the other hand, a bolt is the male part of a fastener system designed to be accepted by a pre-equipped socket (or nut) of exactly the same thread design.

Types of screws and bolts

Threaded fasteners either have a tapered shank or a non-tapered shank. Fasteners with tapered shanks are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shank are designed to mate with a nut or to be driven into a tapped hole.

Fasteners with a tapered shank

American name British name Description
chipboard screw
particle board screw
Similar to a drywall screw except that it has a thinner shank and provides better resistance to pull-out in particle board, while offset against a lower shear strength. The threads on particle board screws are asymmetrical.
concrete screw
masonry screw
Confast screw
multi-material screw
blue screw
self-tapping masonry screw
A stainless or carbon steel screw for fastening wood, metal, or other materials to concrete or masonry. Concrete screws are commonly blue in color, with or without corrosion coating.[24] They may either have a Phillips flat head or a slotted hex washer head. Nominal (thread) sizes range from 0.1875 to 0.375 in (4.763 to 9.525 mm) and lengths from 1.25 to 5 in (32 to 127 mm). Typically an installer uses a hammer drill to make a pilot hole for each concrete screw and a powered impact driver to drive the screw.
deck screw Similar to drywall screw except that it has improved corrosion resistance and is generally supplied in a larger gauge. Most deck screws have a type-17 (auger type) thread cutting tip for installation into decking materials. They have bugle heads that allows the screw to depress the wood surface without breaking it.
Double screw dowel.png double ended screw
dowel screw
hanger bolt
handrail bolt Similar to a wood screw but with two pointed ends and no head, used for making hidden joints between two pieces of wood.
A hanger bolt has wood screw threads on one end and machine threads on the other. A hanger bolt is used when it is necessary to fasten a metal part to a wood surface.
drive screw
hammer drive screw
Chiefly used for attaching manufacturers data plates to equipment. Smooth round or mushroom headed with a multi-start thread on the shank, beneath which is reduced diameter shank that acts as a pilot. The screw is fastened by hitting the head with a hammer and is not intended for removal.[25]
Screw.agr drywall screw Specialized screw with a bugle head that is designed to attach drywall to wood or metal studs, however it is a versatile construction fastener with many uses. The diameter of drywall screw threads is larger than the grip diameter.
Eye bolt wood thread eye screw
screw eye
vine eye
screw eye Screw with a looped head. Larger ones are sometimes called lag eye screws. Designed to be used as attachment point, particularly for something that is hung from it.
A vine eye (in the UK at least) is similar to a screw eye, except that it has a proportionally longer shank and smaller looped head. As the term suggests vine eyes are often used for attaching wire lines across the surface of buildings so that climbing plants can attach themselves.
Tire-fond cropped lag bolt
lag screw[26]
coach screw Similar to a wood screw except that it is generally much larger running to lengths up to 15 in (381 mm) with diameters from 0.25–0.5 in (6.35–12.70 mm) in commonly available (hardware store) sizes (not counting larger mining and civil engineering lags and lag bolts) and it generally has a hexagonal drive head. Lag bolts are designed for securely fastening heavy timbers (post and beams, timber railway trestles and bridges) to one another, or to fasten wood to masonry or concrete. The German standard is DIN 571, Hexagon head wood screws.

Lag bolts are usually used with an expanding insert called a lag in masonry or concrete walls, the lag manufactured with a hard metal jacket that bites into the sides of the drilled hole, and the inner metal in the lag being a softer alloy of lead, or zinc alloyed with soft iron. The coarse thread of a lag bolt and lag mesh and deform slightly making a secure near water tight anti-corroding mechanically strong fastening.

Mirror Screws mirror screw This is a flat-head wood screw with a tapped hole in the head, which receives a screw-in chrome-plated cover. It is usually used to mount a mirror.
Phillips screw sheet metal screw Has sharp threads that cut into a material such as sheet metal, plastic or wood. They are sometimes notched at the tip to aid in chip removal during thread cutting. The shank is usually threaded up to the head. Sheet metal screws make excellent fasteners for attaching metal hardware to wood because the fully threaded shank provides good retention in wood.
Twinfast Twinfast screw A Twinfast screw is a type of screw with two threads (i.e. a twin-start screw), so that it can be driven twice as fast as a normal (i.e. single-start) screw with the same pitch.[27] Dry wall screws designated as fine are the most common screws to use the twinfast style of threads.[28]
Screw for wood wood screw A metal screw with a sharp point designed to attach two pieces of wood together. Wood screws are commonly available with flat, pan or oval-heads. A wood screw generally has a partially unthreaded shank below the head. The unthreaded portion of the shank is designed to slide through the top board (closest to the screw head) so that it can be pulled tight to the board to which it is being attached. Inch-sized wood screws in the U.S. are defined by ANSI-B18.6.1-1981(R2003), while in Germany they are defined by DIN 95 (Slotted raised countersunk (oval) head wood screws), DIN 96 (Slotted round head wood screws), and DIN 97 (Slotted countersunk (flat) head wood screws).
Securityscrew Security head screw These screws are use for security purposes and where vandalism and/or theft is likely. The head of this type of screw is impossible to reverse. It requires special tools or mechanisms like spanners, tri-wings, torxes, square drivers, etc. In some screws, the head can be removed by breaking it after installing the screw.

Fasteners with a non-tapered shank

Vis 6 pans creux coupee
Narrow definition
Tornillo (Tipos de cabeza)
Wide definition
Shiny Ploughshares
Plow bolts in use

Fasteners with built in washers

A fastener with a built in washer is called a SEM or SEMS, short for pre-asSEMbled.[38][39] It could be fitted on either a tapered or non-tapered shank.

Other threaded fasteners

Superbolt, or multi-jackbolt tensioner

A 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. Because of this, the amount of torque required to achieve a given preload is reduced. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.

Bone screws

The field of screws and other hardware for internal fixation within the body is huge and diverse. Like prosthetics, it integrates the industrial and medicosurgical fields, causing manufacturing technologies (such as machining, CAD/CAM, and 3D printing) to intersect with the art and science of medicine. Like aerospace and nuclear power, this field involves some of the highest technology for fasteners, as well as some of the highest prices, for the simple reason that performance, longevity, and quality have to be excellent in such applications. Bone screws tend to be made of stainless steel or titanium, and they often have high-end features such as conical threads, multistart threads, cannulation (hollow core), and proprietary screw drive types (some not seen outside of these applications).

List of abbreviations for types of screws

These abbreviations have jargon currency among fastener specialists (who, working with many screw types all day long, have need to abbreviate repetitive mentions). The smaller basic ones can be built up into the longer ones; for example, if you know that "FH" means "flat head", then you may be able to parse the rest of a longer abbreviation containing "FH".

These abbreviations are not universally standardized across corporations; each corporation can coin their own. The more obscure ones may not be listed here.

The extra spacing between linked terms below helps the reader to see the correct parsing at a glance.

Abbreviation Expansion Comment
BH button head
BHCS button head cap screw
BHMS button head machine screw
CS cap screw
FH flat head
FHCS flat head cap screw
FHP flat head Phillips
FHSCS flat head socket cap screw
FHPMS flat head Phillips machine screw
FT full thread
HHCS hex head cap screw
HSHCS Hexalobular socket head cap screws
MS machine screw
OH oval head
PH Phillips head
RH round head
RHMS round head machine screw
RHP round head Phillips
RHPMS round head Phillips machine screw
SBHCS socket button head cap screw
SBHMS socket button head machine screw
SH socket head Although "socket head" could logically refer to almost any female drive, it refers by convention to hex socket head unless further specified.
SHCS socket head cap screw
SHSS socket head set screw Sometimes Socket Head Shoulder Screw.
SS set screw The abbreviation "SS" more often means stainless steel. Therefore, "SS cap screw" means "stainless steel cap screw" but "SHSS" means "socket head set screw". As with many abbreviations, users rely on context to diminish the ambiguity, although this reliance does not eliminate it.
STS self-tapping screw


Screws and bolts are usually made of steel. Where great resistance to weather or corrosion is required, like in very small screws or medical implants, materials such as stainless steel, brass, titanium, bronze, silicon bronze or monel may be used.

Galvanic corrosion of dissimilar metals can be prevented (using aluminum screws for double-glazing tracks for example) by a careful choice of material. Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation.

Often a surface coating is used to protect the fastener from corrosion (e.g. bright zinc plating for steel screws), to impart a decorative finish (e.g. japanning) or otherwise alter the surface properties of the base material.

Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature.

Bolted joints

Rustybolt th
Rusty hexagonal bolt heads

The American Institute of Steel Construction (AISC) 13th Edition Steel Design Manual section 16.1 chapter J-3 specifies the requirements for bolted structural connections. Structural bolts replaced rivets due to the decreasing cost and increasing strength of structural bolts in the 20th century. Connections are formed with two types of joints: slip-critical connections and bearing connections. In slip-critical connections, movement of the connected parts is a serviceability condition and bolts are tightened to a minimum required pretension. Slip is prevented through friction of the "faying" surface, that is the plane of shear for the bolt and where two members make contact. Because friction is proportional to the normal force, connections must be sized with bolts numerous and large enough to provide the required load capacity. However, this greatly decreases the shear capacity of each bolt in the connection. The second (and more common type) of connection is a bearing connection. In this type of connection, the bolts carry the load through shear and are only tightened to a "snug-fit". These connections require fewer bolts than slip-critical connections and therefore are a less expensive alternative. Slip-critical connections are more common on flange plates for beam and column splices and moment critical connections. Bearing type connections are used in lightweight structures and in member connections where slip is not important and prevention of structural failure is the design constraint. Common bearing type connections include: shear tabs, beam supports, gusset plates in trusses.

Mechanical classifications

The numbers stamped on the head of the bolt are referred to the grade of the bolt used in certain application with the strength of a bolt. High-strength steel bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. And the absence of marking/number indicates a lower grade bolt with low strength. The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the ultimate tensile strength in MPa divided by 100. The number after the point is the multiplier ratio of yield strength to ultimate tensile strength. For example, a property class 5.8 bolt has a nominal (minimum) ultimate tensile strength of 500 MPa, and a tensile yield strength of 0.8 times ultimate tensile strength or 0.8(500) = 400 MPa.

Ultimate tensile strength is the tensile stress at which the bolt fails. Tensile yield strength is the stress at which the bolt will yield in tension across the entire section of the bolt and receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2% offset strain. Proof strength is the usable strength of the fastener. Tension testing of a bolt up to the proof load should not cause permanent set of the bolt and should be conducted on actual fasteners rather than calculated.[40] If a bolt is tensioned beyond the proof load, it may behave in plastic manner due to yielding in the threads and the tension preload may be lost due to the permanent plastic deformations. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region of the bolt material. If a bolt is loaded in tension beyond its proof strength, the yielding at the net root section of the bolt will continue until the entire section is begins to yield and it has exceeded its yield strength. If tension increases, the bolt fractures at its ultimate strength.

Mild steel bolts have property class 4.6, with is 400 MPa ultimate strength and 0.6*400=240 MPa yield strength. High-strength steel bolts have property class 8.8, which is 800 MPa ultimate strength and 0.8*800=640 MPa yield strength or above.

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.[41] 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.[42]


There are many standards governing the material and mechanical properties of imperial sized externally threaded fasteners. Some of the most common consensus standards for grades produced from carbon steels are ASTM A193, ASTM A307, ASTM A354, ASTM F3125, and SAE J429. Some of the most common consensus standards for grades produced from corrosion resistant steels are ASTM F593 & ASTM A193.


The international standards for metric externally threaded fasteners are ISO 898-1 for property classes produced from carbon steels and ISO 3506-1 for property classes produced from corrosion resistant steels.

Screw head shapes

Screw head types
(a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head)
Cross slot screw
Combination flanged-hex/Phillips-head screw used in computers
Pan head
A low disc with a rounded, high outer edge with large surface area[57]
Button or dome head
Cylindrical with a rounded top
Round head
A dome-shaped head used for decoration.[58]
Mushroom or Truss head
Lower-profile dome designed to prevent tampering
Countersunk or flat head
Conical, with flat outer face and tapering inner face allowing it to sink into the material. The angle of the screw is measured as the full angle of the cone.
Oval or raised head
A decorative screw head with a countersunk bottom and rounded top.[58] Also known as "raised countersunk" (UK)
Bugle head
Similar to countersunk, but there is a smooth progression from the shank 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 diameter
Fillister head
Cylindrical, but with a slightly convex top surface. Height to diameter ratio is larger than cheese head.
Flanged head
A flanged head can be any of the above head styles (except the countersunk styles) with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer.

Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and also provides an easily inspectable joint to guarantee proper assembly. An example of this is the shear bolts used on vehicle steering columns, to secure the ignition switch.

Types of screw drives

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 in the US; hex, Robertson, and Torx are also common in some applications, and Pozidriv has almost completely replaced Phillips in Europe. 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.


An electric driver screws a self-tapping phillips head screw into wood

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 but not excessive force is developed by the screw. The hand tool for driving hex head threaded fasteners is a spanner (UK usage) or wrench (US usage), while a nut setter is used with a power screw driver.

Thread standards

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 Unified Thread Standard.

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 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 shank, and the nut or threaded hole is made by tapping threads into a hole of 5 mm diameter (6 mm - 1 mm).

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 (wrench size):

ISO metric thread M1.6 M2 M2.5 M3 M4 M5 M6 M8 M10 M12 M16 M20 M24 M30 M36 M42 M48 M56 M64
Wrench size (mm) 3.2 4.0 5.0 5.5 7.0 8.0 10.0 13.0 17.0 19.0 24.0 30.0 36.0 46.0 55.0 65.0 75.0 85.0 95.0

In addition, the following non-preferred intermediate sizes are specified:

ISO metric thread M7 M14 M18 M22 M27 M33 M39 M45 M52 M60 M68
Wrench size (mm) 11 22 27 32 41 50 60 70 80 90 100


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 too coarse for some applications. The thread angle was 55°, and the depth and pitch varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). Spanners for Whitworth bolts are marked with the size of the bolt, not the distance across the flats of the screw head.

The most common use of a Whitworth pitch nowadays is in all UK scaffolding. Additionally, 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). It is also used for microphone stands and their appropriate clips, again in both sizes, along with "thread adapters" to allow the smaller size to attach to items requiring the larger thread. Note that while 1/4" UNC bolts fit 1/4" BSW camera tripod bushes, yield strength is reduced by the different thread angles of 60° and 55° respectively.

British Association screw thread

British Association (BA) screw threads, named after the British Association for Advancement of Science, were devised in 1884 and standardised in 1903. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK. This equipment made extensive use of odd-numbered BA screws, in order—it may be suspected—to reduce theft. BA threads are specified by British Standard BS 93:1951 "Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A."

While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 6 mm diameter and 1 mm pitch. Other threads in the BA series are related to 0BA in a geometric series with the common factors 0.9 and 1.2. For example, a 4BA thread has pitch  mm (0.65mm) and diameter  mm (3.62mm). Although 0BA has the same diameter and pitch as ISO M6, the threads have different forms and are not compatible.

BA threads are still common in some niche applications. Certain types of fine machinery, such as moving-coil meters and clocks, tend to have BA threads wherever they are manufactured. BA sizes were also used extensively in aircraft, especially those manufactured in the United Kingdom. BA sizing is still used in railway signalling, mainly for the termination of electrical equipment and cabling.

BA threads are extensively used in Model Engineering where the smaller hex head sizes make scale fastenings easier to represent. As a result, many UK Model Engineering suppliers still carry stocks of BA fasteners up to typically 8BA and 10BA. 5BA is also commonly used as it can be threaded onto 1/8 rod.[59]

Unified Thread Standard

The Unified Thread Standard (UTS) is most commonly used in the United States, but is also extensively used in Canada and occasionally in other countries. The size of a UTS screw is described using the following format: X-Y, where X is the nominal size (the hole or slot size in standard manufacturing practice through which the shank of the screw can easily be pushed) and Y is the threads per inch (TPI). For sizes ​14 inch and larger the size is given as a fraction; for sizes less than this an integer is used, ranging from 0 to 16. The integer sizes can be converted to the actual diameter by using the formula 0.060 + (0.013 × number). For example, a #4 screw is 0.060 + (0.013 × 4) = 0.060 + 0.052 = 0.112 inches in diameter. There are also screw sizes smaller than "0" (zero or ought). The sizes are 00, 000, 0000 which are usually referred to as two ought, three ought, and four ought. Most eyeglasses have the bows screwed to the frame with 00-72 (pronounced double ought – seventy two) size screws. To calculate the major diameter of "ought" size screws count the number of 0's and multiply this number by 0.013 and subtract from 0.060. For example, the major diameter of a 000-72 screw thread is .060 – (3 x .013) = 0.060 - 0.039 = .021 inches. For most size screws there are multiple TPI available, with the most common being designated a Unified Coarse Thread (UNC or UN) and Unified Fine Thread (UNF or UF). Note: In countries other than the United States and Canada, the ISO Metric Screw Thread System is primarily used today. Unlike most other countries the United States and Canada still use the Unified (Inch) Thread System. However, both are moving over to the ISO Metric System. It is estimated that approximately 60% of screw threads in use in the United States are still inch based.[60]


Bolt Forming
Screw (bolt) 13-n

There are three steps in manufacturing a screw: heading, thread rolling, and coating. Screws are normally made from wire, which is supplied in large coils, or round bar stock for larger screws. The wire or rod is then cut to the proper length for the type of screw being made; this workpiece is known as a blank. It is then cold headed, which is a cold working process. Heading produces the head of the screw. The shape of the die in the machine dictates what features are pressed into the screw head; for example a flat head screw uses a flat die. For more complicated shapes two heading processes are required to get all of the features into the screw head. This production method is used because heading has a very high production rate, and produces virtually no waste material. Slotted head screws require an extra step to cut the slot in the head; this is done on a slotting machine. These machines are essentially stripped down milling machines designed to process as many blanks as possible.

The blanks are then polished again prior to threading. The threads are usually produced via thread rolling; however, some are cut. The workpiece is then tumble finished with wood and leather media to do final cleaning and polishing. For most screws, a coating, such as electroplating with zinc (galvanizing) or applying black oxide, is applied to prevent corrosion.


Screw making machine, 1871
A lathe of 1871, equipped with leadscrew and change gears for single-point screw-cutting.
A Brown & Sharpe single-spindle screw machine.

While a recent hypothesis attributes the Archimedes' screw to Sennacherib, King of Assyria, archaeological finds and pictorial evidence only appear in the Hellenistic period and the standard view holds the device to be a Greek invention, most probably by the 3rd century BC polymath Archimedes.[61] Though resembling a screw, this is not a screw in the usual sense of the word.

Earlier, the screw had been described by the Greek mathematician Archytas of Tarentum (428–350 BC). By the 1st century BC, wooden screws were commonly used throughout the Mediterranean world in screw presses for pressing olive oil from olives and pressing juice from grapes in winemaking. Metal screws used as fasteners were rare in Europe before the 15th century, if known at all.[62]

Rybczynski has shown[63] that handheld screwdrivers (formerly called "turnscrews" in English, in more direct parallel to their original French name, tournevis[64]) have existed since medieval times (the 1580s at the latest), although they probably did not become truly widespread until after 1800, once threaded fasteners had become commodified, as detailed below.

There were many forms of fastening in use before threaded fasteners became widespread. They tended to involve carpentry and smithing rather than machining, and they involved concepts such as dowels and pins, wedging, mortises and tenons, dovetails, nailing (with or without clenching the nail ends), forge welding, and many kinds of binding with cord made of leather or fiber, using many kinds of knots. Prior to the mid-19th century, cotter pins or pin bolts, and "clinch bolts" (now called rivets), were used in shipbuilding. Glues also existed, although not in the profusion seen today.

The metal screw did not become a common fastener until machine tools for their mass production were developed toward the end of the 18th century. This development blossomed in the 1760s and 1770s[65] along two separate paths that soon converged:[66] the mass production of wood screws (meaning screws made of metal to be used in wood) in a specialized, single-purpose, high-volume-production machine tool; and the low-count, toolroom-style production of machine screws (V-thread) with easy selection among various pitches (whatever the machinist happened to need on any given day).

The first path was pioneered by brothers Job and William Wyatt of Staffordshire, UK,[67] who patented in 1760 a machine that we might today best call a screw machine of an early and prescient sort. It made use of a leadscrew to guide the cutter to produce the desired pitch,[67] and the slot was cut with a rotary file while the main spindle held still (presaging live tools on lathes 250 years later). Not until 1776 did the Wyatt brothers have a wood-screw factory up and running.[67] Their enterprise failed, but new owners soon made it prosper, and in the 1780s they were producing 16,000 screws a day with only 30 employees[68]—the kind of industrial productivity and output volume that would later be characteristic of modern industry but was revolutionary at the time.

Meanwhile, English instrument maker Jesse Ramsden (1735–1800) was working on the toolmaking and instrument-making end of the screw-cutting problem, and in 1777 he invented the first satisfactory screw-cutting lathe.[60] The British engineer Henry Maudslay (1771–1831) gained fame by popularizing such lathes with his screw-cutting lathes of 1797 and 1800, containing the trifecta of leadscrew, slide rest, and change-gear gear train, all in the right proportions for industrial machining. In a sense he unified the paths of the Wyatts and Ramsden and did for machine screws what had already been done for wood screws, i.e., significant easing of production spurring commodification. His firm would remain a leader in machine tools for decades afterward. A misquoting of James Nasmyth popularized the notion that Maudslay had invented the slide rest, but this was incorrect; however, his lathes helped to popularize it.

These developments of the 1760–1800 era, with the Wyatts and Maudslay being arguably the most important drivers, caused great increase in the use of threaded fasteners. Standardization of threadforms began almost immediately, but it was not quickly completed; it has been an evolving process ever since. Further improvements to the mass production of screws continued to push unit prices lower and lower for decades to come, throughout the 19th century.[69]

The American development of the turret lathe (1840s) and of automatic screw machines derived from it (1870s) drastically reduced the unit cost of threaded fasteners by increasingly automating the machine tool control. This cost reduction spurred ever greater use of screws.

Throughout the 19th century, the most commonly used forms of screw head (that is, drive types) were simple internal-wrenching straight slots and external-wrenching squares and hexagons. These were easy to machine and served most applications adequately. Rybczynski describes a flurry of patents for alternative drive types in the 1860s through 1890s,[70] but explains that these were patented but not manufactured due to the difficulties and expense of doing so at the time. In 1908, Canadian P. L. Robertson was the first to make the internal-wrenching square socket drive a practical reality by developing just the right design (slight taper angles and overall proportions) to allow the head to be stamped easily but successfully, with the metal cold forming as desired rather than being sheared or displaced in unwanted ways.[70] Practical manufacture of the internal-wrenching hexagon drive (hex socket) shortly followed in 1911.[71][72]

In the early 1930s, the popular Phillips-head screw was invented by American Henry F. Phillips.[73]

Threadform standardization further improved in the late 1940s, when the ISO metric screw thread and the Unified Thread Standard were defined.

Precision screws, for controlling motion rather than fastening, developed around the turn of the 19th century, were one of the central technical advances, along with flat surfaces, that enabled the industrial revolution.[74] They are key components of micrometers and lathes.

Other fastening methods

Alternative fastening methods are:

See also


  1. ^ Smith 1990, p. 39.
  2. ^ Oberg et al. 2000, p. 1492.
  3. ^ "Cambridge Dictionary of American English". Cambridge University Press. Retrieved 2008-12-03.
  4. ^ "allwords". Retrieved 2008-12-03.
  5. ^ "Merriam Webster Dictionary bolt". Retrieved 2008-12-03.
  6. ^ "Compact Oxford English Dictionary bolt". Oxford. Retrieved 2008-12-03.
  7. ^ "Cambridge Advanced Learner's Dictionary bolt". Cambridge University Press. Retrieved 2008-12-03.
  8. ^ "The Fastener Resource Center - Know your Bolts". Retrieved 2011-03-13.
  9. ^ a b White, Christopher. "Observations on the Development of Wood Screws in North America" (PDF).
  10. ^ "Making 18th c wood screws".
  11. ^ "Iron Age, Volume 44".
  12. ^ Moxon, Joseph (1703). Mechanic Exercises: Or the Doctrine of Handy-Works. Mendham, NJ.
  13. ^ Oberg et al. 2000, pp. 1568–1598.
  14. ^ Oberg et al. 2000, p. 1496.
  15. ^ "Distinguishing Bolts from Screws page 7" (PDF). Retrieved 2018-07-23.
  16. ^ "National Institute of Standards and Technology - NIST". NIST. Archived from the original on 2011-07-21.
  17. ^ B18.2.1 - 1996 Square and Hex Bolts and Screws, Inch Series - Print-Book
  18. ^ " Glossary - lug bolt". Retrieved 2009-01-13.
  19. ^ " Glossary - head bolt". Retrieved 2010-10-13.
  20. ^ Merriam-Webster's Unabridged Dictionary, Merriam-Webster.
  21. ^ Oberg et al. 2000, p. 1497.
  22. ^ U.S. Customs and Border Protection Agency (CBP) (July 2012), What Every Member of the Trade Community Should Know About: Distinguishing Bolts from Screws, An Informed Compliance Publication (2011-02 ed.), Washington, D.C., USA:
  23. ^ a b Dyke's Automobile and Gasoline Engine Encyclopedia page 701, A.L. Dyke, 1919, retrieved 2009-01-13.
  24. ^ Source:
  25. ^ "Tricks of the Trade". Motorcycle Mechanics. London: Fetter Publications. 2 (12): 60. September 1960.
  26. ^ "coach screw definition". Retrieved 2010-01-19.
  27. ^ Soled, Julius (1957), Fasteners handbooks, Reinhold, p. 151.
  28. ^ "Fine thread drywall screws". Mutual Screw & Fastener Supply. Retrieved 2011-03-16.
  29. ^ Oberg 2000, pp. 1599–1605.
  30. ^ Samuel, Andrew (1999), Introduction to Engineering Design, Oxford: Butterworth-Heinemann, p. 213, ISBN 0-7506-4282-3
  31. ^ Anthony, Gardner Chase (1910), Machine Drawing, D. C. Heath, p. 16.
  32. ^ Woolley, Joseph William; Meredith, Roy Brodhead (1913), Shop sketching, McGraw-Hill, pp. 40–41.
  33. ^ "elevator head definition".
  34. ^ Colvin & Stanley 1914, p. 569.
  35. ^ Plow bolts, retrieved 2008-12-25.
  36. ^ The Meaning of "plow head, plow bolt" at
  37. ^ Huth, pp. 166–167.
  38. ^ "All About Screws" (PDF). Curious Inventor. Retrieved 17 October 2013.
  39. ^ "Glossary". Retrieved 17 October 2013.
  40. ^ Brenner, Harry S. (1977). Parmley, Robert O. (ed.). Standard Handbook of Fastening and Joining (5 ed.). New York: McGraw-Hill. p. Chapter 1 page 10. ISBN 0-07-048511-9.
  41. ^ "How to Recognize Metric and SAE Bolts", Chilton DIY, Retrieved April 26, 2016.
  42. ^ "Fraudulent/Counterfeit Electronic Parts", SAE International, Retrieved April 26, 2016.
  43. ^ a b c Bolt grade markings and strength chart, retrieved 2009-05-29.
  44. ^ Mechanical Methods of Joining, retrieved 2009-06-06.
  45. ^ a b c d e f g h i Grade Markings: Carbon Steel Bolts, retrieved 2009-05-30.
  46. ^ a b c d e f Hardware, bulk — Technical information, retrieved 2009-05-30.
  47. ^ a b c d e f g h ASTM, SAE and ISO grade markings and mechanical properties for steel fasteners, retrieved 2009-06-06.
  48. ^ a b c Fastener identification marking (PDF), retrieved 2009-06-23.
  49. ^ a b Other markings may be used to denote atmospheric corrosion resistant material
  50. ^ a b c FastenalTechnicalReferenceGuide (PDF), retrieved 2010-04-30.
  51. ^ Metric Handbook, archived from the original on 2007-10-31, retrieved 2009-06-06.
  52. ^ Mechanical properties of bolts, screws, and studs according DIN-ISO 898, part 1 (PDF), retrieved 2009-06-06.
  53. ^ a b ASTM F568M - 07, 2007, retrieved 2009-06-06.
  54. ^ a b c d Metric structural fasteners, archived from the original on 1999-04-21, retrieved 2009-06-06.
  55. ^ a b ASTM A325M - 09, retrieved 2009-06-13.
  56. ^ a b ASTM A490M - 09, 2009, retrieved 2009-06-06.
  57. ^ "Different Kinds of Bolt and Screw Heads", AFI, Retrieved April 26, 2016.
  58. ^ a b Mitchell, George (1995), Carpentry and Joinery (3rd ed.), Cengage Learning, p. 205, ISBN 978-1-84480-079-7.
  59. ^
  60. ^ a b Rybczynski 2000, pp. 97–99.
  61. ^ Stephanie Dalley and John Peter Oleson (January 2003). "Sennacherib, Archimedes, and the Water Screw: The Context of Invention in the Ancient World", Technology and Culture 44 (1).
  62. ^ Am_Wood_Screws (PDF), retrieved 2010-04-30.
  63. ^ Rybczynski 2000, pp. 34, 66, 90.
  64. ^ Rybczynski 2000, pp. 32–36, 44.
  65. ^ Rybczynski 2000, pp. 75–99.
  66. ^ Rybczynski 2000, p. 99.
  67. ^ a b c Rybczynski 2000, p. 75.
  68. ^ Rybczynski 2000, p. 76.
  69. ^ Rybczynski 2000, pp. 76–78.
  70. ^ a b Rybczynski 2000, pp. 79–81.
  71. ^ U.S. Patent 161,390.
  72. ^ Hallowell 1951, pp. 51–59.
  73. ^ See:
    • Henry F. Phillips and Thomas M. Fitzpatrick, "Screw," U.S. Patent no. 2,046,839 (filed: January 15, 1935 ; issued: July 7, 1936).
    • Henry F. Phillips and Thomas M. Fitzpatrick, "Screw driver," U.S. Patent no. 2,046,840 (filed: January 15, 1935 ; issued: July 7, 1936).
  74. ^ Rybczynski 2000, p. 104.


External links

Archimedes' screw

An Archimedes' screw, also known by the name the Archimedean screw or screw pump, is a machine used for transferring water from a low-lying body of water into irrigation ditches. Water is pumped by turning a screw-shaped surface inside a pipe.

Chopped and screwed

Chopped and screwed (very rarely also called screwed and chopped or slowed and throwed), is a technique of remixing hip hop music which developed in the Houston hip hop scene in the early 1990s. This is accomplished by slowing the tempo down to between 60 and 70 quarter-note beats per minute and applying techniques such as skipping beats, record scratching, stop-time, and affecting portions of the music to make a "chopped-up" version of the original.

DJ Screw is largely recognized as the innovator behind the chopped and screwed genre. DJ Screw's key technique involved playing the same record on both turntables with a delay between them of one beat and quickly moving the crossfader side to side. This created an effect where the words or beats in a song were repeated without interrupting its tempo.


Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections, and to work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms parts with an excellent surface finish.Drawing is a similar process, which uses the tensile strength of the material to pull it through the die. This limits the amount of change which can be performed in one step, so it is limited to simpler shapes, and multiple stages are usually needed. Drawing is the main way to produce wire. Metal bars and tubes are also often drawn.

Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold. Commonly extruded materials include metals, polymers, ceramics, concrete, modelling clay, and foodstuffs. The products of extrusion are generally called "extrudates".

Also referred to as "hole flanging", hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the centre barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die. The material flows around the supports and fuses together to create the desired closed shape.

The extrusion process in metals may also increase the strength of the material.

ISO 965

ISO 965 (ISO general purpose metric screw thread—tolerances) is an International Organization for Standardization (ISO) standard for metric screw thread tolerances. It specifies the basic profile for ISO general purpose metric screw threads (M) conforming to ISO 261.The tolerance system refers to the basic profile in accordance with ISO 68-1. The Field of application and purpose of ISO 965 can be defined as "ISO 965 specifies a tolerance system for screw threads from 1mm diameter upwards.

List of screw drives

A screw drive is a system used to turn a screw. At a minimum, it is a set of shaped cavities and protrusions on the screw head that allows torque to be applied to it. Usually, it also involves a mating tool, such as a screwdriver, that is used to turn it. The following heads are categorized based on commonality, with some of the less-common drives being classified as "tamper-resistant".

Most heads come in a range of sizes, typically distinguished by a number, such as "Phillips #00". These sizes do not necessarily describe a particular dimension of the drive shape, but rather are arbitrary designations.


A micrometer ( my-KROM-i-tər), sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for accurate measurement of components in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital calipers. Micrometers are usually, but not always, in the form of calipers (opposing ends joined by a frame). The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil.

Micrometers are also used in telescopes or microscopes to measure the apparent diameter of celestial bodies or microscopic objects. The micrometer used with a telescope was invented about 1638 by William Gascoigne, an English astronomer. Colloquially the word micrometer is often shortened to mike or mic ( MYKE).


Pandanus is a genus of monocots with some 750 accepted species. They are palm-like, dioecious trees and shrubs native to the Old World tropics and subtropics. Common names include pandan (), screw palm, and screw pine. They are classified in the order Pandanales, family Pandanaceae.


A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by Bernoulli's principle and Newton's third law. Most marine propellers are screw propellers with fixed helical blades rotating around a horizontal (or nearly horizontal) axis or propeller shaft.

Screw cap

A screw cap or closure is a common type of closure for bottles, jars, and tubes.

Screw cap (wine)

A screw cap is a metal cap that screws onto threads on the neck of a wine bottle, generally with a metal skirt down the neck to resemble the traditional wine capsule ("foil"). A layer of plastic (often PVDC), cork, rubber, or other soft material is used as wad to make a seal with the mouth of the bottle. Its use as an alternative wine closure is gaining increasing support as an alternative to cork for sealing wine bottles. In markets such as Australia and New Zealand screw caps on bottles have overtaken cork to become the most common means of sealing bottles.

Screw thread

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a fastener.

The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied, as long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight elastic deformation.


A screwdriver is a tool, manual or powered, for screwing and unscrewing (inserting and removing) screws. A typical simple screwdriver has a handle and a shaft, ending in a tip the user puts into the screw head before turning the handle. The shaft is usually made of tough steel to resist bending or twisting. The tip may be hardened to resist wear, treated with a dark tip coating for improved visual contrast between tip and screw—or ridged or treated for additional 'grip'. Handles are typically wood, metal, or plastic and usually hexagonal, square, or oval in cross-section to improve grip and prevent the tool from rolling when set down. Some manual screwdrivers have interchangeable tips that fit into a socket on the end of the shaft and are held in mechanically or magnetically. These often have a hollow handle that contains various types and sizes of tips, and a reversible ratchet action that allows multiple full turns without repositioning the tip or the user's hand.

A screwdriver is classified by its tip, which is shaped to fit the driving surfaces—slots, grooves, recesses, etc.—on the corresponding screw head. Proper use requires that the screwdriver's tip engage the head of a screw of the same size and type designation as the screwdriver tip. Screwdriver tips are available in a wide variety of types and sizes (List of screw drives). The two most common are the simple 'blade'-type for slotted screws, and Phillips, generically called "cross-recess".

A wide variety of power screwdrivers range from a simple 'stick'-type with batteries, a motor, and a tip holder all inline, to powerful "pistol" type VSR (variable-speed reversible) cordless drills that also function as screwdrivers. This is particularly useful as drilling a pilot hole before driving a screw is a common operation. Special combination drill-driver bits and adapters let an operator rapidly alternate between the two. Variations include impact drivers, which provide two types of 'hammering' force for improved performance in certain situations, and "right-angle" drivers for use in tight spaces. Many options and enhancements, such as built-in bubble levels, high/low gear selection, magnetic screw holders, adjustable-torque clutches, keyless chucks, 'gyroscopic' control, etc., are available.

Screwdriver (cocktail)

A screwdriver is a popular alcoholic highball drink made with orange juice and vodka. While the basic drink is simply the two ingredients, there are many variations. Many of the variations have different names in different parts of the world. The International Bartenders Association has designated this cocktail as an IBA Official Cocktail.

Steam frigate

Steam frigates, also known as screw frigates, and the smaller steam corvettes and steam sloops were steam-powered warships. The first such ships were steam-powered versions of the traditional frigates, corvettes, and sloops.


A steamship, often referred to as a steamer, is a type of steam powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or paddlewheels. The first steamships came into practical usage during the early 1800s; however, there were exceptions that came before. Steamships usually use the prefix designations of "PS" for paddle steamer or "SS" for screw steamer (using a propeller or screw). As paddle steamers became less common, "SS" is assumed by many to stand for "steam ship". Ships powered by internal combustion engines use a prefix such as "MV" for motor vessel, so it is not correct to use "SS" for most modern vessels.

As steamships were less dependent on wind patterns, new trade routes opened up. The steamship has been described as a "major driver of the first wave of trade globalization (1870–1913)" and contributor to "an increase in international trade that was unprecedented in human history."

The Turn of the Screw

The Turn of the Screw is an 1898 horror novella by Henry James that first appeared in serial format in Collier's Weekly magazine (January 27 – April 16, 1898). In October 1898 it appeared in The Two Magics, a book published by Macmillan in New York City and Heinemann in London. Classified as both gothic fiction and a ghost story, the novella focuses on a governess who, caring for two children at a remote estate, becomes convinced that the grounds are haunted.

In the century following its publication, The Turn of the Screw became a cornerstone text of academics who subscribed to New Criticism. The novella has had differing interpretations, often mutually exclusive. Many critics have tried to determine the exact nature of the evil hinted at by the story. However, others have argued that the brilliance of the novella results from its ability to create an intimate sense of confusion and suspense within the reader.

The novella has been adapted numerous times in radio drama, film, stage, and television, including a 1950 Broadway play, and the 1961 film The Innocents.


Torx (pronounced ), developed in 1967 by Camcar Textron, is the trademark for a type of screw head characterized by a 6-point star-shaped pattern. A popular generic name for the drive is star, as in star screwdriver or star bits. The official generic name, standardized by the International Organization for Standardization as ISO 10664, is hexalobular internal. This is sometimes abbreviated in databases and catalogs as 6lobe (starting with the numeral 6, not the capital letter, G). Torx Plus and Torx Paralobe are improved head profiles.

Torx screws are commonly found on automobiles, motorcycles, bicycle brake systems (disc brakes), hard disk drives, computer systems and consumer electronics. Initially, they were sometimes used in applications requiring tamper resistance, since the drive systems and screwdrivers were not widely available; as drivers became more common, tamper-resistant variants, as described below, were developed. Torx screws are also becoming increasingly popular in construction industries.


A vial (also known as a phial or flacon) is a small glass or plastic vessel or bottle, often used to store medication as liquids, powders or capsules. They can also be used as scientific sample vessels; for instance, in autosampler devices in analytical chromatography. Vial-like glass containers date back to classical antiquity; modern vials are often made of plastics such as polypropylene. There are different types of vials such as a single dose vial and multi-dose vials often used for medications. The single dose vial is only used once whereas a multi-dose vial can be used more than once. The CDC sets specific guidelines on multi-dose vials.

Worm drive

A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear). The two elements are also called the worm screw and worm wheel. The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.

Like other gear arrangements, a worm drive can reduce rotational speed or transmit higher torque. A worm is an example of a screw, one of the six simple machines.

One of the major advantages of worm gear drive units are that they can transfer motion in 90 degrees.

Head markings and properties for inch-system hex-head cap screws[43]
Head marking Grade, material and condition Nominal size range (in) Proof strength Yield strength, min. Tensile strength, min. Core hardness (Rockwell)
ksi MPa ksi MPa ksi MPa
Hex cap screw-no markings SAE Grade 0[44] Strength and hardness is not specified
SAE grade 1
ASTM A307[45]
Low carbon steel
14–​1 12 33 230 60 410 B70–100
ASTM A307 - Grade B[45]
Low or medium carbon steel
14–4 60 minimum
100 maximum
410 minimum
690 maximum
SAE grade 2
Low or medium carbon steel
14–​34 55 380 57 390 74 510 B80–100[46]
Greater than ​34 33 230 36 250 60 410 B70–100[46]
SAE grade 4[47]
Medium carbon steel; cold worked
14–​1 12 100 690 115 790
Hex cap screw-grade 3 SAE grade 3[45]
Medium carbon steel; cold worked
14–1 85 590 100 690 B70–100
Hex cap screw-grade 5 SAE grade 5
Medium carbon steel; quench and tempered
14–1 (inc.) 85 590 92 630 120 830 C25–34[46]
1–​1 12 74 510 81 560 105 720 C19–30[46]
ASTM A449 - Type 1[45]
Medium carbon steel; quench and tempered
1–​1 12 (inc.) 74 510 105 720 C19–30
1 12–3 55 380 90 620 Brinell 183–235
Hex cap screw-grade 5.1 SAE grade 5.1[48]
Low or medium carbon steel; quench and tempered
No. 6–​12 85 590 120 830 C25–40
Hex cap screw-grade 5.2 SAE grade 5.2[48]
Low carbon martensitic steel; quench and tempered
14–1 85 590 120 830 C26–36
ASTM A449 - Type 2[48]
Low carbon martensitic steel; quench and tempered
Hex cap screw-ASTM A325or Hex cap screw-ASTM A325 lines ASTM A325 - Type 1[45]
Medium carbon steel; quench and tempered
12–1 (inc.) 85 590 92 630[47] 120 830 C24–35
1–​1 12 74 510 82 570[47] 105 720 C19–31
Hex cap screw-ASTM A325 type 3[49] ASTM A325 - Type 3[45]
Atmospheric corrosion resistant steel; quench and tempered
12–1 85 590 92 630[47] 120 830 C24–35
1–​1 12 74 510 82 570[47] 105 720 C19–31
Hex cap screw-ASTM A354 grade BC ASTM A354 - Grade BC[45]
Medium carbon alloy steel; quench and tempered
14–​2 12 (inc.) 105 720 109 750[47] 125 860 C26–36
2 12–4 95 660 99 680[47] 115 790 C22–33
Hex cap screw-grade 7 SAE grade 7
Medium carbon alloy steel; quench and tempered
14–​1 12 105 720 115 790 133 920
Hex cap screw-grade 8 SAE grade 8
Medium carbon alloy steel; quench and tempered
14–​1 12 120 830 130 900 150 1,000 C32–38[46]
Hex cap screw-ASTM A354 grade BD lines ASTM A354 - Grade BD[50] 14–​2 12 (inc.) 120 830 130 900[50] 150 1,000 C33–39
Hex cap screw-ASTM A354 grade BD 2 12–4 105 720 115 790[50] 140 970 C31–39
Hex cap screw-grade 8.2 SAE grade 8.2[46]
Medium carbon boron martensitic steel; fully kilned, fine grain, quench and tempered
14–1 120 830 150 1,000 C33–39
Hex cap screw-ASTM A490 ASTM A490 - Type 1[45]
Medium carbon alloy steel; quench and tempered
12–​1 12 120 830 130[47] 900 150 minimum
170 maximum
1,000 minimum
1,200 maximum
Hex cap screw-ASTM A490 type 3[49] ASTM A490 - Type 3[45]
Atmospheric corrosion resistant steel; quench and tempered
18/8 Stainless
Stainless steel with 17–19% chromium and 8–13% nickel
14–​58 (inc.) 40 minimum
80–90 typical
280 minimum
550–620 typical
100–125 typical 690–860 typical
58–1 (inc.) 40 minimum
45–70 typical
280 minimum
310–480 typical
100 typical 690 typical
over 1 80–90 typical 550–620 typical
Head markings and properties for metric hex-head cap screws[51]
Head marking Grade, material and condition Nominal size range (mm) Proof strength Yield strength, min. Tensile strength, min. Core hardness (Rockwell)
MPa ksi MPa ksi MPa ksi
Hex cap screw-class 3.6 Class 3.6[52] 1.6–36 180 26 190 28 330 48 B52–95
Hex cap screw-class 4.6 Class 4.6
Low or medium carbon steel
5–100 225 32.6 240 35 400 58 B67–95
Hex cap screw-class 4.8 Class 4.8
Low or medium carbon steel; fully or partially annealed
1.6–16 310 45 340 49 420 61 B71–95
Hex cap screw-class 5.8 Class 5.8
Low or medium carbon steel; cold worked
5–24 380 55 420 61 520 75 B82–95
Hex cap screw-class 8.8 Class 8.8[43]
Medium carbon steel; quench and tempered
Under 16 (inc.) 580 84 640 93 800 120
17–72 600 87 660 96 830 120 C23–34
Hex cap screw-class 8.8 line Class 8.8 low carbon
Low carbon boron steel; quench and tempered
Hex cap screw-class 8.8.3 Class 8.8.3[53]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-ASTM A325M 8S ASTM A325M - Type 1[54][55]
Medium carbon steel; quench and tempered
Hex cap screw-ASTM A325M 8S3 ASTM A325M - Type 3[54][55]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-class 9.8 Class 9.8
Medium carbon steel; quench and tempered
1.6–16 650 94 720 104 900 130 C27–36
Hex cap screw-class 9.8 line Class 9.8 low carbon
Low carbon boron steel; quench and tempered
Hex cap screw-class 10.9 Class 10.9
Alloy steel; quench and tempered
5–100 830 120 940 136 1,040 151 C33–39
Hex cap screw-class 10.9 line Class 10.9 low carbon
Low carbon boron steel; quench and tempered
Hex cap screw-class 10.9.3 Class 10.9.3[53]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-ASTM A490M 10S ASTM A490M - Type 1[54][56]
Alloy steel; quench and tempered
Hex cap screw-ASTM A490M 10S3 ASTM A490M - Type 3[54][56]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-class 12.9 Class 12.9
Alloy steel; quench and tempered
1.6–100 970 141 1,100 160 1,220 177 C38–44
Hex cap screw-A2 A2[43]
Stainless steel with 17–19% chromium and 8–13% nickel
up to 20 210 minimum
450 typical
30 minimum
65 typical
500 minimum
700 typical
73 minimum
100 typical
ISO 3506-1 A2-50
304 stainless steel-class 50 (annealed)
210 30 500 73
ISO 3506-1 A2-70
304 stainless steel-class 70 (cold worked)
450 65 700 100
ISO 3506-1 A2-80
304 stainless steel-class 80
600 87 800 120

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