Screw

This article is about screws used as a fastener. For other uses of the term screw, see screw (disambiguation).

Screws come in a variety of shapes and sizes for different purposes. U.S. quarter coin (diameter 24 mm) shown for scale.

A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.

Often screws have a head, which is a specially formed section on one end of the screw that allows it to be turned, or driven. Common tools for driving 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 have a head smaller than the outer diameter of the screw; and J-bolts do not have a head and are not designed to be driven. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded.

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread. Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to anticlockwise forces (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice.

Differentiation between bolt and screw

A carriage bolt with square nut

A structural bolt

A universally accepted distinction between a screw and a bolt does not exist.

The 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.)

This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw and bolt.

A less authoritative source, the Structural Analysis Reference Library, defines the two differently: {{quote|Bolts are defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as M, MJ, UN, UNR, and UNJ) such that they can accept a nontapered nut. Screws are defined as headed, externally-threaded fasteners that do not meet the above definition of bolts.

By this definition, the essential distinction between a bolt and a screw is that a screw always cuts its own thread (the first time it is used), whereas a bolt almost always does not. (A bolt may in fact cut its own thread, but it is capable of being screwed into a specified matching threaded hole). While this definition of bolts versus screws is not authoritative, it has the advantage of being useful, and non-arbitrary, and eliminates the ambiguity of the Machinery's Handbook distinctions.

Some of the general ambiguity issues are discussed below:

Machine screws

ASME standards specify a variety of "Machine Screws" in diameters ranging up to 0.75 in. These fasteners are often used with nuts and they are often driven into tapped holes. 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 screws can be referred to as stove bolts.

Hex cap screws

ASME standard B18.2.1 -1996 specifies Hex Cap Screws that range in size from 0.25–3 in 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. The Machinery's Handbook refers parenthetically to these fasteners as "Finished Hex Bolts". 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. In 1991 responding to an influx of counterfeit fasteners Congress passed PL 101-592 "Fastener Quality Act" This resulted in the rewriting of specifications by the ASME B18 committee. B18.2.1 was re-written and as a result they eliminated the "Finished Hex Bolts" and renamed them the "Hex Cap Screw".

Lug bolts & 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. This variance, perhaps, originated from common usage ideas that screws are small and bolts are big.

Lag bolt

These are clearly screws based on the Machinery's Handbook distinction. The term has been replaced by "Lag Screw" in the Machinery's Handbook and probably only continues in common usage because of common language notion that bolts are big.

Government standards

The US government made an effort to formalize the difference between a bolt and a screw because different tariffs apply to each. 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.

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. This is now an obsolete distinction.

Controlled vocabulary versus natural language

The distinctions delineated above are enforced in the controlled vocabulary of standards organizations. Nevertheless, there are sometimes differences between the controlled vocabulary and the natural-language usage of the words among machinists, auto mechanics, and other workers. 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 themselves change.) Nonthreaded fasteners predominated in fastening technology 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 is often that screws are smaller than bolts, and that screws are generally tapered and bolts are not. This distinction is not rigorous.

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 (self-threading screws)

A Phillips wood screw being driven into a board with a driver.

Wood screw

Generally has an unthreaded shank below the head. It is designed to attach two pieces of wood together.

Coach screw (UK) or lag screw/bolt (US)

Similar to a wood screw except that it is generally much larger running to lengths up to 15 in (380 mm) with diameters from 0.25–0.5 in (6.4–12.7 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.

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 amalgamated 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.

Sheet metal screw (self-tapping screw, thread cutting screws)

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.

Concrete screw

A stainless or carbon steel screw for fastening wood, metal, or other materials into concrete or masonry. Concrete screws are commonly blue in color, with or without corrosion coating. They may either have a Phillips flat head or a slotted hex washer head. Heads sizes range from 0.1875 to 0.375 in (4.76 to 9.53 mm) and lengths from 1.25 to 5 in (32 to 127 mm).

The concrete screw was patented by Vincent Yotti of Melbourne Beach, Florida in 1975, and was presented as a sheet metal screw or lag bolt used to anchor articles to concrete or related material. The concrete screw would be an inexpensive screw of quick application and of greater holding strength than prior concrete anchoring devices such as conventional sheet metal screws.

In the United States, concrete screws are commonly called Tapcons which refers to the brand name created from the definition of "an anchor that taps its own threads into concrete." Other commercial names for the fastener are masonry screw, confast screw, blue screw, self-tapping screw, and Titen.

Self-drilling screw (Teks screw)

Similar to a sheet metal screw, but it has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. Designed for use in soft steel or other metals. The points are numbered from 1 through 5, the larger the number, the thicker metal it can go through without a pilot hole. A 5 point can drill a 0.5 in (13 mm) of steel, for example.

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 shaft diameter.

Particle board screw (chipboard screw)

Similar to a drywall screw except that it has a thinner shaft 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.

Deck screw

Similar to drywall screw except that it is 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.

Double ended screw (dowel screw)

Similar to a wood screw but with two pointed ends and no head, used for making hidden joints between two pieces of wood.

Screw eye (eye screw)

Screw with a looped head. Larger ones are sometimes call lag eye screws. Designed to be used as attachment point, particularly for something that is hung from it.

Thread rolling screws

These have a lobed (usually triangular) cross-section. They form threads in a pre-drilled hole in the mating workpiece by pushing the material outward during installation.

Fasteners with a non-tapered shank

Breakaway bolt

A bolt with a hollow threaded shank, which is designed to break away upon impact. Typically used to fasten fire hydrants, so they will break away when hit by a car. Also used in aircraft to reduce weight.

Cap screw

In places the term is used interchangeably with bolt. In the past the term cap screw was restricted to threaded fasteners with a shank that is threaded all the way to the head, but this is now a non-standard usage.

Hex cap screw

Cap screw with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shank length. The nature of the tolerance difference allows an ASME B18.2.1 hex cap screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex cap screw is designed in.

Hex bolt

At times the term is used interchangeably with hex cap screw. An ASME B18.2.1 compliant hex bolt is built to different tolerances than a hex cap screw.

Socket cap screw

Also known as a socket head cap screw, socket screw or Allen bolt, this is a type of cap screw with a hexagonal recessed drive. The most common types in use have a cylindrical head whose diameter is nominally 1.5 times (1960 series design) that of the screw shank (major) diameter. Counterbored holes in parts allow the screw head to be flush with the surface or recessed. Other head designs include button head and flat head, the latter designed to be seated into countersunk holes. A hex key (sometimes referred to as an Allen wrench or Allen key) or hex driver is required to tighten or loosen a socket screw. Socket screws are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.

Machine screw

Generally a smaller fastener (less than 1⁄4 inch in diameter) threaded the entire length of its shank that usually has a recessed drive type (slotted, Phillips, etc.). Machine screws are also made with socket heads (see above), in which case they may be referred to as socket head machine screws.

Self-tapping machine screw

Similar to a machine screw except the lower part of the shank is designed to cut threads as the screw is driven into an untapped hole. The advantage of this screw type over a self-tapping screw is that, if the screw is reinstalled, new threads are not cut as the screw is driven.

Set screw (grub screw)

Generally a headless screw but can be any screw used to fix a rotating part to a shaft. The set screw is driven through a threaded hole in the rotating part until it is tight against the shaft. The most often used type is the socket set screw, which is tightened or loosened with a hex key.

Set Bolt

(tap bolt) A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant set/tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.

Stud

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 un-threaded section in the middle. (See also: screw anchor, wedge anchor.)

Eye bolt

A bolt with a looped head.

Toggle bolt

A bolt with a special nut known as a wing. It is designed to be used where there is no access to side of the material where the nut is located. Usually the wing is spring loaded and expands after being inserted into the hole.

Carriage bolt (coach bolt)

Has a domed or countersunk head, and the shank is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.

Stove bolt

A type of machine screw that has a round or flat head and is threaded to the head. They are usually made of low grade steel, have a slot or Philips drive, and are used to join sheet metal parts using a hex or square nut.

Shoulder screw (stripper bolt)

A shoulder screw differs from machine screws in that the shank is ground to a precise diameter, known as the shoulder, and the threaded portion is smaller in diameter than the shoulder. Shoulder bolt specifications call out the shoulder diameter, shoulder length, and threaded diameter; the threaded length is fixed, based on the threaded diameter, and usually quite short. It is usually used for revolving joints in mechanisms and linkages; when used as a guide for the stripper plate in a die set its called a stripper bolt.

Thumb screw

A threaded fastener designed to be twisted into a tapped hole by hand without the use of tools.

Security screw

similar to a standard screw except that once inserted it cannot be easily removed.

Tension control bolt (TC bolt)

Heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shank has a spline on it which is engaged by a special power wrench which prevents the bolt from turning while the nut is tightened. When the appropriate torque is reached the spline shears off.

Plow bolt

A bolt similar to a carriage bolt, except the head is flat or concave. There are many variations, with some not using a square base, but rather a key, a locking slot, or other means. The recess in the mating part must be designed to accept the particular plow bolt.

Spring bolt

A bolt which must be pulled back and which is brought back into place by the spring when the pressure is released. Spring bolts are used in Rubik's Snakes, for example, the wedges of which are pulled apart slightly when twisted and are pulled back together by the spring bolt when shifted back into position.

Threaded rod

A headless fastener threaded on both ends or threaded along the entire length, in which case it is also referred to as "all thread" or "studding".

Other threaded fasteners

Superbolt, or multi-jackbolt tensioner

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.

Hanger screw

A headless fastener that has machine screw threads on one end and self-tapping threads on the other designed to be driven into wood or another soft substrate. Often used for mounting legs on tables.

Materials

Screws and bolts are made from 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 (steel screws can discolor oak and other woods), bronze, monel or silicon 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.

Bolted joints

Rusty hexagonal bolt heads

Main article: Bolted joint

Bolted joints are a common application, where screws are used in tension to secure two items together. Bolted joints are usually designed so that the frictional forces between the items is greater than the shear force present at the joint. To achieve this the screw is torqued to value to tension the bolt and compress the items; in essence creating a spring-like assembly. This applied tension is called a preload.

If the shear forces are greater than the frictional forces induced by the preload then the screw shank must be strong enough to withstand the shear force; in these applications a high-strength screw is used.

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 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 ultimate strength is the stress at which the bolt fails. Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2 % offset strain. 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, since the fastener has suffered permanent plastic deformation.

Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 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. 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.

SAE J429 defines the bolt grades for imperial sized bolts and screws. It defines them by grade, which ranges from 0 to 8, with 8 being the strongest. Higher grades do not exist within the specification. SAE grades 5 and 8 are the most common.

Head markings and strengths for imperial hex-head cap screws

Head marking	Grade, material, and condition	Nominal size range (in)	Proof strength (ksi)	Yield strength, min. (ksi)	Tensile strength, min. (ksi)	Core hardness (Rockwell)
	SAE Grade 0	Strength and hardness is not specified
	SAE grade 1 ASTM A307 Low carbon steel	1⁄4–1-1⁄2	33		60	B70–100
	ASTM A307 - Grade B Low or medium carbon steel	1⁄4–4			60 minimum 100 maximum	B69–95
	SAE grade 2 Low or medium carbon steel	1⁄4–3⁄4	55	57	74	B80–100
		Greater than 3⁄4	33	36	60	B70–100
	SAE grade 4 Medium carbon steel; cold worked	1⁄4–1-1⁄2		100	115
	SAE grade 3 Medium carbon steel; cold worked	1⁄4–1	85		100	B70–100
	SAE grade 5 Medium carbon steel; quench and tempered	1⁄4–1 (inc.)	85	92	120	C25–34
		1–1-1⁄2	74	81	105	C19–30
	ASTM A449 - Type 1 Medium carbon steel; quench and tempered	1–1-1⁄2 (inc.)	74		105	C19–30
		1-1⁄2–3	55		90	Brinell 183–235
	SAE grade 5.1 Low or medium carbon steel; quench and tempered	No. 6–1⁄2	85		120	C25–40
	SAE grade 5.2 Low carbon martensitic steel; quench and tempered	1⁄4–1	85		120	C26–36
	ASTM A449 - Type 2 Low carbon martensitic steel; quench and tempered					C25–34
or	ASTM A325 - Type 1 Medium carbon steel; quench and tempered	1⁄2–1 (inc.)	85	92	120	C24–35
		1–1-1⁄2	74	82	105	C19–31
	ASTM A325 - Type 3 Atmospheric corrosion resistant steel; quench and tempered	1⁄2–1	85	92	120	C24–35
		1–1-1⁄2	74	82	105	C19–31
	ASTM A354 - Grade BC Medium carbon alloy steel; quench and tempered	1⁄4–2-1⁄2 (inc.)	105	109	125	C26–36
		2-1⁄2–4	95	99	115	C22–33
	SAE grade 7 Medium carbon alloy steel; quench and tempered	1⁄4–1-1⁄2	105	115	133
	SAE grade 8 Medium carbon alloy steel; quench and tempered	1⁄4–1-1⁄2	120	130	150	C32–38
	ASTM A354 - Grade BD	1⁄4–2-1⁄2 (inc.)	120	130	150	C33–39
		2-1⁄2–4	105	115	140	C31–39
	SAE grade 8.2 Medium carbon boron martensitic steel; fully kilned, fine grain, quench and tempered	1⁄4–1	120		150	C33–39
	ASTM A490 - Type 1 Medium carbon alloy steel; quench and tempered	1⁄2–1-1⁄2	120	130	150 minimum 170 maximum	C33–38
	ASTM A490 - Type 3 Atmospheric corrosion resistant steel; quench and tempered
	18-8 Stainless Stainless steel with 17–19% chromium and 8–13% nickel	1⁄4–5⁄8 (inc.)		40 minimum 80–90 typical	100–125 typical
		5⁄8–1 (inc.)		40 minimum 45–70 typical	100 typical
		Over 1			80–90 typical

The international standard for metric screws is defined by ISO 898, specifically ISO 898-1. SAE J1199 and ASTM F568M are two North American metric standards that closely mimic the ISO standard. In case of imperial sizes the grade is dictated by the number of radial shapes plus a value of two. And imperial bolts use integer values to indicate grades but metric bolts use numbers with one decimal. The two North American standards use the same property class markings as defined by ISO 898. The ASTM standard only includes the following property classes from the ISO standard: 4.6, 4.8, 5.8, 8.8, 9.8, 10.9, and 12.9; it also includes two extra property classes: 8.8.3 and 10.9.3. ASTM property classes are to be stamped on the top of screws and it is preferred that the marking is raised.

Head markings and strengths for metric hex-head cap screws

Head marking	Grade, material, and condition	Nominal size range (mm)	Proof strength (MPa)	Yield strength, min. (MPa)	Tensile strength, min. (MPa)	Core hardness (Rockwell)
	Class 3.6	1.6–36	180	190	330	B52–95
	Class 4.6 Low or medium carbon steel	5–100	225	240	400	B67–95
	Class 4.8 Low or medium carbon steel; fully or partially annealed	1.6–16	310	340	420	B71–95
	Class 5.8 Low or medium carbon steel; cold worked	5–24	380	420	520	B82–95
	Class 8.8 Medium carbon steel; quench and tempered	Under 16 (inc.)	580	640	800
		17–72	600	660	830	C23–34
	Class 8.8 low carbon Low carbon boron steel; quench and tempered
	Class 8.8.3 Atmospheric corrosion resistant steel; quench and tempered
	ASTM A325M - Type 1 Medium carbon steel; quench and tempered	12–36
	ASTM A325M - Type 3 Atmospheric corrosion resistant steel; quench and tempered
	Class 9.8 Medium carbon steel; quench and tempered	1.6–16	650	720	900	C27–36
	Class 9.8 low carbon Low carbon boron steel; quench and tempered
	Class 10.9 Alloy steel; quench and tempered	5–100	830	940	1040	C33–39
	Class 10.9 low carbon Low carbon boron steel; quench and tempered
	Class 10.9.3 Atmospheric corrosion resistant steel; quench and tempered
	ASTM A490M - Type 1 Alloy steel; quench and tempered	12–36
	ASTM A490M - Type 3 Atmospheric corrosion resistant steel; quench and tempered
	Class 12.9 Alloy steel; quench and tempered	1.6–100	970	1100	1220	C38–44
	A2 Stainless steel with 17–19% chromium and 8–13% nickel	Up to 20		210 minimum 450 typical	500 minimum 700 typical
	ISO 3506-1 A2-50 304 stainless steel-class 50 (annealed)			210	500
	ISO 3506-1 A2-70 304 stainless steel-class 70 (cold worked)			450	700
	ISO 3506-1 A2-80 304 stainless steel-class 80			600	800

Screw head shapes

(a) pan, (b) button, (c) round, (d) truss, (e) flat (countersunk), (f) oval

Combination flanged-hex/Phillips-head screw used in computers

Pan head

A low disc with chamfered outer edge

Button or dome head

Cylindrical with a rounded top

Round head

Dome-shaped, commonly used for machine screws

Truss head

Lower-profile dome designed to prevent tampering

Flat head or countersunk

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

Countersunk with a rounded top

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

A flanged head can be any of the above head styles with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer.

Mirror screw head

Countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for attaching mirrors

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 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; 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 surrounding material.

Cross-head, cross-point, or cruciform

A cross-head screw drive has a cross-shaped recess. They were originally designed for use with mechanical screwing machines.

Phillips

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 manufacture it.

Reed & Prince or Frearson

Similar to a Phillips but has a more pointed 75° V shape. Its advantage over the Phillips drive is that 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. The company is now in business.

JIS

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. Heads are usually identifiable by a single dot to one side of the cross slot. The standard number is JIS B 1012:1985

Pozidriv

similar to cross-head but designed not to slip, or cam out. It 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. Heads are marked with crossed, single lines at 45 degrees to the cross recess, for identification. (Note that doubled lines at 45 are a different recess: a very specialised Phillips screw.)

Supadriv

similar to Pozidriv. but with only two tickmarks instead of four. Can be used with pozidriv tools, designed to accept off-axis use of the hand tool.

French recess

French recess driver bit

Also called BNAE NFL22-070 after its Bureau de Normalisation de l'Aéronautique et de l'Espace standard number.

Square head

A four sided head used for high torque driving with a wrench.

Hex head

Similar to a square head except with six sides.

Socket head

A wide range of screw drives that utilizes a cylindrical head with a shaped hole, known as a socket, in the top. The specialized tool with the mating shape is used turn screw via the socket.

Hexagonal (hex)

Hex socket screws

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.

Robertson (square)

Invented in 1908 by Canadian P.L. Robertson, has a square hole and is driven by a special power-tool bit or screwdriver. In the United States it is referred to as square drive. 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. It is increasingly used in the United States for woodworking applications.

Torx

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-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 "5-star" torx configurations are commonly used in correctional facilities, public facilities and government schools, but can also be found in some electronic devices.

TTAP

an improved "hexalobular" drive for without wobbling and stable stick-fit. TTAP is backward convertible with generic hexalobular (torx) drive.

Tri-Wing

Has a triangular slotted configuration. They have been used by Nintendo on several consoles and accessories, including the Game Boy, Wii, and Wii Remote,and by Nokia on some phones and chargers to discourage home repair, as well as in some pencil sharpeners to prevent removal of the blade.

Torq-set or offset cruciform

May be confused with Phillips; however, the four legs of the contact area are offset in this drive type. This type is commonly used in the aerospace industry.

Spanner

Uses two round holes opposite each other and is designed to prevent tampering. Commonly seen in elevators in the United States. The tool for these is called a spanner wrench in the U.S. and a pin spanner in the UK.

Clutch Type A or standard clutch

Resembles a bow tie. These were common in GM automobiles, trucks and buses of the 1940s and 1950s, particularly for body panels.

Clutch Type G

Resembles a butterfly. This type of screw head is commonly used in the manufacture of mobile homes and recreational vehicles.

TP3

Is a type of tamper-resistant drive that uses a triangular recess in the screw head.

Combination drives

Some screws have heads designed to accommodate more than one kind of driver, sometimes referred to as combo-head or combi-head. The most common of these is a combination of a slotted and Phillips head, often used in attaching knobs to furniture drawer fronts. 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, a torx 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. Quadrex is another Phillips/Robertson drive. Phillips Screw Company offers both Phillips and Pozidriv combo heads with Robertson.

Combined slotted/pozidriv heads are so ubiquitous in electrical switchgear to have earned the nickname 'electricians screws' (the first screwdriver out of the toolbox is used - the user does not have to waste valuable time searching for the correct driver). Their rise to popular use has been in spite of the fact that neither a flat screwdriver or pozidriv screwdriver are fully successful in driving these screws to the required torque. Some screwdriver manufacturers offer matching screwdrivers and call them 'contactor screwdrivers', although the original concept of not needing to search for a particular driver is defeated as a contractor screwdriver is useless for non-combination heads. Slotted/philips (as opposed to slotted/pozidriv) heads occur in some North American made switchgear.

Tamper-resistant screws

The general theory of tamper-resistant fasteners is to make a fastener whose loosening requires a tool that a tamperer is unlikely to have on hand at the time of opportunity for tampering. There is no expectation that it will be impossible for a tamperer to obtain the driver. Rather, the main idea is simply that most tamperers will not bother to seek out and obtain a driver. In the case of end-users, this reduces the incidence of do-it-yourself repair or modifications and any resulting injury, product damage or loss of profit from repairs for the manufacturer. In the cases of vandalism prevention and theft prevention, since most vandalism and theft incidents are simply crimes of easy opportunity, the idea is to "raise the bar" and make the opportunity less convenient.

Protruding obstacle head

Tamper-resistant external-torx driver

Many screw drives, including Phillips, torx, and hex socket, have tamper-resistant variants. These typically have a pin protruding in the center of the screw head, necessitating a special tool for extraction. In some variants the pin is placed slightly off-center, requiring a correspondingly shaped bit. However, the bits for many tamper-resistant screw heads are now readily available from hardware stores, tool suppliers and through the Internet. There are also many commonly used techniques to extract tamper resistant screws without the correct driver — for example, the use of an alternative driver that can achieve enough grip to turn the screw, modifying the head to accept an alternative driver, forming one's own driver by melting an object into the head to mould a driver, or simply turning the screw using a pair of locking pliers. Thus, these special screws offer only modest security. However, it is often enough to discourage the more opportunistic varieties of vandalism.

One-way only head

One-way slotted screw

The slotted screw 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. One-way screws can be removed using a screw extractor like any other safety screw.

Proprietary head

There are specialty fastener companies that make unusual, proprietary head designs, featuring matching drivers available only from them, and only supplied to registered owners. These tend to be confined to industrial uses that the average layperson does not have contact with. But one example familiar to laypersons is the attachment for the wheels and/or spare tires of some types of car; one of the nuts may require a specialized socket (provided with the car) to prevent theft. Security fasteners are also available for bicycle wheels and seats to prevent theft.

Breakaway head

The breakaway bolt is a high-security fastener that is extremely difficult to remove. It consists of a counter-sunk flat head screw, with a thin shank and hex head protruding from the flat head. The hex head is used to drive the bolt into the countersunk hole, then either the wrench or hammer is used to knock the shank and hex head off of the flat head or it is driven until the driving head shears off, 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 screw extractor or a punch and hammer at a sharp angle in a counter-clockwise direction. This type of screw is used primarily in prison door locks and automobile ignition locks, but also has considerable usage in the fastening of street signs to sign posts by municipal public works departments.

Tools

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).

Thread standards

Main article: 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 Unified Thread Standard.

ISO metric screw thread

Main article: 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 in a hole somewhat smaller than 6 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	5	5.5	7	8	10	13	17	19	24	30	36	46	55	65	75	85	95

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

Whitworth

Main article: British Standard Whitworth

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 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 the distance between the flats.

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.

British Association screw thread

Main article: British Association screw threads

A later standard established in the United Kingdom was the British Association (BA) screw threads, named after the British Association for Advancement of Science. 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.

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 factor 0.9. For example, a 4BA thread has diameter (6.0 x 0.9^4) mm and pitch (1.0 x 0.9^4) mm. 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.

Unified Thread Standard

Main article: Unified Thread Standard

The Unified Thread Standard (UTS) is most commonly used in the United States of America, 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 practise through which the shaft of the screw can easily be pushed) and Y is the threads per inch (TPI). For sizes 1⁄4 inch and larger the size is given as a fraction; for sizes less than this an integer is used, ranging from 0 to 16. 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).

History

A lathe of 1871, equipped with leadscrew and change gears for single-point screw-cutting.

A Brown & Sharpe single-spindle screw machine.

In antiquity, the screw was first used as part of the screw pump of Sennacherib, King of Assyria, for the water systems at the Hanging Gardens of Babylon and Nineveh in the 7th century BC.

The screw was later 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 devices such as oil and wine presses. Metal screws used as fasteners did not appear in Europe until the 1400s.

In 1744, the flat-bladed bit for the carpenter's brace was invented, the precursor to the first simple screwdriver. Handheld screwdrivers first appeared after 1800.

Prior to the mid-19th century, cotter pins or pin bolts, and "clinch bolts" (now called rivets), were used in shipbuilding.

The metal screw did not become a common fastener until machine tools for mass production were developed at the end of the 18th century. In the 1770s, English instrument maker Jesse Ramsden (1735-1800) invented the first satisfactory screw-cutting lathe. The British engineer Henry Maudslay (1771-1831) patented a screw-cutting lathe in 1797; a similar device was patented by David Wilkinson in the United States in 1798. These developments 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.

The development of the turret lathe (1840s) and of the screw machine (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 (drive) were simple internal-wrenching slots and external-wrenching squares and hexagons. These were easy to machine and served most applications adequately. The 20th century saw the development of many other types of drive. In 1908, Canadian P. L. Robertson invented the internal-wrenching square drive. The internal-wrenching hexagon drive (hex socket) shortly followed in 1911. In the early 1930s, the Phillips-head screw was invented by Henry F. Phillips.

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

Other fastening methods

Alternative fastening methods are nails, rivets, roll pins, pinned shafts, welding, soldering, brazing, and gluing (including taping), and clinch fastening.

See also

• Gender of connectors and fasteners
• Tap and die
• Thread pitch gauge

References

Bibliography

• Bickford, John H.; Nassar, Sayed (1998), Handbook of bolts and bolted joints, CRC Press, ISBN 9780824799779.
• Colvin, Fred Herbert; Stanley, Frank Arthur (1914), American Machinists' Handbook and Dictionary of Shop Terms (2nd ed.), McGraw-Hill.
• Huth, Mark W. (2003), Basic Principles for Construction, Cengage Learning, ISBN 1401838375.
• Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (2000), Machinery's Handbook (26th ed.), New York: Industrial Press Inc., ISBN 0-8311-2635-3.
• Rybczynski, Witold (2000). One Good Turn: A Natural History of the Screwdriver and the Screw. Toronto, ON, Canada: Harper Collins. ISBN 978-0-00-638603-2.
• Ryffel, Henry H., et al. (eds) (1988), Machinery's Handbook (23rd ed.), New York: Industrial Press, ISBN 978-0-8311-1200-4 {{citation}}: |first= has generic name (help)CS1 maint: multiple names: authors list (link).
• Smith, Carroll (1990), Carroll Smith's Nuts, Bolts, Fasteners, and Plumbing Handbook, MotorBooks/MBI Publishing Company, ISBN 0879384069.

External links

• Screw Information and Specifications
• Metric coarse thread dimensions
• Metric fine thread dimensions
• Detailed metric thread dimensions
• Useful comparison of obsolete british threads from a specialist supplier
• Analysis of bolted joints and threaded fasteners
• Mechanics of screws
• Analysis of power screws
• NASA-RP-1228 Fastener Design Manual