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 it is inserted. Their most common use is to hold objects together or 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 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.

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. Some of these issues are discussed below:

Machine screws

ASME standards specify a variety of “Machine Screws” in diameters ranging up to of an inch. 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 -1981 specifies Hex Cap Screws that range in size from to 3 inches 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.

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.

Other distinctions

Bolts have been 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 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.

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)

Wood screw

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

Coach screw

British English equivalent of Lag screw.

Lag screw (lag bolt)

Similar to a wood screw except that it is generally much larger running to lengths up to with diameters from ¼" to ½" (6.4–12.25 mm) in commonly available (hardware store) sizes (not counting larger mining and civil engineering lags and lag bolts) and it generally has a hexagonal head 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.

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 1/2" 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.

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 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.

Tap bolt

A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant 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

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.

Other threaded fasteners

Thread rolling screws

These have a lobed (usually triangular) cross section. They form threads by pushing outward during installation. They may have tapping threads or machine threads.

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.

Joint analysis

Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. For example, when two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough so that the friction between the two bars can overcome the longitudinal force. If the bars slip, then the bolt may be sheared in half, or friction between the bolt and slipping bars may erode and weaken the bolt (called fretting). For this type of application, high-strength steel bolts are used and should be tightened to a specified torque.

Critical applications of screws and bolts will specify a torque that must be applied when driving it. The main concept is to tension the bolt, and compress parts being held together, creating a spring-like assembly. The stress thus introduced to the bolt is called a preload. When external forces try to separate the parts, the bolt experiences no strain unless the preload force is exceeded.

As long as the preload is never exceeded, the bolt or nut will never come loose (assuming the full strength of the bolt is used). If the full strength of the bolt is not used (for example, a steel bolt threaded into aluminum), then a thread-locking adhesive or insert may be used.

If the preload is exceeded during normal use, the joint will eventually fail. The preload is calculated as a percentage of the bolt's yield strength or the strength of the threads it goes into, or the compressive strength of the clamped layers (plates, washer, gaskets), whichever is least.

The thread on wood screws behave like a wedge.

In some applications joints are designed so that the screw or bolt will intentionally fail before more expensive components. In this case replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus it is generally good practice to replace fasteners with the same grade originally installed.

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. An M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.

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.

Head markings and strengths for imperial hex-head cap screws

Head marking	Grade, material & 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-	33		60	B70–100
	ASTM A307 - Grade B Low or medium carbon steel	–4			60 minimum 100 maximum	B69–95
	SAE grade 2 Low or medium carbon steel	–	55	57	74	B80–100
		Greater than	33	36	60	B70–100
	SAE grade 4 Medium carbon steel; cold worked	–1-		100	115
	SAE grade 3 Medium carbon steel; cold worked	–1	85		100	B70–100
	SAE grade 5 Medium carbon steel; quench and tempered	–1 (inc.)	85	92	120	C25–34
		1–1-	74	81	105	C19–30
	ASTM A449 - Type 1 Medium carbon steel; quench and tempered	1–1- (inc.)	74		105	C19–30
		1- –3	55		90	Brinell 183–235
	SAE grade 5.1 Low or medium carbon steel; quench and tempered	No. 6–	85		120	C25–40
	SAE grade 5.2 Low carbon martensitic steel; quench and tempered	–1	85		120	C26–36
	ASTM A449 - Type 2 Low carbon martensitic steel; quench and tempered					C25–34
	ASTM A325 - Type 1 Medium carbon steel; quench and tempered	–1 (inc.)	85	92	120	C24–35
		1–1-	74	82	105	C19–31
	ASTM A325 - Type 3 Atmospheric corrosion resistant steel; quench and tempered	–1	85	92	120	C24–35
		1–1-	74	82	105	C19–31
	ASTM A354 - Grade BC Medium carbon alloy steel; quench and tempered	–2- (inc.)	105	109	125	C26–36
		2- –4	95	99	115	C22–33
	SAE grade 7 Medium carbon alloy steel; quench and tempered	–1-	105	115	133
	SAE grade 8 Medium carbon alloy steel; quench and tempered	–1-	120	130	150	C32–38
	ASTM A354 - Grade BD Medium carbon alloy steel; quench and tempered	–2- (inc.)	120	130	150	C33–39
		2- –4	105	115	140	C31–39
	SAE grade 8.2 Medium carbon boron martensitic steel; fully kilned, fine grain, quench and tempered	–1	120		150	C33–39
	ASTM A490 - Type 1 Medium carbon alloy steel; quench and tempered	–1-	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	– (inc.)		40 minimum 80–90 typical	100–125 typical
		–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 & 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

Shapes of screw head

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

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.

Socket head: Cylindrical, relatively high, with different types of sockets (hex, square, torx, etc.)

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

Headless (set or grub screw): Has either a socket or slot in one end for driving

Square head: A 4 sided head used for high torque driving with a wrench.

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

; 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

; French recess : also called BNAE NFL22-070 after its Bureau de Normalisation de l'Aéronautique et de l'Espace standard number.

; 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.

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.

Hex socket screws

Hexagonal (hex) socket

Has a hexagonal hole and is driven by a hex wrench, sometimes called an Allen key or Hex key, or by a power tool with a hexagonal bit. Tamper-resistant versions with a pin in the recess are available. Hex sockets are increasingly used for modern bicycle parts because hex wrenches are very light and easily carried tools. They are also frequently used for self-assembled furniture.

Robertson

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.

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. Note that in the UK, "spanner" is the usual word for "wrench".

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 do offer matching screwdrivers and call them 'contactor screwdrivers'. 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 or product damage). 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

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

The slotted screw drive also comes in a tamper-resistant one-way design with sloped edges; the screw can be driven in, but the bit slips out in the reverse direction.

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 the wrench or hammer is used to knock the shank and hex head off of the flat head, leaving only a smooth screw head exposed. Removal is facilitated by drilling a small hole part way into the outer part of the head and using a 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, but also has considerable usage in the fastening of street signs to sign posts by municipal public works departments.

Tools used

The hand tool used to drive in most screws is called a screwdriver. A power tool that does the same job is a power screwdriver; power drills may also be used with screw-driving attachments. Where the holding power of the screwed joint is critical, torque-measuring and torque-limiting screwdrivers are used to ensure sufficient 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

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 , and the SAE 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 known as the "coarse pitch". For some diameters, one or two additional "fine pitch" variants are also specified, for special applications such as threads in thin-walled pipes. ISO metric screw threads are designated by the letter M followed by the major diameter of the thread in millimeters, e.g. "M8". If the thread does not use the normal "coarse pitch" (e.g., 1.25 mm in the case of M8), then the pitch in millimeters is also appended with a multiplication sign, e.g. "M8×1" if the screw thread has an outer diameter of 8 mm and advances by 1 mm per 360° rotation.

The nominal diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shank, and the nut or threaded hole is made by tapping threads in a 5 mm hole.

Metric hexagon bolts, screws and nuts are specified, for example, in British Standard BS 4190 (general purpose screws) and BS 3692 (precision screws). The following table lists the relationship given in these standards between the thread size and the maximal width across the hexagonal flats (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

The first person to create a standard (in about 1841) was the English engineer Sir Joseph Whitworth. Whitworth screw sizes are still used, both for repairing old machinery and where a coarser thread than the metric fastener thread is required. Whitworth became British Standard Whitworth, abbreviated to BSW (BS 84:1956) and the British Standard Fine (BSF) thread was introduced in 1908 because the Whitworth thread was a bit coarse for some applications. The thread angle was 55° and a depth and pitch of thread that varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). The spanner size is determined by the size of the bolt, not 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 threads (BA)

A later standard established in the United Kingdom was the BA system, named after the British Association for Advancement of Science. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used, 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.

Unified Thread Standard

The United States of America has its own system, usually called the Unified Thread Standard, which is also extensively used in Canada and occasionally in other countries. A version of this standard, called SAE for the Society of Automotive Engineers, was used in the American automobile industry. The SAE is still associated with inch-based fasteners by the public, even though the U.S. auto industry (and other heavy industries relying on SAE) have gradually converted to ISO preferred series fasteners for some assemblies from the 1970s onward, because global parts sourcing and product marketing favor international standardization. However, all automobiles sold throughout the world contain both metric (engine assemblies) and Imperial fasteners (for example, spark plugs, oxygen sensors, internal electrical assemblies, body fasteners, lamps, steering, brake and suspension parts).

Machine screws are described as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32, 10-32, 10-24, etc. up to size 16. The first number can be translated to a diameter using a formula, the second is the number of threads per inch. There is a coarse thread and a fine thread for each size, the fine thread being preferred in thin materials or when slightly greater strength is desired.

The numbering system follows a roughly logarithmic series where an increase in each screw number size approximately doubles the tensile strength of the screw and the screw number is found by d=.060+(\# \times .013), where "d" is the nominal diameter. Using this formula a #5 screw has a major diameter of .125" (1/8"), a #10 screw has a diameter of .190" (or 3/16" in practical terms), etc. The formula applies for screw thread numbers #0 and higher, but does NOT apply to smaller Unified miniature screw thread series. Typically screws smaller than size #0 are supplied in the Unified Miniature Series. The formula for number sizes smaller than size #0 is given by d=.060- (\#zerosize \times .013), with the zero size being the number of zeros after the first. So a #00 screw is .047" dia, #000 is .034" dia, etc.

The number series of machine screws once included odd numbers (7, 9, etc.) and extended up to #16 or more. Standardization efforts in the late 19th and the early part of the 20th century reduced the range of sizes considerably. Now, it is less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12.

Sizes 1/4" diameter and larger are designated as 1/4"-20, 1/4"-28, etc. the first number giving the diameter in inches and the second number being threads per inch. Most thread sizes are available in UNC or UC (Unified Coarse Thread, example 1/4"-20) or UNF or UF (Unified Fine Thread, example 1/4"-28).

A Unified Miniature screw thread series is defined in ANSI standard B1.10, for fasteners of 0.3 to 1.4 millimetres (0.0118 to 0.0551 inch) diameter. These sizes are intended for watches, instruments, and miniature mechanisms and are interchangeable with threads made to ISO Standard 68.

Others

Other historical, specialized or obsolescent thread systems include:

• Acme thread form
• BSP (British standard pipe thread which exists in a taper and non taper variant; used for other purposes as well)
• BSC (British Standard Cycle) a 26tpi thread form
• British Standard Buttress Threads (BS 1657:1950),
• British Standard for Spark Plugs BS 45:1972
• CEI (Cycle Engineers Institute, used on bicycles in Britain and possibly elsewhere)
• British Standard Brass a fixed pitch 26tpi thread
• Edison base lamp holder screw thread
• Fire hose connection (NFPA standard 194)
• Hose Coupling Screw Threads (ANSI B2.4-1966) for garden hoses and accessories
• Lowenhertz thread, a German metric thread used for measuring instruments
• NPT (National Pipe Thread) and NPTF (National Pipe Thread Fuel)
• PG (German: "Panzer-Gewinde"), used in thin plate metal, such as for switches and nipples in electrical equipment housings
• Society Thread, a 36 threads/inch Whitworth form standarded by the Royal Microscopical Society of London for microscope objective lenses.

History

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 rivet), 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
• Screw-cutting lathe
• Tap and die
• Thread pitch gauge

References

Notes

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6. The US document can be found here.
7. Huth, pp. 166–167.
8. Colvin, p. 569.
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18. Other markings may be used to denote atmospheric corrosion resistant material
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30. 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).

Bibliography

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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