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A screw is a shaft with a helical groove or threadformed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.

A screw used as a threaded fastener consists of a cylindrical shaft, which in many cases tapers to a point at one end, and with a helical ridge or thread formed on it, and a head at the other end which can be rotated by some means. The thread is essentially an inclined plane wrapped around the shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix using a tap, or the screw may create it when first driven in (a self-tappingscrew). The head is specially shaped to allow a screwdriveror wrench(British English: spanner) to rotate the screw, driving it in or releasing it. The head is of larger diameter than the body of the screw and has no thread so that the screw can not be driven deeper than the length of the shaft, and to provide compression.

Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse of objects.

The vast majority of screws are tightened by clockwise rotation; we speak of a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Left-hand screws are used on rotating items such as the left-hand grinding wheel on a bench grinderor the left hand pedal on a bicycle(both looking towards the equipment) or hub nuts on the left side of some automobiles.

Screws and bolts
Screws and bolts


Threaded fasteners were made by a cutting action such as diesprovide, but recent advances in tooling allow them to be made by rolling an unthreaded rod (the blank) between two specially machined dies which squeeze the blank into the shape of the required fastener, including the thread. This method has the advantages of work hardening the thread and saving material. A rolled thread can be distinguished from a thread formed by a die as the outside diameter of the thread is greater than the diameter of the unthreaded portion of the shaft. Bicycle spokes, which are just bolts with long thin unthreaded portions, always use rolled threads for strength.



Carriage bolt with square nut
Carriage bolt with square nut

 Differentiation between bolt and screw 

A universally accepted distinction between a screw and a bolt (in the context of threaded fasteners) does not exist. The Machinery's Handbook, Edition 26 describes the distinction as follows:

Differentiation between Bolt and Screw -
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 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”[7] in diameters ranging up to ¾ 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 ¼ inch 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. The definition is available online.

Structural bolt DIN 6914 with DIN 6916 washer and UNI 5587 nut.
Structural bolt DIN 6914 with DIN 6916 washer and UNI 5587 nut.

Types of screws and bolts

Threaded fasteners either have a tapered shaft or a non-tapered shaft. Fasteners with tapered shafts 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 shaft are designed to mate with a nut or to be driven into a tapped hole.


Fasteners with a tapered shaft (self-threading screws)


There is not a universally accepted definition of the word, screw. It generally refers to a smaller[15]threaded fastener with a tapered shaft. See the section Differentiation between bolt and screw above for a more detailed discussion.

Wood screw 

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

Lag screw (lag bolt) 

Similar to a wood screw except that it is generally much larger running to lengths up to 15 inches (381 mm) 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.

Coach bolts 

rather like lag bolts, but normally have a square (4 sided) head, rather than a hexagon. Formally used in (horse-drawn) coaches, and have a large thread with good holding force, and an un-threaded part, rather like a giant wood screw.

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 shaft is usually threaded up to the head. Sheet metal screws make excellent fasteners for attaching metal hardware to wood because the fully threaded shaft 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.

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.

Deck screw 

Similar to drywall screw except that it is has improved corrosion resistance and is generally supplied in a larger gauge.

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.

A Phillips wood screw being driven into a board with a drill
A Phillips wood screw being driven into a board with a drill


There is no universally accepted definition of the word bolt. It generally refers to a larger[15]threaded fastener with a non-tapered shaft. See the section Differentiation between bolt and screw above for a more detailed discussion.

Breakaway Bolt 

A Bolt with a hollow threaded shaft. Designed to break away when impacted. Typically used to fasten fire hydrants, so they will break awaywhen hit by a car. Also used in aircraft, not to break away, but to make them lighter in weight.

Cap screw 

In places the term is used interchangeably with bolt. In the past the term cap screwwas restricted to threaded fasteners with a shaft 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 shaft 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. Counterboredholes in parts allow the screw head to be flush with the surface or recessed. Other head designs include buttonhead 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 ¼ inch in diameter) threaded the entire length of its shaft 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 shaft 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 or hex driver.

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.


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 shaft 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 

Screw used for revolving joints in mechanisms and linkages. A shoulder screw consists of the shaft, which is groundto a precise diameter, and a threaded end, which is smaller in diameter than the shaft. Unlike other threaded fasteners, the size of a shoulder screw is defined by the shaft diameter, not the thread diameter. Shoulder screws are also called stripper bolts, as they are often used as guides for the stripper plate(s) in a die set.

Thumb screw 

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

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 shaft 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 fasterners:

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.

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

Shapes of screw head:

Pan head

A low disc with chamferedouter 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 shaft to the angle of the head, similar to the bell of a bugle

Cheese head

Disc with cylindrical outer edge, height approximately half the head 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.

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