Bolt with a nut

A bolt is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to a matching nut. The bolt has an external male thread requiring a matching nut with a pre-formed female thread.[1]

Bolts vs. screws

Main article: Screw § Differentiation between bolt and screw

Bolted joint in vertical section
Screw joint

The distinction between a bolt and a screw is poorly defined. The academic distinction, per Machinery's Handbook,[2] is in their intended purpose: bolts are designed to pass through an unthreaded hole in a component and be fastened with the aid of a nut. Screws in contrast are used in components which contain their own thread, or to cut its own internal thread into them. This definition allows ambiguity in the description of a fastener depending on the application it is actually used for, and the terms screw and bolt are widely used by different people or in different countries to apply to the same or varying fastener.

Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank (called the grip length), as this makes for a better, stronger dowel. The presence of the unthreaded shank has often been given as characteristic of bolts vs. screws,[3][verification needed] but this is incidental to its use, rather than defining.[citation needed]

Where a fastener forms its own thread in the component being fastened, it is called a screw.[2] This is most obviously so when the thread is tapered (i.e. traditional wood screws), precluding the use of a nut,[2] or when a sheet metal screw or other thread-forming screw is used. A screw must always be turned to assemble the joint. Many bolts are held fixed in place during assembly, either by a tool or by a design of non-rotating bolt, such as a carriage bolt, and only the corresponding nut is turned.[2]

Bolt heads

Further information: List of screw drives

Bolts use a wide variety of head designs, as do screws. These are designed to engage with the tool used to tighten them. Some bolt heads instead lock the bolt in place, so that it does not move and a tool is only needed for the nut end.

Common bolt heads include hex, slotted hex washer, and socket cap.

The first bolts had square heads, formed by forging. These are still found, although much more common today is the hexagonal head. These are held and turned by a spanner or socket, of which there are many forms. Most are held from the side, some from in-line with the bolt. Other bolts have T-heads and slotted heads. [4]

Many bolts use a screwdriver head fitting, rather than an external wrench. Screwdrivers are applied in-line with the fastener, rather than from the side. These are smaller than most wrench heads and cannot usually apply the same amount of torque. It is sometimes assumed that screwdriver heads imply a screw and wrenches imply a bolt, although this is incorrect. Coach screws are large square-headed screws with a tapered wood screw thread, used for attaching ironwork to timber. Head designs that overlap both bolts and screws are the Allen or Torx heads; hexagonal or splined sockets. These modern designs span a large range of sizes and can carry a considerable torque. Threaded fasteners with screwdriver-style heads are often referred to as machine screws whether they are being used with a nut or not.[citation needed]

Bolt types

See also: List of screw and bolt types

Terminology of a bolt

Bolt materials

Further information: Screw § Mechanical classifications

Depending on required strength and circumstances, there are several material types can be used for fasteners.[5]

In general, steel is the most commonly used material of all fasteners: 90% or more.[citation needed]

Bolted joints

Main article: Bolted joint

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Rusty hexagonal bolt heads

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

See also


  1. ^ "Bolt | Definition of Bolt by Merriam-Webster". Retrieved 11 April 2016.
  2. ^ a b c d Machinery's Handbook (Twenty-First ed.). New York: Industrial Press. 1980. p. 1131.
  3. ^ Dyke's Automobile and Gasoline Engine Encyclopedia. A. L. Dyke. 1919. p. 701. ISBN 9780140806137. sae uss screw standard.
  4. ^ "What is a bolt?". AALL American Fasteners. Archived from the original on 18 March 2017. Retrieved 13 February 2017.
  5. ^ "Fastener Material Selection".