Nut (hardware)

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An M4 nut threaded onto an Allen key socket head screw M4 Inbusschraube focusstacked.jpg
An M4 nut threaded onto an Allen key socket head screw

A nut is a type of fastener with a threaded hole. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together. The two partners are kept together by a combination of their threads' friction (with slight elastic deformation), a slight stretching of the bolt, and compression of the parts to be held together.


In applications where vibration or rotation may work a nut loose, various locking mechanisms may be employed: lock washers, jam nuts, eccentric double nuts, [1] specialist adhesive thread-locking fluid such as Loctite, safety pins (split pins) or lockwire in conjunction with castellated nuts, nylon inserts (nyloc nut), or slightly oval-shaped threads.

Square nuts, as well as bolt heads, were the first shape made and used to be the most common largely because they were much easier to manufacture, especially by hand. While rare today[ when? ] due to the reasons stated below for the preference of hexagonal nuts, they are occasionally used in some situations when a maximum amount of torque and grip is needed for a given size: the greater length of each side allows a spanner to be applied with a larger surface area and more leverage at the nut.

The most common shape today is hexagonal, for similar reasons as the bolt head: six sides give a good granularity of angles for a tool to approach from (good in tight spots), but more (and smaller) corners would be vulnerable to being rounded off. It takes only one sixth of a rotation to obtain the next side of the hexagon and grip is optimal. However, polygons with more than six sides do not give the requisite grip and polygons with fewer than six sides take more time to be given a complete rotation. Other specialized shapes exist for certain needs, such as wingnuts for finger adjustment and captive nuts (e.g. cage nuts) for inaccessible areas.

Nuts come in many sizes. This one is part of the Sydney Harbour Bridge. SydneyHarbourBridgeNutMilsonsPoint.JPG
Nuts come in many sizes. This one is part of the Sydney Harbour Bridge.


A wide variety of nuts exists, from household hardware versions to specialized industry-specific designs that are engineered to meet various technical standards. Fasteners used in automotive, engineering, and industrial applications usually need to be tightened to a specific torque setting, using a torque wrench. Nuts are graded with strength ratings compatible with their respective bolts; for example, an ISO property class 10 nut will be able to support the bolt proof strength load of an ISO property class 10.9 bolt without stripping. Likewise, an SAE class 5 nut can support the proof load of an SAE class 5 bolt, and so on.

NameAlso known asImageDescription
Acorn nut (cap nut)Crown hex nut, blind nut, cap nut, domed cap nut, or dome nut Domed cap nut.jpg A nut that has a domed end on one side
Barrel nut Steel cross dowel or dowel nut, cross dowel (woodworking) Quergewindebolzen QD-M6-7,5-13x16,2.jpg A round slug or formed sheet metal part with threads perpendicular to the length of the nut
Cage nut Caged nut, captive nut, clip nut Kafigmutter.jpg A (usually square) nut in a spring steel cage which wraps around the nut. The cage has two wings that when compressed allow the cage to be inserted into the square holes
Clip-on nut J-nut or U-nut, sheet metal nut, speed nut (ambiguously) Two spire clips.jpg Designed to be clipped to sheet metal
Coupling nut Extension nut Coupling Nut.svg A threaded fastener for joining two male threads, most commonly a threaded rod,[1][2] but also pipes
Flange nut Collar nut Serrated hex flange nut.jpg Has a wide flange at one end that acts as an integrated washer
HARDLOCK Nut [2] Eccentric Double Nut HARDLOCK nut.PNG A self-lock nut. Improvement of the double nut that uses a wedge principal, eliminating space between threads.
Insert nut Rampamuffe (klein).jpg A threaded socket for a wooden workpiece, similar to a wall anchor.
Knurled nut Thumb nut Knurled nuts, Lexington MA.jpg Has a knurled outside surface which facilitates tightening by hand (thumb nut) or secures the nut into a handle or cover (insertion nut)
Sex bolt Barrel nut, barrel bolt, binding barrel, Chicago screw, post and screw or connector bolt Sleeve nuts.jpg Has a barrel-shaped flange and protruding boss that is internally threaded
Split nut Split lengthwise into two pieces (opposed halves) so that its female thread may be opened and closed over the male thread of a bolt or leadscrew
Sleeve nut
Spring nut Spring Nut.jpg Used with strut to join things to the open face thereof. The grooves in its face engage the lips of the open side of the strut channel, while the spring pushes against the opposite side, to hold the nut in place.
Square nut Square nuts from Ace Hardware, Concord MA.jpg Four-sided nut. Compared to standard hex nuts, square nuts have a greater surface in contact with the part being fastened, and therefore provide greater resistance to loosening (though also greater resistance to tightening)[ citation needed ]
Swage nut Self-clinching nut 17-SwageNuts.JPG Used on sheet metal. It permanently anchors itself to the sheet metal by swaging the surrounding material.
T-nut tee nut, blind nut (ambiguously) Inslagmoer.JPG Used to fasten a wood, particle or composite materials workpiece, leaving a flush surface.
T-slot nut T-groove nut CAD model of a T-Nut 1.png Used with a threaded clamp to position and secure pieces being worked on in a workshop
Wedge nut Wedge nut.png
Weld nut Slab base weld nut.png Designed to be welded to another object
Well nut Rawlnut or Rawl nut (proprietary) Well nut.jpg Used to blindly fasten a piece and to seal the bolt hole
Wing nut Butterfly nut Wingnut-hardware.jpg Has two large metal "wings", one on each side, so it can be easily tightened and loosened by hand without tools


Many specialised types of nut exist to resist loosening of bolted joints, either by providing a prevailing torque against the male fastener or by gripping against the bolted components. These are generally referred to as locknuts.

Standard nut sizes

Metric hex nuts

Nut quotation Nut quote.png
Nut quotation

Note that flat (spanner or wrench) sizes differ between industry standards. For example, spanner or wrench sizes of fastener used in Japanese built cars comply with JIS automotive standard.

Nominal hole
diameter, D (mm)
P (mm)
Across flats,
A/F (mm)
A/C (mm)
H (mm)
CoarseFine ISO DIN JIS Hex
101.51.25 or 116171419.68510
121.751.5 or 1.2518191722.110612
182.52 or 1.5272715918.5
202.52 or 1.530303034.6161020
222.52 or 1.532

SAE hex nuts

Nominal hole
diameter, D
Pitch, PAcross flats,
Across corners,
Height, H
Coarse (UNC)Fine (UNF)Extra fine (UNEF)Hex nutJam nutNylon nut


Mechanical specifications of Metric/English sized nuts [4]
MaterialProof strengthMin. tensile yield strengthMin. tensile ultimate strengthNut markingNut class
ISO 898 (Metric)
Low or medium carbon steel380 MPa (55 ksi)420 MPa (61 ksi)520 MPa (75 ksi) Nut marking Metric Class 5 8.svg 5
Medium carbon steel Q&T 580 MPa (84 ksi)640 MPa (93 ksi)800 MPa (116 ksi) Nut marking Metric Class 8 8.svg 8
Alloy steel Q&T830 MPa (120 ksi)940 MPa (136 ksi)1040 MPa (151 ksi) Nut marking Metric Class 10 9.svg 10
SAE J995 (English)
Low or medium carbon steel55 ksi (379 MPa)57 ksi (393 MPa)74 ksi (510 MPa) Nut marking SAE Grade 2.svg 2
Medium carbon steel Q&T85 ksi (586 MPa)92 ksi (634 MPa)120 ksi (827 MPa) Nut marking SAE Grade 5.svg 5
Alloy steel Q&T120 ksi (827 MPa)130 ksi (896 MPa)150 ksi (1034 MPa) Nut marking SAE Grade 8.svg 8


See also

Related Research Articles

Adjustable spanner A spanner (a wrench) that can be adjusted to handle various sizes of fasteners

An adjustable spanner or adjustable wrench is any of various styles of spanner (wrench) with a movable jaw, allowing it to be used with different sizes of fastener head rather than just one fastener size, as with a conventional fixed spanner.

Pipe wrench A wrench for assembling and disassembling piping made of threaded pipes

A pipe wrench is any of several types of wrench that are designed to turn threaded pipe and pipe fittings for assembly (tightening) or disassembly (loosening). The Stillson wrench, or Stillson-pattern wrench, is the usual form of pipe wrench, especially in North America. The Stillson name is that of the original patent holder, who licensed the design to a number of manufacturers. The patent expired decades ago. Another type of wrench often used on pipes, the plumber wrench, is also called a pipe wrench in some places.

Washer (hardware) Thin plate with a hole, normally used to distribute the load of a threaded fastener

A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring, wear pad, preload indicating device, locking device, and to reduce vibration.

Bolted joint Mechanical joint secured by a threaded fastener

A bolted joint is one of the most common elements in construction and machine design. It consist of a male threaded fastener that captures and joins other parts, secured with a matching female screw thread. There are two main types of bolted joint designs: tension joints and shear joints.

Socket wrench Used to turn the head of a bolt or nut

A socket wrench is a type of spanner, that uses a closed socket format, rather than a typical open wrench/spanner to turn a fastener, typically in the form of a nut or bolt.

Lug nut Fastener, specifically a nut, used to secure a wheel on a vehicle

A lug nut or wheel nut is a fastener, specifically a nut, used to secure a wheel on a vehicle. Typically, lug nuts are found on automobiles, trucks (lorries), and other large vehicles using rubber tires.

Castellated nut

A castellated nut, sometimes referred to as a castle nut, is a nut with slots (notches) cut into one end. The name comes from the nut’s resemblance to the crenellated parapet of a medieval castle. Castellated nuts are sometimes referred to incorrectly as castigated nuts.

Acorn nut Hardware nut

An acorn nut, also referred to as crown hex nut, blind nut, cap nut, domed cap nut, or dome nut (UK), is a nut that has a domed end on one side. When used together with a threaded fastener with an external male thread, the domed end encloses the external thread, either to protect the thread or to protect nearby objects from contact with the thread. In addition, the dome gives a more finished appearance.


A locknut, also known as a lock nut, locking nut, self-locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Prevailing torque nuts have some portion of the nut that deforms elastically to provide a locking action. Free-spinning locknuts exist which carry the advantage of not requiring extra torque until seated.

Nyloc nut Type of lock nut

A nyloc nut, also referred to as a nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut, is a kind of locknut with a nylon collar that increases friction on the screw thread.

Jam nut

A jam nut is a low profile type of nut, typically half as tall as a standard nut. It is commonly used as a type of locknut, where it is "jammed" up against a standard nut to lock the two in place. It is also used in situations where a standard nut would not fit.

Flange nut

A flange nut is a nut that has a wide flange at one end that acts as an integrated washer. This serves to distribute the pressure of the nut over the part being secured, reducing the chance of damage to the part and making it less likely to loosen as a result of an uneven fastening surface. These nuts are mostly hexagonal in shape and are made up of hardened steel and often coated with zinc.

A distorted thread locknut, is a type of locknut that uses a deformed section of thread to keep the nut from loosening from vibrations or rotation of the clamped item. They are broken down into four types: elliptical offset nuts, centerlock nuts, toplock nuts and partially depitched (Philidas) nuts.

Multi-jackbolt tensioners (MJT) are an alternative to traditional bolted joints. Rather than needing to tighten one large bolt, MJTs use several smaller jackbolts to significantly reduce the torque required to attain a certain preload. MJTs range in thread sizes from 34 in (19 mm) to 32 in (810 mm) and can achieve 20 million pounds-force or more. MJTs only require hand-held tools, such as torque wrenches or air/electric impacts, for loading and unloading bolted joints.

A jet nut, also known as a k-nut, is a special type of hex locknut that is commonly used in the aerospace and automotive racing industries.

Screw Type of fastener characterized by a thread wrapped around a cylinder core

A screw and a bolt are similar types of fastener typically made of metal and characterized by a helical ridge, called a male thread. Screws and bolts are used to fasten materials by the engagement of the screw thread with a similar female thread in a matching part.

Positive locking device

A positive locking device is a device used in conjunction with a fastener in order to positively lock the fastener. This means that the fastener cannot work loose from vibrations. The following is a list of positive locking devices:

7-122. GENERAL. The word safetying is a term universally used in the aircraft industry. Briefly, safetying is defined as: "Securing by various means any nut, bolt, turnbuckle etc., on the aircraft so that vibration will not cause it to loosen during operation." These practices are not a means of obtaining or maintaining torque, rather a safety device to prevent the disengagement of screws, nuts, bolts, snap rings, oil caps, drain cocks, valves, and parts. Three basic methods are used in safetying; safety-wire, cotter pins, and self-locking nuts. Retainer washers and pal nuts are also sometimes used.

7-124d. Safety wire must be installed in a manner that will prevent the tendency of the part to loosen.

Bolt (fastener) Threaded fastener with an external male thread requiring a matching female thread

A bolt is a form of threaded fastener with an external male thread requiring a matching pre-formed female thread such as a nut. Bolts are very closely related to screws.

Wrench size

Width across flats is the distance between two parallel surfaces on the head of a screw or bolt, or a nut, mostly for torque transmission by positive locking.


  1. Gong, Hao; Liu, Jianhua; Feng, Huihua (2022-02-01). "Review on anti-loosening methods for threaded fasteners". Chinese Journal of Aeronautics. 35 (2): 47–61. doi:10.1016/j.cja.2020.12.038. ISSN   1000-9361. S2CID   234300791.
  2. "A reliable locking performance using a two nut design". Fastener + Fixing Technology. Retrieved 2022-03-15.
  3. , retrieved 2019-03-08.
  4. Bickford & Nassar 1998 , p. 153.