Basic knowledge of fasteners

What is a fastener?

Fastener is a kind of mechanical parts which is widely used for fastening connection. Fasteners are widely used in various industries, including energy, electronics, electrical appliances, machinery, chemical industry, metallurgy, mold, hydraulic and so on. In various machinery, equipment, vehicles, ships, railways, bridges, buildings, structures, tools, instruments, chemical industry, instruments and supplies, you can see all kinds of fasteners. They are the most widely used mechanical basic parts. It is characterized by a wide variety of specifications, different performance and uses, and a high degree of standardization, serialization and generalization. Therefore, some people refer to a kind of fasteners with national standards as standard fasteners, or referred to as standard parts for short.

Application of fasteners

Fastener is a kind of mechanical parts widely used for fastening and connecting. In all kinds of machinery, equipment, vehicles, ships, railways, bridges, buildings, structures, tools, instruments, meters and supplies, you can see all kinds of fasteners. It is characterized by a wide variety of specifications, different performance and use, and high degree of standardization, serialization and generalization. Therefore, some people refer to a kind of fasteners with national standards as standard fasteners, or referred to as standard parts for short. Fastener is the most widely used mechanical basic parts. With China’s accession to the WTO in 2001, China has entered the ranks of a major international trading country. A large number of Chinese fastener products are exported to all countries in the world, and the fastener products from all countries in the world are constantly pouring into the Chinese market. Fastener is one of the products with large import and export volume in China. It is of great practical and strategic significance to realize the integration with the international market, promote Chinese fastener enterprises to go to the world, and promote fastener enterprises to fully participate in international cooperation and competition. Due to the specification, dimension, tolerance, weight, performance, surface condition, marking method of each specific fastener product, as well as the specific requirements of acceptance inspection, marking and packaging.

Common testing instruments

The most important link in the development and design of fasteners is quality control. There are several gates from the feeding to the delivery of finished products, which have different inspection methods. First of all, the feeding is related to the appearance, size, element, performance, harmful substance detection, etc.; the process is more about the appearance, size, knocking test, forging line; the heat treatment is more about the appearance, hardness, torque, tensile force, metallography, etc.; the surface treatment is more about hydrogen embrittlement test, coating, salt spray, etc., including the shipping has a harmful substance detection. In terms of size and appearance inspection, the common ones are quadratic element, profilometer, CMM and image sorter (this is a total sorter); in terms of mechanical and chemical inspection, there are mainly hardness machine (Rockwell and Vickers), tensile machine and metallographic microscope; in terms of material inspection, there is a spectrum analyzer and salt spray testing machine.

Parts of fasteners

It usually includes the following 12 types of parts:

  • Bolt: a kind of fastener composed of head and screw (cylinder with external thread), which is used to connect two parts with through hole. This kind of connection is called bolt connection. If the nut is unscrewed from the bolt, the two parts can be separated, so the bolt connection is a detachable connection.
  • Stud: a kind of fastener without head and with external thread at both ends. When connecting, one end must be screwed into the part with internal thread hole, and the other end must go through the part with through hole, and then screw on the nut, even if the two parts are tightly connected as a whole. This type of connection is called stud connection, which also belongs to detachable connection. It is mainly used in the occasions where one of the connected parts is thick and requires compact structure, or where bolt connection is not suitable due to frequent disassembly.
  • Screw: it is also a kind of fastener composed of head and screw. It can be divided into three categories according to its application: machine screw, set screw and special purpose screw. The machine screw is mainly used for the fastening connection between a part with a fixed threaded hole and a part with a through hole, without the need of nut (this connection form is called screw connection, which also belongs to detachable connection; it can also be used for the fastening connection between two parts with a through hole.) Set screw is mainly used to fix the relative position between two parts. Special purpose screws such as eyebolts are used for lifting parts.
  • Nut: with internal thread hole, the shape is generally shown as flat hexagonal column, or flat square column or flat cylindrical, with bolts, studs or machine screws, used to fasten and connect two parts, making it a whole.
  • Self tapping screw: similar to machine screw, but the thread on screw is special for self tapping screw. It is used for fastening and connecting two thin metal components to make them into a whole. Small holes need to be made on the component in advance. Because of its high hardness, the screw can be directly screwed into the hole of the component to form a responsive internal thread in the component
  • Wood screw: it is also similar to machine screw, but the screw thread on the screw is a special wood screw thread, which can be directly screwed into the wood component (or part), used to connect a metal (or non-metal) part with a through hole with a wood component. This kind of connection is also detachable.
  • Washer: a kind of fastener with flat circular shape. Placed between the supporting surface of bolt, screw or nut and the surface of connecting parts, it can increase the contact surface area of connected parts, reduce the pressure per unit area and protect the surface of connected parts from damage; another kind of elastic washer can also prevent the nut from loosening.
  • Retaining ring: it is installed in the shaft groove or shaft hole groove of machine and equipment, and plays the role of preventing the parts on the shaft or hole from moving left and right.
  • Pin: it is mainly used for positioning parts, and some can also be used for connecting parts, fixing parts, transferring power or locking fasteners.
  • Rivet: a kind of fastener composed of head and nail rod, which is used to fasten and connect two parts (or components) with holes to make them a whole. This kind of connection is called rivet connection. It is a non detachable link. Because if the two parts connected together are separated, the rivets on the parts must be broken.
  • Assembly and connection pair: assembly is a kind of fastener supplied by combination, such as the combination of some machine screw (or bolt, self supplied screw) and flat washer (or spring washer, lock washer); connection pair refers to a kind of fastener supplied by combination of some special bolt, nut and washer, such as the connection pair of high-strength large hexagon head bolt for steel structure.
  • Welding nail: heterogeneous fastener composed of nail rod and nail head (or no nail head), which connects other parts (or components) by welding, so as to connect with other parts.

Locking of fasteners

Fasteners include: bolt, stud, screw, nut, washer and pin. Locking or biting often occurs on the fasteners made of stainless steel, aluminum alloy and titanium alloy. These metal alloys have anti-corrosion performance, which will produce a thin oxide layer on the metal surface to prevent further corrosion when the surface is damaged. When the stainless steel fasteners are locked, the pressure and heat generated between the teeth will destroy the oxide layer, causing the metal threads to block or shear, and then the phenomenon of adhesion occurs. When this phenomenon continues to occur, the stainless steel fasteners will be completely locked and can no longer be removed or locked. Usually this is a series of blockages- Shear-Adhesion-A series of locking actions take place in just a few seconds, so correct understanding of the use of such fasteners can prevent this phenomenon.

Choose the right products

  • a. Before use, confirm whether the mechanical properties of the product can meet the use requirements, such as the tensile strength of the screw and the guaranteed load of the nut. The length of the screw should be selected properly, with 1-2 pitch exposed after tightening.
  • b. Before use, check whether the thread is rough, whether there are iron filings or dirt between the threads, which often lead to locking.
  • c. The fasteners can be lubricated before use. It is recommended to use grease, molybdenum disulfide, mica, graphite or talc powder for lubrication. Generally, wax impregnation is used to prevent locking.

Pay attention to the usage

  • a. The speed and force of screw in should be appropriate, not too fast and too large. The torque wrench or socket wrench should be selected as far as possible, and the adjustable wrench or electric wrench should be avoided. If the speed is too fast, the temperature will rise rapidly and cause locking.
  • b. In the direction of force application, the nut must be rotated perpendicular to the axis of the screw.
  • c. The use of gasket can effectively prevent the problem of locking too tight.

Standard for fasteners

Standard of stainless steel fastener product size: specify the content of basic product size; products with thread.
The standard of technical conditions of products is not required. It also includes the following standards:

  • Standard of fastener product tolerance: specify the content of product dimension tolerance and geometric tolerance.
  • Standards for mechanical properties of fastener products: specify the marking method of mechanical property grade of products and the contents of mechanical property items and requirements; some fastener products change this content to the contents of material properties or working properties of products.
  • Standards for surface defects of fastener products: specify the types and specific requirements of surface defects.
  • Standards for surface treatment of fastener products: specify the types and requirements of surface treatment.
  • Standard of fastener product test: specify the test contents of the above performance requirements.

Standards for acceptance inspection, marking and packaging of stainless steel fasteners:

  • Standard of fastener product marking method: specify the content of complete marking method and simplified marking method.
  • Other standards of stainless steel fasteners: such as fastener terminology standards, fastener product weight standards, etc

Thread type, fit level and identification symbol

Thread type

  • AmericanNational Coarse
  • American National Fine
  • American Standard United Coarse
  • American Standard United Fine

1. Coarse thread series and fine thread series of the same size have the same nominal thread diameter, but the number of teeth per inch is different.
2. The distribution of tolerances is different; there are also different regulations for threads with a diameter of 1 inch:

  • NF regulations-14 teeth per inch, expressed by 1-14NF
  • UNF regulations-12 teeth per inch, expressed by 1-l2UNF

The level of cooperation

There are also grades of thread. The grade of thread indicates the degree of tightness of the thread pair. It has five grades:

  • Level 1-loose fit (you can easily turn the nut with your fingers);
  • Level 2-free fit (for aviation screws);
  • Level 3-Moderate fit (used by most aviation bolts);
  • Level 4, 5-tight fit (a wrench is required to screw on the nut);
  • The higher the grade number, the tighter the thread pair fits.

Identification symbols

Identification: bolt diameter, length, material, etc.
Note: The unit of measurement for diameter is 1/16in, and the unit of measurement for body bolt length is 1/8 in; the material of bolts is represented by English letters or symbols, such as:

  • “C” means stainless steel;
  • “DD” means 2024 aluminum alloy;
  • “-” means nickel alloy steel.

Standard Aviation Bolt

Universal bolt

A bolt with a hexagonal head, which is widely used in the assembly of aircraft structures and can bear tensile and shear loads. This kind of bolt is manufactured according to the AN model specification standard, and the thread of the bolt is fine thread (NF).
Bolt material: chrome-plated nickel steel, stainless steel, 2024 aluminum alloy
Bolt mark:
Take AN series bolt AN 3 DD 14A as an example:

  • AN-Standard bolt for the U.S. Air Force Navy specification
  • “3”——means body bolt with a bolt diameter of 3/16in
  • DD-means that the bolt is made of 2024 aluminum alloy
  • “14”-means that the bolt rod length is 1.5in
  • A-indicates that there is no safety hole at the end of the screw

AN 6 H 34 A:

  • AN-Standard bolt for the U.S. Air Force Navy specification
  • “6”——means body bolt with a bolt diameter of 6/16in
  • H-indicates that the bolt is used to fix the propeller blade to the flange
  • “34”——means the bolt rod length is 3.5in
  • A-indicates that there is no safety hole in the screw

Engine bolts for head drilling

AN73 to AN81 MS20073 MS20074 as shown:

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The shape of the engine bolt is the same as the body bolt, except that its head is slightly thicker and has a through hole for the fuse. It has the same tensile and shear strength as the body bolt.
The threads of the engine bolts are:

  • Fine thread (without “A” letter)
  • Coarse thread (add the letter “A” after the thread diameter number)

Close tolerance bolts (AN173~AN186 and NAS80~NAS86)

The diameter manufacturing precision of the polished bolt section of the bolt is higher than that of the general type. After the bolt is ground or ground, its diameter error is between 0-0.0005 in.

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Hexagon socket bolts (MS20004~MS20024)

Structure: The head of the bolt is cylindrical or truncated cone, and an inner hexagonal hole is made on the bolt head.
Material: It is made of high-strength steel. Its strength is higher than that of AN series hexagon head bolts. Therefore, it cannot be used interchangeably with AN series hexagon head bolts.
Application: Hexagon socket bolts are mainly used to bear the combined stress of tension and shear. It has rounded corners at the joint between its head and screw to avoid sharp corners and improve fatigue life.

  • 1. When installing on steel parts, the installation hole should be cut so that the rounded corners sink into the hole;
  • 2. When installing on aluminum alloy parts, MS20002C washers should be installed under its head.

Model specification code: MS20004~MS20024
The last two digits are 04, 05, 06, 07, 08, 09, 10, 12, 14, 16, 18, 20, 22, and 24. They indicate that the bolt diameter is 4/16 inches to 24/16 inches. That is, 1/4 inch ~ 3/2 inch.

Slotted oval head bolts (AN2l~AN36)

Features: The head of the shaft pin bolt is round head, and there is a word or cross groove on the round head. There is generally an empty slot between the smooth rod of the screw part and the threaded rod section.

Thread: use fine thread (NF).

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Model specification code:
The second digits “1”, “2” and “3” in AN21, AN22, and AN23 represent “6-40”, “8-36” and “10-32” threads, respectively. The screw diameter from AN24 to AN36 is in units of 1/16 inch. The rod length of the pin bolt is also measured in units of 1/16 inch.
Application: Axle pin bolts can only bear shear load, and are often used as pivot pins in rotating parts; the matching shear nut is closely matched with the bolt thread, and is secured by a split pin.
Example: AN 2 4-14 A

  • “A”-means that there is no safety hole at the end of the bolt rod. In this case, the nut matched with the screw should use a self-locking nut. If there is no “A”, it means there is a safety hole at the end of the bolt rod.
  • “14”-bolt rod length 14/16 inches
  • “-“——The screw material is nickel alloy steel
  • “4”-bolt diameter 4/16 inch
  • “2”——Axle pin bolt
  • “AN”-U.S. Air Force Naval Specification Standard


Standard Aviation Nut

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(1) AN310 castlenut
Name: This type of nut is also called castellated nut
Application: It is used in conjunction with AN3~AN2O body bolts
Thread: NF (fine thread)
Material: Nickel alloy steel, stainless steel and 2024 aluminum alloy “-“, “C” and “D”
Example 1: Mark AN3l0 C 6
Expressed as: stainless steel groove top nut used in conjunction with AN6 bolts;
(2) AN320
Name: Shear castlenut (shear castlenut)
Application: For the part with only shear stress, it is used in conjunction with AN2l~AN36 axle pin bolts.
Construction and manufacturing material: Same as AN310 nut, but the thickness of the nut is slightly thinner.
Example: mark AN320D6
Expressed as: Shear-resistant hexagonal slotted flat nut made of 2024 aluminum alloy for use with AN26 axle pin bolts.
(3) AN315
Name: Ordinary nut
Application: Used for parts subject to high tension. Its disadvantage is that a lock nut or an elastic washer must be used to ensure its tightness.
Material: Same as AN310 nut.
Thread: There are two kinds of right-hand thread and left-hand thread.
Example: Mark AN315-7R
Means: Nickel alloy steel, right-hand threaded nut for use with AN7 bolts.
Mark: AN3l5C4L
Means: stainless steel, left-handed threaded nut with thread diameter of 1/4 inch.
(4) AN316
Name: Lock nut
Features: This is a hexagonal anti-loosening nut with double nut anti-loosening measures. It is used on AN315 ordinary nut to prevent the ordinary nut from loosening. However, due to the increased weight compared to other types of anti-loosening measures, it is currently rarely used in aircraft.
(5) AN340 and AN345
Name: Mechanical screw nut
Application: Used in conjunction with machine screws, therefore, the thread specifications are manufactured according to the screw specifications.
Thread: AN340 is a coarse thread nut; AN345 is a fine thread nut.
Material: There are carbon alloy steel (-), stainless steel (C), brass (B) and 2024 aluminum alloy (DD).
Example: mark AN340B6
Expressed as: a coarse threaded nut made of brass and used with 6-32 mounting screws.
(6) AN350
Name: butterfly nut
Application: It is specially used for assembly parts that are frequently disassembled and assembled and only need to be tightened with fingers. This nut can be used with mounting screws or bolts. The nut matching the mounting screw is given by the number of the screw.
Material: Alloy steel or brass.
Example: mark AN350-6
Means: a nut used in conjunction with 6-40 mounting screws. Nuts used with bolts are given in 1/16 inch bolt specifications with the number 16.

Self-locking nut

(1) Low-temperature self-locking nut and locking principle
Model: AN365, AN364
Locking principle: The low-temperature self-locking nut inserts an elastic ring into the thread, and the inner diameter is slightly smaller than the matched thread. When the nut is turned to make the bolt pass through the bottom thread and then enter the top thread, the resistance of the elastic ring must be overcome to make the thread of the bolt coordinately mesh with the thread of the top section of the nut. After the bolts are fully tightened, the elastic ring section forces the top section threads to bite the bolt threads tightly to achieve the purpose of preventing loosening.
(2) High temperature self-locking nut and locking principle
Model: AN363

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Both of these self-locking nuts are closed and expanded after tightening, and are locked by elasticity. They can be reused many times. Because of their simple manufacturing process, these two kinds of self-locking nuts are increasingly widely used in aircraft.
(3) Application of self-locking nut
Application: fixed anti-friction bearings, pulleys of control systems, engine rocker cover, exhaust pipe.

  • 1. There are restrictions on the use of self-locking nuts on aircraft, which should meet the requirements;
  • 2. The ambient temperature of the low-temperature self-locking nut is T<2500F;
  • 3. When the self-locking nut is screwed into the bolt, there must be enough depth;
  • 4. Self-locking nuts cannot be used for bolts that bear rotating torque.


The role of the washer:

  • 1. Make the force under the nut or bolt head even.
  • 2. It can be used to adjust the distance between the thread and the surface of the component.
  • 3. It has the functions of locking, anti-loosening, protection and pre-tightening instructions.

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1. Ordinary washers
AN960 is the most commonly used common washer (picture (a))
Application: It is widely used under hexagon nuts.
Material: There are carbon, steel, brass, stainless steel and aluminum alloy.
Specifications: From the specifications used with No. 1 mounting screws to the specifications used with 1-inch bolts.
2. AN935 open elastic washer (figure (d))
3. AN936B external tooth elastic washer (picture (e))
4. AN936A type internal tooth elastic washer (figure (f))

AN936 inner and outer tooth elastic washers have uniform elasticity and good anti-loosening effect. There are many applications of external gear.
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The material used to make the screw is often lower than the strength of the material used to make the bolt.
Because it is used in non-structural connections and fairings, detachable panels and other non-mainly stressed structures.

  • 1) Its structure is similar to that of bolts. The screw shaft is generally a fully threaded shaft, and the thread is manufactured with a free fit accuracy of level 2.
  • 2) There are two types of nail tails: flat and pointed.
  • 3) During installation, the screws are usually screwed into the screw holes on the components, and the screws are usually turned to achieve the purpose of fastening.

Classified by purpose: 1. Mechanical screws 2. Structural screws 3. Self-tapping screws.

Mechanical screws

Mechanical screws are often used in the assembly of non-structural parts, such as light and small mechanical parts and aluminum castings such as gear box covers.

  • 1) The screwdriver groove on the end face of the screw head is the same as that of the structural screw.
  • 2) The screw shaft is a fully threaded rod section.
  • 3) There are two types of threads: coarse thread and fine thread.

Material: low carbon steel, stainless steel, brass and aluminum alloy, etc.
Common models are:
(1) AN500 is a coarse-toothed round head screw AN501 is a fine-toothed round head screw
(2) AN505 is a coarse thread 82° countersunk screw AN507 is a fine thread 82° countersunk screw
(3) AN515 is a coarse-toothed round head screw AN526 is a fine-toothed round head screw

Structural screws

Material: made of alloy steel

  • (1) Its tensile and shear strength are the same as AN bolts of the same size and material
  • (2) This type of screw has exactly the same length as the bolt of the same size
  • (3) The nail shaft has a smooth (unthreaded) section, and the thread is NF. The matching accuracy of the screw is also at the same level as the AN-type hexagonal head bolt, so it can be used as a structural bolt.

Screw head type: round head, oval head, 100° countersunk head and other types.
Screwdriver notch of screw head: phillips, Reed-prince slot
The types of common structural screws are:

  • (1) AN509 100° flat head screw.
  • (2) AN525 washer head screw, this type of screw has a large head similar to a washer and has a larger bearing area.
  • (3) AN502 and AN503 round head screws.

AN502 is a fine-toothed round head screw, AN503 is a coarse-toothed round head screw. The round head of these two screws has a slot and a small hole for the fuse.

Self-tapping screws

Self-tapping screws are often used for fastening features of thin metal plates, plastic plates, etc.:

  • 1) Fastened by tapping in the assembly hole by itself.
  • 2) The head types of self-tapping screws are: oval head, 100° countersunk head, large oval head, etc.
  • 3) The nail tail has: pointed (A type) blunt (B type).

Common models are:

  • (1) AN504 and AN506 are commonly used to install removable parts such as signs.
  • (2) AN530 and AN531 are self-tapping screws for sheet metal. AN530 is an oblate head, AN531 is a countersunk head tapping screw-Type B.

Screw model

Mark AN501B-416-7
Means: 416-diameter 4/16 inches, 7-rod length 7/16 inches, B-indicates that the material is brass, AN-hollow, navy standard 501-grooved cylindrical head, fine thread.

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Source: Network Arrangement – China Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Basic knowledge of fasteners
Fastener is a kind of mechanical parts which is widely used for fastening connection.
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