Threaded Fasteners: Bolts, Screws, Washers and Nuts
Spiral line - a moving point on the surface of a cylinder, while rotating around the axis at a constant speed, while moving at a constant speed along the axis.
Thread - The plane figure moves along the helix, and the thread is obtained by keeping the figure passing through the axis of the cylinder during movement.
Thread type, fit class and identification number
1. Thread type
Standard: AmericanNational Coarse
American National Fine
American Standard United Coarse
American Standard United Fine
Differences: 1. The threads of the coarse thread series and the fine thread series of the same size and number have the same nominal thread diameter, but the number of threads per inch of length is different.
2. The distribution of tolerances is different; there are also different specifications for the thread with a diameter of 1 inch:
NF regulations - 14 teeth per inch, expressed as 1 - 14NF
UNF regulations - 12 teeth per inch, expressed as 1 - l2UNF
Second, the level of cooperation
• There are also grades of threads. The grade of the thread indicates the tightness of the thread pair. It has five grades:
• Class 1 - Loose fit (nuts can be turned easily with just fingers)
• Class 2 - Free fit (used by aviation screws)
• Class 3 - Moderate fit (used by most aviation bolts)
• Class 4, 5 - Tight fit (requires a wrench to tighten the nut)
The larger the number of grades, the tighter the fit of the thread pair.
3. Identifier
Identification: bolt diameter, length, material, etc.
Note: The diameter measurement unit is 1/16 inch, and the rod length measurement unit of the body bolt is 1/8 inch; the manufacturing material of the bolt is represented by English letters or symbols, such as
• "C" for stainless steel;
• "DD" means 2024 aluminum alloy;
• "-" indicates nickel alloy steel.
Types and Applications of Threaded Fasteners
Bolts - transmit shear and tensile loads
Application: Large concentrated load or parts that need to be disassembled
Category: Pull-based
mainly sheared
Standard Aviation Bolts
1. Universal bolts
• A bolt with a hexagonal head that is widely used in aircraft structural assembly to withstand tensile and shear loads. The bolts are manufactured according to the AN type specification standard, and the thread of the bolt is fine thread (NF).
•Material of bolts: Chrome Nickel Steel Stainless Steel 2024 Aluminum Alloy
Bolt Marks:
An example of AN series bolts AN 3 DD 14A:
AN—Standard bolts for U.S. Air Force Navy specifications
"3" - indicates a body bolt with a bolt diameter of 3/16 in.
DD——Indicates that the bolt is made of 2024 aluminum alloy
"14" - means the shank is 1.5in long
A——Indicates that there is no safety hole at the end of the screw
AN6H34A:
AN—Standard bolts for U.S. Air Force Navy specifications
"6" - Indicates a body bolt with a bolt diameter of 6/16 in.
H——Indicates that the bolt is used to fix the propeller blade on the flange
"34" - means the shank is 3.5in long
A——Indicates that there is no safety hole at the end of the screw
2. Engine bolts drilled in the head
AN73 to AN81 MS20073 MS20074 as shown in the figure:
The shape of the engine bolt is the same as the body bolt, except that it has a slightly thicker head and 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 (no "A" letter)
Coarse thread (with the letter "A" after the thread diameter number)
3. Tight tolerance bolts (AN173～AN186 and NAS80～NAS86)
The diameter of the polished rod section of the bolt is higher than that of the general-purpose type. After the bolt is ground or ground, its diameter error is between 0 and -0.0005in.
4. Hexagon socket head bolts (MS20004～MS20024)
Construction: The head of the bolt is cylindrical or truncated cone, and a hexagonal inner hole is made on the bolt head.
Material: Made of high-strength steel, its strength is higher than that of AN series hexagon head bolts, so it cannot be used interchangeably with AN series hexagon head bolts.
Application: Hexagon socket head bolts are mainly used for parts that are subjected to tensile and shear composite stresses. The connection between the head and the screw has rounded corners to avoid sharp corners and improve fatigue life.
Note: 1. When installing on steel parts, the installation hole should be dimpled so that the rounded corners sink into the hole;
2. When installed on aluminum alloy parts, MS20002C washer 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, which indicate that the diameter of the bolt is 4/16 inches to 24/16 inches, That is, 1/4 inch to 3/2 inch.
5. Slotted flat head bolts (AN2l～AN36)
Features: The head of the shaft pin bolt is a round head, and there is a word or a cross groove on the round head. There is generally an empty knife groove between the polished rod of the screw part and the threaded rod section.
Thread: Fine thread (NF) is used.
Model specification code:
The second digits "1", "2" and "3" in AN2l, AN22, and AN23 indicate "6-40", "8-36" and "10-32" threads, respectively. Screw diameters from AN24 to AN36 are in 1/16 inch units. The shank length of the pivot bolt is also measured in 1/16 inch.
Application: Axle-pin bolts can only bear shear loads, and are often used as hinge-pins in rotating parts; the mating shear nut fits closely with the bolt thread and is secured with a cotter pin.
Example: AN 2 4 - 14 A
"A"——Indicates 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 that there is a safety hole at the end of the bolt shank.
"14" - 14/16" long shank
"-"——The screw material is nickel alloy steel
"4" - Bolt diameter 4/16"
"2" - axle pin bolt
"AN" - U.S. Air Force Naval Specifications
Nut
1. Standard aviation nut
(1) AN310 groove top nut (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)
Materials: Available in nickel alloy steel, stainless steel and 2024 aluminum alloy
"-", "C" and "D"
Example 1: Mark AN3l0 C 6
It is expressed as: stainless steel groove top nut used with AN6 bolts;
(2) AN320
Name: Shear castlenut
Application: For parts with only shear stress, it is used in conjunction with AN2l~AN36 shaft pin bolts.
Construction and manufacturing materials: the same as the AN310 nut, but the thickness of the nut is slightly thinner.
Example: Mark AN320D6
Indicated 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 tensile force. 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 types of right-hand thread and left-hand thread.
Example: Mark AN315-7R
Indicates: Nickel alloy steel, right-hand thread nut for use with AN7 bolts.
Tagged: AN3l5C4L
Indicates: Stainless steel, left-hand thread nut with 1/4" thread diameter.
(4) AN316
Name: Anti-loosening nut
Features: This is a hexagonal anti-loosening nut with double nut anti-loosening measures. It is used on the AN315 ordinary nut to prevent the ordinary nut from loosening. However, due to the greater weight added than other types of anti-loosening measures, it is rarely used in aircraft at present.
(5) AN340 and AN345
Name: Machine Screw Nut
Application: Used in conjunction with machine screws, therefore, its thread specifications are manufactured according to the specifications of the screw.
Thread: AN340 is a coarse thread nut; AN345 is a fine thread nut.
Materials: Carbon alloy steel (-), stainless steel (C), brass (B) and 2024 aluminum alloy (DD).
Example: Mark AN340B6
Indicated as: Coarse thread nut made of brass for use with 6-32 mounting screws.
(6) AN350
Name: butterfly nut
Application: Specially designed for assemblies that require frequent disassembly and assembly requiring only finger tightening. This nut can be used with mounting screws or bolts. The nut that fits the mounting screw is given by the number of the screw.
Material: Alloy steel or brass.
Example: Mark AN350-6
Indicates: Nuts for use with 6-40 mounting screws. Nuts for use with bolts are given in 1/16-inch bolt sizes, plus the number 16.
2. 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 matching thread. When the nut is turned so that the bolt is screwed through the thread of the bottom section and wants to enter the thread of the top section, the resistance of the elastic ring must be overcome, so that the thread of the bolt and the thread of the top section of the nut can be meshed harmoniously. After the bolts are fully tightened, the elastic ring segment forces the thread of the top segment to tightly bite the thread of the bolt to prevent loosening.
(2) High temperature self-locking nut and locking principle
Model: AN363
These two kinds of self-locking nuts are closed and expanded after tightening, and are locked by elasticity. They can be reused many times, and because of their simple manufacturing process, these two types of self-locking nuts are increasingly used in aircraft.
(3) Application of self-locking nuts
Applications: Fixed anti-friction bearings, pulleys of control systems, engine rocker cover, exhaust pipes.
Precautions:
1. There are restrictions on the use of self-locking nuts in 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 are subjected to rotational torque.
washer
The role of the gasket:
1. It is to make the force under the nut or bolt head uniform.
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.
1. Ordinary gasket
AN960 is the most commonly used ordinary gasket (Figure (a))
Application: It is widely used under hex nuts.
Materials: Available in carbon, steel, brass, stainless steel and aluminum alloys.
Sizes: Available from sizes for use with No. 1 mounting screws to sizes for use with 1-inch bolts.
2. AN935 open elastic washer (Figure (d))
3. AN936B type external tooth elastic washer (Figure (e))
4. AN936A type internal tooth elastic washer (Figure (f))
AN936 inner and outer tooth elastic washer, with uniform elastic force and good anti-loosening effect. There are many applications of external tooth type.
screw
The material used to make the screw is often lower than the strength of the material used to make the bolt.
Removable panels as it applies to non-structural connections and fairings
and other non-main stress structures.
Features:
1) Its structure is similar to that of bolts. The screw shaft is generally a fully threaded shaft, and the threads are manufactured according to the 2-level free fit precision.
2) There are two types of nail tails: flat and pointed.
3) During installation, the screws are generally screwed into the screw holes on the components, and the screws are usually rotated to achieve the purpose of fastening.
Classification by use: 1. Machine screws 2. Structural screws 3. Self-tapping screws.
1. Machine screws
Machine screws are commonly used on non-structural parts, such as light and small parts and the assembly of cast aluminum parts such as gearbox covers.
Features:
1) The screwdriver slot 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 segment
3) There are two types of threads: coarse and fine.
Material: There are mild steel, stainless steel, brass and aluminum alloy, etc.
Common models are:
(1) AN500 is a round head screw with coarse thread AN501 is a round head screw with fine thread
(2) AN505 is 82° countersunk head screw with coarse thread, AN507 is 82° countersunk head screw with fine thread
(3) AN515 is a round head screw with coarse teeth and AN526 is a round head screw with fine teeth
2. Structural screws
Material: Made of alloy steel
Features:
(1) Its tensile and shear strengths are the same as AN-type bolts of the same size and material
(2) This type of screw has the exact same length as the same size bolt
(3) The body of the nail has a polished rod (without thread) section, the thread is NF, and the matching accuracy of the screw is also the same as that of the AN type hexagon head bolt, so it can be used as a structural bolt.
The head type of the screw: there are round head, oblate head, 100° countersunk head and so on.
The screwdriver slot of the screw head: phillips, Reed-prince slot
The types of common structural screws are:
(1) AN509 100° flat head screw
(2) AN525 washer head screw, which has a large head similar to a washer and has a larger bearing area.
(3) AN502 and AN503 round head screws
AN502 is a round head screw with fine thread, and AN503 is a round head screw with coarse thread. The round head of these two types of screws has a slot and a small hole for a fuse.
3. Self-tapping screws
Self-tapping screws are often used for fastening thin metal plates, plastic plates, etc. Features:
1) Fastened by tapping in the assembly hole by itself.
2) The head types of self-tapping screws are: oblate head, 100° countersunk head, large oblate head, etc.
3) The nail tail has: pointed head (A type) blunt head (B type).
Common models are:
(1) AN504 and AN506 They are often used to install removable parts such as signage.
(2) AN530 and AN531 are self-tapping screws for sheet metal. AN530 is oblate head, AN531 is countersunk head tapping screw-B type
4. Screw type
Mark AN501B-416-7
Indicates: 416 - 4/16 inch diameter, 7 - rod length 7/16 inch, B - indicates that the material is brass, AN - empty, navy standard 501 - grooved cylindrical head, fine thread
