Screws are tools that use the physical and mathematical principles of oblique circular rotation and frictional force of objects to gradually tighten mechanical components. Screws are a common term for fasteners, commonly used in everyday language. Screws are an indispensable industrial necessity in daily life, such as tiny screws used in cameras, glasses, clocks, electronics, etc; General screws for television, electrical products, musical instruments, furniture, etc; As for engineering, construction, and bridges, large screws and nuts are used; Transportation equipment, airplanes, trams, cars, etc. use a combination of large and small screws. Screws have important tasks in industry, and as long as industry exists on Earth, the function of screws will always be important. Screws have been a common invention in people's production and life for thousands of years, and according to their application fields, they are the greatest invention of humanity.
The main function of screws is to connect two workpieces together, playing a tightening role.
Screw, also known as "screw" or "screw rod" by some people. In fact, screws are a general term, while screws and screw rods are different from each other. Screws are generally called wooden screws; It is the type with a pointed front end and a larger pitch, usually used for fastening wooden and plastic parts. A screw rod is a machine screw (mechanical screw) with a flat front end, a small and uniform pitch, and is generally used to tighten metal and machine components.
Common specifications
A: Metric screw B: American screw C: English screw
A: Metric mechanical screws:
Ex: M3 x 6-P P B: M3 mechanical screw, 6mm long, cross shaped, round flat head, black plated
Finish Code: Appearance Treatment Specification
Head Code: Head shape.
Screw head shape. Cylindrical head. Half sunk head. Sink your head. Spherical cylindrical head. Pan head. Half round head. Hexagonal head
Material knowledge
There are three main types of standard parts in the market: carbon steel, stainless steel, and copper.
One carbon steel. Distinguish low carbon steel, medium carbon steel, high carbon steel, and alloy steel by their carbon content in carbon steel materials.
Low carbon steel with C% ≤ 0.25% is commonly referred to as A3 steel in China. It is commonly referred to as 10081015181022 in foreign countries. Mainly used for grade 4.8 bolts, grade 4 nuts, small screws and other products without hardness requirements. (Note: The drilling tail nail is mainly made of 1022 material.)
2. Carbon steel 0.25%
3. High carbon steel C% ≥ 0.45%. Basically not used in the market
4-alloy steel: Adding alloying elements to ordinary carbon steel to increase some special properties of the steel, such as 35, 40 chromium molybdenum, SCM435, 10B38. Aromatic screws mainly use SCM435 chromium molybdenum alloy steel, which is mainly composed of C Si,Mn,P,S,Cr,Mo.
Two stainless steels. Performance levels: 45, 50, 60, 70, 80
1. The main components of austenite (18% Cr, 8% Ni) have good heat resistance, corrosion resistance, and weldability. A1,A2,A4
2 martensite and 13% Cr have poor corrosion resistance, high strength, and good wear resistance. C1, C2, C4 ferritic stainless steel. 18% Cr has good forging properties and stronger corrosion resistance than martensite. The main imported materials in the market are from Japan. Mainly divided by level into SUS302, SUS304, and SUS316.
Three copper. The commonly used material is brass... zinc copper alloy. H62, H65, and H68 copper are mainly used as standard parts in the market.
Screw labeling
The annotation format for thread markings is:
Thread code - Thread tolerance zone code (pitch diameter, top diameter) - Spinning length
l: The tolerance zone code is represented by numbers and letters (uppercase letters for internal threads and lowercase letters for external threads), such as 7H, 6g, etc. It should be noted that 7H, 6g, etc. represent thread tolerances, while H7, g6 represent cylinder tolerance codes.
2: The rotation length is specified as short (represented by S), medium (represented by N), and long (represented by L). In general, the thread fitting length is not indicated, and the thread tolerance zone is determined based on the moderate fitting length (N). If necessary, the screw length code S or L can be added, such as "M20-5g6g-L". When there is a special need, the numerical value of the rotation length can be indicated, such as "M20-5g6g-30".
Regular thread
Ordinary coarse thread: feature code M+nominal diameter+rotation direction+thread tolerance zone code (pitch diameter, top diameter) - rotation length
Ordinary fine thread: feature code M+nominal diameter * pitch+direction of rotation+thread tolerance zone code (pitch diameter, top diameter) - turning length
Right-handed threads are omitted without annotation, and left-handed threads are represented by "LH".
M 16-5g6g represents coarse thread ordinary thread, with a nominal diameter of 16 and a right-hand rotation. The thread tolerance zone has a pitch diameter of 5g and a major diameter of 6g, and the thread engagement length is considered to be of medium length.
M16 × 1 LH-6G represents fine pitch ordinary thread, with a nominal diameter of 16 and a pitch of 1. It is left handed and has a tolerance zone of 6G for both medium and major diameters. The length of rotation is considered to be of medium length.
The annotation format is: feature code (represented by G for cylindrical pipe threads and NPT for conical pipe threads)+size code+tolerance level code+rotation direction
G1A -- LH represents British non threaded sealing pipe threads, with a size code of 1 inch, left-hand rotation, and a tolerance level of A.
Rcl/2 represents British standard thread sealing cone pipe thread, size code 1/2in, right-hand rotation.
Common sense of application
1. First, remove the sludge on the surface of the broken screw head and use a center gun to lock the center gun of the section. Then, use an electric drill to install a drill bit with a diameter of 6-8 millimeters to drill holes at the center gun hole of the section, making sure the hole is drilled thoroughly. After drilling through the hole, remove the small drill bit and replace it with a drill bit with a diameter of 16 millimeters. Continue to enlarge and drill through the hole where the bolt is broken.
2. Take welding rods with a diameter of less than 3.2 millimeters and use medium and small currents to weld from inside out in the drilling hole of the broken bolt. Take half of the entire length of the broken bolt at the beginning of the weld. When starting the weld, do not run the arc too long to avoid burning through the outer wall of the broken bolt. After welding to the upper end face of the broken bolt, continue to weld a cylinder with a diameter of 14-16 millimeters and a height of 8-10 millimeters.
3. After welding, use a hammer to strike the end face of the broken bolt to cause vibration along its axis. Due to the heat generated by the previous arc and subsequent cooling, combined with the vibration at this time, the thread between the broken bolt and the body will become loose.
4. Careful observation: When a small amount of rust is found to leak out from the fracture after striking, the M18 nut can be placed on the welded column head and welded together.
5. After welding, use a plum blossom wrench to cover the nut and twist it back and forth while it is still hot. You can also use a small hammer to tap the end face of the nut while twisting it back and forth to remove the broken bolt.
6. After removing the broken bolt, use a suitable tap to process the threads inside the frame once to remove rust and other debris from the holes.
Inspection method
There are two types of surface inspection for screws: one is the inspection before electroplating after production, and the other is the inspection after electroplating, which means the inspection after the screw is hardened and the screw surface is treated. Before electroplating, it is necessary to inspect the screws in all aspects such as size and tolerance after production. Check if it meets national standards or customer requirements. After surface treatment of screws, it is necessary to inspect the electroplated screws, mainly to check the color of the plating and whether there are any damaged screws. When delivering screw products to customers in this way, they can pass the customs smoothly when receiving the goods. Inspection after screw treatment:
Appearance quality requirements
The inspection of screw appearance is carried out from various aspects such as appearance, electroplating layer, etc.
2, Inspection of screw coating thickness
1. Measuring tool method
The required quantities include micrometers, vernier calipers, plug gauges, etc.
2. Magnetic method
The magnetic method is used to measure the thickness of the coating layer, which is a non-destructive measurement of the non-magnetic coating layer on a magnetic substrate using a magnetic thickness gauge.
3. Microscopic method
Microscopic method, also known as metallographic method, involves magnifying a corroded fastener on a metallographic microscope with a micrometer eyepiece to measure the thickness of the coating on the cross-section.
4. Timed liquid flow method
The timed liquid flow method is to use a solution that can dissolve the coating to flow onto the local surface of the coating, and calculate the thickness of the coating based on the time required for the local coating to dissolve. There are also coating droplet method, anodic dissolution Coulomb method, etc.
3, Inspection of adhesion strength of screw coating
There are many methods to evaluate the adhesion between the coating and the substrate metal, usually including the following.
1. Friction polishing test; 2. File method test; 3. Scratch method; 4. Bending test; 5. Thermal shock test; 6. Squeeze method.
4, Inspection of corrosion resistance of screw coatings
The inspection methods for corrosion resistance of coatings include: atmospheric burst test; Neutral salt spray test (NSS test); Acetate spray test (ASS test) and copper accelerated acetate spray test (CASS test); And corrosion paste corrosion test (CORR test) and solution droplet corrosion test; Immersion test, immersion corrosion test, etc.
Scope of application
There are many names for screws, and everyone may have different names. Some people call them screws, some call them screws, some call them standard parts, and some call them fasteners. Although there are so many names, the meaning is the same, they are all screws. Screws are a common term for fasteners. The principle of screws is to use the physical and mathematical principles of the oblique circular rotation and frictional force of an object to gradually tighten the mechanical components of the object.
Screws are indispensable in daily life and industrial production, therefore screws are also known as the rice of industry. The application scope of screws includes electronic products, mechanical products, digital products, power equipment, and electromechanical products. Screws are also used in ships, vehicles, hydraulic engineering, and even chemical experiments.
There are many types of screws, whether they are very small screws for glasses or large screws for heavy electrical engineering. The accuracy of screws is usually 6g level (2 levels, American specification "IFI" is 2A teeth), and rough screws used in construction projects are 1g level.
The wide range of applications of screws has led to a large market for screws, resulting in a large number of manufacturers producing screws. When selecting a professional screw manufacturer, the purchaser needs to first understand some professional basic knowledge of screws, such as screw classification standards and American screw specification sheets.
Usage performance
Self tapping screws
For self tapping screws with diameters ranging from 0.8mm to 12mm. For these types of screws, they generally have high hardness. Self tapping screws need to undergo a screw in test, which involves screwing the screws into a test plate to verify whether the hardness of the screws meets the standard,
Drill tail screw
The tail of a screw is generally in the shape of a drill tail. This type of screw has a very strong hardness, and compared to ordinary screws, it not only has better maintenance ability, but also has a very strong effect in connecting objects. For screws with this performance, it generally does not require auxiliary processing and can directly drill a hole on the object to lock it in. Not only is it very convenient to use, but it can also greatly improve work efficiency. This type of drill tail screw can be said to be the preferred choice for workers in various fields
Anti corrosion technology
Stainless steel screws are made of metal, and there are four main methods for metal corrosion prevention, namely the properties of the material itself, the environment used, the interface between data and environment, and improving the design of the metal structure. If a complete corrosion-resistant alloy is used to manufacture stainless steel screws, unless there is a special need, it is not cost-effective from an economic perspective. It is also impractical to completely isolate the surface of the screw from the environmental factors that cause corrosion. Improving the design of metal structures can improve the impact of special situations under certain conditions, but the design of most stainless steel screws cannot be fully corrected, and their maintenance function is not eternal. Therefore, this method cannot basically solve the problem. As long as it is anti-corrosion on the surface, that is, external anti-corrosion treatment is the most widely used method.
The anti-corrosion treatment on the surface of stainless steel screws refers to the use of various methods to apply a maintenance layer on the metal surface. Its function is to isolate the metal from the corrosive environment, suppress the corrosion process, or reduce the contact between corrosive media and the metal surface, in order to avoid or reduce corrosion.
The maintenance layer should be able to meet the following requirements:
1. Corrosion resistance, wear resistance, high hardness
2. Tightly constructed, intact, and with small pores.
3. Strong separation from the base metal and good adhesion.
4. Spread evenly and have a certain thickness.
The maintenance layer is usually divided into two types: metal coating and non-metal coating. Metal coating refers to the maintenance layer formed by using corrosion-resistant metals or alloys on the surface of easily corroded metals. This type of coating is also called coating. There are many methods and varieties for producing metal coatings, among which the most common is electroplating, followed by molten metal immersion (hot dip) and chemical surface treatment. Non metallic coating refers to the use of organic polymer materials such as paint and inorganic materials such as ceramics to form a maintenance layer on the surface of metal equipment or parts. This maintenance layer can completely isolate the base metal from the environmental medium, avoiding the formation of corrosion of the base metal due to contact corrosion with the stainless steel standard component medium.
Moisture-proof measures
Iron screws are prone to rusting in humid environments. To prevent rusting, screws need to be moisture-proof and moisture-proof.
The methods for moisture-proof and moisture-proof screws are as follows: (1) Use solvent-free paint as much as possible when vibrating machinery. (2) It is best to choose impregnating paints without oxidation components, such as epoxy urethane based or unmodified epoxy based impregnating paints. (3) When using melamine acid impregnating paint, the curing temperature and curing time should be adjusted. The curing temperature should be slightly higher than 130 ℃ (such as 135 ℃) and the curing time should be greater than 180 minutes. The process must be strictly followed, especially in high temperature and humid seasons. From the perspective of rust prevention, the drying (curing) time specified in the paint factory's samples may not be sufficient, and the motor has a specific internal shape. (4) Using paint that does not contain volatile acids. (5) Choose paint with good hydrolysis resistance