Basic knowledge of fastener products (specification section)

The parameters that need to be indicated for fasteners include: product name (standard), specifications, material, strength grade, and surface treatment. For example: DIN912, M4-0.7x8, SCM435, 12.9 grade, blackened.

1. Product Name (Standard)

Note: For screws without standards, which are non-standard parts, drawings are required.

For example, DIN912, Chinese name: Hexagon socket head screw, this is the product name.

However, the most accurate approach is to refer to the standard, as GB70 also has the same product name; But there are many places where the dimensions of the two standards are not consistent.

The most influential standards in the world are: German Standards (DIN), International Standards (ISO), Chinese National Standards (GB), American Standards (ANSI), and Japanese Standards (JIS).

2. Specifications

Generally, screws are referred to as the diameter of the tooth pattern multiplied by the length of the screw.

The commonly used metric diameters for tooth patterns include M2, M3, M4, M5, M6, M8, M10, M12, etc;

The commonly used ones in the American system include: 4 # -40, 6 # -32, 8 # -32, 10 # -24, 1/4-20, 5/16-18, 3/8-16, 1/2-13, etc.

The length of the screw refers to the effective length of the embedded object. For example, countersunk screws are loaded with the total length, semi countersunk screws need to add the length of half the head, while cylindrical screws do not include the head size in their length. For example:

For specifications, it is best to include the tooth spacing in the full name. For example, M4-0.7x8, the outer diameter of 4 teeth is 4mm, 0.7 refers to the distance between two tooth peaks of 0.7mm, and 8 refers to the effective length of 8mm embedded in the loaded object.

For the sake of simplicity, we do not write the tooth spacing, and assume that coarse teeth are standard teeth because they are the most common; So there's no need to label it. This is only available in the metric system, but American products still need to be marked with tooth spacing. Here we will focus on the specifications of American screws, such as 6 # -32 * 3/8. 6 # is the outer diameter of the tooth, which is close to 3.5mm; 32 is 32 teeth per inch of thread length (equivalent to the pitch of a metric screw); 3/8 is the length of the screw (same as metric screws).

Here are two formulas to remember: tooth outer diameter A #=(Ax0.013+0.06) x25.4 (mm), 1 inch=25.4mm.

Among them, 2 #=2.2mm, 4 #=2.9mm, 6 #=3.5mm, 8 # 4.2mm, and 10 #=4.8mm are data that need to be memorized. The number of teeth corresponding to each specification of screws should also be memorized: 2 # -56, 4 # -40, 6 # -32, 8 # -32, 10 # -24, 1/4-20, 5/16-18, 3/8-16, 1/2-13 (American standard teeth).

Note: UNC teeth in the American system are standard teeth, while NDF teeth are fine teeth. We default to coarse teeth as standard teeth.

3. Material

The most common materials are carbon steel, stainless steel, stainless iron, copper, aluminum, etc. Carbon steel is divided into low carbon steel (such as C1008/C1010/C1015/C1018/C1022), medium carbon steel (such as C1035), high carbon steel (C1045/C1050), and alloy steel (SCM435/10B21/40Cr).

Generally, C1008 materials are made of ordinary grade products, such as 4.8 grade screws and ordinary grade nuts; C1015 usually uses hanging ring screws; C1018 is generally used for making machine screws, but there are also those used for making self tapping screws; C1022 is generally used to make self tapping nails; C1035 with 8.8 grade screws; Use grade 10.9 screws on C1045/10B21/40Cr; Use 12.9 grade screws on 40Cr/SCM435.

Stainless steel has SS302/SS304/SS316 as the most common. Of course, a large number of SS201 products are also popular now, even products with lower nickel content, which we call non authentic stainless steel products; The appearance looks similar to stainless steel, but the anti-corrosion performance is much different.

4. Strength level

The strength grade mainly refers to carbon steel fasteners.

The common strength grades for carbon steel screws are 4.8, 5.8, 6.8, 8.8, 10.9, and 12.9. The corresponding nuts are: Grade 4, Grade 6, Grade 8, Grade 10, Grade 12.

Generally, screws below grade 8.8 are called ordinary screws, while screws above grade 8.8 (including grade 8.8) are considered high-strength screws. The difference is that high-strength screws require quenching and tempering heat treatment.

5. Surface treatment

Surface treatment is mainly used to increase anti-corrosion performance, and some also take into account color, so it is mainly for carbon steel products, which generally require surface treatment.

Common surface treatments include blackening, galvanizing, copper plating, nickel plating, chrome plating, silver plating, gold plating, Dacromet, hot-dip galvanizing, etc;

There are many types of galvanizing, including blue and white zinc, blue zinc, white zinc, yellow zinc, black zinc, green zinc, etc., and they are also divided into environmentally friendly and non environmentally friendly types. Each type of galvanizing has multiple coating thicknesses to meet different salt spray testing effects.

Basic knowledge of fastener products (functional section)

1、 In terms of functionality:

1. Screw torque requirements: Outer hexagon screws can withstand relatively greater torque, inner hexagon screws can withstand less torque, and cross groove screws can withstand less torque (therefore, these types of screws are generally ordinary grade screws).

2. The assembly of external hexagonal bolts is generally carried out using flexible wrenches, ring wrenches, and open-end wrenches. The assembly efficiency of the flexible wrench is low, but it has strong versatility and is suitable for various head specifications of hex screws; The plum blossom wrench has the highest efficiency, but it is not suitable in some situations. A plum blossom wrench only has 2 heads, so it can only be used on two types of hex screws with different head sizes; An open-end wrench is similar to a ring wrench, but it can be used with an extended socket.

The smaller the specification of the external hexagonal screw, the higher the requirement for the edges of the external hexagonal bolt, otherwise its head is prone to slipping when subjected to (wrench) force.

Wenzhou people invented the hexagonal concave cavity to save materials. The hexagonal screw outside the concave hole is lightweight, with a thin head thickness that is prone to slipping under stress, and the head may also be unscrewed.

3. Use an Allen wrench to assemble the hex screws. The requirements for this pair of hexagonal holes are very high, with larger holes causing the wrench to slip and smaller holes preventing the wrench from inserting. The smaller the hexagonal specification, the higher the requirements for the hole. For some large-sized hexagon socket screws, as long as one of the opposite sides of the hexagon socket is qualified, it can be assembled normally with a wrench; For some small-sized hex screws, such as the M2 hex fastener, if the wrench is inserted with a little force, the hex will slip, so even if one of the opposite edges is slightly larger, it is not qualified. Therefore, the internal hexagonal screws of M2, M2.5, and M3 (especially tightening products) are prone to slipping during assembly with wrenches.

4. Cross groove screws are assembled with a screwdriver and do not require too much force, so the strength only needs to be 4.8 level. Occasionally, some screws that require high strength only need to undergo carburizing heat treatment.

5. In product matching use, we generally recommend using screws with a grade one level higher than nuts, which is the most cost-effective. For example, grade 8.8 screws are paired with grade 4 nuts; In this way, the next time you change it, just replace the nut.

2、 In terms of heat treatment:

Heat treatment is mainly aimed at carbon steel screws, including quenching and tempering heat treatment and carburizing heat treatment, to meet the strength requirements of screws in different environments.

1. Quenching and tempering heat treatment: Products with a strength grade of 8.8 or above are all products that have undergone quenching and tempering heat treatment. The characteristic of this heat treatment is that the hardness is relatively uniform both inside and outside.

When the same material is subjected to heat treatment, the higher the hardness, the poorer the toughness. So there needs to be a safe match that ensures both hardness and toughness.

2. Carburizing heat treatment: Self tapping screws generally require this treatment, characterized by a very hard surface and a relatively soft core; Because it requires attacking a hard iron plate.

Self tapping nails have a higher risk, for example, self tapping nails often encounter severed heads, which may be due to: ① hydrogen embrittlement; ② Too high or too low hardness may cause twisting and breakage; ③ The cross groove is too deep; ④ The head is too thin; ⑤ Lack of R-angle at the head neck junction leads to stress concentration; ⑥ Improper operation, etc.

3、 In terms of hydrogen embrittlement risk:

1. In general, products with a hardness greater than 32HRC have a risk of hydrogen embrittlement during electroplating. So all products above grade 10.9 (including grade 10.9) and products that have undergone carburizing heat treatment (self tapping nails) will face the risk of hydrogen embrittlement during electroplating.

2. Hydrogen embrittlement refers to the phenomenon where H+ions enter the metal during electroplating, forming bubbles that cause the screw to break continuously on the spot but within 24 hours of use.

3. Products with a risk of hydrogen embrittlement need to be sent to a dehydrogenation furnace within 4 hours after electroplating, and stored at 200 degrees Celsius for about 8 hours; This is called hydrogen treatment.

4. The hydrogen embrittlement treatment method can significantly reduce the risk of hydrogen embrittlement, but cannot completely avoid it. Therefore, when it is necessary to ensure 100% absence of hydrogen embrittlement risk, the use of electroplated products is strictly prohibited; Instead, use surface treatment processes such as Dacromet and sandblasting.

4、 Development direction of fastener performance and processing technology:

1. High strength while ensuring toughness;

2. The same size can reduce weight;

3. Can reduce the volume while ensuring mechanical strength;

4. Under the condition of ensuring strength, it can enhance toughness;

5. When the appearance meets the requirements, it can enhance the anti-corrosion ability;

6. Improvement in accuracy limit;

Any new technology that can meet the requirements at a relatively low cost in any of the existing high processing cost processes mentioned above will open up a new atmosphere in our fastener industry.