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1. Screw torque requirements: the torque of the outer hexagon screw is relatively larger, the torque of the inner hexagon screw is smaller, and the torque of the cross groove is smaller (therefore, these screws are generally ordinary grade screws).
2. External hexagonal bolts are generally assembled with adjustable spanners, ring wrenches and open-ended wrenches. The assembly efficiency of adjustable wrench is low, but its versatility is very strong. It is suitable for all kinds of head specifications of external hexagonal screws; The box wrench is the most efficient, but it is not applicable in some occasions. A box wrench has only two heads, so it can only be used on the external hexagonal screws with two sizes of heads; An open-ended wrench is similar to a box wrench, but it can be used with an extended socket. The smaller the size of the external hexagonal screw, the higher the requirements for the edges and corners of the external hexagonal bolt, otherwise its head is easy to slip under the force (wrench). In order to save materials, Wenzhou people invented the external hexagonal recess. The socket external hexagonal screw is light in weight, with a thin head, which is easy to slip under stress, and the head may also be screwed off.
3. The socket head screws are assembled with an socket head wrench. The requirements for the hexagonal hole are very high. If the hole is larger, the wrench slips, and if the hole is smaller, the wrench cannot be inserted. The smaller the hexagonal socket size, the higher the requirements for holes. For some large hexagon socket screws, as long as one of the opposite sides of the hexagon socket is qualified, they can be assembled normally with a wrench; For some small socket head screws, such as M2 socket head screws, the socket head screws will slip if the wrench is inserted with a little force, so it is unqualified if one of the opposite sides is too large. Therefore, the wrench of M2, M2.5 and M3 hex socket screws (especially the tightening products) is easy to slip during assembly.
4. The cross recessed screws are assembled with a screwdriver and do not need to use too much force, so the strength is only 4.8 grade. Occasionally, some screws that need high strength can be treated by carburizing heat treatment.
5. In the use of products, we generally recommend that the grade of screws is one level higher than that of nuts, which is the most economic benefit. For example, grade 8.8 screws are matched with grade 4 nuts; In this way, it is only necessary to replace the nut next time.
2、 Heat treatment:
Heat treatment is mainly for carbon steel screws, mainly quenching and tempering heat treatment and carburizing heat treatment, to meet the requirements of different environments on screw strength.
1. Quenching and tempering heat treatment: products of strength grade 8.8 and above are all products of quenching and tempering heat treatment. The characteristic of this heat treatment is that the hardness is relatively uniform inside and outside. When the same material is heat treated, the higher the hardness, the worse the toughness. Therefore, it is necessary to have a safe matching. While meeting the hardness, the toughness is also guaranteed.
2. Carburization heat treatment: self tapping screws basically need this treatment, which is characterized by very hard surface and soft core; Because we need to attack the hard iron plate. The risk of self tapping screws is high. For example, self tapping screws often encounter broken ends. The possible reasons are: ① hydrogen embrittlement; ② Too high or too low hardness leads to torsion fracture; ③ The cross groove is too deep; ④ The head is too thin; ⑤ There is no R-angle at the head neck junction leading to stress concentration; ⑥ Nonstandard operation; Etc.
3、 Hydrogen embrittlement risk:
1. Generally, for products with hardness>32hrc, there is a risk of hydrogen embrittlement in electroplating. Therefore, all products above grade 10.9 (including grade 10.9) and products after carburizing heat treatment (self tapping screws) will have the risk of hydrogen embrittlement during electroplating.
2. Hydrogen embrittlement means that when the product is electroplated, h+enters the metal to form bubbles, which causes the screws to break continuously on the spot during use, but delayed fracture occurs within 24 hours.
3. Products with risk of hydrogen embrittlement shall be delivered to the hydrogen removal furnace within 4 hours after electroplating and stored at 200 ℃ for about 8 hours; This is called dehydrogenation.
4. Hydrogen embrittlement treatment can significantly reduce the risk of hydrogen embrittlement, but it can not be completely avoided. Therefore, when it is necessary to ensure that there is no risk of hydrogen embrittlement, electroplating products are strictly prohibited; Instead, use Dacromet, sand blasting and other surface treatment processes.
4、 Fastener performance and processing technology development direction:
1. High strength while ensuring toughness;
2. The same size can reduce the weight;
3. The volume can be reduced when the mechanical strength can be guaranteed;
4. The toughness can be strengthened under the condition of guaranteed strength;
5. The anti-corrosion ability can be enhanced when the appearance meets the requirements;
6. Improvement of accuracy limit;
In terms of any of the above existing processes with high processing costs, new processes with relatively low costs that meet the requirements will open up a new atmosphere in our fastener industry.
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