Screw fasteners are hardware products with high utilization rate in daily life. There are screws on almost every equipment. The following will explain how to use screw fasteners correctly.
Screw specifications are: product name (specification), raw material, strength grade, standard and appearance treatment.
1. In terms of commodity types, the torque of external hexagon screw is relatively larger, that of inner hexagon screw is smaller, and that of cross groove is smaller. In the commodity matching application, we usually advocate that the grade of screw is higher than that of nut, so it has economic benefits.
2. Commodity raw material grade, here we mainly talk about the carbon steel commonly used. According to the concavoconvex of carbon content, we can divide it into: c1008 (corresponding to grade 4.8), c1035 (corresponding to grade 8.8), c1045 (corresponding to grade 10.9) and SCM435 (corresponding to grade 12.9 and 45h). The higher the carbon content, the harder the raw material is. Grade 8.8 and above are high strength screws.
3. As for the standard, for example, M4 × 8, the outer diameter of 4 finger teeth is 4mm, and the useful length of 8-finger embedded object is 8mm. Generally, the countersunk screw is loaded with the total length, and the half head length is added to the half countersunk screw. The pan head screw cross does not include the head
4. When the same material is heat treated, the higher the hardness, the worse the resistance. Heat treatment is required for electroplating of grade 8.8 and above. There are two kinds of heat treatment we do now: high strength screw needs quenching and tempering heat treatment, that is, the hardness of the screw is uniform from the inside to the outside; the self tapping screw needs carburizing heat treatment, that is, the surface of the screw is cemented with a layer of carbon, which is very hard, but the inside is very soft. If carburized to the inside, the screw will be scorched.
5. Generally, the hardness is more than 32hrc, and the electroplating has the risk of hydrogen embrittlement. Hydrogen embrittlement means that H + enters into the metal to form bubbles when acid pickling (oil stain) is carried out. The screw is used continuously on the spot, and cracks are not made until 24 hours later. Therefore, this kind of commodity with hydrogen embrittlement risk needs dehydrogenation treatment after electroplating.