What is the tolerance range of precision screws?
What is the tolerance range of precision screws?
Lock nut is a kind of nut widely used in machinery and other industries. Its working principle is self-locking by the friction between the nut and the bolt. However, the reliability of this self-locking is reduced under dynamic loads. In some important occasions, we will take some anti-loosening measures to ensure the reliability of the nut locking.
Material and process requirements The self-tapping locking screw should be made of high-quality carburized steel cold heading, generally no larger than M12. The finished material should be drawn with a high total reduction rate before cold heading, so that the cold heading must be spheroidized. Annealing, in order to enhance the surface hardness and the toughness of the core, most of them are made of medium and low carbon steel. The chemical composition of the material in the standard is only for guidance. The C content is 0.13%~0.37%, the Mn content is 0.64%~1.71%, and the B content can reach 0.005%. The elements of S, P, Mn and Si in the commonly used steel are generally lower than those of the ordinary bolt steel of the same grade. And the surface quality of the material is strictly controlled to reduce deformation resistance and prevent deformation and cracking. Commonly used grades are 20Mn, 15MnB, SWRCH22A, 1022A and medium carbon steel, medium carbon alloy steel SWRCH35K, SCM435, SCR435, etc. Self-tapping locking screws are threads with an arc-shaped triangular cross-section, so the formulation of cold heading and wire rolling processes, as well as the design and manufacture of cold heading dies and special rolling plates are very important. Self-tapping locking screws require a high-hard surface for cutting and extruding capabilities due to the need for self-tapping low carbon steel. At the same time, there must be sufficient core strength and toughness to prevent twisting and breaking during work. The heat treatment process of this type of screw belongs to shallow carburizing. Regardless of whether it is made of low carbon steel or medium carbon or medium carbon alloy steel, its core hardness must be guaranteed to be within the range of 28~38HRC (9.8 grade) 33~39HRC (10.9 grade) and not less than the minimum tensile load of 930MPa (9.8 grade), 1040MPa (grade 10.9), depending on the material, the minimum tempering temperature is 420℃. In order to ensure that the self-tapping locking screw can be smoothly screwed into the prefabricated cylindrical hole, the end of the screw should be hardened by high-frequency quenching to ensure that at least one to three thread teeth are hardened, and the minimum surface hardness is 45HRC.
There is a standard for the outer hexagon bolt standard, which is a grade standard, which is divided into grade 4.8 and grade 8.8. These two levels are used a lot in the market. Especially grade 4.8 hex head bolts. Because it is much cheaper than 8.8 hex bolts. Of course it is more widely used. But for demanding products. Because of its high requirements in various aspects such as hardness. This requires the use of 8.8 hex bolts. Grade 8.8 hex bolts are harder in terms of hardness and screw torque. It is safer to use the product. Tighter and more stable.
Bolts refer specifically to screws with larger diameters and may also have no heads, such as stud bolts. In general, it is not called a stud bolt but a stud stud. The most common form of stud is threaded at both ends and a polished rod in the middle. The most typical use: anchor bolts, or places similar to anchor bolts, thicker connections, when ordinary bolts cannot be used.
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