What is the tolerance range of precision screws?
What is the tolerance range of precision screws?
Now the screw itself is made of metal, so it is easy to rust, especially screws in harsh weather environments that are exposed to the air for a long time, such as air conditioners, wires, etc. In the existing technology, the shape of the screw has been fixed, and the structure of the screw itself is composed of a shaft with twill in the middle and a polygonal structure with stripes on the outside. OK, but because of the installation of some equipment, it needs to be exposed to the air for a long time. For example, the outdoor unit of the air conditioner must be installed in the outdoor area, and the outdoor can not be sheltered from wind and rain, so after long-term sun and rain, the screw The surface is easy to rust. If the rust is serious to a certain extent, the nut will be embroidered to death. Once the external machine needs to be disassembled one day, the embroidered screw will lose its use value and give workers to the workers. The disassembly and assembly of the screw has caused a lot of trouble, because the embroidered screw cannot be unscrewed directly. The only method used is to cut off the shaft in the middle of the screw with a hacksaw, or discard the entire screw structure. Since the shaft in the screw has been inserted into the middle of the wall, this waste is not good for both aesthetics and safety. To solve the rust problem of the screw, the existing technology is to improve the material of the screw, or to maintain it regularly, but this method itself is time-consuming and labor-intensive, and it is a waste of resources.
When the countersunk head screws and hexagon socket head bolts are produced by the cold heading process, the original structure of the steel will directly affect the forming ability of the cold heading process. In the process of cold heading, the plastic deformation of the local area can reach 60%-80%, so the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor to determine the plasticity. It is generally believed that the coarse flaky pearlite is not conducive to cold heading forming, while the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium carbon steel and medium carbon alloy steel with a large amount of high-strength bolts, spheroidizing (softening) annealing is performed before cold heading, so as to obtain uniform and fine spheroidized pearlite to better meet the actual production needs. For the softening annealing of medium carbon steel wire rods, the heating temperature should be kept above and below the critical point of the steel, and the heating temperature should not be too high, otherwise tertiary cementite will precipitate along the grain boundary, resulting in cold heading cracking. The wire rod of medium carbon alloy steel is annealed by isothermal spheroidization. After heating at AC1+ (20-30%), the furnace is cooled to slightly lower than Ar1, the temperature is about 700 degrees Celsius for an isothermal period, and then the furnace is cooled to about 500 degrees Celsius and air-cooled. The metallographic structure of the steel changes from coarse to fine, from flake to spherical, and the cracking rate of cold heading will be greatly reduced. The general area of softening annealing temperature for 35\45\ML35\SWRCH35K steel is 715-735 degrees Celsius; while the general heating temperature for spheroidizing annealing of SCM435\40Cr\SCR435 steel is 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.
The shear strength of a metal material refers to the ability of the material to withstand shearing force, which refers to the strength of the external force perpendicular to the material axis and shearing the material. The test is usually to install the cylindrical pin into the specified fixture, and the gap between the supporting part and the loading part is required to be no more than 0.15mm, and then force is applied to test the shear strength of the material.
In the field of mechanical design, there are a large number of connector connection structures that need to be fastened and positioned. As shown in Figure 1, the more common positioning and fastening connection methods at present are that pins 5 and fasteners are used for positioning and fastening, respectively. That is, cylindrical pins or taper pins are used for positioning, and fasteners such as screws or double-ended studs are used for fastening.
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