- Recently, they have been promoting the spirit of craftsmen, reporting how rigorous the method of tightening bolts by German mechanics, and how to compare the methods of tightening bolts by Chinese workers. Strictly speaking, in the assembly process of bolts, the tightening method of bolts has been specified at design time. And even what tools are defined, about the screw bolt, what method is used to screw, tighten the process that needs to be followed, not the assembly workers themselves, but the designer defines, what tools to use , all have standard specifications.The question is coming? How to fix a simple operation of screw bolts, is the designer really okay looking for something? In fact, the bolt is not as simple as imagined. The bolt is screwed to obtain an initial pre-tightening force. What effect does this preload have?Why do you want the bolt to get an initial preload? This starts with the role of the bolted joint.1. Transmitting lateral force or torque, the bolt pre-tightening force causes pressure between the contact surfaces of the connected parts, and the greater the pressure between the contact surfaces, the greater the lateral force that can be transmitted. When the contact surface pressure is large enough, the lateral force is transmitted by the frictional force. The bolt only bears the pre-tightening force and is no longer subjected to the shear stress and compressive stress. The safety of the bolt is increased a lot.
2. Sealing function, when the bolt is connected to the pressure pipe or the pressure vessel, the bolt needs to have sufficient pre-tightening force to seal, and the total bolt pre-tightening force cannot be less than the pressure of the fluid between the contact faces.
3. Prevent structural joints. Generally, bolted joints subjected to fatigue loads are not allowed to be seamed by joints. Once gaps are formed between joints, bolt tension will increase rapidly with the increase of external tensile load. For the graph of bolt tension and external load, we can see that in the first half of the curve, there is no slit between the connected parts, the bolt tension increases slowly, and the bolt pull force increases rapidly after the joint is opened.
After the joint is opened, the amplitude of the alternating fatigue load of the bolt will increase rapidly, which will greatly shorten the fatigue strength of the bolt.
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