Selection Of Bolts in Mechanical Design
Jun 21, 2023
The performance grades of bolts for steel structure connections are divided into more than 10 grades such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Strength, 0.8 represents the ratio of yield strength divided by tensile strength, that is, the tensile strength of a grade 4.8 bolt is 400MPa, and its yield strength is 400x0.8=320MPa. In the same way, other bolt grades also mean this.
High-strength bolts and ordinary bolts
High-strength bolts can also be said to be an imported word. The English translation should be high-strength friction pre-tightening bolts. The word friction pre-tightening is omitted. This omission does not matter, and it has caused great confusion to many novices. It can be seen from the grade of bolts that the index to measure the performance of bolts is a strength, whether it is tensile strength or yield strength, in the final analysis is the allowable stress of the material. Some books say that bolts with performance grades greater than or equal to 8.8 are called high-strength bolts. But many ordinary bolts that we usually see also have grade 10.9 and grade 12.9, and the price is also the price of ordinary bolts.
Let's not consider the performance level of high-strength bolts. Let's explain this problem from the point of stress. High-strength bolts rely on clamping two parts, and rely on pre-tightening force to increase the maximum static friction force. For high-strength bolts, it is only In tension, not in shear.
If two objects move relative to each other, it means that the design is invalid and the design is unqualified. This point is very important. It means that the two parts are rigidly connected, which is the same as welding. If it slides, it means that the welding fails. The established mechanical calculation model is not accurate. This is the main point for designers to design with high-strength bolts, and it is also the essential difference between ordinary bolts and high-strength bolts.
The 8.8 grade of ordinary bolts only represents high strength.
Ordinary bolts rely on their own strength to resist shearing force. From the perspective of mechanical analysis, ordinary bolts are hinges in mechanical calculations, which can slide. To exaggerate, two objects can slide without tightening. The role played by bolts It is just the function of connecting two objects, and the two objects can slide, as long as the bolt is pulled continuously.
To sum up, the difference between ordinary bolts and high-strength bolts is the strength of the material.
