The choice of bolts in mechanical design

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. The specific meaning of the bolt grade is, for example, a bolt of grade 4.8, and 4 represents tensile strength. Strength, 0.8 represents the ratio of the yield strength divided by the tensile strength, that is, the tensile strength of a grade 4.8 bolt is 400MPa, and its yield strength is 400x0.8u003d320MPa. The same applies to other bolt grades.

High-strength bolts and ordinary bolts

High-strength bolts can also be said to be imported words. The English translation should be high-strength friction pre-tightening bolts. The word friction pre-tightening is omitted. This omission does not matter, which has caused a lot of confusion to many novices. It can be seen from the grade of bolts that the index to measure the performance of bolts is strength, whether it is tensile strength or yield strength, in the final analysis it is the allowable stress of the material. ??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? Some books say that bolts with a performance level greater than or equal to 8.8 are called high-strength bolts. But many ordinary bolts we usually see are also 10.9 grade, 12.9 grade, and the price is also the price of ordinary bolts. We do not consider the performance level of high-strength bolts, we only illustrate this problem from the point of force.

High-strength bolts rely on clamping two pieces and increase the maximum static friction force by pre-tightening. For high-strength bolts, they are only under tension and not subject to shear force. If two objects move relative to each other, the design is invalid and the design is unqualified. This point is very important. This means that the two pieces are rigidly connected, which is the same as when welding. Sliding means welding failure. The established mechanical calculation model is inaccurate. The rigidity means that it is just connected and can transmit torque. 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. (Guide: Types, specifications, uses and methods of use of embedded expansion nuts)

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 and can slide. To exaggerate, they do not need to be tightened. Two objects can slide, and the bolts The only effect is to connect two objects. The two objects can slide, as long as the bolt is pulled continuously.

Summary of the difference between ordinary bolts and high-strength bolts is the strength of the material.

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