How bolts are divided into strength grades:
The performance grades of steel structure connecting bolts are divided into more than 10 grades, such as 3.6, 4.0, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9. A specific meaning of a bolt grade is, for example, a bolt of grade 4.8 representing tensile strength. 0.8 represents the ratio of the strength divided by the tensile strength, that is, the tensile strength of a 4.8-grade bolt is 400MPa, and the strength is 400×0.8=320MPa. The same is true for other bolt grades.
High strength bolts and ordinary bolts:
High-strength bolts can also be said to be an English word. The English translation should be high-strength friction preload bolts. The term frictional preload is omitted. It can be seen that the index to measure the performance of the bolt 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 rating greater than or equal to 8.8 are called high-strength bolts, but many ordinary bolts we usually see are also 10.9 and 12.9, and the price is also the price of ordinary bolts. We do not consider the performance level of high-strength bolts, and only illustrate the problem from the point of view of force.
High-strength bolts rely on clamping two parts to increase maximum static friction through preload. For high-strength bolts, they are only subjected to tensile forces, not shearing forces. If the two objects move relative to each other, the design doesn't work. Failed design. this point is very important. This means that the two parts are firmly connected, the same as soldering. If they slip, the weld fails. The established mechanical calculation model is inaccurate. Stiffness means just connecting and able to transmit torque. This is the main point for designers to design high-strength bolts, and it is also the essential difference between ordinary bolts and high-strength bolts.
Grade 8.8 ordinary bolts only represent high strength.
Ordinary bolts rely on their own strength to resist shearing forces. From a mechanical analysis point of view, ordinary bolts are hinges in mechanical calculations and can slide. It's an exaggeration to say that two objects may slide without tightening. The purpose of a bolt is simply to connect two objects. As long as the bolt is pulled continuously, the two objects can slide.
In summary, the difference between ordinary bolts and high-strength bolts is the strength of the material.
Post time: Sep-27-2022