Last Updated on April 25, 2021 by Peter Hollens
Bolts are the most used parts in automobiles, and all the mechanical parts of automobiles need bolts. We often see the words 4.8\8.8\12.9 on automobile bolts. In fact, these are the marks of bolt strength grades. The strength grades of bolts refer to the performance grades of bolts used in steel structure connections. Commonly they are grades 4.8, 8.8, 10.9, There are more than 10 levels including 12.9 levels. Among them, bolts of grade 8.8 and above (including grade 8.8) are made of low-carbon alloy steel or medium-carbon steel and are heat-treated (quenched and tempered), collectively referred to as high-strength bolts, and the rest are collectively referred to as ordinary bolts.
The tensile strength of high-strength bolts is generally above 700MPa because they can be mass-produced, the quality is stable, there are sufficient strength and toughness, the material cost is relatively cheap, economically reasonable, and widely used.
Detour direction of the thread
The winding direction of the thread is divided into two types: left-hand thread and right-hand thread. Most of the threads are right-handed threads, and a few special parts use left-handed threads.
Tightening is to connect the parts with bolts and nuts, which cannot be loosened. In order to prevent it from loosening, an appropriate pulling force called “axial force” must be applied to the bolt (tightening force or pre-tightening force). During the tightening process, the initial pre-tightening force that it bears depends on the tightening method and conditions.
When the bolts are tightened, the mating contact surfaces rub and slide against each other under high-stress conditions. This usually brings a certain degree of damage to the nut end surface, the surface of the clamped object, and the tread contact surface. When the bolt is tightened again, this kind of damage will become worse, in the usual sense, it will lead to an increase in the coefficient of friction. Because of the increase in the coefficient of friction, for a certain torque, the resulting bolt axial force will decrease.
For the first tightening, electro-galvanized threads will bring a relatively high coefficient of friction (between 0.2 and 0.3). As discussed earlier, wear conditions will affect friction, that is, more severe wear will result in a higher coefficient of friction. When the surface is worn, the coefficient of friction will increase. In a series of experiments, we found that the friction coefficient of the bolt after ten repeated tightenings has doubled the friction coefficient of the initial tightening. The most direct effect is that under the condition of a fixed torque setting, the bolt axial force is only half of the original.
Reducing the coefficient of friction means reducing the wear rate. When bolts are used repeatedly, it is highly recommended to determine the torque value of each tightening by means of experiments. By increasing lubrication, the coefficient of friction can be reduced, and a relatively stable torsion relationship can be obtained. However, even if lubricant is used, wear cannot be completely avoided. It is still recommended to determine the torque value for each user through experiments.