What Kind of Bolts and Nuts Should Be Selected for Different Load Conditions?
Under different load conditions, flange bolts and nuts should be selected in the order of "determining the bearing capacity first, and then judging the fatigue and anti-loosening requirements". Generally, 8.8 grade flat bolts and nuts can be used for static load connections. A larger load can be considered at level 10.9. The vibration environment can be equipped with toothed flange bolts or reliable anti-loosening structures. For impact and alternating loads, it is necessary to focus on verifying the pretension force, fatigue strength, and stiffness of the connecting components.
The function of the bolts and nuts surface is to expand the head support area, disperse contact pressure, and reduce the need for additional balancing washers in some structures, but it does not automatically increase the tensile or shear strength of the screw body.
How to choose the static load condition?
If the load is stable, the selection of ordinary equipment brackets, enclosures, and sheet metal connections can usually start with 8.8 grade flat bolts and nuts. If the connected component is relatively soft, check whether the flange surface will be compressed. Thin plate structures should also avoid local deformation.
How to choose vibration working conditions?
When engine accessories, construction machinery, and conveyor equipment experience long-term vibration, toothed flange bolts can be considered. Embedding the tooth surface into the contact surface can increase the resistance to rotation, but it may scratch the coating and is not suitable for positions with high requirements for appearance, sealing surface, or insulation layer. Key connections should be matched with all metal anti-loosening nuts and other structures, and cannot rely solely on flange teeth.
How to choose between impact and alternating load?
The impact load not only tests the tensile strength, but also the toughness of the material. Alternating loads can easily form fatigue cracks at the root of the thread and the transition between the head and shaft. Such working conditions should strive to maintain sufficient and stable pre tightening force to reduce stress fluctuations of bolts and nuts directly caused by external loads. Grade 10.9 is not always superior to Grade 8.8 in any situation, and ultimately needs to be determined based on load calculations and connection structures.
ISO 898-1 specifies the mechanical properties of carbon steel and alloy steel bolts, but its grade data is based on ambient temperature testing, and the standard itself does not directly specify fatigue, shear, and torque clamping performance. Therefore, it is not enough to judge the security of a connection solely based on a "10.9 level" rating.
How to match the material and surface treatment?
Galvanized carbon steel bolts and nuts can be used in general indoor environments. Zinc aluminum coating, Dacromet coating, or suitable grades of stainless steel can be considered for outdoor and salt spray environments. When using electroplating technology for high-strength bolts and nuts, attention should also be paid to the risk of hydrogen embrittlement.
The toothed bolts and nuts surface will damage some of the surface coating, and the protection of the tooth contact position needs to be evaluated in corrosive environments.
When assisting in the selection of bolts and nuts, Baike Hardware usually confirms the load type, thread specification, strength grade, flange face form, installation torque, and usage environment at the same time, avoiding purchasing only based on appearance or a single grade. The small series of flange bolts covered by ISO 4162 include categories such as M5 to M16, 8.8, 10.9, and A2-70, which can be used as standardized selection references.
