Bolt connection is one of the main connection methods of power transmission and transformation steel structure, which has the advantages of simple construction operation, quick installation, high bearing capacity and so on. In the process of engineering application, some people have some misunderstandings about the selection and use of bolts, which have a bad impact on the construction and quality of the project. For this reason, this paper analyzes several common problems of bolt connection, and puts forward some suggestions, so as to help relevant technicians to use bolts correctly in engineering practice.





1 common bolt





In the past ten years, with the development of power transmission and transformation engineering to high voltage and large capacity, the steel structure of power transmission and transformation also to high bearing capacity and large-scale development, the application of high-strength bolts is more and more. In steel structure, bolts are divided into ordinary bolts and high-strength bolts. Ordinary bolts refer to hexagon head bolts, and high-strength bolts refer to high-strength large hexagon head bolts. Ordinary bolts are divided into three product grades A, B and C. grade A has the highest accuracy. Grade A and B are refined bolts, grade C is rough bolts, and the performance grades are 4.6 and 4.8.





Class A and B bolt holes are class I holes with high accuracy requirements. The process of drilling on assembled components, drilling on single parts and components, punching or drilling on single parts and components, and then expanding drilling after assembly is adopted. The hole diameter is equal to the nominal diameter of bolt, the tolerance of bolt outer diameter is negative, and the tolerance of hole diameter is positive. Class C bolt hole is class II hole, which is completed by one-time stamping or drilling without drilling die





The diameter of grade a bolt hole is generally 1-2 mm larger than the bolt diameter. The accuracy of class A and B bolt holes shall be H12 as specified in literature [1], and the allowable deviation requirements of class A, B and C bolt hole diameter are shown in Table 1.





Due to the high precision requirements of Grade A and B bolt holes, the production, fabrication and installation are very complex, and the cost is high, so they are rarely used in steel structures, and they are not used in power transmission and transformation steel structures. Even if grade 6.8, 8.8 or higher performance grade A and B ordinary bolts are grade C bolt holes, the bolts specified in literature [2] and literature [3] are galvanized rough bolts (grade C).





Due to the large gap between the bolt and the hole, the joint of grade C bolt hole will produce large slip and large deformation in the connection of bearing shear. Therefore, the common bolt is mainly used for the connection under tension along the bolt bar axis, the secondary shear connection under static load and indirect dynamic load. For bolt connection of components directly bearing dynamic load, if ordinary bolts are used, double nuts or other effective measures to prevent nuts from loosening shall be adopted [4].