Dapeng Town Industrial Park, Tongshan District, Xuzhou City, Jiangsu Province, China
(1)Appearance inspection
Bolt inspection
First check the appearance of high-strength bolts. The bolt surface should be smooth and free of defects such as cracks, burrs, and dents. For large hexagonal high-strength bolts, the shape of the hexagonal head should be regular and the dimensions of each side should meet the standard, so as to ensure that the wrench can apply torque normally. The plum head of the torsion shear type high-strength bolt should also not be damaged, because the plum head plays an important role in torque control during installation.
Nut and washer inspection
The thread of the nut should be complete and fit tightly with the bolt. The surface flatness and thickness of the washer should meet the requirements. Its function is to disperse the pressure of the bolt head or nut and prevent the surface of the connected part from being crushed. If the washer is warped, it will affect the transmission of the bolt’s preload.
(2) Mechanical property inspection
Torque coefficient inspection (large hexagonal high-strength bolts)
The torque coefficient is a key parameter of large hexagonal high-strength bolts. It is determined by a special torque coefficient test. During the test, the bolt, nut and washer are assembled, and the required torque value is measured under a certain pre-tension to calculate the torque coefficient. This coefficient will vary depending on factors such as the surface treatment of the bolt and the thread accuracy. In actual installation, the final tightening torque should be calculated based on the torque coefficient that has passed the inspection.
Pre-tension test (torsion shear type high-strength bolt)
Torsion shear type high-strength bolts mainly test their pre-tension. A certain number of sampled bolts are pre-tensioned using a special tension testing machine to ensure that their pre-tension meets the design requirements. Because the pre-tension of the torsion shear bolt is controlled by cutting the plum head, the accuracy of the pre-tension is directly related to the reliability of the connection.
(1) Over-tightening
Cause analysis
Over-tightening mainly occurs when the torque applied during the tightening of the bolt exceeds the design requirements. This may be due to improper operation by the construction personnel, such as using an inaccurate torque wrench or not operating according to the correct torque value. In addition, if the friction force of the bolt thread is abnormally small (for example, the thread surface is too smooth), it may also cause over-tightening when normal torque is applied.
Hazards
Over-tightening will cause the pre-tension of high-strength bolts to be too large. For large hexagonal head bolts, it may cause the bolt to over-elongate or even break. For torsion shear type bolts, over-tightening may cause the bolt to bear too high a pre-tension after the hexagonal head is sheared off. This will cause excessive pressure on the connected parts, which may cause deformation of the connected parts, and will also reduce the fatigue life of the bolts and increase safety hazards during the use of the structure.
(2) Under-tightening
Cause Analysis
Under-tightening is generally caused by not reaching the specified torque or angle. It may be that the torque wrench is not calibrated, the actual output torque is less than the displayed value, or all tightening steps are not completed during the tightening process (such as not performing final tightening after initial tightening). In addition, the presence of oil, impurities, etc. on the bolt thread increases the friction of the thread, making it impossible to achieve the required pre-tension under normal torque, which will also cause under-tightening.
Hazards
Under-tightening will cause insufficient pre-tension of high-strength bolts. In this way, when the structure is under load, the bolts cannot effectively transmit tension or shear force, which can easily lead to loosening of the connection. Under dynamic load, the under-tightened bolt connection will gradually loosen and eventually lead to connection failure, which will damage the integrity and stability of the entire steel structure.
