1
Bolt connection is the most widely used joint connection method in mechanical engineering, structural engineering and other fields, and it is the most basic structural form of fastener connection. It has the advantages of convenient standardized mass production, simple structure, low cost, convenient installation and interchangeability, and is widely used in modern structural engineering. However, with the different bolt types, mechanical properties and use environment, the importance of bolts is different, and the requirements for locking are also different. Accidents of structural damage caused by bolt looseness are common in history, which shows the importance of bolt looseness to ensure the mechanical characteristics of bolts and the overall safety of the structure. Bolt looseness has become a safety hazard of steel structure connection. It is considered that there are many reasons for bolt looseness, such as initial deformation, axial load and lateral load of bolt connection. In engineering practice, many anti-loosening measures are also put forward, such as accurately applying prestress, taking mechanical measures to prevent loosening, using punching points, gluing bolts and nuts, using self-locking anti-loosening nuts and so on. Therefore, when taking anti-loosening measures for bolts, we must fundamentally understand all aspects of bolt performance.
2 Classification of common bolts and current construction and anti-loosening measures
From the stress mechanism of bolts, bolts can be divided into four types: ordinary bolts, high-strength ordinary bolts bearing vibration loads in high-rise structures, high-strength bolts bearing alternating tensile and compressive loads and high-strength bolts. Ordinary bolts generally bear tension or shear directly and are used to connect unimportant structural members. In the past, people thought that bolts only needed to be tightened in construction. However, at present, the relatively accurate "tightening" torque requirement has been given in the Code for Acceptance of Construction Quality of Power System and Tower Mast Steel Structure Engineering. The second type of bolt is often used in the tower flange of high-rise structure, which bears tension and the structure bears certain vibration. The third kind of high-strength bolt is stressed by applying pre-tension to it first, and then generating friction resistance on the contact surface of the connection part to bear shear force or reducing its pre-pressure to bear tension. Generally used in important industrial and civil buildings such as beam components, this kind of bolt is generally constructed by torque method, and the bolt relies on the friction generated by the compression between threads to prevent loosening. The bolt adopts high-strength ordinary bolt, which can be tightened without pre-tightening, and is locked by double nuts. The fourth type of bolt is a bolt that bears alternating tension and pressure. Generally, this kind of bolt bears fatigue load at the same time, needs pre-tightening and has high locking requirements, and is often used for connecting flanges of wind power towers.
When the tightening torque of the first ordinary bolt reaches the specified value in the specification, the bolt generally does not loosen. The anti-loosening effect of high-strength ordinary bolts bearing certain vibration load in high-rise structures is good. For example, the 336-meter-high Heilongjiang TV Tower completed in 2000 found no bolt looseness. The anti-loosening effect of high-strength bolts in bridge structures is also very good. However, the anti-loosening effect of the fourth bolt shows obvious shortcomings. In the field of wind power generation, it has become an important content of regular maintenance of wind farms to check the pre-tightening force of bolts with torque wrench during the operation of wind turbines, which consumes a lot of manpower and material resources.
3 bolt structure and anti-loosening measures under tension and compression alternating load
For the bolts in the fourth type of wind power tower that bear alternating tension and compression loads, they also bear fatigue loads. The traditional flange form is thick forged flange, which has the advantages of few welds, long bolts, good fatigue resistance and high flange stiffness. However, this kind of flange also has its own shortcomings: high cost, high manufacturing energy consumption, need to mill the end face flat, high material consumption and difficult handling of screw hole deviation. A large number of them are imported, but there is still the problem of loose bolts, which need to be tested and maintained every year. Moreover, regular maintenance can not completely guarantee the reliability of the connection. Repeated tightening of high-strength bolts will change the crystal structure of threads, but due to the increase of torque coefficient, the torque specified in the code can not reach the pre-tightening force specified in the code. In order to achieve pre-tightening, the torque is excessively increased, so that the bolt is plastically deformed or even broken under the action of external force. Once the bolts in key parts fail due to looseness, it may cause huge losses.
The analysis shows that there are three reasons for bolt looseness: first, the anti-corrosion method of bolt is generally dacromet, but this will lead to the unqualified torque coefficient of bolt. Therefore, molybdenum disulfide is applied in engineering to reduce the torque coefficient. However, this will halve the friction coefficient of bolt thread and reduce the self-locking ability of bolt; Secondly, this kind of bolt is generally constructed by torque method. Because the torque method applies pre-tightening force to the bolt by tightening, the bolt is twisted and deformed in this process, and the torque is stored in it. When the torque is removed after the construction is completed, a part of torsional elastic potential energy, that is, rebound torque, will be stored in the bolt. Thirdly, under the action of wind load, the tension of leeward bolt decreases and the pressure on the thread surface decreases. When the friction torque to prevent the bolt from loosening is less than the rebound torque, the bolt will loosen. When the bolt becomes loose, the high-strength bolt becomes an ordinary bolt. Under the action of wind load, the fatigue stress amplitude of the bolt will be significantly improved, and the stress amplitude of the bolt will resist all external bending moments. The tension formula of the high-strength bolt in the fastening state is:
Figure 1 bolt stress diagram (bolt locking measures)
It can be seen that when the area is small, the tension of the screw will increase. The surface of the traditional thick flange is not in full contact, and the edge contact is ensured during initial installation (Figure 2a). When the tower is bent, the tension of the bolt changes from p to (a+b)P/b after the flange plate edge on the tension side is separated (Figure 2b). For the stress of flexible flange, see Article 5.9.4 of Code for Design of Tall Buildings (GB50 135-2006). Same as ordinary bolts. Therefore, the fatigue stress amplitude increases.
Fig. 2 Stress diagram of thick flange (bolt locking measures)
In order to prevent the flange from becoming a "flexible flange" and double the tension of the bolt, the flange is constructed in a shape of tight outside and loose inside (as shown in Figure 2a), but this reduces the base surface Ac of the flange, so the fatigue stress amplitude increases. Especially after the bolt is loosened, the fatigue stress amplitude increases. Bolts will break one by one after enduring fatigue load for a long time. Figs. 3 and 5 are photographs after the bolt is broken. Therefore, it is very necessary to effectively solve the problem of bolt looseness.
Fig. 3 Overall collapse diagram of thick flange tower after bolt fracture (bolt loosening prevention measures)
Fig. 4 Bolt fracture on tower platform
Fig. 5 Bolt fracture diagram (bolt locking measures)
4 new construction and anti-loosening measures
For flange bolts subjected to alternating tension and compression loads, the internal cause of bolt looseness must be fundamentally eliminated. At present, the reverse balance flange used on the wind power tower can effectively solve this problem. The connection relationship between reverse balance flange and general rigid flange and stiffening plate is opposite. The stiffener is in front and the flange is behind, so the bolt length can be increased without increasing the thickness of the large flange, which is convenient for the bolt to apply and control the preload.