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Analysis Of Flange Bolt Anti-Loosening Measures(Middle)

Feb. 12. 2020

3 Bolt construction and anti-loosening measures under tensile and compression alternating loads

For the bolts in the fourth type of wind power tower that are subjected to alternating tension and compression loads, they also bear fatigue loads. Traditional flanges are thick forged flanges. The advantages of such flanges are fewer welds, long bolts, good fatigue resistance and large flange stiffness. However, this type of flange also has its own disadvantages: the cost is too high, the manufacturing energy consumption is large, the end face needs to be milled, the amount of material is large, and the screw hole deviation is not easy to handle. A large number of imported products still have the problem of loose bolts, which need to be inspected and maintained every year. Moreover, regular maintenance cannot completely guarantee the reliability of the connection. Repeated tightening of high-strength bolts will cause the crystal structure of the thread to change. Due to the increase of the torque coefficient, the torque required by the specification will not reach the pre-tension required by the specification. In order to achieve the pre-tensioning force, the torque is excessively increased so that the bolt is plastically deformed or even fractured under the external force. Once the chemical anchor bolt of the key part fails due to loosening, it may cause huge losses.

Chemical Anchor Bolt

Chemical Anchor Bolt

The analysis of the adhesive bolt supplier believes that there are three reasons for the loosening of the bolts: First, the bolt anti-corrosion method is generally used Tucker Crowe anti-corrosion, but this will cause the bolt torque coefficient to be unqualified. Therefore, molybdenum disulfide is applied to reduce the torque coefficient in the project. However, this will reduce the friction coefficient of the bolt thread by half and reduce the bolt's self-locking ability. Second, this bolt is generally constructed by the torque method. Since the torque method applies a pre-tension to the bolt by tightening, the bolt is twisted and deformed in the process. Torque is stored in the internal part. When the torque is removed after construction is completed, a part of the torsional elastic potential energy is stored in the bolt, that is, the rebound torque. Third, under wind load, the tension on the leeward bolt is reduced, and the pressure on the thread surface is reduced. After the friction torque that prevents the bolt from loosening is less than the rebound torque, the bolt will loosen. When the bolts become loose, the high-strength bolts become ordinary bolts. Under the effect of wind load, the fatigue stress amplitude of the bolt will be significantly increased, and the stress amplitude of the bolt resists the action of all external force bending moments. The tension formula of the high-strength bolt in the tightened state is:

It can be seen that when the area is small, the tensile force of the screw will increase. Traditional thick flange plates do not have full contact on the surface. Edge contact is guaranteed during initial installation. When the tower is bent, after the edge of the flange plate on the tension side is disengaged (Figure 2b), the tensile force of the bolt changes from P to (a + b) P / b. Structural Design Code "5.9.4. Same as ordinary bolts. Therefore, the fatigue stress amplitude increases.

In order to avoid the flange becoming a "flexible flange" and doubling the bolt tension, the flange is constructed into an outer dense and loose inner shape, but this reduces the flange base surface Ac and increases the fatigue stress amplitude . Especially after the bolt is loosened, the fatigue stress amplitude increases. Bolts will break one by one after they have been subjected to fatigue loading for a long time. Therefore, it is very necessary to effectively solve the problem of bolt loosening.