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       Above the lower slewing support of the commonly used 60tm and 80tm tower crane are the upper slewing support, jib, balance arm, tower cap, cab, etc., with a weight of 20 ~ 30t. In the working state of the tower crane, there are also the vertical force, bending moment and torque generated by the dynamic load. Therefore, the bolt connection pair must be tightened in strict accordance with the specified torque, and shall not be too tight or too loose or too tight, It will cause the bolt to work at or near the yield strength for a long time, so that the bolt will fatigue quickly and produce cracks and fracture; Too loose, the relative movement between the bolt and the connecting piece, and the bolt head is damaged by repeated impact load, resulting in serious tower crane accident.

   tower cranes are often subjected to vibration, impact and variable load during operation, and there are seasons with great temperature changes. In this way, the friction between threaded inclined planes in bolt connection may disappear or decrease instantaneously. The repeated occurrence of this phenomenon will gradually loosen the connection and affect the rigidity and tightness of the connection, which may occur on any standard section of tower body, Bolt looseness concentrated in the above parts is considered to be a human factor. For the tower lifting operation of the jack up tower crane, loosen the 4 or 8 connecting bolts before the tower ceiling is lifted. After the jacking is completed, after the introduction of the tower body standard section, the introduced standard section is connected with the lower standard section and the upper rotary lower support by bolts. Generally, the bolts connected with the lower standard section are tightened without negligence and are tightened immediately, For the bolt connection and tightening operation with the upper slewing lower support, it is often not tightened immediately because there is more than one standard section for jacking and the jacking needs to be continued. In this way, it is possible that the connecting bolts of this layer are not tightened after the final jacking. What is found in the inspection is an example.

      At present, there is no good way to prohibit the loosening of high-strength bolts of tower crane, which is also an important link and difficult problem of maintenance. According to the requirements of the operation manual of tower crane, it should be checked and tightened once a week, but the actual situation on the construction site is that it is only checked during maintenance every month, which leads to the loosening of many high-strength bolts. The inspection is time-consuming and laborious. What's more, there are hidden dangers, In 2018, according to statistics, a tower crane collapsed every three days in China. The normal maintenance time of a tower crane is about 4 hours (half a working day) and two maintenance personnel. The inspection and fixing of high-strength bolts in standard section takes about 3 hours. The maintenance time is 3 / 4 of the maintenance time. The salary of maintenance personnel is about 400 yuan a day, that is, 600 yuan is required for maintaining only high-strength bolts of a tower crane. A tower crane is maintained 12 times a year, totaling 7200 yuan, excluding the cost of hidden dangers, Once the tower crane collapses due to the loosening of high-strength bolts, the cost is immeasurable.

  High strength bolt connection pairs are widely used in the steel structure of bridges. Although there have long been relevant technical standards and other documents in the world, which have very detailed provisions on the construction of high-strength bolt and nut, some problems still often occur in the construction of high-strength bolt and nut connection, which seriously affect the quality and safety of construction, so bolt connection pairs are extremely important. When the vehicle crosses the bridge at a certain speed, due to the influence of dynamic force, the bridge actually produces the impact of live load stress. At the same time, the vehicle causes bridge span vibration, the impact of wheels on expansion joints and deformation joints, the torsional vibration of the bridge caused by vertical and horizontal irregularities on the bridge deck, the uneven road surface at the bridge head and the round wheels all produce considerable impact, It puts forward very high requirements for bridge special bolt and nut connection structure, so it is an inevitable choice to use higher quality anti vibration nuts.