Why should high-strength bolts be worn from bottom to top?

塔机上φ16以上的螺栓都是高强螺栓，这是因为塔机各部件的联结都是摩擦型联结或是销轴联结（轴销联结对钢结构制造精度要求很高，一般都是螺栓联结），普通螺栓的强度不够，只有高强螺栓才能满足摩擦型的预紧力要求。所有的螺栓和销轴都不得松动，因为一旦松动，就不能满足摩擦型联结的要求（保证至少70%的接触面），也就是仅靠螺栓自身的强度来把塔机联成一体，传递力矩，不能达到强度要求，且晃动后受力更加复杂和更大，螺栓不仅要承受拉力，还要承受剪切力（螺栓的套管直径比螺栓直径大，有间隙。）。由于塔机工作的特性，塔机上所有结构件和螺栓、销轴都是受的交变力。因此，塔机螺栓的防松都是双螺母防松。其它的防松措施都不能在交变力作用下保持防松效果。明白了这些就可以理解我们以前讨论的《高强螺栓的穿装方向》，从下往上穿。尽管国家没有明文规定，但很多安装单位都有自已的安装规范，规定从下往上穿。

Tower crane φ More than 16 bolts are high-strength bolts, because the connection of all components of the tower crane is friction type connection or pin shaft connection (the shaft pin connection has high requirements on the manufacturing accuracy of steel structures, and is generally bolt connection). The strength of ordinary bolts is not enough, and only high-strength bolts can meet the requirements of friction type preload. All bolts and pins shall not be loosened, because once loosened, they cannot meet the requirements of friction type connection (ensuring at least 70% of the contact surface). That is to say, only the strength of the bolts themselves can connect the tower crane together and transmit torque, which cannot meet the strength requirements. Moreover, after shaking, the stress is more complex and greater. The bolts must bear not only tensile force but also shear force (the diameter of the sleeve of the bolts is larger than the diameter of the bolts, and there is a gap). Due to the working characteristics of the tower crane, all structural members, bolts and pins on the tower crane are subjected to alternating forces. Therefore, double nuts are used to prevent looseness of tower crane bolts. Other anti loosening measures can not maintain the anti loosening effect under the action of alternating force. Once you understand these, you can understand the direction of high-strength bolts we discussed earlier, from bottom to top. Although there are no explicit provisions in the country, many installation units have their own installation specifications, which stipulate that they should be worn from bottom to top.

我们先讨论为什么从上往下穿不好。从上往下安装很方便快速，但易漏上和漏拧螺母。漏上和漏拧螺母和不上螺母的危害区别不大（我们不再展开讨论）。这就是很多安装单位禁止螺栓从上往下穿的原因之一，就是您对孔时从上往下穿，上螺母时也要求卸掉重新从下往上穿再预紧紧固。漏拧螺母的危害太大了。

Let's discuss why it's not good to wear it from top to bottom. It is convenient and fast to install from top to bottom, but it is easy to miss the nuts. There is little difference between the hazards of missing nuts and missing nuts and those of missing nuts (we will not discuss them again). This is one of the reasons why many installation units prohibit bolts from being threaded from top to bottom, that is, when you align holes, you should thread them from top to bottom. When you install nuts, you also need to remove them and thread them from bottom to top again before pre tightening them. The danger of missing nuts is too great.

还有一个原因就是，从上往下穿时，当套管孔没对好时，一敲就硬挤进去了，虽然方便但易伤螺牙和螺杆，也就是这个螺栓应该受伤报废禁止继续使用。而从下往上穿，只有对好孔后才能穿进去，很少会伤到螺牙。

Another reason is that when threading from top to bottom, when the casing hole is not aligned properly, it will be forced in by knocking. Although it is convenient, it is easy to damage the screw thread and screw rod, that is, the bolt should be damaged and scrapped. It is forbidden to continue to use. And from the bottom to the top, it can only be inserted after the holes are aligned, which rarely hurts the screw teeth.