In daily installation and use of hot-dip galvanized nuts, we often encounter two situations: one is that the nut is difficult to screw in, and the other is that it is too loose after being screwed in. Why does this happen? Normative documents on hot-dip galvanized nuts in national standards (GB) clearly stipulate that the fit smoothness and torque parameters of hot-dip galvanized nuts during assembly must be consistent with those of standard nuts, and no additional deviations should occur due to hot-dip galvanizing treatment.
First, let's explain why hot-dip galvanized nuts are difficult to screw in. Many enterprises do not fully understand the production logic of hot-dip galvanized nuts and mistakenly believe that they only need to plate a layer of zinc on ordinary national standard nuts. However, the thread size of ordinary national standard nuts is designed for the uncoated state, while hot-dip galvanized coatings have a certain thickness (usually complying with requirements of national standards such as GB/T 30790). If ordinary national standard nuts are directly subjected to hot-dip galvanizing, the zinc coating will fill the thread gaps, resulting in excessively tight thread fit and natural difficulty in screwing in. We often receive test samples sent by users, and inspections show that most of these hard-to-screw-in hot-dip galvanized nuts are delivered directly after galvanizing ordinary national standard nuts. Generally, the core reason why national standard hot-dip galvanized nuts cannot be screwed in is this.
Next, let's analyze in detail the looseness of national standard hot-dip galvanized nuts. Many users mention in consultations that hot-dip galvanized nuts are easy to screw in, but after tightening, there is an obvious loose sound when shaken by hand, and the nut can easily move on the bolt thread-this is a typical case of excessive fit looseness. This situation is mostly caused by poor quality control or improper use, with specific reasons as follows:
Firstly, suppliers pass off inferior products as high-quality ones and illegally expand the pre-expansion drilling size. Hot-dip galvanized nuts require pre-expansion drilling before galvanizing. The drilling parameters must be accurately calculated to ensure that the thickness of the zinc coating after galvanizing can just compensate for the expansion amount, thereby guaranteeing the fit accuracy with bolts. However, some enterprises, to save trouble or due to the lack of precise parameter control capabilities, arbitrarily increase the pre-expansion drilling size of nuts, mistakenly believing that "being able to screw in is better than not being able to". In important occasions requiring fastening strength, such "overly loose" nuts cannot bear the torque strength required by the design, posing serious potential safety hazards for later use.
Secondly, improper selection of product standards or specifications. Hot-dip galvanized nuts are originally designed to be used with hot-dip galvanized bolts. If hot-dip galvanized nuts with expanded pre-drilling are matched with ordinary ungalvanized bolts, looseness will inevitably occur. Especially in scenarios such as automotive equipment installation, some special automotive bolts adopt fine thread specifications with small thread pitches, and this mismatched fit will exacerbate the looseness problem. In addition, confusion in specifications can also cause looseness-for example, using an M10 national standard hot-dip galvanized nut with an M8 hot-dip galvanized bolt will definitely result in severe looseness.
Thirdly, improper installation methods. When installing hot-dip galvanized bolts and nuts, washers are usually used together, which is a common way to ensure connection stability. However, if the selected hot-dip galvanized bolt is too long, and additional washers are randomly added for convenience, this practice is undesirable. After long-term use, too many washers will lead to preload dispersion and reduced stability, eventually causing the nut to loosen naturally and the anti-loosening effect to be greatly reduced. In addition, if there is residual dirt or metal debris on the thread surfaces of hot-dip galvanized nuts or bolts that is not cleaned, during the mutual rotation and installation, it may be mistakenly considered tightened due to the blockage of dirt, but the designed preload is not actually achieved. In subsequent use, after the dirt falls off or shifts, the nut that "seemed tightened" will become loose.
