During daily installation of hot-dip galvanized nuts, two common issues occur: difficulty in screwing in, or excessive looseness after screwing. Why does this happen? National standard specifications for hot-dip galvanized nuts clearly state that their fitting smoothness and torque parameters must match those of standard nuts, without deviations caused by galvanization.
First, let's address why galvanized nuts may be hard to screw in. Many enterprises lack proper understanding, assuming that galvanizing a standard nut is merely coating it with zinc. They overlook that standard nut threads are manufactured based on bare steel dimensions. With hot-dip galvanized layers typically ≥35μm thick, the thread inner diameter shrinks by about 70μm after galvanization, inevitably causing jamming. We often receive sample nuts from users for inspection-most cases of unscrewability result from directly galvanizing standard nuts without adjustment, which fundamentally violates assembly requirements.
Next, the problem of loose fitting in galvanized nuts. Users report that while the nut screws in smoothly, it wobbles with audible rattling after tightening, indicating excessive looseness. The causes mainly relate to quality issues:
If suppliers substitute inferior products, the nuts are likely too loose. Hot-dip galvanized nuts require pre-galvanization reaming, with parameters matching the coating thickness (e.g., 0.07mm reaming for a 70μm layer). However, some enterprises arbitrarily increase reaming size for convenience, leading to excessive clearance. In critical applications requiring torque resistance, this practice compromises performance and poses safety risks.
Mismatched design standards also cause looseness. Hot-dip galvanized nuts must pair with galvanized bolts. Using a reamed galvanized nut with an ungalvanized bolt, or misapplying fine-pitch threads in automotive equipment, creates looseness. Even worse, pairing an M10 nut with an M8 bolt inherently causes looseness due to size mismatch.
During installation, while washers are commonly used, adding excessive washers (over 2) is problematic, as it accelerates natural loosening and reduces anti-loosening performance. Additionally, contaminants on the nut or bolt (e.g., residual flux after galvanization) can cause "false tightening"-when debris lodges in the threads and dislodges during vibration, the pre-tightened nut loosens.
