For every fastener bolt with external threads, there is a corresponding nut fastener with internal threads-this is a standard matching pattern. A single bolt or a single nut alone cannot serve any fastening purpose.
Bolt standards are relatively easy to understand. Hexagon head bolts are the most common and widely used; as long as the material specifications are clarified, mistakes are rarely made. However, nut standards are far more complex than those for bolts. Their numerous subcategories are evident just from their names: GB (Guobiao) nuts, ANSI (American) nuts, JIS (Japanese) nuts, BS (British) nuts, fine-thread nuts, coarse-thread nuts, lock nuts, square nuts, hexagon nuts, zinc-plated nuts, hot-dip galvanized nuts, nylon nuts, weld nuts, etc. It is impossible to list them all at once. If further subdivided by material, the categories become even more extensive.
Specifications and Classification Features of Hot-Dip Galvanized Nuts
A key advantage of nuts is the simplicity of their specification description. For example, an 8mm metric nut only needs to be marked as "M8". For imperial nuts, fractions are used (e.g., "3/8"), and only the internal thread diameter needs to be indicated. Each nut specification has a corresponding thread pitch standard, which differs from bolts: fine-thread bolt specifications are rarely used, and some companies may not use fine-thread bolts for decades. However, fine-thread nuts are quite common. Nevertheless, fine-thread nuts are generally not suitable for hot-dip galvanizing (the thick hot-dip galvanized coating tends to cause jamming in fine-threaded connections), so fine-thread specifications are rarely supplied as standard hot-dip galvanized nuts.
Materials and Strength Grades of Hot-Dip Galvanized Nuts
Common materials and grades of hot-dip galvanized nuts are as follows:
Grade 4 hot-dip galvanized nuts: Made of Q235 steel (formerly known as A3 steel), they require no heat treatment and can be used directly after hot-dip galvanizing production. This is the lowest strength grade.
Grade 8 hot-dip galvanized nuts: Made of 35 steel, they must undergo heat treatment (quenching and tempering) first, followed by hot-dip galvanizing.
It should be noted that Grade 10 nuts are classified as high-strength nuts, and high-strength nuts are generally not suitable for hot-dip galvanizing. The hot-dip galvanizing process may cause hydrogen embrittlement, which reduces their high-strength performance and affects safety in use.
Practical Application and Matching Principles of Hot-Dip Galvanized Nuts
In practical use, whether zinc-plated or hot-dip galvanized, nut threads are rarely damaged. Typically, bolts fail first while nuts remain unaffected, so nuts generally have a longer service life than bolts. This is because during installation and matching, bolts bear significantly more load than nuts. When matching:
For ordinary high-strength bolts (e.g., Grade 8.8), Grade 8 hot-dip galvanized nuts are sufficient.
For high-strength structural steel bolts (e.g., Grade 10.9, Grade 12.9), corresponding high-strength nuts (non-hot-dip galvanized) are required.
The specific selection depends on the service environment and design requirements.
Standard Subdivisions of Hot-Dip Galvanized Nuts
Nuts are categorized into new and old GB standards (e.g., old GB/T 6170-1986 and new GB/T 6170-2015). Among hot-dip galvanized nuts, Grade 8 products are mostly manufactured in accordance with the new GB standard, while Grade 4 products often follow the old GB standard. Imperial and ANSI nuts are based on their respective latest standards.
Nuts are also divided into Type 1 and Type 2. The core difference between the two standards lies in thickness: Type 2 nuts are slightly thicker than Type 1, but their internal thread parameters and hexagon across-flat dimensions are identical. Their load-bearing capacities differ slightly (Type 2 is marginally higher), but users need not overthink this during procurement. Currently, Type 1 nuts are the most widely circulated in the market.
Industry Characteristics of Hot-Dip Galvanized Nuts
The hot-dip galvanized nut industry is relatively "niche". This does not mean low competition, but rather that most knowledge about it is gained through industry practice and rarely covered in school curricula. As a result, those unfamiliar with the field are prone to mistakes.
GB nuts default to right-hand thread (clockwise-thread) nuts. Left-hand thread (counterclockwise-thread) nuts are extremely rarely used. Due to limited market demand, coupled with the fact that the impact of hot-dip galvanizing on their thread fitting is consistent with that of right-hand thread nuts, left-hand thread hot-dip galvanized nuts are rarely available in the market.
