Common Problems Of Steel Structure Fasteners

Many people may know that fasteners are a general term for a class of mechanical parts used when two or more parts or components are connected as a whole. They are characterized by a wide variety of specifications, different performances, and uses, and the degree of standardization, serialization, and universality is also extremely high. Generally, we call the fasteners that have national standards as standard fasteners, short for standard parts.

The common surface treatment methods of fasteners are as follows.

Blackening is the most common type of surface treatment. Its rust-proof ability is general, so rust-proof oil is required when storing fasteners treated with blackening.

Galvanized requires a 48-hour salt spray test. In order to meet the requirements, it is usually treated with a group sealing agent.

The corrosion resistance of mechanical galvanizing is similar to that of hot-dip galvanizing, which is generally replaced by hot-dip galvanizing.

The thickness of Ni must be greater than 2μm to achieve the best effect of chromium plating.

Nickel plating has three requirements: harsh environment, general environment, and light environment. It is usually used in harsh and general environment products.

Why is it often easy to find a performance indicator for tensile strength in drawings or specifications, but no relevant parameter for shear strength?

Tensile strength is the maximum tension that a fastener can support before or when it breaks. For inch fasteners, tensile strength is measured in pounds per square inch (psi), while for metric fasteners, it is measured in megapascals (MPa).

Shear strength is the maximum load perpendicular to the axis of the fastener that a fastener can support before its failure. It is usually divided into single-shear and double-shear. A single shear is a load that occurs in a transverse plane and tends to cut the fastener into two pieces, whereas a double shear will be cut into three pieces after failure. As a sharing of experience, the shear strength of fasteners can be assumed to be 60% of the minimum tensile strength in their specifications. For example, if the minimum tensile strength is 120,000 psi, the shear strength can be safely assumed to be 70,000 psi.

Expanded knowledge:

Shear joints of bolts can be designed for either friction clamping or direct shearing.

If the direct shear joint type is selected, the shear force is applied directly to the shank of the bolt. When friction clamping is used, it is necessary to ensure that the friction generated by the bolts is sufficient to prevent sliding between the plates of the joint, so if the load is dynamic, friction clamping is preferred because it prevents wear.

In most cases, the material strength of fasteners can be found through the technical manual, which is very helpful for analyzing the strength levels of materials in specific applications. For example, to calculate the actual strength of fasteners with a given diameter, the ultimate tensile strength of bolts under this size and material grade can be calculated by multiplying the minimum tensile strength of ASTM grade by the stress area of the diameter.

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