Hardware: How to Choose the Right One for Different Tasks

When working with metal, most tasks involve joining parts together. And if you need to create a disassemblable structure, a bolted connection is the best solution. However, to ensure such a connection is reliable, durable, and can be easily reassembled and disassembled in the future, it's important to consider several factors when selecting bolt types, sizes, and diameters.

A large catalog of bolts is collected on this site. https://traiv-komplekt.ru/catalog/categories/bolt/din_931_bolt_s_shestigrannoy_golovkoy_s_nepolnoy_rezboy_gost_7798_7805_iso_4014/But to choose the right solution for a specific task, it's worth paying attention to several factors that will help you choose the right hardware.

The principle of selecting bolts for connections

Although the calculations used to select bolts for a particular connection are quite extensive, and we will not discuss them within the scope of this article, the whole essence can be conveyed by a relatively simple principle, which consists of a number of steps.

Accuracy class. There are two accuracy classes:

• A - installed in holes without gaps;
• B — a gap of 2.3 mm is allowed, the error in the bolt diameter is no more than 0.52 mm;
• C — a gap of 2.3 mm is allowed, the error in the bolt diameter is no more than 1 mm;

Most designs (structures, etc., not mechanisms) use precision classes B or C. This is because it's not always possible to create high-precision holes and ensure precise fit of parts.

Classification of bolts by strength. The bolt strength class (for carbon steels) is designated by two digits separated by a period. The first digit is the tensile strength, where one unit is 100 MPa/mm.²The second digit is the ratio of the ultimate strength to the yield strength, expressed in tens of percent. The following precision classes are distinguished:

• 6;
• 8;
• 6;
• 8;
• 6;
• 8;
• 6;
• 8;
• 8;
• 9;
• 9.

That is, a bolt of strength class 9.8 has the following characteristics:

• tensile strength 9x100=900 MPa (90 kg/mm²);
• yield strength 900 x 0.8 = 720 MPa.

Based on the expected loads, it is possible to relatively correctly select both the diameter and strength class of the required fasteners.

Determining the length of the bolt. Selecting the right bolt length isn't difficult. The "don't spoil the porridge with butter" principle isn't appropriate here, but it's actually quite simple. To correctly determine the required bolt length, simply add up the thicknesses of all the components it will connect, as well as the thickness of the washers/gross nuts/nuts, and add 0.3D (or 30% diameter) to the resulting figure. The minimum tolerance is that the bolt should protrude from the nut by at least one thread when tightened.