Bolt and Screw Resistance Classes: What You Need to Know

The proper use of bolts and screws in the design of machines, metal structures, or any other special application begins with a good understanding of the elements. For this reason, in this article, we will define everything you need to know about bolt classes, screw resistance classes, the meaning of the markings on the products, and the classification schemes of the production processes.

Knowing all this information is important in order to select the right product based on its final use.

What are the bolt classes?

In order to be in line with national and international guidelines and regulations, it is necessary for the hardware to follow certain classification schemes.

To uniquely recognize a screw or a nut, it is necessary that such elements have the so-called markings, engravings that the manufacturer must apply and which describe the basic characteristics of each element. Generally, the marking is performed on the head of the product by engraving or relief.

Among the fundamental characteristics to pay attention to, which describe the elements of the hardware, is the screw resistance class. The term resistance, and therefore the value indicated on the elements, refers to their ability to withstand yielding, tension, and breaking.

The regulation that governs the mechanical and physical characteristics of the hardware is UNI EN ISO 898-1. This standard distinguishes between two different types of classes: high resistance hardware and medium/low resistance hardware.

The bolt classes are identified through a numeric code or a series of numbers divided from each other by a point.

The most common resistance classes that can be encountered in industrial applications are:

4.6, 4.8, 5.6 (low resistance)
6.8 (medium resistance)
8.8, 10.9, and 12.9 (high resistance).

The values indicated on the elements are not random but have a very specific meaning. In detail, the number to the left of the point, if multiplied by 100, indicates the breaking load of the material. This value is expressed in N/mm² (Newton per square millimeter). The number to the right of the point, if multiplied by 10 times the previous number, provides the value of the yield strength. This second result is also expressed in N/mm² or in Mpa (MegaPascal).

Below is an explanatory table with all the values and calculations just mentioned:

Cosa sono le classi di bulloneria? Tabella-classe-di-resistenza-lim.jpg (Art. corrente, Pag. 2, Foto normale)

It is necessary to underline that, among the different resistance classes, the elements subject to mandatory marking according to the reference regulation are those of high resistance, therefore equal to or higher than 8.8 bolts.

What does 8.8 on bolts mean?

The same type of screw or bolt can be made with steels of different resistances, which are then identified by the abbreviation. For screws, the abbreviation is engraved on the head while, for nuts, on one of the two faces.

As already described in the previous paragraph, the numbers define the degree of resistance of the material (light, medium, or high). The structural grade 8.8 is considered as the most common form of high resistance material.

Items like bolts in class 8.8 are made of medium carbon steel. The same marking is also found on controlled tightening galvanized hardware or other types of products designed for use in shear joints. Those commonly called "8.8 screws" are fastener items made in 8.8 steel and used for various applications ranging from mechanical to construction fields.

In the next paragraph, a rich catalog of items is reported.

Screw Resistance Classes

When talking about high resistance screws, we refer to products that start from class 8.8 up to 12.9 screws. These items are made to allow greater hold in fastening and are designed for different types of installation.

In fact, given the wide use in sectors ranging from industry to craftsmanship, there are products capable of meeting every type of need. Below are some examples of high resistance products available in the catalog:

To meet every need and application, in environments where other types of steel cannot be used, there are also different resistance classes for stainless steels.
Specifically, for austenitic steels, the most widespread resistance classes are A2 and A4, with the subclasses A2-70, A4-70, and A4-80.

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You can contact us at commerciale@univiti.it to request a quote for the range of high resistance screws and bolts available in our catalog.