When we search for suitable screws on the Internet, we always come across the term strength class and numbers such as 4.6, 8.8, 10.9 or 12.9 in the article description. What do these numbers mean? The combination of numbers designates a strength class and makes it possible to compare different screws.
Screws are available with different tensile strength and mating limit. Both values together give the strength class of a screw. The strength class of screws is always indicated by two numbers separated by a dot. The first number is the bolt tensile strength and the second number is the bolt yield strength. What these numbers mean exactly, we will explain here in detail.
The tensile strength of a screw is the maximum stress that the material or material or screw can withstand. If the tensile strength is exceeded, the material fails. If this limit is exceeded, the material or the screw can break off. The tensile strength (stress) is measured in N/mm2 (force per area).
The yield strength of a bolt is the limit to which you can stretch a bolt. From this limit the material of the screw deforms and cannot return to its original shape.
Screws are marked with different strength classes, so that it is very easy to determine the tensile strength (Rm) and the yield strength (Re).
Example: Tensile strength class 8.8 (see picture left)
1. determination of the tensile strength (Rm):
Multiply the first number by 100 to get the tensile strength (Rm).
=> Rm = 8 x 100 = 800 N/mm²
2. determination of yield strength (Re):
Multiply the first number by the second number and obtain the result
multiplied by 10 gives the yield strength (Re).
=> Re = ( 8 x 8 ) x 10 = 640 N/mm²
* All information without guarantee.
The marking of screws with the strength class and the manufacturer's identification mark is usually only carried out by the manufacturers from a screw diameter of 5mm. The marking is usually found on the bolt head or on the side of the bolt head. For nuts, only the tensile strength is indicated with a number, as the yield strength is not relevant. One should always make sure that bolt and nut are made of the same material.
e.g. 8.8 = strenght class and XYZ = producer / manufacturer