The strength of the bolt is determined digit symbol, such as 5.6, 8.8, 12.9, and the like.

How to read the bolt-heads:

For example, Class 5.6:

- the first digit is
*the value of the limit of tensile strength***Rm**- here are**5**x 100 MPa = 500 MPa - the second digit is
*the value of the yield strength*Re recognized as a percentage relative to the tensile strength Rm - here:**6**x 0.1 x 500 MPa = 300 MPa

In this example, the mechanical properties of a class can decipher the bolt as follows: tensile strength Rm = 500 MPa and the yield strength Re = 300 MPa.

Obviously, the higher numbers in marking the greater strength of the bolt.

What does it mean?

Yield stress **Re** is the value beyond which the bolt starts plastically we extend - in other words: if the load is less than this value, the bolt will be elastically stretchable, and after the load returns to its original length.

Tensile strength **Rm** - a tension that can move plastically deforming the bolt until it is completely broken.

How much strength as the bolt?

Appeals to the imagination better concept of "if you can hang on the bolt" than the stress. We must remember that 1 MPa = 1 N / mm^{2} = 0.1 kg / mm^{2}, hence Class 5.6 bolt will parameters: Rm = 500 MPa = 50 kg / mm^{2}, and Re = 300 MPa = 30 kg / mm^{2}. This means that each 1 mm^{2} cross-section of the bolts can hang 30 kg before the bolt starts to, and can hang up to 50 kg before completely broken.

For example, M6 bolt is about 20 mm^{2} cross-sectional area, so on the bolt grade 5.6 **"safe" can hang** 30 kg / mm^{2} x 20 mm^{2} = **600 kg and a maximum of the bolt can we move** 50 kg / mm^{2} x 20 mm^{2} = **1000 kg before it breaks off**.

Stainless steels of type 2 is the following symbols:

- A2-70 - A2 stainless steel, Rm 700 MPa, Re 450 MPa

while the A4-type acid resistant steels:

- A4-80 - A4 stainless steel, RM 800 MPa, Re 600 MPa