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Strength Classifications Of Steel Bolts And Nuts

ISO standards provide a set of classifications regarding strength as a part of mechanical properties. This section explains the strength classifications of steel bolts and nuts. FA catalog '08, page 1111 shows the strength classification of socket head cap screws CB as 12.9 (M3~M20) and 10.9 (M24).

(1) Mechanical properties of steel bolts

1. Mechanical properties of steel bolts
In ISO and JIS, the mechanical properties of bolts are categorized by strength classification numbers. The [Table. 1] is a part of ISO 898-1:1998 standards.


2. How to read strength class in the table
The first number denotes 1/100th of the bolt's minimum tensile strength. The second number denotes a ratio between the tensile strength and the yield point.


3. Items regarding mechanical properties
Minimum tensile strength is the maximum load a bolt can withstand. Applicable to bolts 3mm in diameter and length of 2.5d (2.5 times the diameter). Yield strength is the maximum load a bolt can withstand without any plastic deformation. Elongation property is the amount of bolt elongation before breakage.

(2) Mechanical properties of steel nuts

• The strength classifications for nuts are represented in proof load stress value according to the screw diameter (see [Table. 1] ). Make nut selections to match the strength class of bolts to be used together.
• For instance, combining a nut of class 8 from the table 2 and a bolt of class 8.8 will result in a fastened strength where the bolt shank would break before the meshing threads would yield.
• Numbers on STYLE column in Table 2:
  Style 1: > Relatively high material strength with nut thickness of 0.8d ~ 0.9d
  Style 2: > Relatively low material strength with nut thickness of approx. 1d