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ISO property classes for steel bolts, screws, studs and nuts

The strength of standard ISO metric steel bolts, studs and nuts is readily identified by means of a numerical code.

Property classes for steel bolts, screws and studs
The code is comprised of two numbers separated by a dot. This dot is not a decimal marker but is merely a means of separating the two parts of the code. The number to the left of the dot when multiplied by 100 provides an indication of the Ultimate Strength in Mega Pascals (MPa) while the number to the right when multiplied by 10 times the preceeding number gives the Yield Strength (Mega Pascals - MPa) or Stress at 0,2% Permanent Set (MPa) depending on the property class; e.g. 8,8

Ultimate Strength
8 x100 = 800MPa

Yield Strength or Stress at 0,2% permanent Set
8 x 8 x 10 = 640 MPa

Property class Designation for Nuts

The corresponding code for nuts is a single number and is derived from the left hand number of the property class of the bolt, screw and stud. This number when multiplied by 100 indicates the Proof Load Stress of the nut in MPa. Thus, for the bolt of property class 8,8 in the above example a nut of property class 8 is required. This nut will have a Proof Load Stress of 8 x 100 = 800 MPa

In practice only standard threaded fasteners in the high tensile property classes, i.e. property class 8,8 and above are required to be marked with the appropriate code.

Property class Designation Marking of Bolts, Screws and Studs
Property class 3,6 3,6 4,6 4,8 5,8 6,6 6,8 8,8 10,9 12,9 14,9
Markings 3,6 3,6 4,6 4,8 5,8 6,6 6,8 8,8 10,9 12,9 14,9
Property class Designations of Steel Nuts
Property class 4 5 6 8 10 12          
Proof Load Stress, MPa 400 500 600 800 1000 1200          
Steel Bolts/ Screws/ Stud and Nut Combinations
Property class of Bolt 3,6 4,6 4,8 5,6 (5,8) (6,6) 6,8 8,8 10,9 12,9 (14,9)
Recommended Property class Of Nut 4 4 4 5 5 6 6 8 10 12 14
Note: *Higher property class nuts may be used on lower property class bolts


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