Calculations

Labelling of steel screws

The requirements for the labelling of steel screws are given in the standard ISO 898-1: 2013 ”Fasteners of carbon and alloy steels”. The labelling system for steel screws is indicated by two digits e.g. 8.8.

The first digit indicates 1/100 of the nominal breaking point in N/mm2. In this case 8 x 100 = 800 N/mm2.

The second figure gives the ratio between the nominal yield and ultimate tensile strength of the screw in tenths. Multiplying the figures for the ultimate tensile strength and yield strength of the screw gives the nominal tensile strength: 800 x 0.8 = 640N/mm.

In addition to the strength figures, the screw should be labelled with the manufacturer's trademark. Sometimes you can also see that the screw's thread system is labelled with an M for metric system. Labelling of the thread system is not mandatory.

Designation system for strength

The designation system for strength classes of screws and studs is shown in Table 1. The sketch gives values for nominal ultimate strength Rm in Newtons per square millimetre, while the ordinate gives values for elongation at break A min in per cent.

The strength consists of two numbers

• The first number indicates 1/100 of the nominal tensile strength in N/mm2.
• The second number indicates 10 times the ratio of the lower yield strength ReL (or elongation limit Rp 0,2) to the nominal ultimate strength Rm.

Multiplying the two numbers gives 1/10 of the yield strength in Newtons per square millimetre. Lower yield strength ReL (or elongation limit Rp 0,2) and minimum breaking strength Rm are equal to or greater than the nominal values.