Property Class For Steel Screws

The ISO standard requires that all screws that have a resistance class of 8.8 or higher, must have necessarily marked in the head the name of MANUFACTURER and the RESISTANCE CLASS.

WHAT MEANS RUPTURE TENSION AND ELASTIC TENSILE?

RUPTURE TENSION is measured in kg/mm2 or N/mm2 and is the maximum tensile load applied to the screw and that it supports before entering in rupture, i.e., after this value, the screws can break.

ELASTIC TENSION is measured in kg/mm2 or N/mm2 and is the maximum tensile load applied to the screw causes it to enter in non-elastic or permanent deformation, i.e., after this value is deformed and doesn’t return to its initial form.

HOW TO CALCULATE THE VALUE OF RUPTURE TENSION AND ELASTIC TENSION THROUGH THE RESISTANCE CLASS? (EXAMPLE FOR 8.8 SCREW)

• The first digit (8.8) represents a 1/100 of Rupture Tension in N/mm2 — 800N/mm2
• The second digit (8.8) represents 10 times the Elastic Tension value in percentage (i.e. 0,8x10=80%)
• Multiplying the first digit (8) for the amount of tensile strain (80%), it is obtain 1/10 of the minimum value of the Elastic Tension, that is 640 N/mm2 (= 64Kg/mm2)

We concluded that a 8.8 screw have the following characteristics:

• From 800 N/mm2 (or 80 kg/mm2), which is the value of Rupture Tension, the screw can break or else rupture.
• From 640 N/mm2 (or 64 kg/mm2), which is the limit value of Elastic Tension, the screw can’t recover its initial form, i.e., is deformed.

SUMMARY OF TENSION VALUES FOR DIFFERENT PROPERTY CLASSES

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José Machado

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