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Steel Nuts Strength Designation System

Engineering ANSI Hardware Design Data
Engineering Metric Hardware Design Data
Torque Engineering and Design

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Steel Bolts Strength Designation

Strength Grade Designation System for Steel Nuts:

The strength grade designation system for steel nuts is a number which is one-tenth of the specified proof load stress in kgf/mm². The proof load stress corresponds to the minimum tensile strength of the highest grade of bolt or screw with which the nut can be used.

Strength Grade Designations of Steel Nuts

Strength Grade Designation	4	5	6	8	12	14
Proof Load Stress (kgf/mm2)	40	50	60	80	120	140

Recommended Bolt and Nut Combinations

Grade of Bolt	4.6	4.8	5.6	5.8	6.6	6.8	8.8	10.9	12.9	14.9
Recommended Grade of Nut	4	4	5	5	6	6	8	12	12	14

Note: Nuts of a higher strength grade may be substituted for nuts of a lower strength grade.

Marking: The marking and identification requirements of this Standard are only mandatory for steel bolts, screws and nuts of 6 mm diameter and larger; manufactured to strength grade designations 8.8 (for bolts or screws) and 8 (for nuts) or higher. Bolts and screws are identified as ISO metric by either of the symbols “ISO M” or “M”, embossed or indented on top of the head. Nuts may be indented or embossed by alternative methods depending on their method of manufacture.