Strength & Breaking Force


A fundamental requirement for wire rope is achievement of the minimum breaking force that complies with the crane or winch safe working load.


Rope breaking force can be seen as a function of metallic area, strength and spinning factor. These elements must be carefully combined to confer reliable mechanical properties.

Metallic area depends on the rope’s geometrical construction, diameter oversize and compacting level; strength is dependent on the characteristics of the wire; and spinning factor is dependent on manufacturing skill, geometrical construction and compacting level.

However, it must be emphasised that a high breaking force in itself is not sufficient to ensure safe working conditions. 

For a wire rope to be considered safe, it must be possible to assess, within an acceptable timescale, that it is approaching the end of its service life or if the prescribed payload has been exceeded.

Good quality ropes must be composed of ductile wires, which will break gradually following remarkable plastic deformation. This gradual breakage will be clearly noticeable by a competent person with responsibility for rope integrity management.

Ropes that rely solely on the use of extremely high strength wires for their breaking force can have severe implications in terms of safety, as the wires will have the tendency to break suddenly without giving proper notice of arising problems.




The graph above compares the behaviour of wires with different strengths: the green line represents a brittle trend typical of high strength steel (over 2160 N/mm2). The blue line represents the typical trend of lower strength steel (1770 and 1960 N/mm2).

It is therefore essential to adopt the minimum possible strength level and to achieve the desired breaking force by a combination of high compacting level, finely tuned geometrical construction and manufacturing reliability. 




Metallic cross-section (A): The product of the nominal metallic cross-sectional area factor (C) and the square of the nominal rope diameter.

Fill factor (f ): The ratio between the sum of the nominal metallic cross-sectional areas of all the wires in the rope and the circumscribed area of the rope based on its nominal diameter (d).

Rope grade: A level of requirement of breaking force which is designated by a number (e.g. 1770, 1960). NOTE - it does not imply that the actual tensile strength grades of the wires in the rope are necessarily of this grade.

Wire tensile strength grade: A level of requirement of tensile strength of a wire and its corresponding range. It is designated by the value according to the lower limit of tensile strength and is used when specifying wire and when determining the calculated minimum breaking force or calculated minimum aggregate breaking force of a rope, expressed in N/mm2.

Tensile strength: The ratio between the maximum force obtained in a tensile test and the nominal cross-sectional area of the test piece, expressed in N/mm2.