Three strengths that get mixed up

“Strength” in wire rope is at least three numbers: minimum breaking load (MBL), safe working load (SWL), and the rating of the assembly. Mix them up and a safe rope looks unsafe, or the reverse.

Separate the three before reading a table or certificate, so laboratory data is never treated as lifting capacity.

What MBL is and where to find it

Minimum breaking load is the load at which a rope fails in a tensile test, published by construction, diameter and wire strength grade. It is laboratory data for a new rope, a straight pull and terminations that do not damage the wires.

Our product tables publish MBL in kN, with kg or lbf where useful. Check the full data by construction, diameter and grade in the wire rope breaking strength guide, where each size is listed.

How to calculate SWL: MBL divided by the safety factor

Safe working load equals breaking load divided by a safety factor: SWL = MBL / safety factor. Many lifting applications use a factor of 5, but the factor that applies to your work is set by the standard and jurisdiction you follow.

One kN is about 0.102 tonnes, or 102 kg; one tonne is about 9.81 kN; one kN is about 225 lbf. A one-place unit error changes the answer by an order of magnitude.

Worked example: 20 mm 6x19 IWRC

A 6x19 IWRC rope of 20 mm at 1770 MPa has an MBL of 252 kN. At a safety factor of 5, SWL is about 50.4 kN, or roughly 5.1 tonnes.

The same size at 1960 MPa has an MBL of 279 kN, about 55.8 kN at a factor of 5. The higher grade adds about 11% to the working load, but the same factor still applies.

How much difference 1770 and 1960 MPa make

1770 and 1960 MPa are wire tensile strength grades. At the same construction and diameter, the 1960 grade has a higher published breaking strength, about 11% higher in our tables.

A higher grade does not permit a lower safety factor. The factor comes from regulation and duty, not from wire grade; satisfy the factor first, then compare breaking strength, not the other way round.

Terminations often fail before the rope

Spliced eyes, swaged fittings, wedge sockets and thimbles each have their own efficiency rating. The capacity of a finished sling depends on the termination, not on the rope MBL.

If a supplier quotes one “capacity” without stating the safety factor or the termination, ask whether the figure belongs to the rope or the finished sling. See the wire rope construction guide for assembly details.

Bending, wear, broken wires and corrosion

MBL assumes a new rope, a straight pull and terminations that do not damage the wires. In service, bending, wear, broken wires and corrosion all reduce the remaining strength, often while the rope still looks serviceable.

Retirement is decided by the standard governing the installation, using measured broken wires, diameter loss and corrosion, not by comparing an old rope with the original table. At the selection stage, check the sheave and drum as well; see the wire rope selector, to match the equipment.

What to send for a quotation and calculation

Provide construction, diameter, core, strength grade, termination, the safety factor basis, load type and duty. State the required factor and the standard you follow before the quotation.

We return the published MBL for the specification and the SWL at your stated factor. If we make the termination, we also give the rating based on its efficiency; technical data is in the wire rope technical data, checked by specification.