ISO 2408 Wire Rope Standard — Breaking Force, Construction & Selection Calculator
ISO 2408 — Steel wire ropes — Requirements
ISO 2408 is the international standard that defines the requirements for steel wire ropes for general engineering purposes — covering rope construction designations (e.g. 6×19, 6×37, 35×7), the breaking-force factor K for each construction, minimum breaking force (MBL) as a function of nominal diameter and wire tensile strength grade, and mass per unit length. It is the primary reference for rope selection in lifting, crane, and hoisting applications, and its MBL formula — F_min = K·d²·Rr/1000 — is used by rope manufacturers, lifting-gear inspection bodies and crane engineers worldwide.
In practice, ISO 2408 is applied together with FEM 1.001 (which specifies the duty-class safety factors M3–M8 that the rope must achieve against the MBL) and ISO 4308 (which specifies minimum sheave-to-rope diameter ratios D/d by construction and duty class). MechanixCalc implements the full ISO 2408 rope-selection method in the Wire Rope & Lifting calculator — entering the lifted load, duty class, number of falls and rope construction returns the required MBL, the minimum rope diameter, the selected standard diameter and the achieved safety factor, with the working shown step by step and available as a PDF engineering report.
Calculators that implement ISO 2408
What ISO 2408 covers
- Rope construction designations: strand count, wires per strand, core type (FC = fibre core, IWRC = independent wire rope core) — e.g. 6×19 FC, 6×37 IWRC, 35×7 rotation-resistant
- Breaking-force factor K for each construction, used in the MBL formula F_min = K·d²·Rr/1000 (d in mm, Rr in N/mm², F_min in kN)
- Minimum breaking force (MBL) tables by nominal rope diameter and wire tensile strength grade (Rr = 1570, 1770, 1960 N/mm²)
- Mass per unit length (kg/m) by construction and nominal diameter
- Standard nominal diameters for each rope class (8 mm–60 mm range for general crane/hoist rope)
- Fill factor (ratio of metallic cross-section to the circumscribed circle) by construction — determines metallic area for elongation and stiffness calculations
Governing formulas
F_min (kN) = K · d² (mm²) · Rr (N/mm²) / 1000where K = breaking-force factor for the rope construction (e.g. 0.330 for 6×19 FC, 0.356 for 6×19 IWRC, 0.293 for 8×19 FC, 0.318 for 35×7); d = nominal rope diameter (mm); Rr = nominal tensile strength of wires (N/mm²), e.g. 1570, 1770 or 1960
MBL_req (kN) = (W · φ / n_falls) · SF_dutywhere W = total lifted load (kN); φ = dynamic factor for load pick-up (typically 1.05–1.30); n_falls = number of rope falls (parts of line); SF_duty = duty-class safety factor per FEM 1.001 (M3 = 3.55, M4 = 4.0, M5 = 4.5, M6 = 5.0, M7 = 5.6, M8 = 6.3). The minimum rope diameter d_min follows from d_min = √(MBL_req / (K·Rr/1000)).
Frequently asked questions
What is ISO 2408 used for?
ISO 2408 specifies the requirements for steel wire ropes used in general engineering — defining rope construction designations, the breaking-force factor K for each construction, minimum breaking force (MBL) by diameter and wire grade, and mass per unit length. Engineers use it to select rope diameter and construction for a given load, achieving the minimum safety factor required by the duty class (FEM 1.001). Lifting-gear inspection bodies check rope certificates against ISO 2408 MBL values.
What is the breaking-force factor K in ISO 2408?
K is a dimensionless factor that encodes the rope construction (strand count, wires per strand, core type) and the ratio of metallic cross-section to the circumscribed rope area (fill factor). It is used in the MBL formula F_min = K·d²·Rr/1000 (kN). Typical values: 6×19 FC = 0.330, 6×19 IWRC = 0.356, 6×37 FC = 0.330, 6×37 IWRC = 0.356, 8×19 FC = 0.293, 35×7 rotation-resistant = 0.318. An IWRC core adds roughly 8% more strength than a fibre core for the same diameter and grade.
How does ISO 2408 relate to EN 12385-4 and FEM 1.001?
ISO 2408 and EN 12385-4 use the same breaking-force formula and largely the same construction designations and K values — EN 12385-4 is the European harmonised standard for crane and hoist rope that closely mirrors ISO 2408. FEM 1.001 (Federation Européenne de la Manutention) is a separate document specifying the crane duty classification (M3–M8) and the safety factor the rope MBL must exceed for each class, as well as minimum sheave D/d ratios and load combinations. The three documents are used together in a full rope-selection calculation.
Which wire rope construction should I choose for a crane?
6×19 IWRC (independent wire rope core) is the most common general-purpose crane rope — it offers good flexibility and higher strength than fibre-core variants of the same diameter. 6×37 IWRC is preferred for mobile cranes and longer hoisting heights where very high flexibility is needed (smaller bending radius). 35×7 or 18×7 rotation-resistant ropes are required for single-part reeving on tower cranes and offshore winches where rope torque would cause load spin. The ISO 2408 rope designation encodes the strand and wire count directly; the breaking-force factor K differs by construction.
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