Minimum breaking force of 6×19 fibre-core wire rope in inch and mm sizes.
Minimum breaking force (MBF) is the load a new rope must reach before it fails. It is not a load you may lift. To get a working figure, you divide MBF by a design factor, then allow for the end fittings, the angle of the legs and the rope's condition. The tables give MBF for common 6×19 fibre-core rope in inch and metric sizes. A half-inch rope of this type breaks at about 21,400 lbf (95 kN), and its share of a lift is far lower. These figures are for new rope. They are not sling working loads. Never use this table to lift people.
Minimum breaking force of new rope. This is not a sling working load. Stronger grades and steel cores carry more: a ½ in 6×19 extra improved plow steel rope with a steel core is listed at about 26,600 lbf, so read the data for your exact rope.
| Diameter | Minimum breaking force (lbf) | Minimum breaking force (kN) |
|---|---|---|
| ¼ in | 5,480 | 24.4 |
| ⅜ in | 12,200 | 54.3 |
| ½ in | 21,400 | 95.2 |
| ⅝ in | 33,400 | 148.6 |
| ¾ in | 47,600 | 211.7 |
| ⅞ in | 64,400 | 286.5 |
| 1 in | 83,600 | 371.9 |
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Minimum breaking force of new rope, not a sling working load. EN 12385-4 gives Fmin = K × d² × R ÷ 1000, with d in mm and R the wire grade in N/mm²; for 6×19 with a fibre core, K is 0.330. Grade 1770 is close to US IPS.
| Diameter (mm) | Minimum breaking force, grade 1770 (kN) | Minimum breaking force, grade 1960 (kN) |
|---|---|---|
| 6 | 21.0 | 23.3 |
| 8 | 37.4 | 41.4 |
| 10 | 58.4 | 64.7 |
| 12 | 84.1 | 93.1 |
| 16 | 150 | 166 |
| 20 | 234 | 259 |
| 24 | 336 | 373 |
| 28 | 458 | 507 |
| 32 | 598 | 662 |
| 36 | 757 | 838 |
| 40 | 935 | 1,035 |
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Lifting must follow the rated capacity on the sling tag or the equipment, and the rules that apply where you work.
As a sling flattens, each leg pulls harder for the same load. The load one leg can support vertically is its rating × sin(angle from horizontal). Take two legs, each tagged 2 t for a straight pull. At 60° they lift 3.46 t, at 45° they lift 2.83 t, and at 30° they lift only 2 t: what one leg carries straight. This is geometry, and the tag for your sling comes first.
The first number counts strands and the second the wires in each. Fewer, thicker wires resist abrasion. More, finer wires bend better around sheaves and resist fatigue. So 6×19 suits slings and general rigging, and 6×37 suits drums and sheaves. For the same diameter, strength depends on grade and core as well, so check the maker's table rather than the name. The word strength here means force, unlike material tensile strength, which is a stress.
For wire rope slings, ASME B30.9 removal criteria include 10 randomly broken wires in one rope lay, or 5 in one strand of one lay. Also remove a rope for kinks, crushing, birdcaging, heat damage, severe corrosion or a missing tag. Crane ropes follow their own rules, such as ISO 4309. Ropes lose strength in service, so MBF applies only to new rope.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
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Classification: Minimum breaking force of new rope: formula and catalogue values (verification required)
Sources
EN 12385-4 — Steel wire ropes — Stranded ropes for general lifting applications (minimum breaking force)
CEN (European Committee for Standardisation)
Wire Ropes — Strengths (6×19 FC IPS, inch sizes)
Engineering ToolBox
ASME B30.9 — Slings (design factor and removal criteria)
ASME
Minimum breaking force (MBF) depends on rope construction, the wire grade and the core. Metric values follow the EN 12385-4 formula with K = 0.330 for 6×19 fibre-core rope; inch values are published IPS figures. MBF is a property of new rope and is not a sling working load: use the rope certificate and the sling tag for any lift.
MBF is the minimum load at which a new rope must fail in a proof test, set by the standard as a function of construction and wire grade. Real ropes fail above this minimum. Weight per metre follows from the steel density and the rope’s metallic cross-section.
Classification: Grade- and manufacturer-dependent (verification required)
MBF values are stable as standard minimums, but the applicable value for a specific rope depends on construction, grade and manufacturer. Always use the rope’s test certificate for lifting design — never the tabulated indicative value.
Diameter in inches or millimetres; MBF in pounds-force or kilonewtons.
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