Wire Rope WLL vs Breaking Strength: The 5:1 Safety Factor Explained
WLL = Breaking Strength ÷ 5. Breaking strength is the load at which a new rope fails in a lab. WLL is the maximum load it may carry in service. The two values differ by a factor of 5 — never use breaking strength as a working load.
6×19 IWRC Wire Rope Specs (ISO 2408 / 1960 N/mm²)
Improved plow steel (IPS/EIPS) grade, independent wire rope core (IWRC), right regular lay. WLL at 5:1 design factor.
| DIAMETER | CONSTRUCTION | BREAKING (kN) | WLL (kN) |
|---|---|---|---|
| 6 mm (1/4") | 6×19 IWRC | 29 | 5.8 |
| 8 mm (5/16") | 6×19 IWRC | 46 | 9.2 |
| 10 mm (3/8") | 6×19 IWRC | 65 | 13 |
| 13 mm (1/2") | 6×19 IWRC | 118 | 23.7 |
| 16 mm (5/8") | 6×19 IWRC | 186 | 37.2 |
| 19 mm (3/4") | 6×19 IWRC | 266 | 53.3 |
| 22 mm (7/8") | 6×19 IWRC | 360 | 72 |
| 25 mm (1") | 6×19 IWRC | 463 | 92.6 |
| 32 mm (1-1/4") | 6×19 IWRC | 721 | 144 |
| 38 mm (1-1/2") | 6×19 IWRC | 1032 | 206 |
Reference values. Verify against rope manufacturer test certificate before use.
What Does the 5:1 Safety Factor Cover?
| FACTOR | APPROXIMATE IMPACT |
|---|---|
| Crane start/stop dynamic load | Up to 2× static load |
| Bending fatigue (sheaves/drum) | 20–40% strength loss at low D/d ratios |
| Abrasion & corrosion | In-service capacity can fall to 60% |
| Manufacturing variation | Statistical scatter in breaking load |
Look up wire rope WLL and breaking load
Wire Rope Capacity Calculator →Frequently Asked Questions
What is the difference between WLL and breaking strength?
WLL (Working Load Limit) = Breaking Strength ÷ Design Factor. ASME B30.2 requires a minimum design factor of 5:1. A rope with 100 kN breaking strength has a WLL of 20 kN. Never load a rope to its breaking strength.
Why is the safety factor 5:1 and not 2:1?
The 5:1 factor covers: dynamic loads (crane starts/stops up to 2×), fatigue degradation (a worn rope in service may retain only 60% of original strength), sheave bending losses, and manufacturing variation. A worn in-service rope with 5:1 design factor still has roughly 3:1 real margin.
What is the difference between 6×19 and 6×37 wire rope?
6×19 (15–26 wires per strand): the overhead crane industry standard — best balance of abrasion resistance and flexibility. 6×37 (27–49 wires): more flexible with lower bending losses but shorter fatigue life. For general hoisting, 6×19 IWRC is the default choice.
What D/d ratio does ASME B30.2 require?
ASME B30.2 requires drum and sheave D/d (sheave diameter ÷ rope diameter) ≥ 16:1 for crane hoisting ropes, with 18:1 recommended. Operating below 16:1 accelerates bending fatigue and dramatically reduces rope life.
When should I retire a wire rope?
ASME B30.2 retirement criteria: 6 randomly distributed broken wires per lay or 3 in one strand; corrosion pitting; visible core damage; kinking, bird-caging, or heat marks; more than 1/3 reduction in outer wire diameter; severely worn end fittings.
Based on published standards data. Not a substitute for qualified rigger judgment. Always verify with manufacturer data.