Wire rope strength by size
These figures are for 6x19 class rope with an independent wire rope core in extra improved plough steel, which is the common lifting construction. 6x36 class has more, finer wires and is more flexible at similar strength.
Fibre core rope of the same diameter is roughly 7 percent weaker and should not be used where the rope is crushed on a drum.
| Diameter | Metric | Breaking strength (t) | WLL at 5:1 (t) |
|---|---|---|---|
| 1/4 in | 6.5 mm | 5.8 | 1.16 |
| 5/16 in | 8 mm | 9.0 | 1.80 |
| 3/8 in | 9.5 mm | 12.9 | 2.58 |
| 7/16 in | 11 mm | 17.4 | 3.48 |
| 1/2 in | 13 mm | 22.6 | 4.52 |
| 9/16 in | 14 mm | 28.4 | 5.68 |
| 5/8 in | 16 mm | 35.0 | 7.00 |
| 3/4 in | 19 mm | 50.1 | 10.02 |
| 7/8 in | 22 mm | 67.8 | 13.56 |
| 1 in | 26 mm | 88.1 | 17.62 |
D over d ratio
Bending a rope round something smaller than itself costs capacity, because the outer wires stretch while the inner ones compress. The ratio of the sheave or pin diameter D to the rope diameter d is what governs it.
At a D/d of 25 or more there is effectively no loss. At 10 the rope retains about 86 percent of its strength, at 5 about 75 percent, and at 1, which is a rope bent round something its own diameter, only about half.
This is why a sling choked round a sharp edge fails far below its rating, and why softeners and corner protectors are not optional.
| D/d ratio | Strength retained |
|---|---|
| 25 or more | 100% |
| 20 | 95% |
| 15 | 92% |
| 10 | 86% |
| 5 | 75% |
| 2 | 65% |
| 1 | 50% |
When to retire a rope
Retire running rope when you find six randomly distributed broken wires in one lay length, or three broken wires in one strand in one lay. For standing rope the limits are tighter.
Also retire for a reduction in diameter of more than 5 percent, any kink, crush, birdcage or core protrusion, heat damage, or severe corrosion. A rope with broken wires at an end termination should be retired regardless of count.
Measure diameter across the crowns, not across the valleys, and take the largest reading.
