By Dean McDonald, founder, Crane Rope Solutions
Counting the breaks
The operators of a Liebherr LTM 1250-6.1 at a wind farm found broken wires along its 23 mm rotation-resistant hoist rope. The inspector logged 17 break locations. The rope’s certificate gave its category as RCN 23-2. The limit from the OEM manual for this rope was 2 broken wires over 6d (138 mm) or 4 over 30d (690 mm).
At one location we counted 3 broken wires, the same as the inspector. That alone meets the discard, and the next break within the 30d length added to it. The rope had to be replaced.
Two replacement options
| Partly compacted, 15 outer strands | Fully compacted, 16 outer strands | |
|---|---|---|
| Inner wires | Not compacted, so more flexible and better in bending fatigue | Compacted |
| Rotation resistance at height | Lower | Higher |
| Shock loads and birdcaging | More susceptible | More resistant |
| Profile | Rounder, spreading load on sheaves and on neighbouring wraps on the drum | Round, compacted surface |
| Cost | Lower: less compaction and fewer strands to make | Higher |
One point is easy to miss with the partly compacted design:
“By having softer wires on the inner layers and harder (compacted) wires on the outer layer, you increase the likelihood that the rope will have internal wire breaks while not showing any visible discard criteria.” (Dean McDonald)
The partly compacted rope is OEM on some tower cranes, spools well at high speed, and is cheaper to replace where ropes are usually retired for operator damage rather than wear.
Which one?
- Basic work and lower lift heights: the partly compacted rope is fine, and you won’t notice much difference.
- Long jib, a mix of light and heavy lifts, several parts of line: the fully compacted rope earns its price with more stable loads and better spooling.
See also what is a Category 1 rotation-resistant rope?.
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