By Dean McDonald, founder, Crane Rope Solutions
The rule
New wire ropes are made larger than their nominal size, often by up to 5%. The sheave has to fit the rope you actually have, not the size on the catalogue page.
“Wire ropes can be produced up to 5% larger than their nominal/catalog diameter and sheaves should have 1% clearance between the ACTUAL wire rope diameter and sheave groove diameter.” (Dean McDonald)
Case 1: bulk terminal haulage winch
A rail-car positioner at a bulk terminal needed two new 1-5/8 in ropes urgently. The rope was a rare size, used in a corrosive environment.
The spare rope on offer: another local user had a spare. Its mill certificate showed a minimum breaking load of 122 t, but the OEM drawings called for 132 t, so we recommended against it.
The sheaves: the worn horizontal sheave had been rebuilt. We gauged it with a 43.5 mm gauge, the size of the new rope, and it didn’t fit. The largest gauge that fitted was 41.5 mm, so the “new” sheave had been cut to exactly the nominal 1-5/8 in. The neighbouring vertical sheave measured 44 mm, which was good.
“Using the new, undersized sheave, or the existing worn-out sheave will result in shortened rope life and potentially poor spooling behavior.” (Dean McDonald)
The fix: re-machine the sheaves before the new rope arrives. For a 42 mm rope, an ideal groove is 44.5 mm, and 43.95 to 44 mm is acceptable. Re-use the wedge sockets only if they are in good condition and have passed NDT; otherwise replace them, since they are cheap. The owner also ordered two ropes of the same length, so they couldn’t be mixed up at installation.
What the old rope showed: previous plastic-coated ropes had lost their coating quickly in the potash.
“When my colleague did an MRT on the rope he found about 100:1 internal wire breaks compared to external.” (Dean McDonald)
That is why visual inspection alone can miss a rope’s real condition.
Case 2: container terminal, sheaves worn to 33 mm
On a container crane with 1-1/4 in (31.75 mm) hoist ropes, an inspection found some sheaves worn to 33 mm. With zero clearance, only a rope about 3.9% over nominal would fit. The hoist ropes themselves had internal corrosion and a decrease in diameter adding up to well over 200% of the discard criteria, so they were replaced straight away.
Replacement sheaves would take time. Until they arrived, the stop-gap was to use the size variation between new ropes: new ropes come anywhere from 0 to +5% over nominal.
“Basically, put the smallest rope you have in inventory, on the sheaves with the greatest amount of wear.” (Dean McDonald)
Check the mill certificates of the ropes you have in stock to find the smallest. It is only a band-aid, and the ropes and reeving keep wearing early until the sheaves are replaced.
Takeaways
- Gauge sheaves against the new rope’s actual diameter from its certificate, before it is installed.
- A freshly rebuilt sheave isn’t automatically right. Gauge it too.
- Can a finished rope be galvanized later? No. Each wire is coated at the very start of manufacturing, before stranding.
Photos
More from the field: all case studies