A hoist rope on a gantry crane was being replaced every four months. The rope was 10 mm 6×19 steel core, and the failure was always the same: broken wires concentrated in one small zone, at the point where the rope bent over the sheave. The customer’s first theory was rope quality. The rope was from a reputable mill. The second theory was overloading. The hoist was running at 80% of rated load, which is normal. The actual cause was the sheave diameter. It was too small for the rope, and the bending fatigue was doing the killing.
Why sheave diameter is the main fatigue factor
A wire rope bending over a sheave is a bending fatigue problem. Every pass over the sheave bends the wires, and the bending stress is roughly proportional to the rope diameter divided by the sheave diameter. The smaller the sheave relative to the rope, the higher the bending stress and the fewer cycles the rope survives. The industry rule for a 6×19 rope is a sheave diameter of at least 30 to 40 times the rope diameter. For a 10 mm rope, that means a 300-400 mm sheave. The hoist on this crane had a 150 mm sheave — a D/d ratio of 15. That’s about half the minimum, and it cut the rope’s bending fatigue life by roughly an order of magnitude.
The relationship isn’t linear. A rope on a D/d = 15 sheave will survive maybe 50,000 bends. The same rope on a D/d = 30 sheave will survive several hundred thousand. The failure zone on the failed rope — broken wires in a tight cluster where the rope ran over the sheave — is the textbook signature of bending fatigue at too small a diameter. The rope wasn’t weak. It was being bent too sharply, too often.
The two possible fixes
There were two paths. The right one was to replace the 150 mm sheave with a 300 mm one. That required re-machining the sheave bracket or fitting a bigger sheave unit, a job of a few hours and a few hundred dollars. The other path was to accept the small sheave and derate the hoist to 60% of rated load, which would roughly halve the rope stress and double the life. That made the crane less useful and the problem still wouldn’t go away. The sheave replacement was the honest fix.
The plant chose the 300 mm sheave. The rope life went from four months to over two years. The rope change cost about $180 each time, plus a couple of hours of crane downtime, so the fix paid for itself in the first rope change avoided.
The other sheave factors that matter
Diameter is the big one, but three other things on a sheave affect rope life. First, the groove shape. The groove must be a smooth radius matching the rope diameter (about 1.06x the rope diameter), not a V-groove and not a flat groove. A V-groove pinches the rope and crushes the outer wires. A flat groove lets the rope flatten. The groove also wears: a worn groove that has become too tight pinches the rope, and one that’s too wide lets it deform. The groove should be gauged during rope inspections.
Second, the groove surface finish. A rough or rusted groove surface is like sandpaper on the outer wires. The sheave should be smooth, and the groove should be re-machined if it’s pitted. Third, fleet angle. The rope entering the sheave should be aligned within about 1.5 degrees of the sheave plane. A larger fleet angle makes the rope scrub against the groove side and wears the outer wires on one side. A rope that shows flat spots on one side is almost always a fleet angle or misalignment problem, not a rope problem.
The inspection habit
The daily inspection on this crane now includes running a gloved hand along the rope (with the crane stopped and locked out — never on a moving rope). A rope with a broken wire cluster in one zone gets replaced immediately, not at the next scheduled change. The weekly check includes the sheave groove: wipe it, look for wear, check for a flat spot on the rope that matches the groove. A rope that fails in one place is telling you about a sheave problem. A rope that fails everywhere is telling you about a rope problem. Reading the failure location tells you which part to fix.
The hoist rope failed at the bend, every time, because the sheave was 150 mm on a 10 mm rope — half the minimum D/d ratio. Bending fatigue, not rope quality, was the killer. Fit a 300 mm sheave and the rope life multiplies. Check the groove shape and fleet angle while you’re at it. A rope that dies at the sheave is the sheave’s fault.
