Wire Rope Rejection and Production Pressure on a 10 t Foundry Crane
During a statutory inspection of a 10 t, M5 duty overhead travelling crane in a foundry you record the following. The hoist rope is 6 × 36 IWRC, 16 mm nominal, installed 8 years ago. There are 12 broken wires in a 96 mm length immediately outside one sheave, and 4 more broken wires elsewhere in the same lay. Measured diameter is 15.0 mm. There is light surface corrosion and no visible bird-caging. The sheave groove was checked with a template: radius 9.4 mm, where the nominal for 16 mm rope is about 8.5 mm, and the groove bottom is polished with a visible step at the edges. The group rope standard, applied at all its sites, discards at 10 broken wires in any 6d length, 19 over 30d, 10% diameter reduction or severe corrosion. Production wants the crane released for two weeks at a reduced 4 t load while a replacement rope and sheave are on order; mobile crane hire is available at significant cost.
1. Apply the discard criteria and state your decision on the crane, showing the numbers you used.
Read the worked answer
6d for a 16 mm rope = 6 × 16 = 96 mm, which is exactly the length inspected.
- Broken wires: 12 in 96 mm against a discard limit of 10 in 6d. Exceeded by 20%.
- Diameter: 15.0 mm is a reduction of 1.0 mm = 6.25%, inside the 10% limit on its own, but it is measured on a rope with broken wires and corrosion, so it corroborates loss of metallic area rather than reassuring.
- Corrosion: light in this case.
The governing criterion, 12 > 10, is exceeded, and the breaks are clustered next to a sheave - a fatigue pattern, not random handling damage. The rope is rejected and the crane is taken out of service. The decision is not marginal and does not need a second opinion; what needs engineering is the cause. I would record the measurements, photograph the cluster, tag the rope, and inform the responsible person in writing the same day, because a verbal rejection is routinely overridden on the next shift.
2. Diagnose the cause and list what else must be checked or replaced so the new rope does not fail the same way.
Read the worked answer
The groove radius is 9.4 mm against a nominal 8.5 mm - about 11% oversize. An oversize groove no longer cradles the rope: contact moves to two lines near the groove edges, the rope is bent over an effectively smaller and harder support, and secondary bending plus point contact drives fatigue at exactly the location observed, just outside the sheave. The polished groove with edge steps is consistent with a worn sheave and rope running hard against it.
So replacing the rope alone would repeat the failure in months. Required actions:
- Replace or re-machine the sheave to the correct groove radius and check groove hardness and surface finish; confirm the rope diameter matches the groove series (never fit a rope that sits too deep or too proud in the groove).
- Check all other sheaves and the drum for the same wear, groove pitch and rope-to-groove mismatch, and check drum grooving direction against rope lay.
- Check fleet angles at each sheave and at the drum (excessive fleet angle causes rubbing and uneven spooling), sheave alignment, and rope twist/rotation during installation.
- Check the equaliser/two-fall arrangement and any rope guide or anti-slack device for rubbing.
- Confirm the rope certificate and construction are correct for M5 duty and that the hook block reeving is as designed.
The inspection interval should also be shortened for the first months after the new rope is fitted, since a new rope on a corrected sheave is exactly the case where a latent problem shows up quickly.
3. Production asks for a two-week release at 4 t. Write the engineering response, including what you would authorise instead.
Read the worked answer
I would refuse, and put the refusal in writing with reasons a production manager can act on:
- The broken wires are a fatigue indicator. Residual strength is not proportional to applied load, and 12 broken wires near a sheave means the remaining wires are carrying load they were not designed for, with stress concentrations at every broken end.
- Broken wire ends can catch on sheaves and guides and cause bird-caging or a rope failure under tension; the cluster can migrate. A 4 t limit does not stop the rope from being bent and straightened over the same worn sheave thousands of times in two weeks.
- The design safety factor on the rope is the basis for its rating, not a reserve to be spent on a damaged rope. A foundry also adds shock loading, heat and abrasive dust, all of which reduce rope life.
- My professional obligation is to hold safety of persons above schedule; signing a release against a breached discard criterion would be indefensible after an incident and would also be a breach of the duty owed to the employees working under the crane.
What I will authorise: keep the crane out of service, use the mobile crane for the two-week window, or re-rope immediately if a rope and sheave can be sourced - the sheave change is the critical item. I will provide a signed inspection report supporting the downtime, and I will attend the site meeting with production to explain the diagnosis so the decision is understood rather than merely imposed.