Wire rope inspection before a critical lift
During a pre-lift inspection of a 50 t crawler crane, a 24 mm 6×36 IWRC main hoist rope (4,200 h, 8 years) is found to have seven broken wires within one 144 mm lay length (6d). Three of the breaks are in valleys between strands and two are adjacent to a single strand. The rope diameter measures 22.8 mm at the worst point against a nominal 24 mm, and there is visible crown wear. The crane is due to lift a 9 t prefabricated stair core in two hours. The site’s discard table permits up to six broken wires in 6d for this rope construction, but the inspector notes that valley breaks and diameter loss are not fully captured by the table. Another fitter argues that because the rope has a safety factor of 5, the lift can proceed at reduced speed. The lift plan requires the crane to hold the load over a protected walkway for 20 minutes.
1. Decide whether the rope is fit for the lift. Give your reasoning against the observed defects, not just the table.
Read the worked answer
Remove the rope from service; it is not fit for the lift. The measured seven broken wires in 6d already exceeds the site’s six-wire discard limit, and the location matters: valley breaks and two breaks adjacent to one strand indicate fatigue and local strand damage, not isolated handling damage. The diameter has fallen from 24.0 to 22.8 mm, about 5%, which is significant loss of metallic area and is not captured by the broken-wire count alone. The safety-factor argument is invalid because the factor is a design margin for a new rope; it does not cover broken wires, crown wear, shock loading or a 20-minute static hold over a walkway. A reduced-speed lift still exposes people to a rope that has already lost capacity, and the hold time increases the consequence of failure. Quarantine the rope, tag it, and do not perform the stair-core lift until a new rope is fitted and inspected.
2. What immediate actions and communication are required?
Read the worked answer
Stop the lift and inform the lift supervisor and site manager in writing. Tag the rope and the crane controls ‘out of service’, and remove the rope from the sheave so it cannot be reused. Revise the lift plan: either replace the main hoist rope and re-inspect before the lift, or use a different crane with a valid inspection. Inspect the other ropes on the crane, including boom hoist and auxiliary, because similar duty may have caused the same fatigue. Check the sheave grooves and rope guides for wear that could have caused valley breaks. Brief the rigging crew on why the table alone did not govern the decision. Record the broken-wire count, positions, diameter readings and photographs in the crane log, and do not allow a reduced-speed exception without an engineering assessment.
3. Propose an inspection interval and method for the replacement rope given the duty.
Read the worked answer
Set the replacement rope inspection interval from duty and consequence, not from a generic annual default. For a 50 t crane on repeated lifts, do a visual pre-use check each shift, a detailed inspection weekly, and a competent periodic inspection at least monthly for the first three months. Because this rope failed by fatigue at 4,200 h, arrange non-destructive testing - magnetic rope testing - at about 500 h or three months after installation, then every six months if no deterioration. Pay particular attention to the sections that run over the head sheave and the dead wraps. If the crane does heavy lifts over walkways, shorten the interval and record every lift. Any new valley break, diameter loss over 2 mm, or corrosion should trigger reassessment before the next lift.