A wire rope broke while lifting a load of reinforcing steel estimated to weigh 2.5 - 3 tonnes. The precise sequence of events leading to the failure were not known, but the load did not drop because the rope jammed in the gap between sheave and support bracket. The rope was 2 years old and was stated to be maintained by regular lubrication. Bounce of the load (Activity 1
) was possible during lifting. The rope sheave diameter was 520 mm, and it was known to have been in service longer than the rope.
The insurance loss adjustor (whose job is to advise the insurance company on the quantum of loss and whether the existing policies covered the causes and consequences of the failure) commissioned a failure investigation to ascertain whether the rope was overloaded at the time of failure, or whether poor maintenance had been responsible for premature breakage. The resolution of these issues would determine whether or not insurance cover existed for the various consequences of the accident, and would also indicate if an increase in premium was necessary to offset a possible higher level of risk.
Three pieces of rope were supplied to assist in this investigation - these comprised the two broken ends together with a section of rope taken well away from the failed ends. The purpose of this latter piece was to check the load capacity of the rope, via tensile testing, at the time of failure.
The rope was a general engineering 18 strand non-spin type designed as 12x7(6/1)/6x7(6/1) - see the illustration below.
designation is a shorthand form which summarises the information contained
in the image. Thus this rope had an inner layer of 6 strands of wires,
with each strand comprising 7 wires wrapped as 6 outer wires around 1 inner
wire, while the outer layer is formed by 12 strands of wires wrapped in the
same way. The inner core of the wire is fibre. Resistance to rope spin
is provided by opposing twist directions of the inner layer (anticlockwise)
and outer layer (clockwise), whereby the load-induced torque tends to cancel
out. Further information on wire rope design, damage and the effect of sheave size can be found
by following this hyperlink
and looking at this web page
As received, rope lubrication was deficient to dry, with slight corrosion evident on the outside of the rope. Figure 1
shows the 2 broken ends of the rope. Close inspection of the rope near to the fracture plane showed that a number of wires were cracked in outer and inner strands (Figures
2 & 3
). Cracks on both inner and inner layers were associated with flattened regions on the wires.Cracking was also observed on wires well away from this region (Figure 4
Proceed to second part
of case study.