The hidden lives of wire ropes
When wire ropes fail it can be catastrophic.
‘I begin my life forged in the fires of Hell, drawn through dies, sometimes coated other times bright. I am twisted, turned, bent and distorted. I am subjected to G forces as great as any astronaut, can exist in the driest of deserts and inhabit the deepest of oceans. I am rarely noticed, often taken for granted, and yet I am all around you – from elevators to trawlers, bridges to buildings. I can be as thin as a pin or as wide as the branch of a tree.’
Given the challenges that are faced by a wire rope during its service life, it is little wonder that they should be subject to the strictest of testing and maintenance. Certex (UK ) Ltd – one of the country’s largest distributors of lifting products and services – advise that all wire ropes should be inspected internally on a regular basis to pick up any signs of degradation or failure.
In the case of six or eight stranded ropes, this exercise can be carried out relatively easily by a qualified lifting equipment engineer. Only a few specialised tools are required – a ‘T’ bar, wire rope brush, a steel pick and two correctly sized wire rope opening clamps. The strands of the rope can be carefully parted, residual or excess lubricant removed and a thorough visual inspection of the strands and core carried out.
It is access to the core of the rope, which is vitally important. The core of a six or eight stranded wire rope, designed for engineering applications, only provides around ten per cent of the overall product strength. Yet, its good condition is paramount to the rope’s effectiveness. This is because the core provides support to the outer strands and ensures that strand gaps are maintained reducing the effects of friction or abrasion.
Multi-strand wire rope products offer a much greater challenge to the lifting equipment engineer. As the description implies, they are constructed by ‘closing’ a number of strands over each other, which can be up to three layers over a central core. In these cases, full internal inspection cannot be conducted visually. The final layer of strands can be gently displaced to establish an indication of the rope’s internal condition but it is not wholly accurate.
This problem is exacerbated because, in the case of multi-strand products, up to 50 per cent of the overall efficiency is provided by the core. Therefore, they are far more likely to degrade internally and, if not serviced correctly, failure can be sudden and with catastrophic consequences.
One solution is to use Non-Destructive Test (NDT) procedures. This is a method of internal inspection, which Certex (UK) Ltd has been offering to clients for many years. It is of particular benefit to the highly sensitive and aggressive environment of the offshore industry, where unexpected failure could cost many lives and cause wide-scale destruction of expensive equipment.
The purpose of Non-Destructive Testing, which should only be carried out by suitably qualified personnel, is to aid visual inspection, rather than act as a substitute. It utilises either electromagnetic or permanent magnetic equipment – using magnetic flux and/or magnetic flux leakage principles – and is capable of detecting discontinuities or changes in the cross-sectional area of ferromagnetic wire ropes.
The way magnetic flux leakage instruments work is to use direct current coil or permanent magnets to create a constant magnetism along the wire rope as it passes through the magnetising circuit. The magnetic flux leakage created by a local discontinuity (LD) – e.g. a broken or damaged wire – can be measured by a sensor. The signal from the sensor is then electronically processed and the voltage is recorded on a strip chart recorder. This is described as the ‘LD’ channel. While information on the LD channel is not quantitative, some conclusions can be drawn as to the presence of broken wires, internal corrosion and fretting wires (caused by wire interfacing).
Moving the wires through a test head allows closer visual inspection of specific areas of the rope. It is usual for the test head to be mounted on a pedestal, with the length of rope being drawn through the head by two winches. This enables easy recoil should an internal fault be recognised on the generated trace. The method used by Certex (UK) Ltd has been refined so that the test head fully surrounds the wire rope. This generates the magnetizing field and is known as a full coil test head.
As part of the service, clients may request a reference trace to provide an initial datum point. This can be compared with future test results, giving a significant insight into the product’s gradual deterioration. It is worth remembering that, in time, all wire ropes need to be replaced.
As a final footnote, during a visit to a wildlife park in Singapore, I found myself swinging precariously above the ground in a cable car. Luckily, the system used a constant ‘in line’ NDT facility to monitor the condition of the wire rope. I have never felt safer during such mode of transport. External and internal inspection is one thing, looking deep within is quite another!