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Wire Rope (Cable) Inspection
Wire rope or cable is made by weaving many individual
wires together to produce a product that is both strong and flexible.
Wire rope is used in many safety critical applications in addition to
chair lift and gondola systems. Some of these applications include hoisting
systems, such as cranes and winches; guy wires used in tall antennas
and towers; and mooring lines of oil drilling platforms at sea. A cable
failure in one of these applications could have very serious consequences.
All wire rope eventually wears out making periodic inspections necessary throughout the service life of the rope. Wire rope is prone to damage and wear due to abrasion, fatigue, corrosion, and improper handling. NDT personnel look for localized flaws or loss of metallic cross-sectional area using a variety of inspection methods. The least sophisticated method is visual inspection. Inspector simply look for broken strands, wear and corrosion on the surface of the cable. However, for a more thorough evaluation, a number of instruments have been developed that allow inspectors to assess the internal areas of the cable.
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Most
skiers and snowboarders rate ski resorts by their average powder base
and the overall quality of the slopes. Few likely give serious consideration
to the safety of chair lifts at their favorite mountains. It is likely,
however, that many have looked up at that relatively small cable that
they are dangling from high in the air and hoped that someone had check
to see that it was in good working condition. Luckily, ski resort operators
and governing authorities perform regular inspections and maintenance
of chair lifts to ensure passenger safety. One of the components that gets close scrutiny is the cable.
One
of the more widely used of these instruments uses magnetism to inspect
the rope. The inspection instrument is placed around the wire rope and
moved along the rope or the rope is pulled through the instrument. Strong
permanent magnets or electromagnets are used to create a strong magnetic
field within the rope. The rope is said to be magnetically saturated
because it is caring all the magnet flux that it possibly can. In areas
where the rope is damaged, it can not support as much of the magnet
flux and some of it "leaks" out of the rope. Sensors in the
inspection head detect the magnetic flux leakage caused by the internal
or external defects in the rope. Defects as small as 0.05 % of the rope's
cross-sectional area can often be detected.