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In the second part of a series on detecting faults in wire rope, Roland Verreet andDr Isabel Ridge explain more ways in which wire rope can be damaged, and theeffects

Last month, we explained how wire rope could be damaged through mechanicalwear, bending fatigue breaks, corrosion damage, tensile overload breaks, andshear breaks.

Here, we add to the list of possible causes of damage.

External damage: Steel wire ropes are often mechanically damaged duringservice. The rope might hit a steel structure, thereby locally damaging some outerwires, or it might be dragged along a hard surface, creating a great amount ofmechanical wear.

A wear or damage pattern along the rope's axis or slightly helical to it alwaysindicates that the rope has been dragged along an object. Ropes that have beenpulled over a sharp edge have a tendency to coil when unloaded.

Rope damage can also be cause by, for example, shot-blasting the crane beforerepainting. The metal shot can stick to the lubricant and later got wedged into thestrand.

Another example of external damage include severe plastic deformation causedby the rope hitting a steel structure.

Martensite formation: Martensite is a hard and brittle metal structure formed whensteel is heated above the transformation temperature and then rapidly cooled. Insteel wire ropes, martensite is often found as a thin layer on the crowns of ropewires where these have been dragged over a hard surface. This thin martensitelayer will easily crack when the wire is being bent, initiating a fatigue crack whichwill rapidly propagate.

Martensite formation on the wire surface is difficult to detect. Even in a mountedsample it has to be made visible by etching the microstructure.

Lightning strikes or arcing caused by welding on the crane structure might alsoproduce martensite on wire surfaces. Another way it can be produced is, forexample, if the wire has been dragged along the edge of a cargo hatch severaltimes when unloading a ship. Each time a new layer of martensite is formed, partlytempering the underlying martensite layers.

Heat damage: Steel wire rope is a very good conductor of heat. Therefore, zonesof a wire rope can work in a very hot environment for a limited time as long as theheat absorbed by the rope can be conducted away to other, cooler areas of therope.

If, however, the temperature within the rope wires exceeds approximately 300deg C, the microstructure of the cold drawn wires will recrystallise, losing abouttwo-thirds of the wire tensile strength in the process. If the energy input is muchhigher than the rate at which the rope can conduct or dissipate heat, then the wirerope will heat up very quickly. This will happen, for example, when lightning orarcing strikes a rope locally, heating up the wire rope to temperatures where thesteel will melt.

According to EN 12385-3, stranded rope with fibre cores may be used up to amaximum of 100 deg C. Stranded ropes with steel cores can be used up to 200

More causes of wire rope damage

6 March 2006

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deg C. Special attention must also be given to the temperature limitations of theend connection. Even if the strength of the wire rope is not affected by thetemperature, a reduction in fatigue life must be expected when the workingtemperatures meet or exceed the drop point of the lubricant used.

Next month we will investigate how to spot internal problems with wire ropes, andrun through more causes of damage.

Internal wire breaks: A visual and tactile inspection of a steel wire rope will have torely on the condition of the outer wires. In most ropes, these represent about 40%of the metallic cross section. The outer wires are visible for only about half theirlength. Therefore, a visual and tactile inspection of a steel wire rope will have torely on the condition of about 20% of the metallic cross sectional area only.

Visual rope inspection = 20% evidence + 80% hope

If the contact conditions between the rope elements inside the rope are worsethan the contact conditions on the sheave, the rope wires will fail on the insidefirst. This is dangerous because internal wire breaks are very hard to detect. Non-destructive testing equipment can help in the detection of internal wire failures.

Wire ropes working on plastic sheaves are more likely to fail from the inside outthan ropes working on steel sheaves.

A steel core too small in diameter will lead to insufficient clearance between twoadjacent outer strands, and cause interstrand nicking and so-called valley breaks.

A plastic layer between the steel core and the outer strands will reduce the localpressure between the layers and therefore reduce the risk of internal wire breaks.

Outer wires that have broken at the point of contact with the steel core can bemade visible by severely bending the rope during inspection or by trying to lift upthe outer wires with the aid of a screwdriver.

Internal wire breaks often will display typical fatigue breaks with wire ends twice aslong (valley breaks) or three times as long as those occurring at the crown of theouter wire.

Rotation resistant ropes have relatively good contact conditions on the sheaves.Because of the fact that the rope core is closed in the opposite direction to theouter strands, however, there are many wire crossovers inside the rope.Therefore, rotation resistant ropes tend to develop a great number of internal wirebreaks. Rope cores closed in one operation (parallel lay) or internal plastic layersavoid crossovers and reduce this danger.

In cyclic tension condition, the rope must get longer and shorter during everycycle. The sections lying on drums or sheaves are restricted from adapting theirlength to the line pull. The core will therefore try to get longer while the outerstrands are held back by the drum or sheave surface. This will lead to internal wirebreaks. A drum surface with longitudinal scratch marks is always a clue that therope might fail from the inside out.

As mentioned earlier, non-destructive testing will help to detect internal wirebreaks.

Damage from rotation: A steel wire rope is made up of helical elements. If therope is twisted in the opening direction, these helixes will be opened up andlengthened, if it is twisted in the closing direction, the helixes will be closed andshortened. These changes in geometry will change the wire rope properties,sometimes considerably.

In order to prevent unlaying, non-rotation resistant steel wire ropes must be fixedwith their ends secured against rotation. Rotation resistant ropes, on the otherhand, will not have a tendency to unlay under load, and can therefore be attachedto a swivel. The swivel is even advantageous because it will allow twist caused byother mechanisms to leave the system.

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If the rope is travelling over sheaves or a sheave is travelling over the rope, thelength differences created between the rope elements might be accumulated at asingle point (usually the point where the sheave travel stops).

Twisted rope sections will have a tendency to get rid of some of their twist bysharing it with previously non-twisted rope sections. This is how twist can travelthrough a reeving system, causing problems in areas far distant from the pointwhere it originated.

Because of their helical surface, ropes can also be twisted when pulled throughtight sheaves or rubbing along structural members.

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