Last month, we explained how wire rope could be damaged through mechanical wear, bending fatigue breaks, corrosion damage, tensile overload breaks, and shear breaks.
Here, we add to the list of possible causes of damage.
External damage: Steel wire ropes are often mechanically damaged during service. The rope might hit a steel structure, thereby locally damaging some outer wires, or it might be dragged along a hard surface, creating a great amount of mechanical wear.
A wear or damage pattern along the rope’s axis or slightly helical to it always indicates that the rope has been dragged along an object. Ropes that have been pulled over a sharp edge have a tendency to coil when unloaded.
Rope damage can also be cause by, for example, shot-blasting the crane before repainting. The metal shot can stick to the lubricant and later got wedged into the strand.
Another example of external damage include severe plastic deformation caused by the rope hitting a steel structure.
Martensite formation: Martensite is a hard and brittle metal structure formed when steel is heated above the transformation temperature and then rapidly cooled. In steel wire ropes, martensite is often found as a thin layer on the crowns of rope wires where these have been dragged over a hard surface. This thin martensite layer will easily crack when the wire is being bent, initiating a fatigue crack which will rapidly propagate.
Martensite formation on the wire surface is difficult to detect. Even in a mounted sample it has to be made visible by etching the microstructure.
Lightning strikes or arcing caused by welding on the crane structure might also produce martensite on wire surfaces. Another way it can be produced is, for example, if the wire has been dragged along the edge of a cargo hatch several times when unloading a ship. Each time a new layer of martensite is formed, partly tempering the underlying martensite layers.
Heat damage: Steel wire rope is a very good conductor of heat. Therefore, zones of a wire rope can work in a very hot environment for a limited time as long as the heat absorbed by the rope can be conducted away to other, cooler areas of the rope.
If, however, the temperature within the rope wires exceeds approximately 300 deg C, the microstructure of the cold drawn wires will recrystallise, losing about two-thirds of the wire tensile strength in the process. If the energy input is much higher than the rate at which the rope can conduct or dissipate heat, then the wire rope will heat up very quickly. This will happen, for example, when lightning or arcing strikes a rope locally, heating up the wire rope to temperatures where the steel will melt.
According to EN 12385-3, stranded rope with fibre cores may be used up to a maximum of 100 deg C. Stranded ropes with steel cores can be used up to 200 deg C. Special attention must also be given to the temperature limitations of the end connection. Even if the strength of the wire rope is not affected by the temperature, a reduction in fatigue life must be expected when the working temperatures meet or exceed the drop point of the lubricant used.
Next month we will investigate how to spot internal problems with wire ropes, and run through more causes of damage.
Internal wire breaks: A visual and tactile inspection of a steel wire rope will have to rely 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 their length. Therefore, a visual and tactile inspection of a steel wire rope will have to rely 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 worse than the contact conditions on the sheave, the rope wires will fail on the inside first. 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 out than ropes working on steel sheaves.
A steel core too small in diameter will lead to insufficient clearance between two adjacent 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 local pressure 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 be made visible by severely bending the rope during inspection or by trying to lift up the outer wires with the aid of a screwdriver.
Internal wire breaks often will display typical fatigue breaks with wire ends twice as long (valley breaks) or three times as long as those occurring at the crown of the outer 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 the outer strands, however, there are many wire crossovers inside the rope. Therefore, rotation resistant ropes tend to develop a great number of internal wire breaks. Rope cores closed in one operation (parallel lay) or internal plastic layers avoid crossovers and reduce this danger.
In cyclic tension condition, the rope must get longer and shorter during every cycle. The sections lying on drums or sheaves are restricted from adapting their length to the line pull. The core will therefore try to get longer while the outer strands are held back by the drum or sheave surface. This will lead to internal wire breaks. A drum surface with longitudinal scratch marks is always a clue that the rope might fail from the inside out.
As mentioned earlier, non-destructive testing will help to detect internal wire breaks.
Damage from rotation: A steel wire rope is made up of helical elements. If the rope is twisted in the opening direction, these helixes will be opened up and lengthened, if it is twisted in the closing direction, the helixes will be closed and shortened. 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 fixed with their ends secured against rotation. Rotation resistant ropes, on the other hand, will not have a tendency to unlay under load, and can therefore be attached to a swivel. The swivel is even advantageous because it will allow twist caused by other mechanisms to leave the system.
If the rope is travelling over sheaves or a sheave is travelling over the rope, the length differences created between the rope elements might be accumulated at a single point (usually the point where the sheave travel stops).
Twisted rope sections will have a tendency to get rid of some of their twist by sharing it with previously non-twisted rope sections. This is how twist can travel through a reeving system, causing problems in areas far distant from the point where it originated.
Because of their helical surface, ropes can also be twisted when pulled through tight sheaves or rubbing along structural members.
