Thursday 6th Aug, 2020

Integrity of large steel ropes

Aspec Engineering explains steel wire ropes – how to care for them and their limits of degradation.

Aspec Engineering explains steel wire ropes – how to care for them and their limits of degradation.

A wire rope consists of many wires twisted to make a complex structure combining axial strength and stiffness with flexibility in bending.

Modern ropes are available in a wide range of constructions. They can have different levels of helical complexity and wires of different diameters in combinations to achieve an acceptable performance in a wide range of applications.

The term ‘wire ropes’ includes strands and wires. A strand is a group of wires laid helically in successive layers over a straight central wire. Wire rope, on the other hand, consists of typically six strands laid helically over a central core, which may consist of twisted fibre or a smaller independent wire rope.

The reliable and efficient behaviour of a wire rope and its durability under a given set of working conditions are largely governed by its size (rope diameter size), construction (rope components and arrangement) and method of fabrication, quality (rope tensile strength grade), lay and type of core.

Different configurations of the material, wire, and strand structure will provide different benefits for the specific lifting application, including:

  • Strength
  • Flexibility
  • Abrasion resistance
  • Crushing resistance
  • Fatigue resistance
  • Corrosion resistance
  • Rotation resistance

Ropes are referred to by a diameter size. The true or actual diameter of a wire rope is the diameter of a circumscribed circle that will enclose all the strands.

The construction of a rope for any given application should be suited to the equipment and to the conditions under which it will operate. In general, having many small size wires and strands produces a flexible rope with good resistance to bending fatigue.

Rope lay refers to the way the wires in the strands and the strands in the rope are formed into the completed rope. The core of a wire rope runs through the centre of the rope and supports the strands by maintaining their relative position under loading and bending stresses. A number of core types are available, and each gives specific properties to the rope.

Strand vs rope
For a given cable diameter, there are usually many more wires in a rope than in strand. The smaller wire diameters and the doubly helical wire paths in a rope are responsible for many of the differences between the behaviour of wire rope and strand. Rope is a little more flexible axially than strand but much more flexible in bending. This bending flexibility is the reason why wire rope is widely used as a tractive element over pulleys and winch drums in mines, cranes, and many other machines.

In most tension structures, such as suspension and cable-stayed bridges, fixed length pendants for draglines, and stacker/reclaimers, strands are used because there is no requirement for a low bending stiffness. The advantages of strand for these applications include greater axial stiffness (greater strength to weight ratio) and improved corrosion resistance due to its larger diameter, closely packed wires, and heavier galvanising. Disadvantages include a larger coiling diameter for delivery, careful packing and attention during transport, and more careful handling.

Causes of failures
The types and distribution of wire failures are generally a good indication of the cause of deterioration of a strand. Some typical types of wire failures include tensile, fatigue, corrosion and mechanical.

Whenever tensile failures are detected, the loading on the rope and factor of safety should be checked. Tensile failures can be an indication of an inadequate factor of safety especially if heavy impact loads on the ropes can occur.

Fatigue appears to be a major factor in the failure of ropes. There are not a great deal of test results due to the cost of testing full-size ropes, and available results show a significant amount of scatter between tests on similar ropes.

On heavy strands, the end terminations are generally made from steel sockets to enable load transmittal between the structure and the cable. The load is carried through the socket forging or casting to the rope by adhesion between the rope wires and the material used in the socket.

Typically, the material used in the socket is epoxy resin or zinc metal.
Test results show that even for the same type of end termination, cable fatigue life varies depending on the workmanship of the termination and any secondary effects due to restrained bending and corrosion.

Corrosion failures result from operation in wet, salty, acid, or other chemical conditions, which produce various degrees of corrosion and pitting. Every effort should be made to minimise the effects of corrosion by removing the causes, using galvanised rope where suitable and improving lubrication.

Mechanical damage can be seen as nicking or gouging of outer wires and can be due to careless handling, slapping against obstructions, and collisions and impacts. Also, ‘birdcaging’ effects (the springing of wires away from the core or inner strands) can be due to the sudden release of heavy loading.

Failure strength of strands
Fatigue appears to be a major cause of failure in steel strands. Historically, the majority of failures for strands appear to occur near the end terminations and can be attributed to bending fatigue, possibly accentuated by corrosion.

Bending and twisting of the wires due to loading and unloading, and contact stresses between individual wires can have a significant effect on the fatigue behaviour of strands.

Research has shown that contact stresses can govern fatigue life particularly for large diameter cables.

Dynamic effects such as digging loads on draglines and reclaimers are a common source of stress fluctuations in wire ropes. These can lead to secondary bending stresses at the end terminations as well as fretting fatigue between individual wires. Wire breakages can occur at the internal wires because the contact stresses at the internal wires can be higher than the stresses at the outer wires.

Based on recent work undertaken on materials handling machines (stacker/reclaimers) by Aspec, the fluctuating load in the boom pendants due to digging and material on the belt appears to be about five per cent of the maximum breaking load. The number of digging cycles over a 25-year period is about one million.

Therefore, we would not expect fatigue of the pendants on these types of machines to be a problem. However, secondary effects due to dynamics from machine movement and wind, poor rope condition, lack of lubrication, and the condition of the end terminations may significantly decrease rope fatigue.

High-capacity winch ropes
In order to provide the higher factors of safety for materials handling machines such as shiploaders to the latest standards, higher capacity rope types have been developed. Wire ropes commonly used for shiploaders are Turboplast and Paraplast ropes. Depending on the shiploader specification, a shiploader may have luffing, shuttle and telechute ropes.

These ropes support vertical and horizontal movements of the shiploader boom and telechute to load bulk material into ships. As these ropes are exposed to the salty, marine environment, corrosion protection of ropes, especially the core, is critical.

Wire rope discard recommendations

Failures in wire ropes can occur due to internal or external wire breakages or failure at end terminations. Internal wire failures are not common in areas away from the end terminations, however, appropriate testing methods are required to detect these internal defects.

The grounds for discarding wire ropes are varied depending on the application, the degree of risk if the rope breaks in service, the environmental conditions, and the extent of inspection. Some commonly used criteria include:

  • A percentage loss of tensile strength and therefore factor of safety
  • A maximum number of broken or cracked outer wires
  • A maximum percentage of allowable wear on the outer wires
  • Kinking or ‘birdcaging’ of wires.

As a guide, for a decrease in actual rope diameter greater than 3 per cent for rotation-resistant ropes or 10 per cent for non-rotation-resistant the rope shall be discarded even if no broken wires are visible.

Section 14 of AS 2759:2004, Section 6 of ISO 4309:2017 and A112 of AS 4324.1:2017 outline rope discard criteria.

It is also important to have the knowledge of the performance of previous ropes used in the same application and factor it in the rope discard process.

Care and maintenance
Use of corrosion-resistant galvanised rope, lubrication and checking the condition of end terminations can help avoid potential rope failure. Reducing dynamic effects is beneficial as machine digging or wind can cause the ropes to oscillate excessively.

Inspections and appropriate testing methods are also required to detect internal defects such as internal wire failures that commonly occur in areas away from the end terminations. A non-destructive testing (NDT) system as a supplement to visual inspection can also be employed to monitor internal degradation.

Lubrication
Lubrication is applied during the manufacturing process and should penetrate all the way to the core. This inhibits possible rotting of the fibre core. Lubrication of wire ropes will reduce the resultant friction within the rope as well as the friction between the rope and drum or sheaves.

However, pre-lubrication only lasts for a limited time and should be re-applied periodically during service.

Field lubricants can be applied by spray, brush, dip, drip or pressure boot. Various types of greases are used for wire rope lubrication. There are two types of wire rope lubricants, penetrating and coating. Penetrating lubricants contain a petroleum solvent that carries the lubricant into the core of the wire rope then evaporates, leaving behind a heavy lubricating film to protect and lubricate each strand. Coating lubricants penetrate slightly, sealing the outside of the cable from moisture and reducing wear and fretting corrosion from contact with external bodies.

Both types of wire rope lubricants are widely used. However, most wire ropes tend to fail from the inside which means the centre core needs to receive sufficient lubricant to maintain the rope’s useful life. It is recommended that penetrating lubricants are used either alone or in conjunction with a coating lubricant.

Wire rope lubricants can be petrolatum, asphaltic, grease, petroleum oils, vegetable oil-based or lanolin based. Where environmental considerations govern, lanolin-based lubricants are commonly used.

Lubricating working wire ropes is a difficult proposition, regardless of the construction and composition. Ropes with fibre cores are somewhat easier to lubricate than those made exclusively from steel materials. For this reason, it is important to carefully consider the issue of field relubrication when selecting rope for an application.

The first consideration when changing lubricants is whether the new lubricant and the in-service lubricant are compatible. This issue can impact flushing and change-out decisions as well as result in significant costs.

Inspection
Although wire rope is tough and durable, regular inspection must be carried out for safety and its condition must be maintained to extend its service life. The proper frequency and degree of inspection depends largely on the possible risk to personnel and machinery in the event of rope failure.

A magnetic rope test is a non-destructive testing that uses an electromagnetic instrument to examine the rope. It measures the magnetic flux leakage of a magnetised rope.

For initial inspection, ISO4309 recommends that when it is the intention to use electromagnetic means of NDT as an aid to visual examination, the rope should be subject to an initial electromagnetic NDT examination as soon as possible after the rope has been installed. For subsequent inspections testing frequency should be based on expected rope life operating cycles, operating conditions and rope constructions.

AS 4812:2013 recommends that frequencies do not exceed one sixth of the expected rope life, with a limit of between six and 30 months depending on the type of rope.

Inspection should be carried out following an incident that could have caused damage to the rope and/or its termination, or if a rope has been brought back into operation after dismantling followed by re-assembly.