For more than a century, wire ropes have provided a strong, flexible component for use in crane design and operations. There are two basic types of wire rope – stranded and spiral. Both are important to the lifting industry but, in this article, I want to concentrate on stranded rope, which is the most widely used. This is simply defined as an assembly of several strands laid helically in one or more layers around a core or centre.
There are three main types – single layer; multi-layer; and parallel-closed.
A single layer stranded rope consists of one layer of outer strands (usually six or eight) laid helically over a central core of steel or fibre. A multi-layer stranded rope has more than one layer of outer strands, perhaps two layers.
Two-layer ropes are more rotationally resistant than their single layer counterparts, making them a good choice for hoist ropes, but the more demanding performance requirements of the modern crane has meant that three layer ropes have become more common.
Three-layered stranded ropes have an even greater ability to withstand rotation and tend to be classed as ‘low rotation’. The aim of the low rotation rope is to provide a stable balanced rope that offers greater load control than the old constructions. They are typically used as hoist ropes for mobile, tower, crawler, deck, and offshore pedestal cranes where strength, greater heights of lift and durability are dominant requirements.
Parallel closed ropes can also be referred to as “DSC” (double seal closed) or “parallel laid”. They are strong, and are widely used for overhead cranes and crane derrick hoist ropes. Parallel closed ropes possess poor rotational characteristics, and should therefore only be used where there is minimum fleet angle, and where both ends of the rope are fixed.
The core – usually fibre or steel – maintains the circular section of the rope by supporting the strands around it.
Fibre cores are made from natural materials such as hemp or sisal (FC), or synthetic products such as polypropylene (FFC). Steel cores are sub-divided into wire strand cores (WSC) and independent wire rope core (IWRC). Steel cores are stronger, and are always recommended for crane ropes as they resist crushing. A fibre core rope would only really be used for sling applications. Steel cores are often enhanced by plastic impregnation, which is covered later in this article.
Wire ropes tend to come in a bright (ungalvanised) finish or are zinc-coated (galvanised). The galvanising process makes a rope more resistant to corrosion, and as such is often applied to products used in marine environments. Zinc coating also has a useful lubricating effect on the rope. Galvanised products are more expensive, but the initial outlay is quickly outweighed by the benefits of a longer life.
The tensile strength of the rope is important in determining its overall strength and advances in steel making have led to higher tensiles being used, i.e. up to 2,160 N/sq mm.
In compacted strand rope, the outer strands are subjected to a compacting process such as drawing, rolling, or swaging. As a rule, compacted strand ropes are significantly stronger than their non-compacted equivalents because they have more steel in a given diameter. This gives them a greater breaking load (often more than 20% higher than conventional products) and lower stress levels. Compacting also gives the rope a smooth external periphery, which results in a smoother wearing surface between adjacent wraps of the rope and between the rope and the drum or sheaves. They also have a greater bending fatigue resistance.
These features inevitably make compacted ropes more expensive to buy, but the initial outlay is offset by their longer life.
Today, there are also a number of swaged ropes on the market. This is where a non-compacted or already compacted rope is put through a process that compacts the entire rope. These have been used for decades in applications such as boom hoist ropes. These ropes have an extremely high fill factor, and are excellent at resisting the damage caused on heavily loaded drums, particularly where ropes have been spooled poorly.
Plastic is added to the core of wire ropes to give added protection against corrosion. This is achieved by locking the manufacturing lubricant into the core for longer. Its presence also has a cushioning effect, maintaining the gap between the outer strands. This helps bind the overall construction together, thereby improving structural stability and extending the life of the rope.
The addition of plastic inevitably makes the rope more expensive, but it is particularly recommended in circumstances requiring good fatigue performance or in dirty or dust-laden environments. The plastic will prevent the ingress of debris to the inside of the rope thus retarding internal degradation.
When selecting a wire rope for a particular application, it is essential to take into account a number of factors, including its strength; resistance to rotation and fatigue; and its general ability to withstand wear, abrasion, crushing, and corrosion.
The responsibility for determining the minimum strength of a wire rope used in a given system rests with the manufacturer of the machine, appliance, or lifting equipment. As part of this process, they should have taken into account relevant regulations or codes of practice governing the design of the rope – often referred to as “the coefficient of utilization” – and other factors which might influence the design of the sheaves and drums; the shape of the groove profiles and corresponding radius; the drum pitch; and the angle of fleet – all of which impact on rope performance.
Once the strength (referred to as minimum breaking force or minimum breaking load) has been determined, it is then necessary to consider which type of rope will be most suitable. For instance, does it need to be rotation resistant, have a good fatigue performance, or be able to withstand particular types of abuse or arduous conditions?
Some applications require the use of a low rotation or rotation resistant rope. Such ropes are often referred to as multi-strand ropes. Six or eight strand rope constructions are fine for low lifting heights or those with multiple falls, but the most common choice to minimise load rotation on a single part system, block rotation, or ‘cabling’ on a multi-part reeving system, are low rotation ropes.
When loaded, steel wire ropes will generate ‘torque’ – if both ends of a rope are fixed – or ‘turn’ – if one end is unrestrained. The torque or turn generated will increase as the load applied increases, and the degree to which this happens will be influenced by the construction of the rope. All ropes will rotate to some degree when loaded. For the best ropes, however, this rotation will be minimal.
The tendency for a rope to turn will be greater as the height of lift increases. In a multi-part reeving system, the tendency for the rope to cable will increase as the spacing between the parts of the rope decreases. Selection of the correct rope will help to prevent ‘cabling’ and load rotation.
As a general rule, however, if you don’t need a rotation resistant rope then don’t use one. A six or eight strand rope will always be better able to withstand abuse than their more complex counterparts.
The rope’s fatigue resistance is also an important factor. Steel wire ropes will fatigue when working around a sheave or drum. The rate of deterioration is influenced by the number of sheaves in the system, the diameter of the sheaves and drum, and the loading conditions. If fatigue resistance is an issue, then it is wise to select a rope containing small wires rather than a rope containing larger wires, which is more resistant to wear.
An increased cross-sectional steel area and improved inter-wire contact also ensures that the rope will operate with lower internal stress levels. This ultimately results in greater bending fatigue life, and lower operating costs over the long term. Compacted ropes will have a significantly longer life than conventional ropes where fatigue and wear are the prime factors.
Abrasive wear can take place between wire rope and sheave and between wire rope and drum, but the greatest cause of abrasion is often through ‘interference’ at the drum. If abrasion is determined to be a major factor in rope deterioration, then a wire rope with relatively large outer wires should be selected.
A “Langs Lay structure”, which is when the direction of lay of the wires in the outer strands is the same as that of the outer strands in the rope, also has better wear characteristics than an “Ordinary (or regular) Lay” rope – when the direction of lay of the wires in the outer strands is in the opposite direction to the lay of the outer strands in the rope.
Swaged ropes are particularly crush resistant. and will withstand abuse even when a rope is poorly installed.
In multi-layer coiling applications, where there is more than one layer of rope on the drum, it is essential to install the rope with some back tension. This should be between 2 and 10% of the minimum breaking force of the wire rope. If this is not achieved, or in applications where high pressure on the underlying rope is inevitable (for example, a boom hoist rope raising a boom from the horizontal position) then severe crushing damage can be caused to the underlying layers.
Selection of a steel core rather than a fibre core will help in this situation, and for this reason steel core ropes are always recommended for cranes. A Langs Lay rope also resists interference at the drum more effectively than an Ordinary Lay.
If the wire rope is to be used in a corrosive environment, then a galvanized coating is recommended, and where moisture can penetrate the rope and attack the core, plastic impregnation (PI) should be considered.
In order to minimise the effects of corrosion, it is important to select a wire rope with a suitable manufacturing lubricant, which should be re-applied regularly while the rope is in service.
Any good rope supplier will give you full information on currently available products and help you with the final selection of your wire rope.
