|
| |
HomeCertificationInspectionProduct SupplyProduct HireTrainingMaintenanceNewsContactLocation |
|
Welcome to the Certex News Centre, the only place for information on Certex. Remember to keep returning to this page to keep up-to-date with Certex.How To Select The Best Wire Rope For The JobWire ropes are essential to the crane industry and so it is not surprising that there are dozens to choose from. In these days of precision lifting, load limits and litigation from industrial accidents and injury, it is crucial to select the right one.
Why wire rope?For over a century, wire ropes have provided a strong, flexible component for use in crane design and operations. They offer the required strength and durability demanded in this application – and especially in the more specialist products – are able to withstand high levels of wear, abrasion, crushing and corrosion. Types of wire ropeThere are two basic types of wire rope – stranded and spiral – although this article will concentrate on stranded rope, which is the most widely used type of wire rope in the cranes market. 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, namely 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 typical example of this is Bridon’s Blue Strand® six rope, which is a general purpose rope used mainly for overhead and crawler cranes, and as a standard rope on boom hoists. A multi-layer stranded rope has more than one layer of outer strands, perhaps two layers as with the 17X7 or Dyform® 18 constructions, or three layers like the Endurance® 50 DB and compacted products such as the Dyform® 34LR. 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 may also be referred to as DSC (Double Seale Closed) or Parallel Laid. They have an extremely high strength and are widely used for overhead cranes and crane derrick (J/Boom) hoist ropes. An example is the Endurance Dyform® DSC 8, which is crush resistant, has an exceptional fatigue life, excellent sheave contact and very low elongation properties. 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. Rope coresThe core – usually fibre or steel – maintains the circular section of the rope by supporting the strands around it. Fibre cores are made from either natural materials FC (e.g. hemp, sisal) or synthetic products FFC (e.g. polypropylene). Steel cores are sub-divided into wire strand cores (WSC) and independent wire rope core (IWRC). Steel cores are obviously the much stronger option 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 rope finishesWire ropes tend to come in either 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 those products regularly used in marine and offshore environments. It also has a useful lubricating benefit to the rope. Galvanised products are more expensive but the initial outlay is quickly outweighed by the benefits of a longer life expectancy. TensileThe tensile of the rope is important in determining its overall strength and advances in steel making have lead to higher tensiles being used, i.e. up to 2160 N/mm². Higher tensile steels can affect fatigue life, although with modern good quality steels, this will be minimal. Compacted strand rope 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 gives them 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 expectancy. Today there are 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. Products such as Bridon American’s Constructex® have been used for decades in applications such as boom hoist ropes. These ropes have an extremely high fill factor and are excellent in resisting the damage caused on heavily loaded drums, particularly where ropes have been spooled poorly. Plastic impregnated ropePlastic 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. 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. For instance, a plastic impregnated six or eight strand rope would make a good overhead hoist or boom rope and – in the case of multi-strand ropes – can be beneficial for some marine and offshore applications. Choosing the right ropeWhen 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. StrengthThe 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 any relevant regulations or codes of practice governing the design of the rope – often referred to as the coefficient of utilisation – 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 have an effect 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?
Resistance to rotationSome applications require 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. It should be noted, however, that all ropes will rotate to some degree when loaded. For the best ropes, however, this rotation will be minimal. The tendency for any 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 rotation of the load. ‘Endurance®’ low rotation ropes such as the Endurance® 50 DB and Dyform® 34LR ensure that problems associated with cabling and load rotation are minimised. 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 more robust and better able to withstand excessive fleet angle and abuse than their more complex counterparts. Fatigue resistanceThe 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, e.g. 6X36 WS (14/7 & 7/7/1), as opposed to a rope containing larger wires such as a 6X19 S (9/9/1), which is more resistant to wear. Additional resistance to fatigue can be achieved by selecting a ‘Dyform®’ wire rope. These have a much smoother surface than standard rope, which improves rope to sheave contact leading to reduced wear on both rope and sheave. 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 long-term lower operating costs. Compacted ropes such as the Endurance Dyform® 6 will have a significantly longer life than conventional ropes where fatigue and wear are the prime factors. Resistance to abrasive wearAbrasive 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 and, again, a Dyform® product is recommended. This is because non-Dyform® rope on adjacent drum laps can cause point contact and accelerated wear and the smoother surface of Dyform® rope creates better contact. 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 such as Constructex® are particularly crush resistant and will withstand abuse even when a rope is poorly installed.
Crush resistance ropesIn 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 – e.g. 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 as opposed to a fibre core will help in this situation, and for this reason steel core ropes are always recommended for cranes use. Additional resistance is offered by a Dyform® rope, due to its high steel fill-factor, and Dyform® ropes are also recommended for multi-layer coiling operations where crushing on lower layers is inevitable. It should also be noted that a Langs Lay rope (see above) resists interference at the drum better than an Ordinary Lay. Corrosion resistance ropesIf 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. Ordering a ropeAny good rope supplier will be able to give you full information on currently available products and help you with the final selection of your wire rope. When making your order or enquiry, it is advisable to provide as much background information as possible and the following is a useful checklist to have prepared.
Most of the information above can be found either on the rope certificate or will be contained in the crane manual. Your rope distributor or manufacturer will always be on hand to advise. Common Rope TypesSpiral Rope: An assembly of two or more layers of shaped and/or round wires laid helically over a centre, usually a single round wire. There are three categories of spiral rope: spiral strand, half-locked coil and full-locked coil. Spiral Strand: An assembly of two or more layers of round wires laid helically over a centre, usually a single round wire. Half-locked Coil Rope: A spiral rope type having an outer layer of wires containing alternate half lock and round wires. Full-locked Coil Rope: A spiral rope type having an outer layer of full lock wires. Stranded Rope: An assembly of several strands laid helically in one or more layers around a core or centre. There are three categories of stranded rope, single layer, multi-layer and parallel-closed. Single Layer Rope: Stranded rope consisting of one layer of strands laid helically over a core. Multi-layer Rope: Stranded rope consisting of more than one layer of strands laid helically over a core. Rotation-resistant Rope: Stranded rope having no less than ten outer strands and comprising an assembly of at least two layers of strands laid over a centre, the direction of lay of the outer strands being opposite (i.e. contra - lay) to that of the underlying layer of strands. Low Rotation Rope: Rotation resistant rope having at least fifteen outer strands and comprising an assembly of at least three layers of strands laid over a centre in two operations. Compacted Strand Rope: Rope in which the outer strands, prior to closing of the rope, are subjected to a compacting process such as drawing, rolling or swaging. Compacted Rope: Rope that is subjected to a compacting process after closing, thus reducing its diameter. Solid Polymer Filled Rope: Rope in which the free internal spaces are filled with a solid polymer. The polymer extends to, or slightly beyond, the outer circumference of the rope. Cushioned Rope: Stranded rope in which the inner layers, inner strands or core strands are covered with solid polymers or fibres to form a cushion between adjacent strands or layers of strands. Cushion Core Rope: Stranded rope in which the core is covered (coated) or filled and covered (coated) with a solid polymer. Solid Polymer Covered Rope: Rope that is covered (coated) with a solid polymer. Solid Polymer Covered and Filled Rope: Rope that is covered (coated) and filled with a solid polymer. Preformed Rope: Stranded rope in which the wires in the strands and the strands in the rope have their internal stresses reduced resulting in a rope in which, after removal of any serving, the wires and the strands will not spring out of the rope formation. Cable-laid Rope: An assembly of several (usually six) single layer stranded ropes (referred to as unit ropes) laid helically over a core (usually a seventh single layer stranded rope). Braided Rope: An assembly of several round strands braided in pairs. Electro-mechanical Rope: A stranded or spiral rope containing electrical conductors. Crane Rope Comparisons
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
This is rope where the outer strands are subjected to a compacting process such as drawing, rolling or swaging. Bridon’s products containing compacted strands are identified by the term, ‘Dyform®’ e.g. Endurance Dyform® 6; Endurance Dyform® 34LR; Endurance Dyform® DSC 8 etc. 
