Money for new rope20 July 2018
The new, fifth, edition of ISO 4309 came out in 2017. It deals with ropes and rope safety in hoists. Julian Champkin looks at some of the issues involved.
Wire ropes, says the new ISO 4309, are regarded as an “expendable component.” Which raises the question: how expendable are they? Other questions are how can their life be extended, and when and how should they be replaced?
In the new document “care and maintenance” procedures are distinguished from “inspection and discard”. So what are the best practices and procedures for looking after wire ropes? And as synthetic ropes make headway, how do use and best practice differ between the two?
To start with the basics. Some advice for users of wire rope is straightforward, as manufacturers and users can testify. “Don’t bend it too hard,” says Daniel Calton, who is in charge of sales at Rope Assemblies. “Don’t abuse it. Don’t put it near corrosive chemicals. All of that is basic common sense, but basic common sense is sometimes lacking. ‘Be sensible’ is my best advice. If a rope is overextended, or bent too far, then check it.
“And where ropes pass over shackles and hooks don’t bend them beyond the radius limits. If you are unsure, check with the manufacturer or look at his website, and send follow-up questions via the website contact form.”
Where you use the rope is as important as how you use it. “Use the right rope for the right environment,” he says. “316 is the standard for marine use, but many users are determined to use cheaper galvanised rope if they do not care about its lifetime. It might do the job, but it will not last as long. It might have only a two-month lifespan, but still people do it.”
Casar, a subsidiary of Wire Co World Group, produce a very useful document on maintaining wire ropes in good condition. A potential cause of premature wear, well covered by Casar, are the grooves over which the rope passes on drums or in sheaves. If these are too narrow, a rope under load will be squeezed as it passes through them and very great forces can be generated. These can lead to wire breaks in the inside of the rope—not easy to spot from outside— or to stretching of the outer wire strands—the so-called ‘birdcage deformation’. Too wide a groove means that the rope is unsupported around most of its crosssectional circumference as it passes over the sheave, and all the pressure is concentrated on the small area of contact at the bottom of the groove. This increased pressure, and the additional tensions it generates, leads to intensified deformation of the rope—it becomes oval in cross-section, instead of round—and again reduces service life.
Ideally the grooves should have a diameter that is slightly larger than the effective diameter of the rope. ISO 16625 prescribes that the diameter of the grooves must be a minimum of 5% and a maximum of 10% more than the rope diameter. As an optimum value, the standard specifies a groove diameter of 7.5% more than the rope diameter. For wire rope, lubrication is also an issue. At the factory, it will have received intensive lubrication, against corrosion and to allow strands to slide past each other in use. Generally, the lubrication does not last the entire life of the rope.
“German standard DIN ISO 4309 recommends re-lubricating a wire rope before any signs of dryness and corrosion appear,” says Casar. Special attention should be given to the sections that run over sheaves or onto the drum. Painting, swabbing, or continuous drip methods are all possible methods of re-lubrication. Pressure lubricators are available that can ensure maximum penetration of lubricant into the centre of the rope. “All different methods of relubrication of steel wire ropes should be carried out regularly from the beginning of the service life of the rope and not only after the first damage has been ascertained,” says Casar. Consult the manufacturer about which lubricants to use and how to apply them.
The structure of a wire rope may seem simple—it is, after all, merely strands of wire twisted together—but it contains complexities, which affect the way a new or replacement rope should be installed and used.
To begin with, wire ropes can have wire or fibre cores. The fibre can be natural or synthetic. The outer strands can be wound around the core clockwise or anti-clockwise—so-called right-hand or left-hand lays. But the outer strands are themselves made up of bundles of twisted wires.
In a so-called ‘ordinary lay’ the wires that make those strands twist in the opposite direction to the main rope: the outer strands of a left-hand rope would have a right-hand lay. The combined effect makes the strands of wire point almost straight along the axis of the rope. In a ‘Lang’ lay the wires in the outer strands have the same direction of lay as the strands themselves—which makes the individual wires lie at an angle to the axis of the rope. Choosing the correct direction of lay is important, indeed essential, for the proper functioning of a reeving system. “A wrong direction of lay leads to torque build-up, spooling problems and structural changes in the rope.”
This is because winding drums also come in right- or left-handed versions. A right-hand drum needs a left-hand rope, and vice versa. To determine the handedness of a winding drum, stand at the end of the drum where the rope is fixed to it, look along its axis, and follow the turns of the rope with your finger. If your finger is moving clockwise, then the drum is right-hand and needs a lefthand lay rope; if your finger is moving anti-clockwise, the drum is left hand and needs a right-hand rope. For drums with many layer of rope on them, layers are right- and left-handed alternately. Choose the direction that is unwound most often.
Knowing when to replace a rope is not simple. “There are too many variables to be able to give accurate predictions of how long a rope will last,” says Calton. “Inside a chemical plant, or by the sea, a wire rope will corrode more quickly. So inspect it regularly and often—or if in doubt get a trained inspector. And courses are available to learn how to do it. “There are some visual tests—tell-tale signs of metal degradation. A broken strand in a wire rope does not mean you must replace the rope; it can still be perfectly good and fit for purpose.”
The number of acceptable broken strands depends on the size and classification of the rope. Tables are available giving the maximum number of broken strands per unit length that call for a rope to be discarded. The new ISO for the first time introduces magnetic tests for wire rope and guidance on when to use it.
When a new rope does need to be installed, several precautions are needed. While wire ropes are awaiting installation store them for short times only, either inside or protected from rain and dust, and on a reel to keep them from contact with the ground. If stored outside the reel should be covered with a breathable film that allows condensation to escape.
“Unreel it sensibly,” says Calton. “Don’t just slide it off a reel, or it will kink. Pull it in the right direction. If it is in big loops on the floor and you pull an end, the loops will not come straight, they will just bend into tighter and tighter loops until they kink—which means your new rope is unusable.” Anyone who has unrolled a garden hose will have encountered the same problem.
Casar echo this: “Unwinding the rope for installation requires care. Placing the reel flat on the ground and unrolling, from the inside of the coil or the outside, creates a twist in each turn of rope that is taken from the reel. This can make the rope form loops, which when pulled tight will form kinks that irreparably damage it. The rope must then be discarded.”
Instead, say Casar, make sure that the new coil on its reel is free to turn on its axis by putting the reel on a turntable or frame; or else roll the whole coil along the ground letting the free end pay out as you go—ensure that the ground is clean and dry for this.
In its manufacture and on its delivery reel a rope will have received a preferred direction of bending—this is distinct from its lay. It is important to ensure that the rope as wound onto the drum also bends in this direction. To achieve this, wind from the top of the reel onto the top of the drum—not top to bottom or bottom to top. Otherwise the rope will try to twist between reel and drum, to try to regain its preferred direction; or else it will twist while in service. Either can result in structural change.
When installing a new rope it is important not to introduce added twist. Pulling a new rope through by attaching it to the end of the existing rope can impart torsion into the new rope—the twist that the old rope acquires under load gets transferred into the new rope. Casar manufacture a heavyduty installation eye to overcome this problem: it allows rotation between the old rope and the new. “At first glance, an installation eye does not seem to be spectacular, as its service is ended when rope installation is completed. But every expert knows the errors that can be made during installation and the dramatic impact this can have on the rope,” says the company. An alternative is to use ‘Chinese fingers’—braided cylinders that fit over the ends of the ropes and are kept in place by friction. The ‘fingers’ on the old and new ropes are joined by thin wires, and it is these that absorb the twists.
Once a new rope has been installed, it should be broken in—used under light load only to let it settle into its working position. “The reverse is often done,” says Casar: “All too frequently new ropes are immediately ‘overload tested’ with loads beyond the safe working limits of the system.”
Ropes made with synthetic materials have some applications in the hoist sector. For relatively short lifts, the weight savings of the lighter ropes are largely immaterial—but for the long falls in areas such as sub-sea lifts and mining, they can prove valuable.
Ropes made with Dyneema, a Ultra- High Molecular Weight Polyethylene (UHMWPE) fibre manufactured by DSM Dyneema have proven fit-for-service in a wide variety of applications, and the number of applications in steadily growing. “Synthetic is lighter than steel,” says Michelle Jarvis of Samson Ropes, who use Dyneema extensively in their ropes. “From the point of view of maintenance, synthetic rope will not be damaged by kinking or ‘birdcaging’,” she adds. “There are no wires that can break, which makes it safer to handle because it cannot cause hand injuries.”
Ropes made with Dyneema are among the most resistant to chemical attack. “They are resistant to most chemicals. These ropes will not corrode and are perfect for use in saltwater environments, chemical plants, and so on,” says Jarvis. But various materials do react differently to conditions and chemicals. Jarvis’ advice is to confirm the chemical resistance of the rope you select if there are any specific concerns about its working environment.
“These ropes are hydrophobic—they will not absorb water or increase in weight due to holding water,” says Jarvis. Dyneema is claimed to be up to seven times lighter than wire rope for the same breaking strength. A typical spool can be picked up and carried by one man. “The same amount of steel wire could require equipment to move, or cause back injuries,” says Jarvis.
While ice forming within a wire rope can damage it, Dyneema fibre has been shown to get stronger in sub-zero temperatures. “It is important to know the recommended operating temperatures for any materials you are using, and to stay within them,” says Jarvis.
Choosing the correct rope product for the application and environment is one key to maximising safety and service life. “There are many variables in rope construction to consider,” she says. “Ropes can be jacketed or nonjacketed; they can differ in the number of strands; coatings may or may not have been applied, and a rope may be is loosely or tightly constructed. For example, a loosely constructed rope may have a higher break strength than a firmer construction, but might not be as resistant to abrasion.”
Standard advice is to keep synthetic ropes as clean as possible to avoid dirt entering and possibly causing abrasion from within. To this end, jacketing a rope or sling is frequently recommended. As with wire ropes, bending radiuses— d/D ratios—are important: “Bend-oversheave performance for braided ropes is typically the same or better than for steel wire,” says Jarvis. So, too, as with wire ropes, are groove widths over sheaves.
From the point of view of maintenance, synthetic rope does not require lubrication and will not corrode. Installation of synthetics is simpler than for wire ropes. Being braided rather than made by twisting elements together means that synthetic ropes have no natural twist—so issues of right- and lefthand lay do not arise. “Being inherently torque-neutral they will not induce spin when load is applied,” says Jarvis.
“However, it is important to prevent significant twist in the rope during operation and use. And when installing synthetic ropes the user should take care to smooth and clean any rough or sharp surfaces the rope may be in contact with.”
Guidelines for inspecting, checking or discarding synthetic ropes are based on continuous inspection. “Inspecting your rope should be a continuous process of observation—before, during and after each use,” says Jarvis. “Inspection is done by look and feel of the rope. Items to look for may include cut or pulled strands, glazing, compression, abrasion, discoloration, or inconsistencies in diameter or texture.
“Visual rope inspection guides that can provide detailed instructions and recommendations for retirement based on inspection, and on comparative rope pictures for single- and double-braids, are available from Samson and are recommended.”
Synthetic ropes would seem easier to maintain and care for than wire ones, but even they will repay careful handling by giving longer service life. To quote Jarvis again: “Proper care and use of all ropes will allow for maximum life and safety.”