Rope trick

14 September 2007

While crane designs have developed in leaps and bounds, wire rope for lifting has remained essentially the same for around 100 years. The demands of passenger elevators and mining may bring dramatic changes to lifting ropes, reports Will North

Cranes depend on steel wire ropes to raise their hooks. But new synthetic materials are fast catching up. Otto Grabandt, business manager for marketing and sales in the linear tension members department of synthetic fibre manufacturer Teijin Aramid, says, “In general, steel wire rope has been around a long time. Many people who have used it are looking for other solutions. Two of the key reasons for this are weight reduction and extended rope life, thereby lowering maintenance cost. The development of ropes made out of new materials is driven by efficiency increase requirements, and by the need for enabling technology.

“So, for example, in offshore applications, operators are working in deeper and deeper waters, and steel becomes a limiting factor. Because of the weight of the rope, beyond certain depths, it becomes prohibitive to use steel.

“Products for use in mining and elevators are in active development. In mining, the advantage is weight. Because high performance fibre ropes are much lighter, you can carry much more in each lift. In elevators, the advantage is lifetime. In those well-controlled circumstances, these products increase the rope’s lifetime and reduce maintenance.”

Jorn Boesten, of DSM, the manufacturers of Dyneema, another synthetic fibre, says, “A potential application for synthetic fibres in heavy lifting is replacing steel wire running rigging for cranes to increase the net load of a crane. Crane hoisting lines could potentially be replaced by braided rope made from Dyneema, saving more than 80 percent of the weight of the line.

“This would pay off quickly with tall tower cranes constructing high-rise buildings, as the weight of the steel wire hoisting line starts decreasing the net payload of the crane when these reach a few hundred metres height. Replacing steel wire with rope made from synthetic materials could more than double the payload on the higher cranes.”

The weight reduction from using synthetic materials brings another advantage, Grabandt says, “On mobile cranes and towers, the benefits from weight reduction would mean less changing and shifting of load moments, so less chance of the crane toppling.”

Material differences

Walter Länge, managing director of engineering at Liebherr-Werk Nenzing, explains that the company is watching the market closely for new developments. In an analysis of the different materials that could be used, he highlights some of the potential benefits of these materials. The most obvious of these is the material’s tensile strength versus its weight: what loads a rope can hold, and how heavy the rope will need to be do so.

This isn’t the only quality to consider, though. Rope materials must have low elongation and creep (they should not change their shape, either temporarily or permanently while under load), they should be resistant to bending fatigue (so they can be run over sheaves) and they should keep their shape when reeved in multiple layers on a load bearing drum. Because crane ropes are used outside, they must resist the burning effects of long-term exposure to ultraviolet radiation in sunlight.

Wire rope designer Roland Verreet, of Wire Rope Technology, highlights another issue: crane users need to be able to tell when the rope should be replaced. On standard steel wire ropes, inspectors can look at, or magnetically test, the outer layers of the rope, and assess whether it needs to be replaced. “Currently,” says Verreet, “There are no discard criteria for synthetic-only ropes.”

Liebherr Werk Nenzing premiered a range of heavy duty cranes at Bauma that used carbon fibre, rather than steel, back masts at Bauma 2007. However, while this material may replace steel pendant straps, it is not suitable for use in wire ropes.

Walter Länge, MD of Liebherr-Werk Nenzing explains, “So far, there are no carbon fibre ropes available. Use in running wires seems difficult, because of its low notch toughness. Braiding seems not to be possible. Presently, the only way to use this is for stay ropes as developed by Liebherr.”

Just because synthetic materials have been used in slings for many years, they are not necessarily suitable in ropes, Länge says.

Polyamides, such as Nylon, Perlon, and Nylsuisse, offers high tensile strength, but are prone to elongation, offer low resistance against ultraviolet radiation and acids, and swell in water. Länge says these products are used for lifting straps, but are not suitable for replacing ropes in hoisting gear. Polyester, sold as Diolen, Trevira or Dacron, offers low elongation and reasonable tensile strength, but is not strong enough for wire ropes. Polypropylene, including brands such as Leolene and Softlene, is lightweight but has low resistance against abrasion.

Länge, however, notes three types of material that may have potential for use as crane ropes. He says aramid, sold by Teijin as Twaron and Tecnora, and by DuPont as Kevlar, has very high tensile strength and low elongation, is non-conductive, has excellent resistance against heat and high form-keeping capacity. These features make it suitable to be used to replace steel in wire ropes, and it is already being used for deep sea lowering. Länge says that various rope makers are currently working on wire rope designs that would be suitable for reeving onto drums in multiple layers. This research concentrates on using aramids in combination with other materials.

Länge suggests these could include extra high tensile polyethylenes, such as Dyneema and Spectra, or liquid crystal polymers such as Vectran. Extra high tensile polyethylenes offer very high tensile strength, low weight, and the lowest elongation of all these materials, but are, he says, prone to creep under load. Liquid crystal polymers offer extremely high tensile strength with low elongation, and will not creep under load, but have low resistance to UV.

Lessons from lifts

One company that has pioneered the use of synthetic materials in ropes for lifting is elevator manufacturer Schindler. The steps they have taken may hint at how these materials could soon be used by cranes.

Claudio de Angelis has worked at Schindler SynTec, developing fibre rope technology, for the last 14 years. He is now the division’s managing director. He says, “The properties of the steel wire ropes define the system layout of a lifting system, but the physical possibilities of steel are limited. Polymers offer advantages in terms of weight, strength and bending fatigue. In elevators, the trend is to make all components smaller. This trend is solved by using new materials.”

De Angelis explains how the strength and resistance to bending fatigue of the rope material can affect costs. If they can make the rope smaller, then the sheaves can be smaller. With steel wire rope, sheaves need to be 40 times the rope diameter. With aramid rope technology, which is stronger and more resistant to bending, Schindler has been able to reduce the diameter of the sheave, to between 16–25 times the diameter of the rope. If the sheaves are smaller, then a smaller engine can be fitted, because the torque required to turn a sheave is proportional to its size. A smaller engine costs less than a big one. “If the system developer wants to generate savings in cost and space, it’s a must to reduce the torque of the drive for the lifting system,” he says.

De Angelis continues, “The lower weight of the lifting media has a big impact on the lifting system layout. For improved lifting capacity and travel to greater heights, you have to reduce the weight of the rope. If the lifting equipment and rope is one third of the weight of a system using steel rope, you have a benefit for the payload.”

However, de Angelis says there were barriers to developing alternatives to steel: “In the past the industrial application of the fibre ropes was very often not possible because the rope life end could not be detected, and the end termination was very difficult, making it hard to get more than 80% of the nominal breaking load.”

Schindler SynTec has worked to overcome these issues. De Angelis says, “We have developed a technology where, more or less, we have synthetic wires. It’s a material combination of resin and aramid fibres, like the fibres used in aeroplanes.”

The new rope also offers a solution to the problem of inspecting the rope. De Angelis says, “The quality of the cables in operation is permanently monitored by conductive fibres. The ends of the ropes are connected electrically with a monitoring device. This device could send information to any control or PC via the Internet when the rope end life is detected.”

At Teijin Aramid, Otto Grabandt is familiar with this problem, and has also worked on another approach, developed for a hybrid rope aimed at the mining sector, with a Twaron core and a steel surround. He says, “For several high-risk applications, inspection is a lot easier with steel than with synthetics. The hybrid mining cable has been designed so that the steel will always fail first: it takes the most bending stress. This means you can use the same electromagnetic and visual inspection systems as a traditional steel wire rope.

“For elevators, an advanced detection system has been built into the rope. Other, electrically conductive, fibres are added to the cable. These are, again, designed to fail under elongation or bending first. So, by looking at the electrical effects of these fibres, you can tell when the rope is approaching end-of-life.

“On the safety issue, when you go for synthetic solutions, you need testing time and expensive experiments, and discussions with regulators: this is a barrier to entry. You need to prepare new standards and new operating practices.”

The mining rope described by Grabandt was launched as this article was being written. Grabandt says, “In mining, we have worked with Casar to develop a hybrid rope, with a Twaron core surrounded by steel. This product was introduced at the OIPEEC (Organisation Internationale Pour L'etude De L'endurance Des Cables) 2007 conference in South Africa on September 12. With this new mining cable, you get the weight reduction from the Twaron core, but with a steel surround. The weight reduction will be between 25%–50%, depending on how much of the steel is replaced by Twaron. If the rope is 100% Twaron, it would be 80% lighter; that is, Twaron is about one fifth the weight of steel, for the same tensile strength. This will give users a higher payload on each lift, and an extended lifetime.”

Another problem faced in designing wire rope for lifting is finding a way to store hundreds of metres of rope safely. With steel wire rope, the rope can be stored on a load-bearing drum, reeved in multiple layers. This is possible because the rope retains its radial shape under compression, so that one layer of ropes can support the layer above, without either layer becoming distorted. With synthetic fibres this can be more difficult, as not all fibre rope constructions have the same resistance to compression.

At sea, marine lift specialists ODIM have solved this problem by transferring the load from the storage drum, to a series of tensioning sheaves. As the rope passes through the system, each sheave takes part of the load, so that compression on the stored rope is reduced, allowing layers of soft, braided, rope to be laid on top of each other without becoming deformed. As well as allowing the rope to be stored on a drum, without layers of rope becoming deformed, the system also includes important features such as anti-heave compensation and a rope management system. However, with six tensioning sheaves set in a large frame, the system takes up far more space, and weighs far more, than standard crane hoisting gear.

De Angelis’s solution is to use a laid, not braided, rope construction: “It’s like a steel wire rope, but made from synthetics: the fibres run along the rope, with resin between them. At this stage, the rope is laid, not braided. This is very important because you need stiff rope, with a high compression resistance in the cross direction. This gives you very good form stability.”

Otto Grabandt explains why all of these qualities must be considered when designing a rope for lifting: “One thing you experience when using these ropes, is that they can get warm when the same part of the rope is pulled over the sheaves repeatedly. Heat is generated because of friction inside the rope. You need to be aware of these temperature effects when designing the rope. You need to use fibres that aren’t sensitive to heat: aramids have high heat resistance. In general, polymer materials show creep: this isn’t a reversible effect of the load, like elongation, but an irreversible change. This effect increases at higher temperatures. Aramids have very small creep effects, as well as high heat resistance. Resistance to creep and temperature is very important, but so too is a high modulus. This is the deformation of the rope under mechanical force, from varying loads. Aramids do have a high modulus, albeit not as high as steel.”

Materials companies, rope manufacturers, and end users, are all looking closely at the potential of these materials to improve lifting efficiency. Länge’s best prospect for the future is carbon nanotubes, which are much stronger than today’s fibres and have high notch toughness. “They have all the potential features needed for use in wire ropes.”

A selection of ropes made by Teijin Aramid Teijin ropes Spools of Twaron and Technora fibres Twaron and Technora spools Odim's sheave tensioning system allows synthetic fibre rope to stored on a drum, without compression Odim tensionsing

Teijin ropes Teijin ropes
Twaron and Technora spools Twaron and Technora spools
Odim tensionsing Odim tensionsing