On the comeback trail16 November 2020
Fibre ropes of one kind or another have been used for millennia before iron took over. Steel wire has enjoyed a monopoly in some sectors for almost two centuries. But is fibre about to mount a historic comeback? Simon Hastelow reports.
The 21st century has seen major investment into research and development of fibre ropes. Different raw materials have been tried, tested and evaluated to get us to the point where the choice between wire rope and synthetic rope is no longer simply about which one works. It is about which works best for a particular lift.
You might imagine that traditional wire rope manufacturers have been fighting against the growth in fibre; quite the opposite, several companies, with the cooperation of crane manufacturers, have embraced the technology and clearly see the benefits in both materials.
End users, too, are becoming more aware of the advantages of each product for use in specific circumstances.
“If you have the choice to select from two slings for the same lifting job and one sling only provided with the break load information, whereas the other sling has a break load, proof load, failure load analysis, and all the failure modes are taken care of in the design, manufacturing and testing of the sling. There is much more information available for that sling before you start using it,” says Marc Eijssen, global technical and business development manager for DSM Protective Materials, manufacturer of Dyneema fibre.
“We really focus attention for end users and project owners on that whole ‘Fit-For-Purpose’ approach. Now we see more contractors and project owners buying in to this. They want reliable and safe products and when dealing with safety and reliability we tend to look at more than one parameter called a break load. With slings being the vital link between the payload and the crane, cost can never be the only selection criteria,” Eijssen adds.
The main constituent of the synthetic ropes used in the lifting and crane industry is Ultra-high-molecular-weight polyethylene (UHMWPE), a type of thermoplastic polyethylene. It is also known as high-modulus polyethylene (HMPE). The reason it has become so popular in this field is due to its extremely long molecular chains. This long chain serves to transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. Until recently it was wholly derived from crude oil but DSM have also started producing a bio-based HMPE made from renewable sources.
It is interesting to note that several of the synthetic rope manufacturers use HMPE manufactured by DSM in their own ropes.
Manitowoc was the first crane manufacturer to offer synthetic ropes in its cranes, in collaboration with US rope manufacturer Samson in 2012. The company began supplying K-100 synthetic ropes to rough-terrain cranes in 2014 and now offers them for boom trucks, truck mounted cranes and all terrains.
At Bauma 2019, Liebherr launched the new EC-B series of flat tops tower cranes. This series includes eight cranes, three of which are equipped with soLITE high-tensile fibre rope. SoLITE was developed in collaboration with rope manufacturer Teufelberger and it was tested on Liebherr tower and mobile cranes. SoLITE is unique in the fibre rope sector for its construction, which comprises a wire core with outer layers of fibre.
Polish crane rental company Mazur Zurawie ordered a 10t Liebherr 240 EC-B 10 flat top at the event and took delivery of it earlier this year. The crane has a jib length of 68m and jib tip capacity of 2.85t at 65m.
It was the first to be fitted with soLite fibre rope. Company owner Krzystof Mazur said: “I wanted to be one of the first to own a fibre crane and because of its performance it fits in well with our fleet.”
Last year, tower crane manufacturer Wolffkran acquired a stake in Trowis GmbH, a young German company dedicated to the development and production of high-performance fibre ropes for applications in the lifting and materials handling industry.
Wolffkran got involved early with the company while its ropes were still in the testing and development phase. Trowis high performance fibre ropes feature a doubly redundant, self-monitoring composite fibre construction and sensor elements integrated into the rope allowing it to monitor possible fibre fracture in the load-bearing rope component and alert the crane operator in real time.
“Gradually we are seeing more companies interested in new cranes equipped with fibre rope technology. Because it gives them the opportunity to increase the payload when using the same type of crane,” says Eijssen. So it is clear that the benefits of both materials are recognised and are being exploited even if wire rope still holds 80% of the overall market share in the lifting sector.
Testing and Monitoring
Discussing the merits of wire rope the response is often something along the lines of “Everyone knows where they stand with wire.” There is a lot of empirical experience with wire rope as it has been in use for so long. But this shouldn’t be a barrier to exploring the benefits of synthetics. As discussed above there are areas where each has an advantage.
“In most cases the testing methods are very similar or essentially the same. Synthetic ropes are subject to variations in stretch, making elongation measurement essential. Each has their particulars about cycle loads, number of cycles and terminations, pin sizes (D:d ratios), sheave groove size and shape and speeds,” says a representative from Yale Cordage.
“Many of the synthetic testing methods were originally based on wire but have since been adapted to better fit synthetics. If you are looking to replace wire, elongation is an essential measurement along with setup such as D:d if used in basket for lifting, or sheave D:d along with Grove shape and dimensions for moving rope.”
Eijssen, from DSM adds: “Companies are also asking ‘When do I know when the fibre rope and slings need to be discarded?’ With a wire rope it is visibly easier to see. You can see on the outside of the wire rope when strands are broken. You can use nondestructive testing methods, like magnetic screening of the rope, to give an idea of the condition of the wire.
“With fibre rope technology things are a bit more difficult. Companies have started looking into the use of monitoring tools, temperature sensors, cameras and thermal imaging to picture the appearance of the rope while it is in use. They need to build up this information by dedicated research activities as well as on the fly, in order to be sure that the fibre remains safe and reliable. And the industry needs this information to optimise the use of fibre rope for future generations.”
However the additional monitoring equipment and procedures have presented another issue in an area where much of the plant used is provided by rental companies. Eijssen says: “The crane companies don’t want to add lots of extra components to a crane purely to indicate and monitor the status of a rope.” This is one aspect where wire retains a clear edge.
“WireCo with its special crane rope brands Casar and Oliveira have a long reputation with special and dedicated ropes for the lifting industry. The focus is on rotationresistant ropes of the category 1, which means 15 outer strands and above,” says Christian Schorr- Golsong, director of product marketing at Casar.
“The strongest safety feature our steel wire ropes have is the special design of the ropes and how we produce them. If a rope shows sign for discard there is always a question on how much remaining life is still with that rope.
A safe rope has enough reserves to cover the time between discard and break of the rope so that the user doesn’t get into trouble if the discard point isn’t realised for whatever reason. Our ropes are designed and tested to provide a well balanced life reserve with the ropes.”
With the increasingly complex testing and monitoring procedures being developed and carried out by several different manufacturers, the calls for a standardised approach has led to a FEM guideline and moves towards an ISO are in motion.
“Developing a synthetic rope which is capable in replacing a high-performance rotation resistant rope is a challenge and not easily done. We talk about a very safety-related component in a crane and there is no room for reckless activity. That was one of the main reasons to establish a group of specialists from the crane and rope side in order to develop a standard for the use of synthetic ropes in cranes,” says Schorr- Golsong.
“Based on many discussions, hard work and tests the FEM 5.024 Guideline was developed and published. Now the work has been started on a guideline on ISO level. These are important steps to increase the acceptance and hence the investment in the further development of synthetic ropes for cranes.”
Still to Come
Even going back to the earliest products, synthetic fibre rope have only been in use for little over 20 years, with early adopters being mostly marine and recovery users. In the crane and lifting industry, fibre has a way to go, but new developments and an increased awareness will surely only increase market share.
Yale Cordage’s representative says: “Different end uses will drive innovation of synthetic rope design and will necessitate a clear understanding of the requirement by both the rope manufacturer and the end user. The value proposition presented by synthetics will allow close cooperation in the evolution of new equipment ideas in all types of crane applications. This will let users and machine designers take full advantage of the weight savings, and high strengths offered by today’s synthetics.”
Schorr-Golsong adds: “Today’s market share of synthetic versus steel ropes is still very low. Midterm we see a chance to cover a 20-30% share with synthetics.
The crane market and hence the market for ropes used on this equipment is quite stable and not changing radically. What is always interesting is the improvement of the cranes in order to achieve higher lifting capacity, easier handling, less maintenance, lower cost or a longer life of the used ropes. Any technology which offers improvements here is welcome.”
DSM in particular sees the market expanding for fibre in the engineered lifting sector as opposed to the general purpose construction cranes.
“We differentiate very clearly between general purpose lifts and the engineered lifting operations. General purpose lifts usually require a much higher design factor. But this factor is always influenced by the end user, if they don’t know how to handle a product the design factor is meaningless,” says Eijssen.
“In engineered lifting operations a company typically knows more about the risks and that gives the opportunity to turn risks into safety and reliability, and potentially reducing the design factors without compromising safety and reliability.
The company understands the risks much better and the parameters are clearly set out up front and analysed much better. Using the technical specifications and characteristics of fibre ropes these risks can be mitigated.”
“There are limitations but if you are aware of them they can be dealt with. Abrasion of fibre ropes against rough surfaces is seen as a complication but if you are aware of the issue it can be taken into consideration when designing a sling. For example you can add a specific sleeve for the sling for that particular section where abrasion needs to be addressed. It is integrated into the design and integrated into user manuals. As long as the industry is aware of the issue they can be accommodated.”
Redefining the limits of synthetic cordage
An American shipyard was frustrated that its productivity was weighed down with large, heavy slings composed of steel wire and chain. As they built bigger, better and higher-tech vessels the lifting slings brought workers new frustrations.
Riggers had to hoist assembly parts onto the hull steel-wire slings weighing upwards of 135kg. Heavier lifts took place outdoors with slings rated up to 100t and workers depended on cranes to move them.
The shipyard already used synthetic slings for certain jobs but before they could switch from steel to synthetic for the heaviest lifts concerns needed to be addressed. One of them was abrasion: under a heavy load, bunching on the shackle or pick point or a slight movement of the pick could tear a sling’s sleeve. The other concern was stiffness and the last one was stretch, as lifts could take place under ceiling-height constraints so slings needed low and predictable stretch levels.
After a year of development tests, Yale Cordage produced a synthetic multi-part sling of the same length, stretch characteristics and stiffness as the ones they were using, of the same capacity, but 85% lighter than wire.
Fortis2 is a multi-part sling made of Unitrex XSTM Max Wear synthetic cable, which has a core of Honeywell Spectra HMPE fibre encased with a neoprene coating and overbraided with a tough high-tenacity polyester jacket. Because the core fibre in Unitrex is parallel laid to eliminate torque and then sealed in place with the neoprene layer, the fibre retains its optimal strength and exhibits stiffness comparable to wire.
55ft (17m) Fortis2 slings, rated for 100t and weighing just 75kg and can be lifted and moved around without a crane or forklift. Plus there’s no real operational advantage to stock a range of 60t, 80t and 100t slings, as a 100t can handle all of the yard’s picks.