Hybrid ropes offer ductility and strength
03 January 2018
Cross-sectional cut-view of the 4-strand hybrid rope
A hybrid product is defined as “a thing made by combining two different elements; a mixture.”
Modern day rope applications demand a lightweight product that does not sacrifice the mechanical properties and usability characteristics of a traditional steel wire rope product.
Hybrid rope designs, a blend of steel wires and synthetic fibers, bring synergetic characteristics to both steel and synthetic materials providing the attributes attractive and required in both materials. Using hybrid technology in a four-strand rope configuration results in a high strength-to-weight ratio rope with a very low torque factor while taking advantage of the performance from both materials.
Today’s crane market requires ropes with rotation-resistance and torque-balance behavior in addition to bending fatigue, residual strength and multilayer spooling performance. Hybrid ropes are designed and engineered to be compatible with a full complement of end terminations including wedges and crane buttons to demonstrate a complete compatibility in crane hoist operations. The right choice of rope and termination depends on the type of equipment, application and operator preference.
Ductility and strength
Cross-sectional cut-view of the 4-strand hybrid rope
Even though the strength of steel wire rope has greatly increased over the past decades, there currently exists a technological limitation in making high-carbon steel wires with the required breaking strength needed for manufacturing a lightweight wire rope. Incremental increases of the wire strength may not be sufficient to provide the desired strength-to-weight ratios needed for some near future applications.
The art and science of steel rope wire manufacturing requires a blend of ductility and strength in the resulting steel wire that is maximized with the existing technology. While synthetic fibers are not new to the marketplace, the technology and strength of the products has come to exceed steel products in many applications. A hybrid rope design is an alternative solution for certain engineering applications that demand lighter weight without sacrificing the mechanical properties and usability characteristics of steel. The steel provides protection and abrasion resistance that are detrimental to synthetic fiber ropes in most hoisting applications.
Using history as a guide, we look to the original wire ropes developed by Albert in 1800s. These were of three-strand and four-strand construction. This “simple design concept” has evolved throughout the decades to include improved manufacturing techniques as well as new and improved materials. Blending the original concept with new technology provides the highest strength-to-weight ratio combined with rotation-resistance (torque-balance) for a given diameter. The cross section of the four-strand hybrid rope is shown in Figure 1.
The torque balance of the rope is achieved through careful selection of the lay angles of the steel wire and synthetic fiber materials within each strand and also the final lay angle of the strands within the rope. This rope has a strength-to-weight ratio of 281 (kN/kg/m),
which is significantly higher than a same diameter 100 percent-steel wire rope.
The inspection of hoisting rope in service on crane is extremely important and stressed to the users through training and manuals. A rope that provides simple retirement criteria results in safe practices for field use. All four strands are on the outside of the rope and visible for inspection; therefore a four-strand rope does not have the disadvantages created by multiple strand layers laid in opposing directions. In all internal and external bending fatigue tests, it has been demonstrated that the four-strand hybrid rope reaches removal criteria by exhibiting broken outer steel wires as shown in Figure 2.
The industry is still trying to catch up with the synthetic lifting revolution. Since there are no existing discard criteria standards for hybrid ropes in hoisting or lifting applications, strictly for the purpose of testing the hybrid rope samples, the ISO standard 4309 RCN 21 was used as two broken wires detected in a length of 6Xd and four broken wires detected in a length of 30Xd (where d is the nominal diameter of the rope). The ASME B30.30–Ropes Standard is aimed at bringing the historically established requirements of steel wire ropes into a single document that would also include ropes that utilize high performance synthetics in their design and construction. Hybrid ropes will be included in the steel section with fully synthetic ropes having their own dedicated section.
End terminations are the most important aspect of any rope system as these products see the most abuse and require a secure and efficient connection between the dynamic rope and stationary termination. In laboratory testing, it has also been demonstrated that resin poured buttons can yield 100 percent termination efficiency while certain types of wedge sockets will yield a minimum of 80 percent termination efficiency with a hybrid rope. Figure 3 and 4 shows the two types of end attachments that have been tested.
The efficiencies listed from the testing are average for steel wire rope terminations. Wedging and socketed terminations require friction between the wedge and the rope to produce a bonded strength that is difficult with full synthetic ropes. The hybrid technology with steel outer wires provides a significant benefit in this application which is traditional in the market today.
A flemished eye can also be spliced in the four-strand hybrid rope showing the product to be a viable candidate for lifting slings. (See Figure 5).
Longitudinal view of the rope
In overhead lifting applications slings that are lightweight and user friendly have become important in the lifting market as ergonomically viable. Combine this attribute with high strength, abrasion resistant, and low torque a lifting sling application fits well with hybrid rope technology. As with hoisting the lifting industry does not define the retirement criteria for hybrid rope lifting slings or high-performance fiber lifting slings. The standard retirement criteria as set forth in ASME B30.9 must be applied in the applicable chapter to provide safe practices in the industry.
Human desires, needs and ingenuity drive advancements in all areas of technology. With payload increases, taller buildings and deeper mining, the demand for new technology in high performance ropes with higher strength-to-weight ratios continues to increase opening the door for hybrid and fully synthetic ropes. As new technologies are introduced, industry equipment design, usage and safety standards must follow suit.