Enhancing steel wire rope longevity

Steel wire ropes, renowned for their exceptional durability and strength, are indispensable in heavy-lifting industries such as construction, mining, and maritime. At the annual LEEASA conference, Donald Coward of Engineering Concepts provided valuable insights into optimising rope design and maintenance to extend their lifespan and reliability.

Designed to transmit high tensile loads over large distances efficiently, steel wire ropes demonstrate engineering at its finest. However, to maximise their operational life, a careful approach to design and maintenance is essential. From initial wire drawing to combating wear and fatigue, every step in a rope’s life cycle impacts its performance and resilience, said Coward, emphasising the importance of understanding the complexities of steel wire rope design, deterioration mechanisms, and strategies to optimise their longevity.

Coward explained to delegates that the journey of steel wire rope begins with the production of steel, which is subsequently drawn into rods and wires. “The wire-drawing process not only reduces the wire’s diameter but also aligns its microstructure, thereby increasing tensile strength. Precision at this stage is crucial; over- or under-drawing the wire can negatively impact its integrity and durability,” he said.Following drawing, the wires move to the stranding phase, where they are combined into strands. Stranding requires precision machinery where numerous bobbins feed wire into a central point, forming the desired strand.

Each strand is engineered to meet specific strength, flexibility, and load requirements, using configurations such as parallel, unequal and cross lay. In Parallel Lay ropes, all wires are closed in a single operation, with each wire laid in the same direction. This design results in varying wire lengths across layers. If the the torque is unbalanced it will cause the rope to unlay under load.

In Unequal Lay ropes, each layer is added in a separate operation. The wire lengths are roughly equal, leading to improved load sharing. However, unbalanced torque remains a concern, and these ropes can also unlay under load.

Cross Lay ropes are constructed by adding each layer in an individual operation, with machine rotation alternating for each layer. This method produces nearly equal wire lengths but generates high-stress points where the wires cross over one another.

Key design elements

According to Coward, steel wire ropes can be configured with various design features to meet specific operational needs. Factors such as the number of strands, lay type and wire layering all influence a rope’s suitability for different applications.

For instance, six-strand ropes, commonly used, bear most of the load on two strands. Conversely, eight-strand ropes distribute the load across three strands, improving fatigue resistance.

Rope designs also incorporate a core wire for added stability, with additional layers or fillers enhancing flexibility and load capacity. Advanced rope manufacturing also leverages the geometry of circles and ellipses to calculate optimal wire and strand sizes.

The elliptical theory applies for small lay angles, where long lay lengths and elliptical cross-sections provide enhanced strength. For larger lay angles, ellipsoidal cross-sections can be more effective. “The key consideration is the load being lifted and how it is distributed within the strand. When lifting, slings are often angled, causing the load to decrease at the points of contact.”

“With parallel lay ropes, the tensile load is fully realised within the strand, but when the rope runs over a sheave, the strand may collapse and experience excessive friction.”

“To address this, we twist the strands together, preventing collapse, but this introduces spinning losses.”

“Using finite element analysis, we can now identify high-stress points in a strand or wire and modify the rope design to better distribute stress. The goal is to stabilise the strand design for improved performance and durability,” he said.

Managing deterioration and fatigue

Coward said despite robust design, steel wire ropes are susceptible to wear and fatigue over time. “Routine inspections are critical for detecting signs of wear and damage before they compromise safety. “Tools like magnetic rope tests can identify internal breaks, while electronic monitoring can track stress points along the rope’s length.”

“Industry codes of practice mandate non-destructive testing to evaluate rope health, accounting for different types and functions of ropes,” he said.

He said to mitigate deterioration, both design and maintenance played pivotal roles.

“Abrasion is a major factor in rope deterioration, often caused by contact with rough surfaces. Design solutions include using harder steel grades, shorter lay lengths or larger outer wires. Installation practices, such as avoiding contact with abrasive surfaces and ensuring proper lubrication, can further reduce abrasion.” He said fatigue sets in when wires undergo repeated stress, often due to bending over sheaves. Design adaptations like changing lay ratios or selecting high-grade wire steel could extend fatigue life. Maintenance practices such as periodically cutting and shifting the rope and ensuring proper lubrication at high-stress points would also help mitigate fatigue.

“Corrosion weakens ropes, especially in marine or industrial environments. Using galvanised or plastic-coated wires provides a barrier against moisture and chemicals. Regular in-service lubrication is also essential to combat corrosion over time, said Coward.

“Mechanical damage can also happen. Improper handling can introduce damage that accelerates rope wear. To prevent this, ropes should be unrolled carefully without forming loops, and lifting devices should be used to avoid dragging the rope over sharp edges.”