Choosing the correct wire rope for overhead cranes is essential for worker safety and efficient operation. Learning which criteria to consider when replacing your wire rope will help you avoid the most common causes for failure, which include severe overload, slack rope, and crushing or kinking.
Wire Rope Basics
Before we discuss the key factors you need to consider, let’s start with a quick review of wire rope terminology and construction. While the terms wire rope and wire cable are interchangeable, wire cable has a diameter of 3/8” or less, while wire rope has a diameter larger than 3/8”. Wire ropes or cables include a central core surrounded by multiple strands of wires. They are assigned a class based on the number of strands used. For example, a 7×19 wire rope uses 7 strands of 19 wires twisted together to create the rope or cable.
How the wires are twisted together determines the lay of the wire rope, which falls into three categories.
- The wires are twisted one way for a regular lay wire rope, with the strands wrapped in the opposing direction to minimize kinks and untwisting.
- With the lang lay option, the wires and strands are twisted in the same direction to improve flexibility and abrasion resistance.
- With the alternative lay option, the rope combines regular and long lay cables to maximize flexibility while minimizing potential untwisting concerns.
The grades of wire rope are based on the strength of the steel used and include the following five options.
- Improved Plow Steel – tensile strength ranges from 223 to 258 KSI*.
- Extra Improved Plow Steel – tensile strength ranges from 245 to 284 KSI
- Extra Extra Improved Plow Steel – tensile strength ranges from 269 to 312 KSI.
- Galvanized Improved Steel – improves corrosion resistance.
- Drawn Galvanized Improved Plow Steel – provides superior corrosion resistance and higher tensile strength than GIS wire rope.
*KSI is an abbreviation for Kilopounds per Square Inch. If you have any questions regarding wire rope grades and KSI limits, please call 800-713-0103 or send us an email.
Wire Rope Considerations
Essentially, wire ropes fall into two categories:
- Compacted strand ropes that provide increased strength while reducing wear.
- Rotation-resistant ropes that resist twisting under heavy load.
Choosing the best wire rope for your specific application requires careful consideration of the following factors to provide maximum worker safety.
Strength involves more than the working payload weight. Dynamic loads result from sudden starts and stops, high speeds, and dead weight and must be appropriately accessed.
Crushing resistance describes the ability of the wire rope to resist external pressure as it moves along the sheaves and drums. This pressure distorts the inner wire strands, which restricts wire rope movement.
Fatigue resistance describes how well the wire rope absorbs the shock waves, such as vibration, created during regular operation.
Corrosion resistance is determined by the type of steel and the anti-corrosion additives used by the manufacturer.
Bend ability refers to how well a wire rope recovers from being bent. Over time, repeated bending can cause the outer strands to deform and eventually break.
Metal loss & deformation typically occur at the drum or sheaves but can also happen anywhere the wire rope rubs against another object or itself.
Rotation resistance is critical for hoisting wire ropes where stability and control are essential during lifting operations.
Rotation-Resistant Wire Rope
Each category has significant differences in strength, operating range, and rotation resistance, so it is critical to match the crane requirements with one of the following categories based on the length of the fall.
Category 1
- A minimum of 15 outer strands
- Highest strength and rotation resistance
- Can be used with a swivel
Category 2
- A minimum of 10 outer strands
- Medium strength and rotation resistance
- Should not be used with a swivel
Category 3
- No more than 9 outer strands
- Lowest rotation resistance and strength
- Should not be used with a swivel
Because ASME B30.5 allows boom hoist ropes with a design factor of 3.5:1 and a minimum bending ratio of 15:1, most rotation-resistant ropes cannot be used as boom hoist ropes.
Wire Rope Finish
Besides regular inspections and proper operating procedures, the finish (if any) applied to the wire rope can affect its durability and performance. Wire ropes with a bright finish indicate that the high-carbon steel wires and strands have no protective coating. Those with a galvanized finish have a zinc coating to improve corrosion resistance. Those with a zinc-aluminum coating have three times better corrosion resistance than zinc alone.
Should you have any questions regarding the best wire rope for your specific application and load type, please call the Hi-Speed wire rope experts at 800-713-0103 or send us an email today.