You’ve probably never really noticed wire rope, but it’s there in lift shafts, holding cranes steady, on lorries holding loads, and many, many other tasks where failure is not an option. The surprising thing is how little attention is paid to something so fundamentally critical to modern industry. Take a piece of wire rope, and it looks simple: a collection of metal strands, twisted together like any other cable, doing exactly what you’d expect a cable to do. But there’s a lot more to wire rope than that.
The first step in understanding wire rope is to grasp what makes it work, and, more importantly, what makes it fail, if you’re ever in a position to specify its use in any particular task.
The Effect of Construction on Wire Rope
Wire rope is not a single product, but a family of dozens of different types, each with its own particular needs. At its most basic, a wire rope is a collection of metal strands, with multiple individual wires in each strand, which are then twisted around a core to make a rope. But the specifics of exactly how this is done determine exactly what a wire rope will actually be capable of.
So a 6×19 construction indicates six wires per strand, and about 19 wires per rope. This is a relatively flexible rope that will work very well for bending around smaller diameter sheaves but won’t offer the greatest resistance to abrasion. A 6×36 construction will have even more wires per strand and will be even more flexible but will have a slightly lower breaking strength for a given diameter. Then come the more exotic constructions – 8-strand rope, 19-strand rope, compacted strand rope, and rotation-resistant rope. Each of these constructions solves a special problem. The core of a wire rope is important but not as important as most people think. Fibre cores are made of either polypropylene or natural fibres and are good for use in bending around a drum or sheave. Steel cores have greater strength and greater crush resistance and are better if side loading or compression is a problem. Independent wire rope cores have the greatest strength but are not very flexible.
Matching Rope to Application
The choice of wire rope depends on understanding the forces that the wire rope will have to withstand. Crane ropes have to withstand repeated cycles of bending over sheaves. Static guy wires have to support structures, so they have to have ultimate strength and resistance to corrosion. Winches on vehicles have to have good resistance to abrasion and shock loads.
The choice of size of wire rope depends on balancing the need for sufficient strength against the size of the drums or sheaves. Running wire rope over sheaves that are too small causes rapid degradation of the wire rope. The tighter the bend radius, the shorter the life of the wire rope. There are guidelines for the ratio of sheave diameter to wire rope diameter. The ratio can vary from 18:1 for light-duty applications up to much higher ratios for more severe applications.
The end terminations of wire rope are almost as important as the wire rope itself. Good ferrules, swaging, or splices can retain up to 90% of the full strength of the wire rope. Badly made end terminations can cause problems. They can act as a weak point, which will ultimately cause failure, irrespective of the condition of the wire rope. Wire rope clips, or bulldog grips, are often used for wire rope. They can achieve only 80% efficiency in use, provided they are fitted correctly. Unfortunately, they are rarely fitted correctly.
Inspection Isn’t Optional
It is essential to regularly inspect wire ropes in use. The frequency of inspection depends on the severity of the use. External inspection looks for signs of broken wires, wear, corrosion, or distortion. More detailed inspection can involve internal inspection or even magnetic inspection for signs of internal degradation.
The key thing to remember is that wire rope is a consumable item. It has a finite lifespan. Wire rope should not be treated otherwise. If it is, problems will arise.