Types of Wire Rope Construction
Most riggers will say, there isn’t a “best” wire rope. To understand which wire rope should be used, you should ask, “what is it being used for?”. The construction of the wire rope tells us many things, including the flexibility, fatigue resistance, and lifespan of the wire rope in its specific use case. Using the wrong construction option does not mean the wire rope will fail immediately; the real cost of the wrong choice is hidden and can be significant.
Construction Abbreviations
The construction of wire rope is explained using the number of strands and the number of wires per strand. For example, 7X7 wire rope has 7 strands of wires and each of those strands has 7 wires. So, in total a 7X7 wire rope has 49 wires. 7X19 wire rope has 133 wires. 1X19 has only 19 total wires, since it has 1 strand and no stranding.
With more wires comes the ability to get smaller diameters which means flexibility. A rope that has fewer but larger wires will be less flexible but more resistant to abrasion.
1×19: Stiff, Strong, Not for Tight Bends
1X19 rope is the stiffest rope of the three mentioned. Because the 1X19 rope has laid the 19 wires in a single strand and has no core, the rope has a high break load for its diameter, but the flexibility is essentially zero. Due to this construction, it does not tolerate being bent, wound around a drum, or pulled through a pulley.
Where is it effective? Tension applications, guard wires, balustrade cables, stays, and other applications where the rope runs straight and remains in that position. Common applications include rigging for architecture, stainless steel balustrading, and aerial ropeway guide wires. 1×19 should not be used in any application where the rope will run through bends; the outer wires will fatigue and crack in a short time for those applications.
7×7: The Flexibility Trade Off
7×7 construction is more flexible than 1×19, though it possesses moderate resistance to wear. The arrangement of the seven wires provides more movement allowing the rope to work through moderate sheave or pulley arrangements without incurring fatigue. The relatively large diameter of the wires of 7×7 construction compared to 7×19 construction also improves the surface wear when in contact with guides or abrasives.
This construction is good for general purpose applications. It is well suited to hand winches, light cranes, push-pull control cables and other applications with moderate bending over pulleys. 7×7 construction is not suited for high cycle applications where the rope is constantly tensioned and wound over a drum.
7×19: The Flexible Option
7×19 construction is the most flexible construction of the options and will allow bending with more fatigue than the other constructions. This construction is suited for drum winches, hoists, and any application where the rope will be repeatedly bent around a sheave many hundreds or thousands of times.
The drawback, however, is that 7×19 wire has greater surface area than 7×7 and 1×19 construction wires. Because of this, 7×19 wire has a tendency to wear faster than 7×7 or 1×19 wire. When using 7×19 rope on drum winches, you have to inspect the rope more frequently to check if any of the outer wires are broken.
Concerning wire rope grades (stainless 316 vs stainless 304, or wire that is galvanized vs. wire that is ungalvanized), they have separate considerations from construction. Grade has to do with the environment. Construction is important too and is about the rope’s mechanical properties and the application. For marine and chemically exposed environments rope, the construction is 316 stainless, and for general galvanized outdoor rope, it is galvanized steel rope.
One of the most important specs about wire rope is the minimum breaking load, and this is most commonly neglected. It is most important to compare the breaking load to the dynamic loads a rope will face during actual application, not to the static loads. Especially when rope is used for hoisting and pulling, static loads are often much greater, sometimes by a factor of 2. Construction of the rope is important when considering how the rope will absorb and distribute that load.