Wire rope, often called cable, is sized by diameter (not circumference, as large sizes of fiber rope are). The diameter is measured across the rope, not from flat to flat.
The rope’s construction is described by a designation like “6 × 17”.
Most wire rope is made with six strands. Although wire rope can be made with a wire core, most have a fiber core which is saturated in oil and helps to keep the rope lubricated. Seven strand rope is the same construction as six strand but with a wire core; aircraft cable is an example.
Some commercially-available constructions and the diameters in which consumers might encounter them are:
|6 × 7||haulage rope, transmission, standing rope, aircraft cable.|
|6 × 12||running rope.|
|6 × 19||hoisting rope. diameters of ¼, ⁵⁄₁₆, ³⁄₈, ½ inch.|
|6 × 24|
|6 × 37||special flexible.|
|7 × 7||aircraft cable. Hardware stores sell it in 1/16-inch diameter.|
|7 × 19||aircraft cable, often used in automotive winches. Diameters of ³⁄₃₂, ⅛, ⁵⁄₃₂, ³⁄₁₆, ½ inch.|
|8 × 19||extra flexible.|
|6 × 6 × 7||tiller or hand rope, whose wires are themselves made of wires. Not strong, but very flexible.|
It is also necessary to specify the rope's lay, but this word is used in three different senses in describing wire rope.
First, these lays describe how the wires are twisted to make the strands, and the strands to make the rope.
regular lay rope: the strands are twisted in one direction and the strands are laid into the rope in the opposite direction.
lang lay rope: the strands of the rope are twisted in the same direction as the wires are twisted in the strand. Lang lay wire rope is uncommon; although it resists wear better than regular lay rope does, it also untwists more easily.
right hand lay rope: the twist resembles the twist on an ordinary screw.
left hand lay rope twists in the opposite direction; it is used, for example, in well drilling.
right and left lay rope combines 3 regular lay strands and 3 lang lay strands. Very unusual type.
Second, the word lay is also used to describe the distance, measured along the rope, required for one strand to make one complete turn around the rope, for example, an “18-inch lay.”
Third, while most wire rope is made entirely of wire of the same gauge, a few types combine wires of different gauges, and these are also described as lays.
Seale lay strands have a large inner wire, surrounded by 9 smaller wires, surrounded by 9 large wires.
Warrington lay strands have 7 inside wires surrounded by 12 wires that are alternately large and small.
Wire rope can be damaged by being bent too sharply, so the diameter of the sheave in pulleys or of windlass drums is important. Manufacturer’s recommendations regarding proper sheave size differ. The Coast Guard's rule of thumb is that the diameter of the sheave should not be less than 20 times the diameter of the wire rope. The stiffer the rope, the larger the sheave must be. One manufacturer, for example, suggests that for ¼-inch wire rope, 6 × 19 rope requires a sheave 6 inches in diameter; 6 × 37 requires a 4½-inch sheave; and 8 × 19 requires a 5-inch sheave.
Wire rope is available in galvanized steel. Galvanizing the rope stiffens it and helps it to resist corrosion. Such rope is best used in stationary applications–as guy wires, for example. Running it through pulleys cracks the zinc coating and the rope will corrode even faster than nongalvanized rope.
As wire rope wears, individual wires break, often forming “fishhooks.” If 4% of the individual wires in the length of one lay have broken, the U.S. Navy considers the rope unsafe.
The very large cables used in suspension bridges are often woven in place. Each of the two cables from which the deck of San Francisco's Golden Gate Bridge is hung contains 27,450 wires, made up into 61 strands of 450 wires each. The cable is more than a yard in diameter and weighs 7,125 tons.
John Roebling’s Sons, the firm founded by the man who made the Brooklyn Bridge possible, has an interesting site at www.inventionfactory.com/history/RHAwire/
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Last revised: 11 August 2004.