What are steel strand and steel ropes?

Iron and Steel Wire. Annealed wire of iron or very mild steel has a tensile strength in the range of 310 to 415 MPa (45,000 to 60,000 lb/in2); with increased carbon content, varying amounts of cold drawing, and various heat treatments, the tensile strength ranges all the way from the latter figures up to about 3450 MPa (500,000 lb/in2), but a figure of about 1725 MPa (250,000 lb/in2) represents the ordinary limit for wire for important structural purposes.

For example, see the following paragraph on bridge wire. Wires of high carbon content can be tempered for special applications such as spring wire. The yield strength of cold-drawn steel wire is 65% to 80% of its ultimate strength. For examples showing the effects of drawing and carbon content on wire, see Making, Shaping, and Treating of Steel, U.S. Steel.

Galvanized-Steel Bridge Wire. The manufacture of high-strength bridge wire like that used for the cables and hangers of suspension bridges such as the San Francisco–Oakland Bay Bridge, the Mackinac Bridge in Michigan, and the Narrows Bridge in New York is an excellent example of careful control of processing to produce a quality material.

The wire is a high-carbon product containing 0.75% to 0.85% carbon with maximum limits placed on potentially harmful impurities. Rolling temperatures are carefully specified, and the wire is subjected to a special heat treatment called patenting.

The steel is transformed in a controlled-temperature molten lead bath to ensure an optimal microstructure. This is followed by cold drawing to a minimum tensile strength of 1550 MPa (225,000 lb/in2) and a 4% elongation.

The wire is given a heavy zinc coating to protect against corrosion. Joints or splices are made with cold-pressed sleeves which develop practically the full strength of the wire. Fatigue tests of galvanized bridge wire in reversed bending indicate that the endurance limit of the coated wire is only about 345 to 415 MPa (50,000 to 60,000 lb/in2).

Wire Rope. Wire rope is made of wires twisted together in certain typical constructions and may be either flat or round. Flat ropes consist of a number of strands of alternately right and left lay, sewed together with soft iron to form a band or belt; they are sometimes of advantage in mine hoists.

Round ropes are composed of a number of wire strands twisted around a hemp core or around a wire strand or wire rope. The standard wire rope is made of six strands twisted around a hemp core, but for special purposes, four, five, seven, eight, nine, or any reasonable number of strands may be used.

The hemp is usually saturated with a lubricant, which should be free from acids or corrosive substances;

this provides little additional strength but acts as a cushion to preserve the shape of the rope and helps to lubricate the wires. The number of wires commonly used in the strands are 4, 7, 12, 19, 24, and 37, depending on the service for which the ropes are intended.

When extra flexibility is required, the strands of a rope sometimes consist of ropes, which in turn are made of strands around a hemp core. Ordinarily, the wires are twisted into strands in the opposite direction to the twist of the strands in the rope. The makeup of standard hoisting rope is 6 X 19; extrapliable hoisting rope is 8 X 19 or 6 X 37; transmission or haulage rope is 6 X 7; hawsers and mooring lines are 6 X 12 or 6 X 19 or 6 X 24 or 6 X 37, etc.; tiller or hand rope is 6 X 7; highway guard-rail strand is 3 X 7; galvanized mast-arm rope is 9 X 4 with a cotton center.

The tensile strength of the wire ranges, in different grades, from 415 to 2415 MPa (60,000 to 350,000 lb/in2), depending on the material, diameter, and treatment. The maximum tensile efficiency of wire rope is 90%; the average is about 82.5%, being higher for 6 X 7 rope and lower for 6 X 37 construction.

The apparent modulus of elasticity for steel cables in service may be assumed to be 62 to 83 X 106 kPa (9 to 12 X 106 lb/in2) of cable section. Grades of wire rope are (from historic origins) referred to as traction, mild plow, plow, improved plow, and extra improved plow steel. The most common finish for steel wire is “bright” or uncoated, but various coatings, particularly zinc (galvanized), are used.

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