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.
