Abstract — Over the years, wire rope users have sought high-strength wire ropes as a way to reduce costs. High strength-to-weight ratios reduce the load that wire rope places on the system, thereby reducing equipment costs. On aerial tramways, wire ropes with smaller diameters mean smaller sheaves, smaller bullwheels, smaller gear boxes and so on. In this case, smaller means less expensive.
However, there is an upper limit to the strength available in conventional wire rope. This led to the development of special design ropes like compacted strand wire ropes and ropes with very high tensile strength wires. This paper will present some of the methods used to achieve high tensile strength wire rope and also some of the related problems. Fatigue life, as well as removal criteria for high-tensile strength ropes, will also be discussed.
Early History — Wire rope didn’t just happen; it evolved slowly during the Middle Ages in Europe and flowered with the establishment of the wire-drawing industry. Metal wire was added to the already existing fiber cordage ropes. The lack of technology for making long lengths of metal wire inhibited faster development.
The early developmental period for iron wire rope in Europe and America took place between 1816 and 1850. The Industrial Revolution had created a demand for high-tensile cordage, especially with the rapid growth of the shipbuilding and mining industries. The vegetable fiber ropes were no longer adequate.
The Industrial Revolution started in England, so it is no surprise that England produced the first innovations in ironmaking, which in due time would lead to a wire rope technology.
Henry Cort’s concepts of puddled wrought iron, together with the grooved iron rolling mill for production of long iron rods and wire, created a ready availability of this superior product.
Important also was the new steam-powered cordage-making machine. Prior to this invention, all fiber ropes were hand-made.
The Albert Rope — Wilhelm Albert, of Clausthal, Germany, a government mine official in the Hartz mountains, was the unofficial inventor of stranded wire rope in 1834. Until this time, hemp ropes, along with iron chains, had been used as lifting devices in the mining industry. Hemp ropes were costly because they had to be imported, and they rotted easily in the damp conditions; iron chains were also unreliable as breaks occurred daily.
The Albert rope managed to combine the advantages of hemp rope and iron chain by replacing the fiber in rope with iron wire found in chain. This first successfully stranded rope consisted of three strands, each consisting of four wrought iron wires, with the strands and the rope closed in right lay. Thus the first rope was manufactured in Lang’s lay.
Wilhelm Albert’s wire rope was made of iron with an estimated tensile value of 70,000 psi. Today, wire ropes are manufactured with tensile strengths of 300,000 psi and more.
Development of High Tensile Wire — In 1856 in England, John Fowler invented a steam-powered plow that required high-strength wire ropes; ropes made of iron were not sufficient for heavy machinery. Fortunately, a new type of high-tensile steel wire was appearing on the English market at this time. The new wire, which originally was devised for pianos, proved to be so superior for steam plowing that it became known as Plow Steel, a term still in use.
The new steel wire heat-treating process was invented by James Horsfall in England, for which he received a patent in 1854. This form of heat-treated steel wire is still called “patented” wire everywhere in the world, and the process is called the “patent process” or “patenting.”
Patenting changes the microstructure of the steel. In this process, the steel is heated to a temperature usually in excess of 1,600 degrees Fahrenheit. At this temperature the iron carbide is dissolved in the iron to form a solid solution called Austenite. On cooling under suitable conditions, the carbide plates form once more in the temperature range of 1,300 to 950 degrees F. The lower the temperature at which they form, the thinner the carbide plates are. These thin plates, even though they are made of brittle material, will bend without breaking. This is why patented material can be drawn comparatively easily. The thinner the carbide plates, the higher the tensile strength and the higher the ductility.
Modern Wire Rope — Tom Seale, an enigmatic Irish-American, invented the modern wire rope in 1855. He had made a fortune during the Gold Rush, and, during the 1880’s, was superintendent of the California Street Cable Car Line.
He recognized during this time that wire strands composed of round wires all the same size were not adequate for cable car service because of the severe exterior friction. He was convinced that this could be corrected by using smaller wires inside the strand and larger wires on the outside.
Seale discovered that wire ropes were breaking prematurely, and long before their cross-sectional area diminished to the point of insufficient tensile strength. The cause was cross-over fatigue. Seale realized that the wire breakage in a used cross-laid rope occurred at the cross-over section. He prevented this from happening by devising equal laid ropes where the outer layers always follow the valley created by the inner ones.
Seale patented his invention in 1885, but eventually sold his license.
Extra Extra Improved Plow (EEIP) — Today, three wire rope grades are common for bright steel: Improved Plow (IP), Extra Improved Plow (EIP) and Extra Extra Improved Plow (EEIP). For fiber core wire ropes, each grade is 10 percent stronger than the preceding one.
EEIP was adopted in the 1970’s after several wire rope manufacturers began making wire ropes in strengths above EIP. Paulsen Wire Rope Corp. manufactured a wire rope called XL 20, which was a fiber core 20 percent above the grade of IP. U.S. Steel manufactured another one, Monitor AAA, which was 10 percent above EIP. The American tramway standard, ANSI B77, was one of the first to publish EEIP as a wire rope grade in 1982. Today, EEIP is the highest commonly recognized standard wire rope strength.
Increasing Rope Strength Above EEIP
Following are the factors affecting the strength of wire rope. Along with strength, the factors of economy and safety must also be considered for successful commercialization of wire rope.
Lengthen the Lay
Wires are laid (twisted) into strands and strands are laid into ropes in order to hold them together and to be able to transfer load around sheaves. One strand revolution around a rope is called a lay length. The closer the wires and strands are to paralleling the load axis, the stronger the wire rope, but the poorer the ability to transfer the load around a corner or sheave. The poor ability to transfer load leads to a shorter fatigue life in running ropes.
A typical ratio for running ropes is 6½ d/D. The maximum lay length permitted by the military specification RR-410-D is 6¾ d/D. The average rope with a 6½ ratio has a strength equal to the total of the wire strength less an efficiency factor of about 15 percent. The rope strength can be increased by lengthening the lay at the expense of a shorter fatigue life. An increase to 7½ D/d lay yields over a five percent increase in strength.
Five Ways to Increase Metallic Density
A) Compacting the strands. This is a process of pressing the wires together to form a higher density of the metallic area. One method, invented by Charles Banks of Washington, England, in 1891, is to pull the wires through a die or series of dies to plastically deform the strand or rope. The process was granted a patent which was assigned to British Ropes and came to be known as “Dyform.” Compacted ropes can offer strengths five or more percent above those of standard ropes.
The advantage in using compacted rope is increased strength through the use of increased metallic area for a given rope diameter. Other advantages include a flatter wearing surface for improved sheave liner life, reduced constructional stretch, reduced interstrand nicking and improved diameter stability. A few of the disadvantages are increased cost and only a marginal increase in strength because of lower psi strength, which results from the deformed steel not having the unit strength of round wire.
B) Select a rope construction with higher metallic area. For example, a 1″ dia. 6×19 fiber core Seale construction has a metallic area of .404 sq. in., while the same strand in a Warrington construction has .416. The problem is that the Warrington construction, while three percent stronger, may not provide good service when used as a ski rope because of its smaller outer wires.
C) Use a steel inner rope or strands. The advantage is increased metallic area. The disadvantage is reduced fatigue life because of the interstrand contact, difficulty in inspecting and added weight. A rope with an independent wire rope core (IWRC) has a catalogue breaking strength seven percent higher than fiber core (FC).
D) Manufacture the rope to the maximum allowable oversize. If the tolerance is between minus one and plus five percent oversize, as it is with ANSI B77 standard, manufacture the rope to the higher value. The disadvantage is that this may not be the desired size and problems may develop with grip clearances. In reality this is just a slightly larger wire rope. For a rope designed to plus three percent and changed to five percent oversized, a four percent strength increase is realized.
E) Form round strands into pie shapes to form a higher metallic area. This process, developed by Telford Batchelor in 1884, is called flattened strand. When used as a ski rope, fatigue life may be questionable.
There is also the problem of splicing the strand. Unlike the more common six-strand wire rope, where the strand diameter closely matches the center strand (which it replaces in the ski lift long splice), flattened strands have a much smaller core. Flattened strand wire rope is also more expensive. For flattened strand designs, add 10 percent to the strength.
Increase Wire Tensile Strength
Over the years, one method of increasing wire tensile strength has been to increase the cold working of the high carbon steel. This has often been at the expense of wire ductility.
Fatigue tests done by Southwest Research Institute for the Wire Rope Technical Board have indicated that in some rope constructions, EEIP may have shorter remaining service life after reaching removal criteria than either IP or EIP rope.
To provide good service life, some wire rope companies have manufactured high strength wire for EEIP ropes, with wire torsions meeting the more stringent IP grade. These provide more ductility than ropes made to the lower EEIP torsions. An EEIP wire rope mill run with IP torsions can be, and is, sold as IP, EIP or EEIP grade, depending on market conditions. For ropes made to the lower EEIP torsions, a few percent additional strength may be gained. However, lower ductility will likely provide a shorter fatigue life. It should be noted that comparing wire rope fatigue life in the ski industry is not easy, as haul ropes often last 20 to 30 years.
Another method for increasing wire rope tensile strength is stress relieving, as has been done by U.S. Steel. Their Wire Rope Division manufactured wire ropes 10 percent stronger than EEIP for use in mining applications in the 1970’s. The wire rope was designated Monitor AAA. The extra strength was accomplished by heat treating (stress relieving) the finished strand so all wire is elongated together.
There is not much historical data for wire ropes in service above EEIP grade. For this reason, high-tensile strength wire and wire ropes should be very well studied before being used for transporting people.
Following is a partial list of wire tests currently available for rope wire:
1. The tensile test is the most common, reliable and informative of all wire tests. It is done on a length of wire, usually held by grips according to the requirements of ASTM A370. The specimen is pulled in a pull-test machine until destruction, and the highest strength value recorded.
2. The torsion test is considered a measurement of wire ductility. It is conducted on a single piece of wire held at one end while the other end is rotated about its axis. Torsion values for different wire sizes can be found in the U.S. Federal Specification RR-W-410D and in ANSI B77 1992. Procedures for conducting torsion tests can be found in ASTM E558 and ISO R136.
3. Reduction of area is used to measure ductility of rope in sizes above 0.092 in. (2.34 mm). The reduced section is difficult to measure in sizes below this.
4. The reverse-bend test can be used as a measurement of wire ductility, although it is not popular in the US. It is conducted on a single piece of wire, held in a vise, and bent back and forth over a given radius.
5. With elongation testing, gauge marks are placed on a length of wire, usually 10 inches apart. The specimen is pulled to failure and the distance measured. Experts disagree on the value of the elongation testing on cold drawn wires used in wire rope.
6. Hardness tests are difficult to perform on wire the size typically used in wire rope, and are not recommended.
7. Inspection of the wire rope: Look for loose wires, high helix or high strand or loose strand. Check the original wire rope design for correctness. Check the rope for correctly designed wire sizes, strand sizes, rope and strand lay length and consistent lubrication of the rope.
Conclusion — Today, with new technology, steel mills are producing better quality steel than ever. This represents an opportunity to gain experience with wire rope grades above EEIP, by using higher tensile wire and/or new or rediscovered wire rope designs. However, we must prove that our existing retirement criteria work for high strength ropes, or establish new ones.
