An Introduction to
WIRE ROPE...

Introduction

Wire rope is metal in its strongest form. It consists of a group of strands laid helically around a core. The strands of a wire rope, or cable, consist of a number of individual wires laid about a central wire.

The terms "wire rope" and "cable" are used interchangeably. There is some tendency to use the term "wire rope" for sizes 1/4 in. and larger and "cable" for the smaller sizes. But this is not uniform practice and either name is correct.

Wire rope is versatile. It can be used to transmit motion through almost any plane or angle, to guy or tie down, to hold back, launch or control; to counterbalance; to guide or to lift; or to do hundreds of other jobs. It has a long life and needs little or no maintenance.

Most people do not think of wire rope as a machine, but it is. It is a machine composed of a number of precise, moving parts--all designed to bear a very definite relation to one another. In fact, some wire ropes contain more moving parts than many complicated mechanisms. A six-strand rope consisting of 49 wires per strand, laid around an independent wire rope core, contains a total of 343 individual wires. All of these must be able to blend and move with respect to one another if the rope is to have the flexibility necessary for successful operation.

Construction

Wire rope is composed of wires, strands and a core.

The basic unit is the wire, which is formed into strands.

Various types of wire rope have been designed to meet a wide range of uses and operating conditions. These types are designated by the kind of core; the number of strands; the number, sizes and arrangement of the wires in each strand; and the way in which the wires and strands are wound, or laid, about each other.

Cores

Wire-rope cores are made of fiber, cotton, asbestos, polyvinyl plastic or wire.

Manila or sisal fiber (F.C.) is the type of core often used when loads are not too great. It supports strands in their relative positions and cushions the wires to prevent their nicking each other.

Cotton fiber is used for small ropes such as sash cord and aircraft cord.

Asbestos cores can be furnished for certain operations where the ripe is used in oven operations.

Wire cores are made in two different forms. The one used most extensively is a wire rope of suitable size to serve as a core. It is called as independent wire rope core (IWRC).

The other type of wire core is a wire strand structure (WSC or SC). This consists of a multiple-wire strand, and may be the same construction as the main rope strands.

Plastic cores include the four following general types:

Polypropylene cores, made up of a multiplicity of synthetic filaments extruded from a petrochemical resin. These are similar in physical construction to fiber cores.

Plastic impregnated fibre cores are sisal fiber cores impregnated with polyvinyl chloride (PVC).

Solid plastic cores are rods of PVC plastic.

Plastic covered fibre cores have PVC extruded to a specific thickness around the core.

Fiber cores look like an ordinary hemp fiber rope. But in construction and lubrication they will differ somewhat. Because of its wearing qualities and resiliency this core was for years the most popular center. But wire cores offer less stretch, have better resistance to heavy crushing loads and are not affected by heat.

The IWRC type has about the same flexibility as the fiber core rope and can be used interchangeably without changing sheaves or drums. It increases the rope strength by at least 7.5%.

The problem of selecting the proper rope is simplified by the manufacturers. Their recommendation booklets give the proper wire rope grade and construction for the intended service. Use of these booklets can assure you that you are giving the customer the best wire rope for his job.

Left Lang Lay

Right Lang Lay

Rope Materials

Wire rope, with but few exceptions, is made from cold drawn carbon steel wires. There have been many different strength grades of rope made. Some are listed below in the order of increasing strengths:

Iron (actually low carbon steel)

Traction steel

Mild plow steel

Plow steel

Improved plow steel

Extra improved plow steel

The bulk of commercial wire rope is made from Improved Plow Steel (IPS), a high-carbon steel with a tensile strength of about 260,000 psi. IPS is about 15% stronger than plow steel, which got its name from the original high-carbon crucible furnace steel used to produce plowshares.

An even higher grade is Extra Improved Plow Steel, which is 15% stronger than IPS. Various manufacturers have their own name for this grade. It was developed for applications needing greater safety factors without a diameter increase, such as rotary oil-well drilling--and for maximum resistance to abrasive wear, such as draglines in strip mining through rocky terrain. This premium grade has tensile strength ranging from 280,000 to 340,000 psi.

Each of these grades are usually furnished in "bright" or "self-colored" condition, which means that no special metallic or chemical coating has been applied, except for lubrication.

If operating conditions necessitate greater resistance to corrosion than that provided by the usual bright carbon steel wires, galvanized wires can be used. The resulting wire ropes usually have listed strengths 10% less than corresponding types of bright carbon steel rope. Usually the grease applied to a rope at the point of final twist during manufacture is sufficient to lubricate the rope against internal abrasion and to protect it against normal oxidation.

Two other grades are iron and traction. Iron is a misnomer; it is a low carbon steel wire of about 100,000 psi--very ductile and able to undergo repeated bending stresses around small sheaves. Although largely replaced by traction, it still remains effective for guys, tillers and sash ropes.

Traction steel gets its name from the traction type elevators on which it has found widespread acceptance as a hoisting rope. Its tensile strength ranges from 180,000 to 190,000 psi. Its high resistance to bending fatigue and minimum abrasive force on sheaves and drums are both of extreme importance for long life in elevator service.

In recent years some of the traction sheaves in new elevators have been made with greater wear resistant material. This has permitted the use of higher strength rope, called extra high strength elevator rope.

A limited amount of iron elevator rope is still used for replacements on some of the older machines, but is gradually becoming obsolete.

The grades mention above are the important grades used for construction, mining and industrial equipment. However there are other materials available, such as galvanized aircraft cable and stainless steel aircraft cable, both of which are for specialized applications. Some bronze ropes are also available for marine use, and monel metal has occasionally been used for corrosion resistance.

Wire Rope Lays

Wire ropes have two types of lay. Lay means the direction in which the wires and strands are twisted around the rope.

Regular lay, as opposed to lang lay, denotes the direction of wire twist in the strands. In regular lay rope the wires in each strand lie in the opposite direction from the strands.

In lang lay rope the wires in each strand lie in the same direction as the strands.

Right or left are used to refer to the lay of the strands.

Right regular lay is generally understood to be intended , unless other lay is specified. Most machines on which wire rope is used are designed for right regular lay rope. Exceptions occur in the case of certain types of well-drilling equipment.

In some rope applications, direction of lay is important to proper performance of the job. In operating a clam-shell crane without a tag line to prevent rotation of the bucket--holding and closing lines are frequently opposite in direction of lay.

Right regular lay

Left regular lay

Preforming

Most ropes are preformed. A permanent helical twist is mechanically imparted to each strand before assembly. This eliminates built-in stress in the finished rope, and results in a better performing and longer-lived rope. With few exceptions (some drilling cables, some elevator ropes) almost all wire rope today is made preformed.

Classification

In a numerical classification of rope construction, the first number is the number of strands; the second is the number is the number of strands. Thus, 6x25 means six strands of twenty five wires per strand.

When such numbers are used as designations of standard wire rope classes, the second figure in the designation may be purely nominal, in that the number of wires per strand for various ropes in the class may be slighty less or slightly more than nominal.

For ropes with a wire strand core, a second group of two numbers may be used to indicate the construction of the wire core, such as 1x21, 1x43, etc.

The standard constructions of rope fall into four general classifications: 6x7, 6x19, 6x37, and 8x19.

In the first three, each rope contains six strands. In the fourth, each rope contains eight strands.

Course Laid--The 6x7 class [six strands of seven wires each] is comparatively simple since it represents only one specific wire rope. These wires are generally furnished in right regular lay, and occasionally in lang lay. Cores may be fiber, independent wire rope or wire strand [FC, IWRC or WSC]. Since this is a relatively stiff type of construction, ropes in this class should be used with large sheaves and drums. Rope in this classification has excellent resistance to abrasion and pressure. Heavy hauling, rope transmission, well drilling are common applications.

Flexible, sometimes referred to as standard hoisting rope, covers six-strand rope containing as few as nine, but not greater than 26 wires per strand, with these wires arranged in several different strand patterns. This is the most popular and widely used class of wire ropes. Ropes in this class are furnished on a regular or lang lay. Cores may be fiber, independent wire rope or strand. The different constructions in this classification which have the greater number of wires generally are the more flexible, while the ones containing fewer wires are more resistant to abrasion and pressure.

Extra flexible rope, includes those containing six strands, with the number of wires per strand varying from 27 to 49, although the group is designated 6x37. The group is also referred to as extra flexible hoisting rope. Here, again, the greater number of wires the greater the flexibility, and the less resistance to pressure and abrasion.

Ropes in this grouping, in the popular size range, lend themselves quite readily to applications where they are flexed back and forth over sheaves and drums without being subjected to any great amount of abrasive action. Overhead crane ropes are an example of this type of service.

In the larger diameters, 6x37 rope, lang lay with IWRC, is recommended for shovel, dragline and dredge hoist rope. In these larger sizes the 6x37 rope has sufficient abrasion and pressure resistance for these particular operations.

Ropes in this class are furnished in regular and lang lay with fiber core or independent wire rope core, preformed or not preformed.

Special flexible hoisting rope is a stable smooth-running rope, especially suitable, because of its flexibility, for high speed operation with reverse bends. This 8x19 class has 8 strands, usually around a fiber core. Most of this rope is right lay, regular lay. A large percentage of elevator rope, which is usually in the 1/2 or 5/8 in. size, is supplied in the 8x19 classification because of its suitability for this type of duty.

Strand Wire Arrangement

Most of the different wire arrangements used in the strands of the wire rope can be classified into general types: Filler Wire, Seale and Warrington. The Warrington type differs from the others in that the outside layer of wires in each strand of the wire rope is composed of wires alternately large and small. A Warrington construction of rope therefore can be easily identified by this characteristic.

The outside wires of both the Filler Wire and Seale construction ropes are uniform in size. The fundamental difference between these types is that the layer of wires underneath the outside layer in the Seale type is made up of wires all of the same size. The wires under the outside layer of the Filler Wire rope are made up of a combination of main wires, each of the same size, and smaller filler wires, each of the same size, nested between the main wires. The outside layer of wires, therefore, is supported partly by the main inside wires and partly by the filler wires.

In the 6x37 class, because of the greater number of wires used, combinations of the Filler Wire, Seale and Warrington types are used. Wire sizes would become too large if only one of the three fundamental patterns were used.

In the 6x37 class the strand patterns include:

Filler Wire

Filler Wire Seale

Warrington Seale

Seale Filler Wire

Seale Warrington

Seale Warrington Seale

In the 8x19 class it is customary to use one of the three basic types.

Another construction, called Two Operation, is also available. Its use is confined, with the exception of a few special ropes, to the smaller diameter wire ropes in the 6x19, 8x19 and 6x37 groups.

Special Ropes

For some applications, wire rope constructions different from the usual types are needed to obtain the most satisfactory results.

Ropes used for marines and ship duty, for example, Must be easy to handle and be able to withstand corrosion. If steel ropes are used, a galvanized finish on the individual wires provides corrosion resistance. Relatively large wires are needed to provide enough metal to prolong resistance to corrosive attack. Large wires usually mean less flexibility--which reduces handling ease.

To get a rope with the desired handling properties and corrosion resistance for marine service, the fiber content is increased. Reducing the steel content also reduces the strength, but the sacrifice is necessary to obtain the properties needed for this service.

Coated wire ropes are available. These may be vinyl coated galvanized, nylon coated stainless steel, or similar combination. The coating protects the rope and worker's hands. The plastic coatings are generally transparent so the rope can be visually inspected. Other materials are also used to coat wire ropes.

Marline clad wire rope has individual strands coated with marline before they are laid around the core. Its use is limited to applications where the marline exterior is desired for easier handling, or where additional friction, provided by the marline covering, is needed for friction drives. One such duty is car puller service, where the rope makes several wraps around a power driven capstan and traction is obtained by the friction developed between the marline and the capstan.

Galvanized steel running rope, a 6x12 right lay, regular lay construction with fiber core, is a flexible rope with good corrosion resistance-although not particularly high strength. Each of the six strands has an outer layer of 12 galvanized steel wires. These strands do not have inside wires, such as is customary with standard 6x19 classification rope. Fiber is used in place of these inside wires.

Galvanized steel mooring lines and hawsers are 6x24, right lay, regular lay with fiber core. Sometimes, in even larger sizes of marine ropes, a 6x30 galvanized steel wire rope is used; right lay, regular lay, with fiber core, also having fiber cores in each of the six strands. With ropes this large, however, a conventional 6x37 classification galvanized steel rope with fiber core is sufficiently flexible and durable for the duty, which is usually a towing hawser.

Non-rotating ropes are designed for hoisting applications where a free load is suspended on a single part line. The term "non-rotating" is a misnomer, because all wire rope has some tendency to rotate under load. Torque values can be reduced by special wire and strand arrangements. This type is usually made with 12 outer strands of seven wires each laid left lang lay around a small core. If this small core is in the form of a 7-wire strand, the rope is called a 19x7 non-rotating rope.

The non-rotating constructions have very large centers composed of strands laid opposite to the direction of the outside strands. When this type of rope is placed under load, and the initial rotation occurs, outside strands lengthen their lays, while the center rope strands get tighter and shorter, until forces balance and the rope stops turning.

In the larger sizes, if increased flexibility is required, non-rotating rope can be made with 19 wires in each strand, resulting in an 18x19 with fiber core, or a 19x19 with steel core.

Another type of non-rotating construction is a 34x7-but this is very seldom used in this country.

There are two other types of rope which have lower rotational characteristics than standard wire rope, but more torque than non-rotating. These are special six- and eight- strand ropes with independent wire rope cores laid opposite to the rope strands. Rotation is reduced because of the opposing lays of the IWRC and rope strands.

Of the two, the eight-strand with its larger core will rotate less, but the six-strand will probably withstand more severe operating conditions because of the larger rope strands and wires.

Another different construction is called flattened strand wire rope, in which the individual strands are approximately triangular in cross-section. Although its surface area in contact with the objects over which it operates is greater than round rope, its ability to withstand bending stresses is not as great.

Aircraft cable, designed for operating controls on aircraft, is sometimes called cord. There are two general types. One is composed of six strands of seven wires each, laid around a seven-wire core strand, and is called 7x7. The second type, 7x19, is a more flexible cord, made of six strands of 19 wires each, laid around a core strand of 19 wires.

Wire strands by themselves are sometimes used for certain types of duty. Such strands, in the extremely small sizes, are called aircraft strands and, as the name implies, were originally intended for guys and braces on aircraft. In the smaller sizes these strands contain seven wires, and in the larger sizes, 19 wires.

Writing an Order

The following description, written on an order, might seem confusing until each element is examined by itself:

175' ¾" 6x25FW Pref. IPS Reg. Rt. FC

The factors covered by the above description include:

Length: This order might be for a crane hoist line. The length is 175 ft. Road scrapers, to cite another example, use 500 ft on a spool attached to the machine itself. Oil-well drilling might require 5,000 to 10,000 ft. lengths

Diameter : All wire ropes are intentionally made oversize. They tend to compact in service, so related components such as sheaves, drums, pulleys and guides must take this into account. Normal commercial tolerance for this ¾ in. diameter wire rope is plus 0.031 in.

Constructions: As was explained earlier, the first number refers to the number of strands in the rope; the second the number of wires per strand. So, this 6x25 rope has six strands, each strand has 25 wires. The "FW" denotes "Filler Wire"

Preforming: As was also mentioned earlier, most ropes are preformed--designated by the "Pref." abbreviation.

Grade: IPS means Improved Plow Steel, the material from which the bulk of wire rope is made.

Lay: "Reg," or regular, refers to direction of wire twist in the stands. "RT" refers to the lay of the strands in the rope, in this case, right lay.

Core: "FC" means fiber core.

Wire Rope Assemblies

Wire rope become assembly when its ends are altered by some form of splice, or by addition of fittings. They may be used for operating controls, as a part of machinery and equipment, or for slings and hoists.

Hand-spliced endings have largely been replaced by other methods of end fitting. Because the splice efficiency depends entirely upon the skill of the person doing the splicing, this type does not consistently produce the rope's full strength. Under a tension load the splice may part at a stress lower than the rope's rated strength and surely less than the actual strength of the rope

Zinced endings: There are three types of zinced endings: A cone is formed from molten zinc poured into a mold in which a frayed rope end has been inserted; sometimes a ferrule is used as mold and stays on after pouring the zinc; or sockets are used instead of the ferrule. An open socket has ears to hold a pin and cotter. A closed socket has a loop or "bail." Both are heavy forgings and find widespread use.

All three zinced on endings need a good deal of preparation. The rope's end must be broomed out, cleaned with acid and straightened. Special endings: such as thimbles, clips, and clamps are quicker and easier to apply than a zinc socket, but efficiency is not as high as with other attachments. These are filed attachments and inspection is necessary during service to make sure the nuts on the clips remain tight and provide proper holding power. Clips are U-shaped bolts with a grooved base and nuts to tighten-these and other grooved devices fit around a rope to form loops, or to provide endings similar to zinced sockets. In some cases, special thimbles and bolted clamps are used instead of clips.

Mechanical endings: A mechanical splice consists of a loop in the end of a rope and a sleeve pressed on the rope at the base of the loop to hold the end of the strands in place.

Swaged endings: Swaging is the cold-flowing, under pressure, of metal fittings into the rope body, between strand sand wires. This pressure, applied by press or by rotary swagers, elongates the fitting but forces its metal inward so that the bond becomes permanent and compact, yet as strong as the breaking strength of the rope.

Wire rope slings-Safety is of paramount importance in any wire rope sling. It is important to remember, where a sling with two or more legs is involved, that consideration must be given to the fact that the stress in a sling varies with the angle at which the legs are used.

Some of the simpler sling arrangements, or hitches as they are called, are listed below. Space does not permit a fuller discussion of slings, particularly multiple-leg types, in tjis issue.

Normally, 6x19 class wire rope is recommended where a diameter in the ¼ to 1-1/8 in. range is to be used, and 6x37 class wire rope where a diameter in the 1-1/4 and larger range is to be used. In some cases the 6x19 class may be used even in the larger sizes if resistance to abrasion is of primary importance, and the 6x37 class in the smaller sizes if greater flexibility is desired.

Straight lift hitch is a straight connector between crane hook and load.

Basket hitch may be used with two hooks so that the sides are vertical, or with a single hook with sides at various angles.

Choker hitch is widely used for lifting bundles of items such as bars, poles and pipe. The choker hitch holds these items firmly but the load must be balanced so that it rides safely.

Many slings are constructed by braiding several individual wire ropes together.

Wire Rope Fittings

A wide variety of fittings are available for use with wire rope. The preceding general discussion on endings, also applies to fittings. The list is long and varied, including: thimbles, sockets, chain and anchor shackles, clips, hooks, turnbuckles, swivel hooks, safety hooks and plate hooks. Many of these were dealt with in an earlier article on chain. (See IDN, April 1965, p. 33.)

Many fittings must be swaged to wire ropes at the factory-others are designed for attachment in the field. Portable swaging tools are available for making splices up to 3/16 inches.

Reels, reel lifts, wire rope cutters, splicing vises, along with a broad variety of fittings, and lubricating equipment, add substantially to the wire rope market potential.

Manufacturers' catalogs contain a wealth of more detailed data.