Wire Rope & Strand

Wire Rope & Strand

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Descriptions, Size and Construction

Descriptions, Size and Construction

A wire rope is made up of the basic components illustrated. The terms used to describe these component parts should be strictly adhered to, particularly when reporting on the conditions of ropes. Describing wires as strands and strands as wire can be grossly misleading. For example, a report that a rope has a broken strand in most applications calls for immediate discarding of the rope, and subsequent cessation of operation, while a report that a rope has a broken wire in it should call for early inspection but seldom for discarding the rope.


Composition of wire rope

Wire Rope Description

The properties of a wire rope are derived from its size, construction, quality, lay and type of core.

Size

Ropes are referred to by a diameter size. The correct way to measure wire rope is shown below.

The correct way to measure wire rope

Construction

The main components of a wire rope are shown below.
The main components of a wire rope: wire, strand, rope, core

In the above example, each individual wire is arranged around a central wire to form a 7-wire strand. Six of these strands are formed around a central core to make a wire rope. The rope is specified as 6x7 (6/1) - i.e. six strands each of seven wires.

The size and number of wires in each strand, as well as the size and number of strands in the rope greatly affect the characteristics of the rope. In general, a large number of small-size wires and strands produce a flexible rope with good resistance to bending fatigue. The rope construction is also important for tensile loading (static, live or stock), abrasive wear, crushing, corrosion and rotation.


multiple operation, single operation, rotation resistant, triangular strand, galvanised strand, half locked coil, full locked coil, non-rotating mining

Multiple Operation: Individual strands are composed of successive layers of wire laid up at different lay lengths. This results in a cross laid rope.

This type of construction is now confined to a limited range of products such as ropes below 8mm diameter, and large sling and static ropes.

Single Operation: All wires in the strand are laid up in the one manufacturing operation. This type of rope is standard production, providing an equal laid rope that eliminates internal cross-mating and forms a compact strand of high metallic content. There are three main types: 6x9/9//1, 6x25 FW and 6x36 SW.

Rotation Resistant: The conventional rotation resistant wire rope is composed of a number of strands that are laid up in opposite directions to produce a non-rotating effect.

The 4 strand Mono Track is a complete departure from this convention and is created through theoretical analyses of the working torques.

Triangular Strand: The wires are specially formed to produce a strand with a triangular section - this type of rope is only produced in Lang's lay. This construction has improved wear and crush resistance and has wide application in winding and haulage systems.

Galvanised Strand: These are single strands of concentric layers of wires, some of which are cross laid to produce a non-rotating result.

Half Locked Coil: A strand with the outer layer composed of alternate shaped and round wires covering one or two layers of round wires laid in the opposite direction.

Full Locked Coil: A strand used as a rope and composed of one or two layers of Z-shaped wires laid over layers of half lock coil and/or layers of round wires.

Rotation Resistant Mining Ropes: A rope composed of flattened strands of six or eight wires contra laid over a triangular strand rope to produce a rotation resistant result.
equal laid rope and cross laid rope

Cores & Wire Tensile

Cores & Wire Tensile

Cores

A number of core types are available and each gives specific properties to the rope:

  1. Fibre (FC) - sisal or polypropylene.
  2. Wire Strand (WSC) - Strand usually of the same construction as the outer strands.
  3. Independent Wire Rope Core (IWRC) - a wire rope usually of 6x7 (6/1)/1x7(6/1) construction.


Fibre core in 6x7 wire rope

Fibre Core (FC) in 6 x 7 wire rope

A fibre core, generally sisal, provides a resilient foundation for the strands in the rope structure. Fibre cores are used for ropes that are not subjected to heavy loading and where flexibility in handling is required. Fibre cores are inadequate where wire rope is subjected to heavy loading, prolonged to outdoor exposure and crushing on small drums and sheaves.


wire strand core in 6 by 7 wire rope

Wire Strand Core (WSC) in 6 x 7 wire rope

These cores are used chiefly for standing ropes (guys or rigging), and offer higher tensile strength and, owing to the larger wires in the core, greater resistance to corrosion failure.


independent wire rope core in 6 by 25 wire rope

Independent Wire Rope Core (IWRC) in 6 x 25 FW wire rope

In many instances it is recommended to use a wire rope with an independent wire rope core (I.W.R.C). Such a core is usually made up of 6 strands of 7 wires each plus centre strand.

The independent wire rope core provides:

  1. Permanent support and uniform spacing of the strands laid around it; it is not compressible and has greater wear resistance than fibre core.
  2. Permanent elastic stretch of the wire rope over a longer period of time.
  3. Increased resistance to deterioration and deformation.
  4. Delay of internal corrosion; the lubricant is not squeezed out of the core.
  5. It increases the actual breaking load of the rope by at least 8% in the case of 6-strand ropes and about 25% in the case of 8-strand ropes.
  6. Better performance for operating in very high temperatures.

An independent wire rope core increases the weight of a 6-strand rope by about 10%, and that of an 8-strand rope by approx. 20%.

Although a new rope with I.W.R.C. may be somewhat less flexible that a new rope with fibre core, it retains its relative flexibility whereas a rope with fibre core gradually loses its flexibility during use. Having better resistance to deterioration and deformation, a rope with I.W.R.C. is less susceptible to damage when used on small sheaves and drums than a rope with fibre core, it will also last longer before deterioration and deformation set in when wound on a drum in multiple layers.

Tensile Strength Grades

Wire ropes are usually supplied in the following tensile ranges:

Rope Grade Range of tensile strength grades N/mm2
1570 1370 to 1770
1770 1570 to 1960
1960 1770 to 2160
2160 1960 to 2160

 

Rope Grade Equivalents

Rope Grade Designation Equivalent Rope Grade
IPS 1770
EIPS 1960
EEIPS 2160

 

With the increasing use of heavy-duty and more compact equipment (e.g. power winches on mobile cranes and mine winding) there is a gradual upward trend in the required rope wire tensile range. However, as factors other than strength influence the life of wire rope, the specific application must be kept in mind when tensile strength of wire is selected.

 

Preforming, Postforming and Lay

Preforming, Postforming and Lay

Preforming

A preformed rope is one in which the component strands are shaped to their final helical form before being laid into the rope.

Preforming can be applied to both Ordinary lay and Lang's lay ropes and, unless specifically ordered otherwise, all standard ropes are supplied preformed.
The advantages of preforming are mainly:

  1. Reduction of internal stresses in the rope. This makes the rope easier to handle and install, reduces its tendency to kink and gives better spooling onto drums.
  2. Greatly improved resistance to bending fatigue particularly in operation over small drums and sheaves.
  3. Greater stability and better resistance to shock loading and abrasion.
  4. Improved rope life due to the better equalisation of loading between strands in the rope and reduction of internal stresses in the rope.
  5. Greater safety in handling of ropes as broken wire ends do not protrude. This factor also reduces wear on equipment in contact with the rope.

Postforming

Postforming is a manufacturing process applied to ropes to minimise stretch in service. It reduces the stretch caused by "bedding-in" the wires and strands onto their respective cores. In addition to controlling stretch, postforming produces results closely related to those achieved by preforming.

Postforming is particularly useful in overcoming stretch in long lengths of rope and where take-up adjustment is restricted. It is commonly applied to ropes used in aerial ropeways, guying, chairlifts and control cables.

Lay

This refers to the way the wires in the strands, and the strands in the rope are formed into the completed rope. The wire strands are essentially laid up in a planetary motion with controlled twist being imparted to produce a tightly formed rope.

The term "lay" is used in three ways:

  1. To describe the direction in which the strands are laid in the rope, right or left. In a Right Hand lay strands are laid around the rope core in a clockwise direction - see illustration below. In a Left Hand lay, the strands are laid anti-clockwise - see illustration below. Steel Wire Ropes are conventionally produced Right Hand lay unless special circumstances require Left Hand lay.
  2. To describe the direction in which the wires are stranded in relation to the direction of the strands in the completed rope, e.g. Ordinary lay or Lang's lay.
    Ordinary lay means the wires in a strand are laid in a direction opposite to the direction in which the strands are laid in the final rope.
    Lang's lay is the reverse of Ordinary lay. That is, the wires are laid in the same direction as the strands in the rope.
    Lang's lay ropes have superior properties in resistance to wear, abrasion, fatigue and scuffing. This is illustrated below, where it can be seen that wear on an outer wire is distributed over a far greater area than in Ordinary lay.
  3. "Lay" is also a measure of the pitch of a strand in a rope.


right hand ordinary lay, right hand lang's lay, left hand ordinary lay, left hand lang's lay, right hand alternate lay

Lay Directions & Types

Lay direction of strands for stranded ropes are right (z) or left (s) and correspond to the direction of lay of the outer wires in relation to the longitudinal axis of the strand.

Lay direction of ropes are right (Z) or left (S)and correspond to the direction of lay of outer wires in spiral ropes, the outer strands in a stranded rope or the unit ropes in a cable-laid rope in relation to the longitudinal axis of the rope.
ordinary lay and lang's lay

Care and Use

Care and Use

Breaking in

A wire rope may be looked upon as a machine composed of a large number of moving parts. As such it should be broken in as soon as it is installed, by loading it very lightly for a few cycles and then gradually stepping up the load, to enable both wires and strands to 'bed down' into the working positions, with the load distributed as uniformly as possible.

With strand 6 and 8 stranded ropes, the torque can greatly diminish after breaking in by releasing the connection and allowing the torque to run out. This procedure may have to be repeated until the constructional stretch has been worked out of the rope and it has become neutral.

The use of 'spinners' or swivels should be avoided whenever possible. All ropes should be reeled onto winch drums as tightly and uniformly as possible during the initial installation.

Inspection

Wire rope is tough and durable, but nonetheless expendable and eventually reaches the end of its safe service life. Rope deterioration becomes noticeable through the presence of broken wires, surface wear, corrosion, wire or strand distortion due to mechanical abuse, or drastic reduction in diameter and lengthening of the lay.

Also deterioration can be detected by the use of non-destructive testing techniques. Wire ropes should periodically be inspected for signs of deterioration.

While Statutory Regulations govern the inspection and discarding of certain wire ropes, the same rules cannot be applied to all ropes. The proper frequency and degree of inspection depends largely on the possible risk to personnel and machinery in the event of rope failure.

The determination of the point at which a rope should be discarded for reasons of safety requires judgment and experience in rope inspection in addition to knowledge of the performance of previous ropes used in the same application.

Where the Statutory Regulations are laid down for the inspection and discarding of wire ropes and their attachments, wire rope users should become fully acquainted with the regulations and see that they are carried out.

Sufficient records should be kept to provide a reliable history of the ropes under their control. Inspection of both operated and discarded ropes frequently indicates equipment faults that have a large bearing on the service life and safety of the rope. It is therefore essential to inspect the equipment on which the rope is used as well as the rope itself.

Deterioration

Typical examples of wire rope deterioration

1. Mechanical damage due to rope movement over sharp edges whilst under load Deterioration 1
2. Localised wear due to abrasion on supporting structure. Deterioration 2
3. Narrow path of wire breaks caused by workin in a gorssly oversized groove or over small support rollers. Deterioration 3
4. Severe wear in Lang's Lay, caused by abrasion at
cross-over points on multi-layer coiling application.
Deterioration 4
5. Corrosion of sever degree caused by immersion of rope in water. Deterioration 5
6. Typical wire fractures as a result of bend fatigue. Deterioration 6
7. Wire fractures at the strand, or core interface, as distinct from 'crown' fractures caused by failure of core support. Deterioration 7
8. Typical example of localised wear and deformation created at a previously kinked portion of rope. Deterioration 8
9. Multi-strand rope 'bird caged' due to torsional imbalance. Typical of build-up seen at anchorage end of multi-fall crane application. Deterioration 9
10. Protrusion of IWRC resulting from shock loading. Deterioration 10

 

Lubrication

When a rope is operated over a drum or sheave, the strands and wires move relative to one another. To reduce the resultant friction within the rope as well as the friction between the rope and drum or sheave, ropes are lubricated in manufacture. In addition this lubrication also retards corrosion and inhibits possible rotting of the fibre core. In special applications a combination of lubricants may be called for, e.g., the core and inner wires of the strands may be heavily lubricated while the lighter lubrication may be applied to outer wires and strands.

Wire rope cores are normally heavily lubricated irrespective of the outer strand lubrication.