Technical information steel wire rope

 

The Wire

The basic material is wire rod, which is cold-drawn into wire of different diameters and strength grades.
The most common grades are:

Rope grade
Wire tensile strength grades
Hardness app.
ENAPI 9AMin
Max
BrinelRockwell
kp/mm²N/mm²kp/mm²N/mm²HBHRC
1570PS14013701801770405/42545
1770IPS16015702001960445/47049
1960EIPS (XIP)1801770 2202160470/48051
2160EEIPS (XXIP)2001960 2202160480/50052

Finish
The wire is either untreated (bright), galvanised or stainless. Galvanised wire gives the rope greater protection for corrosive environments. For extreme cases a stainless rope is used. Steel wire ropes can also be provided with plastic impregnation.

Properties of Extension of Steel Wire Ropes

Any assembly of wires spun into a helical formation (either as a strand or a wire rope), when subjected to a tensile load, can extend in three seperate phases, depending on the magnitude of the applied load:

    Phase 1: initial extension
    Phase 2: elastic extension
    Phase 3: permanent extension (thermal elongation and contraction, rotation, wear and corrosion)

Phase 1 and 2 are very important because they are a normal part of the rope bedding in and working of the rope. Phase 3, on the other hand, can be caused by the wrong choice of rope or lack of rope inspection. The phases occure in sequence in all ropes that are exposed to a gradual increased load. Due to this a new rope, when overloaded, will go through phase 1 and 2 before the third phase (permanent extension) begins.

Phase 1: Initial or permanent extension
This phase of extension of the rope depends on the construction of the rope and can be explained as follows:

When loading a new product, extension is created by the bedding down of the assembled wires with a corresponding reduction in overall diameter. This reduction in diameter creates an excess length of wire which is accommodated by a lengthening of the helical lay.

When sufficientely large bearing areas have been generated on adjacent wires, to withstand the circumferential compressive loads, this mechanically created extension ceases and the extension in phase 2 commences.

The initial extention can not be accurately determined and depends on, apart from the strand or the rope construction, the various loads and the current load frequency.

It is not possible to quote any exact values for various constructions but the following approximate values can be used to give reasonably accurate results. 

Load
Safety factorExtension in % of the total length of the rope
Rope with fibre coreRope with steel core
Lightly loaded80,250,125
Normally loaded50,50,25
Heavily loaded30,750,5
Heavily loaded with many "bends"2Up to 2Up to 1

Phase 2: Elastic extension (elongation)
Following Phase 1, the rope extends in a manner which complies approximately with Hookes Law, i.e. stress is proportional to strain.

The proportionality factor normally is a material constant called Modulus of Elasticity (E-modulus). To steel wire ropes the E-modulus is more of a construction constant than a material constant.

The elastic extension can be calculated as follows (Hookes law):

Elastic extension (mm) = (W x L) / (E x A)

W = applied load (kg)
L = rope length (mm)
E = elastic modulus (kg/mm2)
A = area of rope - circumscribed circle (mm2)

The modulus of elasticity varies with different rope constructions. Due to  specific manufacturing factors, wire dimensions and other factors, the E-modulus varies between different wire ropes of the same construction and dimension. If the exact E-modulus value of a certain rope is necessary a specific modulus test needs to be done for that rope.

The elastic extension is valid until the proportionality or elasticity limit is reached. Once the load exceeds this limit , extension according to phase 3 takes place. The elasticity limit is defined as the largest force where the rope returns to its original length when unloaded.

General E-module of the rope construction

Type of steel wire rope
E-module
kp/mm²
Spiral strand, type 1x712600
Spiral strand, type COMPACTED 1x714000
Spiral strand, type 1x1910900
Spiral strand, type COMPACTED 1x1913600
6-part single constructions with fibre core, e.g. 6x7-FC6300
6-part single constructions with steel core, e.g. 6x7-WRC7000
6-part assemblies with fibre core, e.g. 6x36-FC5000
6-part assemblies with steel core, e.g. 6x36-IWRC6000
Compacted constructions with steel core, e.g. ROPETEX 66400
Compacted constructions with steel core, e.g. DSC- DYFORM7900
Multi strand, rotation resistant constructions, e.g. 18x74200
Multi strand, rotating resistant constructions, e.g. 35x76900
Multi strand, rotating resistant compacted constructions, e.g. RopeTex 887200
Elevator rope TRULIFT 8F (8x19S-FC)4000
Elevator rope TRULIFT 8SPC (8x19S-SPC)4300
Elevator rope TRULIFT 8S (8x19S-IWRC)/TRULIFT 9S (9x19S-IWRC)6000

These values are valid for ropes operating with a safety factor of 5:1. With  lower safety factors the modulus of elasticity indicated are increased and for higher safety factors they reduce.

Phase 3: Permanent extension
The permanent, non-elastic extension of the steel caused by tensile loads exceeding the yield point of the material. If the load exceeds the Limit of Proportionality, the rate of extension will accelerate as the load is increased, until a loading is reached at which continuous extension will commence, causing the wire rope to fracture without any further increase of load.

Thermal Expansion and Contraction
The coefficient of linear expansion (∝) of steel wire rope is 0.0000125 = (12.5 x10-6) per o C and therefore the change in length of 1 metre of rope produced by a temperature change of t o C would be;
Change in length Δ| = ∝ |o t
where:
∝ = coefficient of linear expansion
|o = original length of rope (m)
t = temperature change (o C)
The change will be an increase in length if the temperature rises and a decrease in length if the temperature falls.

Extension due to Rotation
The elongation caused by a free rope end being allowed to rotate.

Extension due to Wear
The elongation due to inter-wire wear which reduces the cross-sectional area of steel and produces extra constructional extension.

Example:
The total elongation of a 200 m length of steel wire rope type 28 mm 265-wires (6x36-IWRC) at a tension of 10 tonnes (safety factor 5:1) and with an increase in temperature of 20°C.

According to phase 1:
Permanent constructional extension = 0,25% x total rope length = 0,25% x 200 m = 500 mm.

According to phase 2:
Elastic extension = (W x L) / (E x A) = (10000 x 200000) / (6000 x 615,4) = 540 mm.

According to phase 3:
Thermal expansion = 0,0000125 x L x t = 0,0000125 x 200 x 20 = 50 mm.

Total extension = 500 mm + 540 mm + 5+ mm = 1090 mm

Temperature Affect on Working Load Limit (WLL)

Account should be taken of the maximum temperature that can be reached by the wire rope sling in service. This is difficult in practice but underestimation of the temperature should be avoided.
The table below summarises the necessary de-rated working load limits of a sling due to temperature, taking into account the type of rope termination, the ferrule material and the core of the rope.

When wire rope slings are to be used in temperatures below -40 °C CERTEX should be consulted.

The use of wire rope slings within the permissible temperature ranges given in the table does not require any
permanent reduction in working load limit when the rope is returned to ambient temperature.

Ferrule-
Rope
De-rated working load limit expressed as % of WLL of the sling
material
core
Temperature (t) ºC
-40 - 100101 - 150151 - 200201 - 300301 - 400400<t
AluminiumFibre100Do not useDo not useDo not useDo not useDo not use
AluminiumSteel100100Do not useDo not useDo not useDo not use
SteelFibre100Do not useDo not useDo not useDo not useDo not use
SteelSteel100100907565Do not use
-Fibre100Do not useDo not useDo not useDo not useDo not use
-Steel100100907565Do not use
zinc/alloyFibre-40 - 80ºC 100%
zinc/alloySteel-40 - 120ºC 100%
2-componentFibre-50 - 80ºC 100%
2-componentSteel-50 - 80ºC 100%

Acidic conditions

Wire rope slings should not be used either immersed in acidic solutions or exposed to acid fumes.

Chemical affect

CERTEX should be consulted in case the slings are to be exposed to chemicals combined with high temperature.

The Strand

The strand is built up by individual wires which are laid around the core in one or more layers, typically to one of the following constructions:

Ordinary:
Ordinary lay wires - all wires are in the same size.

Seale (S):

Parallel lay wires - different size, same number of wires in outer and inner layer.

Warrington (W):
Parallel lay wires - the outer layer of wires has two different sizes, twice as many outer wires as inner wires.

Warrington-Seale (WS):
Parallel lay wires - a combination between Seale and Warrington, with three or more layers of wire.

Filler wire (F):
Parallel lay wires - twice as many outer wires as inner wires, with small wires to fill the spaces between the large wires.

Compacted strand:
A strand that has been formed through compaction maintaining the steel area whilst increasing the fill factor.

Rope Lay

Ordinary hand lay
The direction of the lay of the outer layer of wires in the strands is opposite to the direction of lay of the strands in the rope. Ordinary lay is more resistant to kinking and untwisting, and less likely to fail as a result of crushing and distortion.

Lang's lay
The direction of lay of the outer layer of wires in the strands are in the same  direction of lay of the strands in the rope. The advantage of using Lang's lay is that the rope offers a better wearing surface when in use, and therefore can be expected, in many cases to last longer. Lang's lay ropes produce relatively high torque values under working conditions and must not be used on applications where one end of the rope is free to rotate or where problems from rope turn are likely to occur.

Right hand lay
The strands are twisted to the right around the core. Right hand lay is the most common lay direction in a rope.

Left hand lay
The strands are twisted to the left around the core.

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Ordinary right hand lay (sZ)

Lay Length

Lay length is the term used for the length of a "wire- or strand helix". Lay length is determined for each wire rope construction and has to be retained, otherwise the life of the wire rope is considerably reduced.

What is a Steel Wire Rope?

A steel wire rope is made up of individual steel wires spun into a strand. A number of strands are closed over a central core to make up a rope. The number and size of wires will determine the best compromise possible between large wires for maximum corrosion protection and resistance to abrasion, and smaller wires for the required flexibility and handling.

The construction of a steel wire rope is expressed according to the following:

e.g 6x36-FC

     6
is the number of strands in the steel wire rope
     36 is the number of wires in the strand
     FC is the type of the core

The Core

The strands in a rope are laid around a core of either fibre, steel or solid plastic.

Where the strands are laid over a fibre core  FC is used, e.g. 6x19-FC.

Ropes with steel cores should be chosen if the rope is exposed to high working temperature, hard strains, high work rate, hard pressures on drums and blocks etc. The steel core gives better support for the strands, which result in the rope retaining its shape better and giving a better distribution of the stresses in the individual wires. 

If the steel core of the rope consists of a single strand the term -WSC is used, e.g. 6x19-WSC.

If the steel core of the rope consists of a steel wire rope the term -IWRC is used, e.g. 6x19-IWRC.

Some steel wire ropes can have a plastic encapsulated core which provides increased stability to the rope and also reduces the wear and corrosion in and around the core, e.g. Dyform 8 PI.

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6x19-FC

Strand Lay

In a strand construction with several layers of wire of the same diameter the different layers cross each other (cross lay). In an strand construction with unequal wire diameters the layers of wires lie parallel to each other (equal or parellel lay).

In equal laid strands the steel area increases compared to cross lay strands. Other advantages of  parallel laid strands are the improved fatigue and wear resistance resulting from the same wire lay length and strand angle.  

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Cross lay

Preforming

Preforming is a process that puts a helix in the strands to match the helical shape it will assume in the finished rope. Advantages of preforming include:

– The ropes are free from liveliness and twisting tendencies allowing easier installation and handling.
– The ropes can be cut with minimal seizing as exposed ends will not untwist.

Preforming takes place in a preformhead where the strand pass immediately before the closing operation.

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Preforming

From Rod to Rope **

Definitions of Breaking Load

Minimum breaking load
The Minimum breaking force (minimum breaking load), in kilonewtons, is the lowest breaking strain of the rope when tested to destruction.

Calculated breaking load
The value calculated from the product of the sum of the crossectional metallic areas of all the individual wires in the rope and the tensile strength grade(s) of the wires. The total metallic area is directly proportional to the square of the nominal diameter of the rope. A standard spinning loss factor that results from the twisting of strands and wire is then applied.

Special Wire Ropes

Demands for longer operation life and higher breaking loads have contributed to the development of  "special wire ropes". These are to be used in demanding and tough environments when the ropes are used intensively.
The special wire ropes are often manufactured with high tensile wires and the strands are in many cases compacted to give a larger steel area and consequently a higher breaking load.  This results in an increased working load for a specific rope diameter. Compacting of the rope also results in longer life and therefore a lower life cost.

Some areas of use demand wire ropes with better stability and extra low inner friction. For that purpose ropes are manufactured with a plastic coated steel core.

Special wire ropes can be double parallel which means that both wires and strands lay parallel to each other. This prevents wire cross over which can lead to wire indentations. Special wire ropes have a few things in common: less outer and inner friction, narrow tolerances, more flexible, more rounded and a larger contact surface against sheaves and drums. This results in a longer life in tough working conditions.

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Compacted

Rotation Resistance

To minimise  the tendency to rotate especially with high lifting heights "rotation resistant" or "Low Rotation" ropes should be used.

Rotation resistant ropes are made out of many layers of strands. Every layer is laid in the opposite direction to the next layer, so that the torque in the different layers balances the opposing forces in the rope.