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Steel wire ropes
Construction of steel wire ropes
An introduction to the construction of steel wire rope. Including: materials, rope size and types of general wire rope.
Composition of wire rope
Steel wire rope (SWR) is constructed of wires and strands laid around a central core.
Detailed composition of a steel wire rope
It is important to know the difference between wires and strands.
If:
- a strand is broken, the rope is unusable.
- a single wire in a sling is broken, it is still usable unless it is broken immediately below a metal fitting or anchorage. Refer to wire rope discard criteria table.
What is the core of steel wire rope?
The core of steel wire rope can be:
- fibre core (FC)
- independent wire rope core (IWRC)
- plastic core
Example of Fibre core (FC)
Fibre core (FC) provides extra flexibility, normally used in single layer applications.
Example of Independent wire rope core (IWRC)
Wire rope core (WRC) provides extra strength and support in multi-layered applications to prevent crush.
Why is steel strength important?
The tensile strength of steel is how strong something is when pulled in tension under load.
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ASTM A931 - Steel Wire Tensile Testing
Tensile strength is normally measured in megapascals (MPa).
Wire rope with different tensile strengths will break at different loads. The diameter of the rope and what the core is made of will also contribute to the breaking strain.
For example:
- 16mm diameter fibre core 1570 grade has a breaking strain of 113kN, while 16mm diameter wire rope core 1770 grade has a breaking strain of 161kN.
Note: Terms used in USA specifications equal the numerical values used in Australian specifications. See table below:
| Australian Specification | USA Specification |
|---|---|
| 1570 | – |
| 1770 | IPS - Improved plough steel |
| 1960 | EIPS - Extra improved plough steel |
| 2160 | – |
| 2250 | EEIPS - Extra Extra improved plough steel |
Tensile strengths for wire ropes
Tensile strength should be provided with the manufacturer's mill certificate for the:
- wire rope, or
- wire rope sling.
Here is an example of a mill certificate (PDF 236.96KB).
What is the most common tensile strength?
The most commonly used tensile strength for wire rope are 1570, 1770 and 1960 megapascals (MPa).
What is the minimum steel wire rope construction for slings?
A 6/19 (6 strands of 19 wires each) is the minimum steel wire rope construction that can be used for slings.
In the image below, there are 19 wires to the strand and 6 strands around the core making up the rope.
Structure of 6/19 steel wire rope
Rope size
The size of a rope is determined by its diameter.
The smallest diameter steel wire rope that can be used for lifting is 8mm (refer to AS1666.1).
The maximum allowable diameter loss is 5% (AS2759).
How to correctly measure wire rope
Use Vernier callipers to measure wire rope. The correct way to measure wire rope with Vernier callipers is to:
- put the Vernier calliper across the rope
- rotate the Vernier calliper up to 90 degrees until you feel the jaws spread to the maximum diameter of the rope
- then record the measurement.
The image below shows the correct and incorrect way to measure a rope diameter.
The correct way to measure wire rope with Vernier callipers
The incorrect way to measure wire rope with Vernier callipers
General wire rope constructions
Common constructions recommended for use are:
- 6x19 - good resistance to abrasion, stiffest construction for wire rope slings
- 6x25 - good all round rope slings and winching applications
- 6x36 - increased flexibility for winching applications
- 6x41 - maximum flexibility for larger ropes up to 52mm
- 6x49 - maximum flexibility for ropes greater than 52mm.
Round strand wire ropes in either Ordinary or Lang’s lay configuration have been constructed of:
- 6 strands of 19 wires (6x19)
- 6 strands of 24 wires (6x24)
- 6 strands of 25 wires (6x25)
- 6 strands of 36 wires (6x36)
- 6 strands of 41 wires (6x41)
- 8 strands of 19 wires (8x19)
- 8 strands of 25 wires (8x25)
Some 4 strand ropes (like the image below) are also being used in newer cranes and hoists which have the same number of wires as 6 and 8 strand ropes.
4 strand rope
Most flexible steel wire ropes are parallel or equal laid. This means the inner wires in the strand are laid in a longer spiral so that the top wires do not cross the inner wires.
To prevent a different spiral in the inner and outer wires of strands and to obtain parallel lay, different size wires are laid into the same strand. The following standard constructions use this method.
Filler wires
Filler wires have a number of wires laid over a central wire and an equal number of very small wires laid in the valleys of these wires. Filler wires are then laid in the valleys between the large and small wires providing stability and additional tensile strength.
Plastic impregnation
Plastic impregnation, known as cushion core:
- reduces the incident of internal wire breaks
- keeps out water and harmful elements
- stabilises the rope construction during instillation and actual service
- absorbs dynamic energy and reduces the internal stress of rope
- seals in rope lubricant
- prevents steel to steel contact.
Plastic impregnation
The inner layer is plastic coated before the outer layer of strands is put on. This reduces steel to steel contact of the inner and outer layers. This prevents early failure from nicking of wires at such contact points.
Multi stranded wire ropes
Below are examples of typical wire ropes. These have round or compacted wires to improve surface contact with sheaves and drums.
Wire rope and groove contact area examples
Steel wire ropes are constructed to combat fatigue and abrasion. These two destructive forces occur whenever steel wire rope is bent over a system of sheaves.
Wire flexes as it bends over sheaves and drums. Fatigue takes place as the wire bends over the sheave under tension. The outer wires are stretched and the inner wires are compressed against the sheave groove or drum.
The hoist drum of the crane is the pulling mechanism which rotates, hauls in and stores surplus wire. The braking mechanism is connected to either the drum or the gearing, which is joined to the drive mechanism.
The wire passes over the head sheave of the crane and then down to the load.
Wire almost never lays straight into the groove of a sheave. This is because the load slightly swings or the rope vibrates.
This movement causes friction, pressure or abrasion between the sheave and the wires and strand surfaces. This creates points of wear on the outer wires of the strands.