A steel wire rope is defined not only by its basic elements (wires, strands, core), but also by the way in which the individual wires are laid together to create a strand and the way in which the strands are laid around the core, etc. The steel wire rope’s construction is defined when the following criteria have been determined:
- Number of wires in a strand
- Type of strand (strand design)
- Number of strands
- Type of core
- Lay direction (steel wire rope and strand)
The steel wire rope is designated according to the number of strands, the number of wires in each strand, the design (type) of the strand, and the type of core.
- 6x7 Standard with FC (fibre core)
- 8x19 Standard with WSC (steel core)
- 8x19 Seale with IWRC (steel core)
- 6x36 Warrington Seale with FC (fibre core)
Number of Wires in a Strand
The number of wires in a strand varies between three and approx. 139, although there are most commonly 7, 19, 24 or 36 wires. The number of wires and their thickness depend on the design of the strand and affects the characteristic of the steel wire rope.
Types of Strand (Strand Construction)
The type of strand is characterised by the way in which the wires in the strand are arranged. There are four basic types of strand design that are used in all steel wire ropes, either in their original form or as a combination of two or more types. The four basic types are:
The Standard construction (fig. 3) is characterised by the fact that all wires are of equal thickness, although the core wire may be thicker. The wires are also laid together in such a way that all of them, with the exception of the centre wire, are of equal length. In this way all the wires are subjected to an equal distribution of load when pulled straight.
The geometric wire distribution consists of one centre wire, onto which one or more layers are laid. Each layer is produced in a separate operation. If there are several layers, the number of wires increases by six for each layer.
The designation for a Standard strand with e.g. seven wires is (1-6), i.e. one centre wire with six external wires in one operation. If there are 37 wires it is known as (1-6/12/18), i.e. one centre wire with six external wires from the first operation, 12 from the second operation and 18 from the third operation.
The centre wire may be replaced by several wires or a fibre core (fig. 4).
The Seale construction (fig. 5) is characterised by the way in which the strand consists of two layers of wire produced in one operation. Also, the number of wires in the first and second layer is identical.This construction is somewhat stiffer than a corresponding Standard construction (with the same number of wires). This is because the outer wires in the Seale construction are considerably thicker.
A Seale strand with e.g. 19 wires is known as (1-9-9), i.e. one centre wire with nine wires in the first layer and nine wires in the second layer.
The centre wire may be replaced by several wires or a fibre core (fig. 6).
The Filler construction (fig. 7) is characterised by a strand consisting of two layers of wires produced in one operation. Also, the number of wires in the second layer is twice the number in the first layer. This is, however, only possible if filler wires are inserted between the first and the second layers, to prevent the strand becoming hexagonal in shapes.
This construction is more flexible than a corresponding Standard construction and considerably more flexible than a corresponding Seale construction (with the same number of wires excluding filler wires).
A Filler strand with e.g. 25 wires (including 6 filler wires) is known as (1-6+6F-12), i.e. one centre wire with six wires in the first layer and 12 wires in the second layer. There are six filler wires between the first and the second layers.
The centre wire may be replaced by several wires or a fibre core (fig. 8).
The Warrington construction (fig. 9) is characterised by a strand consisting of two layers of wire produced in one operation. The second (outer) layer contains wires of two dimensions, and the number of wires in the second layer is twice the number in the first.
This construction is very compact and flexible. A Warrington strand with e.g. 19 wires is known as (1-6-6+6), i.e. one centre wire with six wires in the first layer and a total of 12 wires of two dimensions in the second layer. The centre wire may be replaced by several wires or a fibre core (fig. 10).
Other Types of Strand
As previously mentioned, there are also strands that are a combination of one or more of the above four basic types of strand. One of these is the Warrington-Seale (fig. 11).
This construction is one of the most widely-used and most flexible constructions compared to the four basic types.
The Warrington-Seale construction is characterised by a strand consisting of three layers of wire produced in one operation. The number of wires in the third (outer) layer matches the number of wires in the second layer. Also, the layers below the outer layer are built as a Warrington construction.
A Warrington-Seale strand with e.g. 36 wires is known as (1-7-7+7-14), i.e. one centre wire with seven wires in the first layer, 14 wires made up of two dimensions in the second layer, and 14 wires in the third layer.
The strands and the wires in the strands do not necessarily have to be round. Examples of this are shown in fig. 12. The strands are special strands (i.a. with profiled wire), designed to meet extremely unusual requirements.
Number of Strands
The number of strands in a steel wire rope varies between three and approx. 36, although most commonly there are six strands. The more strands a steel wire rope contains, the more rounded and flexible it is, although the wires in the strand are also thinner (less durable).
Types of Core
As mentioned in “Core”, page 8-2, there are two types of core for steel wire ropes:
· Fibre core (natural or man-made).
· Steel core (WSC or IWRC).
Fibre cores are the most commonly used, as not only do they provide a good, elastic base but also enable lubrication of the rope from the inside, since it is possible to add oil and/or grease to the fibre core during production. This reduces the risk of rust attacking from the inside. The fibre core is normally produced from polypropylene (PP) or sisal. PP can withstand weaker acids and alkalis and it does not rot. The advantage of a sisal core is that it can absorb oil/grease to a greater degree for lubrication of the steel wire rope from the inside.
The maximum operating temperatures for steel wire ropes with a fibre core can be seen in “Maximum Operating Temperature”, (see page 8-30).
A steel core is formed as either one of the strands (WSC) or as an independent steel wire rope (IWRC).
Randers Reb recommends the use of a steel core, in the event that it is not certain that a fibre core will provide satisfactory support for the strands, e.g. if the steel wire rope is spooled on to a drum in several layers under a considerable load, or at high temperatures.
A steel core increases the steel wire rope’s tensile strength by approx. 10%.
Lay Directions (Steel Wire Rope and Strand)
The word “lay” has more than one meaning in this context. It is used to describe the process of interweaving the wires and strands and also to describe the appearance of the finished steel wire rope. The four most common terms to describe the lay of a steel wire rope are:
Right hand regular lay steel wire rope. In this instance the wires in the strand are laid in the opposite direction to the strands in the rope. The wires are laid helically left, while the strands are laid helically right (see fig. 13).
Left hand regular lay steel wire rope. Here the wires in the strand are laid helically right, and the strands helically left (see fig. 14).
Right hand Lang lay steel wire rope. Here the wires are laid in the same direction as the strands in the rope. The wires in the strands and the strands are laid helically right (see fig. 15).
Left hand Lang lay steel wire rope. The wires in the strands and the strands are laid helically left (see fig. 16).
Other terms used are e.g.:
- Multi layer steel wire rope (low rotation/rotation resistant). Here there are usually two layers of strands, the inner layer as a rule a left hand Lang lay, while the outer layer is a right hand regular lay.
- Alternate lay steel wire rope. This steel wire rope is a combination of regular lay and Lang lay.
- Cable laid steel wire rope. The strands are normally 6-lay steel wire rope with a fibre or steel core. The core is a fibre core or a 6-lay steel wire rope with a fibre or steel core.
- Square braided steel wire rope. The steel wire rope is square braided from strands or steel wire ropes.
- Flat braided steel wire rope. This steel wire rope is flat braided from strands or consists of parallel strands or steel wire ropes that are bound together by sewing (belt strap).
Right hand lay steel wire rope is also known as Z-lay, and left hand as S-lay. Similarly, a right hand lay strand is known as z-lay and left hand as s-lay. Fig. 17 shows why. Of the types of lay described, right hand regular lay is the most common.
“Preformed” refers to steel wire ropes in which the strands have been permanently formed during the laying process (see fig. 18), so that they are completely stress-free within the unloaded steel wire rope.
If a strand is removed from the steel wire rope, it will retain its helical shape, as though it were still in the steel wire rope.
There are many advantages in a preformed steel wire rope, such as:
- The steel wire rope will not untwist during cutting
- It is easier to install, as pre-formed steel wire ropes are stress-free. No tendency to form kinks
- It can run over smaller sheaves
- Less tendency to turn on its own axis
- Less wear and tear
- Better load distribution between strands and wires
- In the event of a wire breaking, less tendency to protrude from the strand. Less tendency to damage adjacent wires and sheaves
All in all, preformed steel wire ropes can offer a longer life expectancy than steel wire ropes that are not pre-formed.
All Randers Reb steel wire ropes are supplied preformed, with the exception of certain individual special constructions (e.g. low-rotation/rotation resistant).