Wire rope
From Wikipedia, the free encyclopedia
Wire rope consists of several strands laid (or 'twisted') together like a helix. Each strand is likewise made of metal wires laid together like a helix. Initially wrought iron wires were used, but today steel is the main material used for wire ropes.
Contents |
[edit] History and materials
Modern wire rope was invented by the German mining engineer Wilhelm Albert in the years between 1831 and 1834 for use in mining in the Harz Mountains in Clausthal, Lower Saxony, Germany. It was quickly accepted because it proved superior to ropes made of hemp or to metal chains, such as had been used before.
Wilhelm Albert's first ropes consisted of wires twisted about a hemp rope core, six such strands then being twisted around another hemp rope core in alternating directions for extra stability. Earlier forms of wire rope had been made by covering a bundle of wires with hemp.
In America wire rope was later manufactured by John A. Roebling, forming the basis for his success in suspension bridge building. Roebling introduced a number of innovations in the design, materials and manufacture of wire rope.
Manufacturing a wire rope is similar to making one from natural fibres. The individual wires are first twisted into a strand, then six or so such strands again twisted around a core. This core may consist of steel, but also of natural fibres such as sisal, manila, henequen, jute, or hemp. This is used to cushion off stress forces when bending the rope.
This flexibility is particularly vital in ropes used in machinery such as cranes or elevators as well as ropes used in transportation modes such as cable cars, cable railways, funiculars and aerial lifts. It is not quite so essential in suspension bridges and similar uses.
[edit] Lay of wire rope
The lay of a wire rope describes the manner in which either the wires in a strand, or the strands in the rope, are laid in a helix.
[edit] Left and right hand lay
Left hand lay or right hand lay describe the manner in which the strands are laid to form the rope. To determine the lay of strains in the rope, look at the rope as it points away from you. If the strands appear to turn in a clockwise direction, or like a right-hand thread, as the strands get further away from you, then the rope is a right hand lay. The picture of steel wire rope on this page shows a rope with right hand lay. If the strands appear to turn in an anti-clockwise direction, or like a left-hand thread, as the strands get further away from you, then the rope is a left hand lay.
[edit] Ordinary, Lang's and alternate lay
Ordinary and Lang's lay describe the manner in which the wires are laid to form a strand of the wire rope. To determine which has been used first identify if left or right hand lay has been used to make the rope. Then identify if a right or left hand lay has been used to twist the wires in each strand.
| Ordinary lay | The lay of wires in each strand is in the opposite direction to the lay of the strands that form the wire. |
| Lang's lay | The lay of wires in each strand is in the same direction as the lay of the strands that form the wire. |
| Alternate lay | The lay of wires in the strands alternate around the rope between being in the opposite and same direction to the lay of the strands that form the wire rope. |
| Regular lay | Alternate term for ordinary lay. |
| Albert's lay | Archaic term for Lang's lay. |
| Reverse lay | Alternate term for alternate lay. |
| Spring lay | This is not a term used to classify a lay as defined in this section. It refers to a specific construction type of wire rope. |
[edit] Construction and specification
The specification of a wire rope type – including the number of wires per strand, the number of strands, and the lay of the rope – is documented using a commonly accepted coding system, consisting of a number of abbreviations.
This is easily demonstrated with a simple example. The rope shown in the figure "Wire rope construction" is designated thus: 6x19 FC RH OL FSWR
| 6 | Number of strands that make up the rope |
| 19 | Number of wires that make up each strand |
| FC | Fibre core |
| RH | Right hand lay |
| OL | Ordinary lay |
| FSWR | Flexible steel wire rope |
Each of the sections of the wire rope designation described above is variable. There are therefore a large number of combinations of wire rope that can be specified in this manner. The following abbreviations are commonly used to specify a wire rope.
| Abbr. | Description |
|---|---|
| FC | Fibre core |
| FSWR | Flexible steel wire rope |
| FW | Filler wire |
| IWR | Independent wire rope |
| IWRC | Independent wire rope core |
| J | Jute (fibre) |
| LH | Left hand lay |
| LL | Lang's lay |
| NR | Non-rotating |
| OL | Ordinary lay |
| RH | Right hand lay |
| S | Seale |
| SF | Seale filler wire |
| SW | Seale Warrington |
| SWL | Safe working load |
| TS | Triangular strand |
| W | Warrington |
| WF | Warriflex |
| WLL | Working load limit |
| WS | Warrington Seale |
[edit] Terminations
The end of a wire rope tends to fray readily, and cannot be easily connected to plant and equipment. A number of different mechanisms exist to secure the ends of wire ropes to make them more useful. The most common and useful type of end fitting for a wire rope is when the end is turned back to form a loop. The loose end is then fixed by any number of methods back to the wire rope.
[edit] Thimbles
When the wire rope is terminated with a loop, there is a risk that the wire rope can bend too tightly, especially when the loop is connected to a device that spreads the load over a relatively small area. A thimble can be installed inside the loop to preserve the natural shape of the loop, and protect the cable from pinching and abrasion on the inside of the loop. The use of thimbles in loops is industry best practice. The thimble prevents the load from coming into direct contact with the wires.
[edit] Wire rope clamps
A wire rope clamp, also called a clip, is used to fix the loose end of the loop back to the wire rope. It usually consists of a u-shaped bolt, a forged saddle and two nuts. The two layers of wire rope are placed in the u-bolt. The saddle is then fitted over the ropes on to the bolt (the saddle includes two holes to fit to the saddle). The nuts secure the arrangement in place. Three or more clamps are usually used to terminate a wire rope.
[edit] Swaged terminations
Swaging is a method of wire rope termination that refers to the installation technique. A metal sleeve is fitted over the two wire rope ends, and a mechanical or hydraulic tool compresses and deforms the fitting, creating a permanent joint. A ferrule is a type of swaged joint.
[edit] Sockets
A socket termination is useful when the fitting needs to be replaced frequently. For example, if the end of a wire rope is in a high-wear region, the rope may be periodically trimmed, requiring the termination hardware to be removed and reapplied. An example of this is on the ends of the drag ropes on a dragline. The end loop of the wire rope enters a tapered opening in the socket, wrapped around a separate component called the wedge. The arrangement is knocked in place, and load gradually eased onto the rope. As the load increases on the wire rope, the wedge become more secure, gripping the rope tighter.
[edit] Eye splice
An eye splice may be used to terminate the loose end of a wire rope when forming a loop. The strands of the end of a wire rope are unwound a certain distance, and plaited back into the wire rope, forming the loop, or an eye, called an eye splice.
[edit] Codes and standards
[edit] Australia
The following Australian Standards apply to wire rope:
- AS 1138-1992 Thimbles for wire rope
- AS 1394-2001 Round steel wire for ropes
- AS 1666.1-1995 Wire-rope slings - Product specification
- AS 1666.2-1995 Wire-rope slings - Care and use
- AS 2076-1996 Wire-rope grips for non-lifting applications
- AS 2759-2004 Steel wire rope - Use, operation and maintenance
- AS 3569-1989 Steel wire ropes
- AS/NZS 4812-2003 Non-destructive examination and discard criteria for wire ropes in mine winding systems
