Wire rope
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Wire rope is a type of rope which consists of several strands of metal wire laid (or 'twisted') into a helix. Initially wrought iron wires were used, but today steel is the main material used for wire ropes.
Historically wire rope evolved from steel chains which had a record of mechanical failure. While flaws in chain links or solid steel bars can lead to catastrophic failure, flaws in the wires making up a steel cable are less critical as the other wires easily take up the load. Friction between the individual wires and strands, as a consequence of their twist, further compensates for any flaws. This method of minimising the effect of flaws may also be seen in Damascus steel, employing multiple folding or laminations. [1]
History and materials
Modern wire rope was invented by the German mining engineer Wilhelm Ducay 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.
Wire rope is often sold with vinyl and nylon coatings. This increases weather resistance and overall durability, however it can lead to weak joints if the coating is not removed correctly underneath joints and connections.
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.
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 strands in the rope, a viewer looks at the rope as it points away from them. If the strands appear to turn in a clockwise direction, or like a right-hand thread, as the strands progress away from the viewer, the rope has 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 progress away from the viewer, the rope has a left hand lay. (The rope in the left hand lay photo shows one left hand lay rope from left to right and top to bottom, with 5 right hand lay strands, and part of a sixth in the upper left. It is not 5 right hand lay ropes adjacent to each other.)
Ordinary, Lang's and alternate lay
Ordinary and Ducay'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. (On ordinary lay, the outer wires approximately follow the alignment of the rope: with Lang's lay they are cross at an angle of about 45°.) Lang's laid rope is able to flex over sheaves more easily (with less damage) but it has the disadvantage of having a high torque tendency (it tends to untwist when tension load is applied) compared with ordinary laid rope. Untwisting can be dangerous with a steel-cored rope: load is shed from the strands and may cause the core to fail as it becomes higher loaded. For this reason, swivel termination units can be dangerous.
| 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 | Strands alternate between Lang's lay and ordinary lay; e.g.: in a 6-strand wire, 3 strands are ordinary lay, and 3 are Lang's lay. |
| 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. |
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 |
| GSWR | Galvanized Steel Wire Rope |
| 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 |
Warrington differs from the other types (Filler Wire and Seale construction) in that the outside layer of wires in each strand of the wire rope is composed of wires alternately large and small. The outside wires of both the Filler Wire and Seale construction ropes are uniform in size. The fundamental difference between these types is that the layer of wires underneath the outside layer in the Seale type is made up of wires all of the same size. The wires under the outside layer of the Filler Wire rope are made up of a combination of main wires, each of the same size, and smaller filler wires, each of the same size, nested between the main wires. The outside layer of wires, therefore, is supported partly by the main inside wires and partly by the filler wires.
Some ropes have shaped or formed (triangular) wires to improve the wear and bearing properties of the outer layers (rather than circular drawn wire).
By having different lay directions of the strands and wire (left and right - also known as S and Z), it is possible to balance the torque value - resulting in a rope that does not tend to untwist when load is applied. This is called torque balanced or non-rotating rope.
Terminations
The end of a wire rope tends to fray readily, and cannot be easily connected to plant and equipment. There are different ways of securing the ends of wire ropes to prevent fraying. The most common and useful type of end fitting for a wire rope is to turn the end back to form a loop. The loose end is then fixed back on the wire rope. Termination efficiencies vary from about 70% for a Flemish eye alone; to nearly 90% for a Flemish eye and splice; to 100% for potted ends and swagings.
Thimbles
When the wire rope is terminated with a loop, there is a risk that it will 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 abrading 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.
Wire rope clamps/clips
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 u-bolt). The nuts secure the arrangement in place. Three or more clamps are usually used to terminate a wire rope. As many as eight may be needed for a 2 in diameter rope. There is an old adage which has over time become the rule; when installing clamps to secure the loop at the end of your wire rope make sure you do not "saddle a dead horse." The saddle portion of the clamp assembly is placed and tightened on the opposite side of the terminal end of the cable (the load bearing or live end). According to the US Navy Manual S9086-UU-STM-010, Chapter 613R3, Wire and Fiber Rope and Rigging, "This is to protect the live or stress-bearing end of the rope against crushing and abuse. The flat bearing seat and extended prongs of the body (saddle) are designed to protect the rope and are always placed against the live end."[2] The US Navy and most regulatory bodies do not recommend the use of such clips as permanent terminations.
Swaged terminations
Swaging is a method of wire rope termination that refers to the installation technique. The purpose of swaging wire rope fittings is to connect two wire rope ends together, or to otherwise terminate one end of wire rope to something else. A mechanical or hydraulic swager is used to compress and deform the fitting, creating a permanent connection. There are many types of swaged fittings. Threaded Studs, Ferrules, Sockets, and Sleeves a few examples. Swaging ropes with fibre cores is not recommended.
Wedge Sockets
A wedge 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.
Potted ends or Poured sockets
Poured sockets are used to make a high strength, permanent termination; they are created by inserting the wire rope into the narrow end of a conical cavity which is oriented in-line with the intended direction of strain. The individual wires are splayed out inside the cone, and the cone is then filled with molten zinc, or now more commonly, an epoxy resin compound.[3]
Eye splice or Flemish eye
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. When this type of rope splice is used specifically on wire rope, it is called a "Molly Hogan", and, by some, a "Dutch" eye instead of a "Flemish" eye.[4]
Codes and standards
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
The following EN Standards apply to wire rope:
The other Parts of EN 12385 are: Part 1: General requirements Part 2: Definitions, designation and classification Part 3: Information for use and maintenance Part 4: Stranded ropes for general lifting applications Part 5: Stranded ropes for lifts Part 6: Stranded ropes for mine shafts Part 7: Locked coil ropes for mine shafts Part 8: Stranded hauling and carrying-hauling ropes for cableway installations designed to carry persons Part 9: Locked coil carrying ropes for cableway installations designed to carry persons Part 10: Spiral ropes for general structural applications
See also
References
- ^ Modern History of Wire Rope - Donald Sayenga
- ^ S9086-UU-STM-010/CH-613R3 NAVAL SHIPS’ TECHNICAL MANUAL, CHAPTER 613, WIRE AND FIBER ROPE AND RIGGING http://www.fas.org/man/dod-101/sys/ship/nstm/ch613.pdf
- ^ http://www.escocorp.com/markets/rigging/socketfast.html
- ^ Primer of Towing / George H. Reid - 3rd ed. Fig. 3-5 p30 - Cornell Maritime Press, 2004. ISBN 0-87033-563-4