Identification and Construction
Wire rope is identified not only by its component parts, but also by its construction, i.e., by the way the wires have been laid to form strands and by the way the strands have been laid around the core.
In the example below, ‘a’ and ‘c’ show strands, as normally laid into the rope the right in a fashion similar to the threading in a right-hand bolt. Conversely, the ‘left lay’ rope strands (illustrations ‘b’ and ‘d’ are laid in the opposite direction.
Again in the example below, the first two (‘a’ and ‘b’), show regular lay ropes. Following these are the types known as Lang lay ropes (‘c’ and ‘d’). Note that the wires in regular lay ropes appear to line up with the axis of the rope; in Lang lay rope the wires form an angle with the axis of the rope. This difference in appearance is a result of variations in manufactoring techniques: regular lay ropes are made so that the direction of the strand lay in the rope; Lang lay ropes are made with both strand lay and rope lay in the same direction. Finally, ‘e’ called alternate lay, consists of alternating regular and Lang lay strands.
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A comparison of typical wire rope lays: a) right regular lay, b) left regular lay, c) right Lang lay, d) left Lang lay, e) right alternate lay
Of all types of wire rope in the current use, right regular lay (RRL) is found in the widest range of applications. Nonetheless, in many equipment applications right Lang lay (RLL) or left Lang lay (ILL) ropes are required. At present, left lay rope is infrequently used. As for alternate lay (R-ALT or L-ALT) ropes, these are only used for special applications.
Compared to other types, the superiority of Lang lay rope in certain applications derives from the fact that when bent over sheaves, its life span is longer than the others. Stated in another way, the advantage of Lang lay rope is its greater fatigue resistance. Yet another claim is made for Lang lay ropes: they are more resistant to abrasion. Broadly speaking, this is true, but there are some reservations that should be taken into account.
It is important to understand the reasons for the advantages of Lang lay rope. To begin with, consider its fatigue and bending properties. Figure a shows, in part, how the Lang lay construction characteristics result in greater fatigue resistance than is found in regular lay ropes. Note how the axis of the wire relates to the axis of the rope in both cases. When the regular lay rope is bent, the same degree of bend is imparted to the crown of the outer wires.
Superior fatigue life in Lang lay rope is also attributable to the longer exposed length of its outer wires. In the upper photograph of a regular lay rope, the valley-to-valley length of individual wires is about 7/8”; the length of the Lang lay wires in the lower photograph is about 1 1/8” or 30% longer. Bending the Lang lay rope results in less axial bending of the outer wires and greater torsional flexure. These combined stresses notwithstanding the Lang lay rope displays 15 25% superiority over regular lay when bending is the principal factor affecting service life.
It is said that Lang lay is more flexible, but flexibility should not be confused with fatigue resistance. These two attributes may under certain circumstances bear some relationship, but they are distinctively separate characteristics. Flexibility defines the relative ease with which a rope ‘flexes’ or bends. Fatigue resistance defines the rope’s ability to endure bending.
A comparison of wear characteristics between regular lay and Lang lay ropes. The lines on a-b, on drawings, indicate the rope axis.
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| A comparison of wear characteristics between regular lay and Lang Lay ropes. The lines on a-b, on drawings, indicate the rope axis. |
Two other factors relate to fatigue; they are discussed here along with abrasion and peening characteristics.
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The figure above illustrates, in drawings and photographs, the wear pattern in regular lay vs. Lang lay ropes drawings (of a single strand) show the wire direction relative to the rope axis in both types. Dimension lines in the upper drawing set off the exposed length of one wire crown in the regular lay rope. The lower drawing shows the corresponding four wire crowns involved in the Lang lay rope. The line a-b shows the relation of the wire crown to the rope axis. Although there is little difference in total contact area between rope and sheave in these two rope types, the forces and wear on the individual wires are quite different.
The fact that the wires of the regular lay rope are subject to higher-pressure increase the rate of wear (abrasion and peening) of both wire and mating surface of the drum or sheave. Moreover, this higher pressure is transmitted to the interior rope structure and this, in turn, decreases fatigue resistance.
Finally, the worn crown of the regular lay wire combined with its shorter exposed length, permits the wire to spring away from the rope axis. Subsequent passage on and off a sheave or drum, results in fatigue breakage.
A note of caution: Lang lay rope has two important limitations. First, if end is not fixed, it will rotate severely when under load and secondly, it is less able to withstand crushing action on a drum or sheave, than is regular lay rope. Hence, Lang lay rope should not be operated without being secured against rotation at both ends; nor should it be operated over minimum-sized sheaves or drums under extreme loads. Additionally, poor drum winding conditions are not well tolerated by Lang lay ropes.
Pre-forming is a wire rope manufacturing process wherein the strands and their wires are formed-during-fabrication to the helical shape that they will ultimately assume in the finished rope or strand.
The wire arrangement in the strands is an important determining factor in the rope’s functional characteristics, i.e., its ability to meet the operating conditions to which it will be subjected. There are many basic strand patterns around which standard ropes are built.
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Wire ropes are identified by a nomenclature that is referred to: 1) the number of strands in the rope, 2) the number (nominal or exact) and arrangement or wires in each strand, and 3) a descriptive word or letter indicating the type of construction, i.e., the geometric arrangement of wires.
Cross-sections of four basic strand constructions are illustrated in the previous figure. Additionally, several possible combinations of these constructions are also shown.
At this point, it would be useful to discuss wire rope nomenclature in somewhat greater detail because the subject may generate some misunderstanding. The reason for this stems from the practice of referring to rope either by class or by its specific construction.
Ropes are classified by the number of strands as well as by the number of wires in each strand, e.g., 6x7, 6x19, 6x37, 8x19, 19x7, etc. However, these are nominal classifications that may or may not reflect the actual construction. For example, the 6x19 class includes constructions such as 6x21 filler wire, 6x25 filler wire, and 6x26 Warrington Seale. Despite the fact that none of the three constructions named have 19 wires, they are designated as being in the 6x19 classification. Hence, a supplier receiving an order for 6x19 rope may assume this to be a class reference and could possibly furnish any construction within this category. But, should the job require the special characteristics of a 6x25 filler wire and a 6x19 Seale is supplied in its stead, a shorter service life may result.
To avoid such misunderstandings, the safest procedure is to order a specific construction. In the event that the specific construction is not known or is in doubt, the rope should be ordered by class along with the description of its end use.
Identification of wire rope in class groups facilitates selection on the basis of strength and weight/foot since it is customary domestic industry practice that all ropes (from a given manufacturer) within a class have the same nominal strength and weight/foot. As for other functional characteristics, these can be obtained by referencing the specific construction within the class.
Only three wire ropes under the 6x19 classification actually have 19 wires: 6x19 two operation (2-op), 6x19 Seale (S), and 6x19 Warrington (W). All the rest have different wire counts. In the 6x37 class there is greater variety of wire constructions. The commonly available construction in the 6x31 Warrington Seale (WS), 6x36 WS, 6x41 Seale Filler Wire (SFW), 6x41 WS, 6x43 Filler Wire Seale (FWS), 6x46 WS, etc, none of which contain exactly 37 wires.
For the users convenience, the most widely used rope classifications are listed and described in the following table. While the interior of a strand is of some significance, its important characteristics relate to the number and in consequence, the size of the outer wires. This is discussed in somewhat greater detail in the selection titled Factors Affecting the Selection of Wire Rope.
| · Rope Description · Direction and type of lay · Length · Finish |
· Size (diameter) · Grade of rope · Preformed (pref) or non-preformed (non-pref) · Type of core |
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If direction and type of lay are omitted from the rope description, it is presumed to be right regular lay. Two other assumptions are made by the supplier: 1) if finish is omitted, this will be presumed to mean uncoated “bright” finish and 2) if no mention is made with reference to preforming, then preforming will be presumed. (Note that an order for elevator rope must have an explicit statement since both pref and non-pref ropes are used extensively.)
When a strand replaces a center wire, it is considered as a single wire and the rope classification remains unchanged. The wire rope cross sections as illustrated in the next page represent some of the most commonly used configurations and are arranged under their respective classification groups. Since these are in greater demand, they are more generally available.
See additional Information on Links below:
| Wire Rope Classification | Rotation Resistant | Elevator Rope |