Wire Rope Constructions

IN previous articles it has been shown that wire rope is a machine, a highly complex machine, composed of many component parts carefully arranged under strict tolerance limits, in order to properly function as a complete mechanism particularly adapted for the work to be performed.

The design, or co-related arrangement of the component parts, consisting of wires, strands and center, is called the "construction." By varying the number, size, and arrangement of the wires and strands, many different wire rope constructions may be formed.

The complex character of wire rope can perhaps be better understood, when it is realized that there are nearly one hundred different constructions in which wire rope has been furnished. Nothing would be gained by enumerating all of these, since many of such constructions are produced for highly specialized duties and do not enter into general wire rope requirements. There are, however, about thirty wire rope constructions in general use. These may be divided into five distinct groups, which may be further sub-divided, according to the general arrangement of the wires and strands.

For simplicity in arrangement and clearness in comparison, we have tabulated thirty-one wire rope constructions. See pages 16 and 17. By referring to this tabulation and accompanying cross-sections, a general idea of the fabrication of these different constructions may be obtained.

The numerical designation states the number of strands placed around the center, as well as the number of wires in each individual strand -- the first number indicating the strand and the second, the wires.

In the column under the caption " Arrangement of Wires," we have indicated the relative position of the individual wires placed in concentric layers about the center wire of each strand. For any construction these numbers should be read from left to right, to correspond with the position of the wires counting from the center wire to the outside wires in each strand.

Cross Sections of Wire Rope Construction

In using the term Reserve Strength we mean the ratio between the metallic area, or strength, of the inside wires and the total metallic area, or strength, of the rope. Reserve strength is that reservoir of safety in a rope which is protected, by the outside wires from surface wear, and can generally be safely relied upon for the entire life of the rope, provided, of course, that internal corrosion is not present. It can, therefore, be seen that this factor is of extreme importance when choosing a rope construction for service in which human life is involved and where a careful surface rope inspection cannot always be obtained.

It will be noted from this tabulation that there are two general types of wire strand construction: one having all the outside wires of the same diameter, and the other having the outside wires in two diameters-half large and half small, spaced alternately. The first type may be subdivided into three groups, namely Coarse Laid Group, Seale Group and Special Seale Group.

In the Coarse Laid Group we have only one layer of wires around each center wire. For equal rope and strand diameter such a small number of wires must be of correspondingly larger diameter, which produces a coarse rope construction of limited flexibility and of maximum resistance to abrasion. It should be noted that ropes in this group have the maximum amount of metal contained in the outside wires in contact with surface wear, and therefore, the minimum reserve strength.

The next, or Seale Group constructions, include ropes fabricated by laying two concentric layers of wires around each center wire. The ropes in this group, while more flexible than those in the coarse laid group, are designed to resist severe abrasion and at the same time be more adaptable to flexure.

The third, or Special Seale Group, containing nine different rope constructions, varying from 6x16 Special Seale to 6x37, represents a range of rope constructions that may be applied to many different classes of service. All of the constructions in this group are fabricated by laying two or three concentric layers of wire around each center wire and filling the spaces, or voids between the outer two layers, with smaller, or spacer wires. It can be seen that between the 6x16 and 6x37 constructions, the intermediate constructions are formed by adding one wire each to the inner layers and two wires each to the outside layer, the general arrangement being the same. It should be noted that as the number of wires increase, the flexibility of the construction and also the reserve strength increases.

In the next group it will be observed that there are two sizes of wire making up the outside layer. With the exception of the Warrington construction, there are two layers of inside wires, totaling three concentric layers of wire around each center wire. While the total number of outside wires and the total number of wires in the ropes of this group are the same as corresponding ropes of the same numerical designation in the Special Seale group, the reserve strength is less for this group than for the Special Seale group, indicating the effect of wire rope design in distributing metal to accomplish a desired result.

The rope constructions listed in the Multiple Hemp Center Groupare designed for special purposes, as indicated by their trade names. It will be noted that each rope construction contains seven hemp centers. In fabricating the 6x12 running rope, which is used mostly for ships' rigging, each strand is composed of 12 wires laid around a hemp core, and six of these strands are then laid around a hemp centerto form the rope. This provides a very flexible construction, with the metallic strength confined entirely to the outside of the rope in contact with surface wear. The tiller rope is formed y laying six individual 6x7 hemp center ropes around a hemp center. It will be further noted that there are 252 wires used in making this construction, which is only used where maximum flexibility is desired, without strength being a major requirement.

In the 8-Strand Group it will be observed that four of the most generally used 6-strand rope constructions have been used in fabricating an 8strand rope. With the strand fabrication the same as used in corresponding 6xl9 ropes, the ropes in this group are formed by laying 8 strands around a hemp center. For equal rope diameters, eight strands taking the place of six, the diameter of the individual strands in this group will be smaller and the size of the hemp center larger. This provides a more flexible rope than the 6-strand constructions, but due to the greater number of strands and fullness of hemp center, these 8strand ropes will not withstand as much scrubbing and intensity of pressures as will 6-strand ropes of corresponding diameters.

As illustrated, the strands in these ropes are roughly triangular in shape. The 6x8 Style D strands are formed by seven wires laid around a triangular center wire, while the 6x30 Style G strands have two layers of 12 wires each around 3 pairs of wires so laid together that a triangular cross section is provided. These ropes are furnished only in the fang lay construction.

These ropes afford a little greater resistance to abrasion than round strand ropes, and in some instances produce less rapid wear to sheaves and drums. In addition, their strength is a little greater for the equivalent size and grade. They are somewhat less flexible however than round st.rand ropes of the same general classification and not as fatigue resistant. For this reason the installations on which this type of rope can be advantageously used are not numerous.

The rope constructions listed in the Multiple Hemp Center Group are designed for special purposes, as indicated by their trade names.

It will be noted that each rope construction contains seven hemp centers. In fabricating the 6x12 Running rope, which is used mostly for ships' rigging, each strand is composed of 12 wires laid around a hemp core, and six of these strands are then laid around a hemp center to form the rope. This provides a very flexible construction, with the metallic strength confined entirely to the outside of the rope in contact with surface wear.

The Tiller rope is formed by laying six individual 6x7 hemp center ropes around a hemp center. It will be further noted that there are 252 w*es used in making this construction. This rope is only used where maximum flexibility is desired, without strength being a major requirement.

The three rope constructions listed in the Miscellaneous Group are each used for special purposes and have no particular relation to each other. The 18x7 Non-rotating rope is fabricated by laying 18 seven-wire strands around a hemp core in two concentric layers, the inside layer consisting of 6 strands laid in one direction and the outside layer consisting of 12 strands laid in the opposite direction. It can therefore be seen that the tendency for twisting between the inside and outside layers is directly opposite to each other, which tends to balance the rope, thereby establishing the non-rotating characteristics. This rope construction is used for hoisting or lowering loads in single part, on derrick equipment, etc., where rotation is to be restricted. These ropes have not the strength of similar size ropes in other constructions of hoisting rope and should be used only upon recommendation of the rope manufacturer. The 7x7 metallic center ropes are similar in construction to the 6x7 coarse laid rope, with the exception that a 7-wire strand is used in place of the hemp center. These ropes are used where flexibility is not an important consideration, and where it is desired to either withstand intense heat or to keep rope elongation to a minimum. The 3x7 Guard Rail rope consists of three 7-wire strands laid together without the use of a center or core, and is extensively used as a protection to moving traffic along state highways. This rope should never be used as a running rope. This construction, while economical, provides an efficient protection in the formation of highway fencing where tension, possible crushing and the ability to absorb impact are the main requirements. The 9x4 Mast-arm rope is composed of nine strands of four wires each laid around a cotton center. It is used for mast-arms, arc lights, or other purposes where operation over small sheaves is intermittent.

The above description of the various rope constructions is intended to give users of wire rope a general idea of wire rope design, and the manner in which wire rope manufacturers assemble and arrange the various component parts of this "complex machine," to meet the many and varied requirements of modern industry. It must, of course, be understood that there are other rope constructions manufactured which we have not listed, but the tabulation made covers the major ones.