Outspinning the Spider: The Story of Wire and Wire Rope

by John Kimberly Mumford

The cut ends of a diversified lot of wire ropes resemble, more than anything else, the eccentric forms of snow flakes, in their regularity and the grouping of their parts around a center. But there is nothing haphazard about the formations. Even the core is figured in the number of days it will add to the rope’s life under varying conditions. The wide difference in ropes consists not only in the materials employed, which have much to do with their resistance to divers strains and the manner of their use, but in skillful selection of sizes in the wire and arrangement in the strands of which they are composed; again in the distribution in the strands, the twists of the strands themselves and the “lay” or manner in which these are twisted to make the rope. It is all the result of careful calculation.

THE CORE

A paramount factor too is the core, in securing the maximum of wear. Its mission, in most ropes, is not to add strength, but pliability, and to serve as a cushion to absorb the impact which the strands make under the tension of service. The fibre cores, for this reason, are usually treated with some lubricant. In the majority of ropes hemp is used for a core but in those intended for stationary service the core may be of steel. This will add from seven to ten per cent to strength and very largely to rigidity.

When we speak of wire rope most of us have a mental picture of a round fabric, but there are flat ropes as well, for use in mines or quarries where the haul is from great depths and twisting is to be avoided. These are made in all widths and thicknesses, and are constructed by placing several strands together, side by side, and sewing them together with soft iron wire. But it is the round rope that supplies the great demand.

THE STRAND

In considering rope, one may start with the strand. Strands, as may be seen from the pictures of transverse sections of ropes, vary infinitely in character, but always with a purpose. They are made up in ordinary practice, of four, seven, twelve, nineteen or thirty-seven wires, according to the work the rope is meant to do. In the rope mills you come upon long, low “stranding machines,” reaching down a long room and carrying in horizontal arrangement, wide apart but in circular formation, the wires that are to form the strand. At a point carefully determined with reference to the strain on each wire, in order to preserve uniformity, all these wires come together and pass through one opening in a twisting machine which whirls them into a unit. The finished strand is wound on bobbins.

The direction of the twist, whether to right or left, is of moment in determining the character of the finished product.

“STANDARD” OR GENERAL PURPOSE ROPE

“Standard rope,” so called, the general purpose rope, is composed of six wire strands and a hemp core, all being practically of the same size; but to secure particular results the number of strands may be four, five, eight, twelve or whatever may be desired. Already it will be apparent that there is wide latitude in rope making for the exercise of skill and the utilization of experimental record. This freedom in selection and adjustment extends through almost every process. For example, in the twists: when wires in the strands and strands in the rope are twisted in the same direction, which ordinarily they are not, the rope has what is known as a “Lang lay,” after a rope man who devised the system. The twist, whether in strand or rope, has distinct effect in service. It may be long or short. If it is long the rope will be stronger and more rigid, if short, it will gain in flexibility. When it comes to the short twist rope, one sees the particular value of the twisting tests which were applied and recorded away back in the wire stage.

TESTING THE ROPE