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.
[Sidenote: "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.
It is singular, but it is true, that the aggregate strength of all the wires that go to make up a rope cannot be retained in the rope, at least in the laboratory on the testing machines. When the rope is tested for breaking strength it is found that no sample will show more than ninety per cent of the total, and the average is about eighty-two. Part of this failure is due to the angle of wires in the strand, with a resultant stress on wires in excess of applied load; therefore, the greater the number of wires in the rope, the lower the efficiency. The other reason is that the contiguous strands in the rope nick each other under high tension, and so are weakened. This, however, may not be important in ordinary working loads under service conditions. These casual truths show with what multiplicity of tendencies the rope maker has to deal in devising a product to give service and safety in the often ticklish jobs it has to do, with great weights in hand, and human lives at stake.