Outspinning the Spider: The Story of Wire and Wire Rope

by John Kimberly Mumford

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.

THE STEAM SHOVEL SHOVELS BY MEANS OF A WIRE ROPE

From molecular condition, as revealed by the microscope, down to the last petty detail in the plan of construction, there is never an end to the problems, and gravity has to be figured into the lifetime of a rope as surely as the elusive trace of sulphuric and muriatic acid producing hydrogen occlusion. Wire rope is a business of exactitude and eternal vigilance. You have to deal with breaking strengths of from 40,000 to 340,000 pounds to the square inch of transverse section, but the wire that will lift weights at the rate of more than a hundred tons has entirely different characteristics than the lower strength material. And the why of that must be traced back to the treatment of the steel when it was passing through the wire stage. Rope makers dealt with molecules once and thought they were taking pains. They found they had to go back to atoms to handle their problems. Today the secret seems to lurk in the electron.

FITTING THE ROPE TO ITS WORK

Of the tricks in making ropes, there is no end. They are fitted for their work like a soldier or a gymnast, and built for it. A tiller rope must be flexible to the last degree, but it must be strong enough so it will stand up under the swift tensions of a storm or in the lightning manœuvers of a race. Therefore, like a few ropes built for other purposes, the composite parts are not mere strands of wire, but little ropes in themselves, complete in all parts. And again, while ropes exposed to weather and stationary, like ships’ standing riggings, are galvanized, those that are subjected to constant bending are not. For every variation, there’s a reason.

To the average man or woman, the elevators in tall buildings suggest danger. The rope engineer counts them highly safe because each elevator is equipped with a multiplicity of ropes and safety devices. What taxes his conscience and spurs him to the last possible effort, is the rope that goes to the “deep shaft” service, where the lives of men going up and down in five thousand feet or more of subterranean darkness, hang on the accuracy of his calculation.

Only now, the Roebling engineers will tell you, is wire rope being perfected. Much of it is in what seem to be small details of construction, which nevertheless go down into the basic principles that make for efficiency. Rope making has been treated as an exact science, because it dealt with materials that were more or less standardized. They are learning now that rope has a large unknown quantity that defies formula past a certain point. For the lack of a better term, they call it “personality.” The labor of today, and many years to come, is to identify these intangible factors and bring them where they can be computed to the end of securing greater endurance and safety.

In the Roebling shops there are men working who got their jobs almost by heredity. Their fathers and grandfathers worked for John A. Roebling.

“You ask them,” said the Chief Engineer, “why they do a thing a certain way. They tell you simply that ‘that’s the way to do it.’” In the old days John A. Roebling figured out the way, and gave it to his workmen in the shape of orders—today somewhat different methods are utilized. To the cumulative experience of over eighty years of wire-rope making, the Roeblings have always availed themselves of the latest engineering skill. With up-to-date research, chemical and metallurgical laboratories, every progress in the art has been incorporated in their product.