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The Case of the Crushed Cable
Kenneth Russell, Professor Emeritus, MIT -- Design News, June 2, 2008
Wire rope or cable is strong, flexible and durable, but must be subjected only to straight forces and, in particular, it must not be kinked. Wire rope is normally coiled on steel sheaves or pulleys. Passing a loaded cable over the rim of the sheave may give a kink and a huge reduction in cable strength.
The current case centered on the painting of a big natural gas tank in a town north of Boston. The painters operated off a metal platform suspended from the “epi,” the piece at the very top of the tank. Wheeled outriders were used to keep the cable from contacting the joint between the top and side of the tank.
The Scene of the Crime
The accident occurred on a day so windy a plastic shroud had been rigged over the platform and painters to allow painting to proceed. The platform was perhaps 30 ft in the air when one of the cables snapped. The ensuing plunge resulted in serious injuries. Neither man was wearing a safety line.
The Investigation
Cable is made out of a high carbon iron alloy called “plow steel.” This steel is heat-treated, then cold drawn to a fine wire of extremely high strength. In this case, the wires were about 1/100-inch in diameter, which is about three times the diameter of a human hair. My attempts to bend back some cut ends of the wire resulted in bloody fingers. The wire was that hard.
In the current case, 19 wires were wound into a single strand and the six strands wound into a 3/16-inch cable. I estimated the breaking strength at about 10,000 lb, the weight of a large bull elephant. The painters were averaged-sized men and the platform did not weigh much. Something had gone badly wrong, but what?
Experts for the various litigants arranged for a joint study on the scanning electron microscope. The photo above is a micrograph of the failed cable. One fractured wire is seen, as well as some sound wires and some battered wires. The fracture is about what one would expect to see in a high-strength steel. But what caused the battering?
Another expert opined that the outriders malfunctioned and let the cable rub against the junction between the top and side of the tank. He also claimed the battering was due to abrasion of the cable against the tank. I bought the first part of his argument, but not the second. The wire is so hard that rubbing it against the mild steel tank would have no more effect than rubbing it against a stick of butter. Some other force was afoot.
The Smoking Gun
A witness stated that the wind raised the platform and dropped it just prior to the accident. The resulting jerk on the cable would propagate a stress wave up the cable that would come to an abrupt halt at the cable: tank contact. The energy of the stress wave would be dissipated at the contact, crushing the cable into itself, as well as the tank. I found it entirely reasonable that the force of such an event was enough to batter even the hard drawn plow steel wires. Anyone who played “crack the whip” as a child knows the effects of traveling stress waves.
My deposition was taken in the case but I never testified. The case was probably settled out of court.
| Author Information |
| Ken Russell (kenruss@mit.edu) is professor emeritus of Metallurgy and Nuclear Engineering at MIT. He specializes in physical metallurgy, forensic metallurgy and failure analysis. Cases presented here are drawn from his actual forensic files. |
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