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Metallurgical Failure Analysis of Wire Rope Cables

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Wire Rope Cable Failure 2



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Michael Fox, PhD.





Metallurgical Failure Analysis of Wire Rope Cables

Chemaxx was asked to investigate the frequent failures of wire rope cables being used at a lumberyard. The wire rope cables were used to move a 40,000-pound lumber cutting carriage back and forth through a saw blade, as seen in the video below:

The logs being cut in the above process are on the order of 10,000-pounds each.

The wire rope cable (WRC) was found to have thermoplastic material throughout and a general lack of lubricant, as seen below:

A detailed (wire-by-wire) examination of one of the outer strands revealed that 11 out of 20 of the wires failed by fatigue (not counting filler wires). The other stands appeared to have the same percentage of fatigue failures - about 50%. As wires break via fatigue, the remaining wires can no longer hold the load and the remaining wires fail by ductile overload and the WRC snaps. Hence, the primary failure mechanism is fatigue followed by ductile overload. A photo of the failed ends of the WRC is below:

Several observations were made. First, the ratio of the drum and sheave diameters to the WRC diameter was too small for the low cycle, high-load type of service. A larger diameter drum and/or sheave were recommended.

Secondly, the particular 7-Strand WRC examined appeared to have a thermoplastic material that is extruded throughout the entire structure, including the independent wire rope core (IWRC). The photo above shows the IWRC that is literally imbedded with the solidified thermoplastic material, which in turn appears to have excluded lubricant. Throughout the WRC there was a general lack of lubricant and the appearance of heat damage that most likely resulted from that lack of lubricant. An uncoated, continuously lubricated, WRC was recommended. The ability of the wires to move over and against each other is essential to extending useful life in this type of application.

A third factor was the high stress caused by the changing momentum of the massive lumber carriage. Chemaxx estimated that the forces are significant at the moment the carriage has to stop and then reverse direction. These forces were in play throughout the day and would facilitate low cycle fatigue. Chemaxx recommended making these transitions slower and smoother to help extend the life of the WRC.

Another suggestion involved new WRC "break in." The first time a new WRC is used, it is beneficial to make several complete round trips as slow as possible. That is followed by several more round trips at about half the normal speed. In fact, it is a good practice to begin each working day with at least one slow round trip and then at least one at half speed. This is particularly true in cold weather and several slow speed and half speed cycles were recommended on the colder days.

Dr. Fox is a nationally recognized metallurgy expert, corrosion expert and failure analysis expert who has published numerous peer-reviewed, scientific papers in these fields. He worked for years as a bench scientist and a research manager in the fields of corrosion, metallurgy and failure analysis. While a research manager, Dr. Fox managed over $100 million worth of research in metallurgy, corrosion and failure analysis.