WIRE ROPE FAILURE ANALYSIS


by
Charles C. Roberts Jr.


Steel cable or wire rope is used throughout industry, providing a variety of tasks such as structural support, lifting and control functions. Figure 1 is a view of a typical wire rope showing many steel wire strands woven in a manner similar to regular hemp rope. Wire rope is very strong for its size and weight and performs many functions well. However, it is susceptible to corrosion, wear and metal fatigue, which can cause failure, economic loss and personal injury.

FIGURE 1



FIGURE 2



Figure 2 shows a wire rope loop, complete with compression sleeve and thimble. The purpose of the thimble is to provide a surface that withstands wear better than the bare wires. Without the thimble, the wire strands would eventually abrade or fatigue causing a failure. The compression sleeve keeps the wire from sliding around the thimble. Sometimes mechanical clamps are used, especially for large wire ropes.

FIGURE 3



FIGURE 4



Figure 3 is a view of a large grain auger that collapsed during a windstorm. Peak wind speeds were approximately 50 MPH, a speed easily resisted by the structure. Figure 4 is a view of the upper portion of the tower. Inspection of the cable indicated that a cable failure had occurred in the upwind cable. Figure 5 shows the cable loop that had failed at a pier in the ground. Several wire strands had been broken. The cable clamp was adequate, but no thimble had been provided. None of the other cables had a thimble at the loop end. Figure 6 is a close-up of failed strands at the end of the loop. Failure analysis of the strands suggested that corrosion fatigue was the likely cause of failure. This was brought on by the lack of a thimble that would have protected the cable end. It was concluded that this failure was a result of improper construction of the tower cable system.


FIGURE 5



FIGURE 6



Overloading of cable systems is a common cause of failure. Serious injury can occur if a cable fails near personnel. For instance, if a wire rope cable fails when lifting or winching a load, the cable often whips violently and can cause serious injury. Figure 7 is a view of a radio tower that collapsed during an ice storm. The cable failure origin showed evidence of severe overloading in an otherwise properly designed and maintained cable system.


FIGURE 7



When inspecting for the cause of a cable failure, it is desirable to save the failed cable, especially the failure origin, for future analysis. Inquire as to the loading on the cable, i.e., wind, lifting, high-speed operation, etc. If abrasion or wear is a cause of the failure, analysis of the design or manufacturing method is in order. Other design considerations may also be a cause of failure, especially in those cases where excessive rope flexing occurs from using cable sheaves (pulleys) that are too small for the wire rope.


FOR TECHNICAL ARTICLES CONTACT CLAIMS MAGAZINE AND ASK

FOR A REPRINT OF A PAST TECHNICAL NOTEBOOK ARTICLE

CLAIMS MAGAZINE



BACK TO C. ROBERTS CONSULTING ENGINEERS HOME PAGE,
WWW.CROBERTS.COM