Forensics

Wire Rope Failures

Introduction

View of a fallen crane due to failed wire rope.

Steel wire ropes find typical applications in the building, construction, marine and elevator industries, involving various types of lifting equipment such as mobile and tower cranes, synchrolift hoist and winch ropes, as well as elevators. Catastrophic failure of these wire ropes can occur without warning, causing damage and endangering lives.

Examination

Above : Corrosion to external wires masked by grease. Above : Severely corroded.
Right : Corrosion on the inside of an elevator cable strand, where it was in contact with the fibrous centre.

Each type of failure has distinct characteristics. However microscopic examination is usually required to ensure that all features are seen. Such examination can be exacting because ropes can be made of hundreds of wires and each fracture has to be individually examined.

A further difficulty is that when a rope fractures, mechanical damage can occur as a result of the event. This can be confused with pre-accident damage unless the examination is thorough.

The extent of damage to elevator rope failures is often severe and it may sometimes be difficult to distinguish between pre-existing damage and damage occurring as a result of the failure. Thus, when a rope fractures, mechanical damage can occur as a result of the event. However close examination of wires away from the failed end can show the true condition of the wire rope prior to failure. Wires that have been worn over a period of time exhibit flat surfaces and fractures often occur across these flats.

Types of Failures

Both wear and corrosion on the same.

Overload : Ropes can fail simply because they are loaded too heavily. This is the least common reason for the failed ropes seen at our laboratory.

Prior Damage : Ropes can come into contact with the side of a building or another crane. A rope might slip out of a sheath and suffer damage. Mechanical damage weakens the rope and failure can occur thereafter.

Wear : In working, the ropes pass over sheaves and are wound onto a drum, providing opportunity for contact and wear to occur. In a dusty construction site the grease can be contaminated by dirt particles, with the most common particle, silica, being very abrasive.

A wire rope that failed by fatigue. The outer wires have become flattened by wear and this weakening has made them more susceptible to fatigue.

Corrosion : Many ropes spend their life outdoors and some are close to the sea and may be affected by chloride contamination. If they are not frequently greased the deterioration can be rapid. Even indoors, humidity can cause corrosion, for example with elevator ropes, particularly in the tropics. Most of the failures we see are a result of wear, corrosion or a combination of the two.

Synchrolift hoist cables used in shipyards are particularly susceptible to corrosion due to the seawater environment. Corrosion to internal wires is often unseen from the outside while external wires are covered with costly grease and there is reluctance to remove it for inspection.

Fatigue : Repeated flexing of ropes can lead to fatigue. Sometimes fatigue follows on from wear.

Our Services

With our experience, we are able to assist in the following areas :

  • Determining the cause of wire rope failure.
  • Non-destructively evaluate the condition of remaining wire to determine its fitness for service.