Common causes of damage and failure

There are various causes of damage and fatigue to steel wire ropes. You should understand these factors before conducting wire rope inspection.

Common causes of damage

The following are some reasons for immediate discard:

• dislodgement of a steel core, generally associated with a sudden release of a load

• wire breaks and wire displacement due to bending in an ordinary lay rope

• bird caging deformation of a multi-strand construction

• bird caging at entry to tight sheave on sudden release of load

• bird caging between load and head sheave on sudden release of load.

Broken wires must be removed by being bent backwards and forwards. This is so they break off in the rope valleys between the strand and are not clipped off on the crowns. You may have to nick the wire to initiate breakage.

Broken wires most often appear on the crowns of the strands. This is caused by fatigue and they are a normal part of rope deterioration.

Broken wires originating in the valleys are more serious. These are usually the result of concentrated internal fatigue or core problems. Ropes should be discarded if such breaks are found.

Refer to the below excerpt from AS2759 – table 14.4.1 covering broken wires, diameter loss and corrosion.

Wire Rope Discard Factors

Excerpt from AS2759 - Table 14.4.1 covering broken wires. diameter loss and corrosion

How to use the above discard criteria:

If broken wires are identified. Use the above discard criteria as follows:

1. Find the mid point of damaged area
2. Measure 3 rope diameters in either direction
3. Count broken wire totals around the circumference over this area.

Causes of wire rope failure

Fatigue and tensile overload

The 2 basic failure modes for wire ropes are fatigue and tensile overload. Fatigue and tensile overload are influenced by:

• the diameter over which the rope is bent
• the tension of the rope
• the number of times the rope is bent
• wear on the outside of the rope
• corrosion
• core collapse
• local distortion or anomaly
• temperature.

Wear

Wear occurs on internal and external wires. It is caused by abrasion, corrosion and erosion.

The following information comes from AS2759 Wire Rope Application standard:

• where the diameter of a wire rope has diminished to 95% or less of the nominal diameter, discard the rope. This applies even if no wire breaks are visible
• where surface wires are reduced in diameter by more than 1/3, replace the rope
• loss of material diameter can also be caused by internal failure of the core due to overload, fatigue and corrosion.

Keep moving ropes away from stationary steelwork such as racks, as wear may result on the rope surface:

Wear on steel wire rope

When the surface wires are worn by 1/3 or more of their diameter replace the rope:

Worn surface wires

Sheave groove shape

Ropes can be damaged beyond repair if operated in incorrectly grooved or damaged sheaves. Always check before fitting a new rope.

Australian Standard AS3569 table 2.2.1.5 tolerance of groove radius is 5% for steel core and 7% for fibre core. Other manufacturers may vary.

A groove that is too big can flatten the rope and also wear down the groove.

When a new rope with the correct diameter is installed, the sharp edges at the bottom of the groove may create fractural impressions that reduce rope life.