Birdcaging:

Source: Dipl. Ing. Verreet: Drahtseile vor Gericht.

Birdcaging means that on a short rope section the outer strands of the wire rope are significantly longer than the inner core. As a result, the outer strands are loosened and this excess continues to build up when running over sheaves or on drums until basket formation occurs. Birdcages are therefore often found at the end point of movement over sheaves. If cage formation has occurred, this always results in discarding of the wire rope. For this reason, it is important to find the cause of the birdcage formation. The following causes can lead to cage formation:

Tight sheaves:

The optimum groove diameter should be rope diameter +6-8% or according to DIN 15020 rope diameter x 1.05 or at least 5% and maximum 10% of the nominal rope diameter according to ISO16625. If the groove diameter is too small, the incoming wire rope will be compressed which can lead to wire breaks and deformations. In addition, the pressure of the lateral flanks may push outer strands further forward and thus cause strands to loosen permanently or lead to the formation of a birdcages .

Source: Dipl. Ing. Verreet: Drahtseile vor Gericht.

Using of a rotating swivels:

Non-rotation-free wire ropes must never work with swivels. Non-rotation or low rotation wire ropes may only work with swivels if the rotation property of the wire rope is less than or equal to 1 rotation /1000d when lifting a load corresponding to 20% of the minimum breaking load. In the case of low rotation wire ropes with poor rotation properties, a swivel may only be used after observing the recommendations of the rope manufacturer and/or the approval of a competent person (definition according to EN12385:3 B1.5).

If a wire rope with insufficient torsional stability is used, the rope will untwist under load and thus loosen the outer strands. If the wire rope runs through sheaves, the excess lengths of the outer strands are carried to a point outside the sheaves and a birdcage is formed.

Source: Dipl. Ing. Verreet: Drahtseile vor Gericht.

Wrong deflection angle:

The deflection angle must be max. 4° for non-rotation-free ropes and max. 2° for rotation-free wire ropes (ISO16625). Otherwise, if the deflection angle is too high, the rope may not enter the sheave at the lowest point, but first touch one of the two outer flanks and then roll into the bottom of the groove. The higher the deflection angle, the more the rope rolls over the flanks into the bottom of the groove, which leads to twisting of the rope and can cause cage formation.

Source: Dipl. Ing. Verreet: Drahtseile vor Gericht.

Damage from rotation

Wrong direction of lay:

The following picture shows a hoisting system with two drums and should consist of a right-handed and a left-handed rope so that different lay directions cancel out the resulting torque. Here, however, two right-handed wire ropes were used with the consequence that the torque could not cancel each other out and was given into the wire rope. The wire rope on the left reel has untwisted while the wire rope on the right drum has twisted in.

Twisting due to heavy abrasion:

When ropes are pulled over stationary rollers, bolts or edges, permanent twisting can occur. In this case, the strands and wires that rest on the inside of the bending object (bolt or fixed roller) are displaced. The wires and strands are forced out of their original helix shape into a straight line. Furthermore, the material of the wire is pushed up in the direction of tension. These material displacements create a tension which deforms the wire rope. This process can be compared to a gift ribbon that is pulled over a pair of scissors or a knife.

Twisting due to jumpy loading:

Jumpy movements can cause twisting in the rope if the rope has been in a loop at the moment of loading, for example. This can happen if a grapple on the rope bends or is at an angle. The following picture shows a wire rope that has been twisted due to a jumpy load.

After removing the outer strands in the rope zone in question, traces of high mechanical pressure were found on the steel core, which had acted on the steel core. This indicates that a jumpy load had been applied.

Corkscrew deformation

When ropes are pulled through sheaves that are too tight or rub against objects, corkscrew-like deformation can occur. The unloaded rope is then deformed into a helix. ISO4309 specifies that ropes with such deformation must be discarded if, on a section that does not run over a sheave or onto a drum, the distance between the deformed underside and a straight surface is 1/3 x d or more, or 1/10 x d or more if this section runs over a sheave or onto a drum.

Source: Dipl. Ing. Verreet: Drahtseile vor Gericht.

Fatgiue break

Fatigue fractures usually occur in the rope zones stressed by bending cycles and are often found on the side of the wire rope that rests on the sheave. Over time, wires begin to crack, which at first is only visible microscopically. With each bend, the crack becomes larger until eventually the wire is completely broken. As soon as the first fatigue breaks are visible, this area must be observed more closely, because more and more wires break in the time sequence (exponential development!) The more frequently a rope is stressed by bending and the higher the load to be carried, the more easily fatigue breaks occur. Small radii of sheaves further increase the risk of fatigue breaks. Regular relubrication, increasing sheave diameters and reducing the working load can prevent fatigue breaks and thus help to extend rope service life.

Fatigue fractures can be easily identified when the affected areas are bent.

Rope damage caused by sharp edges

When ropes are pulled over sharp edges, this can lead to wire, strand or complete rope failure. In the following picture, the breakage pattern clearly shows a pull over a sharp edge, because the breakage runs in a very straight line through the entire rope.

Corrosion Damage

Corroded wires lose breaking strength, flexibility and break much faster than wires without corrosion. Galvanized wires, wire ropes with plastic coating and regularly lubricated wire ropes can protect against corrosion. As corroded steel expands, an increased rope diameter may indicate corrosion inside the rope. The following picture shows a corroded wire rope. This is a running wire rope that had not received any lubrication, thus promoting corrosion.