Bending fatigue in steel wire ropes refers to the process where, as a rope repeatedly passes over sheaves, drums, or guide pulleys, it is subjected to cyclic bending stresses, inter-wire friction, and contact wear. This leads to the gradual development of micro-cracks and the snapping of individual wires, ultimately resulting in a loss of load-bearing capacity or complete rope failure.
As a steel wire rope travels over a sheave, the outer wires undergo repeated tension while the inner wires are subjected to compression, creating alternating bending stresses. Simultaneously, fretting friction and contact wear occur between the wires within the strands, as well as between the strands and the rope core, which accelerates the propagation of cracks. Wire fractures typically appear on the side of the rope subjected to the most severe bending, with the fracture surfaces often appearing relatively flat.
In dependence of the sheave diameter
The applied diameters of the sheave, as well as the diameters of the drums have a huge impact on the lifetime of a rope. In the shown example, a rope running over a sheave with a diameter of 800mm reaches more than 2.000.000 bending cycles, the bending condition reduces by reducing the diameter of the sheave to 400mm to 290.000 bending cycles
In dependence on the groove diameter
According to ISO 16625 the groove of a sheave should have a diameter, that is 5% to 10% bigger than the rope diameter. During the operating time, the rope diameter will decrease. With this decreased diameter the rope will dig itself into the sheave groove and will reduce the groove diameter. Therefore, with the installation of the rope it should be considered, that the groove diameter of the sheave is at least 1% bigger than the measured rope diameter.
A sheave with a too big groove diameter cannot support the rope well and leads to increased surface pressure (between sheave and rope). Consequently the lifetime of the rope decreases steadily with an increasing groove diameter. If the groove diameter is too small, the rope will be squeezed and the lifetime drops extremely.
When using steel or plastic sheaves
The lifetime of a rope is significantly influenced by the sheave material. By the use of plastic sheaves, the bending fatigue rises clearly in comparison to the use of steel sheaves. Therefore the rope inspection must be carried out especially carefully when using plastic sheaves. We recommend plastic sheaves, hence, only in applications where the ropes are checked magnet-inductively or where the rope gets damaged primarily outside like in multi-layer spooling.
In dependence on the working load
An increase in load leads to greater contact pressure between the strands and the individual wires. During repeated bending, this contact pressure translates into more severe fretting wear and frictional heat, which causes the lubricant to fail more readily, thereby accelerating fatigue-induced wire fractures. Operations such as hoisting, braking, emergency stops, and the application of impact loads introduce dynamic load factors, meaning the actual forces experienced are often higher than the static loads. Impact loads cause localized stress to spike instantaneously, making them more likely than steady loads to result in premature wire fractures.
In dependence on the rope construction
In wire rope strands with point contact, the lay lengths of the wires in different layers vary, which can lead to inter-layer compression and additional bending stress under load. In contrast, strands with line contact feature uniform lay lengths across all wire layers; this results in a more compact structure and significantly reduces relative sliding between the wires.
When rope diameter is the same, a higher number of wires—combined with finer outer wires—allows the rope to conform more easily to sheaves and drums, while also reducing the bending stress experienced by individual wires. For example, a 6×36 construction is generally more flexible and offers better resistance to bending fatigue than a 6×19 construction. However, if the outer wires are too fine, the rope's wear resistance will decrease; in cases of severe abrasion, this can actually lead to a shorter service life.
Ungalvanized and galvanized ropes
A comparison of the bending fatigue of ungalvanized and galvanized ropes until the achievement of the discard criteria according to ISO 4309 and until break of the rope shows, that galvanized ropes usually reach more bending cycles. The zinc coating offers better "emergency operating features" when the rope is not lubricated any more and protects the rope from mechanical wear and the resulting corrosion.