In selecting the right steel wire rope, the properties of the various types of steel wire rope must be considered, e.g.:
- Tensile strength
- Abrasion resistance
- Bending fatique resistance
- Corrosion resistance
- Rotation resistance
- Crushing resistance
- Vibration resistance
- Pulsation resistance
- Regular Lay or Lang Lay
In selecting the right steel wire rope, it is important to determine how important the various properties are in relation to the application and then to assign priorities to these. It is also important to be aware of the relevant standards and regulations. If you are in any doubt, please contact our sales consultants or our Technical Department.
The tensile strength of the steel wire rope depends on the rope’s dimensions, the tensile strength of the wires and the construction. The minimum guaranteed tensile strength for the different kinds of rope is shown in the Randers Reb product catalogue.
A steel wire rope should never be subjected to a load exceeding 50% of its breaking load.
The design of the steel wire rope does not significantly affect the tensile strength (up to approx. 5%). A change of core from fibre to steel makes slightly more difference (approx. 10%). The greatest change is achieved by changing the dimensions, usage of Compacted steel wire ropes or tensile strength of the wires (see also fig. 28).
It is often required that the steel wire rope must have a specific SWL value (Safe Working Load), also known as a WLL value (Working Load Limit). This means the steel wire rope’s tensile strength divided by the safety factor required for the relevant application.
NB: There are a number of national and international norms and standards that define the minimum requirements for the safety factor.
Steel wire ropes with thick outer wires (e.g. 6x7 Standard or 6x19 Seale) provide good abrasion resistance. Lang lay ropes provide better abrasion resistance than regular lay steel wire ropes (see also fig. 28). Abrasion resistance can also be increased by using wires with greater tensile strength.
Bending fatique resistance
The greater the number of wires in the strand, the greater the bending fatique resistance and flexibility. Lang lay ropes provide better bending fatique resistance than regular lay steel wire ropes. Bending fatique resistance can also be increased by using pre-formed steel wire ropes (see also fig. 28).
Galvanised and rustproof wires provide excellent protection against corrosion. Lubrication with special types of grease or oil will also increase resistance to corrosion. If the steel wire rope is subjected to significant corrosive influences, it is recommended that strands with thick outer wires are used.
Steel wire ropes with fewer wires (e.g. 1x7 Standard and 1x19 Standard) are subject to the least elongation (have the greatest elasticity modulus). This type of steel wire rope is ideally suited for guy ropes, but is not suitable to be run over sheaves/blocks. If only a small degree of elongation when running over sheaves is required, 6x7 or 6x19 steel wire rope should be used, in each case with a steel core or with certain special constructions. For larger dimensions, 6x36 steel wire rope with a steel core can also be used (see also Elongation and Pre-stretching, page 8-28).
Standard 6-lay and 8-lay steel wire ropes will rotate when they hang free and carry a load. Regular lay steel wire rope provides greater resistance to rotation than lang lay steel wire rope. A steel wire rope with a steel core rotates less than a steel wire rope with a fibre core. The type of rope that provides greatest resistance to rotation is, as the name suggests, low-rotation and rotation-resistant steel wire rope (special constructions, see also ”Low-Rotation and Rotation-Resistant Steel Wire Rope”, page 8-10).
A steel core provides better support for the strands than a fibre core, which is why the risk of flattening is less in a steel wire rope with a steel core. Strands with fewer, thicker wires have greater resistance to flattening/crushing. Also, a 6-lay steel wire rope has greater crushing resistance than an 8-lay rope (see also fig. 28).
Vibrations, from wherever they might come, send shock waves through the steel wire rope, which will be absorbed by the steel wire rope at some point, and in some cases they may cause localised destruction of the steel wire rope (not necessarily on the outside). This may, for example, be at places where the steel wire rope comes into contact with a sheaf/block, or enters the drum, and by the end terminals. In general, those steel wire ropes with the greatest flexibility also have the greatest vibration resistance.
Changes in the tension of a steel wire rope, depending on the size and frequency, will reduce the rope’s life expectancy. In general, steel wire ropes with the greatest flexibility can cope better with intermittent loading. Great care should be taken in the use of end terminals or fittings, as their pulsation resistance is equally as important as the selection of the right steel wire rope.
Regular Lay or Lang Lay
Lang lay steel wire ropes are the ones most suited to running over sheaves and are the most durable, but if they are to be used, three things must be observed:
- Lang lay steel wire ropes must be secured at both ends, otherwise the rope will rotate. The steel wire rope has no resistance to rotation.
- Lang lay steel wire ropes may only be reeled on to the drum in a single layer, as they can easily destroy themselves.
- Lang lay steel wire ropes may not run over small sheaves, as the construction will become unbalanced.
The reason for Lang lay steel wire ropes’ excellent qualities of abrasion resistance and pliability is that the wires are affected/loaded in a different way and have a larger load-bearing surface than a regular lay steel wire rope (see fig. 29). Note that the largest wearing surface is on the Lang lay steel wire rope.