Flexible cables for fixed and moving applications

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What are flexible cables?

The simplest cable is a solid wire with a plastic sheath. It can bend and retains this bending – if you don’t do it too often, because otherwise the wire breaks. Simple cables like these are used in house installations. Once installed, the cable remains in place for decades untouched. Solid wires like these aren’t suitable for many other applications where cables need to be flexible and elastic. Here, the conductors in the cores are made up of strands, fine wire bundles that can be bent millions of times, depending on the design, without breaking and losing their current or data transmission properties.

One of the most annoying locations for a cable is a drag chain. Here, power, servo and data cables are located close together and move back and forth as a machine works. Sometimes faster than five metres per second with more than five times the acceleration of gravity. The cables are laid in the drag chain in such a way that they’re bent in just one direction. However, this is only one of three possible types of movement

  • Bending: The cable is bent, sometimes millions of times;
  • Torsion: The cable is twisted longitudinally. Pure torsional movements are found in wind turbines where the cables run from the rotating nacelle down to the tower. However, they are rare, as cables are both bent and twisted in most applications;
  • Winding and unwinding: This is where cables are unreeled from drums, for instance in stage applications or on live TV, then reeled back onto them and stored after the event.

Special robot cables differ from other robust cables for moving applications in many respects. The key difference: robotic cables withstand both bending and torsion for the entire lifetime. During development, they are fundamentally designed in a different way to a power chain cable, for example. Three parameters are important for a robot cable:

  • Braided conductor class: robot cables that are subject to torsional stress generally contain “fine” strands of class 5. Highly flexible cables such as ÖLFLEX® FD or ÖLFLEX® CHAIN that are subject to purely bending stress, for example in power chains or linear moving axes of portal robots, even contain “extra-fine” strands of class 6. However, the highest braided conductor class 6 is not sufficient for the most severe demands. For cables that need to be highly flexible, at Lapp we use special braids in which the individual wires are just 0.05 millimetres in diameter, considerably thinner than the thinnest standard braided wires.
  • Torsion angle: this angle is specified in degrees per metre of cable length. A typical value is 360°/m, so a cable can be twisted once per metre around its axis without causing any damage. This applies to cables without screening. With screening the value is typically 180° or half a turn per metre.
  • Bending radius: this should be between 4 and 7.5 times the outer diameter and thus in some cases considerably lower than for cables that are only designed for occasional movement. This allows the cables to be routed in tight radii and to be tightly packed in hose assemblies.

In addition to the braided conductor class, there are other aspects that distinguish between a flexible cable and a less flexible one. One is the stranding. In order to understand what this means, here is a comparison that everyone knows: a braid of hair. The more closely you braid it, the thicker the braid becomes; the thicker and thinner areas alternate. If you gather together the same number of strands of hair in a parallel bundle, it is noticeably thinner. It becomes thicker when you twist the bundle of hair. Something similar happens with copper strands in “stranding”. The fine metal wires are twisted because this improves the flexibility – if all the strands and all the cores were parallel, the outer copper wires would be stretched at each bending of the cable and the inner ones would be compressed. This would make the cable very rigid. Thickness and flexibility can be controlled by the length of lay: the distance for a round of twisting. If it is longer, and consequently has less twist, the cable turns out thinner

Cables that are subject to a lot of movement contain a sliding support, which helps the components inside to move against each other with as low friction as possible. They also act as a filler that keeps the cable round. This is important if the cable runs through a gland or into a connector. If the sheath isn’t properly round, there are problems with leaks. Sliding supports can be stranded fine plastic fibres that fit into the gaps between the cores. Thicker cores are often wrapped in a polytetrafluoroethylene film fleece wrapping to make it easier for them to slide against one another, particularly under torsion

Whether a cable can withstand such movements over a long period depends on the sheath material. The material experts face the challenge of combining other properties, such as fire behaviour or resistance to oil, chemicals and cleaning agents, in addition to mobility. PVC continues to dominate the market for sheath materials, but other materials such as thermoplastic elastomers (TPE) or polyurethane have emerged as the first choice for highly dynamic applications, e.g. in the ÖLFLEX® Servo FD 796 CP servo cable. Polypropylene has proved particularly suitable for insulating the cores in moving applications. It has excellent electrical insulation properties, and also has high strength and low density.

Fibre optic cables are the first choice for very high data rates over long distances. They consist of plastic optical fibres (POF) for shorter distances of up to 70 metres, plastic cladded fibres (PCF) for distances of up to 100 metres and glass fibres for even larger distances and applications requiring the highest data rates. In principle, all fibre types are suitable for flexible applications as long as the recommended bending radii are observed. Then you don’t need to be afraid that a glass fibre could split. However, in order to achieve the highest possible transmission performance, the bending radius in fibre optic cables should be at least 15 times greater than the diameter. While a lower bending radius will not cause it to break, it will lead to increased attenuation, meaning that light is lost in the tight curve and the signal quality will suffer. The material enveloping the fibres largely determines how well a fibre optic cable can withstand movements. Aramide fibres, i.e. synthetic fibres that give bulletproof vests or fibre-reinforced plastics their exceptional properties, are often used here. If the cable is stretched, the textile sheath absorbs the tensile force and prevents the fibre optic cable from also being stretched.


Where are flexible cables used?

Except for fixed installation, for example in house installations, almost everywhere. In industry in all applications where something is moving: on moving machine parts or processing stations on production lines, drag chains, robots, wind turbines and oil rigs, in vehicles and motors, on cranes and commercial vehicles, and also in applications where vibrations occur.


What flexible cables are available from LAPP?

Almost all ÖLFLEX® cables and all UNITRONIC® data cables, ETHERLINE® and HITRONIC® brands are flexible. There are differences in the bending radii, which must be adhered to. Some cables only allow occasional bending, while others can bend millions of times. Some cables are specially optimised for torsion. Unfortunately, there is no single cable that covers all applications, but LAPP’s application experts find a solution for all possible and impossible applications. LAPP also offers suitable accessories for connecting and protecting flexible cables in cable ducts and cable conduits. The transition to the connector housing is critical in highly dynamic applications, including with torsion. The housing must hold the cable securely so that it doesn’t slip out and moisture doesn’t penetrate.


Fibre optic cables from LAPP are a good example of how different cables can be optimised. HITRONIC® TORSION was specially designed for high torsion applications such as in wind turbines. They have up to twelve glass fibres for single and multi-mode transmission, strain relief made of aramide fibres and a halogen-free, flame-retardant polyurethane sheath. HITRONIC® HDM is similarly structured, but especially suitable for winding and unwinding on cable drums. And the HITRONIC® HRM FD is suitable for installation in power chains where flexibility is critical, but not torsion.


The tests at LAPP in Stuttgart show that LAPP makes no false promises here. In an old lift shaft, cables for wind turbines are tested for torsion – these tests are a worldwide first. Other manufacturers test shorter cable lengths twisted at more acute angles and extrapolate this data to estimate the figures for longer cable lengths. However, the decisive thing is not what is on paper, but what happens under real conditions.



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