Flexible cables and wires for fixed and moving applications

Titelbild flexible Kabel

What are flexible cables and wires?

The simplest cable is a solid conductor with a plastic sheath. It can be bent and maintains this bend - if you don't do it too often, because then the conductor breaks. Such simple cables are found in domestic installations. Once laid, the cable remains in a place untouched for decades. For the many other applications where cables must be flexible and elastic, such solid wires are not suitable. There, the conductors in the cores consist of strands, bundles of fine wires that can be bent millions of times, depending on the design, without breaking and losing their ability to transmit electricity or data.

One of the most grueling places to use a cable is a drag chain. There, power, servo, and data cables are closely spaced and move back and forth in the working cycle of a machine. Sometimes faster than five meters per second with more than five times the acceleration due to gravity. The cables are laid in the drag chain so that they are only bent in one direction. But this is only one of three possible types of movement:

  • Bending: The cable is bent, sometimes millions of times;
  • Torsion: The cable is twisted in the longitudinal direction. Pure torsional movements can be found in wind turbines with cables that lead from the rotating nacelle down to the tower. However, they are rare, and in most applications, cables are both bent and twisted;
  • winding and unwinding: This applies to cables in event technology or live TV, for example, which are unwound from drums and then rewound and stored after the event. 

Special robot cables differ in many ways from other robust cables for moving applications. The most important difference is that robot cables can withstand both bending and torsion over their entire product life. Three parameters are important for a robot cable: 

  • Stranding class: Robot cables that are subjected to torsional stress usually have "fine-wires" strands of class 5. Highly flexible cables such as ÖLFLEX® FD or ÖLFLEX® CHAIN that are subjected to pure bending stress, for example in energy guiding chains or linearly moved axes of portal robots, even contain "extra-fine wires" of class 6. However, even the highest stranded conductor class 6 is not sufficient for the most demanding requirements. At LAPP, for example, for cables that have to be highly flexible, we used strands outside the standard, where the individual wires with a diameter of up to 0.05 millimeters are considerably thinner than the thinnest stranded wires within the standard. 
  • Torsion angle: This angle is given in degrees per meter of wire length. A typical value is 360°/m, which means that a cable can be twisted once around its axis per meter of length without being damaged. This applies to cables without shielding, with shielding the value is typically 180° or half a turn per meter. 
  • Bending radius: This should be between four and 7.5 times the outer diameter and thus in some cases considerably less than in the case of cables that are only designed for occasional movement. This allows the cables to be guided in tight radii and tightly packed in hose packages.

In addition to the stranding class, there are other aspects that distinguish a flexible cable from a less flexible one. One is stranding. To understand what is meant by this, here is a comparative example that everyone knows: a hair plait. The closer you braid it, the thicker it gets, thicker and thinner parts alternate. If the same number of hairs are simply combined into a parallel bundle, it is noticeably thinner. The thickness increases when you twist the hair bundle. Something similar happens to the copper strands during stranding. The fine metal wires are twisted because this improves flexibility - if all braids and all wires were parallel, each bend in the wire would stretch the outer copper wires and compress the inner ones. This would make the cable very rigid. Thickness and flexibility can be controlled by the lay length: the distance for one revolution of the twist. If it is longer and therefore the twist is less, the cable is thinner.

Cables that are moved a lot contain sliding support: It helps the components inside to move against each other with as little friction as possible. They also serve as a filler that keeps the cable round. This is important when the cable runs through a cable gland or into a connector. If the jacket is not properly round, problems with tightness arise. Sliding supports can be stranded fine plastic fibers that nestle into the spaces between the wires. Thicker cores are often wrapped with a non-woven polytetrafluoroethylene tape to facilitate sliding on each other, especially in torsion.

Whether a cable can withstand such movements for a long time also depends on the outer sheath material. Material experts are faced with the challenge of having to reconcile not only mobility but also other properties such as fire behavior or resistance to oil, chemicals, and cleaning agents. PVC continues to dominate the market for sheathing materials, but other materials have also become established, such as thermoplastic elastomers (TPE) or polyurethane, which is the first choice for highly dynamic applications, for example in the ÖLFLEX® Servo FD 796 CP. Polypropylene in particular has proven its worth as an insulator for the cores in moving applications. With its high strength and low density, it has very good electrical insulation properties.

For very high data rates over long distances, optical fibers are the first choice. They consist of plastic fibers (POF) for shorter distances of up to 70 meters, PCF fibers (plastic-coated glass fibers) for distances of up to 100 meters, and glass fibers for even greater distances and for applications requiring the highest data rates. In principle, all fiber types are suitable for moving applications, provided the recommended bending radii are adhered to. Then there is no need to be afraid that an optical fiber might splinter. However, for the highest transmission rates, a bending radius of 15 times the diameter of optical fibers should not be undercut. Below that, it does not break, but the attenuation increases, which means that light is lost in the narrow curve and the signal quality suffers. How well a fiber optic cable can withstand movement depends largely on the materials that surround the fiber. Often these are aramids, i.e. textile fibers that give bulletproof vests or fiber-reinforced plastics their special properties. If the cable is stretched, the textile sheath absorbs the tensile force and prevents the optical fiber from being stretched as well.

Where are the flexible cables used?

Except for fixed installation, for example in the house installation, almost everywhere. In the industry in all applications where something moves: on moving machine parts or on machining stations on production lines, in drag chains, on robots, in wind turbines and oil drilling platforms, in vehicles and engines, on cranes and commercial vehicles, even in applications where vibrations occur.

What flexible cables are available from LAPP?

Almost all ÖLFLEX® brand cables and all UNITRONIC®, ETHERLINE®, and HITRONIC® brand data cables are flexible. There are differences in the bending radii, which must be observed at all costs. Some cables allow only occasional bending, others can be bent millions of times. Some cables are specially optimized for torsion. Unfortunately, there is no one cable that covers all applications, but the application experts at LAPP find a solution for all possible and impossible applications. LAPP also offers suitable accessories to connect flexible cables and to protect them in cable ducts and protective cable conduits. Especially in highly dynamic applications, even with torsion, the transition at the connector housing is critical. The housing must hold the cable securely so that it cannot slip out or moisture penetrate.


A good example of the different ways in which cables can be optimized are the fiber optic cables from LAPP. HITRONIC® TORSION was specially designed for applications with high torsions, such as in wind power plants. It has up to twelve glass fibers for single and multimode transmission, a strain relief made of aramid fibers, and a halogen-free and flame-retardant jacket made of polyurethane. HITRONIC® HDM has a similar design but is particularly suitable for winding and unwinding on cable drums. And HITRONIC® HRM FD is suitable for installation in cable drag chains where high flexibility is required, but not torsion.


The tests at APP in Stuttgart prove that LAPP is not making false promises here. Cables for wind turbines are tested for torsion in a twelve-meter-high old elevator shaft - a unique procedure worldwide. Other manufacturers test shorter cable sections, which they twist at smaller angles, and extrapolate this to longer cable lengths. The decisive factor, however, is not what is written on paper, but what comes out under real conditions.


Flexible glass optical fibre

Power and control cables

Servo applicationsn