Accelerating and decelerating at several times the acceleration due to gravity and millions of times over - it's hard to imagine what cables that follow the movements of a robot arm or move back and forth 'crammed' into energy supply chains in the work cycle of a machine have to withstand. And with ever higher cycle rates and speeds as well as ever tighter bending radii as a result of the miniaturization of machines, the demands on the mobility of cables have also increased significantly in recent years. Accordingly, polyurethane (PUR) or special thermoplastic elastomers (TPE) are now mainly used as jacket materials for highly dynamic cables instead of PVC. Furthermore, the internal structures of the cables have been adapted accordingly, for example by stranding the individual cores in the cable, constructing shielding braids and optimizing the core insulation materials. And last but not least, the demands on assembly have increased - in other words, complete systems in which all components are optimally matched to one another.

Lapp has recently bundled its system solutions under the name 'Ölflex Connect'. All components are matched to each other, which provides added safety and functionality even in moving applications - as here at machine manufacturer Toshulin.

© U.I. Lapp

When developing fully assembled energy chains, it is always important to set the right priorities: The mechanical design of the energy chain is often designed first and then the cables are adapted - which can result in the chain working very well but the cables failing prematurely. For the user, however, it is not the chains themselves that are important, but the cables or hoses with which they are fitted. Lapp is therefore taking a different approach with its 'Ölflex Connect' assembly initiative and is focusing everything on the optimum and long-term performance of the cables. And if, for example, optimum performance cannot be achieved with a standard cable, a special cable can be developed according to the requirements - it is not uncommon for new standard products to be created from such custom-made products.

In general, electromagnetic compatibility is becoming more important in factories, and this also applies to moving cables. As a rule, these are shielded against electromagnetic influences with a copper braid. The degree of coverage of the braid must usually be over 80 % so that no electromagnetic interference can get through. There must also be no gaps in the braid when it is bent. The braiding angle is crucial here: for highly dynamic applications, the copper wire is laid around the cores at a particularly steep angle - measured to the cable axis - so that it makes a full 360° turn around the cores over a shorter distance.

One example of an Ethernet cable that is ideal for use in energy supply chains is the so-called 'Etherline FD Cat. 6A' from Lapp. Among other things, it is suitable for robot monitoring or for checking production products using camera systems. The 'Torsion' version of the cable can be used for applications where twisting of the cable is to be expected. High mobility is a challenge, especially for data cables with particularly high bandwidths, as these bandwidths require extremely effective shielding - which must remain intact even after millions of movement cycles. Category Cat. 6A Ethernet cables with data transmission rates of up to 10 Gbit/s were therefore previously only available for fixed or slightly moving installations.

Glass is also flexible

If even higher data rates are required, the time has come for fiber optic cables. Today, users can choose between three fiber types: Plastic optical 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 that require the highest data rates. In principle, all fiber types are suitable for moving applications, provided that the recommended bending radii are observed. For maximum transmission performance, however, the bending radius of fiber optic cables should not be less than 15 times the diameter. It does not break below this, but the attenuation increases. In other words: Light is lost in the tight bend and the signal quality deteriorates.

How well a fiber optic cable can withstand movement depends largely on the materials that encase the fiber. These are often aramids, i.e. textile fibers that give bulletproof vests or fiber-reinforced plastics their special properties. The textile sheath absorbs any tensile forces that may occur and prevents the fiber optic cable from being stretched.

Before it is recommended for an application, each cable type undergoes a rigorous test program at Lapp. The engineers test the torsion of fiber optic cables for wind turbines in an old elevator shaft, in which they twist cables over a length of twelve meters. This is unique in the industry. It is customary to test shorter cable sections that are twisted at smaller angles in order to extrapolate the result to longer cable lengths.

As far as further development trends in moving cables are concerned, it is not necessarily a question of even greater accelerations and higher travel speeds - the current standard accelerations of 5 to 8 g will be sufficient for the time being. In other words, cables are not the weakest link in the chain at the moment. Rather, other properties are increasingly required that are not directly related to mobility - such as compatibility with harsh environmental conditions, chemical media or low temperatures, all of which cause plastics to become brittle.

Author:
Bernd Müller is a freelance author from Bonn.