Lapp cables for flexible application

Does a cable break if it is bent or twisted too much? Not if you choose the right cable for the required application. This article explains where the challenges lie and the technical ideas that Lapp is using to overcome these issues as the leading supplier of connection solutions.

Cables have long been high-tech products. There is now a wide range of options when it comes to the material, processing methods and preassembly. This variety is necessary as the demands placed on cables, particularly regarding flexibility, have risen sharply over the last few years. One of the most gruelling places to install a cable is in a power chain. Here servo cables such as the ÖLFLEX® SERVO FD 796 CP and the ETHERLINE® or HITRONIC® fibre optical data cables are placed very close to one another. They move to and fro during a machine’s power cycle, sometimes faster than 5 m/s with accelerations over 50 m/s2. “In highly dynamic applications, several things must be borne in mind in order to accommodate the demand for increased service life while guaranteeing lower space requirements, reduced weight and a small minimum bending radius,” explained Lucas Kehl, Product Manager at U.I. Lapp GmbH.

Three types of movement
There are three different types of movement in cables:

• Torsion: the cable is twisted about a longitudinal axis. Pure torsional movements are found in wind turbines where the cables run from the rotating nacelle down to the tower;
• Drag chain: the cable is bent, sometimes millions of times, in the drag chain;
• 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.

The material, particularly that used in the sheath, determines whether a cable can withstand movements over long periods of time and most combine several properties, e.g. fire behaviour or resistance to oil, chemicals and cleaning agents. PVC continues to dominate the market for sheath materials, but there are other materials such as thermoplastic elastomers (TPE) or polyurethane, which is the first choice for highly dynamic applications, e.g. in the ÖLFLEX® Servo FD 796 CP servo cable. Polypropylene is particularly good at insulating the core in flexible applications as it features excellent electrical insulation properties while also being very strong with a low density.

Flexible braiding
The screening braid plays an important role in a cable’s flexibility. It blocks out interference from other cables, e.g. from motors or live cables. It is important that most of the screening braid is covered; no gaps should appear, even when the cable is bent. The braiding angle is the decisive factor here: the steeper it is when measured agains the cable’s axis, the more turns are made in the wires in the braided shield per metre of cable, the thicker the braiding. Unfortunately, this also makes the cable more expensive as more material is needed.

The ETHERLINE® FD Cat. 6A Ethernet cable, which is used for robot monitoring or inspecting finished products using camera systems, is an example of excellent shielding. Suitable for use in power chains, the ETHERLINE® TORSION Cat. 6A can even be used in applications where the cable is twisted. Until now, cables in accordance with Cat. 6A with data transmission rates of up to 10 Gbit/s were only possible in fixed or slightly flexible installations.

Glass fibres can also bend
Fibre optic cables are the only option if you want even higher data rates. Users can choose between three fibre types: 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. 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 drop.

Textile fabric as strain relief The material enveloping the fibres largely determines whether 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.

Each cable type undergoes rigorous testing at Lapp before being recommended for a particular application. In these tests, Lapp engineers test the torsion of cables for wind power plants in an old lift shaft by twisting the cables over a total length of 12 metres. This process is extraordinary in the cable industry. Other manufacturers test shorter cable lengths twisted at more acute angles and extrapolate this data to estimate the figures for longer cable lengths. “We don’t just rely on what’s written on paper. We check every cable in realistic conditions,” promised Werner Körner, Head of Development and Technology at the U.I. Lapp GmbH.

Everything from one supplier Customers are increasingly requesting preassembled cables, i.e. cables with mounted connectors that are often already built into the energy chain. Lapp Systems, a subsidiary of the Lapp Group, has long provided these kinds of preassembled integrated systems. Customers like to be safe in the knowledge that all components are perfectly matched, particularly in flexible applications. Under the new name of ÖLFLEX® CONNECT, Lapp is now bundling all global preassembly activities for customers, including consultation. To this end, the company is expanding its corresponding production and preassembly capacities in America, Europe and Asia. The customers thus benefit from customised solutions, high quality thanks to the optimum interaction of all components and expert on-site consultation.