Does the stranded wire diameter determine durability?

igu-blog-adm | 19. March 2020

If it were that easy, we at igus could save ourselves a whole lot of research and development. Unfortunately, it’s not. In an article about the class of a stranded wire, I explained how important the wire diameter is in a stranded wire, and that it is inaccurate to simply say that a finer-wired strand will last longer. If we examine durability, we find that many other factors play a role.

The strand’s individual wires do not just lie in the core

In cabling, we often call the structure of a cable its stranding. By this we mean that the cores are wrapped around a centre. If you imagine a cable without its jacket, you will see a steel cable that looks very much like those you have seen on funiculars. The stranding is so that the cable’s components remain together; it also helps compensate for forces applied to the cable. But it is not just the cores in the centre of the cable that are stranded – other high-quality cores are as well. Under their insulation, cores consist of the wires that form the strand, and these wires are stranded around the centre so as to reduce the force on each wire in the strand. Wire stranding is not always the same – it varies by pitch. Technicians speak of pitch length, referring to the number of times an individual wire in the strand passes around the centre per metre of manufactured core. The shorter the pitch length, the more often the wire is wrapped around the core centre.

A strand does not always consist only of wires

Something that is simple with small conductor nominal cross sections is much more complicated in larger ones. In small cross sections, reference is often made to “bunched strands”, or the defined number of wires of a defined diameter stranded under the insulation. As a conductor nominal cross section becomes larger (and igus has single-core cables with cross sections of up to 300mm² in its catalogue), this design is no longer sufficient. It would cause too many crossings of individual wires during processing, which would cause abrasion during movement, breaking the copper. That is why a braided conductor is used. Wires are taken in small groups, and the different groups are braided much as individual wires are braided in a stranded wire. Each group functions as a single wire would in the stranded wire. This increases service life greatly and allows the individual cores to be routed through very small radii on spindles in machine tools and used in highly dynamic motion applications in linear handling robots or on cranes for travels of over 400m.

…but then the core can just be inserted into the cable

No, it can’t. Since each component in the core is wrapped around the core’s axis, the way in which the core with the stranded strands are integrated into the stranding of the entire cable is, of course, important. For instance, the strand in the core can move around it in a clockwise or an anti-clockwise direction. The same is true of cores stranded around the centre element. The combination of the two direction is important here, too. In stranding, we do not use “clockwise” and “anti-clockwise”, but S and Z. You can remember the difference by thinking of which direction each letter goes when you move from its centre to its top end.

igus seems to make stranding very complicated

We don’t make it complicated – unfortunately, it just is this complicated. To avoid confusing users unnecessarily, we provide only the information that is of interest to our customers (further evaluation requires a great deal of experience). We tell you how long the cable will last for a given type of movement. We take care of the rest.