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Standard cables used in cable carriers may risk 'corkscrewing'

Flexible cables, or 'continuous-flex' cables, are cables specially designed to cope with the tight bending radii and physical stress associated with moving applications, such as inside cable carriers.

Due to increasing demands within the field of automation technology in the 1980s, such as increasing loads, moving cables guided inside cable carriers often failed, although the cable carriers themselves did not. In extreme cases, failures caused by "corkscrews" and core ruptures brought entire production lines to a standstill, with high costs incurred as a result.

As a result, specialist, highly flexible cables were developed with unique characteristics to differentiate them from standard designs. These are sometimes known as “chain-suitable”, “high-flex” or “continuous flex” cables.

A higher level of flexibility means the service life of a cable inside a cable carrier can be greatly extended. A normal cable would typically manage 50,000 cycles, but a dynamic cable can complete between 1 and 3 million cycles.

Here the cable conductors are stranded into bundles

[edit] Construction

Flexible cables can be divided into two genres: those with conductors stranded in layers inside the cable and those which have bundled/braided rather than layered conductors.

[edit] Stranding in layers

Stranding in layers, it is significantly easier to produce and is therefore usually offered on the market at low cost. The cable cores are stranded firmly and left relatively long in several layers around the centre and are then enclosed in an extruded tube shaped jacket. In the case of shielded cables, the cores are wrapped up with fleece or foils.

However, this type of cable construction means that, during the bending process, the inner radius of the cable is compressed and the outer radius stretched as the cable core moves. Initially, this works quite well, because the elasticity of the material is still sufficient, but soon material fatigue can set in and cause permanent deformations. The cores move and begin to make their own compressing and stretching zones, which can lead to a “corkscrew” shape and ultimately core rupture.

[edit] Stranding in bundles

The unique cable construction technique of braiding conductors around a tension-proof centre instead of layering them is the second type of construction.

By eliminating multi-layers, a uniform bend radius is guaranteed across each conductor. At any point where the cable is flexed, the path of any core moves quickly from the inside to the outside of the cable. The result is that no single core stays near the inside of the bend to be compressed or near the outside of the bend to be stretched and overall the stresses are much reduced.

An outer jacket is still required to prevent the cores untwisting. A pressure filled rather than simple extruded jacket is preferable here. This fills all the gussets around the cores and ensures that the cores cannot untwist.

The resulting dynamic cable is often stiffer than a standard cable because of this construction, but a cable with this construction will last longer in a constantly flexing application.

[edit] References

• List of flexible cable suppliers
• Another list from the Kellysearch industrial directory