Cables for dynamic applications
In today’s machines, the need for moving power and data supply is increasingly common. But if safe and reliable operation is to be achieved, choosing the correct cable is a critical decision.
In a dynamic application – that is, an application where a cable is used to connect moving parts – cables may have to move backwards and forwards, in a rolling loop or even with a twisting motion. In today’s high performance machines, these movements typically take place at high speed and it is common for them to be repeated thousands or even millions of times in a short timescale.
Standard cables are unsuited to this type of application. They have a very simple construction where each of their copper cores is made up of a small number of relatively large conductor strands. When cables of this type are subjected to repeated bending at a small bend radius, the copper strands making up each core ‘work harden’, become increasingly brittle and then break.
The same process can be demonstrated by bending a paper clip back and forth. Initially it is flexible and bends easily, but after a short time it ‘work hardens’ and breaks. The breaking of conductor cores because of work hardening is a very common failure mode, usually leading to intermittent signals or shorting. There are other modes of failure when an unsuitable type of cable is used in a dynamic application.
In multicore cables, the cores are always twisted together in layers and in standard cables this twist has a long pitch. This means that when the cable bends, the inner cores are compressed while the outer cores are stretched. After a time, this process forces the cable to take up a corkscrew shape which, in effect, shortens it and often pulls the cores out of their terminations or connectors. The deformed cable may also form loops that get trapped in the carrier or in machine parts, damaging the outer jacket and exposing the core conductors.
A third failure mode occurs when the cores untwist under the outer jacket of the cable, due to the forces explained. On a standard cable, this outer jacket is a simple tubular extrusion.
This is beneficial when terminating the cable because the jacket does not grip the cores tightly and is easy to strip back. In dynamic applications, the jacket has the important role of stopping the cores from untwisting and even with dummy filler cores or strings (a common trick used to pack out the inner construction on a standard cable) a simple tubular jacket is not effective in doing this. The result is that the cores untwist within the jacket, and this can be seen as bulges along the length of the cable. Ultimately, the outer jacket will burst under the strain.
With shielded cables there’s yet another problem. The braiding angle of the shield is usually steep, but this means that the braid has a tendency to open up and create gaps in the shielding when the cable is flexed; leading to a deterioration of EMC protection. Also, because of the steep angle, the braid will be extended and compressed under bending, leading to work harden and fracture. This leaves sharp ends that can pierce the insulation of the inner cores leading to earth faults, or splitting the outer jacket.
So how do ‘dynamic’ cables specifically designed for the applications overcome these problems?
First of all, the copper cores are more finely stranded. However, it is a common misconception that the finer the strand the better. The strand size must actually be optimised for maximum flexibility. This type of construction reduces the risk of work hardening and core fractures and can be used to give even large core high-current motor cables the flexibility needed for reliable operation in dynamic applications.
Also, in a dynamic cable, the cores are twisted with a much shorter pitch and in multicore cables, the cores are braided and bundled wherever possible. This means that the path of the core takes it from the outside of the cable to the inside and back again over a very short distance. Because this distance is so short, the compression and tension stresses experienced by the core when the cable is flexed cancel each other out to a large extent, which means that the overall level of stress on the cores is greatly reduced.
Dynamic cables still rely on the outer jacket to prevent the cores from untwisting, but instead of a plain extruded jacket, a good dynamic cable will have a pressure-filled jacket. This fills all of the space around the cores (meaning dummy cores or strings are not needed) and ensures that they cannot untwist. In good quality dynamic cables where EMC shielding is required, this will be of the braided type rather than wound foil and it will have a shallow braid angle to prevent gaps opening up as the cable flexes, which would reduce the effectiveness of the shielding. The shallow angle reduces stress on the strands, whose sizes will also have been chosen to maximise flexibility while minimising the risk of work hardening.
Due to their special construction, dynamic cables often feel stiffer than their standard counterparts. This shows that the myth suggesting that the cables most suitable for dynamic applications are the most flexible is not only wrong but also misleading. A rough and ready test to determine whether or not a cable is properly constructed for dynamic applications is to bend it tightly. A good dynamic cable will immediately return to its original shape when released, but other cables will retain a kink.
Copper cables provide a versatile and convenient solution for providing power and data connections to moving parts of machines. Provided that genuine dynamic cables are used and that they are selected and installed correctly, solutions of this type are safe, reliable and cost effective. However, any attempt to cut corners or trim budgets by using standard cables is doomed to failure, and will without doubt lead to inconvenience and ultimately higher costs.
The conclusion is simple. For dynamic applications, use good quality dynamic cables from a reputable supplier that can provide dependable advice and guidance on getting the best from them.
Justin Leonard is Director of igus UK
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