If the cables you plan to use in your system design require continuous movement, there are several considerations that must be taken into consideration. Carefully pairing continuous-flex cables with cable carrier systems – also known as e-chains – is critical in preventing damage to manufacturing systems and interruptions that result from that damage.
|Cables inside e-chains during high-speed movement tests. Photo courtesy of igus.|
The plastic jacket materials used, in addition to the proper conductor materials, play a central role in the interaction between an e-chain and the cable. This is especially important when combined with external factors that many industrial applications present, such as temperature extremes, oil and chemical exposure, or high-duty cycles.
In the cable industry, there are various well-known test procedures that come from different respected institutions such as UL, CSA, and VDE. Yet these organizations fail to test one key factor - they do not assess the ability for cables to withstand continuous motion in an e-chain. To do that, cables have to be tested in realistic settings that replicate the actual conditions in the field.
Neither the bending cycle test according to VDE, nor the standard abrasion test meet the requirements for testing a true chain-cable combination. In the case of the bending cycle test, the cables are cycled on a series of pulleys, completely different to the movements inside an e-chain. Many cables that reliably meet institutional test requirements can quickly fail e-chain movement tests.
Sandpaper, Needles, and Razor Blades Are Not Enough
Standard abrasion tests that determine the ability of the cable jacket material to resist surface wear use sandpaper, needles, or razor blades. These are adequate for general applications. In these tests, the sandpaper, razor blade, or needles are applied to the cable jacket material under pressure, and dragged along its surface for a pre-defined number of movements. However, these tests do not replicate the real-world situation of cable abrasion inside e-chains, so they may not accurately indicate the durability of the jacket material when used inside a cable carrier. For these types of applications, it is much more important is to test and match the relative movement between the chain material and cable material.
Cable jacket materials need to be tested in combination with e-chains in real-world environmental conditions to most effectively determine their resilience in cable carrier applications. To define a cable’s wear resistance, cables need to be installed and run inside e-chains, causing the same type of abrasion between cable and cross-bar that would occur in the field. These realistic tests combine the cable and e-chain materials to minimize the effects of wear and extend functional reliability.
Realistic Testing Results in Increased Reliability
Chemicals, water, dirt, and temperature extremes and fluxuations should play a central role in the development and testing of jacket materials for continuous-flex cables. The cold bending cable test, for example, determines the flexibility of moving cables at low temperatures. Depending on requirements, this method involves winding up to three test cables around a mandrel and cooling them down to the applicable test temperature. The diameter of the mandrel is selected based on the diameter of the cable to be tested. Once the cable reaches the required test temperature, the cable is unwound from the mandrel; passing the test (at the temperature applied) if no jacket fractures are visible.
While this testing is practical for some applications, it fails to simulate the conditions cables are exposed to while installed inside an e-chains operating at low temperatures. Rigorous testing in a climate chamber, finds that many PVC cable jacket materials do not stand up to realistic low-temperature operations. In order to determine what will happen in the real-world applications, cables need to be tested inside an e-chain in conditions they are expected to encounter in their useful life..
|Cooling test chamber for continuous-flex cable inside e-chains. Photo courtesy of igus.|
Most cable materials and chains are rated for stationary applications or for applications with movement in accordance with UL, VDE, or IEC standards. This isn’t sufficient to match many real-word applications. The chains need to be tested in motion continuously in a predetermined temperature range that replicates actual use. Jacket cracks and fractures can be seen in most cable materials after only a few thousand strokes in rugged testing. Only high-cycle, long term testing in realistic conditions can provide the whole picture of a cable’s reliability and overall service life inside e-chains.
Don Nester is a Chainflex continuous-flex cable expert at igus.
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