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Automation Technologies 5/2016

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Automation Technologies 5/2016

Well bundled and safely

Well bundled and safely routed Rainer Rössel Cables that are used in moving industrial machine parts must withstand tensile and torsional forces as well as external influences such as abrasion or welding sparks. But whilst energy chains offer some protection, the requirements on the cables remain high. Just one of many design factors to achieve this is the braiding of cores into bundles, because it makes the cables perfect for constant movement and hence compatible for e-chains. About igus Igus GmbH is a globally leading manufacturer of energy chain systems and polymer plain bearings. The Cologne-based family business has offices in 35 countries and employs around 2,950 people around the world. In 2015, igus generated a turnover of 552 million euros with motion plastics, plastic components for moving applications. igus operates the largest test laboratories and factories in its sector to offer customers quick turnaround times on innovative products and solutions tailored to their needs. The energy chain can be described as the umbilical cord of a machine. It supplies a machine part with energy, data and media and closely follows its every move. The degree of movement ranges from simple strokes to six-axis robotic applications. The material and design of the cables must be perfectly adapted so that the cables are able to withstand loads many millions of times without failures or “corkscrewing”. Reliable life prediction through long-term tests Real world experience shows that even highly flexible cables often quickly reach their load limits while in moving applications in energy chains. Is it possible to predict their service life? Conventional standard tests, such as those carried out by VDE, IEC or UL, do not provide a clear answer. This is because reliable predictions can only be obtained through long-term tests in an energy chain itself. Applicable standards simulate only the wear through other methods and not directly inside a chain and also don’t refer to the cable materials, with the end result that this cannot be adapted for use in chains. In order to reliably predict the service life, igus the specialist company for 01 Cross-sectional view of braiding in bundles: As with steel cable designs, all of the cores are put under the same amount of stress COMPONENTS AND SOFTWARE Author: Rainer Rössel, Head of Business Unit chainflex Leitungen, igus GmbH, Cologne

02 In the test laboratory with an area of 2,750 square metres, the service life of the products are proven with more than 2 billion test cycles plastics and cables in motion, operates the largest test lab for moving cables in energy chains, with a floor area of the 2,750 square metres. Here the products are tested for their resilience to continuous operation on 58 different test rigs. Since reliability depends on the exact simulation of real working conditions, test rigs are available with a wide variety of travel distances and accelerations or weather conditions. For the testing of large energy chain systems such as those used in cranes, an outdoor test site with a travel distance of up to 240 m travel is available. Here components with 4 m/s and an additional load of 8 kg/m have been successfully tested for a total output of 25,000 km. Demonstrating cold flexibility Likewise temperature cycles of - 40 to + 60 °C are tested. A specially converted shipping container allows these changes to be tested. Unlike the standard cold winding test, in which the test cables are wound onto a mandrel and cooled down to test temperature once, the cables and chains are tested here in appropriate test temperatures under realistic moving conditions. They must withstand millions of strokes and the expected bending stress of real use. A test is considered passed, and thus the necessary cold flexibility proved, when no jacket breaks can be found. The tests are not always about extreme temperatures. Customer queries are usually about cables that must operate reliably at - 5 °C. Therefore, for more than four years, igus has been offering an oil-resistant PVC mixture, which has a high abrasion resistance and a wide temperature range. This is a novelty in the market, because normal PVC mixtures for cables that are suitable for use in chains do not meet these requirements. Another advantage is this: with rather moderate temperatures, it is not absolutely necessary to resort to using more expensive jacket materials such as PUR or TPR. For even more extreme movements, cables with more complex structures are used. The so-called robot cables are mainly used in industrial robots and have to follow extreme movements, under both bending and twisting. Special damping elements allow the cores the necessary freedom of movement within the cable. The tighter the cable is wound the more difficult it becomes to twist the cable. Special shields and outer materials also ensure optimal cable durability under torsion. Calculating cable service life The service life of a cable used in energy chains is dependent on a number of variables that must be considered in both design and material selection. Hence the chainflex product family currently offers 1244 different cables. How long the cables will last in a given application can be calculated by users themselves, because the test findings of over 2 billion test cycles from the laboratory are incorporated every year into a database, which forms the basis of a freely accessible online tool for service life calculation on the Igus website. Photographs: Igus 03 Temperature profiles from - 40 to + 60 °C can be simulated in the cold chamber Avoiding tension and compression The findings from all ongoing tests at igus over the past 25 years are incorporated in the development of its own, steadily growing range of cables. Beside the many new materials, this led to the introduction of bundled designs, a similar concept to that used in steel cables. In a complex process, wires are wound in single bundles consisting of three, four or five wires, and these are then wound with each other to make an braided bundle. For large wound structures, this is done around a strain relief element. The result is a cable that is designed for constant motion and absolutely compatible with chains, because – unlike layer-stranded cables – each of the cores moves same amount on the inner and outer radius avoiding inner cores getting compressed or outer cores getting stretched. AUTOMATION AUTOMATION TECHNOLOGIES TECHNOLOGIES 5/2016 5/2016 27


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