Why selecting the right materials for moving cables is essential
03 August 2017
Proper material selection is very important where the service life of cables is concerned, especially cables in moving applications. Justin Leonard, the e-chain director at igus, considers the durability and lifetime of cables in energy chains.
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Careful matching of cables with the chosen energy chain in a system is of vital importance. The plastics used, in addition to the right conductor materials, play a central role in the interaction of chain and cable, combined with the external factors that may have a negative impact, such as temperature, media and radiation.
In the cable industry, some well-known test procedures come from different normative institutes and are very good. However, they do not relate to the special requirements of continuous motion in an energy chain. Neither the bending cycle test according to VDE, nor the standard abrasion test meet the requirements for testing a real chain-cable combination. In the case of the bending cycle test, the sequence of movements of the cable are completely different to movements in an energy chain. Many cable designs that have reliably met these requirements failed in a very short time in an e-chain movement test carried out according to the igus standard.
Sandpaper, needles and razor blades
Standard abrasion tests, which determine the ability of the cable jacket material to resist surface wear use sandpaper, needles or razor blades, and are fine for general applications. In these tests, the sandpaper, razor blade or needle is applied to the cable jacket material under pressure, and dragged along its surface with a defined number of movements. However, these tests do not replicate the real-world situation of how well cables perform in energy chains and therefore may not indicate the durability of the jacket material in an energy chain accurately. What is much more important is to test and match the relative movement between the chain material and cable material.
In the igus test laboratory, the jacket materials are tested for wear in combination with the materials of the energy chains. To determine the amount of wear, jacket materials are rubbed directly against chain materials. These realistic tests enable igus to match the materials to each other in the best possible way and thus minimise the effects of wear and functional reliability.
Continuous development of specialist PVC materials
Media (gas, water, etc.) and different temperature influences also play a central role in the development and testing of jacket materials for continuously moving cables. In the cable industry, for example, the cold bending test 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 then cooling them down to the applicable test temperature. The diameter of the mandrel is adapted to the diameter of the cable to be tested. Once the cable reaches the required test temperature, the cable is unwound from the mandrel. The cable passes the test if no jacket fractures are visible. After passing the test, this jacket material is regarded as capable of being used at the temperature applied during testing.
However, practice has shown that these tests do not simulate the real conditions in an energy chain at low temperatures. Four years ago, igus decided to develop and test an oil-resistant PVC material that performs its function reliably in a chain at temperatures down to -5°C. The reason is that normal PVC jacket materials for cables that are suitable for use in chains do not meet these requirements up to the present day. The challenge was to develop a new jacket material that was also highly resistant to oil and wear over a wide range of temperatures.
A large number of bending, wear and cold bending tests with a large variety of compounds, as well as the manufacture of numerous prototypes, were necessary in order to finally develop a material with these properties.
Realistic test procedures in the climatic test chamber
In this test, in contrast to the cold bending, the cables continuously move in an energy chain at the test temperatures under realistic conditions - this is not done just once but millions of times.
During the test, the cables are moved continuously in e-chains placed in a climatic test chamber in which temperatures of between -40 and +60°C can be set. Here, they are subjected to continuous bending stress in addition to temperature stress. A cable passes the test if it has no jacket fractures.
The negative test results displayed by jacket materials currently available on the market, e.g. jacket fractures and cracks, occurred after only a few thousand strokes - only the newly developed igus PVC jacket material passed this strict test procedure. This material, especially developed for igus, is now the standard oil-resistant PVC material used on chainflex cables. This provides machine builders assurance that the PVC cables will operate reliably and safely even at 0°C.
Not all jacket materials make the grade
During a series of tests with other materials in the climatic test chamber, it was found that none of the jacket materials available on the market, not even PUR, was able to withstand the indicated temperatures when used on cables that move continuously in energy chains. The result was that igus is now the very first cable supplier to define three different bending and temperature ranges for the jacket materials it uses. Firstly, for stationary cables, secondly for flexibly moving cables in accordance with VDE or IEC standards and, thirdly, for cables moving in energy chains. After all, one thing is certain: only real long-term tests under realistic conditions provide reliable information on the reliability and service life of cables in energy chains.
About the author:
Justin Leonard is director at igus, responsible for the development and management of the plastic energy chains and chainflex cables business roadmap in the UK. A graduate engineer, responsibilities during his 15-year career with igus started with external technical sales then developed into product and project management, before assuming responsibility for the whole divison.
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